The Experts below are selected from a list of 1929 Experts worldwide ranked by ideXlab platform
Robert W Carpick - One of the best experts on this subject based on the ideXlab platform.
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rate and state friction relation for nanoscale contacts thermally activated prandtl tomlinson model with chemical aging
Physical Review Letters, 2018Co-Authors: Kaiwen Tian, D L Goldsby, Robert W CarpickAbstract:Rate and state friction (RSF) laws are widely used empirical relationships that describe macroscale to microscale frictional behavior. They entail a linear combination of the direct effect (the increase of friction with sliding velocity due to the reduced influence of thermal excitations) and the evolution effect (the change in friction with changes in contact "state," such as the real contact area or the degree of interfacial chemical bonds). Recent atomic force microscope (AFM) experiments and simulations found that nanoscale Single-Asperity amorphous silica-silica contacts exhibit logarithmic aging (increasing friction with time) over several decades of contact time, due to the formation of interfacial chemical bonds. Here we establish a physically based RSF relation for such contacts by combining the thermally activated Prandtl-Tomlinson (PTT) model with an evolution effect based on the physics of chemical aging. This thermally activated Prandtl-Tomlinson model with chemical aging (PTTCA), like the PTT model, uses the loading point velocity for describing the direct effect, not the tip velocity (as in conventional RSF laws). Also, in the PTTCA model, the combination of the evolution and direct effects may be nonlinear. We present AFM data consistent with the PTTCA model whereby in aging tests, for a given hold time, static friction increases with the logarithm of the loading point velocity. Kinetic friction also increases with the logarithm of the loading point velocity at sufficiently high velocities, but at a different increasing rate. The discrepancy between the rates of increase of static and kinetic friction with velocity arises from the fact that appreciable aging during static contact changes the energy landscape. Our approach extends the PTT model, originally used for crystalline substrates, to amorphous materials. It also establishes how conventional RSF laws can be modified for nanoscale Single-Asperity contacts to provide a physically based friction relation for nanoscale contacts that exhibit chemical bond-induced aging, as well as other aging mechanisms with similar physical characteristics.
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Multibond Model of Single-Asperity Tribochemical Wear at the Nanoscale
ACS Applied Materials & Interfaces, 2017Co-Authors: Yijie Jiang, Kevin T. Turner, Robert W Carpick, Yuchong Shao, Tevis D. B. Jacobs, Michael L. FalkAbstract:Single-Asperity wear experiments and simulations have identified different regimes of wear including Eyring- and Archard-like behaviors. A multibond dynamics model has been developed based on the friction model of Filippov et al. [Phys. Rev. Lett. 92, 135503 (2004)]. This new model captures both qualitatively distinct regimes of Single-Asperity wear under a unified theoretical framework. In this model, the interfacial bond formation, wearless rupture, and transfer of atoms are governed by three competing thermally activated processes. The Eyring regime holds under the conditions of low load and low adhesive forces; few bonds form between the Asperity and the surface, and wear is a rare and rate-dependent event. As the normal stress increases, the Eyring behavior of wear rate breaks down. A nearly rate-independent regime arises under high load or high adhesive forces, in which wear becomes very nearly, but not precisely, proportional to sliding distance. In this restricted regime, the dependence of wear ra...
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Correcting for Tip Geometry Effects in Molecular Simulations of Single-Asperity Contact
Tribology Letters, 2017Co-Authors: Yijie Jiang, Kathleen E. Ryan, Jeffrey D Schall, Judith A Harrison, Robert W Carpick, Kevin T. TurnerAbstract:Molecular simulation is a powerful tool for studying the nanotribology of Single-Asperity contacts, but computational limits require that compromises be made when choosing tip sizes. To assess and correct for the finite size effects, complementary finite element (FE) and molecular statics (MS) simulations examining the effects of tip size (height and radius) on contact stiffness and stress were performed. MS simulations of contact between paraboloidal tips and a flat, rigid diamond substrate using the 2B-SiCH reactive empirical bond-order potential were used to generate force–displacement curves and stress maps. Tips of various radii and heights, truncated by a rigid boundary, were formed from carbon- and silicon-containing materials so that they possessed differing elastic properties. Results were compared to FE simulations with matching geometries and elastic properties. FE analysis showed that the rigid boundary at the back of the tip influences the contact stiffness strongly, deviating from the Hertz model for small tip heights and radii. By examining the relationships between force, tip height, tip radii, and elastic properties obtained with FE simulations, a map interpolation method is presented that accounts for the effect of tip size and enables the extraction of Young’s modulus from MS force–displacement data. Furthermore, the FE results show that the effect of the finite size of the tip on contact stress is less pronounced than its effect on stiffness. The MS simulations also demonstrate that stress propagation within the tip is significantly impacted by the structure of the tip.
