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GRAPHENE AS AN AQUEOUS
LUBRICANT
Nishant Katyal
Image credits: Stefanie Blendis, CNN
Master of Science Thesis MMK 2017:192 MKN 195
KTH Industrial Engineering and Management
Machine Design
SE-100 44 STOCKHOLM
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Examensarbete MMK 2017:192 MKN 195
Grafen som ett vattenhaltigt smörjmedel
Nishant Katyal
Godkänt
2017-11-06
Examinator
Ulf Sellgren
Handledare
Sergei Glavatskih
Patrick Rohlmann
Uppdragsgivare
Institutionen för Maskinkonstruktion,
KTH
Kontaktperson
Sergei Glavatskih
Sammanfattning
Den möjliga användningen av grafen och dess derivat upplöst i vatten som ett grönt
smörjmedel är en intressant forskningsaveny ur ett tribologiskt perspektiv. I denna studie
späddes en högkoncentrerad grafenlösning med av-joniserat (D.I.) vatten för att alstra
mållösningskoncentrationer av greven mellan 15 µg/ml och 350 µg/ml. Den testade
grafenlösningen hade polyetylenglykol som ytaktivt ämne Proven testades sedan för både
glidande och rullande kontakter. De glidande kontakttesten innefattade användandet av både
en 4-kuletribometer och en triborometer. De rullande kontakttesterna utfördes med en Mini-
traktionsmaskin. De testade proverna uppvisade signifikanta friktions- och förslitningsfördel
jämfört med D.I. vatten och ytaktiva lösningar under samma testförhållanden
Nyckelord: Grafen Smörjning, triborometer, 4-kuletribometer, mini-traktionsmaskin,
friktionsminskning, nötningsreduktion
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Examensarbete MMK 2017:192 MKN 195
Graphene as an aqueous Lubricant
Nishant Katyal
Godkänt
2017-11-06
Examinator
Ulf Sellegren
Handledare
Sergei Glavatskih
Patrick Rohlmann
Uppdragsgivare
Department of Machine Design,
KTH
Kontaktperson
Sergei Glavatskih
Abstract
The possible use of graphene and its derivatives dissolved in water as a green lubricant is an
interesting avenue of research from tribological perspective. In this study, a highly
concentrated stock solution of aqueous Graphene employing Polyethylene Glycol(PEG) as
surfactant was diluted using proportionate volumes of De-Ionized (D.I.) water to generate
target concentrations of Graphene in solution ranging from 15 µg/ml to 350 µg/ml . These
samples were then tested for both sliding and rolling contacts. The sliding contact tests
included the use of both 4-ball Tribometer test rig and triborheometer. The rolling contact
tests were performed on Mini Traction Machine. The tested graphene-PEG-water admixtures
held significant friction and wear advantage over D.I. Water and surfactant solutions under
the same testing conditions.
Keywords: Graphene, Lubrication, triborheometer, 4-ball tribometer, Mini Traction Machine,
friction reduction, wear reduction
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FOREWORD
This chapter acknowledges the help, assistance, cooperation and inspiration important for the
thesis provided by various individuals.
It has been an amazing learning experience here at Machine Design department at KTH for
duration of 8 months. During these months I have grown academically and personally due to
support and guidance from many individuals. I would like to take this opportunity to thank all
of them without whom this thesis would not have been possible.
The first person I would like to thank is my supervisor Prof. Sergei Glavatskih who believed
in my potential and granted me the opportunity to perform this thesis.
I would like to thank Patrick Rohlmann my Ph.D supervisor who constantly supported me
and helped me develop as an engineer and researcher. He answered my every question with
extreme professionalism and eagerness.
I would like to thank Tomas Östberg and all the people from 3D prototyping lab who helped
in manufacturing the components.
My friends and colleagues from Machine Design (Gonzalo, Miguel, Rakesh and Ramtin)
who supported me with their knowledge during the thesis.
I would like to thank my corridor friends (special mention Shashank) who were there trying
to give me suggestions on writing the thesis.
Last but not least my parents as they gave me the opportunity to come to Sweden and be a
part of such an amazing institute.
Nishant Katyal
Stockholm
October 2017
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NOMENCLATURE
This chapter contains Notations, Abbreviations, List of Figures and Tables that are used in
this Thesis.
Notations
Symbol Description
Fn Normal force
M Torque
Fl Actual load
Angle
FF Friction force
d Arm
r Sphere radius
µ Coefficient of friction
η Viscoscity
h Lubricant film thickness
v velocity of motion
A Area of contact
v Poisson’s Ratio
P Hertzian pressure
Fl max Maximum force
Vs Sliding velocity
Kv Velocity geometry constant
Kf Friction force geometry constant
KF Actual load geometry constant
A,B,C,D… Alphabets denoting number of experiments
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Abbreviations
PEO Polyethylene oxide
POE Polyoxyethylene
PEG Polyethylene glycol
ASTM American Standard of the International Association for testing Material
AC Alternate Current
EHD Elastohydrodynamic lubrication
SS Stainless Steel
MTM Mini Traction Machine
LVDT Linear Variable Differential Transformer
RTD Resistance Temperature Detector
PTFE Polytetrafluoroethane
AISI American Iron and Steel Institute
SEM Scanning Electron Microscope
G-X Graphene in X concentration in water
G-L Graphene Liquid
GO Graphene Oxide
GO-L Graphene oxide-liquid
GOT-L Graphene oxide Triton X-100-Liquid
ECR Electrical Contact Resistance
AW Anti wear
D.I De-Ionized Water
SRR Slide to Roll Ratio
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Table of Contents
1 INTRODUCTION ....................................................................................................................................... 13
1.1 Background .................................................................................................................................... 13
1.2 Purpose ......................................................................................................................................... 17
1.3 Delimitations .................................................................................................................................. 17
1.4 Methodolgy...................................................................................................................................... 17
2 Literature Survey ......................................................................................................................................... 19
2.1 Materials ....................................................................................................................................... 19
2.1.1 Graphite .................................................................................................................................... 19
2.1.2 Deionized Water ........................................................................................................................ 21
2.1.3 Polyethylene glycol(PEG) .......................................................................................................... 21
2.2 Tribology ....................................................................................................................................... 21
2.2.1 Friction ..................................................................................................................................... 21
2.2.2 Wear ......................................................................................................................................... 22
2.2.3 Lubrication ............................................................................................................................... 24
2.3 Tribometer ..................................................................................................................................... 25
2.3.1 Sliding contact .......................................................................................................................... 26
2.3.2 Rolling contact .......................................................................................................................... 29
2.4 Measuring microscopes .................................................................................................................. 32
2.4.1 Optical Microscope ....................................................................................................................... 32
2.4.2 Scanning Electron Microscope .................................................................................................... 33
3 EXPERIMENTAL SET-UP AND MATERIALS ......................................................................................... 35
3.1 Sliding contacts .............................................................................................................................. 35
3.1.1 Rheometer ................................................................................................................................. 39
3.1.2 4 ball test rig ................................................................................................................................40
3.2 Tribometer redesign process .......................................................................................................... 41
3.2.1 Concept evaluation and selection .............................................................................................. 45
3.2.2 Analysis of Circular Flexure ..................................................................................................... 54
3.2.3 Final Concept and drawings ..................................................................................................... 57
3.3 Rolling Contact ..............................................................................................................................60
4 RESULTS ................................................................................................................................................... 63
4.1 Friction .......................................................................................................................................... 63
4.1.2 Sliding contact .......................................................................................................................... 63
4.1.2 Rolling Contact ......................................................................................................................... 73
4.2 Wear .............................................................................................................................................. 77
4.2.1 Tribo-rheometer ........................................................................................................................ 77
4.2.2 4 ball tests(tribometer)-old design .............................................................................................. 78
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4.2.3 4 ball test( tribometer)-new design ............................................................................................. 79
5 DISCUSSIONS AND CONCLUSIONS ....................................................................................................... 83
5.1 Discussions ........................................................................................................................................... 83
5.1.1 Friction ...................................................................................................................................... 83
5.1.2 Wear .............................................................................................................................................. 84
5.2 Conclusions........................................................................................................................................... 84
6 FUTURE WORK ......................................................................................................................................... 86
7 REFERENCES ............................................................................................................................................ 87
APPENDIX 1:SUPPLEMENTARY INFORMATION ....................................................................................90
APPENDIX 2:SUPPLEMENTARY INFORMATION .................................................................................... 91
APPENDIX 3:SUPPLEMENTARY INFORMATION .................................................................................... 94
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1 INTRODUCTION
This chapter described the background, the purpose, the limitations and the method(s) used in
the thesis.
1.1 Background
Majority of man-made, biological or mechanical systems generate friction where there is
sliding, rolling or rotating contact interfaces. Energy is consumed in order to overcome the
friction between the contacts. The friction generated at the rubbing interfaces is required to
be controlled effectively, but if that is not the case, more often than not it leads to high friction
and in turn leads to high wear losses, thus reducing the life and the components reliability[1].
The use of lubricants is an efficient and effective way to downregulate the friction at the
interfaces and efficiently save the energy[2].The demand of high efficient coolants and
lubricants has increased because of the growing energy demands, precision manufacturing,
miniaturization, nuclear regulations and critical economies. The liquid lubricants that are
currently being used are commonly based on the by-products of the distillation of non-
renewable petroleum such as mineral oils. The major concern of using these lubricants is the
effect of oils on the environment[1,2].
