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

CARBON 48 (2010) 4289– 4300
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<strong>Synergetic</strong> <strong>effect</strong> <strong>of</strong> <strong>carbon</strong> nan<strong>of</strong>ibers <strong>and</strong> <strong>short</strong> <strong>carbon</strong> <strong>fibers</strong>
on the mechanical <strong>and</strong> fracture properties <strong>of</strong> epoxy resin
G. Zhang a, *, J. Karger-Kocsis b ,J.Zou a
a Institute for Composite Materials, University <strong>of</strong> Kaiserslautern, 67663 Kaiserslautern, Germany
b Department <strong>of</strong> Polymer Technology, Faculty <strong>of</strong> Engineering <strong>and</strong> Built Environment, Tshwane University <strong>of</strong> Technology, Pretoria 0001,
South Africa
ARTICLE
INFO
ABSTRACT
Article history:
Received 27 April 2010
Accepted 22 July 2010
Available online 27 July 2010
The <strong>effect</strong>s <strong>of</strong> <strong>carbon</strong> nan<strong>of</strong>ibers (CNFs) <strong>and</strong> combinations <strong>of</strong> CNFs with microsized <strong>short</strong>
<strong>carbon</strong> <strong>fibers</strong> (SCFs) on the mechanical <strong>and</strong> fracture properties <strong>of</strong> epoxy resin were investigated.
The combined use <strong>of</strong> CNFs <strong>and</strong> SCFs leads to significant synergy in the mechanical
<strong>and</strong> fracture properties. The composites reinforced with the combined multiscale fillers
exhibit much higher modulus, strength <strong>and</strong> fracture toughness than the composites reinforced
solely with either micro- or nan<strong>of</strong>illers. The mechanisms <strong>of</strong> such synergism were
analyzed by fracture studies using scanning electron microscopy. The release <strong>of</strong> overstresses
near to SCFs by stress transfer <strong>and</strong> redistribution, realized by CNFs, was traced to the
synergistic <strong>effect</strong>s observed.
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
Epoxy resins (EPs) are being more <strong>and</strong> more used as matrices
in state-<strong>of</strong>-the-art applications due to many useful properties
such as high modulus <strong>and</strong> strength, high thermal resistance,
<strong>and</strong> resistance to creep. However, EP displays also a highly
undesirable property, i.e. poor resistance to crack initiation
<strong>and</strong> propagation. To simultaneously enhance the mechanical
<strong>and</strong> fracture properties <strong>of</strong> epoxy is <strong>of</strong> great importance.
Microsized rigid inorganic particles [1–4] <strong>and</strong> <strong>short</strong> <strong>fibers</strong>
[5–9] were proven suitable for this task. More recently, EP
composites which are reinforced with nan<strong>of</strong>illers gained
extensive <strong>and</strong> intensive interests both in the academia <strong>and</strong>
industries.
Carbon nanotube (CNT), more specifically a single-walled
<strong>carbon</strong> nanotube (SWCNT), can be considered as a graphene
sheet rolled into a cylinder, where multiple concentric
sheets create a multiwalled <strong>carbon</strong> nanotube (MWCNT). Carbon
nan<strong>of</strong>ibers (CNFs), are a class <strong>of</strong> these materials that
have curved graphene layers or nanocones stacked to form
a quasi-one-dimensional filament, whose internal structure
can be characterized by the angle between the graphene layers
<strong>and</strong> the fiber axis [10]. Due to their exceptionally excellent
thermal, electrical properties <strong>and</strong> their high specific
stiffness <strong>and</strong> strength <strong>and</strong> very large aspect ratios (length/
diameter), CNFs <strong>and</strong> CNTs hold great potential for developing
advanced polymer composites for both structural <strong>and</strong>
functional applications. CNFs typically have diameters on
the order <strong>of</strong> 50–200 nm [11]. CNFs are now produced at high
rates <strong>and</strong> reasonably lower cost than CNTs, <strong>and</strong> thus they
become potential c<strong>and</strong>idates for high-volume applications.
