Yttria stabilised zirconia (YSZ) ceramics are used in a variety of structural and functional applications due to their good mechanical properties over a wide temperature range. In particular, YSZ layers are used as top coat of thermal barrier coatings (TBCs) because of their superior high-temperature properties,such aslow thermal conductivity, high toughness and relatively high thermal expansion coefficient.
One of the most implemented techniques to manufacture TBCs is atmospheric plasma spraying (APS). It consists in the injection within a high-energy plasma flow of a feed material where this later encounters melting and accelerating until it impacts upon substrate. On the substrate, molten (or partially molten) material is quickly cooled forming individual lamellae, the stacking of which forming the coating. Conventional YSZ powders for thermal barrier coatings are obtained by different methods such as fusion and crushing, spray-drying, sol–gel, milling and sintering or spheodisation. The typical coating microstructure obtained from these powders is laminar formed by flattened drops or splats with some microstructural features as voids or cracks which affect the coating properties. The characteristics of the feeding powders strongly affect these microstructural features.
By spraying nanoparticles instead of conventional micrometric powders, coatings can be obtained that exhibit different architectures and enhanced properties. However, nanoparticles cannot be directly sprayed because of their low mass and poor flowability. To overcome these problems, three possible routes can be found in the literature: to reconstitute the individual nanoparticles into spherical micrometre-sized granules; to inject directly a suspension of nanoparticles (suspension plasma spraying or SPS); or to inject a solution precursor of the coating (solution precursor plasma spraying or SPPS).The first route was followed in this work and spray drying from a nanoparticle suspension is one of the most widely used agglomeration method. Often spray-drying is followed by thermal treatment of the resulting nanostructured granules to enhance their sinterability.
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