Spray drying and freeze drying are well established granulation processes for submicron and micron sized particles. In recent years, granulation of nanoparticle suspensions is receiving increased interest for the production of nanostructured materials.In this work the spray drying and freeze drying of different nanosized ceramic materials and the physicochemical characteristics of the obtained granules (size distribution, morphology, surface area, porosity, and flow parameters) are studied. Commercial colloidal suspensions of alumina and titania were studied, as well as a mixture of both with a relative weight ratio of 87/13. On one hand, the influence of temperature, pressure and nozzle diameter on the morphology and characteristics of spray dried granules are studied. On the other hand, the effect of air pressure and nozzle diameter on the morphology and properties of freeze dried granules was evaluated. The influence of solids loading of the starting suspensions has been also studied.It has been demonstrated that these processing parameters have practically no influence on the granules morphology and properties, and the only parameter determining the granules characteristics is the solids content of the suspensions, either in the spraying or in the freezing process. Spray drying leads to a monomodal distribution with higher granule size, while freeze drying produces more porous granules, with a bimodal intragranular distribution. The flowability of spray-dried powder is better than that of the freeze-dried powder. As a result, the characteristics of the spray-dried powder suit better the requirements of a feedstock targeted to obtain coatings by plasma thermal spraying whereas freeze drying can produce high porosity, softer granules.
Atmospheric plasma spraying (APS) is an interesting technique to obtain nanostructured coatings due to its versatility, simplicity and relatively low cost. However, nanometric powders can not be fed into the plume using conventional feeding systems, due to their low mass and poor flowability, and must be adequately reconstituted into sprayable micrometric agglomerates.In this work, Al 2 O 3 -13wt%TiO 2 nanostructured and submicron-nanostructured powders were deposited using APS. The feedstocks were obtained by spray drying from two starting suspensions, prepared by mixing two commercial nanosuspensions of Al 2 O 3 and TiO 2 , or by adding nanosized TiO 2 and submicron-sized Al 2 O 3 powders to water. The spray-dried granules were heat-treated to reduce their porosity and the resultant powders were fully characterised.Optimisation of the deposition conditions enabled the reconstituted powders to be successfully deposited, yielding coatings that were well bonded to the substrate. The coating microstructure, characterised by SEM, was formed by semi-molten feedstock agglomerates surrounded by fully molten particles that act as a binder.Moreover, microhardness, adhesion, and tribological behaviour were determined, and the impact of the granule characteristics on these properties was studied. It was found that changing the feedstock characteristics allows to control the coating quality and properties.2
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