Novel rare-earth hafnate thermal barrier coating materials with improved thermophysical and mechanical properties

To improve the thermophysical performance of hafnate ceramics, we designed and synthesized two innovative high-entropy rare-earth hafnate ceramics, namely (La 0.2 Nd 0.2 Sm 0.2 Eu 0.2 Gd 0.2 ) 2 Hf 2 O 7 and (La 0.2 Nd 0.2 Sm 0.2 Gd 0.2 Yb 0.2 ) 2 Hf 2 O 7 . The factors influencing their thermophysical properties were illustrated. Both ceramics exhibited a pure pyrochlore structure and displayed a favorable long-term phase stability at high temperature. Compared with the single-component La 2 Hf 2 O 7 , the high-entropy (La 0.2 Nd 0.2 Sm 0.2 Gd 0.2 Yb 0.2 ) 2 Hf 2 O 7 material exhibited superior mechanical properties, including enhanced hardness (11.77 ± 0.33 GPa) and fracture toughness (2.23 ± 0.12 MPa·m 1/2 ) values, in addition to a high thermal expansion coefficient (11.1 × 10 −6 K −1 at 1100 °C) and a low thermal conductivity (1.31 W·m −1 ·K −1 at room temperature). The significantly higher thermal expansion coefficient of this ceramic is primarily attributed to the reduced electronegativity difference between its cations and anions, as well as its lower Debye temperature. Furthermore, the large mass and radius differences among the doped cations serve as key phonon scattering sources, effectively lowering the thermal conductivity without compromising the structural stiffness of the material. This work clarifies the pivotal role of elemental diversity and ionic size mismatch in synergistically modulating the thermophysical properties of hafnates, offering a novel material design paradigm for advanced thermal barrier coatings.