A novel single-phase Zr-Hf-Ta-O ceramic with high fracture toughness for thermal barrier coatings

Hf₆Ta₂O₁₇ has attracted great interest in thermal barrier coatings (TBCs) in the field of aero-engine due to the excellent ultra-high temperature thermal stability and low thermal conductivity. However, its lower fracture toughness than that of 7–8 wt % yttria-stabilized zirconia (YSZ) limited the application. Here, ZrO₂ was doped into Hf₆Ta₂O₁₇ to form the Zr-Hf-Ta-O ceramic by solid-phase reaction method to improve its fracture toughness. The effect of the doped Zr on the Hf₆Ta₂O₁₇ structure and mechanical properties of Zr-Hf-Ta-O ceramics were studied by experiment and first-principle simulation. The results showed that the prepared Zr-Hf-Ta-O ceramic exhibited the single orthorhombic phase due to the Zr atom diffusing into the Hf₆Ta₂O₁₇ structure and substituting the Hf atom at high temperatures. The substituted Hf atom might fall on an interstitial site. By increasing the content of ZrO₂, the gradient rise of lattice parameters was realized and the doped Hf₆Ta₂O₁₇ structure grew along the a-axis. Besides, the introduction of ZrO₂ significantly enhanced the hardness and fracture toughness of the Hf₆Ta₂O₁₇ ceramic. The most significant improvement was observed for the Zr-Hf-Ta-O ceramic with the molar ratio of HfO₂, Ta₂O₅, and ZrO₂ of 6:1:3 (2.41 ± 0.05 MPa m^1/2). The introduced Zr atom strengthens the covalency of its adjacent Ta-O and Hf-O bonds in the Zr-Hf-Ta-O structure, thus enhancing its mechanical properties. Through apprehending the role of the doped Zr on the structure and properties of Hf₆Ta₂O₁₇, designed elements might be added to the ceramic structure to further change its mechanical properties.