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Sample records for frictional contact interfaces

  1. Non-Amontons-Coulomb local friction law of randomly rough contact interfaces with rubber

    NASA Astrophysics Data System (ADS)

    Nguyen, Danh Toan; Wandersman, Elie; Prevost, Alexis; Le Chenadec, Yohan; Fretigny, Christian; Chateauminois, Antoine

    2013-12-01

    We report on measurements of the local friction law at a multi-contact interface formed between a smooth rubber and statistically rough glass lenses, under steady-state friction. Using contact imaging, surface displacements are measured, and inverted to extract both distributions of frictional shear stress and contact pressure with a spatial resolution of about 10\\ \\mu\\text{m} . For a glass surface whose topography is self-affine with a Gaussian height asperity distribution, the local frictional shear stress is found to vary sub-linearly with the local contact pressure over the whole investigated pressure range. Such sub-linear behavior is also evidenced for a surface with a non-Gaussian height asperity distribution, demonstrating that, for such multi-contact interfaces, Amontons-Coulomb's friction law does not prevail at the local scale.

  2. Travelling interface waves in a brake-like system under unilateral contact and Coulomb friction

    NASA Astrophysics Data System (ADS)

    Nguyen, Quoc Son; Oueslati, Abdelbacet; Steindl, Alois; Teufel, Andreas; Troger, Hans

    2008-01-01

    This article considers the frictional interface waves generated by the flutter instability of the sliding steady state for an elastic tube in frictional contact with a rigid and rotating shaft. According to the values of the contact pressure, the rotation velocity and the friction coefficient, several periodic dynamical responses can be found under the form of travelling surface waves. Such a periodic solution may be interesting in the study of a possible dynamic transition from the sliding steady state in the spirit of Andronov-Hopf bifurcation. Examples of stick-slip, stick-slip-separation and stick-slip-separation-reverse-slip waves propagating along the contact surface, obtained by various semi-analytical and numerical approaches, are reported here. Some results on the stability of these travelling waves are also indicated. To cite this article: Q.S. Nguyen et al., C. R. Mecanique 336 (2008).

  3. Swing Friction Behavior of the Contact Interface Between CoCrMo and UHMWPE Under Dynamic Loading

    NASA Astrophysics Data System (ADS)

    Chen, Kai; Zhang, Dekun; Yang, Xuehui; Zhang, Xin; Wang, Qingliang; Qi, Jianwei

    2016-12-01

    CoCrMo alloy and UHMWPE have been widely used in knee joint prosthesis implantation materials. In this paper, swing friction behavior of the contact interface between CoCrMo alloy and UHMWPE is studied under dynamic loading. Swing friction characteristic and damage mechanism are discussed. The results show that swing friction coefficients increase with the rising of maximum normal load and swing angular amplitude. Unloading-standing could play alleviative roles in friction and wear to a large degree. As the cycle number gradually increases, the surface roughness of UHMWPE decreases, while the roughness of CoCrMo increases. During the swing friction, the main damage mechanism of CoCrMo is abrasive wear and the main damage mechanisms of UHMWPE are abrasive wear, fatigue wear and plastic deformation. Besides, it is easier to generate surface damages with small angle and heavy load.

  4. Tire/runway friction interface

    NASA Technical Reports Server (NTRS)

    Yager, Thomas J.

    1990-01-01

    An overview is given of NASA Langley's tire/runway pavement interface studies. The National Tire Modeling Program, evaluation of new tire and landing gear designs, tire wear and friction tests, and tire hydroplaning studies are examined. The Aircraft Landing Dynamics Facility is described along with some ground friction measuring vehicles. The major goals and scope of several joint FAA/NASA programs are identified together with current status and plans.

  5. Alternative methods to model frictional contact surfaces using NASTRAN

    NASA Technical Reports Server (NTRS)

    Hoang, Joseph

    1992-01-01

    Elongated (slotted) holes have been used extensively for the integration of equipment into Spacelab racks. In the past, this type of interface has been modeled assuming that there is not slippage between contact surfaces, or that there is no load transfer in the direction of the slot. Since the contact surfaces are bolted together, the contact friction provides a load path determined by the normal applied force (bolt preload) and the coefficient of friction. Three alternate methods that utilize spring elements, externally applied couples, and stress dependent elements are examined to model the contacted surfaces. Results of these methods are compared with results obtained from methods that use GAP elements and rigid elements.

  6. Influence of contact aging on nanoparticle friction kinetics.

    PubMed

    Feldmann, Michael; Dietzel, Dirk; Fuchs, Harald; Schirmeisen, André

    2014-04-18

    One of the oldest concepts in tribology is stick-slip dynamics, where a disruptive sequence of stick and slip phases determine the overall resistance in sliding friction. While the mechanical energy dissipates in the sudden slip phase, the stick phase has been shown to be characterized by contact strengthening mechanisms, also termed contact aging. We present experiments of sliding nanoparticles, where friction is measured as a function of sliding velocity and interface temperature. The resulting complex interdependence is in good agreement with Monte Carlo simulations, in which the energy barrier for contact breaking increases logarithmically with time, at a rate governed by thermal activation.

  7. The evolving quality of frictional contact with graphene.

    PubMed

    Li, Suzhi; Li, Qunyang; Carpick, Robert W; Gumbsch, Peter; Liu, Xin Z; Ding, Xiangdong; Sun, Jun; Li, Ju

    2016-11-24

    -slip behaviour. While the quantity of atomic-scale contacts (true contact area) evolves, the quality (in this case, the local pinning state of individual atoms and the overall commensurability) also evolves in frictional sliding on graphene. Moreover, the effects can be tuned by pre-wrinkling. The evolving contact quality is critical for explaining the time-dependent friction of configurationally flexible interfaces.

  8. Effect of fluorocarbon self-assembled monolayer films on sidewall adhesion and friction of surface micromachines with impacting and sliding contact interfaces

    SciTech Connect

    Xiang, H.; Komvopoulos, K.

    2013-06-14

    A self-assembled monolayer film consisting of fluoro-octyltrichlorosilane (FOTS) was vapor-phase deposited on Si(100) substrates and polycrystalline silicon (polysilicon) surface micromachines. The hydrophobic behavior and structural composition of the FOTS film deposited on Si(100) were investigated by goniometry and X-ray photoelectron spectroscopy, respectively. The effects of contact pressure, relative humidity, temperature, and impact/sliding cycles on the adhesive and friction behavior of uncoated and FOTS-coated polysilicon micromachines (referred to as the Si and FOTS/Si micromachines, respectively) were investigated under controlled loading and environmental conditions. FOTS/Si micromachines demonstrated much lower and stable adhesion than Si micromachines due to the highly hydrophobic and conformal FOTS film. Contrary to Si micromachines, sidewall adhesion of FOTS/Si micromachines demonstrated a weak dependence on relative humidity, temperature, and impact cycles. In addition, FOTS/Si micromachines showed low and stable adhesion and low static friction for significantly more sliding cycles than Si micromachines. The adhesive and static friction characteristics of Si and FOTS/Si micromachines are interpreted in the context of physicochemical surface changes, resulting in the increase of the real area of contact and a hydrophobic-to-hydrophilic transition of the surface chemical characteristics caused by nanoscale surface smoothening and the removal of the organic residue (Si micromachines) or the FOTS film (FOTS/Si micromachines) during repetitive impact and oscillatory sliding of the sidewall surfaces.

  9. Frictional contact of two rotating elastic disks

    NASA Astrophysics Data System (ADS)

    Ostrik, V. I.; Ulitko, A. F.

    2007-10-01

    We study the problem of constrained uniform rotation of two precompressed elastic disks made of different materials with friction forces in the contact region taken into account. The exact solution of the problem is obtained by the Wiener-Hopf method. An important stage in the study of rolling of elastic bodies is the Hertz theory [1] of contact interaction of elastic bodies with smoothly varying curvature in the contact region under normal compression. Friction in the contact region is assumed to be negligible. If there are tangential forces and the friction in the contact region is taken into account, then the picture of contact interaction of elastic bodies changes significantly. Although the normal contact stress distribution strictly follows the Hertz theory for bodies with identical elastic properties and apparently slightly differs from the Hertz diagram for bodies made of different materials, the presence of tangential stresses results in the splitting of the contact region into the adhesion region and the slip region. This phenomenon was first established by Reynolds [2], who experimentally discovered slip regions near points of material entry in and exit from the contact region under constrained rolling of an aluminum cylinder on a rubber base. The theoretical justification of the partial slip phenomenon in the contact region, discovered by Reynolds [2], can be found in Carter [3] and Fromm [4]. Moreover, Fromm presents a complete solution of the problem of constrained uniform rotation of two identical disks. Apparently, Fromm was the first to consider the so-called "clamped" strain and postulated that slip is absent at the point at which the disk materials enter the contact region. Ishlinskii [5, 6] gave an engineering solution of the problem on slip in the contact region under rolling friction. Considering the problem on a rigid disk rolling on an elastic half-plane, we model this problem by an infinite set of elastic vertical rods using Winkler

  10. On uniqueness for frictional contact rate problems

    NASA Astrophysics Data System (ADS)

    Radi, E.; Bigoni, D.; Tralli, A.

    1999-02-01

    A linear elastic solid having part of the boundary in unilateral frictional contact witha stiffer constraint is considered. Bifurcations of the quasistatic velocity problem are analyzed,making use of methods developed for elastoplasticity. An exclusion principle for bifurcation isproposed which is similar, in essence, to the well-known exclusion principle given by Hill, 1958. Sufficient conditions for uniqueness are given for a broad class of contactconstitutive equations. The uniqueness criteria are based on the introduction of linear comparisoninterfaces defined both where the contact rate constitutive equation are piece-wise incrementallylinear and where these are thoroughly nonlinear. Structural examples are proposed which giveevidence to the applicability of the exclusion criteria.

  11. 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.

  12. Adaptive methods, rolling contact, and nonclassical friction laws

    NASA Technical Reports Server (NTRS)

    Oden, J. T.

    1989-01-01

    Results and methods on three different areas of contemporary research are outlined. These include adaptive methods, the rolling contact problem for finite deformation of a hyperelastic or viscoelastic cylinder, and non-classical friction laws for modeling dynamic friction phenomena.

  13. 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.

  14. The Frictional Force with Respect to the Actual Contact Surface

    NASA Technical Reports Server (NTRS)

    Holm, Ragnar

    1944-01-01

    Hardy's statement that the frictional force is largely adhesion, and to a lesser extent, deformation energy is proved by a simple experiment. The actual contact surface of sliding contacts and hence the friction per unit of contact surface was determined in several cases. It was found for contacts in normal atmosphere to be about one-third t-one-half as high as the macroscopic tearing strength of the softest contact link, while contacts annealed in vacuum and then tested, disclosed frictional forces which are greater than the macroscopic strength.

  15. Friction of viscoelastic elastomers with rough surfaces under torsional contact conditions

    NASA Astrophysics Data System (ADS)

    Trejo, Miguel; Fretigny, Christian; Chateauminois, Antoine

    2013-11-01

    Frictional properties of contacts between a smooth viscoelastic rubber and rigid surfaces are investigated using a torsional contact configuration where a glass lens is continuously rotated on the rubber surface. From the inversion of the displacement field measured at the surface of the rubber, spatially resolved values of the steady state frictional shear stress are determined within the nonhomogeneous pressure and velocity fields of the contact. For contacts with a smooth lens, a velocity-dependent but pressure-independent local shear stress is retrieved from the inversion. On the other hand, the local shear stress is found to depend on both velocity and applied contact pressure when a randomly rough (sand-blasted) glass lens is rubbed against the rubber surface. As a result of changes in the density of microasperity contacts, the amount of light transmitted by the transparent multicontact interface is observed to vary locally as a function of both contact pressure and sliding velocity. Under the assumption that the intensity of light transmitted by the rough interface is proportional to the proportion of area into contact, it is found that the local frictional stress can be expressed experimentally as the product of a purely velocity-dependent term, k(v), by a term representing the pressure and velocity dependence of the actual contact area, A/A0. A comparison between k(v) and the frictional shear stress of smooth contacts suggests that nanometer scale dissipative processes occurring at the interface predominate over viscoelastic dissipation at microasperity scale.

  16. The friction behavior of semiconductors Si and GaAs in contact with pure metals

    NASA Technical Reports Server (NTRS)

    Mishina, H.

    1984-01-01

    The friction behavior of the semiconductors silicon and gallium arsenide in contact with pure metals was studied. Five transition and two nontransition metals, titanium, tantalum, nickel, palladium, platinum, copper, and silver, slid on a single crystal silicon (111) surface. Four metals, indium, nickel, copper and silver, slid on a single crystal gallium arsenide (100) surface. Experiments were conducted in room air and in a vacuum of 10 to the minus 7th power N/sq cm (10 to the minus 9th power torr). The results indicate that the sliding of silicon on the transition metals exhibits relatively higher friction than for the nontransition metals in contact with silicon. There is a clear correlation between friction and Schottky barrier height formed at the metal silicon interface for the transition metals. Transition metals with a higher barrier height on silicon had a lower friction. The same effect of barrier height was found for the friction of gallium arsenide in contact with metals.

  17. Contact Hysteresis and Friction of Alkanethiol SAMs on Au

    SciTech Connect

    Houston, J.E.; Kiely, J.D.

    1998-10-14

    Nanoindentation has been combhed with nanometer-scale friction measurements to identi~ dissipative mechanisms responsible for friction in hexadecanethiol self-assembled monolayer on Au. We have demonstrated that friction is primarily due to viscoelastic relaxations within the films, which give rise to contact hysteresis when deformation rates are within the ranges of 5 and 200 k. We observe that this contact hysteresis increases with exposure to air such that the friction coefficient increases from 0.004 to 0.075 when films are exposed to air for 40 days. Both hysteresis and friction increase with probe speed, and we present a model of friction that characterizes this speed dependence and which also predicts a linear dependence of friction on normal force in thin organic films. Finally, we identify several short-term wear regimes and identify that wear changes dramatically when fdms age.

  18. Probing the Contact and Sliding of Elastomer/Polymer Interfaces

    NASA Astrophysics Data System (ADS)

    Yurdumakan, Betul; Nanjundiah, Kumar; Dhinojwala, Ali

    2006-03-01

    In this study, we have designed a novel approach to couple interface sensitive infrared-visible sum frequency generation (SFG) spectroscopy with adhesion and friction experiments. This provides a direct probe of the interfacial structure in terms of orientation and density of molecules during contact and sliding which is important in understanding the molecular origin of adhesion and friction. Here, we show that the friction forces between poly(dimethyl siloxane) (PDMS) lens and glassy poly(styrene) (PS) are 4 times higher than PDMS sliding on surfaces of crystalline alkyl side chain comb polymers. This cannot be explained by the differences in adhesion energy or hysteresis. The in-situ SFG measurements indicate local interdigitation during contact, which is evident from the decrease in the number of oriented phenyl groups at the interface. The local penetration is unexpected at room temperature (TR) that is much below the Tg of PS. For comparison, we have also studied poly(n-butyl methacrylate) and poly(n-propyl methacrylate) having Tg above and below TR, respectively. Both of these polymers show similar adhesion and friction forces as PS. The SFG results indicate that local changes in interfacial structure affect friction, regardless of the bulk Tg. These results also show that the adhesion energy and hysteresis are not sufficient to predict friction, which makes the characterization of the molecular structure during contact and sliding essential.

  19. Effects of water-vapor on friction and deformation of polymeric magnetic media in contact with a ceramic oxide

    NASA Technical Reports Server (NTRS)

    Miyoshi, K.; Buckley, D. H.

    1984-01-01

    The effects of humidity (water-vapor) in nitrogen on the friction and deformation behavior of magnetic tape in contact with a Ni-Zn ferrite spherical pin were studied. The coefficient of friction is markedly dependent on the ambient relative humidity. In elastic contacts the coefficient of friction increased linearly with increasing humidity; it decreased linearly when humidity was lowered. This effect is the result of changes in the chemistry and interaction of tape materials such as degradation of the lubricant. In plastic contacts there was no effect of humidity on friction below 40 percent relative humidity. There is no effect on friction associated with the breakthrough of the adsorbed water-vapor film at the interface of the tape and Ni-Zn ferrite. The coefficient of friction, however, increased rapidly with increasing relative humidity above 40 percent in plastic contacts.

  20. Spanning From Atoms to Micrometers in Simulations of Contact, Adhesion and Friction

    NASA Astrophysics Data System (ADS)

    Robbins, Mark

    Improved understanding of the forces between realistic solid surfaces is needed to optimize adhesion and friction. Modeling these forces is challenging because they arise from interactions between atoms separated by less than a nanometer, but the number and spatial distribution of these contacting atoms depends on surface roughness and deformation on micrometer and larger scales. There are also strong scale effects in the role of elastic deformations along the surface. The talk will first describe a seamless Greens function (GF) method that allows a full treatment of elastic deformations and atomic contact for micrometer scale surfaces and multibody potentials. Next applications of the method to calculations of the contact area, contact stiffness, adhesion and friction for a range of geometries and interactions will be described. The results can be captured with simple analytic expressions and explain why most contacting surfaces do not adhere. Theoretical and experimental studies of single nanometer-scale asperities show that the frictional shear stress depends strongly on whether surfaces are commensurate. A large constant stress is obtained for identical, aligned crystalline surfaces, but the stress averages to zero in the more common case of incommensurate surfaces. The resulting ultralow friction is called superlubricity and is found in experiments and simulations of small contacts. Our simulations reveal dramatic changes in this behavior because different parts of the surface are able to advance independently as the contact radius increases towards micrometer scales. The friction between identical surfaces drops with increasing radius and then saturates at a low value. The force between incommensurate surfaces saturates at a similar value that can be related to the Peierls stress for dislocation motion at the interface. Studies of multiasperity contacts also show that incoherent motion along the interface can lead to pronounced changes in the macroscopic

  1. Speed of fast and slow rupture fronts along frictional interfaces

    NASA Astrophysics Data System (ADS)

    Trømborg, Jørgen Kjoshagen; Sveinsson, Henrik Andersen; Thøgersen, Kjetil; Scheibert, Julien; Malthe-Sørenssen, Anders

    2015-07-01

    The transition from stick to slip at a dry frictional interface occurs through the breaking of microjunctions between the two contacting surfaces. Typically, interactions between junctions through the bulk lead to rupture fronts propagating from weak and/or highly stressed regions, whose junctions break first. Experiments find rupture fronts ranging from quasistatic fronts, via fronts much slower than elastic wave speeds, to fronts faster than the shear wave speed. The mechanisms behind and selection between these fronts are still imperfectly understood. Here we perform simulations in an elastic two-dimensional spring-block model where the frictional interaction between each interfacial block and the substrate arises from a set of junctions modeled explicitly. We find that material slip speed and rupture front speed are proportional across the full range of front speeds we observe. We revisit a mechanism for slow slip in the model and demonstrate that fast slip and fast fronts have a different, inertial origin. We highlight the long transients in front speed even along homogeneous interfaces, and we study how both the local shear to normal stress ratio and the local strength are involved in the selection of front type and front speed. Last, we introduce an experimentally accessible integrated measure of block slip history, the Gini coefficient, and demonstrate that in the model it is a good predictor of the history-dependent local static friction coefficient of the interface. These results will contribute both to building a physically based classification of the various types of fronts and to identifying the important mechanisms involved in the selection of their propagation speed.

  2. Numerical and experimental study of the nonlinear interaction between a shear wave and a frictional interface.

    PubMed

    Blanloeuil, Philippe; Croxford, Anthony J; Meziane, Anissa

    2014-04-01

    The nonlinear interaction of shear waves with a frictional interface are presented and modeled using simple Coulomb friction. Analytical and finite difference implementations are proposed with both in agreement and showing a unique trend in terms of the generated nonlinearity. A dimensionless parameter ξ is proposed to uniquely quantify the nonlinearity produced. The trends produced in the numerical study are then validated with good agreement experimentally. This is carried out loading an interface between two steel blocks and exciting this interface with different amplitude normal incidence shear waves. The experimental results are in good agreement with the numerical results, suggesting the simple friction model does a reasonable job of capturing the fundamental physics. The resulting approach offers a potential way to characterize a contacting interface; however, the difficulty in activating that interface may ultimately limit its applicability.

  3. Molecular dynamics investigation of the effect of copper nanoparticle on the solid contact between friction surfaces

    NASA Astrophysics Data System (ADS)

    Hu, Chengzhi; Bai, Minli; Lv, Jizu; Liu, Hao; Li, Xiaojie

    2014-12-01

    This study investigated the effect of copper (Cu) nanoparticles on the solid contact between friction surfaces by applying a molecular dynamics method to reveal the mechanisms responsible for the favorable friction properties of nanoparticles. Two models were built, which were named model A (without Cu) and model B (with Cu), respectively. The differences in the mechanical properties between these two models were compared. The simulation results demonstrated that the improvement in friction properties by Cu nanoparticles was more obvious at low velocity than at high velocity. At low velocity, a Cu nano-film was formed on the friction surface, which accommodated the velocity gradient and plastic deformation. Due to the good lubrication effect of the nano-film, the plastic deformation, defect structures and friction force of model B were improved compared with model A. Under high velocity conditions, a transfer layer appeared adjacent to the interface in both models. Because of this, the friction forces of the two models decreased with increased velocity. The fluid mechanics theory was used to explain why the friction force in model B was lower than that in model A at high velocity. The effect of the load on friction properties was also analyzed and the results showed that the mechanisms of anti-wear and friction reduction by Cu nanoparticles under a low load were the same as those under a high load.

  4. Analysis of lumped models with contact and friction

    NASA Astrophysics Data System (ADS)

    Migórski, Stanisław; Ochal, Anna; Sofonea, Mircea

    2011-02-01

    We consider two mathematical models that describe the vibrations of spring-mass-damper systems with contact and friction. In the first model, both the contact and frictional boundary conditions are described with subdifferentials of nonconvex functions. In the second model, the contact is modeled with a Lipschitz continuous function, and the restitution force is described by a differential equation involving a Volterra integral term. The two models lead to second-order differential inclusions with and without an integral term, in which the unknowns are the positions of the masses. For each model, we prove the existence of a solution by using an abstract result for first-order differential inclusions in finite dimensional spaces. For the second model, in addition, we prove the uniqueness of the solution by using a fixed point argument. Finally, we provide examples of systems with contact and friction conditions for which our results are valid.

  5. Steady-state propagation speed of rupture fronts along one-dimensional frictional interfaces

    NASA Astrophysics Data System (ADS)

    Amundsen, David Skâlid; Trømborg, Jørgen Kjoshagen; Thøgersen, Kjetil; Katzav, Eytan; Malthe-Sørenssen, Anders; Scheibert, Julien

    2015-09-01

    The rupture of dry frictional interfaces occurs through the propagation of fronts breaking the contacts at the interface. Recent experiments have shown that the velocities of these rupture fronts range from quasistatic velocities proportional to the external loading rate to velocities larger than the shear wave speed. The way system parameters influence front speed is still poorly understood. Here we study steady-state rupture propagation in a one-dimensional (1D) spring-block model of an extended frictional interface for various friction laws. With the classical Amontons-Coulomb friction law, we derive a closed-form expression for the steady-state rupture velocity as a function of the interfacial shear stress just prior to rupture. We then consider an additional shear stiffness of the interface and show that the softer the interface, the slower the rupture fronts. We provide an approximate closed form expression for this effect. We finally show that adding a bulk viscosity on the relative motion of blocks accelerates steady-state rupture fronts and we give an approximate expression for this effect. We demonstrate that the 1D results are qualitatively valid in 2D. Our results provide insights into the qualitative role of various key parameters of a frictional interface on its rupture dynamics. They will be useful to better understand the many systems in which spring-block models have proved adequate, from friction to granular matter and earthquake dynamics.

  6. The role of lubricant entrapment at biological interfaces: reduction of friction and adhesion in articular cartilage.

    PubMed

    Chan, S M T; Neu, C P; Komvopoulos, K; Reddi, A H

    2011-07-28

    Friction and adhesion of articular cartilage from high- and low-load-bearing regions of bovine knee joints were examined with a tribometer under various loads and equilibration times. The effect of trapped lubricants was investigated by briefly unloading the cartilage sample before friction testing, to allow fluid to reflow into the contact interface and boundary lubricants to rearrange. Friction and adhesion of high-load-bearing joint regions were consistently lower than those of low-load-bearing regions. This investigation is the first to demonstrate the regional variation in the friction and adhesion properties of articular cartilage. Friction coefficient decreased with increasing contact pressure and decreasing equilibration time. Briefly unloading cartilage before the onset of sliding resulted in significantly lower friction and adhesion and a loss of the friction dependence on contact pressure, suggesting an enhancement of the cartilage tribological properties by trapped lubricants. The results of this study reveal significant differences in the friction and adhesion properties between high- and low-load-bearing joint regions and elucidate the role of trapped lubricants in cartilage tribology.

  7. Friction Reduction Through Ultrasonic Vibration: Part 1: Modelling Intermittent Contact.

    PubMed

    Vezzoli, Eric; Virdih, Zlatko; Giamundo, Vincenzo; Lemaire-Semail, Betty; Giraud, Frederic; Rodic, Tomaz; Peric, Djordje; Adams, Michael

    2017-02-17

    Ultrasonic vibration is employed to modify the friction of a finger pad in way that induces haptic sensations. A combination of intermittent contact and squeeze film levitation has been previously proposed as the most probable mechanism. In this paper, in order to understand the underlying principles that govern friction modulation by intermittent contact, numerical models based on finite element (FE) analysis and also a spring-Coulombic slider are developed. The physical input parameters for the FE model are optimised by measuring the contact phase shift between a finger pad and a vibrating plate. The spring-slider model assists in the interpretation of the FE model and leads to the identification of a dimensionless group that allows the calculated coefficient of friction to be approximately superimposed onto an exponential function of the dimensionless group. Thus, it is possible to rationalise the computed relative reduction in friction being (i) dependent on the vibrational amplitude, frequency, and the intrinsic coefficient of friction of the device, and the reciprocal of the exploration velocity, and (ii) independent of the applied normal force, and the shear and extensional elastic moduli of the finger skin provided that intermittent contact is sufficiently well developed. Experimental validation of the modelling using real and artificial fingertips will be reported in part 2 of this work, which supports the current modelling.

  8. A constitutive law for finite element contact problems with unclassical friction

    NASA Technical Reports Server (NTRS)

    Plesha, M. E.; Steinetz, B. M.

    1986-01-01

    Techniques for modeling complex, unclassical contact-friction problems arising in solid and structural mechanics are discussed. A constitutive modeling concept is employed whereby analytic relations between increments of contact surface stress (i.e., traction) and contact surface deformation (i.e., relative displacement) are developed. Because of the incremental form of these relations, they are valid for arbitrary load-deformation histories. The motivation for the development of such a constitutive law is that more realistic friction idealizations can be implemented in finite element analysis software in a consistent, straightforward manner. Of particular interest is modeling of two-body (i.e., unlubricated) metal-metal, ceramic-ceramic, and metal-ceramic contact. Interfaces involving ceramics are of engineering importance and are being considered for advanced turbine engines in which higher temperature materials offer potential for higher engine fuel efficiency.

  9. Scale dependence of interface dislocation storage governing the frictional sliding of single asperities

    NASA Astrophysics Data System (ADS)

    Gao, Zhiwen; Zhang, Wei; Gao, Yanfei

    2016-08-01

    Single-asperity friction tests have found a critical dependence of friction stress on the nanoscale contact size, as successfully explained by the nucleation of interface dislocations as opposed to concurrent sliding of all the interfacial atoms in contact. Modeling and simulation results, however, vary when the motion and interactions of multiple dislocations dominate at a larger scale regime. A Rice-Peierls framework is employed to investigate the multiplication and storage of interface dislocations, and the critical conditions for dislocation initiation and steady-state gliding are determined numerically. Our findings identify the key parameters that govern various friction mechanisms in the Hurtado-Kim and Deshpande-Needleman-van der Giessen models.

  10. Effect of barrier height on friction behavior of the semiconductors silicon and gallium arsenide in contact with pure metals

    NASA Technical Reports Server (NTRS)

    Mishina, H.; Buckley, D. H.

    1984-01-01

    Friction experiments were conducted for the semiconductors silicon and gallium arsenide in contact with pure metals. Polycrystalline titanium, tantalum, nickel, palladium, and platinum were made to contact a single crystal silicon (111) surface. Indium, nickel, copper, and silver were made to contact a single crystal gallium arsenide (100) surface. Sliding was conducted both in room air and in a vacuum of 10 to the minus 9th power torr. The friction of semiconductors in contact with metals depended on a Schottky barrier height formed at the metal semiconductor interface. Metals with a higher barrier height on semiconductors gave lower friction. The effect of the barrier height on friction behavior for argon sputtered cleaned surfaces in vacuum was more specific than that for the surfaces containing films in room air. With a silicon surface sliding on titanium, many silicon particles back transferred. In contrast, a large quantity of indium transferred to the gallium arsenide surface.

  11. New Micro- and Macroscopic Models of Contact and Friction

    DTIC Science & Technology

    1993-11-29

    computationally more efficient to use a total formulation of3 the problem which is as follows. Find a displacement field u(x) EV , such thatI Jvj,,Ej1k1Uk,jdfl= v...Effect on Adhesion and Friction," Journ. of Colloid and Interface Science, 58, pp. 36-53 1977. 22. Burwell, J. T., and Rabinowicz , E., "The Nature of...Elasticity with Nonlinear Friction Laws," International Journal of Engineering Science, Vol. 24, No. 11, pp. 1755-1768, 1986. 77. Rabinowicz , E

  12. General contact mechanics theory for randomly rough surfaces with application to rubber friction.

    PubMed

    Scaraggi, M; Persson, B N J

    2015-12-14

    We generalize the Persson contact mechanics and rubber friction theory to the case where both surfaces have surface roughness. The solids can be rigid, elastic, or viscoelastic and can be homogeneous or layered. We calculate the contact area, the viscoelastic contribution to the friction force, and the average interface separation as a function of the sliding speed and the nominal contact pressure. We illustrate the theory with numerical results for the classical case of a rubber block sliding on a road surface. We find that with increasing sliding speed, the influence of the roughness on the rubber block decreases to the extent that only the roughness of the stiff counter face needs to be considered.

  13. Modeling of Instabilities and Self-organization at the Frictional Interface

    NASA Astrophysics Data System (ADS)

    Mortazavi, Vahid

    The field of friction-induced self-organization and its practical importance remains unknown territory to many tribologists. Friction is usually thought of as irreversible dissipation of energy and deterioration; however, under certain conditions, friction can lead to the formation of new structures at the interface, including in-situ tribofilms and various patterns at the interface. This thesis studies self-organization and instabilities at the frictional interface, including the instability due to the temperature-dependency of the coefficient of friction, the transient process of frictional running-in, frictional Turing systems, the stick-and-slip phenomenon, and, finally, contact angle (CA) hysteresis as an example of solid-liquid friction and dissipation. All these problems are chosen to bridge the gap between fundamental interest in understanding the conditions leading to self-organization and practical motivation. We study the relationship between friction-induced instabilities and friction-induced self-organization. Friction is usually thought of as a stabilizing factor; however, sometimes it leads to the instability of sliding, in particular when friction is coupled with another process. Instabilities constitute the main mechanism for pattern formation. At first, a stationary structure loses its stability; after that, vibrations with increasing amplitude occur, leading to a limit cycle corresponding to a periodic pattern. The self-organization is usually beneficial for friction and wear reduction because the tribological systems tend to enter a state with the lowest energy dissipation. The introductory chapter starts with basic definitions related to self-organization, instabilities and friction, literature review, and objectives. We discuss fundamental concepts that provide a methodological tool to investigate, understand and enhance beneficial processes in tribosystems which might lead to self-organization. These processes could result in the ability of a

  14. Friction and shear fracture of an adhesive contact under torsion

    NASA Astrophysics Data System (ADS)

    Chateauminois, Antoine; Fretigny, Christian; Olanier, Ludovic

    2010-02-01

    The shear failure or stiction of an adhesive contact between a poly(dimethylsiloxane) (PDMS) rubber and a glass lens has been investigated using a torsional contact configuration. As compared to linear sliding, torsion presents the advantage of inducing a shear failure under a pure mode III condition, while preserving the cylindrical symmetry of the contact. The surface of the transparent PDMS substrate was marked using a network of dots in order to monitor continuously the in-plane surface displacements during the stiction process. Using a previously developed inversion procedure (A. Chateauminois and C. Fretigny, Eur. Phys. J. E 27, 221 (2008)), the corresponding surface shear stress distributions were obtained from the displacement fields. Stiction was found to involve the progressive shrinkage of a central adhesive zone surrounded by an annular microslip region. Adhesion effects were especially evidenced from a stress overshoot at the boundary of the adhesive zone. The experimental data were analysis using an extension to torsional contact of the Maugis-Dugdale approach’s to adhesive contacts which takes into account frictional effects. This model allowed to extract an effective adhesion energy in the presence of friction, which dependence on kinetics effect is briefly discussed.

  15. Microscopic contact area and friction between medical textiles and skin.

    PubMed

    Derler, S; Rotaru, G-M; Ke, W; El Issawi-Frischknecht, L; Kellenberger, P; Scheel-Sailer, A; Rossi, R M

    2014-10-01

    The mechanical contact between medical textiles and skin is relevant in the health care for patients with vulnerable skin or chronic wounds. In order to gain new insights into the skin-textile contact on the microscopic level, the 3D surface topography of a normal and a new hospital bed sheet with a regular surface structure was measured using a digital microscope. The topographic data was analysed concerning material distribution and real contact area against smooth surfaces as a function of surface deformations. For contact conditions that are relevant for the skin of patients lying in a hospital bed it was found that the order of magnitude of the ratio of real and apparent contact area between textiles and skin or a mechanical skin model lies between 0.02 and 0.1 and that surface deformations, i.e. penetration of the textile surface asperities into skin or a mechanical skin model, range from 10 to 50µm. The performed analyses of textile 3D surface topographies and comparisons with previous friction measurement results provided information on the relationship between microscopic surface properties and macroscopic friction behaviour of medical textiles. In particular, the new bed sheet was found to be characterised by a trend towards a smaller microscopic contact area (up to a factor of two) and by a larger free interfacial volume (more than a factor of two) in addition to a 1.5 times lower shear strength when in contact with counter-surfaces. The applied methods can be useful to develop improved and skin-adapted materials and surfaces for medical applications.

  16. Friction and stress coupling on the subduction interfaces

    NASA Astrophysics Data System (ADS)

    Tan, E.; Lavier, L.; van Avendonk, H.

    2011-12-01

    At a subduction zone, the down-going oceanic plate slides underneath the overriding plate. The frictional resistance to the relative motion between the plates generates great earthquakes along the subduction interface, which can cause tremendous damage in the civil life and property. There is a strong incentive to understand the frictional strength of the subduction interface. One fundamental question of mechanics of subuction is the degree of coupling between the plates, which is linked to the size of earthquakes. It has been noted that the trench-parallel (along-strike) gravity variation correlates positively with the trench-parallel topography anomaly and negatively with the activity of great earthquake (Song and Simons, 2003). Regions with a negative trench-parallel gravity anomaly are more likely to have great earthquakes. The interpretation of such correlation is that strong coupling along subduction interface will drag down the for-arc region of the overriding plate, which generates the gravity and topography anomalies, and could store more strain energy to be released during a great earthquake. We developed a 2D numerical thermo-mechanical code for modeling subduction. The numerical method is based on an explicit finite element method similar to the Fast Lagrangian Analysis of Continua (FLAC) technique. The constitutive law is visco-elasti-plastic with strain weakening. The cohesion and friction angle are reduced with increasing plastic strain after yielding. To track different petrologic phases, Lagrangian particles are distributed in the domain. Basalt-eclogite, sediment-schist and peridotite-serpentinite phase changes are included in the model. Our numerical models show that the degree of coupling negatively correlates with the coefficient of friction. In the low friction case, the subduction interface has very shallow dipping angle, which helps to elastically couple the downing plate with the overriding plate. The topography and gravity anomalies of the

  17. Numerical analysis of piezoelectric active repair in the presence of frictional contact conditions.

    PubMed

    Alaimo, Andrea; Milazzo, Alberto; Orlando, Calogero; Messineo, Antonio

    2013-04-02

    The increasing development of smart materials, such as piezoelectric and shape memory alloys, has opened new opportunities for improving repair techniques. Particularly, active repairs, based on the converse piezoelectric effect, can increase the life of a structure by reducing the crack opening. A deep characterization of the electromechanical behavior of delaminated composite structures, actively repaired by piezoelectric patches, can be achieved by considering the adhesive layer between the host structure and the repair and by taking into account the frictional contact between the crack surfaces. In this paper, Boundary Element (BE) analyses performed on delaminated composite structures repaired by active piezoelectric patches are presented. A two-dimensional boundary integral formulation for piezoelectric solids based on the multi-domain technique to model the composite host damaged structures and the bonded piezoelectric patches is employed. An interface spring model is also implemented to take into account the finite stiffness of the bonding layers and to model the frictional contact between the delamination surfaces, by means of an iterative procedure. The effect of the adhesive between the plies of piezoelectric bimorph devices on the electromechanical response is first pointed out for both sensing and actuating behavior. Then, the effect of the frictional contact condition on the fracture mechanics behavior of actively repaired delaminated composite structures is investigated.

  18. A flexible multi-body approach for frictional contact in spur gears

    NASA Astrophysics Data System (ADS)

    Lundvall, O.; Strömberg, N.; Klarbring, A.

    2004-12-01

    In the present paper, a large rotational approach for dynamic contact problems with friction is proposed. The approach is used for modelling a spur gear pair with shafts and bearings. The model is obtained by superposing small displacement elasticity on rigid-body motions, and postulating tribological laws on the gear flanks. The finite element method is used to model the elastic properties of the gear pair. Shafts and bearings are represented by linear springs. The tribological laws of the contact interface are Signorini's contact law and Coulomb's law of friction. An important feature of the approach is that the difficulties of impacting mass nodes are avoided. The governing equations of the model are numerically treated by use of the augmented Lagrangian approach. In such manner the geometry of the gear flanks are well represented in the numerical simulations. It is possible to study accurately the consequences of different types of profile modifications as well as flank errors. In this work, the dynamic transmission error is studied. For instance, it turns out that the effect from profile modification is less significant for the transmission error when frictional effects are included.

  19. Switchable static friction of piezoelectric composite—silicon wafer contacts

    NASA Astrophysics Data System (ADS)

    van den Ende, D. A.; Fischer, H. R.; Groen, W. A.; van der Zwaag, S.

    2013-04-01

    The meso-scale surface roughness of piezoelectric fiber composites can be manipulated by applying an electric field to a piezocomposite with a polished surface. In the absence of an applied voltage, the tips of the embedded piezoelectric ceramic fibers are below the surface of the piezocomposite and a silicon wafer counter surface rests solely on the matrix region of the piezocomposite surface. When actuated, the piezoelectric ceramic fibers protrude from the surface and the wafer rests solely on these protrusions. A threefold decrease in engineering static friction coefficient upon actuation of the piezocomposite was observed: from μ* = 1.65 to μ* = 0.50. These experimental results could be linked to the change in contact surface area and roughness using capillary adhesion theory, which relates the adhesive force to the number and size of the contacting asperities for the different surface states.

  20. Dynamics of Slip Fronts at Frictional Interfaces: Analysis of Slip Precursors

    NASA Astrophysics Data System (ADS)

    Radiguet, M.; Kammer, D. S.; Molinari, J.

    2012-12-01

    The transition from sticking to sliding of frictional interfaces is a phenomenon of importance for many physical systems in nature as well as in engineering. This transition is marked by the occurrence of local slip events, often called precursors, which appear before the global sliding is observed. Such precursors to global sliding may occur on segments of geophysical faults subject to non uniform shear loading, for example a fault segment located between a locked and steadily slipping region. Sequences of small earthquakes (foreshocks) of identical seismic characteristics have been observed preceding large earthquakes in several regions. The links between the occurrence of these foreshocks and the nucleation process of large earthquakes remains elusive, but has large implications for earthquake prediction and risk assessment. These precursors have been studied experimentally by Rubinstein et al. [2007]. However, the experimental study of interfaces is challenging due to difficulties to access information at the interface. Therefore, numerical simulations are needed in order to give additional information for accurate analysis. First attempts have been undertaken using simple spring-block systems [Maegawa et al. 2010, Tromborg et al. 2011]. In this study however, we use the finite-element method, which allows us to represent accurately the continuum character of the system, and to investigate the onset and evolution of sliding at a frictional interface. The studied setup is similar to the experimental setup used by Ben-David et al. [2010]. It consists of a block of viscoelastic material in contact with a rigid body. A velocity-weakening friction law controls the friction at the interface. Special care is taken to apply appropriate regularization and viscosity in the simulation. We apply a shear load to the block, either on the top surface of the block or on one side. In both cases, the resulting shear tractions at the interface are non-uniform. The stress

  1. Atomic Force Microscopy Study of an Ideally Hard Contact: The Diamond{bold (}111{bold )}/Tungsten Carbide Interface

    SciTech Connect

    Enachescu, M.; van den Oetelaar, R.J.; Carpick, R.W.; Ogletree, D.F.; Flipse, C.F.; Salmeron, M.

    1998-08-01

    A comprehensive nanotribological study of a hydrogen-terminated diamond(111)/tungsten carbide interface has been performed using ultrahigh vacuum atomic force microscopy. Both contact conductance, which is proportional to contact area, and friction have been measured as a function of applied load. We demonstrate for the first time that the load dependence of the contact area in UHV for this extremely hard single asperity contact is described by the Derjaguin-M{umlt u}ller-Toporov continuum mechanics model. Furthermore, the frictional force is found to be directly proportional to the contact area. {copyright} {ital 1998} {ital The American Physical Society}

  2. Interface structure behind a moving contact line

    NASA Astrophysics Data System (ADS)

    He, Mengfei; Nagel, Sidney

    2016-11-01

    When a flat solid substrate straddles the boundary between two fluids (e.g., water and air), there is a contact line where the two fluids and the solid meet. When the substrate is forced to penetrate further in either direction, it distorts the fluid interface and carries along with it a wedge of the trailing fluid. Numerous studies have investigated the onset of the contact-line motion in a two-dimensional geometry where it was assumed that no flows occurred in the direction along the surface of the substrate transverse to its direction of motion. Contrary to this assumption, we discovered that in steady state the fluid interface develops dramatic three-dimensional structure; there are multiple thin and thick regions of the fluid film alternating in the transverse direction. Thus the dynamics behind the contact line is not invariant in the transverse direction suggesting the existence of a new instability. We use interference to map the relative shape of this wedge-shaped region and a new interference technique to identify the absolute thickness of the wedge. It is particularly noteworthy that the same structure appears both in dewetting (when a substrate is removed from a liquid into the air) and in wetting (when it is plunged into the liquid).

  3. Atomic simulations of effects of contact size and interfacial interaction strength on superlubricity in incommensurate sliding interface

    NASA Astrophysics Data System (ADS)

    Zhu, Peng-Zhe; Hu, Yuan-Zhong; Ma, Tian-Bao; Li, Rui; Wang, Hui

    2015-01-01

    Understanding the effects of contact size and interfacial interaction strength on superlubricity in incommensurate sliding interface is critically needed for the design and development of nanoscale ultra-low friction devices. This study uses molecular dynamics simulations to explore the sliding friction behaviors of an incommensurate interface consisting of a diamond slider and a silver substrate. The instantaneous relative lattice constant is proposed to quantitatively describe the commensurability of contacting surfaces in the sliding process. It is found that when the contact size is large, the slider exhibits ultra-low friction force. While for small contact size, superlubricity behavior breaks down,which is due to the transition of incommensurate-commensurate interfacial configuration in the local contact region. It is also found that when the interfacial interaction strength is reduced below a critical value, the obvious stick-slip motion observed for the small slider with large interfacial interaction strength disappears and superlubricity behavior occurs, which results from the incommensurate interfacial configuration in the contact region maintained during the sliding process. These results provide a first demonstration that the instantaneous incommensurate-commensurate transition in the local contact region can result in the breakdown of superlubricity in a realistic three-dimensional sliding system. The obtained results not only may guide the design of nanoscale ultra-low friction devices, but also provide some insights into the origins of friction at macroscopic interfaces which usually consists of many small nanoscale contacts.

  4. The metal to metal interface and its effect on adhesion and friction

    NASA Technical Reports Server (NTRS)

    Buckley, D. H.

    1976-01-01

    The paper considers the interface between two bulk metals and the effect of this interface on adhesive bonding, resistance to tangential displacements, friction and the interfacial transport from one surface to another. Using Auger emission spectroscopy, field ion microscopy, and low energy electron diffraction techniques, the influence of surface orientation, lattice registry, crystal lattice structure and defects, metal surface chemistry and alloying on the characteristics of the interface was studied for noble, platinum, transition, and Group 4B metals. With dissimilar metals in contact, epitaxial transfer of the cohesively weaker to the cohesively stronger metal has been observed. Surface chemical activity of the noble and platinum metals is shown to affect interfacial behavior as does a valence bonding in the transition metals, and the degree of metallic nature in the Group 4B elements. Alloying elements, e.g., Si and Fe, can alter interfacial behavior by segregation to the surface of metals or by altering bulk properties such as crystal transformation kinetics.

  5. 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.

  6. High-vacuum adhesion and friction properties of sliding contact-mode micromachines

    NASA Astrophysics Data System (ADS)

    Xiang, H.; Komvopoulos, K.

    2013-07-01

    The evolution of adhesion and friction in contact-mode micromachines operated in high vacuum was studied by tracking changes in the adhesive pressure, interfacial shear strength, and static coefficient of friction with accumulating sliding cycles. Low adhesion and high static friction observed during the initial stage of sliding were followed by monotonically intensifying adhesion and decreasing friction until reaching an equilibrium stage at steady-state sliding. This trend revealed the existence of two friction regimes in which asperity deformation and adhesion were the dominant friction mechanisms. Scanning electron microscopy and atomic force microscopy observations indicated that sliding resulted in physical and chemical surface changes. The evolution of the adhesion and friction properties with sliding cycles is attributed to the increase of both the real contact area and the work of adhesion due to nanoscale surface smoothening and the removal of contaminant adsorbents, respectively.

  7. Pressurized bellows flat contact heat exchanger interface

    NASA Technical Reports Server (NTRS)

    Voss, Fred E. (Inventor); Howell, Harold R. (Inventor); Winkler, Roger V. (Inventor)

    1990-01-01

    Disclosed is an interdigitated plate-type heat exchanger interface. The interface includes a modular interconnect to thermally connect a pair or pairs of plate-type heat exchangers to a second single or multiple plate-type heat exchanger. The modular interconnect comprises a series of parallel, plate-type heat exchangers arranged in pairs to form a slot therebetween. The plate-type heat exchangers of the second heat exchanger insert into the slots of the modular interconnect. Bellows are provided between the pairs of fins of the modular interconnect so that when the bellows are pressurized, they drive the plate-type heat exchangers of the modular interconnect toward one another, thus closing upon the second heat exchanger plates. Each end of the bellows has a part thereof a thin, membrane diaphragm which readily conforms to the contours of the heat exchanger plates of the modular interconnect when the bellows is pressurized. This ensures an even distribution of pressure on the heat exchangers of the modular interconnect thus creating substantially planar contact between the two heat exchangers. The effect of the interface of the present invention is to provide a dry connection between two heat exchangers whereby the rate of heat transfer can be varied by varying the pressure within the bellows.

  8. Shear response of a frictional interface to a normal load modulation

    NASA Astrophysics Data System (ADS)

    Bureau, L.; Baumberger, T.; Caroli, C.

    2000-11-01

    We study the shear response of a sliding multicontact interface submitted to a harmonically modulated normal load, without loss of contact. We measure, at low velocities (V<100 μm s-1), the average value F¯ of the friction force and the amplitude of its first and second harmonic components. The excitation frequency (f=120 Hz) is chosen much larger than the natural one, associated with the dynamical aging of the interface. We show the following: (i) In agreement with the engineering thumb rule, even a modest modulation induces a substantial decrease of F¯. (ii) The Rice-Ruina state and rate model, though appropriate to describe the slow frictional dynamics, must be extended when dealing with our ``high'' frequency regime. That is, the rheology which controls the shear strength must explicitly account not only for the plastic response of the adhesive junctions between load-bearing asperities, but also for the elastic contribution of the asperities bodies. This ``elastoplastic'' friction model leads to predictions in excellent quantitative agreement with all our experimental data.

  9. Self-similar slip instability on interfaces with rate- and state-dependent friction

    NASA Astrophysics Data System (ADS)

    Viesca, Robert C.

    2016-08-01

    We examine the development of a frictional instability, with diverging sliding rate, at the interface of elastic bodies in contact. Evolution of friction is determined by a slip rate and state dependence. Following Viesca (2016 Phys. Rev. E 93, 060202(R). (doi:10.1103/PhysRevE.93.060202)), we show through an appropriate change of variable, the existence of blow-up solutions that are fixed points of a dynamical system. The solutions show self-similarity of the simple variety: separable dependence of time and space. For an interface with uniform frictional properties, there is a single-problem parameter. We examine the linear stability of these fixed points, as this problem parameter is varied. Specifically, we consider two archetypical elastic settings of the slip surface, in which interactions between points on the surface are either local or non-local. We show that, independent of the nature of elastic interactions, the fixed-points lose stability in the same matter as the parameter is increased towards a limit value: an apparently infinite sequence of Hopf bifurcations. However, for any value of the parameter, the nonlinear development of the instability is attraction, if not asymptotic convergence, towards these fixed points, owing to the existence of stable eigenmodes. For comparison, we perform numerical solutions of the original evolution equations and find precise agreement with the results of the analysis.

  10. 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.

  11. Fundamental considerations in adhesion, friction and wear for ceramic-metal contacts

    NASA Technical Reports Server (NTRS)

    Miyoshi, Kazuhisa

    1990-01-01

    Fundamental studies of friction, wear and adhesion of ceramics in contact with metals are evaluated. It is shown that friction and adhesion are strongly dependent on the ductility of the metals. The surface energy, friction, adhesion and hardness of a metal are related to its Young's modulus and shear modulus, which have a marked dependence on the electron configuration of the metal. Generally, the greater the sheer modulus, the less metal transfer there is to the ceramic.

  12. Effect of capillary-condensed water on the dynamic friction force at nanoasperity contacts

    NASA Astrophysics Data System (ADS)

    Sirghi, L.

    2003-05-01

    A single nanoasperity contact in ambient air is usually wetted by capillary condensation of water vapor and is surrounded by a water meniscus. This phenomenon strongly affects the contact friction, not only by the effect of meniscus loading force (superficial tension and capillary forces), but also by a friction force that accounts for the energy loss in the meniscus movement along with the sliding contact. Occurrence of the water-meniscus-generated friction is experimentally proved by atomic force microscopy measurements of the tip-sample friction force at minimum possible external load (before pull-off). A qualitative explanation for the observed dependence of the friction force on air humidity and solid surface wettability is proposed.

  13. Power optimization of ultrasonic friction-modulation tactile interfaces.

    PubMed

    Wiertlewski, Michael; Colgate, J Edward

    2015-01-01

    Ultrasonic friction-modulation devices provide rich tactile sensation on flat surfaces and have the potential to restore tangibility to touchscreens. To date, their adoption into consumer electronics has been in part limited by relatively high power consumption, incompatible with the requirements of battery-powered devices. This paper introduces a method that optimizes the energy efficiency and performance of this class of devices. It considers optimal energy transfer to the impedance provided by the finger interacting with the surface. Constitutive equations are determined from the mode shape of the interface and the piezoelectric coupling of the actuator. The optimization procedure employs a lumped parameter model to simplify the treatment of the problem. Examples and an experimental study show the evolution of the optimal design as a function of the impedance of the finger.

  14. Modeling of contact mechanics and friction limit surfaces for soft fingers in robotics, with experimental results

    SciTech Connect

    Xydas, N.; Kao, I.

    1999-09-01

    A new theory in contact mechanics for modeling of soft fingers is proposed to define the relationship between the normal force and the radius of contact for soft fingers by considering general soft-finger materials, including linearly and nonlinearly elastic materials. The results show that the radius of contact is proportional to the normal force raised to the power of {gamma}, which ranges from 0 to 1/3. This new theory subsumes the Hertzian contact model for linear elastic materials, where {gamma} = 1/3. Experiments are conducted to validate the theory using artificial soft fingers made of various materials such as rubber and silicone. Results for human fingers are also compared. This theory provides a basis for numerically constructing friction limit surfaces. The numerical friction limit surface can be approximated by an ellipse, with the major and minor axes as the maximum friction force and the maximum moment with respect to the normal axis of contact, respectively. Combining the results of the contact-mechanics model with the contact-pressure distribution, the normalized friction limit surface can be derived for anthropomorphic soft fingers. The results of the contact-mechanics model and the pressure distribution for soft fingers facilitate the construction of numerical friction limit surfaces, and will enable us to analyze and simulate contact behaviors of grasping and manipulation in robotics.

  15. Coulomb frictional contact by explicit projection in the cone for finite displacement quasi-static problems

    NASA Astrophysics Data System (ADS)

    Areias, P.; Rabczuk, T.; de Melo, F. J. M. Queirós; de Sá, J. César

    2015-01-01

    We propose, in this paper, a distinct perspective on the solution of the Coulomb frictional contact problem. By combining the prediction/correction method for the contact force vector with the correction step being a cone projection and writing the friction cone surface in the quadratic form, we directly calculate the contact force. The distance along the friction cone normal is determined by solving a nonlinear problem in closed form. Numerical advantages of this projection are apparent for large values of friction coefficient. Six problems previously indicated as difficult to solve by the node-to-segment discretization and the operator split algorithm are here solved with the new projection algorithm. Discretization follows node-to segment and node-to-face derivations with gap vector defined in a global frame (without tangential and normal decomposition). In addition, we provide source codes for the 2D and 3D contact cases.

  16. Anisotropic friction and wear of single-crystal manganese-zinc ferrite in contact with itself

    NASA Technical Reports Server (NTRS)

    Miyoshi, K.; Buckley, D. H.

    1978-01-01

    Sliding friction experiments were conducted with manganese-zinc ferrite (100), (110), (111), and (211) planes in contact with themselves. Mating the highest-atomic-density directions, (110), of matched crystallographic planes resulted in the lowest coefficients of friction. Mating matched (same) high-atomic-density planes and matched (same)crystallographic directions resulted in low coefficients of friction. Mating dissimilar crystallographic planes, however, did not give significantly different friction results from those with matched planes. Sliding caused cracking and the formation of hexagonal- and rectangular-platelet wear debris on ferrite surfaces, primarily from cleavage of the (110) planes.

  17. Contact-line friction of liquid drops on self-assembled monolayers: chain-length effects.

    PubMed

    Voué, M; Rioboo, R; Adao, M H; Conti, J; Bondar, A I; Ivanov, D A; Blake, T D; De Coninck, J

    2007-04-24

    The static and dynamic wetting properties of self-assembled alkanethiol monolayers of increasing chain length were studied. The molecular-kinetic theory of wetting was used to interpret the dynamic contact angle data and evaluate the contact-line friction on the microscopic scale. Although the surfaces had a similar static wettability, the coefficient of contact-line friction zeta0 increased linearly with alkyl chain length. This result supports the hypothesis of energy dissipation due to a local deformation of the nanometer-thick layer at the contact line.

  18. Contact mechanics and rubber friction for randomly rough surfaces with anisotropic statistical properties

    NASA Astrophysics Data System (ADS)

    Carbone, G.; Lorenz, B.; Persson, B. N. J.; Wohlers, A.

    2009-07-01

    In this paper we extend the theory of contact mechanics and rubber friction developed by one of us (B.N.J. Persson, J. Chem. Phys. 115, 3840 (2001)) to the case of surfaces with anisotropic surface roughness. As an application we calculate the viscoelastic contribution to the rubber friction. We show that the friction coefficient may depend significantly on the sliding direction, while the area of contact depends weakly on the sliding direction. We have carried out experiments for rubber blocks sliding on unidirectionally polished steel surfaces. The experimental data are in a good qualitative agreement with the theory.

  19. 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).

  20. Adhesion and friction of single-crystal diamond in contact with transition metals

    NASA Technical Reports Server (NTRS)

    Miyoshi, K.; Buckley, D. H.

    1980-01-01

    An investigation was conducted to examine the adhesion and friction of single-crystal diamond in contact with various transition metals and the nature of metal transfer to diamond. Sliding friction experiments were conducted with diamond in sliding contact with the metals yttrium, titanium, zirconium, vanadium, iron, cobalt, nickel, tungsten, platinum, rhenium and rhodium. All experiments were conducted with loads of 0.05 to 0.3 N, at a sliding velocity of 0.003 m per minute, in a vacuum of 10 to the -8th Pa, at room temperature, and on the (111) plane of diamond with sliding in the 110 line type direction. The results of the investigation indicate that the coefficient of friction for diamond in contact with various metals is related to the relative chemical activity of the metals in high vacuum. The more active the metal, the higher the coefficient of friction. All the metals examined transferred to the surface of diamond in sliding.

  1. New Micro- and Macroscopic Models of Contact and Friction

    DTIC Science & Technology

    1992-11-01

    formulation of the problem which is as follows. 41 Find a displacement field u(x) EV , such that I j Eijktvi’juk,’dS1 + vi 1 NiNjujds =( Vi1 iN(W-XkN+Pok-Nk...T., and Rabinowicz , E., "The Nature of the Coefficient of Friction," .Journ. Appl. Phys., 24, 2, pp. 136-139, 1953. 23. Bush, A. W., Gibson, R. D...Nonlinear Friction Laws," International Journal of Engineering Science, Vol. 24, No. 11, pp. 1755-1768, 1986. 76. Rabinowicz , E., "The Nature of the

  2. Thermal contact resistance across a copper-silicon interface

    SciTech Connect

    Khounsary, A.M.; Chojnowski, D.; Assoufid, L.; Worek, W.M.

    1997-10-01

    The issue of thermal contact resistance across metallic interfaces has been investigated for many situations over the past several decades. The application in the present case is contact cooling of high heat load optical substrates. High heat load x-ray mirrors and other optical components used at the Advanced Photon Source (APS) are either internally cooled or contact cooled. In the internally cooled mirrors, a coolant flows through passages configured in the optical substrate. In the contact-cooled case, cooling is provided by placing cooling plates in contact with the mirror to extract the heat. Here, an experimental setup to measure the thermal contact conductance across a silicon-copper (Si-Cu) interface is described, and the results obtained are presented. The resulting thermal contact resistance data are used in estimating the thermo-mechanical and optical performance of optical substrates cooled by interfaced copper cooling blocks. Several factors influence the heat transfer across solid interfaces. These include the material properties, interface pressure, flatness and roughness of the contacting surfaces, temperature, and interstitial material, if any. Results presented show the variation of thermal contact conductance as a function of applied interface pressure for a Cu-Si interface. Various interstitial materials investigated include indium foil, silver foil and a liquid eutectic (Ga-In-Sn). As expected, thermal contact resistance decreases as interface pressure increases, except in the case of the eutectic, in which it was nearly constant. The softer the interstitial material, the lower the thermal contact resistance. Liquid metal provides the lowest thermal contact resistance across the Cu-Si interface, followed by the indium foil, and then the silver foil.

  3. Frictional aging, de-aging, and re-aging in a monolayer-coated micromachined interface

    NASA Astrophysics Data System (ADS)

    Corwin, Alex D.; de Boer, Maarten P.

    2010-05-01

    Measurements on monolayer-coated polycrystalline silicon surfaces have shown that the static friction coefficient μs strongly depends on loading parameters including hold time and normal hold force by Corwin and de Boer [J. Microelectromech. Syst. 18, 250 (2009)]. In that work, μs was measured by keeping the tangential force constant and lowering normal force until motion occurred. Results indicated that μs also depends strongly on normal force ramp-down rate. Here, we postulate that if the normal load is lowered instantaneously, the time for the block to begin moving, the “release time” tr , will be greater than the inertial response time, which is on the order of 5μs . We measure the release time and find that it spans nearly six decades from less than 100μs to almost 50 s. Release time depends on the loading and unloading history through all three of the parameters varied: hold time, hold force, and release force. An empirical model incorporating all three of these parameters fits the release time data over the full range. Release time decreases after the contacting surfaces are held together at increasing hold force levels and this qualitatively explains a previous observation that static friction aging is suppressed with increasing normal force at a fixed tangential load in this interfacial system. We further quantitatively relate the previous μs loading dependence on all three parameters to the release time model established here by introducing a “re-aging” parameter. This work firmly establishes that release time is a more fundamental parameter than the static friction coefficient and is the origin of static friction coefficient dependencies in this micromachined interface.

  4. Solute transport across a contact interface in deformable porous media.

    PubMed

    Ateshian, Gerard A; Maas, Steve; Weiss, Jeffrey A

    2012-04-05

    A finite element formulation of neutral solute transport across a contact interface between deformable porous media is implemented and validated against analytical solutions. By reducing the integral statements of external virtual work on the two contacting surfaces into a single contact integral, the algorithm automatically enforces continuity of solute molar flux across the contact interface, whereas continuity of the effective solute concentration (a measure of the solute mechano-chemical potential) is achieved using a penalty method. This novel formulation facilitates the analysis of problems in biomechanics where the transport of metabolites across contact interfaces of deformable tissues may be of interest. This contact algorithm is the first to address solute transport across deformable interfaces, and is made available in the public domain, open-source finite element code FEBio (http://www.febio.org).

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

    SciTech Connect

    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 mismatch 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.

  6. 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

  7. Nanoscale roughness contact in a slider-disk interface.

    PubMed

    Hua, Wei; Liu, Bo; Yu, Shengkai; Zhou, Weidong

    2009-07-15

    The nanoscale roughness contact between molecularly smooth surfaces of a slider-disk interface in a hard disk drive is analyzed, and the lubricant behavior at very high shear rate is presented. A new contact model is developed to study the nanoscale roughness contact behavior by classifying various forms of contact into slider-lubricant contact, slider-disk elastic contact and plastic contact. The contact pressure and the contact probabilities of the three types of contact are investigated. The new contact model is employed to explain and provide insight to an interesting experimental result found in a thermal protrusion slider. The protrusion budget for head surfing in the lubricant, which is the ideal state for contact recording, is also discussed.

  8. Novel Monitoring Techniques for Characterizing Frictional Interfaces in the Laboratory

    PubMed Central

    Selvadurai, Paul A.; Glaser, Steven D.

    2015-01-01

    A pressure-sensitive film was used to characterize the asperity contacts along a polymethyl methacrylate (PMMA) interface in the laboratory. The film has structural health monitoring (SHM) applications for flanges and other precision fittings and train rail condition monitoring. To calibrate the film, simple spherical indentation tests were performed and validated against a finite element model (FEM) to compare normal stress profiles. Experimental measurements of the normal stress profiles were within −7.7% to 6.6% of the numerical calculations between 12 and 50 MPa asperity normal stress. The film also possessed the capability of quantifying surface roughness, an important parameter when examining wear and attrition in SHM applications. A high definition video camera supplied data for photometric analysis (i.e., the measure of visible light) of asperities along the PMMA-PMMA interface in a direct shear configuration, taking advantage of the transparent nature of the sample material. Normal stress over individual asperities, calculated with the pressure-sensitive film, was compared to the light intensity transmitted through the interface. We found that the luminous intensity transmitted through individual asperities linearly increased 0.05643 ± 0.0012 candelas for an increase of 1 MPa in normal stress between normal stresses ranging from 23 to 33 MPa. PMID:25923930

  9. Apparatus and method to reduce wear and friction between CMC-to-metal attachment and interface

    SciTech Connect

    Cairo, Ronald Ralph; Parolini, Jason Robert; Delvaux, John McConnell

    2016-11-22

    An apparatus to reduce wear and friction between CMC-to-metal attachment and interface, including a metal layer configured for insertion between a surface interface between a CMC component and a metal component. The surface interface of the metal layer is compliant relative to asperities of the surface interface of the CMC component. A coefficient of friction between the surface interface of the CMC component and the metal component is about 1.0 or less at an operating temperature between about 300.degree. C. to about 325.degree. C. and a limiting temperature of the metal component.

