J Mater Sci (2009) 44:1750–1756

DOI 10.1007/s10853-009-3290-5

Processing and characterization of MWCNT reinforced

aluminum matrix composites
I. Sridhar Æ Karthic R. Narayanan

Received: 27 May 2008 / Accepted: 21 January 2009 / Published online: 18 February 2009
Ó Springer Science+Business Media, LLC 2009

Abstract Metal matrix composites comprising aluminum alloys are widely used in aerospace, automotive industries
matrix and multi-wall carbon nanotubes (MWCNTs) as as they possess low density, capable of being strengthened
reinforcements are fabricated using cold uniaxial com- by precipitation hardening, have good corrosion resistance,
paction followed by sintering and cold extrusion as high thermal and electrical conductivity [10]. CNTs could
secondary processes. The MWCNTs are pretreated with be an ideal reinforcing phase to design aluminum matrix
sodium dodecyl sulfate for improved adhesion with alu- composites (AMCs) to improve Al alloys wear and creep
minum powder. The effect of sintering temperature on the resistance. Recent research on producing Al matrix com-
microstructure is explored using differential scanning posites reinforced with CNTs has been based on near net-
calorimetric spectrum. The tensile yield and ultimate shape routes, such as cold compaction, melt deposition,
strength of Al-MWCNTs increased to 90% with 2 wt% cold isostatic pressing, hot roll compaction, etc., for the
addition of MWCNTs. Various theories for the strength- primary processing followed by (in some cases) secondary
ening and stiffening of Al-MWCNTs composites are process like hot extrusion to further enhance the properties
explored. of the metal matrix composites [11–16]. In a recent paper,
Loo et al. [17] have fabricated multi-wall carbon nanotubes
(MWCNT) reinforced silica composites using sol–gel route
and they have outlined the importance of surface treatment
Introduction of MWCNTs for their effective dispersion. To the best of
authors’ knowledge, the current experimental data on
Carbon nanotubes (CNTs), discovered by Iijima, are one of Al-MWCNTs are still very much limited and that too cold
the most exciting nanostructural materials of the 20th extrusion processing is not explored. Hence, the aim of this
century due to their superior mechanical, thermal, and paper is to compare the enhancement in mechanical
electrical properties [1–3]. Theoretical and experimental properties of aluminum matrix composites reinforced with
investigations on CNTs have reported their Young’s different weight percentages (0.5, 1.0, and 2.0) of chemi-
modulus and tensile strength to be of the order of 3 TPa cally treated MWCNT to the monolithic 99.96% pure
and 2 GPa, respectively [4–9]. Their excellent mechanical aluminum fabricated by cold compaction as a primary
properties combined with their nano-size and low density process followed by sintering and cold extrusion as sec-
of 2.0 g/cm3 makes them as a viable reinforcing phase in a ondary processes. The secondary process involved here is
variety of polymer, ceramic, and metallic matrices to economical when compared to the initial expenditure
design high-performance composite materials. Aluminum involved in some of the net shape forming routes like spark
plasma sintering and hot isostatic pressing. Various theo-
ries for the strengthening and stiffening of Al-MWCNTs
I. Sridhar
K. R. Narayanan (&) composites are also explored. To date the MWCNTs are
School of Mechanical & Aerospace Engineering,
much cheaper than single-wall carbon nanotubes
Nanyang Technological University, 50 Nanyang Avenue,
Singapore 639798, Singapore (SWCNTs) and hence we will be using MWCNTs in the
e-mail: re0002an@ntu.edu.sg current experimental studies for cost-effectiveness.

