Abstract
The present work deals with a study on the physical characteristics of stoichiometric hydroxyapatite microspheres produced by spray-drying process through different operating conditions. Obtained hydroxyapatite microspheres are intended to be used as powder feedstock in powder bed selective laser processing additive manufacturing technologies where the flowability and the particle size distribution of the powder feedstock are of special importance. The powders produced at different spray-drying operating parameters were evaluated by analysing the particle size, shape, moisture, and agglomerates strength. Spray air pressure (0.25–2 bars) and the solid content of the slurries (10–50% wt.) were the most influent parameters to control the final particle size distribution, whereas the higher process recovery rate (79%) was obtained at specific values of inlet temperature (250 °C), spray air pressure (2 bar), and feed rate (4.9 kg.h−1). A 5 wt% of polyvinyl alcohol as an organic binder to the ceramic part was found to increase the strength of the granules. The flowability of the produced hydroxyapatite microspheres (x50 = 64 μm; span = 1.77) was found to fulfil the defined feedstock requirements (e.g., Hausner ratio < 1.25), without alteration of the physico-chemical properties of the material. These promising results confirm the use of a spray dryer as an efficient method for the production of hydroxyapatite microspheres for powder bed selective laser processing.
Highlights
Spray-drying (SD) process is one of the many methods proposed for the production of ceramic particles with controllable morphology, from which we can distinguish bottom-up and top-down processes [1]
The use of polydisperse size distribution has demonstrated to improve their flowability and packing density, with the smaller particles filling the interstitial void between larger particles [23,24,25]. It makes the SD an excellent process for the production of hydroxyapatite microspheres (HAm) intended for Powder Bed Selective Laser Processing (PBSLP), since the possibility of controlling their particle size distribution (PSD) as well as the morphology allow the tailoring of powders properties such as flowability and packing density
The morphology of powders present in this work was investigated with a scanning electron microscope (SEM) LEO 435 VP (Leica®) equipped with a Ge detector (Imix-PC, Princeton Gamma-Tech) and metallization of the ceramic surface was performed with a thin film of silver using a Scancoat Six sputter coater
Summary
Spray-drying (SD) process is one of the many methods proposed for the production of ceramic particles with controllable morphology, from which we can distinguish bottom-up (precipitation, supercritical fluid...) and top-down (mechanical milling, freeze-drying...) processes [1]. The use of polydisperse size distribution (span > 1) has demonstrated to improve their flowability and packing density, with the smaller particles filling the interstitial void between larger particles [23,24,25] It makes the SD an excellent process for the production of hydroxyapatite microspheres (HAm) intended for PBSLP, since the possibility of controlling their PSD as well as the morphology allow the tailoring of powders properties such as flowability and packing density. The main objective will be the production of HAm powder with a targeted particle size in the micron range (30 μm < x50 < 70 μm) and a certain size polydispersity (span > 1) to fulfil the flowability requirements (e.g., Hausner ratio < 1.25) for its use in a PBSLP process
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