Effects of ageing on mechanical properties and damage behavior of EB-PVD thermal barrier coating systems

Multilayered materials subjected to high temperatures exhibit time and temperature dependent microstructure evolution, which alter the mechanical properties of the respective components and consequently the damage behavior of the layered structure. The here presented investigation was motivated by the observation that in complex thermal mechanical fatigue tests pre aged and as coated specimens of a thermal barrier coating (TBC) system showed completely different damage behaviour. The pre aged specimens displayed perpendicular to the mechanical fatigue load cracks in the thermally grown oxide (TGO) between ceramic top coat and underlying metallic bond coat. The cracks started in the TGO, propagated into top coat and bond coat, entailing a scenario of local oxidation and crack kinking at the bond coat, and finally spallation of the ceramic coating. In contrast, the as coated TBC-system did not show crack formation even after a higher number of test cycles. In order to explain the effect of ageing on the crack initiation, changes of the TBC microstructure and related mechanical properties have been investigated experimentally (scanning electron microscopy, nanoindentation) and by means of finite element analyses. The investigations suggest that only in the aged TBC system tensile stresses can accumulate in the TGO, which are sufficient to initiate the observed cracks