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Materials Science in Additive Manufacturing Alumina platelets additive manufacturing

3. Results when subjected to shearing stress by the coating blade
but also facilitating the formation of bridges between the
Figure 1A–D shows the morphologies of alumina platelets.
platelets and different alumina powders. Examination
of these images revealed that the alumina platelets As shown in Figure 2A–C, the SEM images also revealed
possess an average diameter of 8.19 ± 2.50 µm and a that the NS particles did occupy the space between the
thickness of 0.53 ± 0.12 µm. The nanopowder possessed platelets to provide a form of wheeling mechanism for
a significantly smaller mean diameter of 0.45 ± 0.02 µm the platelets to slide over when stress is applied. When
but was comparable to the thickness of alumina plates. subjected to high shearing stress during the coating
Furthermore, the diameters of MR and MI were measured phase, the platelets slide over the nanoparticles in a way
at 1.09 ± 0.93 µm and 7.36 ± 1.51 µm, respectively. An similar to large and heavy objects being moved on wheels
initial study conducted to examine the characteristics of or rollers by humans in ancient times. Without these
the three types of slurry showed that the agglomerate- nanoparticles, the platelets would be forced to slide against
free nanopowder NS effectively reduces and stabilizes the one another, generating friction that prevented the slurry
viscosity of the slurry to around 0.045 Pa·s even at high from flowing fluidly, as observed in the other two slurries.
shear rates of up to 200 /s (Figure 1E). The other slurries This mechanism, made possible by the addition of the
containing the other two types of powder and without nanopowder, allowed the slurry to inhibit shear thickening
containing any exhibited a shear thickening behavior at under high shearing rates and ensured the alignment of the
high shear rates. platelets during the coating phase of printing.
This shear thickening behavior of a slurry under high As shown in Figure 2A and 2C, at sintering temperatures
shear rates is undesirable in a vat polymerization AM of 1300°C and 1400°C, the nanopowder remained relatively
system, which uses a coating system. During the coating small at approximately 100 nm. As the temperature
phase, the blade moves in at an angle to spread the slurry, increased to 1500°C and above, the nanopowder and
as illustrated in Figure 1E. This angle of attack pushes platelets could be seen to have undergone growth, and
the slurry forward and downward, generating a shearing bridges began to form between the enlarged platelets at
stress between the platelets. The resulting shear thickening 1600°C (Figure 2D and E). In all the samples, the pore
behavior at high shear rates implies that the printing sizes did not seem to differ much, except that they became
process would be slower due to the slower coating speed smaller when sintered at 1600°C. This is due to the growth
required. Furthermore, at a higher shear rate, the slurry in size of the platelets and nanopowder, which leads to a
experiences greater resistance to flow as it hardens. This reduction of the pore size. Moreover, sintering at 1600°C
recorded behavior resembles that of a non-Newtonian promoted the formation of bridges by the nanopowder
cornstarch-water mixture under stress. The addition of between the platelets. The bridge between plates can
the NS powder also showed better stability of the slurry dissipate energy through breakage, thereby enhancing
as compared to the other two. In contrast, the powder toughness. This mechanism is similar to the toughening
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in the other two slurries settled relatively quickly by day mechanism in nacre.
3, as shown in Figure 1F. Based on these prior results, After sintering at 1600°C, the porosity of ceramic parts
we fabricated and sintered a simple sample (Figure 1G) was 0.34 ± 0.02. The sintered parts were then subsequently
with the alumina platelet (PL)-NS slurry to inspect the infiltrated with UV-curable HDDA in a vacuum chamber
microstructure and the orientation of the platelets. As at negative pressure for 24 h and sheltered from lights. The
shown in Figure 1H–J, the alignment of the platelets can degree of infiltration relative to the sintering temperature
clearly be seen under the SEM. Furthermore, the images was then examined at the cross-section under SEM. SEM
also revealed that the nanoparticles formed bridges, which images (Figure 2A–H) show that the UV-curable HDDA
contribute as a toughening mechanism, between the could be infiltrated and cured, regardless of the temperature,
platelets on sintering. The long-range forces generated by the samples were sintered. This is most likely due to the gaps
van der Waals interactions are universal and always attract between the platelets created by the nanopowder, which
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particles of the same kind to each other. By creating a obstructed the sintering of the platelets to one another. The
bridge effect between the platelets, the addition of NS gaps may also offer an explanation about the similarity in
powder can successfully lower the van der Waals forces pore sizes, which indicates that we can sinter at 1600°C to
between them, causing a drop in viscosity. This suggests create a stronger (due to bridges formed) yet porous part
that the NS powder is not only primed for reducing the while allowing the infiltration of a second phase. This also
viscosity effectively and ensuring the alignment of the suggests that, in addition to the mechanical properties
platelets by preventing the shear thickening of the slurry imparted from the infiltrated second phase, the toughness

Volume 3 Issue 1 (2024) 4 https://doi.org/10.36922/msam.2711