  10. Friction and wear of metals in contact with pyrolytic graphite

    NASA Technical Reports Server (NTRS)

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

    1975-01-01

    Sliding friction experiments were conducted with gold, iron, and tantalum single crystals sliding on prismatic and basal orientations of pyrolytic graphite in various environments, including vacuum, oxygen, water vapor, nitrogen, and hydrogen bromide. Surfaces were examined in the clean state and with various adsorbates present on the graphite surfaces. Auger and LEED spectroscopy, SEM, and EDXA were used to characterize the graphite surfaces. Results indicate that the prismatic and basal orientations do not contain nor do they chemisorb oxygen, water vapor, acetylene, or hydrogen bromide. All three metals exhibited higher friction on the prismatic than on the basal orientation and these metals transferred to the atomically clean prismatic orientation of pyrolytic graphite. No metal transfer to the graphite was observed in the presence of adsorbates at 760 torr. Ion bombardment of the graphite surface with nitrogen ions resulted in the adherence of nitrogen to the surface.

  11. Friction of different monolayer lubricants in MEMs interfaces.

    SciTech Connect

    Carpick, Robert W. (University of Wisconsin, Madison, WI); Street, Mark D.; Ashurst, William Robert; Corwin, Alex David

    2006-01-01

    This report details results from our last year of work (FY2005) on friction in MEMS as funded by the Campaign 6 program for the Microscale Friction project. We have applied different monolayers to a sensitive MEMS friction tester called the nanotractor. The nanotractor is also a useful actuator that can travel {+-}100 {micro}m in 40 nm steps, and is being considered for several MEMS applications. With this tester, we can find static and dynamic coefficients of friction. We can also quantify deviations from Amontons' and Coulomb's friction laws. Because of the huge surface-to-volume ratio at the microscale, surface properties such as adhesion and friction can dominate device performance, and therefore such deviations are important to quantify and understand. We find that static and dynamic friction depend on the monolayer lubricant applied. The friction data can be modeled with a non-zero adhesion force, which represents a deviation from Amontons' Law. Further, we show preliminary data indicating that the adhesion force depends not only on the monolayer, but also on the normal load applied. Finally, we also observe slip deflections before the transition from static to dynamic friction, and find that they depend on the monolayer.

  12. The 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 were conducted with various iron-base alloys (alloying elements were Ti, Cr, Ni, Rh, and W) in contact with a single-crystal silicon carbide (0001) surface in vacuum. 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 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.

  13. 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.

  14. Thermal Performance Evaluation of Friction Stir Welded and Bolted Cold Plates with Al/Cu Interface

    NASA Astrophysics Data System (ADS)

    Lakshminarayanan, A. K.; Suresh, M.; Sibi Varshan, M.

    2015-05-01

    An attempt is made to design and fabricate a cold plate with aluminum-copper dissimilar interface joined by friction stir welding. Optimum welding conditions for obtaining sound-quality corner and T joints with an aluminum-copper interface were established. Welded cross sections of the friction stir welded cold plate were analyzed to understand the bonding characteristics. Computational fluid dynamics (CFD) was used to evaluate the fluid-flow characteristics and thermal resistance of friction stir welded cold plate and the resulted are compared with the conventional bolted cold plate configuration. For CFD modeling of a cold plate with a dissimilar interface, a new methodology is proposed. From the CFD analysis and experimental results, it is observed that friction stir welded cold plate offered better thermal performance compared to the bolted cold plate and it is due to the metallurgical bonding at the aluminum-copper interface with the dispersion of copper particles.

  15. Friction and friction-generated temperature at a polymer-metal interface

    NASA Technical Reports Server (NTRS)

    Price, H. L.; Burks, H. D.

    1974-01-01

    Results of friction and thermal tests of molded polyimide and pyrrone polymers are presented. The coefficient of sliding friction up to surface velocities of 2 m/sec and the coefficient of thermal expansion from 300 to 500 K were measured. An apparatus was constructed to measure simultaneously the coefficient of sliding friction and the friction-generated temperature. Measurements were made at a nominal pressure-velocity product of 0.25 MN/msec and at temperatures between 300 and 500 K.

  16. Friction behavior of silicon in contact with titanium, nickel, silver and copper

    NASA Technical Reports Server (NTRS)

    Mishina, H.; Buckley, D. H.

    1984-01-01

    Sliding friction experiments are conducted with the semiconductor silicon in contact with the metals titanium, nickel, copper, and silver. Sliding is on the (111) plane of single-crystal silicon in the 112 crystallographic direction both in dry and lubricated (mineral oil) sliding. The friction coefficient in dry sliding is controlled by adhesion and the surface chemical activity of the metal. The more active the metal the stronger the adhesion and the higher the friction. In lubricated sliding the lubricant absorbs to the surfaces and reduces the importance of metal chemical effects. In lubricated sliding, silicon ceases to behave in a brittle manner and undergoes plastic deformation under load.

  17. Friction and transfer behavior of pyrolytic boron nitride in contact with various metals

    NASA Technical Reports Server (NTRS)

    Buckley, D. H.

    1976-01-01

    Sliding friction experiments were conducted with pyrolytic boron nitride in sliding contact with itself and various metals. Auger emission spectroscopy was used to monitor transfer of pyrolytic boron nitride to metals and metals to pyrolytic boron nitride. Results indicate that the friction coefficient for pyrolytic boron nitride in contact with metals can be related to the chemical activity of the metals and more particularly to the d valence bond character of the metal. Transfer was found to occur to all metals except silver and gold and the amount of transfer was less in the presence than in the absence of metal oxide. Friction was less for pyrolytic boron nitride in contact with a metal in air than in vacuum.

  18. Numerical Modeling of Frictional Stress in the Contact Zone of Direct Extrusion of Aluminum Alloys under Starved Lubrication

    NASA Astrophysics Data System (ADS)

    Tomar, P.; Pandey, R. K.; Nath, Y.

    2013-11-01

    The objective of this article is to investigate numerically frictional stress in the contact zone at the die/billet interface in the direct extrusion of aluminum alloys considering starved lubricated conditions. In the modeling, both the inlet and work zones have been investigated by coupled solution of the governing equations. The influences of the billet material's strain hardening and its heating due to the plastic deformation are accounted for in the numerical computation. The frictional shear stress at the die/billet interface is computed using three different lubricating oils. Numerical results have been presented herein for the various operating parameters viz. starvation factor ( ψ = 0.2-0.6), lubricants' viscosities ( η 0 = 0.05 Pa s-0.2 Pa s), semi die angle ( β = 10°-20°), and material parameter ( G = 0.56-2.25). It has been observed that the frictional stress increases with an increase in the severity of the lubricant's starvation for the given values of semi-die angle, extrusion speed, and material parameter.

  19. A Mortar Segment-to-Segment Frictional Contact Method for Large Deformations

    SciTech Connect

    Puso, M; Laursen, T

    2003-10-29

    Contact modeling is still one of the most difficult aspects of nonlinear implicit structural analysis. Most 3D contact algorithms employed today use node-on-segment approaches for contacting dissimilar meshes. Two pass node-on-segment contact approaches have the well known deficiency of locking due to over constraint. Furthermore, node-on-segment approaches suffer when individual nodes slide out of contact at contact surface boundaries or when contacting nodes slide from facet to facet. This causes jumps in the contact forces due to the discrete nature of the constraint enforcement and difficulties in convergence for implicit solution techniques. In a previous work, we developed a segment-to-segment contact approach based on the mortar method that was applicable to large deformation mechanics. The approach proved extremely robust since it eliminated the overconstraint which caused ''locking'' and provided smooth force variations in large sliding. Here, we extend this previous approach in to treat frictional contact problems. The proposed approach is then applied to several challenging frictional contact problems which demonstrate its effectiveness.

  20. Adhesion and friction of transition metals in contact with nonmetallic hard materials

    NASA Technical Reports Server (NTRS)

    Miyoshi, K.; Buckley, D. H.

    1981-01-01

    Sliding friction experiments were conducted with the metals yttrium, titanium, tantalum, zirconium, vanadium, neodymium, iron, cobalt, nickel, tungsten, platinum, rhenium, ruthenium, and rhodium in sliding contact with single crystal diamond, silicon carbide, pyrolytic boron nitride, and ferrite. Auger electron spectroscopy analysis was conducted with the metals and nonmetals to determine the surface chemistry and the degree of surface cleanliness. The results of the investigation indicate the adhesion and friction of the transition metals in contact with diamond, silicon carbide, boron nitride, and ferrite are related to the relative chemical activity of the metals. The more chemically active the metal, the higher the coefficient of friction and the greater amount of transfer to the nonmetals.

  1. New inequality and functional for contact with friction - The implicit standard material approach

    SciTech Connect

    De saxce, G.; Feng, Z.Q. Compiegne, Universite de Technologie )

    1991-09-01

    The paper is devoted to the analysis of the 2D or 3D elastic contact problem with Coulomb friction, quasi-static equilibrium, and small displacements. The classical approach is based on two minimum principles, or variational inequalities: the first for unilateral contact and the second for friction. In practical applications involve an algorithm of alternately solving the two problems until convergence is achieved. A coupled approach using one principle or one inequality only is presented. The approach, based on a model of material called implicit standard, allows for extension of the notion of a normality law to dissipative behavior with a nonassociated flow rule, such as surface friction. For numerical time integration of the laws, Moreau's implicit method is considered. Nondifferentiable potentials are regularized by means of the augmented Lagrangian technique. 41 refs.

  2. Dynamic model for the wheel-rail contact friction

    NASA Astrophysics Data System (ADS)

    Lee, HyunWook; Sandu, Corina; Holton, Carvel

    2012-02-01

    Accurately estimating the coefficient of friction (CoF) is essential in modelling railroad dynamics, reducing maintenance costs, and increasing safety in rail operations. The typical assumption of a constant CoF is widely used in theoretical studies; however, it has been noticed that the CoF is not constant, but rather depends on various dynamic parameters and instantaneous conditions. In this paper, we present a newly developed three-dimensional nonlinear CoF model for the dry rail condition and test the CoF variation using this model with estimated dynamic parameters. The wheel-rail is modelled as a mass-spring-damper system to simulate the basic wheel-rail dynamics. Although relatively simple, this model is considered sufficient for the purpose of this study. Simulations are performed at a train speed of 20 m/s using rail roughness as an excitation source. The model captures the CoF extremes and illustrates its nonlinear behaviour and instantaneous dependence on several structural and dynamic parameters.

  3. The dryout region in frictionally heated sliding contacts

    NASA Technical Reports Server (NTRS)

    Hendricks, R. C.; Braun, J.; Arp, V.; Giarratano, P. J.

    1982-01-01

    Some conditions under which boiling and two-phase flow can occur in or near a wet sliding contact are determined and illustrated. The experimental apparatus consisted of a tool pressed against an instrumented slider plate and motion picture sequences at 4000 frames/sec. The temperature and photographic data demonstrated surface conditions of boiling, drying, trapped gas evolution (solutions), and volatility of fluid mixture components. The theoretical modeling and analysis are in reasonable agreement with experimental data.

  4. Unilateral contact induced blade/casing vibratory interactions in impellers: Analysis for flexible casings with friction and abradable coating

    NASA Astrophysics Data System (ADS)

    Batailly, Alain; Legrand, Mathias

    2015-07-01

    This contribution addresses the vibratory analysis of unilateral contact induced structural interactions between a bladed impeller and its surrounding flexible casing. It extends the numerical developments exposed in a previous paper to flexible casings. The casing finite element model and the construction of the associated reduced-order model for efficient computations are first exposed in detail along with an extensive presentation of the smoothing strategy implemented on the contact interface. The proposed algorithms embedding unilateral contact conditions together with abradable coating removal are subsequently introduced and validated through a systematic analysis of (1) the nonlinear procedure for the computation of impeller/casing distances, (2) the treatment of three-dimensional friction and contact forces, (3) the correction of the displacements when unilateral contact or abradable removal arises, and (4) the possible hybrid contact scenarii involving localized total removal of the abradable coating. Finally, two illustrative case studies show that the linear interaction condition, commonly considered for the safe design of impellers and casings in turbomachinery, may be advantageously combined with the presented numerical strategy in order to assess the actual importance of predicted critical speeds.

  5. Linear contact interface parameter identification using dynamic characteristic equation

    NASA Astrophysics Data System (ADS)

    Jalali, Hassan

    2016-01-01

    The stiffness characteristics of the contact interfaces in joints or boundary conditions have a great effect on dynamic response of assembled structures. Predictive analytical/numerical modeling of mechanical structures is not possible without representing the contact interfaces accurately. Because of the complex mechanisms involved, contact interfaces introduce difficulties both in modeling the inherent dynamics and identification of the model parameters. In this paper an identification approach employing the dynamic characteristic equation is proposed for linear interface parameters. The proposed method is applicable to both analytical and numerical problems. The accuracy of the proposed method is investigated by simulation results of a beam with elastic boundary support and experimental results of a bolted lap-joint.

  6. Thermal fluctuations of an interface near a contact line.

    PubMed

    Belardinelli, D; Sbragaglia, M; Gross, M; Andreotti, B

    2016-11-01

    The effect of thermal fluctuations near a contact line of a liquid interface partially wetting an impenetrable substrate is studied analytically and numerically. Promoting both the interface profile and the contact line position to random variables, we explore the equilibrium properties of the corresponding fluctuating contact line problem based on an interfacial Hamiltonian involving a "contact" binding potential. To facilitate an analytical treatment, we consider the case of a one-dimensional interface. The effective boundary condition at the contact line is determined by a dimensionless parameter that encodes the relative importance of thermal energy and substrate energy at the microscopic scale. We find that this parameter controls the transition from a partial wetting to a pseudopartial wetting state, the latter being characterized by a thin prewetting film of fixed thickness. In the partial wetting regime, instead, the profile typically approaches the substrate via an exponentially thinning prewetting film. We show that, independently of the physics at the microscopic scale, Young's angle is recovered sufficiently far from the substrate. The fluctuations of the interface and of the contact line give rise to an effective disjoining pressure, exponentially decreasing with height. Fluctuations therefore provide a regularization of the singular contact forces occurring in the corresponding deterministic problem.

  7. Thermal fluctuations of an interface near a contact line

    NASA Astrophysics Data System (ADS)

    Belardinelli, D.; Sbragaglia, M.; Gross, M.; Andreotti, B.

    2016-11-01

    The effect of thermal fluctuations near a contact line of a liquid interface partially wetting an impenetrable substrate is studied analytically and numerically. Promoting both the interface profile and the contact line position to random variables, we explore the equilibrium properties of the corresponding fluctuating contact line problem based on an interfacial Hamiltonian involving a "contact" binding potential. To facilitate an analytical treatment, we consider the case of a one-dimensional interface. The effective boundary condition at the contact line is determined by a dimensionless parameter that encodes the relative importance of thermal energy and substrate energy at the microscopic scale. We find that this parameter controls the transition from a partial wetting to a pseudopartial wetting state, the latter being characterized by a thin prewetting film of fixed thickness. In the partial wetting regime, instead, the profile typically approaches the substrate via an exponentially thinning prewetting film. We show that, independently of the physics at the microscopic scale, Young's angle is recovered sufficiently far from the substrate. The fluctuations of the interface and of the contact line give rise to an effective disjoining pressure, exponentially decreasing with height. Fluctuations therefore provide a regularization of the singular contact forces occurring in the corresponding deterministic problem.

  8. Finite element analysis of the contact interface between trans-femoral stump and prosthetic socket.

    PubMed

    Zhang, Linlin; Zhu, Ming; Shen, Ling; Zheng, Feng

    2013-01-01

    Transfemoral amputees need prosthetic devices after amputation surgery, and the interface pressure between the residual limb and prosthetic socket has a significant effect on an amputee's satisfaction and comfort. The purpose of this study was to build a nonlinear finite element model to investigate the interface pressure between the above-knee residual limb and its prosthetic socket. The model was three-dimensional (3D) with consideration of nonlinear boundary conditions. Contact analysis was used to simulate the friction conditions between skin and the socket. The normal stresses up to 80.57 kPa at the distal end of the soft tissue. The longitudinal and circumferential shear stress distributions at the limb-socket interface were also simulated. This study explores the influences of load transfer between trans-femoral residual limb and its prosthetic socket.

  9. Friction Reduction Through Ultrasonic Vibration Part 2: Experimental Evaluation of Intermittent Contact and Squeeze Film Levitation.

    PubMed

    Sednaoui, Thomas; Vezzoli, Eric; Dzidek, Brygida Maria; Lemaire-Semail, Betty; Chappaz, Cedrick; Adams, Michael

    2017-02-17

    In part 1 of the current study of haptic displays, a finite element (FE) model of a finger exploring a plate vibrating out-of-plane at ultrasonic frequencies was developed as well as a spring-frictional slider model. It was concluded that the reduction in friction induced by the vibrations could be ascribed to ratchet mechanism as a result of intermittent contact. The relative reduction in friction calculated using the FE model could be superimposed onto an exponential function of a dimensionless group defined from relevant parameters. The current paper presents measurements of the reduction in friction, involving real and artificial fingertips, as a function of the vibrational amplitude and frequency, the applied normal force and the exploration velocity. The results are reasonably similar to the calculated FE values and also could be superimposed using the exponential function provided that the intermittent contact was sufficiently well developed, which for the frequencies examined correspond to a minimum vibrational amplitude of 1 μm P-P. It was observed that the reduction in friction depends on the exploration velocity and is independent of the applied normal force and ambient air pressure, which is not consistent with the squeeze film mechanism. However, the modelling did not incorporate the influence of air and the effect of ambient pressure was measured under a limited range of conditions, Thus squeeze film levitation may be synergistic with the mechanical interaction.

  10. Tribo-chemical behavior of eutectoid steel during rolling contact friction

    NASA Astrophysics Data System (ADS)

    Zhou, Y.; Cai, Z. B.; Peng, J. F.; Cao, B. B.; Jin, X. S.; Zhu, M. H.

    2016-12-01

    The tribo-chemical behavior of the eutectoid steel during rolling contact friction is investigated via scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy (XPS) and electron probe X-ray microanalysis. The worn surface is divided into three zones: matrix zone (without friction), tribo-film zone (formed during friction) and delamination zone (tribo-film spalling). The different chemical states of atoms between those three zones and the air were investigated using the XPS analysis. The results showed that the matrix zone is composed of Fe2O3, FeO and metallic Fe, while the tribo-film and delamination zones only contain Fe2O3 and FeO. Where the tribo-film is formed, the absorptive ability of O and C atoms on the top 2-3 atomic layers is probably weakened, while the exposed fresh metal in the delamination zone tends to be continuously oxidized and form tribo-film. The tribo-chemical reaction in the delamination zone is more activated than that in the other two zones. The protective nature of the tribo-film probably maintains a low friction coefficient under rolling contact friction condition.

  11. 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.

  12. Boundary lubrication of heterogeneous surfaces and the onset of cavitation in frictional contacts

    PubMed Central

    Savio, Daniele; Pastewka, Lars; Gumbsch, Peter

    2016-01-01

    Surfaces can be slippery or sticky depending on surface chemistry and roughness. We demonstrate in atomistic simulations that regular and random slip patterns on a surface lead to pressure excursions within a lubricated contact that increase quadratically with decreasing contact separation. This is captured well by a simple hydrodynamic model including wall slip. We predict with this model that pressure changes for larger length scales and realistic frictional conditions can easily reach cavitation thresholds and significantly change the load-bearing capacity of a contact. Cavitation may therefore be the norm, not the exception, under boundary lubrication conditions. PMID:27051871

  13. Frictional unilateral contact for hemitropic solids in micropolar elasticity and boundary element approximation

    NASA Astrophysics Data System (ADS)

    Gwinner, Joachim

    2016-12-01

    This contribution deals with unilateral contact problems with Tresca friction (given friction model) in hemitropic mi-cropolar elasticity. Based on a boundary integral approach such problems can be reduced to boundary variational inequalities. This suggests the use of boundary element methods for their numerical treatment. With higher order approximation this leads to a nonconforming approximation what can numerically be realized by means of Gauss-Lobatto quadrature. The contribution is based on the recent papers [7, 8] of the author and on joint work [3] with A. Gachechiladze, R. Gachechi-ladze, and D. Natroshvili.

  14. The friction and wear of ceramic/ceramic and ceramic/metal combinations in sliding contact

    NASA Technical Reports Server (NTRS)

    Sliney, Harold E.; Dellacorte, Christopher

    1994-01-01

    The tribological characteristics of ceramics sliding on ceramics are compared to those of ceramics sliding on a nickel-based turbine alloy. The friction and wear of oxide ceramics and silicon-based ceramics in air at temperatures from room ambient to 900 C (in a few cases to 1200 C) were measured for a hemispherically-tipped pin on a flat sliding contact geometry. In general, especially at high temperature, friction and wear were lower for ceramic/metal combinations than for ceramic/ceramic combinations. The better tribological performance for ceramic/metal combinations is attributed primarily to the lubricious nature of the oxidized surface of the metal.

  15. The friction and wear of ceramic/ceramic and ceramic/metal combinations in sliding contact

    NASA Technical Reports Server (NTRS)

    Sliney, Harold E.; Dellacorte, Christopher

    1993-01-01

    The tribological characteristics of ceramics sliding on ceramics are compared to those of ceramics sliding on a nickel based turbine alloy. The friction and wear of oxide ceramics and silicon-based ceramics in air at temperatures from room ambient to 900 C (in a few cases to 1200 C) were measured for a hemispherically-tipped pin on a flat sliding contact geometry. In general, especially at high temperature, friction and wear were lower for ceramic/metal combinations than for ceramic/ceramic combinations. The better tribological performance for ceramic/metal combinations is attributed primarily to the lubricious nature of the oxidized surface of the metal.

  16. Computational Methods for Frictional Contact With Applications to the Space Shuttle Orbiter Nose-Gear Tire

    NASA Technical Reports Server (NTRS)

    Tanner, John A.

    1996-01-01

    A computational procedure is presented for the solution of frictional contact problems for aircraft tires. A Space Shuttle nose-gear tire is modeled using a two-dimensional laminated anisotropic shell theory which includes the effects of variations in material and geometric parameters, transverse-shear deformation, and geometric nonlinearities. Contact conditions are incorporated into the formulation by using a perturbed Lagrangian approach with the fundamental unknowns consisting of the stress resultants, the generalized displacements, and the Lagrange multipliers associated with both contact and friction conditions. The contact-friction algorithm is based on a modified Coulomb friction law. A modified two-field, mixed-variational principle is used to obtain elemental arrays. This modification consists of augmenting the functional of that principle by two terms: the Lagrange multiplier vector associated with normal and tangential node contact-load intensities and a regularization term that is quadratic in the Lagrange multiplier vector. These capabilities and computational features are incorporated into an in-house computer code. Experimental measurements were taken to define the response of the Space Shuttle nose-gear tire to inflation-pressure loads and to inflation-pressure loads combined with normal static loads against a rigid flat plate. These experimental results describe the meridional growth of the tire cross section caused by inflation loading, the static load-deflection characteristics of the tire, the geometry of the tire footprint under static loading conditions, and the normal and tangential load-intensity distributions in the tire footprint for the various static vertical loading conditions. Numerical results were obtained for the Space Shuttle nose-gear tire subjected to inflation pressure loads and combined inflation pressure and contact loads against a rigid flat plate. The experimental measurements and the numerical results are compared.

  17. Contact angle distribution of particles at fluid interfaces.

    PubMed

    Snoeyink, Craig; Barman, Sourav; Christopher, Gordon F

    2015-01-27

    Recent measurements have implied a distribution of interfacially adsorbed particles' contact angles; however, it has been impossible to measure statistically significant numbers for these contact angles noninvasively in situ. Using a new microscopy method that allows nanometer-scale resolution of particle's 3D positions on an interface, we have measured the contact angles for thousands of latex particles at an oil/water interface. Furthermore, these measurements are dynamic, allowing the observation of the particle contact angle with high temporal resolution, resulting in hundreds of thousands of individual contact angle measurements. The contact angle has been found to fit a normal distribution with a standard deviation of 19.3°, which is much larger than previously recorded. Furthermore, the technique used allows the effect of measurement error, constrained interfacial diffusion, and particle property variation on the contact angle distribution to be individually evaluated. Because of the ability to measure the contact angle noninvasively, the results provide previously unobtainable, unique data on the dynamics and distribution of the adsorbed particles' contact angle.

  18. Contact damage failure analyses of fretting wear behavior of the metal stem titanium alloy-bone cement interface.

    PubMed

    Zhang, Lanfeng; Ge, Shirong; Liu, Hongtao; Wang, Qingliang; Wang, Liping; Xian, Cory J

    2015-11-01

    Although cemented titanium alloy is not favored currently in the Western world for its poor clinical and radiography outcomes, its lower modulus of elasticity and good biocompatibility are instrumental for its ability supporting and transforming physical load, and it is more suitable for usage in Chinese and Japanese populations due to their lower body weights and unique femoral characteristics. Through various friction tests of different cycles, loads and conditions and by examining fretting hysteresis loops, fatigue process curves and wear surfaces, the current study investigated fretting wear characteristics and wear mechanism of titanium alloy stem-bone cement interface. It was found that the combination of loads and displacement affected the wear quantity. Friction coefficient, which was in an inverse relationship to load under the same amplitude, was proportional to amplitudes under the same load. Additionally, calf serum was found to both lubricate and erode the wear interface. Moreover, cement fatigue contact areas appeared black/oxidative in dry and gruel in 25% calf serum. Fatigue scratches were detected within contact areas, and wear scars were found on cement and titanium surfaces, which were concave-shaped and ring concave/ convex-shaped, respectively. The coupling of thermoplastic effect and minimal torque damage has been proposed to be the major reason of contact damage. These data will be important for further studies analyzing metal-cement interface failure performance and solving interface friction and wear debris production issues.

  19. Beta atomic contacts: identifying critical specific contacts in protein binding interfaces.

    PubMed

    Liu, Qian; Kwoh, Chee Keong; Hoi, Steven C H

    2013-01-01

    Specific binding between proteins plays a crucial role in molecular functions and biological processes. Protein binding interfaces and their atomic contacts are typically defined by simple criteria, such as distance-based definitions that only use some threshold of spatial distance in previous studies. These definitions neglect the nearby atomic organization of contact atoms, and thus detect predominant contacts which are interrupted by other atoms. It is questionable whether such kinds of interrupted contacts are as important as other contacts in protein binding. To tackle this challenge, we propose a new definition called beta (β) atomic contacts. Our definition, founded on the β-skeletons in computational geometry, requires that there is no other atom in the contact spheres defined by two contact atoms; this sphere is similar to the van der Waals spheres of atoms. The statistical analysis on a large dataset shows that β contacts are only a small fraction of conventional distance-based contacts. To empirically quantify the importance of β contacts, we design βACV, an SVM classifier with β contacts as input, to classify homodimers from crystal packing. We found that our βACV is able to achieve the state-of-the-art classification performance superior to SVM classifiers with distance-based contacts as input. Our βACV also outperforms several existing methods when being evaluated on several datasets in previous works. The promising empirical performance suggests that β contacts can truly identify critical specific contacts in protein binding interfaces. β contacts thus provide a new model for more precise description of atomic organization in protein quaternary structures than distance-based contacts.

  20. Triboelectrical charge generated by frictional sliding contact between polymeric materials

    NASA Astrophysics Data System (ADS)

    Zeghloul, T.; Neagoe, M. B.; Prawatya, Y. E.; Dascalescu, L.

    2017-02-01

    The polymers used regularly in mechanical assemblies are brought up in relative sliding. The electrostatic charges generated in these functional conditions are merely known. Many factors are involved in the triboelectric charging process: normal load, the sliding velocity. The aim of this paper is to analyse the influence of these factors in the repartition and evolution of the electric potential at the surface in contact. The tribocharging experiments are carried out with samples cut from three polymers: sample A (5 mm x 15 mm x100 mm) from Acrylonitrile Butadiene Styrene (ABS) or Polypropylene (PP), and sample B (5 mm x 50 mm x 180 mm) from Polyvinyl Chloride (PVC). The normal load is set to four values in the range 2 to 14 N, and the sliding velocity is varied between 70 and 122 mm/s. The results point out that the variation of relative velocity between samples is not changing the average potential for the sample B. The surface potential has a linear increase with the normal load.

  1. Effects of multi-scale roughness and frictional heating on solid body contact deformation

    NASA Astrophysics Data System (ADS)

    Komvopoulos, Kyriakos

    2008-01-01

    Solid body contact is restricted to a discrete number of randomly distributed microscopic areas resulting from the deformation of interacting surface protrusions (asperities). The deformation mode of these asperity contacts can be elastic, elastic-plastic, or fully plastic, depending on the local surface interference, asperity radius of curvature, coefficient of friction, and mechanical properties of the solid surfaces. Traditionally, the surface topography has been described by statistical models which rely on unrealistic simplifications of the shape, height, and size of the asperities. Such assumptions were avoided in contemporary contact mechanics analyses, which use fractal geometry to accomplish a surface topography description over a wide range of length scales. The main objective of this article is to provide an assessment of the role of multi-scale topography (roughness) and frictional heating in contact deformation of elastic-plastic solid bodies. Contact relationships, derived at the asperity level, which include the mechanical properties of surface layer and substrate medium, layer thickness, local surface interference, and equivalent asperity radius of curvature, are presented for different modes of deformation. These asperity-level relationships and a fractal model of the surface topography are incorporated into a numerical integration scheme to analyze multi-scale thermomechanical contact deformation over the entire real contact area of homogeneous and layered media possessing realistic surface topographies. To cite this article: K. Komvopoulos, C. R. Mecanique 336 (2008).

  2. Diffuse interface simulation of ternary fluids in contact with solid

    NASA Astrophysics Data System (ADS)

    Zhang, Chun-Yu; Ding, Hang; Gao, Peng; Wu, Yan-Ling

    2016-03-01

    In this article we developed a geometrical wetting condition for diffuse-interface simulation of ternary fluid flows with moving contact lines. The wettability of the substrate in the presence of ternary fluid flows is represented by multiple contact angles, corresponding to the different material properties between the respective fluid and the substrate. Displacement of ternary fluid flows on the substrate leads to the occurrence of moving contact point, at which three moving contact lines meet. We proposed a weighted contact angle model, to replace the jump in contact angle at the contact point by a relatively smooth transition of contact angle over a region of 'diffuse contact point' of finite size. Based on this model, we extended the geometrical formulation of wetting condition for two-phase flows with moving contact lines to ternary flows with moving contact lines. Combining this wetting condition, a Navier-Stokes solver and a ternary-fluid model, we simulated two-dimensional spreading of a compound droplet on a substrate, and validated the numerical results of the drop shape at equilibrium by comparing against the analytical solution. We also checked the convergence rate of the simulation by investigating the axisymmetric drop spreading in a capillary tube. Finally, we applied the model to a variety of applications of practical importance, including impact of a circular cylinder into a pool of two layers of different fluids and sliding of a three-dimensional compound droplet in shear flows.

  3. Self-organization at the frictional interface for green tribology.

    PubMed

    Nosonovsky, Michael

    2010-10-28

    Despite the fact that self-organization during friction has received relatively little attention from tribologists so far, it has the potential for the creation of self-healing and self-lubricating materials, which are important for green or environment-friendly tribology. The principles of the thermodynamics of irreversible processes and of the nonlinear theory of dynamical systems are used to investigate the formation of spatial and temporal structures during friction. The transition to the self-organized state with low friction and wear occurs through destabilization of steady-state (stationary) sliding. The criterion for destabilization is formulated and several examples are discussed: the formation of a protective film, microtopography evolution and slip waves. The pattern formation may involve self-organized criticality and reaction-diffusion systems. A special self-healing mechanism may be embedded into the material by coupling the corresponding required forces. The analysis provides the structure-property relationship, which can be applied for the design optimization of composite self-lubricating and self-healing materials for various ecologically friendly applications and green tribology.

  4. Rolling-element bearings. [contact sliding friction study of solid bodies

    NASA Technical Reports Server (NTRS)

    Anderson, W. J.

    1980-01-01

    In contrast to hydrodynamic bearings, which depend for low-friction characteristics on a fluid film between the journal and the bearing surfaces, roller-element bearings employ a number of balls or rollers that roll in an annular space. The paper briefly outlines the advantages and disadvantages of roller-element bearings as compared to hydrodynamic bearings. The discussion covers bearing types, rolling friction, friction losses in rolling bearings, contact stresses, deformations, kinematics (normal and high speeds), bearing dynamics including elastohydrodynamics, load distribution, lubrication (grease, solid oil, oil-air mist), specific dynamic capacity and life, specific static capacity, and fatigue or wearout (elastohydrodynamics, wear). Rolling bearing wear factor as a function of operating environment is plotted and discussed.

  5. 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.

  6. 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.

  7. Friction, wear and noise of slip ring and brush contacts for synchronous satellite use.

    NASA Technical Reports Server (NTRS)

    Lewis, N. E.; Cole, S. R.; Glossbrenner, E. W.; Vest, C. E.

    1972-01-01

    A program is being conducted for testing of slip rings for synchronous orbit application. Instrumentation systems necessary for monitoring electrical noise, friction, and brush wear at atmospheric pressure and at less than 50 ntorr have been developed. A multiplex scheme necessary for the simultaneous recording of brush displacement, friction, and electrical noise has also been developed. Composite brushes consisting of silver-molybdenum disulfide-graphite and silver-niobium diselenide-graphite have been employed on rings of coin silver and rhodium plate. Four contact combinations have been tested during an ambient condition run-in at 150 rpm and a humidity sequence at 0.1 rpm. The first six months of the two year vacuum test at 0.1 rpm have been completed. Electrical noise, friction and brush wear data recorded during these periods have been analyzed.

  8. Numerical techniques for rolling rubber wheels: treatment of inelastic material properties and frictional contact

    NASA Astrophysics Data System (ADS)

    Ziefle, M.; Nackenhorst, U.

    2008-08-01

    Arbitrary Lagrangian Eulerian (ALE) methods provide a well established basis for the numerical analysis of rolling contact problems. Whereas the theoretical framework is well developed for elastic constitutive behavior, special measures are necessary for the computation of dissipative effects like inelastic properties and friction because the path of material points is not traced inherently. In this presentation a fractional step approach is suggested for the integration of the evolution equations for internal variables. A Time-Discontinuous Galerkin (TDG) method is introduced for the numerical solution of the related advection equations. Furthermore, a mathematically sound approach for the treatment of frictional rolling within the ALE-description is suggested. By this novel and fully implicit algorithm the slip velocities are integrated along their path-lines. For dissipative effects due to both, inelastic behavior and friction, physical reliable results will be demonstrated as well as the computability of large scaled finite element tire-models.

  9. Experimental Validation of a Differential Variational Inequality-Based Approach for Handling Friction and Contact in Vehicle

    DTIC Science & Technology

    2015-11-20

    Friction and Contact in Vehicle/Granular-Terrain Interaction Daniel Melanza,b, Paramsothy Jayakumara, Dan Negrutb aU.S. Army Tank Automotive Research...Handling Friction and Contact in Vehicle 5a. CONTRACT NUMBER 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 6. AUTHOR(S) Daniel Melanz; Paramsothy...and non-homogeneity. There are multiple formulations of DEM, classified based on how the contact and impact are handled when two bodies collide. This

  10. Molecular mechanistic origin of nanoscale contact, friction, and scratch in complex particulate systems.

    PubMed

    Jalilvand, Soroosh; Shahsavari, Rouzbeh

    2015-02-11

    Nanoscale contact mechanisms, such as friction, scratch, and wear, have a profound impact on physics of technologically important particulate systems. Determining the key underlying interparticle interactions that govern the properties of the particulate systems has been long an engineering challenge. Here, we focus on particulate calcium-silicate-hydrate (C-S-H) as a model system and use atomistic simulations to decode the interplay between crystallographic directions, structural defects, and atomic species on normal and frictional forces. By exhibiting high material inhomogeneity and low structural symmetry, C-S-H provides an excellent system to explore various contact-induced nanoscale deformation mechanisms in complex particulate systems. Our findings provide a deep fundamental understanding of the role of inherent material features, such as van der Waals versus Coulombic interactions and the role of atomic species, in controlling the nanoscale normal contact, friction, and scratch mechanisms, thereby providing de novo insight and strategies for intelligent modulation of the physics of the particulate systems. This work is the first report on atomic-scale investigation of the contact-induced nanoscale mechanisms in structurally complex C-S-H materials and can potentially open new opportunities for knowledge-based engineering of several other particulate systems such as ceramics, sands, and powders and self-assembly of colloidal systems in general.

  11. Friction behavior of a multi-interface system and improved performance by AlMgB14–TiB2–C and diamond-like-carbon coatings

    DOE PAGES

    Qu, Jun; Blau, Peter J.; Higdon, Clifton; ...

    2016-03-29

    We investigated friction behavior of a bearing system with two interfaces involved: a roller component experiencing rolling–sliding interaction against twin cylinders under point contacts while simultaneously undergoing pure sliding interaction against a socket under a conformal contact. Lubrication modeling predicted a strong correlation between the roller's rolling condition and the system's friction behavior. Experimental observations first validated the analytical predictions using steel and iron components. Diamond-like-carbon (DLC) coating and AlMgB14–TiB2 coating with a carbon topcoat (BAMC) were then applied to the roller and twin cylinders, respectively. In conclusion, testing and analysis results suggest that the coatings effectively decreased the slipmore » ratio for the roller–cylinder contact and the sliding friction at both bearing interfaces and, as a result, significantly reduced the system frictional torque.« less

  12. Friction behavior of a multi-interface system and improved performance by AlMgB14–TiB2–C and diamond-like-carbon coatings

    SciTech Connect

    Qu, Jun; Blau, Peter J.; Higdon, Clifton; Cook, Bruce A.

    2016-03-29

    We investigated friction behavior of a bearing system with two interfaces involved: a roller component experiencing rolling–sliding interaction against twin cylinders under point contacts while simultaneously undergoing pure sliding interaction against a socket under a conformal contact. Lubrication modeling predicted a strong correlation between the roller's rolling condition and the system's friction behavior. Experimental observations first validated the analytical predictions using steel and iron components. Diamond-like-carbon (DLC) coating and AlMgB14–TiB2 coating with a carbon topcoat (BAMC) were then applied to the roller and twin cylinders, respectively. In conclusion, testing and analysis results suggest that the coatings effectively decreased the slip ratio for the roller–cylinder contact and the sliding friction at both bearing interfaces and, as a result, significantly reduced the system frictional torque.

  13. Survival of Heterogeneous Stress Distributions Created by Precursory Slip at Frictional Interfaces

    NASA Astrophysics Data System (ADS)

    Radiguet, Mathilde; Kammer, David S.; Gillet, Philippe; Molinari, Jean-François

    2013-10-01

    We study the dynamics of successive slip events at a frictional interface with finite-element simulations. Because of the viscous properties of the material, the stress concentrations created by the arrest of precursory slip are not erased by the propagation of the following rupture but reappear with the relaxation of the material. We show that the amplitude of the stress concentrations follows an exponential decay, which is controlled by the bulk material properties. These results highlight the importance of viscosity in the heterogeneous stress state of a frictional interface and reveal the “memory effect” that affects successive ruptures.

  14. Dissipative interface waves and the transient response of a three-dimensional sliding interface with Coulomb friction

    NASA Astrophysics Data System (ADS)

    Dunham, Eric M.

    2005-02-01

    We investigate the linearized response of two elastic half-spaces sliding past one another with constant Coulomb friction to small three-dimensional perturbations. Starting with the assumption that friction always opposes slip velocity, we derive a set of linearized boundary conditions relating perturbations of shear traction to slip velocity. Friction introduces an effective viscosity transverse to the direction of the original sliding, but offers no additional resistance to slip aligned with the original sliding direction. The amplitude of transverse slip depends on a nondimensional parameter η=csτ0/μv0, where τ0 is the initial shear stress, 2v0 is the initial slip velocity, μ is the shear modulus, and cs is the shear wave speed. As η→0, the transverse shear traction becomes negligible, and we find an azimuthally symmetric Rayleigh wave trapped along the interface. As η→∞, the inplane and antiplane wavesystems frictionally couple into an interface wave with a velocity that is directionally dependent, increasing from the Rayleigh speed in the direction of initial sliding up to the shear wave speed in the transverse direction. Except in these frictional limits and the specialization to two-dimensional inplane geometry, the interface waves are dissipative. In addition to forward and backward propagating interface waves, we find that for η>1, a third solution to the dispersion relation appears, corresponding to a damped standing wave mode. For large-amplitude perturbations, the interface becomes isotropically dissipative. The behavior resembles the frictionless response in the extremely strong perturbation limit, except that the waves are damped. We extend the linearized analysis by presenting analytical solutions for the transient response of the medium to both line and point sources on the interface. The resulting self-similar slip pulses consist of the interface waves and head waves, and help explain the transmission of forces across fracture

  15. The origin of ultrasound-induced friction reduction in microscopic mechanical contacts.

    PubMed

    Hesjedal, Thorsten; Behme, Gerd

    2002-03-01

    We present a study of the origin of ultrasound-induced friction reduction in microscopic mechanical contacts. The effect of friction reduction caused by Rayleigh-type surface acoustic waves (SAWs) is demonstrated for propagating and two-dimensional, standing wave fields using lateral force microscopy (LFM). It is shown that with increasing wave amplitude, friction is completely suppressed. To detect and distinguish between the effect of lateral and vertical surface oscillation components on the cantilever movement, we employed multimode scanning acoustic force microscopy (SAFM). We found that the friction reduction effect is only due to the vertical oscillation component. Because this effect does not appear for purely in-plane polarized Love waves, we concluded that the mechanical diode effect is most probably responsible for the SAW-induced lubrication. This explanation is also supported by vertical and longitudinal SAFM measurements, which show that, in areas where friction is completely suppressed, low frequency vertical cantilever oscillations can still be observed, whereas lateral or torsional oscillations are no longer excited.

  16. An analytical elastic plastic contact model with strain hardening and frictional effects for normal and oblique impacts

    SciTech Connect

    Brake, M. R. W.

    2015-02-17

    Impact between metallic surfaces is a phenomenon that is ubiquitous in the design and analysis of mechanical systems. We found that to model this phenomenon, a new formulation for frictional elastic–plastic contact between two surfaces is developed. The formulation is developed to consider both frictional, oblique contact (of which normal, frictionless contact is a limiting case) and strain hardening effects. The constitutive model for normal contact is developed as two contiguous loading domains: the elastic regime and a transitionary region in which the plastic response of the materials develops and the elastic response abates. For unloading, the constitutive model is based on an elastic process. Moreover, the normal contact model is assumed to only couple one-way with the frictional/tangential contact model, which results in the normal contact model being independent of the frictional effects. Frictional, tangential contact is modeled using a microslip model that is developed to consider the pressure distribution that develops from the elastic–plastic normal contact. This model is validated through comparisons with experimental results reported in the literature, and is demonstrated to be significantly more accurate than 10 other normal contact models and three other tangential contact models found in the literature.

  17. An analytical elastic plastic contact model with strain hardening and frictional effects for normal and oblique impacts

    DOE PAGES

    Brake, M. R. W.

    2015-02-17

    Impact between metallic surfaces is a phenomenon that is ubiquitous in the design and analysis of mechanical systems. We found that to model this phenomenon, a new formulation for frictional elastic–plastic contact between two surfaces is developed. The formulation is developed to consider both frictional, oblique contact (of which normal, frictionless contact is a limiting case) and strain hardening effects. The constitutive model for normal contact is developed as two contiguous loading domains: the elastic regime and a transitionary region in which the plastic response of the materials develops and the elastic response abates. For unloading, the constitutive model ismore » based on an elastic process. Moreover, the normal contact model is assumed to only couple one-way with the frictional/tangential contact model, which results in the normal contact model being independent of the frictional effects. Frictional, tangential contact is modeled using a microslip model that is developed to consider the pressure distribution that develops from the elastic–plastic normal contact. This model is validated through comparisons with experimental results reported in the literature, and is demonstrated to be significantly more accurate than 10 other normal contact models and three other tangential contact models found in the literature.« less

  18. A fast nonlinear conjugate gradient based method for 3D concentrated frictional contact problems

    NASA Astrophysics Data System (ADS)

    Zhao, Jing; Vollebregt, Edwin A. H.; Oosterlee, Cornelis W.

    2015-05-01

    This paper presents a fast numerical solver for a nonlinear constrained optimization problem, arising from 3D concentrated frictional shift and rolling contact problems with dry Coulomb friction. The solver combines an active set strategy with a nonlinear conjugate gradient method. One novelty is to consider the tractions of each slip element in a polar coordinate system, using azimuth angles as variables instead of conventional traction variables. The new variables are scaled by the diagonal of the underlying Jacobian. The fast Fourier transform (FFT) technique accelerates all matrix-vector products encountered, exploiting the matrix' Toeplitz structure. Numerical tests demonstrate a significant reduction of the computational time compared to existing solvers for concentrated contact problems.

  19. Breaking of symmetries for stabilization of rotating continua in frictional contact

    NASA Astrophysics Data System (ADS)

    Spelsberg-Korspeter, Gottfried

    2009-05-01

    Rotating structures subject to frictional contact are susceptible to self-excited vibrations that are responsible for noise problems. In previous work the underlying mechanism has been explained through mathematical-mechanical models. From practical experience it is known that breaking the symmetry of a rotor can have a stabilizing effect. The present paper is devoted to a mathematical justification of this phenomenon. At the same time a method for a quantitative investigation of the influence of asymmetries on the stability behavior is outlined. As an example a rotating annular Kirchhoff plate in contact with friction pads is studied serving as a minimal model for brake squeal. A possible application of the results is the support of the design process for squeal free brake rotors where currently only experimental methods yield information about the tendency of an asymmetric brake rotor to squeal.

  20. Green-Kubo relation for friction at liquid-solid interfaces

    NASA Astrophysics Data System (ADS)

    Huang, Kai; Szlufarska, Izabela

    2014-03-01

    We have developed a Green-Kubo relation that enables accurate calculations of friction at solid-liquid interfaces directly from equilibrium molecular dynamics (MD) simulations and that provides a pathway to bypass the time-scale limitations of typical nonequilibrium MD simulations. The theory has been validated for a number of different interfaces and it is demonstrated that the liquid-solid slip is an intrinsic property of an interface. Because of the high numerical efficiency of our method, it can be used in the design of interfaces for applications in aqueous environments, such as nano- and microfluidics.

  1. Bearing material. [composite material with low friction surface for rolling or sliding contact

    NASA Technical Reports Server (NTRS)

    Sliney, H. E. (Inventor)

    1976-01-01

    A composite material is described which will provide low friction surfaces for materials in rolling or sliding contact and is self-lubricating and oxidation resistant up to and in excess of about 930 C. The composite is comprised of a metal component which lends strength and elasticity to the structure, a fluoride salt component which provides lubrication and, lastly, a glass component which not only provides oxidation protection to the metal but may also enhance the lubrication qualities of the composite.

  2. Contact Interface Verification for DYNA3D Scenario 1: Basic Contact

    SciTech Connect

    McMichael, L D

    2006-05-10

    A suite of test problems has been developed to examine contact behavior within the nonlinear, three-dimensional, explicit finite element analysis (FEA) code DYNA3D (Lin, 2005). The test problems address the basic functionality of the contact algorithms, including the behavior of various kinematic, penalty, and Lagrangian enforcement formulations. The results from the DYNA3D analyses are compared to closed form solutions to verify the contact behavior. This work was performed as part of the Verification and Validation efforts of LLNL W Program within the NNSA's Advanced Simulation and Computing (ASC) Program. DYNA3D models the transient dynamic response of solids and structures including the interactions between disjoint bodies (parts). A wide variety of contact surfaces are available to represent the diverse interactions possible during an analysis, including relative motion (sliding), separation and gap closure (voids), and fixed relative position (tied). The problem geometry may be defined using a combination of element formulations, including one-dimensional beam and truss elements, two-dimensional shell elements, and three-dimensional solid elements. Consequently, it is necessary to consider various element interactions for each contact algorithm being verified. Most of the contact algorithms currently available in DYNA3D are examined; the exceptions are the Type 4--Single Surface Contact and Type 11--SAND algorithms. It is likely that these algorithms will be removed since their functionality is embodied in other, more robust, contact algorithms. The automatic contact algorithm is evaluated using the Type 12 interface. Two other variations of automatic contact, Type 13 and Type 14, offer additional means to adapt the interface domain, but share the same search and restoration algorithms as Type 12. The contact algorithms are summarized in Table 1. This report and associated test problems examine the scenario where one contact surface exists between two

  3. Poly(ethylene oxide) Mushrooms Adsorbed at Silica-Ionic Liquid Interfaces Reduce Friction.

    PubMed

    Sweeney, James; Webber, Grant B; Atkin, Rob

    2016-03-01

    The adsorbed layer conformation and lubricity of 35, 100, and 300 kDa PEO adsorbed to ionic liquid (IL)-silica interfaces from 0.01 wt % solutions have been investigated using colloid probe atomic force microscopy. The ILs used were propylammonium nitrate (PAN) and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), which are protic and aprotic ILs, respectively. Normal force curves reveal steric interactions consistent with adsorbed polymer layers which are best fit using the mushroom model. Friction measurements show that the adsorbed polymer layer markedly reduces friction compared to surfaces sliding in the pure ILs and that lubricity increases with polymer length. When polymer is adsorbed to the sliding surfaces, friction is controlled by the creation and disruption of intermolecular interactions between entangled chains and the dragging of polymer chains through the interpenetration region. These experiments show that added polymer can reduce friction while maintaining the useful properties of ILs as lubricants.

  4. Influence of the Tool Shoulder Contact Conditions on the Material Flow During Friction Stir Welding

    NASA Astrophysics Data System (ADS)

    Doude, Haley R.; Schneider, Judy A.; Nunes, Arthur C.

    2014-09-01

    Friction stir welding (FSWing) is a solid-state joining process of special interest in joining alloys that are traditionally difficult to fusion weld. In order to optimize the process, various numeric modeling approaches have been pursued. Of importance to furthering modeling efforts is a better understanding of the contact conditions between the workpiece and the weld tool. Both theoretical and experimental studies indicate the contact conditions between the workpiece and weld tool are unknown, possibly varying during the FSW process. To provide insight into the contact conditions, this study characterizes the material flow in the FSW nugget by embedding a lead (Pb) wire that melted at the FSWing temperature of aluminum alloy 2195. The Pb trace provided evidence of changes in material flow characteristics which were attributed to changes in the contact conditions between the weld tool and workpiece, as driven by temperature, as the tool travels the length of a weld seam.

  5. Design of biomimetic fibrillar interfaces: 1. Making contact.

    PubMed Central

    Glassmaker, N. J.; Himeno, T.; Hui, C-Y; Kim, J.

    2004-01-01

    This paper explores the contact behaviour of simple fibrillar interfaces designed to mimic natural contact surfaces in lizards and insects. A simple model of bending and buckling of fibrils shows that such a structure can enhance compliance considerably. Contact experiments on poly(dimethylsiloxane) (PDMS) fibrils confirm the model predictions. Although buckling increases compliance, it also reduces adhesion by breaking contact between fibril ends and the substrate. Also, while slender fibrils are preferred from the viewpoint of enhanced compliance, their lateral collapse under the action of surface forces limits the aspect ratio achievable. We have developed a quantitative model to understand this phenomenon, which is shown to be in good agreement with experiments. PMID:16849150

  6. Elastic contact conditions to optimize friction drive of surface acoustic wave motor.

    PubMed

    Kuribayashi Kurosawa, M; Takahashi, M; Higuchi, T

    1998-01-01

    The optimum pressing force, namely the preload, for a slider to obtain superior operation conditions in a surface acoustic wave motor have been examined. We used steel balls as sliders. The preload was controlled using a permanent magnet. The steel balls were 0.5, 1, and 2 mm diameter, with the differences in diameter making it possible to change contact conditions, such as the contact pressure, contact area, and deformation of the stator and the slider. The stator transducer was lithium niobate, 128 degrees rotated, y-cut x-propagation substrate. The driving frequency of the Rayleigh wave was about 10 MHz. Hence, the particle vibration amplitude at the surface is as small as 10 nm. For superior friction drive conditions, a high contact pressure was required. For example, in the case of the 1 mm diameter steel ball at the sinusoidal driving voltage of 180 V(peak), the slider speed was 43 cm/sec, the thrust output force was 1 mN, and the acceleration was 23 times as large as the gravitational acceleration at a contact pressure of 390 MPa. From the Hertz theory of contact stress, the contact area radius was only 3 microm. The estimation of the friction drive performance was carried out from the transient traveling distance of the slider in a 3 msec burst drive. As a result, the deformation of the stator and the slider by the preload should be half of the vibration amplitude. This condition was independent of the ball diameter and the vibration amplitude. The output thrust per square millimeter was 50 N, and the maximum speed was 0.7 m/sec. From these results, we conclude that it is possible for the surface acoustic wave motor to have a large output force, high speed, quick response, long traveling distance, and a thin micro linear actuator.

  7. Ultrasonic Friction Reduction in Elastomer - Metal Contacts and Application to Pneumatic Actuators

    NASA Astrophysics Data System (ADS)

    Pham, The Minh; Twiefel, Jens

    Ultrasonic friction reduction is well known in metal-metal contacts. Due to the vibration, the stick phase in the contact phase vanishes and only sliding occurs. As long as the macroscopic relative velocity of the contact partners is much lower than vibration velocity, the necessary force to move the parts tends to (nearly) zero. If the effect also exists in material combinations with a significant difference in stiffness and damping characteristic has not been investigated in the past. This contribution shows the effect for various material combinations, which are typical for sealings in pneumatic actuators. Further, a novel integrated transducer design for a pneumatic actuator is presented. In this design the transducer also acts as moving part within the pneumatic actuator. The design challenges are the two contact areas on the moving part, where the friction reduction and consequently high vibration amplitudes are needed. The first area is fixed on the transducer geometry, the other is moving along the piston. This novel design has been implemented in the laboratory; detailed experimental results are presented in this contribution.

  8. Fixed contact line helical interfaces in zero gravity

    NASA Astrophysics Data System (ADS)

    Lowry, Brian J.; Thiessen, David B.

    2007-02-01

    Fluid interfaces supported in microgravity by a helical structure are shown to have a more robust stability than more common structures such as liquid bridges. In particular, helical interfaces can take the form of infinite right circular cylinders over a broad range of configurations. In the case of a single fixed contact line support, the infinite cylinder is stable for all cases in which the pitch to diameter ratio is less than π /√3 (more tightly coiled interfaces). When there are two or more equally spaced fixed contact line supports, the infinite cylinder is stable for all configurations. Furthermore, in the two support case (the double helix), stability persists for all volumes from the cylinder to zero volume, when the pitch to diameter ratio is greater than 2.082 (more loosely coiled interfaces). The equivalent to the axisymmetric Young-Laplace equation is derived for helical interfaces. Interfacial stability is determined from equilibrium branch structure following the application of Maddocks' method by Lowry and Steen [Proc. R. Soc. London, Ser. A 449, 411 (1995)]. Perturbations to finite wavelength disturbances are considered for the case of a single helical support. Overall stability envelopes are presented for single and multiple support cases. Limited experimental results verify the infinite length stability limit for the single helical support case.

  9. The metal to metal interface and its effect on adhesion and friction

    NASA Technical Reports Server (NTRS)

    Buckley, D. H.

    1976-01-01

    The nature of the interface, adhesion and friction properties of noble metals, platinum metals, Group IV (B) metals and transition metals were considered. The surface chemical activity of the noble and platinum metals is shown to effect metal to metal interfaces as does a valance bonding in the transition metals. With the Group IV (B) metals the degree of metallic nature of the elements is shown to effect interfacial behavior. The effect of surface segregation of alloy constituents such as silicon in iron and its influence on the metal to metal interface is discussed. In addition the effect of alloy constituents on changes in bulk properties such as transformations in tin are shown to effect interfacial adhesion and friction behavior.

  10. Friction and wear of Si/sub 3/N/sub 4/ ceramic/metal couples in dynamic contact

    SciTech Connect

    Wayne, S.F.; Buljan, S.T.

    1988-01-01

    The tribological behavior of Si/sub 3/N/sub 4/ in sliding contact with itself, nodular cast iron, 316 stainless steel, and Inconel 718 is examined. The role of the environment and of the tribological characteristics breakaway friction, kinetic friction, and wear are measured in vacuum and air. The tribological behavior is shown to be strongly influenced by interactions with metallic alloys, which can lead to metal transfer, high friction, and ceramic wear. Surface oxide and adsorbed films from an air environment can stabilize the metal surfaces and reduce friction and wear. 26 references.

  11. Contact Interface Verification for DYNA3D Scenario 2: Multi-Surface Contact

    SciTech Connect

    McMichael, L D

    2006-05-10

    A suite of test problems has been developed to examine contact behavior within the nonlinear, three-dimensional, explicit finite element analysis (FEA) code DYNA3D (Lin, 2005). The test problems use multiple interfaces and a combination of enforcement methods to assess the basic functionality of the contact algorithms. The results from the DYNA3D analyses are compared to closed form solutions to verify the contact behavior. This work was performed as part of the Verification and Validation efforts of LLNL W Program within the NNSA's Advanced Simulation and Computing (ASC) Program. DYNA3D models the transient dynamic response of solids and structures including the interactions between disjoint bodies (parts). A wide variety of contact surfaces are available to represent the diverse interactions possible during an analysis, including relative motion (sliding), separation and gap closure (voids), and fixed relative position (tied). The problem geometry may be defined using a combination of element formulations, including one-dimensional beam and truss elements, two-dimensional shell elements, and three-dimensional solid elements. Consequently, it is necessary to consider various element interactions during contact. This report and associated test problems examine the scenario where multiple bodies interact with each other via multiple interfaces. The test problems focus on whether any ordering issues exist in the contact logic by using a combination of interface types, contact enforcement options (i.e., penalty, Lagrange, and kinematic), and element interactions within each problem. The influence of rigid materials on interface behavior is also examined. The companion report (McMichael, 2006) and associated test problems address the basic contact scenario where one contact surface exists between two disjoint bodies. The test problems are analyzed using version 5.2 (compiled on 12/22/2005) of DYNA3D. The analytical results are used to form baseline solutions for

  12. A damage mechanics based general purpose interface/contact element

    NASA Astrophysics Data System (ADS)

    Yan, Chengyong

    Most of the microelectronics packaging structures consist of layered substrates connected with bonding materials, such as solder or epoxy. Predicting the thermomechanical behavior of these multilayered structures is a challenging task in electronic packaging engineering. In a layered structure the most complex part is always the interfaces between the strates. Simulating the thermo-mechanical behavior of such interfaces, is the main theme of this dissertation. The most commonly used solder material, Pb-Sn alloy, has a very low melting temperature 180sp°C, so that the material demonstrates a highly viscous behavior. And, creep usually dominates the failure mechanism. Hence, the theory of viscoplasticity is adapted to describe the constitutive behavior. In a multilayered assembly each layer has a different coefficient of thermal expansion. Under thermal cycling, due to heat dissipated from circuits, interfaces and interconnects experience low cycle fatigue. Presently, the state-of-the art damage mechanics model used for fatigue life predictions is based on Kachanov (1986) continuum damage model. This model uses plastic strain as a damage criterion. Since plastic strain is a stress path dependent value, the criterion does not yield unique damage values for the same state of stress. In this dissertation a new damage evolution equation based on the second law of thermodynamic is proposed. The new criterion is based on the entropy of the system and it yields unique damage values for all stress paths to the final state of stress. In the electronics industry, there is a strong desire to develop fatigue free interconnections. The proposed interface/contact element can also simulate the behavior of the fatigue free Z-direction thin film interconnections as well as traditional layered interconnects. The proposed interface element can simulate behavior of a bonded interface or unbonded sliding interface, also called contact element. The proposed element was verified against

  13. Atomic-scale friction experiments reconsidered in the light of rapid contact dynamics

    NASA Astrophysics Data System (ADS)

    Krylov, S. Yu.; Frenken, J. W. M.

    2009-12-01

    We present the first fully quantitative and self-consistent analysis of atomic-scale friction, explicitly taking into account the flexibility and low effective mass of the mechanical nanocontact. In a procedure, which is free of the traditional assumptions with respect to the corrugation of the interaction potential of the contact, the basic but experimentally inaccessible system parameter, we arrive at an excellent description of recent nanotribology experiments, including the transition from stick slip to nearly frictionless sliding. We show that, contrary to original interpretation, the ultralow friction observed in some experiments has been largely due to thermal (thermolubricity) rather than mechanistic effects (superlubricity). Furthermore, we observe the manifestations of two different forms of thermally induced sliding dynamics, namely, true thermolubricity (slipperiness based on thermal excitations) and a specific, low-dissipation type of stick-slip motion.

  14. Friction

    NASA Astrophysics Data System (ADS)

    Matsuo, Yoshihiro; Clarke, Daryl D.; Ozeki, Shinichi

    Friction materials such as disk pads, brake linings, and clutch facings are widely used for automotive applications. Friction materials function during braking due to frictional resistance that transforms kinetic energy into thermal energy. There has been a rudimentary evolution, from materials like leather or wood to asbestos fabric or asbestos fabric saturated with various resins such as asphalt or resin combined with pitch. These efforts were further developed by the use of woven asbestos material saturated by either rubber solution or liquid resin binder and functioned as an internal expanding brake, similar to brake lining system. The role of asbestos continued through the use of chopped asbestos saturated by rubber, but none was entirely successful due to the poor rubber heat resistance required for increased speeds and heavy gearing demands of the automobile industry. The use of phenolic resins as binder for asbestos friction materials provided the necessary thermal resistance and performance characteristics. Thus, the utility of asbestos as the main friction component, for over 100 years, has been significantly reduced in friction materials due to asbestos identity as a carcinogen. Steel and other fibrous components have displaced asbestos in disk pads. Currently, non-asbestos organics are the predominate friction material. Phenolic resins continue to be the preferred binder, and increased amounts are necessary to meet the requirements of highly functional asbestos-free disk pads for the automotive industry. With annual automobile production exceeding 70 million vehicles and additional automobile production occurring in developing countries worldwide and increasing yearly, the amount of phenolic resin for friction material is also increasing (Fig. 14.1). Fig. 14.1 Worldwide commercial vehicle production Computational Studies of Hard Disks: Contact Percolation, Fragility, Frictional Families and Basin Volumes

    NASA Astrophysics Data System (ADS)

    Shen, Tianqi

    This thesis presents four computational and theoretical studies of the structural, mechanical, and vibrational properties of purely repulsive disks, dimer-, and ellipse-shaped particles with and without friction. The first study investigated the formation of interparticle contact networks below jamming onset at packing fraction φJ, where the pressure of the system becomes nonzero. We generated ensembles of static packings of frictionless disks over a range of packing fraction. We find that the network of interparticle contacts forms a system spanning cluster at a critical packing fraction φP < φJ. The contact percolation transition also signals the onset of cooperative non-affine particle motion and non-trivial response to applied stress. For the second project, we performed molecular dynamics simulations of dense liquids composed of bidisperse dimer- and ellipse-shaped particles over a wide range of temperature and packing fraction. We measured structural relaxation times for the translational and rotational degrees of freedom. We find that the slow dynamics for dense liquids composed of dimer- and ellipse-shaped particles are qualitatively the same, despite the fact that zero-temperature static packings of dimers are isostatic, while static packings of ellipses are hypostatic. We also show that the fragility of the structural relaxation time decreases with increasing aspect ratio for both dimer- and ellipse-shaped particles. For the third project, we developed a novel method to calculate and predict the average contact number as a function of the static friction coefficient for disk packings. We employed a novel numerical method that allowed us to enumerate sets of packings with m = N0c -- Nc missing contacts relative to the isostatic value N0c We show that the probability Pm(micro) to obtain a static packing with m missing contacts at micro can be expressed as a power series in micro. Using Pm(micro), we find that the average contact number versus micro

  15. Lattice boltzmann study on the contact angle and contact line dynamics of liquid-vapor interfaces.

    PubMed

    Zhang, Junfeng; Kwok, Daniel Y

    2004-09-14

    The moving contact line problem of liquid-vapor interfaces was studied using a mean-field free-energy lattice Boltzmann method recently proposed [Phys. Rev. E 2004, 69, 032602]. We have examined the static and dynamic interfacial behaviors by means of the bubble and capillary wave tests and found that both the Laplace equation of capillarity and the dispersion relation were satisfied. Dynamic contact angles followed the general trend of contact line velocity observed experimentally and can be described by Blake's theory. The velocity fields near the interface were also obtained and are in good agreement with fluid mechanics and molecular dynamics studies. Our simulations demonstrated that incorporating interfacial effects into the lattice Boltzmann model can be a valuable and powerful alternative in interfacial studies.

