Achieving durable double-layered thermal barrier coatings by tailoring multi-scale structures

Double-layered thermal barrier coatings (DL-TBCs) have been developed to meet multiple service requirements, such as low thermal conductivity, high thermal stability, and high fracture toughness. Conventional DL-TBCs are often designed on the basis of equal total thickness to have long lifespans, which may weaken the thermal insulation. The reason is that the single-scale designed structure often has opposite effects on the thermal and mechanical properties. To enhance both the thermal insulation and lifespan, this work designed durable DL-TBCs at multiple scales under equivalent thermal insulation. The macroscopic thickness ratio of the top layer to the bottom layer was tailored to optimize the total and single thicknesses, and the microscopic pore size in the top layer was tailored to resist sintering. Six groups of samples with different thickness ratios were prepared. The thermal cycling test revealed that the lifespan of DL-TBCs first increases but then decreases with increasing thickness ratio. The optimized thickness ratio is 2:3 for DL-TBCs, which have the largest lifespan among the six groups. The cross-sectional morphologies revealed that the failure mode changed from the spallation of the top layer to the delamination of the total double layers. The long lifespan of the optimized DL-TBCs stems from the cotailored thickness ratio and porous structure in the top layer to lower the total cracking driving force.