Friction and wear performance of boundary-lubricated DLC/DLC contacts in synthetic base oil

The use of diamond-like carbon (DLC) coatings in engine components can be one of the potential solutions for automotive industry to improve fuel efficiency by controlling the friction and wear. The tribological behavior of various types of DLC coatings in liquid lubricated systems and their interaction with lubricants are needed for proper usage of these coatings. In this study, the friction and wear performance of four different DLC coatings in synthetic base oil polyalpha-olefin (PAO) were investigated using pin-on-disc tribometer under boundary lubricated condition. One hydrogen-free ta-C DLC, one hydrogenated a-C:H, one silicon doped a-C:H:Si and one chromium doped a-C:H:Cr were deposited on SUJ2 bearing steel pins and discs for this study. The structural, mechanical and topographical characterizations of these films were analyzed using Raman spectroscopy, Nano indenter and Atomic Force Microscopy (AFM). The results indicate that structure and doping elements greatly affect the tribological performance of DLC coatings. Ta-C coatings gave the lowest boundary friction, while Si-doped a-C:H:Si showed the highest boundary friction. Hydrogenated a-C:H coating exhibited the highest wear rate, but silicon and chromium incorporation in a-C:H coatings greatly increased the wear resistance. Cr-doped a-C:H:Cr showed the lowest wear rate, followed by Si-doped a-C:H:Si coating.