In-Flight Alloying of Nanocrystalline Yttria-Stabilized Zirconia Using Suspension Spray to Produce Ultra-Low Thermal Conductivity Thermal Barriers

Abstract

Previous researchers have shown that it is possible to combine rare-earth oxides with the standard thermal barrier coating material (4.5 mol% Y2O3–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 Nd2O3–1.3 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 ZrO2 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 m2/cm3 for the alloyed coating and ∼3.19 m2/cm3 for the baseline coating. Thermal conductivity (kth) 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, kth 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.