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Materials and methods were filtered and baked at 110 °C for 2 h for drying. When
ionic SDS is used as the surfactant, the negative charges in
The aluminum (Al) powder used is pure Al (99.6 wt% Al) SDS micelles that absorb on MWCNTs prevent re-aggre-
produced by argon gas atomization supplied by Alfa Aesar, gation of MWCNTs.
Germany. The Al powder has nearly spherical shape par-
ticles with many satellite sub-particles with an average Composite preparation
particle size (APS) of 20–30 lm. A micrograph of the
aluminum powder obtained using scanning electron Initially, required amount of Al powder and surface-treated
microscopy (SEM) is shown in Fig. 1a. The MWCNTs MWCNTs were transferred to a horizontal blending
were produced by the catalytic pyrolysis of methane and machine which had air tight metallic containers. The
supplied by Shenzen Nano Tech Port Co. Ltd, China. The powders were mechanically mixed for about 2 h in the
MWCNTs have a nominal diameter of 10 nm, length of blender set at 200 rpm. Previous work showed that the
5–15 lm, and surface area of 40–300 m2/g. The micrograph mixing process and its duration determine the effective
of MWCNTs obtained through field emission scanning dispersion of MWCNTs in the powder [22]. Measures were
electron microscopy (FESEM) is shown in Fig. 1b. The taken to prevent the loss of powder mixture during
FESEM image shows that the CNTs tend to clump together blending. After blending immediately the monolithic Al
due to van der Walls force of attraction between them. and MWCNT reinforced Al, specimens were produced
using cold uniaxial pressing in a 25 ton hydraulic press,
Surface treatment of MWCNTs which can provide the manufacturer suggested 2 ton/cm2
compaction pressure. The cold-pressed cylindrical speci-
Raman spectroscopy with He:Ne laser of 633 nm wave- men are 18 mm in diameter and 30 mm in length. These
length was used to characterize the MWCNTs obtained green compacts are then free-sintered at a heating rate of
from the manufacturer [18, 19]. The spectrum of MWCNTs 10 °C per minute up to 580 °C (based on 660 °C melting
showed a disorder (D-band) at 1348 cm-1 and tangential temperature of Al powder) in a furnace maintained for
G-band at 1590 cm-1, respectively. This G-band Raman 90 min and then cooled to room temperature at a rate of
shift confirmed the presence of amorphous carbon 3 °C per minute in the furnace itself. Finally, the sintered
(\3 wt%) as stated by the manufacturer. The MWCNTs are specimens are extruded in a hot-tool steel die of 45° die
first cleaned by distilled (DI) water and then surface-treated angle at room temperature to 12 mm diameter and length
before mixing with aluminum powder. Initially, MWCNTs of 45 mm using the same 25 ton hydraulic press. The
are sonicated for 4 h in 63 vol.% nitric acid and filtered. extrusion load was calculated to be 22.8 ton with an
The filtered acidic MWCNTs are neutralized with sodium equivalent plastic strain of 0.81. The reduction in area was
hydroxide solution, and then dried by heating them in an 55.55% and extrusion ratio was 2.25. The initial sintering
oven at 110 °C for 2 h. Finally for better adhesion between temperature was set to 580 °C as adopted by George et al.
the MWCNTs and Al powder, they were treated with [12] in their Al-MWCNT composites fabrication by hot
sodium dodecyl sulfate (SDS) surfactant, which decreases extrusion. The differential scanning calorimeter (DSC)
the van der Walls force of attraction between the spectrum of these Al-MWCNT composites in Fig. 2a
MWCNTs [20, 21]. Two grams of SDS was dissolved in shows an exothermic peak after aluminum melting point,
200 mL of water. The nanotubes were added to the SDS indicating the formation of Al4C3 (aluminum carbide)
solution and sonicated for 4 h. These treated nanotubes phase between the Al and MWCNT interface. The presence

Fig. 1 a Scanning electron

micrograph of aluminum
powder and b field emission
micrographs of MWCNTs

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the transition temperature (Ttrans) of AMCs compared

to pristine Al, which shows that the strength of the
Al-MWCNT composite is retained for longer periods. The
optimal sintering temperature needs to be further investi-
gated as Jiang et al. [23] have highlighted the careful
selection of sintering temperature in designing CNT rein-
forced alumina composites.

Results

Density and microstructural analysis

The densities of the extruded Al and Al-MWCNT com-

posites were measured by the Archimedes principle with
deionized water as the immersion medium and are listed in
Table 1. The extruded samples density increased with
increasing MWCNTs weight percentage. This unexpected
result is against the rule of mixtures given that the density of
MWCNTs is less than that of Al. Similar trend is also
reported in the literature [11–14] for CNT reinforced
MMCs. The role of porous MWCNTs in controlling the
densification is beyond the scope of this paper. The density
gradient along the length of the specimen was less than 2%.
The extruded Al-MWCNT composites have a smooth sur-
face finish as shown in Fig. 3a. The microstructure of the
specimen along the extruded direction showed that large
percentage of grains are oriented along the extrusion direc-
tion as shown in Fig. 3b and c and a homogeneous bridging
of MWCNTs in the matrix was evident. The microstructure
indicated some preferential alignment of MWCNTs per-
pendicular to the extruded direction. Further, the grain
structure has certain anisotropy as a result of the lateral
deformation constraint provided by the extrusion die. The
grain size (shown in Fig. 3d) after extrusion decreased due
to high strain rate applied in the cold extrusion process.

