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Materials Science in Additive Manufacturing Alumina platelets additive manufacturing

Figure 3. Fabrication process of the nacre-inspired samples for the three-point bending test.

each print was 100 μm. The dimensions of the samples First, the sintering temperature at 1600°C did not fully
for bending test measure 45 mm × 3 mm × 4 mm. In the densify the printed part, which resulted in poor or little
second part of this study, we sintered the samples at 1600°C bonding between the alumina platelets and nanoparticles.
before infiltration. Moreover, the presence of nanoparticles between the
Figure 4A illustrates a simple three-point bending test platelets also prevented a full densification of the printed
with the applied load perpendicular to the aligned platelets part. Furthermore, obtaining a fully dense part would likely
until failure. The flexural stress and strain (Figure 4B) were restrict the infiltration of a secondary phase. Nevertheless,
then calculated from the applied load and displacement sintering at a higher temperature or reducing the amount
with Equations II and III, respectively. of nanopowder may improve the flexural strength of the
3FL part due to greater mass diffusion and, hence, sintering of
s= (II) the powders.
2bd 2
The second reason for the low strength is due to the type
of polymer used for the secondary phase. Aside from the
6Dd
ε = 2 (III) weaker mechanical properties of HDDA, we suspect that
L the weak interfacial bonding between the alumina phase
Observations from the composite (Figure 4C) under and the polymer phase led to the peeling of the platelets.
the SEM revealed several types of defects that contributed This failure mode is shown in Figure 4D and F. In general,
to the failure of the sample, as depicted in Figure 4D–F. the interface damage mechanism is governed by the
The majority of these defects included platelets pull-out, attraction forces to polymer chains from the alumina phase
platelets peeled away from the polymer phase, and severed 19. When compared to the polymer phase, these attraction
bridges that formed from the sintering of the nanoparticles. forces from the alumina phase to the polymer chains
The absence of fractured platelets suggests that only the are found to be weaker. Therefore, this weak interfacial
platelet-polymer interface and bridges sustained the load, bonding possibly served as a preferential manner of energy
which contributed to the flexural strength of the composite. dissipation and failure when loaded despite there being no
However, since the flexural strength of the HDDA was obvious defects at the interface, as shown in Figure 2F.
reported to be approximately 20 MPa, the calculated stress of
more than 50 MPa suggested that there are other toughening The last probable cause of the weak composite is
mechanisms. For instance, in addition to the energy attributed to the defects from additive manufacturing, such
19
dissipation from the fracturing of the bridges, cracks from as delamination between layers. The delamination between
the polymer phase could be seen deflected by the platelets layers would create huge pores in the part after sintering,
in Figure 4F, which prevented crack propagation directly which would then be filled with the polymer, as depicted in
through the sample. According to the bending tests, the Figure 4G. This resulted in a composite consisting more of a
fabrication of this composite based on the above-described weaker polymer, hence lowering the strength. Furthermore,
additive manufacturing method yielded a strength lower the greater surface contact area between the polymer
than expected. We consider a couple of factors that may and ceramic phase as a result of this defect also offered
have contributed to the low flexural strength. a preferential location for failure, such as peeling of the

Volume 3 Issue 1 (2024) 6 https://doi.org/10.36922/msam.2711