Effects of Interfacial Roughness, Oxide Film Thickness and Heterogeneous Bond Coat Microstructure on Spallation Mechanism in Plasma-Sprayed TBCs

A major failure mechanism in plasma-sprayed thermal barrier coatings is spallation of the top coat due to the top/bond coat thermal expansion mismatch concomitant with deposition-induced interfacial roughness, oxide film growth and creep-induced normal stress reversal at the rough interface’s crest. Reduction of the thermal expansion mismatch through the use of heterogeneous bond coats has been suggested to increase coating durability. This approach is examined using the higher-order theory for functionally graded materials. Specifically, combined effects of a graded bond coat microstructure and oxide film thickness on the crack-tip stress field in the vicinity of a rough top/bond coat interface are investigated during furnace-type thermal cycling in the presence of a local horizontal delamination situated within the homogeneous top coat at the rough interface’s crest. The analysis, which accounts for the creep/relaxation effects within the individual constituents, is conducted in two distinct ways. In the first approach, the bond coat’s heterogeneous microstructure is fully taken into account while in the second approach the bond coat’s microstructure is homogenized. The feasibility of using graded bond coat microstructures to reduce horizontal delamination driving forces is critically examined and the limitations of the homogenization-based approach are highlighted.