  16. Modeling the Electrical Contact Resistance at Steel-Carbon Interfaces

    NASA Astrophysics Data System (ADS)

    Brimmo, Ayoola T.; Hassan, Mohamed I.

    2016-01-01

    In the aluminum smelting industry, electrical contact resistance at the stub-carbon (steel-carbon) interface has been recurrently reported to be of magnitudes that legitimately necessitate concern. Mitigating this via finite element modeling has been the focus of a number of investigations, with the pressure- and temperature-dependent contact resistance relation frequently cited as a factor that limits the accuracy of such models. In this study, pressure- and temperature-dependent relations are derived from the most extensively cited works that have experimentally characterized the electrical contact resistance at these contacts. These relations are applied in a validated thermo-electro-mechanical finite element model used to estimate the voltage drop across a steel-carbon laboratory setup. By comparing the models' estimate of the contact electrical resistance with experimental measurements, we deduce the applicability of the different relations over a range of temperatures. The ultimate goal of this study is to apply mathematical modeling in providing pressure- and temperature-dependent relations that best describe the steel-carbon electrical contact resistance and identify the best fit relation at specific thermodynamic conditions.

  17. Design and calibration of a scanning force microscope for friction, adhesion, and contact potential studies

    NASA Astrophysics Data System (ADS)

    Koleske, D. D.; Lee, G. U.; Gans, B. I.; Lee, K. P.; DiLella, D. P.; Wahl, K. J.; Barger, W. R.; Whitman, L. J.; Colton, R. J.

    1995-09-01

    We present the design and calibration of a scanning force microscope which can be used to study friction, adhesion, and contact potential differences between the cantilever tip and surface. The microscope uses a modular design where the laser, cantilever/sample holder, reflecting mirror, and detector are mounted directly on an optical table. The laser, reflecting mirror, and detector are mounted on translation and rotation stages. With this design the components can be rearranged to calibrate the Z piezo motion as a function of applied voltage. Using the detector micrometers, the detector response (voltage-to-distance relationship) can be determined after each series of measurements. The cantilever/sample holder is constructed such that the components are material matched and thermally compensated from a common reference point. This design feature minimizes thermal drift of the instrument. The instrument can be used in a contact scanning mode where both normal and lateral deflections of the cantilever are measured. In addition, the instrument can be used in frictional force studies, force curve mapping of the surface, and contact potential measurements. We present examples of each, including a detailed account of the instrument design and calibration.

  18. From flash heating to frictional melting: capturing the incipient microstructural development of experimental fault interfaces

    NASA Astrophysics Data System (ADS)

    Hayward, Kathryn; Cox, Stephen; Slagmolen, Bram; Forsyth, Perry; Shaddock, Daniel; Hawkins, Rhys; Fitz Gerald, John

    2016-04-01

    The propagation of seismic slip is often attributed to the activation of weakening mechanisms at high slip velocities, resulting from complex thermo-mechanical changes to asperities at the slip interface. Despite considerable experimental effort being directed towards understanding high-velocity fault behaviour, the mechanisms that drive the onset of instability and formative stages of weakening remain an area of considerable uncertainty. Here we present new evidence for the very rapid onset of microstructural changes on experimental faults surfaces that have been sheared over a range of slip velocities and under stress conditions comparable to upper- to mid-crustal conditions (σn = 92-320MPa). Experiments have been undertaken using pre-ground fault surfaces of Fontainebleau sandstone in a classic triaxial configuration. The traditional high-precision mechanical data of the gas-medium apparatus has been supplemented by the installation of a custom built interferometric sensor that provides microsecond resolution of displacement during stick-slip events. Deformed samples have been analysed using multiple techniques, including FE-SEM and FIB-TEM, that provide insights into physical and structural changes to asperity contacts over a range of scales. Significant findings include recognition of textures that capture the transition from mechanical amorphisation, to flash heating and the early stages of frictional melting. Our results include the formation of a partially amorphous nano-gouge layer during aseismic sliding, the development of amorphous zones at asperity tips and the generation of frictional melt during stick-slip. While the average fault temperature is estimated to remain well below the quartz solidus (~1700°C), we show that extreme localised heating occurs during all stick-slip events, with a clear correlation between slip velocity and the morphology of the amorphous material produced. These microstructural changes occur over slip distances that are

  19. Static and kinetic friction force and surface roughness of different archwire-bracket sliding contacts.

    PubMed

    Carrion-Vilches, Francisco J; Bermudez, María-Dolores; Fructuoso, Paula

    2015-01-01

    The aim of this study was to determine the static and kinetic friction forces of the contact bracket-archwire with different dental material compositions in order to select those materials with lower resistance to sliding. We carried out sliding friction tests by means of a universal testing machine following an experimental procedure as described in ASTM D1894 standard. We determined the static and kinetic friction forces under dry and lubricating conditions using an artificial saliva solution at 36.5ºC. The bracket-archwire pairs studied were: stainless steel-stainless steel; stainless steel-glass fiber composite; stainless steel-Nitinol 60; sapphire-stainless steel; sapphire-glass fiber composite; and sapphire-Nitinol 60. The best performance is obtained for Nitinol 60 archwire sliding against a stainless steel bracket, both under dry and lubricated conditions. These results are in agreement with the low surface roughness of Nitinol 60 with respect to the glass fiber composite archwire. The results described here contribute to establishing selection criteria for materials for dental archwire-brackets.

  1. Asynchronous collision integrators: Explicit treatment of unilateral contact with friction and nodal restraints

    PubMed Central

    Wolff, Sebastian; Bucher, Christian

    2013-01-01

    This article presents asynchronous collision integrators and a simple asynchronous method treating nodal restraints. Asynchronous discretizations allow individual time step sizes for each spatial region, improving the efficiency of explicit time stepping for finite element meshes with heterogeneous element sizes. The article first introduces asynchronous variational integration being expressed by drift and kick operators. Linear nodal restraint conditions are solved by a simple projection of the forces that is shown to be equivalent to RATTLE. Unilateral contact is solved by an asynchronous variant of decomposition contact response. Therein, velocities are modified avoiding penetrations. Although decomposition contact response is solving a large system of linear equations (being critical for the numerical efficiency of explicit time stepping schemes) and is needing special treatment regarding overconstraint and linear dependency of the contact constraints (for example from double-sided node-to-surface contact or self-contact), the asynchronous strategy handles these situations efficiently and robust. Only a single constraint involving a very small number of degrees of freedom is considered at once leading to a very efficient solution. The treatment of friction is exemplified for the Coulomb model. Special care needs the contact of nodes that are subject to restraints. Together with the aforementioned projection for restraints, a novel efficient solution scheme can be presented. The collision integrator does not influence the critical time step. Hence, the time step can be chosen independently from the underlying time-stepping scheme. The time step may be fixed or time-adaptive. New demands on global collision detection are discussed exemplified by position codes and node-to-segment integration. Numerical examples illustrate convergence and efficiency of the new contact algorithm. Copyright © 2013 The Authors. International Journal for Numerical Methods in

  2. Control mechanism of friction by dynamic actuation of nanometer-sized contacts

    NASA Astrophysics Data System (ADS)

    Iizuka, Hiroyuki; Nakamura, Jun; Natori, Akiko

    2009-10-01

    We studied both the mechanism and the condition of dynamic superlubricity actuated in a dynamic way for the atomic contact of a friction force microscope, using dynamical simulation of the Tomlinson model. The superlubricity was achieved by ac modulation of the normal force acting between two contacting bodies at well-defined frequencies corresponding to normal resonances of the combined system [A. Socoliuc , Science 313, 207 (2006)]. The time-averaged friction force depends crucially on the modulation amplitude and the superlubricity occurs above the critical amplitude. The effect on the superlubricity of the corrugation amplitude of surface potential, sliding velocity, a damping coefficient, and temperature are clarified. The superlubricity at zero temperature can be induced by transit of the tip via the “turning point,” the top position of the surface potential without elastic deformation, and it is allowed at low-sliding velocities in the underdamped case. The superlubricity at a room temperature can be actuated efficiently with a much smaller critical amplitude than that at zero temperature and it can be achieved at sufficiently low-sliding velocities in both the underdamped and the overdamped cases, assisted by thermally activated hopping of the tip.

  3. Energy Partition During In-plane Dynamic Rupture on a Frictional Interface

    NASA Astrophysics Data System (ADS)

    Needleman, A.; Shi, Z.; Ben-Zion, Y.

    2007-12-01

    We study properties of dynamic ruptures and the partition of energy between radiation and dissipative mechanisms using two-dimensional in-plane calculations with the finite element method. The model consists of two identical isotropic elastic media separated by an interface governed by rate- and state-dependent friction. Rupture is initiated by gradually overstressing a localized nucleation zone. Our simulations with model parameters representative of Homalite-100 indicate that different values of parameters controlling the velocity dependence of friction, the strength excess parameter and the length of the nucleation zone, can lead to the following four rupture modes: supershear crack-like rupture, subshear crack-like rupture, subshear single pulse and supershear train of pulses. High initial shear stress and weak velocity dependence of friction favor crack-like ruptures, while the opposite conditions favor the pulse mode. The rupture mode can switch from a subshear single pulse to a supershear train of pulses when the width of the nucleation zone increases. The elastic strain energy released over the same propagation distance by the different rupture modes has the following order: supershear crack, subshear crack, supershear train of pulses and subshear single pulse. The same order applies also to the ratio of kinetic energy (radiation) to total change of elastic energy for the different rupture modes. Decreasing the dynamic coefficient of friction increases the fraction of stored energy that is converted to kinetic energy. In the current study we use model parameters representative of rocks instead of Homalite-100, by modeling recent results of Kilgore et al. (2007) who measured and estimated various energy components in laboratory friction experiments with granite. We are also incorporating into the code ingredients that will allow us to study rupture properties and energy partition for cases with a bimaterial interface and dynamic generation of plastic strain

  4. Relationship between the ideal tensile strength and the friction properties of metals in contact with nonmetals and themselves

    NASA Technical Reports Server (NTRS)

    Miyoshi, K.; Buckley, D. H.

    1981-01-01

    The adhesion and friction properties of metals in contact with diamond, boron nitride, silicon carbide, manganese-zinc ferrite, and the metals themselves in vacuum was investigated. An estimate of the ideal uniaxial tensile was obtained in terms of the equilibrium surface energy, interplanar spacing of the planes perpendicular to the tensile axis, and the Young's modulus of elasticity. The coefficient of friction for metals was found to be related to the ideal tensile strength of metals. The higher the strength of the metal, the lower the coefficient of friction.

  5. Study of adhesion and friction properties on a nanoparticle gradient surface: transition from JKR to DMT contact mechanics.

    PubMed

    Ramakrishna, Shivaprakash N; Nalam, Prathima C; Clasohm, Lucy Y; Spencer, Nicholas D

    2013-01-08

    We have previously investigated the dependence of adhesion on nanometer-scale surface roughness by employing a roughness gradient. In this study, we correlate the obtained adhesion forces on nanometer-scale rough surfaces to their frictional properties. A roughness gradient with varying silica particle (diameter ≈ 12 nm) density was prepared, and adhesion and frictional forces were measured across the gradient surface in perfluorodecalin by means of atomic force microscopy with a polyethylene colloidal probe. Similarly to the pull-off measurements, the frictional forces initially showed a reduction with decreasing particle density and later an abrupt increase as the colloidal sphere began to touch the flat substrate beneath, at very low particle densities. The friction-load relation is found to depend on the real contact area (A(real)) between the colloid probe and the underlying particles. At high particle density, the colloidal sphere undergoes large deformations over several nanoparticles, and the contact adhesion (JKR type) dominates the frictional response. However, at low particle density (before the colloidal probe is in contact with the underlying surface), the colloidal sphere is suspended by a few particles only, resulting in local deformations of the colloid sphere, with the frictional response to the applied load being dominated by long-range, noncontact (DMT-type) interactions with the substrate beneath.

  6. 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.

  7. 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

  8. 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.

  9. Modelling the stochastic nature of the available coefficient of friction at footwear-floor interfaces.

    PubMed

    Gragg, Jared; Klose, Ellison; Yang, James

    2016-09-23

    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.

  10. A comparison of Coulomb and pseudo-Coulomb friction implementations: Application to the table contact phase of gymnastics vaulting.

    PubMed

    Jackson, M I; Hiley, M J; Yeadon, M R

    2011-10-13

    In the table contact phase of gymnastics vaulting both dynamic and static friction act. The purpose of this study was to develop a method of simulating Coulomb friction that incorporated both dynamic and static phases and to compare the results with those obtained using a pseudo-Coulomb implementation of friction when applied to the table contact phase of gymnastics vaulting. Kinematic data were obtained from an elite level gymnast performing handspring straight somersault vaults using a Vicon optoelectronic motion capture system. An angle-driven computer model of vaulting that simulated the interaction between a seven segment gymnast and a single segment vaulting table during the table contact phase of the vault was developed. Both dynamic and static friction were incorporated within the model by switching between two implementations of the tangential frictional force. Two vaulting trials were used to determine the model parameters using a genetic algorithm to match simulations to recorded performances. A third independent trial was used to evaluate the model and close agreement was found between the simulation and the recorded performance with an overall difference of 13.5%. The two-state simulation model was found to be capable of replicating performance at take-off and also of replicating key contact phase features such as the normal and tangential motion of the hands. The results of the two-state model were compared to those using a pseudo-Coulomb friction implementation within the simulation model. The two-state model achieved similar overall results to those of the pseudo-Coulomb model but obtained solutions more rapidly.

  11. Evaluation of joint interface of friction stir welding between dissimilar metals using HTS-SQUID gradiometer

    NASA Astrophysics Data System (ADS)

    Mashiko, Y.; Hatsukade, Y.; Yasui, T.; Takenaka, H.; Todaka, Y.; Fukumoto, M.; Tanaka, S.

    2010-11-01

    In this study, we investigated conductive properties of joint interfaces of friction stir welding (FSW) between dissimilar metals, stainless steel SUS304 and aluminum A6063, using a SQUID nondestructive evaluation (NDE) system. With current injection method, the current maps above the FSW specimens jointed under various conditions were measured by a HTS-SQUID gradiometer. The conductivities of the joint interfaces, which were estimated from the current maps, differed between the joint conditions. By destructive tests using optical microscope, large voids were observed on the joint interfaces with low welding speed that generated excess heating. In case of one specimen, which was welded with welding speed of 500 and 200 mm/min, the conductivity of the former was higher than that of the latter, although the inside voids in the respective regions were not much different. From these results, it is suggested that the current maps were influenced not only by the conductivity of the joint interface but also by inside voids. By hardness test on the SUS boards near the interfaces, only the SUS jointed with 200 mm/min was about half softer than its matrix.

  12. Study on the friction in steel/polyamide ball on disk type contacts

    NASA Astrophysics Data System (ADS)

    Lates, M. T.; Gavrila, C. C.; Papuc, R.

    2016-08-01

    The paper presents the experimental study of the friction in the case of steel/polyamide ball on disk type contacts by considering as testing parameters the temperature, the load and the rotational speed. The tests are performed, for two types of polyamides, on a tribometer which allows rotational motions at small and high speeds, with controlled normal loads and temperatures. The tests begin with a one hour running-in at 500 rpm, at ambient temperature. After the running-in process there are made tests, for two types of polyamides at 90°C and 120°C, at loads of 3 N, 5 N, 7 N and at rotational speeds of 5 rpm, 1500 rpm and 3000 rpm. The results are indicating the polyamides behaviour at high temperatures with different loadings, at small and high rotational speeds. The conclusions of the paper offer recommendations regarding the applications of the tested polyamide materials according to temperature, loading and rotational speeds, in the case of ball on disk type contacts.

  13. The analysis of the influence of the material antifrictional layer frictional properties on the parameters of the spherical bearing contact zone

    NASA Astrophysics Data System (ADS)

    Kamenskikh, A. A.; Trufanov, N. A.

    2017-02-01

    The paper presents data on the influence of the frictional properties of a material antifrictional layer on the parameters of the spherical bearing contact zone. The dependences of the friction coefficient from the load were obtained as a result of the study. Series of numerical experiments were conducted to investigate the frictional properties of a materials contact pair in the work. Regularities of the relative contact pressure and relative contact tangential stress were obtained for seven variants of the load-friction coefficient for the spherical bearing with a layer of modified fluoroplastic. The study puts emphasis on the fact that that adhesion area of the contact surface is reduced and the load is increased taking into account the fact that the friction properties of the layer has been fixed in the study.

  14. Interfaces of electrical contacts in organic semiconductor devices

    NASA Astrophysics Data System (ADS)

    Demirkan, Korhan

    Progress in organic semiconductor devices relies on better understanding of interfaces as well as material development. The engineering of interfaces that exhibit low resistance, low operating voltage and long-term stability to minimize device degradation is one of the crucial requirements. Photoelectron spectroscopy is a powerful technique to study the metal-semiconductor interfaces, allowing: (i) elucidation of the energy levels of the semiconductor and the contacts that determine Schottky barrier height, (ii) inspection of electrical interactions (such as charge transfer, dipole formation, formation of induced density of states or formation of polaron/bi-polaron states) that effect the energy level alignment, (iii) determination of interfacial chemistry, and (iv) estimation of interface morphology. In this thesis, we have used photoelectron spectroscopy extensively for detailed analysis of the metal organic semiconductor interfaces. In this study, we demonstrate the use of photoelectron spectroscopy for construction of energy level diagrams and display some results related to chemical tailoring of materials for engineering interfaces with lowered Schottky barriers. Following our work on the energy level alignment of poly(p-phenyene vinylene) based organic semiconductors on various substrates [Au, indium tin oxide, Si (with native oxide) and Al (with native oxide)], we tested controlling the energy level alignment by using polar self assembled molecules (SAMs). Photoelectron spectroscopy showed that, by introducing SAMs on the Au surface, we successfully changed the effective work function of Au surface. We found that in this case, the change in the effective work function of the metal surface was not reflected as a shift in the energy levels of the organic semiconductor, as opposed to the results achieved with different substrate materials. To investigate the chemical interactions at the metal/organic interface, we studied the metallization of poly(2-methoxy-5

  15. Micromechanical modeling of the cement-bone interface: the effect of friction, morphology and material properties on the micromechanical response

    PubMed Central

    Janssen, Dennis; Mann, Kenneth A.; Verdonschot, Nico

    2008-01-01

    In order to gain insight into the micro-mechanical behavior of the cement-bone interface, the effect of parametric variations of frictional, morphological and material properties on the mechanical response of the cement-bone interface were analyzed using a finite element approach. Finite element models of a cement-bone interface specimen were created from micro-computed tomography data of a physical specimen that was sectioned from an in vitro cemented total hip arthroplasty. In five models the friction coefficient was varied (μ= 0.0; 0.3; 0.7; 1.0 and 3.0), while in one model an ideally bonded interface was assumed. In two models cement interface gaps and an optimal cement penetration were simulated. Finally, the effect of bone cement stiffness variations was simulated (2.0 and 2.5 GPa, relative to the default 3.0 GPa). All models were loaded for a cycle of fully reversible tension-compression. From the simulated stress-displacement curves the interface deformation, stiffness and hysteresis were calculated. The results indicate that in the current model the mechanical properties of the cement-bone interface were caused by frictional phenomena at the shape-closed interlock rather than by adhesive properties of the cement. Our findings furthermore show that in our model maximizing cement penetration improved the micromechanical response of the cement-bone interface stiffness, while interface gaps had a detrimental effect. Relative to the frictional and morphological variations, variations in the cement stiffness had only a modest effect on the micromechanical behavior of the cement-bone interface. The current study provides information that may help to better understand the load transfer mechanisms taking place at the cement-bone interface. PMID:18848699

  16. Frictional behaviors of some nitrogen ceramics in conformal contact with tin coated Al-Si alloy, steel and MMC

    NASA Astrophysics Data System (ADS)

    Wang, Q.; Cheng, H. S.; Fine, M. E.

    1994-07-01

    The frictional behavior of certain nitrogen-containing ceramics, such as silicon nitride, alpha sialons, and beta sialons as journal materials were studied in conformal contact with a tin-coated Al-Si alloy (Al-Si/Sn), forged 1141 steel and a cast aluminum matrix composite with silicon carbide reinforcement (cast metal matrix composites (MMC)) as bearing materials while lubricated with SAE 10W30. A case-hardened 1016 steel was also tested with the Al-Si/Sn and cast MMC bearings under the same conditions. The friction values of the ceramic and the steel journal wear pairs were compared and their frictional behaviors were evaluated. Silicon nitride and one of the beta sialons exhibited higher load-supporting capacities than the others when they were in contact with the 1141 steel bearings. The journal surface roughness was found to be very important when the journals were in contact with the Al-Si/Sn bearings. The frictional behavior of the ceramics and cast MMC pairs and the steel and cast MMC pairs were controlled by different wear machanisms, namely for the former, hard particle pull-out and matrix plowing, and for the latter, iron transfer from the journal to the cast MMC bearing surface.

  17. Friction and metal transfer for single-crystal silicon carbide in contact with various metals in vacuum

    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 transition metals (tungsten, iron, rhodium, nickel, titanium, and cobalt), copper, and aluminum. Results indicate the coefficient of friction for a silicon carbide-metal system is related to the d bond character and relative chemical activity of the metal. The more active the metal, the higher the coefficient of friction. All the metals examined transferred to the surface of silicon carbide in sliding. The chemical activity of metal to silicon and carbon and shear modulus of the metal may play important roles in metal transfer and the form of the wear debris. The less active and greater resistance to shear the metal has, with the exception of rhodium and tungsten, the less transfer to silicon carbide.

  18. Microscopic and Macroscopic Dynamic Interface Shapes and the Interpretation of Dynamic Contact Angles.

    PubMed

    Ramé; Garoff

    1996-01-15

    We have studied shapes of dynamic fluid interfaces at distances contact line at capillary numbers (Ca) ranging from 10(-3) to 10(-1). Near the moving contact line where viscous deformation is important, an analysis valid to O(1) in Ca describes the shape of the fluid interface. Static capillarity should describe the interface shape far from the contact line. We have quantitatively determined the extent of the regions described by the analysis with viscous deformation and by a static shape as a function of Ca. We observe a third portion of the interface between the two regions cited above, which is not described by either the analysis with viscous deformation or a static shape. In this third region the interface shape is controlled by viscous and gravitational forces of comparable magnitude. We detect significant viscous deformation even far from the contact line at Ca approximately > 0.01. Our measured dynamic contact angle parameter extracted by fitting the analysis with viscous deformation to the shape near the moving contact line coincides with the contact angle of the static-like shape far from the contact line. We measure and explain the discrepancy between this dynamic contact angle parameter and the apparent contact angles based on meniscus or apex heights. Our observations of viscous effects at large distances from the contact line have implications for dynamic contact angle measurements in capillary tubes.

  19. Determination of time-varying contact length, friction force, torque and forces at the bearings in a helical gear system

    NASA Astrophysics Data System (ADS)

    Kar, Chinmaya; Mohanty, A. R.

    2008-01-01

    This paper deals with determining various time-varying parameters that are instrumental in introducing noise and vibration in a helical gear system. The most important parameter is the contact line variation, which subsequently induces friction force variation, frictional torque variation and variation in the forces at the bearings. The contact line variation will also give rise to gear mesh stiffness and damping variations. All these parameters are simulated for a defect-free and two defective cases of a helical gear system. The defective cases include one tooth missing and two teeth missing in the helical gear. The algorithm formulated in this paper is found to be simple and effective in determining the time-varying parameters.

  20. Onset of frictional sliding of rubber–glass contact under dry and lubricated conditions

    PubMed Central

    Tuononen, Ari J.

    2016-01-01

    Rubber friction is critical in many applications ranging from automotive tyres to cylinder seals. The process where a static rubber sample transitions to frictional sliding is particularly poorly understood. The experimental and simulation results in this paper show a completely different detachment process from the static situation to sliding motion under dry and lubricated conditions. The results underline the contribution of the rubber bulk properties to the static friction force. In fact, simple Amontons’ law is sufficient as a local friction law to produce the correct detachment pattern when the rubber material and loading conditions are modelled properly. Simulations show that micro-sliding due to vertical loading can release initial shear stresses and lead to a high static/dynamic friction coefficient ratio, as observed in the measurements. PMID:27291939

  1. Slow slip and the transition from fast to slow fronts in the rupture of frictional interfaces

    PubMed Central

    Trømborg, Jørgen Kjoshagen; Sveinsson, Henrik Andersen; Scheibert, Julien; Thøgersen, Kjetil; Amundsen, David Skålid; Malthe-Sørenssen, Anders

    2014-01-01

    The failure of the population of microjunctions forming the frictional interface between two solids is central to fields ranging from biomechanics to seismology. This failure is mediated by the propagation along the interface of various types of rupture fronts, covering a wide range of velocities. Among them are the so-called slow fronts, which are recently discovered fronts much slower than the materials’ sound speeds. Despite intense modeling activity, the mechanisms underlying slow fronts remain elusive. Here, we introduce a multiscale model capable of reproducing both the transition from fast to slow fronts in a single rupture event and the short-time slip dynamics observed in recent experiments. We identify slow slip immediately following the arrest of a fast front as a phenomenon sufficient for the front to propagate further at a much slower pace. Whether slow fronts are actually observed is controlled both by the interfacial stresses and by the width of the local distribution of forces among microjunctions. Our results show that slow fronts are qualitatively different from faster fronts. Because the transition from fast to slow fronts is potentially as generic as slow slip, we anticipate that it might occur in the wide range of systems in which slow slip has been reported, including seismic faults. PMID:24889640

  2. Evaluation of Contact Friction in Fracture of Rotationally Bent Nitinol Endodontic Files

    NASA Astrophysics Data System (ADS)

    Haimed, Tariq Abu

    2011-12-01

    The high flexibility of rotary Nitinol (Ni-Ti) files has helped clinicians perform root canal treatments with fewer technical errors than seen with stainless steel files. However, intracanal file fracture can occur, compromising the outcome of the treatment. Ni-Ti file fracture incidence is roughly around 4% amongst specialists and higher amongst general practitioners. Therefore, eliminating or reducing this problem should improve patient care. The aim of this project was to isolate and examine the role of friction between files and the canal walls of the glass tube model, and bending-related maximum strain amplitudes, on Ni-Ti file lifetimes-tofracture in the presence of different irrigant solutions and file coatings. A specifically designed device was used to test over 300 electropolished EndoSequenceRTM Ni-Ti files for number of cycles to failure (NCF) in smooth, bent glass tube models at 45 and 60 degrees during dry, coated and liquid-lubricated rotation at 600rpm. Fractured files were examined under Scanning Electron Microscopy (SEM) afterwards. Four different file sizes 25.04, 25.06, 35.04, 35.06 (diameter in mm/taper %) and six surface modification conditions were used independently. These conditions included, three solutions; (1) a surfactant-based solution, Surface-Active-Displacement-Solution (SADS), (2) a mouth wash proven to remove biofilms, Delmopinol 1%(DEL), and (3) Bleach 6% (vol.%), the most common antibacterial endodontic irrigant solution. The conditions also included two low-friction silane-based coating groups, 3-Hepta-fluoroisopropyl-propoxymethyl-dichlorosilane (3-HEPT) and Octadecyltrichlorosilane (ODS), in addition to an as-received file control group (Dry). The coefficient of friction (CF) between the file and the canal walls for each condition was measured as well as the surface tension of the irrigant solutions and the critical surface tension of the coated and uncoated files by contact angle measurements. The radius of curvature and

  3. Effect of oxygen, methyl mercaptan, and methyl chloride on friction behavior of copper-iron contacts

    NASA Technical Reports Server (NTRS)

    Buckley, D. H.

    1978-01-01

    Sliding friction experiments were conducted with an iron rider on a copper disk and a copper rider on an iron disk. The sputter cleaned iron and copper disk surfaces were saturated with oxygen, methyl mercaptan, and methyl chloride at atmospheric pressure. Auger emission spectroscopy was used to monitor the surfaces. Lower friction was obtained in all experiments with the copper rider sliding on the iron disk than when the couple was reversed. For both iron and copper disks, methyl mercaptan gave the best surface coverage and was most effective in reducing friction. For both iron and copper disks, methyl chloride was the least effective in reducing friction. With sliding, copper transferred to iron and iron to copper.

  4. Investigation of Wear and Friction Properties Under Sliding Conditions of Some Materials Suitable for Cages of Rolling-Contact Bearings

    NASA Technical Reports Server (NTRS)

    Johnson, Robert L; Swikert, Max A; Bisson, Edmond E

    1952-01-01

    An investigation of wear and friction properties of a number of materials sliding against SAE 52100 steel was conducted. These materials included brass, bronze, beryllium copper, monel, nichrome v, 24s-t aluminum, nodular iron, and gray cast iron. The metals investigated may be useful as possible cage (separator or retainer) materials for rolling-contact bearings of high-speed turbine engines. The ability of materials to form surface films that prevent welding is a most important factor in both dry friction and boundary lubrication. On the basis of wear and resistance to welding only, the cast irons were the most promising materials investigated; they showed the least wear and the least tendency to surface failure when run dry, and when boundary lubricated they showed the highest load capacity. On the basis of mechanical properties, nodular iron is superior to gray cast iron. Bronze had the lowest friction coefficient under dry sliding conditions. The results with brass, beryllium copper, and aluminum were poor and these materials do not appear, with regard to friction and wear, to be suitable for cages.

  5. 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.

  6. Friction, wear, and noise of slip ring and brush contacts for synchronous satellite use.

    NASA Technical Reports Server (NTRS)

    Lewis, N. E.; Cole, S. R.; Glossbrenner, E. W.; Vest, C. E.

    1973-01-01

    A program is being conducted for testing of slip rings for synchronous orbit application. Instrumentation systems necessary for monitoring electrical noise, friction, and brush wear at atmospheric pressure and at less than 50 nanotorr have been developed. A multiplex scheme necessary for the simultaneous recording of brush displacement, friction, and electrical noise has also been developed. Composite brushes consisting of silver-molybdenum disulfide-graphite and silver-niobium diselenide-graphite have been employed on rings of coin silver and rhodium plate. Brush property measurements made included measurement of density, electrical resistivity, shear strength, and microstructure.

  7. The adapted augmented Lagrangian method: a new method for the resolution of the mechanical frictional contact problem

    NASA Astrophysics Data System (ADS)

    Bussetta, Philippe; Marceau, Daniel; Ponthot, Jean-Philippe

    2012-02-01

    The aim of this work is to propose a new numerical method for solving the mechanical frictional contact problem in the general case of multi-bodies in a three dimensional space. This method is called adapted augmented Lagrangian method (AALM) and can be used in a multi-physical context (like thermo-electro-mechanical fields problems). This paper presents this new method and its advantages over other classical methods such as penalty method (PM), adapted penalty method (APM) and, augmented Lagrangian method (ALM). In addition, the efficiency and the reliability of the AALM are proved with some academic problems and an industrial thermo-electromechanical problem.

  8. Sugary interfaces mitigate contact damage where stiff meets soft

    NASA Astrophysics Data System (ADS)

    Yoo, Hee Young; Iordachescu, Mihaela; Huang, Jun; Hennebert, Elise; Kim, Sangsik; Rho, Sangchul; Foo, Mathias; Flammang, Patrick; Zeng, Hongbo; Hwang, Daehee; Waite, J. Herbert; Hwang, Dong Soo

    2016-06-01

    The byssal threads of the fan shell Atrina pectinata are non-living functional materials intimately associated with living tissue, which provide an intriguing paradigm of bionic interface for robust load-bearing device. An interfacial load-bearing protein (A. pectinata foot protein-1, apfp-1) with L-3,4-dihydroxyphenylalanine (DOPA)-containing and mannose-binding domains has been characterized from Atrina's foot. apfp-1 was localized at the interface between stiff byssus and the soft tissue by immunochemical staining and confocal Raman imaging, implying that apfp-1 is an interfacial linker between the byssus and soft tissue, that is, the DOPA-containing domain interacts with itself and other byssal proteins via Fe3+-DOPA complexes, and the mannose-binding domain interacts with the soft tissue and cell membranes. Both DOPA- and sugar-mediated bindings are reversible and robust under wet conditions. This work shows the combination of DOPA and sugar chemistry at asymmetric interfaces is unprecedented and highly relevant to bionic interface design for tissue engineering and bionic devices.

  9. Sugary interfaces mitigate contact damage where stiff meets soft.

    PubMed

    Yoo, Hee Young; Iordachescu, Mihaela; Huang, Jun; Hennebert, Elise; Kim, Sangsik; Rho, Sangchul; Foo, Mathias; Flammang, Patrick; Zeng, Hongbo; Hwang, Daehee; Waite, J Herbert; Hwang, Dong Soo

    2016-06-16

    The byssal threads of the fan shell Atrina pectinata are non-living functional materials intimately associated with living tissue, which provide an intriguing paradigm of bionic interface for robust load-bearing device. An interfacial load-bearing protein (A. pectinata foot protein-1, apfp-1) with L-3,4-dihydroxyphenylalanine (DOPA)-containing and mannose-binding domains has been characterized from Atrina's foot. apfp-1 was localized at the interface between stiff byssus and the soft tissue by immunochemical staining and confocal Raman imaging, implying that apfp-1 is an interfacial linker between the byssus and soft tissue, that is, the DOPA-containing domain interacts with itself and other byssal proteins via Fe3(+)-DOPA complexes, and the mannose-binding domain interacts with the soft tissue and cell membranes. Both DOPA- and sugar-mediated bindings are reversible and robust under wet conditions. This work shows the combination of DOPA and sugar chemistry at asymmetric interfaces is unprecedented and highly relevant to bionic interface design for tissue engineering and bionic devices.

  10. Sugary interfaces mitigate contact damage where stiff meets soft

    PubMed Central

    Yoo, Hee Young; Iordachescu, Mihaela; Huang, Jun; Hennebert, Elise; Kim, Sangsik; Rho, Sangchul; Foo, Mathias; Flammang, Patrick; Zeng, Hongbo; Hwang, Daehee; Waite, J. Herbert; Hwang, Dong Soo

    2016-01-01

    The byssal threads of the fan shell Atrina pectinata are non-living functional materials intimately associated with living tissue, which provide an intriguing paradigm of bionic interface for robust load-bearing device. An interfacial load-bearing protein (A. pectinata foot protein-1, apfp-1) with L-3,4-dihydroxyphenylalanine (DOPA)-containing and mannose-binding domains has been characterized from Atrina's foot. apfp-1 was localized at the interface between stiff byssus and the soft tissue by immunochemical staining and confocal Raman imaging, implying that apfp-1 is an interfacial linker between the byssus and soft tissue, that is, the DOPA-containing domain interacts with itself and other byssal proteins via Fe3+–DOPA complexes, and the mannose-binding domain interacts with the soft tissue and cell membranes. Both DOPA- and sugar-mediated bindings are reversible and robust under wet conditions. This work shows the combination of DOPA and sugar chemistry at asymmetric interfaces is unprecedented and highly relevant to bionic interface design for tissue engineering and bionic devices. PMID:27305949

  11. The role of friction in the measurement of slipperiness, Part 1: friction mechanisms and definition of test conditions.

    PubMed

    Chang, W R; Grönqvist, R; Leclercq, S; Myung, R; Makkonen, L; Strandberg, L; Brungraber, R J; Mattke, U; Thorpe, S C

    2001-10-20

    Friction has been widely used as a measure of slipperiness. However, controversies around friction measurements remain. The purposes of this paper are to summarize understanding about friction measurement related to slipperiness assessment of shoe and floor interface and to define test conditions based on biomechanical observations. In addition, friction mechanisms at shoe and floor interface on dry, liquid and solid contaminated, and on icy surfaces are discussed. It is concluded that static friction measurement, by the traditional use of a drag-type device, is only suitable for dry and clean surfaces, and dynamic and transition friction methods are needed to properly estimate the potential risk on contaminated surfaces. Furthermore, at least some of the conditions at the shoe/floor interface during actual slip accidents should be replicated as test conditions for friction measurements, such as sliding speed, contact pressure and normal force build-up rate.

  12. Dilatational viscosity of dilute particle-laden fluid interface at different contact angles

    NASA Astrophysics Data System (ADS)

    Lishchuk, Sergey V.

    2016-12-01

    We consider a solid spherical particle adsorbed at a flat interface between two immiscible fluids and having arbitrary contact angle at the triple contact line. We derive analytically the flow field corresponding to dilatational surface flow in the case of a large ratio of dynamic shear viscosities of two fluids. Considering a dilute assembly of such particles we calculate numerically the dependence on the contact angle of the effective surface dilatational viscosity particle-laden fluid interface. The effective surface dilatational viscosity is proportional to the size and surface concentration of particles and monotonically increases with the increase in protrusion of particles into the fluid with larger shear viscosity.

  13. A substitute model of two-dimensional dry friction exposed to dither generated by rolling contact of wheel and rail

    NASA Astrophysics Data System (ADS)

    Piotrowski, Jerzy

    2012-10-01

    Dither generated by rolling contact of wheel and rail smoothes dry friction damping provided by the primary suspension dampers of freight wagons and it should be taken into account in numerical simulations. But numerically the problem is non-smooth and this leads to long execution time during simulation, especially when the vehicle with friction dampers is modelled in the environment of an multi-body system simulation program, whose solver has to cope with many strong non-linearities. The other difficulty is the necessity of handling within the code a number of big volume files of recorded dither sampled with high frequency. To avoid these difficulties, a substitute model of two-dimensional dry friction exposed to dither is proposed that does not need application of dither during simulation, but it behaves as if dither were applied. Due to this property of the model, the excitation of the vehicle model by track irregularities may be supplied as low-frequency input, which allows fast execution and, the necessity of handling high-volume files of recorded dither is avoided. The substitute model is numerically effective. To identify parameters of the substitute model, a pre-processing employing a sample of the realistic dither is carried-out on a simple two-degrees-of-freedom system. The substitute model is anisotropic, describing anisotropic properties of the two-dimensional friction arising in the presence of one-dimensional dither. The model may be applied in other branches of engineering, for example, in mechatronics and robotics, where application of dither may improve the accuracy of positioning devices.

  14. Friction and Morphology of Magnetic Tapes in Sliding Contact with Nickel-Zinc Ferrite.

    DTIC Science & Technology

    1984-01-01

    microstructure of the tape. Comparisons were made with four binders: nitrocellulose; poly(vinyledene) chloride; cellulose acetate ; and hydroxyl-terminated, low...nitrocellulose binder and increased in the order hydroxyl-terminated, low-molecular-weight polyester resin; poly (vinyledene) chloride; and cellulose acetate . The...order cellulose acetate , poly (vinyledene) chloride, and nitrocellulose. The nature of deformation of the tape was a factor in controlling friction. The

  15. Physical ageing of the contact line on colloidal particles at liquid interfaces.

    PubMed

    Kaz, David M; McGorty, Ryan; Mani, Madhav; Brenner, Michael P; Manoharan, Vinothan N

    2011-12-04

    Young's law predicts that a colloidal sphere in equilibrium with a liquid interface will straddle the two fluids, its height above the interface defined by an equilibrium contact angle. This has been used to explain why colloids often bind to liquid interfaces, and has been exploited in emulsification, water purification, mineral recovery, encapsulation and the making of nanostructured materials. However, little is known about the dynamics of binding. Here we show that the adsorption of polystyrene microspheres to a water/oil interface is characterized by a sudden breach and an unexpectedly slow relaxation. The relaxation appears logarithmic in time, indicating that complete equilibration may take months. Surprisingly, viscous dissipation appears to play little role. Instead, the observed dynamics, which bear strong resemblance to ageing in glassy systems, agree well with a model describing activated hopping of the contact line over nanoscale surface heterogeneities. These results may provide clues to longstanding questions on colloidal interactions at an interface.

  16. 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 coefficient of friction and groove height (corresponding to the wear volume) decrease linearly as the shear strength of the bulk metal increases. The coefficient of friction and groove height generally decrease with an increase in solute content of binary alloys. A separate correlation exists between the solute to iron atomic radius ratio and the decreasing rates of change of coefficient of friction and groove height with increasing solute content. These rates of change are minimum at a solute to iron radius ratio of unity. They increase as the atomic ratio increases or decreases linearly from unity. The correlations indicate that atomic size is an important parameter in controlling friction and wear of alloys.

  17. Particle contact angles at fluid interfaces: pushing the boundary beyond hard uniform spherical colloids.

    PubMed

    Zanini, Michele; Isa, Lucio

    2016-08-10

    Micro and nanoparticles at fluid interfaces have been attracting increasing interest in the last few decades as building blocks for materials, as mechanical and structural probes for complex interfaces and as models for two-dimensional systems. The three-phase contact angle enters practically all aspects of the particle behavior at the interface: its thermodynamics (binding energy to the interface), dynamics (motion and drag at the interface) and interactions with the interface (adsorption and wetting). Moreover, many interactions among particles at the interface also strongly depend on the contact angle. These concepts have been extensively discussed for non-deformable, homogeneous and mostly spherical particles, but recent progress in particle synthesis and fabrication has instead moved in the direction of producing more complex micro and nanoscale objects, which can be responsive, deformable, heterogenous and/or anisotropic in shape, surface chemistry and material properties. These new particles have a much greater potential for applications and new science, and the study of their behavior at interfaces has only very recently started. In this paper, we critically review the current state of the art of the experimental methods available to measure the contact angle of micro and nanoparticles at fluid interfaces, indicating their strengths and limitations. We then comment on new particle systems that are currently attracting increasing interest in relation to their adsorption and assembly at fluid interfaces and discuss if and which ones of the current techniques are suited to investigate their properties at interfaces. Based on this discussion, we will finally try to indicate a direction in which new experimental methods should develop in the future to tackle the new challenges posed by the novel types of particles that more and more often are used at interfaces.

  18. Friction and Wear of Solid-Lubricated Contact in Gas Turbine Engine Bearings.

    DTIC Science & Technology

    1984-11-01

    Norton Corp. Test ball of NC-!32 were provided by WPAFB from a group manufac- tured by SKF Industries, Inc. -2- taking point. A new contact position was...desired contact plane on the disc rim. The contact was carefully aligned . The ball type specimen had the inherent advantage of self- alignment but as...assembly on the load arm. Alignment of the compliant specimen to the disc surface was achieved by adjustment to the loader arm’s universal joint on the

  19. Viscoelastic-damage interface model formulation with friction to simulate the delamination growth in mode II shear

    NASA Astrophysics Data System (ADS)

    Goodarzi, Mohammad Saeed; Hosseini-Toudeshky, Hossein

    2017-02-01

    In this paper a formulation of a viscoelastic-damage interface model with friction in mode-II is presented. The cohesive constitutive law contains elastic and damage regimes. It has been assumed that the shear stress in the elastic regime follows the viscoelastic properties of the matrix material. The three element Voigt model has been used for the formulation of relaxation modulus of the material. Damage evolution proceeds according to the bilinear cohesive constitutive law combined with friction stress consideration. Combination of damage and friction is based on the presumption that the damaged area, related to an integration point, can be dismembered into the un-cracked area with the cohesive damage and cracked area with friction. Samples of a one element model have been presented to see the effect of parameters on the cohesive constitutive law. A comparison between the predicted results with available results of end-notched flexure specimens in the literature is also presented to verify the model. Transverse crack tension specimens are also simulated for different applied displacement velocities.

  20. Atomic Scale Interface Manipulation, Structural Engineering, and Their Impact on Ultrathin Carbon Films in Controlling Wear, Friction, and Corrosion.

    PubMed

    Dwivedi, Neeraj; Yeo, Reuben J; Yak, Leonard J K; Satyanarayana, Nalam; Dhand, Chetna; Bhat, Thirumaleshwara N; Zhang, Zheng; Tripathy, Sudhiranjan; Bhatia, Charanjit S

    2016-07-13

    Reducing friction, wear, and corrosion of diverse materials/devices using <2 nm thick protective carbon films remains challenging, which limits the developments of many technologies, such as magnetic data storage systems. Here, we present a novel approach based on atomic scale interface manipulation to engineer and control the friction, wear, corrosion, and structural characteristics of 0.7-1.7 nm carbon-based films on CoCrPt:oxide-based magnetic media. We demonstrate that when an atomically thin (∼0.5 nm) chromium nitride (CrNx) layer is sandwiched between the magnetic media and an ultrathin carbon overlayer (1.2 nm), it modifies the film-substrate interface, creates various types of interfacial bonding, increases the interfacial adhesion, and tunes the structure of carbon in terms of its sp(3) bonding. These contribute to its remarkable functional properties, such as stable and lowest coefficient of friction (∼0.15-0.2), highest wear resistance and better corrosion resistance despite being only ∼1.7 nm thick, surpassing those of ∼2.7 nm thick current commercial carbon overcoat (COC) and other overcoats in this work. While this approach has direct implications for advancing current magnetic storage technology with its ultralow thickness, it can also be applied to advance the protective and barrier capabilities of other ultrathin materials for associated technologies.

  1. Contact angle and detachment energy of shape anisotropic particles at fluid-fluid interfaces.

    PubMed

    Anjali, Thriveni G; Basavaraj, Madivala G

    2016-09-15

    The three phase contact angle of particles, a measure of its wettability, is an important factor that greatly influences their behaviour at interfaces. It is one of the principal design parameters for potential applications of particles as emulsion/foam stabilizers, functional coatings and other novel materials. In the present work, the effect of size, shape and surface chemistry of particles on their contact angle is investigated using the gel trapping technique, which facilitates the direct visualization of the equilibrium position of particles at interfaces. The contact angle of hematite particles of spherocylindrical, peanut and cuboidal shapes, hematite-silica core-shell and silica shells is reported at a single particle level. The spherocylindrical and peanut shaped particles are always positioned with their major axis parallel to the interface. However, for cuboidal particles at air-water as well as decane-water interfaces, different orientations namely - face-up, edge-up and the vertex-up - are observed. The influence of gravity on the equilibrium position of the colloidal particles at the interface is studied using the hematite-silica core-shell particles and the silica shells. The measured contact angle values are utilized in the calculations of the detachment and surface energies of the hematite particles adsorbed at the interface.

  2. Understanding the fabric of protein crystals: computational classification of biological interfaces and crystal contacts

    PubMed Central

    Capitani, Guido; Duarte, Jose M.; Baskaran, Kumaran; Bliven, Spencer; Somody, Joseph C.

    2016-01-01

    Modern structural biology still draws the vast majority of information from crystallography, a technique where the objects being investigated are embedded in a crystal lattice. Given the complexity and variety of those objects, it becomes fundamental to computationally assess which of the interfaces in the lattice are biologically relevant and which are simply crystal contacts. Since the mid-1990s, several approaches have been applied to obtain high-accuracy classification of crystal contacts and biological protein–protein interfaces. This review provides an overview of the concepts and main approaches to protein interface classification: thermodynamic estimation of interface stability, evolutionary approaches based on conservation of interface residues, and co-occurrence of the interface across different crystal forms. Among the three categories, evolutionary approaches offer the strongest promise for improvement, thanks to the incessant growth in sequence knowledge. Importantly, protein interface classification algorithms can also be used on multimeric structures obtained using other high-resolution techniques or for protein assembly design or validation purposes. A key issue linked to protein interface classification is the identification of the biological assembly of a crystal structure and the analysis of its symmetry. Here, we highlight the most important concepts and problems to be overcome in assembly prediction. Over the next few years, tools and concepts of interface classification will probably become more frequently used and integrated in several areas of structural biology and structural bioinformatics. Among the main challenges for the future are better addressing of weak interfaces and the application of interface classification concepts to prediction problems like protein–protein docking. Supplementary information: Supplementary data are available at Bioinformatics online. Contact: guido.capitani@psi.ch PMID:26508758

  3. Stress-weakening effect on friction and a major revision of evolution law for contact state

    NASA Astrophysics Data System (ADS)

    Nagata, K.; Nakatani, M.; Yoshida, S.

    2009-12-01

    Rate and State Friction (RSF) law comprehensively captures important aspects of rock friction and has explained various aspects of fault motion successfully. However, existing RSF laws are clearly incorrect in describing some aspects of friction firmly established by experiments [Beeler et al., 1994]. The shortfall is that the state evolution law which aptly represents time-dependent strengthening of virtually locked faults systematically mispredicts the slip distance required to complete the state evolution caused by changed slip velocity [Marone, 1998; Nakatani, 2001]. To address the problem of evolution law, we observed state variable in RSF continuously in friction experiments on rough granite surfaces. The observation of state variable was performed in the following two independent ways; 1) by subtracting direct effect from measured shear stress 2) by using an acoustic monitoring technique [Nagata et al., 2008]. The latter method is usable even when slip velocity is so low that the former method cannot be used. The former can be regarded as “semi-direct” measurements of state variable. This method strongly depends on the value of coefficient of direct effect a. Ideally, a is observed as the instantaneous change of applied shear stress (direct effect) upon velocity step. However, the measured shear stress change is much smaller than the real direct effect in reality because the state variable changes considerably before the stress peak. Correction by inferring the change of state using an evolution law is a routine procedure, but the evolution law is in doubt. Hence, before we tackled the evolution law issue, we have designed a special step test where the change of state is minimal and have established that a >0.03. Further, with help of acoustic method, we have identified that a ~0.05. The value is surprisingly large, but agrees very well with the activation volume of silicate lattice. We compared thus observed variation of frictional strength with the

  4. Experimental Study of the Rolling-Sliding Contact Conditions in a PA66/STEEL Gear Using Twin-Disc Test Rig: Friction and Wear Analysis

    NASA Astrophysics Data System (ADS)

    Mbarek, Meftah; Rhaiem, Sadok; Kharrat, Mohamed; Dammak, Maher

    2015-09-01

    This study investigates the effects of sliding ratio on the tribological response of the contact between the teeth of a metal/polymer gear in the regions close to the pitch point. For this purpose, a new twin-disc test rig was developed on the basis of two discs of different diameters rotating one above the other at the same angular speed. Two different materials were used: non-alloyed structural steel (C45) and polyamide (PA66). The effect of the slip ratio (4%, 12%, 20% and 28%) was studied at a constant pressure of 34 MPa and a constant angular speed of 300 rpm. In addition, the contact conditions were controlled with measurements of the two discs surface temperatures. The results indicate that the wear and the friction are closely related to the contact temperature generated by the sliding phenomenon. At low slip ratio (4% and 12%), the coefficient of friction and the temperature are characterized by a quasi-linear increase with time, and the wear increases slowly. At higher slip ratio (20% and 28%), the coefficient of friction and the temperature presents a steady state, and the wear increases dramatically. During the test, a film of transferred PA66 is formed on the steel surface causing the development of adhesive interactions between the contacting discs which increase the friction coefficient and the contact temperature. The high thermal conductivity of steel as compared to that of the polymer can reduce enormously the contact temperature generated by the sliding process.

  5. Friction Mapping as a Tool for Measuring the Elastohydrodynamic Contact Running-in Process

    DTIC Science & Technology

    2015-10-01

    lubricated gear and bearing contacts typically demonstrate a period of wear called “running-in” when they are first put into service, during which time the...values, depending on the ramp direction and extent of mapping range. 15. SUBJECT TERMS elastohydrodynamic, lubrication, wear, gears, bearings 16...1. Introduction Elastohydrodynamic lubricated contacts are common in vehicle transmission gears and bearings . The hydrodynamic pressure built up

  6. Absolute measurement of ultrasonic non-linearity parameter at contact interface

    NASA Astrophysics Data System (ADS)

    Yuan, Maodan; Lee, Taekgyu; Kang, To; Zhang, Jianhai; Song, Sung-Jin; Kim, Hak-Joon

    2015-10-01

    Non-linear interaction of waves with contact interfaces has been widely applied in non-destructive evaluation fields such as bonding quality evaluation, and the detection of closed microcracks and composite delamination. This paper proposes an absolute measurement of the ultrasonic non-linearity parameter using a piezoelectric detection method for two aluminum alloy blocks of different lengths. The results of a two-dimensional finite element method model verified by models for hard and soft contact interfaces, depending on the interface property, were compared with the measured non-linearity parameter. The measured values show good agreement with the modelled results, indicating good potential for measuring the non-linearity parameter at interfaces experimentally and numerically.

  7. Does p-type ohmic contact exist in WSe2-metal interfaces?

    NASA Astrophysics Data System (ADS)

    Wang, Yangyang; Yang, Ruo Xi; Quhe, Ruge; Zhong, Hongxia; Cong, Linxiao; Ye, Meng; Ni, Zeyuan; Song, Zhigang; Yang, Jinbo; Shi, Junjie; Li, Ju; Lu, Jing

    2015-12-01

    Formation of low-resistance metal contacts is the biggest challenge that masks the intrinsic exceptional electronic properties of two dimensional WSe2 devices. We present the first comparative study of the interfacial properties between monolayer/bilayer (ML/BL) WSe2 and Sc, Al, Ag, Au, Pd, and Pt contacts by using ab initio energy band calculations with inclusion of the spin-orbital coupling (SOC) effects and quantum transport simulations. The interlayer coupling tends to reduce both the electron and hole Schottky barrier heights (SBHs) and alters the polarity for the WSe2-Au contact, while the SOC chiefly reduces the hole SBH. In the absence of the SOC, the Pd contact has the smallest hole SBH. Dramatically, the Pt contact surpasses the Pd contact and becomes the p-type ohmic or quasi-ohmic contact with inclusion of the SOC. Therefore, p-type ohmic or quasi-ohmic contact exists in WSe2-metal interfaces. Our study provides a theoretical foundation for the selection of favorable metal electrodes in ML/BL WSe2 devices.Formation of low-resistance metal contacts is the biggest challenge that masks the intrinsic exceptional electronic properties of two dimensional WSe2 devices. We present the first comparative study of the interfacial properties between monolayer/bilayer (ML/BL) WSe2 and Sc, Al, Ag, Au, Pd, and Pt contacts by using ab initio energy band calculations with inclusion of the spin-orbital coupling (SOC) effects and quantum transport simulations. The interlayer coupling tends to reduce both the electron and hole Schottky barrier heights (SBHs) and alters the polarity for the WSe2-Au contact, while the SOC chiefly reduces the hole SBH. In the absence of the SOC, the Pd contact has the smallest hole SBH. Dramatically, the Pt contact surpasses the Pd contact and becomes the p-type ohmic or quasi-ohmic contact with inclusion of the SOC. Therefore, p-type ohmic or quasi-ohmic contact exists in WSe2-metal interfaces. Our study provides a theoretical foundation for

  8. Considerations on the moving contact-line singularity, with application to frictional drag on a slender drop

    NASA Technical Reports Server (NTRS)

    Durbin, P. A.

    1988-01-01

    It has previously been shown that the no-slip boundary conditions leads to a singularity at a moving contact line and that this presumes some form of slip. Present considerations on the energetics of slip due to shear stress lead to a yield stress boundary condition. A model for the distortion of the liquid state near solid boundaries gives a physical basis for this boundary condition. The yield stress condition is illustrated by an analysis of a slender drop rolling down an incline. That analysis provides a formula for the frictional drag resisting the drop movement. With the present boundary condition, the length of the slip region becomes a property of the fluid flow.

  9. Universal Aging Mechanism for Static and Sliding Friction of Metallic Nanoparticles.

    PubMed

    Feldmann, Michael; Dietzel, Dirk; Tekiel, Antoni; Topple, Jessica; Grütter, Peter; Schirmeisen, André

    2016-07-08

    The term "contact aging" refers to the temporal evolution of the interface between a slider and a substrate usually resulting in increasing friction with time. Current phenomenological models for multiasperity contacts anticipate that such aging is not only the driving force behind the transition from static to sliding friction, but at the same time influences the general dynamics of the sliding friction process. To correlate static and sliding friction on the nanoscale, we show experimental evidence of stick-slip friction for nanoparticles sliding on graphite over a wide dynamic range. We can assign defined periods of aging to the stick phases of the particles, which agree with simulations explicitly including contact aging. Additional slide-hold-slide experiments for the same system allow linking the sliding friction results to static friction measurements, where both friction mechanisms can be universally described by a common aging formalism.

  10. Frictional Contact Problems for Thin Elastic Structures and Weak Solutions of Sweeping Processes

    NASA Astrophysics Data System (ADS)

    Ballard, Patrick

    2010-12-01

    The linearized equilibrium equations for straight elastic strings, beams, membranes or plates do not couple tangential and normal components. In the quasi-static evolution occurring above a fixed rigid obstacle with Coulomb dry friction, the normal displacement is governed by a variational inequality, whereas the tangential displacement is seen to obey a sweeping process, the theory of which was extensively developed by Moreau in the 1970s. In some cases, the underlying moving convex set has bounded retraction and, in these cases, the sweeping process can be solved by directly applying Moreau’s results. However, in many other cases, the bounded retraction condition is not fulfilled and this is seen to be connected to the possible event of moving velocity discontinuities. In such a case, there are no strong solutions and we have to cope with weak solutions of the underlying sweeping process.

  11. Method for characterizing the contact resistance of metal-vanadium dioxide thin film interfaces

    SciTech Connect

    Percy, R.; Stan, M.; Weikle, R. M.; Kittiwatanakul, S.; Lu, J.; Wolf, S.

    2014-07-14

    The standard method for determining the contact resistance of planar metal-semiconductor interfaces can underestimate the true contact resistance under normal operating conditions, as it relies on the resistivity of the semiconductor material remaining constant during measurement. However, the strong temperature dependence of the resistivity of VO{sub 2} requires a modified approach that maintains a constant power density dissipated within the film to account for Joule heating. We develop a method for measuring contact resistance in semiconductors with a high thermal coefficient of resistivity, demonstrate this method with an example, and compare the results with the standard technique.

  12. Mesh Size Control of Friction

    NASA Astrophysics Data System (ADS)

    Pitenis, Angela; Uruena, Juan Manuel; Schulze, Kyle D.; Cooper, Andrew C.; Angelini, Thomas E.; Sawyer, W. Gregory

    Soft, permeable sliding interfaces in aqueous environments are ubiquitous in nature but their ability to maintain high lubricity in a poor lubricant (water) has not been well understood. Hydrogels are excellent materials for fundamental soft matter and biotribology studies due to their high water content. While mesh size controls the material and transport properties of a hydrogel, its effects on friction were only recently explored. Polyacrylamide hydrogels slid in a Gemini (self-mated) interface produced low friction under low speeds, low pressures, macroscopic contact areas, and room temperature aqueous environments. The friction coefficients at these interfaces are lowest at low speeds and are speed-independent. This behavior is due to thermal fluctuations at the interface separating the surfaces, with water shearing in this region being the main source of dissipation. We found that mesh size had an inverse correlation with friction. We further investigated a transition from this behavior at higher speeds, and found that the transition speed correlated with the mesh size and relaxation time of the polymer network. Very soft and correspondingly large mesh size Gemini hydrogels show superlubricity under specific conditions with friction being less than 0.005.

  13. Load-Dependent Friction Hysteresis on Graphene.

    PubMed

    Ye, Zhijiang; Egberts, Philip; Han, Gang Hee; Johnson, A T Charlie; Carpick, Robert W; Martini, Ashlie

    2016-05-24

    Nanoscale friction often exhibits hysteresis when load is increased (loading) and then decreased (unloading) and is manifested as larger friction measured during unloading compared to loading for a given load. In this work, the origins of load-dependent friction hysteresis were explored through atomic force microscopy (AFM) experiments of a silicon tip sliding on chemical vapor deposited graphene in air, and molecular dynamics simulations of a model AFM tip on graphene, mimicking both vacuum and humid air environmental conditions. It was found that only simulations with water at the tip-graphene contact reproduced the experimentally observed hysteresis. The mechanisms underlying this friction hysteresis were then investigated in the simulations by varying the graphene-water interaction strength. The size of the water-graphene interface exhibited hysteresis trends consistent with the friction, while measures of other previously proposed mechanisms, such as out-of-plane deformation of the graphene film and irreversible reorganization of the water molecules at the shearing interface, were less correlated to the friction hysteresis. The relationship between the size of the sliding interface and friction observed in the simulations was explained in terms of the varying contact angles in front of and behind the sliding tip, which were larger during loading than unloading.

  14. Vibration transfer in the ball-stem contact interface of artificial hips.

    PubMed

    Hothan, Arne; Lewerenz, Kathrin; Weiss, Cornelius; Hoffmann, Norbert; Morlock, Michael; Huber, Gerd

    2013-10-01

    Audible squeaking has put the reputation of ceramic bearings for total hip replacements into question. Inter-articular friction induces vibrations in the ceramic head which are transferred through the head-taper interface to the femoral stem. If excited to sufficient amplitudes, squeaking can be emitted by the stem. Hence, the damping and amplification properties of this interface have a crucial influence on stem vibrations. The aim of this study was to determine the vibration transfer behavior between the head and the taper of a femoral stem and its dependence on the assembly force, in order to assess its influence on the development of audible squeaking. A ceramic head was assembled on a titanium femoral stem taper with high and low forces. Frequency response functions characterizing the head-stem interface were determined experimentally. The interface demonstrated negligible influence on vibration transfer in the squeaking frequency range (1-5 kHz). However its damping effect in lower and higher frequency ranges was significant and some areas of amplification were also found. Although squeaking vibration was not influenced by the particular interface studied, the method established can be utilized to trace taper designs with dynamic properties that decrease the susceptibility to squeaking.

  15. Improvement and evaluation of thermal, electrical, sealing and mechanical contacts, and their interface materials

    NASA Astrophysics Data System (ADS)

    Luo, Xiangcheng

    Material contacts, including thermal, electrical, seating (fluid sealing and electromagnetic sealing) and mechanical (pressure) contacts, together with their interface materials, were, evaluated, and in some cases, improved beyond the state of the art. The evaluation involved the use of thermal, electrical and mechanical methods. For thermal contacts, this work evaluated and improved the heat transfer efficiency between two contacting components by developing various thermal interface pastes. Sodium silicate based thermal pastes (with boron nitride particles as the thermally conductive filler) as well as polyethylene glycol (PEG) based thermal pastes were developed and evaluated. The optimum volume fractions of BN in sodium silicate based pastes and PEG based pastes were 16% and 18% respectively. The contribution of Li+ ions to the thermal contact conductance in the PEG-based paste was confirmed. For electrical contacts, the relationship between the mechanical reliability and electrical reliability of solder/copper and silver-epoxy/copper joints was addressed. Mechanical pull-out testing was conducted on solder/copper and silver-epoxy/copper joints, while the contact electrical resistivity was measured. Cleansing of the copper surface was more effective for the reliability of silver-epoxy/copper joint than that of solder/copper joint. For sealing contacts, this work evaluated flexible graphite as an electromagnetic shielding gasket material. Flexible graphite was found to be at least comparable to conductive filled silicone (the state of the art) in terms of the shielding effectiveness. The conformability of flexible graphite with its mating metal surface under repeated compression was characterized by monitoring the contact electrical resistance, as the conformability is important to both electromagnetic scaling and fluid waling using flexible graphite. For mechanical contacts, this work focused on the correlation of the interface structure (such as elastic

  16. Effects of friction at the digit-object interface on the digit forces in multi-finger prehension.

    PubMed

    Aoki, Tomoko; Niu, Xun; Latash, Mark L; Zatsiorsky, Vladimir M

    2006-07-01

    The effects of surface friction at the digit-object interface on digit forces were studied when subjects (n=8) statically held an object in a five-digit grasp. The friction conditions were SS (all surfaces are sandpaper), RR (all are rayon), SR (S for the thumb and R for the four fingers), and RS (the reverse of SR). The interaction effects of surface friction and external torque were also examined using five torques (-0.5, -0.25, 0, +0.25, +0.5 Nm). Forces and moments exerted by the digits on a handle were recorded. At zero torque conditions, in the SS and RR (symmetric) tasks the normal forces of the thumb and virtual finger (VF, an imagined finger with the mechanical effect equal to that of the four fingers) were larger for the RR than the SS conditions. In the SR and RS (asymmetric) tasks, the normal forces were between the RR and SS conditions. Tangential forces were smaller at the more slippery side than at the less slippery side. According to the mathematical optimization analysis decreasing the tangential forces at the more slippery sides decreases the cost function values. The difference between the thumb and VF tangential forces, DeltaF (t), generated a moment of the tangential forces (friction-induced moment). At non-zero torque conditions the friction-induced moment and the moment counterbalancing the external torque (equilibrium-necessitated moment) could be in same or in opposite directions. When the two moments were in the same direction, the contribution of the moment of tangential forces to the total moment was large, and the normal forces were relatively low. In contrast, when the two moments were in opposite directions, the contribution of the moment of tangential forces to the total moment markedly decreased, which was compensated by an increase in the moment of normal forces. The apparently complicated results were explained as the result of summation of the friction-related (elemental) and torque-related (synergy) components of the central

  17. Rescore protein-protein docked ensembles with an interface contact statistics.

    PubMed

    Mezei, Mihaly

    2017-02-01

    The recently developed statistical measure for the type of residue-residue contact at protein complex interfaces, based on a parameter-free definition of contact, has been used to define a contact score that is correlated with the likelihood of correctness of a proposed complex structure. Comparing the proposed contact scores on the native structure and on a set of model structures the proposed measure was shown to generally favor the native structure but in itself was not able to reliably score the native structure to be the best. Adjusting the scores of redocking experiments with the contact score showed that the adjusted score was able to move up the ranking of the native-like structure among the proposed complexes when the native-like was not ranked the best by the respective program. Tests on docking of unbound proteins compared the contact scores of the complexes with the contact score of the crystal structure again showing the tendency of the contact score to favor native-like conformations. The possibility of using the contact score to improve the determination of biological dimers in a crystal structure was also explored. Proteins 2017; 85:235-241. © 2016 Wiley Periodicals, Inc.

