Grain Versus Grain-Boundary Contributions to Thermal Conductivity in Prospective Oxide Ceramics for Next-Generation Thermal Barrier Coatings

Thermal barrier coatings (TBCs) require materials with intrinsically low thermal conductivity and high grain-boundary thermal resistance to maximize the temperature gradient across the top coat. In this work, the effective thermal conductivity of more than 40 prospective TBC oxides belonging to seven structural families (YSZ/YSH, pyrochlores/fluorites A2B2O7, defective fluorites A3BO7, fergusonite/monazite ABO4, and perovskites ABO3) was systematically deconvoluted into intrinsic grain thermal conductivity (kgrain) and grain-boundary (Rgb) contributions. It is shown that grain-boundary Kapitza resistance dominates heat transport in virtually all advanced oxides, contributing 60–90% to the total thermal resistance of polycrystalline samples. The lowest kgrain values (4–12 W m−1 K−1) are found for cerates and certain tantalates, while the highest Rgb (up to 7.2 × 10−6 m2 K W−1) are characteristic of high-entropy and heavily doped perovskites. Orthorhombically distorted SrCeO3-based and high-entropy perovskites combine moderate kgrain (4.7–27.9 W m−1 K−1), high Rgb, and tunable thermal-expansion coefficients (10–13 × 10−6 K−1), making them the most promising candidates for next-generation TBCs. These findings provide a rational basis for microstructure engineering and composition design aimed at maximizing the temperature drop across TBC layers while maintaining phase stability and CMAS resistance.