Journal of Minerals & Materials Characterization & Engineering, Vol. 9, No.9, pp.819-830, 2010
jmmce.org Printed in the USA. All rights reserved
819
Fabrication and Investigation of Damping Properties of Nano Particulate
Composites
K. S. Umashankar
1
, K. V. Gangadharan
1
, Vijay Desai
1
and B. Shivamurthy
2*
1
Department of Mechanical Engineering, National Institute of Technology Karnataka, Surathkal,
India-575025.
2
Department of Mechanical and Manufacturing Engineering, Manipal Institute of Technology,
A Constituent Institute of Maniapal University,
Maniapal-576 119, Karnataka, India.
*Coresponding Author : [e-mail]
ABSTRACT
Nano particulate composites with Al-Si alloys (LM6 and LM25) as matrix and Multiwall Carbon
Nanotube (MWNT) as reinforcement (0.25, 0.5, 0.75, 1.0 and 1.5 weight percentage) have been
fabricated by powder metallurgy process. Damping specimens were prepared according to
ASTM E 756-05 standard and the specimens were subjected to free vibration test to investigate
the damping ratio and natural frequency. It is observed from the free vibration test data; both
alloys (LM6 and LM25) have shown significant improvement in damping ratio, natural
frequency and modulus when reinforced with 0.5 weight percentage of MWNT. Increase in
weight percentage of MWNT beyond 0.5 leads to deterioration in damping ratio, natural
frequency and modulus. This is due to agglomeration of reinforcement phase. The same has been
observed in the morphological study using transmission electron microscopy (TEM). In this work
an attempt has been made to also investigate the mechanical properties of the fabricated
composites.
Keywords: Multiwall Carbon Nanotube; LM6; LM25; Damping ratio; Natural frequency;
powder metallurgy process.

820 K. S. Umashankar, K. V. Gangadharan, V. Desai and B. Shivamurthy Vol.9, No.9
1. INTRODUCTION
In aviation, automobile and other structural applications, the demand for materials possessing
superior properties like higher strength to weight ratio, high modulus and high temperature
stability along with good damping ability is continuously increases. However, it is difficult to
achieve all these properties in a single material. This is one of the driving force for the
development of newer and newer composite materials. In this context many researchers all over
the world investigating new composite materials using either polymer matrix or metal matrix
with different reinforcements. For components used at higher operating temperatures, metal
matrix composite materials with a good reinforcement are preferred.
In general, aluminium (Al) and its alloys have emerged as one of the most dominant metal
matrix materials in the 21
st
century [1]. This is because of their attractive mechanical properties
[2, 3]. Among the Al alloys, Aluminium-Silicon (Al-Si) alloys in particular have been utilized
significantly for engineering applications due to their better mechanical and physical properties.
They also possess good manufacturing ability and lower density than Al [4]. This reduces weight
and results in energy savings in automotive and aerospace applications.
Many researchers have studied varieties of reinforcements with aluminum matrix to produce
Metal Matrix Composites (MMCs) to achieve the required properties. Out of these
reinforcements, Carbon Nano Tubes (CNT) is an ideal reinforcement since it possesses
extremely high modulus of elasticity, strength, stiffness, low density and high specific surface
area.
MMCs are generally fabricated through processes such as casting, extrusion and powder
metallurgy (P/M). Out of these, P/M process is regarded as one of the important processing
techniques on account of homogeneity in composition and microstructure, minimum scrap,
consistent porosity and close dimensional tolerances. P/M process also gives ample scope to
produce wide variety of alloying systems and metal matrix composites [5-8]. With this insight it
has been reported that, Al based particle reinforced composites produced by P/M process exhibit
attractive mechanical properties such as increased stiffness, low density, good specific strength
and vibration damping [9-12]. However literature studies indicate that major works have been
carried out on polymer and ceramic based CNT reinforced composites [13]. When appropriate
quantity of nanotubes is mixed with polymers, they show improved mechanical properties
without sacrificing any other base properties [14]. Many researchers have also produced and
characterized the Carbon nanotube based Aluminium metal matrix composites by powder
metallurgy process [15-17] and it has been reported that Powder metallurgy process is one of the
better processing techniques for these kinds of metal matrix composites. In the present work
MWNT reinforced Al-Si alloy (LM6 and LM25) based composites are produced by powder
metallurgy process and their mechanical and damping properties have been investigated.

