Influence of Nano Grease Composite on Rheological Behaviour

Abstract

The aim of this work is to study the rheological behaviors of carbon nanotubes (CNTs) as an additive on lithium grease at different concentrations. The results indicated that the optimum concentrations of the CNTs was 2 %. These experimental investigations were evaluated with a HAAKE Rheovisco RV20, Penetrometer and Measurement of the dropping point. The results indicated that the shear stress and apparent viscosity increase with the increase of CNTs concentration, penetration and consistency not effect of base grease, and the dropping point increasing about 25%. The microstructure of CNTs and lithium grease was examined by high resolution transmission electron microscope (HRTEM) and transmission electron microscope (TEM).

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Influence Of Nano Grease Composite On Rheological Behaviour
Alaa Mohamed A,B , A. Khattab B, T.A. Osmanb, M. Zaki A
a Production Engineering and Printing Technology Department, Akhbar El Yom Academy, Giza, Egypt.
b Mechanical Design and Production Engineering Department, Cairo University, Giza, Egypt.
Abstract
The aim of this work is to study the rheological behaviors of carbon nanotubes (CNTs) as an additive on lithium
grease at different concentrations. The results indicated that the optimum concentrations of the CNTs was 2 %.
These experimental investigations were evaluated with a HAAKE Rheovisco RV20, Penetrometer and
Measurement of the dropping point. The results indicated that the shear stress and apparent viscosity increase
with the increase of CNTs concentration, penetration and consistency not effect of base grease, and the dropping
point increasing about 25%. The microstructure of CNTs and lithium grease was examined by high resolution
transmission electron microscope (HRTEM) and transmission electron microscope (TEM).
Keywords: Carbon nanotubes, Rheological behavior, Lithium grease, Microstructure.
I. INTRODUCTION
Grease is a solid or semi fluid which would
normally have been employed together with a
thickener, additive and anti-oxidant agent. The fluid
lubricant that performs the actual lubrication can be
petroleum (mineral oil), synthetic oil, or vegetable
oil. The thickener gives grease it characteristic
consistency and is sometimes believed as a “sponge”
that holds the oil in place [1]. The majority of greases
on the market are composed of mineral oil blended
with a soap thickener. Additives enhance the
performance and protect the grease and lubricate
surfaces. The influence of the rheological properties
of CNTs additives is very important for all the grease
lubricating bearings. To characterize a lubricant
comprehensively, the rheological properties at all
working conditions, pressures and temperatures have
to be known [2, 3]. Grease is widely used as a
lubricant in the wheel assembly, journal bearings and
rolling element bearings. Grease is also used in other
areas that need occasional service like the brake or
stopper assembly to help keep these fittings rust free
and make removal of dirt and grime easier. Grease is
applied to machines that can be lubricated
infrequently and where lubricating oil would not stay
in position. It also act as a barrier to prevent entering
of water and the incompressible materials. CNTs
used as a performance enhancing additive in gear
lubricants for extended lifetimes, lower operating
costs, and improved power efficiency. Numerous
laboratory investigations and industrial experience
indicate that using of CNTs has significant
advantages compared to conventional solid lubricants
in both mild and extreme pressure conditions [4-6].
Lubricating grease consistency has been
evaluated for years with cone penetration test ASTM-
D217. The test measures the distance in tenths of a
millimeter to which a standard metal cone will
penetrate into the grease surface under standard
conditions. This single numerical value has been
proven to be inadequate to estimate the real
consistency of lubricating grease under dynamic
conditions. It ignores the non-Newtonian flow
behavior characteristic to grease. In the past few
years, rheology has been introduced as a new method
to better understand and evaluate the real behavior of
lubricating grease. Rheology takes into account the
influence of shear rate, shear stress, temperature and
time. By measuring the viscosity with both rotational
and capillary rheometer, it is possible to see the effect
of shear rate on grease consistency which strongly
influences the lubricating capability of greases under
load [7, 8].
The aim of this work is to evaluate the
rheological behaviors of carbon nanotubes (CNTs) as
an additive on lithium grease at different
concentrations and study the microstructure of
lithium grease.
II. EXPERIMENTAL METHODOLOGY
2.1. Syntheses of Carbon Nanotubes and Lithium
Grease CNTs were synthesized by the electric arc
discharge. The arc is generated between two
electrodes (size φ 6 x 100 mm) using distilled water.
The cathode and the anode are from graphite (99.9%
pure), and was performed under AC current, 75 A
and 238 V.
Grease that was used in this work was
commercially available; the main physical-chemical
properties of the grease are presented in Table 1. The
grease is lithium based and has good heat-resistance,
water resistance and mechanical stability. In order to
study the rheological behavior of carbon nanotubes as
an additives on lithium grease, carbon nanotubes
were added into lithium grease at the different
concentrations (0.5, 1, 2, and 3 wt. %). The carbon
RESEARCH ARTICLE OPEN ACCESS

