Materials and Design 32 (2011) 3516–3520
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Materials and Design
journal homepage: www.elsevier.com/locate/matdes
Short Communication
Properties of carbon nano-tubes–Cf/SiC composite by precursor infiltration
and pyrolysis process
Haijiao Yu a,b,⇑, Xingui Zhou a, Wei Zhang a, Huaxin Peng b, Changrui Zhang a, Ke Sun a
a
b
College of Aerospace and Materials Engineering, National University of Defense Technology, Changsha 410073, PR China
Advanced Composites Center for Innovation and Science (ACCIS), Department of Aerospace Engineering, University of Bristol, Bristol BS8 1TR, UK
a r t i c l e
i n f o
Article history:
Received 14 December 2010
Accepted 15 February 2011
Available online 18 February 2011
a b s t r a c t
Carbon nanotubes (CNTs) were introduced into the precursor infiltration and pyrolysis (PIP) carbon fiber
reinforced silicon carbide matrix (Cf/SiC) composite via the infiltration slurry. The weight fraction of CNTs
in the composite was 0.765‰. The fiber–matrix interface coating was prepared through chemical vapor
deposition (CVD) process using methyltrichlorosilane (MTS). Effects of the CNTs on mechanical and thermal properties of the composite were evaluated by three-point bending test, single-edge notched beam
(SENB) test, and laser flash method. Attributed to the introduction of the small quantity of CNTs, flexural
strength and fracture toughness of the Cf/SiC composite both increased by 25%, and thermal conductivity
at room temperature increased by 30%.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
It is well known that silicon carbide (SiC) ceramic has desirable
properties, such as high specific strength, high specific stiffness,
low activity, and excellent wear resistance [1]. However, its use
is limited to a great extent by the inherent brittleness of the material, which can be partially overcomed by reinforcing with fibers
[2,3] or whiskers, etc. [4,5]. The resulting composites usually
possess much higher fracture toughness [6,7].
It is also well known that carbon nanotubes (CNTs), which have
been developed for decades, exist unique and extraordinary
mechanical and thermal properties [8,9]. The name carbon
nanotube is derived from their size which is only a few nanometers
wide. By definition carbon nanotubes are cylindrical carbon
molecules with properties that make them potentially useful in
extremely small scale mechanical and thermal applications
[10,11]. During these decades, it has opened vast areas of research
which include CNTs reinforcements in composites in order to
improve their mechanical [12,13], thermal [14,15] and even
electrical properties, etc. [16,17]. Although the focus of the
research in CNTs based composites has mostly been on polymer
[13] or metal [18,19] composites, the unique properties of carbon
CNTs can also be exploited in ceramic matrix composites. Shimoda
et al. [20] have prepared carbon nanofibers reinforced SiC (CNFs/
⇑ Corresponding author. Address: Key Laboratory of Advanced Ceramic Fibers and
Composites, College of Aerospace and Materials Engineering, National University of
Defense Technology, 47# Yanwachi Street, Changsha 410073, PR China. Tel.: +86 (0)
731 84576397; fax: +86 (0) 731 84573165.
E-mail addresses: poochie@nudt.edu.cn, aexhy@bristol.ac.uk, yunzeyu2000@
163.com, yunzeyu2000@yahoo.com.cn (H. Yu).
0261-3069/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.matdes.2011.02.038
SiC) nanocomposites via transient eutectic route, the optimal one
of which contained 5 wt.% CNFs, showed an about 70% increase
in thermal conductivity, and fracture toughness of 5.7 MPa m1/2.
Ma et al. [21] fabricated CNTs/SiC nanocomposite by hot-press
method, and obtained a toughness increment of 10% over the
monolithic SiC. Morisada et al. [22] have fabricated multi-walled
carbon nanotubes reinforced SiC (MWCNTs/SiC) composites,
which showed superior toughness of 5.4 MPa m1/2 compared to
4.8 MPa m1/2 of the monolithic SiC ceramic. MWCNTS/SiC composite prepared by Lü et al. [23] using aqueous tape casting possessed
the increments of 6.14% and 6.44% in flexural strength and fracture
toughness, respectively when the MWCNTs content was 0.25 wt.%.
