Study on the characteristics of transferred lubricating thin films on ceramic surfaces generated in liquid nitrogen

Ceramic bearings working in liquid nitrogen are mainly lubricated by thin films transferred from the polytetrafluo-roethylene (PTFE) based composite retainers. The thin films on silicon nitride ball surfaces are generated by the friction and interaction among the composite retainer, the balls, and the AISI 440C stainless steel rings. The characteristics of transfer lubricating films on silicon nitride ball surfaces generated during operation in liquid nitrogen at 2.4 and 2.8 GPa were analyzed. The distribution of thin films on the silicon nitride ball surfaces was observed by an optical microscope. The profile of transferred thin films was measured using an atomic force microscope (AFM). The component and structure of transferred thin films were detected by X-ray photoelectron spectroscopy (XPS) and Raman spectrometry. The results show that the distribution of transferred thin films on the silicon nitride ball surfaces is discontinuous and its thickness is about one micrometer. The upper layer of the transfer films, about 100 nm thick, degrades under a heavy load, generating metal fluoride. The bonding strength between the degradation layer and its basis is very weak.