Formation of lubrication film of diamond-like carbon films in water and air environments against stainless steel and Cr-plated balls

Low-friction behaviour of diamond-like carbon (DLC) films in a water environment in regard to their lubrication films was investigated. Previously, these were reported to consist of Fe, Cr, C, and O, when formed against a stainless steel ball. As the role of Cr in lubrication film formation was not clear, a Cr-plated ball was used as a mating ball to investigate it in the present work. The friction coefficient of the DLC/Cr-plated ball system was lower compared with the DLC/steel one, in water and ambient environments. Furthermore, the frictional behaviour at the initial stage against the Cr-plated ball was quite stable comparing with that against the steel ball. However, in long time friction tests (7200 cycles) in water, the friction coefficient was similar for both sliding pairs. Therefore, it is considered that Cr on the surface of mating balls promotes the formation of a lubrication film in a water environment. Difference of the frictional behaviour in water and air is correlated with the formation of lubrication film.