Further increase in MWCNTs content to 0.50 wt.% did not lead to
the increase in mechanical properties. Tian et al. [24] fabricated
ZrB2–20 vol.%SiC ceramics with 2 wt.% CNTs, and obtained 15%
increment in fracture toughness. But hardness, flexural strength
as well as thermal conductivity of the composites had no gain.
To sum up, the CNTs were introduced using several methods and
treatments, and with fairly large amount. However, the mechanical
and thermal properties of the composites seem have room for
improvement.
Even after a decade of their discovery, the full potential of CNTs
in this application has not been realized with experimental
outcomes falling short of predicted values which demand an active
insight in this field. And there are a lot of challenges to be resolved
before they are ready for use in varied industrial applications [25].
The present work was undertaken to combine the advantages of
commercial grade CNTs of small fraction and three dimensional
(3D) carbon fiber reinforced silicon carbide ceramic matrix
(Cf/SiC) composite structure with a technological process as simple
as possible. The characterization and analysis of the resulting
H. Yu et al. / Materials and Design 32 (2011) 3516–3520
composite would supply basis for the industry applications. In the
present work, Cf/SiC composite with dispersed commercial grade
CNTs was prepared without any dispersing agent or surface treatment of the CNTs. It is possible to produce large-sized as well as
complex, thin-walled, high temperature structures via the precursor infiltration and pyrolysis (PIP) process. So PIP was chosen for
the composite preparation and effects of the CNTs on mechanical
and thermal properties of the Cf/SiC composite were investigated.
And to facilitate the industrialized operation and production, CNTs
was introduced through the infiltration slurry.
2. Experimental details
3517
slurry; then the purified CNTs were introduced into this slurry by
virtue of ultrasonic dispersion method. Secondly, the coated carbon
fiber preform was dipped into the slurry mentioned above under
four atm pressure, dried in the air, and heat treated at 1200 °C in
a flowing nitrogen atmosphere. Finally, eight infiltration–pyrolysis
cycles were performed to complete the densification. Blank
samples without CNTs were also prepared for compare.
It should be noted that without any dispersant, the macroscopically uniform distribution of CNTs in precursor slurry was achieved.
And we have found by trial and error that, the threshold fraction
value for non-sedimentation distribution of the CNTs in the
PCS/xylene slurry was 0.6 g/L. When the CNTs fraction exceeded this
value, the CNTs sedimentation would occur within 48 h.
2.1. Material preparation
2.2. Properties evaluation
Polyacrylonitrile (PAN)-based JC-1# carbon fiber bundles
(Jilin Carbon Co., Ltd., China) were used as the reinforcements, single filament diameter, density and tensile strength of which are
7 lm, 1.76 g cm3 and 3.6 GPa, respectively. The 3D carbon fiber
preforms were braided by a four-step method with a fiber volume
fraction of 35% in Yixing Tianniao High Technology Co. Ltd.,
China. Commercial grade MWCNTs were purchased from Shenzhen
Nanotechnologies Co. Ltd., China, with the purity of no less than
95%, out diameter of 20–40 nm, length of 5–15 lm, and thermal
conductivity of 2000 W/(m K). Polycarbosilane (PCS), with the
molecular weight of 1300 and the soften point of 210 °C
(National University of Defense Technology, China) was used as
the precursor of SiC matrix.
The 3D CNTs–Cf/SiC composite was then prepared by PIP process
with a heating temperature of 1200 °C at a heating rate of 20 °C/
min. The flow chart is shown in Fig.1. Firstly, the PCS was
dissolved into xylene at a weight ratio of 1:1 to make the infiltration
The mass density of the composite was determined by mass and
volume of each specimen. The theoretical density was calculated
using the rule of mixtures, volume fraction and individual density
of the constituents of composite. Since the fraction of CNTs is
extremely low (less than 0.1 wt.%) and has little influence on the
porosity of the composite, its influence was not considered in
calculation of the composite porosity. Then, the porosity can be
obtained by subtracting the ratio of real density and theoretical
density of the composite from ‘‘1’’. The three-point bending test
(specimen size of 12 mm 4.0 mm 75 mm) was carried out at
ambient temperature, with the crosshead speed of 0.17 mm/min
and outer support span of 64 mm using servohydraulic machine
(WDW model 100). Specimens were cut parallel to the longitudinal
direction, and the test was conducted following the general guidelines of ASTM standard C1341 [26]. At least five samples were
tested for each composite. A scanning electron microscopy (SEM,
Fig. 1. Flow chart of the fabrication of the CNTs–Cf/SiC composite.
