3D C/SiC复合材料在复杂环境试验中性能演变的两重性

用减压化学气相浸渗法(LPCVI)制备2组3D C/SiC复合材料,其中一组具有不同厚度的PyC界面层,另一组PyC界面层厚度一定,但经过热处理.对C/SiC复合材料在复杂环境中性能演变的两重性,即确定性和随机性进行了研究.结果表明,残余强度及其波动性对评价材料的环境适应性和可靠性是必需的.界面层和涂层是对氧化环境最敏感的微结构控制单元.以敏感度排序,3种环境参数依次是温度、气氛和应力.气氛参数的排序是氧气、水和盐,应力参数的排序是疲劳/蠕变,蠕变和疲劳.应力通过增加涂层裂纹及宽度从而加速复合材料的性能演变.氧化物薄膜有利于涂层裂纹封填,水能促进这种封填,然而疲劳/蠕变应力会使涂层裂纹封填失效.因此包括有氧气、水、疲劳和蠕变的环境是所有环境中最恶劣的.为了使复合材料具有自适应性,PyC的厚度应为最优,以提高热处理的效果;需保持适中的涂层氧化速率,以提高近表面抗氧化性.而适中的氧化速率是由温度和氧化气体分压来控制的.

Dualism in Evolution of 3D C/SiC Composites under Complex Environmental Testing

Three-dimensional (3D) C/SiC composites with a CVD SiC coatings and a PyC interphase were prepared by low-pressure chemical vapor deposition method. The interphase was annealed and changed in thickness. Dualism, certainty and randomicity, in the evolution of C/SiC composites under complicated environments were investigated. The results show that residual strength and its fluctuation are important when evaluating environmental adaptability and reliability. The interphase and coatings are the key microstructural units most sensitive to oxidizing environments. The three parameters to be considered, in order of importance, are temperature, atmosphere and stress sensitivity. The most significant atmosphere is oxygen, followed by water and salt, while the most important stress parameters are fatigue/creep interaction, creep and fatigue. Stress promotes degradation of the composites due to opening of cracks in the coatings. An oxide film on the coatings is best to seal these cracks, and water can accelerate this sealing. However, a fatigue/creep interaction stress causes sealing failure, and then the environment including oxygen, water, fatigue and creep is the worse one of all. For the composites to be self-adaptable, the PyC thickness should be optimum for increasing annealing effect, and the coatings should be oxidized at a moderate rate for increasing resistance to oxidation. The temperature and the partial pressure of oxidizing gases control the moderate rate.