Effect of Degree of Unsaturation in Vegetable Oils on Friction Properties of DLC Coatings

Recently, it has been reported that combinations of various types of vegetable oils containing organic acids and DLC coatings are effective for reducing friction, but there are few reports of detailed investigations into the relationship between differences in the chemical structures of vegetable oil lubricants and friction reduction. Therefore, the authors investigated the influence on friction properties of two types of DLC coatings, a-C:H and ta-C under lubrication with vegetable oils which have different degrees of unsaturation. The ta-C coating displayed markedly lower friction coefficients than the a-C:H coating and the uncoated steel with all of the vegetable oil lubricants used. It was found that the low friction properties of the ta-C coating showed even lower friction coefficients with vegetable oils containing a higher content of monounsaturated fatty acids. Additionally, the sliding surfaces of ta-C coated discs and cylinders were analyzed by XPS and ToF-SIMS analysis. The results confirmed the formation of a surface layer consisting of C-OH bonds, and a lower friction coefficient was seen as the detected intensity of these bonds increased.