Low temperature densification of silicon nitride using Li 2O-based surface coatings

Abstract

For better control of the mechanical properties of Si 3N 4 ceramics, it is necessary to generate homogeneous microstructures, and for this purpose, chemical heterogeneities must be minimised, by careful control of powder processing and the subsequent consolidation steps. Coating of the starting silicon nitride powder is a convenient way 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 up to 5 w/o Li 2O 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 1250 °C. An added observation was that for the coated samples, the final microstructure was more uniform, and showed an absence of large pores.