Carbon Nanotube Reinforced Composites: Metal and Ceramic ...

isostatic pressing. In certain cases, secondary mechanical treatment such as extrusion
or forging is required to shape the compacts into full-density products. The main
drawback of the PM route is the high cost of raw powders. The liquid metal route
offers high production yield and low cost, but suffers from filler segregation and
pore formation. Two liquid processing techniques are currently used to prepare
aluminum composites reinforced with CFs, that is, liquid metal infiltration and
squeeze casting [4]. Melt infiltration involves infiltration of molten aluminum into
a fiber preform in a protective nitrogen atmosphere. However, carbon tends to react
with aluminum and forms a brittle Al4C3 phase due to the high temperature
environment, long processing and solidification times. This leads to poor mechanical
properties of the resulting composites. A deleterious chemical reaction between
carbon and aluminum, poor wetting of carbon by molten aluminum and formation
of an intermetallic phase are the main issues encountered in the processing of the
Al/CF composites by liquid metal processing.
2.2
Importance of Metal-Matrix Nanocomposites
2.2 Importance of Metal-Matrix Nanocompositesj45
Carbon fibers display high tensile strength and stiffness, but exhibit extremely low
fracture strains. The incorporation of a large volume fraction of rigid CFs into metals
deteriorates their tensile ductility and toughness significantly. In this regard, carbon
nanotubes (CNTs) with even higher tensile strength, stiffness and flexibility are
far superior to CFs as reinforcements for metals. The addition of low loading level
of CNTs to metals enhances the strength and stiffness of nanocomposites. Incorporation
of CNTs does not seem to impair the fracture toughness of metals. In certain
cases, a dramatic improvement in tensile ductility in CNT-reinforced nanocomposites
over unreinforced metals has been reported [5]. Furthermore, CNTs with
excellent thermal conductivity are effective heat dissipating fillers for metals for
producing thermal management components in electronic devices. At present,
ceramic particle-reinforced Al-based MMCs are widely used in electronic packaging
and thermal management applications. However, extremely large volume contents
(>50%) of ceramic particulates are needed to achieve the required thermal dissipating
effect [6, 7].
Recently, the escalation of fossil fuel prices and the need to minimize carbon
dioxide emission have driven the search for light-weight structural materials in
aerospace and automotive industries for the reduction of fuel consumption. Conventional
particle-reinforced MMCs for structural applications contain ceramic
fillers up to 25 vol% [8, 9]. Such a large amount of microfiller increases the weight
of composites and degrades their processability and mechanical performance
significantly. The size of ceramic particles generally range from a few to several
hundred micrometers. Fracture of microparticles occurs readily during mechanical
deformation, with cracks perpendicular to the applied load in tension and parallel to
it in compression [10–12]. Therefore, microparticle-reinforced MMCs always
possess low tensile strength and ductility. To minimize or avoid particle cracking,