One of the major concerns in machine such as those used for manufacturing operations is the
generation of large amount of heat which has to be cooled down in order to prevent it from
damaging the machine. For this purpose water based lubricants serve as a perfect
candidate[3]. Water offers many advantages such as good environmental compatibility, high
fluidity and superb thermal conductivity[4]. Furthermore, water based lubricants have the
advantage of being easily available and inexpensive. However, water itself is a poor lubricant
hence, extra additives such as nanoparticle suspensions are necessary for enhancing its
lubricant properties[5-10].Nanolubricants and nanofluids are the two different groups of
lubricants identified by researchers,[2] respectively referring to oil based nanoparticle
suspensions and nanoparticle based aqueous coolants.
Nanoparticles is common for the two groups of lubricants which carry a number of
advantages such as[2]
1) Nanoparticle provide better stability while suspended in base aqueous/ oil as compared
to macro or micro particles.
2) They enter the contact area easily and also provide protective coating against wear.
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3) They do not require an induction period to obtain the desired tribological properties as
they are often efficient at ambient temperature.
4) They possess better thermos-physical properties than the microparticles.
Graphene and its derivatives such as graphene oxide are the most sought after nanoparticle
additives among researchers[5-10]. Two dimensional graphene is a derivative of graphite
which is a 3-D structure. The extremely thin structure of Graphene gives it an edge to easily
form a tribo-film between the contacts. However, graphene is a hydrophobic structure
prompting researchers to prepare its hydrophilic derivatives like graphene oxide as an additive
in water to enhance its lubricating properties .In 2016, H.J-Kim et al. compared frictional
behavior between silicon and steel contacts coated with graphene oxide (GO) in dry sliding
and water lubrication conditions. The GO coated stainless steel ball was allowed to slide
against silicon specimen under dry and water lubrication conditions. The friction coefficient
observed in the sliding test performed under water lubricated conditions could be reduced
12 times in comparison to dry lubrication conditions [5]. It was also observed that the friction
coefficient between the uncoated stainless steel(SS) ball and the silicon(Si) specimen in both
water and dry lubrication condition was higher than the coated ball in water lubrication
condition. The experiment also revealed that the wear of the stainless steel ball slid against the
Si specimen under water lubrication was significantly lower than that of the uncoated SS ball
under the same condition. In 2016, Bhavna Gupta et al. used graphene oxide chemically
linked with PEG molecules through hydrogen bonding to test the effect of the lubricant on the
steel-steel contacts. The authors studied the effect of relative change in concentration of
graphene oxide on the effectiveness of lubricant to form tribofilm. The paper summarizes that
with the increase in the concentration of graphene oxide particles, there was a noticeable
reduction in both friction and wear between the contacts . The authors described that a
specific concentration of graphene oxide(GO) and above resulted in noticeable reduction in
friction between the contacts. They suggested that the concentrations tested below this did
not have sufficient quantity of graphene oxide hence a comparable increase in friction
coefficient was noticed and the residual lubrication effect was due to base liquid in these
cases. Additionally, at high concentration GO agglomerates and the whole region becomes
heavily loaded by graphene flakes which increases the friction and wear in the contacts[6].
Recently, researchers have started using graphene which is hydrophobic in nature[8]. In order
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to make the graphene hydrophilic in nature a surfactant such as the Triton X-100 is used
[8].The surfactant helps in the exfoliation and stabilization of graphene in water and helps in
the formation of graphene protective film in the tribological tests. The paper also describes
that the surfactant eases the deposition procedure of the graphene layer by adjusting the
dispersion wettability of the sliding surfaces, thus leading to the superior tribological
properties. Solutions of graphene-liquid(G-L), graphene oxide-liquid(GO-L) and graphene
oxide-Triton X-100(GOT-L) liquid were taken and compared with deionized water for anti-
friction properties. They concluded that the G-L resulted in the lowest friction and wear
compared to GO-L and graphene oxide-Triton X-100(GOT-L) liquid. The research also
concluded that with the increase in concentration of graphene in the solution until a specific
concentration there was an ample reduction in both friction and wear between the contacts.
Table in appendix1 is presented to compare the research performed for the graphene and it’s
derivatives in order to describe the equipment and the parameters used by the researchers.
From table it can be clearly concluded that with the increase in the concentration of graphene
oxide or graphene the friction and the wear between the contact reduces significantly.
Tasks for the thesis
The first task is to evaluate the effect of different concentrations of graphene in water in the
presence of surfactant on friction and wear in steel on steel contact.
The second task is to check the repeatability of experiments performed in reference 31.
The third task is to redesign parts of the 4-ball tribometer in order to correct the misalignment
problem caused in the journal bearing to improve the test results.
The tribo-testing of all mixtures , DI water and the water-surfactant solution were performed
on three machines namely tribo-rheometer, 4 ball tribometer test rig and mini traction
machine.
Previous experiments performed
The experiments performed in the current research work are a continuation of the work done
by experimenters in the lab [31]. They studied the anti-friction and anti- wear properties of an
inhouse aqueous graphene preparation on the 4- ball tribometer test rig. Highly concentrated
aqueous graphene stock solution employing polyethylene glycol(PEG) as surfactant was
diluted using proportionate volumes of De-Ionized water(D.I. water) to generate target
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solution concentration ranging from 15μg/ml to 150μg/ml of graphene. The diluted graphene
solutions were then tested on the tribometer test rig using AISI52100 Chrome Steel balls of
half inch diameter under 1.25 GPa mean Hertzian contact pressure(generated from the
application of 61.5N axial load) and at 0.075 m/s sliding speed for a sliding distance of 100
m. The results of their experiments are as shown in Fig 1.
The following conclusions can be drawn from the results shown in Fig 1[31].
1. Aqueous graphene solutions hold significant advantage over D.I. water and solutions
with only polyethylene glycol surfactant in both friction and wear characteristics.
2. With the increase in the graphene concentration in the solution the friction coefficient
decreases although the decrease becomes fairly constant at higher concentration
values.
3. The wear characteristics ‘Wear volume’ and ‘Wear coefficient’ are significantly
reduced with higher concentrations of graphene in the solution.
4. Higher graphene concentrations reduces the area of heat affected zones observable and
improve graphene deposition in addition to the measurable improvements in friction
and wear parameters.
Figure1: Results from work done previously[31]
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5. The tested performance of the prepared samples was better than recorded in related
referred studies tested under similar conditions.
1.2 Purpose
The purpose of the project is to vary the different concentrations of graphene in a solution of
water and surfactant and to contemplate the effect of variation of graphene concentration on
the friction and wear in the contact. The goal of the project is to obtain the optimal
concentration of the graphene in the solution of water and surfactant that will provide the
lowest friction and wear for both sliding and rolling conditions.
1.3 Delimitations
The friction results for tribo-rheometer are considered from 0-100m because there are
statistically more experiments conducted for sliding distance of 0-100m compared to 100-
200m. The lubricant in the solution is not able to cover the whole three ball setup
continuously because it goes through the spaces in between the center of the three balls and
settle down thus not being able to cover the contact easily which creates more wear than will
be obtained. The complex physico-chemical interactions between graphene, water and the
surfactant might be a challenge as the graphene might not mix properly as it depends on the
concentration of surfactant.
1.4 Methodology
The thesis started with a pre study in which the information related to the new advancements
of graphite and its derived products were gathered. The study included reading articles related
to the effect of the concentration of graphene or its derivatives on the friction and wear
between the contacts.
Then, based on the literature review and discussions with the supervisor the test plan was
drafted. Then through the test series and the designed test matrix the range of the test
parameters, such as ambient temperature, contact pressure, sliding speed etc. was determined.
Random ordered tests were designed for eliminating the systematic error in experiments.
Different concentration of graphene solutions were obtained by mixing the graphene in the
solution of water and surfactant. The graphene concentrations prepared and used in the tests
were 15 µg/mL, 25 µg/mL, 50 µg/mL, 100 µg/m,150 µg/mL and 350 µg/mL.
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Initially, experiments were performed for the sliding conditions on a 4-ball tribometer in
order to propose the relative effect of changing the concentration of graphene on the friction
and wear between the contacts. Subsequently, the tests were performed for the rolling
conditions on the mini-traction machine and the effect of the test graphene samples on friction
and wear between the contact was observed.
The results gathered from the experiments performed for friction and wear in both sliding
and rolling contact conditions were statistically analyzed using MATLAB and Microsoft
excel. The results were compared with the data gathered from the literature survey.
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2 Literature Survey
The literature survey is a summary of the existing knowledge and former performed research
on the subject. This chapter presents the theoretical literature survey that is necessary for the
performed research, design or product development.
2.1 Materials
The solution used in the thesis is comprised of graphene mixed with surfactant in water.
2.1.1 Graphite
Graphite is the most stable form of carbon under standard conditions. Graphite has a layered
planar structure. The compounds derived from graphite are explained in the subsequent
sections.
