Roughness effect of mating ball on friction of diamond-like carbon film and friction mechanism in water and air environment

The dependence of DLC film friction on the roughness of stainless steel mating balls was investigated in water and air environments. It was found that the steel ball surface parameters of Ra and surface density of summits (Dsum) showed a high correlation with the film friction in a water environment but in an air environment, the correlation was low with the friction coefficients scattered across the parameters of roughness. The difference in the frictional behaviors was explained by the presence of lubrication film that formed in the water environment. Furthermore, it was confirmed that the friction behaviour of DLC film in water environment was interpreted on the basis of boundary lubrication theory proposed by Nakahara.