Abstract
Environmental barrier coatings (EBCs) are required to mitigate ceramic matrix composite (CMC) degradation in gas engine combustion environments, yet the long-term stability of EBCs in turbine applications is not fully understood. It is known that the primary EBC failure mechanisms include reaction with water vapor, steam oxidation of the silicon bond coat, thermomechanical fatigue, corrosion by ingested debris, erosion, and foreign object damage [1], and that all failure modes are connected through the microstructural evolution of the EBC.
This dissertation attempts to better understand EBC-water vapor degradation mechanisms, long term phase stability, thermochemical stability, and criteria for development of next generation EBC candidates through the following objectives:
1. Uncover fundamental mechanisms for high-velocity steam degradation and microstructural evolution of silicate EBC candidates such as barium strontium aluminosilicate (BSAS, Ba0.75Sr0.25Al2Si2O8), hafnon (HfSiO4), and ytterbium disilicate (Yb2Si2O7)
2. Determine fundamental relationships between rare earth elements and rare earth silicate material properties to tailor improved properties such as thermal expansion, phase stability, steam resistance, thermal conductivity, and Young’s Modulus
3. Develop the first figure of merit for a holistic comparison of EBC materials against all major EBC failure modes, for identification and exploration of novel EBC candidates such as ytterbium phosphate (YbPO4), and for advancement of next-generation EBCs
