Non-destructive Microstructural Evaluation Of Yttria Stabilized Zirconia, Nickel Aluminides And Thermal Barrier Coatings Using Electrochemical Impedance Spectroscopy

There has been an urge for increasing the efficiency in advanced gas turbine engines. To fulfill these needs the inlet gas temperatures should be increased in the gas turbine engines, thermal barrier coatings (TBCs) have gained significant applications in increasing the gas inlet temperatures. Insulating characteristics of ceramic TBCs allow the operation at up to 150~250 ˚C higher gas temperatures. Because of the severe turbine engine operating conditions that include high temperature, steep temperature gradient, thermal cycling, oxidation and hot-corrosion, TBCs can fail by spallation at the interface between the metal and ceramic. The lack of understanding in failure mechanisms and their prediction warrant a development of non-destructive evaluation technique that can monitor the quality and degradation of TBCs. In addition, the development of NDE technique must be based on a robust correlation to the characteristics of TBC failure. The objective of this study is to develop electrochemical impedance spectroscopy (EIS) as a Non-destructive evaluation (NDE) technology for application to TBCs. To have a better understanding of the multilayer TBCs using EIS they were divided into individual layers and EIS were performed on them. The individual layers included polycrystalline ZrO2-7~8 wt.%Y2O3 (YSZ) (topcoat) of two different densities were subjected to sintering by varying the sintering temperature and holding time for three different thickness and hot extruded NiAl alloy buttons which were subjected to isothermal oxidation with varying temperature and time. NiAl is as similar to the available commercial bondcoats used in TBCs. Then degradation monitoring with electrolyte penetration was carried out on electron beam physical vapor deposited (EB PVD) TBCs as a function of isothermal exposure. Quality control for air plasma sprayed TBCs were carried out as a function of density, thickness and microstructure. Dense vertically cracked TBCs were tested as a function of vertical crack density and thickness. Electrochemical impedance response was acquired from all specimens at room temperature and analyzed with an AC equivalent circuit based on the impedance response as well as multi-layered structure and micro-constituents of specimens. Physical and microstructural features of these specimens were also examined by optical and electron microscopy. The EIS measurement was carried out in a three-electrode system using a standard Flat Cell (K0235) from Princeton Applied Research™ and IM6e BAS ZAHNER™ frequency response analyzer. The electrolyte employed in this investigation was 0.01M (molar) potassium Ferri/Ferro Cyanide {(K3Fe(CN)6/K4Fe(CN)6·3H2O)}. The thickness and density were directly related to the resistance and capacitance of the polycrystalline YSZ with varying thickness and open pores. As the effective thickness of the YSZ increased with sintering time and temperature, the resistance of the YSZ (RYSZ) increased proportionally. The variation in capacitance of YSZ (CYSZ) with respect to the change in porosity/density and thickness was clearly detected by EIS. The samples with high porosity (less dense) exhibited large capacitance, CYSZ, compared to those with less porosity (high density), given similar thickness. Cracking in the YSZ monoliths resulted in decrease of resistance and increase in capacitance and this was related to the electrolyte penetration. Growth and spallation of TGO scale on NiAl alloys during isothermal oxidation at various temperatures and holding time was also correlated with resistance and capacitance of the TGO scale. With an increase in the TGO thickness, the resistance of the TGO (RTGO) increased and capacitance of the TGO (CTGO) decreased. This trend in the resistance and capacitance of the TGO changed after prolonged heat treatment. This is because of the spallation of the TGO scale from the metal surface. The parabolic growth of TGO during high temperature oxidation was inversely proportional to the capacitance of TGO, excluding the abrupt changes associated with the failure. As a function of isothermal exposure for EB-PVD TBCs, initial increase in the resistance of YSZ with thermal exposure was observed perhaps due to the high temperature sintering of YSZ. The parabolic growth of TGO during high temperature oxidation was inversely proportional to the capacitance of TGO. An explanation based on electrolyte penetration into sub-critical damage is proposed for the gradual decrease in the resistances of YSZ and TGO with prolonged thermal exposure. Observation of exposed metallic bond coat surface on the fracture surface, which readily provides conduction, was related to the abrupt and large increase in the capacitance of YSZ and TGO. A direct relation between the resistance of the YSZ (RYSZ) and density of the YSZ was observed for APS TBCs with varying topcoat density. APS TBCs with varying topcoat chemistry and thickness were tested and directly related to resistance of topcoat. With the increase in the topcoat thickness, the capacitance decreased and the resistance increased. The higher values of CCAT and RCAT compared to that of CYSZ and RYSZ were related to the higher dielectric constant and resistivity of CaTiO3. Dense vertically cracked TBCs were tested with varying crack density were tested and the variation in the resistance was related indirectly to the cracks and directly to the difference in the thickness of the topcoat. EB-PVD TBCs with varying density (dense and columnar) were tested and the variation in resistance was attributed to the dense structure and columnar structure of the topcoat with columnar structure having lower resistance because of more electrolyte penetration through the columnar structure. From this study, EIS showed a potential as a NDE technique for quality assurance and lifetime remain assessment of TBCs. Future work should continue on developing a mathematical model to study the impedance curves and come up with a model for individual layers of TBC and then sum them up to get the multilayered TBC response. The flexible instrument probe of EIS needs to be designed and tested for field evaluation of TBCs