Improvement of oxidation resistance of Si₃N₄ by heat treatment in a wet H₂ atmosphere

To improve the oxidation resistance of Si₃N₄ material, a dense and continuous layer, composed of small Y₂Si₂O₇ crystallites and silicate glass, was formed on the surface. The surface layer was formed by exposing the specimens in a flowing H₂ atmosphere containing 0.1% H₂O at 1450°C for 1 h. For the purpose of comparison, specimens of the same material were heat treated in air at 1450°C for 1 h. Small equiaxed Y₂Si₂O₇ crystallites were formed when the specimen was heat treated in the wet H₂ atmosphere, while relatively large and elongated Y₂Si₂O₇ crystallites developed when the specimens were heat treated in air. The oxidation resistance of Si₃N₄ material was improved remarkably by the heat treatment in the wet H₂, while no improvement in oxidation resistance was observed from the specimen heat treated in air. The improvement was attributed to the retardation of the transport of oxidants through the dense and continuous layer formed on the surface.