The lubrication of DLC coatings with mineral and biodegradable oils having different polar and saturation characteristics

Due to improved performance over the last decade, diamond-like carbon (DLC) coatings are more frequently used in highly loaded mechanical components that sometimes need to operate under boundary- or mixed-lubrication conditions. However, DLC coatings are considered as “inert” coatings with a low surface energy and their lubrication ability according to conventional metal-lubrication mechanisms is therefore questionable. In order to investigate whether the base oil polarity and saturation characteristics play a role in these processes, a tribological investigation of the a-C:H coating lubricated with natural (sunflower) and synthetic (saturated and un-saturated) biodegradable oils that posses different amount of polar components and double bonds was performed. For a comparison we also used mineral base oil with low polar component. The DLC/DLC and steel/steel contacts were tested with base oils without additives and in combination with AW and EP additives. Despite the higher wear compared to steel/steel contacts, the results suggest that the wear of DLC/DLC contacts can be importantly improved by using oils with more polar groups and non-saturated molecules. These findings, together with well-known and even proverbial “low-friction properties”, suggest a great potential for the use of DLC coatings in combination with biodegradable oils, particularly when additives are present.