Properties of silicon nitride ceramics depend on the phases present and their microstructure. In order to obtain ceramics with optimum properties, knowledge of phase relationships in silicon nitride-metal oxides systems and the mechanism and kinetics of microstructure development are necessary.
There are four major stable silicon nitride phases in the Si3N4-metal oxides systems: β-Si3N4, α′-SiAlON, AIN-polytypoids and silicon oxynitride. The morphologies of these phases are different. The β-Si3N4 grains are elongated hexagonal rods; the α′-SiA1ON grains are equiaxed; the A1N-polytypoids are platelets; and the silicon oxynitride are equiaxed. Ceramics with combinations of these phases would have different microstructures and properties. The system Si3N4-SiO2-AIN-Al2O3-YN-Y2O3 will be used to demonstrate the concept of the alloy design. Kinetics of microstructural development will also be discussed.
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