Development and assessment of atomistic models for predicting static friction coefficients

Abstract

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