Local stress around cap-like portions of anisotropically and nonuniformly grown oxide layer in thermal barrier coating system

The intrinsic deformation accompanying the growth of thermally grown oxide (TGO) can induce significant local stress potentially causing interfacial delamination and coatings fracture in a thermal barrier coating system (TBCs). Multiple mechanisms can be involved in a TGO growth process which is sensitive to the reactive elements contained in the coatings, and as a result anisotropic and nonuniform growth deformation can be produced in the TGO layer. The objective of this study is to analytically and numerically investigate the oxide-growth-induced local stress around the cap-like portions of a TGO layer having grown to a certain thickness and furthermore demonstrate the associated micro-crack patterns. A sphere model is proposed to analytically derive the elastic and elastic-plastic solutions of the stress field, which takes into account the anisotropy and nonuniformity of growth strain as well as the yielding of coating materials. On the other hand, finite element analysis is carried out to consider more realistic undulation morphology of the TGO layer and to verify the analytical prediction. It is seen that the through-thickness and lateral components of the anisotropic growth strain compete in the stress generation and there exist critical conditions for the dominance of different growth strains. The effect of growth strain gradient is examined to disclose the consequence of TGO dominant growth at the TGO/BC (bond coat) interface. The effects of the roughness and thickness of TGO and the plastic behaviour of different coating layers are also analysed. Finally, the possible micro-crack patterns due to TGO growth in typical TBC systems are illustrated with suggestions about how to reduce the driving force for the related structural failure.