Definition of Atomic-Scale Contact: What Dominates the Atomic-Scale Friction Behaviors?

The definition of atomic-scale contact is a very ambiguous issue owing to the discrete atomic arrangement, which hinders the development of contact theory and nano-tribological techniques. In this work, we studied the atomic-scale contact area and their correlations with friction force based on three distinct contact definitions (interatomic distance, force, and interfacial chemical bonds) by performing large-scale atomistic simulations on a typical ball-on-disk contact model. In the simulations, the measured contact areas defined by interatomic distance, force, and interfacial chemical bonds (referred as to A_dist, A_force, and A_bond, respectively) are not equivalent at all, while we interestingly clarify that only A_dist is consistent with the one calculated by continuum Hertz contact mechanics, and moreover, only A_bond is proportional to the friction force indicating that A_bond is the dominant one for determining materials' frictional behaviors. The above fundamental insights into the atomic-scale contact problems are useful to deeply understand the origins of tribological phenomena and contribute to the further prediction of atomic-scale friction.