DEVELOPMENT OF HIGH PERFORMANCE SILICON NITRIDE CERAMICS COATED WITH MULLITE

众所周知，Si_3N_4陶瓷易受高温氧化和水热腐蚀的影响。莫来石和Si_3N_4具有非常接近的热膨胀系数，表明可以制备耐久的涂层/基体系统。一般来说，莫来石对高温氧化和水热腐蚀表现出更好的耐腐蚀性。本课题的目的是制备莫来石包覆Si_3N_4陶瓷，并对材料的氧化物/氮化物梯度纳米界面进行表征。采用混合醇盐水解法在Si_3N_4试样和水热腐蚀后的Si_3N_4试样上制备莫来石涂层。结果表明：（1）采用混合醇盐水解法制备的莫来石粉末经900 ℃以上热处理后，X射线衍射分析表明为单相莫来石粉末;（2）采用浸渍法在Si_3N_4基体上成功地制备了莫来石涂层。在1300 ℃下加热后，在Si_3N_4衬底上获得了30 ~ 80 μ m厚的涂层，涂层/衬底系统中存在氧化物/氮化物梯度界面。与Si_3N_4陶瓷相比，其断裂强度和维氏硬度较低。

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.