High temperature oxidation of silicon nitride intergranular phases

Rare earth oxides are used in the fabrication of dense silicon nitride (Si3N4) components. These oxides react at high temperatures with SiO2 (present on the surface of all commercial Si3N4 powders) to form a liquid phase which allows densification by particle rearrangement and solution-reprecipitation. The remaining liquid solidifies and a number of phases, both crystalline and glassy, form. The oxidation behaviour of the intergranular phase plays a significant role at temperatures below 1100 C as oxidation is often accompanied by molar volume increases. At these temperatures, Si3N4 itself is hardly oxidized. This paper deals with the oxidation of N-α-wollastonites (MSiO2N). Dense Y and Ce wollastonite (K-phase) were prepared by hot pressing and gas pressure sintering, respectively. Specimens were exposed to dry synthetic air and the reactions were monitored using Hot Stage Microscopy and Thermogravimetric Analysis. XRD and WDS were used to analyse the materials and oxidation products. (orig.)