Extensive researches were carried out to reinforce polymer
matrices by CNTs or CNFs introducing them in low
amounts. Note that due to the small sizes <strong>of</strong> CNFs <strong>and</strong>
CNTs, the properties <strong>of</strong> these nanoreinforcements can only
be exploited if they are homogeneously dispersed in the related
matrices. How to achieve a homogeneous dispersion <strong>of</strong>
the nan<strong>of</strong>illers in polymers is a challenging task receiving
* Corresponding author: Fax: +49 631 2017 196.
E-mail address: ga.zhang@ivw.uni-kl.de (G. Zhang).
0008-6223/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.<strong>carbon</strong>.2010.07.040

4290 CARBON 48 (2010) 4289– 4300
considerable attention. Different dispersing techniques for
CNTs were documented in recent review papers [12,13].
CNFs <strong>and</strong> CNTs were proven to stiffen <strong>and</strong> toughen EPs even
at their extremely low contents [14–17]. To reinforce EP with
low-loading CNFs or CNTs is <strong>of</strong> practical interest because
the viscosity <strong>of</strong> the resin can be kept on an acceptable level.
The large aspect ratios <strong>and</strong> huge specific surface areas <strong>of</strong>
CNTs or CNFs guarantee an efficient matrix/filler stress
transfer. Available literatures to date <strong>of</strong>fer evidence strong
CNT–polymer interactions. Efficient stress transfer between
CNTs <strong>and</strong> polymer matrix was evidenced in [18–20] <strong>and</strong> simulated
in [21–23]. The stress transfer efficiency between
nanotube <strong>and</strong> matrix was estimated to be at least one order
<strong>of</strong> magnitude larger than in conventional fiber-based composites
[20].
Direct comparisons <strong>of</strong> the mechanical properties between
micr<strong>of</strong>iller- <strong>and</strong> nan<strong>of</strong>iller-reinforced composites are scarce.
A definite answer is still lacking to the basic question: do
nan<strong>of</strong>iller-reinforced composites always display superior
mechanical properties to micr<strong>of</strong>iller-reinforced composites
Microsized particles or <strong>short</strong> <strong>fibers</strong> (micro-scale in diameter
or/<strong>and</strong> in length) were proven to <strong>effect</strong>ively stiffen <strong>and</strong>
toughen EP matrix. However, they strongly lower the ductility
due to the highly constrained deformability <strong>of</strong> the EP matrix
<strong>and</strong> stress concentration occurring near the microsized
filler [7–9,24,25]. Obvious stress concentration near the fillers
can induce critical failure <strong>of</strong> the composite system. Thus, in
many cases, the strength <strong>of</strong> EP can only be moderately improved
<strong>and</strong> even decreased with the addition <strong>of</strong> microsized
fillers. Unlike micr<strong>of</strong>illers, the much smaller size <strong>of</strong> nan<strong>of</strong>illers
might guarantees a good integration with the matrix
<strong>and</strong> avoid obvious stress concentration [25]. The incorporation
<strong>of</strong> low-loading nan<strong>of</strong>illers makes it possible a simultaneous
improvement in E-modulus, strength <strong>and</strong> toughness
<strong>of</strong> EP [15,25,26]. Despite the fact that nan<strong>of</strong>iller-reinforced
polymer composites attracted great attentions in the last
two decades, with low-loading nan<strong>of</strong>illers such as nanoparticles
<strong>and</strong> CNTs, the mechanical properties <strong>of</strong> EP matrix can
only be moderately improved [15,25,26]. However, our previous
work [27] revealed that a combined utilization <strong>of</strong> microsized
<strong>short</strong> <strong>carbon</strong> <strong>fibers</strong> (SCFs) <strong>and</strong> low-loading nano-silica
particles leads to significant synergisms on the mechanical
<strong>and</strong> fracture toughness <strong>of</strong> EP. Combined utilization <strong>of</strong> differently
scaled fillers seems to be a promising route to greatly
enhance the modulus, strength <strong>and</strong> fracture toughness <strong>of</strong>
EP simultaneously.
In this work, we systematically compare the mechanical
<strong>and</strong> fracture properties <strong>of</strong> three series <strong>of</strong> epoxy composites,
i.e. conventional composites reinforced with microsized SCFs
(SCF/EP composites), composites reinforced with CNFs at low
concentrations (CNF/EP composites), <strong>and</strong> composites reinforced
with both SCFs <strong>and</strong> CNFs (SCF/CNF/EP composites).
