Journal of Minerals and Materials Characterization and Engineering, 2012, 11, 938-946
Published Online October 2012 (http://www.SciRP.org/journal/jmmce)
Effect of Micro Size Cenosphere Particles Reinforcement
on Tribological Characteristics of Vinylester Composites
under Dry Sliding Conditions
Santram Chauhan, Sunil Thakur*
Department of Mechanical Engineering National Institute of Technology, Hamirpur (H.P.), India
Received July 5, 2012; revised August 16, 2012; accepted August 27, 2012
ABSTRACT
In this paper the friction and wear characteristics of vinylester and cenosphere reinforced vinylester composites have
been investigated under dry sliding conditions, under different applied normal load and sliding speed. Wear tests were
carried using pin on a rotating disc under ambient conditions. Tests were conducted at normal loads 10, 30, 50 and 70 N
and under sliding velocity of 1.88, 3.14, 4.39 and 5.65 m/s. The results showed that the coefficient of friction decreases
with the increase in applied normal load values under dry conditions. On the other hand for pure vinylester specific
wear rate increases with increase in applied normal load. However the specific wear rate for 2%, 6%, 10% and 15%
cenosphere reinforced vinylester composite decreases with the increase in applied normal load under dry conditions.
The results showed that with increase in the applied normal load and sliding speed the coefficient of friction and spe-
cific wear rate decreases under dry sliding conditions. It is also found that a thin film formed on the counterface seems
to be effective in improving the tribological characteristics. The specific wear rates for pure vinylester and vinylester
composite under dry sliding condition were in the order of 10−6 mm3/Nm. The results showed that the inclusion of
cenosphere as filler materials in vinylester composites will increase the wear resistance of the composite significantly.
SEM analysis has been carried to identify the wear mechanism.
Keywords: Composites; Cenosphere; Coefficient of Friction; Vinylester; Wear
1. Introduction
High performance polymer composite materials are used
increasingly for engineering applications under working
condition. The materials must provide unique mechanical
and tribological properties combined with a low specific
weight and a high resistance to degradation in order to
ensure safety and economic efficiency. Polymer and their
composites are finding ever increasing usage for numer-
ous industrial application such as bearing materials, roll-
ers, seals, gears, cams, wheels and clutches. Composite
mate-rials provide an opportunity to combine different
properties and design materials for applications requiring
multiple functionalities. Polymeric matrices reinforced
with hardand non dissipative fillers can possess high
stiffness and damping, which is ideal for structural prop-
erties [1,2].
Over the past decades, thermoplastic composites have
been increasingly used for numerous mechanical and
tribological purposes such as seals, gears and bearings.
These materials are light in weight and are better alterna-
tives to metalliccomponents [3]. The feature that makes
polymer composites so promising in industrial applica-
tions is the possibility of tailoring their properties with
functional fillers. Polymer composites are more attractive
than conventional metallic system. These are relatively
low density, high corrosion resistance and ability to be
tailored to have stacking sequences that provide high
strength and stiffness in direction of high loading. Ther-
moplastic composites can improve the stiffness, decrease
thermal expansion, improve long-term mechanical per-
formance and reduce costs [4,5]. Polymer composites
consist of resin and a reinforcement chosen according to
the desired mechanical properties. Improved perform-
ance of polymer and their composites in industrial and
structural application by addition of filler materials [6,7].
Polymer based composites materials are the ones used in
such application because of their ever increasing demand
in terms of stability athigher load carrying capacity and
wear rate materials determine their acceptability for in-
dustrial applications. The mechanical properties of com-
mon polymers when compared to metals are not very
good. Considerable attention has thus been paid in the
last 30 years to study the tribological properties of poly-
*Corresponding author.

