Carbon Nanotube Reinforced Composites: Metal and Ceramic ...

46j 2 Carbon Nanotube–Metal Nanocomposites
Table 2.3 Tensile properties of PM Al and its composites.
Materials Yield strength (MPa) Tensile strength (MPa) Elongation (%)
Pure Al 68 103 —
Al/1vol.% Si3N4 (15 nm) 144 180 17.4
Al/15 vol% SiC (3.5 lm) 94 176 14.5
Reproduced with permission from [13]. Copyright Ó (1996) Elsevier.
it is necessary to reduce the size of reinforcing ceramic particulates from micrometer
to nanometer level. Much effort has been directed towards the development of high
performance composites with high strength and modulus by adding ceramic
nanoparticles. Tjong and coworkers [13] demonstrated that the tensile strength of
pure aluminum increases markedly by adding only 1 vol% Si3N4 (15 nm) nanoparticles.
The tensile strength of such a nanocomposite is slightly higher than that of
aluminum composite reinforced with 15 vol% SiC (3.5 mm) particles. Furthermore,
the yield stress of Al/1 vol% Si3N4 nanocomposite is significantly higher than
that of the Al/15 vol% SiCp microcomposite. The enhancement in yield strength
of the Al-based nanocomposite is attributed to Orowan strengthening mechanism.
Moreover, the tensile elongation of nanocomposite is higher than that of microcomposite
(Table 2.3). Apparently, reducing the size of ceramic microparticles to
nanometer level is very effective in avoiding particle cracking, thereby improving
the tensile ductility of nanocomposite. A similar beneficial effect of ceramic nanoparticle
additions in improving the tensile performances of magnesium has also
been reported in the literature [14, 15]. It is considered that the incorporation of CNTs
with larger aspect ratio, higher tensile strength and stiffness as well as better
flexibility, into metals can yield even much better mechanical performances than
ceramic nanoparticles.
2.3
Preparation of Metal-CNT Nanocomposites
Limited research has been conducted in the preparation, structural, physical and
mechanical properties of metal-CNT nanocomposites due to the lack of appropriate
processing technique, difficulty of dispersing CNTs in metal matrices and lack
of proper understanding of the interfacial issue between CNTs and metals. As
mentioned before, CVD-grown MWNTs with large aspect ratios exhibit spaghetti-like
coiled features, and SWNTs often agglomerate into ropes of several tens of nanometers.
Accordingly, CNTs tend to form clusters in metals during processing,
leading to poor mechanical properties. For instance, Kuzumaki et al. studied the
microstructure and tensile properties of Al/5 vol% CNT and Al/10 vol% CNT nanocomposites
prepared by conventional powder mixing, hot pressing followed by
hot extrusion [16]. Very little improvement in tensile strength was found in the
two composites due to inhomogeneous dispersion of CNTs in the metal matrix.