Inertial frictional ratchets and their load bearing efficiencies

NASA Astrophysics Data System (ADS)

Kharkongor, D.; Reenbohn, W. L.; Mahato, Mangal C.

2018-03-01

We investigate the performance of an inertial frictional ratchet in a sinusoidal potential driven by a sinusoidal external field. The dependence of the performance on the parameters of the sinusoidally varying friction, such as the mean friction coefficient and its phase difference with the potential, is studied in detail. Interestingly, under certain circumstances, the thermodynamic efficiency of the ratchet against an applied load shows a non-monotonic behaviour as a function of the mean friction coefficient. Also, in the large friction ranges, the efficiency is shown to increase with increasing applied load even though the corresponding ratchet current decreases as the applied load increases. These counterintuitive numerical results are explained in the text.

The effect of chalk on the finger-hold friction coefficient in rock climbing.

PubMed

Amca, Arif Mithat; Vigouroux, Laurent; Aritan, Serdar; Berton, Eric

2012-11-01

The main purpose of this study was to examine the effect of chalk on the friction coefficient between climber's fingers and two different rock types (sandstone and limestone). The secondary purpose was to investigate the effects of humidity and temperature on the friction coefficient and on the influence of chalk. Eleven experienced climbers took part in this study and 42 test sessions were performed. Participants hung from holds which were fixed on a specially designed hang board. The inclination of the hang board was progressively increased until the climber's hand slipped from the holds. The angle of the hang board was simultaneously recorded by using a gyroscopic sensor and the friction coefficient was calculated at the moment of slip. The results showed that there was a significant positive effect of chalk on the coefficient of friction (+18.7% on limestone and +21.6% on sandstone). Moreover sandstone had a higher coefficient of friction than limestone (+15.6% without chalk, +18.4% with chalk). These results confirmed climbers' belief that chalk enhances friction. However, no correlation with humidity/temperature and friction coefficient was noted which suggested that additional parameters should be considered in order to understand the effects of climate on finger friction in rock climbing.

Friction coefficient of spruce pine on steel -- a note on lubricants

Treesearch

Charles W. McMillin; Truett J. Lemoine; Floyd G. Manwiller

1970-01-01

Generally, the introduction of water and ethanol increased the friction coefficient for ovendry samples but decreased the coeffecient when the samples were saturated. Octanoic acid decreased the coefficient when samples were wet. In the entire experiment, coefficients ranged from 0.14 to 0.78.

Estimation of Road Friction Coefficient in Different Road Conditions Based on Vehicle Braking Dynamics

NASA Astrophysics Data System (ADS)

Zhao, You-Qun; Li, Hai-Qing; Lin, Fen; Wang, Jian; Ji, Xue-Wu

2017-07-01

The accurate estimation of road friction coefficient in the active safety control system has become increasingly prominent. Most previous studies on road friction estimation have only used vehicle longitudinal or lateral dynamics and often ignored the load transfer, which tends to cause inaccurate of the actual road friction coefficient. A novel method considering load transfer of front and rear axles is proposed to estimate road friction coefficient based on braking dynamic model of two-wheeled vehicle. Sliding mode control technique is used to build the ideal braking torque controller, which control target is to control the actual wheel slip ratio of front and rear wheels tracking the ideal wheel slip ratio. In order to eliminate the chattering problem of the sliding mode controller, integral switching surface is used to design the sliding mode surface. A second order linear extended state observer is designed to observe road friction coefficient based on wheel speed and braking torque of front and rear wheels. The proposed road friction coefficient estimation schemes are evaluated by simulation in ADAMS/Car. The results show that the estimated values can well agree with the actual values in different road conditions. The observer can estimate road friction coefficient exactly in real-time and resist external disturbance. The proposed research provides a novel method to estimate road friction coefficient with strong robustness and more accurate.

Surface deformation and friction characteristic of nano scratch at ductile-removal regime for optical glass BK7.

PubMed

Li, Chen; Zhang, Feihu; Ding, Ye; Liu, Lifei

2016-08-20

Nano scratch for optical glass BK7 based on the ductile-removal regime was carried out, and the influence rule of scratch parameters on surface deformation and friction characteristic was analyzed. Experimental results showed that, with increase of normal force, the deformation of burrs in the edge of the scratch was more obvious, and with increase of the scratch velocity, the deformation of micro-fracture and burrs in the edge of the scratch was more obvious similarly. The residual depth of the scratch was measured by atomic force microscope. The experimental results also showed that, with increase of normal force, the residual depth of the scratch increased linearly while the elastic recovery rate decreased. Furthermore, with increase of scratch velocity, the residual depth of the scratch decreased while the elastic recovery rate increased. The scratch process of the Berkovich indenter was divided into the cutting process of many large negative rake faces based on the improved cutting model, and the friction characteristic of the Berkovich indenter and the workpiece was analyzed. The analysis showed that the coefficient of friction increased and then tended to be stable with the increase of normal force. Meanwhile, the coefficient of friction decreased with the increase of scratch velocity, and the coefficients, k ln(v) and μ₀, were introduced to improve the original formula of friction coefficient.

Effect of friction on vibrotactile sensation of normal and dehydrated skin.

PubMed

Chen, S; Ge, S; Tang, W; Zhang, J

2016-02-01

Vibrotactile sensation mediated is highly dependent on surface mechanical and frictional properties. Dehydration of skin could change these properties. To investigate the relationship between friction and vibrotactile sensation of normal and dehydrated skin. Vibrations were firstly measured during surface exploration using a biomimetic sensor. Piglet skin was used as human skin model to study frictional properties for both normal and dehydrated skin using an atomic force microscope on nanoscale and a pin-on-disk tribometer on macroscale. Effect of vibrational frequency on friction and vibrotactile perception was also observed on nano and macro scale for normal and dehydrated skin. The result indicated that dehydrated skin was less sensitive than normal skin. The coefficient of friction of dehydrated skin is smaller than that of normal skin on both nano and macro scale. The coefficient of friction increases as increasing scanning frequencies. There is a positive correlation between coefficient of friction and vibrotactile sensation on nanoscale and macroscale. © 2015 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.

Influence of Laser Shock Texturing on W9 Steel Surface Friction Property

NASA Astrophysics Data System (ADS)

Fan, Yujie; Cui, Pengfei; Zhou, Jianzhong; Dai, Yibin; Guo, Erbin; Tang, Deye

2017-09-01

To improve surface friction property of high speed steel, micro-dent arrays on W9Mo3Cr4V surface were produced by laser shock processing. Friction test was conducted on smooth surface and texturing surface and effect of surface texturing density on friction property was studied. The results show that, under the same condition, friction coefficient of textured surface is lower than smooth surface with dent area density less than 6%, wear mass loss, width and depth of wear scar are smaller; Wear resistance of the surface is the best and the friction coefficient is the smallest when dent area density is 2.2%; Friction coefficient, wear mass loss, width and depth of wear scar increase correspondingly as density of dent area increases when dent area density is more than 2.2%. Abrasive wear and adhesive wear, oxidative wear appear in the wear process. Reasonable control of geometric parameters of surface texturing induced by laser shock processing is helpful to improve friction performance.

Study on the friction of κ-carrageenan hydrogels in air and aqueous environments.

PubMed

Kozbial, Andrew; Li, Lei

2014-03-01

Understanding the friction mechanism of polysaccharide hydrogels, which is the key component of human cartilage that has very low friction coefficient, is critical to develop next generation artificial joint replacement materials. In this study, the friction of the polysaccharide κ-carrageenan hydrogel was investigated to elucidate the effect of external load, cross-linking density, velocity, and environment on friction. Our experimental results show that (1) coefficient of friction (COF) decreases with normal load in air and remains constant in water, (2) increasing cross-linking density concurrently increases friction and is proportional to Young's modulus, (3) COF increases with testing velocity in both air and water, and (4) friction is reduced in aqueous environment due to the lubricating effect of water. The underlying frictional mechanism is discussed on the basis of water transport from bulk to surface and a previously proposed "repulsion-adsorption" model. Copyright © 2013 Elsevier B.V. All rights reserved.

Frictional properties of single crystals HMX, RDX and PETN explosives.

PubMed

Wu, Y Q; Huang, F L

2010-11-15

The frictional properties of single crystals of cyclotetramethylene tetranitramine (HMX), cyclotrimethylene trinitramine (RDX) and pentaerythritol tetranitrate (PETN) secondary explosives are examined using a sensitive friction machine. The explosive crystals used for the measurements are at least 3.5 mm wide. The friction coefficients between crystals of the same explosive (i.e., HMX on HMX, etc.), crystals of different explosives (i.e., HMX on RDX, etc.), and each explosive and a well-polished gauge steel surface are determined. The frictional surfaces are also studied under an environmental scanning electron microscope (ESEM) to analyze surface microstructural changes under increasing loading forces. The friction coefficients vary considerably with increasing normal loading forces and are particularly sensitive to slider shapes, crystal roughness and the mechanical properties of both the slider and the sample. With increasing loading forces, most friction experiments show surface damage, consisting of grooves, debris, and nano-particles, on both the slider and sample. In some cases, a strong evidence of a localized molten state is found in the central region of the friction track. Possible mechanisms that affect the friction coefficient are discussed based on microscopic observations. Copyright © 2010 Elsevier B.V. All rights reserved.

Static coefficient of friction between stainless steel and PMMA used in cemented hip and knee implants.

PubMed

Nuño, N; Groppetti, R; Senin, N

2006-11-01

Design of cemented hip and knee implants, oriented to improve the longevity of artificial joints, is largely based on numerical models. The static coefficient of friction between the implant and the bone cement is necessary to characterize the interface conditions in these models and must be accurately provided. The measurement of this coefficient using a repeatable and reproducible methodology for materials used in total hip arthroplasty is missing from the literature. A micro-topographic surface analysis characterized the surfaces of the specimens used in the experiments. The coefficient of friction between stainless steel and bone cement in dry and wet conditions using bovine serum was determined using a prototype computerized sliding friction tester. The effects of surface roughness (polished versus matt) and of contact pressure on the coefficient of friction have also been investigated. The serum influences little the coefficient of friction for the matt steel surface, where the mechanical interactions due to higher roughness are still the most relevant factor. However, for polished steel surfaces, the restraining effect of proteins plays a very relevant role in increasing the coefficient of friction. When the coefficient of friction is used in finite element analysis, it is used for the debonded stem-cement situation. It can thus be assumed that serum will propagate between the stem and the cement mantle. The authors believe that the use of a static coefficient of friction of 0.3-0.4, measured in the present study, is appropriate in finite element models.

Adhesion and friction of iron-base binary alloys in contact with silicon carbide in vacuum

NASA Technical Reports Server (NTRS)

Miyoshi, K.; Buckley, D. H.

1980-01-01

Single pass sliding friction experiments were conducted with various iron base binary alloys (alloying elements were Ti, Cr, Mn, Ni, Rh, and W) in contact with a single crystal silicon carbide /0001/ surface in vacuum. Results indicate that atomic size and concentration of alloying elements play an important role in controlling adhesion and friction properties of iron base binary alloys. The coefficient of friction generally increases with an increase in solute concentration. The coefficient of friction increases linearly as the solute to iron atomic radius ratio increases or decreases from unity. The chemical activity of the alloying elements was also an important parameter in controlling adhesion and friction of alloys, as these latter properties are highly dependent upon the d bond character of the elements.

Characterization of Skin Friction Coefficient, and Relationship to Stratum Corneum Hydration in a Normal Chinese Population

PubMed Central

Zhu, Y.H.; Song, S.P.; Luo, W.; Elias, P.M.; Man, M.Q.

2011-01-01

Background and Objectives Studies have demonstrated that some cutaneous biophysical properties vary with age, gender and body sites. However, the characteristics of the skin friction coefficient in different genders and age groups have not yet been well established. In the present study, we assess the skin friction coefficient in a larger Chinese population. Methods A total of 633 subjects (300 males and 333 females) aged 0.15–79 years were enrolled. A Frictiometer® FR 770 and Corneometer® CM 825 (C&K MPA 5) were used to measure the skin friction coefficient and stratum corneum hydration, respectively, on the dorsal surface of the hand, the forehead and the canthus. Results In the females, the maximum skin friction coefficients on both the canthus and the dorsal hand skin were observed around the age of 40 years. In the males, the skin friction coefficient on the dorsal hand skin gradually increased from 0 to 40 years of age, and changed little afterward. Skin friction coefficients on some body sites were higher in females than in age-matched males in some age groups. On the canthus and the dorsal hand skin of females, a positive correlation was found between skin friction coefficient and stratum corneum hydration (p < 0.001 and p < 0.0001, respectively). In contrast, in males, the skin friction coefficient was positively correlated with stratum corneum hydration on the forehead and the dorsal hand skin (p < 0.05 and p < 0.0001, respectively). Conclusion The skin friction coefficient varies with age, gender and body site, and positively correlates with stratum corneum hydration on some body sites. PMID:21088455

Characterization of skin friction coefficient, and relationship to stratum corneum hydration in a normal Chinese population.

PubMed

Zhu, Y H; Song, S P; Luo, W; Elias, P M; Man, M Q

2011-01-01

Studies have demonstrated that some cutaneous biophysical properties vary with age, gender and body sites. However, the characteristics of the skin friction coefficient in different genders and age groups have not yet been well established. In the present study, we assess the skin friction coefficient in a larger Chinese population. A total of 633 subjects (300 males and 333 females) aged 0.15-79 years were enrolled. A Frictiometer FR 770 and Corneometer CM 825 (C&K MPA 5) were used to measure the skin friction coefficient and stratum corneum hydration, respectively, on the dorsal surface of the hand, the forehead and the canthus. In the females, the maximum skin friction coefficients on both the canthus and the dorsal hand skin were observed around the age of 40 years. In the males, the skin friction coefficient on the dorsal hand skin gradually increased from 0 to 40 years of age, and changed little afterward. Skin friction coefficients on some body sites were higher in females than in age-matched males in some age groups. On the canthus and the dorsal hand skin of females, a positive correlation was found between skin friction coefficient and stratum corneum hydration (p < 0.001 and p < 0.0001, respectively). In contrast, in males, the skin friction coefficient was positively correlated with stratum corneum hydration on the forehead and the dorsal hand skin (p < 0.05 and p < 0.0001, respectively). The skin friction coefficient varies with age, gender and body site, and positively correlates with stratum corneum hydration on some body sites. Copyright © 2010 S. Karger AG, Basel.

The development of the friction coefficient inspection equipment for skin using a load cell.

PubMed

Song, Han Wook; Park, Yon Kyu; Lee, Sung Jun; Woo, Sam Yong; Kim, Sun Hyung; Kim, Dal Rae

2008-01-01

The skin is an indispensible organ for human because it contributes to the metabolism using its own biochemical functions as well as it protects the human body from the exterior stimuli. Recently, the friction coefficient have been used as the decision index of the progress for the bacterial aliments in the field of the skin physiology and the importance of friction coefficient have been increased in the skin care market because of the needs of the well being times. In addition, the usage of friction coefficient is known to have the big discrimination ability in classification of human constitutions, which is utilized in the alternative medicine. In this study, we designed a system which used the multi axes load cell and hemi-circular probe and tried to measure the friction coefficient of hand skins repeatedly. Using this system, the relative repeatability error for the measurement of the friction coefficient was below 4 %. The coefficient is not concerned in curvatures of tips. Using this system, we will try to establish the standard for classification of constitutions.

Water-vapor effects on friction of magnetic tape in contact with nickel-zinc ferrite

NASA Technical Reports Server (NTRS)

Miyoshi, K.; Buckley, D. H.

1984-01-01

The effects of humidity of moist nitrogen on the friction and deformation behavior of magnetic tape in contact with a nickel-zinc ferrite spherical pin were studied. The results indicate that the coefficient of friction is markedly dependent on the ambient relative humidity. Although the coefficient of friction remains low below 40-percent relative humidity, it increases rapidly with increasing relative humidity above 40 percent. The general ambient environment of the tape does not have any effect on the friction behavior if the area where the tape is in sliding contact with the ferrite pin is flooded with controlled nitrogen. The response time for the friction of the tape to humidity changes is about 10 sec. The effect of friction as a function of relative humidity on dehumidifying is very similar to that on humidifying. A surface softening of the tape due to water vapor increases the friction of the tape.

The effect of friction in coulombian damper

NASA Astrophysics Data System (ADS)

Wahad, H. S.; Tudor, A.; Vlase, M.; Cerbu, N.; Subhi, K. A.

2017-02-01

The study aimed to analyze the damping phenomenon in a system with variable friction, Stribeck type. Shock absorbers with limit and dry friction, is called coulombian shock-absorbers. The physical damping vibration phenomenon, in equipment, is based on friction between the cushioning gasket and the output regulator of the shock-absorber. Friction between them can be dry, limit, mixture or fluid. The friction is depending on the contact pressure and lubricant presence. It is defined dimensionless form for the Striebeck curve (µ friction coefficient - sliding speed v). The friction may damp a vibratory movement or can maintain it (self-vibration), depending on the µ with v (it can increase / decrease or it can be relative constant). The solutions of differential equation of movement are obtained for some work condition of one damper for automatic washing machine. The friction force can transfer partial or total energy or generates excitation energy in damper. The damping efficiency is defined and is determined analytical for the constant friction coefficient and for the parabolic friction coefficient.

Breakdown of Amontons' Law of Friction in Sheared-Elastomer with Local Amontons' Friction

NASA Astrophysics Data System (ADS)

Matsukawa, Hiroshi; Otsuki, Michio

2012-02-01

It is well known that Amontons' law of friction i.e. the frictional force against the sliding motion of solid object is proportional to the loading force and not dependent on the contact area, holds well for various systems. Here we show, however, the breakdown of the Amontons' law for the elastic object which have local friction obeying Amontons' law and is under uniform pressure by FEM calculation The external shearing force applied to the trailing edge of the sample induces local slip. The range of the slip increases with the increasing external force adiabatically at first. When the range reaches the critical magnitude, the slips moves rapidly and reaches the leading edge of the sample then the whole system slides. These behaviors are consistent with the experiment by Rubinstein et.al. (Phys. Rev. Lett. 98, 226103). The static frictional coefficient, the ratio between the static frictional force for the whole system and the loading force, decreases with the increasing pressure. This means the breakdown of Amontons' law. The pressure dependence of the frictional coefficient is caused by the change of the critical length of the local slip. The behaviors of the local slip and the frictional coefficient are well explained by the 1 dimensional model analytically.

Numerical study of the influence of geometrical characteristics of a vertical helical coil on a bubbly flow

NASA Astrophysics Data System (ADS)

Saffari, H.; Moosavi, R.

2014-11-01

In this article, turbulent single-phase and two-phase (air-water) bubbly fluid flows in a vertical helical coil are analyzed by using computational fluid dynamics (CFD). The effects of the pipe diameter, coil diameter, coil pitch, Reynolds number, and void fraction on the pressure loss, friction coefficient, and flow characteristics are investigated. The Eulerian-Eulerian model is used in this work to simulate the two-phase fluid flow. Three-dimensional governing equations of continuity, momentum, and energy are solved by using the finite volume method. The k- ɛ turbulence model is used to calculate turbulence fluctuations. The SIMPLE algorithm is employed to solve the velocity and pressure fields. Due to the effect of a secondary force in helical pipes, the friction coefficient is found to be higher in helical pipes than in straight pipes. The friction coefficient increases with an increase in the curvature, pipe diameter, and coil pitch and decreases with an increase in the coil diameter and void fraction. The close correlation between the numerical results obtained in this study and the numerical and empirical results of other researchers confirm the accuracy of the applied method. For void fractions up to 0.1, the numerical results indicate that the friction coefficient increases with increasing the pipe diameter and keeping the coil pitch and diameter constant and decreases with increasing the coil diameter. Finally, with an increase in the Reynolds number, the friction coefficient decreases, while the void fraction increases.

X-ray photoelectron spectroscopy and friction studies of nickel-zinc and manganese-zinc ferrites in contact with metals

NASA Technical Reports Server (NTRS)

Miyoshi, K.; Buckley, D. H.

1983-01-01

X-ray photoelectron spectroscopy analysis and sliding friction experiments were conducted with hot-pressed, polycrystalline Ni-Zn and Mn-Zn ferrites in sliding contact with various transition metals at room temperature in a vacuum of 30 nPa. The results indicate that the coefficients of friction for Ni-Zn and Mn-Zn ferrites in contact with metals are related to the relative chemical activity in these metals: the more active the metal, the higher is the coefficient of friction. The coefficients of friction for the ferrites correlate with the free energy of formation of the lowest metal oxide. The interfacial bond can be regarded as a chemical bond between the metal atoms and the oxygen anions in the ferrite surfaces. The adsorption of oxygen on clean metal and ferrite surfaces increases the coefficients of friction for the Ni-Zn and Mn-Zn ferrite-metal interfaces.

Description of new dry granular materials of variable cohesion and friction coefficient: Implications for laboratory modeling of the brittle crust

NASA Astrophysics Data System (ADS)

Abdelmalak, M. M.; Bulois, C.; Mourgues, R.; Galland, O.; Legland, J.-B.; Gruber, C.

2016-08-01

Cohesion and friction coefficient are fundamental parameters for scaling brittle deformation in laboratory models of geological processes. However, they are commonly not experimental variable, whereas (1) rocks range from cohesion-less to strongly cohesive and from low friction to high friction and (2) strata exhibit substantial cohesion and friction contrasts. This brittle paradox implies that the effects of brittle properties on processes involving brittle deformation cannot be tested in laboratory models. Solving this paradox requires the use of dry granular materials of tunable and controllable brittle properties. In this paper, we describe dry mixtures of fine-grained cohesive, high friction silica powder (SP) and low-cohesion, low friction glass microspheres (GM) that fulfill this requirement. We systematically estimated the cohesions and friction coefficients of mixtures of variable proportions using two independent methods: (1) a classic Hubbert-type shear box to determine the extrapolated cohesion (C) and friction coefficient (μ), and (2) direct measurements of the tensile strength (T0) and the height (H) of open fractures to calculate the true cohesion (C0). The measured values of cohesion increase from 100 Pa for pure GM to 600 Pa for pure SP, with a sub-linear trend of the cohesion with the mixture GM content. The two independent cohesion measurement methods, from shear tests and tension/extensional tests, yield very similar results of extrapolated cohesion (C) and show that both are robust and can be used independently. The measured values of friction coefficients increase from 0.5 for pure GM to 1.05 for pure SP. The use of these granular material mixtures now allows testing (1) the effects of cohesion and friction coefficient in homogeneous laboratory models and (2) testing the effect of brittle layering on brittle deformation, as demonstrated by preliminary experiments. Therefore, the brittle properties become, at last, experimental variables.

Tribological properties of hydrophilic polymer brushes under wet conditions.

PubMed

Kobayashi, Motoyasu; Takahara, Atsushi

2010-08-01

This article demonstrates a water-lubrication system using high-density hydrophilic polymer brushes consisting of 2,3-dehydroxypropyl methacrylate (DHMA), vinyl alcohol, oligo(ethylene glycol)methyl ether methacrylate, 2-(methacryloyloxy)ethyltrimethylammonium chloride (MTAC), 3-sulfopropyl methacrylate potassium salt (SPMK), and 2-methacryloyloxyethyl phosphorylcholine (MPC) prepared by surface-initiated controlled radical polymerization. Macroscopic frictional properties of brush surfaces were characterized by sliding a glass ball probe in water using a ball-on-plate type tribotester under the load of 0.1-0.49 N at the sliding velocity of 10(-5)-10(-1) m s(-1) at 298 K. A poly(DHMA) brush showed a relatively larger friction coefficient in water, whereas the polyelectrolyte brushes, such as poly(SPMK) and poly(MPC), revealed significantly low friction coefficients below 0.02 in water and in humid air conditions. A drastic reduction in the friction coefficient of polyelectrolyte brushes in aqueous solution was observed at around 10(-3)-10(-2) m s(-1) owing to the hydrodynamic lubrication effect, however, an increase in salt concentration in the aqueous solution led to the increase in the friction coefficients of poly(MTAC) and poly(SPMK) brushes. The poly(SPMK) brush showed a stable and low friction coefficient in water even after sliding over 450 friction cycles, indicating a good wear resistance of the brush film. Copyright 2010 The Japan Chemical Journal Forum and Wiley Periodicals, Inc.

Adhesion, friction, and wear of binary alloys in contact with single-crystal silicon carbide

NASA Technical Reports Server (NTRS)

Miyoshi, K.; Buckley, D. H.

1980-01-01

Sliding friction experiments, conducted with various iron base alloys (alloying elements are Ti, Cr, Mn, Ni, Rh and W) in contact with a single crystal silicon carbide /0001/ surface in vacuum are discussed. Results indicate atomic size misfit and concentration of alloying elements play a dominant role in controlling adhesion, friction, and wear properties of iron-base binary alloys. The controlling mechanism of the alloy properties is as an intrinsic effect involving the resistance to shear fracture of cohesive bonding in the alloy. The coefficient of friction generally increases with an increase in solute concentration. The coefficient of friction increases as the solute-to-iron atomic radius ratio increases or decreases from unity. Alloys having higher solute concentration produce more transfer to silicon carbide than do alloys having low solute concentrations. The chemical activity of the alloying element is also an important parameter in controlling adhesion and friction of alloys.

Behavior of aircraft antiskid braking systems on dry and wet runway surfaces - A velocity-rate-controlled, pressure-bias-modulated system

NASA Technical Reports Server (NTRS)

Stubbs, S. M.; Tanner, J. A.

1976-01-01

During maximum braking the average ratio of drag-force friction coefficient developed by the antiskid system to maximum drag-force friction coefficient available at the tire/runway interface was higher on dry surfaces than on wet surfaces. The gross stopping power generated by the brake system on the dry surface was more than twice that obtained on the wet surfaces. With maximum braking applied, the average ratio of side-force friction coefficient developed by the tire under antiskid control to maximum side-force friction available at the tire/runway interface of a free-rolling yawed tire was shown to decrease with increasing yaw angle. Braking reduced the side-force friction coefficient on a dry surface by 75 percent as the wheel slip ratio was increased to 0.3; on a flooded surface the coefficient dropped to near zero for the same slip ratio. Locked wheel skids were observed when the tire encountered a runway surface transition from dry to flooded, due in part to the response time required for the system to sense abrupt changes in the runway friction; however, the antiskid system quickly responded by reducing brake pressure and cycling normally during the remainder of the run on the flooded surface.

Friction and hardness of gold films deposited by ion plating and evaporation

NASA Technical Reports Server (NTRS)

Miyoshi, K.; Spalvins, T.; Buckley, D. H.

1983-01-01

Sliding friction experiments were conducted with ion-plated and vapor-deposited gold films on various substrates in contact with a 0.025-mm-radius spherical silicon carbide rider in mineral oil. Hardness measurements were also made to examine the hardness depth profile of the coated gold on the substrate. The results indicate that the hardness is influenced by the depth of the gold coating from the surface. The hardness increases with an increase in the depth. The hardness is also related to the composition gradient in the graded interface between the gold coating and the substrate. The graded interface exhibited the highest hardness resulting from an alloy hardening effect. The coefficient of friction is inversely related to the hardness, namely, the load carrying capacity of the surface. The greater the hardness that the metal surface possesses, the lower is the coefficient of friction. The graded interface exhibited the lowest coefficient of friction.

Why Low Bounce Balls Exhibit High Rolling Resistance

ERIC Educational Resources Information Center

Cross, Rod

2015-01-01

A simple experiment is described to measure the coefficient of rolling friction for a low bounce ball rolling on a horizontal surface. As observed previously by others, the coefficient increased with rolling speed. The energy loss due to rolling friction can be explained in terms of the measured coefficient of restitution for the ball, meaning…

Influence of moisture content on physical properties of minor millets.

PubMed

Balasubramanian, S; Viswanathan, R

2010-06-01

Physical properties including 1000 kernel weight, bulk density, true density, porosity, angle of repose, coefficient of static friction, coefficient of internal friction and grain hardness were determined for foxtail millet, little millet, kodo millet, common millet, barnyard millet and finger millet in the moisture content range of 11.1 to 25% db. Thousand kernel weight increased from 2.3 to 6.1 g and angle of repose increased from 25.0 to 38.2°. Bulk density decreased from 868.1 to 477.1 kg/m(3) and true density from 1988.7 to 884.4 kg/m(3) for all minor millets when observed in the moisture range of 11.1 to 25%. Porosity decreased from 63.7 to 32.5%. Coefficient of static friction of minor millets against mild steel surface increased from 0.253 to 0.728 and coefficient of internal friction was in the range of 1.217 and 1.964 in the moisture range studied. Grain hardness decreased from 30.7 to 12.4 for all minor millets when moisture content was increased from 11.1 to 25% db.

Discrete Element Method Simulations of the Inter-Particle Contact Parameters for the Mono-Sized Iron Ore Particles.

PubMed

Li, Tongqing; Peng, Yuxing; Zhu, Zhencai; Zou, Shengyong; Yin, Zixin

2017-05-11

Aiming at predicting what happens in reality inside mills, the contact parameters of iron ore particles for discrete element method (DEM) simulations should be determined accurately. To allow the irregular shape to be accurately determined, the sphere clump method was employed in modelling the particle shape. The inter-particle contact parameters were systematically altered whilst the contact parameters between the particle and wall were arbitrarily assumed, in order to purely assess its impact on the angle of repose for the mono-sized iron ore particles. Results show that varying the restitution coefficient over the range considered does not lead to any obvious difference in the angle of repose, but the angle of repose has strong sensitivity to the rolling/static friction coefficient. The impacts of the rolling/static friction coefficient on the angle of repose are interrelated, and increasing the inter-particle rolling/static friction coefficient can evidently increase the angle of repose. However, the impact of the static friction coefficient is more profound than that of the rolling friction coefficient. Finally, a predictive equation is established and a very close agreement between the predicted and simulated angle of repose is attained. This predictive equation can enormously shorten the inter-particle contact parameters calibration time that can help in the implementation of DEM simulations.

Tribological behaviors of UHMWPE composites with different counter surface morphologies

NASA Astrophysics Data System (ADS)

Wang, Yanzhen; Yin, Zhongwei; Li, Hulin; Gao, Gengyuan

2017-12-01

The influence of counter surface morphologies on hybrid glass fiber (GF) and carbon fiber (CF) filled ultrahigh molecular weight polyethylene (UHMWPE) were studied under various contact pressure and sliding speed against GCr15 steel in dry condition. The goals were to investigate the tribological behavior of GF/CF/UHMWPE composite as a kind of water lubricated journal bearing material. The friction and wear behavior of composites were examined using a pin-on-disc tribometer. The morphologies of the worn surface were examined by scanning electron microscopy (SEM) and laser 3D micro-imaging and profile measurement. Generally, the wear rate and friction coefficient of composites increase as the increment of counter surface roughness. The friction coefficient increases firstly and then decrease with an increase in sliding speed and contact pressure for counterface with Ra=0.2 and 3.5 μm, while the friction coefficient decreased for counterface with Ra=0.6 μm.

Valuation of coefficient of rolling friction by the inclined plane method

NASA Astrophysics Data System (ADS)

Ciornei, F. C.; Alaci, S.; Ciogole, V. I.; Ciornei, M. C.

2017-05-01

A major objective of tribological researches is characterisation of rolling friction, due to various cases encountered in classical engineering applications, like gear transmissions and cam mechanisms or more recent examples met in bioengineering and biomedical devices. A characteristic of these examples consists in reduced dimensions of the contact zones, theoretically zero, the relative motion occurring between the contact points being either sliding or rolling. A characteristic parameter for the rolling motion is the coefficient of rolling friction. The paper proposes a method for estimation of coefficient of rolling friction by studying the motion of a body of revolution on an inclined plane. Assuming the hypothesis that moment of rolling friction is proportional to the normal reaction force, the law of motion for the body on the inclined plane is found under the premise of pure rolling. It is reached the conclusion that there is an uniformly accelerated motion, and thus for a known plane slope, it is sufficient to find the time during which the body runs a certain distance, starting from motionless situation. To obtain accurate results assumes finding precisely the time of motion. The coefficient of rolling friction was estimated for several slopes of the inclined plane and it is concluded that with increased slope, the values of coefficient of rolling friction increase, fact that suggest that the proportionality between the rolling torque and normal load is valid only for domains of limited variations of normal load.

The Influence of Friction Between Football Helmet and Jersey Materials on Force: A Consideration for Sport Safety.

PubMed

Rossi, Anthony M; Claiborne, Tina L; Thompson, Gregory B; Todaro, Stacey

2016-09-01

The pocketing effect of helmet padding helps to dissipate forces experienced by the head, but if the player's helmet remains stationary in an opponent's shoulder pads, the compressive force on the cervical spine may increase. To (1) measure the coefficient of static friction between different football helmet finishes and football jersey fabrics and (2) calculate the potential amount of force on a player's helmet due to the amount of friction present. Cross-sectional study. Laboratory. Helmets with different finishes and different football jersey fabrics. The coefficient of friction was determined for 2 helmet samples (glossy and matte), 3 football jerseys (collegiate, high school, and youth), and 3 types of jersey numbers (silkscreened, sublimated, and stitched on) using the TAPPI T 815 standard method. These measurements determined which helmet-to-helmet, helmet-to-jersey number, and helmet-to-jersey material combination resulted in the least amount of static friction. The glossy helmet versus glossy helmet combination produced a greater amount of static friction than the other 2 helmet combinations (P = .013). The glossy helmet versus collegiate jersey combination produced a greater amount of static friction than the other helmet-to-jersey material combinations (P < .01). The glossy helmet versus silkscreened numbers combination produced a greater amount of static friction than the other helmet-to-jersey number combinations (P < .01). The force of static friction experienced during collisions can be clinically relevant. Conditions with higher coefficients of static friction result in greater forces. In this study, the highest coefficient of friction (glossy helmet versus silkscreened number) could increase the forces on the player's helmet by 3553.88 N when compared with other helmet-to-jersey combinations. Our results indicate that the makeup of helmet and uniform materials may affect sport safety.

Large Electric Field-Enhanced-Hardness Effect in a SiO2 Film

NASA Astrophysics Data System (ADS)

Revilla, Reynier I.; Li, Xiao-Jun; Yang, Yan-Lian; Wang, Chen

2014-03-01

Silicon dioxide films are extensively used in nano and micro-electromechanical systems. Here we studied the influence of an external electric field on the mechanical properties of a SiO2 film by using nanoindentation technique of atomic force microscopy (AFM) and friction force microscopy (FFM). A giant augmentation of the relative elastic modulus was observed by increasing the localized electric field. A slight decrease in friction coefficients was also clearly observed by using FFM with the increase of applied tip voltage. The reduction of the friction coefficients is consistent with the great enhancement of sample hardness by considering the indentation-induced deformation during the friction measurements.

Adhesion, friction, and wear of a copper bicrystal with (111) and (210) grains

NASA Technical Reports Server (NTRS)

Brainard, W. A.; Buckley, D. H.

1973-01-01

Sliding friction experiments were conducted in air with polycrystalline copper and ruby riders sliding against a copper bicrystal. Friction coefficient was measured across the bicrystal surface, and the initiation of adhesive wear was examined with scanning electron microscopy. Results indicate a marked increase in friction coefficient as the copper rider crossed the grain boundary from the (111) plane to the (210) plane of the bicrystal. Adhesion, friction, and initiation of adhesive wear was notably different in the adjacent grains of differing orientation. A slip-band adhesion-generated fracture mechanism for wear particle formation is proposed.

The friction and wear of metals and binary alloys in contact with an abrasive grit of single-crystal silicon carbide

NASA Technical Reports Server (NTRS)

Miyoshi, K.; Buckley, D. H.

1979-01-01

Sliding friction experiments were conducted with various metals and iron-base binary alloys (alloying elements Ti, Cr, Mn, Ni, Rh and W) in contact with single crystal silicon carbide riders. Results indicate that the friction force in the plowing of metal and the groove height (corresponding to the wear volume of the groove) decrease linearly as the shear strength of the bulk metal increases. The coefficient of friction and groove height generally decrease, and the contact pressure increases with an increase in solute content of binary alloys. There appears to be very good correlation of the solute to iron atomic ratio with the decreasing rate of change of coefficient of friction, the decreasing rate of change of groove height and the increasing rate of change of contact pressure with increasing solute content. These rates of change increase as the solute to iron atomic radius ratio increases or decreases from unity.

Ice friction of flared ice hockey skate blades.

PubMed

Federolf, Peter A; Mills, Robert; Nigg, Benno

2008-09-01

In ice hockey, skating performance depends on the skill and physical conditioning of the players and on the characteristics of their equipment. CT Edge have recently designed a new skate blade that angles outward near the bottom of the blade. The objective of this study was to compare the frictional characteristics of three CT Edge blades (with blade angles of 4 degrees, 60, and 8 degrees, respectively) with the frictional characteristics of a standard skate blade. The friction coefficients of the blades were determined by measuring the deceleration of an aluminium test sled equipped with three test blades. The measurements were conducted with an initial sled speed of 1.8 m s(-1) and with a load of 53 kg on each blade. The friction coefficient of the standard blades was 0.0071 (s = 0.0005). For the CT Edge blades with blade angles of 4 degrees, 6 degrees, and 8 degrees, friction coefficients were lower by about 13%, 21%, and 22%, respectively. Furthermore, the friction coefficients decreased with increasing load. The results of this study show that widely accepted paradigms such as "thinner blades cause less friction" need to be revisited. New blade designs might also be able to reduce friction in speed skating, figure skating, bobsledding, and luge.

Dynamics and locomotion of flexible foils in a frictional environment

NASA Astrophysics Data System (ADS)

Wang, Xiaolin; Alben, Silas

2018-01-01

Over the past few decades, oscillating flexible foils have been used to study the physics of organismal propulsion in different fluid environments. Here, we extend this work to a study of flexible foils in a frictional environment. When the foil is oscillated by heaving at one end but is not free to locomote, the dynamics change from periodic to non-periodic and chaotic as the heaving amplitude increases or the bending rigidity decreases. For friction coefficients lying in a certain range, the transition passes through a sequence of N-periodic and asymmetric states before reaching chaotic dynamics. Resonant peaks are damped and shifted by friction and large heaving amplitudes, leading to bistable states. When the foil is free to locomote, the horizontal motion smoothes the resonant behaviours. For moderate frictional coefficients, steady but slow locomotion is obtained. For large transverse friction and small tangential friction corresponding to wheeled snake robots, faster locomotion is obtained. Travelling wave motions arise spontaneously, and move with horizontal speeds that scale as transverse friction coefficient to the power 1/4 and input power that scales as the transverse friction coefficient to the power 5/12. These scalings are consistent with a boundary layer form of the solutions near the foil's leading edge.

Dynamics and locomotion of flexible foils in a frictional environment.

PubMed

Wang, Xiaolin; Alben, Silas

2018-01-01

Over the past few decades, oscillating flexible foils have been used to study the physics of organismal propulsion in different fluid environments. Here, we extend this work to a study of flexible foils in a frictional environment. When the foil is oscillated by heaving at one end but is not free to locomote, the dynamics change from periodic to non-periodic and chaotic as the heaving amplitude increases or the bending rigidity decreases. For friction coefficients lying in a certain range, the transition passes through a sequence of N -periodic and asymmetric states before reaching chaotic dynamics. Resonant peaks are damped and shifted by friction and large heaving amplitudes, leading to bistable states. When the foil is free to locomote, the horizontal motion smoothes the resonant behaviours. For moderate frictional coefficients, steady but slow locomotion is obtained. For large transverse friction and small tangential friction corresponding to wheeled snake robots, faster locomotion is obtained. Travelling wave motions arise spontaneously, and move with horizontal speeds that scale as transverse friction coefficient to the power 1/4 and input power that scales as the transverse friction coefficient to the power 5/12. These scalings are consistent with a boundary layer form of the solutions near the foil's leading edge.

Friction and wear of hydrogenated and hydrogen-free diamond-like carbon films: Relative humidity dependent character

NASA Astrophysics Data System (ADS)

Shi, Jing; Gong, Zhenbin; Wang, Yongfu; Gao, Kaixiong; Zhang, Junyan

2017-11-01

In this study, tribological properties of hydrogenated and hydrogen free diamond-like carbon films at various relative humidity (RH) were investigated to understand the friction mechanism in the presence of water molecules. At normal load of 2N, DLC-H film's friction coefficient was 0.06 at RH14% while DLC film's friction coefficient was 0.19 at RH17%. With the increase of RH, their friction coefficient converged to about 0.15. This character remained unaltered when the normal load was 5N. Results show that low friction of DLC-H film at low RH was attributed to the low shear force aroused by graphitic tribofilm at wear care center. However, the high friction of DLC film was mainly endowed by the high adhesive force aroused by σ dangling bonds. At high RH, solid-to-solid contact was isolated by water molecules confined between the counterfaces, where capillary was a dominant factor for friction. In addition to the capillary force, the absence of tribofilm was also accountable. These two factors lead to the level off of friction coefficient for DLC-H and DLC films. Moreover, for both DLC-H and DLC films, tribo-oxidization was proved to be closely related to wear rate with the assist of H2O molecules during sliding.

Electromechanical imitator of antilock braking modes of wheels with pneumatic tire and its application for the runways friction coefficient measurement

NASA Astrophysics Data System (ADS)

Putov, A. V.; Kopichev, M. M.; Ignatiev, K. V.; Putov, V. V.; Stotckaia, A. D.

2017-01-01

In this paper it is considered a discussion of the technique that realizes a brand new method of runway friction coefficient measurement based upon the proposed principle of measuring wheel braking control for the imitation of antilock braking modes that are close to the real braking modes of the aircraft chassis while landing that are realized by the aircraft anti-skid systems. Also here is the description of the model of towed measuring device that realizes a new technique of runway friction coefficient measuring, based upon the measuring wheel braking control principle. For increasing the repeatability accuracy of electromechanical braking imitation system the sideslip (brake) adaptive control system is proposed. Based upon the Burkhard model and additive random processes several mathematical models were created that describes the friction coefficient arrangement along the airstrip with different qualitative adjectives. Computer models of friction coefficient measuring were designed and first in the world the research of correlation between the friction coefficient measuring results and shape variations, intensity and cycle frequency of the measuring wheel antilock braking modes. The sketch engineering documentation was designed and prototype of the latest generation measuring device is ready to use. The measuring device was tested on the autonomous electromechanical examination laboratory treadmill bench. The experiments approved effectiveness of method of imitation the antilock braking modes for solving the problem of correlation of the runway friction coefficient measuring.

Influences of thickness, scanning velocity and relative humidity on the frictional properties of WS2 nanosheets

NASA Astrophysics Data System (ADS)

Feng, Dongdong; Peng, Jinfeng; Liu, Sisi; Zheng, Xuejun; Yan, Xinyang; He, Wenyuan

2018-01-01

Distinguishing with the traditional cantilever mechanics method, we propose the extended cantilever mechanics method to calibrate the lateral calibration factor by using the normal spring constant obtained from atomic force microscopy (AFM) but not the Young’s modulus and the width of the cantilever, before the influences of thickness, scanning velocity and humidity on the frictional properties are investigated via friction measurement performed by the lateral force mode (LFM) of AFM. Tungsten disulfide (WS2) nanosheets were prepared through hydrothermal intercalation and exfoliation route, and AFM and Raman microscope were used to investigate the frictional properties, thickness and crystalline structure. The friction force and coefficient decrease monotonically with the increase of the nanosheet’s thickness, and the friction coefficient minimum value is close to 0.012 when the thickness larger than 5 nm. The friction property variation on the nanosheet’s thickness can be explained by the puckering effect of tip-sheet adhesion according thickness dependence of bending stiffness in the frame of continuum mechanics. The friction force is a constant value 1.7 nN when the scanning speed larger than the critical value 3.10 μm s-1, while it logarithmically increases for the scanning speed less than the critical value. It is easy to understand through the energy dissipation model and the thermally activated effect. The friction force and friction coefficient increase with the relative humidity at the range of 30%-60%, and the latter is at the range of 0.010-0.013. Influence of relative humidity is discussed via the increasing area of the water monolayer during the water adsorption process. The research can not only enrich nanotribology theory, but also prompt two dimensions materials for nanomechanical applications.

Discrete Element Method Simulations of the Inter-Particle Contact Parameters for the Mono-Sized Iron Ore Particles

PubMed Central

Li, Tongqing; Peng, Yuxing; Zhu, Zhencai; Zou, Shengyong; Yin, Zixin

2017-01-01

Aiming at predicting what happens in reality inside mills, the contact parameters of iron ore particles for discrete element method (DEM) simulations should be determined accurately. To allow the irregular shape to be accurately determined, the sphere clump method was employed in modelling the particle shape. The inter-particle contact parameters were systematically altered whilst the contact parameters between the particle and wall were arbitrarily assumed, in order to purely assess its impact on the angle of repose for the mono-sized iron ore particles. Results show that varying the restitution coefficient over the range considered does not lead to any obvious difference in the angle of repose, but the angle of repose has strong sensitivity to the rolling/static friction coefficient. The impacts of the rolling/static friction coefficient on the angle of repose are interrelated, and increasing the inter-particle rolling/static friction coefficient can evidently increase the angle of repose. However, the impact of the static friction coefficient is more profound than that of the rolling friction coefficient. Finally, a predictive equation is established and a very close agreement between the predicted and simulated angle of repose is attained. This predictive equation can enormously shorten the inter-particle contact parameters calibration time that can help in the implementation of DEM simulations. PMID:28772880

Influence of the pressure dependent coefficient of friction on deep drawing springback predictions

NASA Astrophysics Data System (ADS)

Gil, Imanol; Galdos, Lander; Mendiguren, Joseba; Mugarra, Endika; Sáenz de Argandoña, Eneko

2016-10-01

This research studies the effect of considering an advanced variable friction coefficient on the springback prediction of stamping processes. Traditional constant coefficient of friction considerations are being replaced by more advanced friction coefficient definitions. The aim of this work is to show the influence of defining a pressure dependent friction coefficient on numerical springback predictions of a DX54D mild steel, a HSLA380 and a DP780 high strength steel. The pressure dependent friction model of each material was fitted to the experimental data obtained by Strip Drawing tests. Then, these friction models were implemented in a numerical simulation of a drawing process of an industrial automotive part. The results showed important differences between defining a pressure dependent friction coefficient or a constant friction coefficient.

Tribological behaviour and statistical experimental design of sintered iron-copper based composites

NASA Astrophysics Data System (ADS)

Popescu, Ileana Nicoleta; Ghiţă, Constantin; Bratu, Vasile; Palacios Navarro, Guillermo

2013-11-01

The sintered iron-copper based composites for automotive brake pads have a complex composite composition and should have good physical, mechanical and tribological characteristics. In this paper, we obtained frictional composites by Powder Metallurgy (P/M) technique and we have characterized them by microstructural and tribological point of view. The morphology of raw powders was determined by SEM and the surfaces of obtained sintered friction materials were analyzed by ESEM, EDS elemental and compo-images analyses. One lot of samples were tested on a "pin-on-disc" type wear machine under dry sliding conditions, at applied load between 3.5 and 11.5 × 10-1 MPa and 12.5 and 16.9 m/s relative speed in braking point at constant temperature. The other lot of samples were tested on an inertial test stand according to a methodology simulating the real conditions of dry friction, at a contact pressure of 2.5-3 MPa, at 300-1200 rpm. The most important characteristics required for sintered friction materials are high and stable friction coefficient during breaking and also, for high durability in service, must have: low wear, high corrosion resistance, high thermal conductivity, mechanical resistance and thermal stability at elevated temperature. Because of the tribological characteristics importance (wear rate and friction coefficient) of sintered iron-copper based composites, we predicted the tribological behaviour through statistical analysis. For the first lot of samples, the response variables Yi (represented by the wear rate and friction coefficient) have been correlated with x1 and x2 (the code value of applied load and relative speed in braking points, respectively) using a linear factorial design approach. We obtained brake friction materials with improved wear resistance characteristics and high and stable friction coefficients. It has been shown, through experimental data and obtained linear regression equations, that the sintered composites wear rate increases with increasing applied load and relative speed, but in the same conditions, the frictional coefficients slowly decrease.

Evaluation of Wear Resistance of Friction Materials Prepared by Granulation.

PubMed

Ma, Yunhai; Liu, Yucheng; Menon, Carlo; Tong, Jin

2015-10-21

The tribological properties of friction materials prepared by hot-pressing pellets of different sizes were experimentally investigated. Friction and wear tests of the specimens were performed and morphological analysis was carried out by investigating images acquired with both scanning electron and confocal laser microscopes. The highest friction coefficient of friction materials was obtained with pellets having 1-5 mm size. The lowest wear rate was obtained with pellets having 8-10 mm size. Specimens processed by mixing pellets of different sizes had the highest density and the lowest roughness and were the least expensive to fabricate. The results show that granulation generally enabled increasing the friction coefficient, decreasing the wear rate, and reducing the number of defects on the surface of friction materials.

Poroelasticity-driven lubrication in hydrogel interfaces.

PubMed

Reale, Erik R; Dunn, Alison C

2017-01-04

It is widely accepted that hydrogel surfaces are slippery, and have low friction, but dynamic applied stresses alter the hydrogel composition at the interface as water is displaced. The induced osmotic imbalance of compressed hydrogel which cannot swell to equilibrium should drive the resistance to slip against it. This paper demonstrates the driving role of poroelasticity in the friction of hydrogel-glass interfaces, specifically how poroelastic relaxation of hydrogels increases adhesion. We translate the work of adhesion into an effective surface energy density that increases with the duration of applied pressure from 10 to 50 mJ m -2 , as measured by micro-indentation. A model of static friction coefficient is derived from an area-based rules of mixture for the surface energies, and predicts the friction coefficient changes upon initiation of slip. For kinetic friction, the competition between duration of contact and relaxation time is quantified by a contacting Péclet number, Pe C . A single length parameter on the scale of micrometers fits these two models to experimental micro-friction data. These models predict how short durations of applied pressure and faster sliding speeds, do not disrupt interfacial hydration; this prevailing water maintains low friction. At low speeds where interface drainage dominates, the osmotic suction works against slip for higher friction. The prediction of friction coefficients after adhesion characterization by micro-indentation makes use of the interplay between poroelasticity, adhesion, and friction. This approach provides a starting point for prediction of, and design for, hydrogel interfacial friction.

Torsional Tribological Behavior and Torsional Friction Model of Polytetrafluoroethylene against 1045 Steel

PubMed Central

Wang, Shibo; Niu, Chengchao

2016-01-01

In this work, the plane-on-plane torsional fretting tribological behavior of polytetrafluoroethylene (PTFE) was studied. A model of a rigid, flat-ended punch acting on an elastic half-space was built according to the experimental conditions. The results indicate that the shape of T–θ curves was influenced by both the torsional angle and the normal load. The torsion friction torque and wear rate of PTFE exponentially decreased when the torsion angle rose. The torsional torque increased from 0.025 N·m under a normal load of 43 N to 0.082 N·m under a normal load of 123 N. With sequentially increasing normal load, the value of torque was maintained. With rising normal load, the wear mass loss of PTFE disks was increased and the wear rate was decreased. Good agreement was found with the calculated torque according to the model and the experimental torque except for that under a normal load of 163 N. The difference under a normal load of 163 N was caused by the coefficient of friction. Usually the coefficient of friction of a polymer decreases with increasing normal load, whereas a constant coefficient of friction was applied in the model. PMID:26799324

Large Electric Field–Enhanced–Hardness Effect in a SiO2 Film

PubMed Central

Revilla, Reynier I.; Li, Xiao-Jun; Yang, Yan-Lian; Wang, Chen

2014-01-01

Silicon dioxide films are extensively used in nano and micro–electromechanical systems. Here we studied the influence of an external electric field on the mechanical properties of a SiO2 film by using nanoindentation technique of atomic force microscopy (AFM) and friction force microscopy (FFM). A giant augmentation of the relative elastic modulus was observed by increasing the localized electric field. A slight decrease in friction coefficients was also clearly observed by using FFM with the increase of applied tip voltage. The reduction of the friction coefficients is consistent with the great enhancement of sample hardness by considering the indentation–induced deformation during the friction measurements. PMID:24681517

Friction and wear with a single-crystal abrasive grit of silicon carbide in contact with iron base binary alloys in oil: Effects of alloying element and its content

NASA Technical Reports Server (NTRS)

Miyoshi, K.; Buckley, D. H.

1979-01-01

Sliding friction experiments were conducted with various iron-base binary alloys (alloying elements were Ti, Cr, Mn, Ni, Rh, and W) in contact with a rider of 0.025-millimeter-radius, single-crystal silicon carbide in mineral oil. Results indicate that atomic size and content of alloying element play a dominant role in controlling the abrasive-wear and -friction properties of iron-base binary alloys. The coefficient of friction and groove height (wear volume) general alloy decrease, and the contact pressure increases in solute content. There appears to be very good correlation of the solute to iron atomic radius ratio with the decreasing rate of coefficient of friction, the decreasing rate of groove height (wear volume), and the increasing rate of contact pressure with increasing solute content C. Those rates increase as the solute to iron atomic radius ratio increases from unity.

Surface chemistry, microstructure and friction properties of some ferrous-base metallic glasses at temperatures to 750 C

NASA Technical Reports Server (NTRS)

Miyoshi, K.; Buckley, D. H.

1982-01-01

X-ray photoelectron spectroscopy analysis, transmission electron microscopy, diffraction studies, and sliding friction experiments were conducted with ferrous-base metallic glasses in sliding contact with aluminum oxide at temperatures from room to 750 C in a vacuum of 30 nPa. The results indicate that there is a significant temperature influence on the friction properties, surface chemistry, and microstructure of metallic glasses. The relative concentrations of the various constituents at the surface of the sputtered specimens were very different from the normal bulk compositions. Contaminants can come from the bulk of the material to the surface upon heating and impart boric oxide and silicon oxide at 350 C and boron nitride above 500 C. The coefficient of friction increased with increasing temperature to 350 C. Above 500 C the coefficient of friction decreased rapidly. The segregation of contaminants may be responsible for the friction behavior.

Preload, Coefficient of Friction, and Thread Friction in an Implant-Abutment-Screw Complex.

PubMed

Wentaschek, Stefan; Tomalla, Sven; Schmidtmann, Irene; Lehmann, Karl Martin

To examine the screw preload, coefficient of friction (COF), and tightening torque needed to overcome the thread friction of an implant-abutment-screw complex. In a customized load frame, 25 new implant-abutment-screw complexes including uncoated titanium alloy screws were torqued and untorqued 10 times each, applying 25 Ncm. Mean preload values decreased significantly from 209.8 N to 129.5 N according to the number of repetitions. The overall COF increased correspondingly. There was no comparable trend for the thread friction component. These results suggest that the application of a used implant-abutment-screw complex may be unfavorable for obtaining optimal screw preload.

Optimizing the equal channel angular pressing process (ECAP) operation parameters to produce bulk nanostructure materials

DOE Office of Scientific and Technical Information (OSTI.GOV)

Abushgair, K.

In this work we were interested in doing simulation using finite elements analysis (FEA) to study the equal channel angular pressing process (ECAP), which is currently one of the most popular methods of severe plastic deformation Processes (SPD). for fabricating Ultra-Fine Grained (UFG) materials, because it allows very high strains to be imposed leading to extreme work hardening and microstructural refinement. The main object of this study is to establish the influence of main parameters which effect ECAP process which are magnitude of the die angle and the friction coefficient. The angle studied between (90-135°) degree, and magnitude of themore » friction coefficient μ between (0.12-0.6), and number of pass. The samples were made from aluminum alloy at room temperature with (15X 15) mm cross section and 150 mm length. The simulation result shows that normal elastic strain, shears elastic strain, and max. shear elastic strain increased, when changing the angle from 90° to 100°. and decrease between the angle 110° to 135°. Also the total deformation increased when we change die angle from 90° to 135°. By studding the friction effect on the die and sample we noted that increasing the friction coefficient from 0.12 to 0.6, normal elastic strain, and shear elastic strain increased and increasing the friction coefficient from 0.1 to 0.6 decrease the normal and shear stress.« less

Optimizing the equal channel angular pressing process (ECAP) operation parameters to produce bulk nanostructure materials

NASA Astrophysics Data System (ADS)

Abushgair, K.

2015-03-01

In this work we were interested in doing simulation using finite elements analysis (FEA) to study the equal channel angular pressing process (ECAP), which is currently one of the most popular methods of severe plastic deformation Processes (SPD). for fabricating Ultra-Fine Grained (UFG) materials, because it allows very high strains to be imposed leading to extreme work hardening and microstructural refinement. The main object of this study is to establish the influence of main parameters which effect ECAP process which are magnitude of the die angle and the friction coefficient. The angle studied between (90-135°) degree, and magnitude of the friction coefficient μ between (0.12-0.6), and number of pass. The samples were made from aluminum alloy at room temperature with (15X 15) mm cross section and 150 mm length. The simulation result shows that normal elastic strain, shears elastic strain, and max. shear elastic strain increased, when changing the angle from 90° to 100°. and decrease between the angle 110° to 135°. Also the total deformation increased when we change die angle from 90° to 135°. By studding the friction effect on the die and sample we noted that increasing the friction coefficient from 0.12 to 0.6, normal elastic strain, and shear elastic strain increased and increasing the friction coefficient from 0.1 to 0.6 decrease the normal and shear stress.

Adhesion-dependent negative friction coefficient on chemically modified graphite at the nanoscale

NASA Astrophysics Data System (ADS)

Deng, Zhao; Smolyanitsky, Alex; Li, Qunyang; Feng, Xi-Qiao; Cannara, Rachel J.

2012-12-01

From the early tribological studies of Leonardo da Vinci to Amontons’ law, friction has been shown to increase with increasing normal load. This trend continues to hold at the nanoscale, where friction can vary nonlinearly with normal load. Here we present nanoscale friction force microscopy (FFM) experiments for a nanoscale probe tip sliding on a chemically modified graphite surface in an atomic force microscope (AFM). Our results demonstrate that, when adhesion between the AFM tip and surface is enhanced relative to the exfoliation energy of graphite, friction can increase as the load decreases under tip retraction. This leads to the emergence of an effectively negative coefficient of friction in the low-load regime. We show that the magnitude of this coefficient depends on the ratio of tip-sample adhesion to the exfoliation energy of graphite. Through both atomistic- and continuum-based simulations, we attribute this unusual phenomenon to a reversible partial delamination of the topmost atomic layers, which then mimic few- to single-layer graphene. Lifting of these layers with the AFM tip leads to greater deformability of the surface with decreasing applied load. This discovery suggests that the lamellar nature of graphite yields nanoscale tribological properties outside the predictive capacity of existing continuum mechanical models.

Frictional weakening of Landslides in the Solar System

NASA Astrophysics Data System (ADS)

Lucas, Antoine; Mangeney, Anne; Ampuero, Jean-Paul

2014-05-01

Landslides are an important phenomenon that shapes the surface morphology of solid planetary bodies, including planets and small bodies. In addition, landslide science aims to predict the maximum distance travelled and the maximum velocity reached by a potential landslide in order to quantify the damage it may cause. On the one hand, observations show that the so-called Heim's ratio (i.e. the ratio between the difference of the height of the initial mass and that of the deposit, and the traveling distance) decreases with increasing volume for landslides observed on Earth [1] and other planets like Mars and icy moons like Iapetus [2], but whether this quantity is a good representation of the effective friction during the flow is still a controversial issue. On the other hand, numerical simulations (either continuous or discrete) of real landslides commonly require the assumption of very small friction coefficient to reproduce the extension of deposits [2-5]. We investigate if a common origin can explain the characteristics of landslides in such variety of planetary environments. Based on analytical and numerical solutions for granular flows constrained by remote-sensing observations [3, 7], we developed a consistent method to estimate the effective friction coefficient of landslides, i.e., the constant basal friction coefficient that reproduces their first-order properties. We show that: i) the Heim's ratio is not equivalent to the effective friction coefficient; ii) the friction coefficient decreases with increasing volume or, more fundamentally, with increasing sliding velocity. Inspired by frictional weakening mechanisms thought to operate during earthquakes [8], we propose an empirical velocity-weakening friction law under an unifying phenomenological framework applicable to small to large landslides observed on Earth and beyond (including icy moons of giant planets) whatever the environment and material involved. References: [1] Legros, Eng. Geol. 2002; [2] Lucas, Nat. Geosc. News & Views, 2012. [3] Lucas & Mangeney, GRL, 2007. [4] Pudasaini & Hutter, Springer, 2007. [5] Campbell et al., JGR, 1995. [6] Smart et al., AGU Fall Meeting, 2010. [7] Lucas et al., JGR, 2011. [8] Rice, JGR, 2006. N.B. This work is subject to press embargo.

Self-Regulating Shock Absorber

NASA Technical Reports Server (NTRS)

Wesselski, Clarence J.

1995-01-01

Mechanical shock absorber keeps frictional damping force within tolerable limit. Its damping force does not increase with coefficient of friction between energy-absorbing components; rather, frictional damping force varies only slightly. Relatively insensitive to manufacturing variations and environmental conditions altering friction. Does not exhibit high breakaway friction and consequent sharp increase followed by sharp decrease in damping force at beginning of stroking. Damping force in absorber does not vary appreciably with speed of stroking. In addition, not vulnerable to leakage of hydraulic fluid.

Prediction of Sliding Friction Coefficient Based on a Novel Hybrid Molecular-Mechanical Model.

PubMed

Zhang, Xiaogang; Zhang, Yali; Wang, Jianmei; Sheng, Chenxing; Li, Zhixiong

2018-08-01

Sliding friction is a complex phenomenon which arises from the mechanical and molecular interactions of asperities when examined in a microscale. To reveal and further understand the effects of micro scaled mechanical and molecular components of friction coefficient on overall frictional behavior, a hybrid molecular-mechanical model is developed to investigate the effects of main factors, including different loads and surface roughness values, on the sliding friction coefficient in a boundary lubrication condition. Numerical modelling was conducted using a deterministic contact model and based on the molecular-mechanical theory of friction. In the contact model, with given external loads and surface topographies, the pressure distribution, real contact area, and elastic/plastic deformation of each single asperity contact were calculated. Then asperity friction coefficient was predicted by the sum of mechanical and molecular components of friction coefficient. The mechanical component was mainly determined by the contact width and elastic/plastic deformation, and the molecular component was estimated as a function of the contact area and interfacial shear stress. Numerical results were compared with experimental results and a good agreement was obtained. The model was then used to predict friction coefficients in different operating and surface conditions. Numerical results explain why applied load has a minimum effect on the friction coefficients. They also provide insight into the effect of surface roughness on the mechanical and molecular components of friction coefficients. It is revealed that the mechanical component dominates the friction coefficient when the surface roughness is large (Rq > 0.2 μm), while the friction coefficient is mainly determined by the molecular component when the surface is relatively smooth (Rq < 0.2 μm). Furthermore, optimal roughness values for minimizing the friction coefficient are recommended.

Surface contact and design of fibrillar ‘friction pads’ in stick insects (Carausius morosus): mechanisms for large friction coefficients and negligible adhesion

PubMed Central

Labonte, David; Williams, John A.; Federle, Walter

2014-01-01

Many stick insects and mantophasmids possess tarsal ‘heel pads’ (euplantulae) covered by arrays of conical, micrometre-sized hairs (acanthae). These pads are used mainly under compression; they respond to load with increasing shear resistance, and show negligible adhesion. Reflected-light microscopy in stick insects (Carausius morosus) revealed that the contact area of ‘heel pads’ changes with normal load on three hierarchical levels. First, loading brought larger areas of the convex pads into contact. Second, loading increased the density of acanthae in contact. Third, higher loads changed the shape of individual hair contacts gradually from circular (tip contact) to elongated (side contact). The resulting increase in real contact area can explain the load dependence of friction, indicating a constant shear stress between acanthae and substrate. As the euplantula contact area is negligible for small loads (similar to hard materials), but increases sharply with load (resembling soft materials), these pads show high friction coefficients despite little adhesion. This property appears essential for the pads’ use in locomotion. Several morphological characteristics of hairy friction pads are in apparent contrast to hairy pads used for adhesion, highlighting key adaptations for both pad types. Our results are relevant for the design of fibrillar structures with high friction coefficients but small adhesion. PMID:24554580

Tribological properties of boron nitride synthesized by ion beam deposition

NASA Technical Reports Server (NTRS)

Miyoshi, K.; Buckley, D. H.; Spalvins, T.

1985-01-01

The adhesion and friction behavior of boron nitride films on 440 C bearing stainless steel substrates was examined. The thin films containing the boron nitride were synthesized using an ion beam extracted from a borazine plasma. Sliding friction experiments were conducted with BN in sliding contact with itself and various transition metals. It is indicated that the surfaces of atomically cleaned BN coating film contain a small amount of oxides and carbides, in addition to boron nitride. The coefficients of friction for the BN in contact with metals are related to the relative chemical activity of the metals. The more active the metal, the higher is the coefficient of friction. The adsorption of oxygen on clean metal and BN increases the shear strength of the metal - BN contact and increases the friction. The friction for BN-BN contact is a function of the shear strength of the elastic contacts. Clean BN surfaces exhibit relatively strong interfacial adhesion and high friction. The presence of adsorbates such as adventitious carbon contaminants on the BN surfaces reduces the shear strength of the contact area. In contrast, chemically adsorbed oxygen enhances the shear strength of the BN-BN contact and increases the friction.

Friction and wear of some ferrous-base metallic glasses

NASA Technical Reports Server (NTRS)

Miyoshi, K.; Buckley, D. H.

1983-01-01

Sliding friction experiments, X-ray photoelectron spectroscopy (XPS) analysis, and electron microscopy and diffraction studies were conducted with ferrous base metallic glasses (amorphous alloys) in contact with aluminum oxide at temperatures to 750 C in a vacuum. Sliding friction experiments were also conducted in argon and air atmospheres. The results of the investigation indicate that the coefficient of friction increases with increasing temperature to 350 C in vacuum. The increase in friction is due to an increase in adhesion resulting from surface segregation of boric oxide and/or silicon oxide to the surface of the foil. Above 500 C the coefficient of friction decreased rapidly. The decrease correlates with the segregation of boron nitride to the surface. Contaminants can come from the bulk of the material to the surface upon heating and impart boric oxide and/or silicon oxide at 350 C and boron nitride above 500 C. The segregation of contaminants is responsible for the friction behavior. The amorphous alloys have superior wear resistance to crystalline 304 stainless steel. The relative concentrations of the various constituents at the surfaces of the amorphous alloys are very different from the nominal bulk compositions.

Friction and wear of some ferrous-base metallic glasses

NASA Technical Reports Server (NTRS)

Miyoshi, K.; Buckley, D. H.

1984-01-01

Sliding friction experiments, X-ray photoelectron spectroscopy (XPS) analysis, and electron microscopy and diffraction studies were conducted with ferrous base metallic glasses (amorphous alloys) in contact with aluminium oxide at temperatures to 750 C in a vacuum. Sliding friction experiments were also conducted in argon and air atmospheres. The results of the investigation indicate that the coefficient of friction increases with increasing temperature to 350 C in vacuum. The increase in friction is due to an increase in adhesion resulting from surface segregation of boric oxide and/or silicon oxide to the surface of the foil. Above 500 C the coefficient of friction decreased rapidly. The decrease correlates with the segregation of boron nitride to the surface. Contaminants can come from the bulk of the material to the surface upon heating and impart boric oxide and/or silicon oxide at 350 C and boron nitride above 500 C. The segregation of contaminants is responsible for the friction behavior. The amorphous alloys have superior wear resistance to crystalline 304 stainless steel. The relative concentrations of the various constituents at the surfaces of the amorphous alloys are very different from the nominal bulk compositions.

Measuring internal friction of an ultrafast-folding protein.

PubMed

Cellmer, Troy; Henry, Eric R; Hofrichter, James; Eaton, William A

2008-11-25

Nanosecond laser T-jump was used to measure the viscosity dependence of the folding kinetics of the villin subdomain under conditions where the viscogen has no effect on its equilibrium properties. The dependence of the unfolding/refolding relaxation time on solvent viscosity indicates a major contribution to the dynamics from internal friction. The internal friction increases with increasing temperature, suggesting a shift in the transition state along the reaction coordinate toward the native state with more compact structures, and therefore, a smaller diffusion coefficient due to increased landscape roughness. Fitting the data with an Ising-like model yields a relatively small position dependence for the diffusion coefficient. This finding is consistent with the excellent correlation found between experimental and calculated folding rates based on free energy barrier heights using the same diffusion coefficient for every protein.

The Contact Ageing Effect on Fretting Damage of an Electro-Deposited Coating against an AISI52100 Steel Ball

PubMed Central

Kim, Kyungmok; Ko, Joon Soo

2016-01-01

This article investigates the effect of contact ageing on fretting damage of an epoxy-based cathodic electro-deposited coating for use on automotive seat slide tracks (made of cold-rolled high strength steel). Static normal load was induced at the contact between the coating and an AISI52100 ball for a certain duration. It was identified that plastically deformed contact area increased logarithmically as a function of time when the contact was under static normal load. Fretting tests after various durations of static contact were conducted using a ball-on-flat plate apparatus. All fretting tests were halted when the friction coefficient reached a critical value of 0.5, indicating complete coating failure. The total number of fretting cycles to the critical friction coefficient was found to vary with the duration of static contact before fretting. It was identified that the number of cycles to the critical friction coefficient decreased with the increased duration of static contact. Meanwhile, the friction coefficient at steady-state sliding was not greatly affected by the duration of static contact before fretting. Finally, the relation between coating thickness after indentation creep and the number of cycles to the critical friction coefficient was found to be linear. Obtained results show that the duration of static contact before fretting has an influence on the fretting lifetime of an electro-deposited coating. PMID:28773873

The Contact Ageing Effect on Fretting Damage of an Electro-Deposited Coating against an AISI52100 Steel Ball.

PubMed

Kim, Kyungmok; Ko, Joon Soo

2016-09-03

This article investigates the effect of contact ageing on fretting damage of an epoxy-based cathodic electro-deposited coating for use on automotive seat slide tracks (made of cold-rolled high strength steel). Static normal load was induced at the contact between the coating and an AISI52100 ball for a certain duration. It was identified that plastically deformed contact area increased logarithmically as a function of time when the contact was under static normal load. Fretting tests after various durations of static contact were conducted using a ball-on-flat plate apparatus. All fretting tests were halted when the friction coefficient reached a critical value of 0.5, indicating complete coating failure. The total number of fretting cycles to the critical friction coefficient was found to vary with the duration of static contact before fretting. It was identified that the number of cycles to the critical friction coefficient decreased with the increased duration of static contact. Meanwhile, the friction coefficient at steady-state sliding was not greatly affected by the duration of static contact before fretting. Finally, the relation between coating thickness after indentation creep and the number of cycles to the critical friction coefficient was found to be linear. Obtained results show that the duration of static contact before fretting has an influence on the fretting lifetime of an electro-deposited coating.

The Influence of Friction Between Football Helmet and Jersey Materials on Force: A Consideration for Sport Safety

PubMed Central

Rossi, Anthony M.; Claiborne, Tina L.; Thompson, Gregory B.; Todaro, Stacey

2016-01-01

Context: The pocketing effect of helmet padding helps to dissipate forces experienced by the head, but if the player's helmet remains stationary in an opponent's shoulder pads, the compressive force on the cervical spine may increase. Objective: To (1) measure the coefficient of static friction between different football helmet finishes and football jersey fabrics and (2) calculate the potential amount of force on a player's helmet due to the amount of friction present. Design: Cross-sectional study. Setting: Laboratory. Patients or Other Participants: Helmets with different finishes and different football jersey fabrics. Main Outcome Measure(s): The coefficient of friction was determined for 2 helmet samples (glossy and matte), 3 football jerseys (collegiate, high school, and youth), and 3 types of jersey numbers (silkscreened, sublimated, and stitched on) using the TAPPI T 815 standard method. These measurements determined which helmet-to-helmet, helmet-to-jersey number, and helmet-to-jersey material combination resulted in the least amount of static friction. Results: The glossy helmet versus glossy helmet combination produced a greater amount of static friction than the other 2 helmet combinations (P = .013). The glossy helmet versus collegiate jersey combination produced a greater amount of static friction than the other helmet-to-jersey material combinations (P < .01). The glossy helmet versus silkscreened numbers combination produced a greater amount of static friction than the other helmet-to-jersey number combinations (P < .01). Conclusions: The force of static friction experienced during collisions can be clinically relevant. Conditions with higher coefficients of static friction result in greater forces. In this study, the highest coefficient of friction (glossy helmet versus silkscreened number) could increase the forces on the player's helmet by 3553.88 N when compared with other helmet-to-jersey combinations. Our results indicate that the makeup of helmet and uniform materials may affect sport safety. PMID:27824251

Lubricating Properties of Some Bonded Fluoride and Oxide Coatings for Temperature to 1500 F

NASA Technical Reports Server (NTRS)

Sliney, Harold E.

