Is Ultralow Friction on Graphite Sustainable in Contaminated Environments?

TL;DR

This study uses molecular dynamics simulations to show that ultralow friction on graphite can be maintained in contaminated environments with physisorbed hydrocarbons, challenging previous assumptions about the fragility of superlubricity.

Contribution

It demonstrates that a monolayer of HEX molecules preserves graphite's superlubricity under ambient conditions, revealing the role of molecular orientation and coverage in friction behavior.

Findings

A monolayer of HEX adheres strongly to graphite, maintaining solid-like behavior.

Shear stress depends on molecular orientation and coverage, affecting dissipation mechanisms.

Superlubricity persists despite increased shear stress in contaminated environments.

Abstract

Structural lubricity arises typically at incommensurate, well-defined dry contacts where short-range elastic instability is significantly mitigated. However, under ambient conditions, airborne molecules adsorb onto solid surfaces, forming an intervening viscous medium that alters interfacial properties. Using molecular dynamics simulations with a newly parameterized interfacial potential, we investigate the preservation of ultralow friction on graphite with physisorbed n-hexadecane (HEX) as a model contaminant. Our findings reveal that a well-ordered monolayer of HEX molecules strongly adheres to the graphite surface, replicating its lattice structure and maintaining solid-like behavior, which leads to orientation-dependent shear stresses-an effect absent on a gold (111) surface. As the contaminant film thickens, this orientation effect diminishes. Additionally, as coverage increases from zero to one monolayer, the shear stress-velocity relationship transitions from Coulomb to quasi-Stokesian and then to quasi-Coulomb, highlighting the role of molecular displacement in high-velocity dissipation. Despite a hundredfold increase in shear stress compared to dry sliding, superlubricity on graphite persists under ambient conditions, enhancing our conventional understanding of structural lubricity.