Analysis of Residual Stress State in Thermal Barrier Coatings
Tze-Jer Chuang, Lin-Sien H. Lum
To enhance thermal efficiency of a system in high temperature structural applications, thermal barrier coatings (TBC) are often applied on the metallic substrate to protect the component 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 top of an oxidation-resistant metallic Ni-22Cr-10Al-1Y bond coat. When subjected to thermal cycling in service, however, one additional oxide layer is created, resulting in a 3 layer system. The residual stress field induced by the growing oxide scale plays a very 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 subject 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 the peak stresses occur 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 failure over there. Further, higher thickness and lower radius of curvature of the TGO yield higher level of peak stress. These findings are consistent with experimental observations.