Carbon Nanotube Reinforced Composites: Metal and Ceramic ...

coating material, the plasma gun and component are enclosed in a low pressure
chamber containing an inert atmosphere. Plasma spraying offers improved coating
adhesion and higher density of coating due to its high spraying velocity. The HVOF
method employs a combustion flame of compressed fuel gas and oxygen to heat the
coating material rapidly in an internal combustion at 3000–6000 Candaccelerate
the coating material to high velocities ( 700–1800 ms 1 ) (Figure 2.1(b)). Molten
splats encounter extremely high cooling rates during solidification, that is,
10 6 –10 8 Ks 1 for PSF and 10 3 –10 5 for HVOF. Thermal spray coatings generally
contain pores and consolidation treatment such as sintering, spark plasma sintering,
hot pressing or laser irradiation is needed to remove porosities [22].
Recently, Agarwal and coworkers used both plasma spraying and HVOF techniques
to fabricate Al-23 wt% Si/10 wt% MWNT nanocomposite coatings (with 0.64
and 1.25 mm thicknesses) on the aluminum substrate [23–26]. Gas atomized Al-23
wt% alloy powder (15–45 mm) was blended and mixed with 10 wt% MWNTs in a ball
mill for 48 h to achieve homogeneous mixing prior to spraying. The microstructures
of thermally sprayed coatings are heterogeneous, consisting of splats and pores at
micro-level, and ultrafine primary Si particles and CNTs at submicron/nanolevel.
Moreover, CNTs are located between the splat interfaces and also embedded in the
matrix [25]. Figure 2.2(a) and (b) show cross-sectional SEM micrographs of the PSF
and HVOF sprayed 10 wt% MWNT/Al nanocomposite coatings. It can be seen that
the plasma sprayed coating is more porous, but HVOF sprayed coating is denser
due to its higher spraying velocity. TEM examination of the two coatings reveals the
presence of nanosized a-Al grains with uniformly distributed ultrafine primary
silicon particles [25]. The size of a-Al nanograins for the PSF and HVOF coatings
is 87 nm and 64 nm, respectively. The formation of a-Al nanograins is attributed
Figure 2.2 Cross-sectional SEM images of (a) plasma and (b)
HVOF sprayed MWNT/Al nanocomposite coatings. Reproduced
with permission from [26]. Copyright Ó (2008) Elsevier.
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