  18. Enhanced Locomotion Efficiency of a Bio-inspired Walking Robot using Contact Surfaces with Frictional Anisotropy

    NASA Astrophysics Data System (ADS)

    Manoonpong, Poramate; Petersen, Dennis; Kovalev, Alexander; Wörgötter, Florentin; Gorb, Stanislav N.; Spinner, Marlene; Heepe, Lars

    2016-12-01

    Based on the principles of morphological computation, we propose a novel approach that exploits the interaction between a passive anisotropic scale-like material (e.g., shark skin) and a non-smooth substrate to enhance locomotion efficiency of a robot walking on inclines. Real robot experiments show that passive tribologically-enhanced surfaces of the robot belly or foot allow the robot to grip on specific surfaces and move effectively with reduced energy consumption. Supplementing the robot experiments, we investigated tribological properties of the shark skin as well as its mechanical stability. It shows high frictional anisotropy due to an array of sloped denticles. The orientation of the denticles to the underlying collagenous material also strongly influences their mechanical interlocking with the substrate. This study not only opens up a new way of achieving energy-efficient legged robot locomotion but also provides a better understanding of the functionalities and mechanical properties of anisotropic surfaces. That understanding will assist developing new types of material for other real-world applications.

  19. Enhanced Locomotion Efficiency of a Bio-inspired Walking Robot using Contact Surfaces with Frictional Anisotropy

    PubMed Central

    Manoonpong, Poramate; Petersen, Dennis; Kovalev, Alexander; Wörgötter, Florentin; Gorb, Stanislav N.; Spinner, Marlene; Heepe, Lars

    2016-01-01

    Based on the principles of morphological computation, we propose a novel approach that exploits the interaction between a passive anisotropic scale-like material (e.g., shark skin) and a non-smooth substrate to enhance locomotion efficiency of a robot walking on inclines. Real robot experiments show that passive tribologically-enhanced surfaces of the robot belly or foot allow the robot to grip on specific surfaces and move effectively with reduced energy consumption. Supplementing the robot experiments, we investigated tribological properties of the shark skin as well as its mechanical stability. It shows high frictional anisotropy due to an array of sloped denticles. The orientation of the denticles to the underlying collagenous material also strongly influences their mechanical interlocking with the substrate. This study not only opens up a new way of achieving energy-efficient legged robot locomotion but also provides a better understanding of the functionalities and mechanical properties of anisotropic surfaces. That understanding will assist developing new types of material for other real-world applications. PMID:28008936

  20. The Contribution of Frictional Contacts to the Shear Strength of Coarse Glass Bead Powders and Slurries

    SciTech Connect

    Poloski, Adam P.; Bredt, Paul R.; Daniel, Richard C.; Saez, Avelino E.

    2006-12-01

    The shear strength of powders and slurries containing coarse particles using a vane impeller were examined as a function of vane size, vane immersion depth, container size, and interstitial fluid. Results show that for powders and concentrated slurry systems containing coarse particles, vane immersion depth and container diameter significantly impact the measured shear strength. An equation describing interparticle frictional and cohesive contributions to shear vane measurements was derived in an effort to describe experimental results. A Janssen stress distribution model for granular materials was the basis for this equation. The use of a Janssen stress distribution appears to explain the behavior of shear strength measurements at varying immersion depths with dry cohesionless glass beads, water saturated glass beads, and glass beads dispersed in a non-Newtonian matrix of kaolin clay slurry. The presence of the Janssen stress distribution can affect the interpretation of shear vane results. Rather than shear strength being a material property as is the case with flocculated colloid slurries and polymer solutions, shear strength becomes a process property where vane depth, container size, and container material can result in significant measurement variations. Such parameters should be considered before using the shear vane results on applications involving granular material components.

  1. Non-smooth Hopf-type bifurcations arising from impact–friction contact events in rotating machinery

    PubMed Central

    Mora, Karin; Budd, Chris; Glendinning, Paul; Keogh, Patrick

    2014-01-01

    We analyse the novel dynamics arising in a nonlinear rotor dynamic system by investigating the discontinuity-induced bifurcations corresponding to collisions with the rotor housing (touchdown bearing surface interactions). The simplified Föppl/Jeffcott rotor with clearance and mass unbalance is modelled by a two degree of freedom impact–friction oscillator, as appropriate for a rigid rotor levitated by magnetic bearings. Two types of motion observed in experiments are of interest in this paper: no contact and repeated instantaneous contact. We study how these are affected by damping and stiffness present in the system using analytical and numerical piecewise-smooth dynamical systems methods. By studying the impact map, we show that these types of motion arise at a novel non-smooth Hopf-type bifurcation from a boundary equilibrium bifurcation point for certain parameter values. A local analysis of this bifurcation point allows us a complete understanding of this behaviour in a general setting. The analysis identifies criteria for the existence of such smooth and non-smooth bifurcations, which is an essential step towards achieving reliable and robust controllers that can take compensating action. PMID:25383034

  2. Frictional contact behaviour of the tyre: the effect of tread slip on the in-plane structural deformation and stress field development

    NASA Astrophysics Data System (ADS)

    Tsotras, Achillefs; Mavros, George

    2010-08-01

    The analysis of the in-plane deformation of the tyre in relation to the frictional contact between the road and the tread is a crucial first step in the understanding of its contribution to the longitudinal dynamics of a vehicle. In this work, the physical mechanism of the generation of the two-dimensional contact pressure distribution for a non-rolling tyre is studied. Towards this aim, a physical tyre model is constructed, consisting of an analytical ring under pretension, a non-linear sidewall foundation, and a discretised foundation of viscoelastic elements representing the tread. Tread behaviour is examined first, with focus on the development of shear micro-slip. The tread simulation is enhanced with the combination of radial and tangential tread elements and the benefits of such an approach are identified. Subsequently, the contact of the complete model is examined by implementing an algorithm for transient simulations in the time domain. The effects of the imposed vertical load and sidewall non-linearity on the contact stress and strain fields are identified. The modelling approach is validated by comparison with published experimental results. The physical mechanism that couples the torsional and horizontal/vertical deformations of the carcass with the frictional forces at the tread is identified and discussed in detail. The proposed modelling approach is found appropriate for the description of the development of the two-dimensional contact pressure field as a function of the frictional potential of the contact.

  3. Nanomechanical mapping of the osteochondral interface with contact resonance force microscopy and nanoindentation.

    PubMed

    Campbell, Sara E; Ferguson, Virginia L; Hurley, Donna C

    2012-12-01

    The bone-cartilage, or osteochondral, interface resists remarkably high shear stresses and rarely fails, yet its mechanical characteristics are largely unknown. A complete understanding of this hierarchical system requires mechanical-property information at the length scales of both the interface and the connecting tissues. Here, we combined nanoindentation and atomic force microscopy (AFM) methods to investigate the multiscale mechanical properties across the osteochondral region. The nanoindentation modulus M ranged from that of the subchondral bone (M=22.8±1.8GPa) to that of hyaline articular cartilage embedded in PMMA (M=5.7±1.0GPa) across a narrow transition region <5μm wide. Contact resonance force microscopy (CR-FM), which measures the frequency and quality factor of the AFM cantilever's vibrational resonance in contact mode, was used to determine the relative storage modulus and loss tangent of the osteochondral interface. With better spatial resolution than nanoindentation, CR-FM measurements indicated an even narrower interface width of 2.3±1.2μm. Furthermore, CR-FM revealed a 24% increase in the viscoelastic loss tangent from the articular calcified cartilage into the PMMA-embedded hyaline articular cartilage. Quantitative backscattered electron imaging provided complementary measurement of mineral content. Our results provide insight into the multiscale functionality of the osteochondral interface that will advance understanding of disease states such as osteoarthritis and aid in the development of biomimetic interfaces.

  4. Exploring the Interface Between Religion and Contact with the Dead Among Older Mexican Americans

    PubMed Central

    Krause, Neal; Bastida, Elena

    2011-01-01

    The purpose of the current study is to examine the relationship between religion and post death contact among older Mexican Americans. Four major themes emerged from 52 in-depth interviews that were conducted with Older Mexican Americans residing in Texas. First, many older study participants told us they had contact with the dead, but others indicated this was not possible. Second, the form in which contact with the dead was made varied greatly. Some older Mexican Americans reported they had visual contact with the dead, while others said they only made contact with the dead through dreams. Third, although some older Mexican American study participants believed that it was in the best interests of the dead to contact the living, others felt the dead should instead be in Heaven with God. Fourth, the participants in our study reported that having contact with the dead provides a number of important social and psychological benefits. In the process of discussing these themes, an emphasis is placed on how beliefs and experiences with the dead interface with religion. In addition, we also explore how post death contact may be associated with health and well-being in late life. PMID:21399735

  5. Contact-angle hysteresis on periodic microtextured surfaces: Strongly corrugated liquid interfaces.

    PubMed

    Iliev, Stanimir; Pesheva, Nina

    2016-06-01

    We study numerically the shapes of a liquid meniscus in contact with ultrahydrophobic pillar surfaces in Cassie's wetting regime, when the surface is covered with identical and periodically distributed micropillars. Using the full capillary model we obtain the advancing and the receding equilibrium meniscus shapes when the cross-sections of the pillars are both of square and circular shapes, for a broad interval of pillar concentrations. The bending of the liquid interface in the area between the pillars is studied in the framework of the full capillary model and compared to the results of the heterogeneous approximation model. The contact angle hysteresis is obtained when the three-phase contact line is located on one row (block case) or several rows (kink case) of pillars. It is found that the contact angle hysteresis is proportional to the line fraction of the contact line on pillars tops in the block case and to the surface fraction for pillar concentrations 0.1-0.5 in the kink case. The contact angle hysteresis does not depend on the shape (circular or square) of the pillars cross-section. The expression for the proportionality of the receding contact angle to the line fraction [Raj et al., Langmuir 28, 15777 (2012)LANGD50743-746310.1021/la303070s] in the case of block depinning is theoretically substantiated through the capillary force, acting on the solid plate at the meniscus contact line.

  6. Contact-angle hysteresis on periodic microtextured surfaces: Strongly corrugated liquid interfaces

    NASA Astrophysics Data System (ADS)

    Iliev, Stanimir; Pesheva, Nina

    2016-06-01

    We study numerically the shapes of a liquid meniscus in contact with ultrahydrophobic pillar surfaces in Cassie's wetting regime, when the surface is covered with identical and periodically distributed micropillars. Using the full capillary model we obtain the advancing and the receding equilibrium meniscus shapes when the cross-sections of the pillars are both of square and circular shapes, for a broad interval of pillar concentrations. The bending of the liquid interface in the area between the pillars is studied in the framework of the full capillary model and compared to the results of the heterogeneous approximation model. The contact angle hysteresis is obtained when the three-phase contact line is located on one row (block case) or several rows (kink case) of pillars. It is found that the contact angle hysteresis is proportional to the line fraction of the contact line on pillars tops in the block case and to the surface fraction for pillar concentrations 0.1 -0.5 in the kink case. The contact angle hysteresis does not depend on the shape (circular or square) of the pillars cross-section. The expression for the proportionality of the receding contact angle to the line fraction [Raj et al., Langmuir 28, 15777 (2012), 10.1021/la303070s] in the case of block depinning is theoretically substantiated through the capillary force, acting on the solid plate at the meniscus contact line.

  7. Exploring the Interface Between Religion and Contact with the Dead Among Older Mexican Americans.

    PubMed

    Krause, Neal; Bastida, Elena

    2010-01-01

    The purpose of the current study is to examine the relationship between religion and post death contact among older Mexican Americans. Four major themes emerged from 52 in-depth interviews that were conducted with Older Mexican Americans residing in Texas. First, many older study participants told us they had contact with the dead, but others indicated this was not possible. Second, the form in which contact with the dead was made varied greatly. Some older Mexican Americans reported they had visual contact with the dead, while others said they only made contact with the dead through dreams. Third, although some older Mexican American study participants believed that it was in the best interests of the dead to contact the living, others felt the dead should instead be in Heaven with God. Fourth, the participants in our study reported that having contact with the dead provides a number of important social and psychological benefits. In the process of discussing these themes, an emphasis is placed on how beliefs and experiences with the dead interface with religion. In addition, we also explore how post death contact may be associated with health and well-being in late life.

  8. Numerical simulations of the moving contact line problem using a diffuse-interface model

    NASA Astrophysics Data System (ADS)

    Afzaal, Muhammad; Sibley, David; Duncan, Andrew; Yatsyshin, Petr; Duran-Olivencia, Miguel A.; Nold, Andreas; Savva, Nikos; Schmuck, Markus; Kalliadasis, Serafim

    2015-11-01

    Moving contact lines are a ubiquitous phenomenon both in nature and in many modern technologies. One prevalent way of numerically tackling the problem is with diffuse-interface (phase-field) models, where the classical sharp-interface model of continuum mechanics is relaxed to one with a finite thickness fluid-fluid interface, capturing physics from mesoscopic lengthscales. The present work is devoted to the study of the contact line between two fluids confined by two parallel plates, i.e. a dynamically moving meniscus. Our approach is based on a coupled Navier-Stokes/Cahn-Hilliard model. This system of partial differential equations allows a tractable numerical solution to be computed, capturing diffusive and advective effects in a prototypical case study in a finite-element framework. Particular attention is paid to the static and dynamic contact angle of the meniscus advancing or receding between the plates. The results obtained from our approach are compared to the classical sharp-interface model to elicit the importance of considering diffusion and associated effects. We acknowledge financial support from European Research Council via Advanced Grant No. 247031.

  9. Contact angle determination of micro- and nanoparticles at fluid/fluid interfaces: the excluded area concept.

    PubMed

    Grigoriev, D O; Krägel, J; Dutschk, V; Miller, R; Möhwald, H

    2007-12-28

    A novel and simple method for the determination of the contact angle of nano- and microparticles at the liquid/air interface is proposed. The principle is based on the consideration of differences between the pressure/area isotherms of a 2D single-component system of a surfactant and those of binary systems comprised of the same surfactant and the particles to be studied. The theoretical analysis of the contact-angle behavior in this system upon compression allows the prediction of direction of the particles' squeezing out of the surface layer and therefore the distinction between the particles with high contact angle (Theta(p) > 90 degrees) and low (Theta(p) < 90 degrees) hydrophobicity. The application of this method to microparticles of two different hydrophobicities demonstrates good agreement between the obtained contact angles and the corresponding degrees of hydrophobicity given by the particle provider.

  10. Tuning back contact property via artificial interface dipoles in Si/organic hybrid solar cells

    NASA Astrophysics Data System (ADS)

    Wang, Dan; Sheng, Jiang; Wu, Sudong; Zhu, Juye; Chen, Shaojie; Gao, Pingqi; Ye, Jichun

    2016-07-01

    Back contact property plays a key role in the charge collection efficiency of c-Si/poly(3,4-ethylthiophene):poly(styrenesulfonate) hybrid solar cells (Si-HSCs), as an alternative for the high-efficiency and low-cost photovoltaic devices. In this letter, we utilize the water soluble poly (ethylene oxide) (PEO) to modify the Al/Si interface to be an Ohmic contact via interface dipole tuning, decreasing the work function of the Al film. This Ohmic contact improves the electron collection efficiency of the rear electrode, increasing the short circuit current density (Jsc). Furthermore, the interface dipoles make the band bending downward to increase the total barrier height of built-in electric field of the solar cell, enhancing the open circuit voltage (Voc). The PEO solar cell exhibits an excellent performance, 12.29% power conversion efficiency, a 25.28% increase from the reference solar cell without a PEO interlayer. The simple and water soluble method as a promising alternative is used to develop the interfacial contact quality of the rear electrode for the high photovoltaic performance of Si-HSCs.

  11. Coordinated Water Under Confinement Eases Sliding Friction

    NASA Astrophysics Data System (ADS)

    Defante, Adrian; Dhopotkar, Nishad; Dhinojwala, Ali

    Water is essential to a number of interfacial phenomena such as the lubrication of knee joints, protein folding, mass transport, and adsorption processes. We have used a biaxial friction cell to quantify underwater friction between a hydrophobic elastomeric lens and a hydrophobic self-assembled monolayer in the presence of surfactant solutions. To gain an understanding of the role of water in these processes we have coupled this measurement with surface sensitive sum frequency generation to directly probe the molecular constitution of the confined contact interface. We observe that role of confined coordinated water between two hydrophobic substrates covered with surfactants is the key to obtaining a low coefficient of friction.

  12. Quantitative investigation of the photodegradation of polyethylene terephthalate film by friction force microscopy, contact-angle goniometry, and X-ray photoelectron spectroscopy.

    PubMed

    Hurley, Claire R; Leggett, Graham J

    2009-08-01

    Studies of the UV-induced photodegradation of poly(ethylene terephthalate) (PET) have been carried out using contact-angle goniometry, X-ray photoelectron spectroscopy (XPS), and friction force microscopy (FFM). The advancing contact angle of water, theta, decreased following exposure of free-standing PET films to UV light. Measurements of surface friction by FFM showed that the coefficient of friction mu increased as the degradation proceeded, reaching a limiting value after ca 200 min, in agreement with the contact angle data. Using a modified form of the Cassie equation, a quantitative analysis of the extent of modification could be carried out. There was a very close correlation between the coefficient of friction determined by FFM and the value of cos theta. XPS provided more detailed information on surface bonding that also correlated closely with the FFM data. Although FFM provides quantitative data on surface modification with nanometer-scale spatial resolution, it does not provide detailed structural information such as is provided by XPS. The oxygen content at the surface was found to increase as photo-generated radicals within the PET reacted with atmospheric oxygen. Increases in both ester and carbonyl contributions within XPS data accompanied this increase. It was concluded that the photodegradation process follows mainly Norrish type I reaction pathways, following previous work by Fechine et al and Grosstete et al.

  13. Computational methods for frictional contact with applications to the Space Shuttle orbiter nose-gear tire: Comparisons of experimental measurements and analytical predictions

    NASA Technical Reports Server (NTRS)

    Tanner, John A.

    1996-01-01

    A computational procedure is presented for the solution of frictional contact problems for aircraft tires. A Space Shuttle nose-gear tire is modeled using a two-dimensional laminated anisotropic shell theory which includes the effects of variations in material and geometric parameters, transverse-shear deformation, and geometric nonlinearities. Contact conditions are incorporated into the formulation by using a perturbed Lagrangian approach with the fundamental unknowns consisting of the stress resultants, the generalized displacements, and the Lagrange multipliers associated with both contact and friction conditions. The contact-friction algorithm is based on a modified Coulomb friction law. A modified two-field, mixed-variational principle is used to obtain elemental arrays. This modification consists of augmenting the functional of that principle by two terms: the Lagrange multiplier vector associated with normal and tangential node contact-load intensities and a regularization term that is quadratic in the Lagrange multiplier vector. These capabilities and computational features are incorporated into an in-house computer code. Experimental measurements were taken to define the response of the Space Shuttle nose-gear tire to inflation-pressure loads and to inflation-pressure loads combined with normal static loads against a rigid flat plate. These experimental results describe the meridional growth of the tire cross section caused by inflation loading, the static load-deflection characteristics of the tire, the geometry of the tire footprint under static loading conditions, and the normal and tangential load-intensity distributions in the tire footprint for the various static vertical-loading conditions. Numerical results were obtained for the Space Shuttle nose-gear tire subjected to inflation pressure loads and combined inflation pressure and contact loads against a rigid flat plate. The experimental measurements and the numerical results are compared.

  14. [Design and research of an interface compatible non-contacting respiratory signal detection system].

    PubMed

    Song, Kui; Qi, Jiajun; Lin, Tao; Zhang, Yi

    2011-06-01

    Respiration-induced displacements of organs greatly affect the safety and efficiency of high intensity focused ultrasound (HIFU) tumor therapy system. The key to solve this problem is accurate, real-time detection of respiratory signals. The present study gives a new design of an interface compatible non-contacting respiratory signal detection system using the method of irradiating the laser beam onto certain region of the surface of human body that is intensely influenced by the breathing movements (mostly the breast or the dorsum) at a certain angle, and meanwhile using a camera to acquire information from the location of the laser projection. Then we can draw a curve of the location of laser projection versus time base, that is the respiration curve. This respiratory signal detection method is non-contacting, interface compatible and easy to be integrated into the treatment system.

  15. Formation of a diffusion-based intermetallic interface layer in friction stir welded dissimilar Al-Cu lap joints

    NASA Astrophysics Data System (ADS)

    Marstatt, R.; Krutzlinger, M.; Luderschmid, J.; Zaeh, M. F.; Haider, F.

    2017-03-01

    The joining of dissimilar metals is an important issue in modern lightweight design. Friction Stir Welding (FSW) is suitable for this task since the solidus temperature is usually not exceeded during the process. As a consequence, dissimilar joints can be produced with a minimum of deteriorating intermetallic phases. The latest studies showed the formation of intermetallic layers at the bonding interface with no significant negative influence on the seam quality. In this study, those intermetallic nanolayers at the interface of aluminium / copper lap joints were analysed. For the experiments, the commercially pure alloys EN AW-1050 and CW008A were chosen. The process temperature changed with respect to the parameter setup and was measured at different locations of the seam. The intermetallic layers at the interface were analysed by scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM). The experiments show that the thickness of the interlayer clearly correlates with the process temperature using an Arrhenius equation. It is supposed, that the rotating probe removes the oxide layers of the metal surfaces and a metallic bonding between the Al- and the Cu-phase is formed. Due to the elevated temperature after the probe has passed, the intermetallic layer has emerged by interdiffusion.

  16. Use of Textured Surfaces to Mitigate Sliding Friction and Wear of Lubricated and Non-Lubricated Contacts

    SciTech Connect

    Blau, Peter Julian

    2012-03-01

    If properly employed, the placement of three-dimensional feature patterns, also referred to as textures, on relatively-moving, load-bearing surfaces can be beneficial to their friction and wear characteristics. For example, geometric patterns can function as lubricant supply channels or depressions in which to trap debris. They can also alter lubricant flow in a manner that produces thicker load-bearing films locally. Considering the area occupied by solid areas and spaces, textures also change the load distribution on surfaces. At least ten different attributes of textures can be specified, and their combinations offer wide latitude in surface engineering. By employing directional machining and grinding procedures, texturing has been used on bearings and seals for well over a half century, and the size scales of texturing vary widely. This report summarizes past work on the texturing of load-bearing surfaces, including past research on laser surface dimpling of ceramics done at ORNL. Textured surfaces generally show most pronounced effects when they are used in conformal or nearly conformal contacts, like that in face seals. Combining textures with other forms of surface modification and lubrication methods can offer additional benefits in surface engineering for tribology. As the literature and past work at ORNL shows, texturing does not always provide benefits. Rather, the selected pattern and arrangement of features must be matched to characteristics of the proposed application, bearing materials, and lubricants.

  17. Conduction electrons as dissipation channel in friction experiments at the metal-metal transition of LSMO measured by contact-resonance atomic force microscopy

    NASA Astrophysics Data System (ADS)

    Pfahl, V.; Phani, M. K.; Büchsenschütz-Göbeler, M.; Kumar, A.; Moshnyaga, V.; Arnold, W.; Samwer, K.

    2017-01-01

    We report on friction measurements on a La0.6Sr0.4MnO3 (LSMO) thin film using atomic force microscopy cantilever contact-resonances. There is a contribution to the damping of the cantilever oscillations, which is caused by micro-sliding of the cantilever tip on the surface of the thin film. This frictional part decreases with temperature parallel to the increase in the resistivity of the thin film. The LSMO is well-known for a ferromagnetic to paramagnetic phase transition that occurs without changes in the rhombohedral (R-3c) crystalline structure. The magnetic transition at the Curie temperature TC ˜ 360 K is accompanied by a metal-to-metal transition with a large increase in electrical resistivity. The behavior of the cantilever damping constant demonstrates that there is a direct coupling between mechanical friction and the mobility of the electrons in the LSMO film.

  18. Healing of Experimentally Simulated Fractures: Contact Neck Growth, and Strength Evolution of the Interface

    NASA Astrophysics Data System (ADS)

    Renard, F.; Dysthe, D.; Voisin, C.

    2008-12-01

    To investigate the physical processes operating in active fault zones, we conduct analogue laboratory experiments where we simulated a rough fracture that undergoes healing/shear cycles under dry condition or in the presence of a reactive fluid. This set-up is a surrogate for the healing/sealing of fractures and faults rocks, where fluid-rock interactions are operative at the millennium time scale. A rough slider of sodium chloride was left in contact with a flat glass plate under a constant normal load. The whole set-up was mounted on a microscope and left in a temperature-controlled box. The closure of the interface through several days was measured using high resolution displacement sensors and the contact surface was continuously imaged with a CCD camera. Under dry conditions, a small transient creep displacement perpendicularly to the fracture plane was measured. This deformation last for several minutes and finally stopped. Under fluid saturated conditions, a slow closure of the rough interface was measured over several days. This closure was concomitant with the growth of contact points, driven by surface tension forces. After 50 hours, up to 10% of the fracture surface was healed by this process. The force necessary to break the adhesion forces, allowing the sample to slide, was also measured after several periods of increasing holding times and showed a power law dependence with time. After each experiment, the fracture interface roughness was measured to nanometer resolution using white light interferometry. The morphology of the contacts was characterized by scaling relationships. Finally, we propose a macroscopic constitutive model of fracture closure and strength recovery, related to the dynamics of the contact asperities, which are seen to flatten and expand through time.

  19. Cryo-EM Data Are Superior to Contact and Interface Information in Integrative Modeling

    PubMed Central

    de Vries, Sjoerd J.; Chauvot de Beauchêne, Isaure; Schindler, Christina E.M.; Zacharias, Martin

    2016-01-01

    Protein-protein interactions carry out a large variety of essential cellular processes. Cryo-electron microscopy (cryo-EM) is a powerful technique for the modeling of protein-protein interactions at a wide range of resolutions, and recent developments have caused a revolution in the field. At low resolution, cryo-EM maps can drive integrative modeling of the interaction, assembling existing structures into the map. Other experimental techniques can provide information on the interface or on the contacts between the monomers in the complex. This inevitably raises the question regarding which type of data is best suited to drive integrative modeling approaches. Systematic comparison of the prediction accuracy and specificity of the different integrative modeling paradigms is unavailable to date. Here, we compare EM-driven, interface-driven, and contact-driven integrative modeling paradigms. Models were generated for the protein docking benchmark using the ATTRACT docking engine and evaluated using the CAPRI two-star criterion. At 20 Å resolution, EM-driven modeling achieved a success rate of 100%, outperforming the other paradigms even with perfect interface and contact information. Therefore, even very low resolution cryo-EM data is superior in predicting heterodimeric and heterotrimeric protein assemblies. Our study demonstrates that a force field is not necessary, cryo-EM data alone is sufficient to accurately guide the monomers into place. The resulting rigid models successfully identify regions of conformational change, opening up perspectives for targeted flexible remodeling. PMID:26846888

  20. Adjustments to local friction in multifinger prehension.

    PubMed

    Aoki, Tomoko; Latash, Mark L; Zatsiorsky, Vladimir M

    2007-07-01

    The authors studied the effects of surface friction at the digit-object interface on digit forces and moments when 12 participants statically held an object in a 5-digit grasp. The authors changed low-friction contact (LFC) with rayon and high-friction contact (HFC) with sandpaper independently for each digit in all 32 possible combinations. Normal forces of the thumb and virtual finger (VF), an imagined finger with a mechanical effect equal to that of the 4 fingers, increased with the thumb at LFC or with an increase in the number of fingers at LFC. When the thumb was at LFC, the thumb tangential force decreased. The VF tangential force decreased when the number of fingers at LFC increased. The interaction of the local responses to friction and the synergic responses necessary to maintain the equilibrium explain the coordination of individual digit forces.

  1. Adjustments to Local Friction in Multifinger Prehension

    PubMed Central

    Aoki, Tomoko; Latash, Mark L.; Zatsiorsky, Vladimir M.

    2010-01-01

    The authors studied the effects of surface friction at the digit–object interface on digit forces and moments when 12 participants statically held an object in a 5-digit grasp. The authors changed low-friction contact (LFC) with rayon and high-friction contact (HFC) with sandpaper independently for each digit in all 32 possible combinations. Normal forces of the thumb and virtual finger (VF), an imagined finger with a mechanical effect equal to that of the 4 fingers, increased with the thumb at LFC or with an increase in the number of fingers at LFC. When the thumb was at LFC, the thumb tangential force decreased. The VF tangential force decreased when the number of fingers at LFC increased. The interaction of the local responses to friction and the synergic responses necessary to maintain the equilibrium explain the coordination of individual digit forces. PMID:17664170

  2. Improvement of the electrical contact resistance at rough interfaces using two dimensional materials

    SciTech Connect

    Hu, Jianchen; Pan, Chengbin; Lanza, Mario; Li, Heng; Shen, Panpan; Sun, Hui; Duan, Huiling

    2015-12-07

    Reducing the electronic contact resistance at the interfaces of nanostructured materials is a major goal for many kinds of planar and three dimensional devices. In this work, we develop a method to enhance the electronic transport at rough interfaces by inserting a two dimensional flexible and conductive graphene sheet. We observe that an ultra-thin graphene layer with a thickness of 0.35 nm can remarkably reduce the roughness of a sample in a factor of 40%, avoiding the use of thick coatings, leading to a more homogeneous current flow, and extraordinarily increasing the total current compared to the graphene-free counterpart. Due to its simplicity and performance enhancement, this methodology can be of interest to many interface and device designers.

  3. Atomic-Scale Engineering of Abrupt Interface for Direct Spin Contact of Ferromagnetic Semiconductor with Silicon

    PubMed Central

    Averyanov, Dmitry V.; Karateeva, Christina G.; Karateev, Igor A.; Tokmachev, Andrey M.; Vasiliev, Alexander L.; Zolotarev, Sergey I.; Likhachev, Igor A.; Storchak, Vyacheslav G.

    2016-01-01

    Control and manipulation of the spin of conduction electrons in industrial semiconductors such as silicon are suggested as an operating principle for a new generation of spintronic devices. Coherent injection of spin-polarized carriers into Si is a key to this novel technology. It is contingent on our ability to engineer flawless interfaces of Si with a spin injector to prevent spin-flip scattering. The unique properties of the ferromagnetic semiconductor EuO make it a prospective spin injector into silicon. Recent advances in the epitaxial integration of EuO with Si bring the manufacturing of a direct spin contact within reach. Here we employ transmission electron microscopy to study the interface EuO/Si with atomic-scale resolution. We report techniques for interface control on a submonolayer scale through surface reconstruction. Thus we prevent formation of alien phases and imperfections detrimental to spin injection. This development opens a new avenue for semiconductor spintronics. PMID:26957146

  4. Atomic-Scale Engineering of Abrupt Interface for Direct Spin Contact of Ferromagnetic Semiconductor with Silicon

    NASA Astrophysics Data System (ADS)

    Averyanov, Dmitry V.; Karateeva, Christina G.; Karateev, Igor A.; Tokmachev, Andrey M.; Vasiliev, Alexander L.; Zolotarev, Sergey I.; Likhachev, Igor A.; Storchak, Vyacheslav G.

    2016-03-01

    Control and manipulation of the spin of conduction electrons in industrial semiconductors such as silicon are suggested as an operating principle for a new generation of spintronic devices. Coherent injection of spin-polarized carriers into Si is a key to this novel technology. It is contingent on our ability to engineer flawless interfaces of Si with a spin injector to prevent spin-flip scattering. The unique properties of the ferromagnetic semiconductor EuO make it a prospective spin injector into silicon. Recent advances in the epitaxial integration of EuO with Si bring the manufacturing of a direct spin contact within reach. Here we employ transmission electron microscopy to study the interface EuO/Si with atomic-scale resolution. We report techniques for interface control on a submonolayer scale through surface reconstruction. Thus we prevent formation of alien phases and imperfections detrimental to spin injection. This development opens a new avenue for semiconductor spintronics.

  5. Quantification of the Contact Area at the Head-Stem Taper Interface of Modular Hip Prostheses.

    PubMed

    Witt, Florian; Gührs, Julian; Morlock, Michael M; Bishop, Nicholas E

    2015-01-01

    Corrosion of modular taper junctions of hip implants may be associated with clinical failure. Taper design parameters, as well as the intraoperatively applied assembly forces, have been proposed to affect corrosion. Fretting corrosion is related to relative interface shear motion and fluid ingress, which may vary with contact force and area. It was hypothesised in this study that assembly forces modify the extent and distribution of the surface contact area at the taper interface between a cobalt chrome head and titanium stem taper with a standard threaded surface profile. Local abrasion of a thin gold coating applied to the stem taper prior to assembly was used to determine the contact area after disassembly. Profilometry was then used to assess permanent deformation of the stem taper surface profile. With increasing assembly force (500 N, 2000 N, 4000 N and 8000 N) the number of stem taper surface profile ridges in contact with the head taper was found to increase (9.2±9.3%, 65.4±10.8%, 92.8±6.0% and 100%) and the overall taper area in contact was also found to increase (0.6±0.7%, 5.5±1.0%, 9.9±1.1% and 16.1±0.9%). Contact was inconsistently distributed over the length of the taper. An increase in plastic radial deformation of the surface ridges (-0.05±0.14 μm, 0.1±0.14 μm, 0.21±0.22 μm and 0.96±0.25 μm) was also observed with increasing assembly force. The limited contact of the taper surface ridges at lower assembly forces may influence corrosion rates, suggesting that the magnitude of the assembly force may affect clinical outcome. The method presented provides a simple and practical assessment of the contact area at the taper interface.

  6. Friction. Macroscale superlubricity enabled by graphene nanoscroll formation.

    PubMed

    Berman, Diana; Deshmukh, Sanket A; Sankaranarayanan, Subramanian K R S; Erdemir, Ali; Sumant, Anirudha V

    2015-06-05

    Friction and wear remain as the primary modes of mechanical energy dissipation in moving mechanical assemblies; thus, it is desirable to minimize friction in a number of applications. We demonstrate that superlubricity can be realized at engineering scale when graphene is used in combination with nanodiamond particles and diamondlike carbon (DLC). Macroscopic superlubricity originates because graphene patches at a sliding interface wrap around nanodiamonds to form nanoscrolls with reduced contact area that slide against the DLC surface, achieving an incommensurate contact and substantially reduced coefficient of friction (~0.004). Atomistic simulations elucidate the overall mechanism and mesoscopic link bridging the nanoscale mechanics and macroscopic experimental observations.

  7. In situ assessment of the contact angles of nanoparticles adsorbed at fluid interfaces by multiple angle of incidence ellipsometry.

    PubMed

    Stocco, Antonio; Su, Ge; Nobili, Maurizio; In, Martin; Wang, Dayang

    2014-09-28

    Here multiple angle of incidence ellipsometry was successfully applied to in situ assess the contact angle and surface coverage of gold nanoparticles as small as 18 nm, coated with stimuli-responsive polymers, at water-oil and water-air interfaces in the presence of NaCl and NaOH, respectively. The interfacial adsorption of the nanoparticles was found to be very slow and took days to reach a fairly low surface coverage. For water-oil interfaces, in situ nanoparticle contact angles agree with the macroscopic equilibrium contact angles of planar gold surfaces with the same polymer coatings, whilst for water-air interfaces, significant differences have been observed.

  8. Modification of Silver/Single-Wall Carbon Nanotube Electrical Contact Interfaces via Ion Irradiation.

    PubMed

    Cox, Nathanael D; Cress, Cory D; Rossi, Jamie E; Puchades, Ivan; Merrill, Andrew; Franklin, Aaron D; Landi, Brian J

    2017-03-01

    Introduction of defects via ion irradiation ex situ to modify silver/single-wall carbon nanotube (Ag-SWCNT) electrical contacts and the resulting changes in the electrical properties were studied. Two test samples were fabricated by depositing 0.1 μm Ag onto SWCNT thin films with average thicknesses of 10 and 60 nm, followed by ion irradiation (150 keV (11)B(+) at 5 × 10(14) ions/cm(2)). The contact resistance (Rc) between the Ag and SWCNT thin films was determined using transfer length method (TLM) measurements before and after ion irradiation. Rc increases for both test samples after irradiation, while there is no change in Rc for control structures with thick Ag contacts (1.5 μm), indicating that changes in Rc originate from changes in the SWCNT films and at the Ag-SWCNT interface caused by ion penetration through the Ag contact electrodes. Rc increases by ∼4× for the 60 nm SWCNT structure and increases by ∼2.4× for the 10 nm SWCNT structure. Raman spectroscopy measurements of the SWCNTs under the contacts compared to the starting SWCNT film show that the degradation of the 10 nm SWCNT structure was less significant than that of the 60 nm SWCNT structure, suggesting that the smaller change in Rc for the 10 nm SWCNT structure is a result of the thickness-dependent damage profile in the SWCNTs. Despite the increase in overall contact resistance, further TLM analysis reveals that the specific contact resistance actually decreases by ∼3.5-4× for both test samples, suggesting an enhancement of the electrical properties at the Ag-SWCNT interface. Irradiation simulations provide a physical description of the underlying mechanism, revealing that Ag atoms are forward-scattered into the SWCNTs, creating an Ag/C interfacial layer several nanometers in depth. The collective results indicate competing effects of improvement of the Ag-SWCNT interface versus degradation of the bulk SWCNT films, which has implications for scaled high-performance devices employing

  9. Interface modification in solar cell contact electrode using pre-cleaning treatment chemistries.

    PubMed

    Cui, Yinhua; Kim, Areum; Lee, Seonjea; Choi, Eunmi; Yoon, Sung Pil; Pyo, Sung Gyu

    2014-12-01

    Promoting and employing photovoltaic power as an alternative energy source, the solar cell industry has made rapid strides. However, improving the efficiency of these solar cells using low-cost fabrication processes is still needed. The interface between the Si surface and the electrode plays a very important role in the process of electrode formation of the solar cell. In this study, the electrode interface underwent four different pre-treatments in order to enhance the efficiency of Si-based solar cells. We analyzed the adhesion properties at the interface between the Si wafer and the electrode and conducted an analysis of the variation in contact resistance between the two contact surfaces. To reduce the cost of the entire experiment, we replaced the existing Ag screen printing-based electrode fabrication method with a low-temperature, low-cost Ni/Cu electroless plating method. The test cells exhibited improved adhesion and therefore improved efficiency as compared to cells treated with the currently used diluted HF.

  10. Organic heterojunctions: Contact-induced molecular reorientation, interface states, and charge re-distribution

    PubMed Central

    Opitz, Andreas; Wilke, Andreas; Amsalem, Patrick; Oehzelt, Martin; Blum, Ralf-Peter; Rabe, Jürgen P.; Mizokuro, Toshiko; Hörmann, Ulrich; Hansson, Rickard; Moons, Ellen; Koch, Norbert

    2016-01-01

    We reveal the rather complex interplay of contact-induced re-orientation and interfacial electronic structure – in the presence of Fermi-level pinning – at prototypical molecular heterojunctions comprising copper phthalocyanine (H16CuPc) and its perfluorinated analogue (F16CuPc), by employing ultraviolet photoelectron and X-ray absorption spectroscopy. For both layer sequences, we find that Fermi-level (EF) pinning of the first layer on the conductive polymer substrate modifies the work function encountered by the second layer such that it also becomes EF-pinned, however, at the interface towards the first molecular layer. This results in a charge transfer accompanied by a sheet charge density at the organic/organic interface. While molecules in the bulk of the films exhibit upright orientation, contact formation at the heterojunction results in an interfacial bilayer with lying and co-facial orientation. This interfacial layer is not EF-pinned, but provides for an additional density of states at the interface that is not present in the bulk. With reliable knowledge of the organic heterojunction’s electronic structure we can explain the poor performance of these in photovoltaic cells as well as their valuable function as charge generation layer in electronic devices. PMID:26887445

  11. Metastable nanobubbles at the solid-liquid interface due to contact angle hysteresis.

    PubMed

    Nishiyama, Takashi; Yamada, Yutaka; Ikuta, Tatsuya; Takahashi, Koji; Takata, Yasuyuki

    2015-01-27

    Nanobubbles exist at solid-liquid interfaces between pure water and hydrophobic surfaces with very high stability, lasting in certain cases up to several days. Not only semispherical but also other shapes, such as micropancakes, are known to exist at such interfaces. However, doubt has been raised as to whether or not the nanobubbles are gas-phase entities. In this study, surface nanobubbles at a pure water-highly ordered pyrolytic graphite (HOPG) interface were investigated by peak force quantitative nanomechanics (PF-QNM). Multiple isolated nanobubbles generated by the solvent-exchange method were present on the terraced areas, avoiding the steps of the HOPG surface. Adjacent nanobubbles coalesced and formed metastable nanobubbles. Coalescence was enhanced by the PF-QNM measurement. We determined that nanobubbles can exist for a long time because of nanoscale contact angle hysteresis at the water-HOPG interface. Moreover, the hydrophilic steps of HOPG were avoided during coalescence, providing evidence that the nanobubbles are truly gas phase.

  12. Interface Engineering to Create a Strong Spin Filter Contact to Silicon

    PubMed Central

    Caspers, C.; Gloskovskii, A.; Gorgoi, M.; Besson, C.; Luysberg, M.; Rushchanskii, K. Z.; Ležaić, M.; Fadley, C. S.; Drube, W.; Müller, M.

    2016-01-01

    Integrating epitaxial and ferromagnetic Europium Oxide (EuO) directly on silicon is a perfect route to enrich silicon nanotechnology with spin filter functionality. To date, the inherent chemical reactivity between EuO and Si has prevented a heteroepitaxial integration without significant contaminations of the interface with Eu silicides and Si oxides. We present a solution to this long-standing problem by applying two complementary passivation techniques for the reactive EuO/Si interface: (i) an in situ hydrogen-Si (001) passivation and (ii) the application of oxygen-protective Eu monolayers–without using any additional buffer layers. By careful chemical depth profiling of the oxide-semiconductor interface via hard x-ray photoemission spectroscopy, we show how to systematically minimize both Eu silicide and Si oxide formation to the sub-monolayer regime–and how to ultimately interface-engineer chemically clean, heteroepitaxial and ferromagnetic EuO/Si (001) in order to create a strong spin filter contact to silicon. PMID:26975515

  13. Decrease of contact resistance at the interface of carbon nanotube/electrode by nanowelding

    NASA Astrophysics Data System (ADS)

    Zhao, Bo; Wang, Yanfang; Zhang, Yafei

    2017-03-01

    Reliable interconnection between carbon nanotubes (CNTs) and external circuit is one of the prerequisite in CNT electronics. In this work, ultrasonic nanowelding was used to bond CNTs with metal electrodes. By exerting ultrasonic energy at the interface of CNT/electrode, a reliable joint with negligible contact resistance was obtained between CNTs and electrodes. The performance of welding is susceptible to the ultrasonic parameters such as ultrasonic power and clamping force, as well as the metal type. It is found that the metals with good ductility or low melting point are easier to achieve effective joints. Moreover, interfacial compounds are formed at the welded surface of metal Al and Fe, which is resulted from the interacting and chemical bonding of carbon and metal atoms. After nanowelding, the contact resistance between CNTs and electrode is decreased dramatically, and the two-terminal resistance of the sample approximates to the intrinsic resistance of the CNT itself.

  14. Decrease of contact resistance at the interface of carbon nanotube/electrode by nanowelding

    NASA Astrophysics Data System (ADS)

    Zhao, Bo; Wang, Yanfang; Zhang, Yafei

    2016-12-01

    Reliable interconnection between carbon nanotubes (CNTs) and external circuit is one of the prerequisite in CNT electronics. In this work, ultrasonic nanowelding was used to bond CNTs with metal electrodes. By exerting ultrasonic energy at the interface of CNT/electrode, a reliable joint with negligible contact resistance was obtained between CNTs and electrodes. The performance of welding is susceptible to the ultrasonic parameters such as ultrasonic power and clamping force, as well as the metal type. It is found that the metals with good ductility or low melting point are easier to achieve effective joints. Moreover, interfacial compounds are formed at the welded surface of metal Al and Fe, which is resulted from the interacting and chemical bonding of carbon and metal atoms. After nanowelding, the contact resistance between CNTs and electrode is decreased dramatically, and the two-terminal resistance of the sample approximates to the intrinsic resistance of the CNT itself.

  15. Hole contacts on transition metal dichalcogenides: interface chemistry and band alignments.

    PubMed

    McDonnell, Stephen; Azcatl, Angelica; Addou, Rafik; Gong, Cheng; Battaglia, Corsin; Chuang, Steven; Cho, Kyeongjae; Javey, Ali; Wallace, Robert M

    2014-06-24

    MoOx shows promising potential as an efficient hole injection layer for p-FETs based on transition metal dichalcogenides. A combination of experiment and theory is used to study the surface and interfacial chemistry, as well as the band alignments for MoOx/MoS2 and MoOx/WSe2 heterostructures, using photoelectron spectroscopy, scanning tunneling microscopy, and density functional theory. A Mo(5+) rich interface region is identified and is proposed to explain the similar low hole Schottky barriers reported in a recent device study utilizing MoOx contacts on MoS2 and WSe2.

  16. Brittle Fracture Theory Predicts the Equation of Motion of Frictional Rupture Fronts

    NASA Astrophysics Data System (ADS)

    Svetlizky, Ilya; Kammer, David S.; Bayart, Elsa; Cohen, Gil; Fineberg, Jay

    2017-03-01

    We study rupture fronts propagating along the interface separating two bodies at the onset of frictional motion via high-temporal-resolution measurements of the real contact area and strain fields. The strain measurements provide the energy flux and dissipation at the rupture tips. We show that the classical equation of motion for brittle shear cracks, derived by balancing these quantities, well describes the velocity evolution of frictional ruptures. Our results demonstrate the extensive applicability of the dynamic brittle fracture theory to friction.

  17. Single-asperity friction during quasi-static sliding

    NASA Astrophysics Data System (ADS)

    Sharp, Tristan; Pastewka, Lars; Robbins, Mark

    2015-03-01

    The static friction of an asperity is investigated using atomic-scale simulations. We explore scale effects by varying the sphere radius R and the contact radius a from nanometers to micrometers. We first consider commensurate contact between bare lattices with repulsive interactions across the interface. In small contacts, all contacting atoms move coherently and the friction coefficient μ is independent of contact radius and load. In larger contacts, interfacial slip is mediated by localized dislocations, and the static friction coefficient μ ~ (Ra0/a2) 2 / 3 , where a0 is the nearest-neighbor spacing. In very large contacts μ stops decreasing and begins to increase with a, at fixed R. The results are in sharp contrast to Cattaneo-Mindlin continuum theory where μ is independent of contact size. Separate simulations are performed to connect the results to the dislocation-based models of contact-size effects due to Hurtado and Kim, and Gao, which assume adhesive interactions between surfaces and find μ ~ (a0/a)1/2. Simulations for incommensurate contacts show a transition from superlubricity for rigid contacts to a finite friction associated with the Peierls stress in very large contacts. Support from: DMR-1006805; NSF IGERT-0801471; OCI-0963185; CMMI-0923018

  18. Contact sensing from force measurements

    NASA Technical Reports Server (NTRS)

    Bicchi, Antonio; Salisbury, J. K.; Brock, David L.

    1993-01-01

    This article addresses contact sensing (i.e., the problem of resolving the location of a contact, the force at the interface, and the moment about the contact normals). Called 'intrinsic' contact sensing for the use of internal force and torque measurements, this method allows for practical devices that provide simple, relevant contact information in practical robotic applications. Such sensors have been used in conjunction with robot hands to identify objects, determine surface friction, detect slip, augment grasp stability, measure object mass, probe surfaces, and control collision and for a variety of other useful tasks. This article describes the theoretical basis for their operation and provides a framework for future device design.

  19. The structure of slip-pulses and supershear ruptures driving slip in bimaterial friction

    PubMed Central

    Shlomai, Hadar; Fineberg, Jay

    2016-01-01

    The most general frictional motion in nature involves bimaterial interfaces, when contacting bodies possess different elastic properties. Frictional motion occurs when the contacts composing the interface separating these bodies detach via propagating rupture fronts. Coupling between slip and normal stress variations is unique to bimaterial interfaces. Here we use high speed simultaneous measurements of slip velocities, real contact area and stresses to explicitly reveal this bimaterial coupling and its role in determining different classes of rupture modes and their structures. We directly observe slip-pulses, highly localized slip accompanied by large local reduction of the normal stress near the rupture tip. These pulses propagate in the direction of motion of the softer material at a selected (maximal) velocity and continuously evolve while propagating. In the opposite direction bimaterial coupling favors crack-like ‘supershear' fronts. The robustness of these structures shows the importance of bimaterial coupling to frictional motion and modes of frictional dissipation. PMID:27278687

  20. Interface Induced Gap State Models and ZnO Schottky Contacts

    NASA Astrophysics Data System (ADS)

    Durbin, Steven; Allen, Martin

    2010-03-01

    Practical aspects of fabricating Schottky contacts, such as lateral inhomogeneity, contaminants, and defects, can complicate the comparison of experimentally obtained barrier heights to theoretical predictions. The diode ideality factor η (which should approach unity for laterally homogeneous interfaces, after accounting for image force effects) is also strongly affected by the same issues, and correlations can be observed between barrier height and η when measuring large numbers of devices. ZnO could prove to be an interesting test case for evaluating various theoretical models, as it is significantly more ionic than most semiconductors, resulting in weaker Fermi pinning due to interface states. ZnO also does not require the removal of a native oxide layer for device processing, thereby avoiding often aggressive cleaning procedures. We have fabricated arrays of rectifying metal-ZnO contacts using bulk wafers and a wide variety of metals, using a technique which results in large barrier heights (typically > 0.8 eV) and low η (approaching the image force limit). Using the electrical characteristics of these diodes, we evaluate both Tung's chemical bonding and M"onch's metal induced gap states + electronegativity models. The lack of agreement with either of these popular models raises several questions, including whether predictions for the branch point energy in ZnO --- a parameter relevant to discussions of heterointerfaces as well as doping ability --- are accurate.

  1. Contact angles at the water-air interface of hydrocarbon-contaminated soils and clay minerals

    NASA Astrophysics Data System (ADS)

    Sofinskaya, O. A.; Kosterin, A. V.; Kosterina, E. A.

    2016-12-01

    Contact angles at the water-air interface have been measured for triturated preparations of clays and soils in order to assess changes in their hydrophobic properties under the effect of oil hydrocarbons. Tasks have been to determine the dynamics of contact angle under soil wetting conditions and to reveal the effect of chemical removal of organic matter from soils on the hydrophilicity of preparations. The potentialities of static and dynamic drop tests for assessing the hydrophilic-hydrophobic properties of soils have been estimated. Clays (kaolinite, gumbrine, and argillite) have been investigated, as well as plow horizons of soils from the Republic of Tatarstan: heavy loamy leached chernozem, medium loamy dark gray forest soil, and light loamy soddy-calcareous soil. The soils have been contaminated with raw oil and kerosene at rates of 0.1-3 wt %. In the uncontaminated and contaminated chernozem, capillary water capacity has been maintained for 250 days. The contact angles have been found to depend on the degree of dispersion of powdered preparation, the main type of clay minerals in the soil, the presence and amount of oxidation-resistant soil organic matter, and the soil-water contact time. Characteristic parameters of mathematical models for drop behavior on triturated preparations have been calculated. Contamination with hydrocarbons has resulted in a reliable increase in the contact angles of soil preparations. The hydrophobization of soil surface in chernozem is more active than in soils poorer in organic matter. The complete restoration of the hydrophilic properties of soils after hydrocarbon contamination is due to the oxidation of easily oxidizable organic matter at the low content of humus, or to wetting during several months in the absence of the mazut fraction.

  2. Brittle Asperities and Stick-Slip Motion: Insight from Friction Experiments along A Gabbro/Marble Interface

    NASA Astrophysics Data System (ADS)

    Xu, S.; Takizawa, S.; Fukuyama, E.; Yamashita, F.; Mizoguchi, K.; Kawakata, H.

    2015-12-01

    We conduct a series of meter-scale direct shear experiments along a gabbro/marble fault interface at NIED in Japan. Unlike the transitional behavior from stick-slip to stable sliding along a marble/marble interface under 1.3 MPa normal stress and 0.01 mm/s loading rate, the gabbro/marble case shows persistent stick-slip behavior under the same loading conditions as well as under 2.6 MPa normal stress in subsequent tests. Visual observations of the damage pattern reveal quite different features between the marble/marble case and the gabbro/marble case. For the former, the generated damage typically shows a low aspect ratio between loading-parallel and loading-perpendicular directions, suggesting that some diffusional deformation is effective during slip. For the latter, intruded gabbro pieces with preferred growing direction parallel to loading are distributed on top of the marble side, showing that hard rocks like gabbro can be partially fractured off when sheared against soft rocks like marble. Strain array data show that the apparent friction before failure is high or even above 1 near locations where fractured-off gabbro pieces are later observed, confirming that intact rock strength of gabbro has to be overcome upon the onset of fracture. Although at this moment we do not fully understand the behind mechanism, we believe that the brittleness of gabbro dominates in making the difference. If true, this result will highlight the role of brittle asperities in generating stick-slip fault behavior in a surrounding ductile-like environment. An analogous natural example may be found by the role of seamount in generating earthquakes through or underneath sediments in subduction zones (Cloos, 1992). However, instead of shearing off long-wavelength feature as illustrated by Cloos (1992), our study suggests that the collective behavior of tiny pieces along a nominally flat surface may also generate unstable ruptures macroscopically.

  3. The dynamics of the onset of frictional slip.

    PubMed

    Ben-David, Oded; Cohen, Gil; Fineberg, Jay

    2010-10-08

    The way in which a frictional interface fails is critical to our fundamental understanding of failure processes in fields ranging from engineering to the study of earthquakes. Frictional motion is initiated by rupture fronts that propagate within the thin interface that separates two sheared bodies. By measuring the shear and normal stresses along the interface, together with the subsequent rapid real-contact-area dynamics, we find that the ratio of shear stress to normal stress can locally far exceed the static-friction coefficient without precipitating slip. Moreover, different modes of rupture selected by the system correspond to distinct regimes of the local stress ratio. These results indicate the key role of nonuniformity to frictional stability and dynamics with implications for the prediction, selection, and arrest of different modes of earthquakes.

  4. Understanding Charge Transfer Reactions at the Interface of Plasmas in Contact with Liquids

    NASA Astrophysics Data System (ADS)

    Go, David

    2015-09-01

    Plasmas in and in contact with liquids offer a very rich physical and chemical environment where a multitude of species (electrons, ions, neutrals) and physical phenomena (light, electric fields) intersect. With emerging applications in medicine, environmental remediation, and materials synthesis, it has become paramount to understand the many processes occurring at the interface in order to design and optimize new technologies. Perhaps the most important plasma species is the electron, and it thus reasonable to assume it can play a critical role when plasmas are brought in contact with liquids as well. Over the past several years, our group has focused on deciphering the nature of electron transfer from a plasma to liquid and the subsequent chemistry the electrons induce. Our experimental configuration is the plasma equivalent of an electrochemical or electrolytic cell, where the cathode and anode are submerged in an electrolyte solution and current is carried by reduction reactions at the cathode and oxidation reactions at the anode. When the cathode is replaced by a plasma, the circuit is explicitly completed by the injection of plasma electrons into the solution where they stably solvate before inducing reduction reactions. Recently, we have demonstrated the first direct detection of these stably solvated electrons using a novel total internal reflection absorption spectroscopy experiment, resulting in the first measurement of the optical absorption spectrum for plasma-solvated electrons. Further, we have shown that the lifetime of these electrons can be significantly reduced if suitable solution- and plasma-phase scavengers are used to react quickly with these electrons. These results highlight the complexity of the plasma-liquid interface and how charge-transfer processes often compete with other chemistry that occurs at the plasma-liquid interface, such as the dissolution of plasma species into the liquid. This work was supported by the U.S. Army Research

  5. Micromachine friction test apparatus

    DOEpatents

    deBoer, Maarten P.; Redmond, James M.; Michalske, Terry A.

    2002-01-01

    A microelectromechanical (MEM) friction test apparatus is disclosed for determining static or dynamic friction in MEM devices. The friction test apparatus, formed by surface micromachining, is based on a friction pad supported at one end of a cantilevered beam, with the friction pad overlying a contact pad formed on the substrate. A first electrostatic actuator can be used to bring a lower surface of the friction pad into contact with an upper surface of the contact pad with a controlled and adjustable force of contact. A second electrostatic actuator can then be used to bend the cantilevered beam, thereby shortening its length and generating a relative motion between the two contacting surfaces. The displacement of the cantilevered beam can be measured optically and used to determine the static or dynamic friction, including frictional losses and the coefficient of friction between the surfaces. The test apparatus can also be used to assess the reliability of rubbing surfaces in MEM devices by producing and measuring wear of those surfaces. Finally, the friction test apparatus, which is small in size, can be used as an in situ process quality tool for improving the fabrication of MEM devices.

  6. 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.

  7. Near-Edge X-ray Absorption Fine Structure Imaging of Spherical and Flat Counterfaces of Ultrananocrystalline Diamond Tribological Contacts: A Correlation of Surface Chemistry and Friction

    SciTech Connect

    A Konicek; C Jaye; M Hamilton; W Sawyer; D Fischer; R Carpick

    2011-12-31

    A recently installed synchrotron radiation near-edge X-ray absorption fine structure (NEXAFS) full field imaging electron spectrometer was used to spatially resolve the chemical changes of both counterfaces from an ultra-nanocrystalline diamond (UNCD) tribological contact. A silicon flat and Si{sub 3}N{sub 4} sphere were both coated with UNCD, and employed to form two wear tracks on the flat in a linear reciprocating tribometer. The first wear track was produced using a new, unconditioned sphere whose surface was thus conditioned during this first experiment. This led to faster run-in and lower friction when producing a second wear track using the conditioned sphere. The large depth of field of the magnetically guided NEXAFS imaging detector enabled rapid, large area spectromicroscopic imaging of both the spherical and flat surfaces. Laterally resolved NEXAFS data from the tribological contact area revealed that both substrates had an as-grown surface layer that contained a higher fraction of sp{sup 2}-bonded carbon and oxygen which was mechanically removed. Unlike the flat, the film on the sphere showed evidence of having graphitic character, both before and after sliding. These results show that the graphitic character of the sphere is not solely responsible for low friction and short run-in. Rather, conditioning the sphere, likely by removing asperities and passivating dangling bonds, leads to lower friction with less chemical modification of the substrate in subsequent tests. The new NEXAFS imaging spectroscopy detector enabled a more complete understanding of the tribological phenomena by imaging, for the first time, the surface chemistry of the spherical counterface which had been in continual contact during wear track formation.

  8. Molecular structure of an alkyl-side-chain polymer-water interface: origins of contact angle hysteresis.

    PubMed

    Rangwalla, Hasnain; Schwab, Alexander D; Yurdumakan, Betül; Yablon, Dalia G; Yeganeh, Mohsen S; Dhinojwala, Ali

    2004-09-28

    A new and direct approach to verify surface heterogeneity as the microscopic origin of contact-angle hysteresis is demonstrated. IR-visible sum-frequency-generation spectroscopy (SFG) was used to selectively probe the molecules at the interface of an alkyl-side-chain polymer [poly(vinyl n-octadecyl carbamate-co-vinyl acetate)] with water. The spectra indicate that in contact with water, the polymer surface is heterogeneous (having areas of differing surface energies). This evidence of surface heterogeneity supports the hysteresis observed in the advancing and receding contact angles of the polymer surface with water. The same measurements made for the chemically and structurally similar surface of an octadecyltrichlorosilane self-assembled monolayer indicates a homogeneous surface at the water interface. In this case, contact-angle hysteresis measurements implicate surface roughness as the cause of hysteresis. Atomic force microscopy measurements of roughness for these surfaces further support our conclusions. The polymer-water interface was probed using SFG at above-ambient temperatures, and an order-to-disorder transition (ODT) of alkyl side chains at the interface was observed, which closely follows the melting of crystalline side chains in the bulk. This transition explains the increased wettability of the polymer, by water, when the temperature is raised above the bulk melting temperature. Furthermore, the irreversibility of this ODT suggests that the disordered polymer-water interface is the thermodynamic equilibrium state, whereas the before-heating structure of this interface is a kinetically hindered metastable state.

  9. Interface mobilities for characterization of structure-borne sound sources resiliently mounted via multiple contact points

    NASA Astrophysics Data System (ADS)

    Mathiowetz, S.; Bonhoff, H. A.

    2013-10-01

    This paper introduces an expansion of the interface mobility method for structure-borne sound sources that are resiliently mounted to a receiving structure at multiple discrete contact points. Subsystems of source, receiver and isolators are represented by Fourier transforms of their mobility matrices. Along with the source activity, a formulation for the transmitted power fed into the receiver is obtained. Furthermore, the source descriptor and a coupling function for resiliently mounted multi-point sources are obtained which both form recognized concepts for structure-borne sound source characterization. The method is based on a neglection of cross-order terms which describe a coupling between Fourier coefficients of different order. With regard to a subsystem of discretely placed isolators, cross-order terms were found to account for variations of isolator parameters along the contact points. In a set of numerical studies it was shown that errors introduced by neglecting cross-order terms are most pronounced in the higher frequency region where isolators show resonant behavior due to internal wave effects. A subsystem of identical isolators is completely described by equal-order terms, therefore no error is introduced considering such cases. Moreover, errors inherent in the rigidly mounted case due to cross-order source or receiver mobilities are reduced significantly when isolator mobilities are large compared to those from source and receiver. Findings from theoretical studies were validated in practical settings considering structure-borne sound transmission from a ship diesel engine and a magnetic resonance imaging (MRI) device.

