  Tribological phenomena such as adhesion, friction, and wear can undermine the
functionality of devices and applications based on the diamond-silica
interface. Controlling these phenomena is highly desirable, but difficult since
extrinsic factors, such as the surface termination by adsorbed species, can
deeply affect the reactivity of diamond and its resistance to wear. In this
work, we investigate the effects of diamond oxidation by massive ab initio
molecular dynamics simulations of silica sliding against diamond surfaces
considering different surface orientations, O-coverages, and tribological
conditions. Our findings reveal a dual role of oxygen that depends on coverage.
At full coverage, the adsorbed oxygen is very effective in friction and wear
reduction because the repulsion with the silica counter-surface prevents the
formation of chemical bonds across the interface. At reduced coverage and high
pressure, Si-O-C bonds are anyway established. In this situation the presence
of oxygen results detrimental as it weakens the surface C-C bonds making the
surface more vulnerable to wear. Indeed we observed atomic wear on the C(110)
surface at 50% O-coverage under harsh tribological conditions. The mechanisms
of friction reduction and atomistic wear are explained through the analysis of
the electronic properties and surface-surface interactions. Overall, our
accurate in silico experiments shed light into the effects of adsorbed oxygen
on the tribological behavior of diamond and show how oxidized diamond can be
worn by silica.
