Thermomechanical Stress and Damage Evolution Near Film Cooling Holes in Thermal Barrier Coatings: Role of the Bond-Coat Layer

Abstract Thermal barrier coatings (TBCs) produced by thermal spray processes and combined with film cooling are widely used to protect gas turbine components under severe thermomechanical environments. In such systems, fatigue cracks and interfacial damage frequently initiate near film cooling holes due to complex stress redistribution within the multilayer architecture consisting of a ceramic top coat (TC), a metallic bond coat (BC), and a substrate. Although the effects of cooling holes have been extensively investigated, the mechanical role of the bond-coat layer in stress and damage evolution under thermomechanical fatigue (TMF) remains insufficiently understood. In this study, a three-dimensional finite element framework is developed to analyze TMF-induced stress redistribution, damage accumulation, and crack initiation and propagation near film cooling holes in an air-plasma-sprayed (APS) TBC system. The model incorporates a damage-coupled inelastic constitutive formulation for the TC, temperature-dependent elastoplasticity for a CoNiCrAlY bond coat, and viscoplastic behavior for an IN738LC substrate, together with a nonlocal crack-propagation algorithm. Both in-phase and out-of-phase TMF loadings are examined over a temperature range representative of cooled TBC regions. The results demonstrate that the bond coat plays a dual mechanical role. Inelastic deformation of the bond coat relaxes stresses in the TC and modifies circumferential stress distributions around the cooling hole, while bond-coat-mediated stress redistribution promotes the accumulation of interfacial delamination strain, particularly under thermally dominated TMF conditions. Crack initiation locations rotate by approximately 90° depending on the dominant loading mode, and a characteristic stress-range threshold separates the fatigue lives under in-phase and out-of-phase conditions. These findings provide mechanistic insight into the role of bond-coat plasticity in damage evolution and durability of thermally sprayed TBC systems with cooling holes.