  10. Argonne Engine Friction Study Phase 1 Final Report

    SciTech Connect

    Fox, Issac; Torbeck, Troy; Brogdon, Bill

    2002-01-01

    Argonne National Laboratory (ANL) has developed a process for making near frictionless carbon (NFC) coatings and depositing them on metal substrates. Friction reductions approaching an order of magnitude have been measured in laboratory tests. While there are many potential applications for such coatings, friction reduction in internal combustion engines is of particular interest due to the apparent fuel savings potential. Ricardo has performed a program of work to estimate potential fuel economy improvements due to the application of such a coating at key interfaces within a diesel engine typical of those found in large trucks. Piston, ring pack, and valvetrain simulations have been performed, using existing models of representative engines, with various degrees of friction reduction applied at important interfaces. The simulations were run at 8 specific operating points to allow approximation of engine performance over the FTP test cycle. Reduction in fuel consumption over the cycle was calculated for each reduced friction case. Results show that application of a friction-reducing surface treatment, like the NFC coatings, at the piston rings and skirt, and at key interfaces within the valvetrain, is expected to result in a reduction in fuel consumption of 0.43% to 0.81% over the FTP heavy duty test cycle. The piston skirt and piston rings are the interfaces where the coating can make the most difference, assuming no changes are made to the engine lubricant. Hydrodynamic friction represents a very large fraction of friction losses within the interfaces considered, at all operating conditions, indicating that changes to the engine lubricant, such as reduced viscosity, can result in further improvement. Reduced oil viscosity may result in increased metal-to-metal contact and wear, unless a durable, low friction coating can be applied at key interfaces. Ricardo recommends an analytical evaluation of the potential benefits of reduced oil viscosity, which considers

  11. 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

  12. 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 line type) 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 are observed on the ferrite surfaces as a result of sliding.

  13. 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.

  14. Nanoindentation study of buckling and friction of silicon nanolines

    NASA Astrophysics Data System (ADS)

    Luo, Zhiquan

    Silicon-based nanostructures are essential building blocks for nanoelectronic devices and nano-electromechanical systems (NEMS). As the silicon device size continues to scale down, the surface to volume ratio becomes larger, rendering the properties of surfaces and interfaces more important for improving the properties of the nano-devices and systems. One of those properties is the friction, which is important in controlling the functionality and reliability of the nano-device and systems. The goal of this dissertation is to investigate the deformation and friction behaviors of single crystalline silicon nanolines (SiNLs) using nanoindentation techniques. Following an introduction and a summary of the theoretical background of contact friction in Chapters 1 and 2, the results of this thesis are presented in three chapters. In Chapter 3, the fabrication of the silicon nanolines is described. The fabrication method yielded high-quality single-crystals with line width ranging from 30nm to 90nm and height to width aspect ratio ranging from 10 to 25. These SiNL structures have properties and dimensions well suited for the study of the mechanical and friction behaviors at the nanoscale. In Chapter 4, we describe the study of the mechanical properties of SiNLs using the nanoindentation method. The loading-displacement curves show that the critical load to induce the buckling of the SiNLs can be correlated to the contact friction and geometry of SiNLs. A map was built as a guideline to describe the selection of buckling modes. The map was divided into three regions where different regions correlate to different buckling modes including Mode I, Mode II and sliding-bending of SiNLs. In Chapter 5, we describe the study of the contact friction of the SiNL structures. The friction coefficient at the contact was extracted from the load-displacement curves. Subsequently, the frictional shear stress was evaluated. In addition, the effect of the interface between the indenter and

  15. Nanotribology and Nanoscale Friction

    SciTech Connect

    Guo, Yi; Qu, Zhihua; Braiman, Yehuda; Zhang, Zhenyu; Barhen, Jacob

    2008-01-01

    Tribology is the science and technology of contacting solid surfaces in relative motion, including the study of lubricants, lubrication, friction, wear, and bearings. It is estimated that friction and wear cost the U.S. economy 6% of the gross national product (Persson, 2000). For example, 5% of the total energy generated in an automobile engine is lost to frictional resistance. The study of nanoscale friction has a technological impact in reducing energy loss in machines, in microelectromechanical systems (MEMS), and in the development of durable, low-friction surfaces and ultra-thin lubrication films.

  16. Electronic properties of MoS2 / MoOx interfaces: Implications in Tunnel Field Effect Transistors and Hole Contacts

    DOE PAGES

    Santosh, K. C.; Longo, Roberto; Addou, Rafik; ...

    2016-09-26

    In an electronic device based on two dimensional (2D) transitional metal dichalcogenides (TMDs), finding a low resistance metal contact is critical in order to achieve the desired performance. However, due to the unusual Fermi level pinning in metal/2D TMD interface, the performance is limited. Here, we investigate the electronic properties of TMDs and transition metal oxide (TMO) interfaces (MoS2/MoO3) using density functional theory (DFT). Our results demonstrate that, due to the large work function of MoO3 and the relative band alignment with MoS2, together with small energy gap, the MoS2/MoO3 interface is a good candidate for a tunnel field effectmore » (TFET)-type device. Moreover, if the interface is not stoichiometric because of the presence of oxygen vacancies in MoO3, the heterostructure is more suitable for p-type (hole) contacts, exhibiting an Ohmic electrical behavior as experimentally demonstrated for different TMO/TMD interfaces. Our results reveal that the defect state induced by an oxygen vacancy in the MoO3 aligns with the valance band of MoS2, showing an insignificant impact on the band gap of the TMD. This result highlights the role of oxygen vacancies in oxides on facilitating appropriate contacts at the MoS2 and MoOx (x < 3) interface, which consistently explains the available experimental observations.« less

  17. Electronic properties of MoS2/MoOx interfaces: Implications in Tunnel Field Effect Transistors and Hole Contacts.

    PubMed

    K C, Santosh; Longo, Roberto C; Addou, Rafik; Wallace, Robert M; Cho, Kyeongjae

    2016-09-26

    In an electronic device based on two dimensional (2D) transitional metal dichalcogenides (TMDs), finding a low resistance metal contact is critical in order to achieve the desired performance. However, due to the unusual Fermi level pinning in metal/2D TMD interface, the performance is limited. Here, we investigate the electronic properties of TMDs and transition metal oxide (TMO) interfaces (MoS2/MoO3) using density functional theory (DFT). Our results demonstrate that, due to the large work function of MoO3 and the relative band alignment with MoS2, together with small energy gap, the MoS2/MoO3 interface is a good candidate for a tunnel field effect (TFET)-type device. Moreover, if the interface is not stoichiometric because of the presence of oxygen vacancies in MoO3, the heterostructure is more suitable for p-type (hole) contacts, exhibiting an Ohmic electrical behavior as experimentally demonstrated for different TMO/TMD interfaces. Our results reveal that the defect state induced by an oxygen vacancy in the MoO3 aligns with the valance band of MoS2, showing an insignificant impact on the band gap of the TMD. This result highlights the role of oxygen vacancies in oxides on facilitating appropriate contacts at the MoS2 and MoOx (x < 3) interface, which consistently explains the available experimental observations.

  18. Electronic properties of MoS2/MoOx interfaces: Implications in Tunnel Field Effect Transistors and Hole Contacts

    NASA Astrophysics Data System (ADS)

    K. C., Santosh; Longo, Roberto C.; Addou, Rafik; Wallace, Robert M.; Cho, Kyeongjae

    2016-09-01

    In an electronic device based on two dimensional (2D) transitional metal dichalcogenides (TMDs), finding a low resistance metal contact is critical in order to achieve the desired performance. However, due to the unusual Fermi level pinning in metal/2D TMD interface, the performance is limited. Here, we investigate the electronic properties of TMDs and transition metal oxide (TMO) interfaces (MoS2/MoO3) using density functional theory (DFT). Our results demonstrate that, due to the large work function of MoO3 and the relative band alignment with MoS2, together with small energy gap, the MoS2/MoO3 interface is a good candidate for a tunnel field effect (TFET)-type device. Moreover, if the interface is not stoichiometric because of the presence of oxygen vacancies in MoO3, the heterostructure is more suitable for p-type (hole) contacts, exhibiting an Ohmic electrical behavior as experimentally demonstrated for different TMO/TMD interfaces. Our results reveal that the defect state induced by an oxygen vacancy in the MoO3 aligns with the valance band of MoS2, showing an insignificant impact on the band gap of the TMD. This result highlights the role of oxygen vacancies in oxides on facilitating appropriate contacts at the MoS2 and MoOx (x < 3) interface, which consistently explains the available experimental observations.

  19. Electronic properties of MoS2/MoOx interfaces: Implications in Tunnel Field Effect Transistors and Hole Contacts

    PubMed Central

    K. C., Santosh; Longo, Roberto C.; Addou, Rafik; Wallace, Robert M.; Cho, Kyeongjae

    2016-01-01

    In an electronic device based on two dimensional (2D) transitional metal dichalcogenides (TMDs), finding a low resistance metal contact is critical in order to achieve the desired performance. However, due to the unusual Fermi level pinning in metal/2D TMD interface, the performance is limited. Here, we investigate the electronic properties of TMDs and transition metal oxide (TMO) interfaces (MoS2/MoO3) using density functional theory (DFT). Our results demonstrate that, due to the large work function of MoO3 and the relative band alignment with MoS2, together with small energy gap, the MoS2/MoO3 interface is a good candidate for a tunnel field effect (TFET)-type device. Moreover, if the interface is not stoichiometric because of the presence of oxygen vacancies in MoO3, the heterostructure is more suitable for p-type (hole) contacts, exhibiting an Ohmic electrical behavior as experimentally demonstrated for different TMO/TMD interfaces. Our results reveal that the defect state induced by an oxygen vacancy in the MoO3 aligns with the valance band of MoS2, showing an insignificant impact on the band gap of the TMD. This result highlights the role of oxygen vacancies in oxides on facilitating appropriate contacts at the MoS2 and MoOx (x < 3) interface, which consistently explains the available experimental observations. PMID:27666523

  20. A three-dimensional volume-of-fluid method for reconstructing and advecting three-material interfaces forming contact lines

    NASA Astrophysics Data System (ADS)

    Pathak, Ashish; Raessi, Mehdi

    2016-02-01

    We introduce a piecewise-linear, volume-of-fluid method for reconstructing and advecting three-dimensional interfaces and contact lines formed by three materials. The new method employs a set of geometric constructs that can be used in conjunction with any volume-tracking scheme. In this work, we used the mass-conserving scheme of Youngs to handle two-material cells, perform interface reconstruction in three-material cells, and resolve the contact line. The only information required by the method is the available volume fraction field. Although the proposed method is order dependent and requires a priori information on material ordering, it is suitable for typical contact line applications, where the material representing the contact surface is always known. Following the reconstruction of the contact surface, to compute the interface orientation in a three-material cell, the proposed method minimizes an error function that is based on volume fraction distribution around that cell. As an option, the minimization procedure also allows the user to impose a contact angle. Performance of the proposed method is assessed via both static and advection test cases. The tests show that the new method preserves the accuracy and mass-conserving property of the Youngs method in volume-tracking three materials.

  1. Novel experimental methods for investigating high speed friction of titanium-aluminum-vanadium/tool steel interface and dynamic failure of extrinsically toughened DRA composites

    NASA Astrophysics Data System (ADS)

    Irfan, Mohammad Abdulaziz

    Dynamic deformation, flow, and failure are integral parts of all dynamic processes in materials. Invariably, dynamic failure also involves the relative sliding of one component of the material over the other. Advances in elucidation of these failure mechanisms under high loading rates has been of great interest to scientists working in this area. The need to develop new dynamic mechanical property tests for materials under well characterized and controllable loading conditions has always been a challenge to experimentalists. The current study focuses on the development of two experimental methods to study some aspects of dynamic material response. The first part focuses on the development of a single stage gas gun facility for investigating high-speed metal to metal interfacial friction with applications to high speed machining. During the course of this investigation a gas gun was designed and built capable of accelerating projectiles upto velocities of 1 km/s. Using this gas gun pressure-shear plate impact friction experiments were conducted to simulate conditions similar to high speed machining at the tool-workpiece interface. The impacting plates were fabricated from materials representing the tribo-pair of interest. Accurate measurements of the interfacial tractions, i.e. the normal pressure and the frictional stress at the tribo-pair interface, and the interfacial slip velocity could be made by employing laser interferometry. Normal pressures of the order of 1-2 MPa were generated and slipping velocities of the order of 50 m/s were obtained. In order to illustrate the structure of the constitutive law governing friction, the study included experimental investigation of frictional response to step changes in normal pressure and interfacial shear stress. The results of these experiments indicate that sliding resistance for Ti6Al4V/CH steel interface is much lower than measured under quasi-static sliding conditions. Also the temperature at the interface strongly

  2. Phononic Origins of Friction in Carbon Nanotube Oscillators.

    PubMed

    Prasad, Matukumilli V D; Bhattacharya, Baidurya

    2017-03-01

    Phononic coupling can have a significant role in friction between nanoscale surfaces. We find frictional dissipation per atom in carbon nanotube (CNT) oscillators to depend significantly on interface features such as contact area, commensurability, and by end-capping of the inner core. We perform large-scale phonon wavepacket MD simulations to study phonon coupling between a 250 nm long (10,10) outer tube and inner cores of four different geometries. Five different phonon polarizations known to have dominant roles in thermal transport are selected, and transmission coefficient plots for a range of phonon energies along with phonon scattering dynamics at specific energies are obtained. We find that the length of interface affects friction only through LA phonon scattering and has a significant nonlinear effect on total frictional force. Incommensurate contact does not always give rise to superlubricity: the net effect of two competing interaction mechanisms shown by longitudinal and transverse phonons decides the role of commensurability. Capping of the core has no effect on acoustic phonons but destroys the coherence of transverse optical phonons and creates diffusive scattering. In contrast, the twisting and radial breathing phonon modes have perfect transmission at all energies and can be deemed as the enablers of ultralow friction in CNT oscillators. Our work suggests that tuning of interface geometries can give rise to desirable friction properties in nanoscale devices.

  3. Aftershocks in a frictional earthquake model.

    PubMed

    Braun, O M; Tosatti, Erio

    2014-09-01

    Inspired by spring-block models, we elaborate a "minimal" physical model of earthquakes which reproduces two main empirical seismological laws, the Gutenberg-Richter law and the Omori aftershock law. Our point is to demonstrate that the simultaneous incorporation of aging of contacts in the sliding interface and of elasticity of the sliding plates constitutes the minimal ingredients to account for both laws within the same frictional model.

  4. Friction damping of two-dimensional motion and its application in vibration control

    NASA Technical Reports Server (NTRS)

    Menq, C.-H.; Chidamparam, P.; Griffin, J. H.

    1991-01-01

    This paper presents an approximate method for analyzing the two-dimensional friction contact problem so as to compute the dynamic response of a structure constrained by friction interfaces. The friction force at the joint is formulated based on the Coulomb model. The single-term harmonic balance scheme, together with the receptance approach of decoupling the effect of the friction force on the structure from those of the external forces has been utilized to obtain the steady state response. The computational efficiency and accuracy of the method are demonstrated by comparing the results with long-term time solutions.

  5. Friction reduction using discrete surface textures: principle and design

    NASA Astrophysics Data System (ADS)

    Hsu, Stephen M.; Jing, Yang; Hua, Diann; Zhang, Huan

    2014-08-01

    There have been many reports on the use of dimples, grooves, and other surface textures to control friction in sliding interfaces. The effectiveness of surface textures in friction reduction has been demonstrated in conformal contacts under high speed low load applications such as mechanical seals and automotive water pump seals, etc., resulting in reduced friction and longer durability. For sliding components with higher contact pressures or lower speeds, conflicting results were reported. Reasons for the inconsistency may be due to the differences in texture fabrication techniques, lack of dimple size and shape uniformity, and different tester used. This paper examines the basic principles on which surface textural patterns influence friction under the three principle lubrication regimes: hydrodynamic, elastohydrodynamic, and boundary lubrication regimes. Our findings suggest that each regime requires specific dimple size, shape, depth, and areal density to achieve friction reduction. Control experiments were also conducted to explore mechanisms of friction reduction. The dimple geometric shape and the dimple's orientation with respect to the sliding direction influence friction significantly. The underlying mechanisms for friction control via textures are discussed.

  6. Determination of surface tension and contact angle from the shapes of axisymmetric fluid interfaces without use of apex coordinates.

    PubMed

    Cabezas, M Guadalupe; Bateni, Arash; Montanero, José M; Neumann, A Wilhelm

    2006-11-21

    Drop shape techniques, such as axisymmetric drop shape analysis, are widely used to measure surface properties, as they are accurate and reliable. Nevertheless, they are not applicable in experimental studies dealing with fluid configurations that do not present an apex. A new methodology is presented for measuring interfacial properties of liquids, such as surface tension and contact angles, by analyzing the shape of an axisymmetric liquid-fluid interface without use of apex coordinates. The theoretical shape of the interface is generated numerically as a function of surface tension and some geometrical parameters at the starting point of the interface, e.g., contact angle and radius of the interface. Then, the numerical shape is fitted to the experimental profile, taking the interfacial properties as adjustable parameters. The best fit identifies the true values of surface tension and contact angle. Comparison between the experimental and the theoretical profiles is performed using the theoretical image fitting analysis (TIFA) strategy. The new method, TIFA-axisymmetric interfaces (TIFA-AI), is applicable to any axisymmetric experimental configuration (with or without apex). The versatility and accuracy of TIFA-AI is shown by considering various configurations: liquid bridges, sessile and pendant drops, and liquid lenses.

  7. Characterization of the metal-semiconductor interface of gold contacts on CdZnTe formed by electroless deposition

    NASA Astrophysics Data System (ADS)

    Bell, Steven J.; Baker, Mark A.; Duarte, Diana D.; Schneider, Andreas; Seller, Paul; Sellin, Paul J.; Veale, Matthew C.; Wilson, Matthew D.

    2015-06-01

    Fully spectroscopic x/γ-ray imaging is now possible thanks to advances in the growth of wide-bandgap semiconductors. One of the most promising materials is cadmium zinc telluride (CdZnTe or CZT), which has been demonstrated in homeland security, medical imaging, astrophysics and industrial analysis applications. These applications have demanding energy and spatial resolution requirements that are not always met by the metal contacts deposited on the CdZnTe. To improve the contacts, the interface formed between metal and semiconductor during contact deposition must be better understood. Gold has a work function closely matching that of high resistivity CdZnTe and is a popular choice of contact metal. Gold contacts are often formed by electroless deposition however this forms a complex interface. The prior CdZnTe surface preparation, such as mechanical or chemo-mechanical polishing, and electroless deposition parameters, such as gold chloride solution temperature, play important roles in the formation of the interface and are the subject of the presented work. Techniques such as focused ion beam (FIB) cross section imaging, transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDS), x-ray photoelectron spectroscopy (XPS) and current  -  voltage (I-V) analysis have been used to characterize the interface. It has been found that the electroless reaction depends on the surface preparation and for chemo-mechanically polished (1 1 1) CdZnTe, it also depends on the A/B face identity. Where the deposition occurred at elevated temperature, the deposited contacts were found to produce a greater leakage current and suffered from increased subsurface voiding due to the formation of cadmium chloride.

  8. Real-time deflection and friction force imaging by bimorph-based resonance-type high-speed scanning force microscopy in the contact mode

    PubMed Central

    2014-01-01

    We report herein an alternative high-speed scanning force microscopy method in the contact mode based on a resonance-type piezoelectric bimorph scanner. The experimental setup, the modified optical beam deflection scheme suitable for smaller cantilevers, and a high-speed control program for simultaneous data capture are described in detail. The feature of the method is that the deflection and friction force images of the sample surface can be obtained simultaneously in real time. Images of various samples (e.g., a test grating, a thin gold film, and fluorine-doped tin oxide-coated glass slides) are acquired successfully. The imaging rate is 25 frames per second, and the average scan speed reaches a value of approximately 2.5 cm/s. The method combines the advantages of both observing the dynamic processes of the sample surface and monitoring the frictional properties on the nanometer scale. PACS 07.79.Lh; 07.79.Sp; 68.37.Ps PMID:25593555

  9. Friction and wear of selected metals and alloys in sliding contact with AISI 440 C stainless steel in liquid methane and in liquid natural gas

    NASA Technical Reports Server (NTRS)

    Wisander, D. W.

    1978-01-01

    Aluminum, titanium, beryllium, nickel, iron, copper, and several copper alloys were run in sliding contact with AISI 440C in liquid methane and natural gas. All of the metals run except copper and the copper alloys of tin and tin-lead showed severely galled wear scars. Friction coefficients varied from 0.2 to 1.0, the lowest being for copper, copper-17 wt. % tin, and copper-8 wt. % tin-22 wt. % lead. The wear rate for copper was two orders of magnitude lower than that of the other metals run. An additional order of magnitude of wear reduction was achieved by the addition of tin and/or lead to copper.

  10. Schottky barrier heights at the interfaces between pure-phase InAs nanowires and metal contacts

    SciTech Connect

    Feng, Boyong; Huang, Shaoyun E-mail: hqxu@pku.edu.cn; Wang, Jiyin; Pan, Dong; Zhao, Jianghua; Xu, H. Q. E-mail: hqxu@pku.edu.cn

    2016-02-07

    Understanding of the Schottky barriers formed at metal contact-InAs nanowire interfaces is of great importance for the development of high-performance InAs nanowire nanoelectronic and quantum devices. Here, we report a systematical study of InAs nanowire field-effect transistors (FETs) and the Schottky barrier heights formed at the contact-nanowire interfaces. The InAs nanowires employed are grown by molecular beam epitaxy and are high material quality single crystals, and the devices are made by directly contacting the nanowires with a series of metals of different work functions. The fabricated InAs nanowire FET devices are characterized by electrical measurements at different temperatures and the Schottky barrier heights are extracted from the measured temperature and gate-voltage dependences of the channel current. We show that although the work functions of the contact metals are widely spread, the Schottky barrier heights are determined to be distributed over 35–55 meV, showing a weak but not negligible dependence on the metals. The deduced Fermi level in the InAs nanowire channels is found to be in the band gap and very close to the conduction band. The physical origin of the results is discussed in terms of Fermi level pinning by the surface states of the InAs nanowires and a shift in pinned Fermi level induced by the metal-related interface states.

  11. Adhesion and friction of thin metal films

    NASA Technical Reports Server (NTRS)

    Buckley, D. H.

    1976-01-01

    Sliding friction experiments were conducted in vacuum with thin films of titanium, chromium, iron, and platinum sputter deposited on quartz or mica substrates. A single crystal hemispherically tipped gold slider was used in contact with the films at loads of 1.0 to 30.0 and at a sliding velocity of 0.7 mm/min at 23 C. Test results indicate that the friction coefficient is dependent on the adhesion of two interfaces, that between the film and its substrate and the slider and the film. There exists a relationship between the percent d bond character of metals in bulk and in thin film form and the friction coefficient. Oxygen can increase adhesive bonding of a metal film (platinum) to a substrate.

  12. Electronic properties of MoS2 / MoOx interfaces: Implications in Tunnel Field Effect Transistors and Hole Contacts

    SciTech Connect

    Santosh, K. C.; Longo, Roberto; Addou, Rafik; Wallace, Robert M.; Cho, Kyeongjae

    2016-09-26

    In an electronic device based on two dimensional (2D) transitional metal dichalcogenides (TMDs), finding a low resistance metal contact is critical in order to achieve the desired performance. However, due to the unusual Fermi level pinning in metal/2D TMD interface, the performance is limited. Here, we investigate the electronic properties of TMDs and transition metal oxide (TMO) interfaces (MoS2/MoO3) using density functional theory (DFT). Our results demonstrate that, due to the large work function of MoO3 and the relative band alignment with MoS2, together with small energy gap, the MoS2/MoO3 interface is a good candidate for a tunnel field effect (TFET)-type device. Moreover, if the interface is not stoichiometric because of the presence of oxygen vacancies in MoO3, the heterostructure is more suitable for p-type (hole) contacts, exhibiting an Ohmic electrical behavior as experimentally demonstrated for different TMO/TMD interfaces. Our results reveal that the defect state induced by an oxygen vacancy in the MoO3 aligns with the valance band of MoS2, showing an insignificant impact on the band gap of the TMD. This result highlights the role of oxygen vacancies in oxides on facilitating appropriate contacts at the MoS2 and MoOx (x < 3) interface, which consistently explains the available experimental observations.

  13. The frictional response of patterned soft polymer surfaces

    NASA Astrophysics Data System (ADS)

    Rand, Charles J.

    2008-10-01

    Friction plays an intricate role in our everyday lives, it is therefore critical to understand the underlying features of friction to better help control and manipulate the response anywhere two surfaces in contact move past each other by a sliding motion. Here we present results targeting a thorough understanding of soft material friction and how it can be manipulated with patterns. We found that the naturally occurring length scale or periodicity (lambda) of frictionally induced patterns, Schallamach waves, could be described using two materials properties (critical energy release rate Gc and complex modulus (E*), i.e. lambdainfinity Gc /E*). Following this, we evaluated the effect of a single defect at a sliding interface. Sliding over a defect can be used to model the sliding from one feature to another in a patterned surface. Defects decreased the sliding frictional force by as much as 80% sliding and this decrease was attributed to changes in tangential stiffness of the sliding interface. The frictional response of surface wrinkles, where multiple edges or defects are acting in concert, was also evaluated. Wrinkles were shown to decrease friction (F) and changes in contact area (A) could not describe this decrease. A tangential stiffness correction factor (fx) and changes in the critical energy release rate were used to describe this deviation (F infinity Gc *A*fx/ℓ, where ℓ is a materials defined length scale of dissipation). This scaling can be used to describe the friction of any topographically patterned surface including the Gecko's foot, where the feature size is smaller than ℓ and thus replaces ℓ, increasing the friction compared to a flat surface. Also, mechanically-induced surface defects were used to align osmotically driven surface wrinkles by creating stress discontinuities that convert the global biaxial stress state to local uniaxial stresses. Defect spacing was used to control the alignment process at the surface of the wrinkled rigid

  14. Discrete dislocation plasticity analysis of loading rate-dependent static friction

    NASA Astrophysics Data System (ADS)

    Song, H.; Deshpande, V. S.; Van der Giessen, E.

    2016-08-01

    From a microscopic point of view, the frictional force associated with the relative sliding of rough surfaces originates from deformation of the material in contact, by adhesion in the contact interface or both. We know that plastic deformation at the size scale of micrometres is not only dependent on the size of the contact, but also on the rate of deformation. Moreover, depending on its physical origin, adhesion can also be size and rate dependent, albeit different from plasticity. We present a two-dimensional model that incorporates both discrete dislocation plasticity inside a face-centred cubic crystal and adhesion in the interface to understand the rate dependence of friction caused by micrometre-size asperities. The friction strength is the outcome of the competition between adhesion and discrete dislocation plasticity. As a function of contact size, the friction strength contains two plateaus: at small contact length (≲0.6 μ m), the onset of sliding is fully controlled by adhesion while for large contact length (≳10 μ m), the friction strength approaches the size-independent plastic shear yield strength. The transition regime at intermediate contact size is a result of partial de-cohesion and size-dependent dislocation plasticity, and is determined by dislocation properties, interfacial properties as well as by the loading rate.

  15. Development and Integration of Single-Asperity Nanotribology Experiments & Nanoscale Interface Finite Element Modeling for Prediction and Control of Friction and Damage in Micro- and Nano-mechnical Systems

    SciTech Connect

    R.W. Carpick; M.E. Plesha

    2007-03-03

    This report describes the accomplishments of the DOE BES grant entitled "Development and Integration of Single-Asperity Nanotribology Experiments & Nanoscale Interface Finite Element Modeling for Prediction and Control of Friction and Damage in Micro- and Nano-mechnical Systems". Key results are: the determination of nanoscale frictional properties of MEMS surfaces, self-assembled monolayers, and novel carbon-based films, as well as the development of models to describe this behavior.

  16. Microstructural Characterization of the U-9.1Mo Fuel/AA6061 Cladding Interface in Friction-Bonded Monolithic Fuel Plates Irradiated in the RERTR-6 Experiment

    NASA Astrophysics Data System (ADS)

    Keiser, Dennis D.; Jue, Jan-Fong; Miller, Brandon; Gan, Jian; Robinson, Adam; Medvedev, Pavel; Madden, James; Wachs, Dan; Clark, Curtis; Meyer, Mitch

    2015-09-01

    Low-enrichment (235U < 20 pct) U-Mo monolithic fuel is being developed for use in research and test reactors. The earliest design for this fuel that was investigated via reactor testing consisted of a nominally U-10Mo fuel foil encased in AA6061 (Al-6061) cladding. For a fuel design to be deemed adequate for final use in a reactor, it must maintain dimensional stability and retain fission products throughout irradiation, which means that there must be good integrity at the fuel foil/cladding interface. To investigate the nature of the fuel/cladding interface for this fuel type after irradiation, fuel plates were fabricated using a friction bonding process, tested in INL's advanced test reactor (ATR), and then subsequently characterized using optical metallography, scanning electron microscopy, and transmission electron microscopy. Results of this characterization showed that the fuel/cladding interaction layers present at the U-Mo fuel/AA6061 cladding interface after fabrication became amorphous during irradiation. Up to two main interaction layers, based on composition, could be found at the fuel/cladding interface, depending on location. After irradiation, an Al-rich layer contained very few fission gas bubbles, but did exhibit Xe enrichment near the AA6061 cladding interface. Another layer, which contained more Si, had more observable fission gas bubbles. In the samples produced using a focused ion beam at the interaction zone/AA6061 cladding interface, possible indications of porosity/debonding were found, which suggested that the interface in this location is relatively weak.

  17. Is internal friction friction?

    USGS Publications Warehouse

    Savage, J.C.; Byerlee, J.D.; Lockner, D.A.

    1996-01-01

    Mogi [1974] proposed a simple model of the incipient rupture surface to explain the Coulomb failure criterion. We show here that this model can plausibly be extended to explain the Mohr failure criterion. In Mogi's model the incipient rupture surface immediately before fracture consists of areas across which material integrity is maintained (intact areas) and areas across which it is not (cracks). The strength of the incipient rupture surface is made up of the inherent strength of the intact areas plus the frictional resistance to sliding offered by the cracked areas. Although the coefficient of internal friction (slope of the strength versus normal stress curve) depends upon both the frictional and inherent strengths, the phenomenon of internal friction can be identified with the frictional part. The curvature of the Mohr failure envelope is interpreted as a consequence of differences in damage (cracking) accumulated in prefailure loading at different confining pressures.

  18. The Duality of Nanoscale Friction: Amontons' Law vs. Superlubricity

    NASA Astrophysics Data System (ADS)

    Schwarz, U. D.; Dietzel, D.; Ritter, C.; Monninghoff, T.; Fuchs, H.; Schirmeisen, A.

    2008-03-01

    One of the most fundamental questions in nanotribology is the contact area dependence of frictional forces on the nanometer scale. Unfortunately, conventional friction force microscopy techniques are limited for analyzing this problem due to the unknown and ill-defined tip-sample contact. This limitation can be circumvented by measuring the lateral force signal during the manipulation of nanoscale particles with a well-defined, clean contact to the substrate. In our study, the samples under investigation were metallic islands with diameters between 50-500 nm grown by thermal evaporation of antimony on highly oriented pyrolytic graphite (HOPG). Experiments that included the controlled manipulation of a large number of nanoparticles in ultrahigh vacuum show two distinct frictional states during particle sliding: While some particles show finite friction increasing linearly with interface area, thus reinforcing Amontons' law at the nanoscale, other particles assume a state of frictionless or `superlubric' sliding. Additional experiments revealed a similar result even in air, which can be explained by contamination effects of the interface that alter the frictional properties.

  19. Strongly Modulated Friction of a Film-Terminated Ridge-Channel Structure

    NASA Astrophysics Data System (ADS)

    He, Zhenping; Hui, Chung-Yuen; Levrard, Benjamin; Bai, Ying; Jagota, Anand

    2016-05-01

    Natural contacting surfaces have remarkable surface mechanical properties, which has led to the development of bioinspired surface structures using rubbery materials with strongly enhanced adhesion and static friction. However, sliding friction of structured rubbery surfaces is almost always significantly lower than that of a flat control, often due to significant loss of contact. Here we show that a film-terminated ridge-channel structure can strongly enhance sliding friction. We show that with properly chosen materials and geometrical parameters the near surface structure undergoes mechanical instabilities along with complex folding and sliding of internal interfaces, which is responsible for the enhancement of sliding friction. Because this structure shows no enhancement of adhesion under normal indentation by a sphere, it breaks the connection between energy loss during normal and shear loading. This makes it potentially interesting in many applications, for instance in tires, where one wishes to minimize rolling resistance (normal loading) while maximizing sliding friction (shear loading).

  20. Strongly Modulated Friction of a Film-Terminated Ridge-Channel Structure

    PubMed Central

    He, Zhenping; Hui, Chung-Yuen; Levrard, Benjamin; Bai, Ying; Jagota, Anand

    2016-01-01

    Natural contacting surfaces have remarkable surface mechanical properties, which has led to the development of bioinspired surface structures using rubbery materials with strongly enhanced adhesion and static friction. However, sliding friction of structured rubbery surfaces is almost always significantly lower than that of a flat control, often due to significant loss of contact. Here we show that a film-terminated ridge-channel structure can strongly enhance sliding friction. We show that with properly chosen materials and geometrical parameters the near surface structure undergoes mechanical instabilities along with complex folding and sliding of internal interfaces, which is responsible for the enhancement of sliding friction. Because this structure shows no enhancement of adhesion under normal indentation by a sphere, it breaks the connection between energy loss during normal and shear loading. This makes it potentially interesting in many applications, for instance in tires, where one wishes to minimize rolling resistance (normal loading) while maximizing sliding friction (shear loading). PMID:27226233

  1. In search of quantum-limited contact resistance: understanding the intrinsic and extrinsic effects on the graphene-metal interface

    NASA Astrophysics Data System (ADS)

    Nath, Anindya; Currie, Marc; Boyd, Anthony K.; Wheeler, Virginia D.; Koehler, Andrew D.; Tadjer, Marko J.; Robinson, Zachary R.; Sridhara, Karthik; Hernandez, Sandra C.; Wollmershauser, James A.; Robinson, Jeremy T.; Myers-Ward, Rachael L.; Rao, Mulpuri V.; Gaskill, D. Kurt

    2016-06-01

    Owing to its two-dimensional structure, graphene is extremely sensitive to surface contamination. Conventional processing techniques inevitably modify graphene’s intrinsic properties by introducing adsorbents and/or defects which limit device performance and understanding the intrinsic properties of graphene. Here we demonstrate femtosecond laser direct patterning of graphene microstructures, without the aid of resists or other chemicals, that enables us to study both intrinsic and extrinsic effects on the graphene-metal interface. The pulsed femtosecond laser was configured to ablate epitaxial graphene (EG) on a sub-micrometer scale and form a precisely defined region without damaging the surrounding material or substrate. The ablated area was sufficient to electrically isolate transfer length measurement structures and Hall devices for subsequent transport measurements. Using pristine and systematically contaminated surfaces, we found that Ni does not form bonds to EG synthesized on SiC in contrast to the well-known C-Ni bond formation for graphene synthesized on metals; known as end-contacting. Without end-contacting, the contact resistance (R C) of Ni to pristine and resist-contaminated EG are one and two orders of magnitude larger, respectively, than the intrinsic quantum limited contact resistance. The range of reported R C values is explained using carrier transmission probability, as exemplified by the Landauer-Büttiker model, which is dependent on the presence or absence of end-contacts and dopant/work-function mediated conduction. The model predicts the need for both end-contacts and a clean graphene-metal interface as necessary conditions to approach quantum limited contact resistance.

  2. 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.

  3. Competitive adsorption of surfactants and hydrophilic silica particles at the oil-water interface: interfacial tension and contact angle studies.

    PubMed

    Pichot, R; Spyropoulos, F; Norton, I T

    2012-07-01

    The effect of surfactants' type and concentration on the interfacial tension and contact angle in the presence of hydrophilic silica particles was investigated. Silica particles have been shown to have an antagonistic effect on interfacial tension and contact angle in the presence of both W/O and O/W surfactants. Silica particles, combined with W/O surfactant, have no effect on interfacial tension, which is only dictated by the surfactant concentration, while they strongly affect interfacial tension when combined with O/W surfactants. At low O/W surfactant, both particles and surfactant are adsorbed at the interface, modifying the interface structure. At higher concentration, interfacial tension is only dictated by the surfactant. By increasing the surfactant concentration, the contact angle that a drop of aqueous phase assumes on a glass substrate placed in oil media decreases or increases depending on whether the surfactant is of W/O or O/W type, respectively. This is due to the modification of the wettability of the glass by the oil or water induced by the surfactants. Regardless of the surfactant's type, the contact angle profile was dictated by both particles and surfactant at low surfactant concentration, whereas it is dictated by the surfactant only at high concentration.

  4. Reducing contact resistance at semiconductor to metal or aluminum to metal interfaces

    NASA Technical Reports Server (NTRS)

    Keller, K. R.

    1969-01-01

    Etchant containing chloroplatinous or chloroplatinic acid greatly reduces contact resistance between metallic surfaces. Etching results in a monolayer plating of platinum on the wafer surface, preventing oxide growth.

  5. The adhesion and hysteresis effect in friction skin with artificial materials

    NASA Astrophysics Data System (ADS)

    Subhi, K. A.; Tudor, A.; Hussein, E. K.; Wahad, H. S.

    2017-02-01

    Human skin is a soft biomaterial with a complex anatomical structure and it has a complex material behavior during the mechanical contact with objects and surfaces. The friction adhesion component is defined by means of the theories of Johnson-Kendall-Roberts (JKR), Derjaguin-Muller-Toporov (DMT) and Maugis – Dugdale (MD). We shall consider the human skin entering into contact with a rigid surface. The deformation (hysteresis) component of the skin friction is evaluated with Voigt rheological model for the spherical contact, with the original model, developed in MATHCAD software. The adhesive component of the skin friction is greater than the hysteresis component for all friction parameters (load, velocity, the strength of interface between skin and the artificial material).

  6. Interfacial closure of contacting surfaces

    NASA Astrophysics Data System (ADS)

    Rieutord, F.; Rauer, C.; Moriceau, H.

    2014-08-01

    Understanding the contact between solid surfaces is a long-standing problem which has a strong impact on the physics of many processes such as adhesion, friction, lubrication and wear. Experimentally, the investigation of solid/solid interfaces remains challenging today, due to the lack of experimental techniques able to provide sub-nanometer scale information on interfaces buried between millimeters of materials. Yet, a strong interest exists improving the modeling of contact mechanics of materials in order to adjust their interface properties (e.g., thermal transport, friction). We show here that the essential features of the residual gap between contacting surfaces can be measured using high energy X-ray synchrotron reflectivity. The presence of this nano-gap is general to the contact of solids. In some special case however, it can be removed when attractive forces take over repulsive contributions, depending on both height and wavelength of asperity distributions (roughness). A criterion for this instability is established in the standard case of van der Waals attractive forces and elastic asperity compression repulsive forces (Hertz model). This collapse instability is confirmed experimentally in the case of silicon direct bonding, using high-energy X-ray synchrotron reflectivity and adhesion energy measurements. The possibility to achieve fully closed interfaces at room temperature opens interesting perspectives to build stronger assemblies with smaller thermal budgets.

  7. Effect of Interface Curvature on Turbulent Skin-Friction Drag Reduction with Super-Hydrophobic Micro-Grooves

    NASA Astrophysics Data System (ADS)

    Akhavan, Rayhaneh; Rastegari, Amirreza

    2016-11-01

    Effect of interface curvature on Drag Reduction (DR) with Super-Hydrophobic (SH) Micro-Grooves (MGs) was investigated by DNS with lattice Boltzmann methods. The liquid/gas interfaces in the SH MGs were modeled as curved, stationary, shear-free boundaries, with the interface shape determined from the Young-Laplace equation. The full range of interface protrusion angles, ranging from 0° to -90o , were investigated. DRs of 35% to 63% were realized in DNS, in turbulent channel flows at a Rebulk = 7200 (Reτ0 222) with longitudinal MGs of size 14 <=g+0 <= 56 & g+0 /w+0 = 7 on both walls, where g+0 and w+0 denote the widths and spacings of the MGs, in wall units of the base flow, respectively. The presence of interface curvature led to increases of 2.3% to 4.5% in the magnitude of DR, and drops of -3.5% to -13.5% in the slip velocity, at low protrusion angles, and drops of -2.2% to -12.5% in the magnitude of DR, and either drops of up to -16.5% or increases of up to 6% in the slip velocity, at high protrusion angles, compared to flat interfaces. In addition, the instantaneous pressure fluctuations on curved SH interfaces at low protrusion angles were significantly lower (by a factor of 2) than those on flat interfaces.

  8. Contact variables for exposure to avian influenza H5N1 virus at the human-animal interface.

    PubMed

    Rabinowitz, P; Perdue, M; Mumford, E

    2010-06-01

    Although the highly pathogenic avian influenza H5N1 virus continues to cause infections in both avian and human populations, the specific zoonotic risk factors remain poorly understood. This review summarizes available evidence regarding types of contact associated with transmission of H5N1 virus at the human-animal interface. A systematic search of the published literature revealed five analytical studies and 15 case reports describing avian influenza transmission from animals to humans for further review. Risk factors identified in analytical studies were compared, and World Health Organization-confirmed cases, identified in case reports, were classified according to type of contact reported using a standardized algorithm. Although cases were primarily associated with direct contact with sick/unexpectedly dead birds, some cases reported only indirect contact with birds or contaminated environments or contact with apparently healthy birds. Specific types of contacts or activities leading to exposure could not be determined from data available in the publications reviewed. These results support previous reports that direct contact with sick birds is not the only means of human exposure to avian influenza H5N1 virus. To target public health measures and disease awareness messaging for reducing the risk of zoonotic infection with avian influenza H5N1 virus, the specific types of contacts and activities leading to transmission need to be further understood. The role of environmental virus persistence, shedding of virus by asymptomatic poultry and disease pathophysiology in different avian species relative to human zoonotic risk, as well as specific modes of zoonotic transmission, should be determined.

  9. Friction laws at the nanoscale.

    PubMed

    Mo, Yifei; Turner, Kevin T; Szlufarska, Izabela

    2009-02-26

    Macroscopic laws of friction do not generally apply to nanoscale contacts. Although continuum mechanics models have been predicted to break down at the nanoscale, they continue to be applied for lack of a better theory. An understanding of how friction force depends on applied load and contact area at these scales is essential for the design of miniaturized devices with optimal mechanical performance. Here we use large-scale molecular dynamics simulations with realistic force fields to establish friction laws in dry nanoscale contacts. We show that friction force depends linearly on the number of atoms that chemically interact across the contact. By defining the contact area as being proportional to this number of interacting atoms, we show that the macroscopically observed linear relationship between friction force and contact area can be extended to the nanoscale. Our model predicts that as the adhesion between the contacting surfaces is reduced, a transition takes place from nonlinear to linear dependence of friction force on load. This transition is consistent with the results of several nanoscale friction experiments. We demonstrate that the breakdown of continuum mechanics can be understood as a result of the rough (multi-asperity) nature of the contact, and show that roughness theories of friction can be applied at the nanoscale.

  10. Friction at small displacement.

    NASA Technical Reports Server (NTRS)

    Campbell, W. E.; Aronstein, J.

    1972-01-01

    Low contact resistance between metal surfaces is often observed in spite of interposed lubricant and/or oxide films. To study this effect an apparatus is used with which normal force and tangential microdisplacement are applied between a small lead rider and a gold flat with various surface film conditions. Under nonoxidized and nonlubricated conditions, and with either oxide or stearic acid lubricant film alone, friction is high and contact resistance is low. With oxide and lubricant together, friction is much lower and slide is smooth, but contact resistance remains low and Ohm's law is obeyed. The results are consistent with Greenwood's theory of contact resistance for a cluster of minute metallic contact spots within the load-supporting area. The contact resistance of such a cluster is indistinguishable, for practical purposes, from that given by complete metallic contact.

  11. Atomistics of friction

    NASA Astrophysics Data System (ADS)

    Hirano, M.

    2006-03-01

    When two solid bodies contact and slide against each other, a frictional phenomenon occurs. There have been two models for the origin of the friction forces: the surface roughness model and Tomlinson's model. The surface roughness model explains the origin of the static friction force; contacting solid surfaces are so rough that surface asperities are mechanically locked against the gravitational force. From an atomistic point of view, Tomlinson explained a mechanism of the energy dissipation for the origin of the dynamic friction force. The atomistic mechanisms are described for the origin of the static and the dynamic friction forces, based on the theoretical conclusion that Tomlinson's mechanism is unlikely to occur in realistic frictional systems. The mechanism for the origin of the static friction force resembles the mechanical locking mechanism in a surface roughness model. The origin of the dynamic friction force is formulated as a problem of how the given translational kinetic energy dissipates into the internal relative motions of constituent atoms of bodies during sliding. From studying the available phase space volume of the translational motion becomes negligibly small for a large system size, compared with that of the internal motions, it is concluded that the energy dissipation occurs irreversibly from the translational motion to the internal motions. The comparison of the atomistic mechanisms with the surface roughness model and Tomlinson's model is discussed. A phenomenon of superlubricity, where two solid bodies move relatively with no resistance, is discussed.

  12. An Alternative Frictional Boundary Condition for Computational Fluid Dynamics Simulation of Friction Stir Welding

    NASA Astrophysics Data System (ADS)

    Chen, Gaoqiang; Feng, Zhili; Zhu, Yucan; Shi, Qingyu

    2016-09-01

    For better application of numerical simulation in optimization and design of friction stir welding (FSW), this paper presents a new frictional boundary condition at the tool/workpiece interface for computational fluid dynamics (CFD) modeling of FSW. The proposed boundary condition is based on an implementation of the Coulomb friction model. Using the new boundary condition, the CFD simulation yields non-uniform distribution of contact state over the tool/workpiece interface, as validated by the experimental weld macrostructure. It is found that interfacial sticking state is present over large area at the tool-workpiece interface, while significant interfacial sliding occurs at the shoulder periphery, the lower part of pin side, and the periphery of pin bottom. Due to the interfacial sticking, a rotating flow zone is found under the shoulder, in which fast circular motion occurs. The diameter of the rotating flow zone is smaller than the shoulder diameter, which is attributed to the presence of the interfacial sliding at the shoulder periphery. For the simulated welding condition, the heat generation due to friction and plastic deformation makes up 54.4 and 45.6% of the total heat generation rate, respectively. The simulated temperature field is validated by the good agreement to the experimental measurements.

  13. Metal-graphene-metal sandwich contacts for enhanced interface bonding and work function control.

    PubMed

    Gong, Cheng; Hinojos, David; Wang, Weichao; Nijem, Nour; Shan, Bin; Wallace, Robert M; Cho, Kyeongjae; Chabal, Yves J

    2012-06-26

    Only a small fraction of all available metals has been used as electrode materials for carbon-based devices due to metal-graphene interface debonding problems. We report an enhancement of the bonding energy of weakly interacting metals by using a metal-graphene-metal sandwich geometry, without sacrificing the intrinsic π-electron dispersions of graphene that is usually undermined by strong metal-graphene interface hybridization. This sandwich structure further makes it possible to effectively tune the doping of graphene with an appropriate selection of metals. Density functional theory calculations reveal that the strengthening of the interface interaction is ascribed to an enhancement of interface dipole-dipole interactions. Raman scattering studies of metal-graphene-copper sandwiches are used to validate the theoretically predicted tuning of graphene doping through sandwich structures.

  14. Scalar model for frictional precursors dynamics

    PubMed Central

    Taloni, Alessandro; Benassi, Andrea; Sandfeld, Stefan; Zapperi, Stefano

    2015-01-01

    Recent experiments indicate that frictional sliding occurs by nucleation of detachment fronts at the contact interface that may appear well before the onset of global sliding. This intriguing precursory activity is not accounted for by traditional friction theories but is extremely important for friction dominated geophysical phenomena as earthquakes, landslides or avalanches. Here we simulate the onset of slip of a three dimensional elastic body resting on a surface and show that experimentally observed frictional precursors depend in a complex non-universal way on the sample geometry and loading conditions. Our model satisfies Archard's law and Amontons' first and second laws, reproducing with remarkable precision the real contact area dynamics, the precursors' envelope dynamics prior to sliding, and the normal and shear internal stress distributions close to the interfacial surface. Moreover, it allows to assess which features can be attributed to the elastic equilibrium, and which are attributed to the out-of-equilibrium dynamics, suggesting that precursory activity is an intrinsically quasi-static physical process. A direct calculation of the evolution of the Coulomb stress before and during precursors nucleation shows large variations across the sample, explaining why earthquake forecasting methods based only on accumulated slip and Coulomb stress monitoring are often ineffective. PMID:25640079

  15. Scalar model for frictional precursors dynamics.

    PubMed

    Taloni, Alessandro; Benassi, Andrea; Sandfeld, Stefan; Zapperi, Stefano

    2015-02-02

    Recent experiments indicate that frictional sliding occurs by nucleation of detachment fronts at the contact interface that may appear well before the onset of global sliding. This intriguing precursory activity is not accounted for by traditional friction theories but is extremely important for friction dominated geophysical phenomena as earthquakes, landslides or avalanches. Here we simulate the onset of slip of a three dimensional elastic body resting on a surface and show that experimentally observed frictional precursors depend in a complex non-universal way on the sample geometry and loading conditions. Our model satisfies Archard's law and Amontons' first and second laws, reproducing with remarkable precision the real contact area dynamics, the precursors' envelope dynamics prior to sliding, and the normal and shear internal stress distributions close to the interfacial surface. Moreover, it allows to assess which features can be attributed to the elastic equilibrium, and which are attributed to the out-of-equilibrium dynamics, suggesting that precursory activity is an intrinsically quasi-static physical process. A direct calculation of the evolution of the Coulomb stress before and during precursors nucleation shows large variations across the sample, explaining why earthquake forecasting methods based only on accumulated slip and Coulomb stress monitoring are often ineffective.

  16. Thermal contact resistance at the Nb/Cu interface as a limiting factor for sputtered thin film RF superconducting cavities

    NASA Astrophysics Data System (ADS)

    Palmieri, V.; Vaglio, R.

    2016-01-01

    The ‘Q-slope’ problem has so far strongly limited the application of niobium thin film sputtered copper cavities in high field accelerators. In the present paper, based on experimental evidence, we consider the hypothesis that the Q-slope is related to enhanced thermal boundary resistance R Nb/Cu at the Nb/Cu interface, due to poor thermal contact between film and substrate. We have developed a simple model that directly connects the Q versus E acc curves to the distribution function f(R Nb/Cu) of R Nb/Cu values at the Nb/Cu interface over the cavity surface. Starting from different Q versus E acc experimental curves from different sources, using typical ‘inverse problem’ methods, we deduce the corresponding distribution functions generating those curves. The results show, for all the examined cases, very similar functional dependences of f(R Nb/Cu) and prove that, to describe the experimental Q versus E acc curves, it is sufficient to assume that only a small fraction of the film over the cavity surface is in poor thermal contact with the substrate. The whole body of information and data reported seems to indicate that the main origin of the Q-slope in thin film cavities is related to bad adhesion at the Nb/Cu interface. Strategies to solve the Q-slope problem improving the film adhesion are finally delineated.

  17. Load and Time Dependence of Interfacial Chemical Bond-Induced Friction at the Nanoscale

    NASA Astrophysics Data System (ADS)

    Tian, Kaiwen; Gosvami, Nitya N.; Goldsby, David L.; Liu, Yun; Szlufarska, Izabela; Carpick, Robert W.

    2017-02-01

    Rate and state friction (RSF) laws are widely used empirical relationships that describe the macroscale frictional behavior of a broad range of materials, including rocks found in the seismogenic zone of Earth's crust. A fundamental aspect of the RSF laws is frictional "aging," where friction increases with the time of stationary contact due to asperity creep and/or interfacial strengthening. Recent atomic force microscope (AFM) experiments and simulations found that nanoscale silica contacts exhibit aging due to the progressive formation of interfacial chemical bonds. The role of normal load (and, thus, normal stress) on this interfacial chemical bond-induced (ICBI) friction is predicted to be significant but has not been examined experimentally. Here, we show using AFM that, for nanoscale ICBI friction of silica-silica interfaces, aging (the difference between the maximum static friction and the kinetic friction) increases approximately linearly with the product of the normal load and the log of the hold time. This behavior is attributed to the approximately linear dependence of the contact area on the load in the positive load regime before significant wear occurs, as inferred from sliding friction measurements. This implies that the average pressure, and thus the average bond formation rate, is load independent within the accessible load range. We also consider a more accurate nonlinear model for the contact area, from which we extract the activation volume and the average stress-free energy barrier to the aging process. Our work provides an approach for studying the load and time dependence of contact aging at the nanoscale and further establishes RSF laws for nanoscale asperity contacts.

  18. Partial squeeze film levitation modulates fingertip friction.

    PubMed

    Wiertlewski, Michaël; Fenton Friesen, Rebecca; Colgate, J Edward

    2016-08-16

    When touched, a glass plate excited with ultrasonic transverse waves feels notably more slippery than it does at rest. To study this phenomenon, we use frustrated total internal reflection to image the asperities of the skin that are in intimate contact with a glass plate. We observed that the load at the interface is shared between the elastic compression of the asperities of the skin and a squeeze film of air. Stroboscopic investigation reveals that the time evolution of the interfacial gap is partially out of phase with the plate vibration. Taken together, these results suggest that the skin bounces against the vibrating plate but that the bounces are cushioned by a squeeze film of air that does not have time to escape the interfacial separation. This behavior results in dynamic levitation, in which the average number of asperities in intimate contact is reduced, thereby reducing friction. This improved understanding of the physics of friction reduction provides key guidelines for designing interfaces that can dynamically modulate friction with soft materials and biological tissues, such as human fingertips.

  19. Partial squeeze film levitation modulates fingertip friction

    PubMed Central

    Wiertlewski, Michaël; Fenton Friesen, Rebecca; Colgate, J. Edward

    2016-01-01

    When touched, a glass plate excited with ultrasonic transverse waves feels notably more slippery than it does at rest. To study this phenomenon, we use frustrated total internal reflection to image the asperities of the skin that are in intimate contact with a glass plate. We observed that the load at the interface is shared between the elastic compression of the asperities of the skin and a squeeze film of air. Stroboscopic investigation reveals that the time evolution of the interfacial gap is partially out of phase with the plate vibration. Taken together, these results suggest that the skin bounces against the vibrating plate but that the bounces are cushioned by a squeeze film of air that does not have time to escape the interfacial separation. This behavior results in dynamic levitation, in which the average number of asperities in intimate contact is reduced, thereby reducing friction. This improved understanding of the physics of friction reduction provides key guidelines for designing interfaces that can dynamically modulate friction with soft materials and biological tissues, such as human fingertips. PMID:27482117

  20. The influence of fault geometry and frictional contact properties on slip surface behavior and off-fault damage: insights from quasi-static modeling of small strike-slip faults from the Sierra Nevada, CA

    NASA Astrophysics Data System (ADS)

    Ritz, E.; Pollard, D. D.

    2011-12-01

    Geological and geophysical investigations demonstrate that faults are geometrically complex structures, and that the nature and intensity of off-fault damage is spatially correlated with geometric irregularities of the slip surfaces. Geologic observations of exhumed meter-scale strike-slip faults in the Bear Creek drainage, central Sierra Nevada, CA, provide insight into the relationship between non-planar fault geometry and frictional slip at depth. We investigate natural fault geometries in an otherwise homogeneous and isotropic elastic material with a two-dimensional displacement discontinuity method (DDM). Although the DDM is a powerful tool, frictional contact problems are beyond the scope of the elementary implementation because it allows interpenetration of the crack surfaces. By incorporating a complementarity algorithm, we are able to enforce appropriate contact boundary conditions along the model faults and include variable friction and frictional strength. This tool allows us to model quasi-static slip on non-planar faults and the resulting deformation of the surrounding rock. Both field observations and numerical investigations indicate that sliding along geometrically discontinuous or irregular faults may lead to opening of the fault and the formation of new fractures, affecting permeability in the nearby rock mass and consequently impacting pore fluid pressure. Numerical simulations of natural fault geometries provide local stress fields that are correlated to the style and spatial distribution of off-fault damage. We also show how varying the friction and frictional strength along the model faults affects slip surface behavior and consequently influences the stress distributions in the adjacent material.

  1. Use of separate ZnTe interface layers to form OHMIC contacts to p-CdTe films

    DOEpatents

    Gessert, Timothy A.

    1999-01-01

    A method of improving electrical contact to a thin film of a p-type tellurium-containing II-VI semiconductor comprising: depositing a first undoped layer of ZnTe on a thin film of p-type tellurium containing II-VI semiconductor with material properties selected to limit the formation of potential barriers at the interface between the p-CdTe and the undoped layer, to a thickness sufficient to control diffusion of the metallic-doped ZnTe into the p-type tellurim-containing II-VI semiconductor, but thin enough to minimize affects of series resistance; depositing a second heavy doped p-type ZnTe layer to the first layer using an appropriate dopant; and depositing an appropriate metal onto the outer-most surface of the doped ZnTe layer for connecting an external electrical conductor to an ohmic contact.

  2. Use of separate ZnTe interface layers to form ohmic contacts to p-CdTe films

    DOEpatents

    Gessert, T.A.

    1999-06-01

    A method of is disclosed improving electrical contact to a thin film of a p-type tellurium-containing II-VI semiconductor comprising: depositing a first undoped layer of ZnTe on a thin film of p-type tellurium containing II-VI semiconductor with material properties selected to limit the formation of potential barriers at the interface between the p-CdTe and the undoped layer, to a thickness sufficient to control diffusion of the metallic-doped ZnTe into the p-type tellurium-containing II-VI semiconductor, but thin enough to minimize affects of series resistance; depositing a second heavy doped p-type ZnTe layer to the first layer using an appropriate dopant; and depositing an appropriate metal onto the outer-most surface of the doped ZnTe layer for connecting an external electrical conductor to an ohmic contact. 11 figs.

  3. Research on the relation between the contact angle and the interface curvature radius of electrowetting liquid zoom lens

    NASA Astrophysics Data System (ADS)

    Zhao, Cunhua; Liang, Huiqin; Cui, Dongqing; Hong, Xinhua; Wei, Daling; Gao, Changliu

    2011-08-01

    In the ultralight or ultrathin applied domain of zoom lens, the traditional glass / plastic lens is limited for manufacture technology or cost. Therefore, a liquid lens was put forward to solve the problems. The liquid zoom lens has the merits of lower cost, smaller volume, quicker response, lower energy consumption, continuous zoom and higher accuracy. In liquid zoom lens the precise focal length is obtained by the contact angle changing to affect the curvature radius of interface. In our works, the relations of the exerted voltage, the contact angle, the curvature radius and the focal length were researched and accurately calculated. The calculation of the focal length provides an important theoretical basis for instructing the design of liquid zoom lens.

  4. Investigation and modeling of frictional boundary conditions in oblique cutting of aluminum alloys

    NASA Astrophysics Data System (ADS)

    Kilic, Dursun Sedat

    Friction at the cutting tool interface has been studied for 60 years, yet an accurate model of friction is largely unavailable, especially in operations such as turning, where the interface is inaccessible due the continuous contact between chip and tool. A historical perspective of friction in turning is provided to better understand the purpose of this thesis. The contradictions arising from different frictional boundary condition assumptions in machining were analyzed. Experimental observations were substantiated in the light of the literature review. Friction conditions at the tool chip interface were found to be more complex than the simple models of seizure followed by sliding, which is accepted in most machining models. This thesis investigated the surface topology of cutting tools in conventional turning operation, which is one of the oldest and common machining processes. Two different aluminum alloys Al-2024 and Al-6061 were used in turning experiments with carbide tools to define the frictional conditions as these alloys exhibited a wide range of frictional contacts at different machining conditions. Experiments were conducted using carbide cutting tools at a range of speeds, feed rates, and depths of cut, which are commonly utilized in industrial applications. The analysis of tool chip interface at microscopic levels revealed further details of seizure and sliding zone formation. Newer techniques developed in microscopy and surface characterization were used to characterize the interface in a non-destructive manner. Scanning electron microscopy (SEM), surface profilometer and laser scanning confocal microscopy (LSCM) techniques helped us in the understanding of the frictional boundaries. Analysis of SEM images obtained by turning experiments revealed three distinct regions whose topology is closely related to turning parameters. These different zones were named as primary sticking zone, sliding zone and secondary sticking zone. Furthermore, with the

  5. Properties of ferrites important to their friction and wear behavior

    NASA Technical Reports Server (NTRS)

    Miyoshi, K.; Buckley, D. H.

    1983-01-01

    Environmental, chemical and crystallographical effects on the fundamental nature on friction and wear of the ferrites in contact with metals, magnetic tapes and themselves are reviewed. The removal of adsorbed films from the surfaces of ferrites results in very strong interfacial adhesion and high friction in ferrite to metal and ferrite to magnetic tape contacts. The metal ferrite bond at the interface is primarily a chemical bond between the metal atoms and the large oxygen anions in the ferrite surface, and the strength of these bonds is related to the oxygen to metal bond strength in the metal oxide. The more active the metal, the higher is the coefficient of friction. Not only under adhesive conditions, but also under abrasive conditions the friction and wear properties of ferrites are related to the crystallographic orientation. With ferrite to ferrite contact the mating of highest atomic density (most closely packed) direction on matched crystallographic planes, that is, 110 directions on /110/planes, results in the lowest coefficient of friction.

  6. Perception and Haptic Rendering of Friction Moments.

    PubMed

    Kawasaki, H; Ohtuka, Y; Koide, S; Mouri, T

    2011-01-01

    This paper considers moments due to friction forces on the human fingertip. A computational technique called the friction moment arc method is presented. The method computes the static and/or dynamic friction moment independent of a friction force calculation. In addition, a new finger holder to display friction moment is presented. This device incorporates a small brushless motor and disk, and connects the human's finger to an interface finger of the five-fingered haptic interface robot HIRO II. Subjects' perception of friction moment while wearing the finger holder, as well as perceptions during object manipulation in a virtual reality environment, were evaluated experimentally.

  7. Low-Friction Joint for Robot Fingers

    NASA Technical Reports Server (NTRS)

    Ruoff, C. F.

    1985-01-01

    Mechanical linkage allows adjacent parts to move relative to each other with low friction and with no chatter, slipping, or backlash. Low-friction joint of two surfaces in rolling contact, held in alinement by taut flexible bands. No sliding friction or "stick-slip" motion: Only rolling-contact and bending friction within bands. Proposed linkage intended for finger joints in mechanical hands for robots and manipulators.

  8. Effects of chemical intermixing on electrical and thermal contact conductances at metallized bismuth and antimony telluride interfaces

    SciTech Connect

    Devender,; Mehta, Rutvik J.; Ramanath, Ganpati; Lofgreen, Kelly; Mahajan, Ravi; Yamaguchi, Masashi; Borca-Tasciuc, Theodorian

    2015-03-15

    Tailoring electrical and thermal contact conductivities (Σ{sub c} and Γ{sub c}) across metallized pnictogen chalcogenide interfaces is key for realizing efficient thermoelectric devices. The authors report that Cu, Ni, Ti, and Ta diffusion and interfacial telluride formation with n-Bi{sub 2}Te{sub 3} and p-Sb{sub 2}Te{sub 3} influence both Σ{sub c} and Γ{sub c}. Cu metallization yields the highest Γ{sub c} and the lowest Σ{sub c}, correlating with maximal metal diffusion and copper telluride formation. Ni diffuses less and yields the highest Σ{sub c} with Sb{sub 2}Te{sub 3} due to p-type nickel telluride formation, which diminishes Σ{sub c} improvement with n-Bi{sub 2}Te{sub 3} interfaces. Ta and Ti contacts yield the lowest properties similar to that in Ni-metallized structures. These correlations between interfacial diffusion and phase formation on electronic and thermal transport properties will be important for devising suitable metallization for thermoelectric devices.

  9. Quantitative Contact Resonance Force Microscopy for Viscoelastic Measurement of Soft Materials at the Solid-Liquid Interface.

