Super Low Friction Property of DLC Lubricated With Ester-Containing Oil: Part 2 — Nanometer-Scale Morphological, Structural and Frictional Properties

We have succeeded for the first time anywhere in lowering the friction coefficient of a diamond-like-carbon (DLC) coating to less than 0.01 under boundary lubrication in engine oil [1–3]. This anomalous super-lubrication behavior has been observed for a hydrogen-free DLC-coated (ta-C) disc in an ester-containing oil but not for a hydrogenated DLC (a-C:H) coating. It is thought that some chemical adsorbent may form only on the ta-C sliding surface due to some tribochemical reactions. Our recent studies have suggested that the macro-scale reduction of friction is dependent on nanometer-scale tribological properties [4–6]. The superlow friction behavior seen in a pin-on-disc friction test was taken as the object of this investigation with an eye toward elucidating the mechanism of the anomalous friction reduction. Pin-on-disc tests were conducted by sliding a ta-C/ta-C pair in the presence of poly alpha-olefin based oil containing a modifier additive of glycerol monooleate ester (PAOES1 oil). Nanometer-scale tribological properties were investigated by using atomic force microscopy (AFM), the AFM phase-image technique, and nanoscratch measurements. Attention was focused on the differences in surface roughness, nanostructure and nanofriction coefficient between the sliding and non-sliding areas in an effort to find the origin of the super-lubrication behavior.