Effect of starting Si₃N₄ powder type and sintering conditions on the grain morphology of α-SiAlON ceramics

Nitrogen rich neodymium-ytterbium containing multi-cation α-sialon starting compositions have been prepared by gas pressure sintering using "conventional" α-Si₃N₄ or plasma produced β-Si₃N₄ powder. The effect of α-SiAlON nucleation on growth mechanism was investigated by applying different sintering cycles. X-ray diffraction studies of these compositions after sintering revealed that α-sialon was observed as a matrix phase. Microstructural characterisation of the sintered materials by using α-Si₃N₄ powder as starting composition resulted in a typical equiaxed grain morphology, as expected if the α-SiAlON nucleation step is missed out prior to growth. However, elongated α-SiAlON grains were observed by applying nucleation. In the case of β-Si₃N₄ containing starting composition, in all cases α-SiAlON grain morphology was observed. The effects of starting Si₃N₄ powders and different sintering conditions were discussed in detail.