DEVELOPMENT OF HIGH PERFORMANCE SILICON NITRIDE CERAMICS COATED WITH MULLITE

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It is well known that Si_3N_4 ceramics are readily susceptible to high-temperature oxidation and hydrothermal corrosion. Mullite and Si_3N_4 have very close coefficients of thermal expansion, suggesting a durable coating/substrate system can be made. In general, mullite shows better corrosion resistance to high-temperature oxidation and hydrothermal corrosion. The purpose of this project is to fabricate mullite coated Si_3N_4 ceramics and to characterize the oxide/nitride gradient nano-interface of the materials. Mullite coatings by using the hydrolysis of mixed alkoxides were applied to Si_3N_4 coupons and hydrothermally corroded Si_3N_4 coupons. The following resuls were obtained.1.X-ray diffraction indicated mullite powders to be a single phase, which were prepared by the hydrolysis of mixed alkoxides route and heated at above 900ﾟC.2.Mullite coatings on Si_3N_4 substrates can be successfully made by dipping in the mixed alkoxides solution. Coatings were approximately 30 to 80 mum thick on Si_3N_4 substrates after heating at 1300ﾟC.These materials have a oxide/nitride gradient interface at coating/substrate system.3.Coatings showed good adherence to the Si_3N_4 substrates, but poor density. Because of these features, the fracture strength and vickers hardness were relatively low compared to Si_3N_4 ceramics.