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)

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. •Compatibility of specific friction modifiers (FMs) with a DLC coating was examined.•Boundary lubrication performances of the DLC coating were also evaluated by SRV test.•The FM, GMO reduced friction and wear of the DLC coating and counter surface.•When the MoDTC was a FM, the top a-C:H layer of DLC coating was completely worn out.•GMO was a compatible FM with the DLC coating, while MoDTC was an unsuitable FM.