The effect of oxidation on the mechanical properties and dielectric properties of porous Si3N4 ceramics

The effect of oxidation temperature and time on the microstructures, phase compositions, mechanical properties, and dielectric properties of porous Si3N4 ceramics was investigated in the temperature range from 900 °C to 1300 °C for 1 h, 5 h, and 24 h. The weight gain measured either at lower temperature (900 °C) for long time (24 h) or at higher temperature (1300 °C) for 1 h demonstrated that the porous Si3N4 ceramics were easily oxidized under the current test conditions. Results showed that the amount of open pores, flexural strength, compressive strength, and dielectric constant all decreased with the increase of oxidation temperature independent upon the oxidation time. The oxidation product SiO2 was low-temperature quartz in mild condition (low temperature, short time) and cristobalite in severe condition (high temperature, long time). The existence of cracks on the oxide scale was due to the phase transformation of SiO2 and thermal expansion coefficient mismatch between SiO2 and Si3N4.