Environmental Barrier Coatings (EBCs) for Silicon-Based Ceramics and Composites

Silicon-based ceramics and composites are promising materials for hot-section structural components of high-temperature turbines due to their excellent mechanical properties at elevated temperatures. However, corrosive gas and foreign particles in the service environment, such as water vapor/oxygen and some silicate dusts may attack silicon-based ceramics, which finally result in lifetime recession and sometimes severe failure. Therefore, Environmental Barrier Coatings (EBCs) have been proposed to isolate these corrosive media. EBCs system has developed over several decades to the most common structure consisting of bond coat/interlayer/topcoat. Main performance requirements for EBCs include low-thermal conductivity, thermal expansion coefficients matched with silicon-based Ceramic Matrix Composites (CMCs), and chemical compatibility with CMCs. Meanwhile, higher fracture toughness and bonding strength with CMCs can prolong the service life of EBCs. Therefore, EBCs are usually composed of three layers: the bonding layer, interlayer, and topcoat. Nowadays, EBCs have promoted to the third generation. The topcoat of EBCs in direct contact with the environment is crucial. Rare-earth silicates are considered as the most promising materials of the EBCs topcoats, owing to their appropriate thermophysical properties and excellent chemical compatibility. Additionally, mullites or rare-earth disilicates are usually selected as the interlayer material, meanwhile the bonding layer is generally silicon. During high-temperature service, corrosive media sometimes destroy the integrity of EBCs. In order to improve the service life of EBCs, different strategies have been adopted, such as solid-solutionizing treatment and high-entropy engineering. The thermal protective system of EBCs for silicon-based ceramics and composites has been developed rapidly to the promising rare-earth silicates as topcoats for the requirements of long-term service. Based on explorations of high-temperature corrosion mechanisms under attacks of water vapor and CMAS, scientists are still struggling to perfect EBCs by different strategies in order to meet the needs of practical applications.