    PubMed

    Churnside, Allison B; Tung, Ryan C; Killgore, Jason P

    2015-10-13

    Viscoelastic property measurements made at the solid-liquid interface are key to characterizing materials for a variety of biological and industrial applications. Further, nanostructured materials require nanoscale measurements. Here, material loss tangents (tan δ) were extracted from confounding liquid effects in nanoscale contact resonance force microscopy (CR-FM), an atomic force microscope based technique for observing mechanical properties of surfaces. Obtaining reliable CR-FM viscoelastic measurements in liquid is complicated by two effects. First, in liquid, spurious signals arise during cantilever excitation. Second, it is challenging to separate changes to cantilever behavior due to the sample from changes due to environmental damping and added mass effects. We overcame these challenges by applying photothermal cantilever excitation in multiple resonance modes and a predictive model for the hydrodynamic effects. We demonstrated quantitative, nanoscale viscoelastic CR-FM measurements of polymers at the solid-liquid interface. The technique is demonstrated on a point-by-point basis on polymer samples and while imaging in contact mode on a fixed plant cell wall. Values of tan δ for measurements made in water agreed with the values for measurements in air for some experimental conditions on polystyrene and for all examined conditions on polypropylene.

  10. The frequency response of dynamic friction: Enhanced rate-and-state models

    NASA Astrophysics Data System (ADS)

    Cabboi, A.; Putelat, T.; Woodhouse, J.

    2016-07-01

    The prediction and control of friction-induced vibration requires a sufficiently accurate constitutive law for dynamic friction at the sliding interface: for linearised stability analysis, this requirement takes the form of a frictional frequency response function. Systematic measurements of this frictional frequency response function are presented for small samples of nylon and polycarbonate sliding against a glass disc. Previous efforts to explain such measurements from a theoretical model have failed, but an enhanced rate-and-state model is presented which is shown to match the measurements remarkably well. The tested parameter space covers a range of normal forces (10-50 N), of sliding speeds (1-10 mm/s) and frequencies (100-2000 Hz). The key new ingredient in the model is the inclusion of contact stiffness to take into account elastic deformations near the interface. A systematic methodology is presented to discriminate among possible variants of the model, and then to identify the model parameter values.

  11. Direct evidence of phospholipids in gecko footprints and spatula-substrate contact interface detected using surface-sensitive spectroscopy.

    PubMed

    Hsu, Ping Yuan; Ge, Liehui; Li, Xiaopeng; Stark, Alyssa Y; Wesdemiotis, Chrys; Niewiarowski, Peter H; Dhinojwala, Ali

    2012-04-07

    Observers ranging from Aristotle to young children have long marvelled at the ability of geckos to cling to walls and ceilings. Detailed studies have revealed that geckos are 'sticky' without the use of glue or suction devices. Instead, a gecko's stickiness derives from van der Waals interactions between proteinaceous hairs called setae and substrate. Here, we present surprising evidence that although geckos do not use glue, a residue is transferred on surfaces as they walk-geckos leave footprints. Using matrix-free nano-assisted laser desorption-ionization mass spectrometry, we identified the residue as phospholipids with phosphocholine head groups. Moreover, interface-sensitive sum-frequency generation spectroscopy revealed predominantly hydrophobic methyl and methylene groups and the complete absence of water at the contact interface between a gecko toe pad and the substrate. The presence of lipids has never been considered in current models of gecko adhesion. Our analysis of gecko footprints and the toe pad-substrate interface has significant consequences for models of gecko adhesion and by extension, the design of synthetic mimics.

  12. Area of contact and thermal transport across transfer-printed metal-dielectric interfaces

    SciTech Connect

    Seong, M; Singh, PK; Sinha, S

    2013-01-14

    Recent experiments suggest that the interfacial thermal conductance of transfer printed metal-dielectric interfaces is similar to 45MW/m(2) K at 300 K, approaching that of interfaces formed using physical vapor deposition. We investigate this anomalous result using a combination of theoretical deformation mechanics and nanoscale thermal transport. Our analysis shows that plastic deformation and capillary forces lead to significantly large fractional areal coverage of similar to 0.25. The conductance predicted from theory is on the same order of magnitude (similar to 18MW/m(2) K) as the experimental data and partially explains the temperature trend. There remains a quantitative discrepancy between data and theory that is not explained through deformation of the asperities alone. We suggest that capillary bridges formed in the small asperities contribute significantly to heat conduction. A preliminary analysis shows this to be plausible based on available data. This work shows that metallic interconnects formed using transfer printing are not at a disadvantage compared to ones formed using vapor deposition, in terms of heat flow characteristics. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4773532

  13. The Degradation Interface of Magnesium Based Alloys in Direct Contact with Human Primary Osteoblast Cells

    PubMed Central

    Willumeit-Römer, Regine; Laipple, Daniel; Luthringer, Bérengère; Feyerabend, Frank

    2016-01-01

    Magnesium alloys have been identified as a new generation material of orthopaedic implants. In vitro setups mimicking physiological conditions are promising for material / degradation analysis prior to in vivo studies however the direct influence of cell on the degradation mechanism has never been investigated. For the first time, the direct, active, influence of human primary osteoblasts on magnesium-based materials (pure magnesium, Mg-2Ag and Mg-10Gd alloys) is studied for up to 14 days. Several parameters such as composition of the degradation interface (directly beneath the cells) are analysed with a scanning electron microscope equipped with energy dispersive X-ray and focused ion beam. Furthermore, influence of the materials on cell metabolism is examined via different parameters like active mineralisation process. The results are highlighting the influences of the selected alloying element on the initial cells metabolic activity. PMID:27327435

  14. Effect of Ion Sputtering on Interface Chemistry and Electrical Properties of an Gaas (100) Schottky Contacts

    NASA Technical Reports Server (NTRS)

    Wang, Y. X.; Holloway, P. H.

    1984-01-01

    Auger and electron photoelectron spectroscopy were used to measure the extent of As depletion during 1 keV to 5 keV argon sputtering of GaAs surfaces. This depletion was correlated with a general decrease in the barrier height of the rectifying Au contact deposited in situ. However, nondestructive angle resolved XPS measurements showed As was depleted at the outer surface more by 1 keV than 3 keV argon. These effects are explained based on a combined work effective work function model and creation of a donor like surface damage layer. The donor layer was correlated with As depletion by sputtering. Deep level trap formation and annealing of sputtering effects were studied.

  15. Effect of Partially Wetted Cavities on Superhydrophobic Friction Reduction

    NASA Astrophysics Data System (ADS)

    Kim, Tae Jin; Hidrovo, Carlos

    2011-11-01

    Superhydrophobicity refers to the condition of water-surface contact angle larger than 150°. Such microtextured surfaces with low sliding angles (<5°) involve air pockets trapped underneath the water(Cassie state). This leads to a shear-free flow boundary condition and consequential reduction in frictional losses and pumping power requirements. We investigate the effects of partially wetted microtextured trenches on the friction reduction characteristics in microchannel flow. PDMS based superhydrophobic microchannels with sidewall trenches are used to visualize the water-air interface penetration and its effects on friction reduction. Theoretical models based on actual water layer measurements and shear free/no slip boundary conditions on the interface are used to calculate lower/upper bounds of the effective friction factor. These are compared to experimental values determined from pressure and flow rate data. Results suggest the existence of a ``start-up'' pressure required to overcome capacitance associated with surface tension effects and that friction reduction characteristics of the microchannels are unaffected by partial flooding of the trenches.

  16. Static friction between silicon nanowires and elastomeric substrates.

    PubMed

    Qin, Qingquan; Zhu, Yong

    2011-09-27

    This paper reports the first direct measurements of static friction force and interfacial shear strength between silicon (Si) nanowires (NWs) and poly(dimethylsiloxane) (PDMS). A micromanipulator is used to manipulate and deform the NWs under a high-magnification optical microscope in real time. The static friction force is measured based on "the most-bent state" of the NWs. The static friction and interface shear strength are found to depend on the ultraviolet/ozone (UVO) treatment of PDMS. The shear strength starts at 0.30 MPa without UVO treatment, increases rapidly up to 10.57 MPa at 60 min of treatment and decreases for longer treatment. Water contact angle measurements suggest that the UVO-induced hydrophobic-to-hydrophilic conversion of PDMS surface is responsible for the increase in the static friction, while the hydrophobic recovery effect contributes to the decrease. The static friction between NWs and PDMS is of critical relevance to many device applications of NWs including NW-based flexible/stretchable electronics, NW assembly and nanocomposites (e.g., supercapacitors). Our results will enable quantitative interface design and control for such applications.

  17. Friction and fracture.

    PubMed

    Gerde, E; Marder, M

    2001-09-20

    Consider a block placed on a table and pushed sideways until it begins to slide. Amontons and Coulomb found that the force required to initiate sliding is proportional to the weight of the block (the constant of proportionality being the static coefficient of friction), but independent of the area of contact. This is commonly explained by asserting that, owing to the presence of asperities on the two surfaces, the actual area in physical contact is much smaller than it seems, and grows in proportion to the applied compressive force. Here we present an alternative picture of the static friction coefficient, which starts with an atomic description of surfaces in contact and then employs a multiscale analysis technique to describe how sliding occurs for large objects. We demonstrate the existence of self-healing cracks that have been postulated to solve geophysical paradoxes about heat generated by earthquakes, and we show that, when such cracks are present at the atomic scale, they result in solids that slip in accord with Coulomb's law of friction. We expect that this mechanism for friction will be found to operate at many length scales, and that our approach for connecting atomic and continuum descriptions will enable more realistic first-principles calculations of friction coefficients.

  18. Effect of Dwell Time on Joint Interface Microstructure and Strength of Dissimilar Friction Stir Spot-Welded Al-5083 and St-12 Alloy Sheets

    NASA Astrophysics Data System (ADS)

    Fereiduni, Eskandar; Movahedi, Mojtaba; Kokabi, Amir Hossein; Najafi, Hossein

    2017-04-01

    Joining of Al-5083 alloy sheet to St-12 steel sheet was performed using a new friction stir spot welding (FSSW) technique in which the tool pin tip did not enter lower steel sheet. Effect of dwell time on the microstructure and mechanical properties of the joints was studied by various methods including microhardness measurements, shear test, stereo and light microscopy as well as scanning and transmission electron microscopy (SEM and TEM). Results indicated that compared to the conventional FSSW process, stronger joints can be achieved by this FSSW technique. Cross-sectional observation of the failed specimens indicated the occurrence of final fracture from the circumference of the tool pin where the Al sheet thickness was decreased as a result of the tool pin penetration. However, microhardness measurements introduced these fracture locations as the hardest regions of the Al part of welds. In addition to the Al3Fe and Al5Fe2 intermetallic compounds reported in the literature to form at the interface of dissimilar Al/steel joints, a third layer of AlFe intermetallic compound was also identified adjacent to the steel side of welds. Enhancement of the dwell time from 5 to 15 seconds increased the intermetallic layer thickness from 1.7 to 3 µm and resulted in the formation of harder stirred zone. This consequently increased the strength of the weld.

  19. Effect of Dwell Time on Joint Interface Microstructure and Strength of Dissimilar Friction Stir Spot-Welded Al-5083 and St-12 Alloy Sheets

    NASA Astrophysics Data System (ADS)

    Fereiduni, Eskandar; Movahedi, Mojtaba; Kokabi, Amir Hossein; Najafi, Hossein

    2017-01-01

    Joining of Al-5083 alloy sheet to St-12 steel sheet was performed using a new friction stir spot welding (FSSW) technique in which the tool pin tip did not enter lower steel sheet. Effect of dwell time on the microstructure and mechanical properties of the joints was studied by various methods including microhardness measurements, shear test, stereo and light microscopy as well as scanning and transmission electron microscopy (SEM and TEM). Results indicated that compared to the conventional FSSW process, stronger joints can be achieved by this FSSW technique. Cross-sectional observation of the failed specimens indicated the occurrence of final fracture from the circumference of the tool pin where the Al sheet thickness was decreased as a result of the tool pin penetration. However, microhardness measurements introduced these fracture locations as the hardest regions of the Al part of welds. In addition to the Al3Fe and Al5Fe2 intermetallic compounds reported in the literature to form at the interface of dissimilar Al/steel joints, a third layer of AlFe intermetallic compound was also identified adjacent to the steel side of welds. Enhancement of the dwell time from 5 to 15 seconds increased the intermetallic layer thickness from 1.7 to 3 µm and resulted in the formation of harder stirred zone. This consequently increased the strength of the weld.

  20. Development and analysis of Cu-doped ZnTe for use as a back contact interface for CdS/CdTe solar cells

    NASA Astrophysics Data System (ADS)

    Gessert, T. A.; Coutts, T. J.

    1994-06-01

    It is well known that the losses associated with the back contact of typical CdS/CdTe solar cell devices can be a substantial part of the total external loss. Previous modeling has indicated that these losses will be significant, unless the value of specific contact resistance (rc) at this interface is reduced to ˜0.10 Ω-cm2 or less. Although several studies have inferred values of rc near this level, few have attempted to measure directly the value of rc as a function of various processing conditions. One reason for this situation is the difficulty in fabricating the appropriate patterns for direct analysis of rc. In the following paper, initial characterization studies of sputter-deposited, Cu-doped ZnTe are documented. Additionally, recent attempts to measure directly the contact resistance associated with the two interfaces of the Ni/ZnTe/CdTe contact stack, are presented and discussed. Preliminary testing of these processes has been conducted using sputter-deposited test structures representing the individual interfaces of a typical Ni/ZnTe/CdTe contact stack. Contact resistance analysis of these structures has allowed for the estimation of rc suggesting that, for the conditions studied, the contact stack appears to meet the criterion of yielding an rc value <0.1 Ω-cm2.

  1. Physically representative atomistic modeling of atomic-scale friction

    NASA Astrophysics Data System (ADS)

    Dong, Yalin

    Nanotribology is a research field to study friction, adhesion, wear and lubrication occurred between two sliding interfaces at nano scale. This study is motivated by the demanding need of miniaturization mechanical components in Micro Electro Mechanical Systems (MEMS), improvement of durability in magnetic storage system, and other industrial applications. Overcoming tribological failure and finding ways to control friction at small scale have become keys to commercialize MEMS with sliding components as well as to stimulate the technological innovation associated with the development of MEMS. In addition to the industrial applications, such research is also scientifically fascinating because it opens a door to understand macroscopic friction from the most bottom atomic level, and therefore serves as a bridge between science and engineering. This thesis focuses on solid/solid atomic friction and its associated energy dissipation through theoretical analysis, atomistic simulation, transition state theory, and close collaboration with experimentalists. Reduced-order models have many advantages for its simplification and capacity to simulating long-time event. We will apply Prandtl-Tomlinson models and their extensions to interpret dry atomic-scale friction. We begin with the fundamental equations and build on them step-by-step from the simple quasistatic one-spring, one-mass model for predicting transitions between friction regimes to the two-dimensional and multi-atom models for describing the effect of contact area. Theoretical analysis, numerical implementation, and predicted physical phenomena are all discussed. In the process, we demonstrate the significant potential for this approach to yield new fundamental understanding of atomic-scale friction. Atomistic modeling can never be overemphasized in the investigation of atomic friction, in which each single atom could play a significant role, but is hard to be captured experimentally. In atomic friction, the

  2. Contact resistance tuning at metal/Nb:SrTiO3 interfaces using LaAlO3 interlayers

    NASA Astrophysics Data System (ADS)

    Inoue, Hisashi; Swartz, Adrian; Tachikawa, Takashi; Hikita, Yasuyuki; Hwang, Harold

    2015-03-01

    SrTiO3 (STO) exhibits coexistence of high mobility electrons and possible unconventional superconductivity (SC). Transition metal (TM) contacts to epitaxial insulator/Nb doped STO (NSTO) structures are prototypically used to probe the electronic structure (ES) of n-type STO in tunneling experiments. However, the field dependent permittivity in STO at low temperatures makes it difficult to probe ES when the barrier height (BH) is large. We show that the contact resistance Rc across TM/NSTO interfaces (IF) can be effectively tuned by inserting a thin LaAlO3 (LAO) interlayer (IL) between the TM and NSTO. Change of IL thickness from 0 to 2 u.c. in Co/LAO/NSTO (100) structures results in systematic reduction of Rc by orders of magnitude, as evidenced by a transition from Schottky-type rectifying to nearly ohmic current-voltage curves. This is because the polar nature of the LAO surface, generating an IF dipole IL, lowers the Schottky BH. This is a useful method to optimize Rc for tunneling experiments in doped STO with possible applications for SC and spintronics. It is also important that this effect fails for ex - situ deposited TM and we discuss the distinction.

  3. Exploring of protein - protein interactions at the solid - aqueous interface by means of contact angle measurements.

    PubMed

    Grabowska, I; Dehaen, W; Radecka, H; Radecki, J

    2016-05-01

    In this article we present the results of the studies on interactions between the VC1 domain of the Receptor for Advanced Glycation End Products (RAGE) and its ligand, the S100B protein, performed by contact angle measurements. Histidine-tagged (His6) VC1-RAGE domain was covalently bonded to Cu(II) or Ni(II) complexes with dipyrromethene (DPM) self-assembled on gold surface. The method based on the theory of van Oss was used for the purpose of determining the Lifshitz-van der Waals (γ(LW)) component as well as the electron acceptor-electron donor (the Lewis acid-base, γ(+)-γ(-)) parameters of the VC1-RAGE-S100B complex. Moreover, the surface free energies of the interactions between the VC1 domain attached to the surface and the ligand present in the aqueous phase were determined. The specificity of the VC1- RAGE interactions with the ligand studied was also proved.

  4. Formation of perfect ohmic contact at indium tin oxide/N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine interface using ReO3

    PubMed Central

    Yoo, Seung-Jun; Chang, Jung-Hung; Lee, Jeong-Hwan; Moon, Chang-Ki; Wu, Chih-I; Kim, Jang-Joo

    2014-01-01

    A perfect ohmic contact is formed at the interface of indium tin oxide (ITO) and N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB) using ReO3 as the interfacial layer. The hole injection efficiency is close to 100% at the interface, which is much higher than those for interfacial layers of 1,4,5,8,9,11-hexaazatripheylene hexacarbonitrile (HAT-CN) and MoO3. Interestingly, the ReO3 and MoO3 interfacial layers result in the same hole injection barrier, ≈0.4 eV, to NPB, indicating that the Fermi level is pinned to the NPB polaron energy level. However, a significant difference is observed in the generated charge density in the NPB layer near the interfacial layer/NPB interface, indicating that charge generation at the interface plays an important role in forming the ohmic contact. PMID:24469505

  5. General theory of frictional heating with application to rubber friction

    NASA Astrophysics Data System (ADS)

    Fortunato, G.; Ciaravola, V.; Furno, A.; Lorenz, B.; Persson, B. N. J.

    2015-05-01

    The energy dissipation in the contact regions between solids in sliding contact can result in high local temperatures which may strongly effect friction and wear. This is the case for rubber sliding on road surfaces at speeds above 1 mm s-1. We derive equations which describe the frictional heating for solids with arbitrary thermal properties. The theory is applied to rubber friction on road surfaces and we take into account that the frictional energy is partly produced inside the rubber due to the internal friction of rubber and in a thin (nanometer) interfacial layer at the rubber-road contact region. The heat transfer between the rubber and the road surface is described by a heat transfer coefficient which depends on the sliding speed. Numerical results are presented and compared to experimental data. We find that frictional heating results in a kinetic friction force which depends on the orientation of the sliding block, thus violating one of the two basic Leonardo da Vinci ‘laws’ of friction.

  6. Study of intermolecular contacts in proteins and oligomer interfaces and preliminary investigations into the design and production of nanomaterials from proteins

    NASA Astrophysics Data System (ADS)

    Iyer, Ganesh Hariharan

    The first part of this research involved a study of the nature and extent of nonbonded interactions at crystal and oligomer interfaces. A survey was compiled of several characteristics of intersubunit contacts in 58 different oligomeric proteins, and of the intermolecular contacts in 223 protein crystal structures. Routines written in "S" language were utilized for the generation of the observed and expected contacts. The information in the Protein Data Bank (PDB) was extracted using the database management system, Protein Knowledge Base (PKB). Potentials of mean force for atom-atom contacts and residue-residue contacts were derived by comparison of the number of observed interactions with the number expected by mass action. Preference association matrices and log-linear analyses were applied to determine the different factors that could contribute to the overall interactions at the interfaces of oligomers and crystals. Surface patches at oligomer and crystal interfaces were also studied to further investigate the origin of the differences in their stabilities. Total number of atoms in contact and the secondary structure elements involved are similar in the two types of interfaces. Crystal contacts result from more numerous interactions by polar residues, compared with a tendency toward nonpolar amino acid prominent in oligomer interfaces. Contact potentials indicate that hydrophobic interactions at oligomer interfaces favor aromatic amino acids and methionine over aliphatic amino acids; and that crystal contacts form in such a way as to avoid inclusion of hydrophobic interactions. The second part involved the development of a new class of biomaterials from two-dimensional arrays of ordered proteins. Point mutations were planned to introduce cysteine residues at appropriate locations to enable cross-linking at the molecular interface within given crystallographic planes. Crystallization and subsequent cross-linking of the modified protein would lead to the

  7. Characteristics of Friction Welding Between Solid Bar of 6061 Al Alloy and Pipe of Al-Si12CuNi Al Cast Alloy

    NASA Astrophysics Data System (ADS)

    Kimura, M.; Sakaguchi, H.; Kusaka, M.; Kaizu, K.; Takahashi, T.

    2015-11-01

    This paper describes the characteristics of friction welding between a solid bar of 6061 Al alloy and a pipe of Al-Si12CuNi (AC8A) Al cast alloy. When the joint was made by a continuous drive friction welding machine (conventional method), the AC8A portion of the joint showed heavy deformation and the AA6061 showed minimal deformation. In particular, the joint could not be successfully made with following conditions, because AC8A pipe side crushed due to insufficient friction heat or high pressure: a short friction time such as 0.3 s, high friction pressure such as 100 MPa, or high forge pressure such as 150 MPa. The heavy deformation of AC8A side was caused by increasing friction torque during braking. To prevent braking deformation until rotation stops, a joint was made by a continuous drive friction welding machine that has an electromagnetic clutch. When the clutch was released, the relative speed between both specimens simultaneously decreased to zero. When the joint was made with friction pressure of 25 MPa, friction time of 0.3 s, and forge pressure of 125 MPa, the joining could be successfully achieved and that had approximately 16% efficiency. In addition, when the joint was made with friction pressure of 25 MPa, friction time of 0.7 s, and forge pressure of 125 MPa, it had approximately 54% efficiency. However, all joints showed the fracture between the traveled weld interface and the AC8A side, because the weld interface traveled in the longitudinal direction of AC8A side from the first contacted position of both weld faying surfaces. Hence, it was clarified that the friction welding between a solid bar of AA6061 and a cast pipe of AC8A was not desirable since the traveling phenomena of the weld interface were caused by the combination of the shapes of the friction welding specimens.

  8. Practical determination of friction coefficient of Al 3003 for forming of backward extruded part using simple tip test and inverse finite element analysis

    NASA Astrophysics Data System (ADS)

    Bong, H. J.; Leem, D.; Kim, J. H.; Im, Y. T.; Lee, M. G.

    2016-05-01

    The friction coefficient for aluminum alloy 3003 was determined from a specially designed tip test and finite element (FE) simulations. Measured radial tip distance after the tip test was compared to the FE simulations by iteratively changing friction coefficient and the best fitting friction coefficient was determined. To consider strain rate effect on flow stress response during large plastic deformation, a new combined Hollomon- Voce hardening law was proposed. The friction under three different surface conditions was considered by the proposed inverse FE analysis. The results showed that there was obvious strain rate effect on the predicted punch load in the tip test. Moreover, the different friction coefficients were numerically determined for punch/workpiece and die/workpiece interfaces. Two possible causes of this difference were discussed by the analysis on contact normal pressure and slip velocity distributions of the two interfaces.

  9. A quantitative lateral force Microscopy study of the dolomite (104)-water interface.

    SciTech Connect

    Higgins, S. R.; Hu, X.; Fentert, P.; Wright State Univ.

    2007-08-14

    The friction and lateral stiffness of the contact between an atomic force microscopy (AFM) probe tip and an atomically flat dolomite (104) surface were investigated in contact with two aqueous solutions that were in equilibrium and supersaturated with respect to dolomite, respectively. The two aqueous solutions yielded negligible differences in friction at the native dolomite-water interface. However, the growth of a Ca-rich film from the supersaturated solution, revealed by X-ray reflectivity measurements, altered the probe-dolomite contact region sufficiently to observe distinct friction forces on the native dolomite and the film-covered surface regions. Quantitative friction-load relationships demonstrated three physically distinct load regimes for applied loads up to 200 nN. Similar friction forces were observed on both surfaces below 50 nN load and above 100 nN load. The friction forces on the two surfaces diverged at intermediate loads. Quantitative measurements of dynamic friction forces at low load were consistent with the estimated energy necessary to dehydrate the surface ions, whereas differences in mechanical properties of the Ca-rich film and dolomite surfaces were evidently important above 50 nN load. Attempts to fit the quantitative stiffness-load data using a Hertzian contact mechanical model based on bulk material properties yielded physically unrealistic fitting coefficients, suggesting that the interfacial contact region must be explicitly considered in describing the static and dynamic contact mechanics of this and similar systems.

  10. Quantitative lateral force microscopy study of the dolomite (104)-water interface.

    PubMed

    Higgins, Steven R; Hu, Xiaoming; Fenter, Paul

    2007-08-14

    The friction and lateral stiffness of the contact between an atomic force microscopy (AFM) probe tip and an atomically flat dolomite (104) surface were investigated in contact with two aqueous solutions that were in equilibrium and supersaturated with respect to dolomite, respectively. The two aqueous solutions yielded negligible differences in friction at the native dolomite-water interface. However, the growth of a Ca-rich film from the supersaturated solution, revealed by X-ray reflectivity measurements, altered the probe-dolomite contact region sufficiently to observe distinct friction forces on the native dolomite and the film-covered surface regions. Quantitative friction-load relationships demonstrated three physically distinct load regimes for applied loads up to 200 nN. Similar friction forces were observed on both surfaces below 50 nN load and above 100 nN load. The friction forces on the two surfaces diverged at intermediate loads. Quantitative measurements of dynamic friction forces at low load were consistent with the estimated energy necessary to dehydrate the surface ions, whereas differences in mechanical properties of the Ca-rich film and dolomite surfaces were evidently important above 50 nN load. Attempts to fit the quantitative stiffness-load data using a Hertzian contact mechanical model based on bulk material properties yielded physically unrealistic fitting coefficients, suggesting that the interfacial contact region must be explicitly considered in describing the static and dynamic contact mechanics of this and similar systems.

  11. Theory of Wetting-Induced Fluid Entrainment by Advancing Contact Lines on Dry Surfaces

    NASA Astrophysics Data System (ADS)

    Ledesma-Aguilar, R.; Hernández-Machado, A.; Pagonabarraga, I.

    2013-06-01

    We report on the onset of fluid entrainment when a contact line is forced to advance over a dry solid of arbitrary wettability. We show that entrainment occurs at a critical advancing speed beyond which the balance between capillary, viscous, and contact-line forces sustaining the shape of the interface is no longer satisfied. Wetting couples to the hydrodynamics by setting both the morphology of the interface at small scales and the viscous friction of the front. We find that the critical deformation that the interface can sustain is controlled by the friction at the contact line and the viscosity contrast between the displacing and displaced fluids, leading to a rich variety of wetting-entrainment regimes. We discuss the potential use of our theory to measure contact-line forces using atomic force microscopy and to study entrainment under microfluidic conditions exploiting colloid-polymer fluids of ultralow surface tension.

  12. Theory of wetting-induced fluid entrainment by advancing contact lines on dry surfaces.

    PubMed

    Ledesma-Aguilar, R; Hernández-Machado, A; Pagonabarraga, I

    2013-06-28

    We report on the onset of fluid entrainment when a contact line is forced to advance over a dry solid of arbitrary wettability. We show that entrainment occurs at a critical advancing speed beyond which the balance between capillary, viscous, and contact-line forces sustaining the shape of the interface is no longer satisfied. Wetting couples to the hydrodynamics by setting both the morphology of the interface at small scales and the viscous friction of the front. We find that the critical deformation that the interface can sustain is controlled by the friction at the contact line and the viscosity contrast between the displacing and displaced fluids, leading to a rich variety of wetting-entrainment regimes. We discuss the potential use of our theory to measure contact-line forces using atomic force microscopy and to study entrainment under microfluidic conditions exploiting colloid-polymer fluids of ultralow surface tension.

  13. Friction Stir Welding Development

    NASA Technical Reports Server (NTRS)

    Romine, Peter L.

    1998-01-01

    The research of this summer was a continuation of work started during the previous summer faculty fellowship period. The Friction Stir Welding process (FSW) patented by The Welding Institute (TWI), in Great Britain, has become a popular topic at the Marshall Space Flight Center over the past year. Last year it was considered a novel approach to welding but few people took it very seriously as a near term solution. However, due to continued problems with cracks in the new aluminum-lithium space shuttle external tank (ET), the friction stir process is being mobilized at full speed in an effort to mature this process for the potential manufacture of flight hardware. It is now the goal of NASA and Lockheed-Martin Corporation (LMC) to demonstrate a full-scale friction stir welding system capable of welding ET size barrel sections. The objectives this summer were: (1) Implementation and validation of the rotating dynamometer on the MSFC FSW system; (2) Collection of data for FSW process modeling efforts; (3) Specification development for FSW implementation on the vertical weld tool; (4) Controls and user interface development for the adjustable pin tool; and (5) Development of an instrumentation system for the planishing process. The projects started this summer will lead to a full scale friction stir welding system that is expected to produce a friction stir welded shuttle external tank type barrel section. The success of this could lead to the implementation of the friction stir process for manufacturing future shuttle external tanks.

  14. Solid friction between soft filaments

    NASA Astrophysics Data System (ADS)

    Ward, Andrew; Hilitski, Feodor; Schwenger, Walter; Welch, David; Lau, A. W. C.; Vitelli, Vincenzo; Mahadevan, L.; Dogic, Zvonimir

    2015-06-01

    Any macroscopic deformation of a filamentous bundle is necessarily accompanied by local sliding and/or stretching of the constituent filaments. Yet the nature of the sliding friction between two aligned filaments interacting through multiple contacts remains largely unexplored. Here, by directly measuring the sliding forces between two bundled F-actin filaments, we show that these frictional forces are unexpectedly large, scale logarithmically with sliding velocity as in solid-like friction, and exhibit complex dependence on the filaments’ overlap length. We also show that a reduction of the frictional force by orders of magnitude, associated with a transition from solid-like friction to Stokes’s drag, can be induced by coating F-actin with polymeric brushes. Furthermore, we observe similar transitions in filamentous microtubules and bacterial flagella. Our findings demonstrate how altering a filament’s elasticity, structure and interactions can be used to engineer interfilament friction and thus tune the properties of fibrous composite materials.

  15. Solid friction between soft filaments

    SciTech Connect

    Ward, Andrew; Hilitski, Feodor; Schwenger, Walter; Welch, David; Lau, A. W. C.; Vitelli, Vincenzo; Mahadevan, L.; Dogic, Zvonimir

    2015-03-02

    Any macroscopic deformation of a filamentous bundle is necessarily accompanied by local sliding and/or stretching of the constituent filaments. Yet the nature of the sliding friction between two aligned filaments interacting through multiple contacts remains largely unexplored. Here, by directly measuring the sliding forces between two bundled F-actin filaments, we show that these frictional forces are unexpectedly large, scale logarithmically with sliding velocity as in solid-like friction, and exhibit complex dependence on the filaments’ overlap length. We also show that a reduction of the frictional force by orders of magnitude, associated with a transition from solid-like friction to Stokes’s drag, can be induced by coating F-actin with polymeric brushes. Furthermore, we observe similar transitions in filamentous microtubules and bacterial flagella. In conclusion, our findings demonstrate how altering a filament’s elasticity, structure and interactions can be used to engineer interfilament friction and thus tune the properties of fibrous composite materials.

  16. Solid friction between soft filaments.

    PubMed

    Ward, Andrew; Hilitski, Feodor; Schwenger, Walter; Welch, David; Lau, A W C; Vitelli, Vincenzo; Mahadevan, L; Dogic, Zvonimir

    2015-06-01

    Any macroscopic deformation of a filamentous bundle is necessarily accompanied by local sliding and/or stretching of the constituent filaments. Yet the nature of the sliding friction between two aligned filaments interacting through multiple contacts remains largely unexplored. Here, by directly measuring the sliding forces between two bundled F-actin filaments, we show that these frictional forces are unexpectedly large, scale logarithmically with sliding velocity as in solid-like friction, and exhibit complex dependence on the filaments' overlap length. We also show that a reduction of the frictional force by orders of magnitude, associated with a transition from solid-like friction to Stokes's drag, can be induced by coating F-actin with polymeric brushes. Furthermore, we observe similar transitions in filamentous microtubules and bacterial flagella. Our findings demonstrate how altering a filament's elasticity, structure and interactions can be used to engineer interfilament friction and thus tune the properties of fibrous composite materials.

  17. Frictional behavior of automotive brake materials under wet and dry conditions

    SciTech Connect

    Blau, P.J.; Martin, R.L.; Weintraub, M.H.; Jang, Ho; Donlon, W.

    1996-12-15

    The purpose of this effort was to develop an improved understanding of the relationship between the structure and frictional behavior of materials in the disc brake/rotor interface with a view toward improving the performance of automotive disc brakes. The three tasks involved in this Cooperative Research and Development Agreement (CRADA) were as follows: Task 1. Investigation of Brake Pads and Rotors. Characterize surface features of worn brake pads and rotors, with special attention to the transfer film which forms on them during operation. Ford to supply specimens for examination and other supporting information. Task 2. Effects of Atmosphere and Repeated Applications on Brake Material Friction. Conduct pin-on-disk friction tests at ORNL under controlled moisture levels to determine effects of relative humidity on frictional behavior of brake pad and rotor materials. Conduct limited tests on the characteristics of friction under application of repeated contacts. Task 3. Comparison of Dynamometer Tests with Laboratory Friction Tests. Compare ORNL friction data with Ford dynamometer test data to establish the degree to which the simple bench tests can be useful in helping to understand frictional behavior in full-scale brake component tests. This final report summarizes work performed under this CRADA.

  18. Static friction between rigid fractal surfaces.

    PubMed

    Alonso-Marroquin, Fernando; Huang, Pengyu; Hanaor, Dorian A H; Flores-Johnson, E A; Proust, Gwénaëlle; Gan, Yixiang; Shen, Luming

    2015-09-01

    Using spheropolygon-based simulations and contact slope analysis, we investigate the effects of surface topography and atomic scale friction on the macroscopically observed friction between rigid blocks with fractal surface structures. From our mathematical derivation, the angle of macroscopic friction is the result of the sum of the angle of atomic friction and the slope angle between the contact surfaces. The latter is obtained from the determination of all possible contact slopes between the two surface profiles through an alternative signature function. Our theory is validated through numerical simulations of spheropolygons with fractal Koch surfaces and is applied to the description of frictional properties of Weierstrass-Mandelbrot surfaces. The agreement between simulations and theory suggests that for interpreting macroscopic frictional behavior, the descriptors of surface morphology should be defined from the signature function rather than from the slopes of the contacting surfaces.

  19. Static friction between rigid fractal surfaces

    NASA Astrophysics Data System (ADS)

    Alonso-Marroquin, Fernando; Huang, Pengyu; Hanaor, Dorian A. H.; Flores-Johnson, E. A.; Proust, Gwénaëlle; Gan, Yixiang; Shen, Luming

    2015-09-01

    Using spheropolygon-based simulations and contact slope analysis, we investigate the effects of surface topography and atomic scale friction on the macroscopically observed friction between rigid blocks with fractal surface structures. From our mathematical derivation, the angle of macroscopic friction is the result of the sum of the angle of atomic friction and the slope angle between the contact surfaces. The latter is obtained from the determination of all possible contact slopes between the two surface profiles through an alternative signature function. Our theory is validated through numerical simulations of spheropolygons with fractal Koch surfaces and is applied to the description of frictional properties of Weierstrass-Mandelbrot surfaces. The agreement between simulations and theory suggests that for interpreting macroscopic frictional behavior, the descriptors of surface morphology should be defined from the signature function rather than from the slopes of the contacting surfaces.

  20. Does screw-bone interface modelling matter in finite element analyses?

    PubMed

    MacLeod, Alisdair R; Pankaj, Pankaj; Simpson, A Hamish R W

    2012-06-01

    The effect of screw-bone interface modelling strategies was evaluated in the setting of a tibial mid-shaft fracture stabilised using locking plates. Three interface models were examined: fully bonded interface; screw with sliding contact with bone; and screw with sliding contact with bone in an undersized pilot hole. For the simulation of the last interface condition we used a novel thermal expansion approach to generate the pre-stress that the bone would be exposed to during screw insertion. The study finds that the global load-deformation response is not influenced by the interface modelling approach employed; the deformation varied by less than 1% between different interaction models. However, interface modelling is found to have a considerable impact on the local stress-strain environment within the bone in the vicinity of the screws. Frictional and tied representations did not have significantly different peak strain values (<5% difference); the frictional interface had higher peak compressive strains while the tied interface had higher tensile strains. The undersized pilot hole simulation produced the largest strains. The peak minimum principal strains for the frictional interface were 26% of those for the undersized pilot hole simulation at a load of 770 N. It is concluded that the commonly used tie constraint can be used effectively when the only interest is the global load-deformation behaviour. Different contact interface models, however, alter the mechanical response around screw holes leading to different predictions for screw loosening, bone damage and stress shielding.

  1. Friction of atomically stepped surfaces

    NASA Astrophysics Data System (ADS)

    Dikken, R. J.; Thijsse, B. J.; Nicola, L.

    2017-03-01

    The friction behavior of atomically stepped metal surfaces under contact loading is studied using molecular dynamics simulations. While real rough metal surfaces involve roughness at multiple length scales, the focus of this paper is on understanding friction of the smallest scale of roughness: atomic steps. To this end, periodic stepped Al surfaces with different step geometry are brought into contact and sheared at room temperature. Contact stress that continuously tries to build up during loading, is released with fluctuating stress drops during sliding, according to the typical stick-slip behavior. Stress release occurs not only through local slip, but also by means of step motion. The steps move along the contact, concurrently resulting in normal migration of the contact. The direction of migration depends on the sign of the step, i.e., its orientation with respect to the shearing direction. If the steps are of equal sign, there is a net migration of the entire contact accompanied by significant vacancy generation at room temperature. The stick-slip behavior of the stepped contacts is found to have all the characteristic of a self-organized critical state, with statistics dictated by step density. For the studied step geometries, frictional sliding is found to involve significant atomic rearrangement through which the contact roughness is drastically changed. This leads for certain step configurations to a marked transition from jerky sliding motion to smooth sliding, making the final friction stress approximately similar to that of a flat contact.

  2. On optimal design for the blade-root/hub interface in jet engines

    NASA Technical Reports Server (NTRS)

    Kikuchi, N.; Taylor, J. E.

    1984-01-01

    Two major problems identified with the design of the blade-root/hub interface are discussed. The first is the so-called friction contact problem which has two special features: unilateral contact and Coulomb's friction. One of the difficulties in this problem is that the portions of contact and sticking/sliding surfaces are not known a priori. The second is the shape optimization problem which is characterized either by the minimization of the maximum contact pressure or by the minimization of the equivalent stress on the boundary. Design variables are the shapes of the blade-root and the hub. It is noted that friction contact and shape optimization problems are strongly coupled in the present design problem.

  3. A three-dimensional finite element analysis of a passive and friction fit implant abutment interface and the influence of occlusal table dimension on the stress distribution pattern on the implant and surrounding bone

    PubMed Central

    Sarfaraz, Hasan; Paulose, Anoopa; Shenoy, K. Kamalakanth; Hussain, Akhter

    2015-01-01

    Aims: The aim of the study was to evaluate the stress distribution pattern in the implant and the surrounding bone for a passive and a friction fit implant abutment interface and to analyze the influence of occlusal table dimension on the stress generated. Materials and Methods: CAD models of two different types of implant abutment connections, the passive fit or the slip-fit represented by the Nobel Replace Tri-lobe connection and the friction fit or active fit represented by the Nobel active conical connection were made. The stress distribution pattern was studied at different occlusal dimension. Six models were constructed in PRO-ENGINEER 05 of the two implant abutment connection for three different occlusal dimensions each. The implant and abutment complex was placed in cortical and cancellous bone modeled using a computed tomography scan. This complex was subjected to a force of 100 N in the axial and oblique direction. The amount of stress and the pattern of stress generated were recorded on a color scale using ANSYS 13 software. Results: The results showed that overall maximum Von Misses stress on the bone is significantly less for friction fit than the passive fit in any loading conditions stresses on the implant were significantly higher for the friction fit than the passive fit. The narrow occlusal table models generated the least amount of stress on the implant abutment interface. Conclusion: It can thus be concluded that the conical connection distributes more stress to the implant body and dissipates less stress to the surrounding bone. A narrow occlusal table considerably reduces the occlusal overload. PMID:26929518

  4. Collective behavior of asperities as a model for friction and adhesion

    NASA Astrophysics Data System (ADS)

    Hulikal, Srivatsan

    Understanding friction and adhesion in static and sliding contact of surfaces is important in numerous physical phenomena and technological applications. Most surfaces are rough at the microscale, and thus the real area of contact is only a fraction of the nominal area. The macroscopic frictional and adhesive response is determined by the collective behavior of the population of evolving and interacting microscopic contacts. This collective behavior can be very different from the behavior of individual contacts. It is thus important to understand how the macroscopic response emerges from the microscopic one. In this thesis, we develop a theoretical and computational framework to study the collective behavior. Our philosophy is to assume a simple behavior of a single asperity and study the collective response of an ensemble. Our work bridges the existing well-developed studies of single asperities with phenomenological laws that describe macroscopic rate-and-state behavior of frictional interfaces. We find that many aspects of the macroscopic behavior are robust with respect to the microscopic response. This explains why qualitatively similar frictional features are seen for a diverse range of materials. We first show that the collective response of an ensemble of one-dimensional independent viscoelastic elements interacting through a mean field reproduces many qualitative features of static and sliding friction evolution. The resulting macroscopic behavior is different from the microscopic one: for example, even if each contact is velocity-strengthening, the macroscopic behavior can be velocity-weakening. The framework is then extended to incorporate three-dimensional rough surfaces, long- range elastic interactions between contacts, and time-dependent material behaviors such as viscoelasticity and viscoplasticity. Interestingly, the mean field behavior dominates and the elastic interactions, though important from a quantitative perspective, do not change the

  5. Numerical analysis of human dental occlusal contact

    NASA Astrophysics Data System (ADS)

    Bastos, F. S.; Las Casas, E. B.; Godoy, G. C. D.; Meireles, A. B.

    2010-06-01

    The purpose of this study was to obtain real contact areas, forces, and pressures acting on human dental enamel as a function of the nominal pressure during dental occlusal contact. The described development consisted of three steps: characterization of the surface roughness by 3D contact profilometry test, finite element analysis of micro responses for each pair of main asperities in contact, and homogenization of macro responses using an assumed probability density function. The inelastic deformation of enamel was considered, adjusting the stress-strain relationship of sound enamel to that obtained from instrumented indentation tests conducted with spherical tip. A mechanical part of the static friction coefficient was estimated as the ratio between tangential and normal components of the overall resistive force, resulting in μd = 0.057. Less than 1% of contact pairs reached the yield stress of enamel, indicating that the occlusal contact is essentially elastic. The micro-models indicated an average hardness of 6.25GPa, and the homogenized result for macroscopic interface was around 9GPa. Further refinements of the methodology and verification using experimental data can provide a better understanding of processes related to contact, friction and wear of human tooth enamel.

  6. Earthquake friction

    NASA Astrophysics Data System (ADS)

    Mulargia, Francesco; Bizzarri, Andrea

    2016-12-01

    Laboratory friction slip experiments on rocks provide firm evidence that the static friction coefficient μ has values ∼0.7. This would imply large amounts of heat produced by seismically active faults, but no heat flow anomaly is observed, and mineralogic evidence of frictional heating is virtually absent. This stands for lower μ values ∼0.2, as also required by the observed orientation of faults with respect to the maximum compressive stress. We show that accounting for the thermal and mechanical energy balance of the system removes this inconsistence, implying a multi-stage strain release process. The first stage consists of a small and slow aseismic slip at high friction on pre-existent stress concentrators within the fault volume but angled with the main fault as Riedel cracks. This introduces a second stage dominated by frictional temperature increase inducing local pressurization of pore fluids around the slip patches, which is in turn followed by a third stage in which thermal diffusion extends the frictionally heated zones making them coalesce into a connected pressurized region oriented as the fault plane. Then, the system enters a state of equivalent low static friction in which it can undergo the fast elastic radiation slip prescribed by dislocation earthquake models.

  7. Nanoscale wear and kinetic friction between atomically smooth surfaces sliding at high speeds

    SciTech Connect

    Rajauria, Sukumar Canchi, Sripathi V. Schreck, Erhard; Marchon, Bruno

    2015-02-23

    The kinetic friction and wear at high sliding speeds is investigated using the head-disk interface of hard disk drives, wherein the head and the disk are less than 10 nm apart and move at sliding speeds of 5–10 m/s relative to each other. While the spacing between the sliding surfaces is of the same order of magnitude as various AFM based fundamental studies on friction, the sliding speed is nearly six orders of magnitude larger, allowing a unique set-up for a systematic study of nanoscale wear at high sliding speeds. In a hard disk drive, the physical contact between the head and the disk leads to friction, wear, and degradation of the head overcoat material (typically diamond like carbon). In this work, strain gauge based friction measurements are performed; the friction coefficient as well as the adhering shear strength at the head-disk interface is extracted; and an experimental set-up for studying friction between high speed sliding surfaces is exemplified.

  8. Ultralow Contact Resistivity for a Metal/p-Type Silicon Interface by High-Concentration Germanium and Boron Doping Combined with Low-Temperature Annealing

    NASA Astrophysics Data System (ADS)

    Murakoshi, Atsushi; Iwase, Masao; Niiyama, Hiromi; Koike, Mitsuo; Suguro, Kyoichi

    2013-07-01

    A contact resistivity of 6.9×10-9 Ω.cm2 has been obtained in an AlSi (1 wt %)-Cu (0.5 wt %) alloy/silicon system by using heavy-dose ion implantations of germanium and boron combined with low-temperature annealing. The analysis of the combined state showed that B12 cluster was incorporated and the supersaturation activation layer was formed into the region where germanium separated. Separated germanium is expected to have high interface state density. It is considered that this interface state density also has a Fermi level, and in order to reduce the difference from the Fermi level of the substrate, the charge moves to interface state density from the substrate. As a result, it is not based on a metallic material but a work function becomes small because pinning by which a Fermi level is fixed to interface state density occurs owing to the substrate/metal interface. It is considered to be attributable to the existence of a Ge-rich layer formed by low-temperature annealing, and a supersaturation activation layer that lowers contact resistance was formed.

  9. Investigation of the effect of a diamine-based friction modifier on micropitting and the properties of tribofilms in rolling-sliding contacts

    NASA Astrophysics Data System (ADS)

    Soltanahmadi, Siavash; Morina, Ardian; van Eijk, Marcel C. P.; Nedelcu, Ileana; Neville, Anne

    2016-12-01

    The effect of N-tallow-1,3-diaminopropane (TDP) on friction, rolling wear and micropitting has been investigated with the ultimate objective of developing lubricants with no or minimal environmental impact. A mini traction machine (MTM-SLIM) has been utilised in order to generate tribofilms and observe the effect of TDP on anti-wear tribofilm formation and friction. Micropitting was induced on the surface of specimens using a micropitting rig (MPR). The x-ray photoelectron spectroscopy (XPS) surface analytical technique has been employed to investigate the effect of TDP on the chemical composition of the tribofilm while atomic force microscopy (AFM) was used to generate high resolution topographical images of the tribofilms formed on the MTM discs. Experimental and analytical results showed that TDP delays the zinc dialkyldithiophosphate (ZDDP) anti-wear tribofilm formation. TDP in combination with ZDDP induces a thinner and smoother anti-wear tribofilm with a modified chemical structure composed of mixed Fe/Zn (poly)phosphates. The sulphide contribution to the tribofilm and oxygen-to-phosphorous atomic concentration ratio are greater in the bulk of the tribofilm derived from a combination of TDP and ZDDP compared to a tribofilm derived from ZDDP alone. Surface analyses showed that utilising TDP effectively mitigates micropitting wear in the test conditions used in this study. Reduction of micropitting, relevant to rolling bearing applications, can be attributed to the improved running-in procedure, reduced friction, formation of a smoother tribofilm and modification of the tribofilm composition induced by TDP.

  10. The formulation of dynamical contact problems with friction in the case of systems of rigid bodies and general discrete mechanical systems—Painlevé and Kane paradoxes revisited

    NASA Astrophysics Data System (ADS)

    Charles, Alexandre; Ballard, Patrick

    2016-08-01

    The dynamics of mechanical systems with a finite number of degrees of freedom (discrete mechanical systems) is governed by the Lagrange equation which is a second-order differential equation on a Riemannian manifold (the configuration manifold). The handling of perfect (frictionless) unilateral constraints in this framework (that of Lagrange's analytical dynamics) was undertaken by Schatzman and Moreau at the beginning of the 1980s. A mathematically sound and consistent evolution problem was obtained, paving the road for many subsequent theoretical investigations. In this general evolution problem, the only reaction force which is involved is a generalized reaction force, consistently with the virtual power philosophy of Lagrange. Surprisingly, such a general formulation was never derived in the case of frictional unilateral multibody dynamics. Instead, the paradigm of the Coulomb law applying to reaction forces in the real world is generally invoked. So far, this paradigm has only enabled to obtain a consistent evolution problem in only some very few specific examples and to suggest numerical algorithms to produce computational examples (numerical modeling). In particular, it is not clear what is the evolution problem underlying the computational examples. Moreover, some of the few specific cases in which this paradigm enables to write down a precise evolution problem are known to show paradoxes: the Painlevé paradox (indeterminacy) and the Kane paradox (increase in kinetic energy due to friction). In this paper, we follow Lagrange's philosophy and formulate the frictional unilateral multibody dynamics in terms of the generalized reaction force and not in terms of the real-world reaction force. A general evolution problem that governs the dynamics is obtained for the first time. We prove that all the solutions are dissipative; that is, this new formulation is free of Kane paradox. We also prove that some indeterminacy of the Painlevé paradox is fixed in this

  11. Rolling-Friction Robotic Gripper

    NASA Technical Reports Server (NTRS)

    Vranish, John M.

    1992-01-01

    Robotic gripper using rolling-friction fingers closes in on object with interface designed to mate with rollers somewhat misaligned initially, aligns object with respect to itself, then holds object securely in uniquely determined position and orientation. Operation of gripper causes minimal wear and burring of gripper and object. Exerts minimal friction forces on object when grasping and releasing. Releases object easily and reliably even when side forces and torques are between itself and object.

  12. Phalanx force magnitude and trajectory deviation increased during power grip with an increased coefficient of friction at the hand-object interface.

    PubMed

    Enders, Leah R; Seo, Na Jin

    2011-05-17

    This study examined the effect of friction between the hand and grip surface on a person's grip strategy and force generation capacity. Twelve young healthy adults performed power grip exertions on an instrumented vertical cylinder with the maximum and 50% of maximum efforts (far above the grip force required to hold the cylinder), while normal and shear forces at each phalanx of all five fingers in the direction orthogonal to the gravity were recorded. The cylinder surface was varied for high-friction rubber and low-friction paper coverings. An increase in surface friction by replacing the paper covering with the rubber covering resulted in 4% greater mean phalanx normal force (perpendicular to the cylinder surface) and 22% greater mean phalanx shear force in either the proximal or distal direction of the digits (p<0.05; for both 50% and maximum grip efforts). Consequently, increased friction with the rubber surface compared to the paper surface was associated with a 20% increase in the angular deviation of the phalanx force from the direction normal to the cylinder surface (p<0.05). This study demonstrates that people significantly changed the magnitude and direction of phalanx forces depending on the surface they gripped. Such change in the grip strategy appears to help increase grip force generation capacity. This finding suggests that a seemingly simple power grip exertion involves sensory feedback-based motor control, and that people's power grip capacity may be reduced in cases of numbness, glove use, or injuries resulting in reduced sensation.

  13. Performance enhancement of top contact pentacene-based organic thin-film transistor (OTFT) using perylene interlayer between organic/electrode interface

    NASA Astrophysics Data System (ADS)

    Borthakur, Tribeni; Sarma, Ranjit

    2017-03-01

    We have investigated the effect of perylene interlayer between the organic/electrode interface on the electrical performance of a top contact pentacene-based OTFT. We have found the performance enhancement of the OTFT device. The OTFT devices with perylene layer show better field effect mobility and on-off ratio than that of having only metal electrodes. It also lowers the subthreshold slope and threshold voltage compared to single-layered OTFT devices.

  14. (001) 3C SiC/Ni contact interface: In situ XPS observation of annealing induced Ni2Si formation and the resulting barrier height changes

    NASA Astrophysics Data System (ADS)

    Tengeler, Sven; Kaiser, Bernhard; Chaussende, Didier; Jaegermann, Wolfram

    2017-04-01

    The electronic states of the (001) 3C SiC/Ni interface prior and post annealing are investigated via an in situ XPS interface experiment, allowing direct observation of the induced band bending and the transformation from Schottky to ohmic behaviour for the first time. A single domain (001) 3C SiC sample was prepared via wet chemical etching. Nickel was deposited on the sample in multiple in situ deposition steps via RF sputtering, allowing observation of the 3C SiC/Ni interface formation. Over the course of the experiments, an upward band bending of 0.35 eV was observed, along with defect induced Fermi level pinning. This indicates a Schottky type contact behaviour with a barrier height of 0.41 eV. The subsequent annealing at 850 °C for 5 min resulted in the formation of a Ni2Si layer and a reversal of the band bending to 0.06 eV downward. Thus explaining the ohmic contact behaviour frequently reported for annealed n-type 3C SiC/Ni contacts.

  15. Corrosion effects on friction factors

    SciTech Connect

    Magleby, H.L.; Shaffer, S.J.

    1996-03-01

    This paper presents the results of NRC-sponsored material specimen tests that were performed to determine if corrosion increases the friction factors of sliding surfaces of motor-operated gate valves, which could require higher forces to close and open safety-related valves when subjected to their design basis differential pressures. Friction tests were performed with uncorroded specimens and specimens subjected to accelerated corrosion. Preliminary tests at ambient conditions showed that corrosion increased the friction factors, indicating the need for additional tests duplicating valve operating parameters at hot conditions. The additional tests showed friction factors of corroded specimens were 0.1 to 0.2 higher than for uncorroded specimens, and that the friction factors of the corroded specimens were not very dependent on contact stress or corrosion film thickness. The measured values of friction factors for the three corrosion films tested (simulating three operating times) were in the range of 0.3 to 0.4. The friction factor for even the shortest simulated operating time was essentially the same as the others, indicating that the friction factors appear to reach a plateau and that the plateau is reached quickly.

  16. Three-dimensional finite-element models on the deformation of forearcs caused by aseismic ridge subduction: The role of ridge shape, friction coefficient of the plate interface and mechanical properties of the forearc

    NASA Astrophysics Data System (ADS)

    Zeumann, Stefanie; Hampel, Andrea

    2016-08-01

    Geological and geophysical data show that the forearc of subduction zones experiences strong deformation during the subduction of aseismic oceanic ridges. In order to better understand ridge-related forearc deformation patterns, we performed a series of three-dimensional finite-element models, in which we varied the ridge shape, the friction coefficient of the plate interface and the mechanical strength of the forearc. Experiments were carried out for migrating/non-migrating ridges and accretive/erosive margins, respectively. Our results show that the subducting ridge uplifts the forearc and induces horizontal displacements that alter the strain regime of both erosive and accretive forearcs. Generally, shortening prevails in front of the ridge, while domains of shortening and extension exist above the ridge. Models with stationary ridges show high uplift rates only above the ridge tip, whereas the forearc above migrating ridges experiences uplift above the leading ridge flank and subsequent subsidence above the trailing flank. The height and width of the ridge as well as the friction coefficient of the plate interface have the largest effect on the forearc deformation patterns, whereas the mechanical strength of the forearc plays a lesser role. Forearc indentation at the trench is largest for high and broad ridges, high friction coefficients and/or weak forearc material. Shortening and extension of the forearc above the ridge are more intense for high and narrow ridges. Our model results provide information about the distribution of ridge-induced displacements and strain fields and hence help to identify deformation patterns caused by subducting aseismic ridges in nature.

  17. Thick Low-Friction nc-MeC/a-C Nanocomposite Coatings on Ti-6Al-4V Alloy: Microstructure and Tribological Properties in Sliding Contact with a Ball

    NASA Astrophysics Data System (ADS)

    Zimowski, Sławomir; Moskalewicz, Tomasz; Wendler, Bogdan; Kot, Marcin; Czyrska-Filemonowicz, Aleksandra

    2014-08-01

    In this paper, we show that duplex surface treatment, combining oxygen diffusion hardening with the subsequent deposition of thick, low-friction nanocomposite nc-MeC/a-C coatings to improve the tribological properties of the Ti-6Al-4V alloy. We have synthesized, in a magnetron sputtering process, the nanocomposite nc-MeC/a-C coatings (where Me denotes W or Ti transition metal) consisting of two dissimilar materials (nanocrystallites of transition metal carbides MeC and an amorphous carbon matrix a-C). The nano and microstructure of the substrate material and coatings were examined with the use of scanning and transmission electron microscopy as well as by X-ray diffractometry. It was found that different carbide nanocrystals of the same transition metal were embedded in an amorphous carbon matrix of both coatings. The HRTEM analysis indicated that the volume fraction of tungsten carbides in the nc-WC/a-C coating was equal to 13 pct, whereas in the nc-TiC/a-C one the volume fraction of the titanium carbides was equal to just 3 pct. The tribological properties, hardness, and scratch resistance of the coatings were investigated as well. The coefficient of friction (COF) of the coatings during dry sliding against 6 mm diameter alumina ball reached very low value, 0.05, in comparison with an oxygen-hardened alloy, whose COF was equal to 0.8. This low-friction effect of the coatings has been attributed to the formation of a self-lubricating film in sliding contact. The coatings exhibited similar failure morphology in the scratch tests. Even though the hardness was rather low, the coatings exhibited a very good wear resistance during sliding friction. The wear rate of the nc-WC/a-C coating was equal to 0.08 × 10-6 mm3 N-1 m-1 and for the nc-TiC/a-C one it was 0.28 × 10-6 mm3 N-1 m-1.

  18. Contacts between domestic livestock and wildlife at the Kruger National Park Interface of the Republic of South Africa.

    PubMed

    Brahmbhatt, Dipa P; Fosgate, Geoffrey T; Dyason, Edwin; Budke, Christine M; Gummow, Bruce; Jori, Ferran; Ward, Michael P; Srinivasan, R

    2012-01-01

    One of the most important transboundary animal diseases (TADs) in the southern African region is foot-and-mouth disease (FMD). In this region, a pathway for spread of FMD virus is contacts between cattle and certain species of wildlife. The objective of this study was to evaluate contacts between cattle and wildlife in the Kruger National Park (KNP) and the adjacent Limpopo province for the time periods October 2006 to March 2007 and April to September 2007. In this study, 87 livestock owners and 57 KNP field rangers were interviewed. Fifteen (17%) livestock owners reported contacts between wildlife and cattle. More livestock owners reported observing contacts between cattle and all wildlife species during October-March than April-September (p=0.012). However, no difference was found between these periods for contacts between cattle and individual wildlife species. A total of 18 (32%) field rangers reported contacts between cattle and wildlife. The most common species-specific contacts were between cattle and buffalo (63/year), cattle and impala (17/year) and cattle and lion (10/year). There were no significant differences in rangers reporting observed contacts between cattle and wildlife during October-March versus April-September or between rangers reporting observed contacts outside versus within the KNP. Overall, there was no evidence of higher contact rates between cattle and wildlife in the study area during October-March compared to April-September. Contact data collected in this study can be used to better understand the transmission of FMD virus in this region.

  19. 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

  20. Slow frictional waves

    NASA Astrophysics Data System (ADS)

    Viswanathan, Koushik; Sundaram, Narayan; Chandrasekar, Srinivasan

    Stick-slip, manifest as intermittent tangential motion between two dry solid surfaces, is a friction instability that governs diverse phenomena from automobile brake squeals to earthquakes. We show, using high-speed in situ imaging of an adhesive polymer interface, that low velocity stick-slip is fundamentally of three kinds, corresponding to passage of three different surface waves -- separation pulses, slip pulses and the well-known Schallamach waves. These waves, traveling much slower than elastic waves, have clear distinguishing properties. Separation pulses and Schallamach waves involve local interface separation, and propagate in opposite directions while slip pulses are characterized by a sharp stress front and do not display any interface detachment. A change in the stick-slip mode from separation to slip pulse is effected simply by increasing the normal force. Together, these three waves constitute all possible stick-slip modes in adhesive friction and are shown to have direct analogues in muscular locomotory waves in soft bodied invertebrates. A theory for slow wave propagation is also presented which is capable of explaining the attendant interface displacements, velocities and stresses.

  1. Novel friction law for the static friction force based on local precursor slipping.

    PubMed

    Katano, Yu; Nakano, Ken; Otsuki, Michio; Matsukawa, Hiroshi

    2014-09-10

    The sliding of a solid object on a solid substrate requires a shear force that is larger than the maximum static friction force. It is commonly believed that the maximum static friction force is proportional to the loading force and does not depend on the apparent contact area. The ratio of the maximum static friction force to the loading force is called the static friction coefficient µM, which is considered to be a constant. Here, we conduct experiments demonstrating that the static friction force of a slider on a substrate follows a novel friction law under certain conditions. The magnitude of µM decreases as the loading force increases or as the apparent contact area decreases. This behavior is caused by the slip of local precursors before the onset of bulk sliding and is consistent with recent theory. The results of this study will develop novel methods for static friction control.

  2. Argonne Engine Friction Study Phase 2 Final Report

    SciTech Connect

    Fox, Issac; Torbeck, Troy; Brogdon, Bill

    2002-01-01

    Argonne National Laboratory (ANL) has developed a process for making near frictionless carbon (NFC) coatings and depositing them on metal substrates. Friction reductions approaching an order of magnitude have been measured in laboratory tests. While there are many potential applications for such coatings, friction reduction in internal combustion engines is of particular interest due to the apparent fuel savings potential. Ricardo has performed a program of work to estimate potential fuel economy improvements due to the application of such a coating at key interfaces within a diesel engine typical of those found in large trucks. In the first phase of this work, fuel economy improvements due to the application of coatings without changes to the lubricant were calculated. In the second phase of this work, the combined effects of changes in lubricant viscosity and application of a low-friction coating were calculated. Piston, ring pack, journal bearing, and valvetrain simulations have been performed, using existing models of representative engines, with various degrees of friction reduction applied at important interfaces, for several lubricant viscosity grades. The simulations were run at 8 specific operating points to allow approximation of engine performance over the FTP test cycle. Changes in fuel consumption and predicted metal-to-metal contact severity were calculated for each case. Results from the first phase of the work showed that application of a friction-reducing surface treatment, like the NFC coatings, at the piston rings and skirt, and at key interfaces within the valvetrain, is expected to result in a reduction in fuel consumption of 0.43% to 0.81% over the FTP heavy duty test cycle, with no changes to the engine lubricant. Results from the second phase of the work showed that the combination of reduced lubricant viscosity and reduced asperity friction coefficient can result in fuel economy improvements of nearly 5% over the FTP HD cycle. Metal

  3. Rate-dependent frictional adhesion in natural and synthetic gecko setae.

    PubMed

    Gravish, Nick; Wilkinson, Matt; Sponberg, Simon; Parness, Aaron; Esparza, Noe; Soto, Daniel; Yamaguchi, Tetsuo; Broide, Michael; Cutkosky, Mark; Creton, Costantino; Autumn, Kellar

    2010-02-06

    Geckos owe their remarkable stickiness to millions of dry, hard setae on their toes. In this study, we discovered that gecko setae stick more strongly the faster they slide, and do not wear out after 30,000 cycles. This is surprising because friction between dry, hard, macroscopic materials typically decreases at the onset of sliding, and as velocity increases, friction continues to decrease because of a reduction in the number of interfacial contacts, due in part to wear. Gecko setae did not exhibit the decrease in adhesion or friction characteristic of a transition from static to kinetic contact mechanics. Instead, friction and adhesion forces increased at the onset of sliding and continued to increase with shear speed from 500 nm s(-1) to 158 mm s(-1). To explain how apparently fluid-like, wear-free dynamic friction and adhesion occur macroscopically in a dry, hard solid, we proposed a model based on a population of nanoscopic stick-slip events. In the model, contact elements are either in static contact or in the process of slipping to a new static contact. If stick-slip events are uncorrelated, the model further predicted that contact forces should increase to a critical velocity (V*) and then decrease at velocities greater than V*. We hypothesized that, like natural gecko setae, but unlike any conventional adhesive, gecko-like synthetic adhesives (GSAs) could adhere while sliding. To test the generality of our results and the validity of our model, we fabricated a GSA using a hard silicone polymer. While sliding, the GSA exhibited steady-state adhesion and velocity dependence similar to that of gecko setae. Observations at the interface indicated that macroscopically smooth sliding of the GSA emerged from randomly occurring stick-slip events in the population of flexible fibrils, confirming our model predictions.

