Silicon nitride oxidation behaviour at 1000 and 1200 °C

The oxidation behaviour of Si3N4–Y2Si2O7 ceramics was investigated at 1000 and 1200 °C. 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/cm2 at 1000 and 1200 °C, respectively. The evolution of oxidised layers was analysed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The presence of β-Si3N4, γ-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 1200 °C in atmospheric air, but special attention is required when the time of exposure is long, and temperature and the volume of additives is high.