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

Double-layered thermal barrier coatings (DL-TBCs) were developed to meet multiple requirements in service, such as low thermal conductivity, high thermal stability, and high fracture toughness as well. Conventional DL-TBCs were often designed based on equal total-thickness to have long lifespans, which may weaken the thermal insulations. The cause is that the single-scale designed structure often has opposite effects on the thermal and mechanical properties. To co-enhance the thermal insulation and lifespan, this work designed durable DL-TBCs in multi-scales under equivalent thermal insulation. The macroscopic thickness ratio of top layer to bottom layer was tailored to optimize total and single thicknesses, and the microscopic pore size in top layer was tailored to resist sintering. Six groups of samples with different thickness ratios were prepared. The thermal cyclic test shows that the lifespans of DL-TBCs first increase and then decrease with an increase in thickness ratio. The optimized thickness ratio is 2:3 in DL-TBCs, which have the largest lifespan among the six groups. Cross-section morphologies show that the failure modes changed from the spallation of top-layer to the delamination of total double-layers. The long lifespan of the optimized DL-TBCs stems from the co-tailored thickness ratio and porous structure in top layer to lower the total cracking driving force.