The ta-C Hardness Effect on Wear Properties Under MoDTC Contained Boundary Lubrication

The accelerated wear of hydrogenated amorphous carbon in the presence of molybdenum dithiocarbamate (MoDTC) lubrication is a significant challenge when considering its application in engine components. While it is presumed that the use of high-hardness tetrahedral amorphous carbon (ta-C) can effectively address wear issues, irrespective of chemical wear caused by Mo-degradable materials, we have yet to understand the impact of ta-C coating hardness on wear when combined with MoDTC. This uncertainty arises from the assumption that the quantity and structure of Mo-degradable material may be linked to contact pressure, implying that even with a highly hard ta-C coating, wear may vary with the applied load. In this study, we prepared ta-C coatings with four different hardness levels (such as approximately 18.2, 22.1, 46.6, and 53.6 GPa) and conducted friction tests to elucidate the influence of ta-C hardness on wear characteristics under boundary lubrication with MoDTC. Subsequently, we performed chemical analysis (Raman, EDS, and XPS) to identify the chemical species present in the tribofilm, with a particular focus on the 22.1 GPa ta-C coating as a representative of low-hardness ta-C coatings. Our findings revealed that wear suppression was achieved the contact pressure exceeded 200 MPa, irrespective of the coating hardness. Graphical abstract