Effect of unmodified and surface-modified nano SiO_∞ as additives on the tribological properties of lithium grease

Nanoparticulates of SiO_∞ prepared by means of chemical and physical methods were used as the additives of a commercial lithium grease. The effects of the resulting surface-modified and unmodified SiO_∞ nanoparticulates on the tribological properties of the lithium grease were comparatively examined on a four-ball machine. The morphologies of the worn surfaces of the lower steel balls were observed using a scanning electron microscope, while the chemical states of some typical elements on the worn surfaces of the upper steel balls were analyzed by means of X-ray photoelectron spectroscopy. The results show that the two kinds of SiO_∞ nanoparticulates are effective to improving the friction-reducing and antiwear abilities of the lithium grease, which is attributed to the formation of the boundary lubricating and protective layers on the worn steel surfaces. Namely, both the unmodified and surface-modified SiO_∞ nanoparticulates function to reduce the friction and wear of the steel-steel contact by physical and chemical adsorption on the sliding steel surface, and the former is slightly superior to the latter in terms of the friction-reducing ability, while the latter has stronger antiwear ability at a larger normal load.