2.1.1.1 Graphene
Graphene is a two dimensional material derived from graphite which offers unique friction
and wear properties that is not typically seen in conventional materials Fig 2. Barring its well
established thermal, electrical, optical and mechanical properties ,graphene can serve as solid
lubricant. It’s high shear strength , high chemical inertness and easy shear capability on its
densely packed and atomically smooth surface makes it favorable for tribological
applications. Graphene can be applied to nanoscale and microscale systems with oscillating,
rotating and sliding contacts to reduce stiction, friction and wear because it is ultrathin even
with multilayers. The extreme mechanical strength of graphene suppresses material wear[11].
Lee et al[12] also proved in 2013 that grain boundaries do not affect the overall strength of
the graphene even though the introduction of other kinds of defects(such as oxidation) largely
influence the mechanical properties of graphene and decrease its strength and stiffness.
Graphene is impermeable to liquids and gases[13] such as water or oxygen thus slowing down
the corrosive and oxidative processes that usually cause more damage to rubbing surfaces.
Singh et al. demonstrated that liquid water minimizes friction on graphene. The authors
demonstrated the surface underneath the graphene layer affects the wetting angle however, the
effect of the substrate is modified by the number of layers and is negligible for multiple
layers of graphene. Graphene is a material which is atomically smooth with low surface
energy and is able to replace the thin solid films usually used to reduce adhesion and friction
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of various surfaces. The properties described above consolidate that graphene is useful for
tribological applications to achieve low friction and wear regimes.
Various production methods are available for graphene. The properties of the synthesized
graphene may vary a great deal; in particular the shape, grain size and the density of defect’s
change depends on the type and quality of graphene. Graphene is produced by the mechanical
exfoliation methods or the so called ‘scotch tape’ method[14] where commercially available
adhesive tape is used to peel multiple layers from a highly ordered pyrolytic graphite and to
transfer them by pressing the tape onto the desired substrate. The number of methods for
graphene production have significantly increased in recent years. Some of these include dry
mechanical[15] or chemical exfoliation[16]. In the present work graphene is used as an
additive in water to enhance its properties.
Figure 2: Graphene structure[Characterization of graphene by Raman spectroscopy(CNX.org)]
2.1.1.2 Graphene oxide
Oxidation of graphite using strong oxidizing agents, is performed by introducing oxygenated
functionalities in the graphite structure which not only expand the layer separation, but also
makes the material hydrophilic. The graphite oxide is exfoliated in water using sonication due
to hydrophilicity ultimately producing single or few layers of graphene derivative known as
graphene oxide. Hence, the main difference between graphene oxide and graphite oxide is the
number of layers. Graphite oxide is a multilayer system in comparison to graphene oxide
which is a dispersion of few layered flakes and monolayer flakes [17].
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2.1.2 Deionized Water
There has been a great interest in the water lubrication phenomenon over the recent years with
expectation that it may provide a solution to the environmental issues by replacing the use of
oil lubricants in various sliding applications[3]. Water is highly desirable green lubricant
which can result in huge savings in terms of conservation of natural resources, reduction of
transportation & disposal costs of oil and performance maximization of mechanical systems
in clean environments where oil lubricants cannot be used[3]. However, the water in itself is a
poor lubricant due to its low viscosity which leads to formation of thin films between the 2
bodies[3].
2.1.3 Polyethylene glycol(PEG)
It is a polyether compound with many applications ranging from industrial manufacturing to
medicine. Polyethylene oxide(PEO) or polyoxyethylene(POE) are other names of PEG
depending on its molecular weight. The molecular formula of PEG is H(OCH2CH2)nOH.
The PEG interacts with graphene to form a stable solution in water[18].
Polyethylene glycol acts as a surfactant for the graphene in water mixture. It binds the
graphene to water since, graphene cannot dissolve in water individually thus forming a stable
solution.
2.2 Tribology
Tribology is the study and application of the concepts of friction, wear and lubrication.
2.2.1 Friction
The contact between two solid bodies is affected by its operating conditions, material
parameters, environmental conditions and lubrication. In tribological contact the real contact
area is smaller than the apparent contact area. The contact initially occurs only in few contact
spots and then increases depending on operating and material parameters. The sum of the
areas of all individual contact spots describes the real contact area. The geometry of these
contacts varies depending on the application on a macroscale. There is a non-conformal
contact in the case of rolling between gear and worm teeth. However, there is also a
conformal contact between gear and worm teeth in case of sliding. This contributes to
complex contact geometry. The load applied during relative motion causes high pressure in
the contact spots. This results in plastic deformation of the softer material, causing an increase
in the real contact area until an equilibrium state between load/area is achieved. Dynamic
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friction occurs due to the relative motion between the surfaces. Adhesive component and
plough component constitute the friction. The shear resistance in the real contact spot gives
rise to the adhesive component and resistance to the surface peaks ploughing in the counter
surface contributes to the ploughing component. The coefficient of friction is defined by the
ratio between the friction force FF and normal force FN according to the formula
F
N
F
F
There is a microscopic displacement in the real contact area due to elastic deformation which
is caused by the presence of FF. Hence, the force is expressed as product of shear stress and
the real contact area, FF A which originates from the inter-atomic forces within the
contact spot. Viscous friction force occurs in the form of shear stress within the lubricant and
is determined by lubricant viscosity ƞ, lubricant film thickness h and velocity of the motion v,
poisson’s ratio v and Area of contact A. The friction force with a constant velocity gradient is
defined as
F
vAF
h
2.2.2 Wear
Wear is a phenomenon where there is surface damage including associated loss of
material[20]. The wear can occur in many ways depending on counter surfaces and interaction
in between the contacts. Wear rate also increases in different wear ranges depending on time.
Uneven contact situations causes an increase in local pressure and contact area, which causes
overheating and volume expansion, allowing the friction to increase to severe stage. Wear is
generally related to erosion or sideways displacement of material from its derivative and
original position on the solid surface performed by the action of another surface. The plastic
displacement of surface and the detachment of particles that form wear debris causes wear of
metals to occur. The generated particle size varies from millimeter range down to an ion
range[25].
The different types of wear are described below:
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2.2.2.1 Adhesive wear
When there is sliding contact between two metallic materials, adhesive contact situation
occurs which contributes to an atomic contact. These sliding boundaries are exposed to shear
stress due to sliding motions which causes plastic deformation of surfaces. Shear fracture
between the boundaries has to occur to continue a sliding motion[21]. It is often associated
with high shear deformation of surfaces and severe wear can arise quickly; a mechanism
known as adhesive wear. The original properties of materials at surface e.g. surface
hardening may change with time[21]. During frictional contact adhesive wear can be found
between the surfaces and generally refers to unwanted displacement and attachment of wear
debris and material compounds from one surface to another. Two separate mechanisms
operate between the surfaces.
2.2.2.2 Abrasive wear
Wear occurs when a hard rough surface slides across a softer surface. ASTM international
defines it as the loss of material due to hard particles or hard protuberances that are forced
against and along a solid surface[20].
Abrasive wear is commonly classified according to the type of contact and contact
environment. The type of contact determines the mode of abrasive wear[26]. The two modes
of abrasive wear are defined as two body and three body abrasive wear. The two body
abrasive wear occurs when the grit or the hard body removes the material from the softer
surface. This wear can be related to the analogy of material being removed or displaced by
cutting or plowing operation. Three body abrasion occurs when a harder wear particle
scratches a softer surface. These wear particles are formed during two body abrasion or
adhesive wear.
2.2.2.3 Surface fatigue
It is a process in which the cyclic loading weakens the surface of the material.[20]. The cyclic
track growth of micro cracks on the surface causes the wear particles to get detached which
produce this wear. These microcracks are either superficial cracks or subsurface cracks.
2.2.2.4 Fretting wear
It is the repeated cyclic rubbing between the two surfaces. In this phenomenon the material is
removed from one or both surfaces in contact over a period of time. It occurs typically in
bearings however, most bearings have their surfaces hardened to resist the problem.[20].
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2.2.3 Lubrication
Lubrication is the process or technique employed to reduce friction between surfaces in
proximity and wear of one or both surfaces moving relative to each other, by interposing a
substance called a lubricant in between them. The different categories of lubricants available
are solid, semi solid and fluid lubricants. The following subsection explains the stribeck curve
and different regimes of lubrication
2.2.3.1 Stribeck Curve
The Stribeck curve is an overall view of friction variation in the entire range of lubrication,
including the hydrodynamic, mixed, and boundary lubrication[19].
By means of the Stribeck curve the transitions from boundary lubrication to mixed lubrication
and the transitions from mixed lubrication to elasto-hydrodynamic lubrication can be
predicted and subsequently the lubrication regime in which a particular contact operates can
be determined[20]. Typical Stribeck curve is depicted in Figure 3. It represents the different
regimes of lubrication on friction coefficient Vs lubrication parameter plot.
2.2.3.2 Boundary Lubrication
Boundary lubrication regime originates when there is insufficient pressure in the film to
separate counter surfaces. These counter surfaces are partly in contact and may resist high
loads.The surfaces are covered by boundary films of additives and inhibit the contact between
them, contributing to a decrease in friction. The presence of active surface molecules is
Figure 3: Stribeckcurve[image credit Tribological properties of ionic
liquid( DOI: 10.5772/52595]
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important to maintain the boundary film of additives. Interactions between additives and solid
surfaces contribute to the absorption of molecules to the surface via physisorption,
chemisorption or surface reaction[20].
