Tushar Kulkarni

January 2010
Functionally Graded Mullite Coatings for Gas Turbines
Committee Members: Advisor: Vinod Sarin, MSE/MEE; Chair: Xi Lin, MSE/ME; Srikanth Gopalan, MSE/ME; Soumendra N. Basu, MSE/ME; Dr. William Rhodes

Abstract: The next generation of heat exchangers and gas turbines require high performance materials to operate at higher temperatures for higher efficiency. SiC and Si3N4 are promising candidates as they have excellent high temperature properties. However, when used in combustion environments found in gas-turbine applications, these materials have two major concerns; hot-corrosion and recession. It is well established that environmental barrier coatings can be utilized to overcome these limitations.

Although chemical vapor deposited (CVD) mullite (3Al2O3.2SiO2) coatings developed previously have shown promise in protecting Si-based ceramics, there is concern that the silica content of the mullite coating itself might be susceptible to hot-corrosion and recession during long term exposure to corrosive atmospheres. Thus, there is a strong motivation to substantially reduce or even virtually eliminate the silica component from the surfaces of mullite coatings that are in direct contact with corrosive atmospheres.

In this study, CVD mullite coatings have been developed with potential promise to protect Si-based ceramics for high temperature applications. The composition of these functionally graded mullite coatings was varied from silica-rich close to the coating/substrate (SiC) interface for coefficient of thermal expansion match to alumina-rich towards the outer surface of the coating for hot-corrosion protection. In the process, the highest alumina-rich mullite ever reported has been deposited. These composite coatings typically comprised of three zones: Si-rich nanolayer close to the coating-substrate interface, columnar mullite grains, and a third phase composed of Al-rich nanocrystallites towards the top surface of the coating. The phase stability/transformation of these coatings, which is critical for high temperature applications, has been investigated in the range 1000-1600 °C.

Additionally, the potential of these coatings to protect SiC against hot-corrosion was studied at 1200 °C. CVD based mullite coatings with high alumina content at the top surface of the coating, and therefore reduced SiO2 activity, offered protection to the underlying substrate in corrosive environments and thus show immense potential to protect Si-based ceramics. It is expected that these coatings will have very broad impact by enabling gas turbines to operate at higher temperatures leading to improved fuel efficiency and reduced emissions.