The microstructure of silicate layers formed by a spray coating technique was observed, and the oxidation behavior of Si3N4 with and without the coating layer was investigated. Migration of the silicate phase into the sample was observed when the heat treatment temperature of the coating was higher than 1400oC. Many pores were observed below the silicate layer formed by oxidation of Si3N4, but the pore formation was inhibited in the coated and heat treated (in N2) layer. The oxidized layer contained cristobalite and Y-keivyite, while the formation of cristobalite was suppressed in the coated silicate layer. Oxygen is likely to play an important role for the crystallization of cristobalite in the silicate layer. The SiO2 coating inhibits the rapid oxidation of Si3N4 at 1000oC effectively, and increases the oxidation resistance of Si3N4 up to 1300oC.

Keywords: Si3N4, Oxidation, Spray, Coating, SiO2