Carbon Nanotube Reinforced Composites: Metal and Ceramic ...

This implies the absence of effective load transfer effect from metal matrix to CNTs
during tensile deformation.
In addition to homogeneous dispersion of nanotubes, the interface between the
reinforcement and the matrix plays a crucial role in load transfer and crack
deflection mechanisms. A controlled interface with strong CNT-matrix interactions
generally promotes effective load transfer effect. More recently, Ci et al. investigated
the interfacial reaction between MWNTs and aluminum by sputtering pure Al on the
surface of aligned MWNT arrays The thin film samples were then annealed at
400–950 C [17]. Transmission electron microscopyTEM examination revealed
that aluminum carbide (Al4C3) was formed at the MWNT-Al interfaces and the tips
of the MWNTs after annealing. These carbide particles improve the interfacial
bonding between CNTs and Al layers. As recognized, processing conditions can
affect the structure and property of the nanotube-matrix interface. Generally CNTs
have poor compatibility and wetting with metallic materials. Special surface treatments
for CNTs [18, 19] and/or modification of conventional processing techniques
are adopted by some researchers to achieve stronger interfacial bonding between
CNTs and metals.
Another important factor to be considered in fabricating CNT–metal nanocomposites
is selection of the correct CNT materials. In general, the use of SWNTs as
reinforcements is preferred and beneficial for mechanical properties due to their
higher mechanical strength. From the economical viewpoint, MWNTs and VGCFs
offer distinct advantages over SWNTs. However, the reinforcement efficiency
of MWNTs is far from the expected value. The use of MWNTs as intrinsic reinforcements
for composite materials may not allow the maximum strength to be achieved
due to non-uniform axial deformation inside the tubes [20]. MWNTs are made up
of concentric graphene sheets, linked only by weak van der Waals forces. All the inner
sheets can rotate and slide freely at low applied forces, thus can detach from outer
walls in a sword and sheath mechanism
At present, CNT–metal nanocomposites can be processed by means of thermal
spray forming, liquid metal processing, powder metallurgy, severe plastic deformation,
friction stir processing, electrochemical deposition and molecular-level mixing
techniques. The beneficial effects and drawbacks of these techniques in forming
MMCs are discussed in the following sections.
2.4
Aluminum-Based Nanocomposites
2.4.1
Spray Forming
2.4 Aluminum-Based Nanocompositesj47
Aluminum and its alloys have been selected as the matrix materials for nanocomposites
due to their low melting point, low density, adequate thermal conductivity,
heat treatment capability, processing versatility and low cost. Thermal spraying is an
effective processing technique for the deposition of a wide variety of materials