S. CHAUHAN, S. THAKUR 939
mer composites [8-10].
The advancement in technology and finding of new
technology has increased the need of superior material
for tribological applications. Polymer composites exhib-
its excellent friction and wear characteristics even with-
out external activations and can provide maintenance
freeoperation, excellent corrosion resistance. Wear and
friction originate from multiple sets of complex interac-
tions on microscopic scale between surfaces that are in
mechanical contact and slide against each other. These
interactions depend on the materials, geometrical and
topogical characteristics of the surfaces and overall con-
ditions under which the surfaces are made to slide against
each other e.g. loading, temperature, atmosphere, type of
contact [11]. Wear is defined as damage to a solid sur-
face, generally involving progressive loss of material due
to relative motion between that surface and contacting
substance [12-14]. Abrasive wear is the most important
among all the forms of wear because it contributes al-
most 60% of the total cost of wear. Abrasive wear is
caused due to hard particles that are forced against and
move along a solid surface. Polymer and their compos-
ites are finding ever increasing usage for numerous in-
dustrial applications such as bearing material, rollers,
seals, gears, cams, and clutches [15-17]. Different types
of polymer show different friction and wear behaviour.
The understanding of wear behavior of cenosphere parti-
cle filled vinylester matrix composites is still very lim-
ited. The wear resistance of the composites might be de-
creased or increased depending on the type of particles,
particle size, size distribution, interfacial actions between
particle and matrix resin, particle content and state of
dispersion of the particles in the composites as well as
wear test conditions, i.e. wear mode (pin-on-disc), coun-
terface, sliding velocity, sliding distance, applied load
and humidity [18].
Vinylester resin is widely used in thermosetting resins
because of their low cost, excellent chemical, corrosion
resistance and mechanical properties. Vinylester resins
are stronger than polyester resins and cheaper than epoxy
resins. Vinylester resins utilize a polyester resin type of
cross-linking molecules in the bonding process [19,20].
Vinylester is a hybrid form of polyester resin which has
been toughened with epoxy molecules within the main
molecular structure. Vinylester resins offer better resis-
tance to moisture absorption than polyester resins. Some-
times it won’t cure if the atmospheric conditions are not
right. It also has difficulty in bonding dissimilar and al-
ready-cured materials. It is also known that vinylester
resins bond very well to fiberglass but offer a poor bond
to kevlar and carbon fibers due to the nature of those two
more exotic fibers [21,22]. Vinylester resin have better
thermal performance, excellent mechanical properties
and toughness as compare to polyester resins which is
why vinylester are preferred materials for structure com-
posites.
The word cenosphere is derived from two Greek
words kenos (hollow) and sphaira (sphere) [23]. Ceno-
sphere are lightweight, inert and hollow spheres mainly
consists of silica and alumina are filled with air or gases
and are by-product of the combustion of pulverized coal
at the thermal power plants [24,25]. In general, Ceno-
spheres are hollow spherical particles with shell thick-
nesses of 30 - 250 µm, density varies from 0.3 to 0.6
g/cm3, and shell wall thickness varies from 2 to 10 µm
[26]. Due to their hollow structure these ash particles
float on water when ash is disposed in the slurry form in
ash ponds or lagoons. These shells are porous in nature.
As a result, these cenosphere particles could not make
any significant scratching action over the counter surface.
Cenospheres was used as reinforcing filler in vinylester
resin to developed lightweight composites. Cenospheres
are alumina silicate hollow ceramic particles formed
during the production of electricity by coal burning
power stations [27-29]. They have most of the same pro-
perties as manufactured hollow-sphere products. Ceno-
spheres are primarily used to reduce the weight of plas-
tics, rubbers, resins, cements. Cenospheres are used in a
variety of products, including sports equipments, auto-
mobile bodies, marine craft bodies and heat protection
devices. Providing the advantages of reduces weight, in-
creased filler loadings, better flow characteristics, less
shrinkage and reduces water absorption [30,31]. Deve-
loping light and heat insulating polymer composite mate-
rial filled with cenospheres may also be realized with low
density and spherical alumino-silicate structures which
may offer other additional advantages like enhanced
mechanical properties such as elastic modulus, toughness,
high durability and increased isotropic-compression. Due
to their low density, high strength, good thermal and ele-
ctrical capacities and good tolerance for chemical agents
and high temperatures, cenospheres find tremendous ap-
plication in various industries [32-35].
2. Experimental Details
2.1. Materials and Sample Preparation
The type of resin used in this work is vinylester resin
(density 1.23 gm/cm3) supplied by Northern Polymer
Ltd., Delhi, India. The filler material used in this study is
cenosphere (Hardness 5 - 6 MOH, Density 0.4 - 0.6
gm/cm3), supplied by Cenosphere India Pvt. Ltd. Ceno-
spheres are inert hollow silicate spheres. The shape of
cenosphere is spherical and the colour is light gray. The
chemical composition of cenosphere is SiO2-55%, Al2O3-
34%, Fe2O3-1.5%, TiO2-1.2%, Carbon dioxide-70%,
Nitrogen-30%.
For preparation of composites specimens using hand-