1960-01-01

The lubricating properties of some experimental ceramic coatings, diffusion-bonded fluoride coatings, and ceramic-bonded fluoride coatings were determined. The experiments were conducted in an air atmosphere at a sliding velocity of 430 feet per minute and at temperatures from 75 to 1500 F. Several ceramic coatings provided substantial reductions in friction coefficient and rider wear (compared with the unlubricated metals). For example, a cobaltous oxide (CoO) base coating gave friction coefficients of 0.24 to 0.36 within the temperature range of 75 to 1400 F; serious galling and welding of the metal surfaces were prevented. The friction coefficients were higher than the arbitrary maximum (0.2) usually considered for effective boundary lubrication. However, when a moderately high friction coefficient can be tolerated, this type of coating may be a useful antiwear composition. Diffusion-bonded calcium fluoride (CaF2) on Haynes Stellite 21 and on Inconel X gave friction coefficients of 0.1 to 0.2 at 1500 F. Endurance life was dependent on the thermal history of the coating; life improved with increased exposure time at elevated temperatures prior to running. Promising results were obtained with ceramic-bonded CaF2 on Inconel X. Effective lubrication and good adherence were obtained with a 3 to 1 ratio of CaF2 to ceramic. A very thin sintered and burnished film of CaF2 applied to the surface of this coating further improved lubrication, particularly above 1350 F. The friction coefficient was 0.2 at 500 F and decreased with increasing temperature to 0.06-at 1500 F. It was 0.25 at 75 F and 0.22 at 250 F.

Analysis of friction between articular cartilage and polyvinyl alcohol hydrogel artificial cartilage.

PubMed

Li, Feng; Wang, Anmin; Wang, Chengtao

2016-05-01

Many biomaterials are being used to repair damaged articular cartilage. In particular, poly vinyl alcohol hydrogel has similar mechanical properties to natural cartilage under compressive and shearing loading. Here, three-factor and two-level friction experiments and long-term tests were conducted to better evaluate its tribological properties. The friction coefficient between articular cartilage and the poly vinyl alcohol hydrogel depended primarily on the three factors of load, speed, and lubrication. When the speed increased from 10 to 20 mm/s under a load of 10 N, the friction coefficient increased from 0.12 to 0.147. When the lubricant was changed from Ringer's solution to a hyaluronic acid solution, the friction coefficient decreased to 0.084 with loads as high as 22 N. The poly vinyl alcohol hydrogel was severely damaged and lost its top surface layers, which were transferred to the articular cartilage surface. Wear was observed in the surface morphologies, which indicated the occurrence of surface adhesion of bovine cartilage. Surface fatigue and adhesive wear was the dominant wear mechanism.

The coefficient of friction, particularly of ice

NASA Astrophysics Data System (ADS)

Mills, Allan

2008-07-01

The static and dynamic coefficients of friction are defined, and values from 0.3 to 0.6 are quoted for common materials. These drop to about 0.15 when oil is added as a lubricant. Water ice at temperatures not far below 0 °C is remarkable for low coefficients of around 0.05 for static friction and 0.04-0.02 for dynamic friction, but these figures increase as the temperature diminishes. Reasons for the slipperiness of ice are summarized, but they are still not entirely clear. One hypothesis suggests that it is related to the transient formation of a lubricating film of liquid water produced by frictional heating. If this is the case, some composition melting a little above ambient temperatures might provide a skating rink that did not require expensive refrigeration. Various compositions have been tested, but an entirely satisfactory material has yet to be found.

The behavior of the skin-friction coefficient of a turbulent boundary layer flow over a flat plate with differently configured transverse square grooves

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wahidi, R.; Chakroun, W.; Al-Fahed, S.

2005-11-01

Skin-friction coefficient of turbulent boundary layer flow over a smooth-wall with transverse square grooves was investigated. Four grooved-wall cases were investigated. The four grooved-wall configurations are single 5mm square grooved-wall, and 5mm square grooves spaced 10, 20 and 40 element widths apart in the streamwise direction. Laser-Doppler Anemometer (LDA) was used for the mean velocity and turbulence intensity measurements. The skin-friction coefficient determined from the velocity profile increases sharply just downstream of the groove. This overshoot is followed by an undershoot and then relaxation back to the smooth-wall value. This behavior is observed in most grooved-wall cases. Integrating the skin-frictionmore » coefficient in the streamwise direction indicates that there is an increase in the overall drag in all the grooved-wall cases.« less

Structural and environmental dependence of superlow friction in ion vapour-deposited a-C : H : Si films for solid lubrication application

NASA Astrophysics Data System (ADS)

Chen, Xinchun; Kato, Takahisa; Kawaguchi, Masahiro; Nosaka, Masataka; Choi, Junho

2013-06-01

Understanding the tribochemical interaction of water molecules in humid environment with carbonaceous film surfaces, especially hydrophilic surface, is fundamental for applications in tribology and solid lubrication. This paper highlights some experimental evidence to elucidate the structural and environmental dependence of ultralow or even superlow friction in ion vapour-deposited a-C : H : Si films. The results indicate that both surface density of silicon hydroxyl group (Si-OH) and humidity level (RH) determine the frictional performance of a-C : H : Si films. Ultralow friction coefficient μ (˜0.01-0.055) is feasible in a wide range of RH. The dissociative formation of hydrophilic Si-OH surface and the following nanostructure of interfacial water molecules under contact pressure are the origin of ultralow friction for a-C : H : Si films in humid environment. The correlation between contact pressure and friction coefficient derived from Hertzian contact model is not valid in the present case. Under this nanoscale boundary lubrication, the friction coefficient tends to increase as the contact pressure increases. There even exists a contact pressure threshold for the transition from ultralow to superlow friction (μ ˜ 0.007). In comparison, when tribotested in dry N2, the observed superlow friction (μ ˜ 0.004) in the absence of water is correlated with the formation of a low shear strength tribolayer by wear-induced phase transformation.

Micro- and macroscale coefficients of friction of cementitious materials

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lomboy, Gilson; Sundararajan, Sriram, E-mail: srirams@iastate.edu; Wang, Kejin

2013-12-15

Millions of metric tons of cementitious materials are produced, transported and used in construction each year. The ease or difficulty of handling cementitious materials is greatly influenced by the material friction properties. In the present study, the coefficients of friction of cementitious materials were measured at the microscale and macroscale. The materials tested were commercially-available Portland cement, Class C fly ash, and ground granulated blast furnace slag. At the microscale, the coefficient of friction was determined from the interaction forces between cementitious particles using an Atomic Force Microscope. At the macroscale, the coefficient of friction was determined from stresses onmore » bulk cementitious materials under direct shear. The study indicated that the microscale coefficient of friction ranged from 0.020 to 0.059, and the macroscale coefficient of friction ranged from 0.56 to 0.75. The fly ash studied had the highest microscale coefficient of friction and the lowest macroscale coefficient of friction. -- Highlights: •Microscale (interparticle) coefficient of friction (COF) was determined with AFM. •Macroscale (bulk) COF was measured under direct shear. •Fly ash had the highest microscale COF and the lowest macroscale COF. •Portland cement against GGBFS had the lowest microscale COF. •Portland cement against Portland cement had the highest macroscale COF.« less

Friction and wear behaviour of plasma sprayed Cr2O3-TiO2 coating

NASA Astrophysics Data System (ADS)

Bagde, Pranay; Sapate, S. G.; Khatirkar, R. K.; Vashishtha, Nitesh; Tailor, Satish

2018-02-01

Cr2O3-25TiO2 coating was deposited by atmospheric plasma spray (APS) coating technique. Effect of load (5-30 N) and sliding velocity (0.25, 0.75 m s-1) on friction coefficient and abrasive wear behaviour of the Cr2O3-25TiO2 coating was studied. Mechanical and microstructural characterization of the Cr2O3-25TiO2 coating was carried out. With an increase in sliding velocity, abrasive wear rate and friction coefficient (COF) decreased while wear rate and friction coefficient showed an increasing trend with the load. The worn out surfaces were analyzed by SEM, EDS and XRD. At lower sliding velocity, XRD analysis revealed peaks of Ti2O3, Ti3O5, CrO2 and CrO3. In addition, peak of Ti4O7 was also detected at higher sliding velocity and at 30 N load. At higher sliding velocity medium to severe tribo oxidation was observed. XPS analysis of worn surfaces at both the sliding velocities, showed surface film of oxides of titanium and chromium along with Cr(OH)3. Magneli phase titanium oxides with sub stoichiometric composition, along with surface films of chromium oxides and hydroxides altered the friction and wear behaviour of the coating. The decrease in friction coefficient with an increase in sliding velocity was attributed to tribo oxides and tribochemical reaction films having lower shear strength with good lubricating properties. The mechanism of material removal involved plastic deformation at lower load whereas inter-granular and trans-granular fracture, delamination cracking and splat fracture was observed with an increase load from 10 N to 30 N.

The friction coefficient evolution of a MoS2/WC multi-layer coating system during sliding wear

NASA Astrophysics Data System (ADS)

Chan, T. Y.; Hu, Y.; Gharbi, Mohammad M.; Politis, D. J.; Wang, L.

2016-08-01

This paper discusses the evolution of friction coefficient for the multi-layered Molybdenum Disulphide (MoS2) and WC coated substrate during sliding against Aluminium AA 6082 material. A soft MoS2 coating was prepared over a hard WC coated G3500 cast iron tool substrate and underwent friction test using a pin-on-disc tribometer. The lifetime of the coating was reduced with increasing load while the Aluminium debris accumulated on the WC hard coating surfaces, accelerated the breakdown of the coatings. The lifetime of the coating was represented by the friction coefficient and the sliding distance before MoS2 coating breakdown and was found to be affected by the load applied and the wear mechanism.

Effects of nano-LaF3 on the friction and wear behaviors of PTFE-based bonded solid lubricating coatings under different lubrication conditions

NASA Astrophysics Data System (ADS)

Jia, Yulong; Wan, Hongqi; Chen, Lei; Zhou, Huidi; Chen, Jianmin

2016-09-01

Influence of nanometer lanthanum fluoride (nano-LaF3) on the tribological behaviors of polytetrafluoroethylene (PTFE) bonded solid lubricating coatings were investigated using a ring-on-block friction-wear tester under dry friction and RP-3 jet fuel lubrication conditions. The worn surfaces and transfer films formed on the counterpart steel rings were observed by scanning electron microscope (SEM) and optical microscope (OM), respectively. The microstructures of the nano-LaF3 modified coatings and the distribution states of nano-LaF3 were studied by field-emission scanning electron microscope (FESEM) and transmission electron microscopy (TEM), respectively. The results show that incorporation of nano-LaF3 improves the microhardness and the friction-reduced and anti-wear abilities of PTFE bonded solid lubricating coatings. The wear life of the modified coating is about 6 times longer than that of the coating without nano-LaF3 filler at a relatively low applied load (200 N) and rotary speed (1000 rev/min) under dry friction condition. The friction coefficient and wear life of the modified coating decrease with increase of applied load under dry friction, but the friction coefficient has hardly any variation and wear life decreases under RP-3 jet fuel lubrication condition. In addition, the friction coefficient of the modified coating reduces with the rotary speed increasing under dry sliding but has little change under RP-3 lubrication, the wear life increases firstly and then decreases. The results indicated that the wear failure mechanism is dominated by applied load, which plays an important role in guidance of application of nano-LaF3 modified PTFE bonded coating under different working environment.

Dynamic nature of abutment screw retightening: finite element study of the effect of retightening on the settling effect.

PubMed

Bulaqi, Haddad Arabi; Mousavi Mashhadi, Mahmoud; Safari, Hamed; Samandari, Mohammad Mahdi; Geramipanah, Farideh

2015-05-01

A fundamental problem in fully understanding the dynamic nature of screw loosening is lack of recognition of the entire process of screw tightening and retightening. The purpose of this study was to explain the dynamic nature of abutment screw retightening by using finite element methods to investigate the effect of the coefficient of friction and retightening on the settling effect. Precise computer models were designed of a Straumann dental implant, a directly attached crown, an abutment screw, and the bone surrounding the implant. All threaded interfaces were designed with a spiral thread helix with a specific coefficient of static and kinetic friction, and the surfaces were characterized as fine, regular, and rough. Abaqus software was used for dynamic simulation, which involved applying rotational displacement to the abutment screw and torque controlling during the steps of tightening, relaxation, retightening, and second relaxation and at different coefficients of friction. The obtained torque and preload values were compared to the predicted values. When surfaces changed from fine to rough, the remaining torque and preload decreased, and the settling effect increased. Upon retightening, the remaining torque and preload increased, and the settling effect also decreased. The reduction of the coefficient of friction contributes to increases in the preload and decreases in the settling effect. Retightening reduced the settling effect and had an insignificant effect on the preload. At high coefficients of friction, the retightening effect was intensified. Copyright © 2015 Editorial Council for the Journal of Prosthetic Dentistry. Published by Elsevier Inc. All rights reserved.

Friction Durability of Extremely Thin Diamond-Like Carbon Films at High Temperature

PubMed Central

Miyake, Shojiro; Suzuki, Shota; Miyake, Masatoshi

2017-01-01

To clarify the friction durability, both during and after the high-temperature heating of nanometer-thick diamond-like carbon (DLC) films, deposited using filtered cathodic vacuum arc (FCVA) and plasma chemical vapor deposition (P-CVD) methods, the dependence of the friction coefficient on the load and sliding cycles of the DLC films, were evaluated. Cluster-I consisted of a low friction area in which the DLC film was effective, while cluster-II consisted of a high friction area in which the lubricating effect of the DLC film was lost. The friction durability of the films was evaluated by statistical cluster analysis. Extremely thin FCVA-DLC films exhibited an excellent wear resistance at room temperature, but their friction durability was decreased at high temperatures. In contrast, the durability of the P-CVD-DLC films was increased at high temperatures when compared with that observed at room temperature. This inverse dependence on temperature corresponded to the nano-friction results obtained by atomic force microscopy. The decrease in the friction durability of the FCVA-DLC films at high temperatures, was caused by a complex effect of temperature and friction. The tribochemical reaction produced by the P-CVD-DLC films reduced their friction coefficient, increasing their durability at high temperatures. PMID:28772520

Friction Durability of Extremely Thin Diamond-Like Carbon Films at High Temperature.

PubMed

Miyake, Shojiro; Suzuki, Shota; Miyake, Masatoshi

2017-02-10

To clarify the friction durability, both during and after the high-temperature heating of nanometer-thick diamond-like carbon (DLC) films, deposited using filtered cathodic vacuum arc (FCVA) and plasma chemical vapor deposition (P-CVD) methods, the dependence of the friction coefficient on the load and sliding cycles of the DLC films, were evaluated. Cluster-I consisted of a low friction area in which the DLC film was effective, while cluster-II consisted of a high friction area in which the lubricating effect of the DLC film was lost. The friction durability of the films was evaluated by statistical cluster analysis. Extremely thin FCVA-DLC films exhibited an excellent wear resistance at room temperature, but their friction durability was decreased at high temperatures. In contrast, the durability of the P-CVD-DLC films was increased at high temperatures when compared with that observed at room temperature. This inverse dependence on temperature corresponded to the nano-friction results obtained by atomic force microscopy. The decrease in the friction durability of the FCVA-DLC films at high temperatures, was caused by a complex effect of temperature and friction. The tribochemical reaction produced by the P-CVD-DLC films reduced their friction coefficient, increasing their durability at high temperatures.

Reciprocal Sliding Friction Model for an Electro-Deposited Coating and Its Parameter Estimation Using Markov Chain Monte Carlo Method

PubMed Central

Kim, Kyungmok; Lee, Jaewook

2016-01-01

This paper describes a sliding friction model for an electro-deposited coating. Reciprocating sliding tests using ball-on-flat plate test apparatus are performed to determine an evolution of the kinetic friction coefficient. The evolution of the friction coefficient is classified into the initial running-in period, steady-state sliding, and transition to higher friction. The friction coefficient during the initial running-in period and steady-state sliding is expressed as a simple linear function. The friction coefficient in the transition to higher friction is described with a mathematical model derived from Kachanov-type damage law. The model parameters are then estimated using the Markov Chain Monte Carlo (MCMC) approach. It is identified that estimated friction coefficients obtained by MCMC approach are in good agreement with measured ones. PMID:28773359

Friction and wear study of NR/SBR blends with Si3N4Filler

NASA Astrophysics Data System (ADS)

GaneshKumar, A.; Balaganesan, G.; Sivakumar, M. S.

2018-04-01

The aim of this paper is to investigate mechanical and frictional properties of natural rubber/styrene butadiene rubber (NR/SBR) blends with and without silicon nitride (Si3N4) filler. The rubber is surface modified by silane coupling agent (Si-69) for enhancing hydrophobic property. The Si3N4of percentage 0 1, 3, 5 and 7, is incorporated into NR/SBR rubber compounds with 20% precipitated silica. The specimens with and without fillers are prepared as per standard for tensile and friction testing. Fourier transform infrared (FTIR) spectroscopy test is conducted and it is inferred that the coupling agent is covalently bonded on the surface of Si3N4 particles and an organic coating layer is formed. The co-efficient of friction and specific wear rate of NR/SBR blends are examined using an in-house built friction tester in a disc-on-plate (DOP) configuration. The specimens are tested to find coefficient of friction (COF) against steel grip antiskid plate under dry, mud, wet and oil environmental conditions. It is found that the increase in tensile strength and modulus at low percentage of Si3N4 dispersion. It is also observed that increase in sliding friction co-efficient and decrease in wear rate for 1% of Si3N4 dispersion in NR/SBR blends. The friction tested surfaces are inspected using Scanning Electron Microscope (SEM) and 3D non contact surface profiler.

An eight-legged tactile sensor to estimate coefficient of static friction.

PubMed

Wei Chen; Rodpongpun, Sura; Luo, William; Isaacson, Nathan; Kark, Lauren; Khamis, Heba; Redmond, Stephen J

2015-08-01

It is well known that a tangential force larger than the maximum static friction force is required to initiate the sliding motion between two objects, which is governed by a material constant called the coefficient of static friction. Therefore, knowing the coefficient of static friction is of great importance for robot grippers which wish to maintain a stable and precise grip on an object during various manipulation tasks. Importantly, it is most useful if grippers can estimate the coefficient of static friction without having to explicitly explore the object first, such as lifting the object and reducing the grip force until it slips. A novel eight-legged sensor, based on simplified theoretical principles of friction is presented here to estimate the coefficient of static friction between a planar surface and the prototype sensor. Each of the sensor's eight legs are straight and rigid, and oriented at a specified angle with respect to the vertical, allowing it to estimate one of five ranges (5 = 8/2 + 1) that the coefficient of static friction can occupy. The coefficient of friction can be estimated by determining whether the legs have slipped or not when pressed against a surface. The coefficients of static friction between the sensor and five different materials were estimated and compared to a measurement from traditional methods. A least-squares linear fit of the sensor estimated coefficient showed good correlation with the reference coefficient with a gradient close to one and an r(2) value greater than 0.9.

Effects of Stone-Wales and vacancy defects in atomic-scale friction on defective graphite

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sun, Xiao-Yu; Key Laboratory of Hubei Province for Water Jet Theory and New Technology, Wuhan University, Wuhan 430072; Wu, RunNi

2014-05-05

Graphite is an excellent solid lubricant for surface coating, but its performance is significantly weakened by the vacancy or Stone-Wales (SW) defect. This study uses molecular dynamics simulations to explore the frictional behavior of a diamond tip sliding over a graphite which contains a single defect or stacked defects. Our results suggest that the friction on defective graphite shows a strong dependence on defect location and type. The 5-7-7-5 structure of SW defect results in an effectively negative slope of friction. For defective graphite containing a defect in the surface, adding a single vacancy in the interior layer will decreasemore » the friction coefficients, while setting a SW defect in the interior layer may increase the friction coefficients. Our obtained results may provide useful information for understanding the atomic-scale friction properties of defective graphite.« less

Systematic Breakdown of Amontons' Law of Friction for an Elastic Object Locally Obeying Amontons' Law

PubMed Central

Otsuki, Michio; Matsukawa, Hiroshi

2013-01-01

In many sliding systems consisting of solid object on a solid substrate under dry condition, the friction force does not depend on the apparent contact area and is proportional to the loading force. This behaviour is called Amontons' law and indicates that the friction coefficient, or the ratio of the friction force to the loading force, is constant. Here, however, using numerical and analytical methods, we show that Amontons' law breaks down systematically under certain conditions for an elastic object experiencing a friction force that locally obeys Amontons' law. The macroscopic static friction coefficient, which corresponds to the onset of bulk sliding of the object, decreases as pressure or system length increases. This decrease results from precursor slips before the onset of bulk sliding, and is consistent with the results of certain previous experiments. The mechanisms for these behaviours are clarified. These results will provide new insight into controlling friction. PMID:23545778

Condensation heat transfer and flow friction in silicon microchannels

NASA Astrophysics Data System (ADS)

Wu, Huiying; Wu, Xinyu; Qu, Jian; Yu, Mengmeng

2008-11-01

An experimental investigation was performed on heat transfer and flow friction characteristics during steam condensation flow in silicon microchannels. Three sets of trapezoidal silicon microchannels, with hydraulic diameters of 77.5 µm, 93.0 µm and 128.5 µm respectively, were tested under different flow and cooling conditions. It was found that both the condensation heat transfer Nusselt number (Nu) and the condensation two-phase frictional multiplier (phi2Lo) were dependent on the steam Reynolds number (Rev), condensation number (Co) and dimensionless hydraulic diameter (Dh/L). With the increase in the steam Reynolds number, condensation number and dimensionless hydraulic diameter, the condensation Nusselt number increased. However, different variations were observed for the condensation two-phase frictional multiplier. With the increase in the steam Reynolds number and dimensionless hydraulic diameter, the condensation two-phase frictional multiplier decreased, while with the increase in the condensation number, the condensation two-phase frictional multiplier increased. Based on the experimental results, dimensionless correlations for condensation heat transfer and flow friction in silicon microchannels were proposed for the first time. These correlations can be used to determine the condensation heat transfer coefficient and pressure drop in silicon microchannels if the steam mass flow rate, cooling rate and geometric parameters are fixed. It was also found that the condensation heat transfer and flow friction have relations to the injection flow (a transition flow pattern from the annular flow to the slug/bubbly flow), and with injection flow moving toward the outlet, both the condensation heat transfer coefficient and the condensation two-phase frictional multiplier increased.

Environmental influences on the friction behavior of glasses

NASA Astrophysics Data System (ADS)

Rolf, Jacqueline C.

Two aspects of the friction behavior of glasses were the main focus of this investigation. First, the influence of aqueous inorganic salt solutions on friction and damage on soda-lime-silica, vitreous silica, and an aluminosilicate glass high in alumina content were studied. It was found that the pH of a solution has a higher influence on the friction behavior than the concentration of electrolyte and the size of ions in the solution. A minimum at the i.e.p. (iso-electric point) of the network former of the glass was found, i.e., soda-lime-silica and vitreous silica showed a small minimum in friction at a pH of about 1.8, which corresponds to the i.e.p. of silica. Two small minima were observed for the aluminosilicate in the vicinities of the i.e.p.'s of silica and alumina respectively. The damage created by the frictional contact showed variations with environment. Microindentation experiments on the same glasses were performed in the same environments to compare the responses to the findings of the friction test. For soda-lime-silica and vitreous silica, a maximum in hardness was found at the i.e.p. of the glasses, and for the aluminosilicate, two maxima were found in the vicinity of the i.e.p.'s of silica and alumina respectively, confirming the findings of the friction tests. A data-fitting analysis showed that the major contribution to the observed trends originates from the elastic properties of the surface. A model describing the influence of surface charging on the mechanical properties of the glass surface is suggested. The second major aspect of the study was the influence of temperature on the friction coefficients and resulting surface damage of commercial glasses. Four float glasses were selected, and vitreous silica was tested for comparison. As expected, the coefficients of friction were found to increase, with increasing temperature. Very small differences in composition had an effect on the temperature dependence of the coefficients of friction. Tin and air sides exhibited differences in friction behavior, which were ascribed to chemical differences between the two sides. The float bath seems to have a large effect on friction also, since the air sides showed larger variations in coefficients of friction than the tin sides. A technique for quantitative analysis of surface damage was developed, and coefficients of friction and surface damage were found to correlate very well. Infrared reflection and emission spectroscopy were used to analyze the surface structural changes as a function of temperature. Float B, a glass which exhibited good damage resistance, displayed a very different spectrum than the other float glasses. Contact angle measurements confirmed the results of the IR-spectroscopy work and the friction tests.

Molecular dynamics simulations of metallic friction and of its dependence on electric currents - development and first results

NASA Astrophysics Data System (ADS)

Meintanis, Evangelos Anastasios

We have extended the HOLA molecular dynamics (MD) code to run slider-on-block friction experiments for Al and Cu. Both objects are allowed to evolve freely and show marked deformation despite the hardness difference. We recover realistic coefficients of friction and verify the importance of cold-welding and plastic deformations in dry sliding friction. Our first data also show a mechanism for decoupling between load and friction at high velocities. Such a mechanism can explain an increase in the coefficient of friction of metals with velocity. The study of the effects of currents on our system required the development of a suitable electrodynamic (ED) solver, as the disparity of MD and ED time scales threatened the efficiency of our code. Our first simulations combining ED and MD are presented.

Behavior of aircraft antiskid breaking systems on dry and wet runway surfaces: A slip-ratio-controlled system with ground speed reference from unbraked nose wheel

NASA Technical Reports Server (NTRS)

Tanner, J. A.; Stubbs, S. M.

1977-01-01

An experimental investigation was conducted at the Langley aircraft landing loads and traction facility to study the braking and cornering response of a slip ratio controlled aircraft antiskid braking system with ground speed reference derived from an unbraked nose wheel. The investigation, conducted on dry and wet runway surfaces, utilized one main gear wheel, brake, and tire assembly of a DC-9 series 10 airplane. During maximum braking, the average ratio of the drag force friction coefficient developed by the antiskid system to the maximum drag force friction coefficient available was higher on the dry surface than on damp and flooded surfaces, and was reduced with lighter vertical loads, higher yaw angles, and when new tire treads were replaced by worn treads. Similarly, the average ratio of side force friction coefficient developed by the tire under antiskid control to the maximum side force friction coefficient available to a freely rolling yawed tire decreased with increasing yaw angle, generally increased with ground speed, and decreased when tires with new treads were replaced by those with worn treads.

Solvent friction effects propagate over the entire protein molecule through low-frequency collective modes.

PubMed

Moritsugu, Kei; Kidera, Akinori; Smith, Jeremy C

2014-07-24

Protein solvation dynamics has been investigated using atom-dependent Langevin friction coefficients derived directly from molecular dynamics (MD) simulations. To determine the effect of solvation on the atomic friction coefficients, solution and vacuum MD simulations were performed for lysozyme and staphylococcal nuclease and analyzed by Langevin mode analysis. The coefficients thus derived are roughly correlated with the atomic solvent-accessible surface area (ASA), as expected from the fact that friction occurs as the result of collisions with solvent molecules. However, a considerable number of atoms with higher friction coefficients are found inside the core region. Hence, the influence of solvent friction propagates into the protein core. The internal coefficients have large contributions from the low-frequency modes, yielding a simple picture of the surface-to-core long-range damping via solvation governed by collective low-frequency modes. To make use of these findings in implicit-solvent modeling, we compare the all-atom friction results with those obtained using Langevin dynamics (LD) with two empirical representations: the constant-friction and the ASA-dependent (Pastor-Karplus) friction models. The constant-friction model overestimates the core and underestimates the surface damping whereas the ASA-dependent friction model, which damps protein atoms only on the solvent-accessible surface, reproduces well the friction coefficients for both the surface and core regions observed in the explicit-solvent MD simulations. Therefore, in LD simulation, the solvent friction coefficients should be imposed only on the protein surface.

Solvent friction effects propagate over the entire protein molecule through low-frequency collective modes

DOE Office of Scientific and Technical Information (OSTI.GOV)

Moritsugu, Kei; Kidera, Akinori; Smith, Jeremy C.

2014-06-25

Protein solvation dynamics has been investigated using atom-dependent Langevin friction coefficients derived directly from molecular dynamics (MD) simulations. To determine the effect of solvation on the atomic friction coefficients, solution and vacuum MD simulations were performed for lysozyme and staphylococcal nuclease and analyzed by Langevin mode analysis. The coefficients thus derived are roughly correlated with the atomic solvent-accessible surface area (ASA), as expected from the fact that friction occurs as the result of collisions with solvent molecules. However, a considerable number of atoms with higher friction coefficients are found inside the core region. Hence, the influence of solvent friction propagatesmore » into the protein core. The internal coefficients have large contributions from the low-frequency modes, yielding a simple picture of the surface-to-core long-range damping via solvation governed by collective low-frequency modes. To make use of these findings in implicit-solvent modeling, we compare the all-atom friction results with those obtained using Langevin dynamics (LD) with two empirical representations: the constant-friction and the ASA-dependent (Pastor Karplus) friction models. The constant-friction model overestimates the core and underestimates the surface damping whereas the ASA-dependent friction model, which damps protein atoms only on the solvent-accessible surface, reproduces well the friction coefficients for both the surface and core regions observed in the explicit-solvent MD simulations. Furthermore, in LD simulation, the solvent friction coefficients should be imposed only on the protein surface.« less

Fragility and hysteretic creep in frictional granular jamming.

PubMed

Bandi, M M; Rivera, M K; Krzakala, F; Ecke, R E

2013-04-01

The granular jamming transition is experimentally investigated in a two-dimensional system of frictional, bidispersed disks subject to quasistatic, uniaxial compression without vibrational disturbances (zero granular temperature). Three primary results are presented in this experimental study. First, using disks with different static friction coefficients (μ), we experimentally verify numerical results that predict jamming onset at progressively lower packing fractions with increasing friction. Second, we show that the first compression cycle measurably differs from subsequent cycles. The first cycle is fragile-a metastable configuration with simultaneous jammed and unjammed clusters-over a small packing fraction interval (φ(1)<φ<φ(2)) and exhibits simultaneous exponential rise in pressure and exponential decrease in disk displacements over the same packing fraction interval. This fragile behavior is explained through a percolation mechanism of stressed contacts where cluster growth exhibits spatial correlation with disk displacements and contributes to recent results emphasizing fragility in frictional jamming. Control experiments show that the fragile state results from the experimental incompatibility between the requirements for zero friction and zero granular temperature. Measurements with several disk materials of varying elastic moduli E and friction coefficients μ show that friction directly controls the start of the fragile state but indirectly controls the exponential pressure rise. Finally, under repetitive loading (compression) and unloading (decompression), we find the system exhibits pressure hysteresis, and the critical packing fraction φ(c) increases slowly with repetition number. This friction-induced hysteretic creep is interpreted as the granular pack's evolution from a metastable to an eventual structurally stable configuration. It is shown to depend on the quasistatic step size Δφ, which provides the only perturbative mechanism in the experimental protocol, and the friction coefficient μ, which acts to stabilize the pack.

Postoperative changes in in vivo measured friction in total hip joint prosthesis during walking.

PubMed

Damm, Philipp; Bender, Alwina; Bergmann, Georg

2015-01-01

Loosening of the artificial cup and inlay is the most common reasons for total hip replacement failures. Polyethylene wear and aseptic loosening are frequent reasons. Furthermore, over the past few decades, the population of patients receiving total hip replacements has become younger and more active. Hence, a higher level of activity may include an increased risk of implant loosening as a result of friction-induced wear. In this study, an instrumented hip implant was used to measure the contact forces and friction moments in vivo during walking. Subsequently, the three-dimensional coefficient of friction in vivo was calculated over the whole gait cycle. Measurements were collected from ten subjects at several time points between three and twelve months postoperative. No significant change in the average resultant contact force was observed between three and twelve months postoperative. In contrast, a significant decrease of up to 47% was observed in the friction moment. The coefficient of friction also decreased over postoperative time on average. These changes may be caused by 'running-in' effects of the gliding components or by the improved lubricating properties of the synovia. Because the walking velocity and contact forces were found to be nearly constant during the observed period, the decrease in friction moment suggests an increase in fluid viscosity. The peak values of the contact force individually varied by 32%-44%. The friction moment individually differed much more, by 110%-129% at three and up to 451% at twelve months postoperative. The maximum coefficient of friction showed the highest individual variability, about 100% at three and up to 914% at twelve months after surgery. These individual variations in the friction parameters were most likely due to different 'running-in' effects that were influenced by the individual activity levels and synovia properties.

Friction behaviour of aluminium composites mixed with carbon fibers with different orientations

NASA Astrophysics Data System (ADS)

Caliman, R.

2016-08-01

The primary goal of this study work it was to distinguish a mixture of materials with enhanced friction and wearing behaviour. The composite materials may be differentiated from alloys; which can contain two more components but are formed naturally through different processes such as casting. The load applied on the specimen during the tests, is playing a very important role regarding friction coefficient and also the wearing speed. Sintered composites are gaining importance because the reinforcement serves to reduce the coefficient of thermal expansion and increase the strength and modulus. The friction tests are carried out, at the room temperature in dry condition, on a pin-on-disc machine. The exponentially decreasing areas form graphs, represented to the curves coefficient of friction, are attributed to the formation of lubricant transfer film and initial polishing surface samples. The influence of the orientation of the carbon fibers on the friction properties in the sintered polymer composites may be studied by the use of both mechanical wear tests by microscopy and through the use of phenomenological models.

[Determination of a Friction Coefficient for THA Bearing Couples].

PubMed

Vrbka, M; Nečas, D; Bartošík, J; Hartl, M; Křupka, I; Galandáková, A; Gallo, J

2015-01-01

The wear of articular surfaces is considered one of the most important factors limiting the life of total hip arthroplasty (THA). It is assumed that the particles released from the surface of a softer material induce a complex inflammatory response, which will eventually result in osteolysis and aseptic loosening. Implant wear is related to a friction coefficient which depends on combination of the materials used, roughness of the articulating surfaces, internal clearance, and dimensions of the prosthesis. The selected parameters of the bearing couples tested were studied using an experimental device based on the principle of a pendulum. Bovine serum was used as a lubricant and the load corresponded to a human body mass of 75 kg. The friction coefficient was derived from a curve of slowdown of pendulum oscillations. Roughness was measured with a device working on the principle of interferometry. Clearance was assessed by measuring diameters of the acetabular and femoral heads with a 3D optical scanner. The specimens tested included unused metal-on-highly cross-linked polyethylene, ceramic-on-highly cross-linked polyethylene and ceramic-on-ceramic bearing couples with the diameters of 28 mm and 36 mm. For each measured parameter, an arithmetic mean was calculated from 10 measurements. 1) The roughness of polyethylene surfaces was higher by about one order of magnitude than the roughness of metal and ceramic components. The Protasul metal head had the least rough surface (0.003 μm). 2) The ceramic-on-ceramic couples had the lowest clearance. Bearing couples with polyethylene acetabular liners had markedly higher clearances ranging from 150 μm to 545 μm. A clearance increased with large femoral heads (up to 4-fold in one of the couple tested). 3) The friction coefficient was related to the combination of materials; it was lowest in ceramic-on-ceramic surfaces (0.11 to 0.12) and then in ceramic-on-polyethylene implants (0.13 to 0.14). The friction coefficient is supposed to increase with a decreasing femoral head diameter. However, in the bearing couples with polyethylene liners manufactured by one company, paradoxically, the friction coefficient slightly increased with an increase in femoral head size from 28 mm to 36 mm. 4) The lowest friction moment (< 3.5 Nm) was found for ceramic-on-ceramic implants 28 mm in diameter; the highest values were recorded in metal-on-polyethylene bearing couples 36 mm in diameter (> 7 Nm). Although our study confirmed that the bearing couples produced by different manufacturers varied to some extent in the parameters studied, in our opinion, this variability was not significant because it was not within an order of magnitude in any of the tests. The study showed that both the friction coefficient and the friction moment are affected more by the combination of materials than by the diameter of a femoral head. The best results were achieved in ceramic-on-ceramic implants.

Skin friction under pressure. The role of micromechanics

NASA Astrophysics Data System (ADS)

Leyva-Mendivil, Maria F.; Lengiewicz, Jakub; Limbert, Georges

2018-03-01

The role of contact pressure on skin friction has been documented in multiple experimental studies. Skin friction significantly raises in the low-pressure regime as load increases while, after a critical pressure value is reached, the coefficient of friction of skin against an external surface becomes mostly insensitive to contact pressure. However, up to now, no study has elucidated the qualitative and quantitative nature of the interplay between contact pressure, the material and microstructural properties of the skin, the size of an indenting slider and the resulting measured macroscopic coefficient of friction. A mechanistic understanding of these aspects is essential for guiding the rational design of products intended to interact with the skin through optimally-tuned surface and/or microstructural properties. Here, an anatomically-realistic 2D multi-layer finite element model of the skin was embedded within a computational contact homogenisation procedure. The main objective was to investigate the sensitivity of macroscopic skin friction to the parameters discussed above, in addition to the local (i.e. microscopic) coefficient of friction defined at skin asperity level. This was accomplished via the design of a large-scale computational experiment featuring 312 analyses. Results confirmed the potentially major role of finite deformations of skin asperities on the resulting macroscopic friction. This effect was shown to be modulated by the level of contact pressure and relative size of skin surface asperities compared to those of a rigid slider. The numerical study also corroborated experimental observations concerning the existence of two contact pressure regimes where macroscopic friction steeply and non-linearly increases up to a critical value, and then remains approximately constant as pressure increases further. The proposed computational modelling platform offers attractive features which are beyond the reach of current analytical models of skin friction, namely, the ability to accommodate arbitrary kinematics, non-linear constitutive properties and the complex skin microstructure.

Influence of Boussinesq coefficient on depth-averaged modelling of rapid flows

NASA Astrophysics Data System (ADS)

Yang, Fan; Liang, Dongfang; Xiao, Yang

2018-04-01

The traditional Alternating Direction Implicit (ADI) scheme has been proven to be incapable of modelling trans-critical flows. Its inherent lack of shock-capturing capability often results in spurious oscillations and computational instabilities. However, the ADI scheme is still widely adopted in flood modelling software, and various special treatments have been designed to stabilise the computation. Modification of the Boussinesq coefficient to adjust the amount of fluid inertia is a numerical treatment that allows the ADI scheme to be applicable to rapid flows. This study comprehensively examines the impact of this numerical treatment over a range of flow conditions. A shock-capturing TVD-MacCormack model is used to provide reference results. For unsteady flows over a frictionless bed, such as idealised dam-break floods, the results suggest that an increase in the value of the Boussinesq coefficient reduces the amplitude of the spurious oscillations. The opposite is observed for steady rapid flows over a frictional bed. Finally, a two-dimensional urban flooding phenomenon is presented, involving unsteady flow over a frictional bed. The results show that increasing the value of the Boussinesq coefficient can significantly reduce the numerical oscillations and reduce the predicted area of inundation. In order to stabilise the ADI computations, the Boussinesq coefficient could be judiciously raised or lowered depending on whether the rapid flow is steady or unsteady and whether the bed is frictional or frictionless. An increase in the Boussinesq coefficient generally leads to overprediction of the propagating speed of the flood wave over a frictionless bed, but the opposite is true when bed friction is significant.

Prediction of Very High Reynolds Number Compressible Skin Friction

NASA Technical Reports Server (NTRS)

Carlson, John R.

1998-01-01

Flat plate skin friction calculations over a range of Mach numbers from 0.4 to 3.5 at Reynolds numbers from 16 million to 492 million using a Navier Stokes method with advanced turbulence modeling are compared with incompressible skin friction coefficient correlations. The semi-empirical correlation theories of van Driest; Cope; Winkler and Cha; and Sommer and Short T' are used to transform the predicted skin friction coefficients of solutions using two algebraic Reynolds stress turbulence models in the Navier-Stokes method PAB3D. In general, the predicted skin friction coefficients scaled well with each reference temperature theory though, overall the theory by Sommer and Short appeared to best collapse the predicted coefficients. At the lower Reynolds number 3 to 30 million, both the Girimaji and Shih, Zhu and Lumley turbulence models predicted skin-friction coefficients within 2% of the semi-empirical correlation skin friction coefficients. At the higher Reynolds numbers of 100 to 500 million, the turbulence models by Shih, Zhu and Lumley and Girimaji predicted coefficients that were 6% less and 10% greater, respectively, than the semi-empirical coefficients.

Friction and wear of iron-base binary alloys in sliding contact with silicon carbide in vacuum

NASA Technical Reports Server (NTRS)

Miyoshi, K.; Buckley, D. H.

1980-01-01

Multipass sliding friction experiments were conducted with various iron base binary alloys in contact with a single crystal silicon carbide surface in vacuum. Results indicate that the atomic size and concentration of alloy elements play important roles in controlling the transfer and friction properties of iron base binary alloys. Alloys having high solute concentration produce more transfer than do alloys having low solute concentration. The coefficient of friction during multipass sliding generally increases with an increase in the concentration of alloying element. The change of friction with succeeding passes after the initial pass also increases as the solute to iron, atomic radius ratio increases or decreases from unity.

Low friction wear resistant graphene films

DOE Office of Scientific and Technical Information (OSTI.GOV)

Sumant, Anirudha V.; Berman, Diana; Erdemir, Ali

A low friction wear surface with a coefficient of friction in the superlubric regime including graphene and nanoparticles on the wear surface is provided, and methods of producing the low friction wear surface are also provided. A long lifetime wear resistant surface including graphene exposed to hydrogen is provided, including methods of increasing the lifetime of graphene containing wear surfaces by providing hydrogen to the wear surface.

Development of a New Method to Investigate the Dynamic Friction Behavior of Interfaces Using a Kolsky Tension Bar

DOE PAGES

Sanborn, B.; Song, B.; Nishida, E.

2017-11-02

In order to understand interfacial interaction of a bi-material during an impact loading event, the dynamic friction coefficient is one of the key parameters that must be characterized and quantified. In this study, a new experimental method to determine the dynamic friction coefficient between two metals was developed by using a Kolsky tension bar and a custom-designed friction fixture. Polyvinylidene fluoride (PVDF) force sensors were used to measure the normal force applied to the friction tribo pairs and the friction force was measured with conventional Kolsky tension bar method. To evaluate the technique, the dynamic friction coefficient between 4340 steelmore » and 7075-T6 aluminum was investigated at an impact speed of approximately 8 m/s. Additionally, the dynamic friction coefficient of the tribo pairs with varied surface roughness was also investigated. The data suggest that higher surface roughness leads to higher friction coefficients at the same speed of 8 m/s.« less

Real-Time Dynamic Observation of Micro-Friction on the Contact Interface of Friction Lining

PubMed Central

Zhang, Dekun; Chen, Kai; Guo, Yongbo

2018-01-01

This paper aims to investigate the microscopic friction mechanism based on in situ microscopic observation in order to record the deformation and contact situation of friction lining during the frictional process. The results show that friction coefficient increased with the shear deformation and energy loss of the surfacee, respectively. Furthermore, the friction mechanism mainly included adhesive friction in the high-pressure and high-speed conditions, whereas hysteresis friction was in the low-pressure and low-speed conditions. The mixed-friction mechanism was in the period when the working conditions varied from high pressure and speed to low pressure and speed. PMID:29498677

Atomic scale friction of molecular adsorbates during diffusion.

PubMed

Lechner, B A J; de Wijn, A S; Hedgeland, H; Jardine, A P; Hinch, B J; Allison, W; Ellis, J

2013-05-21

Experimental observations suggest that molecular adsorbates exhibit a larger friction coefficient than atomic species of comparable mass, yet the origin of this increased friction is not well understood. We present a study of the microscopic origins of friction experienced by molecular adsorbates during surface diffusion. Helium spin-echo measurements of a range of five-membered aromatic molecules, cyclopentadienyl, pyrrole, and thiophene, on a copper(111) surface are compared with molecular dynamics simulations of the respective systems. The adsorbates have different chemical interactions with the surface and differ in bonding geometry, yet the measurements show that the friction is greater than 2 ps(-1) for all these molecules. We demonstrate that the internal and external degrees of freedom of these adsorbate species are a key factor in the underlying microscopic processes and identify the rotation modes as the ones contributing most to the total measured friction coefficient.

Required coefficient of friction during turning at self-selected slow, normal, and fast walking speeds.

PubMed

Fino, Peter; Lockhart, Thurmon E

2014-04-11

This study investigated the relationship of required coefficient of friction to gait speed, obstacle height, and turning strategy as participants walked around obstacles of various heights. Ten healthy, young adults performed 90° turns around corner pylons of four different heights at their self selected normal, slow, and fast walking speeds using both step and spin turning strategies. Kinetic data was captured using force plates. Results showed peak required coefficient of friction (RCOF) at push off increased with increased speed (slow μ=0.38, normal μ=0.45, and fast μ=0.54). Obstacle height had no effect on RCOF values. The average peak RCOF for fast turning exceeded the OSHA safety guideline for static COF of μ>0.50, suggesting further research is needed into the minimum static COF to prevent slips and falls, especially around corners. Copyright © 2014 Elsevier Ltd. All rights reserved.

Studies on heat transfer and pressure drop in turbulent flow of silver - water nanofluids through a circular tube at constant wall heat flux

NASA Astrophysics Data System (ADS)

Iyahraja, S.; Rajadurai, J. Selwin; Rajesh, S.; Pandian, R. Seeni Thangaraj; Kumaran, M. Selva; Selvakumar, G.

2018-07-01

In the present study, performance of convective heat transfer and friction factor of silver-water nanofluids in a horizontal circular pipe under turbulent flow were investigated experimentally under uniform heat flux condition. The volume concentration of silver nanoparticles is varied as 0.01, 0.05 and 0.1%. Heat transfer coefficient and friction factor of nanofluids were measured experimentally by varying the Reynolds number from 3000 to 21,000. It is observed that the addition of even low volume fraction of silver nanoparticles increases both Nusselt number and heat transfer coefficient of the nanofluid significantly. Nusselt number of silver-water nanofluid increases up to 32.6% for 0.1% volume fraction at Reynolds number of 21,000. However, the addition of nanoparticles in the base fluid increases the friction factor slightly. New empirical correlations are also proposed for the estimation of Nusselt number and friction factor of silver-water nanofluid based on the data of present experimental investigation. The proposed correlations of Nusselt number and friction factor show good agreement with their experimental data.

Studies on heat transfer and pressure drop in turbulent flow of silver - water nanofluids through a circular tube at constant wall heat flux

NASA Astrophysics Data System (ADS)

Iyahraja, S.; Rajadurai, J. Selwin; Rajesh, S.; Pandian, R. Seeni Thangaraj; Kumaran, M. Selva; Selvakumar, G.

2018-02-01

In the present study, performance of convective heat transfer and friction factor of silver-water nanofluids in a horizontal circular pipe under turbulent flow were investigated experimentally under uniform heat flux condition. The volume concentration of silver nanoparticles is varied as 0.01, 0.05 and 0.1%. Heat transfer coefficient and friction factor of nanofluids were measured experimentally by varying the Reynolds number from 3000 to 21,000. It is observed that the addition of even low volume fraction of silver nanoparticles increases both Nusselt number and heat transfer coefficient of the nanofluid significantly. Nusselt number of silver-water nanofluid increases up to 32.6% for 0.1% volume fraction at Reynolds number of 21,000. However, the addition of nanoparticles in the base fluid increases the friction factor slightly. New empirical correlations are also proposed for the estimation of Nusselt number and friction factor of silver-water nanofluid based on the data of present experimental investigation. The proposed correlations of Nusselt number and friction factor show good agreement with their experimental data.

Study of tribological properties of natural rubber containing carbon nanotubes and carbon black as hybrid fillers

NASA Astrophysics Data System (ADS)

Harea, Evghenii; Stoček, Radek; Storozhuk, Liudmyla; Sementsov, Yurii; Kartel, Nikolai

2018-04-01

Dry friction and wear properties of natural rubber (NR), containing multi-walled carbon nanotubes (MWCNT) and carbon black (CB), were investigated. Natural rubber (NR)-based composites containing all common additives and curatives, and a fixed amount (30 phr—parts per 100 rubber by weight) of hybrid fillers (MWCNT x + CB30-x ) were prepared by simple mixing procedure and tested. The main goal was to study the behaviours of composites at different tribological testing conditions, such as friction speed and normal load. It was found that with an increase of concentration of MWCNT from x = 0 phr to x = 5 phr in studied composites, there was a decrease in the coefficient of friction (COF) with no significant change in wear in the framework of each used combination of testing parameters. Generally, higher friction speed at certain normal force led to the increase of COF of all the samples and wear reflected deliberate value fluctuation. Also, it was established that considerable growth of wear and unexpected reducing of friction coefficient ensued from increasing of applied load for every fixed sliding speed.

The Effect of a Variable Disc Pad Friction Coefficient for the Mechanical Brake System of a Railway Vehicle

PubMed Central

Lee, Nam-Jin; Kang, Chul-Goo

2015-01-01

A brake hardware-in-the-loop simulation (HILS) system for a railway vehicle is widely applied to estimate and validate braking performance in research studies and field tests. When we develop a simulation model for a full vehicle system, the characteristics of all components are generally properly simplified based on the understanding of each component’s purpose and interaction with other components. The friction coefficient between the brake disc and the pad used in simulations has been conventionally considered constant, and the effect of a variable friction coefficient is ignored with the assumption that the variability affects the performance of the vehicle braking very little. However, the friction coefficient of a disc pad changes significantly within a range due to environmental conditions, and thus, the friction coefficient can affect the performance of the brakes considerably, especially on the wheel slide. In this paper, we apply a variable friction coefficient and analyzed the effects of the variable friction coefficient on a mechanical brake system of a railway vehicle. We introduce a mathematical formula for the variable friction coefficient in which the variable friction is represented by two variables and five parameters. The proposed formula is applied to real-time simulations using a brake HILS system, and the effectiveness of the formula is verified experimentally by testing the mechanical braking performance of the brake HILS system. PMID:26267883

The Effect of a Variable Disc Pad Friction Coefficient for the Mechanical Brake System of a Railway Vehicle.

PubMed

Lee, Nam-Jin; Kang, Chul-Goo

2015-01-01

A brake hardware-in-the-loop simulation (HILS) system for a railway vehicle is widely applied to estimate and validate braking performance in research studies and field tests. When we develop a simulation model for a full vehicle system, the characteristics of all components are generally properly simplified based on the understanding of each component's purpose and interaction with other components. The friction coefficient between the brake disc and the pad used in simulations has been conventionally considered constant, and the effect of a variable friction coefficient is ignored with the assumption that the variability affects the performance of the vehicle braking very little. However, the friction coefficient of a disc pad changes significantly within a range due to environmental conditions, and thus, the friction coefficient can affect the performance of the brakes considerably, especially on the wheel slide. In this paper, we apply a variable friction coefficient and analyzed the effects of the variable friction coefficient on a mechanical brake system of a railway vehicle. We introduce a mathematical formula for the variable friction coefficient in which the variable friction is represented by two variables and five parameters. The proposed formula is applied to real-time simulations using a brake HILS system, and the effectiveness of the formula is verified experimentally by testing the mechanical braking performance of the brake HILS system.

Frictional properties of exhumed fault gouges in DFDP-1 cores, Alpine Fault, New Zealand

USGS Publications Warehouse

Boulton, Carolyn; Moore, Diane E.; Lockner, David A.; Toy, Virginia G.; Townend, John; Southerland, Rupert

2014-01-01

Principal slip zone gouges recovered during the Deep Fault Drilling Project (DFDP-1), Alpine Fault, New Zealand, were deformed in triaxial friction experiments at temperatures, T, of up to 350°C, effective normal stresses, σn′, of up to 156 MPa, and velocities between 0.01 and 3 µm/s. Chlorite/white mica-bearing DFDP-1A blue gouge, 90.62 m sample depth, is frictionally strong (friction coefficient, μ, 0.61–0.76) across all experimental conditions tested (T = 70–350°C, σn′ = 31.2–156 MPa); it undergoes a transition from positive to negative rate dependence as T increases past 210°C. The friction coefficient of smectite-bearing DFDP-1B brown gouge, 128.42 m sample depth, increases from 0.49 to 0.74 with increasing temperature and pressure (T = 70–210°C, σn′ = 31.2–93.6 MPa); the positive to negative rate dependence transition occurs as T increases past 140°C. These measurements indicate that, in the absence of elevated pore fluid pressures, DFDP-1 gouges are frictionally strong under conditions representative of the seismogenic crust.

Studies of friction and wear characteristics of various wires for wire-brush skids

NASA Technical Reports Server (NTRS)

Dreher, R. C.

1977-01-01

The friction and wear characteristics of 22 types and sizes of wires for potential use in wire-brush skids were studied. These characteristics were determined by placing brushes made from candidate wires on a belt sander whose moving belt simulated landing roll-out distance. At the same time, the drag force and wear behavior were monitored. Data were obtained over distances up to 3048 m (10,000 ft) at preselected bearing pressures of 172 to 1034 kPa (25 to 150 psi). In general, the friction coefficient developed by the candidate wires was found to be independent of bearing pressure and ranged between 0.4 and 0.6 under the test conditions of this investigation. The friction coefficient was not degraded when the surface was wetted and appears to be independent of wire diameter except perhaps when wire size is relatively large compared with the surface asperities. Generally, the high friction demonstrated by the soft materials was accompanied by high wear rates; conversely, the hard materials provided greater wear resistance but offered lower friction. For all test wires, the wear was shown to increase with increasing bearing pressure, in general, for the same bearing pressure, wear increased with increasing wire diameter and decreased when the surface was wetted.

Modelling clustering of vertically aligned carbon nanotube arrays.

PubMed

Schaber, Clemens F; Filippov, Alexander E; Heinlein, Thorsten; Schneider, Jörg J; Gorb, Stanislav N

2015-08-06

Previous research demonstrated that arrays of vertically aligned carbon nanotubes (VACNTs) exhibit strong frictional properties. Experiments indicated a strong decrease of the friction coefficient from the first to the second sliding cycle in repetitive measurements on the same VACNT spot, but stable values in consecutive cycles. VACNTs form clusters under shear applied during friction tests, and self-organization stabilizes the mechanical properties of the arrays. With increasing load in the range between 300 µN and 4 mN applied normally to the array surface during friction tests the size of the clusters increases, while the coefficient of friction decreases. To better understand the experimentally obtained results, we formulated and numerically studied a minimalistic model, which reproduces the main features of the system with a minimum of adjustable parameters. We calculate the van der Waals forces between the spherical friction probe and bunches of the arrays using the well-known Morse potential function to predict the number of clusters, their size, instantaneous and mean friction forces and the behaviour of the VACNTs during consecutive sliding cycles and at different normal loads. The data obtained by the model calculations coincide very well with the experimental data and can help in adapting VACNT arrays for biomimetic applications.

Frictional strength and heat flow of southern San Andreas Fault

NASA Astrophysics Data System (ADS)

Zhu, P. P.

2016-01-01

Frictional strength and heat flow of faults are two related subjects in geophysics and seismology. To date, the investigation on regional frictional strength and heat flow still stays at the stage of qualitative estimation. This paper is concentrated on the regional frictional strength and heat flow of the southern San Andreas Fault (SAF). Based on the in situ borehole measured stress data, using the method of 3D dynamic faulting analysis, we quantitatively determine the regional normal stress, shear stress, and friction coefficient at various seismogenic depths. These new data indicate that the southern SAF is a weak fault within the depth of 15 km. As depth increases, all the regional normal and shear stresses and friction coefficient increase. The former two increase faster than the latter. Regional shear stress increment per kilometer equals 5.75 ± 0.05 MPa/km for depth ≤15 km; regional normal stress increment per kilometer is equal to 25.3 ± 0.1 MPa/km for depth ≤15 km. As depth increases, regional friction coefficient increment per kilometer decreases rapidly from 0.08 to 0.01/km at depths less than ~3 km. As depth increases from ~3 to ~5 km, it is 0.01/km and then from ~5 to 15 km, and it is 0.002/km. Previously, frictional strength could be qualitatively determined by heat flow measurements. It is difficult to obtain the quantitative heat flow data for the SAF because the measured heat flow data exhibit large scatter. However, our quantitative results of frictional strength can be employed to investigate the heat flow in the southern SAF. We use a physical quantity P f to describe heat flow. It represents the dissipative friction heat power per unit area generated by the relative motion of two tectonic plates accommodated by off-fault deformation. P f is called "fault friction heat." On the basis of our determined frictional strength data, utilizing the method of 3D dynamic faulting analysis, we quantitatively determine the regional long-term fault friction heat at various seismogenic depths in the southern SAF. The new data show that as depth increases, regional friction stress increases within the depth of 15 km; its increment per kilometer equals 5.75 ± 0.05 MPa/km. As depth increases, regional long-term fault friction heat increases; its increment per kilometer is equal to 3.68 ± 0.03 mW/m2/km. The values of regional long-term fault friction heat provided by this study are always lower than those from heat flow measurements. The difference between them and the scatter existing in the measured heat flow data are mainly caused by the following processes: (i) heat convection, (ii) heat advection, (iii) stress accumulation, (iv) seismic bursts between short-term lull periods in a long-term period, and (v) influence of seismicity in short-term periods upon long-term slip rate and heat flow. Fault friction heat is a fundamental parameter in research on heat flow.

Properties data for opening the Galileo's partially unfurled main antenna

NASA Technical Reports Server (NTRS)

Miyoshi, Kazuhisa; Pepper, Stephen V.

1992-01-01

An investigation was conducted into the friction and wear behavior of both unlubricated and dry-film-lubricated (Tiolube 460) titanium alloy (Ti-6Al-4V) in contact with an uncoated high-nickel-content superalloy (Inconel 718) both in vacuum and in air. The acquisition of friction and wear data for this sliding couple was motivated by the need for input data for the 'antenna stuck ribs model' effort to free Galileo's High Gain Antenna. The results of the investigation indicate that galling occurred in the unlubricated system in vacuum and that the coefficient of friction increased to 1.2. The abnormally high friction (1.45) was observed when relatively large wear debris clogged at the sliding interface. The coefficient of friction for the dry-film-lubricated system in vacuum is 0.04, while the value in air is 0.13. The endurance life of the dry-film lubricant is about three orders of magnitude greater in vacuum than in air. The worn surfaces of the dry-film-lubricated Ti-6Al-4V pin and Inconel 718 disk first run in humid air and then rerun in vacuum was completely different from that of the pin and disk run only in vacuum. When galling occurred in the humid-air and vacuum contact, coefficient of friction rose to 0.32 when sliding in humid air and to 1.4 when sliding in vacuum. The galling was accompanied by severe surface damage and extensive transfer of the Ti-6Al-4V to the Inconel 718, or vice versa. When spalling occurred in the dry-film-lubricated Ti-6Al-4V pin run only in vacuum, the coefficient of friction rose to 0.36 or greater. The wear damage caused by spalling can self-heal when rerun in vacuum - the coefficient of friction decreased to 0.05. The friction and wear data obtained can be used for the 'antenna stuck ribs model' effort to free Galileo's high gain antenna.

Evaluation and Description of Friction between an Electro-Deposited Coating and a Ceramic Ball under Fretting Condition

PubMed Central

Kim, Kyungmok

2015-01-01

This article describes fretting behavior of zirconia and silicon nitride balls on an electro-deposited coating. Fretting tests are performed using a ball-on-flat configuration. The evolution of the kinetic friction coefficient is determined, along with slip ratio. Experimental results show that the steady-state friction coefficient between ceramic balls (Si3N4 and ZrO2) and an electro-deposited coating is about 0.06, lower than the value between AISI 52100 ball and the coating. After a steady-state sliding, the transition of the friction coefficient is varied with a ball. The friction coefficient for ZrO2 balls became a critical value after higher fretting cycles than those for Si3N4 and AISI 52100 balls. In addition, it is identified that two parameters can describe the transition of the friction coefficient. Finally, the evolution of the friction coefficient is expressed as an exponential or a power-law form. PMID:28793471

Friction and wear of single-crystal manganese-zinc ferrite

NASA Technical Reports Server (NTRS)

Miyoshi, K.; Buckley, D. H.

1979-01-01

Sliding friction experiments were conducted with single crystal manganese-zinc ferrite in contact with itself and with transition metals. Results indicate mating highest atomic density directions (110) on matched crystallographic planes exhibit the lowest coefficient of friction, indicating that direction is important in the friction behavior of ferrite. Matched parallel high atomic density planes and crystallographic directions at the interface exhibit low coefficients of friction. The coefficients of friction for ferrite in contact with various metals are related to the relative chemical activity of these metals. The more active the metal, the higher the coefficient of friction. Cracking and the formation of hexagon- and rectangular-shaped platelet wear debris due to cleavages of (110) planes are observed on the ferrite surfaces as a result of sliding.

Modifying friction between ultra-high molecular weight polyethylene (UHMWPE) yarns with plasma enhanced chemical vapour deposition (PCVD)

NASA Astrophysics Data System (ADS)

Chu, Yanyan; Chen, Xiaogang; Tian, Lipeng

2017-06-01

Ultra-high molecular weight polyethylene (UHMWPE) yarns are widely used in military applications for protection owing to its high modulus and high strength; however, the friction between UHMWPE yarns is too small, which is a weakness for ballistic applications. The purpose of current research is to increase the friction between UHMWPE yarns by plasma enhanced chemical vapour deposition (PCVD). The changes of morphology and chemical structure were characterised by SEM and FTIR individually. The coefficients of friction between yarns were tested by means of Capstan method. Results from tests showed that the yarn-yarn coefficient of static friction (CSF) has been improved from 0.12 to 0.23 and that of kinetic friction (CSF) increased from 0.11 to 0.19, as the samples exposure from 21 s to 4 min. The more inter-yarn friction can be attributed to more and more particles and more polar groups deposited on the surfaces of yarns, including carboxyl, carbonyl, hydroxyl and amine groups and compounds containing silicon. The tensile strength and modulus of yarns, which are essential to ballistic performance, keep stable and are not affected by the treatments, indicating that PCVD treatment is an effective way to improve the inter-yarn friction without mechanical property degradation.

Experimental rig to estimate the coefficient of friction between tire and surface in airplane touchdown simulations.

PubMed

Li, Chengwei; Zhan, Liwei

2015-08-01

To estimate the coefficient of friction between tire and runway surface during airplane touchdowns, we designed an experimental rig to simulate such events and to record the impact and friction forces being executed. Because of noise in the measured signals, we developed a filtering method that is based on the ensemble empirical mode decomposition and the bandwidth of probability density function of each intrinsic mode function to extract friction and impact force signals. We can quantify the coefficient of friction by calculating the maximum values of the filtered force signals. Signal measurements are recorded for different drop heights and tire rotational speeds, and the corresponding coefficient of friction is calculated. The result shows that the values of the coefficient of friction change only slightly. The random noise and experimental artifact are the major reason of the change.

Experimental research on friction coefficient between grain bulk and bamboo clappers

NASA Astrophysics Data System (ADS)

Tang, Gan; Sun, Ping; Zhao, Yanqi; Yin, Lingfeng; Zhuang, Hong

2017-12-01

A silo is an important piece of storage equipment, especially in the grain industry. The internal friction angle and the friction coefficient between the grain and the silo wall are the main parameters needed for calculating the lateral pressure of the silo wall. Bamboo is used in silo walls, but there are no provisions about the friction coefficient between bulk grain and bamboo clappers in existing codes. In this paper, the material of the silo wall is bamboo. The internal friction of five types of grain and the friction coefficient between the grain and the bamboo clappers were measured with an equal-strain direct shear apparatus. By comparing the experimental result values with the code values, the friction coefficient between the grain bulk and bamboo clappers is lower than that between grain and steel wall and that between grain and concrete wall. The differences in value are 0.21 and 0.09, respectively.

Nonlinear friction model for servo press simulation

NASA Astrophysics Data System (ADS)

Ma, Ninshu; Sugitomo, Nobuhiko; Kyuno, Takunori; Tamura, Shintaro; Naka, Tetsuo

2013-12-01

The friction coefficient was measured under an idealized condition for a pulse servo motion. The measured friction coefficient and its changing with both sliding distance and a pulse motion showed that the friction resistance can be reduced due to the re-lubrication during unloading process of the pulse servo motion. Based on the measured friction coefficient and its changes with sliding distance and re-lubrication of oil, a nonlinear friction model was developed. Using the newly developed the nonlinear friction model, a deep draw simulation was performed and the formability was evaluated. The results were compared with experimental ones and the effectiveness was verified.

Research on the impact of surface properties of particle on damping effect in gear transmission under high speed and heavy load

NASA Astrophysics Data System (ADS)

Xiao, Wangqiang; Chen, Zhiwei; Pan, Tianlong; Li, Jiani

2018-01-01

The vibration and noise from gear transmission have great damage on the mechanical equipment and operators. Through inelastic collisions and friction between particles, the energy can be dissipated in gear transmission. A dynamic model of particle dampers in gear transmission was put forward in this paper. The performance of particle dampers in centrifugal fields under different rotational speeds and load was investigated. The surface properties such as the impact of coefficient of restitution and friction coefficient of the particle on the damping effect were analyzed and the total energy loss was obtained by discrete element method (DEM). The vibration from time-varying mesh stiffness was effectively reduced by particle dampers and the optimum coefficient of restitution was discovered under different rotational speeds and load. Then, a test bench for gear transmission was constructed, and the vibration of driven gear and driving gear were measured through a three-directional wireless acceleration sensor. The research results agree well with the simulation results. While at relatively high speed, smaller coefficient of restitution achieves better damping effect. As to friction coefficient, at relatively high speed, the energy dissipation climbs up and then declines with the increase of the friction coefficient. The results can provide guidelines for the application of particle damper in gear transmission.

Study on Abrasive Wear of Brake Pad in the Large-megawatt Wind Turbine Brake Based on Deform Software

NASA Astrophysics Data System (ADS)

Zhang, Shengfang; Hao, Qiang; Sha, Zhihua; Yin, Jian; Ma, Fujian; Liu, Yu

2017-12-01

For the friction and wear issues of brake pads in the large-megawatt wind turbine brake during braking, this paper established the micro finite element model of abrasive wear by using Deform-2D software. Based on abrasive wear theory and considered the variation of the velocity and load in the micro friction and wear process, the Archard wear calculation model is developed. The influence rules of relative sliding velocity and friction coefficient in the brake pad and disc is analysed. The simulation results showed that as the relative sliding velocity increases, the wear will be more serious, while the larger friction coefficient lowered the contact pressure which released the wear of the brake pad.

Lubricating Properties of Ceramic-Bonded Calcium Fluoride Coatings on Nickel-Base Alloys from 75 to 1900 deg F

NASA Technical Reports Server (NTRS)

Sliney, Harold E.

1962-01-01

The endurance life and the friction coefficient of ceramic-bonded calcium fluoride (CaF2) coatings on nickel-base alloys were determined at temperatures from 75 F to 1900 F. The specimen configuration consisted of a hemispherical rider (3/16-in. rad.) sliding against the flat surface of a rotating disk. Increasing the ambient temperature (up to 1500 F) or the sliding velocity generally reduced the friction coefficient and improved coating life. Base-metal selection was critical above 1500 F. For instance, cast Inconel sliding against coated Inconel X was lubricated effectively to 1500 F, but at 1600 F severe blistering of the coatings occurred. However, good lubrication and adherence were obtained for Rene 41 sliding against coated Rene 41 at temperatures up to 1900 F; no blisters developed, coating wear life was fairly good, and the rider wear rate was significantly lower than for the unlubricated metals. Friction coefficients were 0.12 at 1500 F, 0.15 at 1700 F, and 0.17 at 1800 F and 1900 F. Because of its ready availability, Inconel X appears to be the preferred substrate alloy for applications in which the temperature does not exceed 1500 F. Rene 41 would have to be used in applications involving higher temperatures. Improved coating life was derived by either preoxidizing the substrate metals prior to the coating application or by applying a very thin (less than 0.0002 in.) burnished and sintered overlay to the surface of the coating. Preoxidation did not affect the friction coefficient. The overlay generally resulted in a higher friction coefficient than that obtained without the overlay. The combination of both modifications resulted in longer coating life and in friction coefficients intermediate between those obtained with either modification alone.

Prediction of friction coefficients for gases

NASA Technical Reports Server (NTRS)

Taylor, M. F.

1969-01-01

Empirical relations are used for correlating laminar and turbulent friction coefficients for gases, with large variations in the physical properties, flowing through smooth tubes. These relations have been used to correlate friction coefficients for hydrogen, helium, nitrogen, carbon dioxide and air.

A technique for measuring dynamic friction coefficient under impact loading

NASA Astrophysics Data System (ADS)

Lin, Y. L.; Qin, J. G.; Chen, R.; Zhao, P. D.; Lu, F. Y.

2014-09-01

We develop a novel setup based on the split Hopkinson pressure bar technique to test the dynamic friction coefficient under impact loading. In the setup, the major improvement is that the end of the incident bar near the specimen is wedge-shaped, which results in a combined compressive and shear loading applied to the specimen. In fact, the shear loading is caused by the interfacial friction between specimen and bars. Therefore, when the two loading force histories are measured, the friction coefficient histories can be calculated without any assumptions and theoretical derivations. The geometry of the friction pairs is simple, and can be either cuboid or cylindrical. Regarding the measurements, two quartz transducers are used to directly record the force histories, and an optical apparatus is designed to test the interfacial slip movement. By using the setup, the dynamic friction coefficient of PTFE/aluminum 7075 friction pairs was tested. The time resolved dynamic friction coefficient and slip movement histories were achieved. The results show that the friction coefficient changes during the loading process, the average data of the relatively stable flat plateau section of the friction coefficient curves is 0.137, the maximum normal pressure is 52 MPa, the maximum relative slip velocity is 1.5 m/s, and the acceleration is 8400 m2/s. Furthermore, the friction test was simulated using an explicit FEM code LS-DYNA. The simulation results showed that the constant pressure and slip velocity can both be obtained with a wide flat plateau incident pulse. For some special friction pairs, normal pressure up to a few hundred MPa, interfacial slip velocities up to 10 m/s, and slip movement up to centimeter-level can be expected.

A technique for measuring dynamic friction coefficient under impact loading.

PubMed

Lin, Y L; Qin, J G; Chen, R; Zhao, P D; Lu, F Y

2014-09-01

We develop a novel setup based on the split Hopkinson pressure bar technique to test the dynamic friction coefficient under impact loading. In the setup, the major improvement is that the end of the incident bar near the specimen is wedge-shaped, which results in a combined compressive and shear loading applied to the specimen. In fact, the shear loading is caused by the interfacial friction between specimen and bars. Therefore, when the two loading force histories are measured, the friction coefficient histories can be calculated without any assumptions and theoretical derivations. The geometry of the friction pairs is simple, and can be either cuboid or cylindrical. Regarding the measurements, two quartz transducers are used to directly record the force histories, and an optical apparatus is designed to test the interfacial slip movement. By using the setup, the dynamic friction coefficient of PTFE/aluminum 7075 friction pairs was tested. The time resolved dynamic friction coefficient and slip movement histories were achieved. The results show that the friction coefficient changes during the loading process, the average data of the relatively stable flat plateau section of the friction coefficient curves is 0.137, the maximum normal pressure is 52 MPa, the maximum relative slip velocity is 1.5 m/s, and the acceleration is 8400 m(2)/s. Furthermore, the friction test was simulated using an explicit FEM code LS-DYNA. The simulation results showed that the constant pressure and slip velocity can both be obtained with a wide flat plateau incident pulse. For some special friction pairs, normal pressure up to a few hundred MPa, interfacial slip velocities up to 10 m/s, and slip movement up to centimeter-level can be expected.

Determination of the Frictional Behavior at Compaction of Powder Materials Consisting of Spray-Dried Granules

NASA Astrophysics Data System (ADS)

Staf, Hjalmar; Olsson, Erik; Lindskog, Per; Larsson, Per-Lennart

2018-03-01

The frictional behavior during powder compaction and ejection is studied using an instrumented die with eight radial sensors. The average friction over the total powder pillar is used to determine a local friction coefficient at each sensor. By comparing forces at compaction with forces at ejection, it can be shown that the Coulomb's friction coefficient can be described as a function of normal pressure. Also stick phenomena has been investigated in order to assess its influence on the determination of the local friction coefficient.

Nanomechanical and nanotribological properties of Nb substituted TiN thin films

NASA Astrophysics Data System (ADS)

Krishna, M. Ghanashyam; Vasu, K.; Padmanabhan, K. A.

2012-06-01

Nanomechanical and nanotribological properties of Ti1-xNbxN (0≤x≤1) thin films were investigated as a function x. The films were deposited onto polycrystalline nuclear grade 316LN stainless steel (SS) substrate by radio frequency magnetron sputtering in 100% N2 plasma. The hardness and Young's modulus increased while the friction coefficient and wear volume decreased with increasing Nb substitution. The highest hardness achieved was 31GPa for x=0.77. At the same Nb concentration, the friction coefficient was 0.15 and the elastic recovery was 60%.

Flow and Heat Transfer in a Newtonian Nanoliquid due to a Curved Stretching Sheet

NASA Astrophysics Data System (ADS)

Siddheshwar, Pradeep Ganapathi; Nerolu, Meenakshi; Pažanin, Igor

2017-08-01

Flow of a Newtonian nanoliquid due to a curved stretching sheet and heat transfer in it is studied. The governing nonlinear partial differential equations are reduced to nonlinear ordinary differential equations with variable coefficients by using a similarity transformation. The flow characteristics are studied using plots of flow velocity components and the skin-friction coefficient as a function of suction-injection parameter, curvature, and volume fraction. Prescribed surface temperature and prescribed surface heat flux are considered for studying the temperature distribution in the flow. The thermophysical properties of 20 nanoliquids are considered in the investigation by modeling them through the use of phenomenological laws and mixture theory. The results of the corresponding problem involving a plane stretching sheet is obtained as a particular case of those obtained in the present paper. Skin friction coefficient and Nusselt number are evaluated and it is observed that skin friction coefficient decreases with concentration of nanoparticles in the absence as well as presence of suction where as Nusselt number increases with increase in concentration of nanoparticles in a dilute range.

