Synergetic effect of carbon nanofibers and short carbon fibers on the ...

CARBON 48 (2010) 4289– 4300 4295
As a result, the E-modulus <strong>of</strong> SCFs/EP is much improved with
the addition <strong>of</strong> low-loading CNFs. The homogenized stress
field in the matrix between SCFs enhances not only the E-
modulus (Fig. 3c), but also the ductility (elongation at break)
<strong>of</strong> SCF/CNF/EP composites. The data in Fig. 3e indicates, however,
small enhancement in elongation at break. The reason
behind is the multiple microcracking (cf. Fig. 6) which is
accompanied with an inherently small change in the elongation
at break.
Comparing to combination <strong>of</strong> SiO 2 nanoparticles with
SCFs [27], the synergetic roles <strong>of</strong> CNFs with SCFs are more
prominent. For example, in order to achieve comparable modulus
<strong>and</strong> strength as 10CF0.75NF, at least adding <strong>of</strong> 3 vol.%
nano-SiO 2 is required to the EP with the same amount <strong>of</strong> SCFs
(i.e. 10 vol.%). The high aspect ratio <strong>of</strong> CNFs allows efficient
stress transfer <strong>and</strong> thereby, reduces more <strong>effect</strong>ively the overstress
on SCFs. This is assumed to be the major reason for the
synergetic action <strong>of</strong> CNFs <strong>and</strong> SCFs on the tensile properties.
3.4. Fracture toughness
Fig. 5 – Tensile fracture surfaces <strong>of</strong> 10CF (a) <strong>and</strong> 10CF0.75NF
(b).
plane <strong>of</strong> the SCF/EP composite (10CF). Fig. 6b <strong>and</strong> c shows,
respectively, the micrographs <strong>of</strong> 10CF0.5NF <strong>and</strong> 10CF0.75NF.
Multiple matrix cracks are observed under the fracture planes
<strong>of</strong> SCF/CNF/EP composites (the multiple cracking becomes
less sharp on the graphs due to polishment reason). In the
composites filled both with SCFs <strong>and</strong> CNFs the stress field
in the matrix between SCFs was ‘‘homogenized’’ by the presence
<strong>of</strong> the nan<strong>of</strong>illers. Stress field homogenization means
that between the SCFs now well distributed nan<strong>of</strong>illers (i.e.
CNFs) are present in the matrix <strong>and</strong> they work for efficient
additional stress transfer <strong>and</strong> redistribution.
The stress transfer from matrix to CNFs, followed by the
sub-critical CNF-related failures, is assumed to relieve the
overstress on SCFs <strong>and</strong> reduce the occurrence <strong>of</strong> SCF/matrix
interfacial failures. The formation <strong>of</strong> the sub-critical cracks
due to CNF-related failure events is supposed to be the major
reason for the enhanced ‘‘strain-s<strong>of</strong>tening’’ at a high strain as
revealed above.
For the composites filled only with SCFs the extra stress in
the regions adjacent to a fiber end is associated with large local
strain [7,9,35]. Compared with the composites filled only
with SCFs, CNFs are supposed to relieve the stress concentrations
at the ends <strong>of</strong> SCFs. Therefore, the large local strain
adjacent to SCF ends can be reduced. By this way the neighboring
SCFs can now fulfill their reinforcing role accordingly.
Typical load-line displacement curves measured on the CT
specimens <strong>of</strong> the materials studied are displayed in Fig. 7a.
The curves demonstrate that the incorporation <strong>of</strong> the microor
nan<strong>of</strong>illers increases both the modulus <strong>and</strong> fracture toughness.
The modulus <strong>and</strong> fracture toughness <strong>of</strong> the composite
with the multiscale <strong>carbon</strong> fillers are markedly higher than
those <strong>of</strong> the composites filled solely with the micro- or
nan<strong>of</strong>illers.
The fracture toughness, K IC , <strong>and</strong> the critical energy release
rate, G IC , <strong>of</strong> all materials studied are compared in Fig. 7b <strong>and</strong>
c, respectively. CNFs at low concentrations moderately increase
the K IC <strong>and</strong> G IC <strong>of</strong> the parent EP. With 0.125 vol.% CNFs,
the fracture toughness is increased from 0.62 to 0.70 MPam 1/2 .
With increasing the CNF-loading to 0.25 vol.%, the fracture
toughness is enhanced to 0.74 MPam 1/2 . Nevertheless, further
increase in the CNF-loading till 0.75 vol.% does not lead to significant
change in fracture toughness. The toughening <strong>effect</strong>
<strong>of</strong> CNF in this work is in good agreement with that realized
by CNT, dispersed in a similar EP resin by a three roll calender
[14]. The combination <strong>of</strong> CNFs with nano-SiO 2 also only leads
to moderate improvement in fracture toughness (cf. Fig. 7b
<strong>and</strong> c). By comparing the fracture toughness <strong>of</strong> the CNF/
nano-SiO 2 /EP composites to that <strong>of</strong> the composites filled solely
with CNFs or nano-Al 2 O 3 particles [25], no obvious synergetic
roles are noticed between the two ‘‘size-similar’’
nan<strong>of</strong>illers.
The microsized SCFs significantly improve the K IC <strong>and</strong> G IC
<strong>of</strong> EP (cf. Fig. 7b <strong>and</strong> c). The addition <strong>of</strong> 10 vol.% SCFs doubles
the fracture toughness. In the studied range, the fracture
toughness increases with increasing SCF content. Compared
to CNF, SCF increases more the fracture toughness.
The K IC <strong>and</strong> G IC <strong>of</strong> the SCF/CNF/EP composites are enhanced
with increasing loading <strong>of</strong> CNFs. The composites with
10 vol.% SCFs <strong>and</strong> CNFs with more than 0.5 vol.% possess significantly
higher fracture toughness <strong>and</strong> critical energy release
rate than the superposition <strong>of</strong> the separate
contributions <strong>of</strong> the two respective fillers (cf. Fig. 7b <strong>and</strong> c).
The combined use <strong>of</strong> 10 vol.% SCFs <strong>and</strong> 0.75 vol.% CNFs increases
the fracture toughness <strong>of</strong> epoxy by about 210%. By