S. CHAUHAN, S. THAKUR 940
layup technique of the vinylester resin in four different
percentages (2 wt%, 6 wt%, 10wt%, and 15 wt%). Also
the hardener methyl-ethyl-ketone-peroxide (MEKP) and
accelerator cobalt nephthalate was mixed. This mix was
stirred mechanically for half an hour so that dispersing
cenosphere fillers (90 µm) take place. The curing of
samples was carried at room temperature for 24 hrs.
Slowly poured in glass tubes so as to get cylindrical
specimens (diameter 12 mm, length 120 mm). The hard-
ened composite samples are extracted from the glass tube.
A releasing agent (Silicon spray) is used to facilitate easy
removal of composites from the glass tube after curing.
Specimens of suitable dimension are cut using a diamond
cutter forwear test.
2.2. Friction and Sliding Wear Testing
The friction and sliding wear performance evaluation of
vinylester and its composites C1, C2, C3 and C4 under dry
sliding conditions, wear tests were carried out on a pin-
on-disc type friction and wear monitoring test rig (DU-
COM) as per ASTM G 99. Thecounter body is a disc
made of hardened ground steel (EN-32, hardness 72
HRC). The test was conducted on a track with a diameter
of 120 mm and surface roughness of 0.2 µm by selection
of the test duration, load and sliding speed. The specimen
is held stationary and the disc is rotated while a normal
force is applied through a lever mechanism. This act as
the counterface to wear against the composites coating.
During the test friction force was measured by transducer
mounted on the loading arm. The friction force readings
are taken as the average of 100 readings every 40 se-
conds for therequired period. For this purpose a micro-
processor controlled data acquisition system is used. A
series of test were conducted with four sliding velocity of
1.88, 3.14, 4.39 and 5.65 m/s under four different normal
loads of 10, 30, 50 and 70 N. The environmentcondition
in the laboratory was 23˚C and 43% relative humidity.
Weight loss method was used for finding the specific
wear. Figure 1 shows the photograph of a pin-on-disc
wear tester.
During these experiments initial and final weight of
the specimens were measured. The specimens were
weighted both before and after the tests to an accuracy of
±0.01 mg in a precision balance. The specific wear rate
(mm3/Nm) is then expressed on “volume loss” bases
SN
K
MLF

 (1)
where Ks is the specific wear rate (mm3/Nm), ∆M is the
mass loss in the test duration (gm), ρ is the density of the
composite (gm/cm3), FN is the average normal load (N).
2.4. Scanning Electron Microscopy
Scanning electron microscope (SEM) was used to ana-
Figure 1. Photograph of pin-on-disc wear tester.
lyze the worn surface of the composites. Worn surface
samples were mounted on aluminum stub using conduc-
tive (silver) paint and were sputter coated with gold prior
to SEM examination. The surfaces of the samples were
examined directly by scanning electron microscope FEI
quanta FEG 450.
3. Results and Discussion
The characterization of the composites revels that inclu-
sion of any particles filler has strong influence not only
on the mechanical properties of composites but also on
their wear behaviour. A comparative study of modified
behaviour of the composites against unfilled composites
is presented. The experimental density of the composites
is obtained by the Archimedes principle of weighing
small pieces cut from the large composite panel first in
air and then in water. Theoretical density of composite is
calculated and compared with experimental density in
order to calculate void fraction of the composites. The
theoretical and measured densities along with the corre-
sponding volume fraction of voids are presented in Table
1. The composites under investigations consists of two
components namely matrix and particulate filler. Hence
density of composites can be calculated using rule-of-
mixture as shown in the following expression:



1
tmmpp
WW



(2)
where W and ρ represent the weight fraction and density,
respectively.
The suffix m, p and t stand for the matrix, particulate
filler and the composites materials respectively.
The actual density (ρe) of the composites can be de-
termined experimentally by simple water immersion
technique. The volume fraction of the voids (Vv) in the
composites is calculated using following equation:
vtet
V