Effects of friction layer characteristics on the tribological properties of Ni3Al solid-lubricating composites at different load conditions

NASA Astrophysics Data System (ADS)

Lu, Guanchen; Shi, Xiaoliang; Huang, Yuchun; Liu, Xiyao; Yang, Meijun

2018-05-01

This paper investigates the effects of friction layer characteristics of Ni3Al matrix self-lubricating composites (NMCs) on the tribological properties sliding against ceramic ball Si3N4 at dry friction process at the different load conditions. The characteristics of friction layer are performed in terms of hardness of wear scars, thickness and elemental distributions of friction layer. The results show that the microhardness of wear scars of NMCs increases with the increase of the sliding time and applied load, which results in friction coefficient reduced and wear rate decreased, indicating that the tribological performance of NMCs is obviously affected by microhardness of wear scar. However, under excessive applied load, the performance of friction layer of NMCs is deteriorated for the spalling of wear debris and deformation of contact surface. Therefore, selecting appropriate load conditions during the sliding contact, at the transition to the optimal properties of friction layer maybe occur. NMCs exhibits excellent tribological properties at 15N, which leads to the lowest friction coefficient (0.386) and wear rate (2.48 × 10‑5 mm3 N‑1 m‑1), as well as the smoothest surface of wear track compared with the other load conditions. Meanwhile, the elemental distributions analysis of cross-section of friction layer of NMCs shows that the frictional structures can be divided into three main layers. The thickness of the friction-affected layer varies with the changing of applied load. These results could provide a reference for preparing the solid-lubrication materials with better tribological properties.

Experimental Investigation of Average Heat-Transfer and Friction Coefficients for Air Flowing in Circular Tubes Having Square-Thread-Type Roughness

NASA Technical Reports Server (NTRS)

Sams, E. W.

1952-01-01

An investigation of forced-convection heat transfer and associated pressure drops was conducted with air flowing through electrically heated Inconel tubes having various degrees of square-thread-type roughness, an inside diameter of 1/2 inch, and a length of 24 inches. were obtained for tubes having conventional roughness ratios (height of thread/radius of tube) of 0 (smooth tube), 0.016, 0.025, and 0.037 over ranges of bulk Reynolds numbers up to 350,000, average inside-tube-wall temperatures up to 1950deg R, and heat-flux densities up to 115,000 Btu per hour per square foot. Data The experimental data showed that both heat transfer and friction increased with increase in surface roughness, becoming more pronounced with increase in Reynolds number; for a given roughness, both heat transfer and friction were also influenced by the tube wall-to-bulk temperature ratio. Good correlation of the heat-transfer data for all the tubes investigated was obtained by use of a modification of the conventional Nusselt correlation parameters wherein the mass velocity in the Reynolds number was replaced by the product of air density evaluated at the average film temperature and the so-called friction velocity; in addition, the physical properties of air were evaluated at the average film temperature. The isothermal friction data for the rough tubes, when plotted in the conventional manner, resulted in curves similar to those obtained by other investigators; that is, the curve for a given roughness breaks away from the Blasius line (representing turbulent flow in smooth tubes) at some value of Reynolds number, which decreases with increase in surface roughness, and then becomes a horizontal line (friction coefficient independent of Reynolds number). A comparison of the friction data for the rough tubes used herein indicated that the conventional roughness ratio is not an adequate measure of relative roughness for tubes having a square-thread-type element. The present data, as well as those of other investigators, were used to isolate the influence of ratios of thread height to width, thread spacing to width, and the conventional roughness ratio on the friction coefficient. A fair correlation of the friction data was obtained for each tube with heat addition when the friction coefficient and Reynolds number were defined on the basis of film properties; however, the data for each tube retained the curve characteristic of that particular roughness. The friction data for all the rough tubes could be represented by a single line for the complete turbulence region by incorporating a roughness parameter in the film correlation. No correlation was obtained for the region of incomplete turbulence.

Postoperative Changes in In Vivo Measured Friction in Total Hip Joint Prosthesis during Walking

PubMed Central

Damm, Philipp; Bender, Alwina; Bergmann, Georg

2015-01-01

Loosening of the artificial cup and inlay is the most common reasons for total hip replacement failures. Polyethylene wear and aseptic loosening are frequent reasons. Furthermore, over the past few decades, the population of patients receiving total hip replacements has become younger and more active. Hence, a higher level of activity may include an increased risk of implant loosening as a result of friction-induced wear. In this study, an instrumented hip implant was used to measure the contact forces and friction moments in vivo during walking. Subsequently, the three-dimensional coefficient of friction in vivo was calculated over the whole gait cycle. Measurements were collected from ten subjects at several time points between three and twelve months postoperative. No significant change in the average resultant contact force was observed between three and twelve months postoperative. In contrast, a significant decrease of up to 47% was observed in the friction moment. The coefficient of friction also decreased over postoperative time on average. These changes may be caused by ‘running-in’ effects of the gliding components or by the improved lubricating properties of the synovia. Because the walking velocity and contact forces were found to be nearly constant during the observed period, the decrease in friction moment suggests an increase in fluid viscosity. The peak values of the contact force individually varied by 32%-44%. The friction moment individually differed much more, by 110%-129% at three and up to 451% at twelve months postoperative. The maximum coefficient of friction showed the highest individual variability, about 100% at three and up to 914% at twelve months after surgery. These individual variations in the friction parameters were most likely due to different ‘running-in’ effects that were influenced by the individual activity levels and synovia properties. PMID:25806805

Enhanced surface friction coefficient and hydrophobicity of TPE substrates using an APPJ system

NASA Astrophysics Data System (ADS)

Sainz-García, Elisa; Alba-Elías, Fernando; Múgica-Vidal, Rodolfo; González-Marcos, Ana

2015-02-01

An APPJ system was used to deposit a coating that combines a low friction coefficient with a high water contact angle (WCA) on a thermoplastic elastomer substrate (TPE) that is used in automotive profiling. The main drawback of this research is that groups that improve the hydrophobicity of the surface worsen its tribological properties. To overcome this, this study explored the use of various mixtures of differing proportions of two precursors. They were a siloxane, aminopropyltriethoxysilane (APTES) that was used to reduce the friction coefficient by its content of SiOx and a fluorinated compound, (heptadecafluoro-1,1,2,2-tetrahydrodecyl)trimethoxysilane (FLUSI) that was used to improve the water-repellency characteristics, due to the presence of CF2 long chains. The coatings were characterized by Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR), X-ray Photoelectron Spectroscopy (XPS), dynamic Water Contact Angle (WCA), stability tests and tribological tests. It was found that an increase of the absorbance area under the SiOSi peak and inorganic groups is related to lower friction coefficients. On the other hand, the higher the CF2 percentage is, the higher the WCA is. The sample that was coated with 25% of FLUSI and 75% of APTES combined the improvements of both functional properties, the friction coefficient and the WCA. It has an average friction coefficient that is (0.530 ± 0.050) 51.5% lower and a WCA that is (θadv = 119.8° ± 4.75) 4.4% higher than the uncoated TPE sample. A satisfactory stability in humid ambient for twelve months showed a slight decrease of WCA (4.4%) for this sample. The results of this study permit one to realize the effectiveness of using fluorinated precursors to avoid a significant decrease in the WCA when applying a precursor to anti-friction improvement.

Effect of high energy X-ray irradiation on the nano-mechanical properties of human enamel and dentine.

PubMed

Liang, Xue; Zhang, Jing Yang; Cheng, Iek Ka; Li, Ji Yao

2016-01-01

Radiotherapy for malignancies in the head and neck can cause common complications that can result in tooth damage that are also known as radiation caries. The aim of this study was to examine damage to the surface topography and calculate changes in friction behavior and the nano-mechanical properties (elastic modulus, nanohardness and friction coefficient) of enamel and dentine from extracted human third molars caused by exposure to radiation. Enamel and dentine samples from 50 human third molars were randomly assigned to four test groups or a control group. The test groups were exposed to high energy X-rays at 2 Gy/day, 5 days/week for 5 days (10 Gy group), 15 days (30 Gy group), 25 days (50 Gy group), 35 days (70 Gy group); the control group was not exposed. The nanohardness, elastic modulus, and friction coefficient were analyzed using a Hysitron Triboindenter. The nano-mechanical properties of both enamel and dentine showed significant dose-response relationships. The nanohardness and elastic modulus were most variable between 30-50 Gy, while the friction coefficient was most variable between 0-10 Gy for dentine and 30-50 Gy for enamel. After exposure to X-rays, the fracture resistance of the teeth clearly decreased (rapidly increasing friction coefficient with increasing doses under the same load), and they were more fragile. These nano-mechanical changes in dental hard tissue may increase the susceptibility to caries. Radiotherapy caused nano-mechanical changes in dentine and enamel that were dose related. The key doses were 30-50 Gy and the key time points occurred during the 15th-25th days of treatment, which is when application of measures to prevent radiation caries should be considered.

Calculation of equivalent friction coefficient for castor seed by single screw press

NASA Astrophysics Data System (ADS)

Liu, R.; Xiao, Z.; Li, C.; Zhang, L.; Li, P.; Li, H.; Zhang, A.; Tang, S.; Sun, F.

2017-08-01

Based on the traction angle and transportation rate equation, castor beans were pressed by application of single screw under different cake diameter and different screw speed. The results showed that the greater the cake diameter and screw rotation speed, the greater the actual transmission rate was. The equivalent friction coefficient was defined and calculated as 0.4136, and the friction coefficients between press material and screw, bar cage were less than the equivalent friction coefficient value.

Tire-to-Surface Friction-Coefficient Measurements with a C-123B Airplane on Various Runway Surfaces

NASA Technical Reports Server (NTRS)

Sawyer, Richard H.; Kolnick, Joseph J.

1959-01-01

An investigation was conducted to obtain information on the tire-to-surface friction coefficients available in aircraft braking during the landing run. The tests were made with a C-123B airplane on both wet and dry concrete and bituminous pavements and on snow-covered and ice surfaces at speeds from 12 to 115 knots. Measurements were made of the maximum (incipient skidding) friction coefficient, the full-skidding (locked wheel) friction coefficient, and the wheel slip ratio during braking.

Wear behaviour and morphology of stir cast aluminium/SiC nanocomposites

NASA Astrophysics Data System (ADS)

Tanwir Alam, Md; Arif, Sajjad; Husain Ansari, Akhter

2018-04-01

Wear and friction play a vital role in the service life of components. Aluminium matrix nanocomposites possess tremendous potential for a number of applications in addition to their present uses. It is valuable to the field of newer materials for better performance in tribological applications. In this work, dry sliding wear, friction coefficient and morphology of aluminium alloy (A356) reinforced with silicon carbide nanoparticles (SiCn) were investigated. A356/SiCn nanocomposites (AMNCs) containing 1–5 weight percentage of SiCn were prepared through two-step stir casting process via mechanical ball milling. The wear test was conducted on pin-on-disc test apparatus. Regression analysis was performed to develop mathematical functions to fit the experimental data points. Morphological studies of Al and SiCn as-received, wear debris and worn surfaces were further analysed by SEM along with EDS. The occurrence of oxide layers was observed on worn surfaces. Iron trace was identified by wear debris. It was found that the wear loss and friction coefficient were strongly influenced by mechanical milling and SiCn content. The results exhibited that the friction coefficient reduces with the addition of SiCn as well as with the increase in load. However, wear resistance increases as the reinforcement content increases because of the embedding and wettability effects.

Study on the property of low friction complex graphite-like coating containing tantalum

NASA Astrophysics Data System (ADS)

Wang, Zuoping; Feng, Lajun; Shen, Wenning

2018-03-01

In order to enhance equipment lifetime under low oil or even dry conditions, tantalum was introduced into the graphite-like coating (GLC) by sputtering mosaic targets. The results showed that the introduction of Ta obviously reduced the friction coefficient and hardness of the GLC, while improved the wearability. When the atomic percentage of Ta was larger than 3%, the steady friction coefficient was lower than 0.01, suggesting the coating exhibited super lubricity. When the content of Ta was about 5.0%, the average friction coefficient was 0.02 by a sliding friction test under load of 20 N in unlubricated condition. Its average friction coefficient reduced by 75%, compared with that of control GLC (0.0825).

Friction and morphology of magnetic tapes in sliding contact with nickel-zinc ferrite

NASA Technical Reports Server (NTRS)

Miyoshi, K.; Buckley, D. H.; Bhushan, B.

1984-01-01

Friction and morphological studies were conducted with magnetic tapes containing a Ni-Zn ferrite hemispherical pin in laboratory air at a relative humidity of 40 percent and at 23 C. The results indicate that the binder plays a significant role in the friction properties, morphology, and microstructure of the tape. Comparisons were made with four binders: nitrocellulose; poly (vinyledene) chloride; cellulose acetate; and hydroxyl-terminated, low molecular weight polyester added to the base polymer, polyester-polyurethane. The coefficient of friction was lowest for the tape with the nitrocellulose binder and increased in the order hydroxylterminated, low molecular weight polyester resin; poly (vinyledene) chloride; and cellulose acetate. The degree of enclosure of the oxide particles by the binder was highest for hydroxyl-terminated, low molecular weight polyester and decreased in the order cellulose acetate, poly (vinyledene) chloride, and nitrocellulose. The nature of deformation of the tape was a factor in controlling friction. The coefficient of friction under elastic contact conditions was considerably lower than under conditions that produced plastic contacts.

The role of adsorbed water on the friction of a layer of submicron particles

USGS Publications Warehouse

Sammis, Charles G.; Lockner, David A.; Reches, Ze’ev

2011-01-01

Anomalously low values of friction observed in layers of submicron particles deformed in simple shear at high slip velocities are explained as the consequence of a one nanometer thick layer of water adsorbed on the particles. The observed transition from normal friction with an apparent coefficient near μ = 0.6 at low slip speeds to a coefficient near μ = 0.3 at higher slip speeds is attributed to competition between the time required to extrude the water layer from between neighboring particles in a force chain and the average lifetime of the chain. At low slip speeds the time required for extrusion is less than the average lifetime of a chain so the particles make contact and lock. As slip speed increases, the average lifetime of a chain decreases until it is less than the extrusion time and the particles in a force chain never come into direct contact. If the adsorbed water layer enables the otherwise rough particles to rotate, the coefficient of friction will drop to μ = 0.3, appropriate for rotating spheres. At the highest slip speeds particle temperatures rise above 100°C, the water layer vaporizes, the particles contact and lock, and the coefficient of friction rises to μ = 0.6. The observed onset of weakening at slip speeds near 0.001 m/s is consistent with the measured viscosity of a 1 nm thick layer of adsorbed water, with a minimum particle radius of approximately 20 nm, and with reasonable assumptions about the distribution of force chains guided by experimental observation. The reduction of friction and the range of velocities over which it occurs decrease with increasing normal stress, as predicted by the model. Moreover, the analysis predicts that this high-speed weakening mechanism should operate only for particles with radii smaller than approximately 1 μm. For larger particles the slip speed required for weakening is so large that frictional heating will evaporate the adsorbed water and weakening will not occur.

A Tactile Sensor Using Piezoresistive Beams for Detection of the Coefficient of Static Friction

PubMed Central

Okatani, Taiyu; Takahashi, Hidetoshi; Noda, Kentaro; Takahata, Tomoyuki; Matsumoto, Kiyoshi; Shimoyama, Isao

2016-01-01

This paper reports on a tactile sensor using piezoresistive beams for detection of the coefficient of static friction merely by pressing the sensor against an object. The sensor chip is composed of three pairs of piezoresistive beams arranged in parallel and embedded in an elastomer; this sensor is able to measure the vertical and lateral strains of the elastomer. The coefficient of static friction is estimated from the ratio of the fractional resistance changes corresponding to the sensing elements of vertical and lateral strains when the sensor is in contact with an object surface. We applied a normal force on the sensor surface through objects with coefficients of static friction ranging from 0.2 to 1.1. The fractional resistance changes corresponding to vertical and lateral strains were proportional to the applied force. Furthermore, the relationship between these responses changed according to the coefficients of static friction. The experimental result indicated the proposed sensor could determine the coefficient of static friction before a global slip occurs. PMID:27213374

Research on the Mechanism of In-Plane Vibration on Friction Reduction

PubMed Central

Wang, Peng; Ni, Hongjian; Wang, Ruihe; Liu, Weili; Lu, Shuangfang

2017-01-01

A modified model for predicting the friction force between drill-string and borehole wall under in-plane vibrations was developed. It was found that the frictional coefficient in sliding direction decreased significantly after applying in-plane vibration on the bottom specimen. The friction reduction is due to the direction change of friction force, elastic deformation of surface asperities and the change of frictional coefficient. Normal load, surface topography, vibration direction, velocity ratio and interfacial shear factor are the main influence factors of friction force in sliding direction. Lower driving force can be realized for a pair of determinate rubbing surfaces under constant normal load by setting the driving direction along the minimum arithmetic average attack angle direction, and applying intense longitudinal vibration on the rubbing pair. The modified model can significantly improve the accuracy in predicting frictional coefficient under vibrating conditions, especially under the condition of lower velocity ratio. The results provide a theoretical gist for friction reduction technology by vibrating drill-string, and provide a reference for determination of frictional coefficient during petroleum drilling process, which has great significance for realizing digitized and intelligent drilling. PMID:28862679

Finger pad friction and its role in grip and touch

PubMed Central

Adams, Michael J.; Johnson, Simon A.; Lefèvre, Philippe; Lévesque, Vincent; Hayward, Vincent; André, Thibaut; Thonnard, Jean-Louis

2013-01-01

Many aspects of both grip function and tactile perception depend on complex frictional interactions occurring in the contact zone of the finger pad, which is the subject of the current review. While it is well established that friction plays a crucial role in grip function, its exact contribution for discriminatory touch involving the sliding of a finger pad is more elusive. For texture discrimination, it is clear that vibrotaction plays an important role in the discriminatory mechanisms. Among other factors, friction impacts the nature of the vibrations generated by the relative movement of the fingertip skin against a probed object. Friction also has a major influence on the perceived tactile pleasantness of a surface. The contact mechanics of a finger pad is governed by the fingerprint ridges and the sweat that is exuded from pores located on these ridges. Counterintuitively, the coefficient of friction can increase by an order of magnitude in a period of tens of seconds when in contact with an impermeably smooth surface, such as glass. In contrast, the value will decrease for a porous surface, such as paper. The increase in friction is attributed to an occlusion mechanism and can be described by first-order kinetics. Surprisingly, the sensitivity of the coefficient of friction to the normal load and sliding velocity is comparatively of second order, yet these dependencies provide the main basis of theoretical models which, to-date, largely ignore the time evolution of the frictional dynamics. One well-known effect on taction is the possibility of inducing stick–slip if the friction decreases with increasing sliding velocity. Moreover, the initial slip of a finger pad occurs by the propagation of an annulus of failure from the perimeter of the contact zone and this phenomenon could be important in tactile perception and grip function. PMID:23256185

Development and evaluation of a reinforced polymeric biomaterial for use as an orthodontic wire

NASA Astrophysics Data System (ADS)

Zufall, Scott William

Composite archwires have the potential to provide esthetic and functional improvements over conventional wires. As part of an ongoing effort to bring these materials into general use, composite wires were fabricated using a photo-pultrusion manufacturing technique, and subsequently coated with a 10 mum layer of poly(chloro-p-xylylene). Coated and uncoated composites were subjected to several different evaluations to assess their ability to perform the functions of an orthodontic archwire. An investigation of the viscoelastic behavior of uncoated composite wires was conducted at a physiological temperature of 37°C using a bend stress relaxation test. Over 90 day testing periods, energy losses increased with decreasing reinforcement levels from to 8% of the initial wire stress. Final viscous losses were 1% for all reinforcement levels. Relaxed elastic moduli for the composite wires were comparable to the reported elastic moduli of conventional orthodontic wires that are typically used for initial and intermediate alignment procedures. Frictional characteristics were evaluated in passive and active configurations for uncoated composite wires against three contemporary orthodontic brackets. Kinetic coefficients of friction were the same for all wire-bracket combinations tested and were slightly lower than the reported coefficients of other initial and intermediate alignment wires. Wear patterns on the wires, which were largely caused by sharp leading edges of the bracket slots, were characteristic of plowing and cutting wear behaviors. This wear caused glass fibers to be released from the surface of the wires, presenting a potential irritant. Coated composite wires were subjected to the same frictional analysis as the uncoated wires. A mathematical model of the archwire-bracket system was derived using engineering mechanics, and used to define a coefficient of binding. The coating increased the frictional coefficients of the wires by 72%, yet the binding coefficient was unchanged. When frictional data for initial and intermediate alignment wires were compared, the coated composites had higher friction than all but one couple. However, binding coefficients were comparable. Glass fibers were contained for all testing conditions, although the coating was often damaged by plowing or cutting wear. Overall, the coating improved the clinical acceptability of the composite wires.

Estimation of the Friction Coefficient of a Nanostructured Composite Coating

NASA Astrophysics Data System (ADS)

Shil'ko, S. V.; Chernous, D. A.; Ryabchenko, T. V.; Hat'ko, V. V.

2017-11-01

The frictional-mechanical properties of a thin polymer-ceramic coating obtained by gas-phase impregnation of nanoporous anodic alumina with a fluoropolymer (octafluorocyclobutane) have been investigated. The coefficient of sliding friction of the coating is predicted based on an analysis of contact deformation within the framework of the Winkler elastic foundation hypothesis and a three-phase micromechanical model. It is shown that an acceptable prediction accuracy can be obtained considering the uniaxial strain state of the coating. It was found that, on impregnation by the method of plasmachemical treatment, the relative depth of penetration of the polymer increased almost in proportion to the processing time. The rate and maximum possible depth of penetration of the polymer into nanoscale pores grew with increasing porosity of the alumina substrate.

Influence of Nitrogen Flow Rate on Friction Coefficient and Surface Roughness of TiN Coatings Deposited on Tool Steel Using Arc Method

NASA Astrophysics Data System (ADS)

Hamzah, Esah; Ourdjini, Ali; Ali, Mubarak; Akhter, Parvez; Hj. Mohd Toff, Mohd Radzi; Abdul Hamid, Mansor

In the present study, the effect of various N2 gas flow rates on friction coefficient and surface roughness of TiN-coated D2 tool steel was examined by a commercially available cathodic arc physical vapor deposition (CAPVD) technique. A Pin-on-Disc test was carried out to study the Coefficient of friction (COF) versus sliding distance. A surface roughness tester measured the surface roughness parameters. The minimum values for the COF and surface roughness were recorded at a N2 gas flow rate of 200 sccm. The increase in the COF and surface roughness at a N2 gas flow rate of 100 sccm was mainly attributed to an increase in both size and number of titanium particles, whereas the increase at 300 sccm was attributed to a larger number of growth defects generated during the coating process. These ideas make it possible to optimize the coating properties as a function of N2 gas flow rate for specific applications, e.g. cutting tools for automobiles, aircraft, and various mechanical parts.

Scaling features of the tribology of polymer brushes of increasing grafting density around the mushroom-to-brush transition.

PubMed

Mayoral, E; Klapp, J; Gama Goicochea, A

2017-01-01

Nonequilibrium coarse-grained, dissipative particle dynamics simulations of complex fluids, made up of polymer brushes tethered to planar surfaces immersed in a solvent yield nonmonotonic behavior of the friction coefficient as a function of the polymer grating density on the substrates, Γ, while the viscosity shows a monotonically increasing dependence on Γ. This effect is shown to be independent of the degree of polymerization, N, and the size of the system. It arises from the composition and the structure of the first particle layer adjacent to each surface that results from the confinement of the fluid. Whenever such layers are made up of as close a proportion of polymer beads to solvent particles as there are in the fluid, the friction coefficient shows a minimum, while for disparate proportions the friction coefficient grows. At the mushroom-to-brush transition (MBT) the viscosity scales with an exponent that depends on the characteristic exponent of the MBT (6/5) and the solvent quality exponent (ν=0.5, for θsolvent), but it is independent of the polymerization degree (N). On the other hand, the friction coefficient at the MBT scales as μ∼N^{6/5}, while the grafting density at the MBT scales as Γ∼N^{-6/5} when friction is minimal, in agreement with previous scaling theories. We argue these aspects are the result of cooperative phenomena that have important implications for the understanding of biological brushes and the design of microfluidics devices, among other applications of current academic and industrial interest.

The effect of ion plated silver and sliding friction on tensile stress-induced cracking in aluminum oxide

NASA Technical Reports Server (NTRS)

Sliney, Harold E.; Spalvins, Talivaldis

1991-01-01

A Hertzian analysis of the effect of sliding friction on contact stresses in alumina is used to predict the critical load for crack generation. The results for uncoated alumina and alumina coated with ion plated silver are compared. Friction coefficient inputs to the analysis are determined experimentally with a scratch test instrument employing an 0.2 mm radius diamond stylus. A series of scratches were made at constant load increments on coated and uncoated flat alumina surfaces. Critical loads for cracking are detected by microscopic examination of cross sections of scratches made at various loads and friction coefficients. Acoustic emission (AE) and friction trends were also evaluated as experimental techniques for determining critical loads for cracking. Analytical predictions correlate well with micrographic evidence and with the lowest load at which AE is detected in multiple scratch tests. Friction/load trends are not good indicators of early crack formation. Lubrication with silver films reduced friction and thereby increased the critical load for crack initiation in agreement with analytical predictions.

The effect of ion-plated silver and sliding friction on tensile stress-induced cracking in aluminum oxide

NASA Technical Reports Server (NTRS)

Sliney, Harold E.; Spalvins, Talivaldis

1993-01-01

A Hertzian analysis of the effect of sliding friction on contact stresses in alumina is used to predict the critical load for crack generation. The results for uncoated alumina and alumina coated with ion plated silver are compared. Friction coefficient inputs to the analysis are determined experimentally with a scratch test instrument employing an 0.2 mm radius diamond stylus. A series of scratches were made at constant load increments on coated and uncoated flat alumina surfaces. Critical loads for cracking are detected by microscopic examination of cross sections of scratches made at various loads and friction coefficients. Acoustic emission (AE) and friction trends were also evaluated as experimental techniques for determining critical loads for cracking. Analytical predictions correlate well with micrographic evidence and with the lowest load at which AE is detected in multiple scratch tests. Friction/load trends are not good indicators of early crack formation. Lubrication with silver films reduced friction and thereby increased the critical load for crack initiation in agreement with analytical predictions.

Stretching a Curved Surface in a Viscous Fluid

NASA Astrophysics Data System (ADS)

Sajid, M.; N., Ali; T., Javed; Z., Abbas

2010-02-01

This work is concerned with the viscous flow due to a curved stretching sheet. The similarity solution of the problem is obtained numerically by a shooting method using the Runge-Kutta algorithm. The physical quantities of interest like the fluid velocity and skin friction coefficient are obtained and discussed under the influence of dimensionless curvature. It is evident from the results that dimensionless curvature causes an increase in boundary layer thickness and a decrease in the skin friction coefficient.

The role of crystallographic texture in achieving low friction zinc oxide nanolaminate films

NASA Astrophysics Data System (ADS)

Mojekwu, Nneoma

Metal oxide nanolaminate films are potential high temperature solid lubricants due to their ability to exhibit significant plasticity when grain size is reduced to the nanometer scale, and defective growth structure is achieved by condensation of oxygen vacancies to form intrinsic stacking faults. This is in contrast to conventional microcrystalline and single crystal oxides that exhibit brittle fracture during loading in a sliding contact. This study emphasizes the additional effect of growth orientation, in particular crystallographic texture, on determining the sliding friction behavior in nanocolumnar grain zinc oxide films grown by atomic layer deposition. It was determined that zinc oxide low (0002) versus higher (101¯3) surface energy crystallographic planes influenced the sliding friction coefficient. Texturing of the (0002) grains resulted in a decreased adhesive component of friction thereby lowering the sliding friction coefficient to ˜0.25, while the friction coefficient doubled to ˜0.5 with increasing contribution of surface (101¯3) grains. In addition, the variation of the x-ray grazing incident angle from 0.5° to 5° was studied to better understand the surface grain orientation as a function of ZnO layer thickness in one versus four bilayer nanolaminates where the under layer (seed layer) was load-bearing Zn(Ti,Zr)O3.

Dynamic Stability of the Rate, State, Temperature, and Pore Pressure Friction Model at a Rock Interface

NASA Astrophysics Data System (ADS)

Sinha, Nitish; Singh, Arun K.; Singh, Trilok N.

2018-05-01

In this article, we study numerically the dynamic stability of the rate, state, temperature, and pore pressure friction (RSTPF) model at a rock interface using standard spring-mass sliding system. This particular friction model is a basically modified form of the previously studied friction model namely the rate, state, and temperature friction (RSTF). The RSTPF takes into account the role of thermal pressurization including dilatancy and permeability of the pore fluid due to shear heating at the slip interface. The linear stability analysis shows that the critical stiffness, at which the sliding becomes stable to unstable or vice versa, increases with the coefficient of thermal pressurization. Critical stiffness, on the other hand, remains constant for small values of either dilatancy factor or hydraulic diffusivity, but the same decreases as their values are increased further from dilatancy factor (˜ 10^{ - 4} ) and hydraulic diffusivity (˜ 10^{ - 9} {m}2 {s}^{ - 1} ) . Moreover, steady-state friction is independent of the coefficient of thermal pressurization, hydraulic diffusivity, and dilatancy factor. The proposed model is also used for predicting time of failure of a creeping interface of a rock slope under the constant gravitational force. It is observed that time of failure decreases with increase in coefficient of thermal pressurization and hydraulic diffusivity, but the dilatancy factor delays the failure of the rock fault under the condition of heat accumulation at the creeping interface. Moreover, stiffness of the rock-mass also stabilizes the failure process of the interface as the strain energy due to the gravitational force accumulates in the rock-mass before it transfers to the sliding interface. Practical implications of the present study are also discussed.

Friction and Surface Damage of Several Corrosion-resistant Materials

NASA Technical Reports Server (NTRS)

Peterson, Marshall B; Johnson, Robert L

1952-01-01

Friction and surface damage of several materials that are resistant to corrosion due to liquid metals was studied in air. The values of kinetic friction coefficient at low sliding velocities and photomicrographs of surface damage were obtained. Appreciable surface damage was evident for all materials tested. The friction coefficients for the combinations of steel, stainless steel, and monel sliding against steel, stainless steel, nickel, Iconel, and Nichrome ranged from 0.55 for the monel-Inconel combination to 0.97 for the stainless-steel-nickel combination; for steel, stainless steel, monel, and tungsten carbide against zirconium, the friction coefficient was approximately 0.47. Lower coefficients of friction (0.20 to 0.60) and negligible surface failure at light loads were obtained with tungsten carbide when used in combination with various plate materials.

Hard and low friction nitride coatings and methods for forming the same

DOEpatents

Erdemir, Ali; Urgen, Mustafa; Cakir, Ali Fuat; Eryilmaz, Osman Levent; Kazmanli, Kursat; Keles, Ozgul

2007-05-01

An improved coating material possessing super-hard and low friction properties and a method for forming the same. The improved coating material includes the use of a noble metal or soft metal homogeneously distributed within a hard nitride material. The addition of small amounts of such metals into nitrides such as molybdenum nitride, titanium nitride, and chromium nitride results in as much as increasing of the hardness of the material as well as decreasing the friction coefficient and increasing the oxidation resistance.

Tribological Properties of PVD Ti/C-N Nanocoatnigs

NASA Astrophysics Data System (ADS)

Leitans, A.; Lungevics, J.; Rudzitis, J.; Filipovs, A.

2017-04-01

The present paper discusses and analyses tribological properties of various coatings that increase surface wear resistance. Four Ti/C-N nanocoatings with different coating deposition settings are analysed. Tribological and metrological tests on the samples are performed: 2D and 3D parameters of the surface roughness are measured with modern profilometer, and friction coefficient is measured with CSM Instruments equipment. Roughness parameters Ra, Sa, Sz, Str, Sds, Vmp, Vmc and friction coefficient at 6N load are determined during the experiment. The examined samples have many pores, which is the main reason for relatively large values of roughness parameter. A slight wear is identified in all four samples as well; its friction coefficient values range from 0,.21 to 0.29. Wear rate values are not calculated for the investigated coatings, as no expressed tribotracks are detected on the coating surface.

In-situ-measurement of the friction coefficient in the deep drawing process

NASA Astrophysics Data System (ADS)

Recklin, V.; Dietrich, F.; Groche, P.

2017-09-01

The surface texture plays an important role in the tribological behaviour of deep drawn components. It influences both the process of sheet metal forming as well as the properties for post processing, such as paint appearance, bonding, or corrosion tendency. During the forming process, the texture of the sheet metal and therefore its friction coefficient, changes due to process related strains. This contribution focuses on the development and validation of a tool to investigate the friction coefficient of the flange region of deep drawn components. The influence of biaxial strain on the friction coefficient will be quantified through a comparison of the experimental results with a conventional friction test (stand). The presented method will be applied on a cup drawing test, using a segmented and sensor-monitored blankholder. This setup allows the measurement of the friction coefficient in-situ without simplification of the real process. The experiments were carried out using DX 56D+Z as sheet metal and PL61 as lubricant. The results show a characteristic change in the friction coefficient over the displacement of the punch, which is assumed to be caused by strain induced change of the surface texture.

Characterization of friction and moisture of porcine lingual tissue in vitro in response to artificial saliva and mouthwash solutions.

PubMed

Zundel, J; Ansari, S A; Trivedi, H M; Masters, J G; Mascaro, S

2018-05-07

The purpose of this research is to characterize the effects of mouthwash solutions on oral friction and moisture using a quantitative in vitro approach. The frictional coefficient of in vitro porcine tongue samples was measured using a magnetic levitation haptic device equipped with a custom tactor designed to mimic human skin. A commercially available moisture meter was used to measure moisture content of the samples. Tongue samples were first tested before treatment, then after application of saliva (either human or artificial), and again after application of 1 of 11 different mouthwash solutions. The data indicate that the samples treated with artificial saliva vs real saliva have comparable friction coefficient and moisture content. Furthermore, the moisture and friction coefficient remain relatively constant for up to 60 minutes after exposure to ambient conditions. Samples treated with artificial saliva have an average friction coefficient in the range of 0.70-0.80. Application of mouthwash solutions produced an average friction coefficient of 0.39-0.49 but retained the high moisture content of the artificial salivary layer. Several mouthwash solutions resulted in statistically significant differences in the friction coefficient relative to each other. The results of this study demonstrate that a magnetic levitation device can be an effective tool for in vitro oral tribology and that artificial saliva is an effective substitute for real saliva in extended in vitro experiments. The application of mouthwash generally reduces the coefficient of friction of the tongue samples while preserving a relatively high moisture level, and some mouthwashes reduce friction significantly more than others. © 2018 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.

A study of the 1963 Vajont landslide zonation by means of Lagrangian block modelling

NASA Astrophysics Data System (ADS)

Zaniboni, Filippo; Ausilia Paparo, Maria; Tinti, Stefano

2017-04-01

The 1963 landslide detaching from Mt. Toc (North-East Italy), that crashing on the underlying Vajont reservoir caused a huge wave that killed over 2000 people, is a well-known event that has been extensively and deeply investigated. Recently, studies appeared in the literature suggesting that the landslide dynamics can be explained in terms of a zonation of the moving mass. In this work, an additional support to the zonation hypothesis is given by focusing on the friction coefficient of the sliding surface, which is one of the chief parameters influencing the slide motion. Numerical simulations of the Vajont slide found in the literature assumed a homogenous value of the friction coefficient. We have systematically investigated a set of heterogeneous configurations. More specifically, we have divided the sliding surface into a number N of zones, and let the corresponding friction coefficient vary in the range 0-0.5. For each configuration we have run the numerical simulation via the Lagrangian block-based code UBO-BLOCK2 and have evaluated the configuration goodness by computing the misfit between the observed and the simulated deposits. The number of simulations required by this approach increases exponentially with the number N of zones. The main finding of this research is that a 4-sector zonation provides the best results in terms of deposit misfit. The zones can be roughly described as west-downhill (WD), west uphill (WU), east downhill (ED) and east uphill (EU). It is found that motion is mainly determined by friction in zones WD and EU, that friction coefficients in zone WD is remarkably smaller than in zone EU and that misfit is rather insensitive to the values of the friction coefficients in zones WU and ED.

Friction and Wear Reduction of Eccentric Journal Bearing Made of Sn-Based Babbitt for Ore Cone Crusher

PubMed Central

Amanov, Auezhan; Ahn, Byungmin; Lee, Moon Gu; Jeon, Yongho; Pyun, Young-Sik

2016-01-01

An anti-friction Babbitt alloy-coated bearing made by a casting process is a journal bearing, which is used in an ore cone crusher eccentric. The main purpose of the Babbitt coated eccentric is to provide a low friction to support and guide a rotating shaft. Despite the fact that the Babbitt-coated eccentric offers a low friction coefficient and can be operated without a continuous supply of lubricant, it suffers from mining environments and short service life. In this study, an ultrasonic nanocrystalline surface modification (UNSM) technique was used to further reduce the friction coefficient, to increase the wear resistance, and to extend the service life of the Sn-based Babbitt metal. The friction and wear behavior of the Sn-based Babbitt metal was investigated using a block-on-ring tester under both dry and oil-lubricated conditions. The results of the experiments revealed that the friction and wear behavior of Sn-based Babbitt metal could be improved by the application of the UNSM technique. The friction and wear mechanisms of the specimens were explained and discussed in terms of changes in surface properties—microstructure, surface hardness, surface roughness, etc. PMID:28774070

[Evaluation of orthodontic friction using a tribometer with alternating movement].

PubMed

Pernier, C M; Jablonska-Mazanek, E D; Ponsonnet, L; Grosgogeat, B

2005-12-01

It is essential for orthodontists to control the complex phenomenon of friction. The in vitro techniques, usually dynamometers or tensile testing machines, used to measure the frictional resistance between arch wires and brackets are linear and unidirectional and can be criticised because tooth movements, such as tipping and uprighting, as well everyday oral activities, primarily chewing, are not uni-dimensional but more closely resemble the small amplitude oscillatory phenomena known as fretting. We therefore decided to develop a fretting machine not with linear but with alternating movements better suited to evaluate the frictional behaviour of orthodontic bracket-wire combinations. Once we had completed construction of this device, we proceeded to measure the frictional resistance between one stainless steel bracket (MicroArch GAC) and five wires currently used in orthodontics (Two nickel-titanium shape memory alloys: Neo Sentalloy and Neo Sentalloy with Ionguard GAC--Three titanium-molybdenum alloys: TMA and Low Friction TMA Ormco and Resolve GAC). We were able to set up a classification of the wires according to their coefficient of friction, demonstrating the inefficacy of ion implantation and quantifying the increase in the coefficient of friction which occurs when Resolve wires are placed in the oral environment for approximately one year.

Friction and Wear Reduction of Eccentric Journal Bearing Made of Sn-Based Babbitt for Ore Cone Crusher.

PubMed

Amanov, Auezhan; Ahn, Byungmin; Lee, Moon Gu; Jeon, Yongho; Pyun, Young-Sik

2016-11-22

An anti-friction Babbitt alloy-coated bearing made by a casting process is a journal bearing, which is used in an ore cone crusher eccentric. The main purpose of the Babbitt coated eccentric is to provide a low friction to support and guide a rotating shaft. Despite the fact that the Babbitt-coated eccentric offers a low friction coefficient and can be operated without a continuous supply of lubricant, it suffers from mining environments and short service life. In this study, an ultrasonic nanocrystalline surface modification (UNSM) technique was used to further reduce the friction coefficient, to increase the wear resistance, and to extend the service life of the Sn-based Babbitt metal. The friction and wear behavior of the Sn-based Babbitt metal was investigated using a block-on-ring tester under both dry and oil-lubricated conditions. The results of the experiments revealed that the friction and wear behavior of Sn-based Babbitt metal could be improved by the application of the UNSM technique. The friction and wear mechanisms of the specimens were explained and discussed in terms of changes in surface properties-microstructure, surface hardness, surface roughness, etc.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Shanaghi, Ali, E-mail: alishanaghi@gmail.com; Rouhaghdam, Ali Reza Sabour, E-mail: sabour01@modares.ac.ir; Ahangarani, Shahrokh, E-mail: sh.ahangarani@gmail.com

Highlights: ► The TiC{sub x} nanostructure coatings have been deposited by PACVD method. ► Dominant mechanism of growth structure at 490 °C is island-layer type. ► TiC{sub x} nanostructure coating applied at 490 °C, exhibits lowest friction coefficient. ► Young's moduli are 289.9, 400 and 187.6 GPa for 470, 490 and 510 °C, respectively. ► This higher elastic modulus and higher hardness of nanocoating obtain at 490 °C. -- Abstract: The structure, composition, and mechanical properties of nanostructured titanium carbide (TiC) coatings deposited on H{sub 11} hot-working tool steel by pulsed-DC plasma assisted chemical vapor deposition at three different temperaturesmore » are investigated. Nanoindentation and nanoscratch tests are carried out by atomic force microscopy to determine the mechanical properties such as hardness, elastic modulus, surface roughness, and friction coefficient. The nanostructured TiC coatings prepared at 490 °C exhibit lower friction coefficient (0.23) than the ones deposited at 470 and 510 °C. Increasing the deposition temperature reduces the Young's modulus and hardness. The overall superior mechanical properties such as higher hardness and lower friction coefficient render the coatings deposited at 490 °C suitable for wear resistant applications.« less

Coefficient of friction and wear rate effects of different composite nanolubricant concentrations on Aluminium 2024 plate

NASA Astrophysics Data System (ADS)

Zawawi, N. N. M.; Azmi, W. H.; Redhwan, A. A. M.; Sharif, M. Z.

2017-10-01

Wear of sliding parts and operational machine consistency enhancement can be avoided with good lubrication. Lubrication reduce wear between two contacting and sliding surfaces and decrease the frictional power losses in compressor. The coefficient of friction and wear rate effects study were carried out to measure the friction and anti-wear abilities of Al2O3-SiO2 composite nanolubricants a new type of compressor lubricant to enhanced the compressor performances. The tribology test rig employing reciprocating test conditions to replicate a piston ring contact in the compressor was used to measure the coefficient of friction and wear rate. Coefficient of friction and wear rate effects of different Al2O3-SiO2/PAG composite nanolubricants of Aluminium 2024 plate for 10-kg load at different speed were investigated. Al2O3 and SiO2 nanoparticles were dispersed in the Polyalkylene Glycol (PAG 46) lubricant using two-steps method of preparation. The result shows that the coefficient friction and wear rate of composite nanolubricants decreased compared to pure lubricant. The maximum reduction achievement for friction of coefficient and wear rate by Al2O3-SiO2 composite nanolubricants by 4.78% and 12.96% with 0.06% volume concentration. Therefore, 0.06% volume concentration is selected as the most enhanced composite nanolubricants with effective coefficient of friction and wear rate reduction compared to other volume concentrations. Thus, it is recommended to be used as the compressor lubrication to enhanced compressor performances.

Heterogeneity in friction strength of an active fault by incorporation of fragments of the surrounding host rock

NASA Astrophysics Data System (ADS)

Kato, Naoki; Hirono, Tetsuro

2016-07-01

To understand the correlation between the mesoscale structure and the frictional strength of an active fault, we performed a field investigation of the Atera fault at Tase, central Japan, and made laboratory-based determinations of its mineral assemblages and friction coefficients. The fault zone contains a light gray fault gouge, a brown fault gouge, and a black fault breccia. Samples of the two gouges contained large amounts of clay minerals such as smectite and had low friction coefficients of approximately 0.2-0.4 under the condition of 0.01 m s-1 slip velocity and 0.5-2.5 MP confining pressure, whereas the breccia contained large amounts of angular quartz and feldspar and had a friction coefficient of 0.7 under the same condition. Because the fault breccia closely resembles the granitic rock of the hangingwall in composition, texture, and friction coefficient, we interpret the breccia as having originated from this protolith. If the mechanical incorporation of wall rocks of high friction coefficient into fault zones is widespread at the mesoscale, it causes the heterogeneity in friction strength of fault zones and might contribute to the evolution of fault-zone architectures.

Particle interaction and rheological behavior of cement-based materials at micro- and macro-scales

NASA Astrophysics Data System (ADS)

Lomboy, Gilson Rescober

Rheology of cement based materials is controlled by the interactions at the particle level. The present study investigates the particle interactions and rheological properties of cement-based materials in the micro- and macro-scales. The cementitious materials studied are Portland cement (PC), fly ash (FA), ground granulated blast furnace slag (GGBFS) and densified silica fume (SF). At the micro-scale, aside from the forces on particles due to collisions, interactions of particles in a flowing system include the adhesion and friction. Adhesion is due to the attraction between materials and friction depends on the properties of the sliding surfaces. Atomic Force Microscopy (AFM) is used to measure the adhesion force and coefficient of friction. The adhesion force is measured by pull-off force measurements and is used to calculate Hamaker constants. The coefficient of friction is measured by increasing the deflection set-points on AFM probes with sliding particles, thereby increasing normal loads and friction force. AFM probes were commercial Si3N4 tips and cementitious particles attached to the tips of probe cantilevers. SF was not included in the micro-scale tests due to its limiting size when attaching it to the AFM probes. Other materials included in the tests were silica, calcite and mica, which were used for verification of the developed test method for the adhesion study. The AFM experiments were conducted in dry air and fluid environments at pH levels of 7, 8, 9, 11 and 13. The results in dry air indicate that the Hamaker constant of Class F FA can be similar to PC, but Class C FA can have a high Hamaker constant, also when in contact with other cementitious materials. The results in fluid environments showed low Hamaker constants for Class F fly ashes compared to PC and also showed high Hamaker constants for PC and Class C fly ash. The results for the friction test in dry air indicated that the coefficient of friction of PC is lower than fly ashes, which is attributed to the asperities present on the particle surface. At the macro-scale, flow of cementitious materials may be in its dry or wet state, during transport and handling or when it is used in concrete mixtures, respectively. Hence, the behavior of bulk cementitious materials in their dry state and wet form are studied. In the dry state, the compression, recompression and swell indices, and stiffness modulus of plain and blended cementitious materials are determined by confined uniaxial compression. The coefficients of friction of the bulk materials studied are determined by a direct shear test. The results indicate that shape of particles has a great influence on the compression and shear parameters. The indices for PC blends with FA do not change with FA replacement, while it increases with GGBFS replacement. Replacement with GGBFS slightly decreases coefficient of friction, while replacement with FA significantly decreases coefficient of friction. At low SF replacement, coefficient of friction decreases. In wet state, unary, binary, ternary and quaternary mixes with w/b of 0.35, 0.45 and 0.55 were tested for yield stress, viscosity and thixotropy. It is found that fly ash replacement lowers the rheological properties and replacement with GGBFS and SF increases rheological properties. The distinct element method (DEM) was employed to model particle interaction and bulk behavior. The AFM force curve measurement is simulated to validate the adhesion model in the DEM. The contact due to asperities was incorporated by considering the asperities as a percentage of the radius of the contacting particles. The results of the simulation matches the force-curve obtained from actual AFM experiments. The confined uniaxial compression test is simulated to verify the use of DEM to relate micro-scale properties to macros-scale behavior. The bulk stiffness from the physical experiments is matched in the DEM simulation. The particle stiffness and coefficient of friction are found to have a direct relation to bulk stiffness.

Seismic analysis for translational failure of landfills with retaining walls.

PubMed

Feng, Shi-Jin; Gao, Li-Ya

2010-11-01

In the seismic impact zone, seismic force can be a major triggering mechanism for translational failures of landfills. The scope of this paper is to develop a three-part wedge method for seismic analysis of translational failures of landfills with retaining walls. The approximate solution of the factor of safety can be calculated. Unlike previous conventional limit equilibrium methods, the new method is capable of revealing the effects of both the solid waste shear strength and the retaining wall on the translational failures of landfills during earthquake. Parameter studies of the developed method show that the factor of safety decreases with the increase of the seismic coefficient, while it increases quickly with the increase of the minimum friction angle beneath waste mass for various horizontal seismic coefficients. Increasing the minimum friction angle beneath the waste mass appears to be more effective than any other parameters for increasing the factor of safety under the considered condition. Thus, selecting liner materials with higher friction angle will considerably reduce the potential for translational failures of landfills during earthquake. The factor of safety gradually increases with the increase of the height of retaining wall for various horizontal seismic coefficients. A higher retaining wall is beneficial to the seismic stability of the landfill. Simply ignoring the retaining wall will lead to serious underestimation of the factor of safety. Besides, the approximate solution of the yield acceleration coefficient of the landfill is also presented based on the calculated method. Copyright © 2010 Elsevier Ltd. All rights reserved.

Measuring the Coefficient of Friction of a Small Floating Liquid Marble

PubMed Central

Ooi, Chin Hong; Nguyen, Anh Van; Evans, Geoffrey M.; Dao, Dzung Viet; Nguyen, Nam-Trung

2016-01-01

This paper investigates the friction coefficient of a moving liquid marble, a small liquid droplet coated with hydrophobic powder and floating on another liquid surface. A floating marble can easily move across water surface due to the low friction, allowing for the transport of aqueous solutions with minimal energy input. However, the motion of a floating marble has yet to be systematically characterised due to the lack of insight into key parameters such as the coefficient of friction between the floating marble and the carrier liquid. We measured the coefficient of friction of a small floating marble using a novel experimental setup that exploits the non-wetting properties of a liquid marble. A floating liquid marble pair containing a minute amount magnetite particles were immobilised and then released in a controlled manner using permanent magnets. The capillarity-driven motion was analysed to determine the coefficient of friction of the liquid marbles. The “capillary charge” model was used to fit the experimental results. We varied the marble content and carrier liquid to establish a relationship between the friction correction factor and the meniscus angle. PMID:27910916

Friction force microscopy at a regularly stepped Au(665) electrode: Anisotropy effects

NASA Astrophysics Data System (ADS)

Podgaynyy, Nikolay; Iqbal, Shahid; Baltruschat, Helmut

2015-01-01

Using friction force microscopy, friction was determined for the AFM-tip scanning parallel and vertically to the monoatomic steps of Au(665) electrode for different coverages of Cu in sulfuric acid. When the tip was scanning parallel to the steps, the results were similar to those obtained before for a Au(111) surface: a higher coverage of Cu leads to an increased friction. However, differently from Au(111), no transitions in the friction coefficient were observed with increasing load. Atomic stick slip was observed both for the Au surface and the √{ 3} × √{ 3} honeycomb Cu adlayer with a Cu coverage of 2/3. When the tip was scanning perpendicular to the steps, friction did not depend much on coverage; astonishingly, atomic stick slip was also observed.

Effect of Coulomb friction on orientational correlation and velocity distribution functions in a sheared dilute granular gas.

PubMed

Gayen, Bishakhdatta; Alam, Meheboob

2011-08-01

From particle simulations of a sheared frictional granular gas, we show that the Coulomb friction can have dramatic effects on orientational correlation as well as on both the translational and angular velocity distribution functions even in the Boltzmann (dilute) limit. The dependence of orientational correlation on friction coefficient (μ) is found to be nonmonotonic, and the Coulomb friction plays a dual role of enhancing or diminishing the orientational correlation, depending on the value of the tangential restitution coefficient (which characterizes the roughness of particles). From the sticking limit (i.e., with no sliding contact) of rough particles, decreasing the Coulomb friction is found to reduce the density and spatial velocity correlations which, together with diminished orientational correlation for small enough μ, are responsible for the transition from non-gaussian to gaussian distribution functions in the double limit of small friction (μ→0) and nearly elastic particles (e→1). This double limit in fact corresponds to perfectly smooth particles, and hence the maxwellian (gaussian) is indeed a solution of the Boltzmann equation for a frictional granular gas in the limit of elastic collisions and zero Coulomb friction at any roughness. The high-velocity tails of both distribution functions seem to follow stretched exponentials even in the presence of Coulomb friction, and the related velocity exponents deviate strongly from a gaussian with increasing friction.

Steady sliding frictional contact problem for a 2d elastic half-space with a discontinuous friction coefficient and related stress singularities

NASA Astrophysics Data System (ADS)

Ballard, Patrick

2016-12-01

The steady sliding frictional contact problem between a moving rigid indentor of arbitrary shape and an isotropic homogeneous elastic half-space in plane strain is extensively analysed. The case where the friction coefficient is a step function (with respect to the space variable), that is, where there are jumps in the friction coefficient, is considered. The problem is put under the form of a variational inequality which is proved to always have a solution which, in addition, is unique in some cases. The solutions exhibit different kinds of universal singularities that are explicitly given. In particular, it is shown that the nature of the universal stress singularity at a jump of the friction coefficient is different depending on the sign of the jump.

Friction between a surrogate skin (Lorica Soft) and nonwoven fabrics used in hygiene products

NASA Astrophysics Data System (ADS)

Falloon, Sabrina S.; Cottenden, Alan

2016-09-01

Incontinence pad wearers often suffer from sore skin, and a better understanding of friction between pads and skin is needed to inform the development of less damaging materials. This work investigated friction between a skin surrogate (Lorica Soft) and 13 nonwoven fabrics representing those currently used against the skin in commercial pads. All fabrics were found to behave consistently with Amontons’ law: coefficients of friction did not differ systematically when measured under two different loads. Although the 13 fabrics varied considerably in composition and structure, their coefficients of friction (static and dynamic) against Lorica Soft were remarkably similar, especially for the ten fabrics comprising just polypropylene (PP) fibres. The coefficients of friction for one PP fabric never differed by more than 15.7% from those of any other, suggesting that the ranges of fibre decitex (2.0-6.5), fabric area density (13-30 g m-2) and bonding area (11%-25%) they exhibited had only limited impact on their friction properties. It is likely that differences were largely attributable to variability in properties between multiple samples of a given fabric. Of the remaining fabrics, the one comprising polyester fibres had significantly higher coefficients of friction than the highest friction PP fabric (p < 0.005), while the one comprising PP fibres with a polyethylene sheath had significantly lower coefficients of friction than the lowest friction PP fabric (p < 10-8). However, fabrics differed in too many other ways to confidently attribute these differences in friction properties just to the choice of base polymer.

Effect of Test Parameters on the Friction Behaviour of Anodized Aluminium Alloy

PubMed Central

Khalladi, A.; Elleuch, K.; De-Petris Wery, M.; Ayedi, H. F.

2014-01-01

The tribological behaviour of anodic oxide layer formed on Al5754, used in automotive applications, was investigated against test parameters. The friction coefficient under different normal loads, sliding speeds, and oxide thicknesses was studied using a pin on disc tribometer. Results show that the increase of load and sliding speed increase the friction coefficient. The rise of contact pressure and temperature seems to cause changes in wear mechanism. Glow-discharge optical emission spectroscopy (GDOES) was used to investigate the chemical composition of the oxide layer. Morphology and composition of the wear tracks were analyzed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). On the basis of these characterization techniques, a wear mechanism was proposed. The observed mechanical properties can be related to the morphology and the chemical composition of the layer. PMID:27437452

Cyclic Behavior of Mortarless Brick Joints with Different Interlocking Shapes

PubMed Central

Liu, Hongjun; Liu, Peng; Lin, Kun; Zhao, Sai

2016-01-01

The framed structure infilled with a mortarless brick (MB) panel exhibits considerable in-plane energy dissipation because of the relative sliding between bricks and good out-of-plane stability resulting from the use of interlocking mechanisms. The cyclic behaviors of MB are investigated experimentally in this study. Two different types of bricks, namely non-interlocking mortarless brick (N-IMB) and interlocking mortarless brick (IMB), are examined experimentally. The cyclic behavior of all of the joints (N-IMB and IMB) are investigated in consideration of the effects of interlocking shapes, loading compression stress levels and loading cycles. The hysteretic loops of N-IMB and IMB joints are obtained, according to which a mechanical model is developed. The Mohr–Coulomb failure criterion is employed to describe the shear failure modes of all of the investigated joints. A typical frictional behavior is observed for the N-IMB joints, and a significant stiffening effect is observed for the IMB joints during their sliding stage. The friction coefficients of all of the researched joints increase with the augmentation of the compression stress level and improvement of the smoothness of the interlocking surfaces. An increase in the loading cycle results in a decrease in the friction coefficients of all of the joints. The degradation rate (DR) of the friction coefficients increases with the reduction in the smoothness of the interlocking surface. PMID:28773291

Evaluation of the friction coefficient, the radial stress, and the damage work during needle insertions into agarose gels.

PubMed

Urrea, Fabián A; Casanova, Fernando; Orozco, Gustavo A; García, José J

2016-03-01

Agarose hydrogels have been extensively used as a phantom material to mimic the mechanical behavior of soft biological tissues, e.g. in studies aimed to analyze needle insertions into the organs producing tissue damage. To better predict the radial stress and damage during needle insertions, this study was aimed to determine the friction coefficient between the material of commercial catheters and hydrogels. The friction coefficient, the tissue damage and the radial stress were evaluated at 0.2, 1.8, and 10mm/s velocities for 28, 30, and 32 gauge needles of outer diameters equal to 0.36, 0.31, and 0.23mm, respectively. Force measurements during needle insertions and retractions on agarose gel samples were used to analyze damage and radial stress. The static friction coefficient (0.295±0.056) was significantly higher than the dynamic (0.255±0.086). The static and dynamic friction coefficients were significantly smaller for the 0.2mm/s velocity compared to those for the other two velocities, and there was no significant difference between the friction coefficients for 1.8 and 10mm/s. Radial stress averages were 131.2±54.1, 248.3±64.2, and 804.9±164.3Pa for the insertion velocity of 0.2, 1.8, and 10mm/s, respectively. The radial stress presented a tendency to increase at higher insertion velocities and needle size, which is consistent with other studies. However, the damage work did not show to be a good predictor of tissue damage, which appears to be due to simplifications in the analytical model. Differently to other approaches, the method proposed here based on radial stress may be extended in future studies to quantity tissue damage in vivo along the entire needle track. Copyright © 2015 Elsevier Ltd. All rights reserved.

XPS, AES and friction studies of single-crystal silicon carbide

NASA Technical Reports Server (NTRS)

Miyoshi, K.; Buckley, D. H.

1982-01-01

The surface chemistry and friction behavior of a single crystal silicon carbide surface parallel to the 0001 plane in sliding contact with iron at various temperatures to 1500 C in a vacuum of 3 x 10 nPa are investigated using X-ray photoelectron and Auger electron spectroscopy. Results show that graphite and carbide-type carbon are seen primarily on the silicon carbide surface in addition to silicon at temperatures to 800 C by both types of spectroscopy. The coefficients of friction for iron sliding against a silicon carbide surface parallel to the 0001 plane surface are found to be high at temperatures up to 800 C, with the silicon and carbide-type carbon at maximum intensity in the X-ray photoelectron spectroscopy at 800 C. The concentration of the graphite increases rapidly on the surface as the temperature is increased above 800 C, while the concentrations of the carbide-type carbon and silicon decrease rapidly and this presence of graphite is accompanied by a significant decrease in friction. Preheating the surfaces to 1500 C also gives dramatically lower coefficients of friction when reheating in the sliding temperature range of from room temperature to 1200 C, with this reduction in friction due to the graphite layer on the silicon carbide surface.

Experimental Measurement of the Static Coefficient of Friction at the Ti-Ti Taper Connection in Total Hip Arthroplasty.

PubMed

Bitter, T; Khan, I; Marriott, T; Schreurs, B W; Verdonschot, N; Janssen, D

2016-03-01

The modular taper junction in total hip replacements has been implicated as a possible source of wear. The finite-element (FE) method can be used to study the wear potential at the taper junction. For such simulations it is important to implement representative contact parameters, in order to achieve accurate results. One of the main parameters in FE simulations is the coefficient of friction. However, in current literature, there is quite a wide spread in coefficient of friction values (0.15 - 0.8), which has a significant effect on the outcome of the FE simulations. Therefore, to obtain more accurate results, one should use a coefficient of friction that is determined for the specific material couple being analyzed. In this study, the static coefficient of friction was determined for two types of titanium-on-titanium stem-adaptor couples, using actual cut-outs of the final implants, to ensure that the coefficient of friction was determined consistently for the actual implant material and surface finish characteristics. Two types of tapers were examined, Biomet type-1 and 12/14, where type-1 has a polished surface finish and the 12/14 is a microgrooved system. We found static coefficients of friction of 0.19 and 0.29 for the 12/14 and type-1 stem-adaptor couples, respectively.

Friction and wear life properties of polyimide thin films

NASA Technical Reports Server (NTRS)

Fusaro, R. L.

1972-01-01

A transition in the friction coefficient and wear life properties of Pyralin polyimide (PI) thin films was found to exist at a temperature between 25 deg and 100 deg C. Above this transition, PI thin films gave long wear lives and low friction coefficients. The presence of H2O in air improved the friction and wear life properties at 25 deg C; but at 100 deg C, H2O had a detrimental effect. At 100 deg C and above, a dry argon atmosphere gave lower friction coefficients and longer wear lives than did a dry air atmosphere.

Evaluation of the friction force generated by monocristalyne and policristalyne ceramic brackets in sliding mechanics.

PubMed

Pimentel, Roberta Ferreira; de Oliveira, Roberto Sotto Maior Fortes; Chaves, Maria das Graças Afonso Miranda; Elias, Carlos Nelson; Gravina, Marco Abdo

2013-01-01

To evaluate and compare "in vitro" the maximum friction force generated by three types of esthetic brackets, two types of polycrystalline conventional ceramic brackets (20/40 and InVu) and one type of sapphire monocrystalline bracket (Radiance) in dry and artificial saliva wet settings. Also, to evaluate the influence exerted by artificial saliva on the friction forces of those brackets. Tests were performed in dry and artificial saliva wet setting (Oral Balance) by using an EMIC DL 10000 testing machine, simulating a 2 mm slide of 0.019 x 0.025-in rectangular stainless steel wires over the pre-angulated and pre-torqued (right superior canine, Roth prescription, slot 0.022 x 0.030-in) brackets (n = 18 for each bracket). In order to compare groups in dry and wet settings, the ANOVA was used. For comparisons related to the dry versus wet setting, the student t test was used for each group. The results showed that in the absence of saliva the Radiance monocrystalline brackets showed the highest friction coefficients, followed by the 20/40 and the InVu polycrystalline brackets. In tests with artificial saliva, the Radiance and the 20/40 brackets had statistically similar friction coefficients and both were greater than that presented by the InVu brackets. The artificial saliva did not change the maximum friction force of the Radiance brackets, but, for the others (20/40 and InVu), an increase of friction was observed in its presence. The InVu brackets showed, in the absence and in the presence of saliva, the lowest friction coefficient.

Contrasting frictional behaviour of fault gouges containing Mg-rich phyllosilicates

NASA Astrophysics Data System (ADS)

Sanchez Roa, C.; Faulkner, D.; Jimenez Millan, J.; Nieto, F.

2015-12-01

The clay mineralogy of fault gouges has important implications on frictional properties and stability of fault planes. We studied the specific case of the Galera fault zone where fault gouges containing Mg-rich phyllosilicates appear as hydrothermal deposits related to high salinity fluids enriched in Mg2+. These deposits are dominated by sepiolite and palygorskite, both fibrous clay minerals with similar composition to Mg-smectite. The frictional strengths of sepiolite and palygorskite have not yet been determined, however, as they are part of the clay mineral group, it has been assumed that their frictional behaviour would be in line with platy clay minerals. We performed frictional sliding experiments on powdered pure standards and fault rocks in order to establish the frictional behaviour of sepiolite and palygorskite using a triaxial deformation apparatus with a servo-controlled axial loading system and fluid pressure pump. Friction coefficients for palygorskite and sepiolite as monomineralic samples were found to be 0.65 to 0.7 for dry experiments, and 0.45 to 0.5 for water-saturated experiments. Although these fibrous minerals are part of the phyllosilicates group, they show higher friction coefficients and their mechanical behaviour is less stable than platy clay minerals. This difference is a consequence of their stronger structural framework and the discontinuity of water layers. Our results present a contrast in mechanical behaviour between Mg-rich fibrous and platy clay minerals in fault gouges, where smectite is known to considerably reduce friction coefficients and to increase the stability of the fault plane leading to creeping processes. Transformations between saponite and sepiolite have been previously observed and could modify the deformation regime of a fault zone. Constraining the stability conditions and possible mineral reactions or transformations in fault gouges could help us understand the general role of clay minerals in fault stability.

Local convective heat transfer coefficient and friction factor of CuO/water nanofluid in a microchannel heat sink

NASA Astrophysics Data System (ADS)

Chabi, A. R.; Zarrinabadi, S.; Peyghambarzadeh, S. M.; Hashemabadi, S. H.; Salimi, M.

2017-02-01

Forced convective heat transfer in a microchannel heat sink (MCHS) using CuO/water nanofluids with 0.1 and 0.2 vol% as coolant was investigated. The experiments were focused on the heat transfer enhancement in the channel entrance region at Re < 1800. Hydraulic performance of the MCHS was also estimated by measuring friction factor and pressure drop. Results showed that higher convective heat transfer coefficient was obtained at the microchannel entrance. Maximum enhancement of the average heat transfer coefficient compared with deionized water was about 40 % for 0.2 vol% nanofluid at Re = 1150. Enhancement of the convective heat transfer coefficient of nanofluid decreased with further increasing of Reynolds number.

Rockburst Disaster Prediction of Isolated Coal Pillar by Electromagnetic Radiation Based on Frictional Effect

PubMed Central

Zhao, Tongbin; Yin, Yanchun; Xiao, Fukun; Tan, Yunliang; Zou, Jianchao

2014-01-01

Based on the understanding that charges generated during coal cracking are due to coal particle friction, a microstructure model was developed by considering four different variation laws of friction coefficient. Firstly, the frictional energy release of coal sample during uniaxial compressive tests was investigated and discussed. Then electromagnetic radiation method was used to predict the potential rockburst disaster in isolated coal pillar mining face, Muchengjian Colliery. The results indicate that the friction coefficient of coal particles decreases linearly with the increase of axial loading force. In predicting the strain-type rockburst, the high stress state of coal must be closely monitored. Field monitoring shows that electromagnetic radiation signal became abnormal before the occurrence of rockburst during isolated coal pillar mining. Furthermore, rockburst tends to occur at the early and ending stages of isolated coal pillar extraction. Mine-site investigation shows the occurrence zone of rockburst is consistent with the prediction, proving the reliability of the electromagnetic radiation method to predict strain-type rockburst disaster. PMID:25054186

Rockburst disaster prediction of isolated coal pillar by electromagnetic radiation based on frictional effect.

PubMed

Zhao, Tongbin; Yin, Yanchun; Xiao, Fukun; Tan, Yunliang; Zou, Jianchao

2014-01-01

Based on the understanding that charges generated during coal cracking are due to coal particle friction, a microstructure model was developed by considering four different variation laws of friction coefficient. Firstly, the frictional energy release of coal sample during uniaxial compressive tests was investigated and discussed. Then electromagnetic radiation method was used to predict the potential rockburst disaster in isolated coal pillar mining face, Muchengjian Colliery. The results indicate that the friction coefficient of coal particles decreases linearly with the increase of axial loading force. In predicting the strain-type rockburst, the high stress state of coal must be closely monitored. Field monitoring shows that electromagnetic radiation signal became abnormal before the occurrence of rockburst during isolated coal pillar mining. Furthermore, rockburst tends to occur at the early and ending stages of isolated coal pillar extraction. Mine-site investigation shows the occurrence zone of rockburst is consistent with the prediction, proving the reliability of the electromagnetic radiation method to predict strain-type rockburst disaster.

Microstructure and tribological properties of TiCu2Al intermetallic compound coating

NASA Astrophysics Data System (ADS)

Guo, Chun; Zhou, Jiansong; Zhao, Jierong; Wang, Linqian; Yu, Youjun; Chen, Jianmin; Zhou, Huidi

2011-04-01

TiCu2Al ternary intermetallic compound coating has been in situ synthesized successfully on pure Ti substrate by laser cladding. Tribological properties of the prepared TiCu2Al intermetallic compound coating were systematically evaluated. It was found that the friction coefficient and wear rate was closely related to the normal load and sliding speed, i.e., the friction coefficient of the prepared TiCu2Al intermetallic compound coating decreased with increasing normal load and sliding speed. The wear rate of the TiCu2Al intermetallic compound coating decreased rapidly with increasing sliding speed, while the wear rate first increased and then decreased at normal load from 5 to 15 N.

Tyre-road friction coefficient estimation based on tyre sensors and lateral tyre deflection: modelling, simulations and experiments

NASA Astrophysics Data System (ADS)

Hong, Sanghyun; Erdogan, Gurkan; Hedrick, Karl; Borrelli, Francesco

2013-05-01

The estimation of the tyre-road friction coefficient is fundamental for vehicle control systems. Tyre sensors enable the friction coefficient estimation based on signals extracted directly from tyres. This paper presents a tyre-road friction coefficient estimation algorithm based on tyre lateral deflection obtained from lateral acceleration. The lateral acceleration is measured by wireless three-dimensional accelerometers embedded inside the tyres. The proposed algorithm first determines the contact patch using a radial acceleration profile. Then, the portion of the lateral acceleration profile, only inside the tyre-road contact patch, is used to estimate the friction coefficient through a tyre brush model and a simple tyre model. The proposed strategy accounts for orientation-variation of accelerometer body frame during tyre rotation. The effectiveness and performance of the algorithm are demonstrated through finite element model simulations and experimental tests with small tyre slip angles on different road surface conditions.

Prehension Synergies in the Grasps With Complex Friction Patterns: Local Versus Synergic Effects and the Template Control

PubMed Central

Niu, Xun; Latash, Mark L.; Zatsiorsky, Vladimir M.

2010-01-01

We studied adjustments of digit forces to changes in the friction. The subjects held a handle statically in a three-digit grasp. The friction under each digit was either high or low, resulting in eight three-element friction sets (such grasps were coined the grasps with complex friction pattern). The total load was also manipulated. It was found that digit forces were adjusted not only to the supported load and local friction, but also to friction at other digits (synergic effects). When friction under a digit was low, its tangential force decreased and the normal force increased (local effects). The synergic effects were directed to maintain the equilibrium of the handle. The relation between the individual digit forces and loads agreed with the triple-product model: fin=ki(2)ki(1)L, where fin is normal force of digit i, L is the load (newtons), ki(1) is a dimensionless coefficient representing sharing the total tangential force among the digits (Σki(1)=1.0), and ki(2) is a coefficient representing the relation between the tangential and normal forces of digit i (the overall friction equivalent, OFE). At each friction set, the central controller selected the grasping template—a three-element array of ki(2)ki(1) products—and then scaled the template with the load magnitude. PMID:17493928

Numerical calculation of boundary layers and wake characteristics of high-speed trains with different lengths

PubMed Central

Zhou, Dan; Niu, Jiqiang

2017-01-01

Trains with different numbers of cars running in the open air were simulated using the delayed detached-eddy simulation (DDES). The numbers of cars included in the simulation are 3, 4, 5 and 8. The aim of this study was to investigate how train length influences the boundary layer, the wake flow, the surface pressure, the aerodynamic drag and the friction drag. To certify the accuracy of the mesh and methods, the drag coefficients from numerical simulation of trains with 3 cars were compared with those from the wind tunnel test, and agreement was obtained. The results show that the boundary layer is thicker and the wake vortices are less symmetric as the train length increases. As a result, train length greatly affects pressure. The upper surface pressure of the tail car reduced by 2.9%, the side surface pressure of the tail car reduced by 8.3% and the underneath surface pressure of the tail car reduced by 19.7% in trains that included 3 cars to those including 8 cars. In addition, train length also has a significant effect on the friction drag coefficient and the drag coefficient. The friction drag coefficient of each car in a configuration decreases along the length of the train. In a comparison between trains consisting of 3 cars to those consisting of 8 cars, the friction drag coefficient of the tail car reduced by 8.6% and the drag coefficient of the tail car reduced by 3.7%. PMID:29261758

Surface chemistry, friction, and wear of Ni-Zn and Mn-Zn ferrites in contact with metals

NASA Technical Reports Server (NTRS)

Miyoshi, K.; Buckley, D. H.

1983-01-01

X-ray photoelectron and Auger electron spectroscopy analysis were used in sliding friction experiments. These experiments were conducted with hot-pressed polycrystalline Ni-Zn and Mn-Zn ferrites, and single-crystal Mn-Zn ferrite in contact with various transition metals at room temperature in both vacuum and argon. The results indicate that Ni2O3 and Fe3O4 were present on the Ni-Zn ferrite surface in addition to the nominal bulk constituents, while MnO2 and Fe3O4 were present on the Mn-Zn ferrite surface in addition to the nominal bulk constituents. The coefficients of friction for the ferrites in contact with metals were related to the relative chemical activity of these metals. The more active the metal, the higher is the coefficient of friction. The coefficients of friction for the ferrites were correlated with the free energy of formation of the lowest metal oxide. The interfacial bond can be regarded as a chemical bond between the metal atoms and the oxygen anions in the ferrite surfaces. The adsorption of oxygen on clean metal and ferrite does strengthen the metal-ferrite contact and increase the friction. The ferrites exhibit local cracking and fracture with sliding under adhesive conditions. All the metals transferred to the surfaces of the ferrites in sliding. Previously announced in STAR as N83-19901

Surface chemistry, friction and wear of Ni-Zn and Mn-Zn ferrites in contact with metals

NASA Technical Reports Server (NTRS)

Miyoshi, K.; Buckley, D. H.