Mechanical characterization

The mechanical micro and macro properties of composites

were obtained by microhardness testing and uniaxial ten-
sile tests. The microhardness test was carried out in a
Fig. 2 a DSC spectrums for AL-MWCNTs sintered at 580 °C, b CSMTM microhardness tester using a Berkovich indenter
XRD spectrum of Al and Al-2 wt% MWCNT composite, and c DSC under load control. The indentation load was set to 3 N and
spectrum for Al and Al-MWCNT composite sintered at 520 °C the load-penetrations responses are recorded. The mea-
sured average micro Vickers hardness values in HV (taken
of interfacial compound aluminum carbide deteriorates from four different sampling points) for extruded Al and
composite properties. Hence, the sintering temperature was Al-MWCNT composites are listed in Table 1. The results
reduced to 520 °C and the X-ray diffraction (XRD) spec- show that the hardness values of Al-MWCNTs increase as
trum of Al and Al-MWCNT composites shown in Fig. 2b MWCNTs weight percentage increases. Quantitatively, the
confirmed the absence of aluminum oxide as well as alu- increase in hardness is not so significant: about 8% increase
minum carbide as their reference peaks were absent. The in hardness was observed when MWCNTs weight per-
DSC curves in Fig. 2c indicate the effect of MWCNTs over centage was 2.

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Table 1 Properties of extruded

Description Density Yield stress @ 0.2% Ultimate tensile Average Vickers
aluminum and Al-MWCNT
(g/cm3) off-set strain (MPa) strength (MPa) microhardness (Hv)
composites
Aluminum 2.468 ± 0.01 90.98 98.32 68.70 ± 0.64
Al ? 0.5 wt% MWCNTs 2.512 ± 0.02 114.11 121.62 70.23 ± 0.33
Al ? 1.0 wt% MWCNTs 2.584 ± 0.02 138.69 151.29 71.37 ± 0.45
Al ? 2.0 wt% MWCNTs 2.649 ± 0.015 176.37 184.37 74.16 ± 0.91

Fig. 3 a A photograph of extruded cylindrical AMC specimen, d Grain size reduction observed through optical micrographs of cross-
b, c SEM micrographs of Al-MWCNT composites showing the grain sectioned extruded aluminum sample
orientations and presence of MWCNTs in the extruded direction,

Uniaxial tensile tests were conducted on cylindrical spec- dispersion of MWCNTs and good interfacial bonding
imens of 7 mm diameter and gauge length of 10 mm using a between the matrix and MWCNTs.
Universal Testing Machine under displacement control at The cold extrusion step after sintering stage further
0.2 mm/min. An extensometer was used to measure the strain densifies the part, elongates matrix grain in the axial
accurately and Instron load cell reading provided the load direction, and decreases inter-particle spacing due to par-
values. The measured uniaxial stress and strain response of ticle redistribution. The high strain imposed during cold
pristine aluminum and Al-MWCNT composites are shown in extrusion leads to increase in dislocation density and to
Fig. 4. The measured 0.2% off-set yield stress and ultimate their progressive entanglement. This is the reason for
tensile strength of the composites and pristine Al samples are strain-hardening and strengthening of cold extruded Al.
listed in Table 1. With increasing MWCNT weight percent- Further reasoning for the increase in strength of the com-
age in Al matrix, the composite samples consistently showed posite was attributed to the increased resistance to
increasing stiffness and strength, which indicates effective dislocation motion provided by the deformed MWCNTs

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during the extrusion process. This is evident from the

Raman spectra shown in Fig. 5a and b for MWCNTs alone
and the composite fabricated: the intensity of D-band peak
becomes prominent for the composite. The deformation
might even break-up the MWCNTs and hinder dislocation
motion.
The yield strength and ultimate strength increased on an
average approximately 90% with an addition of 2 wt% of
MWCNTs. Within experimental scatter the tensile strain to
fracture for the extruded samples is in-between 2 and 3%.
It is to be noted that there was no further heat-treatment
carried on the extruded samples. The fractured surfaces of
1 wt% Al-MWCNT composite sample are shown in Fig. 6a
and we notice that even after fracture the MWCNTs were
holding on to the matrix. The matrix failure is also shown
in Fig. 6b.
Fig. 4 Uniaxial tensile stress–strain response of pristine Al and
Al-MWCNT composites

