The role of lubricant and carbon surface in achieving ultra- and superlow friction

In the presented work, tribological interactions of tetrahedral amorphous carbon (ta-C) coatings with variations of sp3 content have been systematically studied in the boundary lubrication regime with emphasis on fatty acid-based lubricants. Manifold phenomena regarding wear and friction have been observed that exceed the conventional understanding of lubricant-surface interaction. Distinct superlubricity behavior of ta-C was found when lubricated with glycerol or oleic acid. It was shown that both the acid function and the double bond in the lubricant molecules are essential features to achieve superlubricity. Superlubricity is not confined to self-mated ta-C surfaces, but also happens in contact to steel. The contact situation has important influence on tribological behavior. Wear rates change by 1-2 orders of magnitude by altering the contact geometry and loading conditions of an oleic-acid-lubricated ta-C/steel pairing. This ta-C wear phenomenon is attributed to tribochemical wear.