  (3)
It can be noticed from Table 1 that composites density

S. CHAUHAN, S. THAKUR 941
Table 1. Comparison of experimental density and theor e tical density.
Sample Composites
specification
Experimental
density (gm/cm3)
Theoretical
density
(gm/cm3)
Void fraction
(%)
Temperature
(˚C)
Humidity
(%) Load (N) Sliding speed (m/s)
C1 Pure vinylester 1.23 1.23 0.000 23 40
10
30
50
70
1.88
3.14
4.39
5.65
C2 2%cenosphere +
vinylester 1.5196 1.5395 1.2926 23 40
10
30
50
70
1.88
3.14
4.39
5.65
C3 6%cenosphere +
vinylester 1.2042 1.2287 1.9939 23 40
10
30
50
70
1.88
3.14
4.39
5.65
C4 10%cenosphere +
vinylester 1.1371 1.1628 2.2101 23 40
10
30
50
70
1.88
3.14
4.39
5.65
C5 15%cenosphere +
vinylester 1.1262 1.2010 6.2281 23 40
10
30
50
70
1.88
3.14
4.39
5.65
values calculated from weight fractions using Equation
(1) are not in agreement with the experimentally deter-
mined values. The difference is a measure of voids and
pores present in the composites. It is clear from the table
that volume fraction of voids is negligible in C1 due to
absence of particulate fillers. With addition of filler ma-
terials voids are more pronounced in the composites.
This can affect composite performance adversely which
may lead to swelling and reduction in density. As filler
content increased from 2 wt% to 15 wt% the volume
fraction increased proportionately for all particulate filled
composites (C2 to C5). This may be due to the fact that
composites material which may entrap air during the
preparation of composite samples in hand layup tech-
nique. The significantly affect some of the mechanical
properties and even the performance of composites.
Higher void contents usually mean lower fatigue resis-
tance, greater susceptibility to water penetration. The
knowledge of void content is desirable for estimation of
the quality of the composites [31,32].
The detailed compositions of the materials taken for
the test conditions and parameters considered for ex-
perimentation scheme are presents in Table 1. Figures 2
(a)-(e) present the variation of coefficients of friction
with applied normal load values (10, 30, 50 and 70 N) at
different sliding velocity of (1.88, 3.14, 4.39 and 5.65
m/s) under dry sliding conditions. The experimental re-
sults show that with increase in the applied normal load,
the coefficient of friction decreases for pure vinylester
and its composites at all sliding speed under dry sliding
condition. From Figure 2(a) it is observed that with in-
creasing applied normal load the coefficient of friction is
decreased. However the coefficient of friction has higher
value at sliding velocity 5.65 m/s and 10 N. Moreover
the friction coefficient of the filled vinylester composites
assumes a little decrease within a mass fraction 2 to 15%
of the cenosphere.
In all test condition the coefficient of friction was
maximum in case of pure vinylester (C1) and minimum
in case of cenosphere filled vinylester composites (C4
and C5). Under dry sliding conditions increasing applied
normal load and sliding speed increases the temperature
at the interface. This increase in temperature causes
thermal penetration to occur, which results in weakness
in bond at the filler-matrix interface. Consequently filler
become the loose in the matrix and shear easily due to
axial thrust. As a result coefficient of friction decreases.
It was also found that the transfer film also plays a very
important role in affecting the friction and wear behave-
iour of fiber reinforced vinylester composites.
Figures 3(a)-(e) present the variation of specific wear
rate for vinylester and its composites (C1, C2, C3, C4and
C5) with applied normal load (10, 30, 50 and 70 N) and
testspeeds (1.88, 3.14, 4.39 and 5.65 m/s) under dry slid-
ing conditions. Figure 3(a) shows that the specific wear
rate for pure vinylester which is influenced by the change
in applied normal load sliding conditions. The specific
wear rate decrease with increase in applied normal load.
The higher the sliding speed the lower is the specific
wear rate in dry conditions. From the observations of
Figures 3(b)-(e) it is seen that the specific wear rate de-
creases with increase in applied normal load conditions.

S. CHAUHAN, S. THAKUR 942
(a) (b)
(c) (d)
(e)
Figure 2. Coefficient of friction vs. applied normal load for sample (a) C1; (b) C2; (c) C3; (d) C4; (e) C5.

S. CHAUHAN, S. THAKUR 943
(a) (b)
(c) (d)
(e)
Figure 3. Specific wear rate vs. applied normal load for sample (a) C1; (b) C2; (c) C3; (d) C4; (e) C5.

