Stress distributions in plasma-sprayed thermal barrier coatings as a function of interface roughness and oxide scale thickness

During thermal cyclic loading, plasma-sprayed thermal barrier coatings (TBCs) often show failure within the top coat close to the interface. In all cases this results from crack propagation of pre-existing cracks near the bond coat (BC)–top coat interface. Stresses developing on a microscopic scale near the BC–TBC interface of plasma-sprayed thermal barrier coatings govern crack growth in an initial phase of the failure process. Using a finite element (FE) method the local dependence of stresses in the vicinity of this rough interface was investigated. Measurements of real roughness profiles provided geometrical parameters needed for the calculations. A significant difference in the stress distributions was found for peak and valley locations of the BC roughness profile. The effect of BC oxidation on stress development was more pronounced in the case of less roughness. Analytical fits of the FE results revealed how the parameters of roughness and the oxide thickness correlate with the stress levels. In the next stage of research these fits will serve as input data for a microstructural based lifetime model.