Construction of a molten CMAS-resistant bilayer-structured apatite layer used for thermal barrier coatings

Molten calcium–magnesium–alumina–silicate (CMAS) is easy to wet and penetrate into thermal barrier coatings (TBCs), causing the coating corrosion and premature failure. Applying a protective layer on the TBC surface is considered a useful method to alleviate CMAS attack. In this study, a bilayer-structured apatite layer was developed, on which molten CMAS has low wettability. It had an acicular upper layer and a compact lower layer, which was constructed by pre-reacting GdPO₄ with CMAS powders through precisely regulating the pre-reaction temperature, time and CMAS concentration. The bilayer-structured apatite layer remained microstructure integrity after heat treatment at 1250 °C for 50 h, and did not crack after 100 thermal cycles. Exposed to CMAS at 1250 °C for 20 h, the layer remained an intact structure and still had function on preventing CMAS penetration. At 1250 °C for 30 min, the CMAS contact angle on the bilayer-structured apatite layer was 17.4 °, approaching three times that on Gd₂Zr₂O₇ which is another promising CMAS-resistant TBC material. Hence, the bilayer-structured apatite layer is quite attractive for addressing the CMAS issue of TBCs.