Abstract
β-SiAlON ceramics with a relative density of 98.0±0.2% were fabricated from carbothermally synthesized SiAlON powders (z=2.45) without any sintering additives by HIPing under N2 gas pressure of 150MPa at 1300 to 1900°C for 2h. It was found by SEM observation that the grain growth hardly occurs with hexagonalshaped grains of 2-4μm in size. The mechanical properties of the ceramics are reported; Young's modulus and Poisson's ratio were obtained using an ultrasonic pulse-echo method while the hardness was measured by the Vickers indentation technique, from which the fracture toughness was also calculated. Oxidation of the ceramics was carried out in the temperature range 1400 to 1600°C in an oxygen-containing atmosphere. XRD and 29Si MAS-NMR spectroscopy indicated that the oxide scale consists of amorphous silica and crystalline mullite. Many pores were included in the oxide scale, probably due to N2 gas formation. The kinetics deduced from both the weight gain and scale thickness suggested that the oxidation rate of these SiAlON ceramics is limited by a nitrogen diffusion process.
