High‐Temperature Evolution of a Core–Shell TBC: Linking Microstructure to CMAS, Sintering, and Fracture Resistance

ABSTRACT Calcia–magnesia–alumino–silicate (CMAS) attack severely limits the durability of thermal barrier coatings (TBCs). We recently introduced a novel core–shell TBC architecture exhibiting exceptional resistance to CMAS attack and fracture. Here, we elucidate the microstructural evolution of this coating during annealing at 1100°C–1300°C and establish its links with key TBC design requirements. The as‐deposited splat shell exhibits a metastable microstructure consisting of alumina‐containing t′‐zirconia and intergranular amorphous alumina. In situ high‐temperature TEM reveals rapid crystallization of the amorphous phase into α‐alumina, accompanied by the precipitation of nanoscale α‐alumina within supersaturated t′‐zirconia. With prolonged exposure, the system progressively coarsens into a thermodynamically equilibrated zirconia‐alumina composite. Despite these pronounced transformations, coating integrity and functional performance are preserved. Remarkably, CMAS resistance remains strong, with infiltration depths remaining more than six times lower than those of the YSZ TBC. This robustness arises because alumina crystallization, coarsening, and redistribution remain confined to the shell region, preserving the multiscale CMAS‐mitigation mechanisms operating along splat interfaces and grain boundaries. The coating exhibits enhanced sintering resistance, as evidenced by its lower modulus, higher porosity, and a defect‐healing activation energy twice that of the YSZ TBC. This behavior is attributed to intergranular alumina that restricts grain‐boundary diffusion and motion. The coating also maintains high fracture resistance, showing toughness 6%–10% higher than the YSZ TBC after annealing at 1100°C–1200°C, which is attributed to increased crack tortuosity and retention of t′‐zirconia. These findings further validate the core–shell design as a robust strategy for achieving durable, CMAS‐resistant TBCs.