The aim of this work, devoted to yttria stabilised zirconia (YSZ) thermal barrier coatings, is to produce by DC plasma spraying a single thick pass macro cracked orthogonally to the substrate. YSZ was plasma sprayed in air atmosphere on Hastelloy X substrates, with a NiCrAlY bond coat. A three-zone microstructure is observed, where lamellae and columns are present. The measurements of deposition stresses during spraying allow explaining the macrocrack formation.

To determine the effect of bond coat oxidation on the coating life, thermal shock testing were performed, using three different thermal cycles. The failure mode and crack paths were investigated in scanning electron microscope. A finite element model was developed to simulate the thermal shock tests. First, transient temperature fields during the thermal cycling were calculated. Second, stresses and strains evolving in the coatings due to thermal expansion mismatches and temperature gradients during the cycling were computed. The stress concentration at the interface due to the roughness of the bond coat was accounted for by using an ideal sinusoidal interface in the model. By adding an oxide layer with and without residual stresses to the model, the influence of the bond coat oxidation was determined. Both the experimental and numerical results revealed that the TBC failed by crack initiating in the ceramic top coat very close to the grown oxide layer at the interface followed by coating fatigue failure. Numerical simulation indicated that bond coat oxidation led to stress concentration at the peak of the asperity of the interface proceeding crack growth. It also showed that bond coat inelasticity and ceramic creep might further enhance the crack growth. There was little effect on coating behavior due to the residual stresses in the oxide layer.

Thermal barrier coatings with a zirconia top coating and a NiCoCrAlY bond coating were plasma sprayed onto a nickelbase alloy. The pre-heating of the bond coated substrates and the cooling during the top coating spraying were varied to produce five different spray sets. A finite element model was developed to predict the heat transfer and the resulting thermal stresses during the spraying. A layer removal technique was used to measure the residual stresses in the as-sprayed samples. The measurements revealed low residual stresses in the top coatings and tensile stresses in the order of 150 MPa in the bond coating. A correlation between the measured top coating residual stresses and the substrate temperature in the end of the top coating spraying was found. In general, good agreement between modelled and measured residual stresses was found. The top coatings were found to contain vertical microcracks and the densities of the cracks were point-counted in the spray sets. A slight increase in microcrack densities was found as the spraying was performed onto a colder substrate. The densities of vertical microcracks were correlated to modelled in-elastic strain in the top coatings.