Effect of fluorine impurity on the oxidation of silicon oxynitride ceramics doped with gadolinium oxide

Impurities in raw Si3N4 powders remain in intergranular glassy phases in Si 3N4 and Si2N2O ceramics and degrade their high-temperature properties. Fluo rine is one of the typical impurities in the raw powders. The oxidation rat e of Si2N2O ceramics doped with Gd2O3 greatly varied with a difference in i mpurity contents (especially F) of the raw Si3N4 powders used. When a high concentration of impurity existed in the intergranular glassy phase, the ra te of oxidation was controlled by O-2- diffusion through the glassy phase i n the partly oxidized scale and unoxidized body; outward diffusion of Gd-3 occurred concurrently. On the other hand, when the impurity contents in th e intergranular glassy phase was very low, the diffusion rate of ions (Gd-3 +, O-2-, etc.) in the glassy phase became very low (substantially zero in t he oxidation at 1300 degreesC). Only cristobalite (SiO2) was formed on the surface. The rate of oxidation was controlled by O-2 diffusion through the cristobalite layer, and was very low.