Thermal fatigue testing and simulation of an APS TBC system in presence of a constant bending load

Thermal Barrier Coatings (TBCs) are used as thermal insulation to protect components operating at high temperature. A multiple failure mechanism may exist in a TBC system associated with diverse in-service conditions. In this paper, Inconel 617 superalloy substrates were coated with 8% Y2O3 stabilized tetragonal ZrO2 powder, deposited on top of a Ni22Cr10Al1Y bond coat by atmospheric plasma spraying (APS). In order to investigate the damage mechanism of the in-service components, experiments and numerical FE analysis were conducted to simulate the loading conditions of the TBC system in a land-based gas turbine combustor component. A specific test rig was designed and manufactured such that coated specimens were simultaneously subjected to constant bending load and thermal cycles. The temperature history was measured on both sides of the specimens and a maximum surface temperature of 1170 °C on the coating surface and a thermal gradient of 500 °C was obtained across the TBC system. The TBC system - failed after 82 thermal cycles. The microstructural observation showed multiple vertical and horizontal cracks confined within the top coat. By increasing the mechanical load, more extensive cracks were observed in a similar pattern. The final failure was governed by delamination at the top coat and bond coat interface. A subsequent finite element analysis confirmed that the application of external mechanical load can change the failure mechanism of the TBC. It also demonstrated that the TBC life is limited by crack propagation instead of crack initiation.