Ý. TOPCU: DETERMINATION OF THE MECHANICAL PROPERTIES OF Al/MWCNT COMPOSITES ...
689–695
DETERMINATION OF THE MECHANICAL PROPERTIES OF
Al/MWCNT COMPOSITES OBTAINED WITH THE
REINFORCEMENT OF CU-COATED MULTIWALL CARBON
NANOTUBES (MWCNTs)
DOLO^ANJE MEHANSKIH LASTNOSTI Al/MWCNT
KOMPOZITOV, OJA^ANIH Z BAKROM OPLA[^ENIMI
VE^STENSKIMI OGLJIKOVIMI NANOCEV^ICAMI (MWCNT)
Ýsmail Topcu
Alanya Alaaddin Keykubat Universty, Engineering Faculty, Metallurgy & Materials Eng. Dep., 07452 Alanya, Turkey
Marmara University, Engineering Faculty, Metallurgy & Materials Eng. Dept., 34722 Göztepe, Ýstanbul, Turkey
Prejem rokopisa – received: 2019-12-25; sprejem za objavo – accepted for publication: 2020-04-26
doi:10.17222/mit.2019.308
In this study, Al/MWCNT composites were produced by milling copper-coated MWCNTs in small amounts (2.5–7.5%) with
aluminum powders using the mechanical alloying method. Then, sintering was applied to the composite material. A phase
analysis and microstructure investigations were performed on the as-sintered samples. A hardness test was conducted and
tribological performances of the unreinforced and MWCNT-reinforced composites were examined at room temperature. Results
showed that the hardness of the base-aluminum alloy was improved. The phase belonging to the MWCNTs was detected with an
X-ray diffraction analysis. The wear rate generally decreased with the addition of MWCNTs in all test conditions. Sintering had
a positive effect, enhancing the wear and hardness behaviour. Abrasive, adhesive, oxidative and thermal wear mechanisms were
observed with a scanning electron microscope (SEM).
Keywords: powder metallurgy, MWCNT, mechanical behaviour, Cu coating
Avtor v ~lanku opisuje {tudijo, v kateri so izdelali vzorce kompozitov tipa Al/MWCNT, oja~ane z majhno vsebnostjo (od 2,5
w/% do 7,5 w/%) oja~itvene faze in dolo~ili njihove mehanske lastnosti. Za oja~itev kovinske osnove iz 99,9 % Al so uporabili
delce z bakrom opla{~enih ve~stenskih ogljikovih nanocevk (MWCNT; angl.: multi-wall carbon nano-tubes). Kompozitne
prahove so izdelali s postopkom mehanskega legiranja ter nato njihovega zgo{~evanja z enoosnim stiskanjem in sintranjem. Na
sintranih vzorcih kompozitov so nato izvedli mikrostrukturne in fazne analize. Pri sobni temperaturi so izmerili {e njihovo
trdoto in izvedli tribolo{ke raziskave. Rezultati testov mikrotrdote so pokazali, da se je le-ta izbolj{ala z dodatkom oja~itvene
faze. Z rentgensko difrakcijsko spektroskopijo (XRD) so zaznali prisotnost MWCNT v kovinski osnovi. Pri vseh pogojih
preizku{anja se je v splo{nem hitrost obrabe zmanj{ala z dodatkom MWCNT delcev. Zgo{~evanje kompozita s sintranjem je
imelo pozitiven u~inek na zvi{anje njegove trdote in izbolj{anje odpornosti proti obrabi. S pomo~jo vrsti~nega elektronskega
mikroskopa (SEM) so opazovali abrazivne, oksidativne in termi~ne u~inke mehanizmov obrabe.
Klju~ne besede: metalurgija prahov, MWCNT, mehanske lastnosti, opla{~enje z bakrom
1 INTRODUCTION
Aluminum and aluminum alloys receive great
attention in the aerospace and automobile sectors due to
their density. Aluminum alloys are known as heat-treat-
able materials, in addition to exhibiting high stiffness
and strength properties.
1
Researchers generally try to use
them as a matrix when developing high-strength com-
posite materials in order to meet the demands of the
industry.
Multiwall carbon nanotubes have become a popular
reinforcement for metal-matrix composites because of
their superior thermal, electrical and mechanical per-
formance. They have an elastic modulus of 1 TPa and
strength of 30–100 GPa.
