Carbon Nanotube Reinforced Composites: Metal and Ceramic ...

64j 2 Carbon Nanotube–Metal Nanocomposites
2.5.3
Friction Stir Processing
Friction stir processing (FSP) is a solid state processing method, developed on the
basic principles of friction stir welding; it is an effective tool to form fine-grained
microstructures in near surface region of metallic materials. In the process, a rotating
pin tool is inserted to the substrate, the resulting frictional heating and the large
processing strain produce microstructural refinement, densification and homogenization
[64]. Mishra et al. fabricated the Al-based surface composites by dispersing
SiC powders (0.7 mm) on the surface of Al 5083 Al (Al-Mg-Mn) alloy plate followed by
FSP treatment [65]. They reported that SiC microparticles were well bonded and
dispersed in the Al matrix of surface composite layer ranged from 50 to 200 mm.
Furthermore, FSP resulted in significant grain refinement in surface layer.
Recently, Morisada et al. fabricated AZ31/MWCNT surface nanocomposite by
using the FSP technique [66]. MWCNTs were initially filled into a groove on the
AZ31 (Mg-3Al-1Zn) alloy plate followed by the insertion of a pin into the groove.
The tool was rotated at a fixed speed of 1500 rpm, but its travel speed varied from
25 to 100 mm min 1 . The dispersion of CNTs depend greatly on the travel speed of
pin tool. Figure 2.18(a)–(f) show SEM micrographs of AZ31/MWCNT surface
nanocomposite specimens prepared from FSP with the pin traveling at different
speeds. At a high travel speed of 100 mm min 1 , entangled CNTs in the form of
large clusters can be readily seen in the surface composite (Figure 2.18(b)). This is
because the high travel speed cannot produce sufficient frictional heat to mix
CNTs properly in the alloy (Figure 2.18(d). Large MWNT clusters tend to break up
into smaller agglomerates by decreasing the travel speed of pin tool. A better
distribution of MWNTs is achieved at a low speed of 25 mm min 1 (Figure 2.18(f)).
Careful examination of this micrograph reveals that CNTs are still agglomerated
into small clusters. Much more work is needed in future to improve the dispersion
of nanotubes using FSP.
2.6
Titanium-Based Nanocomposites
Titanium alloys are structural materials widely used in aerospace, chemical,
biomedical and automotive industries due to their excellent mechanical properties
at room temperature. However, titanium alloys exhibit low mechanical strength
at high temperatures. Continuous SiC fibers and TiC particulates have been
incorporated into Ti alloys to enhance their mechanical performances [67–71].
Very little information is available in the literature regarding the fabrication and
microstructural behavior of Ti-based nanocomposites [72]. The difficulty in dispersing
CNTs in high reactivity titanium matrix hinders the development of Ti/CNT
nanocomposites. This is a big challenge for materials scientists: to produce Ti/CNT
nanocomposites with desired mechanical properties using novel processing
techniques.