Instability of diamond-like carbon (DLC) films during sliding in aqueous environment

The instability of diamond-like carbon (DLC) film deposited on Ti-6Al-4V alloy substrate using the r.f.-PACVD method was investigated under sliding conditions in an aqueous environment. Significant adhesive wear was observed when tested in this environment, while normal abrasive wear occurred in an ambient air of relative humidity about 25%. A critical test was performed to elucidate the reason for the instability which limits the biomedical applications of the DLC coating. By employing a multi-step coating process, it was shown that the instability is closely related to the penetration of water molecules to the interface via through-film defects or pinholes. These results suggest that the stability of DLC film in aqueous environment can be improved by controlling the through-film defects in the DLC coating layer.