Abstract
Low friction of DLC coatings under boundary lubrication with organic friction modifiers, such as fatty acids, is
important for industrial applications. This work aims to make clear the effect of fatty acid chain length on their tribological properties on DLC coatings, a-C:H and ta-C, when fatty acids are used as additives. In this study, hexadecane was used as the base oil, while fatty acids with different carbon number were used as the additives. The friction coefficient was measured using a self-developed pin-on-disk reciprocating nano-tribometer. We found that friction coefficient decreased when fatty acid chain length increased both on SUJ2 steel surface and on a-C:H coating surface. However, friction coefficient increased once the carbon number exceeded 16. The lowest friction coefficient was found when palmitic acid was used, which indicates the importance of correlation of chain length between the base oil and the additive. Although ta-C showed lower friction than a-C:H and SUJ2 for a given lubricant, it seems that fatty acid did not reduce the friction further when used as additives on ta-C coating surface comparing with the base oil.