The main aims <strong>of</strong> the present work are to deepen the underst<strong>and</strong>ing
on the structure–property relationship <strong>of</strong> EP composites
<strong>and</strong> to investigate how the combination <strong>of</strong> the
multiscale <strong>carbon</strong>s, i.e. combination <strong>of</strong> SCFs <strong>and</strong> CNFs, affects
the mechanical properties <strong>and</strong> the fracture toughness <strong>of</strong> the
EP. By revealing the synergetic action <strong>of</strong> the multiscale <strong>carbon</strong>
fillers, we propose a promising composite formulation route
for high-performance applications.
2. Experimental
2.1. Materials <strong>and</strong> preparation
All materials used in the present work were prepared on the
basis <strong>of</strong> a commercially available epoxy resin (DER331 by
DOW) with an epoxide equivalent weight 182–192 g/equiv.
Cycloaliphatic amine hardener (HY 2954; Huntsman) was used
to cure the epoxy materials. Milled PAN-based <strong>carbon</strong> <strong>fibers</strong>
(Tenax â A-385) were supplied by Tenax GmbH (Germany).
The diameter <strong>and</strong> the length <strong>of</strong> the <strong>fibers</strong> are, respectively,
7 lm <strong>and</strong> 40–70 lm. The SCFs were supplied without sizing
treatment <strong>and</strong> were used as-received. CNFs (GANF1) were supplied
by Grupo Antolin (Spain). The CNFs have diameters in
the range <strong>of</strong> 20–80 nm <strong>and</strong> lengths larger than 30 lm. The theoretical
E-modulus <strong>of</strong> the CNFs is 230 GPa <strong>and</strong> the theoretical
strength is 2.7 GPa. Fumed spherical silica nanoparticles
(Aerosil R8200) with hexamethyldisilazane modification were
supplied by Evonik GmbH (Germany).
Three series <strong>of</strong> epoxy composites, i.e. CNF-filled EP composites
(CNF/EP), composites reinforced with the combined
nan<strong>of</strong>illers, i.e. nano-SiO 2 <strong>and</strong> CNFs (CNF/nano-SiO 2 /EP), <strong>and</strong>
composites reinforced with the combined multiscale <strong>carbon</strong>s,
i.e. SCFs <strong>and</strong> CNFs (SCF/CNF/EP), were prepared in the present
work. Pure EP <strong>and</strong> conventional composites filled uniquely
with SCFs were used for references. The loading <strong>of</strong> the CNFs
varied from 0.125 to 0.75 vol.%. The detailed compositions <strong>of</strong>
the materials are disclosed in Table 1. The volume fractions
<strong>of</strong> all the fillers were calculated by considering their weights
<strong>and</strong> (bulk) densities. For simplification purpose, the composite
materials were referenced according to the type <strong>and</strong> the
fraction <strong>of</strong> the fillers as shown in the left column <strong>of</strong> Table 1.
For example, 10CF0.5NF refers to the composite filled with
10 vol.% SCFs <strong>and</strong> 0.5 vol.% CNFs.
A master batch <strong>of</strong> CNF-filled epoxy resin was firstly prepared
by dispersing a CNF/epoxy mixture with a three roll calender
(Exakt 80 E, Exakt GmbH, Germany). Three roll calender
was firstly introduced for dispersing CNTs by Gojny et al. in
2004 [14] <strong>and</strong> received intensive attentions in recent years.
Interestingly, this dispersion technique is well established in
the praxis – for example nano-SiO 2 is traditionally dispersed
in dental composites by such calendering process. The small
gap between the rolls <strong>and</strong> a mismatch in rolling speed can result
in enormous shear forces, which can <strong>effect</strong>ively disentangle
CNTs or CNFs agglomerates without significantly reducing
their aspect ratios. The mismatch between the angular velocity
<strong>of</strong> adjacent rolls was fixed at x 1 :x 2 :x 3 = 1:3:9. The gap distance
between the rolls was adjusted to be 50 lm for
beginning <strong>and</strong> was reduced to 25 lm for the second pass <strong>and</strong>
finally to 5 lm for the third pass. With regard to the SCF/
CNF/EP composites, required amounts <strong>of</strong> SCFs were mixed
into the diluted CNF master batch using a vacuum dissolver
(Dispermat, VMA-Getzmann GmbH, Germany). The CNF/
nano-SiO 2 /EP composites were prepared by mixing <strong>and</strong> diluting
the CNF/EP master batch with the nano-silica/EP master
batch formulation. The nano-silica master batch is prepared
by using the dissolver with an extremely high rotation speed
(5800 rpm). Afterwards, the compounds were blended
with the curing agent HY2954 by stirring in the dissolver
for 15 min. Finally the mixture was poured into release

CARBON 48 (2010) 4289– 4300 4291
Table 1 – Compositions <strong>of</strong> materials studied.
