Carbon Nanotube Reinforced Composites: Metal and Ceramic ...

158j 5 Carbon Nanotube–Ceramic Nanocomposites
Silicon carbide exhibits different crystalline structures from hexagonal (a-SiC),
cubic (b-SiC) to rhombohedral. Of these, cubic b-SiC is particularly important
because of its higher bending strength, hardness, stiffness and fracture toughness
when compared with a-SiC. Because of the high melting point of silicon carbide, PM
method becomes the primary processing technique for making ceramic products.
Further, silicon carbide exhibits poor sinterability due to its strong covalent bonding
and high melting point. Thus, sintering aids must be added to obtain dense ceramic
specimens. SiC-CNT nanocomposites have been fabricated by spray pyrolysis [112],
conventional powder mixing followed by hot pressing [41] or by SPS [113, 114],
microwave synthesis [115] and preceramic polymer precursor methods [116]. Spray
pyrolysis of xylene suspension containing ferrocene and SiC powders into a reactor at
1000 C produces SiC composite flakes with uneven distribution of nanotubes [112].
Ma et al. [41] fabricated the (SiC þ 1% B4C)/10%CNT nanocomposite by blending
SiC nanopowder (80 nm), CNTand sintering aid (B4C) in butylalcohol ultrasonically,
followed by hot-pressing at 2000 C for 1 h. High temperature is required for good
consolidation of these powders into bulk nanocomposite. However, high temperature
sintering can result in severe grain growth and destruction of the integrity of
nanotubes.
Spark plasma sintering with very short processing time is an alternative consolidation
route for SiC-CNT nanocomposites. However, SPS of SiC-based materials
must be carried out at temperatures 1800 C due to the strong covalent bonding of
ceramics. Hirota et al. studied the effect of SPS temperature on the microstructure of
monolithic SiC and its composites reinforced with carbon nanofibers [114]. Monolithic
SiC and its composites were prepared by direct mixing of powder constituents
in methyl alcohol followed by ball milling and SPS. Figure 5.27 shows the density and
average grain size of monolithic b-SiC as a function of plasma sintering temperature.
The relative density of b-SiC sintered at 1700 C is only 80.9% but increases to 96.4%
at 1800 C. The density saturates with further increasing temperature. Further,
the grain size of b-SiC increases with increasing temperature as expected. The
incorporation of carbon nanofibers into monolithic SiC has little effect on the relative
density with the value kept at about 96%. However, the grain size decreases sharply
from 4.2 mm to 1.3 mm by adding 5 vol% VGCF, and reduces slightly to 1.2 mmat
15 vol% VGCF (Figure 5.28).
To preserve the properties of CNTs, it is advantageous to fabricate the SiC/CNT
nanocomposites at lower temperatures, about 1300 C. In this regard, polymer
derived ceramics (PDCs) show promise to make such composites at relatively lower
temperatures. This process involves the initial cross-linking of polymer precursors
followed by a thermal induced polymer to ceramic transformation. The networks of
organoelement compounds are directly converted into covalent bonded ceramics
during pyrolysis. Typical polymer precursors commonly used include polycarbosilane
(PCS), polysiloxane (PSO), and polysilazane (PSZ), producing amorphous Si C,
Si O Si and Si C N ceramics respectively upon pyrolysis. PDCs offer several
advantages over conventional powder mixing and sintering in terms of the ease of
controlling the structure of ceramics by designing the chemistry of polymer precursors,
homogeneous chemical distribution at molecular level, near net shape