Friction and Wear Properties of Hard Coatings on Steel Surfaces under Lubrication with a Fully Formulated Oil with a Mo Additive

When various sliding components of combustion engines protected with hard coatings are slid with automotive lubricants, unexpected interfacial phenomena may occur because typical automotive lubricants have been formulated with additives suitable for steel/steel sliding components. Therefore, understanding the effects of hard coatings on the performance of lubricant additives is crucial for further improving the lubrication properties of fully formulated oils through the optimization of additive formulations for hard coatings. In this study, friction tests were performed with a steel surface against steel surfaces coated with two ceramic materials and two diamond-like carbon materials under lubrication with a fully formulated oil. Atomic force microscopy and X-ray photoelectron spectroscopic (XPS) investigation of the steel surfaces after the friction tests revealed that the tribofilms comprised a bottom layer composed of calcium compounds and a top layer composed of phosphorous, zinc, and sulfur compounds. The thickness of each layer, which is closely related to the wear property, depends on the hard coating material. Moreover, XPS analysis focusing on the molybdenum element suggested that rate of chemical kinetics to form MoS2 differs among the hard coating materials, and dominantly affects the frictional property.