Frictional and Wear Properties of Diamond-Like Carbon Films with Lubricant Additives

Diamond-Like Carbon (DLC) is a promising engine material for reducing friction and wear on sliding parts. By contrast, MoDTC lubricant additives are known to promote the wear of a-C:H films. However, the mechanism that promotes wear and the formation of tribofilms on DLC parts when in contact with molybdenum-based lubricant additives has not been sufficiently studied. The purpose of this research is to determine the wear promotion mechanism and formation of tribofilm on DLC by lubricant additives by comparing friction and wear properties. We conducted friction and wear tests using a tribometer with DLC (ta-C, ta-C:H, a-C, and a-C:H) blocks, FC250 (cast iron) rings, and oils containing lubricant additives (MoDTC, MoDTP, and Mo without DTC ligand) by observing and analyzing the sliding surfaces of specimens. No wear was observed for any of the DLCs (ta-C, ta-C:H, a-C:H, and a-C) in combination with oils containing MoDTP or Mo without DTC ligands. Oil containing MoDTC revealed low frictional properties for all tested DLCs (ta-C, ta-C:H, a-C:H, and a-C), but arc-shaped wear occurred on a-C:H, which has low hardness and contains hydrogen only in combination with MoDTC-containing oil. In addition, MoO3 was most frequently detected on the worn surface of FC250 in combination with a-C:H and MoDTC-containing oil, whereas Mo carbide was seldom detected by XPS analysis of any worn surface or deposit. These results indicated that the constitution of the DLC and the hydrogen content are the main factors driving wear promotion by MoDTC, while MoO3 is the main factor promoting wear in a-C:H.