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Abstract

In the present
day context, the polymer based nano composites finds extensive application both
in engineering and bio-medicine applications. The nano filler addition further
enhances the properties; especially wear and friction which are very well
known. Keeping these points in view, the present work focuses on establishing
the tribological behaviour i.e., both slide wear and friction with various load
applications for different levels of graphene oxide addition (0.5% to 2%) and
MoS2 addition at 0.8%.The specimens were made by compression molding
technique .The test samples were prepared to the required sizes and subjected
to slide wear and friction measurements using pin on disc set up; which is carried
out as per ASTM guidelines. The weight change measurements were done by noting
the initial and final weight of the test samples to get the slide wear loss.
The hardness values were obtained by using Shore D Hardness tester as per ASTM standards.
The weight change measurements to get slide wear loses were done. The
coefficient of friction was computed by dividing frictional load with normal
load. It is very well seen from the test data that the slide wear resistance
increases with increase in graphene oxide content (0.5% to 2%) in steps of 0.5%;
as the hardness level showed an increase with the same level of increase in
graphene oxide content. The co-efficient of friction has shown a declining
trend with increase in graphene content for the load applications of various
loads. The slide wear and friction results have been interpreted based on
logical thinking involving literature reports and supporting tests. Thus, there
are positive responses observed in respect of the graphene oxide addition to
HDPE matrix. This makes the material suitable for wear resistant applications
in automotive industries.

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1. Introduction

The
synthesis of nano particles has gained momentum in recent times from the point
of utilizing in different materials like polymers ceramics and composites .In
this context, the deployment of graphene and graphene oxide in any matrix has
yielded very good results like  enhanced
mechanical and tribological properties 1. The oxidizing agents are used to
oxidize graphite thereby oxygenated functionalities are introduced in the
graphene, thus producing nano layers or few layers of oxygen functionalized
graphene known as graphene oxide. Graphene oxide as a filler material in any
polymer or ceramic matrix has been proved to be highly successful from the
point of achieving very good mechanical, thermal, tribological and other
properties. There is a sizable improvement reported in the literature 2 in
respect of mechanical strength, slide wear and friction, thermal conductivity,
etc. Keeping these aspects in view, a polymer matrix composite development is
envisaged in this work with HDPE as the main matrix and graphene oxide with MoS2
as filler materials for sliding wear applications.

 

1.1  Graphene and graphene oxide

Graphene
is an allotropic form of carbon having two dimensional atomic scale of
hexagonal structure. Graphene has several unusual properties such as 200 times
stronger than steel, conduct heat and electricity, good wear resistance and
hardness .It has tensile strength of about 130GPa with Young’s modulus of 1TPa
3.The Graphene is relatively brittle with a fracture toughness of about
4MPa?m 4.This material can go any other material in right properties so that
the tailor made properties could be achieved. The graphitic oxide or graphite
oxide compound of carbon, oxygen and hydrogen in different ratios produced by
oxidizing graphite, which is layer structured. The dispersion on the bulk
material pertaining to this give rise to nano molecular sheets, which is called
as graphene oxide. This material is highly useful in preparing strong paper
like materials such as membranes, thin films; composite materials etc.This
material serves as an intermediate one to produce graphene by reduction process.
The graphene oxide has specific surface properties and found suitable for
surfactant applications 5.In the emulsion system, the graphene oxide gets
into the interface between the phases 6. It is reported in the literature 7, that the adoption of
graphene and graphene oxide have resulted in much improved
mechanical,electronic,electrical characteristics with a very bright outlook for
certain specific applications.

 

1.2  HDPE matrix and
MoS2

High
Density Polyethylene (HDPE) is an organic material in the thermoplastic
category. This is produced from petroleum product and finds extensive
applications in producing plastic bottles,pipings,membranes,fuel tanks, tubes
etc.It is possessing very good strength to weight ratio with the density ranging
from 930 to 970 kg/m3 . It can be used upto 120ºC for a short time,
opaque in nature ,possessing tensile strength of about 30 to 32Mpa and
resistant to solvents. Molybdenum disulphide (MoS2) is an inorganic
compound consisting of Molybdenum and Sulphur. It is non reactive, used as a
solid lubricant in many engineering applications wherein, it is looked for low
co-efficient of friction. The low co-efficient of friction is attributed to the
weak Vander Waals forces acting between the sulphide atoms.

