Modeling Effects of Material Properties and Three-Dimensional Surface Roughness on Thermal Barrier Coatings
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MODELING EFFECTS OF MATERIAL PROPERTIES AND THREE-DIMENSIONAL SURFACE ROUGHNESS ON THERMAL BARRIER COATINGS Michael L. Glynn, K.T. Ramesh, P.K. Wright* and K.J. Hemker Johns Hopkins University, Mechanical Engineering, Baltimore, MD 21218 *GE-Aircraft Engines, Materials and Process Engineering Department, Cincinnati, OH 45215 ABSTRACT Thermal barrier coatings (TBCs) are known to spall as a result of the residual stresses that develop during thermal cycling. TBC’s are multi-layered coatings comprised of a metallic bond coat, thermally grown oxide and the ceramic top coat, all on top of a Ni-base superalloy substrate. The development of residual stresses is related to the generation of thermal, elastic and plastic strains in each of the layers. The focus of the current study is the development of a finite element analysis (FEA) that will model the development of residual stresses in these layers. Both interfacial roughness and material parameters (e.g.,modulus of elasticity, coefficient of thermal expansion and stress relaxation of the bond coat) play a significant role in the development of residual stresses. The FEA developed in this work incorporates both of these effects and will be used to study the consequence of interface roughness, as measured in SEM micrographs, and material properties, that are being measured in a parallel project, on the development of these stresses. In this paper, the effect of an idealized three-dimensional surface roughness is compared to residual stresses resulting from a grooved surface formed by revolving a sinusoidal wave about an axis of symmetry. It is shown that cylindrical and flat button models give similar results, while the 3-D model results in stresses that are significantly larger than the stresses predicted in 2-D. INTRODUCTION Thermal barrier coating (TBC) systems are comprised of four basic layers. The superalloy substrate, the bond coat, the oxide layer and the top coat all form the TBC system. Because of the interaction between these layers, the mechanical properties of one layer can effect the development of stresses in another layer. For this reason, a finite element model was developed to track the stresses that arise in each layer as a result of thermal cycling. Early TBC development equated durability of the TBC system to bond coat oxidation resistance [1]. However, the mechanical properties of the bond coat can also influence TBC life; a strong correlation between calculated out-of-plane tensile residual stresses in the top coat and TBC life has been presented [2]. In this light, it appears important to determine what contributes to the development of these residual stresses. If all layers remain elastic, no residual stresses will arise. Consequently, at least one layer must behave inelastically in order for residual stresses to develop. Additionally, if all interfaces remain straight, no significant out-of-plane residual stresses will develop. Therefore, it is the combination of an interface roughness and plasticity that leads to significant out-of-plane residual
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