CrTa_0.5Nb_0.5O₄: A solid solution strategy to enhance the mechanical and thermal properties of rutile structured ternaries

CrTaO₄ and CrNbO₄, rutile structured ternary scales formed on refractory high-entropy alloys (RHEAs), protect the RHEAs from oxidation and thermal attack. However, the Vickers hardness (12.20/10.20 GPa for CrTaO₄ and CrNbO₄, respectively) is lower than that of YSZ (14 GPa), making them prone to erosion. In addition, further reduction of thermal conductivity (1.31/1.09 W·m⁻¹·K⁻¹ for CrTaO₄ and CrNbO₄, respectively) benefits to protect the substrate materials from thermal attack. As such, optimizing their properties is urgently needed. To enhance the mechanical properties and further reduce the thermal conductivity of rutile-type ternary oxides CrTaO₄ and CrNbO₄, herein we designed a CrTa_0.5Nb_0.5O₄ solid solution based on the mechanism of solid solution strengthening, and systematically investigated its phase composition, microstructure, mechanical and thermal properties. The HAADF and ABF-STEM analyses confirmed the rutile-structure of CrTa_0.5Nb_0.5O₄, while minor impurities of rutile-structured CrNbO₄ and CrO₂ were also identified. The elastic modulus, bulk modulus, and shear modulus of CrTa_0.5Nb_0.5O₄ are 201, 119, and 175 GPa, respectively. Notably, the mechanical properties of CrTa_0.5Nb_0.5O₄ have been significantly improved via solid solution strengthening, with the Vickers hardness of 13.01 ± 0.2 GPa, fracture toughness of 2.07 ± 0.017 MPa·m^1/2, and flexural strength of 201 ± 12 MPa. The measured melting point of CrTa_0.5Nb_0.5O₄ is 2073 ± 20 K, with an average thermal expansion coefficient of (5.91 ± 0.52) × 10⁻⁶ K⁻¹. The room-temperature thermal conductivity of CrTa_0.5Nb_0.5O₄ is 1.07 W·m⁻¹·K⁻¹, which decreases to 0.57 W·m⁻¹·K⁻¹ at 1473 K, being lower than most of the well-known thermal barrier coating materials. In terms of thermal expansion coefficient matching, CrTa_0.5Nb_0.5O₄ is a qualified thermal barrier material for refractory metals and their alloys and ultra-high temperature ceramics. Therefore, this study has not only successfully developed a thermal barrier coating material with excellent mechanical properties and low thermal conductivity, but also provided new ideas for the research and development of materials in high-temperature fields from the perspective of material design.