  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.
