Carbon Nanotube Reinforced Composites: Metal and Ceramic ...

200j 7 Mechanical Properties of Carbon Nanotube–Ceramic Nanocomposites
[Chap. 5, Ref. 76, Chap. 5, Ref. 77]. The fracture toughness of alumina (A-SD) coating
is 3.22 0.22 MPa m 1/2 and increases to 3.862 0.16 MPa m 1/2 by adding 4 wt%
MWNT (A4C-B) coating. The fracture toughness increases to 4.60 0.27 MPa m 1/2
in the A4C-SD coating due to improved MWNTdispersion. It is further improved to
5.04 0.58 MPa m 1/2 by adding 8 wt% MWNT (A8C-SD). The enhanced toughness
of the alumina/MWNT nanocomposite coating is attributed to the CNT bridge
formation, crack deflection and MWNT pull-out [24].
7.3.2
Silica Matrix
Ning et al. fabricated the SiO 2/MWNT nanocomposites by direct powder mixing in
ethanol and hot pressing at 1300 C under 25 MPa in nitrogen atmosphere for 30 min
[Chap. 5, Ref. 87]. As conventional powder mixing and hot pressing were used, the
dispersion of MWNTs in silica matrix is inhomogeneous (Figure 7.12(a) and (b)).
Agglomeration of nanotubes is more apparent at 10 vol% MWNT. Thus, the bending
strength and fracture toughness of silica can improve by adding only 5 vol% MWNT.
These mechanical properties degrade markedly with further increasing nanotube
content (Figure 7.13). In another study, Ning et al. fabricated the SiO2/5 vol% MWNT
nanocomposite by the sol-gel method with and without C 16TAB surfactant.
The resulting powder mixtures were also hot pressed at 1300 C under 25 MPa in
nitrogen atmosphere [Chap. 5, Ref. 89]. They reported that the bending strength and
fracture toughness of the SiO2/5 vol% MWNT nanocomposite are 70.2 5 MPa and
2.18 0.15 MPa m 1/2 , respectively. And the bending strength and fracture toughness
of the SiO2/(5 vol% MWNT þ C16TAB) nanocomposite are 97.0 10 MPa and
2.46 0.11 MPa m 1/2 , respectively. Compared with the mechanical properties of the
SiO2/5 vol% MWNT nanocomposite prepared by conventional powder mixing as
shown in Figure 7.13, sol-gel prepared SiO 2/(5 vol% MWNT þ C 16TAB) nanocomposite
exhibits higher bending stress and toughness due to better dispersion of CNTs
in silica matrix.
Guo et al. [25] employed colloidal dispersion followed by attrition milling and
SPS to produce the SiO2/5 vol% MWNT and SiO2/10 vol% MWNT nanocomposites.
Figure 7.12 (a) Low and (b) high magnification SEM fractographs
of SiO2/5 vol% MWNT nanocomposite. Reproduced with
permission from [Chap. 5, Ref. 87]. Copyright Ó (2003) Elsevier.