Structural Lubricity and Molecular Contamination: Rejuvenation, Aging, and Friction Switches

TL;DR

This study combines experiments and simulations to explore how contamination influences ultra-small friction in structural lubricity, revealing phenomena like rejuvenation, aging, and friction switches that are transient and affected by contamination layers.

Contribution

It uncovers new transient phenomena in structural lubricity, such as rejuvenation, aging, and switches, and explains their suppression over time due to contamination effects.

Findings

Rejuvenation causes a significant drop in friction shortly after sliding begins.

Aging leads to increased friction after rest periods of 30 minutes or more.

Contamination layers influence friction behavior but do not cause breakdown of lubricity.

Abstract

Using atomic force microscopy experiments and molecular dynamics simulations of gold nanoislands on graphite, we investigate why ultra-small friction commonly associated with structural lubricity can be observed even under ambient conditions. Measurements conducted within a few days after sample synthesis reveal previously undiscovered phenomena in structurally lubric systems: rejuvenation, a drop in kinetic friction of an order of magnitude shortly after the onset of sliding; aging, a significant increase in kinetic friction forces after a rest period of 30 minutes or more; switches, spontaneous jumps between distinct friction branches. These three effects are drastically suppressed a few weeks later. Imaging of a contamination layer and simulations provide a consistent picture of how single- and double-layer contamination underneath the gold nanoislands as well as contamination surrounding the nanoislands affect structural lubricity but not lead to its breakdown.