G. PELIN et al.: MECHANICAL AND TRIBOLOGICAL PROPERTIES OF NANOFILLED PHENOLIC-MATRIX ...
569–575
MECHANICAL AND TRIBOLOGICAL PROPERTIES OF
NANOFILLED PHENOLIC-MATRIX LAMINATED COMPOSITES
MEHANSKE IN TRIBOLO[KE LASTNOSTI FENOLNIH MATRIC V
KOMPOZITIH, PRIDOBLJENIH Z NANOTEHNOLOGIJO
George Pelin1,2, Cristina-Elisabeta Pelin1, Adriana ªtefan1, Ion Dincã1,
Ecaterina Andronescu2, Anton Ficai2, Roxana Truºcã2
1National Institute for Aerospace Research – Elie Carafoli, 220 Iuliu Maniu Blvd, 061126 Bucharest, Romania
2University Politehnica of Bucharest, Faculty of Applied Chemistry and Materials Science, 1-7 Gh. Polizu St., 011061 Bucharest, Romania
ban.cristina@incas.ro
Prejem rokopisa – received: 2016-01-17; sprejem za objavo – accepted for publication: 2016-10-24
doi:10.17222/mit.2016.013
Phenolic-resin composites have attractive properties for applications in various fields from the wood and adhesive industry to
the automotive, aeronautics and aerospace industries. The paper presents the obtaining of SiC-nanofilled phenolic-resin-based
composites reinforced with a bidimensional fabric. Different contents of nanometric silicon carbide (0.5, 1 and 2) % mass frac-
tions) were dispersed into the phenolic-resin matrix, using the ultrasonication method, to ensure the optimum dispersion. Sev-
eral layers of the bidimensional fabric were impregnated with the obtained mixtures and the final laminated composites were
obtained using high-temperature pressing, followed by a multistage temperature program. The obtained laminated
nanocomposites were characterized with FTIR spectroscopy and evaluated in terms of mechanical and tribological properties.
After mechanical testing, fracture cross-sections were characterized with SEM and optical microscopy. The results highlight the
positive effect of the nanometric silicon-carbide addition to the phenolic-resin matrix of the laminated composites, in terms of
mechanical and tribological performance, improving their strength, stiffness and abrasive properties.
Keywords: laminated composites, tensile strength, flexural strength, nanometric silicon carbide, nanocomposites, friction coeffi-
cient
Kompoziti fenolnih smol imajo mo`nosti raznovrstne uporabe na razli~nih podro~jih, tako na podro~ju lesne industrije, v
industriji lepil, kot v avtomobilski in letalski industriji. Prispevek predstavlja pridobitev SiC fenolnih kompozitov na osnovi
smole, oja~anih z bidimenzionalnimi vlakni. Razli~ne vsebine nanosilicijevega karbida (0,5, 1 in 2) % masnega odstotka, so bile
razpr{ene z uporabo ultrazvo~ne metode v matrici, pridobljeni s smolo, da je bila zagotovljena optimalna disperzija. Ve~ plasti
bidimenzionalnih vlaken je bilo impregniranih s pridobljenimi me{anicami in kon~ni laminirani kompoziti so bili pridobljeni z
visokotemperaturnim tla~nim pritiskanjem, kateremu je sledil ve~stopenjski temperaturni program. Pridobljeni laminirani
kompoziti so bili preu~evani s FTIR-spektroskopijo in ovrednoteni glede na mehanske in tribolo{ke lastnosti. Presek zloma po
mehanskem testiranju je bil preu~evan s SEM-mikroskopijo in opti~no mikroskopijo. Rezultati ka`ejo pozitiven u~inek dodatka
nanometri~nih silicijevih karbidov, s smolo pridobljenih matric v ve~plastnih kompozitih, zaradi njihovih mehanskih in
tribolo{kih lastnosti, in ker se tako izbolj{a mo~, togost in abrazivne lastnosti.
