Tribological properties of a steel-steel pair under the lubrication of lithium grease containing various nano-particulates as additives

A four-ball machine was performed to investigate the effect of three kinds of nanoparticles as additives of a commercial lithium grease on the friction and wear behavior of SAE-52100 steel. Thus the friction and wear behavior of the steel-on-steel system under the lubrication of the lithium grease containing nano-particulates of Ni, or LaF₃, or SiO₂ was evaluated. The chemical states of some typical elements on the worn surfaces of the upper steel ball were analyzed by means of X-ray photoelectron spectroscopy, while the morphologies and elemental plane distributions of the worn surfaces of the lower steel balls were observed and determined by means of scanning electron microscopy and energy dispersion spectrometry. It was found that the three kinds of nano-particulates were all effective to improving the friction-reducing and antiwear abilities of the lithium grease, which was attributed to the formation of the boundary lubricating and protective layers on the worn steel surfaces lubricated with the base grease containing different kinds of nano-particales, by way of physico-chemical adsorption and/or tribochemical reactions. The best friction-reducing and antiwear behavior was observed for the lithium grease containing SiO₂ nano-particulates, because in this case the surface protective layer had a reasonable thickness and good uniformity. The worn steel surface was characterized by mild adhesion and scuffing under the lubrication of lithium grease and lithium grease containing various nano-particulates additives, and the adhesion and scuffing were effectively abated in the later cases, which conformed well with the corresponding friction and wear behaviors of the steel-on-steel system under different lubricating conditions.