S. CHAUHAN, S. THAKUR
CJMMCE
944
The highest wear rate is for pure vinylester under dry
sliding conditions with the value of 78.04 × 10−6 mm3/Nm
at 5.65 m/s and load of 10 N. The lowest wear rate is 0.18
× 10−6 mm3/Nm for vinylester composite C5(vinylester +
15wt% cenosphere) composite at 5.65 m/s sliding speed
and applied normal load 70 N. The wear rate of the vi-
nylester composites assumes an obvious decrease with
increasing filler content from 2 wt% to 15 wt%. The spe-
cific wear rate for vinylester and vinylestercomposites is
little influenced by the applied normal load and sliding
velocity.
ture plays a major role in determining the wear mecha-
nism in composites materials. There is a severe deterio-
ration of cenosphere particle surface when applied load is
higher and sliding speeds gets higher. The frictional heat
generated at the interface caused thermal softening of the
matrix and some of the powdery wear debris got embed-
ded into the matrix and formed a protective layer. By
comparing the surfaces of the samples at different pa-
rameter conditions we can found out the wear rate easily.
The optical microscopy examination of worn surfaces of
vinylester composites (C1, C2, C3, C4 and C5) against
steel discs dry sliding conditions under applied load of 70
N and 5.65 m/s sliding speed are shown in Figures 4(a)-
(e). The SEM observation on Figure 4(a) for vinylester
samples (C1) show that conditions matrix is uniformly
4. Morphology Study
The surfaces of specimens are examined directly by
scanning electron microscope. The material microstruc-
(a) (b)
(c) (d)
(e)
Figure 4. SEM pictures of (a) pure vinylester; (b) 2%cenosphere/vinylester; (c) 6% cenosphere/vinylester; (d) 10% ceno-
sphere/vinylester; (e) 15% cenosphere/vinylester.
opyright © 2012 SciRes.

S. CHAUHAN, S. THAKUR 945
spreaded over the surface specimen, cracks in the matrix
and fewer wear debris can be seen that indicates higher
wear rate. As observed from figure it exhibit highest
wear among all applied loads due to cutting mode ofabra-
sive wear is occurred which results in deep grooves that
are clearly visible in micrograph. Cenosphere filled com-
posites (2 wt%, 6 wt%, 10 wt% and 15 wt%) show the
lesser spread of the matrix debris compare to pure vi-
nylester under 70 N load and 5.65 m/s higher sliding ve-
locity. Increasing the filler loading from 2 wt% to 15
wt% into vinylester resin the sample yields surfaces
which shows a less smearing wear of the matrix region
compare to unfilled sample (C1). From Figure 4(b) for
composite samples (C2) the observations show that con-
ditions the matrix is uniformly spreaded over major por-
tion of the specimen in the matrix that indicates lower
wear rate. It is clear from the micrograph that the only-
mechanism that causes wear at this condition is wedge
formation mode of abrasive wear, this can be attributed
to the fact that there is comparatively good adhesion be-
tween the filler and matrix which in turn results into
lowest wear rate. Micrograph shows the existence of
ploughing and wedge formation which is characterized
by wear due to plastic deformation and results into mod-
erate wear rate. The examination of the wear scars
indicated that the damage morphologies for all samples
were similar. The disc worn surfaces for vinylester
composite (C3) show that more of the cenosphere
exposures indi- cating higher wear rate. Figures 4(c) and
(d) represents the micrographs of 6 wt% and 10 wt%
ofvinylester rein- forced composites at a load of 70 N
respectively and both these composites shows moderate
wear which results in breakage of composites at distinct
places due to the com- bination of wedge formation and
ploughing mechanism of abrasive wear. This also
confirmed form Figure 4(c). A wear track is clearly
visible in micrograph. These ob- servations from SEM
very well confirm to the experi- mental results depicted
in Figure 3.
5. Conclusions
The main aim of this research work is to investigate the
influence of cenosphere particle on friction and wear be-
havior of vinylester composites. An experimental study
of friction and wear behavior of vinylester composites at
different sliding speed, applied normal load can reveals
the following:
 The coefficient of friction of vinylester and its com-
posite decreases with increase in applied normal load.
 Pure vinylester has higher specific wear rate due to
small mechanical properties. Therefore incorporation
cenosphere particle in vinylester matrix improves the
wear characteristics.
 The highest wear rate is for pure vinylester under dry
sliding conditions with a value of 78.04 × 10−6
mm3/Nm at 900 rpm and load of 10 N. However the
lowest wear rate is 0.18 × 10−6 mm3/Nm vinylester
composite C2 composite at 700 rpm and applied nor-
mal load 70 N. The incorporation of the micro-size
cenosphere contributed to increase the wear-resis-
tance of the vinylester composites. The vinylester
composites filled with 15% cenosphere recorded the
smallest friction coefficient while that filled with 15%
cenosphere showed the best wear-resistance.
 The specific wear rate for vinylester and vinylester
composites is little influenced by the applied normal
load and sliding velocity.
 For the range of load and sliding velocity in this study
it is observed that load has stronger effect on the fric-
tion and wear than the sliding velocity.
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