Abstract
Thermal–mechanical fatigue (TMF) tests were carried out on specimens made of alloy NiCr22Co12Mo9 which were coated with a LPPS CoNiCrAlY bond-coat (BC) and an electron beam-physical vapour deposition zirconia thermal barrier coating (TBC). The TMF tests simulate the thermally induced loadings at the outer surface of TBC-coated cooled gas turbine blades. In the present paper the cyclic deformation behaviour of the samples is analyzed and a quantitative evaluation of the degradation of the TBC system in two heat treatment states under TMF loadings is given. Segmentation cracks within the TBC as well as fatigue cracks in the BC and in the substrate were observed. Both types of cracks occur predominantly perpendicular to the loading direction. The main reason for this degradation of the TBC system are tensile stresses which are formed at low temperatures (<DBTT). The fatigue crack distribution within the BC and the substrate is mainly influenced by the heat treatment state as well as the applied mechanical strain. A preoxidation treatment before the TMF loading slightly increases the number of lateral cracks at the interface TBC/TGO/BC but does not result in a severe degradation of the TBC system compared to the as-received state. Because of the relatively high mechanical loading of the TBC system which leads to lifetimes of the specimens up to 500 cycles, no wide area spallation or buckling of the TBC was observed.
Published Version
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