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Nanoscale Models of Single Asperity Friction and Wear
Understanding friction and wear behavior is limited in part by our lack of a detailed understanding of the atomic scale processes from which energy dissipation and material transfer arise. This has been particularly well illustrated by recent experiments, primarily employing atomic force microscopy (AFM), investigating tribology of single-asperity contacts during sliding. We have undertaken the molecular dynamics modeling of one such system, a model of an oxidized silicon tip sliding on an oxidized silicon surface in vacuum. Reaching sliding speeds typical of AFM experiments requires the application of acceleration methods known as hyperdynamics. [1,2] Using this method we are able to observe behavior similar to that seen in experiment where at low temperature the friction force increases logarithmically with velocity, while at higher temperature one measures a rate independent friction force. This has led to a theory based on the emergence of intermediate states during the application of force to the asperity. [3] Wear phenomena, even that associated with atom-by-atom adhesive wear, provides further interesting areas for investigation. In this regard we have been investigating the multibond model of Barel and Urbakh. We have modified this friction model to include the influence of wear, and we notice that the model contains within it a transition from classical Archard behavior to thermally activated Eyring behavior. We compare this model to experiment and simulation data from the literature, and we discuss plans to investigate the applicability of this model in our own molecular dynamics simulations.
[1] W.-K. Kim and M.L. Falk, "A Practical Perspective on the Implementation of Hyperdynamics for Accelerated Simulation," Journal of Chemical Physics, Vol. 140, No. 4, Art. No. 044107 (2014).
[2] W.-K. Kim and M.L. Falk, "Accelerated molecular dynamics simulation of low-velocity frictional sliding," Modelling and Simulation in Materials Science and Engineering, Vol. 18, pp. 034003 (2010) invited.
[3] W.-K. Kim and M.L. Falk, "Role of intermediate states in low-velocity friction between amorphous surfaces," Physical Review B, Vol. 84, Art. No. 165422 (2011).