Effect of Organic Acid on Friction and Wear Properties of DLC Coating

Abstract

Diamond-like carbon (DLC) coating provides low friction properties without lubricants and, with lubricants, should provide super low friction. In this study, to find the possibility of the environment-friendly material combination that can provide super low friction (with a coefficient of friction lower than 0.01), the friction and wear properties of DLC coating with lubrication in sliding contact areas were evaluated. The friction and wear properties of the steel pin on DLC coated disk with lubrication were evaluated by using environment-friendly fluids (organic acid or alcohol) as lubricants. In the sliding test of tetrahedral amorphous carbon (ta-C) lubricated with DL(mixture of dextrorotatory and levorotatory molecules)-lactic acid, the super low friction coefficient, 0.01 was obtained at the end of the test, and was much lower than that of the uncoated disk and the a-C:H disk. And the coefficient of friction using DL-lactic acid was lower than that of acetic acid or glycerol. Then, the oxidation film (white layer) of FeO on the pin was formed under DL-lactic acid lubrication at the sliding surface of the pin. This showed that the condition of the steel pin also influenced the coefficient of friction. The main reason of reducing friction was thought that the carboxyl and hydroxyl groups in DL-lactic acid connected strongly with the dangling bond on the surface of the ta-C.