A core-shell thermal barrier coating with strong resistance to molten silicate attack and fracture

We report a new thermal barrier coating (TBC) with strong resistance to calcia–magnesia–alumina–silicate (CMAS) attack and fracture. The design is based on a core-shell microstructure where each building block of the TBC comprises a tough yttria stabilised zirconia (YSZ) core and a CMAS-resistant shell. To demonstrate the feasibility of the design, we select alumina, an established CMAS-resistant ceramic, as the shell material and manufacture core-shell TBCs by first synthesising “YSZ core-alumina shell” powder using the sol-gel method and then thermally spraying the powder to form core-shell TBCs. Microstructural characterisations confirm that a core-shell coating structure is successfully manufactured. However, the melting and mixing of alumina and YSZ in thermal spray result in the formation of a microstructure composed of alumina-alloyed zirconia grains and intergranular alumina, with the overall alumina concentration increasing from the core to the shell. The CMAS penetration depth through the core-shell TBC is over an order of magnitude lower than that through the benchmark YSZ TBC, which is attributed to the multiscale protection of the core-shell microstructure against CMAS infiltration through cracks and grain boundaries. The core-shell TBC has a similar stiffness to the YSZ TBC but exhibits a lower erosion rate and higher fracture toughness, indicating enhanced fracture resistance without compromising strain tolerance. The improved fracture resistance of the core-shell TBC is attributed to its less defective intersplat structure and greater ferroelastic toughening strain. Compared to the YSZ TBC, the core-shell TBC shows lower stiffness and nearly identical fracture toughness after annealing.