Series Material codes Matrix (vol.%) SCF (vol.%) CNFs Nano-SiO 2 (vol.%)
Neat epoxy EP 100 – – –
SCF/EP composites 5CF 95 5 – –
10CF 90 10 – –
15CF 85 15 – –
CNF/EP composites 0.125NF 99.875 – 0.125 –
0.25NF 99.75 – 0.25 –
0.5NF 99.5 – 0.5 –
0.75NF 99.25 – 0.75 –
CNF/nano-SiO 2 /EP composites 0.25NF1Si 98.75 0.25 1
0.25NF3Si 96.75 0.25 3
SCF/CNF/EP composites 10CF0.125NF 89.875 10 0.125 –
10CF0.25NF 89.75 10 0.25 –
10CF0.5NF 89.5 10 0.5 –
10CF0.75NF 89.25 10 0.75 –
agent-coated metallic molds <strong>and</strong> cured at 70 °C for 8 h,
followed by 8 h at 120 °C.
2.2. Characterization <strong>of</strong> CNF network in SCF/CNF/EP
composites
In order to examine the CNF dispersion state <strong>and</strong> CNF network
in the SCF/CNF/EP composites, scanning electron
microscopy (SEM, Zeiss Supra TM 40VP) was utilized to examine
the fracture surface <strong>of</strong> 10CF0.75NF. No conductive coating was
sputtered on the sample surface, in order to avoid any mask
over the CNFs <strong>and</strong> to allow possible charge contrast imaging
between the conductive CNFs <strong>and</strong> EP matrix. Due to the
charge contrast, the overall distribution <strong>of</strong> CNTs or CNFs
can be easily distinguished because <strong>of</strong> the high contrast (in
brightness) between the conductive fillers <strong>and</strong> the EP matrix
[28–31]. A low acceleration voltage <strong>of</strong> 0.5 kV was utilized in order
to reduce electron penetration <strong>and</strong> to view dispersion <strong>of</strong>
CNFs on <strong>and</strong> only slightly below the specimen surface.
2.3. Glass transition temperature
Glass transition temperatures, T g , <strong>of</strong> CNF composites <strong>and</strong> the
SCF/CNF/EP composites were measured by differential scanning
calorimetry (DSC) using a DSC821 apparatus (Mettler-
Toledo GmbH, Switzerl<strong>and</strong>). All tests were performed in
nitrogen atmosphere with a sample mass <strong>of</strong> about 20 mg.
The sample was heated from 20 °C to 200 °C at a rate <strong>of</strong>
10 °C/min.
2.4. Tensile tests
Tensile tests were performed at room temperature (21 °C) on a
Zwick 1474 universal testing machine at a constant crosshead
speed <strong>of</strong> 0.5 mm/min. The measurements followed DIN EN
ISO 527 using dumbbell shaped specimens. The specimens
having a 4 mm thickness were machined from the molded
plates. The displacement <strong>of</strong> each specimen during tension
was accurately measured by an extensometer with an initial
gage length <strong>of</strong> 20 mm. E-moduli were calculated by considering
the load in the 0.05–0.25% strain range. All presented data
corresponds to the average <strong>of</strong> at least five measurements.
