When Is Structural Lubricity Load Independent? The Role of Contact Geometry and Elastic Compliance

TL;DR

This study uses molecular dynamics to identify how contact geometry and elastic compliance influence when structural lubricity results in load-independent friction, revealing that boundary effects and elastic deformation are key factors.

Contribution

It demonstrates that contact geometry and elastic compliance, not normal load, primarily determine the onset and breakdown of load-independent friction in structural lubricity.

Findings

Load independence occurs only in area-filling, infinite contacts.

Finite contacts remain load independent up to a critical load.

Elastic out-of-plane deformation triggers load dependence.

Abstract

Using molecular dynamics simulations of an incommensurate Au(111)/graphite interface, we investigate the conditions under which structural lubricity produces load-independent friction. We show that strict load independence occurs only in laterally infinite, area-filling contacts, where dissipation is governed by phonon-mediated viscous coupling and the shear stress scales linearly with sliding velocity. Finite contacts with explicit boundary terminations exhibit substantially higher friction yet remain load independent up to a critical load. Load dependence arises only when elastic out-of-plane deformation near the contact line exceeds a critical amplitude, activating additional dissipation channels. These results demonstrate that contact geometry and local elastic compliance, rather than normal load itself, determine the onset and breakdown of load-independent structural lubricity.