The impact of the growth of thermally grown oxide layer on the propagation of surface cracks within thermal barrier coatings

Failure mechanisms of thermal barrier coating system (TBCs) are complicated, wherein spalling of ceramic top coat (TC) caused by the interfacial oxidation is the most important factor. The present paper focuses on clarifying the impact of continuous formation of thermally grown oxide (TGO) on the shielding/anti-shielding and deflection of surface cracks within the TC. The growth stress in the TC due to the expansion of TGO layer is taken into account by introducing a new three-layered model in TBCs where the initial residual stress in TC is also presented. The theory of material configurational force is innovatively used to analyze the influence of the growth of TGO layer on the crack-tip driving force, i.e., the J_k-integral. A fracture criterion is employed to predict the deflection of lengthways propagation of the surface crack via the maximum energy release rate associated with J_k-integral. The influences of the key factors such as the initial crack length, the initial crack inclination angle, the magnitude of growth eigenstrain of TGO, the dwelling time of TGO at elevated temperature on the propagation of surface cracks are analyzed. Numerical results verify that the growth of TGO layer significantly impacts the failure of TBCs, particularly the lateral growth behavior of TGO layer is the key factor influencing the initiation and propagation of surface crack at elevated temperature. The lateral expansion growth of TGO layer induced additional tensile stress in TC can accelerate the initiation of surface cracks. Furthermore, the TGO growth can partly suppress/enhance the deflection of surface crack depending on the inclination angle and the suppression/enhancement effect becomes stronger as the dwelling (oxidation) time increases.