2,3
MWCNTs are lightweight and
have a large aspect ratio. When MWCNTs are incorpo-
rated into a matrix material, they significantly increase
the mechanical and thermal properties. The main
challenge is to obtain a homogenous MWCNT distri-
bution in a matrix composite due to the strong Van der
Waals attraction energy between carbon atoms.
4
For this
purpose, different production techniques have been used
so far. In general, powder metallurgy is used as it allows
us to attain high-density materials. It was reported that
high-energy ball milling can be effective for homo-
genous distribution, but after a certain milling time, it
might cause serious damage to the MWCNTs.
5
Although
an agglomeration of the reinforcement particles is
prevented, this technique can cause extra heat, interfacial
reactions and damage to the integrity of carbon.
Nowadays, chemical methods, surface modification
or ultrasonication of MWCNTs in aqueous solutions,
known as semi-powder techniques are tried to fabricate
nanocomposite materials. A study indicates that semi-
powder metallurgy can be practical for obtaining
Materiali in tehnologije / Materials and technology 54 (2020) 5, 689–695 689
UDK 67.017:621.742.4:621.762 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 54(5)689(2020)
*Corresponding author's e-mail:
ismail.topcu@alanya.edu.tr (Ýsmail Topcu)

homogenously embedded carbon-based nano-reinforced
composites with no defects. On the other hand, some
small agglomerations were observed for MWCNTs
compared to graphene and fullerene because of the
structural properties of one-dimensional MWCNTs.
6
In
recent years, friction-stir processing, spark-plasma
sintering and stir-casting methods have been used for the
fabrication of nanocomposites. A melted aluminum ingot
is mixed with nano-reinforcement with a stirring bar in a
stir-casting process, then the melted mixture is poured
into a die and it is solidified. It is possible to produce
large and complex-shape products, but the reinforcement
phase in the matrix cannot be uniform.
7
Al-based MWCNT-reinforced composites are
fabricated with the above techniques and their
mechanical properties are examined in the literature.
Generally, the effect of MWCNTs or their amount during
a room-temperature mechanical performance on alumi-
num were investigated. But few studies can be found
about the high-temperature mechanical or tribological
behaviour of such composites.
8
A multi-walled carbon
nanotube-reinforced aluminum-alloy-matrix composite
was produced with a newly developed technique, which
combines semi-powder metallurgy and stir casting with
an induction furnace for the first time. Furthermore, to
the best of our knowledge, there is no study about the
effect of carbon nanotubes on the wear properties of
aluminum at elevated temperatures. Both the room and
high temperature wear performances of the produced
composite are investigated in detail in this report.
Powder-metallurgy production methods were de-
signed to produce qualified engineering materials using
aluminum powder. In this study, Cu-coated MWCNTs
and aluminum materials chosen as the reinforcement and
matrix were used. The aims of the work were to examine
and improve the mechanical properties and determine
different process conditions such as the temperature,
sintering condition, time, atmosphere and use of the
Al/CNT type composite material.
9
Matrix powder and additional powder were mixed
using attrition milling with the help of the mechanical-
alloying method. The amount of added powder depended
on the experimental results and studies. In accordance
with ASTM standards, the MPIF powder mixture was
shaped using the uniaxial pressing technique. After the
critical temperature, determined with a DTA analysis, the
wet-sample density was measured in different sintering
conditions. Then we examined the microstructure,
chemical and physical properties and size change of the
sintered samples, using XRD and SEM analyses as well
as density, hardness and wear-resistance tests.
2 EXPERIMENTAL PART
2.1 Raw materials and fabrication
Aluminum powder (99.9 % purity, 2.690 g/cm
3
den-
sity, a powder size of 10 μm) and MWCNT powders (a
density of 2.31 g/cm
3
, size of 10–30 nm, from Nanografi
Co. Ltd.) were used to produce a composite in this inves-
tigation. Figure 1 illustrates the fabrication of the
2.5 w/% MWCNT-reinforced Al-matrix composite.
Firstly, the experiment aimed to obtain Cu-coated
MWCNTs, made from CuSO
4
. 2290 mg of CuSO
4
, 200
mL of ethanol, 200 mL of diethylene alcohol and 400 mg
of carbon nanotubes, batched and mixed with magnetic
stirring at 200 °C. The Cu-coated multi-walled carbon
nanotubes (MWCNTs) were prepared with electrodepo-
sition using MWCNTs in an acid-copper plating bath
under optimised bath conditions and process conditions.
The structural and mechanical properties of the
Cu–MWCNT thin film were investigated.