1982-01-01

X-ray photoelectron and Auger electron spectroscopy analysis were used in sliding friction experiments. These experiments were conducted with hot-pressed polycrystalline Ni-Zn and Mn-Zn ferrites, and single-crystal Mn-Zn ferrite in contact with various transition metals at room temperature in both vacuum and argon. The results indicate that Ni2O3 and Fe3O4 were present on the Ni-Zn ferrite surface in addition to the nominal bulk constituents, while MnO2 and Fe3O4 were present on the Mn-Zn ferrite surface in addition to the nominal bulk constituents. The coefficients of friction for the ferrites in contact with metals were related to the relative chemical activity of these metals. The more active the metal, the higher is the coefficient of friction. The coefficients of friction for the ferrites were correlated with the free energy of formation of the lowest metal oxide. The interfacial bond can be regarded as a chemical bond between the metal atoms and the oxygen anions in the ferrite surfaces. The adsorption of oxygen on clean metal and ferrite does strengthen the metal-ferrite contact and increase the friction. The ferrites exhibit local cracking and fracture with sliding under adhesive conditions. All the metals transferred to he surfaces of the ferrites in sliding.

Toward low friction in water for Mo2N/Ag coatings by tailoring the wettability

NASA Astrophysics Data System (ADS)

Dai, Xuan; Wen, Mao; Huang, Keke; Wang, Xin; Yang, Lina; Wang, Jia; Zhang, Kan

2018-07-01

Increasing demands for robust surfaces in harsh conditions, such as erosion, abrasion and sea-water, has stimulated the development of self-lubricated protective coatings. Meanwhile, due to the oil crisis, research in water lubrication again attracts much attention from both academics and practical engineers. Here, a higher hydrophilicity accompanying with a remarkable drop of friction coefficient in water environment was achieved successfully in Mo2N/Ag coatings by increasing Ag content. To do these, the Mo2N/Ag coatings with different Ag content were deposited by co-sputtering, which exhibit a nanocomposite structure consisting of precipitate Ag embedded in the Mo2N matrix. The high hydrophilicity can be ascribed to the combined contributions of the partial oxidation of Mo2N and high polarity of Ag precipitates. The decrease of friction coefficient is illustrated by the colloidal friction products and a mode with electric double layer. In which, enhanced hydrophilicity will result in forming a thin "water film" layer between the interface of counterpart and the coatings. And the MoOx/Ag2Mo4O13 derived from the hydrolysis action of Mo2N/Ag sliding in water could function as lubricant phase. Meanwhile, these negative charged MoOx/Ag2Mo4O13 colloidal particles induce the rearrangement of positive ions in the "water film" and form an electric double layer, which also contributes to the decrease of friction coefficient.

Temperature-Dependent Effect of Boric Acid Additive on Surface Roughness and Wear Rate

NASA Astrophysics Data System (ADS)

Ekinci, Şerafettin

Wear and friction hold an important place in engineering. Currently, scientific societies are struggling to control wear by means of studies on lubricants. Boric acid constitutes an important alternative with its good tribological properties similar to MO2S and graphite alongside with low environmental impacts. Boric acid can be used as a solid lubricant itself whereas it can be added or blended into mineral oils in order to yield better mechanical and tribological properties such as low shear stress due to the lamellar structure and low friction, wear and surface roughness rates. In this study, distinguishing from the literature, boric acid addition effect considering the temperature was investigated for the conventional ranges of internal combustion engines. Surface roughness, wear and friction coefficient values were used in order to determine tribological properties of boric acid as an environmentally friendly additive and mineral oil mixture in the present study. Wear experiments were conducted with a ball on disc experimental setup immersed in an oil reservoir at room temperature, 50∘C and 80∘C. The evolution of both the friction coefficient and wear behavior was determined under 10N load, at 2m/s sliding velocity and a total sliding distance of 9000m. Surface roughness was determined using atomic-force microscopy (AFM). Wear rate was calculated utilizing scanning electron microscope (SEM) visuals and data. The test results showed that wear resistance increased as the temperature increased, and friction coefficient decreased due to the presence of boric acid additive.

High friction interactive aircraft tire-runway systems

NASA Technical Reports Server (NTRS)

Clark, S. K.

1974-01-01

The principle of utilizing geometric interaction between runway asperities and tire pattern design is discussed, and a theoretical basis is presented for substantial enhancement of frictional effects by this process. Test data confirming this is given. First order analytical expressions are given for the increased friction coefficients and for the engagement distances required. High speed friction data on a 7.00 x 8 aircraft tire is presented confirming this. Example design geometries are shown for the tire tread groove pattern, and designs and materials are discussed for the asperity grid and its attachment system.

Oxide Ceramic Films Grown on 55Ni-45Ti for NASA and Department of Defense Applications: Unidirectional Sliding Friction and Wear Evaluation

NASA Technical Reports Server (NTRS)

Miyoshi, Kazuhisa; Lukco, Dorothy; Cytron, Sheldon J.

2004-01-01

An investigation was conducted to examine the friction and wear behavior of the two types of oxide ceramic films furnished by the U.S. Army Research Laboratory, Development and Engineering Center (ARDEC) under Space Act Agreement SAA3 567. These two types of oxide ceramics were grown on 55Ni-45Ti (60 wt% Ni and 40 wt% Ti) substrates: one was a TiO2 with no other species (designated the B film) and the other was a TiO2 with additional species (designated the G film). Unidirectional ball-on-disk sliding friction experiments were conducted with the oxide films in contact with sapphire at 296 K (23 C) in approx. 50-percent relative humidity laboratory air in this investigation. All material characterization and sliding friction experiments were conducted at the NASA Glenn Research Center. The results indicate that both films greatly improve the surface characteristics of 55Ni-45Ti, enhancing its tribological characteristics. Both films decreased the coefficient of friction by a factor of 4 and increased wear resistance by a two-figure factor, though the B film was superior to the G film in wear resistance and endurance life. The levels of coefficient of friction and wear resistance of both films in sliding contact with sapphire were acceptable for NASA and Department of Defense tribological applications. The decrease in friction and increase in wear resistance will contribute to longer wear life for parts, lower energy consumption, reduced related breakdowns, decreased maintenance costs, and increased reliability.

Optimization of wheel-rail interface friction using top-of-rail friction modifiers: State of the art

NASA Astrophysics Data System (ADS)

Khan, M. Roshan; Dasaka, Satyanarayana Murty

2018-05-01

High Speed Railways and Dedicated Freight Corridors are the need of the day for fast and efficient transportation of the ever growing population and freight across long distances of travel. With the increase in speeds and axle loads carried by these trains, wearing out of rails and train wheel sections are a common issue, which is due to the increase in friction at the wheel-rail interfaces. For the cases where the wheel-rail interface friction is less than optimum, as in case of high speed trains with very low axle loads, wheel-slips are imminent and loss of traction occurs when the trains accelerate rapidly or brake all of a sudden. These vast variety of traction problems around the wheel-rail interface friction need to be mitigated carefully, so that the contact interface friction neither ascents too high to cause material wear and need for added locomotive power, nor be on the lower side to cause wheel-slips and loss of traction at high speeds. Top-of-rail friction modifiers are engineered surface coatings applied on top of rails, to maintain an optimum frictional contact between the train wheels and the rails. Extensive research works in the area of wheel-rail tribology have revealed that the optimum frictional coefficients at wheel-rail interfaces lie at a value of around 0.35. Application of top-of-rail (TOR) friction modifiers on rail surfaces add an extra layer of material coating on top of the rails, with a surface frictional coefficient of the desired range. This study reviews the common types of rail friction modifiers, the methods for their application, issues related with the application of friction modifiers, and a guideline on selection of the right class of coating material based on site specific requirements of the railway networks.

Strength of Wet and Dry Montmorillonite

NASA Astrophysics Data System (ADS)

Morrow, C. A.; Lockner, D. A.; Moore, D. E.

2015-12-01

Montmorillonite, an expandable smectite clay, is a common mineral in fault zones to a depth of around 3 km. Its low strength relative to other common fault gouge minerals is important in many models of fault rheology. However, the coefficient of friction is not well constrained in the literature due to the difficulty of establishing fully drained or fully dried states in the laboratory. For instance, in some reported studies, samples were either partially saturated or possibly over pressured, leading to wide variability in reported shear strength. In this study, the coefficient of friction, μ, of both saturated and oven-dried (at 150°C) Na-montmorillonite was measured at normal stresses up to 680 MPa at room temperature and shortening rates from 1.0 to 0.01 μm/s. Care was taken to shear saturated samples slowly enough to avoid pore fluid overpressure in the clay layers. Coefficients of friction are reported after 8 mm of axial displacement in a triaxial apparatus on saw-cut samples containing a layer of montmorillonite gouge, with either granite or sandstone driving blocks. For saturated samples, μ increased from around 0.1 at low pressure to 0.25 at the highest test pressures. In contrast, values for oven-dried samples decreased asymptotically from approximately 0.78 at 10 MPa normal stress to around 0.45 at 400-680 MPa. While wet and dry strengths approached each other with increasing effective normal stress, wet strength remained only about half of the dry strength at 600 MPa effective normal stress. The increased coefficient of friction can be correlated with a reduction in the number of loosely bound lubricating surface water layers on the clay platelets due to applied normal stress under saturated conditions. The steady-state rate dependence of friction, a-b, was positive and dependent on normal stress. For saturated samples, a-b increased linearly with applied normal stress from ~0 to 0.004, while for dry samples a-b decreased with increasing normal stress from 0.008 to 0.002. All values were either neutral or rate strengthening, indicating a tendency for stable sliding.

Numerical study of centrifugal compressor stage vaneless diffusers

NASA Astrophysics Data System (ADS)

Galerkin, Y.; Soldatova, K.; Solovieva, O.

2015-08-01

The authors analyzed CFD calculations of flow in vaneless diffusers with relative width in range from 0.014 to 0.100 at inlet flow angles in range from 100 to 450 with different inlet velocity coefficients, Reynolds numbers and surface roughness. The aim is to simulate calculated performances by simple algebraic equations. The friction coefficient that represents head losses as friction losses is proposed for simulation. The friction coefficient and loss coefficient are directly connected by simple equation. The advantage is that friction coefficient changes comparatively little in range of studied parameters. Simple equations for this coefficient are proposed by the authors. The simulation accuracy is sufficient for practical calculations. To create the complete algebraic model of the vaneless diffuser the authors plan to widen this method of modeling to diffusers with different relative length and for wider range of Reynolds numbers.

Friction and oxidative wear of 440C ball bearing steels under high load and extreme bulk temperatures

NASA Technical Reports Server (NTRS)

Chaudhuri, Dilip K.; Slifka, Andrew J.; Siegwarth, James D.

1993-01-01

Unlubricated sliding friction and wear of 440C steels in an oxygen environment have been studied under a variety of load, speed, and temperature ranging from approximately -185 to 675 deg C. A specially designed test apparatus with a ball-on-flat geometry has been used for this purpose. The observed dependencies of the initial coefficient of friction, the average dynamic coefficient of friction, and the wear rate on load, speed, and test temperatures have been examined from the standpoint of existing theories of friction and wear. High contact temperatures are generated during the sliding friction, causing rapid oxidation and localized surface melting. A combination of fatigue, delamination, and loss of hardness due to tempering of the martensitic structure is responsible for the high wear rate observed and the coefficient of friction.

Calculation of skin-friction coefficients for low Reynolds number turbulent boundary layer flows. M.S. Thesis - California Univ. at Davis

NASA Technical Reports Server (NTRS)

Barr, P. K.

1980-01-01

An analysis is presented of the reliability of various generally accepted empirical expressions for the prediction of the skin-friction coefficient C/sub f/ of turbulent boundary layers at low Reynolds numbers in zero-pressure-gradient flows on a smooth flat plate. The skin-friction coefficients predicted from these expressions were compared to the skin-friction coefficients of experimental profiles that were determined from a graphical method formulated from the law of the wall. These expressions are found to predict values that are consistently different than those obtained from the graphical method over the range 600 Re/sub theta 2000. A curve-fitted empirical relationship was developed from the present data and yields a better estimated value of C/sub f/ in this range. The data, covering the range 200 Re/sub theta 7000, provide insight into the nature of transitional flows. They show that fully developed turbulent boundary layers occur at Reynolds numbers Re/sub theta/ down to 425. Below this level there appears to be a well-ordered evolutionary process from the laminar to the turbulent profiles. These profiles clearly display the development of the turbulent core region and the shrinking of the laminar sublayer with increasing values of Re/sub theta/.

Modulation of Folding Internal Friction by Local and Global Barrier Heights.

PubMed

Zheng, Wenwei; de Sancho, David; Best, Robert B

2016-03-17

Recent experiments have revealed an unexpected deviation from a first power dependence of protein relaxation times on solvent viscosity, an effect that has been attributed to "internal friction". One clear source of internal friction in protein dynamics is the isomerization of dihedral angles. A key outstanding question is whether the global folding barrier height influences the measured internal friction, based on the observation that the folding rates of fast-folding proteins, with smaller folding free energy barriers, tend to exhibit larger internal friction. Here, by studying two alanine-based peptides, we find that systematic variation of global folding barrier heights has little effect on the internal friction for folding rates. On the other hand, increasing local torsion angle barriers leads to increased internal friction, which is consistent with solvent memory effects being the origin of the viscosity dependence. Thus, it appears that local torsion transitions determine the viscosity dependence of the diffusion coefficient on the global coordinate and, in turn, internal friction effects on the folding rate.

Development and assessment of atomistic models for predicting static friction coefficients

NASA Astrophysics Data System (ADS)

Jahangiri, Soran; Heverly-Coulson, Gavin S.; Mosey, Nicholas J.

2016-08-01

The friction coefficient relates friction forces to normal loads and plays a key role in fundamental and applied areas of science and technology. Despite its importance, the relationship between the friction coefficient and the properties of the materials forming a sliding contact is poorly understood. We illustrate how simple relationships regarding the changes in energy that occur during slip can be used to develop a quantitative model relating the friction coefficient to atomic-level features of the contact. The slip event is considered as an activated process and the load dependence of the slip energy barrier is approximated with a Taylor series expansion of the corresponding energies with respect to load. The resulting expression for the load-dependent slip energy barrier is incorporated in the Prandtl-Tomlinson (PT) model and a shear-based model to obtain expressions for friction coefficient. The results indicate that the shear-based model reproduces the static friction coefficients μs obtained from first-principles molecular dynamics simulations more accurately than the PT model. The ability of the model to provide atomistic explanations for differences in μs amongst different contacts is also illustrated. As a whole, the model is able to account for fundamental atomic-level features of μs, explain the differences in μs for different materials based on their properties, and might be also used in guiding the development of contacts with desired values of μs.

A hierarchical estimator development for estimation of tire-road friction coefficient

PubMed Central

Zhang, Xudong; Göhlich, Dietmar

2017-01-01

The effect of vehicle active safety systems is subject to the friction force arising from the contact of tires and the road surface. Therefore, an adequate knowledge of the tire-road friction coefficient is of great importance to achieve a good performance of these control systems. This paper presents a tire-road friction coefficient estimation method for an advanced vehicle configuration, four-motorized-wheel electric vehicles, in which the longitudinal tire force is easily obtained. A hierarchical structure is adopted for the proposed estimation design. An upper estimator is developed based on unscented Kalman filter to estimate vehicle state information, while a hybrid estimation method is applied as the lower estimator to identify the tire-road friction coefficient using general regression neural network (GRNN) and Bayes' theorem. GRNN aims at detecting road friction coefficient under small excitations, which are the most common situations in daily driving. GRNN is able to accurately create a mapping from input parameters to the friction coefficient, avoiding storing an entire complex tire model. As for large excitations, the estimation algorithm is based on Bayes' theorem and a simplified “magic formula” tire model. The integrated estimation method is established by the combination of the above-mentioned estimators. Finally, the simulations based on a high-fidelity CarSim vehicle model are carried out on different road surfaces and driving maneuvers to verify the effectiveness of the proposed estimation method. PMID:28178332

A hierarchical estimator development for estimation of tire-road friction coefficient.

PubMed

Zhang, Xudong; Göhlich, Dietmar

2017-01-01

The effect of vehicle active safety systems is subject to the friction force arising from the contact of tires and the road surface. Therefore, an adequate knowledge of the tire-road friction coefficient is of great importance to achieve a good performance of these control systems. This paper presents a tire-road friction coefficient estimation method for an advanced vehicle configuration, four-motorized-wheel electric vehicles, in which the longitudinal tire force is easily obtained. A hierarchical structure is adopted for the proposed estimation design. An upper estimator is developed based on unscented Kalman filter to estimate vehicle state information, while a hybrid estimation method is applied as the lower estimator to identify the tire-road friction coefficient using general regression neural network (GRNN) and Bayes' theorem. GRNN aims at detecting road friction coefficient under small excitations, which are the most common situations in daily driving. GRNN is able to accurately create a mapping from input parameters to the friction coefficient, avoiding storing an entire complex tire model. As for large excitations, the estimation algorithm is based on Bayes' theorem and a simplified "magic formula" tire model. The integrated estimation method is established by the combination of the above-mentioned estimators. Finally, the simulations based on a high-fidelity CarSim vehicle model are carried out on different road surfaces and driving maneuvers to verify the effectiveness of the proposed estimation method.

Monte Carlo calibration of avalanches described as Coulomb fluid flows.

PubMed

Ancey, Christophe

2005-07-15

The idea that snow avalanches might behave as granular flows, and thus be described as Coulomb fluid flows, came up very early in the scientific study of avalanches, but it is not until recently that field evidence has been provided that demonstrates the reliability of this idea. This paper aims to specify the bulk frictional behaviour of snow avalanches by seeking a universal friction law. Since the bulk friction coefficient cannot be measured directly in the field, the friction coefficient must be calibrated by adjusting the model outputs to closely match the recorded data. Field data are readily available but are of poor quality and accuracy. We used Bayesian inference techniques to specify the model uncertainty relative to data uncertainty and to robustly and efficiently solve the inverse problem. A sample of 173 events taken from seven paths in the French Alps was used. The first analysis showed that the friction coefficient behaved as a random variable with a smooth and bell-shaped empirical distribution function. Evidence was provided that the friction coefficient varied with the avalanche volume, but any attempt to adjust a one-to-one relationship relating friction to volume produced residual errors that could be as large as three times the maximum uncertainty of field data. A tentative universal friction law is proposed: the friction coefficient is a random variable, the distribution of which can be approximated by a normal distribution with a volume-dependent mean.

Effect of strain hardening on friction behavior of iron lubricated with benzyl structures

NASA Technical Reports Server (NTRS)

Buckley, D. H.; Brainard, W. A.

1974-01-01

Sliding friction experiments were conducted with iron, copper, and aluminum in contact with iron in various states of strain. The surfaces were examined in dry sliding and with various benzyl compounds applied as lubricants. Friction experiments were conducted with a hemispherical rider contacting a flat disk at loads of from 50 to 600 grams with a sliding speed of 0.15 cm/min. Results indicate that straining increases friction for dry sliding and for surfaces lubricated with certain benzyl structures such as dibenzyl disulfide. With other benzyl compounds (e.g., benzyl formate), friction coefficients are lower for strained than for annealed iron.

A comparison of roughness parameters and friction coefficients of aesthetic archwires.

PubMed

Rudge, Philippa; Sherriff, Martyn; Bister, Dirk

2015-02-01

Compare surface roughness of 'aesthetic' nickel-titanium (NiTi) archwires with their dynamic frictional properties. Archwires investigated were: four fully coated tooth coloured [Forestadent: Biocosmetic (FB) and Titanol Cosmetic (FT); TOC Tooth Tone (TT); and Hawley Russell Coated Superelastic NiTi (HRC)]; two partially coated tooth coloured [DB Euroline Microcoated (DB) and TP Aesthetic NiTi (TP)]; two rhodium coated [TOC Sentalloy (TS) and Hawley Russell Rhodium Coated Superelastic NiTi (HRR)]; and two controls: stainless steel [Forestadent Steel (FS)] and NiTi archwire [Forestadent Titanol Superelastic (FN)]. Surface roughness [profilometry (Rugosurf)] was compared with frictional coefficients for archwire/bracket/ligature combinations (n = 10). Analysis of variance, Sidak's multiple comparison of means, and Spearman's correlation coefficient were used for analysis. Roughness coefficients were from low to high: FB; FN; TT; FS; TS; HRR; FT; DB; TP; HRC. Friction coefficients were from low to high: TP; FS; FN; HRR; FT; DB; FB; HRC; TS; TT. Coated archwires generally exhibited higher friction than uncoated controls. TP had the lowest friction but this was not statistically significant (P < 0.05). Friction of tooth coloured coated archwires were significantly different for some wires. Spearman's correlation did not demonstrate consistency between surface roughness (R a) and dynamic friction. Aesthetic archwires investigated had either low surface roughness or low frictional resistance but not both properties simultaneously. Causes for friction are likely to be multifactorial and do not appear to be solely determined by surface roughness (measured by profilometry). For selecting the most appropriate aligning archwire, both surface roughness and frictional resistance need to be considered. © The Author 2014. Published by Oxford University Press on behalf of the European Orthodontic Society. All rights reserved. For permissions, please email: journals.permissions@oup.com.

Impact of laser power density on tribological properties of Pulsed Laser Deposited DLC films

NASA Astrophysics Data System (ADS)

Gayathri, S.; Kumar, N.; Krishnan, R.; AmirthaPandian, S.; Ravindran, T. R.; Dash, S.; Tyagi, A. K.; Sridharan, M.

2013-12-01

Fabrication of wear resistant and low friction carbon films on the engineered substrates is considered as a challenging task for expanding the applications of diamond-like carbon (DLC) films. In this paper, pulsed laser deposition (PLD) technique is used to deposit DLC films on two different types of technologically important class of substrates such as silicon and AISI 304 stainless steel. Laser power density is one of the important parameter used to tailor the fraction of sp2 bonded amorphous carbon (a-C) and tetrahedral amorphous carbon (ta-C) made by sp3 domain in the DLC film. The I(D)/I(G) ratio decreases with the increasing laser power density which is associated with decrease in fraction of a-C/ta-C ratio. The fraction of these chemical components is quantitatively analyzed by EELS which is well supported to the data obtained from the Raman spectroscopy. Tribological properties of the DLC are associated with chemical structure of the film. However, the super low value of friction coefficient 0.003 is obtained when the film is predominantly constituted by a-C and sp2 fraction which is embedded within the clusters of ta-C. Such a particular film with super low friction coefficient is measured while it was deposited on steel at low laser power density of 2 GW/cm2. The super low friction mechanism is explained by low sliding resistance of a-C/sp2 and ta-C clusters. Combination of excellent physical and mechanical properties of wear resistance and super low friction coefficient of DLC films is desirable for engineering applications. Moreover, the high friction coefficient of DLC films deposited at 9GW/cm2 is related to widening of the intergrain distance caused by transformation from sp2 to sp3 hybridized structure.

The coupled effects of environmental composition, temperature and contact size-scale on the tribology of molybdenum disulfide

NASA Astrophysics Data System (ADS)

Khare, Harmandeep S.

Liquid lubricants are precluded in an exceedingly large number of consumer as well as extreme applications as a means to reduce friction and wear at the sliding interface of two bodies. The extraterrestrial environment is one such example of an extreme environment which has motivated the development of advanced solid lubricant materials. Mechanical systems for space require fabrication, assembly, transportation and testing on earth before launch and deployment. Solid lubricants for space are expected to not only operate efficiently in the hard vacuum of space but also withstand interactions with moisture or oxygen during the terrestrial storage, transportation and assembly prior to deployment and launch. Molybdenum disulfide (MoS2) is considered the gold standard in solid lubricants for space due to its excellent tribological properties in ultra-high vacuum. However in the presence of environmental species such as water and oxygen or at elevated temperatures, the lubricity and endurance of MoS2 is severely limited. Past studies have offered several hypotheses for the breakdown of lubrication of MoS2 under the influence of water and oxygen, although exact mechanisms remain unknown. Furthermore, it is unclear if temperature acts as a driver solely for oxidation or for thermally activated slip and thermally activated desorption as well. The answers to these questions are of fundamental importance to improving the reliability of existing MoS2-based solid lubricants for space, as well as for guiding the design of advanced lamellar solid lubricant coatings. This dissertation aims to elucidate: (1) the role of water on MoS2 oxidation, (2) the role of water on MoS2 friction, (3) the role of oxygen on MoS2 friction, (4) the contribution of thermal activation to ambient-temperature friction, and (5) effects of length-scale. The results of this study showed that water does not cause oxidation of MoS2. Water increases ambient-temperature friction of MoS2 directly through a combination of both surface adsorption and diffusion into the coating subsurface. Thermally activated desiccation effectively dries the bulk of the coating, yielding low values of friction coefficient even at ambient humidity and temperature. Friction of MoS2 decreases with increasing temperature between 25°C and 100°C in the presence of environmental water and increases in the presence of oxygen alone. At temperatures greater than 100°C, friction generally increases with temperature only in the presence of environmental oxygen; at these elevated temperatures, friction decreases with increasing humidity. The transition from room-temperature increase to elevated-temperature decrease in friction with increasing humidity is found to be a strong function of the contact history as well as coating microstructure. Lastly, the contribution of nanoscale tribofilms to macroscale friction was studied through nanotribometry. Friction measured on the worn MoS2 coating with a nano-scale AFM probe showed direct and quantifiable evidence of sliding-induced surface modification of MoS2; friction measured on the perfectly ordered single crystal MoS2 was nearly an order of magnitude lower than friction on worn MoS2. Although friction coefficients measured with a nanoscale probe showed high surface sensitivity, micron-sized AFM probes gave friction coefficients similar to those obtained in the macroscale, suggesting the formation of surface films in-situ during sliding with the colloidal probe. A reduction in friction is observed after annealing for both the nanoscale and microscale probes, suggesting a strong overriding effect of the desiccated bulk over surface adsorption in driving the friction response at these length-scales.

Prediction of static friction coefficient in rough contacts based on the junction growth theory

NASA Astrophysics Data System (ADS)

Spinu, S.; Cerlinca, D.

2017-08-01

The classic approach to the slip-stick contact is based on the framework advanced by Mindlin, in which localized slip occurs on the contact area when the local shear traction exceeds the product between the local pressure and the static friction coefficient. This assumption may be too conservative in the case of high tractions arising at the asperities tips in the contact of rough surfaces, because the shear traction may be allowed to exceed the shear strength of the softer material. Consequently, the classic frictional contact model is modified in this paper so that gross sliding occurs when the junctions formed between all contacting asperities are independently sheared. In this framework, when the contact tractions, normal and shear, exceed the hardness of the softer material on the entire contact area, the material of the asperities yields and the junction growth process ends in all contact regions, leading to gross sliding inception. This friction mechanism is implemented in a previously proposed numerical model for the Cattaneo-Mindlin slip-stick contact problem, which is modified to accommodate the junction growth theory. The frictionless normal contact problem is solved first, then the tangential force is gradually increased, until gross sliding inception. The contact problems in the normal and in the tangential direction are successively solved, until one is stabilized in relation to the other. The maximum tangential force leading to a non-vanishing stick area is the static friction force that can be sustained by the rough contact. The static friction coefficient is eventually derived as the ratio between the latter friction force and the normal force.

Friction in a Moving Car

ERIC Educational Resources Information Center

Goldberg, Fred M.

1975-01-01

Describes an out-of-doors, partially unstructured experiment to determine the coefficient of friction for a moving car. Presents the equation which relates the coefficient of friction to initial velocity, distance, and time and gives sample computed values as a function of initial speed and tire pressure. (GS)

A hybrid PSO-SVM-based method for predicting the friction coefficient between aircraft tire and coating

NASA Astrophysics Data System (ADS)

Zhan, Liwei; Li, Chengwei

2017-02-01

A hybrid PSO-SVM-based model is proposed to predict the friction coefficient between aircraft tire and coating. The presented hybrid model combines a support vector machine (SVM) with particle swarm optimization (PSO) technique. SVM has been adopted to solve regression problems successfully. Its regression accuracy is greatly related to optimizing parameters such as the regularization constant C , the parameter gamma γ corresponding to RBF kernel and the epsilon parameter \\varepsilon in the SVM training procedure. However, the friction coefficient which is predicted based on SVM has yet to be explored between aircraft tire and coating. The experiment reveals that drop height and tire rotational speed are the factors affecting friction coefficient. Bearing in mind, the friction coefficient can been predicted using the hybrid PSO-SVM-based model by the measured friction coefficient between aircraft tire and coating. To compare regression accuracy, a grid search (GS) method and a genetic algorithm (GA) are used to optimize the relevant parameters (C , γ and \\varepsilon ), respectively. The regression accuracy could be reflected by the coefficient of determination ({{R}2} ). The result shows that the hybrid PSO-RBF-SVM-based model has better accuracy compared with the GS-RBF-SVM- and GA-RBF-SVM-based models. The agreement of this model (PSO-RBF-SVM) with experiment data confirms its good performance.

What information can frictional properties of polymer brushes tell us?

NASA Astrophysics Data System (ADS)

Zhang, Zhenyu; Moxey, Mark; Morse, Andrew; Armes, Steven; Lewis, Andrew; Geoghegan, Mark; Leggett, Graham

2013-03-01

We have used friction force microscopy (FFM) to quantitatively examine surface grown zwitterionic polymer brushes: poly(2-(methacryloyloxy)ethyl phosphorylcholine) (PMPC), and to establish the correlation between its frictional behaviour to other intrinsic properties. In a good solvent, it was found that the coefficient of friction (μ) decreased with increasing film thickness. We conclude that the amount of bound solvent increases as the brush length increases, causing the osmotic pressure to increase and yielding a reduced tendency for the brush layer to deform under applied load. When measured in a series of alcohol/water mixtures, a significant increase in μ was observed for ethanol/water mixtures at a volume fraction of 90%. This is attributed to brush collapse due to co-nonsolvency, leading to loss of hydration of the brush chains and hence substantially reduced lubrication. We show that single asperity contact mechanics is strongly dependent on solvent quality. Friction-load relationship was found linear in methanol (good solvent), but sub-linear in water and ethanol (moderate solvent).

High-velocity frictional properties of Alpine Fault rocks: Mechanical data, microstructural analysis, and implications for rupture propagation

NASA Astrophysics Data System (ADS)

Boulton, Carolyn; Yao, Lu; Faulkner, Daniel R.; Townend, John; Toy, Virginia G.; Sutherland, Rupert; Ma, Shengli; Shimamoto, Toshihiko

2017-04-01

The Alpine Fault in New Zealand is a major plate-bounding structure that typically slips in ∼M8 earthquakes every c. 330 years. To investigate the near-surface, high-velocity frictional behavior of surface- and borehole-derived Alpine Fault gouges and cataclasites, twenty-one rotary shear experiments were conducted at 1 MPa normal stress and 1 m/s equivalent slip velocity under both room-dry and water-saturated (wet) conditions. In the room-dry experiments, the peak friction coefficient (μp = τp/σn) of Alpine Fault cataclasites and fault gouges was consistently high (mean μp = 0.67 ± 0.07). In the wet experiments, the fault gouge peak friction coefficients were lower (mean μp = 0.20 ± 0.12) than the cataclasite peak friction coefficients (mean μp = 0.64 ± 0.04). All fault rocks exhibited very low steady-state friction coefficients (μss) (room-dry experiments mean μss = 0.16 ± 0.05; wet experiments mean μss = 0.09 ± 0.04). Of all the experiments performed, six experiments conducted on wet smectite-bearing principal slip zone (PSZ) fault gouges yielded the lowest peak friction coefficients (μp = 0.10-0.20), the lowest steady-state friction coefficients (μss = 0.03-0.09), and, commonly, the lowest specific fracture energy values (EG = 0.01-0.69 MJ/m2). Microstructures produced during room-dry and wet experiments on a smectite-bearing PSZ fault gouge were compared with microstructures in the same material recovered from the Deep Fault Drilling Project (DFDP-1) drill cores. The near-absence of localized shear bands with a strong crystallographic preferred orientation in the natural samples most resembles microstructures formed during wet experiments. Mechanical data and microstructural observations suggest that Alpine Fault ruptures propagate preferentially through water-saturated smectite-bearing fault gouges that exhibit low peak and steady-state friction coefficients.

Experience-based learning on determining the frictional coefficients of thermoset polymers incorporated with silicon carbide whiskers and chopped carbon fibers at different temperatures

NASA Astrophysics Data System (ADS)

Harrison, Edward; Alamir, Mohammed; Alzahrani, Naif; Asmatulu, Ramazan

2017-04-01

High temperature applications of materials have been increasing for various industrial applications, such as automobile brakes, clutches and thrust pads. The big portion of these materials are made out of the polymeric materials with various reinforcements. In the present study, high temperature polymeric materials were incorporated with SiC whiskers and chopped carbon fibers at 0, 5, 10 and 20wt.% and molded into desired size and shape prior to the curing process. These inclusions were selected because of their high mechanical strengths and thermal conductivity values to easily dissipate the frictional heat energy and sustain more external loads. The method of testing involves a metal ramp with an adjustable incline to find the coefficients of static and kinetic frictions by recording time and the angle of movement at various temperatures (e.g., -10°C and 50°C). The test results indicated that increasing the inclusions made drastic improvements on the coefficients of static and kinetic frictions. The undergraduate students were involved in the project and observed all the details of the process during the laboratory studies, as well as data collection, analysis and presentation. This study will be useful for the future trainings of the undergraduate engineering students on the composite, automobile and other manufacturing industries.

A Simple Measurement of the Sliding Friction Coefficient

ERIC Educational Resources Information Center

Gratton, Luigi M.; Defrancesco, Silvia

2006-01-01

We present a simple computer-aided experiment for investigating Coulomb's law of sliding friction in a classroom. It provides a way of testing the possible dependence of the friction coefficient on various parameters, such as types of materials, normal force, apparent area of contact and sliding velocity.

Interpretation of the human skin biotribological behaviour after tape stripping

PubMed Central

Pailler-Mattei, C.; Guerret-Piécourt, C.; Zahouani, H.; Nicoli, S.

2011-01-01

The present study deals with the modification of the human skin biotribological behaviour after tape stripping. The tape-stripping procedure consists in the sequential application and removal of adhesive tapes on the skin surface in order to remove stratum corneum (SC) layers, which electrically charges the skin surface. The skin electric charges generated by tape stripping highly change the skin friction behaviour by increasing the adhesion component of the skin friction coefficient. It has been proposed to rewrite the friction adhesion component as the sum of two terms: the first classical adhesion term depending on the intrinsic shear strength, τ0, and the second term depending on the electric shear strength, τelec. The experimental results allowed to estimate a numerical value of the electric shear strength τelec. Moreover, a plan capacitor model with a dielectric material inside was used to modelize the experimental system. This physical model permitted to evaluate the friction electric force and the electric shear strength values to calculate the skin friction coefficient after the tape stripping. The comparison between the experimental and the theoretical value of the skin friction coefficient after the tape stripping has shown the importance of the electric charges on skin biotribological behaviour. The static electric charges produced by tape stripping on the skin surface are probably able to highly modify the interaction of formulations with the skin surface and their spreading properties. This phenomenon, generally overlooked, should be taken into consideration as it could be involved in alteration of drug absorption. PMID:21227961

Internal rotor friction instability

NASA Technical Reports Server (NTRS)

Walton, J.; Artiles, A.; Lund, J.; Dill, J.; Zorzi, E.

1990-01-01

The analytical developments and experimental investigations performed in assessing the effect of internal friction on rotor systems dynamic performance are documented. Analytical component models for axial splines, Curvic splines, and interference fit joints commonly found in modern high speed turbomachinery were developed. Rotor systems operating above a bending critical speed were shown to exhibit unstable subsynchronous vibrations at the first natural frequency. The effect of speed, bearing stiffness, joint stiffness, external damping, torque, and coefficient of friction, was evaluated. Testing included material coefficient of friction evaluations, component joint quantity and form of damping determinations, and rotordynamic stability assessments. Under conditions similar to those in the SSME turbopumps, material interfaces experienced a coefficient of friction of approx. 0.2 for lubricated and 0.8 for unlubricated conditions. The damping observed in the component joints displayed nearly linear behavior with increasing amplitude. Thus, the measured damping, as a function of amplitude, is not represented by either linear or Coulomb friction damper models. Rotordynamic testing of an axial spline joint under 5000 in.-lb of static torque, demonstrated the presence of an extremely severe instability when the rotor was operated above its first flexible natural frequency. The presence of this instability was predicted by nonlinear rotordynamic time-transient analysis using the nonlinear component model developed under this program. Corresponding rotordynamic testing of a shaft with an interference fit joint demonstrated the presence of subsynchronous vibrations at the first natural frequency. While subsynchronous vibrations were observed, they were bounded and significantly lower in amplitude than the synchronous vibrations.

Interpretation of the human skin biotribological behaviour after tape stripping.

PubMed

Pailler-Mattei, C; Guerret-Piécourt, C; Zahouani, H; Nicoli, S

2011-07-06

The present study deals with the modification of the human skin biotribological behaviour after tape stripping. The tape-stripping procedure consists in the sequential application and removal of adhesive tapes on the skin surface in order to remove stratum corneum (SC) layers, which electrically charges the skin surface. The skin electric charges generated by tape stripping highly change the skin friction behaviour by increasing the adhesion component of the skin friction coefficient. It has been proposed to rewrite the friction adhesion component as the sum of two terms: the first classical adhesion term depending on the intrinsic shear strength, τ(0), and the second term depending on the electric shear strength, τ(elec). The experimental results allowed to estimate a numerical value of the electric shear strength τ(elec). Moreover, a plan capacitor model with a dielectric material inside was used to modelize the experimental system. This physical model permitted to evaluate the friction electric force and the electric shear strength values to calculate the skin friction coefficient after the tape stripping. The comparison between the experimental and the theoretical value of the skin friction coefficient after the tape stripping has shown the importance of the electric charges on skin biotribological behaviour. The static electric charges produced by tape stripping on the skin surface are probably able to highly modify the interaction of formulations with the skin surface and their spreading properties. This phenomenon, generally overlooked, should be taken into consideration as it could be involved in alteration of drug absorption.

Effect of load on the tribological properties of hypereutectic Al-Si alloy under boundary lubrication conditions

NASA Astrophysics Data System (ADS)

Kumar, Parveen; Wani, M. F.

2017-11-01

Researchers reported that the IC engine components (piston, cylinder liner etc) fail due to the friction losses (~45%) and wear losses (~25-40%). So the demand of light weight, low friction and wear resistance alloys increases day by day, which reduces the emission and increases the efficiency of the IC engine. In this connection, tribological tests on hypereutectic Al-25Si alloy were performed using a ball-on-disk configuration under dry and lubricated sliding conditions. Hypereutectic Al-25Si alloy was prepared by rapid solidification process with T6 condition. T6 condition improves the friction, wear and mechanical properties of the alloy. Friction coefficient and wear rate of the alloy was measured under high loads ranging from 100 to 300 N. It was found that the friction coefficient (COF) and wear rate of hypereutectic Al-25Si alloy/steel tribo pair increased with increase in load. Significant reduction in COF and wear rate was accomplished with SAE20W50 engine oil and Si particles act as solid lubricant. Optical microscope, 3D surface profilometer and scanning electron microscope (SEM) coupled with an energy dispersive spectrometer (EDS) were used for characterization the worn surface morphologies. The morphology, size and distribution of high Si particles due to its fabrication process caused the improvements in COF and wear rate under lubricated conditions. Adhesive wear, abrasive wear and plastic deformation acted as the dominant wear mechanism for hypereutectic Al-25Si alloy.

Non-equilibrium phase behavior and friction of confined molecular films under shear: A non-equilibrium molecular dynamics study.

PubMed

Maćkowiak, Sz; Heyes, D M; Dini, D; Brańka, A C

2016-10-28

The phase behavior of a confined liquid at high pressure and shear rate, such as is found in elastohydrodynamic lubrication, can influence the traction characteristics in machine operation. Generic aspects of this behavior are investigated here using Non-equilibrium Molecular Dynamics (NEMD) simulations of confined Lennard-Jones (LJ) films under load with a recently proposed wall-driven shearing method without wall atom tethering [C. Gattinoni et al., Phys. Rev. E 90, 043302 (2014)]. The focus is on thick films in which the nonequilibrium phases formed in the confined region impact on the traction properties. The nonequilibrium phase and tribological diagrams are mapped out in detail as a function of load, wall sliding speed, and atomic scale surface roughness, which is shown can have a significant effect. The transition between these phases is typically not sharp as the external conditions are varied. The magnitude of the friction coefficient depends strongly on the nonequilibrium phase adopted by the confined region of molecules, and in general does not follow the classical friction relations between macroscopic bodies, e.g., the frictional force can decrease with increasing load in the Plug-Slip (PS) region of the phase diagram owing to structural changes induced in the confined film. The friction coefficient can be extremely low (∼0.01) in the PS region as a result of incommensurate alignment between a (100) face-centered cubic wall plane and reconstructed (111) layers of the confined region near the wall. It is possible to exploit hysteresis to retain low friction PS states well into the central localization high wall speed region of the phase diagram. Stick-slip behavior due to periodic in-plane melting of layers in the confined region and subsequent annealing is observed at low wall speeds and moderate external loads. At intermediate wall speeds and pressure values (at least) the friction coefficient decreases with increasing well depth of the LJ potential between the wall atoms, but increases when the attractive part of the potential between wall atoms and confined molecules is made larger.

In-flight and laboratory vacuum-friction test results

NASA Technical Reports Server (NTRS)

Devine, E. J.; Evans, H. E.; Leasure, W. A.

1973-01-01

Coefficient of friction measurements were made for six unlubricated metal couples exposed to the space environment aboard the OV-1-13 spacecraft and exposed to laboratory vacuum. Materials studied included mutually soluble, partially soluble, and insoluble metal combinations. Two samples of each material couple were tested in space and in the laboratory using the disk and rider technique. Linear velocity was 0.10 cm/s (2.5 in/min) and rider normal load was 4.45 N (1 lb) for the gold versus silver couples and 8.90 N (2lb) for the other combinations. Results showed that friction data obtained in a clean ion-pumped laboratory vacuum of 10 to the minus 10 power materials with low mutual solubility can be correlated to operation in the vicinity of a typical scientific spacecraft that is exposed to an ambient pressure as low as 10 to the minus 12 power torr. The expected increase in coefficient of friction with solubility was shown. Material couples with high mutual solubility present the hazard of unpredictable drastic friction increase in orbit which may not be evident in laboratory testing at levels down to 10 to the minus 10 power torr. It was also shown that gross cold welding of unlubricated metals exposed to a satellite environment does not occur.

Assembling of carbon nanotubes film responding to significant reduction wear and friction on steel surface

NASA Astrophysics Data System (ADS)

Zhang, Bin; Xue, Yong; Qiang, Li; Gao, Kaixong; Liu, Qiao; Yang, Baoping; Liang, Aiming; Zhang, Junyan

2017-11-01

Friction properties of carbon nanotubes have been widely studied and reported, however, the friction properties of carbon nanotubes related on state of itself. It is showing superlubricity under nanoscale, but indicates high shear adhesion as aligned carbon nanotube film. However, friction properties under high load (which is commonly in industry) of carbon nanotube films are seldom reported. In this paper, carbon nanotube films, via mechanical rubbing method, were obtained and its tribology properties were investigated at high load of 5 to 15 N. Though different couple pairs were employed, the friction coefficients of carbon nanotube films are nearly the same. Compared with bare stainless steel, friction coefficients and wear rates under carbon nanotube films lubrication reduced to, at least, 1/5 and 1/(4.3-14.5), respectively. Friction test as well as structure study were carried out to reveal the mechanism of the significant reduction wear and friction on steel surface. One can conclude that sliding and densifying of carbon nanotubes at sliding interface contribute to the sufficient decrease of friction coefficients and wear rates.

Contact line friction of electrowetting actuated viscous droplets

NASA Astrophysics Data System (ADS)

Vo, Quoc; Tran, Tuan

2018-06-01

We examine the contact line friction coefficient of viscous droplets spreading and retracting on solid surfaces immersed in ambient oil. By using the electrowetting effect, we generate a surface tension imbalance to drive the spreading and the retracting motion of the three-phase contact line (TCL). We show that neither the driving force intensity nor TCL direction significantly influences the friction coefficient. Instead, the friction coefficient depends equivalently on the viscosity of liquid droplets and the surrounding oil. We derive and experimentally verify a transient timescale that can be used to characterize both the spreading and retracting dynamics.

Tribological and microstructural characteristics of ion-nitrided steels

NASA Technical Reports Server (NTRS)

Spalvins, T.

1983-01-01

Three steels AISI 4140, AISI 4340 and AISI 304 stainless steel were ion nitrided in a plasma consisting of a 75:25 mixture of H2:N2, sometimes with a trace of CH4. Their surface topography was characterized by SEM and two distinct compound phases were identified: the gamma and the epsilon. The core-case hardness profiles were also established. The low Cr alloy steels have an extended diffusion zone in contrast to the 304 stainless steels which have a sharp interface. The depth of ion-nitriding is increased as the Cr content is decreased. Friction tests reveal that the gamma surface phase has a lower coefficient of friction than the epsilon phase. The lowest coefficient of friction is achieved when both the rider and the specimen surface are ion nitrided.

Effect of T6 treatment on the coefficient of friction of Al25Mg2Si2Cu4Fe alloy

NASA Astrophysics Data System (ADS)

Sondur, D. G.; Mallapur, D. G.; Udupa, K. Rajendra

2018-04-01

Effect of T6 treatment on the coefficient of friction of Al25Mg2Si2Cu4Fe alloy was evaluated by conducting wear test on pin on disc wear testing machine. Wear test parameters such as the load and the speed were varied by keeping one constant and varying the other respectively. It was observed that the coefficient of friction is high for as cast condition due to the brittle microstructure. After T6 heat treatment the precipitates formed such as the Chinese scripts and the Mg2Si blocks got modified that lead to improvement in the hardness and the wear resistance. This reduces the coefficient of friction.

Length scale effects of friction in particle compaction using atomistic simulations and a friction scaling model

NASA Astrophysics Data System (ADS)

Stone, T. W.; Horstemeyer, M. F.

2012-09-01

The objective of this study is to illustrate and quantify the length scale effects related to interparticle friction under compaction. Previous studies have shown as the length scale of a specimen decreases, the strength of a single crystal metal or ceramic increases. The question underlying this research effort continues the thought—If there is a length scale parameter related to the strength of a material, is there a length scale parameter related to friction? To explore the length scale effects of friction, molecular dynamics (MD) simulations using an embedded atom method potential were performed to analyze the compression of two spherical FCC nickel nanoparticles at different contact angles. In the MD model study, we applied a macroscopic plastic contact formulation to determine the normal plastic contact force at the particle interfaces and used the average shear stress from the MD simulations to determine the tangential contact forces. Combining this information with the Coulomb friction law, we quantified the MD interparticle coefficient of friction and showed good agreement with experimental studies and a Discrete Element Method prediction as a function of contact angle. Lastly, we compared our MD simulation friction values to the tribological predictions of Bhushan and Nosonovsky (BN), who developed a friction scaling model based on strain gradient plasticity and dislocation-assisted sliding that included a length scale parameter. The comparison revealed that the BN elastic friction scaling model did a much better job than the BN plastic scaling model of predicting the coefficient of friction values obtained from the MD simulations.

Dry Sliding Tribological Studies of AA6061-B4C-Gr Hybrid Composites

NASA Astrophysics Data System (ADS)

Monikandan, V. V.; Joseph, M. A.; Rajendrakumar, P. K.

2016-10-01

The dry sliding behavior of stir-cast AA6061-10 wt.% B4C composites containing 2.5, 5 and 7.5 wt.% graphite particles was studied as a function of applied load, sliding speed and sliding distance on a pin-on-disk tribotester. The wear rate and friction coefficient increased with increase in applied load and sliding distance. The increase in graphite addition reduced the increase in wear rate and friction coefficient in the sliding speed range 2-2.5 m/s. Scanning electron microscopy of the worn pin revealed a graphite tribolayer, and transmission electron microscopy revealed overlapping deformation bands under 30 N applied load. Upon increasing the applied load to 40 N, welded region with fine crystalline structure was formed due to dynamic recrystallization of AA6061 alloy matrix.

Nonlinear shear wave interaction at a frictional interface: energy dissipation and generation of harmonics.

PubMed

Meziane, A; Norris, A N; Shuvalov, A L

2011-10-01

Analytical and numerical modeling of the nonlinear interaction of shear wave with a frictional interface is presented. The system studied is composed of two homogeneous and isotropic elastic solids, brought into frictional contact by remote normal compression. A shear wave, either time harmonic or a narrow band pulse, is incident normal to the interface and propagates through the contact. Two friction laws are considered and the influence on interface behavior is investigated: Coulomb's law with a constant friction coefficient and a slip-weakening friction law which involves static and dynamic friction coefficients. The relationship between the nonlinear harmonics and the dissipated energy, and the dependence on the contact dynamics (friction law, sliding, and tangential stress) and on the normal contact stress are examined in detail. The analytical and numerical results indicate universal type laws for the amplitude of the higher harmonics and for the dissipated energy, properly non-dimensionalized in terms of the pre-stress, the friction coefficient and the incident amplitude. The results suggest that measurements of higher harmonics can be used to quantify friction and dissipation effects of a sliding interface. © 2011 Acoustical Society of America

Estimation of real-time runway surface contamination using flight data recorder parameters

NASA Astrophysics Data System (ADS)

Curry, Donovan

Within this research effort, the development of an analytic process for friction coefficient estimation is presented. Under static equilibrium, the sum of forces and moments acting on the aircraft, in the aircraft body coordinate system, while on the ground at any instant is equal to zero. Under this premise the longitudinal, lateral and normal forces due to landing are calculated along with the individual deceleration components existent when an aircraft comes to a rest during ground roll. In order to validate this hypothesis a six degree of freedom aircraft model had to be created and landing tests had to be simulated on different surfaces. The simulated aircraft model includes a high fidelity aerodynamic model, thrust model, landing gear model, friction model and antiskid model. Three main surfaces were defined in the friction model; dry, wet and snow/ice. Only the parameters recorded by an FDR are used directly from the aircraft model all others are estimated or known a priori. The estimation of unknown parameters is also presented in the research effort. With all needed parameters a comparison and validation with simulated and estimated data, under different runway conditions, is performed. Finally, this report presents results of a sensitivity analysis in order to provide a measure of reliability of the analytic estimation process. Linear and non-linear sensitivity analysis has been performed in order to quantify the level of uncertainty implicit in modeling estimated parameters and how they can affect the calculation of the instantaneous coefficient of friction. Using the approach of force and moment equilibrium about the CG at landing to reconstruct the instantaneous coefficient of friction appears to be a reasonably accurate estimate when compared to the simulated friction coefficient. This is also true when the FDR and estimated parameters are introduced to white noise and when crosswind is introduced to the simulation. After the linear analysis the results show the minimum frequency at which the algorithm still provides moderately accurate data is at 2Hz. In addition, the linear analysis shows that with estimated parameters increased and decreased up to 25% at random, high priority parameters have to be accurate to within at least +/-5% to have an effect of less than 1% change in the average coefficient of friction. Non-linear analysis results show that the algorithm can be considered reasonably accurate for all simulated cases when inaccuracies in the estimated parameters vary randomly and simultaneously up to +/-27%. At worst-case the maximum percentage change in average coefficient of friction is less than 10% for all surfaces.

A Digital Image-Based Discrete Fracture Network Model and Its Numerical Investigation of Direct Shear Tests

NASA Astrophysics Data System (ADS)

Wang, Peitao; Cai, Meifeng; Ren, Fenhua; Li, Changhong; Yang, Tianhong

2017-07-01

This paper develops a numerical approach to determine the mechanical behavior of discrete fractures network (DFN) models based on digital image processing technique and particle flow code (PFC2D). A series of direct shear tests of jointed rocks were numerically performed to study the effect of normal stress, friction coefficient and joint bond strength on the mechanical behavior of joint rock and evaluate the influence of micro-parameters on the shear properties of jointed rocks using the proposed approach. The complete shear stress-displacement curve of the DFN model under direct shear tests was presented to evaluate the failure processes of jointed rock. The results show that the peak and residual strength are sensitive to normal stress. A higher normal stress has a greater effect on the initiation and propagation of cracks. Additionally, an increase in the bond strength ratio results in an increase in the number of both shear and normal cracks. The friction coefficient was also found to have a significant influence on the shear strength and shear cracks. Increasing in the friction coefficient resulted in the decreasing in the initiation of normal cracks. The unique contribution of this paper is the proposed modeling technique to simulate the mechanical behavior of jointed rock mass based on particle mechanics approaches.

Surface Design and Engineering Toward Wear-Resistant, Self-Lubricating Diamond Films and Coatings

NASA Technical Reports Server (NTRS)

Miyoshi, Kazuhisa

1999-01-01

The tribological properties of chemical-vapor-deposited (CVD) diamond films vary with the environment, possessing a Jekyll-and-Hyde character. CVD diamond has low coefficient of friction and high wear resistance in air but high coefficient of friction and low wear resistance in vacuum. Improving the tribological functionality of materials (such as achieving low friction and good wear resistance) was an aim of this investigation. Three studies on the surface design, surface engineering, and tribology of CVD diamond have shown that its friction and wear are significantly reduced in ultrahigh vacuum. The main criteria for judging whether diamond films are an effective wear-resistant, self-lubricating material were coefficient of friction and wear rate, which must be less than 0.1 and on the order of 10(exp 6) cu mm/N(dot)m, respectively. In the first study the presence of a thin film (less than 1 micron thick) of amorphous, nondiamond carbon (hydrogenated carbon, also called diamondlike carbon or DLC) on CVD diamond greatly decreased the coefficient of friction and the wear rate. Therefore, a thin DLC film on CVD diamond can be an effective wear-resistant, lubricating coating in ultrahigh vacuum. In the second study the presence of an amorphous, nondiamond carbon surface layer formed on CVD diamond by ion implantation significantly reduced the coefficient of friction and the wear rate in ultrahigh vacuum. Therefore, such surface layers are acceptable for effective self-lubricating, wear-resistant applications of CVD diamond. In the third study CVD diamond in contact with cubic boron nitride exhibited low coefficient of friction in ultra high vacuum. Therefore, this materials combination can provide an effective self-lubricating, wear-resistant couple in ultrahigh vacuum.

A study of the rheology and micro-structure of dumbbells in shear geometries

NASA Astrophysics Data System (ADS)

Mandal, Sandip; Khakhar, D. V.

2018-01-01

We study the flow of frictional, inelastic dumbbells made of two fused spheres of different aspect ratios down a rough inclined plane and in a simple shear cell, using discrete element simulations. At a fixed inclination angle, the mean velocity decreases, and the volume fraction increases significantly with increasing aspect ratio in the chute flow. At a fixed solid fraction, the shear stress and pressure decrease significantly with increasing aspect ratio in the shear cell flow. The micro-structure of the flow is characterized. The translational diffusion coefficient in the normal direction to the flow is found to scale as Dy y=b γ ˙ d2, independent of aspect ratio, where b is a constant, γ ˙ is the shear rate, and d is the diameter of the constituent spheres of the dumbbells. The effective friction coefficient (μ, the ratio of shear stress to pressure) increases by 30%-35% on increasing the aspect ratio λ, from 1.0 to 1.7, for a fixed inertial number I. The volume fraction (ϕ) also increases significantly with increasing aspect ratio, especially at high inertial numbers. The effective friction coefficient and volume fraction are found to follow simple scalings of the form μ = μ(I, λ) and ϕ = ϕ(I, λ) for all the data from both systems, and the results are in reasonable agreement with kinetic theory predictions at low I. The computational results are in reasonable agreement with the experimental data for flow in a rotating cylinder.

Wind-tunnel investigation at supersonic speeds of a remote-controlled canard missile with a free-rolling-tail brake torque system

NASA Technical Reports Server (NTRS)

Blair, A. B., Jr.

1985-01-01

Wind tunnel tests were conducted at Mach numbers 1.70, 2.16, and 2.86 to determine the static aerodynamic characteristics of a cruciform canard-controlled missile with fixed or free rolling tailfin afterbodies. Mechanical coupling effects of the free-rolling-tail afterbody were investigated by using an electronic electromagnetic brake system providing arbitrary tail-fin brake torques with continuous measurements of tail-to-mainframe torque and tail roll rate. Remote-controlled canards were deflected to provide pitch, yaw, and roll control. Results indicate that the induced rolling moment coefficients due to canard yaw control are reduced and linearized for the free-rolling-tail (free-tail) configuration. The canards of the latter provide conventional roll control for the entire angle-of-attack test range. For the free-tail configuration, the induced rolling moment coefficient due to canard yaw control increased and the canard roll control decreased with increases in brake torque, which simulated bearing friction torque. It appears that a compromise in regard to bearing friction, for example, low-cost bearings with some friction, may allow satisfactory free-tail aerodynamic characteristics that include reductions in adverse rolling-moment coefficients and lower tail roll rates.

Shear Model Development of Limestone Joints with Incorporating Variations of Basic Friction Coefficient and Roughness Components During Shearing

NASA Astrophysics Data System (ADS)

Mehrishal, Seyedahmad; Sharifzadeh, Mostafa; Shahriar, Korosh; Song, Jae-Jon

2017-04-01

In relation to the shearing of rock joints, the precise and continuous evaluation of asperity interlocking, dilation, and basic friction properties has been the most important task in the modeling of shear strength. In this paper, in order to investigate these controlling factors, two types of limestone joint samples were prepared and CNL direct shear tests were performed on these joints under various shear conditions. One set of samples were travertine and another were onyx marble with slickensided surfaces, surfaces ground to #80, and rough surfaces were tested. Direct shear experiments conducted on slickensided and ground surfaces of limestone indicated that by increasing the applied normal stress, under different shearing rates, the basic friction coefficient decreased. Moreover, in the shear tests under constant normal stress and shearing rate, the basic friction coefficient remained constant for the different contact sizes. The second series of direct shear experiments in this research was conducted on tension joint samples to evaluate the effect of surface roughness on the shear behavior of the rough joints. This paper deals with the dilation and roughness interlocking using a method that characterizes the surface roughness of the joint based on a fundamental combined surface roughness concept. The application of stress-dependent basic friction and quantitative roughness parameters in the continuous modeling of the shear behavior of rock joints is an important aspect of this research.

Micro-scale abrasive wear behavior of medical implant material Ti-25Nb-3Mo-3Zr-2Sn alloy on various friction pairs.

PubMed

Wang, Zhenguo; Huang, Weijiu; Ma, Yanlong

2014-09-01

The micro-scale abrasion behaviors of surgical implant materials have often been reported in the literature. However, little work has been reported on the micro-scale abrasive wear behavior of Ti-25Nb-3Mo-3Zr-2Sn (TLM) titanium alloy in simulated body fluids, especially with respect to friction pairs. Therefore, a TE66 Micro-Scale Abrasion Tester was used to study the micro-scale abrasive wear behavior of the TLM alloy. This study covers the friction coefficient and wear loss of the TLM alloy induced by various friction pairs. Different friction pairs comprised of ZrO2, Si3N4 and Al2O3 ceramic balls with 25.4mm diameters were employed. The micro-scale abrasive wear mechanisms and synergistic effect between corrosion and micro-abrasion of the TLM alloy were investigated under various wear-corrosion conditions employing an abrasive, comprised of SiC (3.5 ± 0.5 μm), in two test solutions, Hanks' solution and distilled water. Before the test, the specimens were heat treated at 760°C/1.0/AC+550°C/6.0/AC. It was discovered that the friction coefficient values of the TLM alloy are larger than those in distilled water regardless of friction pairs used, because of the corrosive Hanks' solution. It was also found that the value of the friction coefficient was volatile at the beginning of wear testing, and it became more stable with further experiments. Because the ceramic balls have different properties, especially with respect to the Vickers hardness (Hv), the wear loss of the TLM alloy increased as the ball hardness increased. In addition, the wear loss of the TLM alloy in Hanks' solution was greater than that in distilled water, and this was due to the synergistic effect of micro-abrasion and corrosion, and this micro-abrasion played a leading role in the wear process. The micro-scale abrasive wear mechanism of the TLM alloy gradually changed from two-body to mixed abrasion and then to three-body abrasion as the Vickers hardness of the balls increased. Copyright © 2014 Elsevier B.V. All rights reserved.

Determination of the coefficient of dynamic friction between coatings of alumina and metallic materials

NASA Astrophysics Data System (ADS)

Santos, A.; Córdoba, E.; Ramírez, Z.; Sierra, C.; Ortega, Y.

2017-12-01

This project aims to determine the coefficient of dynamic friction between micrometric size coatings of alumina and metallic materials (Steel and aluminium); the methodology used to achieve the proposed objective consisted of 4 phases, in the first one was developed a procedure that allowed, from a Pin on Disk machine built based on the specifications given by the ASTM G99-05 standard (Standard test method for wear tests with a Pin on Disk machine), to determine the coefficient of dynamic friction between two materials in contact; subsequently the methodology was verified through tests between steel-steel and steel-aluminium, due to these values are widely reported in the literature; as a third step, deposits of alumina particles of micrometric size were made on a steel substrate through thermal spraying by flame; finally, the tests were carried out between pins of steel of aluminium and alumina coating to determine the coefficients of dynamic friction between these two surfaces. The results of the project allowed to verify that the developed methodology is valid to obtain coefficients of dynamic friction between surfaces in contact since the percentages of error were of 3.5% and 2.1% for steel-steel and aluminium-steel, respectively; additionally, it was found that the coefficient of friction between steel-alumina coatings is 0.36 and aluminium-alumina coating is 0.25.

Friction Coefficient Determination by Electrical Resistance Measurements

ERIC Educational Resources Information Center

Tunyagi, A.; Kandrai, K.; Fülöp, Z.; Kapusi, Z.; Simon, A.

2018-01-01

A simple and low-cost, DIY-type, Arduino-driven experiment is presented for the study of friction and measurement of the friction coefficient, using a conductive rubber cord as a force sensor. It is proposed for high-school or college/university-level students. We strongly believe that it is worthwhile planning, designing and performing Arduino…

An Application of Calculus: Optimum Parabolic Path Problem

ERIC Educational Resources Information Center

Atasever, Merve; Pakdemirli, Mehmet; Yurtsever, Hasan Ali

2009-01-01

A practical and technological application of calculus problem is posed to motivate freshman students or junior high school students. A variable coefficient of friction is used in modelling air friction. The case in which the coefficient of friction is a decreasing function of altitude is considered. The optimum parabolic path for a flying object…

Grip Analysis of Road Surface and Tire Footprint Using FEM

NASA Astrophysics Data System (ADS)

Sabri, M.; Abda, S.

2018-02-01

Road grip involve a touch between road pavement and the tire tread pattern. The load bearing surface, which depends on pavement roughness and local pressures in the contact patch. This research conducted to develop a Finite element model for simulating the experimentally testing of asphalt in Jl. AH Nasution Medan, North Sumatera Indonesia base on the value of grip coefficient from various tire loads and the various speed of the vehicle during contact to the road. A tire model and road pavement are developed for the analyses the geometry of tire footprint. The results showed that the greater the mass of car will increase grip coefficient. The coefficient of grip on the road surface contact trough the tire footprint strongly influence the kinetic coefficient of friction at certain speeds. Experimentally show that Concrete road grip coefficient of more than 34% compared to the asphalt road at the same IRI parameters (6-8). Kinetic friction coefficient more than 0.33 was obtained in a asphalt path at a speed of 30-40 Km/hour.

A km-scale "triaxial experiment" reveals the extreme mechanical weakness and anisotropy of mica-schists (Grandes Rousses Massif, France)

NASA Astrophysics Data System (ADS)

Bolognesi, Francesca; Bistacchi, Andrea

2018-02-01

The development of Andersonian faults is predicted, according to theory and experiments, for brittle/frictional deformation occurring in a homogeneous medium. In contrast, in an anisotropic medium it is possible to observe fault nucleation and propagation that is non-Andersonian in geometry and kinematics. Here, we consider post-metamorphic brittle/frictional deformation in the mechanically anisotropic mylonitic mica-schists of the Grandes Rousse Massif (France). The role of the mylonitic foliation (and of any other source of mechanical anisotropy) in brittle/frictional deformation is a function of orientation and friction angle. According to the relative orientation of principal stress axes and foliation, a foliation characterized by a certain coefficient of friction will be utilized or not for the nucleation and propagation of brittle/frictional fractures and faults. If the foliation is not utilized, the rock behaves as if it was isotropic, and Andersonian geometry and kinematics can be observed. If the foliation is utilized, the deviatoric stress magnitude is buffered and Andersonian faults/fractures cannot develop. In a narrow transition regime, both Andersonian and non-Andersonian structures can be observed. We apply stress inversion and slip tendency analysis to determine the critical angle for failure of the metamorphic foliation of the Grandes Rousses schists, defined as the limit angle between the foliation and principal stress axes for which the foliation was brittlely reactivated. This approach allows defining the ratio of the coefficient of internal friction for failure along the mylonitic foliation to the isotropic coefficient of friction. Thus, the study area can be seen as a km-scale triaxial experiment that allows measuring the degree of mechanical anisotropy of the mylonitic mica-schists. In this way, we infer a coefficient of friction μweak = 0.14 for brittle-frictional failure of the foliation, or 20 % of the isotropic coefficient of internal friction.

Role of friction in vertically oscillated granular materials

NASA Astrophysics Data System (ADS)

Moon, Sung Joon; Swift, J. B.; Swinney, Harry L.

2002-11-01

We use a previously validated molecular dynamics simulation of vertically oscillated granular layers to study how the contact friction affects standing wave patterns. Our collision model follows Walton(O. R. Walton, in Particulate Two-Phase Flow), edited by M. C. Roco (Butterworth-Heinemann, Boston, 1993), p. 884.: Dissipation in the normal component of colliding velocity is characterized by the normal coefficient of restitution e (0<= e < 1), and interaction in the tangential component by the tangential coefficient of restitution β = β(μ,e,Φ), where -1<= β <= β_0, μ is the static coefficient of friction on the surface of grains, Φ is the collision angle, and β0 corresponds to the crossover between static and sliding friction. We varied the above parameters independently for the grain-grain collisions and for the grain-wall collisions. The grain-grain friction changes the phase diagram of patterns significantly, and the patterns become fuzzy as the friction is decreased. The grain-wall friction is necessary to stabilize the patterns.

The Friction of Piston Rings

NASA Technical Reports Server (NTRS)

Tischbein, Hans W

1945-01-01

The coefficient of friction between piston ring and cylinder liner was measured in relation to gliding acceleration, pressure, temperature, quantity of oil and quality of oil. Comparing former lubrication-technical tests, conclusions were drawn as to the state of friction. The coefficients of friction as figured out according to the hydrodynamic theory were compared with those measured by tests. Special tests were made on "oiliness." The highest permissible pressure was measured and the ratio of pressure discussed.

Effect of graphite target power density on tribological properties of graphite-like carbon films

NASA Astrophysics Data System (ADS)

Dong, Dan; Jiang, Bailing; Li, Hongtao; Du, Yuzhou; Yang, Chao

2018-05-01

In order to improve the tribological performance, a series of graphite-like carbon (GLC) films with different graphite target power densities were prepared by magnetron sputtering. The valence bond and microstructure of films were characterized by AFM, TEM, XPS and Raman spectra. The variation of mechanical and tribological properties with graphite target power density was analyzed. The results showed that with the increase of graphite target power density, the deposition rate and the ratio of sp2 bond increased obviously. The hardness firstly increased and then decreased with the increase of graphite target power density, whilst the friction coefficient and the specific wear rate increased slightly after a decrease with the increasing graphite target power density. The friction coefficient and the specific wear rate were the lowest when the graphite target power density was 23.3 W/cm2.

Velocity and stress autocorrelation decay in isothermal dissipative particle dynamics

NASA Astrophysics Data System (ADS)

Chaudhri, Anuj; Lukes, Jennifer R.

2010-02-01

The velocity and stress autocorrelation decay in a dissipative particle dynamics ideal fluid model is analyzed in this paper. The autocorrelation functions are calculated at three different friction parameters and three different time steps using the well-known Groot/Warren algorithm and newer algorithms including self-consistent leap-frog, self-consistent velocity Verlet and Shardlow first and second order integrators. At low friction values, the velocity autocorrelation function decays exponentially at short times, shows slower-than exponential decay at intermediate times, and approaches zero at long times for all five integrators. As friction value increases, the deviation from exponential behavior occurs earlier and is more pronounced. At small time steps, all the integrators give identical decay profiles. As time step increases, there are qualitative and quantitative differences between the integrators. The stress correlation behavior is markedly different for the algorithms. The self-consistent velocity Verlet and the Shardlow algorithms show very similar stress autocorrelation decay with change in friction parameter, whereas the Groot/Warren and leap-frog schemes show variations at higher friction factors. Diffusion coefficients and shear viscosities are calculated using Green-Kubo integration of the velocity and stress autocorrelation functions. The diffusion coefficients match well-known theoretical results at low friction limits. Although the stress autocorrelation function is different for each integrator, fluctuates rapidly, and gives poor statistics for most of the cases, the calculated shear viscosities still fall within range of theoretical predictions and nonequilibrium studies.

Tribological and Mechanical Behaviors of Polyamide 6/Glass Fiber Composite Filled with Various Solid Lubricants

PubMed Central

Li, Duxin; Xie, Ying; Li, Wenjuan; You, Yilan; Deng, Xin

2013-01-01

The effects of polytetrafluoroethylene (PTFE), graphite, ultrahigh molecular weight polyethylene (UHMWPE), and their compounds on mechanical and tribological properties of glass-fiber-reinforced polyamide 6 (PA6/GF) were studied. The polymeric materials were blended using twin-screw extruder and subsequently injection molded for test samples. Mechanical properties were investigated in terms of hardness, tensile strength, and impact strength. Friction and wear experiments were run under ambient conditions at a rotating speed of 200 rpm and load of 100 N. The morphologies of the worn surfaces were also observed with scanning electron microscope. The results showed that graphite could increase the tensile strength of PA6/GF-15 composite, but the material became soft. Graphite/UHMWPE complex solid lubricants were effective in increasing the already high impact strength of PA6/GF-15 composite. 5% PTFE gave the maximum reduction in the coefficient of friction. However, PTFE/UHMWPE complex solid lubricants were the best choice for improving both friction and wear behaviors due to the lower friction coefficient and mass wear rate. Moreover, the worn surface of PA6 composites revealed that adhesive wear, abrasive wear, and fatigue wear occurred in this study. PMID:23766687

Kinetic theory for identical, frictional, nearly elastic disks

NASA Astrophysics Data System (ADS)

Yoon, David K.; Jenkins, James T.

2005-08-01

We develop kinetic theory for slightly frictional and nearly elastic disks. The tangential interaction is modeled by two parameters: a Coulomb friction coefficient and a tangential restitution coefficient. Assuming Maxwellian velocity distribution functions for both translational and rotational velocities, we derive exact expressions for the rates of dissipation of translational and rotational fluctuation energies per unit area. Setting the rotational dissipation rate to zero, as in a steady, homogeneous shearing flow, we find the ratio of the rotational temperature to the translational. In the case of small friction, this is used to determine an effective coefficient of normal restitution. In this way, the effects of small friction can be incorporated into the theory, thereby dispensing with the need to separately consider the complete balances for the momentum and the energy of the rotational motion.

Friction and wear of single-crystal and polycrystalline maganese-zinc ferrite in contact with various metals

NASA Technical Reports Server (NTRS)

Miyoshi, K.; Buckley, D. H.

1977-01-01

Sliding friction experiments were conducted with single-crystal (SCF) and hot-pressed polycrystalline (HPF) manganese-zinc ferrite in contact with various metals. Results indicate that the coefficients of friction for SCF and HPF are related to the relative chemical activity of those metals in high vacuum. The more active the metal, the higher the coefficient of friction. The coefficients of friction for both SCF and HPF were the same and much higher in vacuum than in argon at atmospheric pressure. All the metals tested transferred to the surface of both SCF and HPF in sliding. Both SCF and HPF exhibited cracking and fracture with sliding. Cracking in SCF is dependent on crystallographic characteristics. In HPF, cracking depends on the orientation of the individual crystallites.

Determination of the static friction coefficient from circular motion

NASA Astrophysics Data System (ADS)

Molina-Bolívar, J. A.; Cabrerizo-Vílchez, M. A.

2014-07-01

This paper describes a physics laboratory exercise for determining the coefficient of static friction between two surfaces. The circular motion of a coin placed on the surface of a rotating turntable has been studied. For this purpose, the motion is recorded with a high-speed digital video camera recording at 240 frames s-1, and the videos are analyzed using Tracker video-analysis software, allowing the students to dynamically model the motion of the coin. The students have to obtain the static friction coefficient by comparing the centripetal and maximum static friction forces. The experiment only requires simple and inexpensive materials. The dynamics of circular motion and static friction forces are difficult for many students to understand. The proposed laboratory exercise addresses these topics, which are relevant to the physics curriculum.

Steady and transient sliding under rate-and-state friction

NASA Astrophysics Data System (ADS)

Putelat, Thibaut; Dawes, Jonathan H. P.