2.5. Fracture toughness
The fracture toughness <strong>of</strong> the materials studied was determined
by means <strong>of</strong> compact tension (CT) tests following the
st<strong>and</strong>ard ISO 13586 on at least five specimens at a crosshead
speed <strong>of</strong> 0.5 mm/min. The CT specimens were cured in a
metallic mold <strong>and</strong> then both sides were polished by abrasive
papers until all visible marks disappeared. A notch was machined
<strong>and</strong> then sharpened by tapping a fresh razor blade
(tip radius: 2 lm) into the material, so that a sufficiently sharp
crack was initiated. The thickness <strong>and</strong> the width <strong>of</strong> specimens
were 4 mm <strong>and</strong> 30 mm, respectively. In order to analyze
failure mechanisms, the fracture surfaces after tensile <strong>and</strong> CT
tests were inspected using SEM after being coated a thin gold
layer.
3. Results <strong>and</strong> discussions
3.1. Dispersion state <strong>of</strong> CNFs <strong>and</strong> structure <strong>of</strong> SCF/CNF/EP
composites
Fig. 1 displays at different magnifications SEM images <strong>of</strong> the
non-coated fracture surface <strong>of</strong> 10CF0.75NF. From Fig. 1a <strong>and</strong>
b, it is seen that CNFs are globally well distributed in the EP
matrix between SCFs. With a low magnification (cf. Fig. 1a
<strong>and</strong> b), the overall structure <strong>of</strong> the SCF/CNF/EP composite
is clear due to the high contrast between CNFs <strong>and</strong> EP.
The high contrast between CNFs <strong>and</strong> EP matrix is related
to a charge contrast <strong>effect</strong> [28–31], besides topography contrast.
With respect to the micron SCFs which protrudes from
the fractured EP surface, the topography contrast is believed
to be important. Charge contrast mechanisms are currently
under investigation. Fig. 1c illustrates the fracture surface
with a high magnification, with some <strong>of</strong> the CNFs indicated
by arrows on the surface. Besides discretely dispersed CNFs,
small bundles <strong>and</strong> agglomerates <strong>of</strong> CNFs can be resolved
locally.

4292 CARBON 48 (2010) 4289– 4300
144
140
136
T g
( o C)
132
128
124
EP + CNFs
EP + SCFs + CNFs
120
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Content <strong>of</strong> CNFs (vol.%)
Fig. 2 – Effect <strong>of</strong> CNF-loading on the T g <strong>of</strong> CNFs/EP <strong>and</strong> CNFs/
SCF/EP composites.
Fig. 1 – SEM images <strong>of</strong> the non-coated fracture surface <strong>of</strong>
10CF0.75NF.
As reviewed in a recent paper [32], widespread <strong>and</strong> contradictory
results regarding the glass transition temperature <strong>of</strong>
CNTs or CNFs filled EP were reported by different researchers.
The mechanisms how CNTs or CNFs modify the T g are rather
complex <strong>and</strong> remain unclear. It is generally believed that the
change in T g can be related to some restrictions <strong>of</strong> the polymer
chains’ movements <strong>and</strong> to changes in the cure degree
<strong>of</strong> the EP in the corresponding interphase. Due to the large
matrix/filler interface area, the nan<strong>of</strong>illers could limit the
mobility <strong>of</strong> polymer chains <strong>and</strong> provide an increment in T g .
Moreover, a higher cure degree can enhance the T g <strong>and</strong> vice
versa. The addition <strong>of</strong> CNFs could earlier initiate the crosslinking
reaction due to the presence <strong>of</strong> catalyst particles or
hydrogen-bond donor molecules on CNF [32,33]. However,a
higher conversion obtained in the interphase may be associated
with a decrease <strong>of</strong> the final cure in the following diffusion-controlled
stage [32,33]. Note that selective absorption
<strong>of</strong> the resin or the hardener at nan<strong>of</strong>iller surface can lead to
an <strong>of</strong>f-stoichiometry in the interphase which strongly affects
the T g . The mechanisms, how CNF content influences the T g s
<strong>of</strong> EP <strong>and</strong> SCF/EP, cannot be clarified by the present work.
However, it is assumed that the content <strong>of</strong> CNFs can influence
the weighting <strong>of</strong> aforementioned competing factors. Thus,
the trend <strong>of</strong> T g with CNF content is the outcome <strong>of</strong> the abovementioned
competing factors.
3.3. Tensile properties
3.2. Effect <strong>of</strong> CNFs on T g
Fig. 2 shows the T g <strong>of</strong> the CNF/EP composites <strong>and</strong> SCF/CNF/EP
composites as a function <strong>of</strong> CNF-loading. The increase <strong>of</strong>
CNF-loading first leads to a slight enhancement in T g . When
the CNF-loading is increased from 0.5 to 0.75 vol.%, the T g declines.