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mechanisms of antiwear tribofilm growth revealed in situ by Single Asperity sliding contacts
Science, 2015Co-Authors: Nitya Nand Gosvami, J A Bares, Filippo Mangolini, Andrew R Konicek, D G Yablon, Robert W CarpickAbstract:Zinc dialkyldithiophosphates (ZDDPs) form antiwear tribofilms at sliding interfaces and are widely used as additives in automotive lubricants. The mechanisms governing the tribofilm growth are not well understood, which limits the development of replacements that offer better performance and are less likely to degrade automobile catalytic converters over time. Using atomic force microscopy in ZDDP-containing lubricant base stock at elevated temperatures, we monitored the growth and properties of the tribofilms in situ in well-defined Single-Asperity sliding nanocontacts. Surface-based nucleation, growth, and thickness saturation of patchy tribofilms were observed. The growth rate increased exponentially with either applied compressive stress or temperature, consistent with a thermally activated, stress-assisted reaction rate model. Although some models rely on the presence of iron to catalyze tribofilm growth, the films grew regardless of the presence of iron on either the tip or substrate, highlighting the critical role of stress and thermal activation.
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Local Nanoscale Heating Modulates Single-Asperity Friction
Nano Letters, 2010Co-Authors: Jonathan R. Felts, Christian Greiner, Zhenting Dai, William P. King, Robert W CarpickAbstract:We demonstrate measurement and control of Single-Asperity friction by using cantilever probes featuring an in situ solid-state heater. The heater temperature was varied between 25 and 650 °C (tip temperatures from 25 ± 2 to 120 ± 20 °C). Heating caused friction to increase by a factor of 4 in air at ∼ 30% relative humidity, but in dry nitrogen friction decreased by ∼ 40%. Higher velocity reduced friction in ambient with no effect in dry nitrogen. These trends are attributed to thermally assisted formation of capillary bridges between the tip and substrate in air, and thermally assisted sliding in dry nitrogen. Real-time friction measurements while modulating the tip temperature revealed an energy barrier for capillary condensation of 0.40 ± 0.04 eV but with slower kinetics compared to isothermal measurements that we attribute to the distinct thermal environment that occurs when heating in real time. Controlling the presence of this nanoscale capillary and the associated control of friction and adhesion offers new opportunities for tip-based nanomanufacturing.
Polina Prokopovich - One of the best experts on this subject based on the ideXlab platform.
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multi Asperity elliptical jkr model for adhesion of a surface with non axially symmetric asperities
Tribology International, 2015Co-Authors: Polina Prokopovich, Stefano PerniAbstract:Surfaces can present high levels of topographic asymmetry and, therefore, theories based on the assumption of symmetry cannot be effectively employed. A new multi-Asperity adhesion model that assumes that asperities are not perfectly hemispherical is presented here, this model is based on the elliptical JKR model for a Single Asperity. The adhesion between a soft tissue with asperities greatly asymmetric and a polymer was modelled, the predicted adhesion forces were successfully validated against experimentally obtained data. Moreover, simulations with a simpler model, which assumes symmetrical asperities, have been also carried out; these results were significantly different from those obtained, using both the newly developed model and those determined experimentally. This highlights the importance of the model presented in this work.
