Analysis of Residual Stress State in Thermal Barrier Coatings

Abstract

To enhance thermal efficiency of high-temperature structural system, thermal barrier coatings (TBC) are often applied on the cooled metallic substrate to extend the life of a component and/or to protect it from deterioration while increasing the operating temperature of the system. Current TBCs utilize a two-layer design methodology. Usually it consists of a zirconia ceramic top coat on an oxidation-resistant metallic (Ni-22Cr-10A1-1Y, for example) bond coat. When subjected to thermal cycling in service, however, an additional oxide layer is created on the bond coat at an interface with a topcoat, resulting in a 3 layer system. The residual stress field induced by the growing oxide scale can play an important role in spalling, delamination and failure of the TBC. A finite element analysis is performed to study the morphological effects of the thermally grown oxide (TGO) on the residual stress state of this three-layer heterogeneous elastic system subjected to cool down conditions. Specifically, the effects of the thickness and radii of curvature of the TGO on the maximum normal stress as well as first principal stress in the TBC have been systematically investigated. The results indicated that enhanced stresses are built-up at the interfaces between the TGO and the ceramic top coat as well as between TGO and the metallic bond coat, indicating the propensity of cracking over those locations. Further, higher thickness and lower radius of curvature of the TGO zone yield higher level of peak stress. These findings are consistent with experimental observations. The developed finite-element code can be used to predict reliability of the TBC in service against thermal fatigue.