2.2.3.3 Mixed Lubrication
The mixed lubrication regime is situated between the full hydrodynamic lubrication and
boundary lubrication. It is a transition regime from boundary lubrication to full film
lubrication where the counter surfaces are separated compared to boundary lubrication
regime. The load carrying capacity depends partly on the pressure in the lubricant film and
partly on the counter surfaces in contact. This leads to friction dependence on shear within the
lubricant and between the lubricant and the boundary films, respectively[21]. In this operating
state the surface roughness significantly affects the performance of the contact. It may occur
with the conformal contact lubrication such as the journal bearing lubrication[22]. The
generated lubricant film is not enough to separate the bodies completely but hydrodynamic
effects are considerable[23].
2.2.3.4 Hydrodynamic lubrication:
Within the full film lubrication regime the pressure in the film is sufficiently high to separate
the surfaces. Elastohydrodynamic(EHD) lubrication is a well-known mechanism in this
regime, where elastic deformation of the surfaces have a significant impact. The initial contact
load, between non-conformal contacts, provides an elastic deformation, which increases the
contact area and further EHD lubrication. The lubricant must form a film, thick enough, to
separate the surfaces[21].It is the lubrication regime where the contact surfaces motion and
the exact design of the bearing is used to pump lubricant around the bearing to maintain the
lubricant film. This design of bearing may wear when started, stopped or reversed as the
lubricant film breaks down. Reynolds equation forms the basis of the hydrodynamic theory of
lubrication[24].
2.3 Tribometer
The tribometers are classified based on the sliding and rolling contacts. Tribometer measures
traction coefficient and coefficient of friction. Traction can be defined as the friction between
a rolling body( in this case a ball) and the surface it moves upon. It is the amount of force a
ball can apply to a surface before it slips. Whereas, friction is the force resisting the relative
26
motion of two surfaces in contact. The following sections describe the tribometers classified
on sliding and rolling contacts.
2.3.1 Sliding contact
The sliding contact is a special type of contact which allows displacement tangential to the
contact surface but no relative movement along the normal direction.
2.3.1.1 4 ball tribometer
The rig comprises of three bottom balls and one top ball making a stable and repeatable
contact thus, allowing test results to be repeatable. Four ball test rig is used to determine the
wear preventive properties, extreme pressure properties and friction behavior of lubricants.
The test rig is an excellent choice to benchmark products because of its wide acceptance. The
advantage of using four ball test rig is its ability to produce quick repeatable results at
substantially low cost.
Four ball machines are used to perform weld point tests for lubricating grease or oil under
specific axial loads and speeds. The load carrying properties of a lubricant at high load are
also determined by using the rig. It can be used to perform both wear preventive and extreme
pressure analysis for measuring the wear and friction characteristic. In the case of wear,
preventive and extreme pressure lubricants the ability to resist scuffing is a major concern.
Figure 4 shows the structure of the rig. The load is applied on the three balls through the
fourth ball by hooking the weights on the leverage. The upper ball is fastened at the end of the
shaft and rotationally driven by an AC motor. The speed of the engine is controlled by tuning
the frequency inventor. The torque is transferred between the rotating and stationary balls.
The test load, duration temperature and rotational speed are set according to the experiments
required to be performed. The tests performed on the four ball tribometer are all for the
sliding conditions. In the case of wear primitive tests also called the anti-wear tests (AW) the
average scar diameter in the bottom three balls is reported. The ability of the lubricant to
prevent wear depends on the size of scar. A smaller diameter points out to a superior wear
preventive property while a larger diameter indicates poor wear preventive property.
The test rig used in this thesis to perform the experiments is as shown in the Figure 4[28].By
hooking the weights on the leverage the load can be applied to the four ball sample. The upper
ball of the sample is fastened in the end of the shaft and rotationally driven by an AC motor.
27
The frequency inventor is tuned which is used to control the speed of the engine. The
coupling starts clicking when welding occur thus protects the engine.
Figure 4: Test Rig 1=AC motor 2=coupling, 3=shaft, 4=four ball sample, 5=platform, 6=frame[20]
There is another function served by the test rig which is to perform bearing test. The platform
was designed to be vertically rotatable as shown in the Figure 5. There is a face contact to the
upper labyrinth sealing which rides on a thrust ball bearing[28]. For friction measurement a
pair of panel mounted force cells is used. While conducting bearing tests one needs to replace
the four ball sample with a bearing housing. The rotational motion is transferred to the upper
face of the bearing sample by using an adapter. The angular misalignment caused by the
manufacturing precision is compensated by the sphere to cone contact. The friction torque
transmitted from the bearing, through the platform to the sensors will be detected by the pair
of a panel mounted force cells. This arrangement allows the friction torque to be continuously
measured as long as the sensors are not overheated.
28
Figure 5: 1 = shaft, 2 = four-ball sample, 3 = platform, 4 = support thrust ball bearing, 5 = force sensor[20]
2.3.1.2 Rheometer and 4 ball test setup in rheometer
Rheometer is a laboratory device used to measure the way in which the liquid, suspension or
slurry flows in response to applied forces. It is used for those fluids which cannot be defined
by a single value of viscosity and requires more parameters to be set and measured in case of
a viscometer.
Rheometer can also be used as a setup to perform four ball tests. The setup can be explained
from the image shown in the Figure 6
4 ball test geometry
The tests geometry consists of ball as the upper substrate and three balls arranged in a circle
as the lower substrate. The 3 lower spheres are held in place using a supporting frame. An
important application of this geometry is for testing the lubricity of asphalt. The spheres have
a diameter of 1/2” (1.27 mm). The upper ball provides a point contact with the 3 lower balls. 4
ball tribo-rheometry geometry is designed for lubrication testing. Because of the point contact
between the substrates, the contact surface depends on the modulus of the spheres; as a result,
the stresses are difficult to compute. Therefore only friction force and load are calculated for
this geometry. However, the stress and normal stress geometry constants can be manually
edited to calculate friction stress and normal stress if desired
29
Figure 6: Ball on three ball test geometry[ image credit TA instruments:Trios V3]
2.3.2 Rolling contact
Any contact between rotating bodies such that the relative velocity of two contacting surfaces
is zero at the point of contact.
2.3.2.1 Mini Traction Machine
The Mini Traction Machine combines the mechanical and electrical components in a compact
package. In the standard configuration the test specimens are 19.05 mm(3/4 inch) steel ball
and a 46 mm disc. The ball is loaded on the disc and the ball and the disc are rotated
independently to create a mixed rolling/sliding contact. Force transducer measures the
frictional force between the ball and the disc. The applied load, the lubricant temperature and
(optionally) the electrical contact resistance(ECR) between the specimens and the relative
wear between them is measured by additional sensors.[27]
The Mini Traction machine contains built in electronics to control the mechanical unit and
send signals to and from the PC through the cable connection. The main functions of the
electronic unit on the instrument are as follows[27]:
1. Provide Power for the ball, disc and the stepper motors.
2. Provide power for the heaters.
3. Process the signals from the force transducer and load feedback.
4. Process signals from LVDT.
5. Process the signals from the temperature RTD’s.
30
6. Provide the bias voltage for the electrical contact resistance(ECR) measurement(if
fitted) .
A schematic diagram of the mechanical unit is shown in Figure7. The main parts described in
the Figure 7 are as follows:
Lubricant Pot : It is a single piece stainless steel machined block that contains an
internal reservoir to accommodate the test lubricant. Two electrical cartridge heaters
which are located in the channels heat the pot. Cooling galleries rapidly cool the pot
between the tests. The lubricant pot lid is also made out of stainless steel. Pot and the
lid are both shrouded in the white PTFE thermal insulating jacket which must always
be kept in place before the testing commences. Both pot and lid are shrouded in a
white PTFE thermal insulating jacket which must always be in place before testing
commences.[27]
Standard MTM disc: It is a disposable, highly polished steel disc(AISI 52100) and
during the test the ball is loaded onto its upper polished surface. The disc is attached to
the vertical driveshaft in the lubricant pot. Discs of different material and sizes are
available which are attached to the vertical driveshaft in the lubricant pot.[27]
Test ball: It is a super finished AISI 52100 (535A99) ball. An important role for the
repeatability of test results is played by the highly polished surface of the test results
and in reducing disc wear during each test. Each ball should be used only once. The
drilled hole allows the ball to be attached to the ball drive stub shaft for the traction
measurements .Balls of different materials are available. [27]
Two DC servo motors drive the disc and the ball specimens independently to allow
high precision speed control particularly at low slide/roll ratios. The motors are
controlled by PC and follow a user defined program. [27]
The vertical shaft and drive system which supports the 46mm diameter disc specimen
is fixed. However, the gimbal arrangement which can be rotated around two
orthogonal axis is supported by the 19.05 mm diameter ball specimen which is
supported by the shaft and the drive system. One axis is normal to the load direction
and the other axis is normal to the traction force direction. Application of the load and
restraint of the traction force is made through high stiffness force tranducers. These
are appropriately mounted in the gimbal arrangement to minimize the overall support
system deflections. The output from force transducers is monitored directly by the PC.
