Microstructure, elastic/mechanical and thermal properties of CrTaO₄: A new thermal barrier material?

CrTaO₄ (or Cr_0.5Ta_0.5O₂) has been unexpectedly found to play a decisive role in improving the oxidation resistance of Cr and Ta-containing refractory high-entropy alloys (RHEAs). This rarely encountered complex oxide can effectively prevent the outward diffusion of metal cations from the RHEAs. Moreover, the oxidation kinetics of CrTaO₄-forming RHEAs is comparable to that of the well-known oxidation resistant Cr₂O₃- and Al₂O₃-forming Ni-based superalloys. However, CrTaO₄ has been ignored and its mechanical and thermal properties have yet to be studied. To fill this research gap and explore the untapped potential for its applications, here we report for the first time the microstructure, mechanical and thermal properties of CrTaO₄ prepared by hot-press sintering of solid-state reaction synthesized powders. Using the HAADF and ABF-STEM techniques, rutile crystal structure was confirmed and short range ordering was directly observed. In addition, segregation of Ta and Cr was identified. Intriguingly, CrTaO₄ exhibits elastic/mechanical properties similar to those of yttria stabilized zirconia (YSZ) with Young’s modulus, shear modulus, and bulk modulus of 268, 107, and 181 GPa, respectively, and Vickers hardness, flexural strength, and fracture toughness of 12.2±0.44 GPa, 142±14 MPa, and 1.87±0.074 MPa·m^1/2. The analogous elastic/mechanical properties of CrTaO₄ to those of YSZ has spurred inquiries to lucrative leverage it as a new thermal barrier material. The measured melting point of CrTaO₄ is 2103±20 K. The anisotropic thermal expansion coefficients are α_a = (5.68±0.10)×10⁻⁶ K⁻¹, α_c = (7.81±0.11)×10⁻⁶ K⁻¹, with an average thermal expansion coefficient of (6.39±0.11)×10⁻⁶ K⁻¹. The room temperature thermal conductivity of CrTaO₄ is 1.31 W·m⁻¹·K⁻¹ and declines to 0.66 W·m⁻¹·K⁻¹ at 1473 K, which are lower than most of the currently well-known thermal barrier materials. From the perspective of matched thermal expansion coefficient, CrTaO₄ pertains to an eligible thermal barrier material for refractory metals such as Ta, Nb, and RHEAs, and ultrahigh temperature ceramics. As such, this work not only provides fundamental microstructure, elastic/mechanical and thermal properties that are instructive for understanding the protectiveness displayed by CrTaO₄ on top of RHEAs but also outreaches its untapped potential as a new thermal barrier material.