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

CARBON 48 (2010) 4289– 4300 4297
Load (mm)
K IC
(MPa m 1/2 )
G IC
(KJ/m 2 )
120
100
80
60
40
20
a
EP
0.5NF
10CF
10CF0.5NF
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
2.0
1.6
1.2
0.8
0.4
0.0
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
b
EP
EP
EP
c
EP
EP
EP
CNF/EP
0.125NF
0.25NF
0.125NF
0.25NF
CNF/EP
0.5NF
0.75NF
0.5NF
Displacement (mm)
CNF/
nano-SiO 2
/
EP
0.25NF1Si
0.75NF
0.25NF1Si
0.25NF3Si
SCF/EP
10CF
5CF
10CF
15CF
Materials
CNF/
nano-SiO 2
/
EP
0.25NF3Si
10CF
5CF
Materials
SCF/EP
10CF
15CF
10CF0.125NF
SCF/CNF/EP
10CF0.25NF
SCF/CNF/EP
10CF0.125NF
10CF0.25NF
10CF0.5NF
10CF0.5NF
10CF0.75NF
Fig. 7 – CT load–displacement curves <strong>of</strong> typical materials
studied (a); fracture toughness, K IC , (b) <strong>and</strong> critical energy
release rate, G IC , (c) <strong>of</strong> all materials studied. Dash lines in (b)
<strong>and</strong> (c) indicating the levels <strong>of</strong> two reference materials, i.e.
pure EP <strong>and</strong> EP + 10 vol.% SCFs, are added for easier
comparisons.
10CF0.75NF
The crack in SCF-filled composites produces a typical zigzag
pattern, as was intensively analyzed by Karger-Kocsis
<strong>and</strong> Friedrich [7–9]. Fig. 8c shows that the EP matrix fails in
a brittle manner in the SCF/EP composite (10CF). With respect
to the SCF/CNF/EP composite (i.e. 10CF0.75NF), the fracture
plane <strong>of</strong> the matrix is much rougher <strong>and</strong> numerous microcracks
appear, as is seen from Fig. 8d. Fig. 8e <strong>and</strong> f demonstrates
the fracture surfaces <strong>of</strong> the SCF/CNF/EP composites
at high magnifications. Extensive deformation <strong>of</strong> the matrix
is observed on the fracture surface due to the presence <strong>of</strong>
CNFs. For the conventional composites filled with SCFs, due
to the stress concentration occurring near fiber ends a limited
plastic zone (also termed damage zone) develops near the
fracture plane as schematized in Fig. 9a. The stress transfer
realized by CNFs, followed by pull-out, breakage <strong>and</strong> debonding
<strong>of</strong> CNFs, can participate in the stress redistribution process
<strong>and</strong> thus can relieve the overstress on SCF ends. In
addition, CNF-related failures trigger extensive microcracks
<strong>of</strong> matrix in the CNF–CNF ligaments <strong>and</strong> the CNF–SCF ligaments
as a result <strong>of</strong> shear yielding <strong>of</strong> the matrix. As a consequence,
a larger plastic zone develops because other SCFs can
now be involved in the stress transfer, as schematically illustrated
in Fig. 9b. This is assumed to be the major reason for
the synergetic roles <strong>of</strong> the multiscale <strong>carbon</strong> fillers. In comparison
to spherical nanoparticles (i.e. nano-SiO 2 as reported
in [27]), the high aspect ratio CNF is more efficient to enhance
the fracture toughness <strong>of</strong> SCF-filled EP. That is, in order to get
comparable fracture toughness, a smaller fraction <strong>of</strong> CNFs
(than spherical nanoparticles) is required to be combined
with SCFs. This can be attributed to the high aspect ratio <strong>of</strong>
CNFs which allow an efficient stress transfer <strong>and</strong> thereby,
an efficient release <strong>of</strong> stress concentrations occurring near
to the SCF ends.
4. Conclusions
The <strong>effect</strong>s <strong>of</strong> CNFs (nanosized), <strong>and</strong> the combination <strong>of</strong> CNFs
<strong>and</strong> SCFs (microsized) on the tensile <strong>and</strong> fracture properties
<strong>of</strong> EP were studied. The loading <strong>of</strong> CNFs <strong>and</strong> SCFs varied from
0.125 to 0.75 vol.% <strong>and</strong> from 5 to 15 vol.%, respectively. The
tensile properties <strong>and</strong> the fracture mechanical properties <strong>of</strong>
SCF/EP, CNF/EP, <strong>and</strong> SCF/CNF/EP composites were compared.
It was revealed that the SCF/CNF/EP composites exhibit much
higher fracture toughness <strong>and</strong> much better mechanical properties
than the composites filled uniquely with CNFs or SCFs.
The multiscale <strong>carbon</strong> fillers improved the tensile <strong>and</strong> fracture
mechanical properties <strong>of</strong> EP synergistically. Based on
the work done the following conclusions can be drawn:
1. Filled uniquely with CNFs, the E-modulus <strong>of</strong> the EP was
slightly increased. The tensile strength <strong>and</strong> fracture
toughness were moderately enhanced with the addition
<strong>of</strong> CNFs. Filled with microsized SCFs, the modulus <strong>and</strong>
the fracture toughness <strong>of</strong> EP are strongly improved. However,
the tensile strength decreased with the addition <strong>of</strong>
SCFs.
2. The combined use <strong>of</strong> CNFs <strong>and</strong> SCFs (multiscale <strong>carbon</strong>
fillers) leads to significant synergetic <strong>effect</strong>s on the
mechanical <strong>and</strong> fracture properties <strong>of</strong> epoxy. The contributions
<strong>of</strong> the combined fillers to the E-modulus, tensile
strength <strong>and</strong> fracture toughness are significantly higher
than the superposition <strong>of</strong> the separate contributions <strong>of</strong>
the respective fillers achieved in the composites filled
uniquely with SCFs or CNFs. The distinctly different
scales <strong>of</strong> SCFs <strong>and</strong> CNFs are believed to be crucial for the