Thermal barrier coatings (TBCs) are applied on the metallic substrate materials in gas turbines for stationary energy conversion and for airplane propulsion to protect them from the harsh conditions. Therefore, TBCs must meet several properties: low thermal conductivity, high compatibility with Al₂O₃, high fracture toughness, corrosion resistance and phase stability at high temperatures. Besides this, phase transformations should be avoided in the temperature window of the application since cracks in the coating can be formed due to volume changes, which endanger the integrity of the protective layer.^[1]

The state-of-the-art material is the yttria-stabilized zirconia. Yttria stabilizes the high temperature polymorph tetragonal due to the formation of oxygen vacancies for charge compensation.^[1] The tetragonality of the ZrO₂ lattice can be increased by doping with smaller cations or higher oxidation states than Zr⁴⁺ (e.g. Ti⁴⁺, Nb⁵⁺, Ta⁵⁺) which is beneficial for the ferroelastic toughening mechanism strengthening the material at elevated temperature. Adding Hf⁴⁺ leads to an increased fracture toughness and phase and structural stability of the ZrO₂ lattice up to 1700 °C and Hf⁴⁺ low thermal conductivity above 1100 °C.^[2] Therefore, the ZrO₂-HfO₂-Y₂O₃-Ta₂O₅ material system is of great interest for future TBCs applications.

In this work, the ZrO₂-HfO₂-Y₂O₃-Ta₂O₅ system and its sub-systems^[3-5] are investigated using CALPHAD approach, including experimental and computational thermodynamics. Key compositions are synthesized using reverse co-precipitation and are subjected to heat treatments at selected temperatures. Phase and chemical compositions are determined using X-ray diffraction and scanning electron microscopy/energy dispersive spectroscopy. Phase stabilities are investigated using thermal analysis and heat capacities are measured using differential scanning calorimetry. The experimental data is used as an input for thermodynamic modelling of the material system.

References

[1] X. Song, M. Xie, S. An, X. Hao, R. Mu \textit{Scr. Mater.}, \textbf{2010}, \textit{62}, 879-882.

[2] O. Doronin, P. Artemenko, P. Stekhov, P. Marakhovskii, V. Stolyarova, V. Vorozhtsov \textit{Russ. J. Inorg. Chem.}, \textbf{2022}, \textit{67}, 732-739.

[3] M. Löffler, P. Hutterer, O. Fabrichnaya, M. Lepple \textit{J. Am. Ceram. Soc.}, \textbf{2023}, \textit{43}, 7668-7681.

[4] M. Lepple, K. Lilova, C. Levi, A. Navrotsky \textit{J. Mater. Res.}, \textbf{2019}, \textit{34}, 3343-3350.

[5] M. Lepple, S. Ushakov, K. Lilova, C. Macauley, A. Fernandez, C. Levi, A. Navrotsky, \textit{J. Eur. Ceram. Soc.}, \textbf{2021}, \textit{41}, 1629-1638.