Synthesis and tribology of Sialon/TiB2 ceramic composites

The inclusion of TiB2 particles in a Si3N4 or sialon matrix has the potential to improve the materials properties over those of the Si3N4 alone. The combination of the excellent properties of sialon and the high hardness of the TiB2 makes these materials potentially suitable for tribological applications. This research has investigated the use of chemical reactions that can take place during sintering to synthesise TiB2 from TiN and BN. The reactions not only produce TiB2 but simultaneously synthesise the matrix material. The sialon matrix is used as a flexible receptor for excess nitrogen and oxygen. Using such reactions, sialonlTiB 2 composites were successfully produced by both hot pressing and pressureless sintering. It was shown that these materials achieve densities close to the theoretical and contain only the required phases of TiB2 and ~ sialon. Pressureless sintered materials also contain residual sintering aid as an intergranular phase. The composites wee characterised with respect to their microstructure, hardness, fracture toughness and tribological properties. A specially designed wear test apparatus was constructed for the tribological testing. The pin-on-disc apparatus was capable of testing the small volumes of material (minimum diameter of disc ~ 15mm) typically produced at the development stage. The materials were found to have high hardness and reasonable fracture toughness which was attributed primarily to the properties of the matrix phase with some possible toughening from the presence of TiB 2 . Tribological testing revealed high dry sliding coefficients of friction (Jl) for the composites sliding on themselves and reasonable values for the wear coefficient (k) were measured. Wear was found to be dominated by tribochemical reactions forming tribofilms which act to reduce wear of the disc. However, the presence of TiB2 was not seen to contribute directly to the tribological properties of these materials and the usefulness of this type of composite is questioned