10
Secondly, different amounts (2.5–7.5 %) of Cu-
coated MWCNTs and aluminum particles were
mechanically alloyed in a Turbula mixer at 450 min
–1
for
four hours.
11,12
The agitation of a powder (especially
powders with different bulk densities) may result in a
migration of smaller particles downwards and larger
ones upwards. Another problem is the segregation whose
main cause is the difference in the particle size, density
shape and resilience. The Turbula device used is a
Kenics-type static mixer. Static mixers save energy,
prevent segregation and influence particle migration.
13
Powders were pressed at 200 MPa, then sintered in a
high-purity argon atmosphere at 600 °C and 650 °C for
one hour. The horizontal-tube furnace arrangement of the
sintering process is shown in Figure 2.
Ý. TOPCU: DETERMINATION OF THE MECHANICAL PROPERTIES OF Al/MWCNT COMPOSITES ...
690 Materiali in tehnologije / Materials and technology 54 (2020) 5, 689–695
Figure 2: Sintering process of a horizontal-tube furnace system
Figure 1: Fabrication stages for the MWCNT-reinforced Al-matrix
composite

2.2 Characterization of the fabricated composites
The sintered samples were prepared for microstruc-
ture and phase analyses. For this purpose, grinding
(180–1200 SiC grit paper) and polishing (3- and
1-micron diamond solutions, respectively) were applied.
An X-ray diffraction machine (Rigaku Ultimate) was
operated to observe the present phases in the material.
Then the samples were etched with a Keller solution for
microstructural investigations. Scanning electron
microscopy using energy dispersive X-ray spectroscopy
(EDX) was used to observe the microstructure of both
the as-cast and extruded samples.
Dry-sliding wear tests were carried out using a
pin-on-disk device (a UTS tribometer, ball-on-disc test
apparatus, as shown in Figure 3).
The 52100 steel was chosen as the counter-face
material. The wear tests were conducted under a normal
load of 10 N at room temperature. To determine the wear
rate, the wear volume of the samples was calculated and
this value was multiplied by the stroke distance. Then,
the calculated values were divided by the sliding distance
in order to obtain the units in mm
3
/m. Scanning electron
microscopy was used to display worn surfaces and
understand the wear mechanisms of the samples versus
the applied load and test temperature.
The hardness measurements of the Al/MWCNT sam-
ples were measured on a Future Tech-700 microhardness
testing machine using a Vickers 136° diamond tip and
50-g test load. The hardness value indicates the hardness
of the composite. The hardness-test results are the aver-
age of 10 consecutive measurements.
2,12
Figure 4 shows the Vickers images produced at the
sample surface as a result of the measurement made with
the hardness-measurement method.
Abrasion testing is used to test the abrasive resistance
of solid materials. The dimensions of the composite
samples included a diameter of 13 mm of and length of
30–40 mm. The abrasion testing wheel had a diameter of
75 mm and rotation rate of 520 min
–1
. It showed that the
abrasion resistance under a dynamic load changes with
distance.
3 RESULTS
3.1 X-ray diffraction
X-ray diffraction results from Figure 5 show the
X-ray diffraction patterns of the sintered MWCNT-rein-
forced composite. An X-ray diffraction machine with a
fixed monochromatic filter was operated in a range of
10–90° at 40 kV and 40 mA. Four obvious peaks corres-
ponding to the Al reflections can be seen on the graph. It
may be concluded that the MWCNT addition can react
with the matrix material and new phases, such as Al
4
C
3
,
which may weaken the bonding between the aluminium
alloy and reinforcement, may occur.
11,12
The XRD anal-
ysis of the phases is shown in Figure 6.
However, no intermetallic phases were detected with
the measurement. Because the reinforcement based on a
low amount of carbon was added to the matrix, Al4C3
could not be observed. During continuous scanning in
the range of defined angles, the MWCNT peak could not
be seen. This can be attributed to the fact that MWCNTs
might get amorphized during production stages. It can be
claimed that carbon peaks may overlap with strong inten-
sity peaks of the Al solid solution phase.
13
Furthermore,
previous studies showed that a low amount of carbo-
naceous reinforcement may be below the detection
limit.
6,14
In this study, the fixed-time method, i.e., very
slow scanning in defined ranges was used to reveal the
presence of MWCNTs in the composite material. It is
known that commercial MWCNTs generally have a
Ý. TOPCU: DETERMINATION OF THE MECHANICAL PROPERTIES OF Al/MWCNT COMPOSITES ...
