Sialon/TiB2 ceramic composites: Synthesis, properties and tribology

The addition of titanium diboride particulates to a sialon matrix offers the possibility of ultrahard ceramic materials suitable for tribological applications (e.g. ball bearings, cutting tools etc.). Sialon matrix composites have been fabricated by hot pressing and pressureless sintering with TiB2 added by both direct addition and in-situ reaction sintering. Both processes require a dual atmosphere sintering cycle of argon followed by nitrogen at the sintering temperature. Hot pressing has been used to produce materials which approach full density independent of the amount of TiB2 present. However, the densification of pressureless sintered materials is inhibited by the presence of TiB2. The study of hardness, indentation fracture toughness, microstructure and tribology has shown significant improvements in properties. At >40vol.% TiB2 the low resistivity of these materials allows the possibility of electro-discharge machining (EDM). A mini pin-on-disc apparatus was constructed to measure the coefficient of friction (mu) and the wear rate (k) of the composites. Worn surfaces were studied by electron microscopy. The wear behaviour is dominated by tribe-chemical reactions, forming oxygen rich tribofilms adhered to the discs surface. Wear then occurs by degradation of the tribofilm and the polishing action of wear debris. Inherent problems with this type of wear test are discussed