Tribological characteristics of Cr/CrN/a-C:H:W/a-C:H coating under boundary lubrication conditions with glycerol mono-oleate (GMO) and molybdenum dithiocarbamate (MoDTC)

Abstract

In order to employ advanced coatings and thin films in many practical engineering systems, they must be compatible with lubricants. Therefore short-stroke, reciprocating sliding (SRV) tests were performed under boundary lubrication conditions, to investigate the compatibility between two friction modifiers (FMs) and the following coating: Cr/CrN/a-C:H:W/a-C:H. The counter surface of coating was a AISI 52100 steel ball. Three types of fully-formulated SAE 5W-20 oils that consisted of mineral base oils and various lubricant additives were used as fluid lubricants. The lubricating oils were classified by the blended FMs: no FM, glycerol mono-oleate (GMO) and molybdenum dithiocarbamate (MoDTC). Both FMs reduced the friction coefficient. When the MoDTC-containing oil was used, the wear rates of coatings and AISI 52100 steel balls were the highest and the top a-C:H layer was completely worn out. Whereas the wear rates of coatings and AISI 52100 steel balls lubricated with the GMO-containing oil were the lowest in most cases. From these results, it was concluded that the GMO was a compatible FM with the coating and improved tribological characteristics of both sliding surfaces. While the MoDTC was not suitable with the coating, since it remarkably promoted the wear of a-C:H layer.