Abstract
This work evaluates the high-temperature oxidation behavior of thermal barrier coatings by means of impedance spectroscopy. TBCs consisting of YSZ topcoats and NiCoCrAlYTa bond coats were deposited on Ni-based superalloy substrates by atmospheric plasma spraying. Test specimens were heated in air at 1000 °C for different periods of time from 5 h to 250 h. SEM-EDS analysis of the thermally grown oxide (TGO) shows that it mainly contains alumina and grew at a parabolic rate with increasing oxidation time. The resistance of the TGO, as determined by impedance spectroscopy, was found to increase at similar rate. Impedance spectroscopy also revealed an increase in YSZ grain boundary resistance corresponding to grain boundary cracking that occurred in the first 50 h of heating. The YSZ grain boundary resistance remained relatively constant over the interval of 50 to 150 h, but showed a slight decrease beyond 150 h mainly due to sintering effects.
