Design of CMAS-resistant Gd2O3/YSZ bilayer thermal barrier coatings via apatite phase control

CaO-MgO-Al2O3-SiO2 (CMAS) corrosion remains a critical challenge for thermal barrier coatings (TBCs) in aero-engines. To address this, we design a novel bilayer TBC system by depositing a Gd2O3 top layer on yttria-stabilized zirconia (YSZ) via atmospheric plasma spraying (APS). Interfacial reactions between Gd2O3 and CMAS at 1300 °C trigger rapid formation of a multicomponent sealant layer dominated by apatite phases. Crucially, in situ phase evolution occurs. Metastable Gd9.33(SiO4)6O2 initially nucleates, then transforms into thermodynamically stable Ca2Gd8(SiO4)6O2 driven by dynamic Ca2+/Gd3+ redistribution. Subsequent garnet crystallization within inter-apatite gaps densifies the reaction layer. This synergistic barrier effectively inhibits CMAS infiltration, demonstrated by a self-limiting reaction layer whose thickness stabilizes beyond 10 h. Consequently, no CMAS penetration into the underlying YSZ occurs even after 15 h of exposure. First-principles calculations confirm the preferential apatite formation (lower degree of distortion/formation enthalpy) and phase stability hierarchy. Our work provides a rationally designed TBC architecture with exceptional CMAS resistance through controlled interfacial phase engineering.