Through-thickness dependence of in-plane cracking behavior in plasma-sprayed thermal barrier coatings
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I. INTRODUCTION
ADVANCED multilayered coating systems, made up of a ceramic thermal barrier coating (TBC) and metallic bond coating (BC), have been commonly applied by a plasma spraying (PS) method to protect hot-section parts in aerospace and land-based gas turbines. Yittria-stabilized zirconia (YSZ) has been used as a TBC material because of its low thermal conductivity and high toughness.[1] The TBCs processed by the PS method delaminate adjacent to the interface under thermomechanical stress cycling while in-service.[2,3,4] The TBC spallation causes a loss of the integrity of hightemperature components as the result of a sudden increase in the operating temperature of substrates and the exposure of BCs to elevated temperature environmental attack. As schematically illustrated in Figure 1, there are several factors controlling the TBC delamination. The PS processing requires the roughened interface of TBC and BC for its strong adhesion and results in the formation of splats, associated with in-plane and out-of-plane microcracks, due to pancakelike solidification of molten droplets.[5,6] The TBC possesses high porosity to further enhance the thermal shielding effect. In-service operation induces not only TBC densification and microcrack healing, leading to stiffening, but also formation of thermally ZHEHUA ZHANG, formerly Research Associate, United States Department of Energy, Ames Laboratory, Ames, IA 50011, is Project Engineer, Smith International Inc., Houston, TX 77205. J. KAMEDA, formerly Metallurgist, United States Department of Energy, Ames Laboratory, and Associate Director, United States Office of Naval Research Global–Asia, Tokyo 106, Japan, is Senior Fellow, Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104. Contact e-mail: [email protected] S. SAKURAI, Chief Engineer, is with Power System of Hitachi Ltd., Hitachi 317, Japan. M. SATO, formerly Director, Hachinohe Power Plant, Tohoku Electric Power Co., Hachinohe 031, Japan, is Director, Australian Coal, Japan Coal Development Co. Ltd., 105 Tokyo. Manuscript submitted March 8, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
grown oxide (TGO) near wavy TBC/BC interfaces due to high permeability of oxygen in porous YSZ. It is recognized that in-plane biaxial residual stresses in the TBC are generated during in-service thermal cycling, due to the difference in the coefficient of thermal expansion (CTE) between the ceramic YSZ and metallic BC/substrate.[7,8] Because of the complexity of TBC systems and in-service degradation, there is a need to design a special mechanical testing methodology, which is capable of examining the influence of the above-mentioned factors on the TBC delamination and ultimately leading to the development of better coating processing and systems. The TBC delamination is considered to occur as a sequence of two types of out-of-plane and in-plane cracking. In-plane TBC cracking near the interface is a more critical event for the delamination because the driving force is not as s
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