Thermochemical compatibility between Hf6Ta2O17 and Al2O3 at high temperatures for thermal barrier coatings

Abstract

Hf₆Ta₂O₁₇ is a potential high-temperature thermal barrier coating (TBC) material with better high-temperature stability and calcium magnesium aluminosilicate (CMAS) resistance than yttria-stabilized zirconia (YSZ). However, the high-temperature compatibility between Hf₆Ta₂O₁₇ and the thermally grown oxide (mainly α-Al₂O₃) has not been clarified. The thermochemical compatibility between Hf₆Ta₂O₁₇ and Al₂O₃ was investigated by annealing Hf₆Ta₂O₁₇-Al₂O₃ composite powders and Al₂O₃/Hf₆Ta₂O₁₇ diffusion couples at 1300–1600 °C. At 1300 °C, Hf₆Ta₂O₁₇ and Al₂O₃ do not react. Above 1400 °C, Hf₆Ta₂O₁₇ reacts with Al₂O₃ to form AlHf₃TaO₁₀, forming an intermediate phase layer at the Al₂O₃/Hf₆Ta₂O₁₇ interface. The layer thickness gradually increases as the annealing time rises, and Al diffusion causes micro-voids at the Al₂O₃/AlHf₃TaO₁₀ interface. The thermal expansion coefficient of AlHf₃TaO₁₀ is much lower than that of Hf₆Ta₂O₁₇ and Al₂O₃, resulting in high interfacial thermal stresses. The thermochemical incompatibility between Hf₆Ta₂O₁₇ and Al₂O₃ above 1400 °C may exacerbate the failure of Hf₆Ta₂O₁₇-based TBCs.