SLIDING WEAR AND FRICTION OF SI3N4-SIC-BASED CERAMIC COMPOSITES CONTAINING HEXAGONAL BORON-NITRIDE

The friction and wear behaviour of a series of new Si3N4-SiC-based cer amic composites, intended for face seal applications, has been investi gated with cylinder-on-disc equipment. In particular, the influence of water and vapour on the friction, wear and tribe-film formation has b een studied. The ceramics consisted of Si3N4 and SiC in the proportion s 7:3, to which 0, 4 or 8 wt % of hexagonal boron nitride was added as a solid lubricant. All specimens were produced by hot isostatic press ing. X-ray diffraction and scanning electron microscopy were used to r eveal the phase composition and microstructure respectively The hardne ss was assessed using Vickers indentation. The chemical composition of the tribo-films was studied with X-ray photoelectron spectroscopy. Th e major results are that both friction and wear showed distinct low an d high levels. The high friction level corresponded to friction coeffi cients between 0.4 and 0.9 and wear rates between 10(4) and 10(5) mu m (3)/Nm. The low level corresponded to friction coefficients between 0. 02 and 0.1, and wear rates between 10 and 10(3) mu m(3)/Nm. When water or saturated vapour was present in the tribe-system, both friction an d wear were low. All surfaces produced in the low level were smooth, c overed by a thin well-adhering tribe-film while the high level resulte d in strongly oxidized and patchy tribe-film covered by wear particles .