Is ultralow friction on graphite sustainable in contaminated environments?

Abstract

Structural lubricity typically occurs in incommensurate, dry contacts where short-range elastic instability is minimized. Under ambient conditions, however, airborne molecules can adsorb onto solid surfaces, forming a viscous medium that alters interfacial properties. We hypothesize that despite the presence of physisorbed contaminants, structural lubricity on graphite can persist due to molecular ordering. Molecular dynamics simulations were performed with a newly parameterized interfacial potential to study 𝑛-hexadecane as a model contaminant on graphite surfaces. We investigated the effects of 𝑛-hexadecane coverage on shear stress, comparing behavior on graphite and gold (111). Results reveal that a monolayer of 𝑛-hexadecane molecules adheres strongly to graphite, replicating its lattice and maintaining solid-like behavior, which leads to orientation-dependent shear stresses. This behavior is absent on gold. As the contaminant film thickens, the orientation effect diminishes, and the shear stress-velocity relationship transitions from Coulomb to quasiStokesian and back to quasi-Coulomb as coverage increases. Despite a substantial increase in shear stress under ambient conditions, ultralow friction, or superlubricity, remains on graphite.