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

4294 CARBON 48 (2010) 4289– 4300
the range studied, the content <strong>of</strong> CNFs does not play an
important role on the tensile properties <strong>of</strong> the CNF/EP
composites.
Compared with the CNF/EP composites, the SCF/EP composites
exhibit much higher E-moduli <strong>and</strong> the modulus is linearly
increased with increasing SCF content (cf. Fig. 3c). With
SCFs incorporation the tensile strength decreased (Fig. 3d).
The ultimate elongation is markedly lowered with the addition
<strong>of</strong> the micr<strong>of</strong>illers (cf. Fig. 3e).
A combination <strong>of</strong> the multiscale <strong>carbon</strong> fillers, i.e. SCFs
<strong>and</strong> CNFs, leads to impressive improvements in modulus
<strong>and</strong> tensile strength (cf. Fig. 3c <strong>and</strong> d). The modulus <strong>and</strong> the
strength <strong>of</strong> SCF/CNF/EP composites increase steadily with
increasing CNF-loading. For example, compared with the conventional
composite 10CF, incorporation <strong>of</strong> only 0.75% CNFs
into the matrix (10CF0.75NF) increases the modulus from
3.70 to 5.03 GPa <strong>and</strong> the tensile strength from 80.0 to
91.4 MPa. It is interesting to note that with CNF contents higher
than 0.25%, the enhancements in modulus <strong>and</strong> strength
achieved by the combined multiscale fillers are significantly
higher than superposition <strong>of</strong> separate contributions <strong>of</strong> the
two fillers, as realized in the SCF/EP <strong>and</strong> CNF/EP systems (cf.
Fig. 3c <strong>and</strong> d). Therefore, the multiscale fillers play synergetic
roles on stiffening <strong>and</strong> strengthening the EP matrix. The elongation
at break values <strong>of</strong> SCF/CNF/EP composites are slightly
higher than that <strong>of</strong> the composite filled solely with SCFs (cf.
Fig. 3e). Fig. 4a <strong>and</strong> b shows the failure modes <strong>of</strong> EP <strong>and</strong>
0.75NF after the tensile test. The fracture propagation directions
are indicated by arrows in Fig. 4. Compared with pure
EP, the CNF/EP composite is less brittle. Fig. 4c <strong>and</strong> d shows
the fracture surface <strong>of</strong> 0.75NF at high magnifications. Similar
to SCF/EP composites, pull-out, breakage <strong>and</strong> debonding
(from the EP matrix) <strong>of</strong> CNFs are noticed in the fracture surface
(cf. Fig. 4d).
Regardless <strong>of</strong> the efficient stiffening by microsized SCFs,
the ductility <strong>of</strong> the matrix is radically reduced <strong>and</strong> the
strength <strong>of</strong> the composites is also usually lowered. Due to
the much higher modulus <strong>of</strong> SCFs than that <strong>of</strong> the matrix,
stress concentration can occur near the <strong>fibers</strong>, especially at
the ends <strong>of</strong> SCFs. As a result, the SCF/matrix interface, especially
at the zone near SCF ends, can initiate failures. The
fracture surfaces <strong>of</strong> 10CF <strong>and</strong> 10CF0.75NF are compared in
Fig. 5. The matrix between the <strong>fibers</strong> is very brittle in case
<strong>of</strong> the conventional composites filled uniquely with SCFs.
With respect to the SCF/CNF/EP composites, the matrix
undergoes pronounced deformation as a result <strong>of</strong> pull-out,
breakage, <strong>and</strong> debonding <strong>of</strong> CNFs (cf. Fig. 5b).
The broken tensile specimens were polished in order to
study with an optic microscope the morphologies under the
fracture planes. The diagram associated with Fig. 6 shows
the location from which observations were made. As is seen
in Fig. 6a, no obvious cracking is noticed under the fracture
Fig. 4 – Tensile fracture surfaces <strong>of</strong> EP (a) <strong>and</strong> EP filled with 0.75% CNFs (b)–(d).