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Residual Stress Change in Thermal Barrier Coating Due to Thermal Exposure Evaluated by Curvature Method Tomoyuki Hayase1 • Hiroyuki Waki2 • Kanta Adachi2

Submitted: 29 November 2019 / in revised form: 27 March 2020 Ó ASM International 2020

Abstract 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 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. Vladimir Luzin, Australian Centre for Neutron Scattering; Dr. Seiji Kuroda, National Institute of Materials Science; Dr. Shuo Yin, Trinity College Dublin; and Dr. Andrew Ang, Swinburne University of Technology. & Hiroyuki Waki [email protected] 1

Graduate School of Engineering, Iwate University, 4-3-5 Ueda, Morioka, Iwate 020-8551, Japan

2

Department of Systems Innovation Engineering, Iwate University, 4-3-5 Ueda, Morioka, Iwate 020-8551, Japan

changes in coating stress, including temperature and time dependency, were consistently explained by stress relaxation in the BC using the FEA model. Keywords curvature method  creep  finite element modeling  MCrAlY  residual stress  thermal barrier coating  Yttria-stabilized zirconia

Introduction Thermal barrier coatings (TBCs) play a critical role in the hot sections of gas turbines and jet engines. They protect the underlying substrate from hot combustion gas by maintaining lower temperatures as a result of their low thermal conductivity. This leads to a more efficient turbine engine owing to the higher gas inlet and operating temperatures. A typical TBC system comprises of a substrate, metallic bond coat (BC), and ceramic top coat (TC), where MCrAlY alloys (M represents Co and/or Ni) and yttriastabilized zirconia (YSZ) are used as BC and TC, respectively (Ref 1, 2). The residual stress has a strong influence on coating durability through its impacts on characterist