Evolution of residual stress in environmental barrier coatings for ceramic matrix composites

陶瓷基复合材料（CMCs）是一种令人兴奋的用于燃气轮机发动机的新材料，其中需要一种环境屏障涂层（EBC）来保护CMCs免受水蒸气的侵害。该涂层还可作为热障涂层（TBC）。涂层的作用是增加使用寿命，也可以提供更高的工作温度，提高燃油效率。值得注意的是，环境屏障涂层是一种主要的依赖涂层（不像TBC），在使用中不能失败，因为没有它，CMC组件在恶劣的燃气涡轮发动机运行环境中的寿命将非常有限。众所周知，控制环境屏障涂层寿命的一个主要因素是其残余应力状态，这是由其通过热喷涂工艺制造的淬火过程和喷涂后的冷却造成的。涂层的应力状态也随着热处理和发动机运行等热漂移而变化。这是由于热膨胀系数（CTE）不匹配、热梯度、涂层结晶、相变和烧结等因素造成的。然而，对于这些因素是如何结合起来改变应力状态的，人们知之甚少，没有这些知识，目前就不可能预测它们的进化，也不可能预测对EBC寿命的影响。该项目的目的是为涂层应力随时间和温度的演变发展新的认识和模型。目标包括对EBC结晶、相变、烧结和热机械载荷对涂层内应力及其完整性的影响的新观察。该项目将采用新颖的实验方法，以高分辨率研究应力和应变状态，包括同步加速器x射线衍射，同步加速器和实验室x射线计算机断层扫描，以及使用聚焦离子束铣削的微尺度钻孔，通过2D和3D数字图像相关分析。该研究将主要使用热处理涂层的非原位观察，但在英国国家设施使用同步加速器x射线可以实现原位研究。结果将通过模拟中的有限元方法建模来解释，该模拟将包含热物理过程，例如基材和EBC之间的粘结层的蠕变。该项目属于EPSRC材料工程-陶瓷研究领域，在本网站https://epsrc.ukri.org/research/ourportfolio/themes/上列出的工程主题中。这是一个iCASE项目，得到了劳斯莱斯公司的支持。

Ceramic matrix composites (CMCs) are exciting new materials for use in gas turbine engines, in which an environmental barrier coating (EBC) is needed to protect the CMCs from water vapour attack. The coating also serves as a thermal barrier coating (TBC). The coating acts to increase the operating lifetime and can also offer higher operating temperature with improved fuel efficiency. Significantly, the environmental barrier coating is a prime reliant coating (unlike a TBC), that must not fail in service, since without it the CMC components would have very limited life in the harsh gas turbine engine operating environments. It is known that one major factor governing the life of an environmental barrier coating is its residual stress state, which results from the quenching process of its manufacture via the thermal spray process and the cooling after spraying. The coating stress state also changes with thermal excursions such as heat-treatment and engine operations. This is because of factors that include the mismatch of coefficients of thermal expansion (CTE), thermal gradients, coating crystallization, phase change and sintering, among others. However, there is little knowledge of how these combine to change the stress state, without which it is not currently possible to predict their evolution, nor the effect on the EBC lifetime. The aim of this project is to develop new understanding and models for the evolution of coating stress with time and temperature. The objectives include new observations of the effects of EBC crystallization, phase change, sintering, and thermal mechanical loading on the stresses within the coating and its integrity. The project will apply novel experimental methodologies to investigate the state of stress and strain at high resolution, including synchrotron X-ray diffraction, synchrotron and laboratory X-ray computed tomography, and micro-scale hole-drilling using focused ion beam milling, analysed by 2D and 3D digital image correlation. The study will mostly use ex situ observations of thermally treated coatings, but in situ studies may be achieved using synchrotron X-rays at UK National Facilities. The results will be interpreted using modelling by the finite element method in simulations that will incorporate thermophysical processes such as creep of the bond coat between the substrate and the EBC. This project falls within the EPSRC Materials engineering - ceramics research area' in the Engineering theme listed on this website https://epsrc.ukri.org/research/ourportfolio/themes/ This is an iCASE project, with the support of Rolls-Royce Plc.