Materiali in tehnologije / Materials and technology 54 (2020) 5, 689–695 691
Figure 5: XRD patterns of 2.5–7.5 % MWCNT particles in the Al
matrix
Figure 4: a) image of the sample surface and b) a magnified view of
the sample surface
Figure 3: Schematic of the pin-on-disc wear device

strong peak at 2 equal to 40–50°.
15
The counting time
and step width were determined as 120 s and 0.05°,
respectively, for 2 of 42.6°. The highest peak obtained
belonged to the MWCNTs at around 2 of 42.6°. As
shown in Figure 5, this peak indicates the existence of
MWCNTs in the material. Figure 6 shows the XRD
analysis results for the samples.
3.2 Microstructure analysis
Sintering changes the grain structure and orientation.
Grain refinement can be obtained with sintering, which
provides the enhancement of the mechanical behaviour
of a composite. MWCNTs cannot be recognized from
the figure due to their high surface area and low amount.
No cracks or macroporosity can be seen on the compo-
sites.
The powder morphologies and microstructures of the
sintered samples were examined using a SEM (JEOL
Ltd., JSM-5910LV). Microstructural analyses of pure
aluminium and the composite of 7.5 % MWCNT/Al are
shown in Figures 6a to 6c. The structure of the com-
posite material (7.5 % of MWCNT) is given in Figure
6b as a representative image. It consists of black and
grey areas where the former represents the MWCNT
structure and the latter is the Al matrix. The black area
became larger as the MWCNT content increased.
Additionally, the EDS analysis of the phases shown in
Figure 6c demonstrates the presence of C and Al.
The microscopic examination showed that the
MWCNT powders were distributed homogenously in the
matrix and that there was no segregation in the copper-
coated MWCNTs.
16
3.3 Density and hardness results
The density of the composites reinforced with diffe-
rent amounts of MWCNTs is shown in Figure 7. The
sample density approached the theoretical density with
the increasing reinforcement amount and sintering
temperature.
17
Theoretical densities of the composite specimens
were calculated with the following equation:
12,18
1

c
f
f
m
m
=+
w w
(1)
where subscripts m, f and c refer to the matrix, rein-
forcement fibres and composite, respectively.
This reduction in the grain size and a uniform
dispersion of Cu-MWCNTs in the Al matrix obtained
Ý. TOPCU: DETERMINATION OF THE MECHANICAL PROPERTIES OF Al/MWCNT COMPOSITES ...
692 Materiali in tehnologije / Materials and technology 54 (2020) 5, 689–695
Figure 6: MWCNT (7.5 %) reinforced Al matrix: a) pure Al, b) SEM image, c) EDS analysis
Figure 8: Hardness vs. CNT amount for the coated and non-coated
samples
Figure 7: Density of the Cu-coated samples at different sintering tem-
peratures

with the electrodeposition method was also confirmed by
the XRD and EDS studies. The Vickers-hardness tests
were carried out on different composites and at two
different sintering temperatures. Ten hardness measure-
ments were taken in different regions of the samples. The
hardness values are shown in Figure 8.
Increasing the weight percentage of MWCNTs and
the sintering temperature increased the hardness of the
composites. The increase in the hardness of the com-
posites due to the addition of MWCNTs can be attributed
to the dispersion of the strengthening effect. The hard-
ness-test results revealed that the MWCNT addition
increases the hardness of the aluminium alloy. The
hardness increased as a result of sintering the alumi-
nium-matrix composites produced with 2.5–7.5 % of
MWCNTs.
This can be based on the presence of harder rein-
forcements in the aluminium alloy. MWCNTs probably
affect the dislocation motion, especially in the grain
boundaries and an improvement in the material strength
might be obtained.
19
Therefore, as a result of the restric-
tion in the dislocation with the MWCNT reinforcements,
the hardness begins to increase. This is due to the
grain-size reduction in the Cu–MWCNT/Al composite.
Such a very small crystalline size of pure Al and
Cu–MWCNT/Al composite causes an increase in the
microhardness of the composite.
10
3.4 Abrasion tests of the produced samples
In Figure 9, abrasion-test results are plotted as a
function of the MWCNT amount for different sintering
temperatures. The weight loss for all the samples was
calculated with Equation (2) and the abrasion resistance
for all the samples was calculated with Equation (3).
