Carbon Nanotube Reinforced Composites: Metal and Ceramic ...

materials [17]. This is because VIF result cannot be reproduced, hence does not yield
a reliable KICvalue. He compared the KICvalues of silicon nitride specimens obtained
from standardized fracture toughness test and VIF technique. He found that
Antis equation underestimates the fracture toughness whilst Niihara s equation
overestimates the toughness. Although Miyoshi equation yields a result close to
that of standardized fracture toughness test, a calibration constant is needed to fit
the data. Therefore, the fracture toughness of brittle ceramics should be determined
from the standardized fracture toughness test. The standard fracture test produces
reliable fracture toughness values because the specimen with a well-defined single
crack is subjected to a well-defined mechanical loading condition. On the contrary,
VIF test has a complex three-dimensional cracks and ill-defined crack arrest condition.
For new ceramic materials with unknown fracture toughness value, VIF test
should not be used to determine the toughness. This is because VIF toughness can
deviate up to 48% from the true fracture toughness value [18]. Despite these
deficiencies, the VIF test is used extensively to determine the fracture toughness
of ceramic-CNT nanocomposites.
7.2
Toughening and Strengthening Mechanisms
7.2 Toughening and Strengthening Mechanismsj187
The mechanisms responsible for toughening of CNT-reinforced ceramics include
crack deflection, crack bridging and nanotube pull-out. The toughening behavior
can be explored from the Vickers indentation of ceramic-CNT nanocomposites
synthesized from AAO membranes. Such nanocomposite coatings contain a
highly ordered array of parallel nanotubes in an alumina matrix. Xia et al. prepared
highly aligned alumina-MWNT nanocomposites using AAO coatings of different
thicknesses (20 and 90 mm) through template synthesis [Chap. 5, Ref. 81]. After
multi-step anodizing, Co or Ni catalyst particles were deposited into the bottom of
nanopores of amorphous alumina coatings. CVD treatment at 645 C was employed
to grow MWNTs up the pore walls. Controlling the composition of reactant gases and
deposition time produces MWNTs with different wall thickness. The nanotubes
formed inside the AAO coating with 90 mm thickness have a larger diameter and a
thinner wall thickness than those formed with 20 mm. The nanocomposites were
then subjected to nanoindentation using a Berkovitch indenter. Interestingly, alumina-MWNT
nanocomposites exhibit all the toughening features similar to those
observed in conventional fiber-reinforced composites, that is, crack deflection, crack
bridging and crack pull-out (Figure 7.1(a)–(c)). Consequently, interface debonding
and sliding can occur in composite materials with nanostructures. Furthermore, the
nanocomposite having nanotubes with thinner wall thickness and larger diameter
is more resistant to indentation damage. This is demonstrated by the collapse of
CNTs into shear bands, leading to the absorption of large energy during indentation.
No racking is observed during deformation of the nanocomposite coating
(Figure 7.2). The shear band formation is somewhat similar to shear yielding
observed in polymers. For polymeric materials, shear banding often initiates from