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

Abstract

Nitrogen rich multication alpha-SiAlON ceramics doped with Y-Ce have been densified by gas pressure sintering using alpha-Si3N4 or mixed beta/alpha-Si3N4 starting powder. The effects of alpha-SiAlON nucleation and growth mechanisms were investigated by using different sintering cycles. X-ray diffraction studies after sintering revealed that 21 R polytype phase was present in addition to alpha-SiAlON matrix phase. Microstructural characterisation of sintered materials prepared using alpha-Si3N4 powder in the starting composition revealed a typical equiaxed grain morphology, as expected if the alpha-SiAlON nucleation step was not applied before grain growth. However, needlelike alpha-SiAlON grains were observed if a nucleation step was carried out before final sintering. In starting powders containing mixed beta/alpha-Si3N4, needlelike grain morphology was also observed. The effects of different Si3N4 starting powders and sintering conditions on the grain morphology and mechanical properties are discussed.