Biomedical Engineering Reference
In-Depth Information
Hardness
The hardness of CNT-glass/glass-ceramic matrix composites has been
investigated several times, but the actual effect of CNTs on hardness is still
unclear. In the absence of a hardness value of individual CNTs, their effect
on the hardness of composites is difficult to predict. As a result, three
different hardness trends have been shown in the literature and can be
explained as follows.
(a) An increase in hardness has been associated with dense matrices
and good bonding between the CNTs and the glass matrix,
although no evidence of a strong interface is usually available other
than the existence of a simple mechanical bond.
(b) A continuous decrease in hardness has been associated with
inhomogeneous dispersion of CNTs in matrices and poor densities
of sintered composites.
(c) An initial increase followed by a decrease in hardness has been
explained by a combination of (a) and (b), i.e. good interfacial
bonding and/or uniform dispersion at low CNTs loading and
inhomogeneous distribution of CNTs at higher contents, leading to
the formation of CNT agglomerates and poor composite densities.
An early investigation on CNT-SiO
2
glass composites containing 4wt%
CNTs showed an increase in hardness of 28% in comparison to that of
unreinforced silica glass (de Andrade et al., 2007). Another study on CNT-
SiO
2
glass composites used a high-pressure technique to densify composites
and showed 44% improvement in hardness of composites by adding only
0.075wt% CNTs (de Andrade et al., 2008). In contrast, borosilicate glass
matrix composites containing 10wt% CNTs demonstrated a 22% decrease
in hardness, which was attributed to inhomogeneous dispersion of CNTs as
a direct mixing technique was used to prepare the composite powder
(Boccaccini et al., 2007). Another study on borosilicate glass matrix
composites, prepared by the sol-gel technique, demonstrated an increase in
hardness of 13% by adding 2wt% CNTs but further addition of CNTs (3wt
%) was reported to produce agglomerates and a decreased hardness to a
value 7% less than that of pure borosilicate glass (Thomas et al., 2009).
CNT composites containing glass-ceramic matrices have also shown a mixed
trend in hardness with CNT content. In a study on CNT-aluminobor-
osilicate glass-ceramic composites, an increase in hardness of 10% and 23%
was observed on adding MWCNTs and SWCNTs, respectively (Chu et al.,
2008). However, another study by the same group of researches showed a
31% decrease in hardness in 15wt% CNT-aluminoborosilicate composites
(Mukhopadhyay et al., 2010).
A model composite system has recently been developed by uniformly
