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Fabrication of Yttria-Stabilized-Zirconia (YSZ) Coatings by Electrophoretic Deposition (EPD)
[Thesis]. Manchester, UK: The University of Manchester; 2010.
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Abstract
Yttria stabilized zirconia (YSZ) coatings were produced from a YSZ suspension in acetylacetone (ACAC) using electrophoretic deposition (EPD) and then consolidated via the natural drying and isothermal sintering with the constraint of the metal substrates. Before EPD, the operational pH of the suspension was adjusted by addition of acetic acid or organic bases. The effect of suspension pH on the deposition of EPD coatings was studied with respect to the suspension stability, coating density and microstructure both for a mono-sized system and micro-nano binary systems. The constrained drying process of the deposits was examined via the measurement of the critical cracking thickness (CCT). The sinterability of coatings was evaluated by micro-hardness and microstructure. For a mono-sized (0.26μm) suspension, results showed that the zeta potential had a high positive value on both sides of the isoelectric point (IEP). This probably resulted from the adsorption of base molecules triethanolamine (TEA), detected by fourier transform infrared spectroscopy. Three alkalis with different molecular structure were compared and the effect of their molecule length on the interparticle repulsion was discussed. Accordingly, the double layer thickness of the particles can be estimated. Based on this, particle interactions were estimated for different pH suspensions. The reduced particle coagulation increased the packing density of the EPD coatings from 38 % at pH 7.4 to 53 % at pH 8.4. Therefore, subsequent sintering of coatings was promoted. After sintering at 1200 °C, coatings made in pH 8.4 suspensions obtained a much higher hardness and had fewer big pores than coatings fabricated in pH 7.4 suspensions. The CCT of the latter is slightly higher than the former which might be ascribed to its particle network structure.In a binary suspension composed of the coarse (1μm) and fine (with average size of 100 nm or 10 nm, content varied in 0-30 wt. % to the powder mixture) YSZ powders, interactions between different species can be tuned by the zeta potential of individual component. Binary particles can be well dispersed at pH 4 when both of the coarse and fine powders reached their highest zeta potentials. Heterocoagulation occurred between them to form a haloing structure with fine powders covered on the coarse particle surfaces when they exhibited zeta potentials of the opposite sign at pH 8.6. Particle interactions were estimated and the microstructures of the binary coatings were examined to discuss how the different fine particle sizes influenced the particle packing after EPD. At pH 4, there existed a “stability window” for the 10 nm fines at 10 wt. % whereas no noticeable the border of the window can be observed for 100 nm fines within the measuring range. 10 nm and 100 nm fine powders gave similar overall densities of binary EPD coatings which were independent of the fine powder content. For heterocoagulation coatings made at pH 8.6, although the adsorption of fine particles reduce the agglomeration of coarse powder, the low zeta potential of the halos led to a loose structure of the “skeleton” ( the packing of the coarse powder) in the final binary coatings. 10 nm fine powders was observed to give a higher CCT and denser particle packing than 100 nm fine powders especially in a pre-saturated heterocoagulated binary coatings at 20 wt. % fine powder content. In order to further improve the sintering of the EPD coatings at low temperature sintering, a layer of CuO was applied on the coarse powder surface. With the addition of 30 wt. % fine powders, the hardness of EPD coatings after sintering 2 hours at 1150°C increased from 6 to 61 Vickers. With the presence of CuO, the hardness values were enhanced by 2.5-4.25 times. The density measurements indicated that the CuO layer not only served as a sintering aid, the CuO layer also helped with the binary particle packing particularly in the heterocoagulation condition because of the stronger particle interactions between the fine powders and CuO modified coarse powders. It seems that CuO had no significant impact on the cracking resistance of the binary coatings during drying, however t-m phase transformation was observed during sintering possibly due to the liquid phase induce by CuO.