A fundamental mechanism for carbon-film lubricity identified by means of ab initio molecular dynamics

TL;DR

This study uses ab initio molecular dynamics to reveal that water-induced surface hydroxylation and a water film formation are key to the low friction of carbon coatings, explaining their humidity dependence.

Contribution

It provides atomistic insight into the mechanism of carbon film lubricity, highlighting the role of water layers and surface hydroxylation, which was previously not well understood.

Findings

Water molecules form a thin bound layer on hydroxylated surfaces.

Silicon doping influences surface hydroxylation and water interaction.

Water layer reduces direct contact, enabling low friction.

Abstract

Different hypotheses have been proposed to explain the mechanism for the extremely low friction coefficient of carbon coatings and its undesired dependence on air humidity. A decisive atomistic insight is still lacking because of the difficulties in monitoring what actually happens at the buried sliding interface. Here we perform large-scale ab initio molecular dynamics simulations of both undoped and silicon-doped carbon films sliding in the presence of water. We observe the tribologically-induced surface hydroxylation and subsequent formation of a thin film of water molecules bound to the OH-terminated surface by hydrogen bonds. The comparative analysis of silicon-incorporating and clean surfaces, suggests that this two-step process can be the key phenomenon to provide high slipperiness to the carbon coatings. The water layer is, in fact, expected to shelter the carbon surface from direct solid-on-solid contact and make any counter surface slide extremely easily on it. The present insight into the wettability of carbon-based films can be useful for designing new coatings for biomedical and energy-saving applications with environmental adaptability.