Thermodynamics and Phase Relations of High Performance Materials for Next Generation Thermal Barrier Coatings in the System ZrO2-HfO2-Y2O3-Ta2O5

• Applicants: Privatdozentin Dr. Olga Fabrichnaya, Ph.D.; Dr.-Ing. Maren Lepple
• Subject Area: Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials
• Term: since 2016
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 314579101

In order to increase the efficiency of turbines by raising the operating temperature and thus reduce fuel consumption and CO2 emissions, new high-temperature stable ceramics are required for the application as thermal barrier coatings. Thermal barrier coatings protect the metallic structural materials in the hottest zones of the turbine. For such high-temperature applications, a comprehensive understanding of the materials system used is essential, including knowledge on the phase stabilities and the driving forces for the formation of metastable and stable phases. Therefore, the aim of this project is to thermodynamically investigate and model the promising material system ZrO2-HfO2-Y2O3-Ta2O5 in order to elucidate the stabilization mechanisms. Advanced thermodynamic modelling is used for this purpose. By combining key experiments with thermodynamic models of phases possibly existing in the system, a consistent thermodynamic description of the multi-component system can be generated and is therefore an invaluable tool for material development as an integral part of "Integrated Computational Materials Engineering" (ICME).Compositions in the material system ZrO2-Y2O3-Ta2O3 are promising materials for next generation thermal barrier coatings due to their attractive properties such as low thermal conductivity, phase stability up to at least 1500 °C and mechanical properties comparable to the state-of-the-art thermal barrier coating material yttria-stabilized zirconia. The stabilization mechanisms of the phases along the equimolar doping line ZrO2-YTaO4 were elucidated by a combination of thermochemical and phase diagram investigations during the first funding period. In the continuation of the project, the material system is extended by HfO2, since this allows the generation of compositions with even lower thermal conductivity, an adapted coefficient of thermal expansion and improved thermochemical stability at high temperatures. The phase equilibria and phase stabilities in the material system ZrO2-HfO2-Y2O3-Ta2O5 and its subsystems are determined by X-ray diffraction, thermal analysis and electron microscopy with elemental analysis. Standard formation enthalpies of stable and metastable samples are derived from solution enthalpies obtained by high temperature solution calorimetry. Heat capacities are measured by differential scanning calorimetry, which reflect the temperature dependence of Gibbs energies and thus contribute directly to the understanding of the thermal behavior of the compositions. For each stable and metastable phase in the multi-component material system, a thermodynamic model is selected based on crystallographic information and thermodynamic parameters are assessed using the CALPHAD method in order to obtain a deep understanding of the stabilization effects, phase relations and constitution. The experimental results are directly incorporated into the thermodynamic modeling.

• DFG Programme: Research Grants