Biomedical Engineering Reference
In-Depth Information
through different mechanisms, which will be explained later in the
chapter.
.
When ceramic particles approach nanosize, their thermal properties
change due to the increase in surface energy and different interatomic
spacing.
.
When the reinforcing phase scales down to nanometersize, it leads to
difficulties in material testing technology. TEM instead of SEM is
commonly used for particles observation. Motta et al. (2004) illustrated
different methods to observe the distribution of nanoparticles in Cu/
Al
2
O
3
nanocomposites.
However, even with all these difficulties, MMNCs have shown other
promising characteristics that still attract researchers in this field. For
example, the ductility of the metal matrix is expected to remain the same
while other properties of MMNCs would be enhanced considerably (e.g.
improved strength and creep properties, better elevated temperature
performance, and better machinability (Cao et al., 2008a)). Also low-
volume nanoparticles can help keep the thermal and electrical conductivity
of the MMC whilst improving the mechanical properties. This character is
essential for functional composites such as Cu/Al
2
O
3
, used as electronic
packaging material. He et al. (2008) successfully produced nanodiamond
(ND)/copper nanocomposite, which maintained the electrical conductivity
of copper whilst increasing the mechanical properties of the metal matrix.
Furthermore, nanoparticles in a metal matrix will affect the solidification
and heat treatment process, which will have a strong effect on micro-
structure and the mechanical properties of the MMNC. A good example is
that, with the existence of nano SiC particles, the Mg matrix grain size is
refined, which will lead to balanced mechanical properties, as reported by
Cao et al. (2008a). The assumption is that nanoparticle could act as a
nucleate during the solidification process. Also, the presence of nanopar-
ticles can suppress the softening processes (i.e. dislocation annihilations) and
pinned grain boundaries, resulting in a more equiaxed grain structure in the
composite. The dispersion of Al
2
O
3
particles into a copper matrix produces
Cu/Al
2
O
3
nanocomposites with excellent resistance to high-temperature
annealing while maintaining high thermal and electrical conductivities
(Motta et al., 2004).
The most commonly used metals in MMNC research are aluminum,
copper, magnesium, titanium, and iron. The reinforcing constituent is
normally ceramic, including borides, carbides, nitrides, oxides, and their
mixtures. Mechanical, chemical, and physical properties of both matrix and
reinforcement should be considered when selecting the material system
(Ibrahim et al., 1991). For in-situ production processes, the thermal stability
of different ceramic particles is the main factor that determines the
