GOALI: Multilayered Oxide Fiber Coating Concept for Environmentally Durable SiC/SiC Composites

该GALI项目是史蒂文斯理工学院和通用电气公司研发部门的合作项目，将探索一种新的纤维涂层概念，使碳化硅/碳化硅陶瓷基复合材料能够在飞机发动机、空间推进系统、通用燃气轮机和工业热交换器中遇到的高温环境中可靠使用。与传统的脆性陶瓷材料不同，碳化硅/碳化硅复合材料通过引入BN纤维涂层作为弱界面来偏转纤维/基体界面区域的裂纹，从而使其具有韧性和损伤容忍性。首席研究员和他的合作者最近发现，氧化物纤维涂层可以为碳化硅/碳化硅复合材料提供所需的弱界面行为，而不会明显降低纤维强度。氧化物基纤维涂层的这种特性以前从未报道过。这一新发现的基础是一个相当非传统的概念，即使用两种不同但热化学兼容的氧化物材料来形成多层氧化物涂层。例如，对由化学气相沉积(CVD)制备的含有SiO_2/ZrO_2/SiO_2层的纤维涂层的初步结果显示，确凿的证据表明，多层界面区域内存在“优雅”的拉伸行为和广泛的裂纹偏转，这可能是由于SiO_2和ZrO_2层的热膨胀系数(CTE)严重不匹配所致。此外，与氧化物基纤维涂层的预期一样，复合材料的大部分特性在应力氧化后仍保持不变。从技术角度来看，这种新的涂层概念可能为应力氧化问题提供独特的解决方案，特别是如果在实际开发之前可以测试该概念的一般性。这项目标项目将：(1)证明明显的弱界面行为是由CTE失配引起的，(2)评估如何通过选择合适的氧化物成分将这种类型的主动界面控制保持到1200℃，以及(3)确定CVD SiO_2层对纤维强度的影响。本着GOALI计划的精神，史蒂文斯和GECRD的独特研究技能和设施得到了整合。涂层工艺优化和界面特性研究将由教授进行。李和利伯拉分别在史蒂文斯。GECRD将在工业科学家王博士的指导下，向史蒂文斯的研究生提供一台气氛控制的拉伸测试仪。更重要的是，GECRD在制造碳化硅/碳化硅元件方面的工业专业知识将被用来预测这种高风险、高收益涂层概念的可行性，以解决美国工业的应力氧化问题。该项目由材料研究部的陶瓷项目、国家科学基金会数学和物理科学委员会的多学科活动办公室以及莱特-帕特森空军基地空军研究实验室的陶瓷研究小组共同资助。

This GOALI project, a collaboration between Stevens Institute of Technology and General Electric Corporate Research and Development, will explore a novel fiber coating concept which may enable the reliable use of SiC/SiC ceramic matrix composites in high-temperature environments encountered in aircraft engines, space propulsion systems, utility gas turbines, and industrial heat exchangers. Unlike traditionally brittle ceramic materials, SiC/SiC composites are made to be tough and damage-tolerant by incorporating a BN fiber coating which serves as a weak interface to deflect cracks at the fiber/matrix interface region. The Principal Investigator and his collaborators have recently made intriguing observations that an oxide fiber coating can provide the desired weak interface behavior for SiC/SiC composites without apparent degradation of fiber strength. Such characteristics have never been reported previously for an oxide-based fiber coating. The basis of this new discovery was a rather unconventional concept of using two dissimilar, but thermochemically compatible oxide materials to form a multilayered oxide coating. For example, initial results with a fiber coating containing SiO2/ZrO2/SiO2 layers prepared by chemical vapor deposition (CVD) showed definitive evidence for "graceful" tensile behavior and extensive crack-deflection within the multilayered interface region, presumably due to the significant mismatch in the coefficient of thermal expansion (CTE) of the SiO2 and ZrO2 layers. Furthermore, as expected of the oxide-based fiber coating, much of the composite characteristics was retained after stress oxidation. From a technological perspective, this new coating concept may offer an unique solution to the stress oxidation problem, particularly if the generality of the concept can be tested prior to practical development. This GOALI project will: (1) prove that the apparent weak interface behavior is caused by the CTE mismatch, (2) evaluate how this type of proactive interface control can be sustained up to 1200 degrees C by selecting appropriate oxide constituents, and (3) determine the effect of the CVD SiO2 layer on fiber strength. In the spirit of the GOALI program, unique research skills and facilities at Stevens and GECRD are integrated. Coating process optimization and interface characterization studies will be conducted by Profs. Lee and Libera, respectively, at Stevens. An atmosphere-controlled tensile tester at GECRD will be available to Stevens' graduate students under the direction of Dr. Wang, the industrial scientist. More importantly, GECRD's industrial expertise in manufacturing SiC/SiC components will be used to project the viability of this high-risk, high-gain coating concept for solving the stress oxidation problem for the U.S. industry. This project is co-funded by the Ceramics Program of the Division of Materials Research, the Office of Multidisciplinary Activities of the Mathematical and Physical Sciences Directorate, both at NSF, and the Ceramics Research Group at the Air Force Research Laboratory at Wright-Patterson Air Force Base.***