An Overview on Failure Behavior Study of Thermal Barrier Coatings Based on Cohesive Zone Model

Thermal barrier coatings (TBCs) hold widespread applications in gas turbines and aero engines because of their excellent thermal insulation, wear and corrosion resistance, which can improve engine thermal efficiency and prolong the service life of turbine blades. Generally, the failure of thermal barrier coatings is caused by the crack propagation, and the major failure modes consist of surface cracking and interface dela-mination. Concerning the crack propagation behavior of coatings, the most important and direct research approaches is to numerically simulate the entire failure process of the thermal barrier coatings, so as to acquire deep understanding of the mechanism of coating failure. The cohesive zone model (CZM) is capable of describing the interface cracking problem accurately, reducing or even relieving the stress singularity at the crack tip, and simulating the propagation of any crack without defining the pre-crack. When CZM is employed to simulate the process of surface cracking and interfacial delamination of the thermal barrier coatings, the cohesive unit is usually embedded between solid elements where cracks may occur. Once the material is destroyed, the cracks will form and expand along the direction of the cohesive unit. Nevertheless, the damage failure of TBCs are affected by diverse factors accompanied by complex failure mechanism. The variety of failure modes and the insufficiency of the CZM itself make CZM fail to describe the whole picture in study of thermal barrier coatings failure. At present, the simulation of damage failure process of TBCs has been realized by CZM, including the surface cracking process and the interface layer failure process. However, the majority of current studies regard the TBCs as homogenous materials and neglect the micro-defects inside the coating. Moreover, there is still some problems in CZM itself, such as the determination of parameters. With the advance of the TBCs and CZM, and the deepening of understanding to CZM, the simulation of failure process of TBCs by the CZM has been developed and improved continuously. Regarding to the simulation of surface cracking, the CZM is embedded vertically in the ceramic coating to simulate the expansion beha-vior of the internal surface cracks in the ceramic layer. Concerning the simulation of the crack growth behavior inside the ceramic coatings, the extended finite element method is commonly adopted, while CZM is seldom used. The CZM can effectively solve the problem of interface cracking, especially the bonding layer/ceramic interface cracking problem, it is widely employed in the study of interface failure problem of TBCs. In this article, we give a brief introduction of the CZM, summarize the progress of CZM in simulating the failure behavior of the TBCs, and finally point out problems in the current research and the future development direction. © 2019, Materials Review Magazine. All right reserved.