Interaction of Fatigue and Ageing in TBC Testing

Kurzfassung

Testing for reliable lifetime assessment of components that have to survive long term service has to be accelerated in comparision to real loading time in service. For example, it is impractical to perform realistic thermo-mechanical fatigue tests for turbine blade materials in real time until failure, if the blade or the thermal barrier coating (TBC) system on the blade is supposed to function 5,000 to 10,000 flights of 1 to 10 h. Accelerated thermo-mechanical fatigue testing has been performed on EB-PVD TBC systems by using high loading rates as well as high heating and cooling rates. In order to capture time-at-high-temperature-dependent effects despite reduced operating time in the fatigue testing facility, specimens have been pre-aged separately in a furnace. The resulting damage features in the pre-aged specimens were significantly different to those in the as-coated specimens. A specific type of fatigue cracks, so-called ‘smiley cracks,’ initiating from a TGO crack perpendicular to the mechanical tensile load, evolved only in pre-aged specimens. The pre-aged samples had formed a coarse-grained, dense TGO, which was not present in the ‘as-coated’ samples. The analysis of the results suggests that the loading rate was too high to allow relaxation processes in the dense TGO of pre-aged specimens but allowed relaxation in the thin fine-grained TGO of the as-coated specimens. Thus, high tensile stresses develop only in the pre-aged TGO, entailing fracture of the TGO.