Influence of environmental parameters on the frictional behavior of DLC coatings

It was shown in our previous studies that diamond-like carbon (DLC) films possess low friction coefficient (f) and excellent wear resistance. The reduction in f was found to be consistent with a 'wear-induced graphitization' mechanism of the DLC structure proposed recently by Liu and Meletis (J. Mater. Sci., 32 (1997) 3491). A recent study showed that operational parameters (sliding velocity and loading level) influence the tribological behavior of DLC film through control of the kinetics of the graphitization process. The objective of the present study was to investigate the influence of environmental parameters (humidity and temperature) on the tribological behavior of DLC film and test the wear-induced graphitization mechanism. Ion-beam deposition was utilized to deposit DLC on a SiC substrate. Pin-on-disk experiments were conducted by varying humidity (0%, 40% and 100%) and temperature (-10 degrees C and 25 degrees C). As-deposited DLC and wear debris was characterized by transmission electron microscopy. It was found that lower humidity increases the graphitization rate more than likely due to the reduction in the effect by the water molecules. A decreased graphitization rate was observed at lower temperature and higher humidity, and can be attributed to suppression of temperature rise at 'hot spots'. The present findings are consistent and further support the wear-induced graphitization mechanism. (C) 1997 Elsevier Science S.A.