Klju~ne besede: laminirani kompoziti, natezna trdnost, upogibna trdnost, nanosilicijevi karbidi, nanokompoziti, koeficient trenja
1 INTRODUCTION
Due to their high strength and rigidity combined with
low density, fiber-reinforced polymeric composites
(FRP) are extensively used in a wide variety of fields,
from sports equipment1–2 to the automotive, civil engi-
neering,3–5 military,6–7 aeronautics and aerospace8–10 in-
dustries. The oldest thermoset polymers are phenolic res-
ins. This type of resins are mostly used as heat insulation
in aerospace applications, due to their low cost and facile
processability11 as well as their attractive properties such
as chemical, heat and friction resistance and superior
thermal insulation characteristics.12 They are generally
used in combination with other materials such as powder
fillers and short or long fiber reinforcements. Pheno-
lic-resin-impregnated fibers (glass, carbon, aramid) re-
sult in phenolic-resin-laminated composites and are
mostly formed with compression molding.12–13 During
the obtaining process of this kind of materials, vola-
tile-compound management is crucial in producing
high-quality laminates.14 An efficient management of
volatile compounds during the development stage of
phenolic-resin/fiber laminates leads to void-free compos-
ites and, consequently, fewer stress-concentration sites
that contribute to creating a stronger interface. The fi-
ber/matrix interface is a decisive factor for the final me-
chanical and physical properties of the composite materi-
als.15
Composite materials based on phenolic resins/carbon
fibers have been used by NASA as the standard material
for high-temperature applications. These composites
have different micronic fillers added to the matrix for the
stabilization of charred polymer during the combustion.16
However, the use of nanofillers instead of the micronic
ones allows a weight reduction in the aerospace systems,
and it also leads to thinner protection layers with better
ablative properties.16 Studies show that mechanical, ther-
mal and friction properties of phenolic-matrix/fiber com-
Materiali in tehnologije / Materials and technology 51 (2017) 4, 569–575 569
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
UDK 67.017:620.168:620.3 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(4)569(2017)
posites can be improved by adding nanometric fillers
such as carbon nanotubes,17–18 layered silicates (nano-
clays),19 POSS compounds,20 silica21–22 or silicon car-
bide23–25 to the phenolic resin. In the case of carbon-phe-
nolic composites with nanoclays, studies show that
higher nanoclay contents decrease the erosion rate, sur-
face temperature and insulation index,26 and can improve
some properties, such as the flexural strength, stiffness
and glass-transition temperature.27 On the contrary, there
are studies that report a decrease in the flexural strength
and stiffness.16 Literature also presents studies that exam-
ine the effect of montmorillonite nanoclays on the me-
chanical and morphological characteristics of the glass-
fiber-reinforced composites based on the novolac phe-
nol- formaldehyde matrix, showing that a nanoclay addi-
tion enhances the mechanical properties when a strong
matrix-fiber interface is observed.11
The other interesting nanofiller is silicon carbide
(nSiC) that is mostly used to improve the thermo-oxida-
tive resistance25 and wear resistance23–24 of carbon/pheno-
lic ablative materials. However, literature reports no
studies involving carbon- and/or glass-fiber-fabric-lami-
nated composites based on the nSiC-modified phenolic
resin. The aim of this study is the evaluation of the effect
of a silicon-carbide nanofiller addition to the pheno-
lic-resin matrix of carbon- and glass-fabric-reinforced
composites, taking into consideration morphological,
mechanical and tribological properties. The paper pres-
ents the obtaining of nanometric silicon-carbide-filled
phenolic-resin-matrix composites reinforced with glass
or carbon-fiber bidimensional fabric. The obtained lami-
nates based on different nanofiller contents were charac-
terized in terms of chemical, mechanical and tribological
properties. The results highlighted the positive effect of
nSiC on the laminated composites, when added in the
optimum weight content relative to the phenolic-resin
matrix.
