Residual α–Si₃N₄ in O′ Crystals in CeO₂‐Doped O′+β′ SiAlON Ceramics

The microstructure of a pressureless sintered (1605°C, 90 min) O′+β′ SiAlON ceramic with CeO₂ doping has been investigated. It is duplex in nature, consisting of very large, slablike elongated O′ grains (20–30 μm long), and a continuous matrix of small rodlike β′ grains (< 1.0 μm in length). Many α‐Si₃N₄ inclusions (0.1–0.5 μm 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 α‐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 α–Si₃N₄ from the reacting liquid, resulting in a delay in the α→β‐Si₃N₄ transformation. The large O′ grains and the α‐Si₃N₄ inclusions have a pronounced effect on the strength degradation of O′+β′ ceramics.