Vol.9, No.9 Fabrication and Investigation of Damping Properties 821
2. MATERIALS AND METHODS
2.1 Materials
Two types of nano particulate reinforced Al-Si composites were manufactured by using LM6
and LM25 powders of 200 mesh size as a matrix and MWNT as reinforcement. LM6 and LM25
were supplied by M/s Metal Powder. Co Ltd, Chennai, and the research grade MWNT was
supplied by M/s Sigma Aldrich, Bangalore. The properties of as supplied MWNTs are given in
Table.1.
Table.1: Properties of MWNT
Properties Values
Purity Carbon > 90% (trace metal basis)
OD × ID × L 10-15 nm × 2-6 nm × 0.1-10 µm
Total Impurities Amorphous carbon, none detected by transmission electron
microscope (TEM))
Melting Point 3652-3697 °C
Density ~2.1 g/ml at 25 °C
2.2 Methods
2.2.1 Establishment of compaction load
In order to achieve optimal compaction density, trial compactions were done at different loads
for both LM6 and LM25 powders. A cylindrical die of 25.4 mm diameter was used to obtain
cylindrical specimen as per ASTM B-925. A coating of zinc stearate was applied to the die and
punch to minimize friction during the compaction process. 35 grams of metal powder was taken
in the die and load was applied through a hydraulic press of 40 Ton capacity at the rate of 2
Ton/min. After compaction, the specimen was ejected and its volume was measured. The density
was determined by mass-volume relation. The variation of density versus load for LM6 and
LM25 are shown in Figure 1. From the densification trend it is observed that optimum density
obtained at a compaction load of around 160 KN and 140 KN for LM6 and LM25 respectively.

822 K. S. Umashankar, K. V. Gangadharan, V. Desai and B. Shivamurthy Vol.9, No.9
6080100 120 140 160 180 200
2000
2100
2200
2300
2400
2500
Density in kg/m
3
Load in kN
LM6
6080100 120 140 160 180 200
2000
2100
2200
2300
2400
2500
Density in kg/m
3
Load in kN
LM25
Figure 1. Variation of density with compaction load.
2.2.2 Preparation of composite
The CNT powder was initially purified by mixing it in concentrated Nitric acid, filtering and
washing with de-ionized water and drying at 120
o
C. This is done to remove the impurities such
as graphitic particles, amorphous carbon or any other impurities present. MWNT of 0.25, 0.5,
0.75, 1.0 and 1.5 weight percentage was mixed with LM6 powder in ethanol solution. The
mixing was done in a ultrasonic shaker for 30 min. Finally, the mixed powders were dried at
120°C in vacuum (less than 10
−2
Pa) and further ball milled using Retsch PM100 high speed
planetary ball mill apparatus. The process of mixing is continued for duration of 10 min at 200
rpm in order to get uniform mixing. The mixture of a particular weight percentage of MWNT
and LM6 was compacted in the die assembly using a 40 Ton capacity universal testing machine.
The standardized load (160 KN) was applied at the rate of 2 Ton/min as per ASTM B-925. After
ejecting, the green specimen was sintered in nitrogen atmosphere for 1 hour at 490
o
C. The
sintered specimen was subjected to hot extrusion at a temperature of 350
o
C to produce the
rectangular strips. Figure 2 & Figure 3 shows the photographs of specimen after compaction and
extrusion respectively.
The same procedure was repeated to produce specimens of rectangular extruded strips of
different weight percent content of MWNT with LM25 matrix. While compacting LM25 matrix
based composite, a compaction load of 140KN and a sintering temperature of 515
o
C were used.
3. EXPERIMENTATION RESULTS AND DISCUSSIONS
3.1 Mechanical Properties
3.1.1 Hardness

Vol.9, No.9 Fabrication and Investigation of Damping Properties 823
Figure 2. Cylindrical Specimen after compaction. Figure 3. Specimen c/s after extrusion.
Hardness of specimen of base alloys (LM6 and LM25) and MWNT reinforced composites were
determined by using Rockwell Hardness Testing apparatus as per ASTM B-925. The results are
tabulated in Table 2. It is found that both for LM6 and LM25 based composites, the hardness
increases with the addition of MWNT upto 0.5 wt % of MWNT and then the hardness decreases
beyond 0.5 wt%. However, the hardness of LM6 composites is slightly on higher side compared
to LM25 composites. This is due to the contribution of metallurgical composition and structure
of base material LM6.
Table 2. Hardness values of green and sintered compacts.
Material Load
(Ton)
Hardness
(RHN)
Before
sintering
Hardness
(RHN)
After
sintering
For
0.25 wt
%
MWNT
For
0.50 wt
%
MWNT
For
0.75 wt
%
MWNT
LM6 160 18 33 39 48 44
LM25 140 17 30 35 43 39
3.1.2. Density
Density of the LM6 and LM25 base alloys and their composite before and after sintering were
computed by mass-volume relation and plotted against wt% of CNT. The variation in density is
shown in Figure 4. The density decreases with an increase in weight percentage of MWNT in the
composites, both before and after sintering. It has also been observed that, the density decreased
remarkably in both LM6 and LM25 based composites at above 1.0 weight percent of
reinforcement. This is due to agglomeration of MWNT in the matrix. The agglomeration of CNT
is well supported by the TEM images, shown in Figure 5.