2 EXPERIMENTAL PART
2.1 Materials
The matrix used was the resole-type phenolic resin
ISOPHEN 215 SM 57 % (PR) provided by ISOVOLTA
S.A. Bucharest, with a density of 1135 g/cm3. The
nanofiller used was the -type nanometric silicon carbide
(nSiC) purchased from Nanostructured & Amorphous
Materials Inc., USA, with the following characteristics: a
purity of 97.5 %, the average particle size of 45–55 nm,
the specific surface area of 34-40 m2/g and true density
of 3.22 g/cm3. The reinforcing materials were the carbon
fiber (CARP/T 193, produced by Chemie Craft, France)
and E-glass-fiber bidimensional fabrics.
2.2 Method for obtaining nanofilled-laminated com-
posites
The development of the nanofilled-laminated com-
posites was a 3-stage process. The first stage consisted of
nSiC additions of different contents (0, 0.5, 1 and 2) %
of mass fractions to the phenolic resin (PR), bulk ho-
mogenization achieved with mechanical stirring for
approx. 5 min and nanofiller dispersion carried out with
the ultrasonication technique, using a Bandelin Sonopuls
probe for 15 min. In the second stage, 5 layers of carbon
fiber (CF) and glass fiber (GF), respectively, were im-
pregnated with the obtained mixtures. The soaked layers
were maintained at 25 °C for 30 h for a better impregna-
tion, then they were subjected to methanol-solvent elimi-
nation at 70 °C for 30 min.28 The final stage was the
heat-curing process that took place under pressure using
a CARVER hydraulic press. Along with pressing, the
temperature was raised from 25 °C to 150 °C at a
30 °C/min heating rate, followed by a 30 min dwell pe-
riod at 150 °C and cooling under pressure down to room
temperature. Laminated composite plates were obtained;
from them, dumbbell and rectangular specimens for me-
chanical and tribological tests were cut. Figure 1 shows
two of the specimens used for tensile tests, illustrating
the dumbell geometry.
2.3 Characterization techniques
The phenolic-based nanocomposite laminates were
subjected to a spectroscopy analysis using a Nicolet iS50
spectrometer (operated in the ATR mode) and scanning
electron microscopy (SEM) using a QUANTA INSPECT
F microscope with a field emission gun and 1.2 nm reso-
lution, and an energy-dispersive X-ray spectrometer
(EDS). The materials were tested in terms of mechanical
performance with an INSTRON 5982 machine. The ten-
sile tests carried out on dumbbell specimens were per-
formed according to SR EN ISO 527-229 using a
5 mm/min tensile rate, while flexural (3-point bending)
tests carried out on rectangular specimens were per-
formed according to SR EN ISO 1412530 using a
2 mm/min speed, conventional deflection and the nomi-
nal span length (16× the specimen thickness). Tribo-
logical tests were performed using CETR UMT 3 (Uni-
versal Macro Materials Tester) – the block-on ring
module – on a 35 mm (diameter) × 11 mm (width)
stamped steel roller (A4138 Timken outer rolling bearing
ring), in the counterclockwise testing direction under a
10 N normal force, using samples with dimensions of
16 mm (length) × 6.5 mm (width) × 2 (thickness) mm. A
G. PELIN et al.: MECHANICAL AND TRIBOLOGICAL PROPERTIES OF NANOFILLED PHENOLIC-MATRIX ...
570 Materiali in tehnologije / Materials and technology 51 (2017) 4, 569–575
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 1: a) glass-fiber and b) carbon-fiber-PR-based composites;
specimens for tensile tests
test was performed for 60 s using two different rotation
speeds: 1000 min–1 (1.75 m/s) and 1500 min–1 (2.62 m/s).
The tests were conducted on a minimum of 3 specimens
per sample, at the standard atmospheric conditions,
(25 °C and 45–55 % relative humidity).
3 RESULTS AND DISCUSSION
3.1 FTIR-spectroscopy
The nanofilled phenolic matrix of the laminated com-
posites was subjected to FTIR spectroscopy analyses af-
ter polishing the analyzed area. The nanofilled-ma-
trix-based laminated composite samples were compared
with the control sample to evaluate the interactions be-
tween nSiC and the phenolic resin that could generate
peak-position shifting and/or peak-intensity variations.