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adhesion models from Single to multiple Asperity contacts
Advances in Colloid and Interface Science, 2011Co-Authors: Polina Prokopovich, Victor StarovAbstract:This review presents a summary of the current adhesion models available to date, between real rough surfaces, starting from Single Asperity models and expanding to multiple Asperity contacts. The focus is made on multi-Asperity contact interactions. Both van der Waals and contact mechanics approaches have been considered and relevant adhesion models are reviewed and discussed. The influence of the meniscus forces on adhesion has been considered, along with a summary of the various meniscus models. The effect of surface geometry, its topography and environmental conditions on meniscus action are also discussed along with its integration into multi-Asperity adhesion models.
Clare Mccabe - One of the best experts on this subject based on the ideXlab platform.
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Investigating Alkylsilane Monolayer Tribology at a Single-Asperity Contact with Molecular Dynamics Simulation
Langmuir, 2017Co-Authors: Andrew Z. Summers, Christopher R. Iacovella, Peter T. Cummings, Clare MccabeAbstract:Chemisorbed monolayer films are known to possess favorable characteristics for nanoscale lubrication of micro- and nanoelectromechanical systems (MEMS/NEMS). Prior studies have shown that the friction observed for monolayer-coated surfaces features a strong dependence on the geometry of contact. Specifically, tip-like geometries have been shown to penetrate into monolayer films, inducing defects in the monolayer chains and leading to plowing mechanisms during shear, which result in higher coefficients of friction (COF) than those observed for planar geometries. In this work, we use molecular dynamics simulations to examine the tribology of model silica Single-Asperity contacts under shear with monolayer-coated substrates featuring various film densities. It is observed that lower monolayer densities lead to reduced COFs, in contrast to results for planar systems where COF is found to be nearly independent of monolayer density. This is attributed to a liquid-like response to shear, whereby fewer defects ar...
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Investigating Alkylsilane Monolayer Tribology at a Single-Asperity Contact with Molecular Dynamics Simulation
2017Co-Authors: Andrew Z. Summers, Christopher R. Iacovella, Peter T. Cummings, Clare MccabeAbstract:Chemisorbed monolayer films are known to possess favorable characteristics for nanoscale lubrication of micro- and nanoelectromechanical systems (MEMS/NEMS). Prior studies have shown that the friction observed for monolayer-coated surfaces features a strong dependence on the geometry of contact. Specifically, tip-like geometries have been shown to penetrate into monolayer films, inducing defects in the monolayer chains and leading to plowing mechanisms during shear, which result in higher coefficients of friction (COF) than those observed for planar geometries. In this work, we use molecular dynamics simulations to examine the tribology of model silica Single-Asperity contacts under shear with monolayer-coated substrates featuring various film densities. It is observed that lower monolayer densities lead to reduced COFs, in contrast to results for planar systems where COF is found to be nearly independent of monolayer density. This is attributed to a liquid-like response to shear, whereby fewer defects are imparted in monolayer chains from the Asperity, and chains are easily displaced by the tip as a result of the higher free volume. This transition in the mechanism of molecular plowing suggests that liquid-like films should provide favorable lubrication at Single-Asperity contacts
Andreas A Polycarpou - One of the best experts on this subject based on the ideXlab platform.
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A Single Asperity sliding contact model for molecularly thin lubricant
Microsystem Technologies-micro-and Nanosystems-information Storage and Processing Systems, 2016Co-Authors: Youfeng Zhang, Andreas A PolycarpouAbstract:Molecularly thin lubricants are important in protecting the recording head and the rotating disk in a magnetic storage hard disk drive from mechanical damage induced by contact. Direct contact is more likely to occur at lower head-media spacing, which is the distance between the rotating magnetic disk and the head that reads/writes the data, and is highly desirable to be extremely low (in the nanometer range) for Terabit/in2 areal densities. Solid contact mechanics of the head-disk interface has been well addressed through extensive experiments and modeling, by neglecting the effect of the molecularly thin lubricant. Considering the important role of the lubricant, it is necessary to investigate the contact mechanics that include the lubricant. The present study develops a mechanics-based model to account for the nanoscale phenomena including slippage, nonlinear viscosity and bonded lubricant fraction, thus providing a measure to bridge the hydrodynamic lubrication and solid contact regimes at the nanoscale. Results show that the friction coefficient increases with the bonded ratio, which correlates with experimental observations.
