Friction and wear characteristics of hydrogenated diamond-like carbon films formed on the roughened stainless steel surface

The effects of the surface roughness of hydrogenated diamond-like carbon (DLC) films on friction and wear properties have been studied using a ball-on-disk type tribometer in air and water environments. Rough DLC films were formed by depositing DLC with the plasma-enhanced chemical vapor deposition method on roughened stainless steel substrate prepared by argon plasma sputter etching. DLC films deposited on a smooth stainless steel substrate showed unstable friction behavior in both air and water environments. The final friction coefficient values ranged from 0.10 to 0.15 in air and 0.15 to 0.20 in water. Line-shaped large-scale delamination of DLC film from stainless steel substrate was observed for smooth DLC films rubbed in water. In an air environment, large-scale delamination was not observed, but wear of the DLC film could be seen. The wear scar formed on the stainless steel ball under water lubrication was larger than that formed in air environment. In contrast to smooth DLC film, DLC films having a rough surface showed stable frictional behavior. The friction coefficients in water and air environments were approximately 0.1 and 0.2, respectively. No large-scale delamination of DLC film was observed for rough DLC films. Although friction characteristics were improved by roughing the surface of the DLC films, the wear scars forming on the ball surface became larger with increases in the surface roughness of the DLC film. From the results of the tribotest in n-decane, which has almost the same viscosity as water, it was suggested that not only the hydrodynamic effect but also the interaction between DLC films and water affect the friction and wear behavior. DLC films changed from hydrophobic to hydrophilic by tribochemical reaction in water environment. The low friction and wear of DLC films under water lubrication is considered to be caused by hydrophilication of the DLC films.