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

为了通过提高工作温度来提高涡轮机的效率，从而降低燃料消耗和二氧化碳排放，需要新的高温稳定陶瓷作为热障涂层。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，全面了解所使用的材料系统是必不可少的，包括有关相稳定性以及亚稳相和稳定相形成的驱动力的知识。因此，本项目的目的是对有前途的材料体系ZrO2-HfO2-Y2O3-Ta2O5进行热力学研究和建模，以阐明其稳定机制。先进的热力学模型用于此目的。通过将关键实验与系统中可能存在的相的热力学模型相结合，可以生成多组分系统的一致热力学描述，因此是材料开发的宝贵工具，是“集成计算材料工程”（ICME）的一个组成部分。材料体系中的组合物ZrO2-Y2O3-Ta2O3是下一代热障涂层的有前途的材料，因为它们具有诱人的性能，如低导热性，高达至少1500°C的相稳定性以及可与最先进的热障涂层材料氧化钇稳定氧化锆相媲美的机械性能。通过热化学和相图研究，阐明了ZrO2-YTaO4等摩尔掺杂线上相的稳定机制。在项目的延续中，HfO2扩展了材料系统，因为这允许产生具有更低导热系数的组合物，适应热膨胀系数并改善高温下的热化学稳定性。采用x射线衍射、热分析和电子显微镜元素分析等方法测定了ZrO2-HfO2-Y2O3-Ta2O5材料体系及其子系统的相平衡和相稳定性。稳定和亚稳样品的标准生成焓由高温溶液量热法得到的溶液焓导出。热容是通过差示扫描量热法测量的，它反映了吉布斯能的温度依赖性，从而直接有助于理解组合物的热行为。针对多组分材料体系中的每一个稳定相和亚稳相，基于晶体学信息选择了热力学模型，并利用calphhad方法对热力学参数进行了评估，以深入了解稳定效应、相关系和组成。实验结果直接纳入热力学模型。

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.