Carbon Nanotube Reinforced Composites: Metal and Ceramic ...

Table 2.4 Changes in the size and volume fraction of the porosity
and primary silicon in thermally sprayed Al/MWNT
nanocomposites after sintering at 400 C for different time
periods.
As sprayed
However, the presence of pores in plasma- and HVOF sprayed coatings has
deleterious effects on the hardness and stiffness. This in turn leads to lower
tribological performance. In this regard, post-spraying sintering is needed and
found to be very effective in removing pores in plasma and HVOF sprayed coatings
(Table 2.4). Moreover, sintering treatments also increase the volume fraction of
primary Si particles for both nanocomposite coatings.
In a recent study, Agarwal and coworkers also attempted to deposit Al-12Si/MWNT
coatings with thickness up to 500 mm using cold spraying [28]. Cold spraying is an
emerging coating process involving the injection of powder particles into a supersonic
speed gas jet onto a substrate through a de-Laval type of nozzle. The coating is
formed through plastic deformation of sprayed particles in the solid state during
impact at temperatures well below the melting point of spray materials. Thus the
deleterious effects inherent in conventional thermal spraying processes such as
phase transformation, grain growth and oxidation can be minimized or avoided.
From microstructural observation, CNTs are also found at the splat interfaces of
the coatings. The nanotubes are shortened in length due to the high velocity impact
of the spray particles.
2.4.2
Powder Metallurgy Processing
24 h sintering
at 400 C
72 h sintering
400 C
PSF HVOF PSF HVOF PSF HVOF
Pore
Size(mm) 1.7–7.8 1.1–2.4 0.8–5.3 1.2–2.1 0.9–4.3 1.0–2.8
%VPore 6.7 0.4 3.2 0.3 4.3 0.5 2.5 0.2 3.2 0.3 2.1 0.03
Primary Si
Size(mm) 0.8–2.0 1.2–2.4 0.8–3.3 1.5–4.2 1.1–2.7 0.9–3.5
%V Si 14.1 1.9 14.9 1.0 17.3 1.3 18.3 1.1 17.7 1.5 18.1 1.5
Reproduced with permission from [26]. Copyright Ó (2008) Elsevier.
2.4 Aluminum-Based Nanocompositesj51
Powder metallurgy processing is a versatile method commonly used to make Al–CNT
nanocomposites due to its simplicity. Aluminum alloy powders of micro- or nanometer
sizes are blended ultrasonically with CNTs in an organic solvent (e.g., alcohol),
followed by solvent evaporation, sintering or hot consolidation of powdered mixture.
However, materials scientists have encountered a number of difficulties during
PM processing of metal-matrix nanocomposites. For instance, sintering and/or