Influence of slurry characteristics on the morphology of spray-dried alumina powders

Among all the parameters that must be managed to produce plasma sprayed coatings with controlled properties, the powder feedstock is a crucial point. The shape, density, morphology, composition and size distribution are the major characteristics of the powders that influence the coating properties and depend on the way of production. In the present work we focused on the spray drying which is a convenient process for producing sprayable ceramic powders. A systematic study has been performed to determine how the alumina slurry formulation (e.g. dispersant level, pH, binder addition) affects the granule characteristics. Aqueous slurries consisting of 30 vol.% of alumina particles, 0–1.2 wt.% ammonium polyacrylate as a dispersant and 15 wt.% styrene–ester acrylic copolymer as a binder were investigated using settling experiments and rheological analyses. Correlations appeared between sediment volume, zeta potential, yield stress and, viscosity of the slurries. At pH 4 and for 0.08 wt.% added PAA, the maximum flocculated suspension (without binder) is achieved as shown by the highest values of the sediment height (ratio of 0.85), yield stress and viscosity and the lowest zeta potential value. In contrast, at pH 9 and with the same dispersant concentration, the slurry, which is characterised by the lowest sediment ratio (0.45) and viscosity as well as no yield stress, is considered fully dispersed. Several spray dryer operating conditions were also investigated. Granules prepared from pH 4 slurry (flocculated) have solid, spherical shapes while the ones obtained from pH 9 slurry (dispersed) are hollow. It appears clearly that a close link exists between the floc structure and the granules morphology.