MICROSTRUCTURE AND PROPERTY-CONTROL IN SiAlON-CONTAINING MONOLITHIC AND COMPOSITE CERAMICS

Recent research into alpha'/beta' Sialon microstructures and the application of Sialon ceramics in tribology is described. A study of the influence of rare-earth additives, Gd, Yb and Nd, on the sinterability and microstructure of alpha'/beta' monoliths shows the benefit of mixed additions; Gd/Nd or Yb/Nd additives may be fully densified on pressureless-sintering of high alpha' compositions and there is greater control of alpha'/beta' phase ratios, dictated by the relative solubility of the cations in alpha'. Re-partitioning of the mixed cations occurs during post-sintering heat-treatment, due to crystallisation of rare-earth containing intergranular phases. Hardness follows the normal trend, increasing with alpha' content, with an accompanying reduction in toughness due to loss of the bridging/pull-out contribution of prismatic beta', with further reduction during crystallising heat-treatment. The wear behaviour of alpha'/beta' microstructures was compared with standard bearing grade silicon nitride and with a TiB2 dispersion-containing Y-Sialon. All Sialon ceramics exhibit improved wear coefficients, and the TiB2 content is shown to be beneficial in relation to the formation of an adherent tribochemical surface film. Some remaining constitutional and structural problems in alpha'/beta' are reviewed. 18 refs.