Atomistic Simulations of Friction at Sliding Diamond Interfaces
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These models predict that the dissipation of the work of sliding into vibrations of the oscillators can result in wearless friction. Because of the simple form of these models, the onset and magnitude of friction can be directly related to the spring constants of the oscillators and the curvature and magnitude of the potential energy wells.15 The main prediction of these models, namely wearless friction at the atomic scale, has recently been confirmed by AFM experiments.6 While the older analytic models and newer ones by both Hirano et al.12 and by Sokoloff13 have provided much insight into the phenomenon of wearless friction at the atomic scale, they can only provide part of the story. Recently, molecular dynamics simulations have expanded on the ideas put forth in these models, confirming and expanding their conclusions.1"1 In our work, we have been using molecular dynamics simulations to study mechanisms of energy dissipation and the resulting friction between sliding diamond interfaces.19"21 While still in their initial stages, these studies have revealed some novel mechanisms of energy dissipation that are directly related to surface structures, and have provided some clues as to how friction can be reduced by effectively smoothing the surface with chemisorbed hydrocarbon chains. In this paper we present a brief review of our use of molecular dynamics simulations to investigate atomic-scale mechanisms of friction between two diamond (111) surfaces.19"21 Diamond surfaces were chosen in our initial studies both because they have the potential to display rich dynamics, and because of the increasing in-
terest in using diamond as a low-friction and wear-resistant coating. Specifically, our simulations can predict and examine the behavior of the friction coefficient, fi, as a function of normal load, temperature, crystallographic sliding direction, sliding speed, and surface morphology. We can also correlate these properties with specific mechanisms of mechanical excitation with unprecedented atomic detail. We believe that these and related studies bring us one step closer to understanding and ultimately controlling friction and wear. Molecular Dynamics Simulations Our molecular dynamics simulations are carried out in the usual way.1^21 Once the positions and velocities of all the atoms in the simulation are specified, the positions of the atoms evolve in time according to an interatomic potential energy function and classical equations of motion. The interatomic forces used in these simulations were derived from a manybody classical potential energy function that was originally developed to model the deposition of diamond films.22 This potential consists of pair-additive repulsive terms plus pair-additive attractive terms coupled to a many-body analytic bondorder function. This bond-order function includes angle bending terms necessary to model hydrocarbon interactions. Both solid-state and molecular interactions are modeled on an equal basis; therefore, the same potential is used for the diamond substrate an
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