Effect of environmental condition on tribological properties of a DLC film

Tribological properties between a diamond-like carbon (DLC) film and steel ball (SUJ2) were investigated under ambient air and synthetic air with controlled relative humidity. The DLC film was deposited onto an SKD11 substrate by unbalanced magnetron sputtering of graphite under Ar+CH4 atmosphere. The friction tests were performed using a ball-on-plate type reciprocating friction tester. In synthetic air, friction coefficients of the film were 0.1-0.2 and the wear loss decreased significantly with an increase in relative humidity. In ambient, the wide range of friction coefficient (0.1-1.0) was obtained and the wear loss was independent of relative humidity. The transfer layer was observed on the wear scar of the ball surface when low friction coefficient was obtained. Micro-laser Raman spectra showed that the transfer layer was wear debris with graphite-like structure transformed from DLC. Wear behaviors of the DLC film and the steel ball were explained by cross-sectional shapes of wear tracks of the film. It is seemed that not only relative humidity but also clean level of testing environment influences the formation of the transfer layer on the ball surface.