Carbon Nanotube Reinforced Composites: Metal and Ceramic ...

52j 2 Carbon Nanotube–Metal Nanocomposites
consolidation of nanosized metal powders at high temperatures always lead to fast
grain growth. The size of matrix grains can grow up to micrometer scale, depending
upon processing temperature and time. In general, it is advantageous to retain the
size of matrix grains of composites in nanometer region because significant
improvement in hardness, yield strength and wear resistance of composites can be
obtained in the presence of nanosized grains. For instance, Zhong et al. attempted
to prepare nano-Al/SWNT composites having a nanograined matrix [29]. The
composites were prepared by mixing Al nanopowders (53 nm) and purified SWNTs
in alcohol ultrasonically. The blended material was dried, cold compacted into
disks at room temperature, followed by hot consolidation at 260–480 C under a
pressure of 1 GPa. Excessive grain growth occurred during hot compression of Al
nanopowders. The grain size of the matrix of composite grew up to 800 nm during
hot compaction at 480 C. The nanotubes have little effects in restricting the grain
growth of matrix grains. Furthermore, SWNTs dispersed as bundles rather than
individual nanotubes in the Al matrix.
Another complication arising from PM processing of metal-matrix nanocomposites
is agglomeration of CNTs. Homogeneous dispersion of nanotubes in Al
matrix cannot be achieved by conventional mixing, pressing, sintering of Al
powders and CNTs [16]. Effort has been made toward uniform dispersion of CNTs
in the metal matrix by modifying conventional PM process. For example, Esawi and
El Borady used a powder can rolling technique to fabricate the Al/MWNT
nanocomposites [30]. In the process, Al powders and MWCNTs were mixed in a
planetary mill for 5 h without using milling media. The mixture was encapsulated
in copper cans under an argon atmosphere, and subjected to hot rolling (Figure 2.5).
The rolled cans were then sintered in a vacuum furnace at 300 Cfor3h.Thecans
were removed, and the strips were sintered in an air furnace at 550 C for 45 min.
This technique can yield better dispersion of CNTs in aluminum matrix by adding
very low MWCNTcontent, that is, 0.5 wt% (Figure 2.6(a) and (b)). Spherical dimples
associated with ductile fracture can be readily seen in these micrographs. At 1 wt%
MWCNT, nanotubes tend to agglomerate into clusters, leading to inferior mechanical
properties (Figure 2.7).
Figure 2.5 Powder blending and can rolling process. Reproduced
with permission from [30]. Copyright Ó (2008) Elsevier.