Improvement of oxidation resistance of Si 3 N 4 by heat treatment in a wet H 2 atmosphere
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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.
I. INTRODUCTION
Silicon nitride (Si3N4) exhibits excellent thermomechanical properties such as high strength, hardness, and resistance to creep at elevated temperatures. For these reasons, Si3N4 is regarded as one of the most promising materials for high-temperature structural applications.1,2 For those applications, oxidation resistance is another important factor that should be considered. Many investigations have been made on the oxidation of Si3N4.3 It is well known that the resistance of pure Si3N4 to oxidation is excellent due to the formation of continuous SiO2 on the surface4–8 or Si2ON2 at the interface during oxidation.7,8 These layers effectively suppress oxygen transport to the sample surface, protecting the material from further oxidation. The oxidation resistance of polycrystalline Si3N4 is strongly influenced by the composition and crystalline state of the secondary phase.9–15 When sintering aids such as MgO, Al2O3, and Y2O3 are added for the densification, the pre-existing SiO2 reacts with the oxides to form a eutetic liquid, which results in the formation of an amorphous phase at the grain boundaries upon cooling. When such Si3N4 is exposed to air at elevated temperatures, those grain boundary phases react with silica (produced as an oxidation product of Si3N4) to form various types of silicate. As the grain boundary phases are removed via migration to the sample surface, the strength of the material decreases markedly after the oxidation.16–18 Even though a large amount of research has been carried out to investigate the oxidation mechanism of Si3N4, there are relatively few reports about the improvement of J. Mater. Res., Vol. 17, No. 9, Sep 2002
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the oxidation resistance of the material.19 One approach to improve the oxidation resistance of polycrystalline Si3N4 is to crystallize the grain boundary phase through the modification of the composition or through heat treatments.13,20,21 Another possible method is to coat the Si3N4 specimen with some oxidation-resistant materials. In this case, the
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