20
W
G
dmS
a
=
⋅⋅
Δ
(2)
W
W
r
a
=
1
(3)
W
a
: percantage of abrassion, G: weight loss, M: force,
S: distance, d: density
Increasing the weight percentage of the MWCNTs
and the sintering temperature increased the abrasion
resistance of the composite. The composite amount is
much more effective than the sintering temperature.
However, there are some differences between the
samples: those with 2.5 % and5%ofthecopper-coated
MWCNTs were sintered at 650 °C; the samples with
7.5 % of MWCNTs exhibited a lower abrasion resistance
than the others.
Low-reinforcement samples exhibited the lowest
wear rate under the load of 10 N. When the time
increased, the wear began to increase. After the applied
load, there was a dramatic increase in the wear rate,
especially for the low-reinforcement alloy. Among the
samples, the one exposed to a high sintering temperature
and with a high reinforcement showed the best wear
performance. The wear behaviour improved as the
Cu-MWCNT/Al composite obtained a higher relative
texture than pure aluminum.
21
Once the as-sintered and reinforced composites are
evaluated, the grain-size refinement and internal poro-
sities might help us to determine the wear behaviour.
Furthermore, the compressive residual stress, formed
during the fabrication process affects the wear perform-
ance.
22,23
When the compressive residual stress increases,
the mechanical and wear behaviour of the composite
begin to develop, preventing the micro-crack growth.
When a sample is sintered at a high vacuum, a com-
pressive residual stress begins to occur.
24,25
The SEM was used to understand the wear phenome-
non at different temperatures of the sintered MWCNT-
reinforced composite. As shown in Figure 10, the effects
of different reinforcement rates (2.5–7.5 % of
MWCNTs) and test temperatures on the wear mecha-
nism were investigated. Obvious marks can be seen on
the figure and they are parallel to the sliding direction.
This is a result of the abrasive-wear mechanism. These
marks are more clearly recognized when the applied load
increases due to the higher pressure of the load on the
contact surface of the sample.
4 DISCUSSION
As mentioned above, the sample with copper-coated
MWCNTs has a higher hardness than the low-reinforce-
Ý. TOPCU: DETERMINATION OF THE MECHANICAL PROPERTIES OF Al/MWCNT COMPOSITES ...
Materiali in tehnologije / Materials and technology 54 (2020) 5, 689–695 693
Figure 10: Worn surfaces of the produced samples: a) 2.5 %
MWCNT/Al wear resistance, b) 7.5 % MWCNT/Al wear resistance
Figure 9: Display of the wear rates of composites with different
reinforcements

ment sample. It is noted that an enhancement of the
hardness improves the load-bearing capacity of the
matrix material so that the wear resistance of the alloy
increases.
26,27
The wear behaviour of the sintered MWCNT-rein-
forced composites with different amounts of
reinforcement (2.5–7.5 %) was studied. Low-reinforce-
ment samples exhibit the lowest wear rate under the load
of 10 N. When the applied distance and time increase,
the wear rate begins to increase. There is a dramatic
increase in the wear resistance, especially for the 7.5 %
MWCNT-reinforced alloy. Among the samples, the
highly reinforced composite sintered at a high tempera-
ture shows the best wear performance. Once the as-sin-
tered composites are evaluated, the grain-size refinement
and internal porosities might help determine the wear
behavior.
28
5 CONCLUSIONS
A Cu-MWCNT reinforced Al-matrix composite was
fabricated and sintering was performed successfully.
Based on the experimental tests, the following findings
were obtained:
• No intermetallic phases such as Al
2
Cu and Al
4
C
3
were found according to the XRD analysis.
• The Al/MWCNT composite has a higher hardness
versus wear resistance compared to the unreinforced
aluminum.
• The wear rate significantly decreases with an
addition of MWCNTs.
• The weight loss increases directly with the applied
time and distance.
• The abrasive, adhesive and oxidative wear are the
main mechanisms at room temperature.
• A homogeneous distribution of the MWCNTs in the
matrix increases the load-transfer ability due to the
formation of carbide on the material surface.
29
• The Cu–MWCNT/Al composite show higher
microhardness and wear properties than pure alumi-
num.
Acknowledgments
This work was supported by the Scientific Research
Project Program of the Marmara University
(FEN-K-DRP-070317-0107).
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Ý. TOPCU: DETERMINATION OF THE MECHANICAL PROPERTIES OF Al/MWCNT COMPOSITES ...
Materiali in tehnologije / Materials and technology 54 (2020) 5, 689–695 695