824 K. S. Umashankar, K. V. Gangadharan, V. Desai and B. Shivamurthy Vol.9, No.9
0.0 0.5 1.0 1.5
2050
2100
2150
2200
2250
2300
2350
2400
2450
2500
2550
2600
Density (Kg/m3)
CNT reinforcement (% wt fraction)
before sintering
after extrusion
0.0 0.5 1.0 1.5 2.0
2340
2360
2380
2400
2420
2440
2460
2480
2500
2520
2540
2560
2580
2600
2620
Density (Kg/m3)
CNT reinforcement (% wt fraction
before sinterin
after sintering
(a) (b)
Figure 4. Density trend of (a) MWNT-LM6 and (b) MWNT-LM25
Before and after sintering
(a) (b)
Figure 5. TEM image: (a) LM6 - 0.5 wt.% MWNT composite and (b) LM6 -1.5wt.% MWNT
composite
3.1.3 Young’s modulus (E)
The Young’s modulus of the LM6 and LM25 alloys without MWNT and with MWNT
reinforced were examined as per ASTM standard E-8 by using computer interfaced electronic
load cell controlled Lloyds universal testing machine. The young’s moduli of composites are
given in Table 3. It is observed that LM6 composite containing 0.5 wt percentage MWNT
exhibits highest (69.33GPa) value of E followed by LM25 composite containing 0.5 wt
percentage MWNT (64.2 GPa). Further, increase in MWNT in both the composites is not
beneficial from the point of view of modulus. This reduction in modulus is again attributed to the
agglomeration of MWNT in the matrix at higher content.

Vol.9, No.9 Fabrication and Investigation of Damping Properties 825
Table 3. Young’s modulus of LM6 and LM25 composites
3.1.4 SEM and EDS of composites.
The EDS of 0.5 weight percentage reinforced LM6 and LM25 composites is shown in Figure 6.
is performed to ascertain the correctness of composition and fabrication steps. The EDS image is
shown particularly for 0.5 wt% of MWNT, as the best properties were obtained at this wt% for
both LM6 & LM25. The composition obtained from EDS results are tabulated in Table 4.
Table 4. Constituents of MWNT-LM6 and MWNT- LM25
Material Constituents
Si Cu Mg Fe Zn Pb Sn C Al
LM6 % 12.1 0.1 0.1 0.6 0.1 0.1 0.05 0.48 Remaining
LM25 % 6.9 0.1 0.3 0.5 0.1 0.1 0.05 0.47 Remaining
Figure 6. EDS image of (a) LM6-0.5 wt % MW NT and (b) LM25-0.5 wt % MWNT
Material combination E (GPa) Material combination E (GPa)
LM6 64.2 LM25 59.8
LM6- 0.25 wt % MWNT 60.99 LM25-0.25 wt %MWNT 57.8
LM6-0.5 wt % MW NT 69.33 LM25-0.5 wt % MWNT 64.2
LM6-0.75 wt % MWNT 62.63 LM25-0.75 wt % MWNT 58.1
LM6-1.0 wt % MWNT 60.34 LM25-1.0 wt % MWNT 55.6
LM6-1.5 wt % MWNT 56.49 LM25-1.5 wt % MWNT 51.2
0.00 1.00 2.00 3.00 4.00 5.00 6.007.00 8.00 9.0010.00
keV
001
0
30
60
90
120
150
180
210
240
270
300
330
360
CPS
CKa
MgKa AlKa
SiKa
FeLl FeLa FeKesc FeKa FeKb
NiLl NiLa
NiKa NiKb
CuLlCuLa
CuKa
CuKb
ZnLl
ZnLa
ZnKa
ZnKb SnLl SnLa SnLb SnLb2 SnLr SnLr2,