  4. Multiscale physics-based modeling of friction

    NASA Astrophysics Data System (ADS)

    Eriten, Melih

    Frictional contacts between solids exist in nature and in a wide range of engineering applications. Friction causes energy loss, and it is the main source of wear and surface degradation which limits the lifetime of mechanical systems. Yet, friction is needed to walk, run, accelerate, slow down or stop moving systems. Whether desirable or not, friction is a very complex physical phenomenon. The behavior of systems with friction is nonlinear, and the physical mechanisms governing friction behavior span a wide range of spatial and temporal scales. A thorough study of friction should employ experimentalists and theoreticians in chemistry, materials science, tribology, mechanics, dynamics, and structural engineering. High spatial and temporal resolutions are required to capture and model essential physics of a frictional contact. However, such a detailed model is impractical in large-scale structural dynamics simulations; especially since frictional contacts can be numerous in a given application. Reduced-order models (ROMs) achieve broader applicability by compromising several aspects and accounting for the important physics. Hence, rather simple Coulomb friction is still the most ubiquitous model in the modeling and simulation literature. As an alternative, a reduced-order friction model built-up from micromechanics of surfaces is proposed in this work. Continuum-scale formulation of pre-sliding friction behavior is combined with material-strength-based friction coefficients to develop a physics-based friction model at asperity-scale. Then, the statistical summation technique is utilized to build a multiscale modeling framework. A novel joint fretting setup is designed for friction experiments in a practical setting, and the developed models are tested. Both asperity and rough surface friction models show good agreement with experimental data. The influences of materials, surface roughness and contact contamination on the friction are also studied. Finally, the

  5. Evidence of contact epitaxy in the self-assembly of HgSe nanocrystals formed at a liquid–liquid interface

    NASA Astrophysics Data System (ADS)

    Maiti, Santanu; Sanyal, Milan K.; Jana, Manoj K.; Runge, Benjamin; Murphy, Bridget M.; Biswas, Kanishka; Rao, C. N. R.

    2017-03-01

    The grazing incidence x-ray scattering results presented here show that the self-assembly process of HgSe nanocrystals formed at a liquid–liquid interface is quite different along the in-plane direction and across the interface. In situ x-ray reflectivity and ex situ microscopy measurements suggest quantized out-of-plane growth for HgSe nanoparticles of a size of about 1.0+/- 0.2 nm initially. Grazing incidence small-angle x-ray scattering measurements for films transferred from the water–toluene interface at various stages of reaction show that these nanoparticles first form random clusters with an average radius of 2.2 nm, giving rise to equally spaced rings of several orders. Finally, these clusters self-organize into face-centered cubic superstructures, giving sharp x-ray diffraction peaks oriented normal to the liquid–liquid interface with more than 100 nm-coherent domains. We also observed the x-ray diffraction pattern of the HgSe crystalline phase, with the superlattice peaks in these grazing incidence measurements of the transferred films. The electron microscopy and atomic force microscopy results support the x-ray observation of the self-organization of HgSe nanocrystals into close-packed superlattices. These results show that capillary wave fluctuation promotes the oriented attachment of clusters at the liquid–liquid interface, giving direct experimental evidence of contact epitaxy.

  6. Evidence of contact epitaxy in the self-assembly of HgSe nanocrystals formed at a liquid-liquid interface.

    PubMed

    Maiti, Santanu; Sanyal, Milan K; Jana, Manoj K; Runge, Benjamin; Murphy, Bridget M; Biswas, Kanishka; Rao, C N R

    2017-03-08

    The grazing incidence x-ray scattering results presented here show that the self-assembly process of HgSe nanocrystals formed at a liquid-liquid interface is quite different along the in-plane direction and across the interface. In situ x-ray reflectivity and ex situ microscopy measurements suggest quantized out-of-plane growth for HgSe nanoparticles of a size of about [Formula: see text] nm initially. Grazing incidence small-angle x-ray scattering measurements for films transferred from the water-toluene interface at various stages of reaction show that these nanoparticles first form random clusters with an average radius of 2.2 nm, giving rise to equally spaced rings of several orders. Finally, these clusters self-organize into face-centered cubic superstructures, giving sharp x-ray diffraction peaks oriented normal to the liquid-liquid interface with more than 100 nm-coherent domains. We also observed the x-ray diffraction pattern of the HgSe crystalline phase, with the superlattice peaks in these grazing incidence measurements of the transferred films. The electron microscopy and atomic force microscopy results support the x-ray observation of the self-organization of HgSe nanocrystals into close-packed superlattices. These results show that capillary wave fluctuation promotes the oriented attachment of clusters at the liquid-liquid interface, giving direct experimental evidence of contact epitaxy.

  7. Frictional properties of jointed welded tuff

    SciTech Connect

    Teufel, L.W.

    1981-07-01

    The results of the experiments on simulated joints in welded tuff from the Grouse Canyon Member of the Belted Range Tuff warrant the following conclusions: (1) The coefficient of friction of the joints is independent of normal stress at a given sliding velocity. (2) The coefficient of friction increases with both increasing time of stationary contact and decreasing sliding velocity. (3) Time and velocity dependence of friction is due to an increase in the real area of contact on the sliding surface, caused by asperity creep. (4) Joints in water-saturated tuff show a greater time and velocity dependence of friction than those in dehydrated tuff. (5) The enhanced time and velocity dependence of friction with water saturation is a result of increased creep at asperity contacts, which is in turn due to a reduction in the surface indentation hardness by hydrolytic weakening and/or stress corrosion cracking.

  8. A Model for Static and Dry Friction

    NASA Astrophysics Data System (ADS)

    Daly, Christopher

    2005-03-01

    It will be shown that the Muser-Robbins (MR) model, consisting of mobile molecules trapped between two incommensurate crystalline solids, exhibits many of the qualitative features of friction between macroscopic solids, such as the result that the static friction is greater than the kinetic friction, stick-slip motion and a force of static friction which increases as a function of the time that the two solids are in contact and stationary. At zero temperature, the kinetic friction is highly sensitive to the direction of sliding, but this sensitivity decreases markedly as the temperature rises. At low temperatures (with the surfaces stationary for a relatively long time), the model gives a static friction approximately 3 times larger than the kinetic friction for sufficiently slow velocities, but this ratio decreases steadily as the temperature is increased.

  9. Interfacial deformation and friction heating in ultrasonic Al ribbon bonding

    NASA Astrophysics Data System (ADS)

    Takahashi, Yasuo; Maeda, Masakatsu; Ando, Masaya; Yamaguchi, Eito

    2014-08-01

    The interfacial deformation and friction behavior between an Al ribbon and an electric pad (or substrate) during ultrasonic bonding is analyzed, based on numerical simulation and experimental results. The friction heating is estimated by the friction slip work at the bonding interface between the ribbon and pad. The temperature rise of the bonding interface is calculated by the numerical simulation and compared with the experimental results. It is suggested that the electric pad reduces the temperature rise, as compared to the bonding process without a pad. The shear stress at the bonding interface increases as the bonding progresses. The frictional slip due to adhesion increases stress and heats the bond interface.

  10. Effect of Operating Conditions on Tribological Response of Al Al Sliding Electrical Interface

    SciTech Connect

    Bansal, Dinesh G; Streator, Jeffrey L

    2011-01-01

    Aluminum is widely used in electrical contacts due to its electrical properties and inexpensiveness when compared to copper. In this study, we investigate the influence of operating conditions like contact load (pressure), sliding speed, current, and surface roughness on the electrical and tribological behavior of the interface. The tests are conducted on a linear, pin-on-flat tribo-simulator specially designed to investigate electrical contacts under high contact pressures and high current densities. Control parameters include sliding speed, load, current, and surface roughness. The response of the interface is evaluated in the light of coefficient of friction, contact resistance, contact voltage, mass loss of pins, and interfacial temperature rise. As compared to sliding speed, load, and roughness, current is found to have the greatest influence on the various measured parameters. Under certain test conditions, the interface operates in a voltage saturation regime, wherein increase in current do not result in any increase in contact voltage. Within the voltage saturation regime the coefficient of friction tends to be lower, a result that is attributed to the higher temperatures associated with the higher voltage (and resulting material softening). Higher interfacial temperatures also appear to be responsible for the higher wear rates observed at higher current levels as well as lower coefficients of friction for smoother surfaces in the presence of current.

  11. Mixtures of latex particles and the surfactant of opposite charge used as interface stabilizers--influence of particle contact angle, zeta potential, flocculation and shear energy.

    PubMed

    Deleurence, Rémi; Parneix, Caroline; Monteux, Cécile

    2014-09-28

    We investigate the stabilization of air-water interfaces by mixtures of negatively charged latex particles (sulfate polystyrene) and cationic surfactants (alkyl trimethylammonium bromides). First we report results concerning the binding of surfactant molecules to the latex particles. As the surfactant concentration increases, the charge of the particles reverses, from negative to positive, because CnTAB first binds electrostatically to the latex particles and then through hydrophobic interaction with the monolayer already adsorbed on the particles as well as directly with the hydrophobic surface of the latex. Over a large range of surfactant concentrations around the charge inversion, a strong flocculation is observed and 100 μm large aggregates form in the suspension. Unlike previous studies published on mixtures of inorganic particles with oppositely charged surfactants, we show that we can vary the sign of the zeta potential of the particles without changing the contact angle of the particles over a large range of surfactant concentrations. Indeed, the latex particles that we study are more hydrophobic than inorganic particles, hence adding moderate concentrations of the surfactant results in a weak variation of the contact angle while the charge of the particles can be reversed. This enables decoupling of the effect of zeta potential and contact angle on the interfacial properties of the mixtures. Our study shows that the contact angle and the charge of the particles are not sufficient parameters to control the foam properties, and the key-parameters are the flocculation state and the shear energy applied to produce the foam. Indeed, flocculated samples, whatever the sign of the zeta potential, enable production of a stable armour at the interface. The large aggregates do not adsorb spontaneously at the interface because of their large size, however when a large shear energy is used to produce the foam very stable foam is obtained, where particles are trapped

  12. On the dependency of friction on load: Theory and experiment

    NASA Astrophysics Data System (ADS)

    Braun, O. M.; Steenwyk, B.; Warhadpande, A.; Persson, B. N. J.

    2016-03-01

    In rubber friction studies it is often observed that the kinetic friction coefficient depends on the nominal contact pressure. This is usually due to frictional heating, which softens the rubber, increases the area of contact, and (in most cases) reduces the viscoelastic contribution to the friction. In this paper we present experimental results showing that the rubber friction also depends on the nominal contact pressure at such low sliding speed that frictional heating is negligible. This effect has important implications for rubber sliding dynamics, e.g., in the context of the tire-road grip. We attribute this effect to the viscoelastic coupling between the macroasperity contact regions, and present a simple earthquakelike model and numerical simulations supporting this picture. The mechanism for the dependency of the friction coefficient on the load considered is very general, and is relevant for non-rubber materials as well.

  13. Friction and roughness of a melting rock surface

    NASA Astrophysics Data System (ADS)

    Nielsen, S.; di Toro, G.; Griffith, W. A.

    2010-07-01

    Under extreme conditions like those encountered during earthquake slip, frictional melt is likely to occur. It has been observed on ancient faults that the melt is mostly extruded toward local extensional jogs or lateral tension cracks. In the case of laboratory experiments with a rotary shear apparatus, melt is extruded from the sample borders. When this happens, a thin and irregular melt layer is formed whereby the normal load is still in part supported by contact asperities under an incipient yield condition (as in dry friction models), but also, in the interstices between asperities, by the pressure of the viscous fluid wetting the interface. In addition, roughness of the surface is dynamically reshaped by the melting process of an inhomogeneous material (polymineralic rock). In particular, we argue that the roughness of the melting surface decreases with melting rate and temperature gradient perpendicular to the fault. Taking into account the above conditions, we obtain an expression for the average melt layer thickness and viscous pressure that may be used in estimates of friction in the presence of melt. We argue that the ratio of melt thickness to roughness depends on sliding velocity; such a ratio may be used as a gauge of slip-rate during fossil earthquakes on faults bearing pseudotachylite (solidified melt). Finally, we derive an improved analytical solution for friction in the presence of melt including the effect of roughness evolution.

  14. Friction and roughness of a melting rock surface

    NASA Astrophysics Data System (ADS)

    Nielsen, Stefan; di Toro, Giulio; Griffith, Ashley

    2010-05-01

    Under extreme conditions like those encountered during earthquake slip, frictional melt is likely to occur. It has been observed on fossil faults that the melt is mostly extruded toward local extensional jogs or lateral tension cracks. A similar condition is reproduced in laboratory experiments with a rotary shear apparatus. When this happens, a thin and irregular melt layer is formed whereby the normal load is still in part supported by contact asperities under an incipient yield condition (as in dry friction models), but also, in the interstices between asperities, by the pressure of the viscous fluid wetting the interface. In addition, roughness of the surface is dynamically reshaped by the melting process of an inhomogeneous material (polymineral rock). In particular, we argue that the roughness decreases with temperature gradient and the melting rate. Taking into account the above conditions, we obtain an expression for the average melt layer thickness and viscous pressure that may be used in estimates of friction in the presence of melt. We argue that the ratio of melt thickness to roughness depends on sliding velocity; such a ratio may be used as a gauge of slip-rate during fossil earthquakes on faults bearing pseudotachylite (solidified melt). Finally, we derive an improved analytical solution for friction in the presence of melt including the effect of roughness evolution.

  15. The CAD-score web server: contact area-based comparison of structures and interfaces of proteins, nucleic acids and their complexes.

    PubMed

    Olechnovič, Kliment; Venclovas, Ceslovas

    2014-07-01

    The Contact Area Difference score (CAD-score) web server provides a universal framework to compute and analyze discrepancies between different 3D structures of the same biological macromolecule or complex. The server accepts both single-subunit and multi-subunit structures and can handle all the major types of macromolecules (proteins, RNA, DNA and their complexes). It can perform numerical comparison of both structures and interfaces. In addition to entire structures and interfaces, the server can assess user-defined subsets. The CAD-score server performs both global and local numerical evaluations of structural differences between structures or interfaces. The results can be explored interactively using sortable tables of global scores, profiles of local errors, superimposed contact maps and 3D structure visualization. The web server could be used for tasks such as comparison of models with the native (reference) structure, comparison of X-ray structures of the same macromolecule obtained in different states (e.g. with and without a bound ligand), analysis of nuclear magnetic resonance (NMR) structural ensemble or structures obtained in the course of molecular dynamics simulation. The web server is freely accessible at: http://www.ibt.lt/bioinformatics/cad-score.

  16. Theoretical model for adhesive friction between elastomers and rough solid surfaces

    NASA Astrophysics Data System (ADS)

    Momozono, Satoshi; Nakamura, Kenya; Kyogoku, Keiji

    2010-03-01

    A theoretical model for the adhesive friction between elastomers and rough solid surfaces is proposed on the basis of opening crack propagation processes at the boundary of the contact interfaces and the rate processes of formation of molecular bonds on the solid surface. This model, which is expressed as a product of the terms related to the two abovementioned processes, requires some measurable and fitted parameters such as the frictional shear strength expressed as a function of viscoelastic dissipation, rate-dependent elasticity, density of bonded molecular chains at a contact junction, critical velocity related to viscoelastic relaxation, and critical velocity related to the rate process of formation of molecular bonds on the solid surface. The friction-velocity relationship exhibits a remarkable fit to previously obtained experimental results for polymers such as engineering rubber, gels, and plastics (glassy polymers), and all fitting parameters are physically reasonable. The viscoelastic index "n" is also related to the "glass-to-rubber transition" of a nanometer-thick polymer layer for frictional behavior. Thus, from a practical viewpoint, this model can be used effectively for fitting the adhesive friction behavior of polymers.

  17. 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.

  18. Elastic contact mechanics: percolation of the contact area and fluid squeeze-out.

    PubMed

    Persson, B N J; Prodanov, N; Krick, B A; Rodriguez, N; Mulakaluri, N; Sawyer, W G; Mangiagalli, P

    2012-01-01

    The dynamics of fluid flow at the interface between elastic solids with rough surfaces depends sensitively on the area of real contact, in particular close to the percolation threshold, where an irregular network of narrow flow channels prevails. In this paper, numerical simulation and experimental results for the contact between elastic solids with isotropic and anisotropic surface roughness are compared with the predictions of a theory based on the Persson contact mechanics theory and the Bruggeman effective medium theory. The theory predictions are in good agreement with the experimental and numerical simulation results and the (small) deviation can be understood as a finite-size effect. The fluid squeeze-out at the interface between elastic solids with randomly rough surfaces is studied. We present results for such high contact pressures that the area of real contact percolates, giving rise to sealed-off domains with pressurized fluid at the interface. The theoretical predictions are compared to experimental data for a simple model system (a rubber block squeezed against a flat glass plate), and for prefilled syringes, where the rubber plunger stopper is lubricated by a high-viscosity silicon oil to ensure functionality of the delivery device. For the latter system we compare the breakloose (or static) friction, as a function of the time of stationary contact, to the theory prediction.

  19. Observation of a Dislocation-Related Interfacial Friction Mechanism in Mobile Solid 4He

    NASA Astrophysics Data System (ADS)

    Eyal, Anna; Livne, Ethan; Polturak, Emil

    2016-04-01

    We report a study of the temperature and stress dependence of the friction associated with a relative motion of crystallites of solid 4He in contact with each other. A situation where such motion exists emerges spontaneously during a disordering of a single crystal contained inside an annular sample space of a torsional oscillator (TO). Under the torque applied by the oscillating walls of the TO these crystallites move relative to each other, generating measurable dissipation at their interface. We studied this friction between 0.5 and 1.8 K in solid samples grown from commercially pure 4He and from a 100 ppm 3He-4He mixture. The data were analyzed by modeling the TO as a driven harmonic oscillator. In this model, an analysis of the resonant frequency and amplitude of the TO yields the temperature dependence of the friction coefficient. By fitting the data to specific forms, we found that over our temperature range, the dominant friction mechanism associated with the interfacial motion of the crystallites results from climb of individual dislocations. The characteristic energy scale associated with this friction can be 3 or 6 K, depending on the sample. The motion of the solid in the presence of such friction can perhaps be described as the low limit of "slip-stick" motion.

  20. Annealing behaviors of vacancy-type defects near interfaces between metal contacts and GaN probed using a monoenergetic positron beam

    NASA Astrophysics Data System (ADS)

    Uedono, Akira; Fujishima, Tatsuya; Piedra, Daniel; Yoshihara, Nakaaki; Ishibashi, Shoji; Sumiya, Masatomo; Laboutin, Oleg; Johnson, Wayne; Palacios, Tomás

    2014-08-01

    Vacancy-type defects near interfaces between metal contacts and GaN grown on Si substrates by metal organic chemical vapor deposition have been studied using a monoenergetic positron beam. Measurements of Doppler broadening spectra of the annihilation radiation for Ti-deposited GaN showed that optically active vacancy-type defects were introduced below the Ti/GaN interface after annealing at 800 °C. Charge transition of those defects due to electron capture was observed and was found to correlate with a yellow band in the photoluminescence spectrum. The major defect species was identified as vacancy clusters such as three to five Ga-vacancies coupled with multiple nitrogen-vacancies. The annealing behaviors of vacancy-type defects in Ti-, Ni-, and Pt-deposited GaN were also examined.

  1. Annealing behaviors of vacancy-type defects near interfaces between metal contacts and GaN probed using a monoenergetic positron beam

    SciTech Connect

    Uedono, Akira Yoshihara, Nakaaki; Fujishima, Tatsuya; Piedra, Daniel; Palacios, Tomás; Ishibashi, Shoji; Sumiya, Masatomo; Laboutin, Oleg; Johnson, Wayne

    2014-08-04

    Vacancy-type defects near interfaces between metal contacts and GaN grown on Si substrates by metal organic chemical vapor deposition have been studied using a monoenergetic positron beam. Measurements of Doppler broadening spectra of the annihilation radiation for Ti-deposited GaN showed that optically active vacancy-type defects were introduced below the Ti/GaN interface after annealing at 800 °C. Charge transition of those defects due to electron capture was observed and was found to correlate with a yellow band in the photoluminescence spectrum. The major defect species was identified as vacancy clusters such as three to five Ga-vacancies coupled with multiple nitrogen-vacancies. The annealing behaviors of vacancy-type defects in Ti-, Ni-, and Pt-deposited GaN were also examined.

  2. The silicon/zinc oxide interface in amorphous silicon-based thin-film solar cells: Understanding an empirically optimized contact

    SciTech Connect

    Gerlach, D.; Wilks, R. G.; Wimmer, M.; Felix, R.; Gorgoi, M.; Lips, K.; Rech, B.; Wippler, D.; Mueck, A.; Meier, M.; Huepkes, J.; Lozac'h, M.; Ueda, S.; Sumiya, M.; Yoshikawa, H.; Kobayashi, K.; Baer, M.

    2013-07-08

    The electronic structure of the interface between the boron-doped oxygenated amorphous silicon 'window layer' (a-SiO{sub x}:H(B)) and aluminum-doped zinc oxide (ZnO:Al) was investigated using hard x-ray photoelectron spectroscopy and compared to that of the boron-doped microcrystalline silicon ({mu}c-Si:H(B))/ZnO:Al interface. The corresponding valence band offsets have been determined to be (-2.87 {+-} 0.27) eV and (-3.37 {+-} 0.27) eV, respectively. A lower tunnel junction barrier height at the {mu}c-Si:H(B)/ZnO:Al interface compared to that at the a-SiO{sub x}:H(B)/ZnO:Al interface is found and linked to the higher device performances in cells where a {mu}c-Si:H(B) buffer between the a-Si:H p-i-n absorber stack and the ZnO:Al contact is employed.

  3. Towards an Accurate Measurement of Thermal Contact Resistance at Chemical Vapor Deposition-Grown Graphene/SiO2 Interface Through Null Point Scanning Thermal Microscopy.

    PubMed

    Chung, Jaehun; Hwang, Gwangseok; Kim, Hyeongkeun; Yang, Wooseok; Kwon, Ohmyoung

    2015-11-01

    In the development of graphene-based electronic devices, it is crucial to characterize the thermal contact resistance between the graphene and the substrate precisely. In this study, we demonstrate that the thermal contact resistance between CVD-grown graphene and SiO2 substrate can be obtained by measuring the temperature drop occurring at the graphene/SiO2 interface with null point scanning thermal microscopy (NP SThM), which profiles the temperature distribution quantitatively with nanoscale spatial resolution (-50 nm) without the shortcomings of the conventional SThM. The thermal contact resistance between the CVD-grown graphene and SiO2 substrate is measured as (1.7 ± 0.27) x 10(-6) M2K/W. This abnormally large thermal contact resistance seems to be caused by extrinsic factors such as ripples and metal-based contamination, which inevitably form in CVD-grown graphene during the production and transfer processes.

  4. Friction characteristics of Cd-rich carbonate films on calcite surfaces: implications for compositional differentiation at the nanometer scale

    PubMed Central

    2009-01-01

    Lateral Force Microscopy (LFM) studies were carried out on cleaved calcite sections in contact with solutions supersaturated with respect to otavite (CdCO3) or calcite-otavite solid solutions (SS) as a means to examine the potential for future application of LFM as a nanometer-scale mineral surface composition mapping technique. Layer-by-layer growth of surface films took place either by step advancement or by a surface nucleation and step advancement mechanisms. Friction vs. applied load data acquired on the films and the calcite substrate were successfully fitted to the Johnson Kendall Roberts (JKR) model for single asperity contacts. Following this model, friction differences between film and substrate at low loads were dictated by differences in adhesion, whereas at higher load they reflect differences in contact shear strength. In most experiments at fixed load, the film showed higher friction than the calcite surface, but the friction-load dependence for the different surfaces revealed that at low loads (0–40 nN), a calcian otavite film has lower friction than calcite; a result that is contrary to earlier LFM reports of the same system. Multilayer films of calcian-otavite displayed increasing friction with film thickness, consistent with the expectation that the film surface composition will become increasingly Cd-rich with increasing thickness. Both load- and thickness-dependence trends support the hypothesis that the contact shear strength correlates with the hydration enthalpy of the surface ions, thereby imparting friction sensitivity in the LFM to mineral-water interface composition. PMID:19549312

  5. Contact resistivity decrease at a metal/semiconductor interface by a solid-to-liquid phase transitional metallo-organic silver.

    PubMed

    Shin, Dong-Youn; Seo, Jun-Young; Kang, Min Gu; Song, Hee-eun

    2014-09-24

    We present a new approach to ensure the low contact resistivity of a silver paste at a metal/semiconductor interface over a broad range of peak firing temperatures by using a solid-to-liquid phase transitional metallo-organic silver, that is, silver neodecanoate. Silver nanoclusters, thermally derived from silver neodecanoate, are readily dissolved into the melt of metal oxide glass frit even at low temperatures, at which point the molten metal oxide glass frit lacks the dissociation capability of bulk silver into Ag(+) ions. In the presence of O(2-) ions in the melt of metal oxide glass frit, the redox reaction from Ag(+) to Ag(0) augments the noble-metal-assisted etching capability to remove the passivation layer of silicon nitride. Moreover, during the cooling stage, the nucleated silver atoms enrich the content of silver nanocolloids in the solidified metal oxide glass layer. The resulting contact resistivity of silver paste with silver neodecanoate at the metal/semiconductor interface thus remains low-between 4.12 and 16.08 mΩ cm(2)-whereas without silver neodecanoate, the paste exhibits a contact resistivity between 2.61 and 72.38 mΩ cm(2) in the range of peak firing temperatures from 750 to 810 °C. The advantage of using silver neodecanoate in silver paste becomes evident in that contact resistivity remains low over the broad range of peak firing temperatures, thus providing greater flexibility with respect to the firing temperature required in silicon solar cell applications.

  6. Thermodynamics of a Block Sliding across a Frictional Surface

    ERIC Educational Resources Information Center

    Mungan, Carl E.

    2007-01-01

    The following idealized problem is intended to illustrate some basic thermodynamic concepts involved in kinetic friction. A block of mass m is sliding on top of a frictional, flat-topped table of mass M. The table is magnetically levitated, so that it can move without thermal contact and friction across a horizontal floor. The table is initially…

  7. Measurement of Gear Tooth Dynamic Friction

    NASA Technical Reports Server (NTRS)

    Rebbechi, Brian; Oswald, Fred B.; Townsend, Dennis P.

    1996-01-01

    Measurements of dynamic friction forces at the gear tooth contact were undertaken using strain gages at the root fillets of two successive teeth. Results are presented from two gear sets over a range of speeds and loads. The results demonstrate that the friction coefficient does not appear to be significantly influenced by the sliding reversal at the pitch point, and that the friction coefficient values found are in accord with those in general use. The friction coefficient was found to increase at low sliding speeds. This agrees with the results of disc machine testing.

  8. Structure and interface chemistry of MoO{sub 3} back contacts in Cu(In,Ga)Se{sub 2} thin film solar cells

    SciTech Connect

    Simchi, Hamed; Shafarman, William N.; McCandless, Brian E.; Meng, T.

    2014-01-21

    Molybdenum oxide (MoO{sub 3}) is considered as a possible primary back contact for Cu(InGa)Se{sub 2} thin film solar cells for its potential as a transparent back contact for superstrate and bifacial devices. MoO{sub 3} films were deposited on Mo or ITO-coated soda lime glass substrates by reactive rf sputtering in an ambient of Ar + O{sub 2} with O{sub 2}/(O{sub 2} + Ar) = 35% on which Cu(In{sub 0.7}Ga{sub 0.3})Se{sub 2} alloy absorber layers were deposited using multi-source elemental evaporation. Scanning Electron Microscopy studies showed uniform coverage of the as-deposited MoO{sub 3} layer and good adhesion was obtained in all cases. X-ray Photoelectron Spectroscopy depth profile analysis showed that MoSe{sub 2} was not formed at the Cu(InGa)Se{sub 2} interface with either the Mo-MoO{sub 3} or ITO-MoO{sub 3} back contacts. Determination of the valence band offsets showed that the MoO{sub 3} layer at the interface changes the energy band alignment with Cu(InGa)Se{sub 2}, producing a primary contact with lower valence band offset than ITO. Cu(In,Ga)Se{sub 2} thin film solar cells prepared using an as-deposited Mo-MoO{sub 3} back contact yielded a best conversion efficiency of 14%, with V{sub OC} = 647 mV, J{sub SC} = 28.4 mA/cm{sup 2}, and FF = 78.1%. Cells with ITO-MoO{sub 3} back contact showed a best efficiency of 12%, with V{sub OC} = 642 mV, J{sub SC} = 26.8 mA/cm{sup 2}, and FF = 69.2%.

  9. High temperature ceramic interface study

    NASA Technical Reports Server (NTRS)

    Lindberg, L. J.

    1984-01-01

    Monolithic SiC and Si3N4 are susceptible to contact stress damage at static and sliding interfaces. Transformation-toughened zirconia (TTZ) was evaluated under sliding contact conditions to determine if the higher material fracture toughness would reduce the susceptibility to contact stress damage. Contact stress tests were conducted on four commercially available TTZ materials at normal loads ranging from 0.455 to 22.7 kg (1 to 50 pounds) at temperatures ranging from room temperature to 1204C (2200 F). Static and dynamic friction were measured as a function of temperature. Flexural strength measurements after these tests determined that the contact stress exposure did not reduce the strength of TTZ at contact loads of 0.455, 4.55, and 11.3 kg (1, 10, and 25 pounds). Prior testing with the lower toughness SiC and Si3N4 materials resulted in a substantial strength reduction at loads of only 4.55 and 11.3 kg (10 and 25 pounds). An increase in material toughness appears to improve ceramic material resistance to contact stress damage. Baseline material flexure strength was established and the stress rupture capability of TTZ was evaluated. Stress rupture tests determined that TTZ materials are susceptible to deformation due to creep and that aging of TTZ materials at elevated temperatures results in a reduction of material strength.

  10. System dynamic instabilities induced by sliding contact: A numerical analysis with experimental validation

    NASA Astrophysics Data System (ADS)

    Brunetti, J.; Massi, F.; Saulot, A.; Renouf, M.; D`Ambrogio, W.

    2015-06-01

    Mechanical systems present several contact surfaces between deformable bodies. The contact interface can be either static (joints) or in sliding (active interfaces). The sliding interfaces can have several roles and according to their application they can be developed either for maximizing the friction coefficient and the energy dissipation (e.g. brakes) or rather to allow the relative displacement at joints with a maximum efficiency. In both cases the coupling between system and local contact dynamics can bring to system dynamics instabilities (e.g. brake squeal or squeaking of hip prostheses). This results in unstable vibrations of the system, induced by the oscillation of the contact forces. In the literature, a large number of works deal with such kind of instabilities and are mainly focused on applied problems such as brake squeal noise. This paper shows a more general numerical analysis of a simple system constituted by two bodies in sliding contact: a rigid cylinder rotating inside a deformable one. The parametrical Complex Eigenvalue Analysis and the transient numerical simulations show how the friction forces can give rise to in-plane dynamic instabilities due to the interaction between two system modes, even for such a simple system characterized by one deformable body. Results from transient simulations highlight the key role of realistic values of the material damping to have convergence of the model and, consequently, reliable physical results. To this aim an experimental estimation of the material damping has been carried out. Moreover, the simplicity of the system allows for a deeper analysis of the contact instability and a balance of the energy flux among friction, system vibrations and damping. The numerical results have been validated by comparison with experimental ones, obtained by a specific test bench developed to reproduce and analyze the contact friction instabilities.

  11. Adsorption at the solid-liquid interface as the source of contact angle dependence on the curvature of the three-phase line.

    PubMed

    Ward, C A; Sefiane, K

    2010-12-15

    We review the thermodynamic approach to determining the surface tension of solid-fluid interfaces. If the pressure is in the narrow range where the contact angle, θ, can exist, then for isothermal systems, adsorption at the solid-liquid interface affects γ(SL) or θ, but γ(SV) is very nearly equal γ(LV), the surface tension of the adsorbing fluid. For a liquid partially filling a cylinder, the pressure in the liquid phase at the three-phase line, x(3)(L), depends on the curvature of the three-phase line, C(cl), but the line tension can play no role, since it acts perpendicular to the cylinder wall. C(cl) is decreased as the cylinder diameter is increased; x(3)(L) is increased; and θ increases. For a given value of C(cl), x(3)(L) can be changed by rotating the cylinder or by changing the height of the three-phase line in a gravitational field. In all cases, for water in borosilicate glass cylinders, the value of θ is shown to increase as x(3)(L) is increased. This behaviour requires the Gibbsian adsorption at the solid-liquid interface to be negative, indicating the liquid concentration in the interphase is less than that in the bulk liquid. For sessile droplets, the value of θ depends on both x(3)(L) and C(cl). If the value of θ for spherical sessile droplets is measured as a function of C(cl), the adsorption at the solid-liquid interface that would give that dependence can be determined. It is unnecessary to introduce the line tension hypothesis to explain the dependence of θ on C(cl). Adsorption at the solid-liquid interface gives a full explanation.

  12. Modeling of friction-induced deformation and microstructures.

    SciTech Connect

    Michael, Joseph Richard; Prasad, Somuri V.; Jungk, John Michael; Cordill, Megan J.; Bammann, Douglas J.; Battaile, Corbett Chandler; Moody, Neville Reid; Majumdar, Bhaskar Sinha (New Mexico Institure of Mining and Technology)

    2006-12-01

    Frictional contact results in surface and subsurface damage that could influence the performance, aging, and reliability of moving mechanical assemblies. Changes in surface roughness, hardness, grain size and texture often occur during the initial run-in period, resulting in the evolution of subsurface layers with characteristic microstructural features that are different from those of the bulk. The objective of this LDRD funded research was to model friction-induced microstructures. In order to accomplish this objective, novel experimental techniques were developed to make friction measurements on single crystal surfaces along specific crystallographic surfaces. Focused ion beam techniques were used to prepare cross-sections of wear scars, and electron backscattered diffraction (EBSD) and TEM to understand the deformation, orientation changes, and recrystallization that are associated with sliding wear. The extent of subsurface deformation and the coefficient of friction were strongly dependent on the crystal orientation. These experimental observations and insights were used to develop and validate phenomenological models. A phenomenological model was developed to elucidate the relationships between deformation, microstructure formation, and friction during wear. The contact mechanics problem was described by well-known mathematical solutions for the stresses during sliding friction. Crystal plasticity theory was used to describe the evolution of dislocation content in the worn material, which in turn provided an estimate of the characteristic microstructural feature size as a function of the imposed strain. An analysis of grain boundary sliding in ultra-fine-grained material provided a mechanism for lubrication, and model predictions of the contribution of grain boundary sliding (relative to plastic deformation) to lubrication were in good qualitative agreement with experimental evidence. A nanomechanics-based approach has been developed for characterizing the

  13. Microstructural Characterization of the U-9.1Mo Fuel/AA6061 Cladding Interface in Friction-Bonded Monolithic Fuel Plates Irradiated in the RERTR-6 Experiment

    SciTech Connect

    Keiser, Dennis D.; Jue, Jan-Fong; Miller, Brandon; Gan, Jian; Robinson, Adam; Medvedev, Pavel; Madden, James; Wachs, Dan; Clark, Curtis; Meyer, Mitch

    2015-09-03

    Low-enrichment (U-235 < 20%) U-Mo monolithic fuel is being developed for use in research and test reactors. The earliest design for this fuel that was investigated via reactor testing was comprised of a nominally U-10Mo fuel foil encased in AA6061 (Al-6061) cladding. For a fuel design to be deemed adequate for final use in a reactor, it must maintain dimensional stability and retain fission products throughout irradiation, which means that there must be good integrity at the fuel foil/cladding interface. To investigate the nature of the fuel/cladding interface for this fuel type after irradiation, fuel plates that were tested in INL's Advanced Test Reactor (ATR) were subsequently characterized using optical metallography, scanning electron microscopy, and transmission electron microscopy. Results of this characterization showed that the fuel/cladding interaction layers present at the U-Mo fuel/AA6061 cladding interface after fabrication became amorphous during irradiation. Up to two main interaction layers, based on composition, could be found at the fuel/cladding interface, depending on location. After irradiation, an Al-rich layer contained very few fission gas bubbles, but did exhibit Xe enrichment near the AA6061 cladding interface. Another layer, which contained more Si, had more observable fission gas bubbles. Adjacent to the AA6061 cladding were Mg-rich precipitates, which was in close proximity to the region where Xe is observed to be enriched. In samples produced using a focused ion beam at the interaction zone/AA6061 cladding interface were possible indications of porosity/debonding, which suggested that the interface in this location is relatively weak.

  14. Microstructural Characterization of the U-9.1Mo Fuel/AA6061 Cladding Interface in Friction-Bonded Monolithic Fuel Plates Irradiated in the RERTR-6 Experiment

    DOE PAGES

    Keiser, Dennis D.; Jue, Jan-Fong; Miller, Brandon; ...

    2015-09-03

    Low-enrichment (U-235 < 20%) U-Mo monolithic fuel is being developed for use in research and test reactors. The earliest design for this fuel that was investigated via reactor testing was comprised of a nominally U-10Mo fuel foil encased in AA6061 (Al-6061) cladding. For a fuel design to be deemed adequate for final use in a reactor, it must maintain dimensional stability and retain fission products throughout irradiation, which means that there must be good integrity at the fuel foil/cladding interface. To investigate the nature of the fuel/cladding interface for this fuel type after irradiation, fuel plates that were tested inmore » INL's Advanced Test Reactor (ATR) were subsequently characterized using optical metallography, scanning electron microscopy, and transmission electron microscopy. Results of this characterization showed that the fuel/cladding interaction layers present at the U-Mo fuel/AA6061 cladding interface after fabrication became amorphous during irradiation. Up to two main interaction layers, based on composition, could be found at the fuel/cladding interface, depending on location. After irradiation, an Al-rich layer contained very few fission gas bubbles, but did exhibit Xe enrichment near the AA6061 cladding interface. Another layer, which contained more Si, had more observable fission gas bubbles. Adjacent to the AA6061 cladding were Mg-rich precipitates, which was in close proximity to the region where Xe is observed to be enriched. In samples produced using a focused ion beam at the interaction zone/AA6061 cladding interface were possible indications of porosity/debonding, which suggested that the interface in this location is relatively weak.« less

  15. Different contact formations at the interfaces of C60/LiF/Al and C60/LiF/Ag

    NASA Astrophysics Data System (ADS)

    Jeon, Pyungeun; Kang, Seong Jun; Lee, Hyunbok; Lee, Jeihyun; Jeong, Kwangho; Lee, JinWoo; Yi, Yeonjin

    2012-04-01

    C60 has been used as an electron accepting and transporting material in various organic electronic devices. In such devices, Al and Ag have been adopted as a common cathode in combination with electron injection layers (EIL), e.g., LiF. We found that the initial interface formations of C60/LiF/Al and C60/LiF/Ag are quite different in terms of interfacial electronic structures. We measured the interfacial electronic structures with photoemission spectroscopy and found that LiF works well as an EIL on Al but performs poorly on Ag. The origin of this difference could be attributed to the larger interface dipole on Al, highlighting the importance of the choice of cathode materials.

  16. Friction coefficient dependence on electrostatic tribocharging

    NASA Astrophysics Data System (ADS)

    Burgo, Thiago A. L.; Silva, Cristiane A.; Balestrin, Lia B. S.; Galembeck, Fernando

    2013-08-01

    Friction between dielectric surfaces produces patterns of fixed, stable electric charges that in turn contribute electrostatic components to surface interactions between the contacting solids. The literature presents a wealth of information on the electronic contributions to friction in metals and semiconductors but the effect of triboelectricity on friction coefficients of dielectrics is as yet poorly defined and understood. In this work, friction coefficients were measured on tribocharged polytetrafluoroethylene (PTFE), using three different techniques. As a result, friction coefficients at the macro- and nanoscales increase many-fold when PTFE surfaces are tribocharged, but this effect is eliminated by silanization of glass spheres rolling on PTFE. In conclusion, tribocharging may supersede all other contributions to macro- and nanoscale friction coefficients in PTFE and probably in other insulating polymers.

  17. Friction coefficient dependence on electrostatic tribocharging.

    PubMed

    Burgo, Thiago A L; Silva, Cristiane A; Balestrin, Lia B S; Galembeck, Fernando

    2013-01-01

    Friction between dielectric surfaces produces patterns of fixed, stable electric charges that in turn contribute electrostatic components to surface interactions between the contacting solids. The literature presents a wealth of information on the electronic contributions to friction in metals and semiconductors but the effect of triboelectricity on friction coefficients of dielectrics is as yet poorly defined and understood. In this work, friction coefficients were measured on tribocharged polytetrafluoroethylene (PTFE), using three different techniques. As a result, friction coefficients at the macro- and nanoscales increase many-fold when PTFE surfaces are tribocharged, but this effect is eliminated by silanization of glass spheres rolling on PTFE. In conclusion, tribocharging may supersede all other contributions to macro- and nanoscale friction coefficients in PTFE and probably in other insulating polymers.

  18. Model of the Frictional Heating of Inconel 718 and Titanium (ti-6al-4v) in Helium

    NASA Technical Reports Server (NTRS)

    Skowlund, Christopher T.

    1991-01-01

    A computer model of the frictional heating of metals in an inert environment has been developed. The model incorporates the effects of the heat loss from the samples due to conduction, radiation, and convection to the surroundings. This model allows the measured temperatures to be used to determine the amount of heat produced at the interface during the experiment by the sliding contact of two different metallic samples. The results of the simulation for an experiment run at the NASA White Sands Test Facility (WSTF) show that for the same heat production at the interface the heat losses have a significant effect on the temperatures in the samples. But, the heat losses do not significantly affect the different calculated heat flows (or friction coefficients), at the interface, that are necessary to correlate the measured temperatures.

  19. Solid friction between soft filaments

    DOE PAGES

    Ward, Andrew; Hilitski, Feodor; Schwenger, Walter; ...

    2015-03-02

    Any macroscopic deformation of a filamentous bundle is necessarily accompanied by local sliding and/or stretching of the constituent filaments. Yet the nature of the sliding friction between two aligned filaments interacting through multiple contacts remains largely unexplored. Here, by directly measuring the sliding forces between two bundled F-actin filaments, we show that these frictional forces are unexpectedly large, scale logarithmically with sliding velocity as in solid-like friction, and exhibit complex dependence on the filaments’ overlap length. We also show that a reduction of the frictional force by orders of magnitude, associated with a transition from solid-like friction to Stokes’s drag,more » can be induced by coating F-actin with polymeric brushes. Furthermore, we observe similar transitions in filamentous microtubules and bacterial flagella. In conclusion, our findings demonstrate how altering a filament’s elasticity, structure and interactions can be used to engineer interfilament friction and thus tune the properties of fibrous composite materials.« less

  20. Local friction in polyolefin blends

    NASA Astrophysics Data System (ADS)

    Luettmer-Strathmann, Jutta

    2005-07-01

    Processes on different length scales affect the dynamics of chain molecules. The friction experienced by a short chain segment depends on both small-scale chain properties and on the local environment of the segment. As a consequence, the (monomeric) friction coefficients of the two components of a binary polymer blend will, in general, differ from each other and from the friction coefficients of the corresponding melts. In this work, we investigate local friction in polyolefin blends with the aid of a small-scale simulation approach. The polymer chains, in united atom representation, are assumed to occupy the sites of a partially filled simple cubic lattice. The simulation focuses on short chain sections with straight backbones and enumerates all possible binary contacts and relative movements of such sections. By evaluating the exact enumeration results in conjunction with equations of state for the blends, we are able to make predictions about the variation of the friction coefficients with local chain architecture and thermodynamic state (temperature, pressure, and composition). We calculate relative values of friction coefficients at temperatures well above the glass transition for blends of PEP, an alternating copolymer of polyethylene and polypropylene, with polyethylene and polyisobutylene and for blends of polyethylene and atactic polypropylene. We also investigate a blend of PEP with head-to-head polypropylene and compare our results with experimental data.

  1. Solid friction between soft filaments

    PubMed Central

    Ward, Andrew; Hilitski, Feodor; Schwenger, Walter; Welch, David; Lau, A.W. C.; Vitelli, Vincenzo; Mahadevan, L.; Dogic, Zvonimir

    2015-01-01

    Any macroscopic deformation of a filamentous bundle is necessarily accompanied by local sliding and/or stretching of the constituent filaments1,2. Yet the nature of the sliding friction between two aligned filaments interacting through multiple contacts remains largely unexplored. Here, by directly measuring the sliding forces between two bundled F-actin filaments, we show that these frictional forces are unexpectedly large, scale logarithmically with sliding velocity as in solid-like friction, and exhibit complex dependence on the filaments’ overlap length. We also show that a reduction of the frictional force by orders of magnitude, associated with a transition from solid-like friction to Stokes’s drag, can be induced by coating F-actin with polymeric brushes. Furthermore, we observe similar transitions in filamentous microtubules and bacterial flagella. Our findings demonstrate how altering a filament’s elasticity, structure and interactions can be used to engineer interfilament friction and thus tune the properties of fibrous composite materials. PMID:25730393

  2. Area of Interest 1, CO2 at the Interface. Nature and Dynamics of the Reservoir/Caprock Contact and Implications for Carbon Storage Performance

    SciTech Connect

    Mozley, Peter; Evans, James; Dewers, Thomas

    2014-10-31

    We examined the influence of geologic features present at the reservoir/caprock interface on the transmission of supercritical CO2 into and through caprock. We focused on the case of deformation-band faults in reservoir lithologies that intersect the interface and transition to opening-mode fractures in caprock lithologies. Deformation-band faults are exceeding common in potential CO2 injection units and our fieldwork in Utah indicates that this sort of transition is common. To quantify the impact of these interface features on flow and transport we first described the sedimentology and permeability characteristics of selected sites along the Navajo Sandstone (reservoir lithology) and Carmel Formation (caprock lithology) interface, and along the Slickrock Member (reservoir lithology) and Earthy Member (caprock lithology) of the Entrada Sandstone interface, and used this information to construct conceptual permeability models for numerical analysis. We then examined the impact of these structures on flow using single-phase and multiphase numerical flow models for these study sites. Key findings include: (1) Deformation-band faults strongly compartmentalize the reservoir and largely block cross-fault flow of supercritical CO2. (2) Significant flow of CO2 through the fractures is possible, however, the magnitude is dependent on the small-scale geometry of the contact between the opening-mode fracture and the deformation band fault. (3) Due to the presence of permeable units in the caprock, caprock units are capable of storing significant volumes of CO2, particularly when the fracture network does not extend all the way through the caprock. The large-scale distribution of these deformation-bandfault-to-opening-mode-fractures is related to the curvature of the beds, with greater densities of fractures in high curvature regions. We also examined core and outcrops from the Mount Simon Sandstone and Eau Claire

  3. 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

  4. Noncontact friction via capillary shear interaction at nanoscale

    PubMed Central

    Lee, Manhee; Kim, Bongsu; Kim, Jongwoo; Jhe, Wonho

    2015-01-01

    Friction in an ambient condition involves highly nonlinear interactions of capillary force, induced by the capillary-condensed water nanobridges between contact or noncontact asperities of two sliding surfaces. Since the real contact area of sliding solids is much smaller than the apparent contact area, the nanobridges formed on the distant asperities can contribute significantly to the overall friction. Therefore, it is essential to understand how the water nanobridges mediate the ‘noncontact' friction, which helps narrow the gap between our knowledge of friction on the microscopic and macroscopic scales. Here we show, by using noncontact dynamic force spectroscopy, the single capillary bridge generates noncontact friction via its shear interaction. The pinning–depinning dynamics of the nanobridge's contact line produces nonviscous damping, which occurs even without normal load and dominates the capillary-induced hydrodynamic damping. The novel nanofriction mechanism may provide a deeper microscopic view of macroscopic friction in air where numerous asperities exist. PMID:26066909

  5. Noncontact friction via capillary shear interaction at nanoscale.

    PubMed

    Lee, Manhee; Kim, Bongsu; Kim, Jongwoo; Jhe, Wonho

    2015-06-12

    Friction in an ambient condition involves highly nonlinear interactions of capillary force, induced by the capillary-condensed water nanobridges between contact or noncontact asperities of two sliding surfaces. Since the real contact area of sliding solids is much smaller than the apparent contact area, the nanobridges formed on the distant asperities can contribute significantly to the overall friction. Therefore, it is essential to understand how the water nanobridges mediate the 'noncontact' friction, which helps narrow the gap between our knowledge of friction on the microscopic and macroscopic scales. Here we show, by using noncontact dynamic force spectroscopy, the single capillary bridge generates noncontact friction via its shear interaction. The pinning-depinning dynamics of the nanobridge's contact line produces nonviscous damping, which occurs even without normal load and dominates the capillary-induced hydrodynamic damping. The novel nanofriction mechanism may provide a deeper microscopic view of macroscopic friction in air where numerous asperities exist.

  6. Electronic Polarization at Pentacene/Polymer Dielectric Interfaces: Imaging Surface Potentials and Contact Potential Differences as a Function of Substrate Type, Growth Temperature, and Pentacene Microstructure

    NASA Astrophysics Data System (ADS)

    Wu, Yanfei; Haugstad, Greg; Frisbie, C. Daniel; University of Minnesota Team

    2014-03-01

    Interfaces between organic semiconductors and dielectrics may exhibit interfacial electronic polarization, which is equivalently quantified as a contact potential difference (CPD), an interface dipole, or a vacuum level shift. Here we report quantitative measurements by Scanning Kelvin Probe Microscopy (SKPM) of surface potentials and CPDs across ultrathin (1-2 monolayer) crystalline islands of the benchmark semiconductor pentacene thermally deposited on a variety of polymer dielectrics (e.g., poly(methyl methacrylate), polystyrene). The CPDs between the pentacene islands and the polymer substrates are in the range of -10- +50 mV, they depend strongly on the polymer type and deposition temperature, and the CPD magnitude is correlated with the dipole moment of the characteristic monomers. Surface potential variations within 2 monolayer (3 nm) thick pentacene islands are approximately15 mV and may be ascribed to microstructure (epitaxial) differences. Overall, the microscopy results reveal both strong variations in interfacial polarization and lateral electrostatic heterogeneity; these factors ultimately should affect the transport properties of these interfaces in devices.

  7. Cu(In,Ga)Se2 absorber thinning and the homo-interface model: Influence of Mo back contact and 3-stage process on device characteristics

    NASA Astrophysics Data System (ADS)

    Leonard, E.; Arzel, L.; Tomassini, M.; Zabierowski, P.; Fuertes Marrón, D.; Barreau, N.

    2014-08-01

    Thinning the absorber layer is one of the possibilities envisaged to further decrease the production costs of Cu(In,Ga)Se2 (CIGSe) thin films solar cell technology. In the present study, the electronic transport in submicron CIGSe-based devices has been investigated and compared to that of standard devices. It is observed that when the absorber is around 0.5 μm-thick, tunnelling enhanced interface recombination dominates, which harms cells energy conversion efficiency. It is also shown that by varying either the properties of the Mo back contact or the characteristics of 3-stage growth processing, one can shift the dominating recombination mechanism from interface to space charge region and thereby improve the cells efficiency. Discussions on these experimental facts led to the conclusions that 3-stage process implies the formation of a CIGSe/CIGSe homo-interface, whose location as well as properties rule the device operation; its influence is enhanced in submicron CIGSe based solar cells.

  8. Cu(In,Ga)Se{sub 2} absorber thinning and the homo-interface model: Influence of Mo back contact and 3-stage process on device characteristics

    SciTech Connect

    Leonard, E.; Arzel, L.; Tomassini, M.; Barreau, N.; Zabierowski, P.; Fuertes Marrón, D.

    2014-08-21

    Thinning the absorber layer is one of the possibilities envisaged to further decrease the production costs of Cu(In,Ga)Se{sub 2} (CIGSe) thin films solar cell technology. In the present study, the electronic transport in submicron CIGSe-based devices has been investigated and compared to that of standard devices. It is observed that when the absorber is around 0.5 μm-thick, tunnelling enhanced interface recombination dominates, which harms cells energy conversion efficiency. It is also shown that by varying either the properties of the Mo back contact or the characteristics of 3-stage growth processing, one can shift the dominating recombination mechanism from interface to space charge region and thereby improve the cells efficiency. Discussions on these experimental facts led to the conclusions that 3-stage process implies the formation of a CIGSe/CIGSe homo-interface, whose location as well as properties rule the device operation; its influence is enhanced in submicron CIGSe based solar cells.

  9. 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.

  10. A contact stress model for multifingered grasps of rough objects

    NASA Technical Reports Server (NTRS)

    Sinha, Pramath Raj; Abel, Jacob M.

    1990-01-01

    The model developed utilizes a contact-stress analysis of an arbitrarily shaped object in a multifingered grasp. The fingers and the object are all treated as elastic bodies, and the region of contact is modeled as a deformable surface patch. The relationship between the friction and normal forces is nonlocal and nonlinear in nature and departs from the Coulomb approximation. The nature of the constraints arising out of conditions for compatibility and static equilibrium motivated the formulation of the model as a nonlinear constrained minimization problem. The model is able to predict the magnitude of the inwardly directed normal forces and both the magnitude and direction of the tangential (friction) forces at each finger-object interface for grasped objects in static equilibrium.

  11. Friction experiments with a capstan

    NASA Astrophysics Data System (ADS)

    Levin, Eugene

    1991-01-01

    The force of static friction on a cord wrapped through an angle θ around a rough-surfaced cylinder increases exponentially with θ. A small tension T1 at one end of the cord, maintaining contact between cord and cylinder, provides the condition for a substantially larger tension, of maximum value T2=T1 exp(μsθ), at the other end. The validity of this relationship is studied for the kinetic friction case. The suitability of this system for an undergraduate experiment is discussed, together with practical applications—the donkey engine and the capstan—with a digression on sea chanties.

  12. Experimental verification and theoretical prediction of cartilage interstitial fluid pressurization at an impermeable contact interface in confined compression.

    PubMed

    Soltz, M A; Ateshian, G A

    1998-10-01

    Interstitial fluid pressurization has long been hypothesized to play a fundamental role in the load support mechanism and frictional response of articular cartilage. However, to date, few experimental studies have been performed to verify this hypothesis from direct measurements. The first objective of this study was to investigate experimentally the hypothesis that cartilage interstitial fluid pressurization does support the great majority of the applied load, in the testing configurations of confined compression creep and stress relaxation. The second objective was to investigate the hypothesis that the experimentally observed interstitial fluid pressurization could also be predicted using the linear biphasic theory of Mow et al. (J. Biomech. Engng ASME, 102, 73-84, 1980). Fourteen bovine cartilage samples were tested in a confined compression chamber fitted with a microchip piezoresistive transducer to measure interstitial fluid pressure, while simultaneously measuring (during stress relaxation) or prescribing (during creep) the total stress. It was found that interstitial fluid pressure supported more than 90% of the total stress for durations as long as 725 +/- 248 s during stress relaxation (mean +/- S.D., n = 7), and 404 +/- 229 s during creep (n = 7). When comparing experimental measurements of the time-varying interstitial fluid pressure against predictions from the linear biphasic theory, nonlinear coefficients of determination r2 = 0.871 +/- 0.086 (stress relaxation) and r2 = 0.941 +/- 0.061 (creep) were found. The results of this study provide some of the most direct evidence to date that interstitial fluid pressurization plays a fundamental role in cartilage mechanics; they also indicate that the mechanism of fluid load support in cartilage can be properly predicted from theory.

  13. Experiments and numerical simulations of nonlinear vibration responses of an assembly with friction joints - Application on a test structure named "Harmony"

    NASA Astrophysics Data System (ADS)

    Claeys, M.; Sinou, J.-J.; Lambelin, J.-P.; Todeschini, R.

    2016-03-01

    In presence of friction, the frequency response function of a metallic assembly is strongly dependent on the excitation level. The local stick-slip behavior at the friction interfaces induces energy dissipation and local stiffness softening. These phenomena are studied both experimentally and numerically on a test structure named "Harmony". Concerning the numerical part, a classical complete methodology from the finite element and friction modeling to the prediction of the nonlinear vibrational response is implemented. The well-known Harmonic Balance Method with a specific condensation process on the nonlinear frictional elements is achieved. Also, vibration experiments are performed to validate not only the finite element model of the test structure named "Harmony" at low excitation levels but also to investigate the nonlinear behavior of the system on several excitation levels. A scanning laser vibrometer is used to measure the nonlinear behavior and the local stick-slip movement near the contacts.

  14. Deposition and electrical, chemical, and microstructural characterization of the interface formed between platinum, gold and silver rectifying contacts on cleaned n-type gallium nitride (0001) surfaces

    NASA Astrophysics Data System (ADS)

    Tracy, Kieran Mark

    2000-10-01

    The characteristics of clean n-type GaN surfaces and the interface between this surface and Pt, Au and Ag, have been investigated. Gallium-terminated (0001) surfaces of GaN, free of carbon and oxygen within the detection limits of XPS have been achieved by annealing in ammonia at 860°C for 15 minutes. Additional, in-situ surface analysis indicated a flat, stoichometric, and unreconstructed surface free of other contaminants. The electron affinity of this surface was 3.1 +/- 0.2 eV. The valence band maximum was located 3.0 +/- 0.1 eV below the Fermi level, indicating the presence of a surface state near the valence band maximum. Individual layers of Pt, Au or Ag were deposited in-situ on the cleaned surface and the interfaces characterized using XPS, UPS, LEED and TEM. All as-deposited meta/GaN interfaces were abrupt and unreacted; the Pt and Au were deposited epitaxially. The Schottky barrier heights obtained from photoemission measurements were 1.2, 0.9 and 0.5 +/- 0.2 eV for Pt, Au and Ag, respectively. Values of the metal work function from UPS results were 5.7, 5.3 and 4.4 +/- 0.2 eV for Pt, Au and Ag, respectively. Schottky barrier heights determined via ex-situ current-voltage measurements were 1.15, 0.88 and 0.56 +/- 0.05 eV for Pt, Au and Ag, respectively. Capacitance-voltage measurements yielded barrier heights of 1.25 and 0.96 +/- 0.05 eV, for Pt and Au, respectively. These results indicate that the Fermi level of the cleaned surface is not pinned. Upon annealing the aforementioned contacts from 400 to 800°C for 3 minutes each. The rectifying behavior of the Pt and Au contacts degraded as a function of temperature during annealing at 400, 600 and 800°C for 3 minutes each until they became ohmic. This was correlated with TEM of the annealed interfaces, which displayed increased chemical reaction and roughening as a function of temperature.

  15. Diffuse interface modeling of three-phase contact line dynamics on curved boundaries: A lattice Boltzmann model for large density and viscosity ratios

    NASA Astrophysics Data System (ADS)

    Fakhari, Abbas; Bolster, Diogo

    2017-04-01

    We introduce a simple and efficient lattice Boltzmann method for immiscible multiphase flows, capable of handling large density and viscosity contrasts. The model is based on a diffuse-interface phase-field approach. Within this context we propose a new algorithm for specifying the three-phase contact angle on curved boundaries within the framework of structured Cartesian grids. The proposed method has superior computational accuracy compared with the common approach of approximating curved boundaries with stair cases. We test the model by applying it to four benchmark problems: (i) wetting and dewetting of a droplet on a flat surface and (ii) on a cylindrical surface, (iii) multiphase flow past a circular cylinder at an intermediate Reynolds number, and (iv) a droplet falling on hydrophilic and superhydrophobic circular cylinders under differing conditions. Where available, our results show good agreement with analytical solutions and/or existing experimental data, highlighting strengths of this new approach.

  16. 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.

  17. Interfacial melting of ice under a high-speed slider: real-time visualization and friction modeling

    NASA Astrophysics Data System (ADS)

    Kim, Hyung-Seok; Yun, Chang-Ho; Kim, Dong-Jo; Kim, Ho-Young

    2016-11-01

    When a solid plate slides on ice, frictional heat melts asperities on the ice surface causing the real contact area to increase. Previous studies indicate the significance of contact area growth for ice friction, yet its quantitative understanding is far from clear mainly because the direct observation of the melting process at the interface has been extremely difficult. Here we describe a novel experimental setup that visualizes the interface of a rapidly rotating ice disc (up to the linear velocity of 10 m/s) and a transparent quartz surface in real time using the total internal reflection. We find that the melted area of the ice surface is a sensitive function of both sliding speed and temperature. We rationalize such quantitative measurements numerically and analytically, which allows us to predict the friction coefficient of ice as a function of relative velocity and temperature. This work can be used to develop friction-controlling mechanisms on ice surface, which are important in traffic safety as well as winter sports.

  18. Preliminary Study of a Pull Plug Friction Weld

    NASA Technical Reports Server (NTRS)

    Buchanan, George R.

    1999-01-01

    A pull plug friction weld, simply defined, comprises inserting a rotating cone-shaped cylinder into a somewhat cone-shaped hole in a plate. The rotating plug makes contact with the edge of the plate and the resulting friction generates heat. The temperature of the plate material eventually reaches a magnitude that will cause the plate material at the edge of the hole to flow. This can be termed a temperature dependent plastic flow. The rotation of the plug is terminated, additional pressure is applied and the metal at the interface of the two materials cools and welding occurs. This preliminary study addresses only three aspects of a complete analysis that is multi-faceted. The transient temperature distribution for different pull plug configurations has been studied in some detail even though the initial conditions and boundary conditions may still be deemed tentative. The stress distribution within the pull plug caused by the heating pressure was studied along with a preliminary analysis of the thermoelastic stress distribution caused by friction heating. There are no definitive results for the stress analysis. Additional study will be required.

  19. Adsorption of submicrometer-sized cationic sterically stabilized polystyrene latex at the air-water interface: contact angle determination by ellipsometry.

    PubMed

    Hunter, Timothy N; Jameson, Graeme J; Wanless, Erica J; Dupin, Damien; Armes, Steven P

    2009-04-09

    Near-monodisperse, sterically stabilized cationic polystyrene latexes of either 122 or 310 nm diameter were prepared by aqueous emulsion polymerization using cheap, readily available reagents. At low pH, these latexes stabilized foams prepared by either hand-shaking or by using a foam column. SEM studies confirmed that the dried foam mainly comprised well-defined bilayers, which suggests that each air bubble is stabilized with a latex monolayer. Adsorption of the same latexes at the planar air-water interface was studied using the Langmuir-Blodgett trough technique. Surface pressure isotherms confirmed particle desorption from the interface on repeated compression of the latex monolayers. For the 122 nm latex at pH 2, ellipsometric analysis enabled a contact angle of approximately 43 degrees to be calculated from a simple two-layer model, which suggests that these particles have only moderate wettability. Similar results were obtained for the 310 nm latex, but the data were much less reliable in this case due to additional background particle scattering.

