In-flight alloying of nanocrystalline Yttria-stabilized zirconia using suspension spray to produce ultra-low thermal conductivity thermal barriers

Previous researchers have shown that it is possible to combine rare-earth oxides with the standard thermal barrier coating material (4.5 mol% Y ₂O₃-ZrO₂ or YSZ) to form ultra-low thermal conductivity coatings using a standard powder manufacturing route. A similar approach to making low thermal conductivity coatings by adding rare-earth oxides is discussed presently, but a different manufacturing route was used. This route involved dissolving hydrated ytterbium and neodymium nitrates into a suspension of 80 nm diameter 4.5 mol% YSZ powder and ethanol. Suspension plasma spray was then used to create coatings in which the YSZ powders were alloyed with rare-earth elements while the plasma transported the melted powders to the substrate. Mass spectrometry measurements showed a YSZ coating composition, in mol%, of ZrO₂-4.4 Y₂O₃-1.4 Nd₂O ₃-1.3 Yb₂O₃. The amount of Yb³⁺ and Nd³⁺ ions in the final coating was ∼50% of that added to the starting suspension. Wide-angle X-ray diffraction revealed a cubic ZrO ₂ phase, consistent with the incorporation of more stabilizer into the zirconia crystal structure. The total porosity in the coatings was ∼35-36%, with a bulk density of 3.94 g/cm³. Small-angle X-ray scattering measured an apparent void specific surface area of ∼2.68 m ²/cm³ for the alloyed coating and ∼3.19 m ²/cm³ for the baseline coating. Thermal conductivity (k_th) of the alloyed coating was ∼0.8 W/m/K at 800°C, as compared with ∼1.5 W/m/K at 800°C for the YSZ-only baseline coating. After 50 h at 1200°C, k_th increased to ∼1.1 W/m/K at 800°C for the alloyed samples, with an associated decrease in the apparent void specific surface area to ∼1.55 m²/cm³.