Rare-earth tantalates (RETaO₄) have been extensively investigated as thermal protective materials, but their application as environmental barrier coatings (EBCs) for ceramic matrix composites (CMCs) is limited by their high thermal expansion coefficients (TECs ≥ 9.0×10⁻⁶ K⁻¹). High-entropy design provides a feasible route to tailor the thermal properties of RETaO₄; however, most reports focus on equimolar RETaO₄ high-entropy ceramics (HECs). In this study, a series of nonequimolar monoclinic-prime (m') RETaO₄ HECs are designed and synthesized to clarify the composition-structure-property relationships. The nonequimolar design is employed to regulate configurational complexity, polyhedral distortion, and lattice strain in m'-RETaO₄ within the same phase. Atomic-scale transmission electron microscopy (TEM) and geometric phase analysis (GPA) characterizations reveal the associated local distortion and lattice strain. These observations link nonequimolar composition with local structural distortion and macroscopic thermal transport behavior. The lowest thermal conductivity reaches 1.52–2.68 W·m⁻¹·K⁻¹ at 25–900 °C, and this is mainly attributed to RE-site disorder, lattice strain, and polyhedral distortion. Polyhedral distortion also suppresses thermal expansion by restricting atomic anharmonic vibrations. Among the designed compositions, the optimized nonequimolar m'-RETaO₄ HEC (Sc_0.2Y_0.2Tm_0.2Ho_0.2Dy_0.1Gd_0.1)TaO₄ exhibits the lowest TEC of 6.2×10⁻⁶ K⁻¹ at 1500 °C, which is closer to that of SiC-based CMCs than the other compositions, indicating its potential as a candidate EBC material. This work proposes that polyhedral distortion and lattice strain are important structural factors for tailoring the thermal properties of nonequimolar RETaO₄ HECs, providing guidance for the design of complex oxides.

Failure is a main disadvantage of thermal barrier coatings (TBCs). Typically, whether failure occurs depends on competition between the cracking driving force (negatively related to strain tolerance) and cracking resistance. However, it is difficult to simultaneously enhance these two properties in conventional monostructures since they often need totally opposite structures. In this work, a novel structure was designed to co-enhance strain tolerance and cracking resistance in thick TBCs. The novel structure appears to have trimodal features. The macro-columnar structure was tailored to enhance the strain tolerance, the mesogradient structure in individual columns was porosity-distributed from bottom to top to enhance the cracking resistance, and the microlamellar structure was deposited to further tolerate strain and to prevent heat flux. First, the trimodal-featured structure was tailored in 2000-μm-thick TBCs. Thermal cyclic tests showed that the lifespan was nearly 4 and 9 times that of the conventional one-way designs of columnar and gradient TBCs, respectively. Second, a finite element model was developed to investigate the mechanism responsible for the long lifespan. Cooperation in lowering the driving force and increasing the cracking resistance significantly retards the cracking behavior in thick coatings. Finally, dominant factors of trimodal-featured structures were discussed and optimized to further extend the lifespan of thick TBCs. Overall, the matching design between strain tolerance and cracking resistance provides a fundamental method for durable protection in thick TBCs.

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.