 

2.0 Materials
and Methods

2.1
Materials

 The polymer matrix and filler additions used
are detailed in Table 1. Further five different compositions employed in the
present work have been designated in the same table.

Table 1

Sl. No.

Sample Designation/Code

Test Sample Description

Matrix

Filler Wt. (%)

Graphene Oxide

MoS2

1

HDPE

HDPE

2

HDPE (0.5+0.8)

0.5

0.8

3

HDPE (1.0+0.8)

1.0

4

HDPE (1.5+0.8)

1.5

5

HDPE (2.0+0.8)

2.0

 

2.2Methods

The test samples were prepared by adopting
the procedures of Brabending and Compression Moulding Technique (CMT).A brief
procedure in respect of Brabending and Compression moulding methods are as
discussed. The Brabender basically operates on the feed system in the form of
drive units as it provides the drive motor for the processing of modules and
contains direct torque measurement system. Initially the temperature and the
rotation speed for mixing is set. Raw materials in the form of granules is
poured into the mixing attachment; placed between the two rotating rotors.
Then; the obtained material is sent for further processing via Compression
molding technique. Compression moulding is a notable technique used to develop
and manufacture variety of various composites. It is a closed moulding process
involving high pressure application. A couple of matched metal molds are used
to fabricate composites. In compression molder, base plate is fixed while upper
plate is easily movable. Reinforcement and matrix are placed in the metallic
mold and the entire assembly is placed in between the compression molder.
Application of Heat and pressure is as per the standard requirements of
composite for a specified period of time. Due to application of pressure and heat,
the material placed in between the molding plates flow and acquires the shape
of the mold cavity with high dimensional accuracy. This depends upon mold design.
Curing of the composite may be carried out either at room temperature or at
elevated temperature. After curing, mold is opened and composite product is removed.
If required it is sent for further processing. In principle, a compression
molding machine is a form of press which is oriented vertically with two
molding halves (top and bottom halves). Basically, hydraulic mechanism is used
for pressure application in compression molding.

 

2.2.1.
Slide wear and friction

Dry sliding wear tests were performed using Pin-on-disk apparatus,
as per ASTM             G99-95standards.All
the tests were conducted with test duration of 10 min and adopted a varying
load from 40N to 70N with a sliding velocity from 0.44m/s to 0.78m/s.Wear loss
was measured in the steady state regime using linear variable differential
transducer (LVDT) of accuracy 1?m at the end of 10 min. The wear rates were
calculated from height loss data.

 

2.2.2
Hardness test

Durometer is one of the
instruments used to measure the hardness of a material. There are
several scales of durometer, used for materials with different properties. The
two most common scales using slightly different measurement systems – are the ASTM
D2240 Type A and Type D scales. Type D scale is used to measure the Hardness in
the present assessment as per the standards used for testing polymers.

 

3.0
Results and Discussion

The
hardness (Shore D) values, slide wear and coefficient of friction data are
shown in fig. 1, 2 & 3 respectively.

Fig. 1 Shows Hardness vs HDPE and
Polymer Composite Samples

 

A
maximum improvement of 7% is observed for HDPE (2.0+0.8) polymer composites
when compared with HDPE for 0.5% graphene addition and the base material.

 

 

Fig. 2 Shows Wear Rate v/s Load for HDPE
Samples

 

At a maximum load of 70N, a reduction of 40% in the wear rates for
HDPE (2.0+0.8) composites when compared with HDPE for 0.5% graphene addition
and compared to base material

 

 

 

Fig. 3 Shows Coefficient of Friction v/s
Load for HDPE Samples

 

A
maximum improvement of 17% is observed for HDPE (2.0+0.8) composites when
compared with HDPE for 0.5% graphene addition when compared to base material
with load 70N.

