Hierarchical Formation of Intrasplat Cracks in Thermal Spray Ceramic Coatings
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Lin Chen, Guan-Jun Yang, Cheng-Xin Li, and Chang-Jiu Li (Submitted January 31, 2016; in revised form April 19, 2016) Intrasplat cracks, an essential feature of thermally sprayed ceramic coatings, play important roles in determining coating properties. However, final intrasplat crack patterns are always considered to be disordered and irregular, resulting from random cracking during splat cooling, since the detailed formation process of intrasplat cracks has scarcely been considered. In the present study, the primary formation mechanism for intrasplat cracking was explored based on both experimental observations and mechanical analysis. The results show that the intrasplat crack pattern in thermally sprayed ceramic splats presents a hierarchical structure with four sides and six neighbors, indicating that intrasplat crack patterns arise from successive domain divisions due to sequential cracking during splat cooling. The driving forces for intrasplat cracking are discussed, and the experimental data quantitatively agree well with theoretical results. This will provide insight for further coating structure designs and tailoring by tuning of intrasplat cracks.
Keywords
crack morphology, crack patterns, hierarchical structure, intrasplat cracks, successive domain divisions
1. Introduction Thermal spraying is one of the most important approaches for depositing thermal barrier coatings, such as low-thermal-conductivity lanthanum zirconate (LZO) coating, for advanced gas turbines due to the low thermal conductivity resulting from a lamellar structure. Numerous intrasplat cracks and intersplat pores between lamellae are two essential features of thermally sprayed ceramic coatings (Ref 1, 2). In addition to the well-documented intersplat pores resulting from limited interlamellar bonding, intrasplat cracks also play an important role in determining coating properties, including thermal and mechanical properties. Although conventional coating structure models do not consider intrasplat cracks (Ref 3), their contribution to the thermal resistivity is estimated to be as high as 42-57% based on cutting of heat flow paths for a typical thermally sprayed ceramic coating (Ref 4). The much lower elastic modulus of coatings in the inplane directions is also mainly attributed to intrasplat cracks (Ref 5). In addition, the elastic modulus of coatings in the out-of-plane direction is determined by the pore network composed of both intersplat pores and intrasplat cracks (Ref 5). Therefore, study of the crack formation Lin Chen, Guan-Jun Yang, Cheng-Xin Li, and Chang-Jiu Li, State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xian Jiaotong University, Xian 710049, Shaanxi, Peoples Republic of China. Contact e-mail: [email protected].
Journal of Thermal Spray Technology
mechanism and cracking control approaches is essentially to meet the requirements of different coating applications. Although intrasplat cracks can be widely found in many publications, in-depth understanding of
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