Whereas, in the whole range studied the CNFs gives
rise to the T g in comparison to pure EP. The SCF/CNF/EP composites
present slightly lower T g than the CNF/EP composites.
With respect to the SCF/CNF/EP composites, the CNFs slightly
increase the T g .
Fig. 3a shows representative stress–strain curves <strong>of</strong> materials
studied, i.e. pure EP, CNF/EP composites, SCF/EP composites
<strong>and</strong> SCF/CNF/EP composites. Pure EP first exhibits a constant
stiffness zone (stain 0–0.3%) with the initial modulus <strong>of</strong> about
2.71 GPa. The transition from linear to nonlinear behavior was
interpreted in terms <strong>of</strong> ‘‘strain-s<strong>of</strong>tening’’ [34,35]. The nan<strong>of</strong>illers
do not significantly change the elongation rate at break
while the micron fillers significantly decrease the ultimate
elongation. Compared with the composites filled only with
SCFs, the SCF/CNF/EP composites show a tendency to yield.
Fig. 3b shows the slopes <strong>of</strong> the local points on the curves in
Fig. 3a as a function <strong>of</strong> strain, which was defined in [35] secant

CARBON 48 (2010) 4289– 4300 4293
Stress (MPa)
100
90 a 10CF0.75NF
0.75NF
5
b
80
10CF0.75NF
10CF
4 10CF
EP
70
60
3 0.75NF
50
40
2 EP
30
1
20
10
0
0
0 1 2 3 4 5 6 0 1 2 3 4 5 6 7
Strain (%)
Strain (%)
E sm
(GPa)
Modulus (MPa)
5500
5000
4500
4000
3500
3000
2500
2000
c
Neat Nanocomposites
epoxy
EP
EP
0.125NF
0.25NF
0.5NF
0.75NF
Microcomposites
10SCF
5CF
10CF
15CF
10CF0.125NF
Materials
Multiscale
composites
10CF0.25NF
10CF0.5NF
10CF0.75NF
Tensile strength (MPa)
95
90
85
80
75
70
d
Neat
epoxy
EP
EP
0.125NF
Nanocomposites
0.25NF
0.5NF
0.75NF
Microcomposites
10SCF
5CF
10CF
15CF
Materials
Multiscale
composites
10CF0.125NF
10CF0.25NF
10CF0.5NF
10CF0.75NF
Elogation at break (%)
7
6
5
4
3
2
1
0
e
Neat
epoxy
EP
Nanocomposites
EP
0.125NF
0.25NF
0.5NF
0.75NF
Microcomposites
10SCF
5CF
10CF
15CF
10CF0.125NF
Materials
Multiscale
composites
10CF0.25NF
10CF0.5NF
10CF0.75NF
Fig. 3 – (a) Stress–strain curves <strong>of</strong> typical materials studied, (b) secant moduli <strong>of</strong> the materials in (a), <strong>and</strong> the tensile
characteristics: (c) E-modulus, (d) tensile strength <strong>and</strong> (e) elongation rate at break <strong>of</strong> EP composites. Dash lines in (c)–(e)
indicating the levels <strong>of</strong> two reference materials, i.e. EP <strong>and</strong> SCFs/EP, are added for easier comparisons.
modulus E sm . The data with strain from 0% to 0.3% was not
shown due to the scattering <strong>of</strong> the data. The tendency <strong>of</strong>
the secant modulus is associated to the debonding <strong>of</strong> fillers
from polymer matrix with certain pre-strains [35]. From
Fig. 3b, at a low strain the secant modulus <strong>of</strong> the CNF/EP composite
(0.75NF) is higher than that <strong>of</strong> EP. However, when the
strain is higher than 4.5%, the two materials have similar secant
moduli. With respect to the SCF/EP composite, the secant
modulus declines rapidly due to debonding <strong>of</strong> SCFs. The
‘‘strain-s<strong>of</strong>tening’’ <strong>of</strong> 10CF0.75NF is enhanced compared to
10CF.
