Thermal barrier coatings (TBCs) are used to protect the hot sections of gas turbine and jet engines. A TBC system comprises of a substrate, bond coat (BC), and TBC top coat (TC). The residual stress development mechanism by high temperature exposure in TBC is important in designing a high-performance TBC. However, quantitative studies of the stress change and its modeling are few because of its difficulty. The objective of this study is to reveal the changing mechanism of coating stress under high temperature exposure. For this purpose, we applied a three-layered beam model to evaluate the TBC's residual stress using the curvature change. Time-dependent residual stresses in the TC and BC thermally exposed at 600-1000°C were evaluated by the curvature method. Subsequently, we investigated the stress-generating mechanism of the coatings by using a finite element analytical (FEA) model that reproduces the measured curvatures. Our experimental result revealed that the residual stress in the BC changed from tensile to compressive by thermal exposure. However, thermal exposure had an insignificant effect on the residual stress in the TC. These changes in coating stress, including temperature and time dependency, were consistently explained by stress relaxation in the BC using the FEA model.This article is part of a special topical focus in the Journal of Thermal Spray Technology on Advanced Residual Stress Analysis in Thermal Spray and Cold Spray Processes. This issue was organized by Dr.
A thermal barrier coating (TBC) system consists of a TBC topcoat (TC), a bond coat (BC), and a substrate. Residual stress acting on a coating is a important parameter for materials mechanics and hence need to be evaluated. We propose a simple method for evaluating the residual stresses acting on each layer of a TBC system by using a curvature method. First, a three-layered model was constructed based on the misfit strain between the coatings and the substrate. This method uses the curvatures of the substrate with a blast treatment, the BC system specimen with the blast and BC, and the TBC system specimen with the blast, BC, and TC instead of a change in curvature during thermal spraying. Next, the residual stresses acting on the TBC system specimen with varying thicknesses were evaluated by using the proposed method. It was confirmed that the proposed method provides reasonable residual stresses in the BC and TC. Finally, the thermal stress after the coating deposition was theoretically evaluated, and the quenching stress of the coating during the deposition was evaluated by using the difference between the experimentally determined residual stress and the theoretical thermal stress. The mechanism for generating the residual stress in the TBC system during thermal spraying was investigated using the quenching stress and thermal stress.
Thermal barrier coating (TBC) is a key technology for prolonging the life of the hot sections of gas turbine and airplane engines. A TBC system has a three-layered structure comprising a substrate, bond coat (BC), and TBC topcoat (TC). Young's moduli and Poisson's ratios of the coatings are important for calculating the parameters of material mechanics in the TBC system. However, research on Poisson's ratio is scarce owing to the difficulty of its evaluation. The objective of this paper is to investigate the effect of high temperature exposure on Poisson's ratios of yttria-stabilized zirconia TC and CoNiCrAlY BC. For this purpose, we evaluated the time-dependent Young's moduli and Poisson's ratios of coatings thermally treated at 600°C to 1000°C using the bending method. Our measurement revealed that an increase in Poisson's ratios of the TC and BC as well as Young's moduli depended on the temperature and length of exposure. The increasing rate in Poisson's ratios was lower than that of Young's moduli for both the TC and BC. These phenomena were explained by focusing on the microcracks and pores of the coatings.
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