Figure 2 presents the spectra of the nSiC nanofiller pow-
der and simple nSiC-filled phenolic resin from the lami-
nated composites; all the composite samples present the
characteristic peaks of the cured resin. The peak at
3290 cm–1 corresponds to the OH stretching from resole,
the one at 2920 cm–1 is due to the CH2 stretching, while
the peaks in the 1600–1470 cm–1 range are due to the
-C=C stretching in the aromatic ring. The CH2 bending
vibration generated the peaks at 1435 cm–1 and
1330 cm–1, the C-O stretching at approximately
1190 cm–1 and the C-O-C stretching at 998 cm–1.31 The 3
peaks between 900–600 cm–1 are due to ortho-disub-
stituted, meta-disubstituted and monosubstituted ben-
zenes.31–32 The meta-disubstituted benzene peak at
815 cm–1 overlaps with the nSiC characteristic peak, as-
signed to the Si-C vibration that appears at approxi-
mately 800–815 cm–1 33–34 so that the nSiC presence is
more difficult to highlight. However, it is noticed that the
peak intensity increases with the nSiC content in the PR
matrix of the laminated composites, which could be due
to the higher nSiC content interacting with the phenolic
resin.35 The absorption of a diamond crystal generates a
noise between 1900–2300 cm–1, which, therefore, must
be ignored.
3.2 SEM analysis
A SEM analysis was performed on the fracture
cross-sections of the mechanically tested laminated sam-
ples to evaluate the fiber/matrix interface at high magni-
fication levels. SEM images of carbon and glass-fab-
ric-based control samples (Figure 3a to 3c) illustrate that
the resin did not embed the entire surface of the fibers
that the fabric is made of, and that the stress induced dur-
ing the mechanical testing generated both a detachment
of the polymer from the fibers as well as the matrix mi-
cro-cracking. In the case of the 1 % nSiC-filled matrix, it
can be observed that the polymer layer covering the fi-
bers is more compact, suggesting that the matrix was
able to undergo the mechanical stress without being sub-
jected to a detachment or micro-cracking. This could be
due to the strengthening of the matrix caused by the
nSiC presence that, along with a proper fiber/matrix in-
terface, helps sustain a better mechanical load transfer
within the composite. Figure 4 illustrates high-magnifi-
cation images of the samples with 1 % and 2 % nSiC
contents added to the phenolic resin of the carbon-fi-
ber-reinforced composites. A higher agglomeration ten-
dency of the nanoparticles can be observed in the sam-
ples with a higher nSiC content. Also, in the case of
5CF/PR+1% nSiC (Figure 4a) the polymer layer uni-
formly covers the fiber surface, embedding the nano-
particles, while in the case of 5CF/PR+2% nSiC (Figure
4b) the agglomerated nanoparticles produce voids
around that area. The areas composed of voids and ag-
glomerated nanoparticles can act as stress-concentration
sites, sustaining crack propagation, which influences the
mechanical behavior.
3.3 Mechanical testing
The mechanical-performance evaluation was done
through tensile and three-point bending tests. Mechani-
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MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 2: FTIR spectra of PR and nSiC/PR from the laminated com-
posites
Figure 3: SEM images of: a) 5CF/PR, b) 5CF/PR+1% nSiC,
c) 5GF/PR, d) 5GF/PR+1% nSiC
cal test results (Table 1) show the fact that an nSiC addi-
tion to the phenolic-resin matrix significantly improves
the mechanical performance of both carbon- and glass-fi-
ber-reinforced laminated composites. Overall, all the
nanofilled samples exhibited higher strength and stiff-
ness compared with the control samples.
