Formation of AlN polytype ceramics and some of their properties.

Formation of 27 R-AlN polytype ceramics and their mechanical properties were investigated in the typical composition of the AlN-SiO2. Densified 27 R-AlN polytype ceramics consisted of plate like grains without low temperature melting compounds at grain-boundaries were obtained by 2000°C presintering and subsequent 2100°C hot-pressing of 90wt% AlN-10wt% SiO2 under a pressure of 500kg/cm2. The ceramics showed approximately constant strength values at room temperature to 1700°C. In the fracture of the AlN polytype, crack propagates predominantly by tranversing the grains. However, fractographs largely differ between at room temperature and at high temperatures. In particular, it is noticeable that that transgranular fracture with subsidiary crack growth was observed at high temperatures. To study grain morphology effect, AlN-Y2O3 and AlN-SiO2-Y2O3 were also investigated.