Oxidation of Silicon Nitride Hot‐Pressed with Ceria

Short‐term (<30 h) oxidation of CeO 2 ‐doped hot‐pressed Si 3 N 4 , studied at 773 to 1623 K in flowing dry air, resulted in parabolic‐weight gain curves and in oxidation‐rate constants nearly independent of the amount of grain‐boundary second phase. Exolution of CeO 2 crystals occurs from the oxide layer; their morphology depends on oxidation temperature. Either a direct redox reaction between CeO 2 and Si 3 N 4 or solution of Si 3 N 4 in the glassy silica‐rich oxide layer, followed by its oxidation by dissolved oxygen, have been proposed as possible oxidation mechanisms, both appearing strongly dependent on the low solubility of cerium oxides hi silicate melts. The value of the activation energy for oxidation of 385 kj·mol −1 suggests additive and impurity migration from the bulk to the Si 3 N 4 /oxide reaction interface as the most probable rate‐limiting step.