Silicon nitride ceramics with celsian as an additive

Four different Si₃N₄ ceramics using 10 wt% celsian (BaAl₂Si₂O₈) as an additive, have been prepared by hot pressing. Different celsian sources and hot-pressing conditions were used and their effects on densification, in situ crystallization of celsian, to phase transformation, microstructures and properties, were examined. The use of a presynthesized celsian as a raw material was found to enhance the rates of densification and phase transformation due to the excess silica from raw Si₃N₄ powders which, however, did not change the microstructure and crystallization behaviour of celsian. Increases in hot-pressing temperatures and times increased the total number of large elongated grains and resulted in coincidental increases in strength and toughness until the to transformation was complete. An intermediate quench-and-reheat step during hot pressing made the microstructure finer and more uniform by greatly reducing the large-sized elongated grains. The quenching step also disrupted complete crystallization of celsian which led to more grain-boundary glassy phase and compromised the material's high-temperature properties.