Discussion

The mechanical characterizations of Al-MWCNT com-

posites reveal that the adopted powder compaction route is
a viable method for their manufacture. There is a consistent
increase in the modulus and tensile strength of the fabri-
cated composites with increasing weight percentage of
MWCNTs. SDS seems to be an effective surfactant in
dispersing the MWCNTs in Al matrix. In the following, we
briefly discuss various mechanisms that have contributed
to the improved mechanical behavior of Al-MWCNT
composites.
George et al. [12] have elucidated three different
mechanisms for the strengthening of composite materials.
They include thermal mismatch, Orowan looping, and
shear lag theory. Shear lag model [24] has been used to
describe the stiffening effect of MWCNTs in Al-MWCNT
Fig. 5 Raman spectra of a MWCNTs after surface treatment and composites. According to this theory, the Young’s modulus
b extruded Al-MWCNT composite (Ec) of the composite is given by

Fig. 6 a Fractured surface

of Al/1 wt% MWCNT and
b fracture surface fractograph

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Ec ¼ Vf Ef ð1  tanhðnsÞÞ=ðnsÞ þ ð1  Vf ÞEm ð1Þ nanometer-sized carbon nanotubes, leading to their

pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi bending between the CNTs. This produces a back
where n ¼ ð2Em =Ef ð1 þ tm ÞÞ lnð1=Vf Þ, Ef is the Young’s stress, which will prevent further dislocation move-
modulus of MWCNTs, Em is the aluminum Young’s ment leading to an increase in yield strength. The
modulus, s the aspect ratio of MWCNTs (*100), extrusion operation after the free sintering makes it
Vf the reinforcement volume fraction, and tm is the alumi- difficult to make a direct comparison of composite
num matrix Poisson’s ratio. The modulus measured from the strength with Orowan looping theory.
initial slope of the stress–strain curve is compared with the
shear lag model of Eq. 1 in Table 2. Consistently the con-
servative upper-bound shear lag model overestimates the
Young’s modulus value by about 12%. Conclusion
The possible reasons for the strengthening of
Al-MWCNTs composites could be as follows: Aluminum matrix composites reinforced with 0.5, 1.0, and
2.0 wt% of MWCNTs were successfully manufactured
1. As noted before, CNTs are nanometer dimensions: in using cold compaction followed by sintering and cold
metal matrix composites, the strengthening of metals extrusion techniques to near net shape. Careful controlling
by a given volume fraction of hard particles is greater of sintering temperature has prevented the formation of
for small particles than for large particles as it intermetallic compounds such as aluminum carbide. The
increases their rate of work-hardening. microhardness and uniaxial tensile tests have revealed
2. During the sintering process, due to the large mismatch enhanced mechanical properties of Al-MWCNT compos-
in the coefficient of thermal expansion between the ites, indicating that the proposed manufacturing route is a
aluminum matrix and MWCNTs results in prismatic viable cost-effective one. There was no observed percola-
punching of dislocations at their interface leading to tion of MWCNTs up to the 2 wt% volume fraction. It
the work-hardening of the aluminum matrix. would be further interesting to study the wear and creep
When a stress is applied to a material the interplanar properties of these composites and also explore strength-
spacing (d) changes. Using Bragg’s law one can ening mechanisms.
determine the interplanar spacing using XRD tech-
nique. By measuring the interplanar spacing of pristine Acknowledgements The authors acknowledge many fruitful dis-
Al, Al-MWCNT composites for different incident cussions they had with Prof. Raju Ramanujan and Prof. Tan Ming Jen.
angles of X-ray, the process-induced residual stresses The experimental help rendered by Mr. Seah Kheng Wee, Mr. Lim
Yee Wee, and Mr. Sa’Don Ahmad is much appreciated. The authors
can be estimated [25]. In our current experimentation, are grateful for the financial support from NTU Academic Research
due to instrument limitations we could only measure Fund (AcRF) via project number RG 19/06.
the interplanar spacing (d) for normal incidence. The
measured interplanar spacing for pristine Al was
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