Carbon Nanotube Reinforced Composites: Metal and Ceramic ...

50j 2 Carbon Nanotube–Metal Nanocomposites
Figure 2.3 Reactive wetting kinetics at MWNT and hypereutectic
Al-Si alloy, showing gradual improvement in wettability with
formation of SiC layer. Reproduced with permission from [24].
Copyright Ó (2007) Elsevier.
to the fragmentation of dendritic structure under the heavy impact of spayed
molten droplets and restriction of grain growth as a result of very high cooling rates
during solidification. The primary silicon nanoparticles are produced by diffusing
out of silicon from the hypereutectic Al-Si alloy during solidification.
The wetting behavior between molten Al and the CNTs could lead to the generation
of new interface layer. During the initial stage of interfacial reaction, C from MWNTs
reacts with Si atoms of Al-Si droplet to form a thin b-SiC layer on the MWNTsurface.
The contact angle (q primary) is large at this stage as the Al-Si liquid droplet is in contact
with MWNTsurface. The SiC layer then spreads and grows laterally until the steadystate
reaction is achieved. The contact angle (qsteady) then decreases as the Al-Si alloy
flows over the SiC layer (Figure 2.3). TEM examination reveals the formation of b-SiC
layer at the nanotube-matrix interface. Plasma sprayed coating exhibits a thicker
SiC layer ( 5 nm) than the HVOF nanocomposite ( 2 nm) (Figure 2.4(a) and (b)).
The formation of b-SiC interfacial layer is responsible for the improved wettability
of the molten Al-Si alloy matrix with MWNTs [25].
The high strength and stiffness, and excellent flexibility of CNTs make them
excellent fillers to improve the wear resistance of bulk aluminum or coatings [19, 27].
Figure 2.4 Presence of silicon carbide layer on CNT surface in (a)
plasma and (b) HVOF formed hypereutectic Al-Si composite
reinforced with MWCT. Reproduced with permission from [24].
Copyright Ó (2007) Elsevier.