AN INVESTIGATION OF THE RELATIONSHIP BETWEEN GRAPHITIZATION AND FRICTIONAL BEHAVIOR OF DLC COATINGS

In our recent studies, diamond-like carbon (DLC) films were found to p ossess low coefficient of friction (f<0.1) and excellent wear resistan ce. The reduction in f was found to be consistent with wear-induced gr aphitization of the DLC structure. The purpose of the present work was to study the effect of load and sliding velocity on the frictional be havior and graphitization process occurring in DLC during wear. Pin-on -disc experiments were conducted on DLC-coated SIC substrates at slidi ng velocities between 0.06 and 1.6 m s(-1) under 1 and 10 N loading le vels using ZrO2 balls as the pin material. Analytical transmission ele ctron microscopy was used to characterize the structure and microstruc ture of the wear debris after testing. The results showed that both sl iding velocity and contact load influence the graphitization process. Higher sliding velocities increase the contact frequency and the rate of temperature rise that may facilitate the release of hydrogen atoms from the sp(3) structure. Higher loading enhances shear deformation an d transformation of the weakened hydrogen-depleted DLC structure into graphite [10]. The present findings are consistent with our earlier pr oposed wear-induced graphitization mechanism for these films. An equat ion was developed to describe the transformation kinetics of DLC into graphite as a function of sliding velocity and applied stress.