High-pressure lubricity at the meso- and nanoscale

TL;DR

This paper explores how high pressure can cause a decrease in friction at the mesoscale and nanoscale due to structural transitions in confined lubricant films, challenging the typical friction increase with load.

Contribution

It introduces the concept that pressure-induced layering and solidification transitions can lead to friction drops, supported by high pressure sliding simulations.

Findings

Friction can decrease under high pressure due to layering transitions.

Friction drops are linked to changes in lubricant structure and shear localization.

Simulations suggest relevance to boundary lubrication behavior.

Abstract

The increase of sliding friction upon increasing load is a classic in the macroscopic world. Here we discuss the possibility that friction rise might sometimes turn into a drop when, at the mesoscale and nanoscale, a confined lubricant film separating crystalline sliders undergoes strong layering and solidification. Under pressure, transitions from N to N-1 layers may imply a change of lateral periodicity of the crystallized lubricant sufficient to alter the matching of crystal structures, influencing the ensuing friction jump. A pressure-induced friction drop may occur as the shear gradient maximum switches from the lubricant middle, marked by strong stick-slip with or without shear melting, to the crystalline slider-lubricant interface, characterized by smooth superlubric sliding. We present high pressure sliding simulations to display examples of frictional drops, suggesting their possible relevance to the local behavior in boundary lubrication.