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

为了提高透平的运行温度以提高效率，从而降低燃油消耗和二氧化碳排放，需要新型高温稳定的陶瓷材料作为热障涂层。热障涂层保护涡轮机最热区域的金属结构材料。对于这种高温应用，对所使用的材料系统的全面了解是必不可少的，包括关于相稳定性和亚稳态和稳定相形成的驱动力的知识。因此，本项目的目的是对具有应用前景的材料体系--ZrO2-HfO2-Y2O3-Ta2O5进行热力学研究和模型化，以阐明其稳定机理。为此，使用了先进的热力学模型。通过将关键实验与体系中可能存在的相的热力学模型相结合，可以得到对多元体系的一致的热力学描述，因此是作为“综合计算材料工程”(ICME)组成部分的材料开发的无价工具。在材料体系中，由于其具有低导热系数、相稳定性高达至少1500°C以及可与最先进的热障涂层材料Y_2O_3稳定的氧化锆相媲美的性能，在材料体系中有望成为下一代热障涂层材料。在第一个资助期，通过热化学和相图研究相结合的方法，阐明了等摩尔掺杂线上的相的稳定机制。在该项目的继续进行中，材料系统被HfO2扩展，因为这允许产生具有更低的导热系数、适应的热膨胀系数和改善高温下的热化学稳定性的组合物。用X射线衍射法、热分析法和电子显微镜结合元素分析等方法测定了ZrO2-HfO2-Y2O3-Ta2O5及其各亚系的相平衡和相稳定性。稳定和亚稳态样品的标准生成热是由高温溶液量热法得到的溶液生成热计算出来的。热容是用差示扫描量热法测量的，它反映了吉布斯能量的温度依赖关系，因此直接有助于了解组合物的热行为。对于多组分材料体系中的每个稳定相和亚稳态相，根据结晶学信息选择热力学模型，并使用CALPHAD方法估算热力学参数，以深入了解稳定化效果、相关系和组成。实验结果直接纳入到热力学模型中。

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.