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nanotube particles were dispersed well in the grease
in an ultrasonic bath.
Table 1: Composition of the tested grease
Base oil
Mineral oil
Soap thickener
Lithium
Penetration (1/10 mm at
25°C)
280
Dropping point
180 °C
Viscosity of base oil at
40°C
150 cSt
2.2. Structural Characterization
The size and morphology of carbon
nanotube were characterized with high resolution
transmission electron microscopy (HRTEM) (JEOL
JEM 2100) with an accelerating voltage of 200 kV.
The grease structure was investigated
transmission electron microscopy (TEM) (JEOL JEM
2100) with an accelerating voltage of 200 kV.
Transmission Electron Microscopy (TEM)
observations were conducted after a classical sample
preparation. A small amount of grease was placed on
a carbon coated sample grid and immersed for
several minutes in hexane to remove its base oil. It
was then dried for 15 minutes in an oven at 30° -
40°C.
2.3. Viscometer (HAAKE Rheovisco RV20)
This experimental investigation employed a
commercial rotational viscometer, HAAKE
Rheovisco RV20. The instrument consists of the base
unit of Rotovisco RV20, the Rheocontroller RC20
which acts as an interface between the computer and
Rotovisco RV20, and the measuring system M5
utilizing a cone and plate configuration. A HAAKE
circulator provides precise temperature control for
the samples. The operation principle of the
instrument is illustrated in Fig 1.
After placing the grease sample in the gap
between the cone and the stationary plate, the cone is
driven to rotate at programmable speeds by a DC
motor with a feed back loop for accurate speed
control. The rotation of the cone leads to a uniform
shear rate in the sample. The resistance of the sample
to flow gives rise to a very small distortion in a
torsion bar, mounted between the motor and the
driven shaft. This distortion is detected by a
transducer. Signals proportional to the speed and the
torque are respectively transmitted to the control unit
for processing. A flow curve plotted as shear stress
vs. shear rate, which indicates the flow characteristics
and is regarded as the rheological ‘fingerprint’ of the
sample, is obtained. With a carefully designed test
scheme, much more information about the sample’s
rheological properties can be collected.
Out of consideration for thixotropy, the test
procedure should include a set holding time with the
aim of degrading the thixotropic structure after
measuring the flow curve from zero to a
predetermined maximum shear rate, and then
measuring a flow curve back to zero shear rate. If a
hysteresis exists between the ascending and
descending curves, the substance can be referred to as
thixotropic and the area between the curves
corresponds to the extent of thixotropy.
In the rheological measurements of grease
with a cone and plate configuration there may be
some anomalous phenomena such as slip at the wall,
fracture and flow disturbance, which make the
experimental results unreliable and should be avoided
as much as possible.
Observations showed that slip at the wall
begins to appear at shear rates lower than about 10 s-1
and becomes greater when the applied shear rate
decreases. For a shear rate greater than 10 s-1, the
contribution of slip at the wall to the total strain
becomes low compared with viscous deformation [9].
A characteristic of a measurement exhibiting slip at
the wall is that the flow curve will shift if measured
with a different sensor system geometry. It has been
demonstrated to be a quick and efficient method for
judging the presence of slip flow to measure the same
sample under constant conditions with different
sensor systems [10].
Fracture occurs systematically when the
shear rate increases [9]. When a free surface forms in
the grease film at the edge of the gap, the effective
radius of sheared grease is reduced; as a result, the
calculated shear stresses are erroneously low. The
magnitude of the reduction in grease radius can be
estimated from the size of the undisturbed annulus
around the periphery of the grease film after
withdrawing the cone from the plate [11]. It seems
that when the angle and radius of the cone is small
(i.e. the gap is narrow), the influence of fracture on
the measurements is tolerable.
The flow disturbance is caused by the
normal stress and inertia of the sample. It is
negligible when the cone angle is small enough; the
shear rate is not very high and the elasticity of the
sample is not very significant [12].
In this investigation, the cone radius was 10
mm; the cone angle was 1°; another cone angle of 5°
was employed for examining the validity of the test
results. In addition, the tested grease was
experimentally verified without significant elastic
effect. The flow disturbance can therefore be
neglected. Furthermore, the test was schemed and
carried out with discretion. All the results were
examined carefully to detect the influence of slip and
fracture. Only those which were apparently not
disturbed are presented here.

