Tribochemistry of tetrahedral hydrogen‐free amorphous carbon coatings in the presence of OH‐containing lubricants

Abstract

AbstractDiamond‐like carbon (DLC) films are gaining attention for its use in a wide range of tribological applications because of their low friction coefficient and high wear resistance. Recently, ultra‐low friction or superlubricity as low as 0.006 has been observed between hydrogen‐free DLC films lubricated with oil containing a glycerol mono‐oleate (GMO) additive. Consequently, there are many efforts to understand their detailed tribological behaviour. In this study, first, a characterisation of a pristine hydrogen‐free tetrahedral amorphous DLC coating (denoted as ta‐C) is presented. The technique used for ta‐C characterisation is the energy‐filtered transmission electron microscopy studied on a focus ion beam cross section of the coating. Then, to simulate the action of GMO, which is used as an additive in engine oils, and to understand its mechanism of action in boundary lubrication conditions, simple and shorter molecules such as pure glycerol and hydrogen peroxide are used in friction tests. These two molecules and GMO have the same alcohol chemical function. Friction tests in the presence of pure glycerol and hydrogen peroxide are presented. Moreover, to understand the properties of these two molecules, the overtone of GMO and their reaction mechanism in boundary lubrication, liquid phase lubrication was simulated by gas phase lubrication. Results show that very low friction coefficients are obtained with no apparent wear. Finally, the mechanisms of ultra‐low friction are investigated by the wetting method and the X‐ray photoelectron spectroscopy (XPS) technique. These two techniques are used to identify the physical (wetting method) and chemical (XPS) changes occurring at the ta‐C surface after friction. Copyright © 2007 John Wiley & Sons, Ltd.