Evaluation of microstructural evolution in thermal barrier coatings during thermal cycling using impedance spectroscopy

Air plasma sprayed thermal barrier coatings are thermally cycled in air up to 1030 °C and evaluated using impedance spectroscopy in conjunction with scanning electron microscopy. When the number of cycles is less than 15, impedance measurements cannot be used to detect the thermally grown oxide (TGO, usually alumina) scale because it does not fully cover the top coat-bond coat interface and the YSZ (yttria-stabilised zirconia used as the top coat) is much more electrically conductive than the alumina, leading to most of the current passing through the YSZ rather than the alumina. After the specimens are subjected to 100 until 250 cycles, impedance measurements show that a continuous alumina scale is formed. In the impedance spectra, there are four relaxation processes, which correspond to the YSZ grains, the YSZ grain boundaries, the TGO, and the electrode effect. Impedance analyses demonstrate that the resistance of the alumina scale increases and the capacitance decreases with increasing cycling. When the specimen is subjected to 400 cycles, the impedance response to the continuous TGO vanishes due to the TGO degradation.