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Materials Science in Additive Manufacturing Cold spray additive manufacturing of Cu-based materials

A B distribution of CNTs in the copper matrix. The MWCNT
and copper had good clean and closed interfaces,
contributing to low thermal resistance. Furthermore, these
were moderately damaged by the ball milling process,
which helped retain the desirable properties in the coatings.

In another study, Pialago and Park further increased the
[44]
amount of CNTs in the copper composite powder feedstock
C D to 15 vol.%. They successfully deposited copper CNT
composite powders via cold spray process after ball milling.
According to the report, with the increase in the volume
percent of CNT reinforcement particles in the feedstock
(ranging from 5 to 15 vol.%), the deposition efficiency
decreased. Furthermore, the particle size decreases with
increasing ceramic content, mostly because the embedded
Figure 11. Scanning electron microscopy micrographs of wear cross- CNTs initiated the cracks during ball milling. This process
section regions in copper-based coatings: (A) Pure copper, (B) copper- enhanced the fracturing against the cold welding process of
alumina (10 wt.%), (C) copper-alumina (10 wt.%)-copper-coated graphite the particles. The cold welding and fracturing of the powder
(10 wt.%), and (D) energy-dispersive spectrum of copper-alumina particles may also vary with different milling parameters
(10 wt.%)-copper-coated graphite (10 wt.%) tribolayer . (Reprinted such as powder composition, size of the balls used, ball-to-
[31]
from Journal of Thermal Spray Technology, 27(8), Chen, W., Yu, Y., Cheng, [44]
J., Wang, S., Zhu, S., Liu, W., and Yang, J., Microstructure, Mechanical powder ratio, and time of milling .
Properties and Dry Sliding Wear Behavior of Cu-Al2O3-Graphite Solid- Furthermore, from the study by Pialago et al. , they
[45]
Lubricating Coatings Deposited by Low-Pressure Cold Spraying, 1652 –
1663, 2018, with permission from Springer Nature). fabricated copper-based metal matrix composites with
CNT and SiC additions (Figure 12). The compositions they
made were copper 5%CNT, copper 5%CNT-10%SiC, and
coated graphite (10 wt.%) cold-sprayed coating, a tribolayer copper 5%CNT-20%SiC along with pure copper coatings.
of copper, alumina, and graphite enhanced the solid As expected, it was reported that with the increase in
lubrication, which resulted in no crack formation . ceramic content, the fracturing of powder particles (while
[31]
4.2. Copper-carbon nanotube (CNTs) coatings milling) decreased compared to cold welding, possibly
because the CNT and SiC served as crack initiators.
The CNTs have excellent properties such as stiffness, Interestingly, the copper 5%CNT-20%SiC was an exception
strength, elastic modulus, electrical, and thermal as there was an agglomeration in these powder particles
conductivities. Because of the properties of CNTs, the because of increased surface energy due to a reduction in
studies on the incorporation of CNTs in composites are the size of particles. A similar trend was seen in deposition
widely pursued [6,32-37] . CNTs can be used as reinforcements efficiency, where pure copper had the highest deposition
in improving strength of composites [38-40] . In 2012, Cho efficiency. However, the deposition efficiency decreased
et al. prepared multi-walled CNT (MWCNT) reinforced with increasing ceramic content for copper 5%CNT and
[41]
copper coatings by cold spray method. As reported, they copper 5%CNT-10%SiC. Note that copper 5%CNT-
could substantially avoid the structural damage caused to 20%SiC had deposition efficiency and coating thickness
the CNTs by the low-pressure cold spray process compared almost equivalent to pure copper coatings. The reason
to other thermal and high-pressure fabrication processes. reported for this exception was the increased amount of
In their work, the ball milling process employed to mix the ceramics that enhance metal deformation and increase
CNTs with copper powder ensured a satisfactory level of deposition efficiency. Furthermore, copper 5%CNT-
mixing of the particles before spraying. The 3 vol.% multi- 20%SiC coatings had the highest hardness, around 240
walled CNT in copper powder feedstock was successfully Hv , as compared to all other coatings .
[45]
sprayed onto the aluminum substrate, and the coatings 0.1
produced had high thermal diffusivity compared to pure The cold spray ternary coatings of copper-CNT-AlN
copper coatings. The CNTs were homogeneously dispersed composites were also studied in the literature . These
[46]
in the copper matrix, which accounted for high thermal composites were produced by the ball milling mechanical
diffusivity. In some previous reports, the presence of alloying technique. The coating compositions made were
bundled or non-homogeneous dispersion of CNT decreases copper-5CNT, copper-5CNT-10AlN, copper-10CNT-
thermal diffusivity [42,43] . Therefore, Cho et al. employed ball 20AlN, and pure copper (Figure 13). The deposition
milling as an effective strategy to enhance the homogeneous efficiency reported was highest at around 0.2 for pure

Volume 1 Issue 2 (2022) 9 https://doi.org/10.18063/msam.v1i2.12