2015-05-01

The physics of dry friction is often modelled by assuming that static and kinetic frictional forces can be represented by a pair of coefficients usually referred to as μs and μk, respectively. In this paper we re-examine this discontinuous dichotomy and relate it quantitatively to the more general, and smooth, framework of rate-and-state friction. This is important because it enables us to link the ideas behind the widely used static and dynamic coefficients to the more complex concepts that lie behind the rate-and-state framework. Further, we introduce a generic framework for rate-and-state friction that unifies different approaches found in the literature. We consider specific dynamical models for the motion of a rigid block sliding on an inclined surface. In the Coulomb model with constant dynamic friction coefficient, sliding at constant velocity is not possible. In the rate-and-state formalism steady sliding states exist, and analysing their existence and stability enables us to show that the static friction coefficient μs should be interpreted as the local maximum at very small slip rates of the steady state rate-and-state friction law. Next, we revisit the often-cited experiments of Rabinowicz (J. Appl. Phys., 22:1373-1379, 1951). Rabinowicz further developed the idea of static and kinetic friction by proposing that the friction coefficient maintains its higher and static value μs over a persistence length before dropping to the value μk. We show that there is a natural identification of the persistence length with the distance that the block slips as measured along the stable manifold of the saddle point equilibrium in the phase space of the rate-and-state dynamics. This enables us explicitly to define μs in terms of the rate-and-state variables and hence link Rabinowicz's ideas to rate-and-state friction laws. This stable manifold naturally separates two basins of attraction in the phase space: initial conditions in the first one lead to the block eventually stopping, while in the second basin of attraction the sliding motion continues indefinitely. We show that a second definition of μs is possible, compatible with the first one, as the weighted average of the rate-and-state friction coefficient over the time the block is in motion.

Slip Potential for Commonly Used Inclined Grated Metal Walkways

PubMed Central

Pollard, Jonisha P.; Heberger, John R.; Dempsey, Patrick G.

2016-01-01

Background No specific guidelines or regulations are provided by the Mine Safety and Health Administration for the use of inclined grated metal walkways in mining plants. Mining and other companies may be using walkway materials that do not provide sufficient friction, contributing to slip and fall injuries. Purpose The purpose of this study was to determine if there are significant differences in the required friction for different grated metal walkways during walking in diverse conditions. Methods The normalized coefficients of friction were measured for 12 participants while walking up and down an instrumented walkway with different inclinations (0°, 5°, 10°, 15°, and 20°) and with and without the presence of a contaminant (glycerol). Self-reported slip events were recorded and the required coefficients of friction were calculated considering only the anterior/posterior components of the shear forces. Additionally, the available coefficients of friction for these walkway materials were measured at the 0° orientation using a tribometer, with and without the presence of the contaminant, using a boot heel as well as Neolite as the test feet. Results The number of slips increased when the inclination angle reached 10° and above. Of all materials tested, the diamond weave grating was found to have the best performance at all inclines and when contaminated or dry. A high number of slips occurred for the perforated grating and serrated bar grating at 20° when contaminated. Conclusions Results of this study suggest that the diamond weave grating provides significantly better friction compared to serrated bar and perforated gratings, especially at inclines greater than 10°. PMID:26779388

Effects of high-temperature hydrogenation treatment on sliding friction and wear behavior of carbide-derived carbon films.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Erdemir, A.; Kovalchenko, A.; McNallan, M. J.

2004-01-01

In this study, we investigated the effects of a high-temperature hydrogenation treatment on the sliding friction and wear behavior of nanostructured carbide-derived carbon (CDC) films in dry nitrogen and humid air environments. These films are produced on the surfaces of silicon carbide substrates by reacting the carbide phase with chlorine or chlorine-hydrogen gas mixtures at 1000 to 1100 C in a sealed tube furnace. The typical friction coefficients of CDC films in open air are in the range of 0.2 to 0.25, but in dry nitrogen, the friction coefficients are 0.15. In an effort to achieve lower friction on CDCmore » films, we developed and used a special hydrogenation process that was proven to be very effective in lowering friction of CDC films produced on SiC substrates. Specifically, the films that were post-hydrogen-treated exhibited friction coefficients as low as 0.03 in dry nitrogen, while the friction coefficients in humid air were 0.2. The wear of Si{sub 3}N{sub 4} counterface balls was hard to measure after the tests, while shallow wear tracks had formed on CDC films on SiC disks. Detailed mechanical and structural characterizations of the CDC films and sliding contact surfaces were done using a series of analytical techniques and these findings were correlated with the friction and wear behaviors of as-produced and hydrogen-treated CDC films.« less

Corrosion and tribological properties of basalt fiber reinforced composite materials

NASA Astrophysics Data System (ADS)

Ha, Jin Cheol; Kim, Yun-Hae; Lee, Myeong-Hoon; Moon, Kyung-Man; Park, Se-Ho

2015-03-01

This experiment has examined the corrosion and tribological properties of basalt fiber reinforced composite materials. There were slight changes of weight after the occurring of corrosion based on time and H2SO4 concentration, but in general, the weight increased. It is assumed that this happens due to the basalt fiber precipitate. Prior to the corrosion, friction-wear behavior showed irregular patterns compared to metallic materials, and when it was compared with the behavior after the corrosion, the coefficient of friction was 2 to 3 times greater. The coefficient of friction of all test specimen ranged from 0.1 to 0.2. Such a result has proven that the basalt fiber, similar to the resin rubber, shows regular patterns regardless of time and H2SO4 concentration because of the space made between resins and reinforced materials.

Tribological and microstructural characteristics of ion-nitrided steels

NASA Technical Reports Server (NTRS)

Spalvins, T.

1983-01-01

Three steels AISI 4140, AISI 4340 and AISI 304 stainless steel were ion nitrided in a plasma consisting of a 75:25 mixture of H2:N2, sometimes with a trace of CH4. Their surface topography was characterized by SEM and two distinct compound phases were identified: the gamma and the epsilon. The core-case hardness profiles were also established. The low Cr alloy steels have an extended diffusion zone in contrast to the 3034 stainless steels which have a sharp interface. The depth of ion-nitriding is increased as the Cr content is decreased. Friction tests reveal that the gamma surface phase has a lower coefficient of friction than the epsilon phase. The lowest coefficient of friction is achieved when both the rider and the specimen surface are ion nitrided. Previously announced in STAR as N83-24635

Pad-mode-induced instantaneous mode instability for simple models of brake systems

NASA Astrophysics Data System (ADS)

Oberst, S.; Lai, J. C. S.

2015-10-01

Automotive disc brake squeal is fugitive, transient and remains difficult to predict. In particular, instantaneous mode squeal observed experimentally does not seem to be associated with mode coupling and its mechanism is not clear. The effects of contact pressures, friction coefficients as well as material properties (pressure and temperature dependency and anisotropy) for brake squeal propensity have not been systematically explored. By analysing a finite element model of an isotropic pad sliding on a plate similar to that of a previously reported experimental study, pad modes have been identified and found to be stable using conventional complex eigenvalue analysis. However, by subjecting the model to contact pressure harmonic excitation for a range of pressures and friction coefficients, a forced response analysis reveals that the dissipated energy for pad modes is negative and becomes more negative with increasing contact pressures and friction coefficients, indicating the potential for instabilities. The frequency of the pad mode in the sliding direction is within the range of squeal frequencies observed experimentally. Nonlinear time series analysis of the vibration velocity also confirms the evolution of instabilities induced by pad modes as the friction coefficient increases. By extending this analysis to a more realistic but simple brake model in the form of a pad-on-disc system, in-plane pad-modes, which a complex eigenvalue analysis predicts to be stable, have also been identified by negative dissipated energy for both isotropic and anisotropic pad material properties. The influence of contact pressures on potential instabilities has been found to be more dominant than changes in material properties owing to changes in pressure or temperature. Results here suggest that instantaneous mode squeal is likely caused by in-plane pad-mode instabilities.

Measurements of Heat-Transfer and Friction Coefficients for Helium Flowing in a Tube at Surface Temperatures up to 5900 Deg R

NASA Technical Reports Server (NTRS)

Taylor, Maynard F.; Kirchgessner, Thomas A.

1959-01-01

Measurements of average heat transfer and friction coefficients and local heat transfer coefficients were made with helium flowing through electrically heated smooth tubes with length-diameter ratios of 60 and 92 for the following range of conditions: Average surface temperature from 1457 to 4533 R, Reynolds numbe r from 3230 to 60,000, heat flux up to 583,200 Btu per hr per ft2 of heat transfer area, and exit Mach numbe r up to 1.0. The results indicate that, in the turbulent range of Reynolds number, good correlation of the local heat transfer coefficients is obtained when the physical properties and density of helium are evaluated at the surface temperature. The average heat transfer coefficients are best correlated on the basis that the coefficient varies with [1 + (L/D))(sup -0,7)] and that the physical properties and density are evaluated at the surface temperature. The average friction coefficients for the tests with no heat addition are in complete agreement with the Karman-Nikuradse line. The average friction coefficients for heat addition are in poor agreement with the accepted line.

Effect of friction on shear jamming

NASA Astrophysics Data System (ADS)

Wang, Dong; Ren, Jie; Dijksman, Joshua; Behringer, Robert

2014-03-01

Shear Jamming of granular materials was first found for systems of frictional disks, with a static friction coefficients μs ~= 0 . 6 . Jamming by shear is obtained by starting from a zero-stress state with a packing fraction ϕS <= ϕ <=ϕJ between ϕJ (isotropic jamming) and a lowest ϕS for shear jamming. This phenomenon is associated with strong anisotropy in stress and the contact network in the form of ``force chains,'' which are stabilized and/or enhanced by the presence of friction. We address experimentally how reducing friction affects shear jamming by using either teflon disks of teflon wrapped photoelastic particles. The teflon disks were placed in a wall driven 2D shear apparatus, in which we can probe shear stresses mechanically. Teflon-wrapped disks were placed in a bottom driven 2D shear apparatus (Ren et al., PRL 2013). Both apparatuses provide uniform simple shear. In all low- μ experiments, the shear jamming occurred, as observed through stress increases on the packing. However, the low- μ differences observed for ϕJ -ϕS were smaller than for higher friction particles. Ongoing work is studying systems using hydrogel disks, which have a lower friction coefficient than teflon. We acknowledge support from NSF Grant No. DMR12-06351, ARO Grant No. W911NF-1-11-0110, and NASA Grant No. NNX10AU01G.

A finite element study on rail corrugation based on saturated creep force-induced self-excited vibration of a wheelset-track system

NASA Astrophysics Data System (ADS)

Chen, G. X.; Zhou, Z. R.; Ouyang, H.; Jin, X. S.; Zhu, M. H.; Liu, Q. Y.

2010-10-01

The present work proposes friction coupling at the wheel-rail interface as the mechanism for formation of rail corrugation. Stability of a wheelset-track system is studied using the finite element complex eigenvalue method. Two models for a wheelset-track system on a tight curved track and on a straight track are established. In these two models, motion of the wheelset is coupled with that of the rail by friction. Creep force at the interface is assumed to become saturated and approximately equal to friction force, which is equal to the normal contact force multiplied by dynamic coefficient of friction. The rail is supported by vertical and lateral springs and dampers at the positions of sleepers. Numerical results show that there is a strong propensity of self-excited vibration of the wheelset-track system when the friction coefficient is larger than 0.21. Some unstable frequencies fall in the range 60-1200 Hz, which correspond to frequencies of rail corrugation. Parameter sensitivity analysis shows that the dynamic coefficient of friction, spring stiffness and damping of the sleeper supports all have important influences on the rail corrugation formation. Bringing the friction coefficient below a certain level can suppress or eliminate rail corrugation.

Friction behavior of 304 stainless steel of varying hardness lubricated with benzene and some benzyl structures

NASA Technical Reports Server (NTRS)

Buckley, D. H.

1974-01-01

The lubricating properties of some benzyl and benzene structures were determined by using 304 stainless steel surfaces strained to various hardness. Friction coefficients and wear track widths were measured with a Bowden-Leben type friction apparatus by using a pin-on-disk specimen configuration. Results obtained indicate that benzyl monosulfide, dibenzyl disulfide, and benzyl alcohol resulted in the lowest friction coefficients for 304 stainless steel, while benzyl ether provided the least surface protection and gave the highest friction. Strainhardening of the 304 stainless steel prior to sliding resulted in reduced friction in dry sliding. With benzyl monosulfide, dibenzyl disulfide, and benzyl alcohol changes in 304 stainless steel hardness had no effect upon friction behavior.

The influence of bed friction variability due to land cover on storm-driven barrier island morphodynamics

USGS Publications Warehouse

Passeri, Davina L.; Long, Joseph W.; Plant, Nathaniel G.; Bilskie, Matthew V.; Hagen, Scott C.

2018-01-01

Variations in bed friction due to land cover type have the potential to influence morphologic change during storm events; the importance of these variations can be studied through numerical simulation and experimentation at locations with sufficient observational data to initialize realistic scenarios, evaluate model accuracy and guide interpretations. Two-dimensional in the horizontal plane (2DH) morphodynamic (XBeach) simulations were conducted to assess morphodynamic sensitivity to spatially varying bed friction at Dauphin Island, AL using hurricanes Ivan (2004) and Katrina (2005) as experimental test cases. For each storm, three bed friction scenarios were simulated: (1) a constant Chezy coefficient across land and water, (2) a constant Chezy coefficient across land and depth-dependent Chezy coefficients across water, and (3) spatially varying Chezy coefficients across land based on land use/land cover (LULC) data and depth-dependent Chezy coefficients across water. Modeled post-storm bed elevations were compared qualitatively and quantitatively with post-storm lidar data. Results showed that implementing spatially varying bed friction influenced the ability of XBeach to accurately simulate morphologic change during both storms. Accounting for frictional effects due to large-scale variations in vegetation and development reduced cross-barrier sediment transport and captured overwash and breaching more accurately. Model output from the spatially varying friction scenarios was used to examine the need for an existing sediment transport limiter, the influence of pre-storm topography and the effects of water level gradients on storm-driven morphodynamics.

Tribology of thin wetting films between bubble and moving solid surface.

PubMed

Karakashev, Stoyan I; Stöckelhuber, Klaus W; Tsekov, Roumen; Phan, Chi M; Heinrich, Gert

2014-08-01

This work shows a successful example of coupling of theory and experiment to study the tribology of bubble rubbing on solid surface. Such kind of investigation is reported for the first time in the literature. A theory about wetting film intercalated between bubble and moving solid surface was developed, thus deriving the non-linear evolution differential equation which accounted for the friction slip coefficient at the solid surface. The stationary 3D film thickness profile, which appears to be a solution of the differential equation, for each particular speed of motion of the solid surface was derived by means of special procedure and unique interferometric experimental setup. This allowed us to determine the 3D map of the lift pressure within the wetting film, the friction force per unit area and the friction coefficient of rubbing at different speeds of motion of the solid surface. Thus, we observed interesting tribological details about the rubbing of the bubble on the solid surface like for example: 1. A regime of mixed friction between dry and lubricated friction exists in the range of 6-170 μm/s, beyond which the rubbing between the bubble and solid becomes completely lubricated and passes through the maximum; 2. The friction coefficient of rubbing has high values at very small speeds of solid's motion and reduces substantially with the increase of the speed of the solid motion until reaching small values, which change insignificantly with the further increase of the speed of the solid. Despite the numerous studies on the motion of bubble/droplet in close proximity to solid wall in the literature, the present investigation appears to be a step ahead in this area as far as we were able to derive 3D maps of the bubble close to the solid surface, which makes the investigation more profound. © 2013.

Stability and tribological performances of fluid phospholipid bilayers: effect of buffer and ions.

PubMed

Dekkiche, F; Corneci, M C; Trunfio-Sfarghiu, A-M; Munteanu, B; Berthier, Y; Kaabar, W; Rieu, J-P

2010-10-15

We have investigated the mechanical and tribological properties of supported Dioleoyl phosphatidylcholine (DOPC) bilayers in different solutions: ultrapure water (pH 5.5), saline solution (150 mM NaCl, pH 5.8), Tris buffer (pH 7.2) and Tris saline buffer (150 mM NaCl, pH 7.2). Friction forces are measured using a homemade biotribometer. Lipid bilayer degradation is controlled in situ during friction tests using fluorescence microscopy. Mechanical resistance to indentation is measured by force spectroscopy with an atomic force microscope. This study confirms that mechanical stability under shear or normal load is essential to obtain low and constant friction coefficients. In ultrapure water, bilayers are not resistant and have poor lubricant properties. On the other hand, in Tris saline buffer, they fully resist to indentation and exhibit low (micro=0.035) and stable friction coefficient with no visible wear during the 50 min of the friction test. The unbuffered saline solution improves the mechanical resistance to indentation but not the lubrication. These results suggest that the adsorption of ions to the zwiterrionic bilayers has different effects on the mechanical and tribological properties of bilayers: higher resistance to normal indentation due to an increase in bilayer cohesion, higher lubrication due to an increase in bilayer-bilayer repulsion. Copyright (c) 2010 Elsevier B.V. All rights reserved.

Bottom friction models for shallow water equations: Manning’s roughness coefficient and small-scale bottom heterogeneity

NASA Astrophysics Data System (ADS)

Dyakonova, Tatyana; Khoperskov, Alexander

2018-03-01

The correct description of the surface water dynamics in the model of shallow water requires accounting for friction. To simulate a channel flow in the Chezy model the constant Manning roughness coefficient is frequently used. The Manning coefficient nM is an integral parameter which accounts for a large number of physical factors determining the flow braking. We used computational simulations in a shallow water model to determine the relationship between the Manning coefficient and the parameters of small-scale perturbations of a bottom in a long channel. Comparing the transverse water velocity profiles in the channel obtained in the models with a perturbed bottom without bottom friction and with bottom friction on a smooth bottom, we constructed the dependence of nM on the amplitude and spatial scale of perturbation of the bottom relief.

Consideration of Wear Rates at High Velocities

DTIC Science & Technology

2010-03-01

Strain vs. Three-dimensional Model . . . . . . . . . . . . 57 3.11 Example Single Asperity Wear Rate Integral . . . . . . . . . . 58 4.1 Third Stage...Slipper Accumulated Frictional Heating . . . . . . 67 4.2 Surface Temperature Third Stage Slipper, ave=0.5 . . . . . . . 67 4.3 Melt Depth Example...64 A3S Coefficient for Frictional Heat Curve Fit, Third Stage Slipper 66 B3S Coefficient for Frictional Heat Curve Fit, Third

Dry friction of microstructured polymer surfaces inspired by snake skin.

PubMed

Baum, Martina J; Heepe, Lars; Fadeeva, Elena; Gorb, Stanislav N

2014-01-01

The microstructure investigated in this study was inspired by the anisotropic microornamentation of scales from the ventral body side of the California King Snake (Lampropeltis getula californiae). Frictional properties of snake-inspired microstructured polymer surface (SIMPS) made of epoxy resin were characterised in contact with a smooth glass ball by a microtribometer in two perpendicular directions. The SIMPS exhibited a considerable frictional anisotropy: Frictional coefficients measured along the microstructure were about 33% lower than those measured in the opposite direction. Frictional coefficients were compared to those obtained on other types of surface microstructure: (i) smooth ones, (ii) rough ones, and (iii) ones with periodic groove-like microstructures of different dimensions. The results demonstrate the existence of a common pattern of interaction between two general effects that influence friction: (1) molecular interaction depending on real contact area and (2) the mechanical interlocking of both contacting surfaces. The strongest reduction of the frictional coefficient, compared to the smooth reference surface, was observed at a medium range of surface structure dimensions suggesting a trade-off between these two effects.

Studies on centrifugal clutch judder behavior and the design of frictional lining materials

NASA Astrophysics Data System (ADS)

Li, Tse-Chang; Huang, Yu-Wen; Lin, Jen-Fin

2016-01-01

This study examines the judder behavior of a centrifugal clutch from the start of hot spots in the conformal contact, then the repeated developments of thermoelastic instability, and finally the formation of cyclic undulations in the vibrations, friction coefficient and torque. This behavior is proved to be consistent with the testing results. Using the Taguchi method, 18 kinds of frictional lining specimens were prepared in order to investigate their performance in judder resistance and establish a relationship between judder behavior and the Ts/Td (Ts: static torque; Td: dynamic torque) and dμ/dVx (μ: friction coefficient; Vx: relative sliding velocity of frictional lining and clutch drum) parameters. These specimens are also provided to examine the effects and profitability with regard to the centrifugal clutch, and find the relative importance of the various control factors. Theoretical models for the friction coefficient (μ), the critical sliding velocity (Vc) with clutch judder, and the contact pressure ratio p* /pbar (p*: pressure undulation w.r.t. pbar; pbar: mean contact pressure) and temperature corresponding to judder behavior are developed. The parameters of the contact pressure ratio and temperature are shown to be helpful to explain the occurrence of judder. The frictional torque and the rotational speeds of the driveline, clutch, and clutch drum as functions of engagement time for 100 clutch cycles are obtained experimentally to evaluate dμ/dVx and Ts/Td. A sharp rise in the maximum p* /pbar occurred when the relative sliding velocity reached the critical velocity, Vc. An increase in the maximum p* /pbar generally led to an increase of the (initially negative) dμ/dVx value, and thus the severity of judder. The fluctuation intensity of dμ/dVx becomes a governing factor of the growth of dμ/dVx itself in the engagement process. The mean values of dμ/dVx and Ts/Td for the clutching tests with 100 cycles can be roughly divided into three groups dependent on the fluctuation intensities of these two parameters, for each of which there is a linear relationship.

Tribological behaviour of orthodontic archwires under dry and wet sliding conditions in-vitro. I--Frictional behaviour.

PubMed

Berradja, Abdenacer; Willems, Guy; Celis, Jean-Pierre

2006-05-01

To evaluate the frictional behaviour of orthodontic archwires in dry and wet conditions in-vitro. Two types of archwire materials were investigated: stainless steel and NiTi. A fretting wear tribometer fitted with an alumina ball was operated at 23 degrees C in three different environments: ambient air with 50 per cent relative humidity, 0.9 wt. per cent sodium chloride solution, and deionised water. NiTi archwires sliding against alumina exhibited high coefficients of friction (about 0.6) in the three environments. Stainless steel archwires sliding against alumina had relatively low coefficients of friction (0.3) in the solutions, but high coefficients (0.8) in air. The low frictional forces of the stainless steel wires sliding against alumina in the solutions were due to a lubricating effect of the solutions and corrosion-wear debris. The high frictional forces between the NiTi wires and alumina are attributed to an abrasive interfacial transfer film between the wires and alumina.

Tribological Analysis of Copper-Coated Graphite Particle-Reinforced A359 Al/5 wt.% SiC Composites

NASA Astrophysics Data System (ADS)

Lin, C. B.; Wang, T. C.; Chang, Z. C.; Chu, H. Y.

2013-01-01

Copper-coated graphite particles can be mass-produced by the cementation process using simple equipment. Graphite particulates that were coated with electroless copper and 5 wt.% SiC particulates were introduced into an aluminum alloy by compocasting to make A359 Al/5 wt.% SiC(p) composite that contained 2, 4, 6, and 8 wt.% graphite particulate composite. The effects of SiC particles, quantity of graphite particles, normal loading, sliding speed and wear debris on the coefficient of friction, and the wear rate were investigated. The results thus obtained indicate that the wear properties were improved by adding small amounts of SiC and graphite particles into the A359 Al alloy. The coefficient of friction of the A359 Al/5 wt.% SiC(p) composite that contained 6.0 wt.% graphite particulates was reduced to 0.246 and the amount of graphite film that was released on the worn surface increased with the graphite particulate content. The coefficient of friction and the wear rate were insensitive to the variation in the sliding speed and normal loading.

Preparation, deformation, and failure of functional Al-Sn and Al-Sn-Pb nanocrystalline alloys

NASA Astrophysics Data System (ADS)

Noskova, N. I.; Vil'Danova, N. F.; Filippov, Yu. I.; Churbaev, R. V.; Pereturina, I. A.; Korshunov, L. G.; Korznikov, A. V.

2006-12-01

Changes in the structure, hardness, mechanical properties, and friction coefficient of Al-30% Sn, Al-15% Sn-25% Pb, and Al-5% Sn-35% Pb (wt %) alloys subjected to severe plastic deformation by equal-channel angular pressing (with a force of 40 tonne) and by shear at a pressure of 5 GPa have been studied. The transition into the nanocrystalline state was shown to occur at different degrees of plastic deformation. The hardness exhibits nonmonotonic variations, namely, first it increases and subsequently decreases. The friction coefficient of the Al-30% Sn, Al-15% Sn-25% Pb, and Al-5% Sn-35% Pb alloys quenched from the melt was found to be 0.33; the friction coefficients of these alloys in the submicrocrystalline state (after equal-channel angular pressing) equal 0.24, 0.32, and 0.35, respectively. The effect of disintegration into nano-sized powders was found to occur in the Al-15% Sn-25% Pb, and Al-5% Sn-35% Pb alloys after severe plastic deformation to ɛ = 6.4 and subsequent short-time holding.

Static-dynamic friction transition of FRP esthetic orthodontic wires on various brackets by suspension-type friction test.

PubMed

Suwa, N; Watari, F; Yamagata, S; Iida, J; Kobayashi, M

2003-11-15

A new testing apparatus for the measurement of frictional properties was designed and the frictional coefficients were obtained and compared with each other in various combinations of brackets and orthodontic wires, including esthetic fiber-reinforced plastic (FRP) wire that was especially designed and manufactured. Three kinds of wires (stainless steel, nickel-titanium, and FRP) and four brackets (single-crystal alumina, polycrystalline alumina, polycarbonate, and stainless steel) were used. The testing was done under dry and wet conditions. The friction testing equipment was designed to attach the bracket to a C-shaped bar suspended with a variable mass, and sliding along a fixed wire. The transition between static and dynamic friction was measured as a breakaway force, with the use of a universal test machine. In addition to material properties, this testing fixture eliminates geometrical factors, such as the rotational moment at the edge of the bracket slot, deflection of the orthodontic wire, and tension of the ligature wire. Nearly ideal frictional properties between materials are obtained. The frictional properties of FRP wire were similar to those of metal wires on all brackets, except the polycrystalline alumina bracket. The frictional coefficient between the polycrystalline ceramic bracket and FRP wire was larger than that of other combinations. There was little difference in frictional coefficients between dry and wet conditions. Copyright 2003 Wiley Periodicals, Inc.

Surface Texturing of Polyimide Composite by Micro-Ultrasonic Machining

NASA Astrophysics Data System (ADS)

Qu, N. S.; Zhang, T.; Chen, X. L.

2018-03-01

In this study, micro-dimples were prepared on a polyimide composite surface to obtain the dual benefits of polymer materials and surface texture. Micro-ultrasonic machining is employed for the first time for micro-dimple fabrication on polyimide composite surfaces. Surface textures of simple patterns were fabricated successfully with dimple depths of 150 μm, side lengths of 225-425 μm, and area ratios of 10-30%. The friction coefficient of the micro-dimple surfaces with side lengths of 325 or 425 μm could be increased by up to 100% of that of non-textured surfaces, alongside a significant enhancement of wear resistance. The results show that surface texturing of polyimide composite can be applied successfully to increase the friction coefficient and reduce wear, thereby contributing to a large output torque.

Study on induced strain in direct nanoimprint lithography

NASA Astrophysics Data System (ADS)

Watanabe, Kenta; Iida, Tatsuya; Yasuda, Masaaki; Kawata, Hiroaki; Hirai, Yoshihiko

2018-06-01

The induced shear strain distribution in a polymer film is investigated by computational study in a direct nanoimprint process. The effects of the polymer thickness, mold pattern shape such as rectangular, triangular or overcut pattern shape, and the coefficient of friction between the mold and the polymer are studied by computational work. As the coefficient of friction increases, the induced shear strain increases along the mold surface. Depending on the polymer thickness, the shear strain is induced in the residual and/or pattern area. In the triangular pattern, the strain is induced in the pattern central area. The results suggest that shear stress remains in the triangular pattern area in the direct nanoimprint process. On the other hand, the rectangular pattern is suitable for suppressing the induced strain inside the pattern.

Estimation of Dynamic Friction Process of the Akatani Landslide Based on the Waveform Inversion and Numerical Simulation

NASA Astrophysics Data System (ADS)

Yamada, M.; Mangeney, A.; Moretti, L.; Matsushi, Y.

2014-12-01

Understanding physical parameters, such as frictional coefficients, velocity change, and dynamic history, is important issue for assessing and managing the risks posed by deep-seated catastrophic landslides. Previously, landslide motion has been inferred qualitatively from topographic changes caused by the event, and occasionally from eyewitness reports. However, these conventional approaches are unable to evaluate source processes and dynamic parameters. In this study, we use broadband seismic recordings to trace the dynamic process of the deep-seated Akatani landslide that occurred on the Kii Peninsula, Japan, which is one of the best recorded large slope failures. Based on the previous results of waveform inversions and precise topographic surveys done before and after the event, we applied numerical simulations using the SHALTOP numerical model (Mangeney et al., 2007). This model describes homogeneous continuous granular flows on a 3D topography based on a depth averaged thin layer approximation. We assume a Coulomb's friction law with a constant friction coefficient, i. e. the friction is independent of the sliding velocity. We varied the friction coefficients in the simulation so that the resulting force acting on the surface agrees with the single force estimated from the seismic waveform inversion. Figure shows the force history of the east-west components after the band-pass filtering between 10-100 seconds. The force history of the simulation with frictional coefficient 0.27 (thin red line) the best agrees with the result of seismic waveform inversion (thick gray line). Although the amplitude is slightly different, phases are coherent for the main three pulses. This is an evidence that the point-source approximation works reasonably well for this particular event. The friction coefficient during the sliding was estimated to be 0.38 based on the seismic waveform inversion performed by the previous study and on the sliding block model (Yamada et al., 2013), whereas the frictional coefficient estimated from the numerical simulation was about 0.27. This discrepancy may be due to the digital elevation model, to the other forces such as pressure gradients and centrifugal acceleration included in the model. However, quantitative interpretation of this difference requires further investigation.

Intermonolayer Friction and Surface Shear Viscosity of Lipid Bilayer Membranes

PubMed Central

den Otter, W. K.; Shkulipa, S. A.

2007-01-01

The flow behavior of lipid bilayer membranes is characterized by a surface viscosity for in-plane shear deformations, and an intermonolayer friction coefficient for slip between the two leaflets of the bilayer. Both properties have been studied for a variety of coarse-grained double-tailed model lipids, using equilibrium and nonequilibrium molecular dynamics simulations. For lipids with two identical tails, the surface shear viscosity rises rapidly with tail length, while the intermonolayer friction coefficient is less sensitive to the tail length. Interdigitation of lipid tails across the bilayer midsurface, as observed for lipids with two distinct tails, strongly enhances the intermonolayer friction coefficient, but hardly affects the surface shear viscosity. The simulation results are compared against the available experimental data. PMID:17468168

Unsteady Convection Flow and Heat Transfer over a Vertical Stretching Surface

PubMed Central

Cai, Wenli; Su, Ning; Liu, Xiangdong

2014-01-01

This paper investigates the effect of thermal radiation on unsteady convection flow and heat transfer over a vertical permeable stretching surface in porous medium, where the effects of temperature dependent viscosity and thermal conductivity are also considered. By using a similarity transformation, the governing time-dependent boundary layer equations for momentum and thermal energy are first transformed into coupled, non-linear ordinary differential equations with variable coefficients. Numerical solutions to these equations subject to appropriate boundary conditions are obtained by the numerical shooting technique with fourth-fifth order Runge-Kutta scheme. Numerical results show that as viscosity variation parameter increases both the absolute value of the surface friction coefficient and the absolute value of the surface temperature gradient increase whereas the temperature decreases slightly. With the increase of viscosity variation parameter, the velocity decreases near the sheet surface but increases far away from the surface of the sheet in the boundary layer. The increase in permeability parameter leads to the decrease in both the temperature and the absolute value of the surface friction coefficient, and the increase in both the velocity and the absolute value of the surface temperature gradient. PMID:25264737

Unsteady convection flow and heat transfer over a vertical stretching surface.

PubMed

Cai, Wenli; Su, Ning; Liu, Xiangdong

2014-01-01

This paper investigates the effect of thermal radiation on unsteady convection flow and heat transfer over a vertical permeable stretching surface in porous medium, where the effects of temperature dependent viscosity and thermal conductivity are also considered. By using a similarity transformation, the governing time-dependent boundary layer equations for momentum and thermal energy are first transformed into coupled, non-linear ordinary differential equations with variable coefficients. Numerical solutions to these equations subject to appropriate boundary conditions are obtained by the numerical shooting technique with fourth-fifth order Runge-Kutta scheme. Numerical results show that as viscosity variation parameter increases both the absolute value of the surface friction coefficient and the absolute value of the surface temperature gradient increase whereas the temperature decreases slightly. With the increase of viscosity variation parameter, the velocity decreases near the sheet surface but increases far away from the surface of the sheet in the boundary layer. The increase in permeability parameter leads to the decrease in both the temperature and the absolute value of the surface friction coefficient, and the increase in both the velocity and the absolute value of the surface temperature gradient.

Effect of time derivative of contact area on dynamic friction

NASA Astrophysics Data System (ADS)

Arakawa, Kazuo

2014-06-01

This study investigated dynamic friction during oblique impact of a golf ball by evaluating the ball's angular velocity, contact force, and the contact area between the ball and target. The effect of the contact area on the angular velocities was evaluated, and the results indicated that the contact area plays an important role in dynamic friction. In this study, the dynamic friction force F was given by F = μN + μη dA/dt, where μ is the coefficient of friction, N is the contact force, dA/dt is the time derivative of the contact area A, and η is a coefficient associated with the contact area.

Effect of pendent chains on the interfacial properties of thin polydimethylsiloxane (PDMS) networks.

PubMed

Landherr, Lucas J T; Cohen, Claude; Archer, Lynden A

2011-05-17

The interfacial properties of end-linked polydimethylsiloxane (PDMS) films on silicon are examined. Thin cross-linked PDMS films (∼10 μm thick) were synthesized over a self-assembled monolayer supported on a silicon wafer. By systematically varying the concentration of monofunctional PDMS in a mixture with telechelic precursor molecules, structures ranging from near-ideal elastic networks to poorly cross-linked networks composed of a preponderance of dangling/pendent chains were synthesized. Lateral force microscopy (LFM) employing bead probes was used to quantify the effect of network structure on the interfacial friction coefficient and residual force. Indentation measurements employing an AFM in force mode were used to characterize the elastic modulus and the pull-off force for the films as a function of pendent chain content. These measurements were complemented with conventional mechanical rheometry measurements on similar thick network films to determine their bulk rheological properties. All networks studied manifested interfacial friction coefficients substantially lower than that of bare silicon. PDMS networks with the lowest pendent chain content displayed friction coefficients close to 1 order of magnitude lower than that of bare silicon, whereas networks with the highest pendent chain content manifested friction coefficients about 3 times lower than that of bare silicon. At intermediate sliding velocities, a crossover in the interfacial friction coefficient was observed, wherein cross-linked PDMS films with the least amount of pendent chains exhibit the highest friction coefficient. These observations are discussed in terms of the structure of the films and relaxation dynamics of elastic strands and dangling chains in tethered network films.

Sedimentation Coefficient, Frictional Coefficient, and Molecular Weight: A Preparative Ultracentrifuge Experiment for the Advanced Undergraduate Laboratory.

ERIC Educational Resources Information Center

Halsall, H. B.; Wermeling, J. R.

1982-01-01

Describes an experiment using a high-speed preparative centrifuge and calculator to demonstrate effects of the frictional coefficient of a macromolecule on its rate of transport in a force field and to estimate molecular weight of the macromolecule using an empirical relationship. Background information, procedures, and discussion of results are…

Reduction of friction stress of ethylene glycol by attached hydrogen ions

PubMed Central

Li, Jinjin; Zhang, Chenhui; Deng, Mingming; Luo, Jianbin

2014-01-01

In the present work, it is shown that the friction stress of ethylene glycol can decrease by an order of magnitude to achieve superlubricity if there are hydrogen ions attached on the friction surfaces. An ultra-low friction coefficient (μ = 0.004) of ethylene glycol between Si3N4 and SiO2 can be obtained with the effect of hydrogen ions. Experimental result indicates that the hydrogen ions adsorbed on the friction surfaces forming a hydration layer and the ethylene glycol in the contact region forming an elastohydrodynamic film are the two indispensable factors for the reduction of friction stress. The mechanism of superlubricity is attributed to the extremely low shear strength of formation of elastohydrodynamic film on the hydration layer. This finding may introduce a new approach to reduce friction coefficient of liquid by attaching hydrogen ions on friction surfaces. PMID:25428584

Turbulent Friction in the Boundary Layer of a Flat Plate in a Two-Dimensional Compressible Flow at High Speeds

NASA Technical Reports Server (NTRS)

Frankl, F.; Voishel, V.

1943-01-01

In the present report an investigation is made on a flat plate in a two-dimensional compressible flow of the effect of compressibility and heating on the turbulent frictional drag coefficient in the boundary layer of an airfoil or wing radiator. The analysis is based on the Prandtl-Karman theory of the turbulent boundary later and the Stodola-Crocco, theorem on the linear relation between the total energy of the flow and its velocity. Formulas are obtained for the velocity distribution and the frictional drag law in a turbulent boundary later with the compressibility effect and heat transfer taken into account. It is found that with increase of compressibility and temperature at full retardation of the flow (the temperature when the velocity of the flow at a given point is reduced to zero in case of an adiabatic process in the gas) at a constant R (sub x), the frictional drag coefficient C (sub f) decreased, both of these factors acting in the same sense.

In Situ High Pressure Hydrogen Tribological Testing of Common Polymer Materials Used in the Hydrogen Delivery Infrastructure.

PubMed

Duranty, Edward R; Roosendaal, Timothy J; Pitman, Stan G; Tucker, Joseph C; Owsley, Stanley L; Suter, Jonathan D; Alvine, Kyle James

2018-03-31

High pressure hydrogen gas is known to adversely affect metallic components of compressors, valves, hoses, and actuators. However, relatively little is known about the effects of high pressure hydrogen on the polymer sealing and barrier materials also found within these components. More study is required in order to determine the compatibility of common polymer materials found in the components of the hydrogen fuel delivery infrastructure with high pressure hydrogen. As a result, it is important to consider the changes in physical properties such as friction and wear in situ while the polymer is exposed to high pressure hydrogen. In this protocol, we present a method for testing the friction and wear properties of ethylene propylene diene monomer (EPDM) elastomer samples in a 28 MPa high pressure hydrogen environment using a custom-built in situ pin-on-flat linear reciprocating tribometer. Representative results from this testing are presented which indicate that the coefficient of friction between the EPDM sample coupon and steel counter surface is increased in high pressure hydrogen as compared to the coefficient of friction similarly measured in ambient air.

Properties of Interfacial Tribo-Films

DTIC Science & Technology

1993-06-01

cf these rods is such as to have the center of gravity of or the attraction of water into the re-entrant peripheral gap the whole sample as close as...difference between the fluid dynamics, acoustic effects in stringed musical static and the kinetic friction coefficients increases with instruments...interfacial fluid molecules to static minimize oscillations, the center of gravity of the sample friction have been explored and, in this regard, adsorbed

Skin-Friction Measurements in Incompressible Flow

NASA Technical Reports Server (NTRS)

Smith, Donald W.; Walker, John H.

1959-01-01

Experiments have been conducted to measure the local surface-shear stress and the average skin-friction coefficient in Incompressible flow for a turbulent boundary layer on a smooth flat plate having zero pressure gradient. Data were obtained for a range of Reynolds numbers from 1 million to 45 million. The local surface-shear stress was measured by a floating-element skin-friction balance and also by a calibrated total head tube located on the surface of the test wall. The average skin-friction coefficient was obtained from boundary-layer velocity profiles.

Investigation on the oxidation behavior of AlCrVxN thin films by means of synchrotron radiation and influence on the high temperature friction

NASA Astrophysics Data System (ADS)

Tillmann, Wolfgang; Kokalj, David; Stangier, Dominic; Paulus, Michael; Sternemann, Christian; Tolan, Metin

2018-01-01

Friction minimization is an important topic which is pursued in research and industry. In addition to the use of lubricants, friction-reducing oxide phases can be utilized which occur during. These oxides are called Magnéli phases and especially vanadium oxides exhibit good friction reducing properties. Thereby, the lubrication effect can be traced back to oxygen deficiencies. AlCrN thin films are being used as coatings for tools which have to withstand high temperatures. A further improvement of AlCrN thin films concerning their friction properties is possible by incorporation of vanadium. This study analyzes the temperature dependent oxidation behavior of magnetron sputtered AlCrVN thin films with different vanadium contents up to 13.5 at.-% by means of X-ray diffraction and X-ray absorption near-edge spectroscopy. Up to 400 °C the coatings show no oxidation. A higher temperature of 700 °C leads to an oxidation and formation of Magnéli phases of the coatings with vanadium contents above 10.7 at.-%. Friction coefficients, measured by ball-on-disk test are correlated with the oxide formation in order to figure out the effect of vanadium oxides. At 700 °C a decrease of the friction coefficient with increasing vanadium content can be observed, due to the formation of VO2, V2O3 and the Magnéli phase V4O7.

Adhesive Wear Performance of CFRP Multilayered Polyester Composites Under Dry/wet Contact Conditions

NASA Astrophysics Data System (ADS)

Danaelan, D.; Yousif, B. F.

The tribo-performance of a new engineering composite material based on coconut fibers was investigated. In this work, coconut fibers reinforced polyester (CFRP) composites were developed. The tribo-experiments were conducted by using pin-on-disc machine under dry and wet sliding contact condition against smooth stainless steel counterface. Worn surfaces were observed using optical microscope. Friction coefficient and specific wear rate were presented as a function of sliding distance (0-0.6 km) at different sliding velocities (0.1-0.28 m/s). The effect of applied load and sliding velocity was evaluated. The results showed that all test parameters have significant influence on friction and wear characteristics of the composites. Moreover, friction coefficient increased as the normal load and speed increased, the values were about 0.7-0.9 under dry contact condition. Meanwhile, under wet contact condition, there was a great reduction in the friction coefficient, i.e. the values were about 0.1-0.2. Furthermore, the specific wear rates were found to be around 2-4 (10-3) mm3/Nm under dry contact condition and highly reduced under wet condition. In other words, the presence of water as cleaner and polisher assisted to enhance the adhesive wear performance of CFRP by about 10%. The images from optical microscope showed evidence of adhesive wear mode with transition to abrasive wear mode at higher sliding velocities due to third body abrasion. On the other hand, optical images for wet condition showed less adhesive wear and smooth surfaces.

The frictional properties of a simulated gouge having a fractal particle distribution

USGS Publications Warehouse

Biegel, R.L.; Sammis, C.G.; Dieterich, J.H.

1989-01-01

The frictional properties of a layer of simulated Westerly granite fault gouge sandwiched between sliding blocks of Westerly granite have been measured in a high-speed servo-controlled double-direct shear apparatus. Most gouge layers were prepared to have a self-similar particle distribution with a fractal dimension of 2.6. The upper fractal limit was varied between 45 and 710 ??m. Some gouges were prepared with all particles in the range between 360 and 710 ??m. In each experiment the sliding velocity was cyclically alternated between 1 and 10 ??ms-1 and the coefficient of friction ??m and its transient parameters a, b and Dc were measured as functions of displacement. In addition to the particle size distribution, the following experimental variables were also investigated: the layer thickness (1 and 3 mm), the roughness of the sliding surfaces (Nos 60 and 600 grit) and the normal stress (10 and 25 MPa). Some of the sample assemblies were epoxy impregnated following a run so the gouge structure could be microscopically examined in thin section. We observed that gouges which were initially non-fractal evolved to a fractal distribution with dimension 2.6. Gouges which had an initial fractal distribution remained fractal. When the sliding blocks had smooth surfaces, the coefficient of friction was relatively low and was independent of the particle distribution. In these cases, strong velocity weakening was observed throughout the experiment and the transient parameters a, b and Dc, remained almost constant. When the sliding blocks had rough surfaces, the coefficient of friction was larger and more dependent on the particle distribution. Velocity strengthening was observed initially but evolved to velocity weakening with increased sliding displacement. All three transient parameters changed with increasing displacement. The a and b values were about three times as large for rough surfaces as for smooth. The characteristic displacement Dc was not sensitive to surface roughness but was the only transient parameter which was sensitive to the normal stress. For the case of rough surfaces, the coefficient of friction of the 1 mm thick gouge was significantly larger than that for the 3 mm thick layers. Many of these observations can be explained by a micromechanical model in which the stress in the gouge layer is heterogeneous. The applied normal and shear stresses are supported by 'grain bridges' which span the layer and which are continually forming and failing. In this model, the frictional properties of the gouge are largely determined by the dominant failure mode of the bridging structures. ?? 1989.

Heat transfer and pressure drop of condensation of hydrocarbons in tubes

NASA Astrophysics Data System (ADS)

Fries, Simon; Skusa, Severin; Luke, Andrea

2018-03-01

The heat transfer coefficient and pressure drop are investigated for propane. Two different mild steel plain tubes and saturation pressures are considered for varying mass flux and vapour quality. The pressure drop is compared to the Friedel-Correlation with two different approaches to determine the friction factor. The first is calculation as proposed by Friedel and the second is through single phase pressure drop investigations. For lower vapour qualities the experimental results are in better agreement with the approach of the calculated friction factor. For higher vapour qualities the experimental friction factor is more precise. The pressure drop increases for a decreasing tube diameter and saturation pressure. The circumferential temperature profile and heat transfer coefficients are shown for a constant vapour quality at varying mass fluxes. The subcooling is highest for the bottom of the tube and lowest for the top. The average subcooling as well as the circumferential deviation decreases for rising mass fluxes. The averaged heat transfer coefficients are compared to the model proposed by Thome and Cavallini. The experimental results are in good agreement with both correlations, however the trend is better described with the correlation from Thome. The experimental heat transfer coefficients are under predicted by Thome and over predicted by Cavallini.

Measurements and Characterizations of Mechanical Properties of Human Skins

NASA Astrophysics Data System (ADS)

Song, Han Wook; Park, Yon Kyu

A skin is an indispensible organ for humans because it contributes to metabolism using its own biochemical functions and protects the human body from external stimuli. Recently, mechanical properties such as a thickness, a friction and an elastic coefficient have been used as a decision index in the skin physiology and in the skin care market due to the increased awareness of wellbeing issues. In addition, the use of mechanical properties is known to have good discrimination ability in the classification of human constitutions, which are used in the field of an alternative medicine. In this study, a system that measures mechanical properties such as a friction and an elastic coefficient is designed. The equipment consists of a load cell type (manufactured by the authors) for the measurements of a friction coefficient, a decompression tube for the measurement of an elastic coefficient. Using the proposed system, the mechanical properties of human skins from different constitutions were compared, and the relative repeatability error for measurements of mechanical properties was determined to be less than 2%. Combining the inspection results of medical doctors in the field of an alternative medicine, we could conclude that the proposed system might be applicable to a quantitative constitutional diagnosis between human constitutions within an acceptable level of uncertainty.

Angular circulation speed of tablets in a vibratory tablet coating pan.

PubMed

Kumar, Rahul; Wassgren, Carl

2013-03-01

In this work, a single tablet model and a discrete element method (DEM) computer simulation are developed to obtain the angular circulation speed of tablets in a vibratory tablet coating pan for range of vibration frequencies and amplitudes. The models identify three important dimensionless parameters that influence the speed of the tablets: the dimensionless amplitude ratio (a/R), the Froude number (aω2/g), and the tablet-wall friction coefficient, where a is the peak vibration amplitude at the drum center, ω is the vibration angular frequency, R is the drum radius, and g is the acceleration due to gravity. The models predict that the angular circulation speed of tablets increases with an increase in each of these parameters. The rate of increase in the angular circulation speed is observed to decrease for larger values of a/R. The angular circulation speed reaches an asymptote beyond a tablet-wall friction coefficient value of about 0.4. Furthermore, it is found that the Froude number should be greater than one for the tablets to start circulating. The angular circulation speed increases as Froude number increases but then does not change significantly at larger values of the Froude number. Period doubling, where the motion of the bed is repeated every two cycles, occurs at a Froude number larger than five. The single tablet model, although much simpler than the DEM model, is able to predict the maximum circulation speed (the limiting case for a large value of tablet-wall friction coefficient) as well as the transition to period doubling.

Wear Behaviour of Al-6061/SiC Metal Matrix Composites

NASA Astrophysics Data System (ADS)

Mishra, Ashok Kumar; Srivastava, Rajesh Kumar

2017-04-01

Aluminium Al-6061 base composites, reinforced with SiC particles having mesh size of 150 and 600, which is fabricated by stir casting method and their wear resistance and coefficient of friction has been investigated in the present study as a function of applied load and weight fraction of SiC varying from 5, 10, 15, 20, 25, 30, 35 and 40 %. The dry sliding wear properties of composites were investigated by using Pin-on-disk testing machine at sliding velocity of 2 m/s and sliding distance of 2000 m over a various loads of 10, 20 and 30 N. The result shows that the reinforcement of the metal matrix with SiC particulates up to weight percentage of 35 % reduces the wear rate. The result also show that the wear of the test specimens increases with the increasing load and sliding distance. The coefficient of friction slightly decreases with increasing weight percentage of reinforcements. The wear surfaces are examined by optical microscopy which shows that the large grooved regions and cavities with ceramic particles are found on the worn surface of the composite alloy. This indicates an abrasive wear mechanism, which is essentially a result of hard ceramic particles exposed on the worn surfaces. Further, it was found from the experimentation that the wear rate decreases linearly with increasing weight fraction of SiC and average coefficient of friction decreases linearly with increasing applied load, weight fraction of SiC and mesh size of SiC. The best result has been obtained at 35 % weight fraction and 600 mesh size of SiC.

A method for the determination of the coefficient of rolling friction using cycloidal pendulum

NASA Astrophysics Data System (ADS)

Ciornei, M. C.; Alaci, S.; Ciornei, F. C.; Romanu, I. C.

2017-08-01

The paper presents a method for experimental finding of coefficient of rolling friction appropriate for biomedical applications based on the theory of cycloidal pendulum. When a mobile circle rolls over a fixed straight line, the points from the circle describe trajectories called normal cycloids. To materialize this model, it is sufficient that a small region from boundary surfaces of a moving rigid body is spherical. Assuming pure rolling motion, the equation of motion of the cycloidal pendulum is obtained - an ordinary nonlinear differential equation. The experimental device is composed by two interconnected balls rolling over the material to be studied. The inertial characteristics of the pendulum can be adjusted via weights placed on a rod. A laser spot oscillates together to the pendulum and provides the amplitude of oscillations. After finding the experimental parameters necessary in differential equation of motion, it can be integrated using the Runge-Kutta of fourth order method. The equation was integrated for several materials and found values of rolling friction coefficients. Two main conclusions are drawn: the coefficient of rolling friction influenced significantly the amplitude of oscillation but the effect upon the period of oscillation is practically imperceptible. A methodology is proposed for finding the rolling friction coefficient and the pure rolling condition is verified.

Lithology-dependent minimum horizontal stress and in-situ stress estimate

NASA Astrophysics Data System (ADS)

Zhang, Yushuai; Zhang, Jincai

2017-04-01

Based on the generalized Hooke's law with coupling stresses and pore pressure, the minimum horizontal stress is solved with assumption that the vertical, minimum and maximum horizontal stresses are in equilibrium in the subsurface formations. From this derivation, we find that the uniaxial strain method is the minimum value or lower bound of the minimum stress. Using Anderson's faulting theory and this lower bound of the minimum horizontal stress, the coefficient of friction of the fault is derived. It shows that the coefficient of friction may have a much smaller value than what it is commonly assumed (e.g., μf = 0.6-0.7) for in-situ stress estimate. Using the derived coefficient of friction, an improved stress polygon is drawn, which can reduce the uncertainty of in-situ stress calculation by narrowing the area of the conventional stress polygon. It also shows that the coefficient of friction of the fault is dependent on lithology. For example, if the formation in the fault is composed of weak shales, then the coefficient of friction of the fault may be small (as low as μf = 0.2). This implies that this fault is weaker and more likely to have shear failures than the fault composed of sandstones. To avoid the weak fault from shear sliding, it needs to have a higher minimum stress and a lower shear stress. That is, the critically stressed weak fault maintains a higher minimum stress, which explains why a low shear stress appears in the frictionally weak fault.

Friction on a granular-continuum interface: Effects of granular media

NASA Astrophysics Data System (ADS)

Ecke, Robert; Geller, Drew

We consider the frictional interactions of two soft plates with interposed granular material subject to normal and shear forces. The plates are soft photo-elastic material, have length 50 cm, and are separated by a gap of variable width from 0 to 20 granular particle diameters. The granular materials are two-dimensional rods that are bi-dispersed in size to prevent crystallization. Different rod materials with frictional coefficients between 0 . 04 < μ < 0 . 5 are used to explore the effects of inter-granular friction on the effective friction of a granular medium. The gap is varied to test the dependence of the friction coefficient on the thickness of the granular layer. Because the soft plates absorb most of the displacement associated with the compressional normal force, the granular packing fractions are close to a jamming threshold, probably a shear jamming criterion. The overall shear and normal forces are measured using force sensors and the local strain tensor over a central portion of the gap is obtained using relative displacements of fiducial markers on the soft elastic material. These measurements provide a good characterization of the global and local forces giving rise to an effective friction coefficient. Funded by US DOE LDRD Program.

Comprehensive tire-road friction coefficient estimation based on signal fusion method under complex maneuvering operations

NASA Astrophysics Data System (ADS)

Li, L.; Yang, K.; Jia, G.; Ran, X.; Song, J.; Han, Z.-Q.

2015-05-01

The accurate estimation of the tire-road friction coefficient plays a significant role in the vehicle dynamics control. The estimation method should be timely and reliable for the controlling requirements, which means the contact friction characteristics between the tire and the road should be recognized before the interference to ensure the safety of the driver and passengers from drifting and losing control. In addition, the estimation method should be stable and feasible for complex maneuvering operations to guarantee the control performance as well. A signal fusion method combining the available signals to estimate the road friction is suggested in this paper on the basis of the estimated ones of braking, driving and steering conditions individually. Through the input characteristics and the states of the vehicle and tires from sensors the maneuvering condition may be recognized, by which the certainty factors of the friction of the three conditions mentioned above may be obtained correspondingly, and then the comprehensive road friction may be calculated. Experimental vehicle tests validate the effectiveness of the proposed method through complex maneuvering operations; the estimated road friction coefficient based on the signal fusion method is relatively timely and accurate to satisfy the control demands.

Electroresponsive Aqueous Silk Protein As “Smart” Mechanical Damping Fluid

PubMed Central

2015-01-01

Here we demonstrate the effectiveness of an electroresponsive aqueous silk protein polymer as a smart mechanical damping fluid. The aqueous polymer solution is liquid under ambient conditions, but is reversibly converted into a gel once subjected to an electric current, thereby increasing or decreasing in viscosity. This nontoxic, biodegradable, reversible, edible fluid also bonds to device surfaces and is demonstrated to reduce friction and provide striking wear protection. The friction and mechanical damping coefficients are shown to modulate with electric field exposure time and/or intensity. Damping coefficient can be modulated electrically, and then preserved without continued power for longer time scales than conventional “smart” fluid dampers. PMID:24750065

Rheology of concentrated suspensions of non-colloidal rigid fibers

NASA Astrophysics Data System (ADS)

Guazzelli, Elisabeth; Tapia, Franco; Shaikh, Saif; Butler, Jason E.; Pouliquen, Olivier

2017-11-01

Pressure and volume-imposed rheology is used to study suspensions of non-colloidal, rigid fibers in the concentrated regime for aspect ratios ranging from 3 to 15. The suspensions exhibit yield-stresses. Subtracting these apparent yield-stresses reveals a viscous scaling for both the shear and normal stresses. The variation in aspect ratio does not affect the friction coefficient (ratio of shear and normal stresses), but increasing the aspect ratio lowers the maximum volume fraction at which the suspension flows. Constitutive laws are proposed for the viscosities and the friction coefficient close to this maximum flowable fraction. The scaling of the stresses near this jamming transition are found to differ substantially from that of a suspension of spheres.

Calibration of discrete element model parameters: soybeans

NASA Astrophysics Data System (ADS)

Ghodki, Bhupendra M.; Patel, Manish; Namdeo, Rohit; Carpenter, Gopal

2018-05-01

Discrete element method (DEM) simulations are broadly used to get an insight of flow characteristics of granular materials in complex particulate systems. DEM input parameters for a model are the critical prerequisite for an efficient simulation. Thus, the present investigation aims to determine DEM input parameters for Hertz-Mindlin model using soybeans as a granular material. To achieve this aim, widely acceptable calibration approach was used having standard box-type apparatus. Further, qualitative and quantitative findings such as particle profile, height of kernels retaining the acrylic wall, and angle of repose of experiments and numerical simulations were compared to get the parameters. The calibrated set of DEM input parameters includes the following (a) material properties: particle geometric mean diameter (6.24 mm); spherical shape; particle density (1220 kg m^{-3} ), and (b) interaction parameters such as particle-particle: coefficient of restitution (0.17); coefficient of static friction (0.26); coefficient of rolling friction (0.08), and particle-wall: coefficient of restitution (0.35); coefficient of static friction (0.30); coefficient of rolling friction (0.08). The results may adequately be used to simulate particle scale mechanics (grain commingling, flow/motion, forces, etc) of soybeans in post-harvest machinery and devices.

Estimation of internal friction angle of subduction zone in northeast of Japan by using seismic focal mechanisms

NASA Astrophysics Data System (ADS)

Miyakawa, A.; Sato, K.; Otsubo, M.

2017-12-01

Physical properties, such as friction angle of the material, is important to understand the interplate earthquake of a subduction zone. Coulomb wedge model (Davis et al., 1983, JGR) is successfully revealed the relationship between a geometry of an accretionary wedge in a subduction zone and the physical properties of the material composing the accretionary wedge (e.g. Dahlen, 1984, JGR). An internal friction angle of the wedge and the frictional strength of the plate boundary fault control the wedge angle according to the Coulomb wedge model. However, the internal friction angle of the wedge and the frictional strength of the plate boundary fault are hard to estimate. Many previous works assumed the internal friction angle of the wedge on the basis of the laboratory experiments. Then, the frictional strength of the plate boundary fault, which is usually most interested, were evaluated from the observed wedge angle and the assumed internal friction angle of the wedge. Consequently, we should be careful of the selection of the internal friction angle of the wedge, otherwise, the uncertain an inappropriate internal friction angle may mislead the frictional strength of the plate boundary fault. In this study, we employed the newly developed technique to evaluate the internal friction angle of the wedge from the earthquake focal mechanisms occurred in the wedge along Japan Trench, northeast Japan. We used 650 earthquake mechanisms determined by NIED, Japan for the stress and friction coefficient inversion. The stress and friction coefficient inversion method is modified to handle the earthquake focal mechanisms from a computerized method to estimate the friction coefficient from the orientation distribution of faults (Sato, 2016, JSG). Finally, we obtained 25 degrees of internal friction angle of the wedge from the inversion. This value of friction angle is lower than usually assumed internal friction angle (30 degrees) (Byerlee, 1978, PAGEOPH). This lower internal friction angle leads to lower frictional strength of plate boundary fault ( 0.35) according to the Coulomb wedge model. These constrained physical parameters can contribute to understanding the interplate earthquake at each subduction zones.

Modeling and Investigation of the Wear Resistance of Salt Bath Nitrided Aisi 4140 via ANN

NASA Astrophysics Data System (ADS)

Ekinci, Şerafettin; Akdemir, Ahmet; Kahramanli, Humar

2013-05-01

Nitriding is usually used to improve the surface properties of steel materials. In this way, the wear resistance of steels is improved. We conducted a series of studies in order to investigate the microstructural, mechanical and tribological properties of salt bath nitrided AISI 4140 steel. The present study has two parts. For the first phase, the tribological behavior of the AISI 4140 steel which was nitrided in sulfinuz salt bath (SBN) was compared to the behavior of the same steel which was untreated. After surface characterization using metallography, microhardness and sliding wear tests were performed on a block-on-cylinder machine in which carbonized AISI 52100 steel discs were used as the counter face. For the examined AISI 4140 steel samples with and without surface treatment, the evolution of both the friction coefficient and of the wear behavior were determined under various loads, at different sliding velocities and a total sliding distance of 1000 m. The test results showed that wear resistance increased with the nitriding process, friction coefficient decreased due to the sulfur in salt bath and friction coefficient depended systematically on surface hardness. For the second part of this study, four artificial neural network (ANN) models were designed to predict the weight loss and friction coefficient of the nitrided and unnitrided AISI 4140 steel. Load, velocity and sliding distance were used as input. Back-propagation algorithm was chosen for training the ANN. Statistical measurements of R2, MAE and RMSE were employed to evaluate the success of the systems. The results showed that all the systems produced successful results.

Excellent vacuum tribological properties of Pb/PbS film deposited by RF magnetron sputtering and ion sulfurizing.

PubMed

Guozheng, Ma; Binshi, Xu; Haidou, Wang; Shuying, Chen; Zhiguo, Xing

2014-01-08

Soft metal Pb film of 3 μm in thickness was deposited on AISI 440C steel by RF magnetron sputtering, and then some of the Pb film samples were treated by low-temperature ion sulfurizing (LTIS) and formed Pb/PbS composite film. Tribological properties of the Pb and Pb/PbS films were tested contrastively in vacuum and air condition using a self-developed tribometer (model of MSTS-1). Scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were adopted to analyze the microstructure and chemical construction of the films and their worn surfaces. The results show that a mass of Pb was changed to PbS during the process of LTIS. In air condition, owing to the severe oxidation effect, pure Pb film showed relatively high friction coefficients (0.6), and Pb/PbS composite film also lost its friction-reduction property after sliding for a short time. In a vacuum, the average friction coefficients of Pb film were about 0.1, but the friction coefficient curve fluctuated obviously. And the Pb/PbS composite film exhibited excellent tribological properties in vacuum condition. Its friction coefficients keep stable at a low value of about 0.07 for a long time. If takes the value of friction coefficients exceeding 0.2 continuously as a criterion of lubrication failure, the sliding friction life of Pb/PbS film was as long as 3.2 × 10(5) r, which is 8 times of that of the Pb film. It can be concluded that the Pb/PbS film has excellent vacuum tribological properties and important foreground for applying in space solid lubrication related fields.

Cerium Addition Improved the Dry Sliding Wear Resistance of Surface Welding AZ91 Alloy

PubMed Central

Zhao, Zhihao; Zhu, Qingfeng; Wang, Gaosong; Tao, Kai

2018-01-01

In this study, the effects of cerium (Ce) addition on the friction and wear properties of surface welding AZ91 magnesium alloys were evaluated by pin-on-disk dry sliding friction and wear tests at normal temperature. The results show that both the friction coefficient and wear rate of surfacing magnesium alloys decreased with the decrease in load and increase in sliding speed. The surfacing AZ91 alloy with 1.5% Ce had the lowest friction coefficient and wear rate. The alloy without Ce had the worst wear resistance, mainly because it contained a lot of irregularly shaped and coarse β-Mg17Al12 phases. During friction, the β phase readily caused stress concentration and thus formed cracks at the interface between β phase and α-Mg matrix. The addition of Ce reduced the size and amount of Mg17Al12, while generating Al4Ce phase with a higher thermal stability. The Al-Ce phase could hinder the grain-boundary sliding and migration and reduced the degree of plastic deformation of subsurface metal. Scanning electron microscopy observation showed that the surfacing AZ91 alloy with 1.5% Ce had a total of four types of wear mechanism: abrasion, oxidation, and severe plastic deformation were the primary mechanisms; delamination was the secondary mechanism. PMID:29415492

Friction is Fracture: a new paradigm for the onset of frictional motion

NASA Astrophysics Data System (ADS)

Fineberg, Jay

Friction is generally described by a single degree of freedom, a `friction coefficient'. We experimentally study the space-time dynamics of the onset of dry and lubricated frictional motion when two contacting bodies start to slide. We first show that the transition from static to dynamic sliding is governed by rupture fronts (closely analogous to earthquakes) that break the contacts along the interface separating the two bodies. Moreover, the structure of these ''laboratory earthquakes'' is quantitatively described by singular solutions originally derived to describe the motion of rapid cracks under applied shear. We demonstrate that this framework quantitatively describes both earthquake motion and arrest. This framework also providing a new window into the hidden properties of the micron thick interface that governs a body's frictional properties. Using this window we show that lubricated interfaces, although ``slippery'', actually becomes tougher; lubricants significantly increase dissipated energy during rupture. The results establish a new (and fruitful) paradigm for describing friction. Israel Science Foundation, ERC.

Tactile friction of topical formulations.

PubMed

Skedung, L; Buraczewska-Norin, I; Dawood, N; Rutland, M W; Ringstad, L

2016-02-01

The tactile perception is essential for all types of topical formulations (cosmetic, pharmaceutical, medical device) and the possibility to predict the sensorial response by using instrumental methods instead of sensory testing would save time and cost at an early stage product development. Here, we report on an instrumental evaluation method using tactile friction measurements to estimate perceptual attributes of topical formulations. Friction was measured between an index finger and an artificial skin substrate after application of formulations using a force sensor. Both model formulations of liquid crystalline phase structures with significantly different tactile properties, as well as commercial pharmaceutical moisturizing creams being more tactile-similar, were investigated. Friction coefficients were calculated as the ratio of the friction force to the applied load. The structures of the model formulations and phase transitions as a result of water evaporation were identified using optical microscopy. The friction device could distinguish friction coefficients between the phase structures, as well as the commercial creams after spreading and absorption into the substrate. In addition, phase transitions resulting in alterations in the feel of the formulations could be detected. A correlation was established between skin hydration and friction coefficient, where hydrated skin gave rise to higher friction. Also a link between skin smoothening and finger friction was established for the commercial moisturizing creams, although further investigations are needed to analyse this and correlations with other sensorial attributes in more detail. The present investigation shows that tactile friction measurements have potential as an alternative or complement in the evaluation of perception of topical formulations. © 2015 John Wiley & Sons A/S. Published by John Wiley & Sons Ltd.

Wear Potential of Dental Ceramics and its Relationship with Microhardness and Coefficient of Friction.

PubMed

Freddo, Rafael Augusto; Kapczinski, Myriam Pereira; Kinast, Eder Julio; de Souza Junior, Oswaldo Baptista; Rivaldo, Elken Gomes; da Fontoura Frasca, Luis Carlos

2016-10-01

To evaluate, by means of pin-on-disk testing, the wear potential of different dental ceramic systems as it relates to friction parameters, surface finish, and microhardness. Three groups of different ceramic systems (Noritake EX3, Eris, Empress II) with 20 disks each (10 glazed, 10 polished) were used. Vickers microhardness (Hv) was determined with a 200-g load for 30 seconds. Friction coefficients (μ) were determined by pin-on-disk testing (5 N load, 600 seconds, and 120 rpm). Wear patterns were assessed by scanning electron microscopy (SEM). The results were analyzed using one-way ANOVA and Tukey's test, with the significance level set at α = 0.05. The coefficients of friction were as follows: Noritake EX3 0.28 ± 0.12 (polished), 0.33 ± 0.08 (glazed); Empress II 0.38 ± 0.08 (polished), 0.45 ± 0.05 (glazed); Eris 0.49 ± 0.05 (polished), 0.49 ± 0.06 (glazed). Microhardness measurements were as follows: Noritake EX3 530.7 ± 8.7 (polished), 525.9 ± 6.2 (glazed); Empress II 534.1 ± 8 (polished), 534.7 ± 4.5 (glazed); Eris, 511.7 ± 6.5 (polished), 519.5 ± 4.1 (glazed). The polished and glazed Noritake EX3 and polished and glazed Eris specimens showed statistically different friction coefficients. SEM image analysis revealed more surface changes, such as small cracks and grains peeling off, in glazed ceramics. Wear potential may be related to the coefficient of friction in Noritake ceramics, which had a lower coefficient than Eris ceramics. Within-group analysis showed no differences in polished or glazed specimens. The differences observed were not associated with microhardness. © 2015 by the American College of Prosthodontists.

Quantifying the Frictional Forces between Skin and Nonwoven Fabrics

PubMed Central

Jayawardana, Kavinda; Ovenden, Nicholas C.; Cottenden, Alan

2017-01-01

When a compliant sheet of material is dragged over a curved surface of a body, the frictional forces generated can be many times greater than they would be for a planar interface. This phenomenon is known to contribute to the abrasion damage to skin often suffered by wearers of incontinence pads and bed/chairbound people susceptible to pressure sores. Experiments that attempt to quantify these forces often use a simple capstan-type equation to obtain a characteristic coefficient of friction. In general, the capstan approach assumes the ratio of applied tensions depends only on the arc of contact and the coefficient of friction, and ignores other geometric and physical considerations; this approach makes it straightforward to obtain explicitly a coefficient of friction from the tensions measured. In this paper, two mathematical models are presented that compute the material displacements and surface forces generated by, firstly, a membrane under tension in moving contact with a rigid obstacle and, secondly, a shell-membrane under tension in contact with a deformable substrate. The results show that, while the use of a capstan equation remains fairly robust in some cases, effects such as the curvature and flaccidness of the underlying body, and the mass density of the fabric can lead to significant variations in stresses generated in the contact region. Thus, the coefficient of friction determined by a capstan model may not be an accurate reflection of the true frictional behavior of the contact region. PMID:28321192

Polyelectrolyte brushes: a novel stable lubrication system in aqueous conditions.

PubMed

Kobayashi, Motoyasu; Terada, Masami; Takahara, Atsushi

2012-01-01

Surface-initiated controlled radical copolymerizations of 2-dimethylaminoethyl methacrylate (DMAEMA), 2-(methacryloyloxy)ethyl phosphorylcholine (MPC), 2-(methacryloyloxy)ethyltrimethylammonium chloride) (MTAC), and 3-sulfopropyl methacrylate potassium salt (SPMK) were carried out on a silicon wafer and glass ball to prepare polyelectrolyte brushes with excellent water wettability. The frictional coefficient of the polymer brushes was recorded on a ball-on-plate type tribometer by linear reciprocating motion of the brush specimen at a selected velocity of 1.5 x 10(-3) m s-1 under a normal load of 0.49 N applied to the stationary glass ball (d = 10 mm) at 298 K. The poly(DMAEMA-co-MPC) brush partially cross-linked by bis(2-iodoethoxy)ethane maintained a relatively low friction coefficient around 0.13 under humid air (RH > 75%) even after 200 friction cycles. The poly(SPMK) brush revealed an extremely low friction coefficient around 0.01 even after 450 friction cycles. We supposed that the abrasion of the brush was prevented owing to the good affinity of the poly(SPMK) brush for water forming a water lubrication layer, and electrostatic repulsive interactions among the brushes bearing sulfonic acid groups. Furthermore, the poly(SPMK-co-MTAC) brush with a chemically cross-linked structure showed a stable low friction coefficient in water even after 1400 friction cycles under a normal load of 139 MPa, indicating that the cross-linking structure improved the wear resistance of the brush layer.

Showing Area Matters: A Work of Friction

ERIC Educational Resources Information Center

Van Domelen, David

2010-01-01

Typically, we teach the simplified friction equation of the form F[subscript s] = [mu][subscript s]N for static friction, where F[subscript s] is the maximum static friction, [mu][subscript s] is the coefficient of static friction, and "N" is the normal force pressing the surfaces together. However, this is a bit too simplified, and…

Friction behavior of a microstructured polymer surface inspired by snake skin.

PubMed

Baum, Martina J; Heepe, Lars; Gorb, Stanislav N

2014-01-01

The aim of this study was to understand the influence of microstructures found on ventral scales of the biological model, Lampropeltis getula californiae, the California King Snake, on the friction behavior. For this purpose, we compared snake-inspired anisotropic microstructured surfaces to other microstructured surfaces with isotropic and anisotropic geometry. To exclude that the friction measurements were influenced by physico-chemical variations, all friction measurements were performed on the same epoxy polymer. For frictional measurements a microtribometer was used. Original data were processed by fast Fourier transformation (FFT) with a zero frequency related to the average friction and other peaks resulting from periodic stick-slip behavior. The data showed that the specific ventral surface ornamentation of snakes does not only reduce the frictional coefficient and generate anisotropic frictional properties, but also reduces stick-slip vibrations during sliding, which might be an adaptation to reduce wear. Based on this extensive comparative study of different microstructured polymer samples, it was experimentally demonstrated that the friction-induced stick-slip behavior does not solely depend on the frictional coefficient of the contact pair.

Tribo-electrochemical characterization of hafnium multilayer systems deposited on nitride/vanadium nitride AISI 4140 steel

NASA Astrophysics Data System (ADS)

Mora, M.; Vera, E.; Aperador, W.

2016-02-01

In this work is presented the synergistic behaviour among corrosion/wear (tribocorrosion) of the multilayer coatings hafnium nitride/vanadium nitride [HfN/VN]n. The multilayers were deposited on AISI 4140 steel using the technique of physical vapor deposition PVD magnetron sputtering, the tests were performed using a pin-on-disk tribometer, which has an adapted potentiostat galvanostat with three-electrode electrochemical cell. Tribocorrosive parameters such as: Friction coefficient between the coating and the counter body (100 Cr6 steel ball); Polarization resistance by means of electrochemical impedance spectroscopy technique and corrosion rate by polarization curves were determined. It was observed an increase in the polarization resistance, a decrease in the corrosion rate and a low coefficient of friction in comparison with the substrate, due to an increase on the number of bilayers.

