Carbon Nanotube Reinforced Composites: Metal and Ceramic ...

prevention of agglomeration of CNTs is of particular importance for nanocomposites
because most of their unique properties are only associated with individual
nanotubes. As mentioned before, fabrication technologies play a dominant role in
the uniform dispersion of nanotubes within the metal matrix. For CNT-Al nanocomposites
prepared by conventional powder mixing, hot pressing and hot extrusion, the
tensile strength of nanocomposites is inferior to that of pure Al due to the clustering of
nanotubes [Chap. 2, Ref. 16]. This demonstrates the absence of the load transfer effect
across the nanotube–matrix interface during tensile deformation.
George et al. [13] prepared the Al/MWNT and Al/SWNT nanocomposites by ball
milling composite mixtures followed by sintering at 580 C and hot extrusion at
560 C. Pure aluminum was also fabricated under the same processing conditions.
All these specimens were then subjected to tensile testing. The pure Al specimen
exhibits low yield strength of 80 MPa and Young s modulus of 70 GPa. The experimental
Young s modulus, yield strength and tensile strength of such nanocomposites
are summarized in Table 4.1. Apparently, the elastic modulus of Al increases by
12 and 23% by adding 0.5 and 2 vol% MWNT, respectively. The elastic modulus
values of the Al/0.5 vol% MWNT and Al/2 vol% MWNT nanocomposites correlate
reasonably with those predicted from the shear-lag model assuming an aspect
ratio of 100. Similarly, SWNT additions also enhance the stiffness and strength
of aluminum as expected. The enhanced yield strength of nanotube-reinforced
composites demonstrates that the applied load is effectively transferred across the
nanotube–matrix interface.
Deng et al. prepared the 2024 Al/MWNT nanocomposites by PM followed by cold
isostatic pressing and hot extrusion [Chap. 2, Ref. 35]. In the process, CNTs and 2024
Al powder (4.20 wt% Cu, 1.47 wt% Mg, 0.56 wt% Mn, 0.02 wt% Zr, 0.40 wt% Fe,
0.27 wt% Si) were dispersed in ethanol under sonication. The powder mixtures were
dried, mechanically milled, followed by cold isostatic pressing and hot extrusion at
450 C. Figure 4.2 shows the variations of relative density and Vickers hardness of
2024 Al/MWNT nanocomposites with nanotube content. The relative density and
hardness increase with increasing nanotube content up to 1 wt%. At 2 wt% MWNT,
the relative density and hardness decrease sharply due to the agglomeration
of nanotubes. The Young s modulus and tensile strength of 2024 Al/MWNT
nanocomposites also reach an apparent maximum at 1 wt% (Figure 4.3). The tensile
Table 4.1 Comparison of theoretical and experimental tensile data
for Al/MWNT and Al/SWNT nanocomposites.
Materials
Shear lag
Young s
modulus (GPa)
Experimental
Young s
modulus (GPa)
Experimental
yield strength
(MPa)
Al/0.5 vol%MWNT 74.31 78.1 86 134
Al/2 vol% MWNT 87.38 85.85 99 138
Al/1 vol% SWNT 79.17 70 79.8 141
Al/2 vol% SWNT 88.36 79.3 90.8 134
Reproduced with permission from [13]. Copyright Ó (2005) Elsevier.
4.2 Tensile Deformation Behaviorj107
Experimental
ultimate tensile
strength (MPa)