Carbon Nanotube Reinforced Composites: Metal and Ceramic ...
Carbon Nanotube Reinforced Composites: Metal and Ceramic ...
Carbon Nanotube Reinforced Composites: Metal and Ceramic ...
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Table 7.5 Mechanical properties of monolithic Si3N4 <strong>and</strong> Si3N4/1<br />
wt% MWNT nanocomposite spark plasma sintered at different<br />
temperatures.<br />
Materials<br />
7.6<br />
Wear Behavior<br />
Sintering<br />
temperature ( C)<br />
Density<br />
(g cm 3 )<br />
Vickers<br />
hardness<br />
(GPa)<br />
Fracture<br />
toughness<br />
(MPa m 1/2 )<br />
Si3N4 1500 3.23 20.1 0.9 5.2<br />
Si 3N 4 1650 3.24 18.3 0.5 6.5<br />
Si3N4/1 wt% MWNT 1500 3.17 16.6 0.4 5.3<br />
Si 3N 4/1 wt% MWNT 1650 3.19 19.1 0.6 4.4<br />
Reproduced with permission from [Chap. 5, Ref. 120]. Copyright Ó (2005) Elsevier.<br />
7.6 Wear Behaviorj207<br />
Hard <strong>and</strong> superhard ceramic nanocomposite coatings have attracted increasing<br />
attention due to their potential applications in diverse areas such as cutting tools,<br />
bearings, micro-electro-mechanical systems (MEMS), magnetic disk drives, <strong>and</strong> so<br />
on. Hard coatings improve the durability of substrate materials in hostile environments<br />
against severe wear, thus prolonging the materials life. Typical ceramic<br />
nanocomposite coatings are composed of nanocrystalline transition metal carbides<br />
embedded in amorphous covalent nitride (e.g., Si3N4, or BN) matrix [26].<br />
Hard nanocomposite coatings are often very brittle, which strongly limits their<br />
practical use. Tough nanocomposite coatings can be realized by embedding solid<br />
lubricant soft phase such as transition metal dichalgonenides (MoS2, WS2, NbSe2,<br />
etc.) in an amorphous ceramic matrix [27] as shown in Figure 7.19(a) <strong>and</strong> (b).<br />
As mentioned before, CNTs also exhibit self-lubrication behavior. In combination<br />
with their other attractive mechanical <strong>and</strong> physical properties, CNTs have consistently<br />
outperformed their transition metal dichalgonenide rivals. <strong>Ceramic</strong>-CNT<br />
nanocomposites have been shown to exhibit low friction coefficient <strong>and</strong> wear rate,<br />
making them useful as wear-resistant materials for structural components in<br />
industrial sectors. In general, the distribution of CNTs in ceramic matrix affects<br />
the wear behavior of ceramic-CNTnanocomposites considerably. Recently, Lim et al.<br />
investigated the effect of CNT dispersion on tribological behavior of the of Al2O3/<br />
MWNT nanocomposites [28]. Two processing routes were adopted to prepare<br />
nanocomposites. The first processing route consisted of ball milling of CNTs <strong>and</strong><br />
alumina powders in ethanol followed by hot pressing at 1850 C. The second route<br />
was ball milling of slurry consisting of a mixture of MWNT, alumina, methyl-isobutyl<br />
ketone, poly(vinyl)butyral <strong>and</strong> dibutyl phthalate. These organic agents acted as<br />
solvent, dispersant, binder <strong>and</strong> plasticizer, respectively. The slurry was poured into<br />
a tape-casting equipment, followed by lamination <strong>and</strong> hot pressing at 1850 C. Tape<br />
casting gives better distribution of CNTs in alumina matrix, resulting in the<br />
formation of dense nanocomposites (Figure 7.20). However, the r.d. of hot-pressed<br />
nanocomposites decreases continuously with increasing nanotube content.