Tribological properties of the babbit B83-based composite materials fabricated by powder metallurgy

NASA Astrophysics Data System (ADS)

Kalashnikov, I. E.; Bolotova, L. K.; Bykov, P. A.; Kobeleva, L. I.; Katin, I. V.; Mikheev, R. S.; Kobernik, N. V.

2016-07-01

Technological processes are developed to fabricate composite materials based on B83 babbit using hot pressing of a mixture of powders in the presence of a liquid phase. As a result, the structure of the matrix B83 alloy is dispersed, the morphology of intermetallic phases is changed, and reinforcing micro- and nanosized fillers are introduced and uniformly distributed in the matrix. The tribological properties of the synthesized materials are studied. The friction of the B83 babbit + 0.5 wt % MSR + 3 wt % SiC (MSR is modified schungite rock) composite material at high loads is characterized by an increase in the stability coefficient, and the wear resistance of the material increases by a factor of 1.8 as compared to the as-cast alloy at comparable friction coefficients.

Friction between footwear and floor covered with solid particles under dry and wet conditions.

PubMed

Li, Kai Way; Meng, Fanxing; Zhang, Wei

2014-01-01

Solid particles on the floor, both dry and wet, are common but their effects on the friction on the floor were seldom discussed in the literature. In this study, friction measurements were conducted to test the effects of particle size of solid contaminants on the friction coefficient on the floor under footwear, floor, and surface conditions. The results supported the hypothesis that particle size of solids affected the friction coefficient and the effects depended on footwear, floor, and surface conditions. On dry surfaces, solid particles resulted in friction loss when the Neolite footwear pad was used. On the other hand, solid particles provided additional friction when measured with the ethylene vinyl acetate (EVA) footwear pad. On wet surfaces, introducing solid particles made the floors more slip-resistant and such effects depended on particle size. This study provides information for better understanding of the mechanism of slipping when solid contaminants are present.

Friction Anisotropy with Respect to Topographic Orientation

PubMed Central

Yu, Chengjiao; Wang, Q. Jane

2012-01-01

Friction characteristics with respect to surface topographic orientation were investigated using surfaces of different materials and fabricated with grooves of different scales. Scratching friction tests were conducted using a nano-indentation-scratching system with the tip motion parallel or perpendicular to the groove orientation. Similar friction anisotropy trends were observed for all the surfaces studied, which are (1) under a light load and for surfaces with narrow grooves, the tip motion parallel to the grooves offers higher friction coefficients than does that perpendicular to them, (2) otherwise, equal or lower friction coefficients are found under this motion. The influences of groove size relative to the diameter of the mating tip (as a representative asperity), surface contact stiffness, contact area, and the characteristic stiction length are discussed. The appearance of this friction anisotropy is independent of material; however, the boundary and the point of trend transition depend on material properties. PMID:23248751

Tribological changes in the articular cartilage of a human femoral head with avascular necrosis.

PubMed

Seo, Eun-Min; Shrestha, Suman K; Duong, Cong-Truyen; Sharma, Ashish Ranjan; Kim, Tae-Woo; Vijayachandra, Ayyappan; Thompson, Mark S; Cho, Myung Guk; Park, Sungchan; Kim, Kwanghoon; Park, Seonghun; Lee, Sang-Soo

2015-06-29

The present study evaluated the tribological properties of the articular cartilage surface of the human femoral head with postcollapse stage avascular necrosis (AVN) using atomic force microscopy. The cartilage surface in the postcollapse stage AVN of the femoral head was reported to resemble those of disuse conditions, which suggests that the damage could be reversible and offers the possibilities of success of head-sparing surgeries. By comparing the tribological properties of articular cartilage in AVN with that of osteoarthritis, the authors intended to understand the cartilage degeneration mechanism and reversibility of AVN. Human femoral heads with AVN were explanted from the hip replacement surgery of four patients (60-83 years old). Nine cylindrical cartilage samples (diameter, 5 mm and height, 0.5 mm) were sectioned from the weight-bearing areas of the femoral head with AVN, and the cartilage surface was classified according to the Outerbridge Classification System (AVN0, normal; AVN1, softening and swelling; and AVN2, partial thickness defect and fissuring). Tribological properties including surface roughness and frictional coefficients and histochemistry including Safranin O and lubricin staining were compared among the three groups. The mean surface roughness Rq values of AVN cartilage increased significantly with increasing Outerbridge stages: Rq = 137 ± 26 nm in AVN0, Rq = 274 ± 49 nm in AVN1, and Rq = 452 ± 77 nm in AVN2. Significant differences in Rq were observed among different Outerbridge stages in all cases (p < 0.0001). The frictional coefficients (μ) also increased with increasing Outerbridge stages. The frictional coefficient values were μ = 0.115 ± 0.034 in AVN0, μ = 0.143 ± 0.025 in AVN1, and μ = 0.171 ± 0.039 in AVN2. Similarly to the statistical analysis of surface roughness, significant statistical differences were detected between different Outerbridge stages in all cases (p < 0.05). Both surface roughness and frictional coefficient of cartilage, which were linearly correlated, increased with increasing Outerbridge stages in postcollapse AVN. The underlying mechanism of these results can be related to proteoglycan loss within the articular cartilage that is also observed in osteoarthritis. With regard to the tribological properties, the cartilage degeneration mechanism in AVN was similar to that of osteoarthritis without reversibility.

Temperature-Dependent Friction and Wear Behavior of PTFE and MoS 2

DOE Office of Scientific and Technical Information (OSTI.GOV)

Babuska, T. F.; Pitenis, A. A.; Jones, M. R.

2016-06-16

We present an investigation of the temperature-dependent friction behavior of PTFE, MoS 2, and PTFE-on- MoS 2. Friction behavior was measured while continuously varying contact temperature in the range -150 to 175°C while sliding in dry nitrogen, as well as for self-mated PTFE immersed in liquid nitrogen. These results contrast with previous reports of monotonic inverse temperature dependent friction behavior, as well as reported high-friction transitions and plateaus at temperatures below about -20°C that were not observed, providing new insights about the molecular mechanisms of macro-scale friction. The temperature-dependent friction behavior characteristic of self-mated PTFE was found also on themore » PTFE-on-MoS 2 sliding contact, suggesting that PTFE friction was defined by sub-surface deformation mechanisms and internal friction even when sliding against a lamellar lubricant with extremely low friction coefficient (μ ~ 0.02). The various relaxation temperatures of PTFE were found in the temperature-dependent friction behavior, showing excellent agreement with reported values acquired using torsional techniques measuring internal friction. Additionally, hysteresis in friction behavior suggests an increase in near-surface crystallinity at upon exceeding the high temperature relaxation, T α~ 116°C.« less

Orthogonal Simulation Experiment for Flow Characteristics of Ore in Ore Drawing and Influencing Factors in a Single Funnel Under a Flexible Isolation Layer

NASA Astrophysics Data System (ADS)

Chen, Qingfa; Zhao, Fuyu; Chen, Qinglin; Wang, Yuding; Zhong, Yu; Niu, Wenjing

2017-12-01

A study on the flow characteristics of ore and factors that influence these characteristics is important to master ore flow laws. An orthogonal ore-drawing numerical model was established and the flow characteristics were explored. A weight matrix was obtained and the effect of the factors was determined. It was found that (1) the entire isolation-layer interface presents a Gaussian curve morphology and marked particles in each layer show a funnel morphology; (2) the drawing amount, Q, and the isolation layer half-width, W, are correlated positively with the fall depth, H, of the isolation layer; (3) factors that affect the characteristics sequentially include the particle friction coefficient, the interface friction coefficient, the isolation layer thickness, and the particle radius, and (4) the optimal combination is an isolation layer thickness of 0.005 m, an interface friction coefficient of 0.8, a particle friction coefficient of 0.2, and a particle radius of 0.007 m.

Friction behavior of network-structured CNT coating on pure titanium plate

NASA Astrophysics Data System (ADS)

Umeda, Junko; Fugetsu, Bunshi; Nishida, Erika; Miyaji, Hirofumi; Kondoh, Katsuyoshi

2015-12-01

Friction behavior of the network-structured CNTs coated pure Ti plate was evaluated by ball-on-disk wear test using SUS304 ball specimen under dry condition. The friction coefficient was significantly low and stable compared to the as-received Ti plate with no coating film. CNTs coating film had two important roles; self-lubrication and bearing effects to reduce the friction coefficient and carbon solid-solution hardening to improve the abrasive wear property of Ti plate. The annealing treatment at higher temperature (1123 K) was more effective to reduce the friction coefficient than that at lower temperature (973 K) because the Ti plate surface was uniformly covered with CNTs film even after sliding wear test. This is due to TiC interlayer formation via a reaction between Ti plate and carbon elements originated from CNTs during annealing. As a result, a strong interface bonding between CNTs film and Ti plate surface was obtained by higher temperature annealing treatment, and obstructed the detachment of CNTs film during wear test.

Tribology behavior on scratch tests: Effects of yield strength

DOE PAGES

Feng, Biao

2017-03-07

In this paper, a three-dimensional (3D) scratch model is proposed to investigate the effects of yield strength of both coatings and substrates. With the help of combined Coulomb and plastic friction, the obtained results comprehensively interpret the experimental phenomena in most metals that with the growth of hardness after heat treatment the scratch friction coefficient (SFC) increases. This interpretation could not be done before. Scratch tests on the surface with or without the coating are discussed. Without the coating the SFC increases due to the decrease of the area with plastic slippage and/or the increase of friction stress during themore » increase of the yield strength in the material. With a softer substrate the friction stress decreases but the SFC increases, which is caused by the growth of the entire contact area and surface deformation. Conversely, with a stronger substrate the SFC decreases due to an intensified plastic slippage In conclusion, the obtained results pave a new way to understanding the effects of yield strength on scratch tests, interpret experimental phenomena, and should be helpful for an optimum design in experiments.« less

Tribology behavior on scratch tests: Effects of yield strength

DOE Office of Scientific and Technical Information (OSTI.GOV)

Feng, Biao

In this paper, a three-dimensional (3D) scratch model is proposed to investigate the effects of yield strength of both coatings and substrates. With the help of combined Coulomb and plastic friction, the obtained results comprehensively interpret the experimental phenomena in most metals that with the growth of hardness after heat treatment the scratch friction coefficient (SFC) increases. This interpretation could not be done before. Scratch tests on the surface with or without the coating are discussed. Without the coating the SFC increases due to the decrease of the area with plastic slippage and/or the increase of friction stress during themore » increase of the yield strength in the material. With a softer substrate the friction stress decreases but the SFC increases, which is caused by the growth of the entire contact area and surface deformation. Conversely, with a stronger substrate the SFC decreases due to an intensified plastic slippage In conclusion, the obtained results pave a new way to understanding the effects of yield strength on scratch tests, interpret experimental phenomena, and should be helpful for an optimum design in experiments.« less

Pendulum Mass Affects the Measurement of Articular Friction Coefficient

PubMed Central

Akelman, Matthew R.; Teeple, Erin; Machan, Jason T.; Crisco, Joseph J.; Jay, Gregory D.; Fleming, Braden C.

2012-01-01

Friction measurements of articular cartilage are important to determine the relative tribologic contributions made by synovial fluid or cartilage, and to assess the efficacy of therapies for preventing the development of post-traumatic osteoarthritis. Stanton’s equation is the most frequently used formula for estimating the whole joint friction coefficient (μ) of an articular pendulum, and assumes pendulum energy loss through a mass-independent mechanism. This study examines if articular pendulum energy loss is indeed mass independent, and compares Stanton’s model to an alternative model, which incorporates viscous damping, for calculating μ. Ten loads (25-100% body weight) were applied in a random order to an articular pendulum using the knees of adult male Hartley guinea pigs (n = 4) as the fulcrum. Motion of the decaying pendulum was recorded and μ was estimated using two models: Stanton’s equation, and an exponential decay function incorporating a viscous damping coefficient. μ estimates decreased as mass increased for both models. Exponential decay model fit error values were 82% less than the Stanton model. These results indicate that μ decreases with increasing mass, and that an exponential decay model provides a better fit for articular pendulum data at all mass values. In conclusion, inter-study comparisons of articular pendulum μ values should not be made without recognizing the loads used, as μ values are mass dependent. PMID:23122223

Pendulum mass affects the measurement of articular friction coefficient.

PubMed

Akelman, Matthew R; Teeple, Erin; Machan, Jason T; Crisco, Joseph J; Jay, Gregory D; Fleming, Braden C

2013-02-01

Friction measurements of articular cartilage are important to determine the relative tribologic contributions made by synovial fluid or cartilage, and to assess the efficacy of therapies for preventing the development of post-traumatic osteoarthritis. Stanton's equation is the most frequently used formula for estimating the whole joint friction coefficient (μ) of an articular pendulum, and assumes pendulum energy loss through a mass-independent mechanism. This study examines if articular pendulum energy loss is indeed mass independent, and compares Stanton's model to an alternative model, which incorporates viscous damping, for calculating μ. Ten loads (25-100% body weight) were applied in a random order to an articular pendulum using the knees of adult male Hartley guinea pigs (n=4) as the fulcrum. Motion of the decaying pendulum was recorded and μ was estimated using two models: Stanton's equation, and an exponential decay function incorporating a viscous damping coefficient. μ estimates decreased as mass increased for both models. Exponential decay model fit error values were 82% less than the Stanton model. These results indicate that μ decreases with increasing mass, and that an exponential decay model provides a better fit for articular pendulum data at all mass values. In conclusion, inter-study comparisons of articular pendulum μ values should not be made without recognizing the loads used, as μ values are mass dependent. Copyright © 2012 Elsevier Ltd. All rights reserved.

Biological characterization of the skin of shortfin mako shark Isurus oxyrinchus and preliminary study of the hydrodynamic behaviour through computational fluid dynamics.

PubMed

Díez, G; Soto, M; Blanco, J M

2015-07-01

This study characterized the morphology, density and orientation of the dermal denticles along the body of a shortfin mako shark Isurus oxyrinchus and identified the hydrodynamic parameters of its body through a computational fluid-dynamics model. The study showed a great variability in the morphology, size, shape, orientation and density of dermal denticles along the body of I. oxyrinchus. There was a significant higher density in dorsal and ventral areas of the body and their highest angular deviations were found in the lower part of the mouth and in the areas between the pre-caudal pit and the second dorsal and pelvic fins. A detailed three-dimensional geometry from a scanned body of a shark was carried out to evaluate the hydrodynamic properties such as drag coefficient, lift coefficient and superficial (skin) friction coefficient of the skin together with flow velocity field, according to different roughness coefficients simulating the effect of the dermal denticles. This preliminary approach contributed to detailed information of the denticle interactions. As the height of the denticles was increased, flow velocity and the effect of lift decreased whereas drag increased. The highest peaks of skin friction coefficient were observed around the pectoral fins. © 2015 The Fisheries Society of the British Isles.

Multiscale physics of rubber-ice friction

NASA Astrophysics Data System (ADS)

Tuononen, Ari J.; Kriston, András; Persson, Bo

2016-09-01

Ice friction plays an important role in many engineering applications, e.g., tires on icy roads, ice breaker ship motion, or winter sports equipment. Although numerous experiments have already been performed to understand the effect of various conditions on ice friction, to reveal the fundamental frictional mechanisms is still a challenging task. This study uses in situ white light interferometry to analyze ice surface topography during linear friction testing with a rubber slider. The method helps to provide an understanding of the link between changes in the surface topography and the friction coefficient through direct visualization and quantitative measurement of the morphologies of the ice surface at different length scales. Besides surface polishing and scratching, it was found that ice melts locally even after one sweep showing the refrozen droplets. A multi-scale rubber friction theory was also applied to study the contribution of viscoelasticity to the total friction coefficient, which showed a significant level with respect to the smoothness of the ice; furthermore, the theory also confirmed the possibility of local ice melting.

Effect of Substrate Bias on Friction Coefficient, Adhesion Strength and Hardness of TiN-COATED Tool Steel

NASA Astrophysics Data System (ADS)

Hamzah, Esah; Ali, Mubarak; Toff, Mohd Radzi Hj. Mohd

In the present study, TiN coatings have been deposited on D2 tool steel substrates by using cathodic arc physical vapor deposition technique. The objective of this research work is to determine the usefulness of TiN coatings in order to improve the micro-Vickers hardness and friction coefficient of TiN coating deposited on D2 tool steel, which is widely used in tooling applications. A Pin-on-Disc test was carried out to study the coefficient of friction versus sliding distance of TiN coating deposited at various substrate biases. The standard deviation parameter during tribo-test result showed that the coating deposited at substrate bias of -75 V was the most stable coating. A significant increase in micro-Vickers hardness was recorded, when substrate bias was reduced from -150 V to zero. Scratch tester was used to compare the critical loads for coatings deposited at different bias voltages and the adhesion achievable was demonstrated with relevance to the various modes, scratch macroscopic analysis, critical load, acoustic emission and penetration depth. A considerable improvement in TiN coatings was observed as a function of various substrate bias voltages.

Friction and wear behaviour of ion beam modified ceramics

NASA Technical Reports Server (NTRS)

Lankford, J.; Wei, W.; Kossowsky, R.

1987-01-01

In the present study, the sliding friction coefficients and wear rates of carbide, oxide, and nitride materials for potential use as sliding seals (ring/liner) were measured under temperature, environmental, velocity, and loading conditions representative of a diesel engine. In addition, silicon nitride and partially stabilized zirconia discs were modified by ion mixing with TiNi, nickel, cobalt and chromium, and subsequently run against carbide pins, with the objective of producing reduced friction via solid lubrication at elevated temperature. Unmodified ceramic sliding couples were characterized at all temperatures by friction coefficients of 0.24 and above. However, the coefficient at 800 C in an oxidizing environment was reduced to below 0.1, for certain material combinations, by the ion implantation of TiNi or cobalt. This beneficial effect was found to derive from lubricious titanium, nickel, and cobalt oxides.

Friction coefficient determination by electrical resistance measurements

NASA Astrophysics Data System (ADS)

Tunyagi, A.; Kandrai, K.; Fülöp, Z.; Kapusi, Z.; Simon, A.

2018-05-01

A simple and low-cost, DIY-type, Arduino-driven experiment is presented for the study of friction and measurement of the friction coefficient, using a conductive rubber cord as a force sensor. It is proposed for high-school or college/university-level students. We strongly believe that it is worthwhile planning, designing and performing Arduino and compatible sensor-based experiments in physics class in order to ensure a better understanding of phenomena, develop theoretical knowledge and multiple experimental skills.

Comments on Static vs Kinetic Friction

NASA Astrophysics Data System (ADS)

Kessler, Gabriel

2009-09-01

I'm writing to comment on the article published in the March edition of The Physics Teacher titled "Choose Wisely: Static or Kinetic Friction—The Power of Dimensionless Plots." As I was reading the article, something caught my eye that I couldn't reconcile with. It was the phrase on page 160 in the first column near the bottom. The statement was that the experimental value for the coefficient of kinetic friction was "unexpectedly greater than the coefficient of static friction!"

Effects of humidity and interlayer cations on the frictional strength of montmorillonite

NASA Astrophysics Data System (ADS)

Tetsuka, Hiroshi; Katayama, Ikuo; Sakuma, Hiroshi; Tamura, Kenji

2018-04-01

We developed a humidity control system in a biaxial friction testing machine to investigate the effect of relative humidity and interlayer cations on the frictional strength of montmorillonite. We carried out the frictional experiments on Na- and Ca-montmorillonite under controlled relative humidities (ca. 10, 30, 50, 70, and 90%) and at a constant temperature (95 °C). Our experimental results show that frictional strengths of both Na- and Ca-montmorillonite decrease systematically with increasing relative humidity. The friction coefficients of Na-montmorillonite decrease from 0.33 (at relative humidity of 10%) to 0.06 (at relative humidity of 93%) and those of Ca-montmorillonite decrease from 0.22 (at relative humidity of 11%) to 0.04 (at relative humidity of 91%). Our results also show that the frictional strength of Na-montmorillonite is higher than that of Ca-montmorillonite at a given relative humidity. These results reveal that the frictional strength of montmorillonite is sensitive to hydration state and interlayer cation species, suggesting that the strength of faults containing these clay minerals depends on the physical and chemical environment.[Figure not available: see fulltext.

Friction and wear of iron and nickel in sodium hydroxide solutions

NASA Technical Reports Server (NTRS)

Rengstorff, G. W. P.; Miyoshi, K.; Buckley, D. H.

1982-01-01

A loaded spherical aluminum oxider rider was made to slide, while in various solutions, on a flat iron or nickel surface reciprocate a distance of 1 cm. Time of experiments was 1 hr during which the rider passed over the rider passed over the center section of the track 540 times. Coefficients of friction were measured throughout the experiments. Wear was measured by scanning the track with a profilometer. Analysis of some of the wear tracks included use of the SEM (scanning electron microscrope) and XPS (X-ray photoelectron spectroscopy). Investigated were the effect of various concentractions of NaOH and of water. On iron, increasing NaOH concentration above 0.01 N caused the friction and wear to decrease. This decrease is accompanied by a decrease in surface concentration of ferric oxide (Fe2O3) while more complex iron-oxygen compounds, not clearly identified, also form. At low concentrations of NaOH, such as 0.01 N, where the friction is high, the wear track is badely torn up and the surface is broken. At high concentration, such as 10 N, where the friction is low, the wear track is smooth. The general conclusion is that NaOH forms a protective, low friction film on iron which is destroyed by wear at low concentrations but remains intact at high concentrations of NaOH. Nickel behaves differently than iron in that only a little NaOH gives a low coefficient of friction and a surface which, although roughened in the wear track, remains intact.

Effects of Heat Treatment on Corrosion and Wear Behaviors of Mg-6Gd-2Zn-0.4Zr Alloy in Simulated Body Fluid

NASA Astrophysics Data System (ADS)

Zhao, Li; Chen, Wei; Dai, Jianwei; Wang, Zhangzhong; Zhang, Xiaobo

2017-11-01

Mg-6Gd-2Zn-0.4Zr (wt.%, GZ62K) alloy was processed by solution treatment under different temperatures. The microstructure, hardness, corrosion and wear behaviors in simulated body fluid (SBF) have been studied. The results indicate that the (Mg, Zn)3Gd phase decreases, the precipitated phases gradually increase, and the long-period stacking ordered structure disappears with the increase of solution temperature. The alloy has better corrosion resistance after solution treatment, and that solution treated at 490 °C for 12 h shows the best corrosion resistance. The friction coefficient of the alloy under dry sliding condition decreases slightly, but the mass loss increases with increasing the solution temperature. The alloy solution treated at 460 °C for 12 h exhibits the lowest friction coefficient and mass loss in SBF, and it also has the best wear resistance under dry sliding condition.

Tribological performance of polycrystalline tantalum-carbide-incorporated diamond films on silicon substrates

NASA Astrophysics Data System (ADS)

Ullah, Mahtab; Rana, Anwar Manzoor; Ahmed, E.; Malik, Abdul Sattar; Shah, Z. A.; Ahmad, Naseeb; Mehtab, Ujala; Raza, Rizwan

2018-05-01

Polycrystalline tantalum-carbide-incorporated diamond coatings have been made on unpolished side of Si (100) wafer by hot filament chemical vapor deposition process. Morphology of the coatings has been found to vary from (111) triangular-facetted to predominantly (111) square-faceted by increasing the concentration of tantalum carbide. The results have been compared to those of a diamond reference coating with no tantalum content. An increase in roughness has been observed with the increase of tantalum carbide (TaC) due to change in morphology of the diamond films. It is noticed that roughness of the coatings increases as grains become more square-faceted. It is found that diamond coatings involving tantalum carbide are not as resistant as diamond films with no TaC content and the coefficient of friction for such coatings with microcrystalline grains can be manipulated to 0·33 under high vacuum of 10-7 Torr. Such a low friction coefficient value enhances tribological behavior of unpolished Si substrates and can possibly be used in sliding applications.

Microstructure and tribological properties of TiAg intermetallic compound coating

NASA Astrophysics Data System (ADS)

Guo, Chun; Chen, Jianmin; Zhou, Jiansong; Zhao, Jierong; Wang, Linqian; Yu, Youjun; Zhou, Huidi

2011-10-01

TiAg intermetallic compound coating has been in situ synthesized successfully on pure Ti substrate by laser cladding using Ag powder as the precursor. It has been found that the prepared coating mainly comprised TiAg and Ti phases. The high resolution transmission electron microscopy results further conform the existence of TiAg intermetallic compound in the prepared coating. The magnified high resolution transmission electron microscopy images shown that the laser cladding coating contains TiAg nanocrystalline with the size of about 4 nm. Tribological properties of the prepared TiAg intermetallic compound coating were systematically evaluated. It was found that the friction coefficient and wear rate was closely related to the normal load and sliding speed, i.e., the friction coefficient of the prepared TiAg intermetallic compound coating decreased with increasing normal load and sliding speed. The wear rate of the TiAg intermetallic compound coating decreased rapidly with increasing sliding speed, while the wear rate increased as the normal load increased.

The Mohr-Coulomb criterion for intact rock strength and friction - a re-evaluation and consideration of failure under polyaxial stresses

NASA Astrophysics Data System (ADS)

Hackston, Abigail; Rutter, Ernest

2016-04-01

Darley Dale and Pennant sandstones were tested under conditions of both axisymmetric shortening and extension normal to bedding. These are the two extremes of loading under polyaxial stress conditions. Failure under generalized stress conditions can be predicted from the Mohr-Coulomb failure criterion under axisymmetric shortening conditions, provided the best form of polyaxial failure criterion is known. The sandstone data are best reconciled using the Mogi (1967) empirical criterion. Fault plane orientations produced vary greatly with respect to the maximum compressive stress direction in the two loading configurations. The normals to the Mohr-Coulomb failure envelopes do not predict the orientations of the fault planes eventually produced. Frictional sliding on variously inclined saw cuts and failure surfaces produced in intact rock samples was also investigated. Friction coefficient is not affected by fault plane orientation in a given loading configuration, but friction coefficients in extension were systematically lower than in compression for both rock types. Friction data for these and other porous sandstones accord well with the Byerlee (1978) generalization about rock friction being largely independent of rock type. For engineering and geodynamic modelling purposes, the stress-state-dependent friction coefficient should be used for sandstones, but it is not known to what extent this might apply to other rock types.

Pollutant Plume Dispersion in the Atmospheric Boundary Layer over Idealized Urban Roughness

NASA Astrophysics Data System (ADS)

Wong, Colman C. C.; Liu, Chun-Ho

2013-05-01

The Gaussian model of plume dispersion is commonly used for pollutant concentration estimates. However, its major parameters, dispersion coefficients, barely account for terrain configuration and surface roughness. Large-scale roughness elements (e.g. buildings in urban areas) can substantially modify the ground features together with the pollutant transport in the atmospheric boundary layer over urban roughness (also known as the urban boundary layer, UBL). This study is thus conceived to investigate how urban roughness affects the flow structure and vertical dispersion coefficient in the UBL. Large-eddy simulation (LES) is carried out to examine the plume dispersion from a ground-level pollutant (area) source over idealized street canyons for cross flows in neutral stratification. A range of building-height-to-street-width (aspect) ratios, covering the regimes of skimming flow, wake interference, and isolated roughness, is employed to control the surface roughness. Apart from the widely used aerodynamic resistance or roughness function, the friction factor is another suitable parameter that measures the drag imposed by urban roughness quantitatively. Previous results from laboratory experiments and mathematical modelling also support the aforementioned approach for both two- and three-dimensional roughness elements. Comparing the UBL plume behaviour, the LES results show that the pollutant dispersion strongly depends on the friction factor. Empirical studies reveal that the vertical dispersion coefficient increases with increasing friction factor in the skimming flow regime (lower resistance) but is more uniform in the regimes of wake interference and isolated roughness (higher resistance). Hence, it is proposed that the friction factor and flow regimes could be adopted concurrently for pollutant concentration estimate in the UBL over urban street canyons of different roughness.

Internal structure of inertial granular flows.

PubMed

Azéma, Emilien; Radjaï, Farhang

2014-02-21

We analyze inertial granular flows and show that, for all values of the inertial number I, the effective friction coefficient μ arises from three different parameters pertaining to the contact network and force transmission: (1) contact anisotropy, (2) force chain anisotropy, and (3) friction mobilization. Our extensive 3D numerical simulations reveal that μ increases with I mainly due to an increasing contact anisotropy and partially by friction mobilization whereas the anisotropy of force chains declines as a result of the destabilizing effect of particle inertia. The contact network undergoes topological transitions, and beyond I≃0.1 the force chains break into clusters immersed in a background "soup" of floating particles. We show that this transition coincides with the divergence of the size of fluidized zones characterized from the local environments of floating particles and a slower increase of μ with I.

Internal Structure of Inertial Granular Flows

NASA Astrophysics Data System (ADS)

Azéma, Emilien; Radjaï, Farhang

2014-02-01

We analyze inertial granular flows and show that, for all values of the inertial number I, the effective friction coefficient μ arises from three different parameters pertaining to the contact network and force transmission: (1) contact anisotropy, (2) force chain anisotropy, and (3) friction mobilization. Our extensive 3D numerical simulations reveal that μ increases with I mainly due to an increasing contact anisotropy and partially by friction mobilization whereas the anisotropy of force chains declines as a result of the destabilizing effect of particle inertia. The contact network undergoes topological transitions, and beyond I≃0.1 the force chains break into clusters immersed in a background "soup" of floating particles. We show that this transition coincides with the divergence of the size of fluidized zones characterized from the local environments of floating particles and a slower increase of μ with I.

Methods to Measure, Predict and Relate Friction, Wear and Fuel Economy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gravante, Steve; Fenske, George; Demas, Nicholas

High-fidelity measurements of the coefficient of friction and the parasitic friction power of the power cylinder components have been made for the Isuzu 5.2L 4H on-highway engine. In particular, measurements of the asperity friction coefficient were made with test coupons using Argonne National Lab’s (ANL) reciprocating test rig for the ring-on-liner and skirt-on-liner component pairs. These measurements correlated well with independent measurements made by Electro-Mechanical Associates (EMA). In addition, surface roughness measurements of the Isuzu components were made using white light interferometer (WLI). The asperity friction and surface characterization are key inputs to advanced CAE simulation tools such as RINGPAKmore » and PISDYN which are used to predict the friction power and wear rates of power cylinder components. Finally, motored friction tests were successfully performed to quantify the friction mean effective pressure (FMEP) of the power cylinder components for various oils (High viscosity 15W40, low viscosity 5W20 with friction modifier (FM) and specially blended oil containing consisting of PAO/ZDDP/MoDTC) at 25, 50, and 110°C.« less

High-velocity frictional strength across the Tohoku-Oki megathrust determined from surface drilling torque

NASA Astrophysics Data System (ADS)

Ujiie, K.; Inoue, T.; Ishiwata, J.

2015-12-01

Frictional strength at seismic slip rates is a key to evaluate fault weakening and rupture propagation during earthquakes. The Japan Trench First Drilling Project (JFAST) drilled through the shallow plate-boundary thrust, where huge displacements of ~50 m occurred during the 2011 Tohoku-Oki earthquake. To determine the downhole frictional strength at drilled site (Site C0019), we analyzed surface drilling data. The equivalent slip rate estimated from the rotation rate and inner and outer radiuses of the drill bit ranges from 0.8 to 1.3 m/s. The measured torque includes the frictional torque between the drilling string and borehole wall, the viscous torque between the drilling string and seawater/drilling fluid, and the drilling torque between the drill bit and sediments. We subtracted the former two from the measured torque using the torque data during bottom-up rotating operations at several depths. Then, the shear stress was calculated from the drilling torque taking the configuration of the drill bit into consideration. The normal stress was estimated from the weight on bit data and the projected area of the drill bit. Assuming negligible cohesion, the frictional strength was obtained by dividing shear stress by normal stress. The results show a clear contrast in high-velocity frictional strength across the plate-boundary thrust: the friction coefficient of frontal prism sediments (hemipelagic mudstones) in hanging wall is 0.1-0.2, while that in subducting sediments (hemipelagic to pelagic mudstones and chert) in footwall increases to 0.2-0.4. The friction coefficient of smectite-rich pelagic clay in the plate-boundary thrust is ~0.1, which is consistent with that obtained from high-velocity (1.3 m/s) friction experiments and temperature measurements. We conclude that surface drilling torque provides useful data to obtain a continuous downhole frictional strength.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Chang, Shyy Woei; Yang, Tsun Lirng; Liou, Jin Shuen

An experimental study measuring the axial heat transfer distributions and the pressure drop coefficients of the tube fitted with a broken twisted tape of twist ratio 1, 1.5, 2, 2.5 or {infinity} is performed in the Re range of 1000-40,000. This type of broken twisted tape is newly invented without previous investigations available. Local Nusselt numbers and mean Fanning friction factors in the tube fitted with the broken twisted tape increase as the twist ratio decreases. Heat transfer coefficients, mean Fanning friction factors and thermal performance factors in the tube fitted with the broken twisted tape are, respectively, augmented tomore » 1.28-2.4, 2-4.7 and 0.99-1.8 times of those in the tube fitted with the smooth twisted tape. Empirical heat transfer and pressure drop correlations which evaluate the local Nusselt number and the mean Fanning friction factor for the tube with the broken twisted tape insert are generated to assist the industrial applications. (author)« less

Heat transfer and flow friction correlations for perforated plate matrix heat exchangers

NASA Astrophysics Data System (ADS)

Ratna Raju, L.; Kumar, S. Sunil; Chowdhury, K.; Nandi, T. K.

2017-02-01

Perforated plate matrix heat exchangers (MHE) are constructed of high conductivity perforated plates stacked alternately with low conductivity spacers. They are being increasingly used in many cryogenic applications including Claude cycle or Reversed Brayton cycle cryo-refrigerators and liquefiers. Design of high NTU (number of (heat) transfer unit) cryogenic MHEs requires accurate heat transfer coefficient and flow friction factor. Thermo-hydraulic behaviour of perforated plates strongly depends on the geometrical parameters. Existing correlations, however, are mostly expressed as functions of Reynolds number only. This causes, for a given configuration, significant variations in coefficients from one correlation to the other. In this paper we present heat transfer and flow friction correlations as functions of all geometrical and other controlling variables. A FluentTM based numerical model has been developed for heat transfer and pressure drop studies over a stack of alternately arranged perforated plates and spacers. The model is validated with the data from literature. Generalized correlations are obtained through regression analysis over a large number of computed data.

Shear thinning in non-Brownian suspensions.

PubMed

Chatté, Guillaume; Comtet, Jean; Niguès, Antoine; Bocquet, Lydéric; Siria, Alessandro; Ducouret, Guylaine; Lequeux, François; Lenoir, Nicolas; Ovarlez, Guillaume; Colin, Annie

2018-02-14

We study the flow of suspensions of non-Brownian particles dispersed into a Newtonian solvent. Combining capillary rheometry and conventional rheometry, we evidence a succession of two shear thinning regimes separated by a shear thickening one. Through X-ray radiography measurements, we show that during each of those regimes, the flow remains homogeneous and does not involve particle migration. Using a quartz-tuning fork based atomic force microscope, we measure the repulsive force profile and the microscopic friction coefficient μ between two particles immersed into the solvent, as a function of normal load. Coupling measurements from those three techniques, we propose that (1) the first shear-thinning regime at low shear rates occurs for a lubricated rheology and can be interpreted as a decrease of the effective volume fraction under increasing particle pressures, due to short-ranged repulsive forces and (2) the second shear thinning regime after the shear-thickening transition occurs for a frictional rheology and can be interpreted as stemming from a decrease of the microscopic friction coefficient at large normal load.

Numerical Modeling of Mechanical Behavior for Buried Steel Pipelines Crossing Subsidence Strata

PubMed Central

Han, C. J.

2015-01-01

This paper addresses the mechanical behavior of buried steel pipeline crossing subsidence strata. The investigation is based on numerical simulation of the nonlinear response of the pipeline-soil system through finite element method, considering large strain and displacement, inelastic material behavior of buried pipeline and the surrounding soil, as well as contact and friction on the pipeline-soil interface. Effects of key parameters on the mechanical behavior of buried pipeline were investigated, such as strata subsidence, diameter-thickness ratio, buried depth, internal pressure, friction coefficient and soil properties. The results show that the maximum strain appears on the outer transition subsidence section of the pipeline, and its cross section is concave shaped. With the increasing of strata subsidence and diameter-thickness ratio, the out of roundness, longitudinal strain and equivalent plastic strain increase gradually. With the buried depth increasing, the deflection, out of roundness and strain of the pipeline decrease. Internal pressure and friction coefficient have little effect on the deflection of buried pipeline. Out of roundness is reduced and the strain is increased gradually with the increasing of internal pressure. The physical properties of soil have a great influence on the mechanical properties of buried pipeline. The results from the present study can be used for the development of optimization design and preventive maintenance for buried steel pipelines. PMID:26103460

Ultra low friction carbon/carbon composites for extreme temperature applications

DOEpatents

Erdemir, Ali; Busch, Donald E.; Fenske, George R.; Lee, Sam; Shepherd, Gary; Pruett, Gary J.

2001-01-01

A carbon/carbon composite in which a carbon matrix containing a controlled amount of boron or a boron compound is reinforced with carbon fiber exhibits a low coefficient of friction, i.e., on the order of 0.04 to 0.1 at temperatures up to 600.degree. C., which is one of the lowest frictional coefficients for any type of carbonaceous material, including graphite, glassy carbon, diamond, diamond-like carbon and other forms of carbon material. The high degree of slipperiness of the carbon composite renders it particularly adapted for limiting friction and wear at elevated temperatures such as in seals, bearings, shafts, and flexible joints

Predicting a contact's sensitivity to initial conditions using metrics of frictional coupling

DOE PAGES

Flicek, Robert C.; Hills, David A.; Brake, Matthew Robert W.

2016-09-29

This paper presents a method for predicting how sensitive a frictional contact’s steady-state behavior is to its initial conditions. Previous research has proven that if a contact is uncoupled, i.e. if slip displacements do not influence the contact pressure distribution, then its steady-state response is independent of initial conditions, but if the contact is coupled, the steady-state response depends on initial conditions. In this paper, two metrics for quantifying coupling in discrete frictional systems are examined. These metrics suggest that coupling is dominated by material dissimilarity due to Dundurs’ composite material parameter β when β ≥ 0.2, but geometric mismatchmore » becomes the dominant source of coupling for smaller values of β. Based on a large set of numerical simulations with different contact geometries, material combinations, and friction coefficients, a contact’s sensitivity to initial conditions is found to be correlated with the product of the coupling metric and the friction coefficient. For cyclic shear loading, this correlation is maintained for simulations with different contact geometries, material combinations, and friction coefficients. Furthermore, for cyclic bulk loading, the correlation is only maintained when the contact edge angle is held constant.« less

Predicting a contact's sensitivity to initial conditions using metrics of frictional coupling

DOE Office of Scientific and Technical Information (OSTI.GOV)

Flicek, Robert C.; Hills, David A.; Brake, Matthew Robert W.

This paper presents a method for predicting how sensitive a frictional contact’s steady-state behavior is to its initial conditions. Previous research has proven that if a contact is uncoupled, i.e. if slip displacements do not influence the contact pressure distribution, then its steady-state response is independent of initial conditions, but if the contact is coupled, the steady-state response depends on initial conditions. In this paper, two metrics for quantifying coupling in discrete frictional systems are examined. These metrics suggest that coupling is dominated by material dissimilarity due to Dundurs’ composite material parameter β when β ≥ 0.2, but geometric mismatchmore » becomes the dominant source of coupling for smaller values of β. Based on a large set of numerical simulations with different contact geometries, material combinations, and friction coefficients, a contact’s sensitivity to initial conditions is found to be correlated with the product of the coupling metric and the friction coefficient. For cyclic shear loading, this correlation is maintained for simulations with different contact geometries, material combinations, and friction coefficients. Furthermore, for cyclic bulk loading, the correlation is only maintained when the contact edge angle is held constant.« less

New Effective Material Couple--Oxide Ceramic and Carbon Nanotube-- Developed for Aerospace Microsystem and Micromachine Technologies

NASA Technical Reports Server (NTRS)

Miyoshi, Kazuhisa; VanderWal, Randall L.; Tomasek, Aaron J.; Sayir, Ali; Farmer, Serene C.

2004-01-01

The prime driving force for using microsystem and micromachine technologies in transport vehicles, such as spacecraft, aircraft, and automobiles, is to reduce the weight, power consumption, and volume of components and systems to lower costs and increase affordability and reliability. However, a number of specific issues need to be addressed with respect to using microsystems and micromachines in aerospace applications--such as the lack of understanding of material characteristics; methods for producing and testing the materials in small batches; the limited proven durability and lifetime of current microcomponents, packaging, and interconnections; a cultural change with respect to system designs; and the use of embedded software, which will require new product assurance guidelines. In regards to material characteristics, there are significant adhesion, friction, and wear issues in using microdevices. Because these issues are directly related to surface phenomena, they cannot be scaled down linearly and they become increasingly important as the devices become smaller. When microsystems have contacting surfaces in relative motion, the adhesion and friction affect performance, energy consumption, wear damage, maintenance, lifetime and catastrophic failure, and reliability. Ceramics, for the most part, do not have inherently good friction and wear properties. For example, coefficients of friction in excess of 0.7 have been reported for ceramics and ceramic composite materials. Under Alternate Fuels Foundation Technologies funding, two-phase oxide ceramics developed for superior high-temperature wear resistance in NASA's High Operating Temperature Propulsion Components (HOTPC) project and new two-layered carbon nanotube (CNT) coatings (CNT topcoat/iron bondcoat/quartz substrate) developed in NASA's Revolutionary Aeropropulsion Concepts (RAC) project have been chosen as a materials couple for aerospace applications, including micromachines, in the nanotechnology lubrication task because of their potential for superior friction and wearf properties in air and in an ultrahigh vacuum, spacelike environment. At the NASA Glenn Research Center, two-phase oxide ceramic eutectics, Al2O3/ZrO2(Y2O3), were directionally solidified using the laser-float-zone process, and carbon nanotubes were synthesized within a high-temperature tube furnace at 800 C. Physical vapor deposition was used to coat all quartz substrates with 5-nm-thick iron as catalyst and bondcoat, which formed iron islands resembling droplets and serving as catalyst particles on the quartz. A series of scanning electron micrographs showing multiwalled carbon nanotubes directionally grown as aligned "nanograss" on quartz is presented. Unidirectional sliding friction eperiments were conducted at Glenn with the two-layered CNT coatings in contact with the two-phase Al2O3/ZrO2(Y2O3) eutectics in air and in ultrachigh vacuum. The main criteria for judging the performance of the materials couple for solid lubrication and antistick applications in a space environment were the coefficient of friction and the wear resistance (reciprocal of wear rate), which had to be less than 0.2 and greater than 10(exp 5) N(raised dot)/cubic millimetes, respectively, in ultrahigh vacuum. In air, the coefficient of friction for the CNT coatings in contact with Al2O3/ZrO2 (Y2O3) eutectics was 0.04, one-fourth of that for quartz. In an ultrahigh vacuum, the coefficient of friction for CNT coatings in contact with Al2O3/ZrO2 (Y2O3) was one-third of that for quartz. The two-phase Al2O3/ZrO2 (Y2O3) eutectic coupled with the two-layered CNT coating met the coefficient of friction and wear resistance criteria both in air and in an ultrahigh vacuum, spacelike environment. This material's couple can dramatically improve the stiction (or adhesion), friction, and wear resistance of the contacting surfaces, which are major issues for microdevices and micromachines.

Friction Properties of Polished Cvd Diamond Films Sliding against Different Metals

NASA Astrophysics Data System (ADS)

Lin, Zichao; Sun, Fanghong; Shen, Bin

2016-11-01

Owing to their excellent mechanical and tribological properties, like the well-known extreme hardness, low coefficient of friction and high chemical inertness, chemical vapor deposition (CVD) diamond films have found applications as a hard coating for drawing dies. The surface roughness of the diamond films is one of the most important attributes to the drawing dies. In this paper, the effects of different surface roughnesses on the friction properties of diamond films have been experimentally studied. Diamond films were fabricated using hot filament CVD. The WC-Co (Co 6wt.%) drawing dies were used as substrates. A gas mixture of acetone and hydrogen gas was used as the feedstock gas. The CVD diamond films were polished using mechanical polishing. Polished diamond films with three different surface roughnesses, as well as the unpolished diamond film, were fabricated in order to study the tribological performance between the CVD diamond films and different metals with oil lubrication. The unpolished and polished CVD diamond films are characterized with scanning electron microscope (SEM), atomic force microscope (AFM), surface profilometer, Raman spectrum and X-ray diffraction (XRD). The friction examinations were carried out by using a ball-on-plate type reciprocating friction tester. Low carbide steel, stainless steel, copper and aluminum materials were used as counterpart balls. Based on this study, the results presented the friction coefficients between the polished CVD films and different metals. The friction tests demonstrate that the smooth surface finish of CVD diamond films is beneficial for reducing their friction coefficients. The diamond films exhibit low friction coefficients when slid against the stainless steel balls and low carbide steel ball, lower than that slid against copper ball and aluminum ball, attributed to the higher ductility of copper and aluminum causing larger amount of wear debris adhering to the sliding interface and higher adhesive strength between the contacting surfaces.

Internally architectured materials with directionally asymmetric friction

PubMed Central

Bafekrpour, Ehsan; Dyskin, Arcady; Pasternak, Elena; Molotnikov, Andrey; Estrin, Yuri

2015-01-01

Internally Architectured Materials (IAMs) that exhibit different friction forces for sliding in the opposite directions are proposed. This is achieved by translating deformation normal to the sliding plane into a tangential force in a manner that is akin to a toothbrush with inclined bristles. Friction asymmetry is attained by employing a layered material or a structure with parallel ‘ribs’ inclined to the direction of sliding. A theory of directionally asymmetric friction is presented, along with prototype IAMs designed, fabricated and tested. The friction anisotropy (the ξ-coefficient) is characterised by the ratio of the friction forces for two opposite directions of sliding. It is further demonstrated that IAM can possess very high levels of friction anisotropy, with ξ of the order of 10. Further increase in ξ is attained by modifying the shape of the ribs to provide them with directionally dependent bending stiffness. Prototype IAMs produced by 3D printing exhibit truly giant friction asymmetry, with ξ in excess of 20. A novel mechanical rectifier, which can convert oscillatory movement into unidirectional movement by virtue of directionally asymmetric friction, is proposed. Possible applications include locomotion in a constrained environment and energy harvesting from oscillatory noise and vibrations. PMID:26040634

Tribological properties of self-lubricating Ta-Cu films

NASA Astrophysics Data System (ADS)

Qin, Wen; Fu, Licai; Zhu, Jiajun; Yang, Wulin; Li, Deyi; Zhou, Lingping

2018-03-01

In this paper, Ta and TaCu films were deposited by using magnetron sputtering, and the tribological properties of the films against Si3N4 balls were investigated under the loads of 2 N and 5 N. The average grain sizes of both films are below 25 nm. Ta and TaCu films have approximate hardness. While the wear rate of TaCu film is much smaller than that of Ta film. Post-wear testing XRD, Raman and XPS revealed the formation of tantalum oxide on the worn surface of both Ta and TaCu films. Tantalum oxidation is effectively lubricating to reduce friction coefficient. So the friction coefficient of both Ta and TaCu film is about 0.45 under different applied loads. Meanwhile, the addition of Cu could increase the toughness of the film, and avoid the generation of wear debris, resulting in a significant increase in wear resistance.

Interference assembly and fretting wear analysis of hollow shaft.

PubMed

Han, Chuanjun; Zhang, Jie

2014-01-01

Fretting damage phenomenon often appears in the interference fit assembly. The finite element model of hollow shaft and shaft sleeve was established, and the equivalent stress and contact stress were computed after interference assembly. The assembly body of hollow shaft and shaft sleeve was in whirling bending load, and the contact status (sticking, sliding, and opening) and the distribution of stress along one typical contact line were computed under different loads, interferences, hollow degrees, friction coefficient, and wear quantity. Judgment formula of contact state was fixed by introducing the corrected coefficient k. The computation results showed that the "edge effect" appears in the contact surface after interference fit. The size of slip zone is unchanged along with the increase of bending load. The greater the interference value, the bigger the wear range. The hollow degree does not influence the size of stick zone but controls the position of the junction point of slip-open. Tangential contact stress increases with the friction coefficient, which has a little effect on normal contact stress. The relationship between open size and wear capacity is approximately linear.

The effect of angular velocity and cycle on the dissipative properties of the knee during passive cyclic stretching: a matter of viscosity or solid friction.

PubMed

Nordez, A; McNair, P J; Casari, P; Cornu, C

2009-01-01

The mechanisms behind changes in mechanical parameters following stretching are not understood clearly. This study assessed the effects of joint angular velocity on the immediate changes in passive musculo-articular properties induced by cyclic stretching allowing an appreciation of viscosity and friction, and their contribution to changes in torque that occur. Ten healthy subjects performed five passive knee extension/flexion cycles on a Biodex dynamometer at five preset angular velocities (5-120 deg/s). The passive torque and knee angle were measured, and the potential elastic energy stored during the loading and the dissipation coefficient were calculated. As the stretching velocity increased, so did stored elastic energy and the dissipation coefficient. The slope of the linear relationship between the dissipation coefficient and the angular velocity was unchanged across repetitions indicating that viscosity was unlikely to be affected. A difference in the y-intercept across repetitions 1 and 5 was indicative of a change in processes associated with solid friction. Electromyographical responses to stretching were low across all joint angular velocities. Torque changes during cyclic motion may primarily involve solid friction which is more indicative of rearrangement/slipping of collagen fibers rather than the redistribution of fluid and its constituents within the muscle. The findings also suggest that it is better to stretch slowly initially to reduce the amount of energy absorption required by tissues, but thereafter higher stretching speeds can be undertaken.

Growth and tribological properties of diamond films on silicon and tungsten carbide substrates

NASA Astrophysics Data System (ADS)

Radhika, R.; Ramachandra Rao, M. S.

2016-11-01

Hot filament chemical vapor deposition technique was used to deposit microcrystalline diamond (MCD) and nanocrystalline diamond (NCD) films on silicon (Si) and tungsten carbide (WC-6Co) substrates. Friction coefficient of larger diamond grains deposited on WC-6Co substrate shows less value approximately 0.2 while this differs marginally on films grown on Si substrate. The study claims that for a less friction coefficient, the grain size is not necessarily smaller. However, the less friction coefficient (less than 0.1 saturated value) in MCD and NCD deposited on Si is explained by the formation of graphitized tribolayer. This layer easily forms when diamond phase is thermodynamically unstable.

Friction and wear behaviour of Mo-W doped carbon-based coating during boundary lubricated sliding

NASA Astrophysics Data System (ADS)

Hovsepian, Papken Eh.; Mandal, Paranjayee; Ehiasarian, Arutiun P.; Sáfrán, G.; Tietema, R.; Doerwald, D.

2016-03-01

A molybdenum and tungsten doped carbon-based coating (Mo-W-C) was developed in order to provide low friction in boundary lubricated sliding condition at ambient and at high temperature. The Mo-W-C coating showed the lowest friction coefficient among a number of commercially available state-of-the-art DLC coatings at ambient temperature. At elevated temperature (200 °C), Mo-W-C coating showed a significant reduction in friction coefficient with sliding distance in contrast to DLC coatings. Raman spectroscopy revealed the importance of combined Mo and W doping for achieving low friction at both ambient and high temperature. The significant decrease in friction and wear rate was attributed to the presence of graphitic carbon debris (from coating) and 'in situ' formed metal sulphides (WS2 and MoS2, where metals were supplied from coating and sulphur from engine oil) in the transfer layer.

Anisotropic frictional heating and defect generation in cyclotrimethylene-trinitramine molecular crystals

NASA Astrophysics Data System (ADS)

Rajak, Pankaj; Mishra, Ankit; Sheng, Chunyang; Tiwari, Subodh; Kalia, Rajiv K.; Nakano, Aiichiro; Vashishta, Priya

2018-05-01

Anisotropic frictional response and corresponding heating in cyclotrimethylene-trinitramine molecular crystals are studied using molecular dynamics simulations. The nature of damage and temperature rise due to frictional forces is monitored along different sliding directions on the primary slip plane, (010), and on non-slip planes, (100) and (001). Correlations between the friction coefficient, deformation, and frictional heating are established. We find that the friction coefficients on slip planes are smaller than those on non-slip planes. In response to sliding on a slip plane, the crystal deforms easily via dislocation generation and shows less heating. On non-slip planes, due to the inability of the crystal to deform via dislocation generation, a large damage zone is formed just below the contact area, accompanied by the change in the molecular ring conformation from chair to boat/half-boat. This in turn leads to a large temperature rise below the contact area.

Psychophysical evaluation of a variable friction tactile interface

NASA Astrophysics Data System (ADS)

Samur, Evren; Colgate, J. Edward; Peshkin, Michael A.

2009-02-01

This study explores the haptic rendering capabilities of a variable friction tactile interface through psychophysical experiments. In order to obtain a deeper understanding of the sensory resolution associated with the Tactile Pattern Display (TPaD), friction discrimination experiments are conducted. During the experiments, subjects are asked to explore the glass surface of the TPaD using their bare index fingers, to feel the friction on the surface, and to compare the slipperiness of two stimuli, displayed in sequential order. The fingertip position data is collected by an infrared frame and normal and translational forces applied by the finger are measured by force sensors attached to the TPaD. The recorded data is used to calculate the coefficient of friction between the fingertip and the TPaD. The experiments determine the just noticeable difference (JND) of friction coefficient for humans interacting with the TPaD.

Friction and wear behavior of single-crystal silicon carbide in contact with titanium

NASA Technical Reports Server (NTRS)

Miyoshi, K.; Buckley, D. H.

1977-01-01

Sliding friction experiments were conducted with single crystal silicon carbide in sliding contact with titanium. Results indicate that the friction coefficient is greater in vacuum than in argon and that this is due to the greater adhesion or adhesive transfer in vacuum. Thin films of silicon carbide transferred to titanium also adhered to silicon carbide both in argon at atmospheric pressure and in high vacuum. Cohesive bonds fractured on both the silicon carbide and titanium surfaces. The wear debris of silicon carbide created by fracture plowed the silicon carbide surface in a plastic manner. The friction characteristics of titanium in contact with silicon carbide were sensitive to the surface roughness of silicon carbide, and the friction coefficients were higher for a rough surface of silicon carbide than for a smooth one. The difference in friction results was due to plastic deformation (plowing of titanium).

The influence of micro-scale dimples and nano-sized grains on the fretting characteristics generated by laser pulses.

PubMed

Amanov, Auezhan; Watabe, Tsukasa; Sasaki, Shinya

2013-12-01

The tribological characteristics of micro-scale dimpled Cu-based alloy specimen generated using a laser surface texturing (LST) were assessed and compared with that of the untextured specimen. The objective of this study is to improve the tribological characteristics of internal combustion engine (ICE) bearings and bushings made of Cu-based alloy by generating micro-scale dimples using an LST. Fretting wear tests were performed by sliding a hardened SAE52100 steel ball against the untextured and LSTed specimens at a normal load of 5 N under oil-lubricated conditions. The friction force and relative movement between the specimens were measured simultaneously during the fretting tests. The test results showed that the LSTed specimens showed a reduction in friction coefficient and an enhancement in fretting wear resistance compared to that of the untextured specimen. The friction coefficient and fretting wear volume increased with increasing frequency for both untextured and LSTed specimens. The improved tribological properties of the LSTed specimen may be attributed to the micro-scale dimples, refined grain size and high lattice strain. In addition, a model for the nanocrystallization mechanism of the LSTed specimen was proposed.

Angle-dependent lubricated tribological properties of stainless steel by femtosecond laser surface texturing

NASA Astrophysics Data System (ADS)

Wang, Zhuo; Li, Yang-Bo; Bai, Feng; Wang, Cheng-Wei; Zhao, Quan-Zhong

2016-07-01

Lubricated tribological properties of stainless steel were investigated by femtosecond laser surface texturing. Regular-arranged micro-grooved textures with different spacing and micro-groove inclination angles (between micro-groove path and sliding direction) were produced on AISI 304L steel surfaces by an 800 nm femtosecond laser. The spacing of micro-groove was varied from 25 to 300 μm, and the inclination angles of micro-groove were measured as 90° and 45°. The tribological properties of the smooth and textured surfaces with micro-grooves were investigated by reciprocating ball-on-flat tests against Al2O3 ceramic balls under starved oil lubricated conditions. Results showed that the spacing of micro-grooves significantly affected the tribological property. With the increase of micro-groove spacing, the average friction coefficients and wear rates of textured surfaces initially decreased then increased. The tribological performance also depended on the inclination angles of micro-grooves. Among the investigated patterns, the micro-grooves perpendicular to the sliding direction exhibited the lowest average friction coefficient and wear rate to a certain extent. Femtosecond laser-induced surface texturing may remarkably improve friction and wear properties if the micro-grooves were properly distributed.

Experimental study on the flow/ heat transfer performance of micro-scale pin fin coating with super-hydrophobic surface adding Nano particle

NASA Astrophysics Data System (ADS)

Hua, Junye; Duan, Yuanyuan; Li, Gui; Xu, Qiong; Li, Dong; Wu, Wei; Zhao, Xiaobao; Qiu, Delai

2018-02-01

The experimental studies on heat transfer and flow resistance characteristics of ellipse-shape micro pin fin have been conducted which is drafted with hydrophobic material, holding the various contact angles fulfilled by adjusting the amount of Nano particle. The results show that with the increases of contact angle(83°,99.5°, 119.5°and 151.5°), the bottom wall temperature rises under the same flow rate. Under a certain heating condition with heating power as 100 W, the average convective heat transfer coefficient decreases with the increase of contact angle with the same Re. The value of Nu for ellipse-shape micro pin fin increases with a higher Re, with the maximum value under experimental condition of Nu as 25. Besides, the friction coefficient of micro pin fin experimental section drafted hydrophobicity treatment significantly decreases, compared with the smooth micro pin fin experimental section (θ = 83°). While the higher contact angle has obvious positive influences on friction coefficient under the same Re. Generally, the flow resistance performance of ellipse-shape micro pin fin drafted with hydrophobic material is better than that without any treatment.

Effect of sliding velocity on the tribological behavior of copper and associated nanostructure development

NASA Astrophysics Data System (ADS)

Emge, Andrew

The unlubricated sliding of metals is important in many mechanical devices covering a wide range of sliding velocities. However, the effect of sliding velocity on the tribological behavior of unlubricated metals has not been widely studied. Similarly, the relationship between microstructures developed at high sliding velocities and tribological behavior has not been studied in depth. Microstructures produced at low sliding velocities have been studied extensively and commonly include nanocrystalline or fine grained material near the sliding surface with heavily deformed microstructures further from the surface. The current research relates two aspects of the sliding friction of ductile metals, the effect of sliding velocity and the production of nanocrystalline tribomaterial. The project focused on the effects of sliding velocity on the frictional behavior of oxygen free high conductivity (OFHC) copper sliding against 440C stainless steel, Nitronic 40 stainless steel, and copper. Low velocity tests were performed with a pin on disk tribometer. High velocity tests were performed with a rotating barrel gas gun (RBGG) which combined impact with sliding. The RBGG provides sliding velocities as high as 5.5 m/s and impact velocities as high as 12 m/s while maintaining sliding times on the order of tens of microseconds. Changes in the coefficient of friction, microstructure, and composition were studied. Surface and subsurface microstructures of the worn samples were characterized with a range of instruments including scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS), focused ion beam (FIB) milling and imaging, transmission electron microscopy (TEM) with EDS, orientation imaging microscopy (OIM), and nanoindentation. In the case of self-mated copper the sliding velocity had little effect on the coefficient of friction for both experimental apparatuses. For the case of copper sliding against 440C stainless steel on the pin on disk system the friction was found to increase with sliding velocity and was strongly influenced by material transfer from the copper to the steel pin. An increase in the coefficient of friction with sliding velocity was observed for the sliding of OFHC copper against Nitronic 40 steel in RBGG tests. The increase in the coefficient of friction was correlated to an increase in subsurface plastic deformation and grain refinement. The growth of the nanocrystalline tribolayer in copper after sliding against 440C stainless steel at varying times was studied at sliding velocities of 0.05 and 1.0 m/s. A sliding velocity of 0.05 m/s produced a consistent nanocrystalline layer in as little as 10 s. The thickness of the nanocrystalline layer grew to an average thickness of 3 microm after 10 ks of sliding, but large variations in thickness were observed. A sliding velocity of 1.0 m/s produced a continuous nanocrystalline layer after 10 s of sliding. Ledges developed on the wear tracks at longer sliding times which greatly influenced the tribolayer thickness making it difficult to quantify. Dynamic recrystallization of the tribolayer also led to difficulties in measuring its thickness.

Determination of mechanical behavior of nanoscale materials using molecular dynamics simulations

NASA Astrophysics Data System (ADS)

Heo, Seongjun

It is important to understand the mechanical properties of nanometer-scale materials for use in such applications as microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS). These properties are difficult to measure directly using experimental methods due to their small sizes. Computational simulations provide important insights that complement experimental data and lead to improved understanding of the mechanical properties of nanometer-scale systems. Molecular dynamics (MD) simulations, which are used to investigate the properties of materials at the atomic scale, is used in my research to determine (1) best thermostat managing way for acceptable mechanical behavior of nanoscale systems; (2) filling effect on the bending and compressive properties of carbon nanotubes (CNTs); (3) vibrational behavior of bridged and cantilevered CNT bombarded by external fluid atoms; (4) frictional behavior of filled CNT bundles and the effect of external molecules on friction; (5) effect of sliding orientations on the tribological properties of polyethylene (PE). In all the simulations the reactive empirical bond-order (REBO) potential combined with the Lennard Jones potential is applied to control inter-atomic interactions. During the MD simulations, thermostats are used to maintain the system temperature at a constant value. Tests indicate that the simulations describe the mechanical behavior of CNTs differently depending on the type of thermostat used, and the relative fraction of the system to which the thermostat is applied. The results indicate that Langevin and velocity rescaling thermostats are more reliable for temperature control than the Nose-Hoover thermostat. In examining CNT bending and compression, the simulations predict filled CNTs are more resistant to external bending and compressive forces than hollow CNTs. The mechanical properties deteriorate with increases in temperature and number of CNT wall defects. MD simulations of the vibrational behavior of bridged and cantilevered CNTs are found to match the results of continuum mechanics calculations. The principal vibration frequency of the CNT is predicted to decrease with increasing nanotube length, gas pressure, and the atomic mass of the external fluid. In studies of CNT tribology, simulations show that two layers of filled CNTs are more resistant to compressive forces and exhibit lower friction coefficients during sliding than unfilled CNTs. The friction coefficient increases with the thickness of the CNT layer due to the increase in effective friction interface. The addition of an external, molecular fluid of benzene molecules is predicted to reduce the friction coefficient of CNTs because of the lubricity of the molecules. Lastly, simulation results illustrate the effect of relative orientation on the tribological properties of polyethylene (PE) sliding surfaces. The friction coefficient of perpendicular sliding is much higher than that of parallel sliding based on the polymer chain orientation. The PE exhibits stick-slip motion during sliding regardless of the sliding orientation. In addition, the PE shows no surface morphology change due to the higher strength of the PE bonds, which is in contrast to the behavior of other polymers, such as polytetrafluoroethylene (PTFE), which exhibits bond breaking and realignment of surface chains along the sliding direction in the less favorable orientation.

Uncertainty Analysis for Oil-Film Interferometry Skin-Friction Measurement Techniques

NASA Technical Reports Server (NTRS)

Naughton, Jonathan W.; Brown, James L.; Merriam, Marshal (Technical Monitor)

1996-01-01

Over the past 20 years, the use of oil-film interferometry to measure the skin friction coefficient (C(sub f) = tau/q where tau is the surface shear stress and q is the dynamic pressure) has increased. Different forms of this oil-film technique with various levels of accuracy and ease of use have been successfully applied in a wide range of flows. The method's popularity is growing due to its relative ease of implementation and minimal intrusiveness as well as an increased demand for C(sub f) measurements. Nonetheless, the accuracy of these methods has not been rigorously addressed to date. Most researchers have simply shown that the skin-friction measurements made using these techniques compare favorably with other measurements and theory, most of which are only accurate to within 5-20%. The use of skin-friction data in the design of commercial aircraft, whose drag at cruise is 50% skin-friction drag, and in the validation of computational fluid dynamics programs warrants better uncertainty estimates. Additional information is contained in the original extended abstract.

Spectroscopic Analysis of Perfluoropolyether Lubricant Degradation During Boundary Lubrication

NASA Technical Reports Server (NTRS)

Herrera-Fierro, Pilar; Shogrin, Bradley A.; Jones, William R., Jr.

1996-01-01

The degradation of a branched perfluoropolyether (PFPE) under boundary lubrication conditions was studied using mu-FTIR and mu-Raman spectroscopies. Stainless steel (440C) discs coated with thin (600A), uniform films of the PFPE were tested in a ball-on-disc apparatus until various levels of friction coefficient were attained. Discs were then examined using the above techniques. When the friction coefficient surpassed the value obtained with an un-lubricated control, the lubricant film had either been physically displaced or partially transformed in to a 'friction polymer'. Infrared analysis of this 'friction polymer' indicated the presence of a polymeric fluorinated acid species (R(sub f)COOH). Raman spectroscopy indicated the presence of amorphous carbon in the wear track and in the friction polymer. Some reaction mechanisms are suggested to explain the results.

Stress sensitivity of fault seismicity: A comparison between limited-offset oblique and major strike-slip faults

USGS Publications Warehouse

Parsons, T.; Stein, R.S.; Simpson, R.W.; Reasenberg, P.A.

1999-01-01

We present a new three-dimensional inventory of the southern San Francisco Bay area faults and use it to calculate stress applied principally by the 1989 M = 7.1 Loma Prieta earthquake and to compare fault seismicity rates before and after 1989. The major high-angle right-lateral faults exhibit a different response to the stress change than do minor oblique (right-lateral/thrust) faults. Seismicity on oblique-slip faults in the southern Santa Clara Valley thrust belt increased where the faults were unclamped. The strong dependence of seismicity change on normal stress change implies a high coefficient of static friction. In contrast, we observe that faults with significant offset (>50-100 km) behave differently; microseismicity on the Hayward fault diminished where right-lateral shear stress was reduced and where it was unclamped by the Loma Prieta earthquake. We observe a similar response on the San Andreas fault zone in southern California after the Landers earthquake sequence. Additionally, the offshore San Gregorio fault shows a seismicity rate increase where right-lateral/oblique shear stress was increased by the Loma Prieta earthquake despite also being clamped by it. These responses are consistent with either a low coefficient of static friction or high pore fluid pressures within the fault zones. We can explain the different behavior of the two styles of faults if those with large cumulative offset become impermeable through gouge buildup; coseismically pressurized pore fluids could be trapped and negate imposed normal stress changes, whereas in more limited offset faults, fluids could rapidly escape. The difference in behavior between minor and major faults may explain why frictional failure criteria that apply intermediate coefficients of static friction can be effective in describing the broad distributions of aftershocks that follow large earthquakes, since many of these events occur both inside and outside major fault zones.

Effect of Ti3SiC2 on Tribological Properties of M50 Matrix Self-Lubricating Composites from 25 to 450 °C

NASA Astrophysics Data System (ADS)

Deng, Xiaobin; Shi, Xiaoliang; Liu, Xiyao; Huang, Yuchun; Yan, Zhao; Yang, Kang; Wang, Yufu

2017-09-01

The tribological performance is a key factor for M50 steel that is widely used in aero-engine main-shaft bearings. In this study, the tribological properties of M50 matrix self-lubricating composites with different contents of Ti3SiC2 against Si3N4 ceramic counterpart are investigated at 15 N-0.2 m/s from 25 to 450 °C. The results showed that M50 with 10 wt.% Ti3SiC2 (MT10) exhibits the lower friction coefficients (0.21-0.78) and less wear rates (1.78-3.14 × 10-6 mm3 N-1 m-1) at 25-450 °C. Especially at 350 °C, MT10 shows the lowest friction coefficient and wear rate owing to the formation of smooth lubricating layer containing Ti3SiC2 and oxides. Ti3SiC2 and compacted Ti-Si-oxides are uniformly distributed in the lubricating layer, which can well improve the anti-friction and anti-wear performance of MT10. The mechanically mixed layer containing massive Ti3SiC2 can sustain the lubricating layer, resulting in the increase of anti-wear performance of MT10. MT10 could be applied under the practical conditions of friction and wear for its outstanding anti-friction and anti-wear performance.

A Real-Time Method for Estimating Viscous Forebody Drag Coefficients

NASA Technical Reports Server (NTRS)

Whitmore, Stephen A.; Hurtado, Marco; Rivera, Jose; Naughton, Jonathan W.

2000-01-01

This paper develops a real-time method based on the law of the wake for estimating forebody skin-friction coefficients. The incompressible law-of-the-wake equations are numerically integrated across the boundary layer depth to develop an engineering model that relates longitudinally averaged skin-friction coefficients to local boundary layer thickness. Solutions applicable to smooth surfaces with pressure gradients and rough surfaces with negligible pressure gradients are presented. Model accuracy is evaluated by comparing model predictions with previously measured flight data. This integral law procedure is beneficial in that skin-friction coefficients can be indirectly evaluated in real-time using a single boundary layer height measurement. In this concept a reference pitot probe is inserted into the flow, well above the anticipated maximum thickness of the local boundary layer. Another probe is servomechanism-driven and floats within the boundary layer. A controller regulates the position of the floating probe. The measured servomechanism position of this second probe provides an indirect measurement of both local and longitudinally averaged skin friction. Simulation results showing the performance of the control law for a noisy boundary layer are then presented.

Study on Material Parameters Identification of Brain Tissue Considering Uncertainty of Friction Coefficient

NASA Astrophysics Data System (ADS)

Guan, Fengjiao; Zhang, Guanjun; Liu, Jie; Wang, Shujing; Luo, Xu; Zhu, Feng

2017-10-01

Accurate material parameters are critical to construct the high biofidelity finite element (FE) models. However, it is hard to obtain the brain tissue parameters accurately because of the effects of irregular geometry and uncertain boundary conditions. Considering the complexity of material test and the uncertainty of friction coefficient, a computational inverse method for viscoelastic material parameters identification of brain tissue is presented based on the interval analysis method. Firstly, the intervals are used to quantify the friction coefficient in the boundary condition. And then the inverse problem of material parameters identification under uncertain friction coefficient is transformed into two types of deterministic inverse problem. Finally the intelligent optimization algorithm is used to solve the two types of deterministic inverse problems quickly and accurately, and the range of material parameters can be easily acquired with no need of a variety of samples. The efficiency and convergence of this method are demonstrated by the material parameters identification of thalamus. The proposed method provides a potential effective tool for building high biofidelity human finite element model in the study of traffic accident injury.

Determination of oral mucosal Poisson's ratio and coefficient of friction from in-vivo contact pressure measurements.

PubMed

Chen, Junning; Suenaga, Hanako; Hogg, Michael; Li, Wei; Swain, Michael; Li, Qing

2016-01-01

Despite their considerable importance to biomechanics, there are no existing methods available to directly measure apparent Poisson's ratio and friction coefficient of oral mucosa. This study aimed to develop an inverse procedure to determine these two biomechanical parameters by utilizing in vivo experiment of contact pressure between partial denture and beneath mucosa through nonlinear finite element (FE) analysis and surrogate response surface (RS) modelling technique. First, the in vivo denture-mucosa contact pressure was measured by a tactile electronic sensing sheet. Second, a 3D FE model was constructed based on the patient CT images. Third, a range of apparent Poisson's ratios and the coefficients of friction from literature was considered as the design variables in a series of FE runs for constructing a RS surrogate model. Finally, the discrepancy between computed in silico and measured in vivo results was minimized to identify the best matching Poisson's ratio and coefficient of friction. The established non-invasive methodology was demonstrated effective to identify such biomechanical parameters of oral mucosa and can be potentially used for determining the biomaterial properties of other soft biological tissues.

Dependence of frictional strength on compositional variations of Hayward fault rock gouges

USGS Publications Warehouse

Morrow, Carolyn A.; Moore, Diane E.; Lockner, David A.

2010-01-01

The northern termination of the locked portion of the Hayward Fault near Berkeley, California, is found to coincide with the transition from strong Franciscan metagraywacke to melange on the western side of the fault. Both of these units are juxtaposed with various serpentinite, gabbro and graywacke units to the east, suggesting that the gouges formed within the Hayward Fault zone may vary widely due to the mixing of adjacent rock units and that the mechanical behavior of the fault would be best modeled by determining the frictional properties of mixtures of the principal rock types. To this end, room temperature, water-saturated, triaxial shearing tests were conducted on binary and ternary mixtures of fine-grained gouges prepared from serpentinite and gabbro from the Coast Range Ophiolite, a Great Valley Sequence graywacke, and three different Franciscan Complex metasedimentary rocks. Friction coefficients ranged from 0.36 for the serpentinite to 0.84 for the gabbro, with four of the rock types having coefficients of friction ranging from 0.67-0.84. The friction coefficients of the mixtures can be predicted reliably by a simple weighted average of the end-member dry-weight percentages and strengths for all samples except those containing serpentinite. For the serpentinite mixtures, a linear trend between end-member values slightly overestimates the coefficients of friction in the midcomposition ranges. The range in strength for these rock admixtures suggests that both theoretical and numerical modeling of the fault should attempt to account for variations in rock and gouge properties.

