Jinnestrand, Magnus. Delamination in APS applied thermal barrier coatings: life modelling. Linköping Studies in Sci. Tech. Dissertation No. 902, 2004
Thermal barrier coatings (TBCs) are used in gas turbines as a thermal shield resulting in lower temperature in coated components. The decrease of temperature allows higher gas temperatures in the turbine, which increases the efficiency. The bimaterial construction with an outer ceramic layer applied onto a metallic material gives rise to problems during thermal cycling. Thermally induced stresses will gradually break down the coating. The ceramic layer will delaminate from the substrate, resulting in spallation, and the component will break down due to overheating.
The delamination process is investigated in this thesis by finite element simulations. The growth of an internal alumina layer in the top/bond coat interface is investigated by 3D finite element simulations which show that the local stress state changes in such a way that the alumina growth helps nucleation and growth of small delamination cracks. Finite element simulations, in which the energy release rate and stress intensity factors are calculated, investigate the growth of small delamination cracks in or close to the top/bond coat interface. Experiments show that these cracks grow parallel to or in the sinusoidal top/bond coat interface and the results of the simulations show that the mode mixity changes as the delamination cracks grow.
A new delamination life model is proposed which is based on results of the fracture mechanical simulations and experimental observations. The model predicts the growth of small cracks in the TBC before they form a large delamination crack. The model is based on a modified Paris law where a mode mixity dependence on the crack growth rate is included, giving lower crack growth rate in mode 2 load compared with mode 1. Parameters of the model are obtained by optimisation of the model against experimental data, describing the delamination damage evolution in the TBC. The data are obtained from interrupted thermal cycling tests and the prediction of the model corresponds well with these data.
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