31
The stepper motor controlled from the PC applies the load and the load
feedback/check is sent from a strain gauge system mounted on the load arm. [27]
(a)
(b)
Lube temperature sensor
Pot temperature sensor
Mini pot Small disc
½ ” ball
Liquid
sample
Figure7: Mini Traction Machine[19] a) Cross section view b) Overall machine 2D drawing
32
2.4 Measuring microscopes
A microscope is an instrument used to see objects that are too small to be seen by the naked
eye.
2.4.1 Optical Microscope
Non-contact measurements are of a specimen’s X-Y axis or any planar dimension in the
microscope field is obtained by measuring microscope. Measuring microscopes has either
binocular or trinocular options. Optional digimatic indicators for X or Y, Z and an optional
mounting bracket, and mount adaptors are also available from Microscope World.
The wear tests in this thesis is performed on NIKON MM-60 measuring microscope which is
shown in the Figure8.
Figure 8: NIKON MM-60 [image credit NIKON]
33
2.4.2 Scanning Electron Microscope
It is a type of electron microscope in which a focused beam of electrons scans the surface to
produce the images of a sample. The electrons interact with atoms in the sample, producing
various signals that contain information about the sample's surface topography and
composition. The beam is scanned in a pattern called the raster scan pattern and the beam’s
position is combined with the detected signal to produce an image. SEM can achieve a
resolution better than 1nm. Specimens can be observed in high vacuum in conventional SEM
or in low vacuum or wet conditions in variable pressure or environmental SEM and at a wide
range of cryogenic or elevated temperatures with specialized instruments[29]. Detection of
secondary electrons emitted by atoms excited by the electron beam is the most common SEM
mode of detection. The number of secondary electrons that can be detected depends, among
other things, on surface topography. By collecting the secondary electrons that are emitted
using a special detector and scanning the sample, an image displaying the topography of the
surface is created. The signals that derive from electron-sample interactions reveal
information about the sample including external morphology (texture), chemical composition,
and crystalline structure and orientation of materials making up the sample. In most
applications, data are collected over a selected area of the surface of the sample, and a 2-
dimensional image is generated that displays spatial variations in these properties. Areas
ranging from approximately 1 cm to 5 microns in width can be imaged in a scanning mode
using conventional SEM techniques (magnification ranging from 20X to approximately
30,000X, spatial resolution of 50 to 100 nm). The SEM is also capable of performing analyses
of selected point locations on the sample; this approach is especially useful in qualitatively or
semi-quantitatively determining chemical compositions, crystalline structure, and crystal
orientations[30].
34
Figure 9: Scanning electron Microscope[ image credit Susan Swapp, University of Wyoming]
35
3 EXPERIMENTAL SET-UP
In this chapter the working process is described. A structured set up is often called a
methodology and its purpose is to help the researcher/developer/designer to reach the goals
for the project.
3.1 Sliding contacts
Before explaining about the experimentation on rheometer it is necessary to describe how the
parameters which are required to perform the 4 ball tests are obtained. This is explained in the
subsequent section.
4-ball geometry
The arrangement of 4 balls in 4 ball tribometer is shown in Figure 10.
Figure10: Ball on three balls arrangement
The parameters for the four ball arrangement depicted in the above figure are described in
detail in this section.
36
Figure 11: a) Triangle formed between three balls b) Triangle formed between upper ball and three balls c)
Triangle formed from the center of the three base balls d) triangle formed from the center of the fourth ball and
three base balls
From Figure 11 it can be seen that IBC is the Equilateral prism connecting the center points of
all the balls where IJ is the direction of the normal force acting from the fourth ball. ABC is
a triangle formed by connecting the center point of the three connecting balls having 2r as the
length of each side. The ABC is the triangle formed inside the base circle which is the circle
passing through the three center points of the balls kept in the same plane. EFG is the triangle
formed inside the contact circle which passes through the points K,F,G. Considering the base
circle triangle ABC. Length BD is
cos(30)
rX (1.1)
37
Triangle IBD and IFH are similar triangles from AAA similarity. Hence, from the formula for
similar triangles
2
cos(30)
2cos(30)
r FH
rr
rFH
(1.2)
Contact angle β is
1
sin( )
sin
FH FH
IF r
FH
r
(1.3)
Hence,
o 35.264
Now, all the formulae used according to the contact angle are described as shown below:
From triangle IFH
Actual Load FL:
F ( )
with
35.2644 1/cos( ) 1.2247
F 1.2247F
NL
L N
F
Cos
(1.4)
38
Friction Force FF:
F
F
F sin( )
with
35.2644 1/sin( ) 1.8165
1.8165F
M
r
M
r
(1.5)
Sliding Speed vs:
v sin( )
with
35.2644 sin( ) 0.57735
v 0.57735
s
s
r
r
(1.6)
Friction Coefficient µ:
cos( ) 1.4832
sin( ) N N
M M
r F rF
(1.7)
Dimensions
Sphere radius: r=0.00635m
arm d: 0.007398 =35.2644°
Geometry constants
sin( ) 0.00732
1/ sin( ) 136.382
1/ cos( ) 1.22474
v
f
F
K r
K r
K
(1.8)
39
3.1.1 Rheometer
Rheometer as explained in the frame of reference section is a device that measures the way a
liquid, suspension or slurry flows in response to the applied forces. However, the rheometer
can also act as a 4 ball tribometer as explained in the frame of references section. The
rheometer was used to conduct the tests because it is more precise than the 4 ball tribometer
test rig.
The aim of the experiment is to find the optimal concentration of graphene in the aqueous
solution of graphene containing PEG as surfactant by performing sliding tests on rheometer.
The procedure for the experiments performed is detailed below
1. Already prepared aqueous graphene solutions having graphene concentration of 25
μg/ml, 50 μg/ml, 100 μg/ml and 150 μg/ml and 350 μg/ml along with water and
solution of DI water and surfactant were used as testing lubricants.
2. 30 AISI52100 chrome steel balls of half inch diameter were cleaned in acetone for an
hour by changing the cleaning agent after 30 minutes.
3. Three balls were placed in the cup holder and the fourth ball was attached to the upper
shaft by means of the short single ball upper geometry holder.
4. The final setup looks like the figure shown in section 2.3.3.1 Figure 6.
5. Then, the experiments were conducted with an axial load of 45N at 0.075m/sec sliding
speed for a sliding distance of both 100m and 200m.
6. The wear scar diameter was also observed under the microscope and the results were
tabulated in an excel file.
7. The results are compiled in section 4.1 and 4.2.
All the parameters were kept same as described above except for the axial load which was
kept 45N because of the limitation of rheometer as it cannot apply more than 45N load on the
ball. In addition to the tests performed for a sliding distance of 100m, 200m. The reason to
perform sliding tests at 200m was because the wear scar on the ball was not visible with a
40
sliding distance of 100m. Since, the experiments performed in [31] was for axial load of
61.5N and they were performed on 4 ball test rig, next set of experiments were performed on
the rig.
3.1.2 4 ball test rig
The procedure to conduct tests for 4 ball test rig is as explained below
1. Place the cup on the jig and place 3 clean balls.
2. Lock balls in the cup with holder and the cup with nut.
3. Add lubricant (10ml) to cup & place cup on the machine base matching the slot.
4. Place the fourth ball in the holder and tighten carefully.
5. Check the load for zero, if not adjusted offset value to set the load to zero.
6. Place the required dead weight on the weights holder.
7. Switch on the fan and torque meter.
8. Set the required rotation speed.
9. Set the sampling rate and click measure to start measurements.
10. Run the machine and perform the test (check time with a stopwatch).
11. Monitor the lubricant level in the cup.
12. Switch off the machine, torque meter and fan once the test is completed.
13. Remove added dead weights.
14. Remove the cup and fourth ball with care not locking the setup.
15. Ensure samples and machine are dry (use cleaning agent/compressed air/tissue paper).
Cleaning agent: Ethanol
16. Save necessary measurements/graphs.
The experiments were conducted with an axial load of 61.5N at 0.075m/sec sliding speed for
a sliding distance of 200m
From the results shown in the next chapter it can be concluded that design improvements must
be made on the test rig. The problem identified in the test rig was the misalignment of the
shaft as shown in Figure 13. The next section addresses the problem with the sliding in the
shaft that causes variation in the axial load while the test is running.
41
3.2 Tribometer redesign process
The 4 ball tribometer as shown in the Figure 12 is currently being used at KTH to perform
sliding tests. From the previous section it became clear that the four ball tribometer
encountered some problems. This section involves the discussion of the new designs of four
ball tribometer in order to rectify the problem. The problem with the four ball tribometer is
shown in the Figure13.
Figure 12: 4 Ball tribometer
Motor
Outer shell
covering
Place for
mounting the balls
Threaded shaft
42
Figure 13: Tribometer cross section
43
Figure 14: Force distribution due to misalignment in the shaft
The shaft in Figure13 is connecting the torque sensor to the load sensor. The load sensor is
placed directly below the shaft and the torque sensor is placed above the shaft sitting on top of
the lower flange. The shaft slides up and down along the fixed surface which generates sliding
friction in the shaft (as shown in the figure) contributing to the instability in the friction
curves.