Table 1: Mechanical properties of nSiC-filled 5CF/PR and 5GF/PR
laminated composites
nSiC
(w/%)
Tensile
strength
(MPa)
Tensile
modulus
(GPa)
Tensile
strain
(%)
Flexural
strength
(MPa)
Flexural
modulus
(GPa)
Elonga-
tion (%)
5CF/PR based laminated composites
0 318.0 47.8 0.8 382.1 43.9 1.22
0.5 344.5 50.5 0.75 420.3 48.6 1.09
1 384.5 68.6 0.7 458.0 56.3 1.04
2 331.9 52.9 0.48 440.2 45.0 1.02
5GF/PR based laminated composites
0 300.3 22.0 2.02 350.7 27.4 2.41
0.5 314.3 27.1 0.83 363.2 32.7 1.29
1 342.5 37.5 0.77 423.6 34.6 1.23
2 293.3 22.1 0.71 352.0 33.3 1.17
The same trend was observed in both carbon- and
glass-fiber laminates in terms of the nSiC-content effect.
The highest results were exhibited by the samples based
on the 1 % nSiC-filled matrix, in the case of 5CF/PR,
where the 1 % content samples showed an increase of
20 % in the tensile and flexural strength and an increase
of 30–40 % in the tensile and flexural modulus, while for
5GF/PR, the 1 % nanofiller led to an increase of
15–20 % in the tensile and flexural strength and an in-
crease of 30–70 % in the tensile and flexural modulus.
The nanoparticles embedded into the phenolic-resin ma-
trix that covers the fibers (Figure 4a) could act as a
crack-propagation hindering agent, enhancing the matrix
strength and stiffness and, consequently, the mechanical
properties of the laminated composite based on this ma-
trix. This phenomenon was observed also in the case of
phenolic-resin/nSiC nanocomposites presented in a pre-
vious study.35 At higher contents (2 %), the properties
decrease compared to 1 %, probably due to the existent
nanofiller agglomeration, showed by SEM images (Fig-
ure 4b), that could lead to stress-concentration sites
and/or crack-initiation sites that sustain earlier mechani-
cal failures and contribute to the decrease in the mechan-
ical-properties.
In both tensile and flexural tests, elongation de-
creases with the nSiC content increase due to the brittle
nature of nSiC that enhances the composite rigidity.
3.4 Fractography
Fractography represents the fracture-mode analysis
through light microscopy. Being an important tool in a
material investigation related to a failure analysis and
quality control,36 light microscopy was the main method
used to analyze the damage after the mechanical testing
and to establish the failure mode type and mechanism.
Light microscopy was the first method used to investi-
gate the failure mode as it allows a visualization of the
entire fractured area along the whole length of a sample.
Figure 5 illustrates the fractured areas of the repre-
sentative specimens of laminated composite samples.
Both carbon- and glass-fiber-based composites showed
the same behavior as the nSiC content in the phenolic
matrix. All the samples exhibited a certain degree of
delamination, generated by crack propagation. While the
control samples (Figure 5a and 5e) and the PR-matrix
laminates including 0.5 % nSiC (Figure 5b and 5f)
showed more extended delaminated areas, the 1 %
nSiC-content-based sample showed the lowest delamina-
tion degree in both carbon and glass-fabric laminates
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572 Materiali in tehnologije / Materials and technology 51 (2017) 4, 569–575
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 5: Optical-microscopy images of the tensile-test specimens:
a), b), c), d) 5CF/PR+ 0; 0.5; 1; 2 % nSiC and e), f), g), h) 5GF/PR+ 0;
0.5; 1; 2 % nSiC
Figure 4: High-magnification SEM images of: a) 5CF/PR+1% nSiC,
b) 5CF/PR+2% nSiC
(Figure 5c and 5g), where only the external layers exhib-
ited detachment.
In the case of the 2 % nSiC laminates (Figure 5d and
5h), all the fabric layers ruptured after the tensile testing,
showing delaminated layers only in the fracture regions.
The optical-microscopy method was further sup-
ported with higher-magnification images of the fracture
cross-sections, captured with SEM (Figure 6). Opti-
cal-microscopy images provide valuable information
when choosing the most appropriate locations to be visu-
alized with scanning electronic microscopy.36 SEM im-
ages of the entire fracture cross-sections of the samples
show that the control samples (Figure 6a and 6c) were
subjected to a higher damage extent; delaminated areas
are still visible on the 0.5% nSiC-based sample (Figure
6b), while the image of the 1% nSiC sample (Figure 6d)
suggests that the fabric-layer fracture occurred due to the
strain and that the fibers broke on the same fracture sec-
tion line.