The
worn surface features of the test samples using scanning electron microscope
(SEM) pertaining to HDPE, HDPE (0.5+0.8), HDPE (1.0+0.8), HDPE (1.5+0.8) and
HDPE (2.0+0.8) are shown in figures 4 to 8 respectively at a magnification of
250x. The input parameters employed for SEM examinations are 70N load, 2.4m/s
sliding speed, sliding distance of   1445
m. The counter surface being EN31 is an alloy steel of HRC 62 in the heat
treated condition. The surface roughness of the counter surface is about 1 µm.

Fig.4 HDPE @ (250X)

Fig.5  0.5%
Graphene  @  (250X)

Fig.6 1% Graphene
 @ (250X)

Fig.7 1.5% Graphene  @ (250X)

Fig.8  2%Graphene  @ (250X)

As
regards the trend in hardness level for the samples tested, it is very evident
from fig. 1 that the increase in graphene oxide content from 0.5 to 2%, the
value increases almost linearly. This is on the expected line the introduction
of any oxide into the matrix results in increase in hardness 8.Thus there is
one to one correspondence between the literature report and the present work on
hardness.

The
data on slide wear loss obtained for the graphene oxide based HDPE composite
shows that it increases with increase in load (Fig. 2). It is also observed
from Fig.2 that the slide wear loss increases with increase in load
application. This is quite logical since the highest addition of graphene oxide
in the polymer matrix has shown the best hardness and for the pure HDPE it has shown
the least. The SEM pictures also support these findings on wear loss data. The
Fig.4 pertaining to test sample HDPE is showing higher matrix damage, increased
debris formation and a few visible cracks compared to the test samples HDPE 2.0+0.8
(Fig. 8) which is displaying lesser matrix distortion, lesser debris and less
cracks. The other test samples HDPE (0.5+0.8), HDPE (1.0+0.8), HDPE (1.5+0.8) are
showing worn surface features of intermediate in nature. Thus, there is one to
one correspondence between the SEM features and slide wear
data, thus complimenting each other. Hence SEM gives credence to the slide wear
data. The
work reported by other researchers 9, 10, 11 with
the use of graphene oxide or any other oxides reveal that the addition to a
matrix helps to improve the slide wear resistance due to the increase in
hardness level compared to the oxide free matrix.

The
coefficient of friction is another parameter which has been determined for
addition of graphene oxide at four levels (0.5, 1.0, 1.5 and 2.0%) & MoS2
at 0.8% to HDPE matrix. From Fig.3 it is very well seen that COF decreases with
increase in load for all the samples including pure HDPE sample. The highest
COF is found to be for pure HDPE and lowest for HDPE (2.0+0.8) irrespective of
the load application (40, 50, 60, 70N). The decrease in COF may be attributed
to addition of solid lubricant MoS2 and also the graphene oxide being
a good lubricant. The increase in graphene oxide addition has shown decrease in
COF. Similarly work reported by other 12 have shown similar trend in COF
data. The graphene oxide and MoS2 addition have been proved to be
beneficial in reducing friction as well as slide wear loss. Thus for lower wear
loss and low friction applications it would be ideal to employ graphene oxide
at 2.0% and MoS2 at 0.8 shows that this combination may be tried
especially in automobile applications for better field performance.

4.0 Conclusion

Hardness
of the polymer composite with signature HDPE (2.0+0.8) improved by 7% hinting
the active distribution of reinforcement effectively.HDPE (2.0+0.8) composites exhibited
lower coefficient of friction (17%) and lower wear rate (40%) when compared
with HDPE under all the loads and sliding velocities studied. Significantly
reduced wear rate can be attributed to the destruction of the possible MML
layer formed and material softening due to increased temperature with increased
sliding velocity during the sliding process.SEM morphology of the worn surfaces
reveals that the extent of damage is least for HDPE (2.0+0.8) when compared
with the base material.

 

 

 

 

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