The above problem can be resolved by using two methods:
1. Placing the load sensor before the torque sensor(Figure15a)
2. Placing the torque sensor before the load sensor(Figure15b)
The advantage of the two method described are that the sliding friction fluctuations from the
shaft can be removed.
The first method is chosen because less parts needs to be repositioned while changing the
position of the load sensor than changing the position of the torque sensor.
44
a)
b)
Figure 15: Changing position of sensors a) load cell between upper flange and lower flange b) load cell between
upper flange column and upper flange
45
3.2.1 Concept evaluation and selection
Various solutions were generated for solving the problem using the morphological chart. The
morphological chart is as described in the Table 1. The new concept should satisfy the
following functions
1. The application of force on the sensor
2. The resistance to relative torque(transmitted by the rotation of the shaft) between the
load sensor and the part attached to it.
Table 1: Morphological chart
Amongst the various solutions generated from the chart, the best ones are chosen for the
concept. The best concepts are as follows:
C1- RestingGeometrical Locking
The upper flange connector sits on the sensor and a locking mechanism as shown in the
Figure16 prevents the relative torque between the sensor and the upper flange connector.
46
Figure 16: Concept 1
The above concept is evaluated by listing advantages and disadvantages. The concept is
simple and easy to build but the concept cannot be selected because of the compressive stress
and strain applied on the load cell which the load cell cannot sustain.
Table 2: Advantages and disadvantages of concept 1
Advantages Disadvantages
No relative movement between the planar and
axial direction.
The geometrical locking ring placed around the
button will induce compressive stress and strain
on the load cell which it cannot take.
Axial Force is directly on the top part(button)
of load cell.
All the weight of the upper column flange on
the sensor which may affect the strength and
life of the sensor
No extra friction force
Simple Design
Cheap to build
47
C2- RestingClamp
In the second concept the upper flange connector sits on the load sensor and the force is
transferred from the upper flange connector to the load cell. The relative torque between the
two is restricted by the use of hose clamp which clamps the two bodies together. The concept
is as shown in the Figure 17 below.
Figure 17: Concept2
The above concept is evaluated by listing advantages and disadvantages Table 3. The concept
is simple and easy to build but the concept cannot be selected because of the compressive
stress and strain applied on the load cell which the load cell cannot sustain.
48
Table 3: Advantages and disadvantages of Concept 2
Advantages Disadvantages
No relative movement The geometrical locking ring placed around
the button will induce compressive stress
and strain on the load cell which it cannot
take.
Axial Force is acting on the button of load
cell.
All the weight of the upper column flange on
the sensor which may affect the strength and
life of the sensor
No extra friction force The hose clamp has to be bought because of
standard dimension
Simple Design More time taken to build
C3- Resting Screws
In the third concept the screws are used to lock the rotation between the upper flange column
and the load sensor. However, the problem associated with the concept is the sliding friction
that might occur between upper flange column and the screw which would contribute to the
additional force in the sensor. The concept is aptly explained by Figure 18. The advantages
and disadvantages of the concept are listed in the Table 4.
Figure 18: Concept 3
49
Table 4: Advantages and disadvantages of Concept 3
Advantages Disadvantages
No relative movement The friction force may affect the force value
Simple Design
Cheap
C4- Resting New Sensor
Concept 4 is using a new sensor futek lcf 451 as shown in the figure below.In order to use the
sensor the design has been modified accordingly(Figure 19). The advantages and
disadvantages of the design are as described in the Table5. The design was discarded because
it was expensive.
Figure 19: Concept 4
50
Table 5: Advantages and disadvantages of Concept 4
C-5 Resting Disc bellow couplings
The next concept considered was the upper flange column resting on the load sensor and the
relative torque is prevented by using disc bellow couplings as shown in Figure 20. Due to the
bellow’s thin walls, the coupling is able to flex easily while remaining rigid under torsional
loads. Parallel misalignment, angular misalignment and axial motion are accommodated by
the bellows coupling.The main disadvantage of the concept is a standard disc bellow coupling
is needed. Unfortunately, the disc bellow coupling dimensions required for this particular
design was not available.The advantages and disadvantages of the concept is detailed in the
Table6.
Advantages Disadvantages
No Relative movement Expensive
Simple Design
Less time to build it completely
51
Figure 20: Disc bellow coupling
Table 6: Advantages and disadvantages of Concept 5
Advantages Disadvantages
No Relative movement Standard dimension for disc bellow coupling
Simple Design
Less time to build it completely
C6 Resting Flexure
In this concept(Figure 21) the load is distributed to the sensor by clamping the upper part and
lower part between which the circular flexure is placed. The circular flexure can flex easily in
the vertical direction while remaining rigid under torsional loads. Thus, the flexure prevents
the relative torque between the sensor and the upper flange column. The advantages and
disadvantages of the concept is described in the Table7.
Upper flange
column
Upper part
52
Upper flange
column
Upper part
clamping
Lower part
clamping
Load sensor
Plate Holder Upper
Flange
Torque
Sensor
Table 7: Advantages and disadvantages of Concept 6
Advantages Disadvantages
No Relative movement Head connecting the upper flange to the
sensor interferes with the torque sensor
Simple Design
Cheap
C-7Resting Circular Flexure
The selected concept is as shown in the Figure22. The load sensor is attached in inverted
position to the upper flange column using three screws. A circular flexure is placed in
between the upper flange and the load sensor which can flex easily in the vertical direction
while remaining rigid under torsional loads. Thus, the flexure prevents the relative torque
between the sensor and the upper flange column. The concept shown is the best possible
design as proved by the Weighted matrix(Table8). However, because of the manufacturing
constraint and unavailability of Mr. Tomas Östberg in manufacturing lab, the parts had to be
Figure 21: Concept-6
53
slightly modified (the final concept remained the same) in order to use water jet cutting to
manufacture them. The final concept is summarized in the next section.
Table 8: Weighted matrix
From the above table it can be clearly observed that C-7 or concept7 is the best possible
design.
0-Low scale
5-High scale C1 C2 C3 C4 C5 C6 C7
Easy to
manufacture
(5%)
5 5 4 4 4 3 4
Simple Design
(40%) 5 5 5 5 5 4 5
Cheap (25%) 3 3 5 2 3 5 5
Time taken to
build (5%) 4 4 5 4 4
5
5
stress on sensor
(5%) 3 3 5 5 5 5 5
Additional force
measured by
sensor due to
sliding (20%)
5 5 2 5 5 5 5
Net score(out of
5) 4.35 4.35 4.35 4.15 4.4 4.5 4.95
Figure 22: a) dimensioning of the concept b) another section view of the same concept-7
54
3.2.2 Analysis of Circular Flexure
The load requirement for the experiment conducted for graphene is 61.5N. The purpose of the
circular flexure is to allow the load cell to measure the load exactly as applied. In order to
achieve that there is a need to calculate how much load the circular flexure should take to
keep the loss in load as less as possible. Hence, the bending in circular flexure should be equal
to the bending in the load cell for that amount of load. In order to determine this deflection in
the load cell for 100N will be calculated.
Hence, the deflection in the load cell for 100N load is 5.6 microns. Now, the circular flexure
too should have the same deflection so that the load cell can take as much load as possible. In
order to determine the deflection in the circular flexure ANSYS is used.
The circular flexure experiences a torsional as well as bending moment. The bending moment
is due to the 0.15N (this force gives the same deflection as that of the load cell) applied on the
circular flexure in the center as shown in the Figure23. The torsional moment will be due to
the relative movement between the circular flexure, upper flange connector and the load cell.
So, the maximum torsional moment that the flexure can undergo is equal to the maximum
torque that the torque sensor can take, which is 1Nm. The constraints set for the analysis of
the flexure in ANSYS are shown in Figure 23.
maxMaximum Load(F )=445
Maximum Deflection(x)=0.025
Stiffness of load cell= 17800 N/mm
100Deflection in load cell for 100N= 0.0056 mm 5.6 microns
17800
l N
mm
F
x
55
Figure 23: Circular Flexure and constraints
The reason behind taking the circular flexure as described in Figure 23 is that it would be
difficult to place the constraints using the circular flexure shown in the assembly in Figure 22.
From, the assembly described in Figure 22 it becomes clear that the diameter of the circular
flexure for normal load applications and torsional load applications will be as shown in Figure
24. For the case of normal load application the contact point begins from 38mm and for the
torsional moment application the contact begins from the place where screws are placed
which is 25.4 mm.
56
Figure 24: Circular flexure dimensions
The stress and deflection plot for 0.45 mm width circular flexure plate in ANSYS is as shown
in Figure 25 and Figure 26.
Figure 25: Deformation and stress plot for load application
57
As it can be observed from Figure 23 for a width of 0.45 mm circular flexure plate only
0.15N load is required to give it the same deflection as the load cell hence only 0.0015% of
the total load goes into the bending of circular flexure and the remaining load is measured by
the load cell. In addition to it, from Figure 26 it can be seen that the circular flexure is
extremely rigid in the torsional moment direction.
3.2.3 Final Concept and drawings
As described in the previous section that due to manufacturing constraints the parts had to be
modified in order to manufacture them using water jet cut. Upper flange column was the only
part that was not changed as it was manufactured. The final assembly is shown in Figure 27.