The E-moduli, tensile strengths <strong>and</strong> elongations at break
<strong>of</strong> pure EP, SCF/EP composites, CNF/EP composites, SCF/
CNF/EP composites, are collated in Fig. 3c–e. The modulus <strong>of</strong>
EP is only slightly increased (by less than 10%) by adding CNFs
(cf. Fig. 3c). The tensile strength is moderately improved with
the addition <strong>of</strong> the nan<strong>of</strong>illers (cf. Fig. 3d), whereas the elongation
at break is not changed significantly (cf. Fig. 3e). In

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).

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

4296 CARBON 48 (2010) 4289– 4300
Fig. 6 – Optic micrographs <strong>of</strong> polished sections <strong>of</strong> specimens after tensile tests, perpendicular <strong>and</strong> near to the fracture planes,
<strong>of</strong> 10CF (a), 10CF0.5NF (b) <strong>and</strong> 10CF0.75NF (c); the diagram associated shows the location observed.
contrast the superposition <strong>of</strong> the separate contributions <strong>of</strong>
the two fillers to K IC , as, respectively, achieved in 10CF <strong>and</strong>
0.75NF, result in a value <strong>of</strong> 120% (additive rule). With respect
to G IC , the combination <strong>of</strong> the two multiscale <strong>carbon</strong> fillers
leads to an improvement by 427%. Whereas, the superposition
<strong>of</strong> the separate contributions <strong>of</strong> the two fillers to G IC ,
as, respectively, achieved in 10CF <strong>and</strong> 0.75NF, result in a value
<strong>of</strong> 232%. Based on the above results, it can be concluded that
the multiscale <strong>carbon</strong> fillers play synergetic roles on the linear
elastic fracture mechanical parameters <strong>of</strong> EP.
The fracture surface <strong>of</strong> pure EP is very smooth manifesting
the brittle deformation <strong>of</strong> the EP [25]. This explains why the
fracture toughness <strong>and</strong> fracture energy <strong>of</strong> pure EP are low.
Fig. 8a <strong>and</strong> b shows the fracture surface <strong>of</strong> a CNF/EP composite
(0.75NF) with different magnifications. The fracture propagation
direction is downward as indicated by an arrow, which
is identical to all the other CT fracture surfaces presented in
this paper. Similar to the toughening mechanisms <strong>of</strong> microsized
SCFs, pull-out, breakage <strong>and</strong> debonding <strong>of</strong> CNFs, as noticed
in Fig. 8b, are assumed to be the major energy
dissipation mechanisms <strong>of</strong> CNFs. In addition, some tiny
CNF bundles or agglomerates cause crack pinning <strong>effect</strong> as
evidenced by the ‘‘tail’’ formation behind them (cf. Fig. 8a
<strong>and</strong> b).

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

4298 CARBON 48 (2010) 4289– 4300
Fig. 8 – CT fracture surfaces <strong>of</strong> 0.75NF (a) <strong>and</strong> (b), 10CF (c) <strong>and</strong> 10CF0.75NF (d)–(f).
synergetic <strong>effect</strong>s found. The stress transfer realized by
CNFs, followed by sub-critical cracks produced due to
CNF-related failure events, can reduce the stress concentrations
on microsized SCFs, especially near to their ends.
As consequence, the compliance <strong>of</strong> the EP in the region
near fiber ends is decreased <strong>and</strong> the SCF/matrix interfacial
failures are reduced. Due to the release <strong>of</strong> overstress on
SCFs by the addition <strong>of</strong> CNFs, a larger plastic zone
develops along the fracture plane as other SCFs are overtaking
the stress. This is believed to be the main reason
for the synergetic roles on fracture toughness <strong>and</strong> fracture
energy.
3. Compared to spherical silica nanoparticles [27], the synergetic
roles <strong>of</strong> CNFs with SCFs are more prominent. This is
attributed to the high aspect ratio <strong>of</strong> CNFs which allows a
highly efficient stress transfer from the matrix to the CNFs,
<strong>and</strong> being well dispersed in the EP, they contribute to an
efficient release <strong>of</strong> the overstresses at SCFs.

CARBON 48 (2010) 4289– 4300 4299
a
Fracture plane
Initial-crack tip
b
Fracture plane
Initial-crack tip
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