Microstructural and numerical analysis of fracture mechanisms in a thermal barrier coating system on Ni-based superalloys

Abstract

The numerical analysis of factors governing the magnitude and distribution of residual thermal stress in an oxide/metal system was carried out regarding industrial applications of Ni-based superalloys protected with the thermal barrier coating system (TBC). The particular emphasis was paid on the microstructural characterization of damaging behavior of the α-Al2O3 thermally grown oxide (TGO) and the integrity along the TGO/NiCoCrAlY-bond coat and TGO/ZrO2-Y2O3 top coat interfaces which are the key to successful application of TBC systems. The cross-sections of samples after the high temperature cyclic oxidation tests at 1100 °C in air were characterised by SEM-EDS to study the fracture mechanisms and to model the TBC system. The data on the TGO thickness, its uniformity, chemical and phase compositions, spallation occurrence, and geometry of the interfaces were obtained. The numerical analysis of residual thermal stress was run for five different cooling rates using a finite element model. The following parameters influencing the stress state developed during cooling from the oxidation temperature were considered: physical and mechanical properties of the components, geometry of the interface including roughness and thickness. All the material layers were assumed to creep at elevated temperature. Finally results have been discussed in relation with creep mechanisms for TBCs layer.