Analysis of spallation mechanism in thermal barrier coatings with graded bond coats using the higher-order theory for FGMs

A major failure mechanism in plasma-sprayed thermal barrier coatings (TBCs) 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 interfaceʹs crest. Experiments indicate that reduction of the thermal expansion mismatch by embedding alumina particles in the bond coat may increase coating durability. This approach is examined using the higher-order theory for functionally graded materials (FGMs). 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 thermal cycling in the presence of a local horizontal delamination situated within the homogeneous top coat at the rough interfaceʹs crest. The analysis, which accounts for the high-temperature creep/relaxation effects within the individual constituents, is conducted in two distinct ways which reflect the present dichotomy in the modeling of FGMs. The coupled approach explicitly accounts for the micro–macrostructural interaction due to the particlesʹ presence and is an intrinsic feature of the higher-order theory. The uncoupled approach, which is the prevailing paradigm in the mechanics/materials communities, replaces the actual graded microstructure with a fictitious material having continuously or discretely varying homogenized properties. These homogenized properties are then used in the structural analysis of a TBC without regard for particle–particle or particle–interface interactions. 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.