Mechanical properties of carbon nanotube–alumina nanocomposites synthesized
by chemical vapor deposition and spark plasma sintering
T. Zhang
a
, Latha Kumari
a
, G.H. Du
a
, W.Z. Li
a,
*
, Q.W. Wang
b
, K. Balani
c
, A. Agarwal
c
a
Department of Physics, Florida International University, Miami, FL 33199, United States
b
Agiltron Inc., 15 Cabot Road, Woburn, MA 01801, United States
c
Mechanical and Materials Engineering, Florida International University, Miami, FL 33174, United States
article info
Article history:
Received 6 March 2008
Received in revised form 9 October 2008
Accepted 13 October 2008
Keywords:
A. Nano-structures
B. Mechanical properties
D. Electron Microscopy
E. Chemical vapor deposition
abstract
Carbon nanotubes–alumina (CNT–Al
2
O
3
) nanocomposites with variable CNT content were directly syn-
thesized by chemical vapor deposition (CVD). The as-grown CNT–Al
2
O
3
mixture was densified by spark
plasma sintering (SPS) at 1150 and 1450 °C. Vickers hardness of 9.98 GPa and fracture toughness of
4.7 MPam
1/2
were obtained for 7.39 wt.% CNT–Al
2
O
3
nanocomposite. The addition of CNTs gives rise to
8.4% increase in hardness and 21.1% increase in toughness over that of the pure Al
2
O
3
. The optimum
amount of CNTs is considered to be able to significantly enhance the mechanical property of ceramics
in composites.
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
Carbon nanotubes in the form of single-walled (SWCNTs) or
multiwalled (MWCNTs) assemblies have been the focus of consid-
erable scientific research. The unusual mechanical properties of
CNTs make them an ideal class of reinforcement for composite
materials [1]. CNTs have great potential for use as a toughening
agent due to their very large aspect ratio (1000–10,000) [2], low
density, high rigidity (Young’s modulus of the order of 1 TPa)
[3,4], and high tensile strength (up to 60 GPa) [5]. In addition,
the excellent electrical conductivity (10
6
S/m at 300 K for SWCNT,
>10
5
S/m for MWCNT) [6,7] and thermal conductivity (6600 W/mK
for an individual SWCNT and >3000 W/mK for an individual
MWCNT) [8,9] make CNTs suitable candidates to synthesize nan-
composites with new functional properties. Ceramics can sustain
high temperature and high hardness, but the most noted short-
coming of ceramics is the inherent brittleness, which has limited
its extensive applications [10]. Thus the incorporation of CNTs with
ceramics will provide an unparalleled opportunity to form a new
class of CNT–ceramics materials with enhanced mechanical
property.
Many attempts have been made to improve the mechanical
properties of ceramics through incorporating CNTs in ceramic ma-
trix. Siegel et al. [11] reported that the addition of 10 vol.%
MWCNTs to monolithic Al
2
O
3
could lead to 24% increase in the
fracture toughness. Ma et al. [12] showed that a SiC composite con-
taining 10 vol.% MWNTs had a 10% increase in bending strength
and toughness compared with the monolithic SiC. Zhan et al.
[13] obtained a fracture toughness of 9.7 MPam
1/2
from 10 vol.%
SWNT–Al
2
O
3
composite, which was nearly three times higher than
that of pure Al
2
O
3
. However, Laurent et al. [14] reported a contra-
dictory result in CNT/Fe–Al
2
O
3
composites, where the CNT
reinforcement did not show any significant effect on the CNT/Fe–
Al
2
O
3
composites and the fracture toughness of CNT/Fe–Al
2
O
3
composites was almost similar to that of carbon-free Fe–Al
2
O
3
composites. Earlier work by Wang et al. suggested that the CNT–
Al
2
O
3
nanocomposites with 10 vol.% CNT are as brittle as bulk
Al
2
O
3
, showing almost the same toughness [15]. Recently, Balani
et al. synthesized CNT reinforced Al
2
O
3
nanocomposite coating
by plasma spray technique and showed an increase in the fracture
toughness of 43% [16] and elastic modulus of 200% as compared to
pure Al
2
O
3
[17]. In the present work, the synthesis and consolida-
tion of as-grown CNT–Al
2
O
3
composites are discussed in detail. The
Vickers hardness and fracture toughness of the CNT–Al
2
O
3
com-
posites as a function of the CNT content and sintering tempera-
tures are investigated.
2. Experimental method
CNT–Al
2
O
3
mixture was synthesized by CVD. CNTs are directly
grown on Al
2
O
3
nanoparticles by using Co(NO
3
)
2
Á6H
2
O as a catalyst
precursor [18]. To upload the catalyst precursor on the Al
2
O
3
nano-
particles, Co(NO
3
)
2
Á6H
2
O (98+%, Sigma–Aldrich) and Al
2
O
3
powder
1359-835X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.compositesa.2008.10.003
* Corresponding author. Tel.: +1 305 348 7257; fax: +1 305 348 6700.
E-mail address: Wenzhi.Li@fiu.edu (W.Z. Li).
Composites: Part A 40 (2009) 86–93
Contents lists available at ScienceDirect
Composites: Part A
journal homepage: www.elsevier.com/locate/compositesa