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Fig. 1: The principle of HAAKE Rheovisco RV 20
2.4 Penetration and Consistency
The most important feature of grease is its
stiffness or consistency. For oils the viscosity is
measured to assess oil fluidity. For greases the
penetration or the consistency indicates whether the
grease is softer or more solid or stiff. Grease
consistency depends on the type and amount of
thickener used and the viscosity of its base oil.
Grease’s consistency is its resistance to deformation
by an applied force. For use greases the consistency
is measured by a Penetrometer as shown in fig. 2
with a quarter cone. The penetration is used as an
identifier and provides information whether it can be
pumped by a central lubrication system or used for a
certain application [13].
2.4.1 Test Principle
2 g of the grease sample is filled at room
temperature into a standard beaker. The tip of a
standardized double cone touching the surface. Over
a 5 second period how deep the cone penetrates into
the grease is measured. Soft greases will have higher
penetrations than hard greases.
Fig. 2: Penetrometer
2.5 Dropping Point
The dropping point is the temperature at
which the grease passes from a semisolid to a liquid
state under the conditions of the test. The test shows
the end point of a softening process under static
conditions [14].
Dropping point indicates the upper
temperature limit at which grease retains its structure,
not the maximum temperature at which grease may
be used. They are not thinned in a uniform way, they
get softer dependent on the thickener type. For the
determination of the operating temperature of the
grease, the oxidation of the base oil and the
destruction of the thickener but not the dropping
point are more relevant (Fig. 3).
2.5.1 Test Principle
A small sample volume of approximately
0.5 g is filled into a nipple has an associated
thermometer. The test unit is heated until a drop is
formed on the bottom opening of the nipple. The
drop, consisting of a thickener and oil will fall into
the test tube. The temperature, at which the drop
formation starts, is recorded as "dropping point". The
test unit operates up to 300 °C.
Fig. 3: Measurement of the dropping point
III. RESULTS AND DISCUSSION
3.1. Structural Characterization of Carbon
Nanotubes
High resolution transmission electron
microscope (HRTEM) image of CNTs shown in Fig.
4 show the presence of different structures in the
sample and the average size of the nanoparticles is
about 10 nm in diameter and 1-25 µm in length.
Figure 5 shows the SEM image of CNTs
dispersed in lithium hydroxystearate (soap) fiber. It
can be seen that there is no apparent aggregation of
CNTs, indicating that the CNTs could be well
dispersed in lithium grease, and it can be observed
that the microscopic structure of lithium grease

