Effect of oil additives on the durability of hydrogenated DLC coating under boundary lubrication conditions

Diamond-like carbon (DLC) coatings have became accepted non-ferrous coatings for automotive tribo-components as they offer excellent tribological properties resulting in improved fuel economy and reduced dependence on existing lubricant additives which can be harmful to catalytic converters and ultimately to the environment. Obtaining optimum durability (wear) as well as high fuel economy (low friction) using DLC-coated parts relies in part on the compatibility between surface and lubricant additives. The objective of this study is to understand the role of friction modifiers and antiwear additives on the durability of DLC coating under boundary lubrication conditions. Experiments were performed using a pin-on-plate tribotester using plates coated by 30 at.% hydrogen containing DLC (HDLC) sliding against cast iron (CI) pins. The physical observation of the wear scar, formed on the HDLC coating by low friction and/or antiwear additives, was performed using both optical and scanning electron microscopes. X-ray photoelectron spectroscopy analysis was performed on the tribofilms to help to understand the tribochemical interactions between oil additives and the HDLC coating. Based on the physical observations and tribochemical analysis of the wear scar, the mechanisms of failure/wear of the HDLC coating are proposed and the requirement for designing optimal additive packages for the HDLC coating is discussed.