INTERPRETATION OF FRICTION AND WEAR OF CERAMICS IN TERMS OF SURFACE-ANALYSIS

Selected combinations of ceramic tribomaterials (SiC-SiC, Si3N4-Si3N4, h-BN-h-BN) were tested in an Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) analytical tribotester, under ultrahigh vacuum, and under partial pressures of gases (O2,N2,H2O, C3H8, air) respectively. The main purpose of the work was to clarify the correlations existing between the composition and microstructure of friction-induced interface films and the reduction of friction and wear of ceramics caused by ambient gases. In addition to conventional AES, XPS and secondary ion mass spectrometry analyses, electron microscopy (both transmission and STEM), electron energy loss spectroscopy, and extended electron energy loss fine structures were used to characterize the tribochemical reactions on the ceramic surfaces. Under ultrahigh vacuum, ceramic surfaces exhibit high friction (f(k) > 0.7) which is associated with friction-induced disorder in the interface material. Adsorbed gases did not "lubricate" these interfaces. Reactive gases (O2) formed films in the contact region which provided lubrication and lower friction. This lubrication was observed with selected, carbon-containing systems and was associated with the formation of sp2-hybridized carbon in the interface films. The wide range of physical and chemical phenomena observed for sliding ceramic contacts and the implications for their use in severe environments is discussed.