Silicon nitride oxidation behaviour at 1000 and 1200 °C

The oxidation behaviour of Si3N4-Y2Si2O7 ceramics was investigated at 1000 and 1200degreesC. These temperatures were chosen because most of the applications of Si3N4 occur at these temperatures. The Si3N4 samples were sintered by liquid phase sintering using 7 and 14 vol.% of Y2Si2O7 as a sintering additive. The density of the sintered samples reached 99%. The samples were heated under stationary air for different periods of time between 0 and 256 h. The kinetic of oxidation was studied in function of the temperature and time of exposure, mainly during a short period of oxidation. The weight gain during the oxidative process reached values of 0.1 and 0.2 mg/cm(2) at 1000 and 1200degreesC, respectively. The evolution of oxidised layers was analysed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The presence of beta-Si3N4, gamma-Y2Si2O7, SiO2 and Si2N2O phases was observed on the surface of samples by XRD. It was concluded that the Si3N4 shows a high oxidation resistance at 1000 and 1200degreesC in atmospheric air, but special attention is required when the time of exposure is long, and temperature and the volume of additives is high. (C) 2004 Elsevier B.V. All rights reserved.