FRG: Functionally Graded High-Al Mullite Environmental Barrier Coatings

硅基陶瓷，如碳化硅和氮化硅，在高温下暴露于含有高压蒸汽的腐蚀环境时，需要热腐蚀和耐衰退的环境屏障涂层（ebc）。本研究将重点开发一种功能分级的富铝莫来石（3Al2O3.2SiO2） EBC体系，用于保护硅基陶瓷免受此类腐蚀环境的影响。这些涂层将通过化学气相沉积（CVD）沉积，这允许在复杂的基材上沉积致密、均匀的涂层。α -氧化铝涂层具有优异的耐热腐蚀和抗衰退性能，但由于耐热性差，可能会失效。相比之下，莫来石涂层具有优异的抗热震性，与硅基陶瓷的CTE匹配密切，但它含有二氧化硅，在腐蚀环境中长期暴露可能导致涂层本身的热腐蚀和衰退。我们建议生长功能梯度涂层，其成分从涂层/衬底界面的近化学计量莫来石到涂层表面的高富铝莫来石不等。因此，预计这种涂层将具有α -氧化铝涂层的抗氧化、抗腐蚀和抗衰退性能，同时具有si基基上化学计量莫来石涂层的抗热震性能。拟议的研究将包括开发一种工艺来生长成分梯度CVD莫来石涂层，并评估其氧化，腐蚀和抗衰退能力。此外，这些涂层的机械性能将被评估，其中使用新颖的无损评价（NDE）技术。该知识库将用于帮助系统设计功能分级的CVD莫来石涂层，以在特定的工业服务环境中获得最佳性能。预计本研究的成功完成将导致硅基陶瓷复杂表面致密、均匀和粘附性强的涂层，具有良好的抗开裂、抗剥落、抗氧化、抗热腐蚀和抗衰退性能。这些涂层将对燃气轮机在更高温度下运行产生关键影响，从而提高燃油效率并减少排放。这项研究还将推进涂层材料无损评估的最新技术，包括测量渐变涂层的弹性性能（作为深度的函数）、监测涂层降解以及在热循环过程中涂层/基体界面处裂纹扩展的检测。该研究项目将培养3名研究生在加工、结构和性能评估领域作为一个团队工作，并使他们能够与来自工业、国家实验室、其他学术机构的研究人员以及波士顿大学的本科生进行互动。

Si-based ceramics such as silicon carbide and silicon nitride require hot-corrosion and recession resistant environmental barrier coatings (EBCs) when exposed to corrosive environments containing high-pressure steam at elevated temperatures. This research will focus on developing a functionally graded Al-rich mullite (3Al2O3.2SiO2) EBC system for the protection of Si-based ceramics from such corrosive environments. These coatings will be deposited by chemical vapor deposition (CVD), which allows for the deposition of dense, uniform coatings over complex substrates. It is well established that alpha-alumina coatings, which have excellent hot-corrosion and recession resistance, can fail due to poor thermal shock resistance. In contrast, mullite coatings, which have excellent thermal shock resistance and a close CTE match with the Si-based ceramics, contains silica, which may lead to hot-corrosion and recession attack of the coatings themselves under long term exposures in corrosive environments. We propose to grow functionally graded coatings whose composition is varied from near-stoichiometric mullite at the coating/substrate interface to a highly Al-rich mullite at the coating surface. It is therefore expected that such coatings will have the oxidation, corrosion and recession resistance of alpha-alumina coatings, with the thermal shock resistance of stoichiometric mullite coatings on Si-based substrates. The proposed research will involve developing a process to grow compositionally graded CVD mullite coatings and to evaluate their oxidation, corrosion, and recession resistance. Additionally, mechanical properties of these the coatings will be evaluated, using among others, novel non-destructive evaluation (NDE) techniques. This knowledge base will be used to aid in the systematic design of functionally graded CVD mullite coatings for optimal performance in specific industrial service environments.It is projected that the successful completion of this research will lead to dense, uniform and adherent coatings on complex surfaces of Si-based ceramics, that are highly resistant to cracking, spallation, oxidation, hot-corrosion and recession. These coatings will have a critical impact by enabling gas turbines to operate at higher temperatures leading to improved fuel efficiency and reduced emissions. This research will also advance the state-of-the-art in non-destructive evaluation of coating materials including measurement of elastic properties of the graded coatings as a function of depth, monitoring coating degradation, and detection of crack growth at the coating/substrate interface during thermal cycling. The research program will train 3 graduate students to work as a team in the areas of processing, structure and property evaluations and will enable them to interact with researchers from industry, national laboratories, other academic institutions, as well as with undergraduates from Boston University.