Spray Pyrolysis of Thermal Barrier Coatings
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Thermal barrier coatings are multilayer systems developed to increase the operating temperature and lifetime of modern gas-turbine engines by thermally insulating the underlying superalloy components. Thick oxide thermal barrier coatings are currently deposited by complex and expensive physical deposition methods, whereas wet chemistry deposition processes using liquid precursor solutions as a feedstock appear as an attractive low-cost and simpler alternative. Nevertheless, these wet chemistry deposition techniques have mainly been focused on the deposition of thin films until now, and the deposition of thicker coatings, even though challenging, would be beneficial to countless applications. The aim of this work has consequently been to evaluate the feasibility of depositing thermal barrier coatings via wet chemistry deposition methods. Special emphasis was given to the spray pyrolysis technique, and the deposition mechanisms and the cracking behavior of thick oxide coatings produced by spray pyrolysis have been investigated. Finally, the thermal and mechanical properties of thick and porous crack-designed lanthanum zirconate coatings were determined. In Paper I, nanostructured yttria-stabilized zirconia coatings were deposited on stainless steel by dip-coating and spray-coating of water based suspensions. Porous and crack-free coatings were achieved by both methods. Nevertheless the achieved coatings remained relatively thin, and the dip-coating and spray-coating techniques were not considered further. In Paper II, an aqueous nitrate based precursor solution was deposited on preheated stainless steel substrates by spray pyrolysis to produce thick lanthanum zirconate coatings. The influence of the substrate temperature on the coating formation and microstructure has been carefully studied for a better understanding of the deposition mechanisms. A temperature range corresponding to the deposition of an ionic salt precipitate was found to be critical to obtain thick crack-free green coatings. Further heat treatment was necessary to decompose the nitrate species into an oxide coating with a fine porous microstructure. In Paper III, the crack mechanisms of thick lanthanum zirconate coatings deposited by spray pyrolysis from aqueous nitrate based precursor solutions are presented. Cracks were formed during the decomposition of the nitrate species due to the associated volume change. The thickness of the coatings demonstrated the largest impact on the crack pattern. The crack opening and the crack spacing varied linearly with increasing thickness, leading to small delamination at the interface. The cracks were stable after the crystallization of the coatings by further heat treatment. Finally, it was proposed that coatings with a designed crack pattern can be deposited based on the knowledge of the influence of the different parameters. In Paper IV, the influence of the chemistry of the precursor solution on the deposition of thick oxide coatings by air blast spray pyrolysis is reported. Coatings of four different oxide materials were deposited using aqueous lanthanum-zirconyl, yttrium-zirconyl, aluminum, and aluminum-magnesium nitrate based precursor solutions. The most suitable deposition temperature interval was determined for each selected precursor solution system and correlated to thermogravimetric analyses of the as-deposited green coatings. The quality of these green coatings was further characterized in terms of microstructure, porosity, and crack-freeness. The effect of the concentration of the precursor solutions on the deposition temperature intervals and the microstructure of the green coatings was explained in relation to the salt saturation limit. A trend was established based on the results obtained for the four systems, and general guidelines for successful deposition of thick coatings by spray pyrolysis were given. In Paper V, the thermal and mechanical properties of thick crack-designed lanthanum zirconate coatings are presented. The green coatings crystallized from 600 °C and the cubic pyrochlore structure was formed after heat treatment at 1000 °C for 2 h. Crystalline lanthanum zirconate multilayered coatings with small crack spacing and crack opening exhibited a higher density, a higher hardness, a lower thermal diffusivity of ~0.33 mm2/s, and a higher thermal conductivity of ~0.34 W/(mK) at elevated temperatures compared to crystalline monolayered coatings of similar thickness with large crack spacing and crack opening. The thermal diffusivity of the coatings was similar to the lowest values reported for yttriastabilized zirconia plasma sprayed coatings.