Figure 26: Deformation and stress plot for torsional moment case
58
Figure 27: Final Concept (Assembly drawings)
59
Figure 27 comprises of two figures; the top figure shows the centering of the flexure plate
using pins and the bottom figure describes all the parts in the assembly. The final part
drawings for the concept are shown in appendix2.
The parts were manufactured as per the drawings in manufacturing lab using stainless steel.
The final manufactured parts are shown in Figure 28.
The test rig after design modifications is as shown in Figure29.
Figure 28: Manufactured Parts
60
Figure 29: 4 ball test rig after design modifications
4-ball test rig after design tests
The tests were conducted on the test rig (Figure 29) after the design modifications. The
procedure for the 4 ball test is the same as described in section 3.1.2. Following the
procedure the tests are performed. The final results obtained are described in the results
section.
3.3 Rolling Contact
Mini Traction Machine
The aim of the experiment is to observe the relative effect of solutions of water, graphene
100µg/ml and 350µg/ml as lubricant in rolling contacts by observing the different test
parameters (e.g. SRR, load) in mini traction machine.
Outer shell
covering
Place for
mounting the balls
Threaded shaft
Final assembly
with new
manufactured
parts
61
The parameters chosen for performing the tests and the test conditions are described below
1. Step chosen as stribeck step
2. 5 𝑁, 10 𝑁, 20 𝑁, 30 𝑁 and 40 𝑁 applied load.
3. Entrainment speed (i.e. rolling speed) varying from 0 to 200mm/sec in steps of
20mm/sec for every load.
4. Tests performed at different slide to roll ratio (SRR) for each load which are as
follows 1%, 5%, 10%, 15%, 20%, 25%, 30%, 35%
5. Tests performed on pure water, graphene 100µg/ml (G-100) and graphene 350µg/ml
(G-350).
Following procedure is adopted to perform the experiments
1. The mini traction machine was calibrated before starting the experiment series.
2. Mini pot (10ml sample volume) was used in order to conduct the tests. Half inch ball
on disc (size 6 mm) was used in order to perform the experiments.
3. The experiments were performed for 5 𝑁, 10 𝑁, 20 𝑁, 30 𝑁 and 40 𝑁 load. The
measurements were taken by varying the SRR values from 1% to 35% as mentioned in
the test parameters for each individual load. The SRR values started from 1% because
the mini traction machine showed uncertainty at 0% due to the issue that both sides
need to rotate with the same speed. Hence, there were 8 stribeck steps for each load.
4. For a single stribeck step the SRR value was kept constant for a constant load and the
entrainment speed was varied as mentioned in the tests parameters.
5. The total number of steps followed in the stribeck test was 40. A matrix for tests
parameters is described in Table 9.
6. During the experiments the values were recorded and curves were plotted in excel
sheet after the experiments. They are described in the results section.
62
Table 9: Stribeck test steps
SRR(%) Load(N)
5 10 20 30 40
1
Entrainment speed 0-200mm/sec in steps of 20mm/sec
5
10
15
20
25
30
35
Total 8 8 8 8 8
Net total 40
63
4 RESULTS
In the results chapter the results that are obtained with the methods described in the
Experimental setup and materials are compiled, and analyzed and compared with the existing
knowledge and theory presented in the frame of reference chapter.
4.1 Friction
The friction results for sliding tests and rolling tests are described in this subsection
4.1.2 Sliding contact
The experiments for sliding tests include trib-rheometer tests and 4-ball tribometer tests
before and after redesigning.
4.1.1.2 Tribo-rheometer
The average coefficient of friction plots for tests A and B obtained from the experiment
performed in section 4.1.1 are as shown in the figure below. The solutions of pure water,
surfactant (PEG), graphene 25μg/ml, graphene 50 μg/ml, graphene 100μg/ml, graphene 150
μg/ml and graphene 350 μg/ml(pure graphene) are compared for a sliding distance of 200m.
64
Figure 30: Overview for tests on triborheometer
65
The observations obtained from Figure 28 are pointed out below
1. The surfactant solution has the highest coefficient of friction upto 100m and from 100-
200m water has the highest friction coefficient.
2. It can also be observed from the box plot that with the increase in the graphene
concentration the coefficient of friction decreases till graphene concentration of
150µg/ml.
3. For the graphene concentration of 25µg/ml, the solution is a good lubricant till 40m
however after 40m the friction curve increases for G-25.
4. Similarly, for a graphene concentration of 50µg/ml, the solution holds itself until 80m
and after 80m it increases.
5. Trend is the same for G-100 and G-150. Although, the coefficient of friction increases
after 140m for G-100 and after 160m in the case of G-150.
6. The coefficient of friction for G-350 is high in the beginning but decreases after 100m.
7. From the plots shown in appendix3 it can be concluded that the results from sliding
distance of 0-100m are more reliable than from 100-200m since, they are an average
of 4 plots whereas, the values after 100m are a combination of 2 plots.
Figure 31 and Figure 32 are shown as an example to demonstrate the repeatability of the test
A and B performed on the triborheometer.
Figure 31 and 32 has sliding distances of 100m and 200m. For the case of 100m four tests
were averaged and included whereas in the case of 200m only two were considered. The
sliding distance of 200m include 100m too hence, two tests for 100m and 200m(values up to
sliding distance of 100m) are averaged in order to compute values for 100m sliding distance.
Hence, statistically 100m will provide better insights compared to 200m.
66
Figure 31: Test A and B for pure water
Figure 32: Test A and B comparison for G-25
In order to observe the trend for coefficient of friction at higher loads specifically at 61.5N
load the tests were performed on 4 ball tribometer test rig.
67
4.1.1.2 4 ball tests(tribometer)-old design
The experiments were only performed on pure water since the results were showing
discrepancy as it can be observed from Figure 33&34 . From the axial load plot it can be
clearly observed that there is a variation in the load although the load is kept constant. From
the coefficient of friction plot also it can be concluded that the curves are not following a
smooth trend there is a lot of turbulence in the curve. Hence from the observations it is
concluded
1. There is sliding in the shaft because of which friction acts on the shaft thus
contributing an additional force which may cause the variation in the load.
2. The solution is destroyed due to the chemical changes in solution leading to change in
its properties.
Since, it is easier to fix the mechanical part of the problem the solution to the first hypothesis
is addressed in section 4.1.3.1
Figure 34: Coefficient of friction Vs sliding distance for pure water
Figure 33: Axial load Vs sliding distance for pure water
68
4.1.1.3 4 ball tests(tribometer)-new design
The results for the experiments performed on the test rig after modification is described in this
section. The repeatability for test A, test B and test C is not observed. However, the mean
value of the coefficient of friction curve is much more stable as compared to the tests
performed on the test rig before modification. The following box plots depict the results
obtained from the experiments
69
Figure 35: Axial load Vs sliding distance
70
Figure 36: Coefficient of friction Vs Sliding Distance
71
From the above plots the conclusions that can be drawn out are shown as below
1. Coefficient of friction curve for pure water is really noisy for all the tests.
2. Solution of pure water and surfactant has the highest friction coefficient.
3. The mean value of the coefficient of the friction force curve is stable however, if
observed more carefully it increases a bit.
4. For test B and test C G-25 has the lowest coefficient of friction. However, for test A
G-100 has the lowest friction coefficient.
5. There is a sharp increase in friction value for G-15 in test-A.
72
Figure 37: Overview of Coefficient of friction Vs Sliding Distance
73
From the above box plot following conclusions can be drawn
1. Water and surfactant mixture has the highest friction
2. With the addition of graphene the friction reduces significantly.
3. The coefficient of friction reduces until it reaches an optimal concentration of 25µg/ml
after that the coefficient of friction again increases.
The reason for the above conclusion is that there is an optimal concentration of graphene at
which the coefficient of friction is minimum. However, above or below that concentration the
coefficient of friction increases[4].
4.1.2 Rolling Contact
The results are plotted for entrainment speeds of 40mm/sec (Figure39), 100mm/sec
(Figure40) and 200mm/sec (Figure41).
In order to observe the trends; low, medium and high speed values were considered so that a
final conclusion can be achieved. For the low speed values 40mm/sec was considered because
a. Speed of 0mm/sec would be pure sliding.
b. The trend was not conclusive with 20mm/sec.
c. Example for 10N - H2O (Figure38)
74
Figure 38: H2O plots for different entrainment speeds for 10N
In Figure 38 the shape of the curve for entrainment speed of 1mm/sec is first increasing then
decreasing in the opposite direction because initially the ball moves away from the sensor and
then it starts moving towards the sensor thus, contributing to the change in direction.
Speed of 100mm/sec was chosen because it is the mid-way value for the entrainment speeds
and 200mm/sec is the highest value of speed.
In the plots shown(Figure 39,40) below the intensity of the color specifies the increasing load
values. Increasing color intensities represent increasing loads.
75
Figure 39: Traction coefficient as function of SRR for all liquids for 40mm/s at different loads ranging from 5N
to 40N. H2O in red; G100 in orange and G350 in blue. Intensity of color varies from 20% to 100% in increasing
order for increasing loads.
Figure 40: Traction coefficient as function of SRR for all liquids for 100 mm/s at different loads ranging from
5N to 40N. H2O in red; G100 in orange and G350 in blue. Intensity of color varies from 20% to 100% in
increasing order for increasing loads.
