|Thesis Advisor||魏悦广 ; 梁立红|
|Place of Conferral||北京|
|Keyword||陶瓷涂层体 ,断裂机制 内聚力模型 损伤演化 ceramic coating system fracture mechanism cohesive zone model damage evolution|
陶瓷涂层由于具有良好的隔热性能被广泛应用在发动机的叶片、近空间飞行器的表面上，保护金属基底以防基底在高温下失效。陶瓷涂层服役环境中经常有热震现象发生，极易导致涂层内部开裂和界面层裂，一旦涂层剥落，基底将会暴露在高温工作环境中。因此研究陶瓷涂层体系的失效机制和损伤演化规律非常重要。以往的研究表明涂层厚度和微结构对陶瓷涂层体系抗热震性能有很大的影响。本文通过设计相关实验，应用热力等效原理，用机械载荷作用下的弹性失配来模拟热载荷作用下的热失配。通过常温下的在位弯曲实验以及数值模拟来研究不同涂层厚度和微结构下体系的损伤演化规律和失效模式，对陶瓷涂层体系的设计提供一定的指导。主要的结论如下：(1) 通过在位梁弯曲实验，同步监测陶瓷涂层体系裂纹扩展过程。实验结果表明体系的破坏模式依赖于涂层的厚度，三点弯曲实验下，薄涂层体系主要由横向裂纹主导失效；厚涂层体系主要由界面层裂主导失效。而四点弯曲实验下，薄厚涂层体系纯弯曲段均为横向裂纹主导失效，随着涂层厚度增加，横向裂纹数目减少，平均裂纹间距增大。(2) 通过在涂层内部和界面处布置大量内聚力单元，利用内聚力模型的弱化特征来表征体系失效的全过程。本文建立了一系列不同层厚度的计算模型，来研究陶瓷涂层体系在弯曲载荷作用下破坏模式的厚度依赖性。模拟的载荷位移曲线、破坏模式与实验结果一致，表明内聚力模型能有效地模拟陶瓷涂层体系的断裂特征。(3) 通过在位圆薄板弯曲实验，同步监测陶瓷涂层体系裂纹扩展过程。实验结果表明体系的破坏模式依赖于涂层的厚度，薄涂层为拉伸引起的表面失稳（径向和环向）主导失效，厚涂层为径向表面拉伸失稳和界面层裂共同主导。随着涂层厚度增加，径向和环向裂纹数目减少，当涂层厚度达到一定值时，涂层内部只径向开裂且界面发生层裂。随着载荷增大，径向和环向裂纹向外扩展，涂层表面破坏区域增大。通过类比Mises屈服条件，建立了涂层表面拉伸失稳的破坏准则，来表征破坏区域半径随载荷的变化规律。体系的破坏模式依赖于涂层的微结构，纳米颗粒涂层体系的抗弯性能高于传统微米颗粒涂层体系，因此涂层中的纳米结构有助于延长其使用寿命。灾变损伤模型和基于能量分析得到的损伤模型都能有效刻划陶瓷涂层体系的损伤演化规律。两种模型均表明在临界载荷附近，损伤急剧增加，表现出灾变失效特征。研究结果表明损伤速率依赖于应力状态及应力局部化程度。剪切主导的界面层裂比拉伸主导的涂层强度破坏损伤快；三点弯曲应力局部化程度比四点弯曲高，损伤快。
Thermal barrier coatings (TBC) with low thermal conductivity provide excellent thermal protection and wear resistance and have been widely used in aircraft and blades of gas turbines to protect alloy substrates from the high–temperature environment. Thermal shock usually occurs among service conditions of TBC, which leads to cracking within the coating and interfacial delamination. Once spallation of the TBC occurs, the substrate becomes exposed to the high–temperature environment and operation becomes impossible. Therefore, investigating the damage evolution behavior and fracture mechanism of TBC systems is important. Previous studies have shown that the coating thickness and microstructure of TBC have a great impact on thermal shock resistance. In this paper, some mechanical loading tests were designed based on the principle of equivalent thermal and mechanical energy, therefore, the thermal mismatch under thermal loadings can be simulated by elastic mismatch under mechanical loadings. The damage evolution behavior and failure modes of TBC systems with different coating thicknesses and microstructures were studied by in situ bending experiments at room temperature and numerical simulations, which provided some guidance for the design of ceramic coating systems. The main conclusions are as follows:(1) The process of crack propagation of ceramic coating system was monitored synchronously by in situ bending experiments of beams. The experimental results showed that failure modes of coating systems depended on the coating thickness. The thin coating systems were dominated by transverse cracks, and the thick ones were dominated by interfacial delamination under three–point bending tests. While the pure bending section of both the thin and thick coating systems were dominated by transverse cracks under four–point bending tests. With increasing coating thickness, the number of transverse cracks decreased and the average crack spacing increased.(2) A large number of CZM elements were inserted in the coating as well as on the interface to characterize the whole cracking process of the system by taking advantage of the weakening part of CZM. In this paper, a series of calculation models with different coating thicknesses were established to study the thickness dependence on the failure mode of ceramic coating systems under bending loadings. The simulated load–displacement curves and failure modes were consistent with the experimental results, which indicated that CZM can effectively simulate the fracture characteristics of the ceramic coating systems.(3) The process of crack propagation of ceramic coating system was monitored synchronously by in situ bending experiments of circle thin plate. The experimental results showed that failure modes of coating systems depended on the coating thickness, the thin coating systems were dominated by surface instability (radial and circumferential directions) caused by tensile stress, and the thick coating systems were dominated by surface instability of radial direction and interfacial delamination. With increasing coating thickness, the number of radial and circumferential cracks decreased, when the coating thickness reached a certain value, the coating only cracked in the radial direction, and interface cracking occurred. With increasing load, radial and circumferential cracks propagated so the failure area of coating increased. The failure criterion of tensile instability of coating surface was established by an analogy of Mises yield condition to characterize the variation of the radius of the failure region with applied load. The failure modes of the systems depended on the microstructure of the coatings. Bending resistance of the nanostructured coating systems is stronger than that of the conventional coating systems. Therefore, nanostructures in the coating can help to prolong its service life.(4) Both the catastrophic damage model and damage model based on the energy analysis can effectively characterize the damage evolution of the ceramic coating systems. Both models showed that the damage increased sharply near the critical load and exhibited catastrophic failure characteristics. The results showed that the damage rate depended on the stress state and degree of stress localization. Interfacial cracking dominated by shear stress damaged faster than that of cracking in coating dominated by tensile stress. Stress localization under three–point bending was larger than that of four–point bending, so it damaged quickly.
|刘海燕. 微纳米颗粒陶瓷涂层体系损伤演化规律与失效机制研究[D]. 北京. 中国科学院大学,2016.|
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