In-Plane Cracking Behavior Near and Away from Interface of Thermal Barrier Coatings and Thermally Grown Oxides
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In-Plane Cracking Behavior Near and Away From Interface of Thermal Barrier Coatings and Thermally Grown Oxides Z. Zhang*, J. Kameda*#, A. H. Swanson* and S. Sakurai$ *Ames Laboratory, Iowa State University, Ames IA, USA # Office of Naval Research International Field Office, Tokyo, Japan $ Mechanical Engineering Research Laboratory, Hitachi Ltd., Hitachi, Japan ABSTRACT The initiation characteristics of in-plane cracks near and away from the interface of thermal barrier coatings (TBC) and thermally grown oxides (TGO) have been studied using a protruded four-point bend testing technique together with a finite element analysis. In-plane TBC cracks were initiated near and away from the TBC/TGO interface, respectively, in protruded specimens without and with grooved substrates. It was shown that the onset of in-plane TBC cracks near or away from the interface in the protruded TBC tests was controlled by the out-of-plane tensile stress but not by the principal tensile stress acting upon an inclined plane to the interface. The critical local tensile stress for the initiation of TBC cracks near the interface was found to be 20% lower than that away from the interface. The TBC cracking near and away from the TBC/TGO interface is discussed in light of the residual stress distribution through the TBC thickness. INTRODUCTION Thermal barrier coatings (TBC), made up of Y2O3 stabilized ZrO2, (YSZ), and metallic bond coatings (BC) commonly have been applied by a plasma spraying (PS) method to protect hot-section components in aerospace and land-based gas turbines. It is recognized [1-3] that TBC processed by the PS method delaminates while in-service under thermal/mechanical stress cycles as a result of in-plane cracking in TBC adjacent to the TBC/BC interface. There are two major reasons for the TBC delamination. First, the build-up of residual stresses occurs during thermal cycling due to the difference in the thermal expansion coefficient between TBC and BC/ substrate. Secondly, high oxygen permeability of YSZ results in the formation of thermally grown oxide (TGO), made up of alumina, near the TBC/BC interface. In order to maintain the integrity of advanced gas turbines and to develop better TBC systems, it is necessary to examine the characteristics of TBC delamination induced while in-service. For this purpose, several mechanical testing methods, such as notched bending test [4], pull test [5], and protruded compressive test [6] have been carried out. Yet, the mechanisms of TBC delamination, such as the effects of the residual stress and TGO, have not been fully understood, due to lack of experiments controlling the in-plane TBC cracking and detailed mechanistic analysis. In this paper, in-plane TBC cracking behavior is studied using a protruded four-point bend testing method recently developed [7-10]. Different features of TBC cracking near and away from the TBC/BC interface are shown using protruded TBC specimens without and with grooved substrates. By applying a finite element method (FEM), the critical local tensile stres
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