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.