3.5 Tribological testing
The positive effect of the nSiC addition was also re-
flected on the tribological test results. The laminated
composites were tested at two speed rates: 1000 cm–1
(1.75 m/s) and 1500 cm–1 (2.62 m/s), on a minimum of
three specimens per sample, over a short period of time
(60 s) to evaluate the friction-coefficient trend at the ini-
tial stage of the friction-force action. Figure 7 and Fig-
ure 8 illustrate the friction-coefficient function of the
nSiC weight content in the matrix of the carbon- and
glass-fabric-laminated composites. It can be observed
that there is a significant increase in the friction coeffi-
cient with a nanofiller content increase in the matrix,
where substantial increments are obtained at the higher
testing speed.
In the case of carbon-fiber-based composites, the
friction coefficient increases by approximately 10–16 %
when adding 0.5 % nSiC, 30–50 % for 1 % nSiC and
70–90 % for the 2 % nanofiller compared with the con-
trol sample.
For glass-fiber-based composites, the friction-coeffi-
cient increments are not as significant as in the case of
carbon fiber, as nSiC generates an increase of 14 % in
the case of the 0.5 % addition, 30–37 % in the case of
the 1 % addition and 40–45 % in the case of the 2 % ad-
dition.
The differences between the GF- and CF-based sam-
ples regarding the friction-coefficient values are due to
different tribological characteristics of the two fiber
types, where the carbon fiber is an anti-friction mater-
ial37, while the glass fiber promotes friction.38
4 CONCLUSIONS
The paper presents the obtaining and characterization
of glass- or carbon-fiber bidimensional, reinforced com-
posites based on the phenolic-resin matrix with different
nSiC contents. The obtained results showed that the ad-
dition of nanometric silicon carbide improved the tensile
and flexural strength and the modulus of the laminated
composites, and that the best results were obtained with
the 1 % mass fractions of the nanofiller contents for both
carbon- and glass-fiber-reinforced composites. The ten-
G. PELIN et al.: MECHANICAL AND TRIBOLOGICAL PROPERTIES OF NANOFILLED PHENOLIC-MATRIX ...
Materiali in tehnologije / Materials and technology 51 (2017) 4, 569–575 573
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 6: SEM images of fracture cross-sections of: a) 5CF/PR,
b) 5CF/PR+0.5% nSiC, c) 5GF/PR, d) 5GF/PR+1% nSiC
Figure 8: Friction-coefficient values for 5GF/PR/nSiC composites
Figure 7: Friction-coefficient values for 5CF/PR/nSiC composites
sile and flexural strength increased by 15–20 % with the
1 % nSiC content in the PR matrix for both glass- and
carbon-fiber-reinforced composites, while the tensile and
flexural modulus increased by 30–40 % for the car-
bon-fiber and 20–30 % for the glass-fiber composites.
In terms of the tribological behavior, the friction co-
efficient increased with the nanofiller-content increase.
For the glass-fiber-based composites, nSiC generated an
increase of 30–37 % when the 1 % content was added
and an increase of 40–45 % for the 2 % weight contents.
In the case of the carbon-fiber-based composites, the
control sample had a very low friction coefficient (0.13);
therefore, the nanofilled sample’s friction coefficient in-
creased significantly: by 30–50 % for 1 % nSiC and by
70–90 % for the 2 % nanofiller. It was observed that
higher speed rates led to higher friction-coefficient val-
ues.
The results highlight that 1 % nSiC by weight is the
optimum content for phenolic/fabric composites in order
to obtain both the maximum mechanical and tribological
improvements as, at this content, the optimum nanopart-
icle embedment into the phenolic resin is achieved.
Acknowledgments
The work was funded by the Sectorial Operational
Programme Human Resources Development 2007–2013
of the Ministry of European Funds through the Financial
Agreement POSDRU/159/1.5/S/134398.
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