Up to now, the development of environmental barrier coatings on silicon-based ceramics has generally gone through three generations, with respective focuses on mullite/YSZ (Yttria-Stabilized Zirconia), mullite/BSAS and rare earth silicates. Mullite (3Al2O3∙2SiO2) was extensively studied as EBCs for Si3N4 and SiC in the early days of coating development due to its low CTE (5-6×10-6 K-1), low thermal conductivity and chemical compatibility with silicon-based ceramics. However, its key durability limitations include through-thickness cracking, weak bonding between mullite and Si-based ceramics and a relatively high silica activity, which results in selective volatilization of silica due to its reaction with water vapor.
Yttria-stabilized zirconia (YSZ) top coat was added on the top of mullite to improve water vapor resistance, which made the first generation water-vapor-resistant EBC. This EBC only lasted a few hundred hours at ~1300°C before the large CTE mismatch between YSZ and mullite caused severe cracking and delamination of the coating.
For the second generation EBCs, the YSZ top coat was replaced with BSAS (1-xBaO-x SrO-Al2O3-2SiO2, 0≤x≤1), whose lower CTE and lower elastic modulus brought superior crack resistance. A silicon bond coat was also added to improve adherence. Thus, the second generation EBC was made up of a Si bond coat, a mullite or a mullite + BSAS intermediate coat, and a BSAS top coat. There are two key durability issues with this EBC: (1) the volatilization of BSAS top coat in high velocity combustion environments (a BSAS recession of ~70 μm was predicted after 1000 h at 1400 °C , 6 atm total pressure, and 24 m/s gas velocity); (2) the chemical reaction between BSAS and silica (formed by the oxidation of Si bond), which generates a relatively low-temperature melting (~1300°C ) glass.
After decades of research on mullite coatings, recent research suggested that rare earth disilicates (Re2Si2O7), which have relatively low CTE, good phase stability and low recession rate in water vapor, are promising alternatives to the previous generations of EBCs. The third generation of EBC system NASA designed for SiC/CMC is made up of a Si bond coat, a mullite intermediate layer and a rare earth silicate top coat .
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