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

Abstract

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 . Measurements of hardness, indentation fracture toughness, microstructural examination, and tribological behaviour showed significant improvements as a result of the TiB₂ addition. At >40 vol.-%TiB₂ the low resistivity of these materials also allows the possibility of electrodischarge matching (EDM). A mini pin-on-disc apparatus was constructed to measure the coefficient of friction μ and the wear rate k of the composites. Worn surfaces were studied by electron microscopy. The wear behaviour is dominated 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 wear test are discussed.