826 K. S. Umashankar, K. V. Gangadharan, V. Desai and B. Shivamurthy Vol.9, No.9
3.2. Investigation of Damping Ratio
Internal damping results from mechanical energy dissipation within the material due to various
microscopic and macroscopic processes. Internal damping of materials originates from the
energy dissipation associated with microstructure defects such as grain boundaries and
impurities, thermo-elastic effects caused by local temperature gradients, non-uniform stresses as
in vibrating beams, eddy current effects in ferromagnetic materials, dislocation motion in metals
and chain motion in polymers. It is observed that the material damping is the energy dissipated
within the materials of construction and is due to internal hysteresis in materials arising from
non-linear stress-strain behaviour, intergranular friction and thermo-elasticity [18].
In this research, specimens size of 150 mm X 15 mm X 5 mm were prepared from LM6 and
LM25 based MWNT composites. The specimen were subjected to free vibration according the
standard test methods for measuring Vibration-Damping properties of materials ASTM E 756-
05. The specimens were treated as self supporting materials with cantilever beam configuration.
The experiment was conducted on the specimen considering the dimensions of 100mm X 15mm
X 5mm. This allows sufficient length for fixing the beam as cantilever. The experimental setup
consists of an accelerometer A&D 3101 with sensitivity 9.8mV/g which is used to measure the
beam response. Data acquisition is through National Instruments eight channel industrial
platform sound and vibration measurement module having 24 bit resolution and acquisition rate
capability of 1024 kS/s. Signal conditioning and analysis is done through National Instruments
LabVIEW 8.6 software. The Experiment setup and LabVIEW block diagram for vibration
measurement is shown in Figure 7.
Figure 7. Free vibration test setup and LabVIEW block diagram

Vol.9, No.9 Fabrication and Investigation of Damping Properties 827
Three sample specimens were subjected to testing in each composite and average value was
taken to compute damping ratio and the natural frequency. The damping ratio is calculated and
tabulated in Table 5.
A typical free vibration response for a Single degree freedom system is shown in Figure 8. The
Logarithmic decrement is computed from the amplitudes using the following Equation 3.1.
Logarithmic decrement,








=








=
−1
1
2
1
1
n
X
X
nX
X
δ (3.1)
Where, X1 and X2 are first two successive amplitudes and n is the number of cycles.
The damping ratio is computed by using the Equation 3.2.
2 2
4
δ
ζ
π δ
=
+
(3.2)
x
n+1
x
n
x
2
x
1
Amplitude(x)
Time(t)
x
0
Figure 8. Free vibration response of a single degree freedom system.
From the experimental data mentioned in Table 5, it is evident that 0.5 weight percent MWNT
reinforced LM6 and LM25 exhibit excellent damping and natural frequencies in comparison with
other weight percentages. 0.5 weight percent MWNT composite of both LM6 and LM25 also
exhibit high natural frequency (416 Hz and 406 Hz). This is due to composites possessing high
stiffness on account of high modulus of MWNT and its uniform distribution. The improvements
in damping ratio for LM6 and LM25 with 0.5 weight percent MWNT with respect to base alloys
are 50.9% and 38.1% respectively. At lower weight percentage of MWNT in the composites, the
improvement in the damping ratio and natural frequency is very marginal. Higher weight percent
of MWNT in the composites leads to decrease in damping ratio and natural frequency.
Clustering of MWNT at higher weight percentage causes the decrease in natural frequency and
damping ratio. At 0.5 weight percentage, the MWNT is uniformly distributed. This results in
good bonding between the reinforcement and the matrix. This provides large interfacial area

828 K. S. Umashankar, K. V. Gangadharan, V. Desai and B. Shivamurthy Vol.9, No.9
between matrix and MWNT. This increases the modulus value as well as energy dissipation at
interface.
Table 5. Natural frequencies and damping ratios of LM6 and LM25 based MWNT reinforced
composites.
4. CONCLUSION
LM6 and LM25 composites with MWNT in weight percentages of 0.25, 0.5, 0.75, 1.0 and 1.5 as
reinforcement were produced through powder metallurgy route. The extruded specimens were
subjected to free vibration test to evaluate the damping ability and natural frequency (stiffness).
From the investigation, following points are concluded.
 LM6 has better damping ability than LM25.
 Both alloys show marked improvement in damping ratio when reinforced with 0.5 weight
percentage of MWNT.
 More than 0.5 weight percentage of MWNT leads to clustering and thus the damping and
stiffness properties are reduced remarkably.
 From the experimental observation, it is concluded that the mixing and compaction
parameters for densification need to be standardized for each base material and MWNT
weight percentage in order to further realize improvement in damping and stiffness
properties.
Material
combination
Natural
Frequency
(Hz)
Dampin
g Ratio
Material
combination
Natural
Frequency
(Hz)
Damping
Ratio
LM6 405 0.0055 LM25 399 0.0042
LM6- 0.25 wt %
MWNT
402 0.0056 LM25-0.25 wt
%MWNT
397 0.0045
LM6-0.5 wt %
MW NT
416 0.0083 LM25-0.5 wt %
MWNT
406 0.0058
LM6-0.75 wt %
MWNT
406 0.0052 LM25-0.75 wt %
MWNT
400 0.0043
LM6-1.0 wt %
MWNT
395 0.0043 LM25-1.0 wt %
MWNT
386 0.0040
LM6-1.5 wt %
MWNT
380 0.0041 LM25-1.5 wt %
MWNT
374 0.0038

Vol.9, No.9 Fabrication and Investigation of Damping Properties 829
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