Synergistic regulation for crack resistant and strain tolerant regions for a matching design in double-layered thermal barrier coatings with long life span

Double-layered thermal barrier coatings (DL-TBCs), which combine a top layer (with low thermal conductivity and high-temperature stability) and a bottom layer (with high fracture toughness), offer superior thermal insulation potential compared to single-layer coatings. This advantage stems from the functional synergy between the two layers. However, the introduction of a new heterogeneous interface often leads to interfacial stress concentration, which becomes a critical bottleneck limiting the coating's service life. To resolve the conflicting structural requirements—where the ceramic layer near the bond coat must be dense to resist cracking, while the interface between the two ceramic layers requires porosity to relieve stress. This study proposes a design approach based on multiscale strain partitioning and gradient porosity regulation. The design uses high-entropy rare-earth zirconate (HEFO) as the top layer (for low thermal conductivity and high-temperature stability) and yttria-stabilized zirconia (YSZ) as the bottom layer (for stress buffering). Through integrated optimization of materials, structure, and processing, a functionally graded pore structure was achieved across different regions. The region near the bond coat was densified, with a porosity of approximately 7.3%. In contrast, the double ceramic layers exhibited a gradual increase in porosity along the thickness direction: from 7.3% to 17.3%. Meanwhile, the proportion of unmelted regions increased from 5.8% to 34.2%, forming a continuously transitional microstructure. The lifespan of the DL-TBCs under this design is approximately 1.7 times longer than that of the conventional DL-TBCs. Further investigation into the structural evolution during high-temperature service revealed that the graded pore structure can effectively mitigate interfacial thermal expansion mismatch, reduce stress concentration at the interface, and enhance the bonding strength of heterogeneous interfaces, thereby delaying the process of interfacial cracking. The “structure–stress–life” relationship established in this study offers a new paradigm for designing DL-TBCs.