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presents a more regular and homogeneous
network structure, with long fibers, which confirm
the rheological stability.
Fig. 4: HRTEM images of CNTs
Fig. 5: TEM image of grease with (a) base grease
(b) 0.5 % (c) 1 % (d) 2 %
3.2. Rheological Behavior of Carbon Nanotubes as
an Additives on Lithium Grease
Many models are available to describe
rheological properties of lithium grease such as;
Bingham model, Herschel - Bulkley model, Casson
model, Bauer model, Balan model, Papanastasiou
model, Dorier and Tichy model.
The rheological results from the
measurements with the cone and plate rheometer, that
are shown in figures 6 and 7 represent the effect of
carbon nanotube additives on lithium grease with
shear stress and viscosity.
Figures 6 and 7 give the shear stress and
apparent viscosity as a function of shear rate for
lithium grease alone and that containing different
concentrations (0.5, 1, 2, and 3 wt. %) of CNTs. It
can be seen that the shear stress and apparent
viscosity of the lithium grease containing 2 wt. %
CNTs are much higher and more stable than that of
pure lithium grease at all shear rates. At this point,
the shear stress and apparent viscosity could be
increased by 67.3 % and 81.8 %, respectively. The
shear stress of base grease and the grease containing
CNTs become larger with the increase of shear rate
and with the increase of the percentage of carbon
nanotube additives on lithium grease.
The apparent viscosity of base grease and
the grease containing CNTs becomes larger with the

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decrease of shear rate and increases with increasing
the percentage of carbon nanotube additives on
lithium grease. These experiments were carried out
under stationary conditions, to avoid thixotropic
behavior. Therefore, the result indicates that all the
samples show a large shear thinning behaviour. At
low strain rates, the values of apparent viscosity
follow quite well the classification found for the yield
stress.
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
0
20
40
60
80
100
120
140
Shear rate (1/s)
Shear stress (Pa)
base
0.5% CNTs
1% CNTs
2% CNTs
3% CNTs
Fig. 6: Shear stress of the grease samples
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
0
50
100
150
200
250
Shear rate (1/s)
Apparent viscosity (Pa.s)
base
0.5% CNTs
1% CNTs
2% CNTs
3% CNTs
Fig. 7: Apparent viscosity of the grease samples
3.3 Penetration and Consistency
The consistency of the grease characterizes
its ability to be deformed in an application. The
consistency is grouped in NLGl classes from 000 to
6. If the used grease penetration is compared to the
fresh grease, following information can be gathered:
1. The penetration will be higher if there is by
water or other liquid contamination
2. The grease will be softer if it is sheered by
mechanical stress in a bearing. This destroys the
soap structure and shears its long fibered
components.
3. The penetration is lower and the grease gets
harder if it contains less base oil and more
thickener. This may happen if base oil is lost by
bleeding out because of vibrations or if it is
vaporized by high temperature or oxidation.
Penetration and Consistency of CNTs added
into lithium grease is the same of base grease,
because thickener gives grease its characteristic
consistency not additives. Therefore, the results
indicating that the CNTs as an additive not an effect
of base grease.
3.4 Dropping Point
The dropping point only indicates whether
grease is running at a specific operating temperature.
The maximum operating temperature for a grease
should be always far below the dropping point
temperature. The base oil type and the thickener will
determine how far below the dropping point the
operating temperature can be. Usually the dropping
point should be at least 50 °C higher than the
operating temperature.
Dropping point of CNTs added into lithium
grease could be increased 25% at 2 wt. %. Therefore,
the results indicating that the CNTs as an additive are
effective in improving the dropping point of base
grease.

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IV. CONCLUSIONS
According to the above results and
discussion, the conclusions can be summarized as:
1. CNTs were successfully synthesized by electric
arc discharge method. The synthesized CNTs
have an average diameter of 10 nm and could be
well dispersed in lithium grease.
2. A rheological characterization, including
apparent viscosity, shear stress and shear rate
was carried out at different concentrations of
CNTs. The grease response was studied at
constant temperature and time, which led to a
real mechanical spectroscopic investigation.
3. The microstructure of lithium grease at the
different concentrations was confirmed by
scanning electron microscope (SEM). The results
indicated that the microscopic structure of the
lithium grease presents a more regular and closer
network structure with long fibers, which
confirms the rheological stability.
4. CNTs as an additive are effective in improving
the dropping point of base grease about 25%.
5. The optimum percentage of the CNTs in the
grease composites was 2 %.
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