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effect of Asperity interactions on rough surface elastic contact behavior hard film on soft substrate
Tribology International, 2010Co-Authors: Chang-dong Yeo, Raja R Katta, Jungkyu Lee, Andreas A PolycarpouAbstract:Abstract An improved elastic micro-contact model of rough surfaces accounting for Asperity interactions is proposed. The contact behavior of a Single Asperity system is composed of a stiffer hemi-spherical Asperity deformation and bellowing softer substrate deformation, which is then extended to rough surface contact including Asperity interactions. Using the solution of substrate deformation, normal positions of individual asperities are adjusted during quasi-static contact, from which surface interactive forces are obtained. Analytical simulations are performed using the proposed rough surface contact model, whose results are compared to Greenwood–Williamson-based models and with experimental measurements.
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Improved Elastic Contact Model Accounting for Asperity and Bulk Substrate Deformation
Tribology Letters, 2009Co-Authors: Chang-dong Yeo, Raja R Katta, Andreas A PolycarpouAbstract:An improved elastic contact model for a Single Asperity system is proposed accounting for both the effects of bulk substrate and Asperity deformations. The Asperity contact stiffness is based on the Hertzian solution for spherical contact, and the bulk substrate stiffness on the solution of Hertzian pressure on a circular region of the elastic half-space. Depending on the magnitude of the applied load, as well as the geometrical and physical properties of the Asperity and bulk materials, the bulk substrate could have considerable contribution to the overall contact stiffness. The proposed Single Asperity model is generalized using two parameters based on physical and geometrical properties, and is also verified using finite element analysis. A parametric study for a practical range of geometric and physical parameters is performed using finite element analysis to determine the range of validity of the proposed model and also to compare it with the Hertz contact model. The Single Asperity model is extended to rough surfaces in contact and the contact stiffness from the proposed model and the simpler Greenwood–Williamson Asperity model are compared to experimental measurements.
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Elastic Contact Model Accounting for Both Asperity and Substrate Compliance With Application to Patterned Media
ASME STLE 2007 International Joint Tribology Conference Parts A and B, 2007Co-Authors: Chang-dong Yeo, Andreas A PolycarpouAbstract:An improved elastic contact stiffness model for a Single Asperity system is proposed to account for the effects of both bulk substrate and Asperity deformations between two contacting surfaces. Depending upon the applied load, as well as the geometrical and physical properties of the Asperity and bulk material, the bulk substrate can have a considerable contribution to the overall contact stiffness. Finite element analysis is performed to verify the proposed analytical model. The Single Asperity model is extended to rough surfaces in contact. The contact stiffness values from the proposed model are compared to those from the GW model. The proposed contact model can be directly relevant to analyze the contact behavior of modern patterned media.© 2007 ASME
Michael L. Falk - One of the best experts on this subject based on the ideXlab platform.
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Multibond Model of Single-Asperity Tribochemical Wear at the Nanoscale
ACS Applied Materials & Interfaces, 2017Co-Authors: Yijie Jiang, Kevin T. Turner, Robert W Carpick, Yuchong Shao, Tevis D. B. Jacobs, Michael L. FalkAbstract:Single-Asperity wear experiments and simulations have identified different regimes of wear including Eyring- and Archard-like behaviors. A multibond dynamics model has been developed based on the friction model of Filippov et al. [Phys. Rev. Lett. 92, 135503 (2004)]. This new model captures both qualitatively distinct regimes of Single-Asperity wear under a unified theoretical framework. In this model, the interfacial bond formation, wearless rupture, and transfer of atoms are governed by three competing thermally activated processes. The Eyring regime holds under the conditions of low load and low adhesive forces; few bonds form between the Asperity and the surface, and wear is a rare and rate-dependent event. As the normal stress increases, the Eyring behavior of wear rate breaks down. A nearly rate-independent regime arises under high load or high adhesive forces, in which wear becomes very nearly, but not precisely, proportional to sliding distance. In this restricted regime, the dependence of wear ra...
