Carbon Nanotube Reinforced Composites: Metal and Ceramic ...

54j 2 Carbon Nanotube–Metal Nanocomposites
homogeneous dispersion of ceramic microparticles in metals [42–45]. Since MA is a
non-equilibrium process, it produces a variety of supersaturated solid solutions,
metastable crystallites and amorphous metal alloys. The process involves loading
the powders into a high-energy ball mill containing a grinding medium such as
stainless steel or alumina balls. The total milling energy can be manipulated by
varying the charge ratio (ratio of the weight of balls to the powder), ball mill design,
milling atmosphere, time, speed and temperature. Various types of ball mills
consisting of vials/tanks and grinding balls can be used for this purpose. These
include the Spex shaker mill, platenary ball mill, and attritors [44]. For the shaker mill,
the motor vigorously shakes the vials, resulting in high energy impacts between the
balls and powder materials. A platenary ball mill employs strong centrifuge forces to
develop high-energy milling action inside the vial. In general, large quantity of
powders can be processed using an attritor mill consisting of a tank container,
rotating impeller and grinding ball.
During mechanical alloying, the powders experience repeated fracturing, welding
and plastic deformation as a result of high-energy collision of powders with the
ball media during milling, thus inducing structural changes and chemical reactions
at room temperature. Consequently, the microstructure of milled powders becomes
finer and can be reduced to nanometer range depending on the processing conditions.
In certain cases, a process control agent (PCA), which is mostly organic
compound, is added to the powder mixture during milling. The PCA adsorbs onto the
surface of the powder particles and minimizes cold welding between powder
particles, thereby suppressing agglomeration [44]. MA powders are finally consolidated
into full density using hot pressing, hipping or extrusion.
The degree of homogenous dispersion of CNTs in metals using MA technique
depends on several processing variables such as type of mill, ball-to-powder weight
ratio, milling time, process control agent, and so on. Esawi and Morsi investigated
the effects of MA time and CNT content on the dispersion of MWNTs in
aluminum powder [36, 37]. SEM micrographs showing microstructural evolution
of the Al/2 wt% MWNT and Al/5 wt% MWNT powders during mechanical alloying
for different periods of time are shown in Figure 2.8. In the process, Al powders
(75 mm) and MWNTs (average diameter of 140 nm, length of 3–4 mm) of the
correct proportion were placed in stainless steel jars containing stainless steel balls
with a ball-to-powder weight ratio of 10 : 1. The jars were filled with argon and then
agitated using a planetary ball mill at 200 rpm for various milling times. For the
Al/2 wt% MWNT sample, aluminum powders were first flattened and formed
flakes after milling for 0.5 h due to the high energy collision with stainless steel
balls. With increasing milling time to 3 h, the flakes began to weld together,
forming large particles of rougher surface. After 6 h, large particles of smooth
surface were developed. These particles reached a size of 1.6 mm and 3 mm,
respectively after 18 h and 24 h of milling. Thus, particle welding is pronounced
for Al/2 wt% MWNT sample due to the excellent ductility of Al in this composite
and dynamic recovery process during milling. In contrast, large particles are
not observed in Al/5 wt% MWNT sample. The incorporation of 5 wt% MWNT
into Al impairs its tensile ductility. The large powder sizes obtained for the