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Materials Science in Additive Manufacturing Alumina platelets additive manufacturing

limited deformation induced by various mechanisms such on the polymer phase. In another study, four-dimensional
as breaking of mineral bridges, inhibited sliding by nano- (4D) printing of alumina platelets was accomplished
asperities, and bonding between the two phases. These with direct ink writing method. The resultant alumina
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6,7
mechanisms were established to be the main reasons that platelets, comprising a high platelet content of 95 wt.%,
crack deflection occurred during crack propagation, which exhibited excellent mechanical properties and thermal
dissipated energy to toughen the shell. Such exceptional properties.
properties of nacre have encouraged many studies and In the present study, we investigated the concept of
efforts to synthesize materials (from one-dimensional to using alumina powder (nano- to micron-sized) to achieve
three-dimensional [3D]) with microstructures replicating various characteristics observed in a nacre. The first
that of a nacre. 8 objective was to reduce the viscosity of a slurry containing
Additive manufacturing comprises technologies that alumina platelets to additively manufacture nacre-inspired
enable the freeform fabrication of various materials, such structures. This concept explored the use of powders of
as metals and plastics, into 3D objects, overcoming design different morphologies added into the slurry to separate
and manufacturing constraints caused by geometrical and act as wheels to reduce sliding friction between the
complexities faced in conventional manufacturing. platelets when a shearing force is applied on the slurry
However, the direct additive manufacturing of ceramics during the coating phase of a vat polymerization printing
remains a challenge due to the inherent properties process. The second objective involved determining a
of ceramics, such as brittleness and high melting suitable sintering temperature to promote the formation
temperatures. As such, indirect additive manufacturing, of bridges to mimic the mineral bridges present in a
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such as binder jetting, direct ink writing, and vat nacre. Furthermore, the powder was added to create
polymerization, are the most common methods used to spaces between the platelets such that a secondary phase
fabricate ceramic parts at present. For instance, we can can be infiltrated, in essence, to mimic the organic phase
infuse a solvent such as ultraviolet (UV)-curable resin with of a nacre. Last but not least, we aimed to prepare a
ceramic filler to create a ceramic slurry before curing it UV-curable slurry with an appropriate platelet-to-powder
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with a suitable light to obtain a 3D part. The printed part ratio, based on the best result composition, suitable for
is then subjected to debinding and sintering in a furnace vat polymerization. To align the platelets such that they
to finally obtain a ceramic part. However, such a process is replicate the microstructure of a nacre, we used a vat
not without its issues. Infusing a resin with ceramic filler polymerization printer equipped with a coating blade to
often results in the thickening of the slurry and increasing spread material to every layer.
the viscosity. 12,13 This increased viscosity complicates the
process as the slurry resists to flow during the printing 2. Materials and methods
process, unless a coating system is used in the printer. As In this study, four types of slurry were prepared. Of the four,
a result, the amount of ceramic filler that can be used is three slurries contain the alumina platelets and an alumina
limited which also results in shrinkages after sintering. powder obtained from either Taimei (100 nm, regular and

Nevertheless, additive manufacturing is still an agglomerate-free nanopowder [NS]), NLM (1 μm, regular
adequate method for fabricating ceramic parts, especially micron-sized powder [MR]), and Sigma (10 μm, irregular
for nature-inspired structures. By capitalizing on the micron-sized powder [MI]) and one contains purely the
advantages of a suitable additive manufacturing process, platelets. Figure 1A–D depicts the size and morphology
nature-inspired structures such as nacre can be mimicked. of the powders used in this study. The alumina platelets
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A study that closely exemplifies the mimicking of nature- (purity ≥ 99%; White Sapphire RonaFlair , Merck KGaA,
inspired structures has been reported, investigating the Germany) were obtained from Sigma Aldrich and had a
effect of alumina platelets on the rheological properties of particle size (d50) of approximately 10.5 μm. The effects of
a ceramic slurry. However, this study prepared relatively these powders were then compared by characterizing the
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low platelets to the nanopowder ratio in the slurry to print rheological properties of the slurry. Each slurry contains
a nacre-mimicking structure. Furthermore, shear thinning alumina platelets (PL) and one type of powder (NS or MR
in such a case is expected as the slurry contains a higher or MI) in a ratio of 3:1. In addition, 1 wt.% of dispersant
proportion of regular-shaped nanoparticles. Another and 0.3 wt.% of photo-initiator were added to reduce the
study used boron nitride platelets to prepare a slurry, viscosity and make the slurry UV-curable, respectively.
which exhibited relatively high viscosity to 3D-print a The preparation of each slurry begins with dissolving
nacre-inspired structure. However, a pre-sintering test the dispersant in the photo-monomer HDDA. Afterward,
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revealed that the mechanical properties were largely reliant the powders (NS, MI, and MR) were each added and

Volume 3 Issue 1 (2024) 2 https://doi.org/10.36922/msam.2711