Elastic modulus evolution and behaviour of Si/Mullite/BSAS-based environmental barrier coatings exposed to high temperature in water vapour environment

摘要

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 stateof- the-art EBCs multilayer system comprises a silicon (Si) bond coat, mullite (3Al2O3 2SiO2) interlayer and (1 2 x)BaO xSrO Al2O3 2SiO2, \ua3 x \ua3 1 (BSAS) top coat. In this article, the role of hightemperature exposure (1300 \ub0C) 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.