Modeling of Stick-Slip Behavior in Sheared Granular Fault Gouge Using the Combined Finite-Discrete Element Method

DOE Office of Scientific and Technical Information (OSTI.GOV)

Gao, Ke; Euser, Bryan J.; Rougier, Esteban

Sheared granular layers undergoing stick-slip behavior are broadly employed to study the physics and dynamics of earthquakes. In this paper, a two-dimensional implementation of the combined finite-discrete element method (FDEM), which merges the finite element method (FEM) and the discrete element method (DEM), is used to explicitly simulate a sheared granular fault system including both gouge and plate, and to investigate the influence of different normal loads on seismic moment, macroscopic friction coefficient, kinetic energy, gouge layer thickness, and recurrence time between slips. In the FDEM model, the deformation of plates and particles is simulated using the FEM formulation whilemore » particle-particle and particle-plate interactions are modeled using DEM-derived techniques. The simulated seismic moment distributions are generally consistent with those obtained from the laboratory experiments. In addition, the simulation results demonstrate that with increasing normal load, (i) the kinetic energy of the granular fault system increases; (ii) the gouge layer thickness shows a decreasing trend; and (iii) the macroscopic friction coefficient does not experience much change. Analyses of the slip events reveal that, as the normal load increases, more slip events with large kinetic energy release and longer recurrence time occur, and the magnitude of gouge layer thickness decrease also tends to be larger; while the macroscopic friction coefficient drop decreases. Finally, the simulations not only reveal the influence of normal loads on the dynamics of sheared granular fault gouge, but also demonstrate the capabilities of FDEM for studying stick-slip dynamic behavior of granular fault systems.« less

Modeling of Stick-Slip Behavior in Sheared Granular Fault Gouge Using the Combined Finite-Discrete Element Method

DOE PAGES

Gao, Ke; Euser, Bryan J.; Rougier, Esteban; ...

2018-06-20

Sheared granular layers undergoing stick-slip behavior are broadly employed to study the physics and dynamics of earthquakes. In this paper, a two-dimensional implementation of the combined finite-discrete element method (FDEM), which merges the finite element method (FEM) and the discrete element method (DEM), is used to explicitly simulate a sheared granular fault system including both gouge and plate, and to investigate the influence of different normal loads on seismic moment, macroscopic friction coefficient, kinetic energy, gouge layer thickness, and recurrence time between slips. In the FDEM model, the deformation of plates and particles is simulated using the FEM formulation whilemore » particle-particle and particle-plate interactions are modeled using DEM-derived techniques. The simulated seismic moment distributions are generally consistent with those obtained from the laboratory experiments. In addition, the simulation results demonstrate that with increasing normal load, (i) the kinetic energy of the granular fault system increases; (ii) the gouge layer thickness shows a decreasing trend; and (iii) the macroscopic friction coefficient does not experience much change. Analyses of the slip events reveal that, as the normal load increases, more slip events with large kinetic energy release and longer recurrence time occur, and the magnitude of gouge layer thickness decrease also tends to be larger; while the macroscopic friction coefficient drop decreases. Finally, the simulations not only reveal the influence of normal loads on the dynamics of sheared granular fault gouge, but also demonstrate the capabilities of FDEM for studying stick-slip dynamic behavior of granular fault systems.« less

Effect of Applied Load and Sliding Speed on Tribological Behavior of TiAl-Based Self-Lubricating Composites

NASA Astrophysics Data System (ADS)

Liu, Xiyao; Shen, Qiao; Shi, Xiaoliang; Zou, Jialiang; Huang, Yuchun; Zhang, Ao; Yan, Zhao; Deng, Xiaobin; Yang, Kang

2018-01-01

This article was dedicated to explore the combined lubrication of silver, MoS2 and carbon nanotubes (CNTs) based on the changes in applied loads and sliding speeds. The results showed that the formed lubricating films played the major role in undertaking the equivalent stress, as well as effectively reduced friction resistance and material loss. It led to small friction coefficient and less wear rate at 1.2 m/s. At 1.2 m/s-16 N, an integrated lubricating film containing Ag, CNTs and MoS2 was continuously formed, which well provided the excellent lubricating property, resulting in lower friction coefficient (0.19) and less wear rate (1.56 × 10-5 mm3/N m). The formation of Ag and CNTs enriched islands acted as the bearing areas and played the major role in resisting friction resistance. Meanwhile, solid lubricant MoS2 was enriched in the lubricating film and effectively protected lubricating film from being destroyed, resulting in small friction coefficient and less wear rate at 1.2 m/s-16 N.

Observation instrument of dynamic frictional interface of gel engineering materials with polarized optical microscopic

NASA Astrophysics Data System (ADS)

Yamada, Naoya; Wada, Masato; Kabir, M. Hasnat; Gong, Jin; Furukawa, Hidemitsu

2013-03-01

Gels are soft and wet materials that differ from hard and dry materials like metals, plastics and ceramics. These have some unique characteristic such as low frictional properties, high water content and materials permeability. A decade earlier, DN gels having a mechanical strength of 30MPa of the maximum breaking stress in compression was developed and it is a prospective material as the biomaterial of the human body. Indeed it frictional coefficient and mechanical strength are comparable to our cartilages. In this study, we focus on the dynamic frictional interface of hydrogels and aim to develop a new apparatus with a polarization microscope for observation. The dynamical interface is observed by the friction of gel and glass with hudroxypropylcellulose (HPC) polymer solution sandwiching. At the beginning, we rubbed hydrogel and glass with HPC solution sandwiching on stage of polarization microscope. Second step, we designed a new system which combined microscope with friction measuring machine. The comparison between direct observation with this instrument and measurement of friction coefficient will become a foothold to elucidate distinctive frictional phenomena that can be seen in soft and wet materials.

Anisotropic frictional heat dissipation in cyclotrimethylene trinitramine

NASA Astrophysics Data System (ADS)

Rajak, Pankaj; Kalia, Rajiv; Nakano, Aiichiro; Vashishta, Priya

Anisotropic frictional response and corresponding heat dissipation from different crystallographic planes of RDX crystal is studied using molecular dynamics simulations. The effect of frictional force on the nature of damage and system temperature is monitored along different directions on primary slip plane, (010), of RDX and on non-slip planes, (100) and (001). The correlation between the friction coefficient, deformation and the frictional heating in these system is determined. It is observed that friction coefficients on slip planes are smaller than those of non-slip planes. In response to friction on slip plane, RDX crystal deforms via dislocation formation and shows less heating. On non-slip planes due to the inability of the system to deform by dislocation formation, large temperature rise is observed in the system just below the contact area of two surfaces. Frictional sliding on non-slip planes also lead to the formation of damage zone just below the contact area of two surfaces due to the change in RDX ring conformation from chair to boat/half-boat. This research is supported by the AFOSR Grant: FA9550-16- 1-0042.

On the residual stress modeling of shot-peened AISI 4340 steel: finite element and response surface methods

NASA Astrophysics Data System (ADS)

Asgari, Ali; Dehestani, Pouya; Poruraminaie, Iman

2018-02-01

Shot peening is a well-known process in applying the residual stress on the surface of industrial parts. The induced residual stress improves fatigue life. In this study, the effects of shot peening parameters such as shot diameter, shot speed, friction coefficient, and the number of impacts on the applied residual stress will be evaluated. To assess these parameters effect, firstly the shot peening process has been simulated by finite element method. Then, effects of the process parameters on the residual stress have been evaluated by response surface method as a statistical approach. Finally, a strong model is presented to predict the maximum residual stress induced by shot peening process in AISI 4340 steel. Also, the optimum parameters for the maximum residual stress are achieved. The results indicate that effect of shot diameter on the induced residual stress is increased by increasing the shot speed. Also, enhancing the friction coefficient magnitude always cannot lead to increase in the residual stress.

Plume Dispersion over Idealized Urban-liked Roughness with Height Variation: an LES Approach

NASA Astrophysics Data System (ADS)

Wong, Colman Ching Chi; Liu, Chun-Ho

2013-04-01

Human activities (e.g. vehicular emission) are the primary pollutant sources affecting the health and living quality of stakeholders in modern compact cities. Gaussian plume dispersion model is commonly used for pollutant distribution estimate that works well over rural areas with flat terrain. However, its major parameters, dispersion coefficients, exclude the effect of surface roughness that unavoidably prone to error handling the pollutant transport in the urban boundary layer (UBL) over building roughness. Our recent large-eddy simulation (LES) has shown that urban surfaces affect significantly the pollutant dispersion over idealized, identical two-dimensional (2D) street canyons of uniform height. As an extension to our on-going effort, this study is conceived to investigate how rough urban surfaces, which are constructed by 2D street canyons of non-uniform height, modify the UBL pollutant dispersion . A series of LESs with idealized roughness elements of non-uniform heights were performed in neutral stratification. Building models with two different heights were placed alternatively in the computational domain to construct 2D street canyons in cross flows. The plume dispersion from a ground-level passive pollutant source over more realistic urban areas was then examined. Along with the existing building-height-to-street-width (aspect) ratio (AR), a new parameter, building-height variability (BHV), is used to measure the building height unevenness. Four ARs (1, 0.5, 0.25 and 0.125) and three BHVs (20%, 40% and 60%) were considered in this study. Preliminary results show that BHV greatly increases the aerodynamic roughness of the hypothetical urban surfaces for narrow street canyons. Analogous to our previous findings, the air exchange rate (ACH) of street canyons increases with increasing friction factor, implying that street-level ventilation could be improved by increasing building roughness via BHV. In addition, the parameters used in dispersion coefficient estimates are related to the friction factor in the way similar to that of uniform street canyons, i.e. they are linear functions of friction factor when the roughness is small and become insensitive to friction factor thereafter over very rough surfaces. It is thus suggested that aerodynamic resistance is the key factor affecting the air quality in urban areas. Moreover, the friction factor could be used to parameterize the dispersion coefficients over different roughness elements.

Consequences of the presence of a weak fault on the stress and strain within an active margin

NASA Astrophysics Data System (ADS)

Conin, M.; Henry, P.; Godard, V.; Bourlange, S.

2009-12-01

Accreting margins often display an outer thrust and fold belt and an inner forearc domain overlying the subduction plate. Assuming that this overlying material behaves as Coulomb material, the outer wedge and the inner wedge are classically approximated as a critical state and a stable state Coulomb wedge, respectively. Critical Coulomb wedge theory can account for the transition from wedge to forearc. However, it cannot be used to determine the state of stress in the transition zone, nor the consequences of a discontinuity within the margin. The presence of a discontinuity such as a splay fault having a low effective friction coefficient should affect the stress state within the wedge, at least locally around the splay fault. Moreover, the effective friction coefficient of the seismogenic zone is expected to vary during the seismic cycle, and this may influence the stability of the Coulomb wedges. We use the ADELI finite element code (Chery and Hassani, 2000) to model the quasi-static stress and strain of a decollement and splay fault system, within a two dimensional elasto-plastic wedge with Drucker-Prager rheology. The subduction plane, the basal decollement of the accretionary wedge and the splay fault are modeled with contact elements. The modeled margin comprises an inner and an outer domain with distinct tapers and basal friction coefficients. For a given splay fault geometry, we evaluate the friction coefficient threshold for splay fault activation as a function of the basal friction coefficients, and examine the consequences of motion along the splay fault on stress and strain within the wedge and on the surface slope at equilibrium. Friction coefficients are varied in time to mimic the consequence of the seismic cycle on the static stress state and strain distribution. Results show the possibility of coexistence of localized extensional regime above the splay fault within a regional compressional regime. Such coexistence is consistent with stress orientation estimation made from breakouts in the Nankai accretionary prim (Kinoshita et al, 2009).

Ice Friction in the Sport of Bobsleigh

NASA Astrophysics Data System (ADS)

Poirier, Louis

The primary objective of this work is to examine the effect of the bobsleigh runner profile on ice / runner friction. The work is centered on a computational model (F.A.S.T. 3.2b) which calculates the coefficient of friction between a steel blade and ice. The first step was to analyze runners used in the sport of bobsleigh. This analysis was performed using a handheld rocker gauge, a device used in speed skating. The size of the device was optimized for hockey, short and long track speed skating, and bobsleigh. A number of runners were measured using the gauge and it was found that the portion of the runner contacting the ice generally has a rocker value of (20--50) m. Next, the hardness of athletic ice surfaces was analyzed. The ice hardness was determined by dropping steel balls varying in mass from (8--540) g onto the ice surface, from a height of (0.3--1.2) m, and measuring the diameter of the indentation craters. The ice hardness was found to be P¯(T) = ((--0.6 +/- 0.4) T + 14.7 +/- 2.1) MPa and the elastic recovery of the ice surface was found to be negligible. The F.A.S.T. model was adapted from a speed skate model to calculate the coefficient of friction between a bobsleigh runner and a flat ice surface. The model predicts that maximum velocities are obtained for temperatures between --10 and --20°C, in agreement with observations on the Calgary bobsleigh track. The model for flat ice suggests that the flattest runners produce the lowest coefficient of friction and that the rocker affects friction more than the cross-sectional radius. The coefficient of friction between runners and ice and the drag performance of 2-men bobsleighs were determined from radar speed measurements taken at the Calgary Olympic Oval and at Canada Olympic Park: at the Ice House and on the bobsleigh track during a World Cup competition. The mean coefficient of friction was found to be mu = (5.3 +/- 2.0) x 10--3 and the mean drag performance was CdA = (0.18 +/- 0.02) m2.

The Mohr-Coulomb criterion for intact rock strength and friction - a re-evaluation and consideration of failure under polyaxial stresses

NASA Astrophysics Data System (ADS)

Hackston, A.; Rutter, E.

2015-12-01

Abstract Darley Dale and Pennant sandstones were tested under conditions of both axisymmetric shortening and extension normal to bedding. These are the two extremes of loading under polyaxial stress conditions. Failure under generalized stress conditions can be predicted from the Mohr-Coulomb failure criterion under axisymmetric compression conditions provided the best form of polyaxial failure criterion is known. The sandstone data are best reconciled using the Mogi (1967) empirical criterion. Fault plane orientations produced vary greatly with respect to the maximum compression direction in the two loading configurations. The normals to the Mohr-Coulomb failure envelopes do not predict the orientations of the fault planes eventually produced. Frictional sliding on variously inclined sawcuts and failure surfaces produced in intact rock samples was also investigated. Friction coefficient is not affected by fault plane orientation in a given loading configuration, but friction coefficients in extension were systematically lower than in compression for both rock types and could be reconciled by a variant on the Mogi (1967) failure criterion. Friction data for these and other porous sandstones accord well with the Byerlee (1977) generalization about rock friction being largely independent of rock type. For engineering and geodynamic modelling purposes, the stress-state dependent friction coefficient should be used for sandstones, but it is not known to what extent this might apply to other rock types.

Turning behaviour depends on frictional damping in the fruit fly Drosophila.

PubMed

Hesselberg, Thomas; Lehmann, Fritz-Olaf

2007-12-01

Turning behaviour in the fruit fly Drosophila depends on several factors including not only feedback from sensory organs and muscular control of wing motion, but also the mass moments of inertia and the frictional damping coefficient of the rotating body. In the present study we evaluate the significance of body friction for yaw turning and thus the limits of visually mediated flight control in Drosophila, by scoring tethered flies flying in a flight simulator on their ability to visually compensate a bias on a moving object and a visual background panorama at different simulated frictional dampings. We estimated the fly's natural damping coefficient from a numerical aerodynamic model based on both friction on the body and the flapping wings during saccadic turning. The model predicts a coefficient of 54 x 10(-12) Nm s, which is more than 100-times larger than the value estimated from a previous study on the body alone. Our estimate suggests that friction plays a larger role for yaw turning in Drosophila than moments of inertia. The simulator experiments showed that visual performance of the fruit fly collapses near the physical conditions estimated for freely flying animals, which is consistent with the suggested role of the halteres for flight stabilization. However, kinematic analyses indicate that the measured loss of flight control might be due predominantly to the limited fine control in the fly's steering muscles below a threshold of 1-2 degrees stroke amplitude, rather than resulting from the limits of visual motion detection by the fly's compound eyes. We discuss the impact of these results and suggest that the elevated frictional coefficient permits freely flying fruit flies to passively terminate rotational body movements without producing counter-torque during the second half of the saccadic turning manoeuvre.

Fault Frictional Stability in a Nuclear Waste Repository

NASA Astrophysics Data System (ADS)

Orellana, Felipe; Violay, Marie; Scuderi, Marco; Collettini, Cristiano

2016-04-01

Exploitation of underground resources induces hydro-mechanical and chemical perturbations in the rock mass. In response to such disturbances, seismic events might occur, affecting the safety of the whole engineering system. The Mont Terri Rock Laboratory is an underground infrastructure devoted to the study of geological disposal of nuclear waste in Switzerland. At the site, it is intersected by large fault zones of about 0.8 - 3 m in thickness and the host rock formation is a shale rock named Opalinus Clay (OPA). The mineralogy of OPA includes a high content of phyllosilicates (50%), quartz (25%), calcite (15%), and smaller proportions of siderite and pyrite. OPA is a stiff, low permeable rock (2×10-18 m2), and its mechanical behaviour is strongly affected by the anisotropy induced by bedding planes. The evaluation of fault stability and associated fault slip behaviour (i.e. seismic vs. aseismic) is a major issue in order to ensure the long-term safety and operation of the repository. Consequently, experiments devoted to understand the frictional behaviour of OPA have been performed in the biaxial apparatus "BRAVA", recently developed at INGV. Simulated fault gouge obtained from intact OPA samples, were deformed at different normal stresses (from 4 to 30 MPa), under dry and fluid-saturated conditions. To estimate the frictional stability, the velocity-dependence of friction was evaluated during velocity steps tests (1-300 μm/s). Slide-hold-slide tests were performed (1-3000 s) to measure the amount of frictional healing. The collected data were subsequently modelled with the Ruina's slip dependent formulation of the rate and state friction constitutive equations. To understand the deformation mechanism, the microstructures of the sheared gouge were analysed. At 7 MPa normal stress and under dry conditions, the friction coefficient decreased from a peak value of μpeak,dry = 0.57 to μss,dry = 0.50. Under fluid-saturated conditions and same normal stress, the friction coefficient decreased from a peak value of μpeak,sat = 0.45 to μss,sat = 0.34. Additionally, it has been observed that the weakening distance Dw is smaller under fluid- saturated conditions (˜4 mm) compared to dry conditions (˜6 mm). Results showed a linear decrease of both peak friction and steady state friction when normal stress increases. When fluid- saturation degree of gouges is reduced, gouge samples underwent a transition from velocity strengthening to velocity weakening behaviour, thus indicating a potentially unstable frictional behaviour of the fault. Furthermore, under both saturated and dry conditions, the frictional healing rate showed a low recovery of the friction coefficient under different holding times. Our experiments indicate that the frictional behaviour of Opalinus Clay is characterized by complex processes depending upon normal stress, sliding velocity, and saturation degree of the samples. This complexity highlights the need for further experiments in order to better evaluate the seismic risk during long-term nuclear waste disposal within the OPA clay formation.

Analysis of a Compressible Fluid Soft Recoil (CFSR) Concept Applied to a 155 MM Howitzer

DTIC Science & Technology

1979-03-01

Nitrile or Buna-N ( NBR ) rubber with ’ backup rings of nylotron. HITRILE NVLOTRON Piston seals An unresolved problem is that the coefficient of...fluid at atmospheric pressure Poisson’s ratio for Nitrile rubber dynamic coefficient of friction for rubber mass of recoiling parts weight of...Greene, tweed 5 Co. Palmetto catalog.) 43 [i^ - 0.50 = coefficient of friction (An approximate figure for rubber supplied by RIA Rubber

Wear behavior of pressable lithium disilicate glass ceramic.

PubMed

Peng, Zhongxiao; Izzat Abdul Rahman, Muhammad; Zhang, Yu; Yin, Ling

2016-07-01

This article reports effects of surface preparation and contact loads on abrasive wear properties of highly aesthetic and high-strength pressable lithium disilicate glass-ceramics (LDGC). Abrasive wear testing was performed using a pin-on-disk device in which LDGC disks prepared with different surface finishes were against alumina pins at different contact loads. Coefficients of friction and wear volumes were measured as functions of initial surface finishes and contact loads. Wear-induced surface morphology changes in both LDGC disks and alumina pins were characterized using three-dimensional laser scanning microscopy, scanning electron microscopy, and energy dispersive X-ray spectroscopy. The results show that initial surface finishes of LDGC specimens and contact loads significantly affected the friction coefficients, wear volumes and wear-induced surface roughness changes of the material. Both wear volumes and friction coefficients of LDGC increased as the load increased while surface roughness effects were complicated. For rough LDGC surfaces, three-body wear was dominant while for fine LDGC surfaces, two-body abrasive wear played a key role. Delamination, plastic deformation, and brittle fracture were observed on worn LDGC surfaces. The adhesion of LDGC matrix materials to alumina pins was also discovered. This research has advanced our understanding of the abrasive wear behavior of LDGC and will provide guidelines for better utilization and preparation of the material for long-term success in dental restorations. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 104B: 968-978, 2016. © 2015 Wiley Periodicals, Inc.

In vivo measured joint friction in hip implants during walking after a short rest.

PubMed

Damm, Philipp; Bender, Alwina; Duda, Georg; Bergmann, Georg

2017-01-01

It has been suspected that friction in hip implants is higher when walking is initiated after a resting period than during continuous movement. It cannot be excluded that such increased initial moments endanger the cup fixation in the acetabulum, overstress the taper connections in the implant or increase wear. To assess these risks, the contact forces, friction moments and friction coefficients in the joint were measured in vivo in ten subjects. Instrumented hip joint implants with telemetric data transmission were used to access the contact loads between the cup and head during the first steps of walking after a short rest. The analysis demonstrated that the contact force is not increased during the first step. The friction moment in the joint, however, is much higher during the first step than during continuous walking. The moment increases throughout the gait cycle were 32% to 143% on average and up to 621% individually. The high initial moments will probably not increase wear by much in the joint. However, comparisons with literature data on the fixation resistance of the cup against moments made clear that the stability can be endangered. This risk is highest during the first postoperative months for cementless cups with insufficient under-reaming. The high moments after a break can also put taper connections between the head and neck and neck and shaft at a higher risk. During continuous walking, the friction moments individually were extremely varied by factors of 4 to 10. Much of this difference is presumably caused by the varying lubrication properties of the synovia. These large moment variations can possibly lead to friction-induced temperature increases during walking, which are higher than the 43.1°C which have previously been observed in a group of only five subjects.

A comparison of the lubrication behavior of whey protein model foods using tribology in linear and elliptical movement.

PubMed

Campbell, Caroline L; Foegeding, E Allen; van de Velde, Fred

2017-08-01

Lubrication is an important factor in the sensory evaluation of food products. Tribology provides a theoretical framework and instrumental methods for evaluating frictional properties between two moving surfaces and the lubrication behavior of products between these surfaces. Relating frictional measurements to sensory properties detected during oral processing requires careful and pertinent choices in surface materials and testing conditions. The aims of this study were to investigate: (a) differences in lubrication behavior of a range of food textures and (b) the differences between linear and elliptical movement and added saliva to understand the contribution of food structure to friction. Six whey protein model food samples, ranging in texture from fluid to semisolid to soft solid, were analyzed using a pin on disk tribometer to determine the coefficient of friction (COF) across a range of sliding speeds. The samples were analyzed in their initial form and post-oral processing (n = 4) in both linear and elliptical movements. Elliptical movement slightly decreased coefficients of friction and extended the shape of the friction curve. Increases in test food viscosity decreased the COF but differences in viscosity were not apparent when test foods were mixed with saliva. Data correction for viscosity shifted the friction curves horizontally, indicating that lubrication had a greater impact upon friction than viscosity. This study provides initial insights for further comparison of linear and elliptical movement with a variety of sample compositions. Sensory perception of smoothness and creaminess are often major contributors to overall hedonic food liking and are a major reason why products high in fat and sugar are more highly preferred over other foods. These parameters are influenced by friction and lubrication between the tongue, palate, teeth, food products, and saliva during oral processing. Tribology provides an instrumental method to evaluate friction between moving surfaces that mimic oral surfaces and the lubrication behavior of foods. Trends in frictional measurements can be correlated with sensory ratings of the same foods to better understand why preferences exist for certain foods or food compositions and how to effectively improve the acceptability and enjoyment of healthier foods. © 2017 Wiley Periodicals, Inc.

Friction behavior for clay minerals during dehydration process: implication for unstable friction at shallow portion along subducting plate

NASA Astrophysics Data System (ADS)

Kubo, T.; Katayama, I.

2016-12-01

Along plate boundary subduction thrusts, the transformation of smectite to illite within fault gouge at temperatures around 100 - 200 °C is one of the key mineralogical changes thought to control the updip limit of seismicity (Hyndman et al., 1997). Since hydration state of clay minerals is possible to vary from moment to moment in nature, it is important to investigate the effect of dehydration and hydrate state on frictional properties with progression of a removal of water is rare. In this study, we focus on the effect of dehydration of water on the frictional properties of clay minerals by temperature-rising test. For the friction experiments, starting materials we used are Ca-montmorillonite, which were placed on the simulated fault surface and two side blocks were placed together to produce a double-direct shear configuration. The sample assembly was heated by an external furnace up to 400 °C that is monitored by thermocouples located in the central part of sample assembly. After steady-state friction at room temperature we started to elevate the temperature around the specimen at a constant heating rate of 1, 3, and 10 °C/min. Ca-montmorillonite gouge showed unique friction behavior development as elevated temperature, which is divided into three stages; (1) friction coefficient decreased at relative low temperature, (2) friction coefficient increased at middle temperature, and (3) stick-slip behavior occurred at high temperature. Stick-slip behavior as elevated temperature implies to have a potential of velocity weakening behavior. Observed stick-slip behavior occurs at a temperature of 320 °C, which is extremely higher from a temperature range of occurring dehydration for Ca-montmorillonite (100 - 200 °C). However, at low heating rate the temperature that stick-slip behavior occurs shifted to lower temperature. Our preliminary results suggest that the observed systematical shift suggest that these frictional behavior is likely to be controlled by dehydration reaction kinetics. Dehydration of clay minerals change friction behavior, and play a key role for the occurrence of earthquakes along subducting plate.

Study on friction coefficient of soft soil based on particle flow code

NASA Astrophysics Data System (ADS)

Lei, Xiaohong; Zhang, Zhongwei

2017-04-01

There has no uniform method for determining the micro parameters in particle flow code, and the corresponding formulas obtained by each scholar can only be applied to similar situations. In this paper, the relationship between the micro parameters friction coefficient and macro parameters friction angle is established by using the two axis servo compression as the calibration experiment, and the corresponding formula is fitted to solve the difficulties of determining the PFC micro parameters which provide a reference for determination of the micro parameters of soft soil.

Giant and Tunable Anisotropy of Nanoscale Friction in Graphene

NASA Astrophysics Data System (ADS)

Capaz, Rodrigo; Menezes, Marcos; Almeida, Clara; de Cicco, Marcelo; Achete, Carlos; Fragneaud, Benjamin; Cançado, Luiz Gustavo; Paupitz, Ricardo; Galvão, Douglas; Prioli, Rodrigo

The nanoscale friction between an atomic force microscopy tip and graphene is investigated using friction force microscopy (FFM). During the tip movement, friction forces are observed to increase and then saturate in a highly anisotropic manner. As a result, the friction coefficient of graphene is highly dependent on the scanning direction: Under some conditions, the energy dissipated along the armchair direction can be 80% higher than along the zigzag direction. In comparison, for highly-oriented pyrolitic graphite (HOPG), the friction anisotropy between armchair and zigzag directions is only 15%. This giant friction anisotropy in graphene results from anisotropies in the amplitudes of flexural deformations of the graphene sheet driven by the tip movement, not present in HOPG. The effect can be seen as a novel manifestation of the classical phenomenon of Euler buckling at the nanoscale, which provides the non-linear ingredients that amplify friction anisotropy. Simulations based on a novel version of the 2D Tomlinson model (modified to include the effects of flexural deformations), as well as fully atomistic molecular dynamics simulations and first-principles density-functional theory (DFT) calculations, are able to reproduce and explain the experimental observations.

Wood variables affecting the friction coefficient of spruce pine on steel

Treesearch

Truett J. Lemoine; Charles W. McMillin; Floyd G. Manwiller

1970-01-01

Wood of spruce pine, Pinus glabra Walk., was factorially segregated by moisture content (0, 10, and 18 percent), specific gravity (less than 0.45 and more than 0.45), and extractive content (unextracted and extractive-freE), and the kinetic coefficient of friction on steel (having surface roughness of 9 microinches RMS) determined for tangential...

Coefficient of friction of dry slash pine and southern red oak on three tension-grip facings

Treesearch

T.J. Lemoine; P. Koch

1975-01-01

A urethane material proved to have nine times higher static friction coefficient (0.9) than smooth steel (0.1) on radial and tangential wood surfaces pulled parallel to the grain. It is probably superior to 22O-grit garnet paper or sand coatings for tension-grip facings in lumber testing machines.

Coefficient of friction of dry slash pine and southern red oak on three tension-grip facings

Treesearch

Truett J. Lemoine; Peter Koch

1974-01-01

A urethane material proved to have nine times higher static friction coefficient (0.9) than smooth steel (0.1) on radial and tangential wood surfaces pulled parallel to the grain. It is probably superior to 220-grit garnet paper or sand coatings for tension-grip facings in lumber testing machines.

Determination of the Static Friction Coefficient from Circular Motion

ERIC Educational Resources Information Center

Molina-Bolívar, J. A.; Cabrerizo-Vílchez, M. A.

2014-01-01

This paper describes a physics laboratory exercise for determining the coefficient of static friction between two surfaces. The circular motion of a coin placed on the surface of a rotating turntable has been studied. For this purpose, the motion is recorded with a high-speed digital video camera recording at 240 frames s[superscript-1], and the…

Investigation of the Maximum Spin-Up Coefficients of Friction Obtained During Tests of a Landing Gear Having a Static-Load Rating of 20,000 Pounds

NASA Technical Reports Server (NTRS)

Batterson, Sidney A.

1959-01-01

An experimental investigation was made at the Langley landing loads track to obtain data on the maximum spin-up coefficients of friction developed by a landing gear having a static-load rating of 20,000 pounds. The forward speeds ranged from 0 to approximately 180 feet per second and the sinking speeds, from 2.7 feet per second to 9.4 feet per second. The results indicated the variation of the maximum spin-up coefficient of friction with forward speed and vertical load. Data obtained during this investigation are also compared with some results previously obtained for nonrolling tires to show the effect of forward speed.

Effect of bone-soft tissue friction on ultrasound axial shear strain elastography

NASA Astrophysics Data System (ADS)

Tang, Songyuan; Chaudhry, Anuj; Kim, Namhee; Reddy, J. N.; Righetti, Raffaella

2017-08-01

Bone-soft tissue friction is an important factor affecting several musculoskeletal disorders, frictional syndromes and the ability of a bone fracture to heal. However, this parameter is difficult to determine using non-invasive imaging modalities, especially in clinical settings. Ultrasound axial shear strain elastography is a non-invasive imaging modality that has been used in the recent past to estimate the bonding between different tissue layers. As most elastography methods, axial shear strain elastography is primarily used in soft tissues. More recently, this technique has been proposed to assess the bone-soft tissue interface. In this paper, we investigate the effect of a variation in bone-soft tissue friction coefficient in the resulting axial shear strain elastograms. Finite element poroelastic models of bone specimens exhibiting different bone-soft tissue friction coefficients were created and mechanically analyzed. These models were then imported to an ultrasound elastography simulation module to assess the presence of axial shear strain patterns. In vitro experiments were performed to corroborate selected simulation results. The results of this study show that the normalized axial shear strain estimated at the bone-soft tissue interface is statistically correlated to the bone-soft tissue coefficient of friction. This information may prove useful to better interpret ultrasound elastography results obtained in bone-related applications and, possibly, monitor bone healing.

Effect of bone-soft tissue friction on ultrasound axial shear strain elastography.

PubMed

Tang, Songyuan; Chaudhry, Anuj; Kim, Namhee; Reddy, J N; Righetti, Raffaella

2017-07-12

Bone-soft tissue friction is an important factor affecting several musculoskeletal disorders, frictional syndromes and the ability of a bone fracture to heal. However, this parameter is difficult to determine using non-invasive imaging modalities, especially in clinical settings. Ultrasound axial shear strain elastography is a non-invasive imaging modality that has been used in the recent past to estimate the bonding between different tissue layers. As most elastography methods, axial shear strain elastography is primarily used in soft tissues. More recently, this technique has been proposed to assess the bone-soft tissue interface. In this paper, we investigate the effect of a variation in bone-soft tissue friction coefficient in the resulting axial shear strain elastograms. Finite element poroelastic models of bone specimens exhibiting different bone-soft tissue friction coefficients were created and mechanically analyzed. These models were then imported to an ultrasound elastography simulation module to assess the presence of axial shear strain patterns. In vitro experiments were performed to corroborate selected simulation results. The results of this study show that the normalized axial shear strain estimated at the bone-soft tissue interface is statistically correlated to the bone-soft tissue coefficient of friction. This information may prove useful to better interpret ultrasound elastography results obtained in bone-related applications and, possibly, monitor bone healing.

Sliding Seal Materials for Adiabatic Engines, Phase 2

NASA Technical Reports Server (NTRS)

Lankford, J.; Wei, W.

1986-01-01

An essential task in the development of the heavy-duty adiabatic diesel engine is identification and improvements of reliable, low-friction piston seal materials. In the present study, the sliding friction coefficients and wear rates of promising carbide, oxide, and nitride materials were measured under temperature, environmental, velocity, and loading conditions that are representative of the adiabatic engine environment. In addition, silicon nitride and partially stabilized zirconia disks were ion implanted with TiNi, Ni, Co, and Cr, and subsequently run against carbide pins, with the objective of producing reduced friction via solid lubrication at elevated temperature. In order to provide guidance needed to improve materials for this application, the program stressed fundamental understanding of the mechanisms involved in friction and wear. Electron microscopy was used to elucidate the micromechanisms of wear following wear testing, and Auger electron spectroscopy was used to evaluate interface/environment interactions which seemed to be important in the friction and wear process. Unmodified ceramic sliding couples were characterized at all temperatures by friction coefficients of 0.24 and above. The coefficient at 800 C in an oxidizing environment was reduced to below 0.1, for certain material combinations, by the ion implanation of TiNi or Co. This beneficial effect was found to derive from lubricious Ti, Ni, and Co oxides.

Stick-slip friction and wear of articular joints

PubMed Central

Lee, Dong Woog; Banquy, Xavier; Israelachvili, Jacob N.

2013-01-01

Stick-slip friction was observed in articular cartilage under certain loading and sliding conditions and systematically studied. Using the Surface Forces Apparatus, we show that stick-slip friction can induce permanent morphological changes (a change in the roughness indicative of wear/damage) in cartilage surfaces, even under mild loading and sliding conditions. The different load and speed regimes can be represented by friction maps—separating regimes of smooth and stick-slip sliding; damage generally occurs within the stick-slip regimes. Prolonged exposure of cartilage surfaces to stick-slip sliding resulted in a significant increase of surface roughness, indicative of severe morphological changes of the cartilage superficial zone. To further investigate the factors that are conducive to stick-slip and wear, we selectively digested essential components of cartilage: type II collagen, hyaluronic acid (HA), and glycosaminoglycans (GAGs). Compared with the normal cartilage, HA and GAG digestions modified the stick-slip behavior and increased surface roughness (wear) during sliding, whereas collagen digestion decreased the surface roughness. Importantly, friction forces increased up to 2, 10, and 5 times after HA, GAGs, and collagen digestion, respectively. Also, each digestion altered the friction map in different ways. Our results show that (i) wear is not directly related to the friction coefficient but (ii) more directly related to stick-slip sliding, even when present at small amplitudes, and that (iii) the different molecular components of joints work synergistically to prevent wear. Our results also suggest potential noninvasive diagnostic tools for sensing stick-slip in joints. PMID:23359687

Kalker's algorithm Fastsim solves tangential contact problems with slip-dependent friction and friction anisotropy

NASA Astrophysics Data System (ADS)

Piotrowski, J.

2010-07-01

This paper presents two extensions of Kalker's algorithm Fastsim of the simplified theory of rolling contact. The first extension is for solving tangential contact problems with the coefficient of friction depending on slip velocity. Two friction laws have been considered: with and without recuperation of the static friction. According to the tribological hypothesis for metallic bodies shear failure, the friction law without recuperation of static friction is more suitable for wheel and rail than the other one. Sample results present local quantities inside the contact area (division to slip and adhesion, traction) as well as global ones (creep forces as functions of creepages and rolling velocity). For the coefficient of friction diminishing with slip, the creep forces decay after reaching the maximum and they depend on the rolling velocity. The second extension is for solving tangential contact problems with friction anisotropy characterised by a convex set of the permissible tangential tractions. The effect of the anisotropy has been shown on examples of rolling without spin and in the presence of pure spin for the elliptical set. The friction anisotropy influences tangential tractions and creep forces. Sample results present local and global quantities. Both extensions have been described with the same language of formulation and they may be merged into one, joint algorithm.

Slip rates on San Francisco Bay area faults from anelastic deformation of the continental lithosphere

USGS Publications Warehouse

Geist, E.L.; Andrews, D.J.

2000-01-01

Long-term slip rates on major faults in the San Francisco Bay area are predicted by modeling the anelastic deformation of the continental lithosphere in response to regional relative plate motion. The model developed by Bird and Kong [1994] is used to simulate lithospheric deformation according to a Coulomb frictional rheology of the upper crust and a dislocation creep rheology at depth. The focus of this study is the long-term motion of faults in a region extending from the creeping section of the San Andreas fault to the south up to the latitude of Cape Mendocino to the north. Boundary conditions are specified by the relative motion between the Pacific plate and the Sierra Nevada - Great Valley microplate [Argus and Gordon, 2000]. Rheologic-frictional parameters are specified as independent variables, and prediction errors are calculated with respect to geologic estimates of slip rates and maximum compressive stress directions. The model that best explains the region-wide observations is one in which the coefficient of friction on all of the major faults is less than 0.15, with the coefficient of friction for the San Andreas fault being approximately 0.09, consistent with previous inferences of San Andreas fault friction. Prediction error increases with lower fault friction on the San Andreas, indicating a lower bound of ??SAF > 0.08. Discrepancies with respect to previous slip rate estimates include a higher than expected slip rate along the peninsula segment of the San Andreas fault and a slightly lower than expected slip rate along the San Gregorio fault.

Friction and Wear Properties of Selected Solid Lubricating Films

NASA Technical Reports Server (NTRS)

Miyoshi, Kazuhisa; Iwaki, Masanori; Gotoh, Kenichi; Obara, Shingo; Imagawa, Kichiro

1999-01-01

To evaluate commercially developed solid film lubricants for aerospace bearing applications, we investigated the friction and wear behavior of bonded molybdenum disulfide (MoS2), magnetron-sputtered MoS2 and ion-plated silver films in sliding contact with 6-mm-diameter American Iron and Steel Institute (AISI) 440 C stainless steel balls. Unidirectional sliding friction experiments were conducted with a load of 5.9 N (600 g), a mean Herizian contact pressure of 0.79 GPa maximum 1.19 GPa), and a sliding velocity of 0.2 m/s at room temperature in three environments: ultrahigh vacuum (7x10 (exp -7Pa)), humid air (approx. 20 percent humidity), and dry nitrogen (less than 1 percent humidity). The resultant films were characterized by scanning electron microscopy, energy-dispersive x-ray spectroscopy, and surface profilometry. Marked differences in friction and wear resulted front the environmental conditions and the film materials. The main criteria for judging the performance were coefficient of friction and wear rate, which had to be less than 0.3 and on the order of 10 (exp -6mm exp 3/Nm or less), respectively. The bonded MoS2 and magnetron-sputtered MoS2 films met the criteria in all three environments. Also, the wear rates of the counterpart AISI 440 C stainless steel balls met that criterion in all three environments. The ion-plated silver films met the criteria only in ultrahigh vacuum. In ultrahigh vacuum the bonded MoS2 films were superior. In humid air the bonded MoS2 films had higher coefficient of friction and shorter wear life than did the magnetron-sputtered MoS2 films. The ion-plated silver films had a high coefficient of friction in humid air but relatively low coefficients of friction in the nonoxidative environments. Adhesion and plastic deformation played important roles in all three environments. All sliding involved adhesive transfer of materials.

A Projectile for a Rectangular Barreled Rail Gun

DTIC Science & Technology

1999-12-01

fins Cfb ’ skin friction drag coefficient for nose/body combination modified for equations of motion Cff’ skin friction drag coefficient for fins...occasionally referred to as the last point method, uses a loop , shown in simplified form in Figure (2) as a flow chart. The program loop takes the final... Cfb ’ = CfofCf(Sno/s+Sbs)/ScsS (4.11) Cff = CfffcfSw/Scss (4.12) 2. Form Drag The form drag coefficients are determined by the methods from

Tribological Behavior of Oil-Lubricated Laser Textured Steel Surfaces in Conformal Flat and Non-Conformal Contacts

DOE Office of Scientific and Technical Information (OSTI.GOV)

Kovalchenko, A. M.; Erdemir, A.; Ajayi, O. O.

Changing the surface texture of sliding surfaces is an effective way to manipulate friction and wear of lubricated surfaces. Having realized its potential, we have done very extensive studies on the effects of laser surface texturing (LST, which involves the creation of an array of microdimples on a surface) on friction and wear behavior of oil-lubricated steel surfaces in the early 2000s. In this paper, we reviewed some of our research accomplishments and assessed future directions of the laser texturing field in many diverse industrial applications. Our studies specifically addressed the impact of laser texturing on friction and wear ofmore » both the flat conformal and initial non-conformal point contact configurations using a pin-on-disk test rig under fully-flooded synthetic oil lubricants with different viscosities. Electrical resistance measurement between pin and LST disks was also used to determine the operating lubrication regimes in relation to friction. In conformal contact, we confirmed that LST could significantly expand the operating conditions for hydrodynamic lubrication to significantly much higher loads and slower speeds. In particular, with LST and higher viscosity oils, the low-friction full hydrodynamic regime was shifted to the far left in the Stribeck diagram. Overall, the beneficial effects of laser surface texturing were more pronounced at higher speeds and loads and with higher viscosity oil. LST was also observed to reduce the magnitude of friction coefficients in the boundary regime. For the non-conformal contact configuration, we determined that LST would produce more abrasive wear on the rubbing counterface compared to the untreated surfaces due to a reduction in lubricant fluid film thickness, as well as the highly uneven and rough nature of the textured surfaces. However, this higher initial wear rate has led to faster generation of a conformal contact, and thus transition from the high-friction boundary to lower friction mixed lubrication regime, resulting in a rapid reduction in the friction coefficient with increased ball wear. Higher density of LST, lower oil viscosity, and hardness of counterface steel surface facilitate an increase of the initial wear, which promotes friction reduction. This phenomenon can be beneficial if the initial accelerated wear on the counterface is acceptable in intended applications. This paper summarizes our experimental investigation of the effect of LST on friction properties and lubrication regime transitions in a unidirectional sliding contact.« less

An Ice Track Equipped with Optical Sensors for Determining the Influence of Experimental Conditions on the Sliding Velocity

NASA Astrophysics Data System (ADS)

Lungevics, J.; Jansons, E.; Gross, K. A.

2018-02-01

The ability to slide on ice has previously focused on the measurement of friction coefficient rather than the actual sliding velocity that is affected by it. The performance can only be directly measured by the sliding velocity, and therefore the objective was to design and setup a facility to measure velo-city, and determine how experimental conditions affect it. Optical sensors were placed on an angled ice track to provide sliding velocity measurements along three sections and the velocity for the total sliding distance. Experimental conditions included the surface roughness, ambient temperature and load. The effect of roughness was best reported with a Criterion of Contact that showed a similar sliding velocity for metal blocks abraded with sand paper smoother than 600 grit. Searching for the effect of temperature, the highest sliding velocity coincided with the previously reported lowest coefficient of ice friction. Load showed the greatest velocity increase at temperatures closer to the ice melting point suggesting that in such conditions metal block overcame friction forces more easily than in solid friction. Further research needs to be conducted on a longer ice track, with larger metal surfaces, heavier loads and higher velocities to determine how laboratory experiments can predict real-life situations.

Open System Tribology and Influence of Weather Condition.

PubMed

Lyu, Yezhe; Bergseth, Ellen; Olofsson, Ulf

2016-08-30

The tribology of an open system at temperatures ranging between 3 °C and -35 °C, with and without snow, was investigated using a pin-on-disc tribometer mounted in a temperature-controlled environmental chamber. The relationship between the microstructure and ductility of the materials and the tribology at the contacting surfaces was investigated. The study shows that during continuous sliding, pressure causes snow particles to melt into a liquid-like layer, encouraging the generation of oxide flakes on the contact path. The friction coefficient and wear rate are dramatically reduced through an oxidative friction and wear mechanism. In the absence of snow, the tribological process is controlled by the low temperature brittleness of steel in the temperature range from 3 °C to -15 °C. At these temperatures, cracks are prone to form and extend on the worn surfaces, resulting in the spalling of bulk scraps, which are crushed into debris that increases the friction coefficient and wear rate due to strong abrasion. When the temperature falls to -25 °C, an ice layer condenses on the metal surfaces and relaxes the tribological process in the same way as the added snow particles, which significantly decreases the friction and wear.

Open System Tribology and Influence of Weather Condition

PubMed Central

Lyu, Yezhe; Bergseth, Ellen; Olofsson, Ulf

2016-01-01

The tribology of an open system at temperatures ranging between 3 °C and −35 °C, with and without snow, was investigated using a pin-on-disc tribometer mounted in a temperature-controlled environmental chamber. The relationship between the microstructure and ductility of the materials and the tribology at the contacting surfaces was investigated. The study shows that during continuous sliding, pressure causes snow particles to melt into a liquid-like layer, encouraging the generation of oxide flakes on the contact path. The friction coefficient and wear rate are dramatically reduced through an oxidative friction and wear mechanism. In the absence of snow, the tribological process is controlled by the low temperature brittleness of steel in the temperature range from 3 °C to −15 °C. At these temperatures, cracks are prone to form and extend on the worn surfaces, resulting in the spalling of bulk scraps, which are crushed into debris that increases the friction coefficient and wear rate due to strong abrasion. When the temperature falls to −25 °C, an ice layer condenses on the metal surfaces and relaxes the tribological process in the same way as the added snow particles, which significantly decreases the friction and wear. PMID:27573973

Explicit Solvent Simulations of Friction between Brush Layers of Charged and Neutral Bottle-Brush Macromolecules

DOE Office of Scientific and Technical Information (OSTI.GOV)

Carrillo, Jan-Michael; Brown, W Michael; Dobrynin, Andrey

2012-01-01

We study friction between charged and neutral brush layers of bottle-brush macromolecules using molecular dynamics simulations. In our simulations the solvent molecules were treated explicitly. The deformation of the bottle-brush macromolecules under the shear were studied as a function of the substrate separation and shear stress. For charged bottle-brush layers we study effect of the added salt on the brush lubricating properties to elucidate factors responsible for energy dissipation in charged and neutral brush systems. Our simulations have shown that for both charged and neutral brush systems the main deformation mode of the bottle-brush macromolecule is associated with the backbonemore » deformation. This deformation mode manifests itself in the backbone deformation ratio, , and shear viscosity, , to be universal functions of the Weissenberg number W. The value of the friction coefficient, , and viscosity, , are larger for the charged bottle-brush coatings in comparison with those for neutral brushes at the same separation distance, D, between substrates. The additional energy dissipation generated by brush sliding in charged bottle-brush systems is due to electrostatic coupling between bottle-brush and counterion motion. This coupling weakens as salt concentration, cs, increases resulting in values of the viscosity, , and friction coefficient, , approaching corresponding values obtained for neutral brush systems.« less

Tribological Properties of Water-lubricated Rubber Materials after Modification by MoS2 Nanoparticles

PubMed Central

Dong, Conglin; Yuan, Chengqing; Wang, Lei; Liu, Wei; Bai, Xiuqin; Yan, Xinping

2016-01-01

Frictional vibration and noise caused by water-lubricated rubber stern tube bearings, which are generated under extreme conditions, severely threaten underwater vehicles’ survivability and concealment performance. This study investigates the effect of flaky and spherical MoS2 nanoparticles on tribological properties and damping capacity of water-lubricated rubber materials, with the aim of decreasing frictional noise. A CBZ-1 tribo-tester was used to conduct the sliding tests between rubber ring-discs and ZCuSn10Zn2 ring-discs with water lubrication. These materials’ typical mechanical properties were analysed and compared. Coefficients of friction (COFs), wear rates, and surface morphologies were evaluated. Frictional noise and critical velocities of generating friction vibration were examined to corroborate above analysis. Results showed that spherical MoS2 nanoparticles enhanced rubber material’s mechanical and tribological properties and, in turn, reduced the friction noise and critical velocity. Flaky MoS2 nanoparticles reduced COF but did not enhance their mechanical properties, i.e., the damping capacity, wear resistance property; thus, these nanoparticles did not reduce the critical velocity obviously, even though increased the frictional noise at high load. The knowledge gained in the present work will be useful for optimizing friction pairs under extreme conditions to decrease frictional noise of water-lubricated rubber stern tube bearings. PMID:27713573

Coefficient of Friction Patterns Can Identify Damage in Native and Engineered Cartilage Subjected to Frictional-Shear Stress

PubMed Central

Whitney, G. A.; Mansour, J. M.; Dennis, J. E.

2015-01-01

The mechanical loading environment encountered by articular cartilage in situ makes frictional-shear testing an invaluable technique for assessing engineered cartilage. Despite the important information that is gained from this testing, it remains under-utilized, especially for determining damage behavior. Currently, extensive visual inspection is required to assess damage; this is cumbersome and subjective. Tools to simplify, automate, and remove subjectivity from the analysis may increase the accessibility and usefulness of frictional-shear testing as an evaluation method. The objective of this study was to determine if the friction signal could be used to detect damage that occurred during the testing. This study proceeded in two phases: first, a simplified model of biphasic lubrication that does not require knowledge of interstitial fluid pressure was developed. In the second phase, frictional-shear tests were performed on 74 cartilage samples, and the simplified model was used to extract characteristic features from the friction signals. Using support vector machine classifiers, the extracted features were able to detect damage with a median accuracy of approximately 90%. The accuracy remained high even in samples with minimal damage. In conclusion, the friction signal acquired during frictional-shear testing can be used to detect resultant damage to a high level of accuracy. PMID:25691395

Estimation of wheel-rail friction for vehicle certification

NASA Astrophysics Data System (ADS)

Petrov, Vladislav; Berg, Mats; Persson, Ingemar

2014-08-01

In certification of new rail vehicles with respect to running characteristics, a wide variety of operating conditions needs to be considered. However, in associated test runs the wheel-rail friction condition is difficult to handle because the friction coefficient needs to be fairly high and the friction is also generally hard to assess. This is an issue that has been studied in the European project DynoTRAIN and part of the results is presented in this paper. More specifically, an algorithm for estimating the wheel-rail friction coefficient at vehicle certification tests is proposed. Owing to lack of some measurement results, the algorithm here is evaluated in a simulation environment which is also an important step towards practical implementation. A quality measure of the friction estimate is suggested in terms of estimated wheel-rail spin and total creep. It is concluded that, tentatively, the total creep should exceed 0.006 and the spin should be less than 1.0 m-1 for the algorithm to give a good friction estimate. Sensitivity analysis is carried out to imitate measurement errors, but should be expanded in further work.

The effect of friction in the hold down post spherical bearings on hold down post loads

NASA Technical Reports Server (NTRS)

Richardson, James A.

1990-01-01

The effect of friction at the connection of the Solid Rocket Booster (SRB) aft skirt and the mobile launch platform (MLP) hold down posts was analyzed. A simplified model of the shuttle response during the Space Shuttle Main Engine (SSME) buildup was constructed. The model included the effect of stick-slip friction for the rotation of the skirt about the spherical bearing. Current finite element models assume the joint is completely frictionless in rotation and therefore no moment is transferred between the skirt and the hold down posts. The model was partially verified against test data and preliminary parameter studies were performed. The parameter studies indicated that the coefficient of friction strongly influenced the moment on the hold down posts. The coefficient of friction had little effect on hold down post vertical loads, however. Further calibration of the model is necessary before the effect of friction on the hold down post horizontal loads can be analyzed.

Design of tyre force excitation for tyre-road friction estimation

NASA Astrophysics Data System (ADS)

Albinsson, Anton; Bruzelius, Fredrik; Jacobson, Bengt; Fredriksson, Jonas

2017-02-01

Knowledge of the current tyre-road friction coefficient is essential for future autonomous vehicles. The environmental conditions, and the tyre-road friction in particular, determine both the braking distance and the maximum cornering velocity and thus set the boundaries for the vehicle. Tyre-road friction is difficult to estimate during normal driving due to low levels of tyre force excitation. This problem can be solved by using active tyre force excitation. A torque is added to one or several wheels in the purpose of estimating the tyre-road friction coefficient. Active tyre force excitation provides the opportunity to design the tyre force excitation freely. This study investigates how the tyre force should be applied to minimise the error of the tyre-road friction estimate. The performance of different excitation strategies was found to be dependent on both tyre model choice and noise level. Furthermore, the advantage with using tyre models with more parameters decreased when noise was added to the force and slip ratio.

Direct measurement of friction of a fluctuating contact line

NASA Astrophysics Data System (ADS)

Guo, Shuo; Gao, Min; Xiong, Xiaomin; Wang, Yong Jian; Wang, Xiaoping; Sheng, Ping; Tong, Penger

2013-03-01

What happens at a moving contact line, where one fluid displaces another (immiscible) fluid over a solid surface, is a fundamental issue in fluid dynamics. In this presentation, we report a direct measurement of the friction coefficient in the immediate vicinity of a fluctuating contact line using a micron-sized vertical glass fiber with one end glued to an atomic force microscope (AFM) cantilever beam and the other end touching a liquid-air interface. By measuring the broadening of the resonance peak of the cantilever system with varying liquid viscosity η, we obtain the friction coefficient ξc associated with the contact line fluctuations on the glass fiber of diameter d and find it has the universal form, ξc = 0 . 8 πdη , independent of the contact angle. The result is further confirmed by using a soap film system whose bulk effect is negligibly small. This is the first time that the friction coefficient of a fluctuating contact line is measured. *Work supported by the Research Grants Council of Hong Kong SAR.

Confinement-Dependent Friction in Peptide Bundles

PubMed Central

Erbaş, Aykut; Netz, Roland R.

2013-01-01

Friction within globular proteins or between adhering macromolecules crucially determines the kinetics of protein folding, the formation, and the relaxation of self-assembled molecular systems. One fundamental question is how these friction effects depend on the local environment and in particular on the presence of water. In this model study, we use fully atomistic MD simulations with explicit water to obtain friction forces as a single polyglycine peptide chain is pulled out of a bundle of k adhering parallel polyglycine peptide chains. The whole system is periodically replicated along the peptide axes, so a stationary state at prescribed mean sliding velocity V is achieved. The aggregation number is varied between k = 2 (two peptide chains adhering to each other with plenty of water present at the adhesion sites) and k = 7 (one peptide chain pulled out from a close-packed cylindrical array of six neighboring peptide chains with no water inside the bundle). The friction coefficient per hydrogen bond, extrapolated to the viscous limit of vanishing pulling velocity V → 0, exhibits an increase by five orders of magnitude when going from k = 2 to k = 7. This dramatic confinement-induced friction enhancement we argue to be due to a combination of water depletion and increased hydrogen-bond cooperativity. PMID:23528088

Movements of a Sphere Moving Over Smooth and Rough Inclines

NASA Astrophysics Data System (ADS)

Jan, Chyan-Deng

1992-01-01

The steady movements of a sphere over a rough incline in air, and over smooth and rough inclines in a liquid were studied theoretically and experimentally. The principle of energy conservation was used to analyze the translation velocities, rolling resistances, and drag coefficients of a sphere moving over the inclines. The rolling resistance to the movement of a sphere from the rough incline was presumed to be caused by collisions and frictional slidings. A varnished wooden board was placed on the bottom of an experimental tilting flume to form a smooth incline and a layer of spheres identical to the sphere moving over them was placed on the smooth wooden board to form a rough incline. Spheres used in the experiments were glass spheres, steel spheres, and golf balls. Experiments show that a sphere moving over a rough incline with negligible fluid drag in air can reach a constant translation velocity. This constant velocity was found to be proportional to the bed inclination (between 11 ^circ and 21^circ) and the square root of the sphere's diameter, but seemingly independent of the sphere's specific gravity. Two empirical coefficients in the theoretical expression of the sphere's translation velocity were determined by experiments. The collision and friction parts of the shear stress exerted on the interface between the moving sphere and rough incline were determined. The ratio of collision to friction parts appears to increase with increase in the bed inclination. These two parts seem to be of the same order of magnitude. The rolling resistances and the relations between the drag coefficient and Reynolds number for a sphere moving over smooth and rough inclines in a liquid, such as water or salad oil, were determined by a regression analysis based on experimental data. It was found that the drag coefficient for a sphere over the rough incline is larger than that for a sphere over the smooth incline, and both of which are much larger than that for a sphere in free fall. The relative magnitudes of the shear stresses due to drag, collision, and friction were also determined in terms of the Reynolds number.

Turbulent Plume Dispersion over Two-dimensional Idealized Urban Street Canyons

NASA Astrophysics Data System (ADS)

Wong, C. C. C.; Liu, C. H.

2012-04-01

Human activities are the primary pollutant sources which degrade the living quality in the current era of dense and compact cities. A simple and reasonably accurate pollutant dispersion model is helpful to reduce pollutant concentrations in city or neighborhood scales by refining architectural design or urban planning. The conventional method to estimate the pollutant concentration from point/line sources is the Gaussian plume model using empirical dispersion coefficients. Its accuracy is pretty well for applying to rural areas. However, the dispersion coefficients only account for the atmospheric stability and streamwise distance that often overlook the roughness of urban surfaces. Large-scale buildings erected in urban areas significantly modify the surface roughness that in turn affects the pollutant transport in the urban canopy layer (UCL). We hypothesize that the aerodynamic resistance is another factor governing the dispersion coefficient in the UCL. This study is thus conceived to study the effects of urban roughness on pollutant dispersion coefficients and the plume behaviors. Large-eddy simulations (LESs) are carried out to examine the plume dispersion from a ground-level pollutant source over idealized 2D street canyons in neutral stratification. Computations with a wide range of aspect ratios (ARs), including skimming flow to isolated flow regimes, are conducted. The vertical profiles of pollutant distribution for different values of friction factor are compared that all reach a self-similar Gaussian shape. Preliminary results show that the pollutant dispersion is closely related to the friction factor. For relatively small roughness, the factors of dispersion coefficient vary linearly with the friction factor until the roughness is over a certain level. When the friction factor is large, its effect on the dispersion coefficient is less significant. Since the linear region covers at least one-third of the full range of friction factor in our empirical analysis, urban roughness is a major factor for dispersion coefficient. The downstream air quality could then be a function of both atmospheric stability and urban roughness.

Improving friction performance of cast iron by laser shock peening

NASA Astrophysics Data System (ADS)

Feng, Xu; Zhou, Jianzhong; Huang, Shu; Sheng, Jie; Mei, Yufen; Zhou, Hongda

2015-05-01

According to different purpose, some high or low friction coefficient of the material surface is required. In this study, micro-dent texture was fabricated on cast iron specimens by a set of laser shock peening (LSP) experiments under different laser energy, with different patterns of micro dimples in terms of the depth over diameter. The mechanism of LSP was discussed and surface morphology of the micro dimples were investigated by utilizing a Keyence KS-1100 3D optical surface profilometer. The tests under the conditions of dry and lubricating sliding friction were accomplished on the UMT-2 apparatus. The performance of treated samples during friction and wear tests were characterized and analyzed. Based on theoretical analysis and experimental study, friction performance of textured and untextured samples were studied and compared. Morphological characteristics were observed by scanning electron microscope (SEM) and compared after friction tests under dry condition. The results showed that friction coefficient of textured samples were obvious changed than smooth samples. It can be seen that LSP is an effective way to improve the friction performance of cast iron by fabricating high quality micro dimples on its surface, no matter what kind of engineering application mentioned in this paper.

In-flight Compressible Turbulent Boundary Layer Measurements on a Hollow Cylinder at a Mach Number of 3.0

NASA Technical Reports Server (NTRS)

Quinn, R. D.; Gong, L.

1978-01-01

Skin temperatures, shearing forces, surface static pressures, and boundary layer pitot pressures and total temperatures were measured on a hollow cylinder 3.04 meters long and 0.437 meter in diameter mounted beneath the fuselage of the YF-12A airplane. The data were obtained at a nominal free stream Mach number of 3.0 and at wall-to-recovery temperature ratios of 0.66 to 0.91. The free stream Reynolds number had a minimal value of 4.2 million per meter. Heat transfer coefficients and skin friction coefficients were derived from skin temperature time histories and shear force measurements, respectively. Boundary layer velocity profiles were derived from pitot pressure measurements, and a Reynolds analogy factor of 1.11 was obtained from the measured heat transfer and skin friction data. The skin friction coefficients predicted by the theory of van Driest were in excellent agreement with the measurements. Theoretical heat transfer coefficients, in the form of Stanton numbers calculated by using a modified Reynolds analogy between skin friction and heat transfer, were compared with measured values. The measured velocity profiles were compared to Coles' incompressible law-of-the-wall profile.

Ion-Implanted Diamond Films and Their Tribological Properties

NASA Technical Reports Server (NTRS)

Wu, Richard L. C.; Miyoshi, Kazuhisa; Korenyi-Both, Andras L.; Garscadden, Alan; Barnes, Paul N.

1993-01-01

This paper reports the physical characterization and tribological evaluation of ion-implanted diamond films. Diamond films were produced by microwave plasma, chemical vapor deposition technique. Diamond films with various grain sizes (0.3 and 3 microns) and roughness (9.1 and 92.1 nm r.m.s. respectively) were implanted with C(+) (m/e = 12) at an ion energy of 160 eV and a fluence of 6.72 x 10(exp 17) ions/sq cm. Unidirectional sliding friction experiments were conducted in ultrahigh vacuum (6.6 x 10(exp -7)Pa), dry nitrogen and humid air (40% RH) environments. The effects of C(+) ion bombardment on fine and coarse-grained diamond films are as follows: the surface morphology of the diamond films did not change; the surface roughness increased (16.3 and 135.3 nm r.m.s.); the diamond structures were damaged and formed a thin layer of amorphous non-diamond carbon; the friction coefficients dramatically decreased in the ultrahigh vacuum (0.1 and 0.4); the friction coefficients decreased slightly in the dry nitrogen and humid air environments.

Control system for maximum use of adhesive forces of a railway vehicle in a tractive mode

NASA Astrophysics Data System (ADS)

Spiryagin, Maksym; Lee, Kwan Soo; Yoo, Hong Hee

2008-04-01

The realization of maximum adhesive forces for a railway vehicle is a very difficult process, because it involves using tractive efforts and depends on friction characteristics in the contact zone between wheels and rails. Tractive efforts are realized by means of tractive torques of motors, and their maximum values can provide negative effects such as slip and skid. These situations usually happen when information about friction conditions is lacking. The negative processes have a major influence on wearing of contact bodies and tractive units. Therefore, many existing control systems for vehicles use an effect of a prediction of a friction coefficient between wheels and rails because measuring a friction coefficient at the moment of running vehicle movement is very difficult. One of the ways to solve this task is to use noise spectrum analysis for friction coefficient detection. This noise phenomenon has not been clearly studied and analyzed. In this paper, we propose an adhesion control system of railway vehicles based on an observer, which allows one to determine the maximum tractive torque based on the optimal adhesive force between the wheels (wheel pair) of a railway vehicle and rails (rail track) depending on weight load from a wheel to a rail, friction conditions in the contact zone, a lateral displacement of wheel set and wheel sleep. As a result, it allows a railway vehicle to be driven in a tractive mode by the maximum adhesion force for real friction conditions.

Friction and lubrication of pleural tissues.

PubMed

D'Angelo, Edgardo; Loring, Stephen H; Gioia, Magda E; Pecchiari, Matteo; Moscheni, Claudia

2004-08-20

The frictional behaviour of rabbit's visceral pleura sliding against parietal pleura was assessed in vitro while oscillating at physiological velocities and amplitudes under physiological normal forces. For sliding velocities up to 3 cm s(-1) and normal compressive loads up to 12 cm H2O, the average value of the coefficient of kinetic friction (mu) was constant at 0.019 +/- 0.002 (S.E.) with pleural liquid as lubricant. With Ringer-bicarbonate solution, mu was still constant, but significantly increased (Deltamu = 0.008 +/- 0.001; P < 0.001). Under these conditions, no damage of the sliding pleural surfaces was found on light and electron microscopy. Additional measurements, performed also on peritoneum, showed that changes in nominal contact area or strain of the mesothelia, temperature in the range 19-39 degrees C, and prolonged sliding did not affect mu. Gentle application of filter paper increased mu approximately 10-fold and irreversibly, suggesting alteration of the mesothelia. With packed the red blood cells (RBC) between the sliding mesothelia, mu increased appreciably but reversibly on removal of RBC suspension, whilst no ruptures of RBC occurred. In conclusion, the results indicate a low value of sliding friction in pleural tissues, partly related to the characteristics of the pleural liquid, and show that friction is independent of velocity, normal load, and nominal contact area, consistent with boundary lubrication.

A novel pendulum test for measuring roller chain efficiency

NASA Astrophysics Data System (ADS)

Wragge-Morley, R.; Yon, J.; Lock, R.; Alexander, B.; Burgess, S.

2018-07-01

This paper describes a novel pendulum decay test for determining the transmission efficiency of chain drives. The test involves releasing a pendulum with an initial potential energy and measuring its decaying oscillations: under controlled conditions the decay reveals the losses in the transmission to a high degree of accuracy. The main advantage over motorised rigs is that there are significantly fewer sources of friction and inertia and hence measurement error. The pendulum rigs have an accuracy around 0.6% for the measurement of the coefficient of friction, giving an accuracy of transmission efficiency measurement around 0.012%. A theoretical model of chain friction combined with the equations of motion enables the coefficient of friction to be determined from the decay rate of pendulum velocity. The pendulum rigs operate at relatively low speeds. However, they allow an accurate determination of the coefficient of friction to estimate transmission efficiency at higher speeds. The pendulum rig revealed a previously undetected rocking behaviour in the chain links at very small articulation angles. In this regime, the link interfaces were observed to roll against one another rather than slide. This observation indicates that a very high-efficiency transmission can be achieved if the articulation angle is very low.

Friction and wear performance of bearing ball sliding against diamond-like carbon coatings

NASA Astrophysics Data System (ADS)

Wu, Shenjiang; Kousaka, Hiroyuki; Kar, Satyananda; Li, Dangjuan; Su, Junhong

2017-01-01

We have studied the tribological properties of bearing steel ball (Japan standard, SUJ2) sliding against tetrahedral amorphous carbon (ta-C) coatings and amorphous hydrogenated carbon (a-C:H) coatings. The reciprocating sliding testes are performed with ball-on-plate friction tester in ambient air condition. Analysis of friction coefficient, wear volume and microstructure in wear scar are carried out using optical microscopy, atom force morphology (AFM) and Raman spectroscopy. The results show the SUJ2 on ta-C coating has low friction coefficient (around 0.15) but high wear loss. In contrast, the low wear loss of SUJ2 on a-C:H coating with high (around 0.4) and unsteady friction coefficient. Some Fe2O3, FeO and graphitization have been found on the wear scar of SUJ2 sliding against ta-C coating. Nearly no oxide materials exist on the wear scar of SUJ2 against a-C:H coating. The mechanism and hypothesis of the wear behavior have been investigated according to the measurement results. This study will contribute to proper selection and understand the tribological performance of bearing steels against DLC coatings.

Quantum Drude friction for time-dependent density functional theory

NASA Astrophysics Data System (ADS)

Neuhauser, Daniel; Lopata, Kenneth

2008-10-01

Friction is a desired property in quantum dynamics as it allows for localization, prevents backscattering, and is essential in the description of multistage transfer. Practical approaches for friction generally involve memory functionals or interactions with system baths. Here, we start by requiring that a friction term will always reduce the energy of the system; we show that this is automatically true once the Hamiltonian is augmented by a term of the form ∫a(q ;n0)[∂j(q,t)/∂t]ṡJ(q)dq, which includes the current operator times the derivative of its expectation value with respect to time, times a local coefficient; the local coefficient will be fitted to experiment, to more sophisticated theories of electron-electron interaction and interaction with nuclear vibrations and the nuclear background, or alternately, will be artificially constructed to prevent backscattering of energy. We relate this term to previous results and to optimal control studies, and generalize it to further operators, i.e., any operator of the form ∫a(q ;n0)[∂c(q,t)/∂t]ṡC(q)dq (or a discrete sum) will yield friction. Simulations of a small jellium cluster, both in the linear and highly nonlinear excitation regime, demonstrate that the friction always reduces energy. The energy damping is essentially double exponential; the long-time decay is almost an order of magnitude slower than the rapid short-time decay. The friction term stabilizes the propagation (split-operator propagator here), therefore increasing the time-step needed for convergence, i.e., reducing the overall computational cost. The local friction also allows the simulation of a metal cluster in a uniform jellium as the energy loss in the excitation due to the underlying corrugation is accounted for by the friction. We also relate the friction to models of coupling to damped harmonic oscillators, which can be used for a more sophisticated description of the coupling, and to memory functionals. Our results open the way to very simple finite grid description of scattering and multistage conductance using time-dependent density functional theory away from the linear regime, just as absorbing potentials and self-energies are useful for noninteracting systems and leads.

Rheological effects on friction in elastohydrodynamic lubrication

NASA Technical Reports Server (NTRS)

Trachman, E. G.; Cheng, H. S.

1973-01-01

An analytical and experimental investigation is presented of the friction in a rolling and sliding elastohydrodynamic lubricated contact. The rheological behavior of the lubricant is described in terms of two viscoelastic models. These models represent the separate effects of non-Newtonian behavior and the transient response of the fluid. A unified description of the non-Newtonian shear rate dependence of the viscosity is presented as a new hyperbolic liquid model. The transient response of viscosity, following the rapid pressure rise encountered in the contact, is described by a compressional viscoelastic model of the volume response of a liquid to an applied pressure step. The resulting momentum and energy equations are solved by an iterative numerical technique, and a friction coefficient is calculated. The experimental study was performed, with two synthetic paraffinic lubricants, to verify the friction predictions of the analysis. The values of friction coefficient from theory and experiment are in close agreement.

High friction on ice provided by elastomeric fiber composites with textured surfaces

NASA Astrophysics Data System (ADS)

Rizvi, R.; Naguib, H.; Fernie, G.; Dutta, T.

2015-03-01

Two main applications requiring high friction on ice are automobile tires and footwear. The main motivation behind the use of soft rubbers in these applications is the relatively high friction force generated between a smooth rubber contacting smooth ice. Unfortunately, the friction force between rubber and ice is very low at temperatures near the melting point of ice and as a result we still experience automobile accidents and pedestrian slips and falls in the winter. Here, we report on a class of compliant fiber-composite materials with textured surfaces that provide outstanding coefficients of friction on wet ice. The fibrous composites consist of a hard glass-fiber phase reinforcing a compliant thermoplastic polyurethane matrix. The glass-fiber phase is textured such that it is aligned transversally and protruding out of the elastomer surface. Our analysis indicates that the exposed fiber phase exhibits a "micro-cleat" effect, allowing for it to fracture the ice and increase the interfacial contact area thereby requiring a high force to shear the interface.

An assessment of the skid resistance effect on traffic safety under wet-pavement conditions.

PubMed

Pardillo Mayora, José M; Jurado Piña, Rafael

2009-07-01

Pavement-tire friction provides the grip that is required for maintaining vehicle control and for stopping in emergency situations. Statistically significant negative correlations of skid resistance values and wet-pavement accident rates have been found in previous research. Skid resistance measured with SCRIM and crash data from over 1750km of two-lane rural roads in the Spanish National Road System were analyzed to determine the influence of pavement conditions on safety and to assess the effects of improving pavement friction on safety. Both wet- and dry-pavement crash rates presented a decreasing trend as skid resistance values increased. Thresholds in SCRIM coefficient values associated with significant decreases in wet-pavement crash rates were determined. Pavement friction improvement schemes were found to yield significant reductions in wet-pavement crash rates averaging 68%. The results confirm the importance of maintaining adequate levels of pavement friction to safeguard traffic safety as well as the potential of pavement friction improvement schemes to achieve significant crash reductions.

Surface effects of corrosive media on hardness, friction, and wear of materials

NASA Technical Reports Server (NTRS)

Miyoshi, K.; Buckley, D. H.; Rengstorff, G. W. P.; Ishigaki, H.

