层级热障涂层中弱界面裂纹的相互竞争机理

The application of multi-ceramic-layer thermal barrier coating (TBC) is an important way of solving the thermal barrier problems in gas turbine engines operating at ultra-high temperature condition. However, the weak interfaces between ceramic layers are one of the main sources leading to the failure of coating system. The competitive behavior of these weak-interface cracks makes the failure mechanism of hierarchical TBC much more complex than the conventional single-ceramic-layer TBC. In this proposal, we aim to understand the competitive mechanism of weak-interface cracks in double-ceramic-layer TBC, which is composed of a conventional ceramic layer and a new thermal insulation material top-coated layer, and to further preliminarily establish failure criteria for the coatings. We will develop a crack propagation numerical algorithm to deal with problems of multiple interfacial cracks propagating along arbitrary path in heterogeneous materials. The numerical analysis and in-situ fracture experiments will be performed for investigation of initiation and propagation behavior of the weak-interface cracks, and the influence of hierarchical component features such as the geometrical, material and interfacial morphology on the crack propagation path. The interaction of crack tip driving force among different weak interfaces will be analyzed to realize the mechanism controlling the cracks competing with each other. Understanding competitive mechanism of weak-interface cracks will provide theoretical basis for the structural design and reliability evaluation of advanced multi-ceramic-layer TBC.

层级热障涂层是解决重型燃气轮机高温部件超高温热障的重要手段，陶瓷层级间的弱界面会对涂层系统的失效机理产生显著影响。因此，研究涂层中弱界面裂纹间的竞争机理对分析涂层系统失效至关重要。本项目以新型耐高温材料涂覆于传统陶瓷涂层上形成的双陶瓷层热障涂层为对象，发展处理异质材料界面多裂纹任意路径扩展数值算法，结合原位观测断裂实验，研究层级热障涂层中弱界面裂纹的萌生及扩展行为，考察层级组元的几何、材料、界面形貌等特征对弱界面裂纹扩展路径的影响机制，分析不同弱界面处裂纹裂尖驱动力的演变规律，揭示弱界面裂纹间的竞争机理，初步建立层级热障涂层失效准则。本项目的研究可为新型热障涂层结构设计和强度评价提供理论基础。

层级热障涂层中陶瓷层间的弱界面显著影响涂层内裂纹的萌生与扩展，不同弱界面裂纹的相互竞争将改变涂层系统的脱粘失效机制。本项目采用数值与试验相结合的方法，研究了热生长氧化物（TGO）形貌、涂层/基底蠕变、服役条件等因素对层级热障涂层应力演化的影响规律，获得了裂纹萌生机理；开发了多界面裂纹扩展数值算法，阐明了层级热障涂层组元特征对弱界面裂纹扩展行为的影响，建立了涂层系统增韧结构设计机制图。.研究表明：（1）层级热障涂层在长时氧化过程中，由于高温蠕变的作用，将使涂层系统在高温下形成无应力状态，而在服役冷却阶段，残余应力几乎不受TGO生长的影响，TGO生长导致的非弹性变形才是涂层裂纹萌生的主要原因，而非传统认为的高温生长应力；（2）粗糙界面导致的应力集中主要集中在局部区域范围，当内陶瓷层厚度足够厚时（大于90 μm），TGO引起的应力集中区域与陶瓷层间粗糙界面的应力集中区域互不影响；（3）双陶瓷层热障涂层热震寿命、剥离速率及失效模式均与两层陶瓷层的厚度比密切相关，随着外陶瓷层与内陶瓷层厚度比增加，涂层系统的失效模式逐渐从外陶瓷层的层状剥落，转变成陶瓷层间弱界面附近处的剥离；（4）表面裂纹密度是影响层级热障涂层界面裂纹扩展行为的关键因素，当表面裂纹密度较低时，界面裂纹尖端驱动力对陶瓷层之间的材料性能和几何参数十分敏感，即陶瓷层总厚度较厚、外陶瓷层的厚度比较高、或外陶瓷层模量较高时，均会使得界面裂纹驱动力增加，导致涂层过早剥落；然而，当垂直裂纹密度足够高时，上述影响将会变的十分微弱；因此，在双陶瓷层热障涂层内主动预制足够密集的垂直裂纹，可允许使用更厚和刚度更高的外陶瓷层，从而获得更好的隔热性能;（5）当粘结层具有中等程度的断裂韧性时，陶瓷层内的垂直裂纹首先偏转进入陶瓷层与粘接层界面，在该界面裂纹扩展过程中，粘接层内会发生垂直裂纹萌生，使两个界面同时发生开裂，弱界面裂纹的竞争扩展可一定程度提高涂层应变容限。