  20. A technique to determine friction at the fingertips.

    PubMed

    Savescu, Adriana V; Latash, Mark L; Zatsiorsky, Vladimir M

    2008-02-01

    This article proposes a technique to calculate the coefficient of friction for the fingertip- object interface. Twelve subjects (6 males and 6 females) participated in two experiments. During the first experiment (the imposed displacement method), a 3-D force sensor was moved horizontally while the subjects applied a specified normal force (4 N, 8 N, 12 N) on the surface of a sensor covered with different materials (sandpaper, cotton, rayon, polyester, and silk). The normal force and the tangential force (i.e., the force due to the sensor motion) were recorded. The coefficient of friction (mu(d)) was calculated as the ratio between the tangential force and the normal force. In the second experiment (the beginning slip method), a small instrumented object was gripped between the index finger and the thumb, held stationary in the air, and then allowed to drop. The weight (200 g, 500 g, and 1,000 g) and the surface (sandpaper, cotton, rayon, polyester, and silk) in contact with the digits varied across trials. The same sensor as in the first experiment was used to record the normal force (in a horizontal direction) and the tangential force (in the vertical direction). The slip force (i.e., the minimal normal force or grip force necessary to prevent slipping) was estimated as the force at the moment when the object just began to slip. The coefficient of friction was calculated as the ratio between the tangential force and the slip force. The results show that (1) the imposed displacement method is reliable; (2) except sandpaper, for all other materials the coefficient of friction did not depend on the normal force; (3) the skin-sandpaper coefficient of friction was the highest mu(d) =0.96+/-0.09 (for 4-N normal force) and the skin-rayon rayon coefficient of friction was the smallest mu(d) =0.36+/-0.10; (4) no significant difference between the coefficients of friction determined with the imposed displacement method and the beginning slip method was observed. We view

  1. Fingerprints are unlikely to increase the friction of primate fingerpads.

    PubMed

    Warman, Peter H; Ennos, A Roland

    2009-07-01

    It is generally assumed that fingerprints improve the grip of primates, but the efficiency of their ridging will depend on the type of frictional behaviour the skin exhibits. Ridges would be effective at increasing friction for hard materials, but in a rubbery material they would reduce friction because they would reduce contact area. In this study we investigated the frictional performance of human fingertips on dry acrylic glass using a modified universal mechanical testing machine, measuring friction at a range of normal loads while also measuring the contact area. Tests were carried out on different fingers, fingers at different angles and against different widths of acrylic sheet to separate the effects of normal force and contact area. The results showed that fingertips behaved more like rubbers than hard solids; their coefficients of friction fell at higher normal forces and friction was higher when fingers were held flatter against wider sheets and hence when contact area was greater. The shear stress was greater at higher pressures, suggesting the presence of a biofilm between the skin and the surface. Fingerprints reduced contact area by a factor of one-third compared with flat skin, however, which would have reduced the friction; this casts severe doubt on their supposed frictional function.

  2. Frictional Properties of Single Crystalline and Quasicrystalline Surfaces

    NASA Astrophysics Data System (ADS)

    Gellman, Andrew

    2000-03-01

    undergo plastic deformation when brought into contact and yet it is possible for these ultra-thin layers of adsorbed molecules to reduce friction and lubricate the interface. There are a number of measurements which show that friction and adhesion between crystalline materials can be anisotropic in the sense that they depend on the relative crystallographic orientations of the two surfaces. For the most part these have been made between surfaces in elastic contact and the understanding of the source of the anisotropy is based on the commensurability of the overlapping crystal surface lattices. In the case of metal surfaces which undergo plastic deformation when brought into contact it is not so clear that the commensurability of the surface lattices is the determining factor in friction anisotropy. Three sets of experiments have been performed in which pairs of Ni(100) surfaces have been brought onto contact and sheared with different orientations of the surface lattices. In the first experiment the Ni(100) surfaces were clean. In the second they were modified by adsorption of 1/2 monolayer of sulfur atoms ordered into a c(2x2) lattice. In this case the two surface lattices are rotated by 45o with respect to the original Ni(100) lattice. Finally, in the third set of experiments the surfaces were modified by adsorption of 2 monolayers of ethanol. Adsorbed ethanol forms an overlayer with no long range order. The basic point of these surface modifications is that they either rotate or eliminate the surface lattice periodicity while leaving the bulk lattices of the Ni(100) crystals in the same relative orientations. In all three cases friction anisotropy is observed as the two bulk crystal lattices are rotated with respect to one another. The interesting thing is that a minimum in the friction is observed when the two are rotated by 45^o with respect to one another. The fact that this is observed for all three modifications of the surface lattice suggests that it is the

  3. A local model for the thermomechanical conditions in friction stir welding

    NASA Astrophysics Data System (ADS)

    Schmidt, H.; Hattel, J.

    2005-01-01

    The conditions under which the deposition process in friction stir welding is successful are not fully understood. However, it is known that only under specific thermomechanical conditions does a weld formation occur. If these conditions are not present, void formation will occur leading to a faulty weld. The objective of the present work is to analyse the primary conditions under which the cavity behind the tool is filled. For this, a fully coupled thermomechanical three-dimensional FE model has been developed in ABAQUS/Explicit using the arbitrary Lagrangian-Eulerian formulation and the Johnson-Cook material law. The model accounts for the compressibility by including the elastic response of the aluminium matrix. The contact forces are modelled by Coulomb's Law of friction, making the contact condition highly solution dependent. Furthermore, separation between the workpiece and the tool is allowed. This is often neglected in other models. Once non-recoverable separation is estimated by the model, a void develops. This is suggested as a preliminary criterion for evaluating the success of the deposition process. Of special interest is the contact condition along the tool/matrix interface, which controls the efficiency of the deposition process. In most models presented previously in the literature, the material flow at the tool interface is prescribed as boundary conditions. In all other contact models, the material is forced to keep contact with the tool. Therefore, the models are unable to predict when the suitable thermomechanical conditions and welding parameters are present. In the present work, the quasi-stationary thermomechanical state in the workpiece is established by modelling the dwell and weld periods. The different thermomechanical states in the colder, stiffer far-field matrix and the hotter, softer near-field matrix (under the tool) result in contact at the tool/matrix interface, thus, no void formation is observed. The steady-state model results

  4. Contact mechanics of piezoelectric ultrasonic motors

    NASA Astrophysics Data System (ADS)

    Wallaschek, Jörg

    1998-06-01

    Piezoelectric ultrasonic motors are driven by tangential stresses in the interface between stator and rotor. These stresses are generated by the elliptical motion of the material points of the stator or rotor surface and depend on frictional processes in the contact area. The contact mechanics of piezoelectric ultrasonic motors determines the operational characteristics, like rotational speed and torque or transmitted mechanical power and efficiency. Wear properties and lifetime of piezoelectric ultrasonic motors are also determined by contact mechanics. The goal of the present paper is to summarize the state of the art in the understanding of some fundamental processes governing the contact mechanics of piezoelectric ultrasonic motors. After a short introduction, a survey of publications devoted to the subject will be given. Then, an attempt will be made to classify the mechanical models, which were developed in order to explain the contact mechanics of piezoelectric ultrasonic motors, according to the physical effects which have been taken into account in their derivation. Some results concerning the choice of proper contact materials, wear and lifetime of ultrasonic motors will be addressed in a separate section. Finally a summary and outlook will be given and open questions for future research will be formulated.

  5. 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.

  6. Friction Phenomenon in Contact Stress Problems

    DTIC Science & Technology

    1988-07-01

    specimen. A virtual reference grating created by interference of two coherent beams A and B is superimposed on the specimen grating. The specimen and...where f is the frequency of the virtual reference grating, x is the wavelength of the light employed, a is the angle of incidence (Fig. 3), N is the...orders the test case was run again. This corresponds to a similar order of interpolation error which might occur with a virtual reference grating of 240

  7. 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.

  8. Friction and plasticity between self-affine surfaces

    NASA Astrophysics Data System (ADS)

    Luan, Binquan; Robbins, Mark; Harrison, Judith

    2006-03-01

    Simulations are used to study the contact area and adhesion between two amorphous solids with self-affine fractal surfaces, and the results are compared to continuum calculations. The friction between non-adhesive surfaces is proportional to load, but the coefficient of friction increases with roughness. The friction is much higher than expected for elasticallly deforming surfaces,^* and substantial plastic deformation is observed. Indeed, friction forces for different surface roughness collapse when plotted against the number of plastic rearrangements per unit sliding distance. Including adhesion leads to an increase in both friction and plasticity. ^* M. H. Müser, L. Wenning, and M. O. Robbins, Phys. Rev. Lett. 86, 1295 (2001).

  9. Friction of ice measured using lateral force microscopy

    SciTech Connect

    Bluhm, Hendrik; Inoue, Takahito; Salmeron, Miquel

    2000-03-15

    The friction of nanometer thin ice films grown on mica substrates is investigated using atomic force microscopy (AFM). Friction was found to be of similar magnitude as the static friction of ice reported in macroscopic experiments. The possible existence of a lubricating film of water due to pressure melting, frictional heating, and surface premelting is discussed based on the experimental results using noncontact, contact, and lateral force microscopy. We conclude that AFM measures the dry friction of ice due to the low scan speed and the squeezing out of the water layer between the sharp AFM tip and the ice surface. (c) 2000 The American Physical Society.

  10. A frictional sliding algorithm for liquid droplets

    NASA Astrophysics Data System (ADS)

    Sauer, Roger A.

    2016-12-01

    This work presents a new frictional sliding algorithm for liquid menisci in contact with solid substrates. In contrast to solid-solid contact, the liquid-solid contact behavior is governed by the contact line, where a contact angle forms and undergoes hysteresis. The new algorithm admits arbitrary meniscus shapes and arbitrary substrate roughness, heterogeneity and compliance. It is discussed and analyzed in the context of droplet contact, but it also applies to liquid films and solids with surface tension. The droplet is modeled as a stabilized membrane enclosing an incompressible medium. The contact formulation is considered rate-independent such that hydrostatic conditions apply. Three distinct contact algorithms are needed to describe the cases of frictionless surface contact, frictionless line contact and frictional line contact. For the latter, a predictor-corrector algorithm is proposed in order to enforce the contact conditions at the contact line and thus distinguish between the cases of advancing, pinning and receding. The algorithms are discretized within a monolithic finite element formulation. Several numerical examples are presented to illustrate the numerical and physical behavior of sliding droplets.

  11. A comparison of the peri-implant bone stress generated by the preload with screw tightening between 'bonded' and 'contact' model.

    PubMed

    Matsuzaki, Maki; Ayukawa, Yasunori; Sakai, Nobuo; Matsuzaki, Tatsuya; Matsushita, Yasuyuki; Koyano, Kiyoshi

    2017-03-01

    A number of finite element analyses (FEAs) for the dental implant were performed without regard for preload and with all interfaces 'fixed-bonded'. The purpose of this study was comparing the stress distributions between the conventional FEA model with all contacting interfaces 'fixed-bonded' (bonded model) and the model with the interfaces of the components in 'contact' with friction simulated as a preloaded implant (contact model). We further verified the accuracy of the result of the FEA using model experiment. In the contact model, the stress was more widely distributed than in the bonded model. From the model study, the preload induced by screw tightening generated strain at the peri-implant bone, even before the application of external force. As a result, the bonded model could not reproduce the mechanical phenomena, whereas the contact model is considered to be appropriate for analysing mechanical problems.

  12. A distributed mechanical joint contact model with slip/slap coupling effects

    NASA Astrophysics Data System (ADS)

    Ahmadian, Hamid; Mohammadali, Mohsen

    2016-12-01

    This paper introduces a zero thickness interface model that considers hysteresis effects in both normal and shear directions of a contact. The model is rate independent and represents coupling effects between normal and shear displacements. Contact effects are included through a segment-to-segment contact model which considers stick, micro-slip, slide and slap behaviors at every point within the contact interface. The model has six parameters and three memory variables without the need for integration during response computations. Behavior of the model is validated using the available mechanical joint records in the literature and it is successfully employed for model identification and dynamic response prediction of an internally resonating test structure with frictional support.

  13. Study on friction behaviour of brake shoe materials for mining hoist

    NASA Astrophysics Data System (ADS)

    Ungureanu, M.; Ungureanu, N. S.; Crăciun, I.

    2017-02-01

    The friction coefficient in the brake linkages has an important influence on the braking efficiency and safety of machines. The paper presents a method for the study of the friction coefficient of the friction couple brake shoe-drum for mining hoist. In this context, it is interesting to define the friction coefficient, not just according to the materials in contact, but according to the entire ensemble of tribological factors of the friction couple.

  14. Microstructure analysis in friction welding of copper and aluminum

    NASA Astrophysics Data System (ADS)

    Wibowo, A. G. Wahyu; Ismail, Rifky; Jamari, J.

    2016-04-01

    The Friction welding is a welding method with utilizing heat generated due to friction. Surfaces of two materials to be joined, one rotates the other being idle, is contacted by a pressure force. Friction on the second contact surface is done continuously so that the heat generated by the continuous friction will continue to rise. With the heat and the pressure force on the second surface to the second meeting of the material reaches its melting temperature then there is the process of welding. This paper examines the influence of the pressure force, rotational speed and contact time on friction welding of Aluminum (Al) and Copper (Cu) to the quality of welded joints. Friction welding process is performed on a friction welding machine that is equipped with the loading mechanism. The parameters used are the pressure force, rotational speed and friction time. Determination of the quality of welding is done by testing the tensile strength, hardness, and micro structure on the weld joint areas. The results showed that the friction welding quality is very good, this is evidenced by the results of a tensile strength test where the fault occurs outside the weld joint and increased violence in the weld joint. On the results visually cuts the welding area did not reveal any porosity so that it can be concluded that each metal contacts have melted perfectly and produce a connection with good quality.

  15. Gas desorption during friction of amorphous carbon films

    NASA Astrophysics Data System (ADS)

    Rusanov, A.; Fontaine, J.; Martin, J.-M.; Mogne, T. L.; Nevshupa, R.

    2008-03-01

    Gas desorption induced by friction of solids, i.e. tribodesorption, is one of the numerous physical and chemical phenomena, which arise during friction as result of thermal and structural activation of material in a friction zone. Tribodesorption of carbon oxides, hydrocarbons, and water vapours may lead to significant deterioration of ultra high vacuum conditions in modern technological equipment in electronic, optoelectronic industries. Therefore, knowledge of tribodesorption is crucial for the performance and lifetime of vacuum tribosystems. Diamond-like carbon (DLC) coatings are interesting materials for vacuum tribological systems due to their high wear resistance and low friction. Highly hydrogenated amorphous carbon (a-C:H) films are known to exhibit extremely low friction coefficient under high vacuum or inert environment, known as 'superlubricity' or 'superlow friction'. However, the superlow friction period is not always stable and then tends to spontaneous transition to high friction. It is supposed that hydrogen supply from the bulk to the surface is crucial for establishing and maintaining superlow friction. Thus, tribodesorption can serve also as a new technique to determine the role of gases in superlow friction mechanisms. Desorption of various a-C:H films, deposited by PECVD, ion-beam deposition and deposition using diode system, has been studied by means of ultra-high vacuum tribometer equipped with a mass spectrometer. It was found that in superlow friction period desorption rate was below the detection limit in the 0-85 mass range. However, transition from superlow friction to high friction was accompanied by desorption of various gases, mainly of H2 and CH4. During friction transition, surfaces were heavily damaged. In experiments with DLC films with low hydrogen content tribodesorption was significant during the whole experiment, while low friction was not observed. From estimation of maximum surface temperature during sliding contact it was

  16. Mechanisms for force adjustments to unpredictable frictional changes at individual digits during two-fingered manipulation.

    PubMed

    Birznieks, I; Burstedt, M K; Edin, B B; Johansson, R S

    1998-10-01

    Previous studies on adaptation of fingertip forces to local friction at individual digit-object interfaces largely focused on static phases of manipulative tasks in which humans could rely on anticipatory control based on the friction in previous trials. Here we instead analyze mechanisms underlying this adaptation after unpredictable changes in local friction between consecutive trials. With the tips of the right index and middle fingers or the right and left index fingers, subjects restrained a manipulandum whose horizontal contact surfaces were located side by side. At unpredictable moments a tangential force was applied to the contact surfaces in the distal direction at 16 N/s to a plateau at 4 N. The subjects were free to use any combination of normal and tangential forces at the two fingers, but the sum of the tangential forces had to counterbalance the imposed load. The contact surface of the right index finger was fine-grained sandpaper, whereas that of the cooperating finger was changed between sandpaper and the more slippery rayon. The load increase automatically triggered normal force responses at both fingers. When a finger contacted rayon, subjects allowed slips to occur at this finger during the load force increase instead of elevating the normal force. These slips accounted for a partitioning of the load force between the digits that resulted in an adequate adjustment of the normal:tangential force ratios to the local friction at each digit. This mechanism required a fine control of the normal forces. Although the normal force at the more slippery surface had to be comparatively low to allow slippage, the normal forces applied by the nonslipping digit at the same time had to be high enough to prevent loss of the manipulandum. The frictional changes influenced the normal forces applied before the load ramp as well as the size of the triggered normal force responses similarly at both fingers, that is, with rayon at one contact surface the normal forces

  17. Stress evaluations under rolling/sliding contacts

    NASA Technical Reports Server (NTRS)

    Kannel, J. W.; Tevaarwerk, J. L.

    1981-01-01

    The state of stress beneath traction drive type of contacts were analyzed. Computing stresses and stress reversals on various planes for points beneath the surface were examined. The effect of tangential and axial friction under gross slip conditions is evaluated with the models. Evaluations were performed on an RC (rolling contact) tester configuration and it is indicated that the classical fatigue stresses are not altered by friction forces typical of lubricated contact. Higher values of friction can result in surface shear reversal that exceeds the stresses at the depth of maximum shear reversal under rolling contact.

  18. Investigation of dynamic dry friction between stainless steel and aluminium alloy

    NASA Astrophysics Data System (ADS)

    Keightley, Peter; Winter, Ron; Stirk, Stewart

    2009-06-01

    Previous workers studied dynamic friction by using an impacting copper plate to drive a tapered aluminium alloy plug into a matching hole in a stainless steel outer sheath. The velocity of the back surface of the plug was measured using velocity interferometry. We have performed experiments on a version of this basic configuration that has been enlarged so that the sliding surfaces remain in contact for a longer time than with the original configuration. By comparing our results with computer simulations we conclude that the frictional forces between the inner cone and the steel outer are initially high but decrease significantly as the sliding proceeds. This effect is assumed to result from thermal softening of the material at the sliding interface. The study is supported by metallography of the recovered components.

  19. Turbine blade friction damping study

    NASA Technical Reports Server (NTRS)

    Dominic, R. J.

    1985-01-01

    A lumped parameter method, implemented on a VAX 11/780 computer shows that the primary parameters affecting the performance of the friction damper of the first stage turbine of the SSME high pressure fuel pump are: the damper-blade coefficient of friction; the normal force applied to the friction interface; the amplitude of the periodic forcing function; the relative phase angle of the forcing functions for adjacent blades bridged by a damper (effectively, the engine order of the forcing function); and the amount of hysteretic damping that acts to limit the vibration amplitude of the blade in its resonance modes. The low order flexural resonance vibration modes of HPFTP blades without dampers, with production dampers, and with two types of lightweight experimental dampers were evaluated in high speed spin pit tests. Results agree with those of the analytical study in that blades fitted with production friction dampers experienced the airfoil-alone flexural resonance mode, while those without dampers or with lighter weight dampers did not. No blades fitted with dampers experienced the whole blade flexural resonance mode during high speed tests, while those without dampers did.

  20. Computing Contact Stresses In Gear Teeth

    NASA Technical Reports Server (NTRS)

    Oswald, Fred B.; Somprakit, Paisan; Huston, Ronald L.

    1995-01-01

    Improved method of computing contact stresses in gear teeth accounts for complicating effects like those of static and sliding friction. Provides iterative procedure for determination of contact region and nodal contact forces along with contact stresses. Method based on equations and computational procedure incorporating these effects routinely.

  1. Friction phenomena and their impact on the shear behaviour of granular material

    NASA Astrophysics Data System (ADS)

    Suhr, Bettina; Six, Klaus

    2017-01-01

    In the discrete element simulation of granular materials, the modelling of contacts is crucial for the prediction of the macroscopic material behaviour. From the tribological point of view, friction at contacts needs to be modelled carefully, as it depends on several factors, e.g. contact normal load or temperature to name only two. In discrete element method (DEM) simulations the usage of Coulomb's law of friction is state of the art in modelling particle-particle contacts. Usually in Coulomb's law, for all contacts only one constant coefficient of friction is used, which needs to reflect all tribological effects. Thus, whenever one of the influence factors of friction varies over a wide range, it can be expected that the usage of only one constant coefficient of friction in Coulomb's law is an oversimplification of reality. For certain materials, e.g. steel, it is known that a dependency of the coefficient of friction on the contact normal load exists. A more tribological tangential contact law is implemented in DEM, where the interparticle friction coefficient depends on the averaged normal stress in the contact. Simulations of direct shear tests are conducted, using steel spheres of different size distributions. The strong influence of interparticle friction on the bulk friction is shown via a variation of the constant interparticle friction coefficient. Simulations with constant and stress-dependent interparticle friction are compared. For the stress-dependent interparticle friction, a normal stress dependency of the bulk friction is seen. In the literature, measurements of different granular materials and small normal loads also show a stress dependency of the bulk friction coefficient. With increasing applied normal stress, the bulk friction coefficient reduces both in the experiments and in the simulations.

  2. Static and dry friction due to multiscale surface roughness.

    PubMed

    Sokoloff, J B

    2008-09-01

    It is shown on the basis of scaling arguments that a disordered interface between two elastic solids will quite generally exhibit static and dry friction (i.e., kinetic friction which does not vanish as the sliding velocity approaches zero) because of Tomlinson-model instabilities that occur for small-length-scale asperities. This provides a possible explanation for why static and dry friction are virtually always observed, and superlubricity almost never occurs.

  3. Static and dry friction due to multiscale surface roughness

    NASA Astrophysics Data System (ADS)

    Sokoloff, J. B.

    2008-09-01

    It is shown on the basis of scaling arguments that a disordered interface between two elastic solids will quite generally exhibit static and dry friction (i.e., kinetic friction which does not vanish as the sliding velocity approaches zero) because of Tomlinson-model instabilities that occur for small-length-scale asperities. This provides a possible explanation for why static and dry friction are virtually always observed, and superlubricity almost never occurs.

  4. Surface sliding friction of negatively charged polyelectrolyte gels.

    PubMed

    Kagata, Go; Gong, Jian Ping

    2007-04-15

    The friction between two polyelectrolyte gels carrying the same or opposite sign of charges has been investigated using a rheometer. It is found that the friction was strongly dependent on the interfacial interaction between two gel surfaces. In the repulsive interaction case, especially, the friction was extremely low. The friction behavior is attempted to be described in terms of the hydrodynamic lubrication of the solvent layer between two like-charged gel surfaces, which is formed due to the electrostatic repulsion of the two gel surfaces. From the theoretical analysis (hydrodynamic mechanism), the friction behaviors were explained qualitatively, all of the experimental results, nevertheless, could not be understood well. The viscoelastic feature of the gel and the non-Newtonian behavior of water at the friction interface are considered to be important to elucidate the gel friction.

  5. Analytical and experimental evaluation of biaxial contact stress

    NASA Technical Reports Server (NTRS)

    Richerson, D. W.; Finger, D. G.; Wimmer, J. M.

    1981-01-01

    Contact stress analysis was conducted for ceramic-metal and ceramic-ceramic interfaces using a finite element model. Ceramics investigated included NC-132 hot-pressed silicon nitride, NC-350 reaction-bonded silicon nitride, Hexaloy SA SiC, and RBN104 reaction-bonded silicon nitride. The results are shown to be well correlated with closed-form solution both for normal and normal-tangential loading. The latter load condition is found to be especially critical for ceramic materials due to the presence of a high tensile stress at the trailing edge of the ceramic surface contact zone. It is shown that during sliding contact or biaxial loading, the magnitude of this tensile stress increases with the coefficient of friction.

  6. Modeling and data analysis of the NASA-WSTF frictional heating apparatus - Effects of test parameters on friction coefficient

    NASA Technical Reports Server (NTRS)

    Zhu, Sheng-Hu; Stoltzfus, Joel M.; Benz, Frank J.; Yuen, Walter W.

    1988-01-01

    A theoretical model is being developed jointly by the NASA White Sands Test Facility (WSTF) and the University of California at Santa Barbara (UCSB) to analyze data generated from the WSTF frictional heating test facility. Analyses of the data generated in the first seconds of the frictional heating test are shown to be effective in determining the friction coefficient between the rubbing interfaces. Different friction coefficients for carobn steel and Monel K-500 are observed. The initial condition of the surface is shown to affect only the initial value of the friction coefficient but to have no significant influence on the average steady-state friction coefficient. Rotational speed and the formation of oxide film on the rotating surfaces are shown to have a significant effect on the friction coefficient.

  7. PREFACE: The International Conference on Science of Friction

    NASA Astrophysics Data System (ADS)

    Miura, Kouji; Matsukawa, Hiroshi

    2007-07-01

    The first international conference on the science of friction in Japan was held at Irago, Aichi on 9-13 September 2007. The conference focused on the elementary process of friction phenomena from the atomic and molecular scale view. Topics covered in the conference are shown below.:

  8. Superlubricity and friction>
  9. Electronic and phononic contributions to friction>
  10. Friction on the atomic and molecular scales
  11. van der Waals friction and Casimir force
  12. Molecular motor and friction>
  13. Friction and adhesion in soft matter systems
  14. Wear and crack on the nanoscale
  15. Theoretical studies on the atomic scale friction and energy dissipation
  16. Friction and chaos
  17. Mechanical properties of nanoscale contacts
  18. Friction of powder
  19. The number of participants in the conference was approximately 100, registered from 11 countries. 48 oral and 29 poster talks were presented at the conference. This volume of Journal of Physics: Conference Series includes 23 papers devoted to the above topics of friction. The successful organization of the conference was made possible by the contribution of the members of the Organizing Committee and International Advisory Committee. The conference was made possible thanks to the financial support from Aichi University of Education and the Taihokogyo Tribology Research Foundation (TTRF), and moreover thanks to the approval societies of The Physical Society of Japan, The Surface Science Society of Japan, The Japanese Society of Tribologists and Toyota Physical and Chemical Research Institute. The details of the conference are available at http://www.science-of-friction.com . Finally we want to thank the speakers for the high quality of their talks and all participants for coming to Irago, Japan and actively contributing to the conference. Kouji Miura and Hiroshi Matsukawa Editors

  20. DSPC/DLPC mixed films supported on silica: a QCM-D and friction force study.

    PubMed

    Oguchi, Takakuni; Sakai, Kenichi; Sakai, Hideki; Abe, Masahiko

    2011-01-01

    The membrane properties of phospholipid mixtures supported on silica were studied by means of a quartz crystal microbalance with dissipation monitoring (QCM-D) technique, in situ soft-contact atomic force microscopy (AFM), and friction force microscopy (FFM). The phospholipids used in this study were di-stearoylphosphatidylcholine (DSPC) and dilauroylphosphatidylcholine (DLPC). The phospholipid films were prepared by a vesicle-fusion method, in which DSPC/DLPC mixed liposomes dispersed in an aqueous medium are adsorbed on silica and their structure is transformed into a bilayer on the substrate. The changes in QCM-D (frequency and dissipation) and friction responses of DSPC single systems (gel state at 25°C) are relatively large compared with those of DLPC single systems (liquid-crystalline state at 25°C) and those of mixed DSPC/DLPC systems. This suggests that (i) the gel-state DSPC liposomes are somewhat flattened on the silica, by keeping their solid-like molecular rigidity, whereas (ii) both the liquid-crystalline DLPC and mixed liposomes experience instantaneous structural transformation at the silica/water interface and form a normally flattened bilayer on the substrate. The friction force response is dependent on the phase state of the phospholipids, and the liquid-crystalline DLPC has a more significant impact on the overall membrane properties (i.e., the degree of swelling and the friction response on the surface) than does the gel-state DSPC.

  21. The Use of Microscale Geometry to Tailor Stimulus-Responsive Surface Friction

    NASA Astrophysics Data System (ADS)

    Han, Lin; Yin, Jie; Wang, Lifeng; Chia, Khek-Khiang; Cohen, Robert; Rubner, Michael; Boyce, Mary; Ortiz, Christine

    2012-02-01

    The capability to tailor stimulus-responsive surface friction, including sensitivity profile, range, temporal response and deformation mechanisms, holds great potential for an array of engineering and biomedical applications. In this study, the pH-dependent friction of layer-by-layer assemblies of poly(allylamine hydrochloride) and poly(acrylic acid) (PAH/PAA) were quantified for structures of a continuous planar film and anisotropic microtube forests via lateral force microscopy. By comparing experiments to microstructure-specific finite element modeling, a mechanistic change from surface adhesion-dominated friction (μ=0.11) to viscoelasticity-governed shear (=0.017) was predicted upon ionic crosslink density reduction of PAH/PAA from pH 5.5 to 2.0 for the film (6.5x decrease). The responsiveness of μ was further tuned by the tube forest geometry to be 3.5x. At pH 5.5, μ (=0.094) was lower than the film due to discrete tube bending/buckling and smaller tip-sample interface stress. At pH 2.0, μ (=0.027) was higher because of inter-tube contact and weaker substrate effect. This study provides an excellent platform to quantitatively access and design dynamic substrates with tailorable stimulus-responsive surface friction.

  22. Reduction of friction using piezoelectrically excited ultrasonic vibrations

    NASA Astrophysics Data System (ADS)

    Littmann, Walter; Storck, Heiner; Wallaschek, Joerg

    2001-07-01

    Piezoelectric materials are an important class of smart materials for the generation of mechanical ultrasonic vibrations. In industrial applications (for example ultrasonic cutting) the frictional contact of the vibrating tool with the workpiece is of special importance. A common observation at the contact zone is that frictional forces can be significantly reduced by superposition of ultrasonic vibrations. In this report we present a theoretical explanation for the reduction of friction. A basic system, consisting of a longitudinal ultrasonic vibrator sliding on a plane, is investigated. It is shown that a modification of Coulomb's friction law can be applied to this kind of vibrating friction contact. The macroscopically observed friction-force with ultrasonic vibration depends on the sliding velocity and the velocity of vibration: For sliding velocities higher than the vibration-amplitude the frictional force is not changed by vibration. But for small sliding velocities the friction-coefficient is significantly reduced and almost approaches zero for very slow sliding-velocity. The theoretical results were confirmed systematically by experimental investigations done on a specially designed test-rig. Energy considerations are used to calculate the ultrasonic energy which is required to achieve a prescribed reduction of the frictional forces. The model is also used for sensing the vibration-amplitude as well as the sliding-velocity without an additional sensor.

  23. 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.

  24. Friction Pull Plug and Material Configuration for Anti-Chatter Friction Pull Plug Weld

    NASA Technical Reports Server (NTRS)

    Littell, Justin Anderson (Inventor)

    2016-01-01

    A friction pull plug is provided for use in forming a plug weld in a hole in a material. The friction pull plug includes a shank and a series of three frustoconical sections. The relative sizes of the sections assure that a central one of the sections defines the initial contact point between the hole's sides. The angle defined by the central one of the sections reduces or eliminates chatter as the plug is pulled into the hole.

  25. Effects of antimony trisulfide (Sb2S3) on sliding friction of automotive brake friction materials

    NASA Astrophysics Data System (ADS)

    Lee, Wan Kyu; Rhee, Tae Hee; Kim, Hyun Seong; Jang, Ho

    2013-09-01

    The effect of antimony trisulfide (Sb2S3) on the tribological properties of automotive brake friction materials was investigated using a Krauss type tribometer and a 1/5 scale dynamometer with a rigid caliper. Results showed that Sb2S3 improved fade resistance by developing transfer films on the disc surface at elevated temperatures. On the other hand, the rubbing surfaces of the friction material exhibited contact plateaus with a broader height distribution when it contained Sb2S3, indicating fewer contact junctions compared to the friction material with graphite. The friction material with Sb2S3 also exhibited a lower stick-slip propensity than the friction material with graphite. The improved fade resistance with Sb2S3 is attributed to its lubricating capability sustained at high temperatures, while the lower stick-slip propensity of the friction material with Sb2S3 is associated with the slight difference between its static and kinetic coefficients of friction and high normal stiffness.

  26. Molecular origin of friction

    NASA Astrophysics Data System (ADS)

    Wang, Hui; Zhang, Tao; Hu, Yuanzhong

    2004-01-01

    The wearless friction originating from molecular interactions has been discussed in this paper. We find that the frictional properties are closely related to the structural match of two surfaces in relative motion. For the surfaces with incommensurate structure and week inter-surface interaction, zero static and kinetic friction can be achieved. In a sliding considered as in a quasi-static state, the energy dissipation initiates when interfacial particles move in a discontinuous fashion, which gives rise to a finite kinetic friction. The state of superlubricity is a result of computer simulations, but the prediction will encourage people to look for a technical approach to realizing the state of super low friction.

  27. Study of the effect of static/dynamic Coulomb friction variation at the tape-head interface of a spacecraft tape recorder by non-linear time response simulation

    NASA Technical Reports Server (NTRS)

    Mukhopadhyay, A. K.

    1978-01-01

    A description is presented of six simulation cases investigating the effect of the variation of static-dynamic Coulomb friction on servo system stability/performance. The upper and lower levels of dynamic Coulomb friction which allowed operation within requirements were determined roughly to be three times and 50% respectively of nominal values considered in a table. A useful application for the nonlinear time response simulation is the sensitivity analysis of final hardware design with respect to such system parameters as cannot be varied realistically or easily in the actual hardware. Parameters of the static/dynamic Coulomb friction fall in this category.

  28. Adhesion, friction, and wear behavior of clean metal-ceramic couples

    NASA Technical Reports Server (NTRS)

    Miyoshi, Kazuhisa

    1995-01-01

    When a clean metal is brought into contact with a clean, harder ceramic in ultrahigh vacuum, strong bonds form between the two materials. The interfacial bond strength between the metal and ceramic surfaces in sliding contact is generally greater than the cohesive bond strength in the metal. Thus, fracture of the cohesive bonds in the metal results when shearing occurs. These strong interfacial bonds and the shearing fracture in the metal are the main causes of the observed wear behavior and the transfer of the metal to the ceramic. In the literature, the surface energy (bond energy) per unit area of the metal is shown to be related to the degree of interfacial bond strength per unit area. Because the two materials of a metal-ceramic couple have markedly different ductilities, contact can cause considerable plastic deformation of the softer metal. It is the ductility of the metal, then, that determines the real area of contact. In general, the less ductile the metal, the smaller the real area of contact. The coefficient of friction for clean surfaces of metal-ceramic couples correlates with the metals total surface energy in the real area of contact gamma A (which is the product of the surface energy per unit area of the metal gamma and the real area of contact (A)). The coefficient of friction increases as gamma A increases. Furthermore, gamma A is associated with the wear and transfer of the metal at the metal-ceramic interface: the higher the value of gamma A, the greater the wear and transfer of the metal.

  29. Noise of sliding rough contact

    NASA Astrophysics Data System (ADS)

    Le Bot, Alain

    2017-01-01

    This article is a discussion about the origin of friction noise produced when rubbing solids having rough surfaces. We show that noise emerges from numerous impacts into the contact between antagonist asperities of surfaces. Prediction of sound sources reduces to a statistical problem of contact mechanics. On the other hand, contact is also responsible of dissipation of vibration. This leads to the paradoxical result that the noise may not be proportional to the number of sources.

  30. Hydrodynamic skin-friction reduction

    NASA Technical Reports Server (NTRS)

    Reed, Jason C. (Inventor); Bushnell, Dennis M. (Inventor); Weinstein, Leonard M. (Inventor)

    1991-01-01

    A process for reducing skin friction, inhibiting the effects of liquid turbulence, and decreasing heat transfer in a system involving flow of a liquid along a surface of a body includes applying a substantially integral sheet of a gas, e.g., air, immediately adjacent to the surface of the body, e.g., a marine vehicle, which has a longitudinally grooved surface in proximity with the liquid and with a surface material having high contact angle between the liquid and said wall to reduce interaction of the liquid, e.g., water, with the surface of the body, e.g., the hull of the marine vehicle.

  31. Adiabatic molecular-dynamics-simulation-method studies of kinetic friction

    NASA Astrophysics Data System (ADS)

    Zhang, J.; Sokoloff, J. B.

    2005-06-01

    An adiabatic molecular-dynamics method is developed and used to study the Muser-Robbins model for dry friction (i.e., nonzero kinetic friction in the slow sliding speed limit). In this model, dry friction between two crystalline surfaces rotated with respect to each other is due to mobile molecules (i.e., dirt particles) adsorbed at the interface. Our adiabatic method allows us to quickly locate interface potential-well minima, which become unstable during sliding of the surfaces. Since dissipation due to friction in the slow sliding speed limit results from mobile molecules dropping out of such unstable wells, our method provides a way to calculate dry friction, which agrees extremely well with results found by conventional molecular dynamics for the same system, but our method is more than a factor of 10 faster.

  32. High fidelity frictional models for MEMS.

    SciTech Connect

    Carpick, Robert W.; Reedy, Earl David, Jr.; Bitsie, Fernando; de Boer, Maarten Pieter; Corwin, Alex David; Ashurst, William Robert; Jones, Reese E.; Subhash, Ghatu S.; Street, Mark D.; Sumali, Anton Hartono; Antoun, Bonnie R.; Starr, Michael James; Redmond, James Michael; Flater, Erin E.

    2004-10-01

    The primary goals of the present study are to: (1) determine how and why MEMS-scale friction differs from friction on the macro-scale, and (2) to begin to develop a capability to perform finite element simulations of MEMS materials and components that accurately predicts response in the presence of adhesion and friction. Regarding the first goal, a newly developed nanotractor actuator was used to measure friction between molecular monolayer-coated, polysilicon surfaces. Amontons law does indeed apply over a wide range of forces. However, at low loads, which are of relevance to MEMS, there is an important adhesive contribution to the normal load that cannot be neglected. More importantly, we found that at short sliding distances, the concept of a coefficient of friction is not relevant; rather, one must invoke the notion of 'pre-sliding tangential deflections' (PSTD). Results of a simple 2-D model suggests that PSTD is a cascade of small-scale slips with a roughly constant number of contacts equilibrating the applied normal load. Regarding the second goal, an Adhesion Model and a Junction Model have been implemented in PRESTO, Sandia's transient dynamics, finite element code to enable asperity-level simulations. The Junction Model includes a tangential shear traction that opposes the relative tangential motion of contacting surfaces. An atomic force microscope (AFM)-based method was used to measure nano-scale, single asperity friction forces as a function of normal force. This data is used to determine Junction Model parameters. An illustrative simulation demonstrates the use of the Junction Model in conjunction with a mesh generated directly from an atomic force microscope (AFM) image to directly predict frictional response of a sliding asperity. Also with regards to the second goal, grid-level, homogenized models were studied. One would like to perform a finite element analysis of a MEMS component assuming nominally flat surfaces and to include the effect of

  1. Foreword: Quasicrystals at Interfaces

    NASA Astrophysics Data System (ADS)

    Fournée, Vincent; Ledieu, Julian; Thiel, Patricia

    2008-08-01

    lattice is important in many ways. This special issue of Journal of Physics: Condensed Matter approaches various aspects of interfaces involving a quasicrystal and a crystalline counterpart. The first two articles [1, 2] discuss quasicrystal-crystal interfaces in bulk materials and their influence on materials strength. The next two articles [3, 4] introduce the general principles that govern such interfaces: coincidence of reciprocal lattice site and locking into registry of the two half-crystal atomic structures. Articles [5-7] deal with interfaces created by depositing overlayers on clean quasicrystalline surfaces prepared in ultra high vacuum (UHV). This type of study can be regarded as a situation where interfaces are formed and monitored step-by-step during the growth of the deposited film. Articles [8, 9] present similar studies where the formation of crystalline alloys as overlayers is observed on top of a quasicrystalline substrate, also in UHV environment. Articles [10, 11] deal with surface and interface energies and their role in the wetting of liquids on quasicrystalline substrates. Finally, [12] discusses the remarkable friction and adhesion properties of quasicrystal surfaces, involving a dynamical interface. References [1] Chang H J et al 2008 Microstructural evolution and role of interface in Mg-Zn-Y alloys with high strength and formability J. Phys.: Condens. Matter 20 314001 [2] Singh A et al 2008 Quasicrystal-crystal interfaces in bulk materials J. Phys.: Condens. Matter 20 314002 [3] Widjaja P E J and Marks L D 2008 Models for quasicrystal-crystal epitaxy J. Phys.: Condens. Matter 20 314003 [4] Theis W and Franke K J 2008 Epitaxial interfaces between half-crystals of quasicrystalline and periodic material J. Phys.: Condens. Matter 20 314004 [5] Smerdon J A, Sharma H R, Ledieu J and McGrath R 2008 Nucleation and growth of pseudomorphic monolayers on quasicrystal surfaces J. Phys.: Condens. Matter 20 314005 [6] Burkardt S, Deloudi S, Erbudak M, Kortan A

  2. Friction-Induced Fluid Heating in Nanoscale Helium Flows

    SciTech Connect

    Li Zhigang

    2010-05-21

    We investigate the mechanism of friction-induced fluid heating in nanoconfinements. Molecular dynamics simulations are used to study the temperature variations of liquid helium in nanoscale Poiseuille flows. It is found that the fluid heating is dominated by different sources of friction as the external driving force is changed. For small external force, the fluid heating is mainly caused by the internal viscous friction in the fluid. When the external force is large and causes fluid slip at the surfaces of channel walls, the friction at the fluid-solid interface dominates over the internal friction in the fluid and is the major contribution to fluid heating. An asymmetric temperature gradient in the fluid is developed in the case of nonidentical walls and the general temperature gradient may change sign as the dominant heating factor changes from internal to interfacial friction with increasing external force.

  3. A systems based experimental approach to tactile friction.

    PubMed

    Masen, M A

    2011-11-01

    This work focuses on the friction in contacts where the human finger pad is one of the interacting surfaces. This 'tactile friction' requires a full understanding of the contact mechanics and the behaviour of human skin. The coefficient of friction cannot be considered as a property of the skin alone, but depends on the entire tribo-system. In this work, frictional forces were measured using a commercially available load cell. Parameters such as the hydration of the skin, the normal load on the contact and the roughness of the contacting surfaces were varied, whilst keeping the other parameters constant. The tests were performed under controlled environmental conditions. The total friction force is a combination of forces related to adhesion and to deformation. A commonly made assumption is that, to describe the friction of human skin, the deformation component can be ignored and only the adhesive behaviour has to be taken into account. However, in this study it was found that the forces related to the (micro-scale) deformation of skin can have a significant contribution to the total friction force; this is valid both for dry conditions and in the presence of water, when hydration of the skin causes softening.

  4. The friction and wear of TPS fibers

    NASA Technical Reports Server (NTRS)

    Bascom, W. D.; Wong, S.

    1987-01-01

    The sliding friction behavior of single filaments of SiO2, SiC, and an aluminoborosilicate has been determined. These fibers are used in thermal protection systems (TPS) and are subject to damage during weaving and aero-maneuvering. All fibers exhibited stick-slip friction indicating the successive formation and rupture of strong junctions between the contacting filaments. The static frictional resistance of the sized SiC filament was 4X greater than for the same filament after heat cleaning. This result suggests that the sizing is an organic polymer with a high shear yield strength. Heat cleaning exposes the SiC surface and/or leaves an inorganic residue so that the adhesional contact between filaments has a low fracture energy and frictional sliding occurs by brittle fracture. The frictional resistances of the sized and heat cleaned SiO2 and glass filaments were all comparable to that of the heat cleaned SiC. It would appear that the sizings as well as the heat cleaned surfaces of the silica and glass have low fracture energies so that the sliding resistance is determined by brittle fracture.

  5. Status of Stellite 6 friction testing

    SciTech Connect

    Watkins, J.C.; DeWall, K.G.; Weidenhamer, G.H.

    1998-06-01

    For the past several years, researchers at the Idaho National Engineering and Environmental Laboratory, under the sponsorship of the US Nuclear Regulatory Commission, Office of Nuclear Regulatory Research, have been investigating the performance of motor-operated valves subjected to design basis flow and pressure loads. Part of this research addresses the friction that occurs at the interface between the valve disc and the valve body seats during operation of a gate valve. In most gate valves, these surfaces are hardfaced with Stellite 6, a cobalt-based alloy. Analytical methods exist for predicting the thrust needed to operate these valves at specific pressure conditions. To produce accurate valve thrust predictions, the analyst must have a reasonably accurate, though conservative, estimate of the coefficient of friction at the disc-to-seat interface. One of the questions that remains to be answered is whether, and to what extent, aging of the disc and seat surfaces effects the disc-to-seat coefficient of friction. Specifically, does the environment in a nuclear plants piping system cause the accumulation of an oxide film on these surfaces that increases the coefficient of friction; and if so, how great is the increase? This paper presents results of specimen tests addressing this issue, with emphasis on the following: (1) the characteristics and thickness of the oxide film that develops on Stellite 6 as it ages; (2) the change in the friction coefficient of Stellite 6 as it ages, including the question of whether the friction coefficient eventually reaches a plateau; and (3) the effect in-service cycling has on the characteristics and thickness of the oxide film and on the friction coefficient.

  6. Physicochemical processes of frictional healing: Effects of water on stick-slip stress drop and friction of granular fault gouge

    NASA Astrophysics Data System (ADS)

    Scuderi, Marco M.; Carpenter, Brett M.; Marone, Chris

    2014-05-01

    Understanding the micromechanical processes that dictate the evolution of fault strength during the seismic cycle is a fundamental problem in earthquake physics. We report on laboratory experiments that investigate the role of water during repetitive stick-slip frictional sliding, with particular emphasis on the grain-scale and atomic-scale mechanisms of frictional restrengthening (healing). Our experiments are designed to test underlying concepts of rate and state friction laws. We sheared layers of soda-lime glass beads in a double direct shear configuration at a constant normal stress of 5 MPa. Shear stress was applied via a constant displacement rate from 0.3 to 300 µm/s. During each experiment, relative air humidity (RH) was kept constant at values of 5, 50, or 100%. Our data show a systematic increase in maximum friction (μmax), stick-slip friction drop (Δμ), and frictional healing rate, with increasing RH. The highest values of interevent dilation occur at 100% RH. Postexperiment scanning electron microscope observations reveal details of contact junction processes, showing a larger grain-to-grain contact area at higher RH. We find that the evolution of contact area depends inversely on slip velocity and directly on RH. Our results illuminate the fundamental processes that dictate stick-slip frictional sliding and provide important constraints on the mechanisms of rate and state friction.

  7. Transition from stick-slip to continuous sliding in atomic friction: entering a new regime of ultralow friction.

    PubMed

    Socoliuc, A; Bennewitz, R; Gnecco, E; Meyer, E

    2004-04-02

    A transition from stick-slip to continuous sliding is observed for atomically modulated friction by means of a friction force microscope. When the stick-slip instabilities cease to exist, a new regime of ultralow friction is encountered. The transition is described in the framework of the Tomlinson model using a parameter eta which relates the strength of the lateral atomic surface potential and the stiffness of the contact under study. Experimentally, this parameter can be tuned by varying the normal load on the contact. We compare our results to a recently discussed concept called superlubricity.

  8. Transition from Stick-Slip to Continuous Sliding in Atomic Friction: Entering a New Regime of Ultralow Friction

    NASA Astrophysics Data System (ADS)

    Socoliuc, A.; Bennewitz, R.; Gnecco, E.; Meyer, E.

    2004-04-01

    A transition from stick-slip to continuous sliding is observed for atomically modulated friction by means of a friction force microscope. When the stick-slip instabilities cease to exist, a new regime of ultralow friction is encountered. The transition is described in the framework of the Tomlinson model using a parameter η which relates the strength of the lateral atomic surface potential and the stiffness of the contact under study. Experimentally, this parameter can be tuned by varying the normal load on the contact. We compare our results to a recently discussed concept called superlubricity.

  9. Origin of Subglacial Debris-bed Friction

    NASA Astrophysics Data System (ADS)

    Cohen, D. O.; Byers, J.; Iverson, N. R.

    2011-12-01

    Numerical models of glaciers sliding on hard beds assume that basal flow resistance is controlled entirely by viscous drag on bedrock bumps. However, observations and measurements indicate that basal ice can contain large concentrations of rock debris that exert significant frictional resistance: for example, locally high shear stress (˜500 kPa) was measured below 200 m of ice on a smooth rock tablet at the bed of Engabreen, Norway. This value of shear stress is an order of magnitude greater than estimated by leading theories. To better understand the origin of debris-bed friction, we built a new laboratory apparatus that recorded the contact force between a clast and a hard bed as a function of ice velocity toward the bed. An independent experiment with the same apparatus in which the clast is isolated from the bed was used to obtain the ice viscosity. After correcting for cavity formation and ice flow geometry, results indicate that the contact force between a clast and a hard bed is about twice the drag force on the same clast estimated using Stokes's law. This value is insufficient to explain the high debris-bed friction measured beneath Engabreen. An alternative explanation is that longitudinal ice extension caused by ice flowing over the rough topography near the smooth rock tablet increased the rate of ice convergence with the bed by a factor of 5. Our measurements confirm that debris-bed friction is controlled by contact forces caused by flow of ice towards the bed due to basal melting and longitudinal ice extension. This form of frictional drag has yet to be included in models of ice flow. Inclusion of debris-bed friction may prove important to properly estimating rates of basal sliding, energy dissipation and meltwater production at the bed, and, more importantly, to quantifying the stick-slip behavior of hard-bedded glaciers.

  10. Modeling of rock friction 1. Experimental results and constitutive equations

    USGS Publications Warehouse

    Dieterich, J.H.

    1979-01-01

    Direct shear experiments on ground surfaces of a granodiorite from Raymond, California, at normal stresses of ??6 MPa demonstrate that competing time, displacement, and velocity effects control rock friction. It is proposed that the strength of the population of points of contacts between sliding surfaces determines frictional strength and that the population of contacts changes continuously with displacements. Previous experiments demonstrate that the strength of the contacts increases with the age of the contacts. The present experiments establish that a characteristic displacement, proportional to surface roughness, is required to change the population of contacts. Hence during slip the average age of the points of contact and therefore frictional strength decrease as slip velocity increases. Displacement weakening and consequently the potential for unstable slip occur whenever displacement reduces the average age of the contacts. In addition to this velocity dependency, which arises from displacement dependency and time dependency, the experiments also show a competing but transient increase in friction whenever slip velocity increases. Creep of the sliding surface at stresses below that for steady state slip is also observed. Constitutive relationships are developed that permit quantitative simulation of the friction versus displacement data as a function of surface roughness and for different time and velocity histories. Unstable slip in experiments is controlled by these constitutive effects and by the stiffness of the experimental system. It is argued that analogous properties control earthquake instability. Copyright ?? 1979 by the American Geophysical Union.

  11. Friction velocity and aerodynamic roughness of conventional and undercutter tillage within the Columbia Plateau, USA

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Friction velocity and aerodynamic roughness are characteristics of the soil-plant-atmosphere interface which affect wind erosion. Although exchange of momentum at the interface can be altered by land management practices, no attempts have been made to quantify the effect of tillage on friction veloc...

  12. Friction-Stir Processing

    DTIC Science & Technology

    2006-01-01

    fatigue results for friction stir welded 2219 aluminum in the following conditions: 1) milled, 2) milled + LPB, 3) milled + 100 hours in a salt...same alloy following friction stir processing. Increased fatigue life in 5083-H321 aluminum fusion welds It will not be possible to friction...fine grain and weld defects near the surface will be eliminated. Potential benefits include both increased corrosion resistance and fatigue life

  13. Friction of rocks

    USGS Publications Warehouse

    Byerlee, J.

    1978-01-01

    Experimental results in the published literature show that at low normal stress the shear stress required to slide one rock over another varies widely between experiments. This is because at low stress rock friction is strongly dependent on surface roughness. At high normal stress that effect is diminished and the friction is nearly independent of rock type. If the sliding surfaces are separated by gouge composed of Montmorillonite or vermiculite the friction can be very low. ?? 1978 Birkha??user Verlag.

  14. An analytical model of dynamic sliding friction during impact

    NASA Astrophysics Data System (ADS)

    Arakawa, Kazuo

    2017-01-01

    Dynamic sliding friction was studied based on the angular velocity of a golf ball during an oblique impact. This study used the analytical model proposed for the dynamic sliding friction on lubricated and non-lubricated inclines. The contact area A and sliding velocity u of the ball during impact were used to describe the dynamic friction force Fd = λAu, where λ is a parameter related to the wear of the contact area. A comparison with experimental results revealed that the model agreed well with the observed changes in the angular velocity during impact, and λAu is qualitatively equivalent to the empirical relationship, μN + μη‧dA/dt, given by the product between the frictional coefficient μ and the contact force N, and the additional term related to factor η‧ for the surface condition and the time derivative of A.

  15. An analytical model of dynamic sliding friction during impact

    PubMed Central

    Arakawa, Kazuo

    2017-01-01

    Dynamic sliding friction was studied based on the angular velocity of a golf ball during an oblique impact. This study used the analytical model proposed for the dynamic sliding friction on lubricated and non-lubricated inclines. The contact area A and sliding velocity u of the ball during impact were used to describe the dynamic friction force Fd = λAu, where λ is a parameter related to the wear of the contact area. A comparison with experimental results revealed that the model agreed well with the observed changes in the angular velocity during impact, and λAu is qualitatively equivalent to the empirical relationship, μN + μη′dA/dt, given by the product between the frictional coefficient μ and the contact force N, and the additional term related to factor η′ for the surface condition and the time derivative of A. PMID:28054668

  16. Rubber friction directional asymmetry

    NASA Astrophysics Data System (ADS)

    Tiwari, A.; Dorogin, L.; Steenwyk, B.; Warhadpande, A.; Motamedi, M.; Fortunato, G.; Ciaravola, V.; Persson, B. N. J.

    2016-12-01

    In rubber friction studies it is usually assumed that the friction force does not depend on the sliding direction, unless the substrate has anisotropic properties, like a steel surface grinded in one direction. Here we will present experimental results for rubber friction, where we observe a strong asymmetry between forward and backward sliding, where forward and backward refer to the run-in direction of the rubber block. The observed effect could be very important in tire applications, where directional properties of the rubber friction could be induced during braking.

  17. Friction. Tuning friction atom-by-atom in an ion-crystal simulator.

    PubMed

    Bylinskii, Alexei; Gangloff, Dorian; Vuletić, Vladan

    2015-06-05

    Friction between ordered, atomically smooth surfaces at the nanoscale (nanofriction) is often governed by stick-slip processes. To test long-standing atomistic models of such processes, we implemented a synthetic nanofriction interface between a laser-cooled Coulomb crystal of individually addressable ions as the moving object and a periodic light-field potential as the substrate. We show that stick-slip friction can be tuned from maximal to nearly frictionless via arrangement of the ions relative to the substrate. By varying the ion number, we also show that this strong dependence of friction on the structural mismatch, as predicted by many-particle models, already emerges at the level of two or three atoms. This model system enables a microscopic and systematic investigation of friction, potentially even into the quantum many-body regime.

  18. Mussel Coating Protein-Derived Complex Coacervates Mitigate Frictional Surface Damage.

    PubMed

    Miller, Dusty Rose; Das, Saurabh; Huang, Kuo-Ying; Han, Songi; Israelachvili, Jacob N; Waite, J Herbert

    2015-11-09

    The role of friction in the functional performance of biomaterial interfaces is widely reckoned to be critical and complicated but poorly understood. To better understand friction forces, we investigated the natural adaptation of the holdfast or byssus of mussels that live in high-energy surf habitats. As the outermost covering of the byssus, the cuticle deserves particular attention for its adaptations to frictional wear under shear. In this study, we coacervated one of three variants of a key cuticular component, mussel foot protein 1, mfp-1 [(1) Mytilus californianus mcfp-1, (2) rmfp-1, and (3) rmfp-1-Dopa], with hyaluronic acid (HA) and investigated the wear protection capabilities of these coacervates to surfaces (mica) during shear. Native mcfp-1/HA coacervates had an intermediate coefficient of friction (μ ∼0.3) but conferred excellent wear protection to mica with no damage from applied loads, F⊥, as high as 300 mN (pressure, P, > 2 MPa). Recombinant rmfp-1/HA coacervates exhibited a comparable coefficient of friction (μ ∼0.3); however, wear protection was significantly inferior (damage at F⊥ > 60 mN) compared with that of native protein coacervates. Wear protection of rmfp-1/HA coacervates increased 5-fold upon addition of the surface adhesive group 3,4-dihydroxyphenylalanine, (Dopa). We propose a Dopa-dependent wear protection mechanism to explain the differences in wear protection between coacervates. Our results reveal a significant untapped potential for coacervates in applications that require adhesion, lubrication, and wear protection. These applications include artificial joints, contact lenses, dental sealants, and hair and skin conditioners.

  19. Mussel Coating Protein-Derived Complex Coacervates Mitigate Frictional Surface Damage

    PubMed Central

    2015-01-01

    The role of friction in the functional performance of biomaterial interfaces is widely reckoned to be critical and complicated but poorly understood. To better understand friction forces, we investigated the natural adaptation of the holdfast or byssus of mussels that live in high-energy surf habitats. As the outermost covering of the byssus, the cuticle deserves particular attention for its adaptations to frictional wear under shear. In this study, we coacervated one of three variants of a key cuticular component, mussel foot protein 1, mfp-1 [(1) Mytilus californianus mcfp-1, (2) rmfp-1, and (3) rmfp-1-Dopa], with hyaluronic acid (HA) and investigated the wear protection capabilities of these coacervates to surfaces (mica) during shear. Native mcfp-1/HA coacervates had an intermediate coefficient of friction (μ ∼0.3) but conferred excellent wear protection to mica with no damage from applied loads, F⊥, as high as 300 mN (pressure, P, > 2 MPa). Recombinant rmfp-1/HA coacervates exhibited a comparable coefficient of friction (μ ∼0.3); however, wear protection was significantly inferior (damage at F⊥ > 60 mN) compared with that of native protein coacervates. Wear protection of rmfp-1/HA coacervates increased 5-fold upon addition of the surface adhesive group 3,4-dihydroxyphenylalanine, (Dopa). We propose a Dopa-dependent wear protection mechanism to explain the differences in wear protection between coacervates. Our results reveal a significant untapped potential for coacervates in applications that require adhesion, lubrication, and wear protection. These applications include artificial joints, contact lenses, dental sealants, and hair and skin conditioners. PMID:26618194

  20. Contact dermatitis

    MedlinePlus

    Dermatitis - contact; Allergic dermatitis; Dermatitis - allergic; Irritant contact dermatitis; Skin rash - contact dermatitis ... There are 2 types of contact dermatitis. Irritant dermatitis: This ... can be by contact with acids, alkaline materials such as soaps ...

  1. Analysis of the NASA White Sands Test Facility (WSTF) Test System for Friction-Ignition of Metallic Materials

    NASA Technical Reports Server (NTRS)

    Shoffstall, Michael S.; Wilson, D. Bruce; Stoltzfus, Joel M.

    2000-01-01

    Friction is a known ignition source for metals in oxygen-enriched atmospheres. The test system developed by the NASA White Sands Test Facility in response to ASTM G-94 has been used successfully to determine the relative ignition from friction of numerous metallic materials and metallic materials pairs. These results have been ranked in terms of a pressure-velocity product (PV) as measured under the prescribed test conditions. A high value of 4.1(exp 8) watts per square meter for Inconel MA 754 is used to imply resistance to friction ignition, whereas a low value of 1.04(exp 8) watts per square meter for stainless steel 304 is taken as indicating material susceptible to friction ignition. No attempt has been made to relate PV values to other material properties. This work reports the analysis of the WSTF friction-ignition test system for producing fundamental properties of metallic materials relating to ignition through friction. Three materials, aluminum, titanium, and nickel were tested in the WSTF frictional ignition instrument system under atmospheres of oxygen or nitrogen. Test conditions were modified to reach a steady state of operation, that is applied, the force was reduced and the rotational speed was reduced. Additional temperature measurements were made on the stator sample. The aluminum immediately galled on contact (reproducible) and the test was stopped. Titanium immediately ignited as a result of non-uniform contact of the stator and rotor. This was reproducible. A portion of the stator sampled burned, but the test continued. Temperature measurements on the stator were used to validate the mathematical model used for estimating the interface (stator/rotor) temperature. These interface temperature measurements and the associate thermal flux into the stator were used to distinguish material-phase transitions, chemical reaction, and mechanical work. The mechanical work was used to analyze surface asperities in the materials and to estimate a

  2. A study of frictional property of the human fingertip using three-dimensional finite element analysis.

    PubMed

    Yoshida, Hiroaki; Tada, Mitsunori; Mochimaru, Masaaki

    2011-03-01

    Since the tactile perception detects skin deformation due to the contact of an object, it is important to understand contact mechanics, especially, frictional behavior of the human fingertip. The coefficient of friction is recently modeled as a function of the applied normal load in which case the traditional Coulomb's law does not provide a description for the skin surface. When a surface is a rubber-like material, the frictional behavior follows the frictional law of the rubber-like material. Therefore, we developed a three-dimensional Finite Element model of the fingertip and analyzed frictional behavior based on the frictional law of rubber-like material. We proposed a combined technique using both experimental and Finite Element analyses in order to investigate the frictional property of the fingertip. A three-dimensional Finite Element model of the fingertip was developed using MRI images. We hypothesized a frictional equation of the critical shear stress. Squared differences between equivalent coefficient of friction of the FE analysis and the coefficient of kinetic friction of the experiment while sliding was decreased and the Finite Element analysis iterated until the error was minimized, and thus the frictional equation was determined. We obtained the equation of the critical shear stress and simulated kinetic friction of the fingertip while sliding under arbitrary normal loading condition by using the Finite Element analysis. We think this study is an appropriate method for understanding the frictional property of the human fingertip using the Finite Element analysis.

  3. Multimodal Friction Ignition Tester

    NASA Technical Reports Server (NTRS)

    Davis, Eddie; Howard, Bill; Herald, Stephen

    2009-01-01

    The multimodal friction ignition tester (MFIT) is a testbed for experiments on the thermal and mechanical effects of friction on material specimens in pressurized, oxygen-rich atmospheres. In simplest terms, a test involves recording sensory data while rubbing two specimens against each other at a controlled normal force, with either a random stroke or a sinusoidal stroke having controlled amplitude and frequency. The term multimodal in the full name of the apparatus refers to a capability for imposing any combination of widely ranging values of the atmospheric pressure, atmospheric oxygen content, stroke length, stroke frequency, and normal force. The MFIT was designed especially for studying the tendency toward heating and combustion of nonmetallic composite materials and the fretting of metals subjected to dynamic (vibrational) friction forces in the presence of liquid oxygen or pressurized gaseous oxygen test conditions approximating conditions expected to be encountered in proposed composite material oxygen tanks aboard aircraft and spacecraft in flight. The MFIT includes a stainless-steel pressure vessel capable of retaining the required test atmosphere. Mounted atop the vessel is a pneumatic cylinder containing a piston for exerting the specified normal force between the two specimens. Through a shaft seal, the piston shaft extends downward into the vessel. One of the specimens is mounted on a block, denoted the pressure block, at the lower end of the piston shaft. This specimen is pressed down against the other specimen, which is mounted in a recess in another block, denoted the slip block, that can be moved horizontally but not vertically. The slip block is driven in reciprocating horizontal motion by an electrodynamic vibration exciter outside the pressure vessel. The armature of the electrodynamic exciter is connected to the slip block via a horizontal shaft that extends into the pressure vessel via a second shaft seal. The reciprocating horizontal

  4. Friction induced rail vibrations

    NASA Astrophysics Data System (ADS)

    Kralov, Ivan; Sinapov, Petko; Nedelchev, Krasimir; Ignatov, Ignat

    2012-11-01

    A model of rail, considered as multiple supported beam, subjected on friction induced vibration is studied in this work using FEM. The model is presented as continuous system and the mass and elastic properties of a real object are taken into account. The friction forces are nonlinear functions of the relative velocity during slipping. The problem is solved using Matlab Simulink.