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.

This study compares the dielectric properties of annealed forsterite (Mg 2 SiO 4 ) and alumina coatings deposited on mild steel substrates by atmospheric plasma spraying. As-sprayed coating samples were electrically characterized then submitted to a series of one-hour annealing treatments at temperatures from 300 to 800 °F. After each treatment, impedance measurements were recorded over a frequency range of 30 to 100 kHz. An electrical model was fitted to Nyquist data (Im Z vs. Re Z) using a least-mean-square algorithm with a weighting function. Although impedance spectroscopy measurements were obtained at different temperatures, this paper focuses on the acquisition, modeling, and comparison of room temperature properties, particularly electrical resistivity and dielectric constant. It also compares the microstructure of as-sprayed and annealed forsterite and alumina coatings and discusses coating degradation mechanisms stemming from differences in CTE.

In this study, the influence of spray parameters on the electrical resistivity of thermally sprayed ceramic coatings from the system Cr 2 O 3 -TiO 2 was investigated. Fused and crushed feedstock powders with contents of 10 wt. % and 20 wt. % chromium oxide were deposited by APS and HVOF. Temperature and velocity of the particles in the spray jet as well as the coating surface temperature were analyzed during the deposition process. Impedance spectroscopy was used to investigate the electrical resistivity of the coatings and the results were correlated to coating microstructure and phase composition. It was found that phase transformations occur during the spray process. In the coatings a high temperature phase (n-phase) and rutile were observed. Though, the ratio of rutile depends on the spray methods employed for coating deposition. The electrical resistivity of coatings obtained by HVOF can be correlated to the content of chromium oxide. Furthermore, the surface temperature of the coating during deposition also shows some influence. Concerning the coatings resulting from APS, the different mixtures of the plasma gases (Ar-H 2 and Ar-N 2 ) are supposed to have the most important influence on the electric resistivity.

Nanostructured YSZ+NiO functional layers for SOFC anodes were developed by air plasma spraying. Spray processing was controlled to conserve finely porous and nanostructure of the feedstock in the deposit. The optimized deposits exhibiting high gas permeability, suitable high temperature electronic conductivity, enhanced triple phase boundaries and catalytic activity. The results were compared with conventional NiO+YSZ and Ni-C+YSZ anodes. YSZ electrolyte layer was deposited onto the anodes for electrochemical testing at 800°C in static conditions. Impedance spectroscopy measurements were performed to collect data on the polarization resistance and catalytic behavior of these anode layers. It was established that enlarged reaction zone, provided by high specific surface area of the nanostructured anodes, and finely porous microstructure, led to lower polarizations and enhanced cell performance by more than 60% compared to conventional cells.

In the continuing progress of fuel cell technology, CeO 2 double doped electrolytes appears to be promising for lowering the SOFC's working temperatures. Ceria electrolytes have better ionic conductivities than YSZ but, at low oxygen partial pressures, the chemical reduction of ceria leads to increasing electronic conduction. Double doping of the ceria increases the electrolytic conduction range without changing its conductivity. To avoid stress development within the ceria crystallographic structure, the dopants mix must have a mean ionic radius as close as possible to the critical ionic radius. Ceria electrolytes with various compositions and dopant concentrations are synthesized with a combinatorial chemistry approach. To synthesize new electrolytes, solution plasma spraying with nitrate salt precursor is used. The reaction is completed and nanocrystalline thin layers of ceramic are formed in the plasma. Comparative studies of plasma spraying techniques, with YSZ powder plasma spraying as electrolyte reference, were performed. Also, comparative impedance spectroscopy measurements are to be performed to validate the double doping hypothesis and thence to identify the best electrolytes in the suite of over 300 new materials.