High temperature stability of CeO2-Y2O3 stabilized zirconia plasma spray powder. XPS, STEM, and DTA investigations

Advanced investigations on high temperature stability of commercial prealloyed ceria-yttria-zirconia powder, suitable for the manufacturing of Thermal Barrier Coatings (TBC), have been carried out by means of X-ray Photoelectron Spectroscopy (XPS), Scanning Transmission Electron Microscopy (STEM), Differential Thermal Analysis (DTA), and Thermogravimetry (TGA) with a view to foresee their behaviour in the plasma jet atmosphere and oxidizing service environment. Thermal treatments of the powder were performed in a thermobalance in both oxidizing (static air) and reducing (flowing N2/H2 = 10%) atmospheres up to 1873 K and then cooled down. Under static air conditions, two exothermal reactions taking place at 700 and 1300 K were disclosed by DTA experiments. On the contrary, under reducing conditions the powder was affected by a weight loss owing to the Ce (IV) reduction to Ce (III). XPS results lead to the evaluation of the effect of the abovementioned thermal treatments on cerium valence state, which is the only element in the powder that changes its chemical state. In particular, the analysis of the characteristic satellite structures and peak lineshapes in the Ce 3d region demonstrated that cerium in the starting powder was a mixture of Ce (IV) and Ce(III) oxides in the outermost layers, and after thermal treatments, it was either fully reduced to Ce (III) or fully oxidized to Ce (IV).