Influence of carbon black in engine oil on wear of H-free diamond-like carbon coatings

Wear tests were conducted on hydrogen-free DLC coatings lubricated with engine oil containing carbon black (CB) in order to clarify their wear resistance. The influence of CB on accelerating wear was noticeable in the region where the minimum oil film thickness was less than the averaged secondary particle diameter of CB aggregates. According to a Raman spectroscopic analysis, the amount of DLC coating wear when lubricated in oil without CB tended to increase with increasing oil temperature and with an increasing Id/Ig ratio in the coating surface layer. In contrast, the Id/Ig ratio of the DLC coatings tested in oil containing CB tended to decrease, though the amount of DLC coating wear increased, which indicated a different wear mechanism.