RESIDUAL ALPHA-SI3N4 IN O' CRYSTALS IN CEO2-DOPED O'+BETA' SIALON CERAMICS

The microstructure of a pressureless sintered (1605-degrees-C, 90 min) O' + beta' SiAlON ceramic with CeO2 doping has been investigated. It is duplex in nature, consisting of very large, slablike elongated O' grains (20-30 mum long), and a continuous matrix of small rodlike beta' grains (< 1.0 mum in length). Many alpha-Si3N4 inclusions (0.1-0.5 mum in size) were found in the large O' grains. CeO2-doping and its high doping level as well as the high Al2O3 concentration were thought to be the main reasons for accelerating the reaction between the alpha-Si3N4 and the Si-Al-O-N liquid to precipitate O'-SiAlON. This caused the supergrowth of O' grains. The rapid growth of O' crystals isolated the remnant alpha-Si3N4 from the reacting liquid, resulting in a delay in the alpha --> beta-Si3N4 transformation. The large O' grains and the alpha-Si3N4 inclusions have a pronounced effect on the strength degradation of O' + beta' ceramics.