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H. Yu et al. / Materials and Design 32 (2011) 3516–3520
model JSM-6300, JEOL, Tokyo, Japan) was employed to investigate
the fracture surface of specimens after the bending test.
Thermal diffusivities of the specimens (nominally 10 mm in
diameter and 2 mm in thickness) were measured at room temperature through a laser flash method using thermal diffusivity
analyzer (model JR-3, laser thermal conduction laboratory, Central
South University, China). The thermal conductivity was then calculated using the mass density and specific heat of the composite,
which was estimated by the rule of mixtures using literature values of the constituents specific heats [27]. Then, a high frequency
infrared analyzer of carbon and sulfur (model HW 2000, Wuxi
Yingzhicheng High Speed Analytical Instruments Co. Ltd., China)
was employed to determine the composition information of the
composite.
Fig. 2. Typical stress–strain curves recorded during the three-point bending test of
the composites with and without CNTs.
3. Results and discussion
As the commercial grade CNTs are usually accompanied by carbonaceous or metallic impurities, the purification is an essential
issue to be addressed for exerting their properties to the largest
extent. During these years, a number of purification methods have
been developed for obtaining CNTs with desired purity [28]. We
chose the liquid phase oxidation, which belongs to the chemical
oxidation methods, to purify the CNTs. The oxidants selected were
NaOH and HNO3.
The measured density of the infiltration slurry was about
0.95 g/ml, and the CNTs fraction in the PCS/xylene slurry was
0.6 g/L, so the calculated weight fraction of CNTs in the composite
was about 0.765‰. Table 1 lists the composition information of the
Cf/SiC composites with and without dispersed CNTs examined by
the high frequency infrared analyzer of carbon. The sample was
pulverized and placed in a ceramic crucible in the high frequency
induction furnace to be heated at a programmable temperature.
After dust and moisture removal, gases produced during the combustion were then analyzed using four Infrared detectors. If the
oxygen is supplied during this process, the analysis of CO2 and
CO determines the carbon fraction; and it determines the oxygen
fraction of the composite if the oxygen supply is isolated; then,
the silicon fraction can be calculated. As listed in Fig.1, with the
introduction of CNTs, mol fraction of the free carbon increases from
37.6% to 46.6%, and of the oxygen decreases from 9.3% to 7.4%.
Typical stress–strain curves recorded during the three-point
bending test of the Cf/SiC composites with and without CNTs are
shown in Fig. 2. The curve for the composite without CNTs gives
a initial linear behavior and then a sharp decrease, indicating a
lower flexural strength and a catastrophic failure. In contrast, the
curve for the CNTs–Cf/SiC composite shows a similar initial linear
behavior, following an extended tail with much larger area under
it. It is clear that the flexural strength and fracture toughness of
the CNTs–Cf/SiC is much higher than those of the composite without CNTs.
Properties of the Cf/SiC composites with and without CNTs are
given in Table 2. The density seems to be independent with the
introduction of CNTs. Meanwhile, since the fraction of CNTs is
Table 1
Composition of the Cf/SiC composites with and without dispersed
CNTs.
Composite
Composite without
CNTs (mol%)
Composite with
CNTs (mol%)
SiC
C
O
53.1
37.6
9.3
46.0
46.6
7.4
Table 2
Properties of the Cf/SiC composites without and with dispersed CNTs.
Properties
Without CNTs
With CNTs
Density (g cm3)
Porosity (%)
Flexural strength (MPa)
Fracture toughness (MPa m1/2)
Thermal conductivity (W m 1 K
1.69
16.7
246.9 ± 10.0
10.94 ± 0.02
1.39
1.72
16.4
311.4 ± 3.5
13.70 ± 0.01
1.81
1
)
relatively small, the volume fraction of the carbon fiber is surely
not influenced either.
