Action of oil additives when used in DLC coated contacts

Under boundary lubrication conditions, different oil additives are used to prevent metal to metal contact by boundary film formation, thus reducing friction and wear of contact surfaces. With the introduction of diamond-like carbon (DLC) coatings, the major concern is the compatibility of coated surfaces with existing oils and additives, originally designed for uncoated metallic surfaces. The aim of the present work was to investigate the influence of extreme pressure, antiwear and friction modifier additives and their concentration on the tribological performance of DLC coatings in the boundary lubrication regime. Results show that antiwear and friction modifier additives have only minor effect on the frictional behaviour of DLC coatings. On the other hand, depending on the contact conditions, lubrication of steel/W doped DLC contacts with oils containing S based extreme pressure additives can result in greatly improved tribological performance caused by WS₂ tribofilm formation.