FRG: Functionally Graded High-Al Mullite Environmental Barrier Coatings

碳化硅和氮化硅等硅基陶瓷在高温下暴露在含有高压蒸汽的腐蚀性环境中时，需要耐热腐蚀和抗衰退的环境屏障涂层(EBCs)。本研究将致力于开发一种功能梯度富铝莫来石(3Al_2O_3·2SiO_2)EBC系统，以保护硅基陶瓷免受这种腐蚀环境的影响。这些涂层将通过化学气相沉积(CVD)进行沉积，这种方法允许在复杂的衬底上沉积致密、均匀的涂层。众所周知，α-氧化铝涂层具有良好的抗热腐蚀和耐热衰退性能，但由于耐热震性较差，可能会失效。相比之下，莫来石涂层具有优异的抗热震性，与硅基陶瓷的热膨胀系数接近，含有二氧化硅，这可能导致涂层本身在腐蚀环境中长期暴露时的热腐蚀和衰退。我们建议生长功能梯度涂层，其组成从涂层/基材界面的近化学计量比的莫来石到涂层表面的高度富铝的莫来石。因此，预计这种涂层将具有α-氧化铝涂层的抗氧化、耐腐蚀和抗衰退性能，以及硅基衬底上化学计量比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的CVD莫来石涂层，并评估其抗氧化性、耐腐蚀性和抗退性。此外，还将使用新的无损评估(NDE)技术来评估这些涂层的机械性能。该知识库将用于系统地设计CVD莫来石功能梯度涂层，使其在特定的工业环境中具有最佳的性能。预计这项研究的成功完成将导致在硅基陶瓷的复杂表面上获得致密、均匀和附着力的涂层，具有很高的抗裂性、层裂、氧化、热腐蚀和衰退性能。这些涂层将产生关键影响，使燃气轮机能够在更高的温度下运行，从而提高燃料效率和减少排放。这项研究还将推动涂层材料无损评估方面的最新进展，包括测量梯度涂层的弹性性能随深度的变化，监测涂层退化，以及检测涂层/基材界面在热循环过程中的裂纹扩展。该研究计划将培训3名研究生在加工、结构和财产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.