1985-01-01

Hardness, friction, and wear experiments were conducted with magnesium oxide exposed to various corrosive media and also with elemental iron and nickel exposed to water and NaOH. Chlorides such as MgCl2 and sodium containing films were formed on cleaved magnesium oxide surfaces. The MgCl2 films softened the magnesium oxide surfaces and caused high friction and great deformation. Hardness was strongly influenced by the pH value of the HCl-containing solution. The lower the pH, the lower the microhardness. Neither the pH value of nor the immersion time in NaOH containing, NaCl containing, and HNO3 containing solutions influenced the microhardness of magnesium oxide. NaOH formed a protective and low friction film on iron surfaces. The coefficient of friction and the wear for iron were low at concentrations of NaOH higher than 0.01 N. An increase in NaOH concentration resulted in a decrease in the concentration of ferric oxide on the iron surface. It took less NaOH to form a protective, low friction film on nickel than on iron.

Influence of the chemical surface structure on the nanoscale friction in plasma nitrided and post-oxidized ferrous alloy

DOE Office of Scientific and Technical Information (OSTI.GOV)

Freislebem, Márcia; Menezes, Caren M.; Cemin, Felipe

2014-09-15

Friction is a ubiquitous phenomenon in everyday activities spanning from vehicles where efficient brakes are mandatory up to mechanical devices where its minimum effects are pursued for energy efficiency issues. Recently, theoretical models succeed correlating the friction behavior with energy transference via phonons between sliding surfaces. Therefore, considering that the energy losses by friction are prompted through phonons, the chemical surface structure between sliding surfaces is very important to determine the friction phenomenon. In this work, we address the issue of friction between a conical diamond tip sliding on different functionalized flat steel surfaces by focusing the influence of themore » chemical bonds in the outermost layers on the sliding resistance. This geometry allows probing the coupling of the sharp tip with terminator species on the top and underneath material surface at in-depth friction measurements from 20 to 200 nm. Experimentally, the friction coefficient decreases when nitrogen atoms are substituted for oxygen in the iron network. This effect is interpreted as due to energy losses through phonons whilst lower vibrational frequency excitation modes imply lower friction coefficients and a more accurate adjustment is obtained when a theoretical model with longitudinal adsorbate vibration is used.« less

Automated vehicle location, data recording, friction measurement and applicator control for winter road maintenance.

DOT National Transportation Integrated Search

2010-02-01

The first part of this project conducted a detailed evaluation of the ability of a new friction measurement system to : provide an accurate measure of road conditions. A system that records friction coefficient as a function of road : location was de...

Aluminum runway surface as possible aid to aircraft braking

NASA Technical Reports Server (NTRS)

Miller, C. D.; Pinkel, I. I.

1973-01-01

Several concepts are described for use singly or in combination to improve aircraft braking. All involve a thin layer of aluminum covering all or part of the runway. Advantage would derive from faster heat conduction from the tire-runway interface. Heating of tread surface with consequent softening and loss of friction coefficient should be reduced. Equations are developed indicating that at least 99 percent of friction heat should flow into the aluminum. Preliminary test results indicate a coefficient of sliding friction of 1.4, with predictably slight heating of tread. Elimination of conventional brakes is at least a remote possibility.

Skin maintenance in the bed-ridden patient.

PubMed

Flam, E

1990-01-01

The skin of a patient at risk of developing pressure ulcers can resist deterioration if the conditions that weaken it are controlled. The purpose of this study is to determine the relationships between hydration level, skin temperature, and friction in patients at risk of development or reoccurrence of pressure ulcers and in patients with newly created surgical flaps. Two systems were considered: the standard hospital mattress covered with a thick occlusive plastic film and a 50/50 cotton/polyester bed sheet and the KinAir and the TheraPulse support systems with nylon/High Air Loss GORE-TEX (n/HAL) laminate cushions and coverlets. The moisture vapor management and aeration capabilities of the support system materials were determined, and the frictional force generated against the skin was measured. The results revealed that excessive hydration increases the level of friction against the skin while at the same time reducing the mechanical properties of the protective skin layers. The n/HAL laminate coverlet also had a significantly lower skin friction coefficient than the 50/50 cotton/polyester bed sheet. The significance of these findings is that over-hydration accelerates the abrading action on the skin by increasing the frictional force and decreasing the shear resistance of the skin.

Plasma-assisted physical vapor deposition surface treatments for tribological control

NASA Technical Reports Server (NTRS)

Spalvins, Talivaldis

1990-01-01

In any mechanical or engineering system where contacting surfaces are in relative motion, adhesion, wear, and friction affect reliability and performance. With the advancement of space age transportation systems, the tribological requirements have dramatically increased. This is due to the optimized design, precision tolerance requirements, and high reliability expected for solid lubricating films in order to withstand hostile operating conditions (vacuum, high-low temperatures, high loads, and space radiation). For these problem areas the ion-assisted deposition/modification processes (plasma-based and ion beam techniques) offer the greatest potential for the synthesis of thin films and the tailoring of adherence and chemical and structural properties for optimized tribological performance. The present practices and new approaches of applying soft solid lubricant and hard wear resistant films to engineering substrates are reviewed. The ion bombardment treatments have increased film adherence, lowered friction coefficients, and enhanced wear life of the solid lubricating films such as the dichalcogenides (MoS2) and the soft metals (Au, Ag, Pb). Currently, sputtering is the preferred method of applying MoS2 films; and ion plating, the soft metallic films. Ultralow friction coefficients (less than 0.01) were achieved with sputtered MoS2. Further, new diamond-like carbon and BN lubricating films are being developed by using the ion assisted deposition techniques.

Transduction channels' gating can control friction on vibrating hair-cell bundles in the ear.

PubMed

Bormuth, Volker; Barral, Jérémie; Joanny, Jean-François; Jülicher, Frank; Martin, Pascal

2014-05-20

Hearing starts when sound-evoked mechanical vibrations of the hair-cell bundle activate mechanosensitive ion channels, giving birth to an electrical signal. As for any mechanical system, friction impedes movements of the hair bundle and thus constrains the sensitivity and frequency selectivity of auditory transduction. Friction is generally thought to result mainly from viscous drag by the surrounding fluid. We demonstrate here that the opening and closing of the transduction channels produce internal frictional forces that can dominate viscous drag on the micrometer-sized hair bundle. We characterized friction by analyzing hysteresis in the force-displacement relation of single hair-cell bundles in response to periodic triangular stimuli. For bundle velocities high enough to outrun adaptation, we found that frictional forces were maximal within the narrow region of deflections that elicited significant channel gating, plummeted upon application of a channel blocker, and displayed a sublinear growth for increasing bundle velocity. At low velocity, the slope of the relation between the frictional force and velocity was nearly fivefold larger than the hydrodynamic friction coefficient that was measured when the transduction machinery was decoupled from bundle motion by severing tip links. A theoretical analysis reveals that channel friction arises from coupling the dynamics of the conformational change associated with channel gating to tip-link tension. Varying channel properties affects friction, with faster channels producing smaller friction. We propose that this intrinsic source of friction may contribute to the process that sets the hair cell's characteristic frequency of responsiveness.

Transduction channels’ gating can control friction on vibrating hair-cell bundles in the ear

PubMed Central

Bormuth, Volker; Barral, Jérémie; Joanny, Jean-François; Jülicher, Frank; Martin, Pascal

2014-01-01

Hearing starts when sound-evoked mechanical vibrations of the hair-cell bundle activate mechanosensitive ion channels, giving birth to an electrical signal. As for any mechanical system, friction impedes movements of the hair bundle and thus constrains the sensitivity and frequency selectivity of auditory transduction. Friction is generally thought to result mainly from viscous drag by the surrounding fluid. We demonstrate here that the opening and closing of the transduction channels produce internal frictional forces that can dominate viscous drag on the micrometer-sized hair bundle. We characterized friction by analyzing hysteresis in the force–displacement relation of single hair-cell bundles in response to periodic triangular stimuli. For bundle velocities high enough to outrun adaptation, we found that frictional forces were maximal within the narrow region of deflections that elicited significant channel gating, plummeted upon application of a channel blocker, and displayed a sublinear growth for increasing bundle velocity. At low velocity, the slope of the relation between the frictional force and velocity was nearly fivefold larger than the hydrodynamic friction coefficient that was measured when the transduction machinery was decoupled from bundle motion by severing tip links. A theoretical analysis reveals that channel friction arises from coupling the dynamics of the conformational change associated with channel gating to tip-link tension. Varying channel properties affects friction, with faster channels producing smaller friction. We propose that this intrinsic source of friction may contribute to the process that sets the hair cell’s characteristic frequency of responsiveness. PMID:24799674

Surface modification of Monel K-500 as a means of reducing friction and wear in high-pressure oxygen

NASA Technical Reports Server (NTRS)

Gunaji, Mohan; Stoltzfus, Joel M.; Schoenman, Leonard; Kazaroff, John

1989-01-01

A study is conducted of the tribological characteristics of Monel K-500 during rubbing in a high pressure oxygen atmosphere, upon surface treatment by ion-implanted oxygen, chromium, lead, and silver, as well as electrolyzed chromium and an electroless nickel/SiC composite. The electrolyzed chromium dramatically increased total sample wear, while other surface treatments affected sample wear only moderately. Although the ion-implant treatments reduced the average coefficient of friction at low contact pressure, higher contact pressures eliminated this improvement.

Dynamic Friction Performance of a Pneumatic Cylinder with Al2O3 Film on Cylinder Surface.

PubMed

Chang, Ho; Lan, Chou-Wei; Wang, Hao-Xian

2015-11-01

A friction force system is proposed for accurately measuring friction force and motion properties produced by reciprocating motion of piston in a pneumatic cylinder. In this study, the proposed system is used to measure the effects of lubricating greases of different viscosities on the friction properties of pneumatic cylinder, and improvement of stick-slip motion for the cylinder bore by anodizing processes. A servo motor-driven ball screw is used to drive the pneumatic cylinder to be tested and to measure the change in friction force of the pneumatic cylinder. Experimental results show, that under similar test conditions, the lubricating grease with viscosity VG100 is best suited for measuring reciprocating motion of the piston of pneumatic cylinder. The wear experiment showed that, in the Al2O3 film obtained at a preset voltage 40 V in the anodic process, the friction coefficient and hardness decreased by 55% and increased by 274% respectively, thus achieving a good tribology and wear resistance. Additionally, the amplitude variation in the friction force of the pneumatic cylinder wall that received the anodizing treatment was substantially reduced. Additionally, the stick-slip motion of the pneumatic cylinder during low-speed motion was substantially improved.

Skin friction related behaviour of artificial turf systems.

PubMed

Tay, Sock Peng; Fleming, Paul; Hu, Xiao; Forrester, Steph

2017-08-01

The occurrence of skin friction related injuries is an issue for artificial turf sports pitches and remains a barrier to their acceptance. The purpose of this study was to evaluate the current industry standard Securisport® Sports Surface Tester that measures skin surface related frictional behaviour of artificial turf. Little research has been published about the device and its efficacy, despite its widespread use as a standard FIFA test instrument. To achieve a range of frictional behaviours, several "third generation" (3G) carpet and infill combinations were investigated; friction time profiles throughout the Securisport rotations were assessed in combination with independent measurements of skin roughness before and after friction testing via 3D surface scanning. The results indicated that carpets without infill had greatest friction (coefficients of friction 0.97-1.20) while those completely filled with sand or rubber had similar and lower values independent of carpet type (coefficient of friction (COF) ≈0.57). Surface roughness of a silicone skin (s-skin) decreased after friction testing, with the largest change on sand infilled surfaces, indicating an "abrasive" polishing effect. The combined data show that the s-skin is damaged in a surface-specific manner, thus the Securisport COF values appear to be a poor measure of the potential for skin abrasion. It is proposed that the change in s-skin roughness improves assessment of the potential for skin damage when players slide on artificial turf.

New method of extrapolation of the resistance of a model planing boat to full size

NASA Technical Reports Server (NTRS)

Sottorf, W

1942-01-01

The previously employed method of extrapolating the total resistance to full size with lambda(exp 3) (model scale) and thereby foregoing a separate appraisal of the frictional resistance, was permissible for large models and floats of normal size. But faced with the ever increasing size of aircraft a reexamination of the problem of extrapolation to full size is called for. A method is described by means of which, on the basis of an analysis of tests on planing surfaces, the variation of the wetted surface over the take-off range is analytically obtained. The friction coefficients are read from Prandtl's curve for turbulent boundary layer with laminar approach. With these two values a correction for friction is obtainable.

Characterization of Friction Joints Subjected to High Levels of Random Vibration

NASA Technical Reports Server (NTRS)

deSantos, Omar; MacNeal, Paul

2012-01-01

This paper describes the test program in detail including test sample description, test procedures, and vibration test results of multiple test samples. The material pairs used in the experiment were Aluminum-Aluminum, Aluminum- Dicronite coated Aluminum, and Aluminum-Plasmadize coated Aluminum. Levels of vibration for each set of twelve samples of each material pairing were gradually increased until all samples experienced substantial displacement. Data was collected on 1) acceleration in all three axes, 2) relative static displacement between vibration runs utilizing photogrammetry techniques, and 3) surface galling and contaminant generation. This data was used to estimate the values of static friction during random vibratory motion when "stick-slip" occurs and compare these to static friction coefficients measured before and after vibration testing.

Analysis Method of Friction Torque and Weld Interface Temperature during Friction Process of Steel Friction Welding

NASA Astrophysics Data System (ADS)

Kimura, Masaaki; Inoue, Haruo; Kusaka, Masahiro; Kaizu, Koichi; Fuji, Akiyoshi

This paper describes an analysis method of the friction torque and weld interface temperature during the friction process for steel friction welding. The joining mechanism model of the friction welding for the wear and seizure stages was constructed from the actual joining phenomena that were obtained by the experiment. The non-steady two-dimensional heat transfer analysis for the friction process was carried out by calculation with FEM code ANSYS. The contact pressure, heat generation quantity, and friction torque during the wear stage were calculated using the coefficient of friction, which was considered as the constant value. The thermal stress was included in the contact pressure. On the other hand, those values during the seizure stage were calculated by introducing the coefficient of seizure, which depended on the seizure temperature. The relationship between the seizure temperature and the relative speed at the weld interface in the seizure stage was determined using the experimental results. In addition, the contact pressure and heat generation quantity, which depended on the relative speed of the weld interface, were solved by taking the friction pressure, the relative speed and the yield strength of the base material into the computational conditions. The calculated friction torque and weld interface temperatures of a low carbon steel joint were equal to the experimental results when friction pressures were 30 and 90 MPa, friction speed was 27.5 s-1, and weld interface diameter was 12 mm. The calculation results of the initial peak torque and the elapsed time for initial peak torque were also equal to the experimental results under the same conditions. Furthermore, the calculation results of the initial peak torque and the elapsed time for initial peak torque at various friction pressures were equal to the experimental results.

Time- & Load-Dependence of Triboelectric Effect.

PubMed

Pan, Shuaihang; Yin, Nian; Zhang, Zhinan

2018-02-06

Time- and load-dependent friction behavior is considered as important for a long time, due to its time-evolution and force-driving characteristics. However, its electronic behavior, mainly considered in triboelectric effect, has almost never been given the full attention and analyses from the above point of view. In this paper, by experimenting with fcc-latticed aluminum and copper friction pairs, the mechanical and electronic behaviors of friction contacts are correlated by time and load analyses, and the behind physical understanding is provided. Most importantly, the difference of "response lag" in force and electricity is discussed, the extreme points of coefficient of friction with the increasing normal loads are observed and explained with the surface properties and dynamical behaviors (i.e. wear), and the micro and macro theories linking tribo-electricity to normal load and wear (i.e. the physical explanation between coupled electrical and mechanical phenomena) are successfully developed and tested.

Tribological Behavior of IN718 Superalloy Coating Fabricated by Laser Additive Manufacturing

NASA Astrophysics Data System (ADS)

Zhang, Yaocheng; Pan, Qiyong; Yang, Li; Li, Ruifeng; Dai, Jun

2017-12-01

The tribological behavior of laser manufactured IN718 superalloy coating are investigated with different applied loads, sliding speeds and lubricating mediums. The wear resistance of laser manufactured IN718 coating is increased by heat treatment due to higher microhardness and homogeneous brittle phase distribution. The principal factors for the wear rate are applied load and lubricating medium. The worn surface of laser manufactured IN718 coating consists of the grooves, crack, wear debris and material delamination generated by the fatigue wear associated with adhesive wear and abrasive wear. The friction coefficients are influenced by the tribological noise decrescence by the tribo-oxidant and the liquid lubricant. The real contact temperature between coating sample and frictional counterpart is higher than the solid-solution temperature of IN718 superalloy, and the effect of surface contact temperature on the orientational microstructure and wear resistance for dry friction and wet friction process is indistinct.

Jet impinging onto a laser drilled tapered hole: Influence of tapper location on heat transfer and skin friction at hole surface

NASA Astrophysics Data System (ADS)

Shuja, S. Z.; Yilbas, B. S.

2013-02-01

Jet emerging from a conical nozzle and impinging onto a tapered hole in relation to laser drilling is investigated and the influence taper location on the heat transfer and skin friction at the hole wall surface is examined. The study is extended to include four different gases as working fluid. The Reynolds stress model is incorporated to account for the turbulence effect in the flow field. The hole wall surface temperature is kept at 1500 K to resemble the laser drilled hole. It is found that the location of tapering in the hole influences the heat transfer rates and skin friction at the hole wall surface. The maximum skin friction coefficient increases for taper location of 0.25 H, where H is the thickness of the workpiece, while Nusselt number is higher in the hole for taper location of 0.75 H.

History-dependent friction and slow slip from time-dependent microscopic junction laws studied in a statistical framework

NASA Astrophysics Data System (ADS)

Thøgersen, Kjetil; Trømborg, Jørgen Kjoshagen; Sveinsson, Henrik Andersen; Malthe-Sørenssen, Anders; Scheibert, Julien

2014-05-01

To study how macroscopic friction phenomena originate from microscopic junction laws, we introduce a general statistical framework describing the collective behavior of a large number of individual microjunctions forming a macroscopic frictional interface. Each microjunction can switch in time between two states: a pinned state characterized by a displacement-dependent force and a slipping state characterized by a time-dependent force. Instead of tracking each microjunction individually, the state of the interface is described by two coupled distributions for (i) the stretching of pinned junctions and (ii) the time spent in the slipping state. This framework allows for a whole family of microjunction behavior laws, and we show how it represents an overarching structure for many existing models found in the friction literature. We then use this framework to pinpoint the effects of the time scale that controls the duration of the slipping state. First, we show that the model reproduces a series of friction phenomena already observed experimentally. The macroscopic steady-state friction force is velocity dependent, either monotonic (strengthening or weakening) or nonmonotonic (weakening-strengthening), depending on the microscopic behavior of individual junctions. In addition, slow slip, which has been reported in a wide variety of systems, spontaneously occurs in the model if the friction contribution from junctions in the slipping state is time weakening. Next, we show that the model predicts a nontrivial history dependence of the macroscopic static friction force. In particular, the static friction coefficient at the onset of sliding is shown to increase with increasing deceleration during the final phases of the preceding sliding event. We suggest that this form of history dependence of static friction should be investigated in experiments, and we provide the acceleration range in which this effect is expected to be experimentally observable.

History-dependent friction and slow slip from time-dependent microscopic junction laws studied in a statistical framework.

PubMed

Thøgersen, Kjetil; Trømborg, Jørgen Kjoshagen; Sveinsson, Henrik Andersen; Malthe-Sørenssen, Anders; Scheibert, Julien

2014-05-01

To study how macroscopic friction phenomena originate from microscopic junction laws, we introduce a general statistical framework describing the collective behavior of a large number of individual microjunctions forming a macroscopic frictional interface. Each microjunction can switch in time between two states: a pinned state characterized by a displacement-dependent force and a slipping state characterized by a time-dependent force. Instead of tracking each microjunction individually, the state of the interface is described by two coupled distributions for (i) the stretching of pinned junctions and (ii) the time spent in the slipping state. This framework allows for a whole family of microjunction behavior laws, and we show how it represents an overarching structure for many existing models found in the friction literature. We then use this framework to pinpoint the effects of the time scale that controls the duration of the slipping state. First, we show that the model reproduces a series of friction phenomena already observed experimentally. The macroscopic steady-state friction force is velocity dependent, either monotonic (strengthening or weakening) or nonmonotonic (weakening-strengthening), depending on the microscopic behavior of individual junctions. In addition, slow slip, which has been reported in a wide variety of systems, spontaneously occurs in the model if the friction contribution from junctions in the slipping state is time weakening. Next, we show that the model predicts a nontrivial history dependence of the macroscopic static friction force. In particular, the static friction coefficient at the onset of sliding is shown to increase with increasing deceleration during the final phases of the preceding sliding event. We suggest that this form of history dependence of static friction should be investigated in experiments, and we provide the acceleration range in which this effect is expected to be experimentally observable.

Velocity space scattering coefficients with applications in antihydrogen recombination studies

NASA Astrophysics Data System (ADS)

Chang, Yongbin; Ordonez, C. A.

2000-12-01

An approach for calculating velocity space friction and diffusion coefficients with Maxwellian field particles is developed based on a kernel function derived in a previous paper [Y. Chang and C. A. Ordonez, Phys. Plasmas 6, 2947 (1999)]. The original fivefold integral expressions for the coefficients are reduced to onefold integrals, which can be used for any value of the Coulomb logarithm. The onefold integrals can be further reduced to standard analytical expressions by using a weak coupling approximation. The integral expression for the friction coefficient is used to predict a time scale that describes the rate at which a reflecting antiproton beam slows down within a positron plasma, while both species are simultaneously confined by a nested Penning trap. The time scale is used to consider the possibility of achieving antihydrogen recombination within the trap. The friction and diffusion coefficients are then used to derive an expression for calculating the energy transfer rate between antiprotons and positrons. The expression is employed to illustrate achieving antihydrogen recombination while taking into account positron heating by the antiprotons. The effect of the presence of an electric field on recombination is discussed.

Skin-friction measurements in high-enthalpy hypersonic boundary layers

NASA Astrophysics Data System (ADS)

Goyne, C. P.; Stalker, R. J.; Paull, A.

2003-06-01

Skin-friction measurements are reported for high-enthalpy and high-Mach-number laminar, transitional and turbulent boundary layers. The measurements were performed in a free-piston shock tunnel with air-flow Mach number, stagnation enthalpy and Reynolds numbers in the ranges of 4.4 6.7, 3 13 MJ kg(-1) and 0.16× 10(6) 21× 10(6) , respectively. Wall temperatures were near 300 K and this resulted in ratios of wall enthalpy to flow-stagnation enthalpy in the range of 0.1 0.02. The experiments were performed using rectangular ducts. The measurements were accomplished using a new skin-friction gauge that was developed for impulse facility testing. The gauge was an acceleration compensated piezoelectric transducer and had a lowest natural frequency near 40 kHz. Turbulent skin-friction levels were measured to within a typical uncertainty of ± 7%. The systematic uncertainty in measured skin-friction coefficient was high for the tested laminar conditions; however, to within experimental uncertainty, the skin-friction and heat-transfer measurements were in agreement with the laminar theory of van Driest (1952). For predicting turbulent skin-friction coefficient, it was established that, for the range of Mach numbers and Reynolds numbers of the experiments, with cold walls and boundary layers approaching the turbulent equilibrium state, the Spalding & Chi (1964) method was the most suitable of the theories tested. It was also established that if the heat transfer rate to the wall is to be predicted, then the Spalding & Chi (1964) method should be used in conjunction with a Reynolds analogy factor near unity. If more accurate results are required, then an experimentally observed relationship between the Reynolds analogy factor and the skin-friction coefficient may be applied.

Friction and wear characteristics of wire-brush skids

NASA Technical Reports Server (NTRS)

Dreher, R. C.

1979-01-01

The testing technique consisted of towing the skids with a ground test vehicle over asphalt and concrete surfaces at ground speeds up to 80 km/hr (50 mph) and bearing pressures up to 689 kPa (100 psi) over sliding distances up to 1585 m (5200 ft). Results indicate that the friction coefficient developed by wire brush skids is essentially independent of ground speed, is slightly increased with increasing bearing pressure, is noticeably affected by surface texture, and is not degraded by surface wetness. Skid wear is shown to increase with increasing bearing pressure and with increasing ground speed and is dependent on the nature of the surface. Runway surface damage caused by the skids was in the form of an abrasive scrubbing action rather than physical damage.

Rupture Dynamics along Thrust Dipping Fault: Inertia Effects due to Free Surface Wave Interactions

NASA Astrophysics Data System (ADS)

Vilotte, J. P.; Scala, A.; Festa, G.

2017-12-01

We numerically investigate the dynamic interaction between free surface and up-dip, in-plane rupture propagation along thrust faults, under linear slip-weakening friction. With reference to shallow along-dip rupture propagation during large subduction earthquakes, we consider here low dip-angle fault configurations with fixed strength excess and depth-increasing initial stress. In this configuration, the rupture undergoes a break of symmetry with slip-induced normal stress perturbations triggered by the interaction with reflected waves from the free surface. We found that both body-waves - behind the crack front - and surface waves - at the crack front - can trigger inertial effects. When waves interact with the rupture before this latter reaches its asymptotic speed, the rupture can accelerate toward the asymptotic speed faster than in the unbounded symmetric case, as a result of these inertial effects. Moreover, wave interaction at the crack front also affects the slip rate generating large ground motion on the hanging wall. Imposing the same initial normal stress, frictional strength and stress drop while varying the static friction coefficient we found that the break of symmetry makes the rupture dynamics dependent on the absolute value of friction. The higher the friction the stronger the inertial effect both in terms of rupture acceleration and slip amount. When the contact condition allows the fault interface to open close to the free surface, the length of the opening zone is shown to depend on the propagation length, the initial normal stress and the static friction coefficient. These new results are shown to agree with analytical results of rupture propagation in bounded media, and open new perspectives for understanding the shallow rupture of large subduction earthquakes and tsunami sources.

Direct Measurements of Skin Friction

NASA Technical Reports Server (NTRS)

Dhawan, Satish

1953-01-01

A device has been developed to measure local skin friction on a flat plate by measuring the force exerted upon a very small movable part of the surface of the flat plate. These forces, which range from about 1 milligram to about 100 milligrams, are measured by means of a reactance device. The apparatus was first applied to measurements in the low-speed range, both for laminar and turbulent boundary layers. The measured skin-friction coefficients show excellent agreement with Blasius' and Von Karman's results. The device was then applied to high-speed subsonic flow and the turbulent-skin-friction coefficients were determined up to a Mach number of about 0.8. A few measurements in supersonic flow were also made. This paper describes the design and construction of the device and the results of the measurements.

Micromechanics of Friction in a Detailed Study of Mg-rich Phyllosilicates

NASA Astrophysics Data System (ADS)

Sanchez Roa, C.; Faulkner, D.; Boulton, C. J.; Jimenez Millan, J.; Nieto, F.

2016-12-01

Phyllosilicate minerals commonly occur within faults, which may accommodate slip either aseismically via creep mechanisms or seismically in earthquakes. The Mg-rich phyllosilicates talc, saponite, sepiolite, and palygorskite have different crystallography and habits. Sepiolite and palygorskite are fibrous due to their discontinuous tetrahedral layers, while saponite and talc are platy due to the continuity of their TOT and water layers. Friction experiments were conducted in a triaxial apparatus under 95 MPa effective normal stress with water and argon as pore fluids. Results show a marked contrast between friction coefficients of fibrous phyllosilicates, 0.57 to 0.63 for argon experiments and 0.4 to 0.5 for water-saturated experiments, and platy Mg-rich phyllosilicates, as low as 0.22 for argon experiments and 0.04 for water-saturated experiments. During velocity steps (where sliding velocity is increased or decreased by one order of magnitude), the two mineral groups exhibit distinctly dissimilar behaviours. After the direct effect of the change in sliding rate, fibrous phyllosilicates show a rapid exponential decay towards a new friction coefficient (a positive b value). Meanwhile, the friction coefficient of the platy phyllosilicates has a more linear evolution (a zero, or negative b value). This effect could be related to a difference in the sliding strength of the contact asperities which would be much higher for crystal surfaces of fibrous minerals with an indented surface due to the silicon tetrahedra inversions. The fibre-shaped crystals may consequently require higher amounts of volumetric work against the normal stress (dilatancy). SEM and TEM observations of the deformed samples showed a well-developed network of R1 Riedel shears in the fibrous materials; planar phyllosilicates show a more homogeneous matrix and incipient development of P foliation. Planar phyllosilicate grains align on their basal planes facilitating intergranular sliding, in contrast, the fibrous phyllosilicates appear to form an interlocking grid-like network that may promote dilatancy during velocity steps. The contrasting strength of Mg-rich phyllosilicates and analysis of their microstructures imply that phyllosilicate habit strongly influences the micromechanics of frictional sliding.

A tribological and biomimetic study of potential bone joint repair materials

NASA Astrophysics Data System (ADS)

Ribeiro, Rahul

This research investigates materials for bone-joint failure repair using tribological and biomimicking approaches. The materials investigated represent three different repairing strategies. Refractory metals with and without treatment are candidates for total joint replacements due to their mechanical strength, high corrosion resistance and biocompatibility. A composite of biodegradable polytrimethylene carbonate, hydroxyl apatite, and nanotubes was investigated for application as a tissue engineering scaffold. Non-biodegradable polymer polyimide combined with various concentrations of nanotubes was investigated as a cartilage replacement material. A series of experimental approaches were used in this research. These include analysis of material surfaces and debris using high-resolution techniques and tribological experiments, as well as evaluation of nanomechanical properties. Specifically, the surface structure and wear mechanisms were investigated using a scanning electron microscope and an atomic force microscope. Debris morphology and structure was investigated using a transmission electron microscope. The debris composition was analyzed using an X-ray diffractometer. Nanoindentation was incorporated to investigate the surface nanomechanical properties. Polytrimythelene carbonate combined with hydroxyapatite and nanotubes exhibited a friction coefficient lower than UHMWPE. The nanoindentation response mimicked cartilage more closely than UHMWPE. A composite formed with PI and nanotubes showed a varying friction coefficient and varying nanoindentation response with variation in nanotube concentration. Low friction coefficients corresponded with low modulus values. A theory was proposed to explain this behavior based on surface interactions between nanotubes and between nanotubes and PI. A model was developed to simulate the modulus as a function of nanotube concentration. The boronized refractory metals exhibited brittleness and cracking. Higher friction coefficients were associated with the formation of amorphous debris. The friction coefficient for boronized Cr (˜0.06) under simulated body fluid conditions was in the range found in natural joints.

The stochastic distribution of available coefficient of friction for human locomotion of five different floor surfaces.

PubMed

Chang, Wen-Ruey; Matz, Simon; Chang, Chien-Chi

2014-05-01

The maximum coefficient of friction that can be supported at the shoe and floor interface without a slip is usually called the available coefficient of friction (ACOF) for human locomotion. The probability of a slip could be estimated using a statistical model by comparing the ACOF with the required coefficient of friction (RCOF), assuming that both coefficients have stochastic distributions. An investigation of the stochastic distributions of the ACOF of five different floor surfaces under dry, water and glycerol conditions is presented in this paper. One hundred friction measurements were performed on each floor surface under each surface condition. The Kolmogorov-Smirnov goodness-of-fit test was used to determine if the distribution of the ACOF was a good fit with the normal, log-normal and Weibull distributions. The results indicated that the ACOF distributions had a slightly better match with the normal and log-normal distributions than with the Weibull in only three out of 15 cases with a statistical significance. The results are far more complex than what had heretofore been published and different scenarios could emerge. Since the ACOF is compared with the RCOF for the estimate of slip probability, the distribution of the ACOF in seven cases could be considered a constant for this purpose when the ACOF is much lower or higher than the RCOF. A few cases could be represented by a normal distribution for practical reasons based on their skewness and kurtosis values without a statistical significance. No representation could be found in three cases out of 15. Copyright © 2013 Elsevier Ltd and The Ergonomics Society. All rights reserved.

Structure and Mechanical Properties of CrTiAlN/TiAlN Composite Coatings Deposited by Multi-Arc Ion Plating

NASA Astrophysics Data System (ADS)

Yan, Shaojian; Tian, Canxin; Huang, Zhihong; Yang, Bing; Fu, Dejun

2014-10-01

CrTiAlN/TiAlN composite coatings were deposited on cemented carbide by using a home-made industrial scale multi-arc ion plating system. The samples were studied by X-ray diffraction, scanning electron microscopy (SEM), microhardness and ball-on-disk testing. The properties of the CrTiAlN/TiAlN coatings were significantly influenced by the microstructure and the deposition time ratio of TiAlN over CrTiAlN layers. With the increase of deposition time ratio, the microhardness of CrTiAlN/TiAlN increased from 28.6 GPa to 37.5 GPa, much higher than that of CrTiAlN coatings. The friction coefficients of the CrTiAlN/TiAlN coatings were higher than those of CrTiAlN coatings against a cemented carbide ball. The microhardness of the CrTiAlN/TiAlN coatings was changed after annealing at 800°C, and the friction coefficients of the annealed coatings were increased against the cemented carbide ball.

Layer growth kinetics and wear resistance of martensitic precipitation hardening stainless steel plasma nitrocarburized at 460°C with rare earth addition

NASA Astrophysics Data System (ADS)

Liu, R. L.; Qiao, Y. J.; Yan, M. F.; Fu, Y. D.

2013-09-01

To study the effect of rare earth (RE) addition on low temperature plasma nitrocarburizing of martensitic precipitation hardening stainless steel, 17-4PH stainless steel was plasma nitrocarburized at 460 °C for different times with RE addition. The modified layers were tested by optical microscope, scanning electron microscope, X-ray diffraction, microhardness tester and pin-on-disc tribometer. The experimental results show that the layer depth of plasma RE nitrocarburized layer can be increased up to 56% compared with plasma nitrocarburizing without RE addition. Incorporation of RE element is beneficial to the formation of nitrogen and carbon expanded martensite (α'N). The surface microhardness of plasma RE nitrocarburized layer can be increased to 1286 HV and higher up to 80 HV than that obtained from the conventional treated one. The friction coefficient of martensitic stainless steel can be dramatically decreased by low temperature plasma nitrocarburizing with RE addition, and the friction coefficient of the modified specimens decrease gradually with increasing process time in the present test condition.

The friction coefficient of shoulder joints remains remarkably low over 24 h of loading.

PubMed

Jones, Brian K; Durney, Krista M; Hung, Clark T; Ateshian, Gerard A

2015-11-05

The frictional response of whole human joints over durations spanning activities of daily living has not been reported previously. This study measured the friction of human glenohumeral joints during 24 h of reciprocal loading in a pendulum testing device, at moderate (0.2 mm/s, 4320 cycles) and low (0.02 mm/s, 432 cycles) sliding speeds, under a 200 N load. The effect of joint congruence was also investigated by testing human humeral heads against significantly larger mature bovine glenoids. Eight human joints and six bovine joints were tested in four combinations: human joints tested at moderate (hHCMS, n=6) and low speed (hHCLS, n=3), human humeral heads tested against bovine glenoids at moderate speed (LCMS, n=3), and bovine joints tested at moderate speed (bHCMS, n=3). In the first half hour the mean±standard deviation of the friction coefficient was hHCMS: 0.0016±0.0011, hHCLS: 0.0012±0.0002, LCMS: 0.0008±0.0002 and bHCMS: 0.0024±0.0008; in the last four hours it was hHCMS: 0.0057±0.0025, hHCLS: 0.0047±0.0017, LCMS: 0.0012±0.0003 and bHCMS: 0.0056±0.0016. The initial value was lower than the final value (p<0.0001). The value in LCMS was significantly lower than in hHCMS and bHCMS (p<0.01). No visual damage was observed in any of the specimens. These are the first results to demonstrate that the friction coefficient of natural human shoulders remains remarkably low (averaging as little as 0.0015 and no greater than 0.006) for up to 24 h of continuous loading. The sustained low friction coefficients observed in incongruent joints (~0.001) likely represent rolling rather than sliding friction. Copyright © 2015. Published by Elsevier Ltd.

Direct measurements of bed stress under swash in the field

NASA Astrophysics Data System (ADS)

Conley, Daniel C.; Griffin, John G.

2004-03-01

Utilizing flush mounted hot film anemometry, the bed stress under swash was measured directly in a field experiment conducted on Barret Beach, Fire Island, New York. The theory, development, and calibration of the instrument package are discussed, and results from the field experiment are presented. Examples of bed stress time series throughout a swash cycle are presented, and an ensemble averaged swash bed stress cycle is calculated. Strong asymmetry is observed between the uprush and backwash phases of the swash flow. The maximum bed shear stress exerted by the uprush is approximately double that of the backwash, while the duration of the backwash is 135% greater than that of the uprush. Friction coefficients in the swash zone are observed to be similar in magnitude to those from steady flow, with the mean observed friction coefficient equal to 0.0037. Swash friction coefficients derived from the current measurements exhibit a Reynolds number dependence similar to that observed for other flows. A systematic difference between coefficients for uprush and backwash is suggested.

Fretting properties of biodegradable Mg-Nd-Zn-Zr alloy in air and in Hank’s solution

NASA Astrophysics Data System (ADS)

Li, Wenting; Li, Nan; Zheng, Yufeng; Yuan, Guangyin

2016-11-01

Fretting is a significant cause for the failure of orthopedic implants. Currently, since magnesium and its alloys have been developed as promising biodegradable implant materials, the fretting behavior of the Mg alloys is of great research significance. In this study, a Mg-Nd-Zn-Zr alloy (hereafter, denoted as JDBM alloy) was selected as experimental material, and its fretting behaviors were evaluated under 5 N, 10 N and 20 N normal loads with a displacement of 200 μm under the frequency of 10 Hz at 37 °C in air and in Hank’s solution, respectively. The results indicated that while the friction coefficient decreased with the increment of the normal load, the wear volume of the alloy increased with the increment of the normal load both in air and in Hank’s solution. Both the friction coefficients and the wear volume of the fretting in Hank’s solution were much lower than those in air environment. The evolution trend of friction coefficients with time had different performance in air environment and the Hank’s solution group. Although oxidation occurred during the fretting tests in Hank’s solution, the damage of JDBM alloy was still reduced due to the lubrication effects of Hank’s solution. Moreover, the addition of Fetal bovine serum (FBS) could act as lubrication and result in the reduction of the fretting damage.

Strength anisotropy of shales deformed under uppermost crustal conditions

NASA Astrophysics Data System (ADS)

Bonnelye, Audrey; Schubnel, Alexandre; David, Christian; Henry, Pierre; Guglielmi, Yves; Gout, Claude; Fauchille, Anne-Laure; Dick, Pierre

2017-01-01

Conventional triaxial tests were performed on three sets of samples of Tournemire shale along different orientations relative to bedding (0°, 45°, and 90°). Experiments were carried out up to failure at increasing confining pressures ranging from 2.5 to 160 MPa, at strain rates ranging between 3 × 10-7s-1 and 3 × 10-5s-1. This allowed us to determine the entire anisotropic elastic compliance matrix as a function of confining pressure. Results show that the orientation of principal stress relative to bedding plays an important role on the brittle strength, with 45° orientation being the weakest. We fit our results with a wing crack micromechanical model and an anisotropic fracture toughness. We found low values of internal friction coefficient and apparent friction coefficient in agreement with friction coefficient of clay minerals (between 0.2 and 0.3) and values of KIc comparable to that already published in the literature. We also showed that strain rate has a strong impact on peak stress and that dilatancy appears right before failure and hence highlighting the importance of plasticity mechanisms. Although brittle failure was systematically observed, stress drops and associated slips were slow and deformation always remained aseismic (no acoustic emission were detected). This confirms that shales are good lithological candidates for shallow crust aseismic creep and slow slip events.

Evaluation of tribological properties of selected engine oils during operation of the friction pairs of steel-on-steel

NASA Astrophysics Data System (ADS)

Barszcz, Marcin; Józwik, Jerzy; Dziedzic, Krzysztof; Stec, Kamil

2017-10-01

The paper includes an assessment of the tribological properties of mineral and synthetic Lotos oil marked SAE 15W/40 and SAE 5W/40 at ambient temperature and 100 °C. The evaluation was based on the analysis of the tribological properties of friction couple consumables. Tribological tests were performed using the Anton Paar THT 1000 high temperature tribotester according to ASTM G133. Tribological properties were investigated using the "ball on disc" method. The change of friction coefficient, friction couple temperature, volume wear of samples and counter-samples and Hertz stresses were evaluated. In addition, hardness tests of the friction couple materials as well as surface roughness before and after friction were performed. On the basis of tribological studies, it was noted that Lotos Synthetic 5W/40 oil has better cooling properties compared. For both oils the coefficient of friction was lower at ambient temperature than at 100 °C. The highest value of volume wear of the sample was noted for the combination lubricated with Mineral Oil 15W/40 at 100 °C (0.0143 mm3) while for counter-sample lubricated with synthetic oil at ambient temperature (0.0039 mm3). The highest sample wear coefficient was recorded for the mineral oil lubricated at temperature of 100 °C (3.585*10-7 mm3/N/m) while for counter-sample lubricated with synthetic oil at ambient temperature (9.8768*10-8 mm3/N/m). The Hertz stress for each test couple had a value of 1.787 GPa.

Influences of surface modification of nano-silica by silane coupling agents on the thermal and frictional properties of cyanate ester resin

NASA Astrophysics Data System (ADS)

Chuang, Wang; Geng-sheng, Jiao; Lei, Peng; Bao-lin, Zhu; Ke-zhi, Li; Jun-long, Wang

2018-06-01

The surface of nano-silicon dioxide (nano-SiO2) particles was modified by small molecular coupling agent KH-560 and macromolecular coupling agent SEA-171, respectively, to change the surface activity and structure. The modified nano-SiO2 was then used for reinforcing cyanate ester resin (CE). Influences of the content of nano-SiO2 and the interfacial structure over the thermal and frictional properties of nano-SiO2/CE composites were investigated. The mechanism of the surface modification of silicon dioxide by KH-560 and SEA-171 was discussed. The experimental results show that the addition of coupling agents increased the interfacial bonding between nano-SiO2 particles and the CE resin so that the heat resistance and friction properties of the composites were improved. After surface treatment of nano-SiO2 by SEA-171, the thermal decomposition temperature of the 3.0 wt% nano-SiO2/CE composites increased nearly by 75 °C and the frictional coefficient was reduced by 25% compared with that of the pure CE, and the wear resistance increased by 77%.

Friction law and hysteresis in granular materials

PubMed Central

Wyart, M.

2017-01-01

The macroscopic friction of particulate materials often weakens as the flow rate is increased, leading to potentially disastrous intermittent phenomena including earthquakes and landslides. We theoretically and numerically study this phenomenon in simple granular materials. We show that velocity weakening, corresponding to a nonmonotonic behavior in the friction law, μ(I), is present even if the dynamic and static microscopic friction coefficients are identical, but disappears for softer particles. We argue that this instability is induced by endogenous acoustic noise, which tends to make contacts slide, leading to faster flow and increased noise. We show that soft spots, or excitable regions in the materials, correspond to rolling contacts that are about to slide, whose density is described by a nontrivial exponent θs. We build a microscopic theory for the nonmonotonicity of μ(I), which also predicts the scaling behavior of acoustic noise, the fraction of sliding contacts χ, and the sliding velocity, in terms of θs. Surprisingly, these quantities have no limit when particles become infinitely hard, as confirmed numerically. Our analysis rationalizes previously unexplained observations and makes experimentally testable predictions. PMID:28811373

Friction law and hysteresis in granular materials

NASA Astrophysics Data System (ADS)

DeGiuli, E.; Wyart, M.

2017-08-01

The macroscopic friction of particulate materials often weakens as the flow rate is increased, leading to potentially disastrous intermittent phenomena including earthquakes and landslides. We theoretically and numerically study this phenomenon in simple granular materials. We show that velocity weakening, corresponding to a nonmonotonic behavior in the friction law, μ(I), is present even if the dynamic and static microscopic friction coefficients are identical, but disappears for softer particles. We argue that this instability is induced by endogenous acoustic noise, which tends to make contacts slide, leading to faster flow and increased noise. We show that soft spots, or excitable regions in the materials, correspond to rolling contacts that are about to slide, whose density is described by a nontrivial exponent θs. We build a microscopic theory for the nonmonotonicity of μ(I), which also predicts the scaling behavior of acoustic noise, the fraction of sliding contacts χ, and the sliding velocity, in terms of θs. Surprisingly, these quantities have no limit when particles become infinitely hard, as confirmed numerically. Our analysis rationalizes previously unexplained observations and makes experimentally testable predictions.

Friction and wear behaviour of Ni-Cr-B hardface coating on 316LN stainless steel in liquid sodium at elevated temperature

NASA Astrophysics Data System (ADS)

Kumar, Hemant; Ramakrishnan, V.; Albert, S. K.; Bhaduri, A. K.; Ray, K. K.

2017-11-01

The sliding friction and wear behaviour of Ni-Cr-B hardface coating made on 316LN stainless steel were evaluated in liquid sodium at 823 K by using a fabricated reciprocating-type tribometer. The test parameters have been selected based on operational conditions prevailing in the Indian sodium cooled fast breeder reactors (FBRs). Accordingly, the tests were carried out at sliding speeds of 2 and 16 mm/s under contact stresses of 10 and 40 MPa respectively using Ni-Cr-B coated pin and disc specimens. The static and dynamic friction coefficients are found to be in the ranges of 0.03-0.07 and 0.01-0.02 respectively under the imposed test conditions. The estimated wear rates (WR) are found to be in the range of 0.62 × 10-12 - 3.07 × 10-12 m3/m; the magnitude of WR increases with increase in the contact stress. The examination of the worn disc specimens by confocal laser scanning microscopy indicated higher damage in specimens tested at 40 MPa compared to that in specimens tested at 10 MPa; the quantitative estimation of damage was made by the number of scars and their depth. These observations corroborate well with the morphological features of the worn surfaces of the pin specimens examined by scanning electron microscopy. The results unambiguously indicate superior friction coefficients and wear resistance of Ni-Cr-B coatings in liquid sodium compared to that in air under identical test conditions.

Contact Behavior of Composite CrTiSiN Coated Dies in Compressing of Mg Alloy Sheets under High Pressure

PubMed Central

Yang, T.S.; Yao, S.H.; Chang, Y.Y.; Deng, J.H.

2018-01-01

Hard coatings have been adopted in cutting and forming applications for nearly two decades. The major purpose of using hard coatings is to reduce the friction coefficient between contact surfaces, to increase strength, toughness and anti-wear performance of working tools and molds, and then to obtain a smooth work surface and an increase in service life of tools and molds. In this report, we deposited a composite CrTiSiN hard coating, and a traditional single-layered TiAlN coating as a reference. Then, the coatings were comparatively studied by a series of tests. A field emission SEM was used to characterize the microstructure. Hardness was measured using a nano-indentation tester. Adhesion of coatings was evaluated using a Rockwell C hardness indentation tester. A pin-on-disk wear tester with WC balls as sliding counterparts was used to determine the wear properties. A self-designed compression and friction tester, by combining a Universal Testing Machine and a wear tester, was used to evaluate the contact behavior of composite CrTiSiN coated dies in compressing of Mg alloy sheets under high pressure. The results indicated that the hardness of composite CrTiSiN coating was lower than that of the TiAlN coating. However, the CrTiSiN coating showed better anti-wear performance. The CrTiSiN coated dies achieved smooth surfaces on the Mg alloy sheet in the compressing test and lower friction coefficient in the friction test, as compared with the TiAlN coating. PMID:29316687

Contact Behavior of Composite CrTiSiN Coated Dies in Compressing of Mg Alloy Sheets under High Pressure.

PubMed

Yang, T S; Yao, S H; Chang, Y Y; Deng, J H

2018-01-08

Hard coatings have been adopted in cutting and forming applications for nearly two decades. The major purpose of using hard coatings is to reduce the friction coefficient between contact surfaces, to increase strength, toughness and anti-wear performance of working tools and molds, and then to obtain a smooth work surface and an increase in service life of tools and molds. In this report, we deposited a composite CrTiSiN hard coating, and a traditional single-layered TiAlN coating as a reference. Then, the coatings were comparatively studied by a series of tests. A field emission SEM was used to characterize the microstructure. Hardness was measured using a nano-indentation tester. Adhesion of coatings was evaluated using a Rockwell C hardness indentation tester. A pin-on-disk wear tester with WC balls as sliding counterparts was used to determine the wear properties. A self-designed compression and friction tester, by combining a Universal Testing Machine and a wear tester, was used to evaluate the contact behavior of composite CrTiSiN coated dies in compressing of Mg alloy sheets under high pressure. The results indicated that the hardness of composite CrTiSiN coating was lower than that of the TiAlN coating. However, the CrTiSiN coating showed better anti-wear performance. The CrTiSiN coated dies achieved smooth surfaces on the Mg alloy sheet in the compressing test and lower friction coefficient in the friction test, as compared with the TiAlN coating.

Effects of intraoral aging of arch-wires on frictional forces: An ex vivo study.

PubMed

Kumar, Avinash; Khanam, Arifa; Ghafoor, Hajra

2016-01-01

Archwires act as gears to move teeth with light, continuous forces. However, the intraoral use of orthodontic archwires is liable to surface deposits which alter the mechanical properties of archwires, causing an increase in the friction coefficient. To evaluate the surface changes of the stainless steel archwires after 6 weeks of intraoral use and its influence on frictional resistance during sliding mechanics. As-received rectangular 0.019" × 0.025" stainless steel orthodontic archwires (control) were compared with the archwires retrieved after the final phase of leveling and alignment stage of orthodontic treatment collected after 6 weeks of intraoral exposure (test samples) from 10 patients undergoing treatment. The control and test samples were used to evaluate surface debris using Scanning Electron Microscopy, surface roughness was assessed using Atomic Force Microscope and frictional forces were measured using Instron Universal Testing Machine in the buccal inter-bracket region that slides through the molar tube for space closure. Unpaired t -test and Pearson correlation tests were used for statistical analysis ( P < 0.05 level of significance). Significant increase was observed in the level of debris ( P = 0.0001), surface roughness ( P = 0.0001), and friction resistance ( P = 0.001) of orthodontic archwires after their intraoral exposure. Significant positive correlations ( P < 0.05) were also observed between these three variables. Stainless steel test archwires showed a significant increase in the degree of debris and surface roughness, increasing the frictional forces between the archwire-bracket interfaces which would considerably reduce the normal orthodontic forces. Thus, continuing the same archwire after levelling and alignment for space closure is not recommended.

Effect of roughness on stiction

NASA Astrophysics Data System (ADS)

Fuadi, Zahrul; Zahouani, Hassan; Takagi, Toshiyuki; Miki, Hiroyuki

2018-05-01

In this paper, the viscoelastic material was used to investigate the effect of roughness on stiction. The material is chosen because it is highly deformable so that contact during friction can be fully elastic. The soft surfaces were prepared by casting the silicon material on metal surfaces having smooth and unidirectional grooved texture. Two tests were conducted, indentation and friction, to find out the effect of roughness on parameters of normal contact stiffness, friction force and the difference between static and kinetic friction coefficient, μs-μk. As the results, it is found that all parameters are related to the surface roughness. Smoother surface tends to have a higher value of normal contact stiffness and higher value of friction force thus resulting in a larger difference between the static and kinetic coefficient of friction. Since the value of μs-μk is commonly related to the stick-slip motion, the smoother surface tends to have a larger propensity of stiction. It is shown by the result that the texture can reduce the stiction because it reduces the value of normal contact stiffness, resulting in a lower value of μs-μk.

Steady-state wear and friction in boundary lubrication studies

NASA Technical Reports Server (NTRS)

Loomis, W. R.; Jones, W. R., Jr.

1980-01-01

A friction and wear study was made at 20 C to obtain improved reproducibility and reliability in boundary lubrication testing. Ester-base and C-ether-base fluids were used to lubricate a pure iron rider in sliding contact with a rotating M-50 steel disk in a friction and wear apparatus. Conditions included loads of 1/2 and 1 kg and sliding velocities of 3.6 to 18.2 m/min in a dry air atmosphere and stepwise time intervals from 1 to 250 min for wear measurements. The wear rate results were compared with those from previous studies where a single 25 min test period was used. Satisfactory test conditions for studying friction and wear in boundary lubrication for this apparatus were found to be 1 kg load; sliding velocities of 7.1 to 9.1 m/min (50 rpm disk speed); and use of a time stepwise test procedure. Highly reproducible steady-state wear rates and steady-state friction coefficients were determined under boundary conditions. Wear rates and coefficients of friction were constant following initially high values during run-in periods.

Minimizing of the boundary friction coefficient in automotive engines using Al2O3 and TiO2 nanoparticles

NASA Astrophysics Data System (ADS)

Ali, Mohamed Kamal Ahmed; Xianjun, Hou; Elagouz, Ahmed; Essa, F. A.; Abdelkareem, Mohamed A. A.

2016-12-01

Minimizing of the boundary friction coefficient is critical for engine efficiency improvement. It is known that the tribological behavior has a major role in controlling the performance of automotive engines in terms of the fuel consumption. The purpose of this research is an experimental study to minimize the boundary friction coefficient via nano-lubricant additives. The tribological characteristics of Al2O3 and TiO2 nano-lubricants were evaluated under reciprocating test conditions to simulate a piston ring/cylinder liner interface in automotive engines. The nanoparticles were suspended in a commercially available lubricant in a concentration of 0.25 wt.% to formulate the nano-lubricants. The Al2O3 and TiO2 nanoparticles had sizes of 8-12 and 10 nm, respectively. The experimental results have shown that the boundary friction coefficient reduced by 35-51% near the top and bottom dead center of the stroke (TDC and BDC) for the Al2O3 and TiO2 nano-lubricants, respectively. The anti-wear mechanism was generated via the formation of protective films on the worn surfaces of the ring and liner. These results will be a promising approach for improving fuel economy in automotive.

[Process study on hysteresis of vegetation cover influencing sand-dust events].

PubMed

Xu, Xing-Kui; Wang, Xiao-Tao; Zhang, Feng

2009-02-15

Data analysis from satellite and weather stations during 1982-2000 shows nonlinear relationship between vegetation cover and sand-dust events is present in most part of China. Vegetation cover ratio in summer can impact significantly on the frequency of sand-dust storms from winter to spring in the source regions of sand-dust events. It is not quite clear about the hysteresis that vegetation cover in summer influence sand-dust events during winter and spring. A quasi-geostrophic barotropic model is used under the condition of 3 magnitude of frictional coefficient to investigate the cause of the hysteresis. Wind velocity shows a greatest decline at 90% during 72 h as initial wind velocity is 10 m/s for magnitude of frictional coefficient between atmosphere and water surface, greatest decline at 100% during 18 h for magnitude of frictional coefficient between atmosphere and bare soil and a 100% reduction of wind speed during 1 h for magnitude of frictional coefficient between atmosphere and vegetation cover. Observation and simulation prove that residual root and stem from summervegetation are one of factors to influence sand-dust events happened during winter and spring. Air inhibition from residual root and stem is a most important reason for hysteresis that vegetation cover influence sand-dust events.

Tribological behavior of 440C martensitic stainless steel from -184 C to 750 C

NASA Technical Reports Server (NTRS)

Slifka, A. J.; Compos, R.; Morgan, T. J.; Siegwarth, J. D.; Chaudhuri, Dilip K.

1992-01-01

Characterization of the coefficient of friction and wear rate of 440C stainless steel is needed to understand the effects of frictional heating in the bearings of the High Pressure Oxygen Turbopump of the Space Shuttle Main Engine. The coefficient of friction and wear rate have been measured over a range of temperature varying from liquid oxygen temperature (-184 C) to 750 C. The normal load has also been varied resulting in a variation of Hertzian stress from 0.915 to 3.660 GPa while the surface velocity has been varied from 0.5 to 2.0 m/s.

Effect of friction on shear jamming

NASA Astrophysics Data System (ADS)

Wang, Dong; Ren, Jie; Dijksman, Joshua; Behringer, Robert

2014-11-01

Shear Jamming of granular materials was first found for systems of frictional disks, with a static friction coefficients μs ~= 0 . 6 . Jamming by shear is obtained by starting from a zero-stress state with a packing fraction ϕS <= ϕ <=ϕJ between ϕJ (isotropic jamming) and a lowest ϕS for shear jamming. This phenomenon is associated with strong anisotropy in stress and the contact network in the form of ``force chains,'' which are stabilized and/or enhanced by the presence of friction. The issue that we address experimentally is how reducing friction affects shear jamming. We use photoelastic disks that have been wrapped with Teflon, lowering the friction coefficient substantially from previous experiments. The Teflon-wrapped disks were placed in a well-studied 2D shear apparatus (Ren et al., PRL, 110, 018302 (2013)), which provides uniform simple shear without generating shear bands. Shear jamming is still observed, but the difference ϕJ -ϕS is smaller than for higher friction particles. With Teflon-wrapped disks, we observe larger anisotropies compared to the previous experiment with higher friction particles at the same packing fraction, which indicates force chains tending to be straight in the low friction system. We acknowledge support from NSF Grant No. DMR12-06351, ARO Grant No. W911NF-1-11-0110, and NASA Grant No. NNX10AU01G.

Effects of wall friction on flow in a quasi-2D hopper

NASA Astrophysics Data System (ADS)

Shah, Neil; Birwa, Sumit; Carballo-Ramirez, Brenda; Pleau, Mollie; Easwar, Nalini; Tewari, Shubha

Our experiments on the gravity-driven flow of spherical particles in a vertical hopper examine how the flow rate varies with opening size and wall friction. We report here on a model simulation using LAMMPS of the experimental geometry, a quasi-2D hopper. Keeping inter-particle friction fixed, the coefficient of friction at the walls is varied from 0.0 to 0.9 for a range of opening sizes. Our simulations find a steady rate of flow at each wall friction and outlet size. The Janssen effect attributes the constant rate of flow of a granular column to the column height independence of the pressure at the base, since the weight of the grains is borne in part by friction at the walls. However, we observe a constant flow regime even in the absence of wall friction, suggesting that wall friction may not be a necessary condition for pressure saturation. The observed velocities of particles near the opening are used to extrapolate their starting positions had they been in free fall. In contrast to scaling predictions, our data suggest that the height of this free-fall arch does not vary with opening size for higher frictional coefficients. We analyze the velocity traces of particles to see the range over which contact interactions remain collisional as they approach the hopper outlet.

Correlation between Mechanical Properties with Specific Wear Rate and the Coefficient of Friction of Graphite/Epoxy Composites

PubMed Central

Alajmi, Mahdi; Shalwan, Abdullah

2015-01-01

The correlation between the mechanical properties of Fillers/Epoxy composites and their tribological behavior was investigated. Tensile, hardness, wear, and friction tests were conducted for Neat Epoxy (NE), Graphite/Epoxy composites (GE), and Data Palm Fiber/Epoxy with or without Graphite composites (GFE and FE). The correlation was made between the tensile strength, the modulus of elasticity, elongation at the break, and the hardness, as an individual or a combined factor, with the specific wear rate (SWR) and coefficient of friction (COF) of composites. In general, graphite as an additive to polymeric composite has had an eclectic effect on mechanical properties, whereas it has led to a positive effect on tribological properties, whilst date palm fibers (DPFs), as reinforcement for polymeric composite, promoted a mechanical performance with a slight improvement to the tribological performance. Statistically, this study reveals that there is no strong confirmation of any marked correlation between the mechanical and the specific wear rate of filler/Epoxy composites. There is, however, a remarkable correlation between the mechanical properties and the friction coefficient of filler/Epoxy composites. PMID:28793431

Modelling the stochastic nature of the available coefficient of friction at footwear-floor interfaces.

PubMed

Gragg, Jared; Klose, Ellison; Yang, James

2017-07-01

The available coefficient of friction (ACOF) is a measure of the friction available between two surfaces, which for human gait would be the footwear-floor interface. It is often compared to the required coefficient of friction (RCOF) to determine the likelihood of a slip in gait. Both the ACOF and RCOF are stochastic by nature meaning that neither should be represented by a deterministic value, such as the sample mean. Previous research has determined that the RCOF can be modelled well by either the normal or lognormal distributions, but previous research aimed at determining an appropriate distribution for the ACOF was inconclusive. This study focuses on modelling the stochastic nature of the ACOF by fitting eight continuous probability distributions to ACOF data for six scenarios. In addition, the data were used to study the effect that a simple housekeeping action such as sweeping could have on the ACOF. Practitioner Summary: Previous research aimed at determining an appropriate distribution for the ACOF was inconclusive. The study addresses this issue as well as looking at the effect that an act such as sweeping has on the ACOF.

Correlation between Mechanical Properties with Specific Wear Rate and the Coefficient of Friction of Graphite/Epoxy Composites.

PubMed

Alajmi, Mahdi; Shalwan, Abdullah

2015-07-08

The correlation between the mechanical properties of Fillers/Epoxy composites and their tribological behavior was investigated. Tensile, hardness, wear, and friction tests were conducted for Neat Epoxy (NE), Graphite/Epoxy composites (GE), and Data Palm Fiber/Epoxy with or without Graphite composites (GFE and FE). The correlation was made between the tensile strength, the modulus of elasticity, elongation at the break, and the hardness, as an individual or a combined factor, with the specific wear rate (SWR) and coefficient of friction (COF) of composites. In general, graphite as an additive to polymeric composite has had an eclectic effect on mechanical properties, whereas it has led to a positive effect on tribological properties, whilst date palm fibers (DPFs), as reinforcement for polymeric composite, promoted a mechanical performance with a slight improvement to the tribological performance. Statistically, this study reveals that there is no strong confirmation of any marked correlation between the mechanical and the specific wear rate of filler/Epoxy composites. There is, however, a remarkable correlation between the mechanical properties and the friction coefficient of filler/Epoxy composites.

Development of Nanofluids as Lubricant to Study Friction and Wear Behavior of Stainless Steels

NASA Astrophysics Data System (ADS)

Sahoo, Rashmi Ranjan; Bhattacharjee, Santu; Das, Tuhin

A number of nanofluids have been prepared to study the effect of lubrication properties of nanofluids on stainless steels taking Kaolin and Boron Nitride (BN) as the lubricant particles and Sodium Dodecyl Sulfate (SDS), Cetyl Trimethyl Ammonium Bromide (CTAB), Sodium Hexa Meta Phosphate (SHMP) as dispersants in the same liquid medium i.e. water. A pin on disc tribometer is being used to access the tribological behaviour of the prepared nanofluids. The particle size of these particle dispersions are examined with a nanoparticle size analyzer. It has been found that the use of dispersants significantly control the particle size and tribological behavior of the nanofluids as for Boron Nitride particle with Sodium Dodecyl Sulfate (SDS) as dispersant has got a very low value of coefficient of friction being equal to 0.142 while without dispersant the value is 0.498. Similarly, in case of Kaolin water with SDS as dispersant the value of coefficient of friction obtained is 0.161 and without dispersant it is 0.333. Sodium Dodecyl Sulfate (SDS) as dispersant has resulted a very low coefficient of friction compared to other dispersants tested even though it doesn’t always assure a least particle size. The role of SDS in yielding the lowest friction has pursued significant attention for further investigation.

Phase transformation and tribological properties of Ag-MoO3 contained NiCrAlY based composite coatings fabricated by laser cladding

NASA Astrophysics Data System (ADS)

Wang, Lingqian; Zhou, Jiansong; Xin, Benbin; Yu, Youjun; Ren, Shufang; Li, Zhen

2017-08-01

Ag-MoO3 contained NiCrAlY based composite coating was successfully prepared on GH4169 stainless steel substrate by high energy ball milling and laser cladding. The microstructure and phase transformation were investigated by scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) and X-ray diffraction spectrum (XRD). The tribological behavior and mechanism from room temperature to 800 °C were investigated. Results showed that MoO3 in the composite powders transformed to Mo2C reinforcement under the high energy density of laser, and a series of opposite transformation occurred during friction process. The coating showed the lowest friction coefficient and low wear rate at 600 °C and 800 °C due to the generation of Ag2MoO4 during tribo-chemical reactions and the formation of lubrication glaze on the worn surface. Ag made effective lubrication when the temperature rose up to 200 °C. The coating displayed a relatively high friction coefficient (about 0.51) at 400 °C, because though MoO3 (oxidation products of Mo2C) and Ag2MoO4 were detected on the worn surface, they could not realize effective lubrication at this temperature. Abrasive wear, adhesive wear and plastic deformation contributed to the increased friction and wear.

In vivo measured joint friction in hip implants during walking after a short rest

PubMed Central

Damm, Philipp; Bender, Alwina; Duda, Georg; Bergmann, Georg

2017-01-01

Introduction It has been suspected that friction in hip implants is higher when walking is initiated after a resting period than during continuous movement. It cannot be excluded that such increased initial moments endanger the cup fixation in the acetabulum, overstress the taper connections in the implant or increase wear. To assess these risks, the contact forces, friction moments and friction coefficients in the joint were measured in vivo in ten subjects. Instrumented hip joint implants with telemetric data transmission were used to access the contact loads between the cup and head during the first steps of walking after a short rest. Results The analysis demonstrated that the contact force is not increased during the first step. The friction moment in the joint, however, is much higher during the first step than during continuous walking. The moment increases throughout the gait cycle were 32% to 143% on average and up to 621% individually. The high initial moments will probably not increase wear by much in the joint. However, comparisons with literature data on the fixation resistance of the cup against moments made clear that the stability can be endangered. This risk is highest during the first postoperative months for cementless cups with insufficient under-reaming. The high moments after a break can also put taper connections between the head and neck and neck and shaft at a higher risk. Discussion During continuous walking, the friction moments individually were extremely varied by factors of 4 to 10. Much of this difference is presumably caused by the varying lubrication properties of the synovia. These large moment variations can possibly lead to friction-induced temperature increases during walking, which are higher than the 43.1°C which have previously been observed in a group of only five subjects. PMID:28350858

Tactile texture and friction of soft sponge surfaces.

PubMed

Takahashi, Akira; Suzuki, Makoto; Imai, Yumi; Nonomura, Yoshimune

2015-06-01

We evaluated the tactile texture and frictional properties of five soft sponges with various cell sizes. The frictional forces were measured by a friction meter containing a contact probe with human-finger-like geometry and mechanical properties. When the subjects touched these sponges with their fingers, hard-textured sponges were deemed unpleasant. This tactile feeling changed with friction factors including friction coefficients, their temporal patterns, as well as mechanical and shape factors. These findings provide useful information on how to control the tactile textures of various sponges. Copyright © 2015 Elsevier B.V. All rights reserved.

Tribological Behavior of Electroless Ni-P Coatings in Various Corrosive Environments

NASA Astrophysics Data System (ADS)

Panja, Bikash; Das, Suman Kalyan; Sahoo, Prasanta

2016-04-01

The present paper deals with the study of tribological characteristics, viz. friction and wear, of electroless Ni-P coating in corrosive environments (brine, acidic and alkaline) by varying different coating process parameters as well as varying the tribological testing parameters, viz. applied load and speed. The optimized results of coating process parameters for minimum friction and wear performance of the coating are presented. Moreover, a detailed study of the tribological behavior of the coating is undertaken individually for the three corrosive environments. The results obtained are compared among each other and also with the dry condition test of the coating. It is found that the friction coefficient of Ni-P coating decreases with increase in load for all environments. In case of wear, the wear rate of Ni-P coating gradually increases with increase in load for all mediums but the same decreases after 40N in brine and alkaline mediums. However, for acidic solution, the wear rate shows a continuous increasing trend. It is observed that alkaline and brine environments are favorable from friction and wear point of view of the coating, respectively. Microstructure study of the coatings is also performed and the coating is found to be of cauliflower-like morphology. The coating also exhibits amorphous structure in as-deposited condition, which gradually turns crystalline with heat treatment.

Influence of friction forces on the motion of VTOL aircraft during landing operations on ships at sea

NASA Technical Reports Server (NTRS)

Howard, J. C.; Chin, D. O.

1981-01-01

Equations describing the friction forces generated during landing operations on ships at sea were formulated. These forces depend on the platform reaction and the coefficient of friction. The platform reaction depends on the relative sink rate and the shock absorbing capability of the landing gear. The friction coefficient varies with the surface condition of the landing platform and the angle of yaw of the aircraft relative to the landing platform. Landings by VTOL aircraft, equipped with conventional oleopneumatic landing gears are discussed. Simplifications are introduced to reduce the complexity of the mathematical description of the tire and shock strut characteristics. Approximating the actual complicated force deflection characteristic of the tire by linear relationship is adequate. The internal friction forces in the shock strut are included in the landing gear model. A set of relatively simple equations was obtained by including only those tire and shock strut characteristics that contribute significantly to the generation of landing gear forces.

On the Link Between Kolmogorov Microscales and Friction in Wall-Bounded Flow of Viscoplastic Fluids

NASA Astrophysics Data System (ADS)

Ramos, Fabio; Anbarlooei, Hamid; Cruz, Daniel; Silva Freire, Atila; Santos, Cecilia M.

2017-11-01

Most discussions in literature on the friction coefficient of turbulent flows of fluids with complex rheology are empirical. As a rule, theoretical frameworks are not available even for some relatively simple constitutive models. In this work, we present a new family of formulas for the evaluation of the friction coefficient of turbulent flows of a large family of viscoplastic fluids. The developments combine an unified analysis for the description of the Kolmogorov's micro-scales and the phenomenological turbulence model of Gioia and Chakraborty. The resulting Blasius-type friction equation has only Blasius' constant as a parameter, and tests against experimental data show excellent agreement over a significant range of Hedstrom and Reynolds numbers. The limits of the proposed model are also discussed. We also comment on the role of the new formula as a possible benchmark test for the convergence of DNS simulations of viscoplastic flows. The friction formula also provides limits for the Maximum Drag Reduction (MDR) for viscoplastic flows, which resembles MDR asymptote for viscoelastic flows.

Friction and wear behavior of single-crystal silicon carbide in sliding contact with various metals

NASA Technical Reports Server (NTRS)

Miyoshi, K.; Buckley, D. H.

1978-01-01

Sliding friction experiments were conducted with single-crystal silicon carbide in contact with various metals. Results indicate the coefficient of friction is related to the relative chemical activity of the metals. The more active the metal, the higher the coefficient of friction. All the metals examined transferred to silicon carbide. The chemical activity of the metal and its shear modulus may play important roles in metal transfer, the form of the wear debris and the surface roughness of the metal wear scar. The more active the metal, and the less resistance to shear, the greater the transfer to silicon carbide and the rougher the wear scar on the surface of the metal. Hexagon shaped cracking and fracturing formed by cleavage of both prismatic and basal planes is observed on the silicon carbide surface.

Diffusion in different models of active Brownian motion

NASA Astrophysics Data System (ADS)

Lindner, B.; Nicola, E. M.

2008-04-01

Active Brownian particles (ABP) have served as phenomenological models of self-propelled motion in biology. We study the effective diffusion coefficient of two one-dimensional ABP models (simplified depot model and Rayleigh-Helmholtz model) differing in their nonlinear friction functions. Depending on the choice of the friction function the diffusion coefficient does or does not attain a minimum as a function of noise intensity. We furthermore discuss the case of an additional bias breaking the left-right symmetry of the system. We show that this bias induces a drift and that it generally reduces the diffusion coefficient. For a finite range of values of the bias, both models can exhibit a maximum in the diffusion coefficient vs. noise intensity.

Effect of Polishing on the Friction Behaviors and Cutting Performance of Boron-Doped Diamond Films on WC-Co Inserts

NASA Astrophysics Data System (ADS)

Wang, Liang; Shen, Bin; Sun, Fanghong; Zhang, Zhiming

2014-04-01

Boron doped (B-doped) diamond films are deposited onto WC-Co inserts by HFCVD with the mixture of acetone, trimethyl borate (C3H9BO3) and H2. The as-deposited B-doped diamond films are characterized with scanning electron microscope (SEM), X-ray diffraction (XRD) spectroscopy, Raman spectroscopy, 3D surface topography based on white-light interferometry and Rockwell hardness tester. The effects of mechanical polishing on the friction behavior and cutting performance of B-doped diamond are evaluated by ball-on-plate type reciprocating tribometer and turning of aluminum alloy 7075 materials, respectively. For comparison, the same tests are also conducted for the bare WC-Co inserts with smooth surface. Friction tests suggest that the unpolished and polished B-doped diamond films possess relatively low fluctuation of friction coefficient than as-received bare WC-Co samples. The average stable friction coefficient for B-doped diamond films decreases apparently after mechanical polishing. The values for WC-Co sample, unpolished and polished B-doped diamond films are approximately 0.38, 0.25 and 0.11, respectively. The cutting results demonstrate that the low friction coefficient and high adhesive strength of B-doped diamond films play an essential role in the cutting performance enhancement of the WC-Co inserts. However, the mechanical polishing process may lower the adhesive strength of B-doped diamond films. Consequently, the polished B-doped diamond coated inserts show premature wear in the machining of adhesive aluminum alloy materials.

A robust control scheme for flexible arms with friction in the joints

NASA Technical Reports Server (NTRS)

Rattan, Kuldip S.; Feliu, Vicente; Brown, H. Benjamin, Jr.

1988-01-01

A general control scheme to control flexible arms with friction in the joints is proposed in this paper. This scheme presents the advantage of being robust in the sense that it minimizes the effects of the Coulomb friction existing in the motor and the effects of changes in the dynamic friction coefficient. A justification of the robustness properties of the scheme is given in terms of the sensitivity analysis.