Edge Effect on Crack Patterns in Thermally Sprayed Ceramic Splats
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Edge Effect on Crack Patterns in Thermally Sprayed Ceramic Splats Lin Chen1 • Guan-Jun Yang1 • Cheng-Xin Li1 • Chang-Jiu Li1
Submitted: 22 June 2016 / in revised form: 27 September 2016 / Published online: 19 December 2016 Ó ASM International 2016
Abstract To explore the edge effect on intrasplat cracking of thermally sprayed ceramic splats, crack patterns of splats were experimentally observed and investigated through mechanical analysis. Both the polycrystalline splats and single-crystal splats showed obvious edge effects, i.e., preferential cracking orientation and differences in domain size between center fragments and edge fragments. In addition, substrate/interface delamination on the periphery was clearly observed for single-crystal splats. Mechanical analysis of edge effect was also carried out, and it was found that both singular normal stress in the substrate and huge peeling stress and shear stress at the interface were induced. Moreover, effective relief of tensile stress in splats is discussed. The good correspondence between experimental observations and mechanical analysis is elaborated. The edge effect can be used to tailor the pattern morphology and shed further light on coating structure design and optimization. Keywords crack morphology edge effect preferential orientation substrate stress shear and peeling stress
& Guan-Jun Yang [email protected] 1
State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, People’s Republic of China
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Introduction Because of the low thermal conductivity of lamellar coating structures, the thermal spray technique has become one of the most important approaches to depositing thermal barrier coatings for advanced gas turbines. Numerous interlamellar pores and intrasplat cracks are two essential characteristics of thermally sprayed ceramic coatings (Ref 1, 2). Although there have been many studies on intrasplat cracks, an in-depth understanding of the formation mechanism is not yet clear. To date, it is merely known that the existence of intrasplat cracks is usually due to stress relaxation during rapid cooling of ceramic splat (Ref 3-5). However, the final crack pattern is often considered to be disordered, because the segments are frequently regarded as having irregular morphology owing to the random appearance of intrasplat cracks (Ref 1, 6-8). Also, there is little information on how to effectively tailor the segment size. Where and how intrasplat cracks are initiated and propagated are also rarely reported. In a previous study, intrasplat crack patterns were generally found to present a hierarchical structure, with four sides and six neighbors, arising from successive domain divisions owing to both sequential cracking and geometric constraints (Ref 9). The intrasplat crack patterns of a single-crystal splat presented a perfect hierarchy, whereas the hierarchy was somewhat different for polycrystalline splat crack patterns owing to inter
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