Sialon/TiB2 ceramic composites: synthesis, properties, and tribology

The addition of titanium diboride particulates to a sialon matrix offers th e possibility of ultrahard ceramic materials suitable for tribological appl ications (e.g. ball bearings, cutting tools, etc.). Sialon matrix composite s have been fabricated by hot pressing and pressureless sintering with TiB2 added by both direct addition and in situ reaction sintering. Both process es require a dual atmosphere sintering cycle of argon followed by nitrogen at the sintering temperature. Hot pressing has been used to produce materia ls which approach full density independent of the amount of TiB2 present. H owever, the densification of pressureless sintered materials is inhibited b y the presence of TiB2. Measurements of hardness, indentation fracture toug hness, microstructural examination, and tribological behaviour showed signi ficant improvements as a result of the TiB2 addition. At >40 vol.-%TiB2 the low resistivity of these materials also allows the possibility of electrod ischarge matching (EDM). A mini pin-on-disc apparatus was constructed to me asure the coefficient of friction mu and the wear rate k of the composites. Worn surfaces were studied by electron microscopy. The wear behaviour is d ominated by tribochemical reactions, forming oxygen rich tribofilms adhered to the disc surface. Wear then occurs by degradation of the tribofilm and the polishing action of wear debris. Inherent problems with this type of we ar test are discussed.