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 ceramic composites, intended for face seal applications, has been investigated with cylinder-on-disc equipment. In particular, the influence of water and vapour on the friction, wear and tribe-film formation has been studied. The ceramics consisted of Si3N4 and SiC in the proportions 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 pressing. X-ray diffraction and scanning electron microscopy were used to reveal the phase composition and microstructure respectively The hardness was assessed using Vickers indentation. The chemical composition of the tribo-films was studied with X-ray photoelectron spectroscopy. The major results are that both friction and wear showed distinct low and high levels. The high friction level corresponded to friction coefficients 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 and wear were low. All surfaces produced in the low level were smooth, covered by a thin well-adhering tribe-film while the high level resulted in strongly oxidized and patchy tribe-film covered by wear particles.