Kinetic Friction and Atomistic Instabilities in Boundary-Lubricated Systems

TL;DR

This study uses molecular dynamics to explore how atomic-scale instabilities influence kinetic friction in boundary-lubricated systems, revealing the roles of interface properties and lubricant characteristics.

Contribution

It derives a relationship between kinetic friction and velocity distribution, analyzing how interface dimensionality, coverage, and molecular bonds affect friction.

Findings

Kinetic friction is highly sensitive to lubricant coverage and sliding velocity.

Velocity distributions exhibit exponential tails not describable by temperature.

Commensurate surfaces show greater susceptibility to parameter changes.

Abstract

The contribution of sliding-induced, atomic-scale instabilities to the kinetic friction force is investigated by molecular dynamics. For this purpose, we derive a relationship between the kinetic friction force $F_{\rm k}$ and the non-equilibrium velocity distribution $P(v)$ of the lubricant particles. $P(v)$ typically shows exponential tails, which cannot be described in terms of an effective temperature. It is investigated which parameters control the existence of instabilities and how they affect $P(v)$ and hence $F_{\rm k}$. The effects of the interfaces' dimensionality, lubricant coverage, and internal degrees of freedom of lubricant particles on $F_{\rm k}$ are studied explicitly. Among other results we find that the kinetic friction between commensurate surfaces is much more susceptible to changes in $(i)$ lubricant coverage, $(ii)$ sliding velocity, and $(iii)$ bond length of lubricant molecules than incommensurate surfaces.