Boundary film formation from overbased calcium sulfonate additives during running-in process of steel–DLC contact

The initial stage of boundary film formation from overbased calcium sulfonate at steel–DLC (diamond like carbon) contact was studied. A running-in process was clearly observed by the combination of fresh DLC with fresh steel or fresh DLC with rubbed steel. On the other hand, rubbed DLC does not possess the running-in process. Surface analysis by TOF-SIMS was applied to investigate the formation process and structure of films. It was found that the transfer of iron from the steel ball on to the DLC surface plays a significant role in the formation of boundary film. The depth profile of the film was also obtained by means of an etching technique. The boundary film on steel ball and DLC are mainly composed of iron and calcium oxide. However the depth profiles indicate that distribution of calcium oxide on steel ball is different from that on DLC coated disk.