MICROSTRUCTURE AND DENSIFICATION BEHAVIOUR OF SURFACE­ COATED SILICON NITRIDE POWDER

Abstract

For better control of the mechanical properties of Si3N4 ceramics, it is necessary to generate homogeneous niicrostructures, and for this purpose, chemical heterogeneities must be minimised, by careful control of pO\\'der processing and the subsequent consolidation steps. Coating of the starting silicon nitride po7der is a convenient '\-vay of incorporating a liquid forming sintering aid more homogeneously than can be achieved by current commercial methods such as ball-milling. Thin layers of oxides, corresponding to additions of 5 w/o MgO have been deposited on the surface of grains of a commercial silicon nitride powder using alcoholic solutions containing appropriate amounts of the metal alkoxide. The resulting powders have been densified by pressurelesss sintering techniques, and their sintering characteristics identified in comparison with equivalent materials produced by adding the oxide in particulate form. In every case, a better sintering performance was observed at lower temperatures for the oxide-coated materials, with fully dense pressureless-sintered materials being obtained at temperatures as low as 1525 °C. Microstructures were observed using a s ... 2400 Hitachi Scanning Electron Microscope (SEM). and final microstructure was more uniform than that obtained by conventional method