-1- Effect of interfacial defect on surface crack behavior of an air plasma sprayed thermal barrier coating Xu Rong, Zhang Weixu, Fan Xueling, Wang Tiejun* State Key Laboratory for Strength and Vibration of Mechanical Structures, School of Aerospace Engineering, Xi’an Jiaotong University, Xi’an 710049, China ∗ Corresponding author: wangtj@mail.xjtu.edu.cn Abstract Multiple surface cracking has significant effect on the failure of thermal barrier coating system (TBCs). The surface cracking is influenced by interface defects. It is of significance to study the relationship between the interface defect and the surface crack. In present work, the effect of interface defect on the surface crack behavior of TBCs has been investigated by using finite element method. The strain energy release rate (SERR) of the surface vertical crack is obtained as functions of the interfacial defect length and location. The results show that both the defect location and the defect length have a significant influence on the driving force of surface crack. It is concluded that the interface defect under the surface crack may induce a more intense facilitation on the surface cracking behavior than the remote defect. Moreover, a large scale interface defect may also induce a larger SERR of the surface crack which may promote its propagation. Keywords Thermal barrier coating, Surface crack, Interfacial delamination, Strain energy release rate 1. Introduction Thermal barrier coating systems (TBCs) are widely used in gas turbines or diesel engines due to their excellent heat-insulating property, which can efficiently prolong the life of the turbines blades[1, 2]. The major role contributing to the heat-insulating is the top layer of the TBCs, a ceramic coat with low thermal conductivity, which provides a temperature reduction of the metallic substrate[3, 4]. In order to protect the substrate from oxidation and make a better bond strength of the top coat to the substrate, a metallic bond coat is deposited between the top coat and the substrate. The top ceramic coat, metallic bond coat and substrate build up the typical TBCs. Due to the mismatch of the geometric dimensions and the material properties, some fracture phenomena are inevitable in the TBCs[5, 6]. The common fracture behaviors observed during the service are the multiple surface cracks and the interface cracks[7, 8]. Considerable work was focused on the surface cracking behavior in the film/substrate system. Schulze and Erdogan[9] analyzed the periodic cracking of an elastic coating bonded to a homogeneous substrate. Vlassak[10] studied the channel cracking in thin films on substrates of finite thickness in order to simulate the segmentation of the ceramic layer of TBCs. Recently, by using finite element method, Fan et al.[11, 12] and Zhang et al.[13] built a three-layer model to investigate the effects of several important characteristic parameters, e.g. the interface roughness, thermally grown oxide (TGO), etc., on the periodic surface crack behavior. However, it is assumed in most of the aforementioned investigations that the ceramic/bond coat interface of TBCs is perfect, which ignores the fact that interface cracks may be initiated due to stress concentration. The interface cracks have been frequently observed during the service, which lead to the spalling of ceramic coating and eventual failure of TBCs. The analyses and explanations for the appearances of interface cracks were presented by many researchers. Rice[14] developed the elastic fracture mechanics concepts for interfacial cracks between dissimilar solids, which made a great contribution to the interface cracks analysis in TBCs. Mumm and Evans[15] focused on the interface crack induced by the TGO growth. In their study, the morphological imperfections in the TGO played a great role in the failure of TBCs. Another significant reason for the initiation of interface cracks is the natural restrictions of depositing process especially the air plasma spraying. Currently, it is generally accepted that the ceramic coating of air plasma spraying TBCs exhibits laminar structural characteristic. Pores and micro-cracks are inevitable due to the internal thermal
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