Elastic Modulus Evolution and Behavior of Si/Mullite/BSAS-Based Environmental Barrier Coatings Exposed to High Temperature in Water Vapor Environment

Abstract

Si-based ceramics (e.g., SiC and Si3N4) are known as promising high-temperature structural materials in various components where metals/alloys reached their ultimate performances (e.g., advanced gas turbine engines and structural components of future hypersonic vehicles). To alleviate the surface recession that Si-based ceramics undergo in a high-temperature environmental attack (e.g., H2O vapor), appropriate refractory oxides are engineered to serve as environmental barrier coatings (EBCs). The current state-of-the-art EBCs multilayer system comprises a silicon (Si) bond coat, mullite (3Al2O3·2SiO2) interlayer and (1 − x)BaO·xSrO·Al2O3·2SiO2, 0 ≤ x ≤ 1 (BSAS) top coat. In this article, the role of high-temperature exposure (1300 °C) performed in H2O vapor environment (for time intervals up to 500 h) on the elastic moduli of air plasma sprayed Si/mullite/BSAS layers deposited on SiC substrates was investigated via depth-sensing indentation. Laser-ultrasonics was employed to evaluate the E values of as-sprayed BSAS coatings as an attempt to validate the indentation results. Fully crystalline, crack-free, and near-crack-free as-sprayed EBCs were engineered under controlled deposition conditions. The absence of phase transformation and stability of the low elastic modulus values (e.g., ~60-70 GPa) retained by the BSAS top layers after harsh environmental exposure provides a plausible explanation for the almost crack-free coatings observed. The relationships between the measured elastic moduli of the EBCs and their microstructural behavior during the high-temperature exposure are discussed.