Data from Table 2 also illustrates the relationship between
mechanical and thermal properties of the composites and the
introduction of the CNTs. Compared with the composite without
CNTs, although the fraction of CNTs is relatively small, the flexural
strength, fracture toughness and Z-directional thermal conductivity of the CNTs–Cf/SiC composite were enhanced considerably.
Especially, the 0.765 wt.‰ CNTs lead to 25% increment of the fracture toughness of the composite, is preferable to 24% increment
with 5 wt.% CNTs in Ref. [20], 10% increment with 10 wt.% CNTs
in Ref. [21], 12.5% increment with 0.25 wt.% CNTs in Ref. [22],
6.44% increment with 0.25 wt.% CNTs in Ref. [23], and 15% increment with 2 wt.% CNTs in Ref. [24].
The fracture surface morphologies of Cf/SiC composites with
and without dispersed CNTs are presented in Figs. 3 and 4. In
Fig. 3a and b, the similar widespread smooth surfaces with a few
of fiber pullouts indicate that the two composites both failed in
the brittle modes.
In Fig. 4a, lots of residues are observed adhered to the surface of
pull-out fibers. It implies that the SiC coatings have made sense to
the interface debonding, cracks deflection and the consequent energy consuming, which are believed to be in favor of the composite
toughening. While in Fig. 4b, some straight and thick CNTs are on
the fracture surface of the composite. Fig. 4c and d showed the high
magnification image of the observed CNTs and the as-received
CNTs, respectively for compare. The straight and thick form of
the CNTs may be attributed to the favorable adsorption between
the CNTs and the infiltration slurry, or/and the difference of the
coefficient of thermal expansion between CNTs and SiC matrix. In
the former case, as the expansion would occur during the pyrolysis
of SiC matrix, the CNTs would puff and become thick and straight;
while in the latter case, the thermal expansion dismatch would
introduce a residue stress and tension on the CNTs. However, it
is not proved yet. Moreover, it seems that it is impossible for the
small quantity of straight CNTs to connect to each other to form
a network in the composite.
H. Yu et al. / Materials and Design 32 (2011) 3516–3520
3519
Fig. 3. Low magnification SEM micrographs showing fracture surfaces of the composites: (a) without and (b) with dispersed CNTs.
Fig. 4. Fracture surfaces of the composites: (a) without and (b) with dispersed CNTs, (c) high magnification image of the observed CNTs, and (d) the as-received CNTs.
In the present study, because there is no notable difference
between density, porosity and fracture surface morphology of the
two composites, it is assumed that the advanced CNTs had endured
a portion of load through various of enhancement mechanisms,
and the mechanical properties of the CNTs-composite were
considerably improved. As shown in Table 2, flexural strength
and fracture toughness of the CNTs–Cf/SiC composite were both
25% higher than the composite without CNTs.
Also in Table 2, the thermal conductivity of the CNTs–Cf/SiC
specimens is about 30% higher than that of the Cf/SiC composite.
It is assumed that the high thermal conductivity-CNTs formed
series of thermal flux shortcuts in the Cf/SiC composite, which
can be well understood by the parallel model, and made the
thermal diffuseness easier and faster. However, the crosslinked
CNTs were not observed in the fracture surface of the CNTs–Cf/
SiC composite. Thus, with a quite small quantity of CNTs, more
remarkable increments than the reported work in flexural strength
[23,24], fracture toughness [20–24] and thermal conductivity [24]
of the composite were obtained.
4. Conclusions
(1) CNTs were introduced into the precursor infiltration and PIP
Cf/SiC composite via the infiltration slurry.
(2) Flexural strength and fracture toughness of the CNTs–Cf/SiC
composite were both 25% higher than those of the composite without CNTs, and the Z-directional thermal conductivity
was 30% higher, although only straight and thick CNTs were
observed on the fracture surface.
(3) Although their weight fraction in the PIP Cf/SiC composite
was only 0.765‰, and the network has not been formed in
the composite, the dispersed CNTs have laid enhancement
mechanisms and shortcuts for load transfer and thermal
conduction in the composite.
Acknowledgments
The authors would like to thank Prof. Renchao Che of Fudan
University for supplying test instruments for this study. HJY’s work
at Bristol Uni. is supported by China Scholarship Council (CSC,
China).
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