76
Figure 41: Traction coefficient as function of SRR for all liquids for 200 mm/s at different loads ranging from
5N to 40N. H2O in red; G100 in orange and G350 in blue. Intensity of color varies from 20% to 100% in
increasing order for increasing loads.
Observations
Following observations are drawn from the plots.
1. Water is a having quite low traction coefficient values over the range of 1% SRR to
35 % for all loads.
2. As the load and the speed increases the traction coefficient values decreases for all the
solutions with quite a drastic change observed for G-350. This trend is observed for
all the loads and speeds.
3. At low load values of 5N, 10N and 20N it can be observed that G-350 has the highest
traction coefficient while water has the lowest traction coefficient. In addition to it, G-
100 is having coefficient of traction values between G-350 and water.
77
4. At higher loads (> 30N), G-350 and water are showing similar traction coefficient
with increasing SRR. However, water is having slightly lower traction coefficient
values. Traction coefficient values for G-100 are highest.
5. As the entrainment speed increases the curve becomes smoother.
At entrainment speeds of 100mm/sec and 200mm/sec there is a sharp rise in the traction
coefficient values for water as at higher speeds the water started to splash. There is no
splashing observed for G-100 and G-350.
4.2 Wear
Due to the sliding motion of the ball on ball wear is generated on all the four balls. The
rotating ball has a wear track that is why it is line shaped. The stationary balls kept in the
holder have a wear scar hence, the scar is point shaped. In this section the wear results for the
experiments are described.
4.2.1 Tribo-rheometer
The wear scar was analyzed by using optical microscope NIKON MM-60. The measured
wear scar on the three balls along with wear track on the fourth ball is represented by the bar
plots in (Figure 42 and 43) for both 100m and 200m sliding distance.
78
Figure 42: Wear plots for all liquids at sliding distance 100m
From the above bar plots it becomes clear that there is no pattern being observed for the wear
values. It is not possible to conclude the impact of the lubricants on the wear. Thus, the
conclusion drawn from the bar plots are same as that observed from the friction plots.
4.2.2 4 ball tests(tribometer)-old design
The wear data for the 4 ball tests performed on the tribometer test rig is described below:
Figure 43: Wear plots for all liquids at sliding distance 200m
79
Figure 44: Wear plot for old tribometer design for water
The bar plot is only for water hence, no trend can be concluded from it.
4.2.3 4 ball test( tribometer)-new design
The sliding tracks on the chromium steel balls which were obtained from the sliding motion
on the 4 ball test rig. It is indicated that the sliding tracks between the balls mainly consists of
oval pattern as shown in the Figure45. The figure shows the wear pattern for every liquid
generated on the ball.
80
Figure 45: Representation of wear track on the balls for all the liquids a) Pure water to G-25 b) G-50 till G-350
From the above figure it can be clearly observed that at higher concentrations of graphene the
deposition of graphene is really high. The wear track for G-100 is only visible but got G-150
and G-350 the wear track is not visible at all because of the graphene deposition.
The following bar plots depict the results for the wear tests for 4 ball tribometer with
upgraded design.
81
Figure 46: Wear track width for test A and B for all the solutions
Figure 47: Wear scar diameter for test A and B for all the solutions
From the above bar plots the following can be clearly observed
82
Test A
1. Surfactant has the highest wear whereas, G-350 has the lowest wear
2. The wear is reducing as the concentration of graphene increases only exception being
G-350
Test-B
3. Pure water has the highest wear whereas, G-350 has the lowest wear
4. Wear reduces till G-15 and then it increases with G-350 being the exception.
Test-C
1. Surfactant has the highest wear whereas, G-100 has the lowest wear
The overview of the wear is shown in the plot below
Figure 48: Overview of wear
The overview of wear concludes that no clear trend in wear is observed however, it shows
that the wear reduces with the addition of graphene.
83
5 DISCUSSIONS AND CONCLUSIONS
A discussion of the results and the conclusions that the authors have drawn during the Master
of Science thesis are presented in this chapter. The conclusions are based from the analysis
with the intention to answer the formulation of questions that is presented in Chapter 1.
5.1 Discussions
5.1.1 Friction
In this section the friction results are discussed in detail.
5.1.1.1 Sliding Contact
The discussions for the case of sliding contacts are described in the subsequent sections.
Tribo-rheometer
The behavior of liquids for the experiments conducted on tribo-rheometer is that with the
increase in concentration of graphene the friction reduces up to the concentration of
150μg/ml. The reasoning behind this observation is that as the concentration of graphene
increases there is enough deposition of graphene in between the contacts which causes
separation between the steel balls and hence, reducing friction. In solid graphene low shear
strength which is acting between the graphene sheets mediates lubrication. The structure and
morphology of graphene is stable and shear mechanism is active when graphene is well
dispersed in the water. Undispersed graphene make aggregated particles and shear ability is
disrupted thus increasing friction.
Tribometer
Oscillation of the coefficient of friction plots for pure water as observed in the results is
probably due to the asperities on contact surfaces and the insufficient load-carrying capability
of water. Solution of PEG and water has the highest friction coefficient because being a
surfactant it is an active ion it easily gets deposited on the ball and thus attracts graphene
which further gets deposited in the asperities. Also, it can be concluded that if the
concentration of graphene deposition is less or more compared to optimal then the friction
increases. For less concentration of graphene, PEG will be more which will prevent graphene
to form layer. Similarly, if Graphene is more and it is non homogenous there will be interlayer
interaction which will again increase friction.
84
It can also be observed from the results that the lubricant has a limitation on its duration of
working in terms of distances. This is due to the surfactant quantity in the lubricant solution.
The surfactant holds the solution of graphene and water together and if the surfactant quantity
is less than what is required to hold the solution together then the lubricant solution would not
be sufficient to decrease the friction over the specific distance.
1.2.1.1 Rolling Contact
The results shown for the rolling contact can be explained in the following points
1. Water has the lowest friction at low loads and the friction increases at higher loads this
may be due to water has a poor load carrying capacity it gets easily destroyed hence, at
lower loads it can survive but as the load is increased the lubricant film formed from
water is not able to hold under high loads leading to increase in friction coefficient.
2. In the case of aqueous solution of graphene, low loads are not able to separate
surfactant and graphene from water thus, it is difficult for surfactant and graphene to
deposit at the asperities and form a separating layer. However, at higher loads the PEG
and graphene can be easily separated from water and form layers to separate the
rolling contact. Thus, the friction coefficient is reduced at higher loads in the case of
aqueous graphene solution.
5.1.2 Wear
From the wear plots it can only be concluded that there is a reduction in the wear with the
addition of graphene although, there is no clear trend observed between the concentration of
graphene and wear reduction. It can be concluded from Figure 45 that the wear developed is
of the type abrasive wear.
Due to the unavailability of SEM the wear scar was not observed on SEM.
5.2 Conclusions
1. Aqueous graphene solutions hold significant advantage over pure water or deionized
water and water + surfactant solution.
2. The friction results with both the tribometer suggest that friction is significantly
reduced with the addition of graphene.
3. There is a trend observed with the tests performed with both the tribometer and tribo-
rheometer. In the case of tribo-rheometer the reduction of friction is directly
85
proportional to the concentration of graphene solution. However, in the case of
tribometer the friction increases before and after an optimal concentration of graphene.
4. Higher graphene concentrations reduce the area of heat affected zones observable and
improve graphene deposition in addition to the measurable improvements in friction
and wear parameters.
5. The aqueous graphene solution is destroyed after a certain distance according to the
concentration of graphene.
6. Water has the lowest friction at low loads in the case of rolling contacts and the
friction coefficient increases with increase in the load. However, the opposite is true in
the case of aqueous solution of graphene.
7. Although the wear is reduced significantly compared to pure water and the solution of
pure water and surfactant there is no clear trend observed.
86
6 FUTURE WORK
In this chapter, future work in this field is presented.
There can be some improvements related to the work performed in this thesis. They are
detailed as follows:
Further in depth study of the mechanism of lubrication can be assessed.
The stability of solution could be improved by using a high grade surfactant or
increasing the concentration of the surfactant in the solution.
Effect of ageing on the friction and wear properties can also be assessed.
The 4 ball tribometer could be improved by changing the design of the bearing and the
shaft connecting the load and torque sensor. The tribometer can also be improved by
lubricating the journal bearing supporting the shaft.
More rolling tests with more liquids can be performed to determine the effect of
aqueous graphene on rolling contact.
87
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APPENDIX 1:SUPPLEMENTARY INFORMATION
91
APPENDIX 2:SUPPLEMENTARY INFORMATION
Figure 1:Upperflangecolumn
Figure 2: Circular flexure
92
Figur 4: Disc cutout
Figure 3: Flexure holder
93
Figure 5: Upper Flange
94
APPENDIX 3:SUPPLEMENTARY INFORMATION
Figure 1: H2O Test A and B
95
Figure 2: H2O+surf Test A and B
96
Figure 3: G-25 Test A and B
97
Figure 4: G-50 Test A and B
98
Figure5: G-100 Test A and B
99
Figure6: G-150 Test A and B
100
Figure7: G-350 Test A and B
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