Graphene as an aqueous Lubricant

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 1 2 Examensarbete MMK 2017:192 MKN 195 Grafen som ett vattenhaltigt smörjmedel Nishant Katyal Godkänt Examinator Handledare 2017-11-06 Ulf Sellgren Sergei Glavatskih Patrick Rohlmann Uppdragsgivare Kontaktperson Institutionen för Maskinkonstruktion, Sergei Glavatskih KTH 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 Minitraktionsmaskin. De testade proverna uppvisade signifikanta friktions- och förslitningsfördel jämfört med D.I. vatten och ytaktiva lösningar under samma testförhållanden Grafen Smörjning, triborometer, 4-kuletribometer, mini-traktionsmaskin, friktionsminskning, nötningsreduktion Nyckelord: 3 4 Examensarbete MMK 2017:192 MKN 195 Graphene as an aqueous Lubricant Nishant Katyal Godkänt Examinator Handledare 2017-11-06 Ulf Sellegren Sergei Glavatskih Patrick Rohlmann Uppdragsgivare Department KTH Kontaktperson of Machine Design, 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 5 6 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 7 8 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 9 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 10 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 3.1.1 Sliding contacts .............................................................................................................................. 35 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 11 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 12 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 nonrenewable 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. 13 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 14 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 antifriction 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 15 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. Figure1: Results from work done previously[31] 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. 16 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 100200m. 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. 17 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. 18 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 19 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]. 20 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 21 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  FF FN 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 FF   vA 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: 22 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]. 23 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. Figure 3: Stribeckcurve[image credit Tribological properties of ionic liquid( DOI: 10.5772/52595] 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 24 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 25 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. 26 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. 27 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 28 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. 29 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. 30 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] Small disc Mini pot Liquid sample Lube temperature sensor ½ ” ball Pot temperature sensor (a) (b) Figure7: Mini Traction Machine[19] a) Cross section view b) Overall machine 2D drawing 31 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] 32 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 2dimensional 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]. 33 Figure 9: Scanning electron Microscope[ image credit Susan Swapp, University of Wyoming] 34 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. 35 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 X  r cos(30) (1.1) 36 Triangle IBD and IFH are similar triangles from AAA similarity. Hence, from the formula for similar triangles r 2r FH r cos(30) r FH  2 cos(30)  (1.2) Contact angle β is FH FH  IF r  FH   sin 1    r  sin(  )   (1.3) Hence,   35.264o Now, all the formulae used according to the contact angle are described as shown below: From triangle IFH Actual Load FL: FL  FN Cos(  ) with   35.2644  1/cos( )  1.2247 FL  1.2247FN (1.4) 37 Friction Force FF: FF  M r sin(  ) with   35.2644  1/sin( )  1.8165 FF  (1.5) 1.8165M r Sliding Speed vs: v s  r sin(  ) with (1.6)   35.2644  sin(  )  0.57735 v s  0.57735r Friction Coefficient µ:   M cos(  ) 1.4832M  r sin(  ) FN rFN (1.7) Dimensions Sphere radius: r=0.00635m arm d: 0.007398 =35.2644° Geometry constants Kv  r sin(  )  0.00732 Kf  1/ r sin(  )  136.382 (1.8) K F  1/ cos(  )  1.22474 38 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 39 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. 40 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. Motor Outer shell covering Threaded shaft Place for mounting the balls Figure 12: 4 Ball tribometer 41 Figure 13: Tribometer cross section 42 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. 43 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 44 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- RestingGeometrical 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. 45 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 46 C2- RestingClamp 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. 47 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. 48 Figure 18: Concept 3 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. 49 Figure 19: Concept 4 Table 5: Advantages and disadvantages of Concept 4 Advantages Disadvantages No Relative movement Expensive Simple Design Less time to build it completely 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. 50 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. 51 Upper flange column Upper part clamping Lower part clamping Load sensor Plate Holder Upper Flange Torque Sensor Figure 21: Concept-6 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 52 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. Figure 22: a) dimensioning of the concept b) another section view of the same concept-7 Table 8: Weighted matrix 0-Low scale 5-High scale Easy to manufacture (5%) Simple Design (40%) Cheap (25%) Time taken to build (5%) stress on sensor (5%) Additional force measured by sensor due to sliding (20%) Net score(out of 5) C1 C2 C3 C4 C5 C6 C7 5 5 4 4 4 3 4 5 5 5 5 5 4 5 3 3 5 2 3 5 4 4 5 4 4 5 5 3 3 5 5 5 5 5 5 5 2 5 5 5 5 4.35 4.35 4.35 4.15 4.4 4.5 4.95 5 From the above table it can be clearly observed that C-7 or concept7 is the best possible design. 53 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. Maximum Load(Fl max )=445 N Maximum Deflection(x)=0.025mm F  17800 N/mm x 100 Deflection in load cell for 100N=  0.0056 mm  5.6 microns 17800 Stiffness of load cell= 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. 54 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. 55 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 56 Figure 26: Deformation and stress plot for torsional moment case 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. 57 Figure 27: Final Concept (Assembly drawings) 58 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. Figure 28: Manufactured Parts The test rig after design modifications is as shown in Figure29. 59 Outer shell covering Threaded shaft Place for mounting the balls Final assembly with new manufactured parts 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. 60 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. 61 Table 9: Stribeck test steps SRR(%) 5 1 5 10 15 20 25 30 35 Total Net total 10 Load(N) 20 30 40 Entrainment speed 0-200mm/sec in steps of 20mm/sec 8 8 8 8 8 40 62 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. 63 Figure 30: Overview for tests on triborheometer 64 The observations obtained from Figure 28 are pointed out below 1. The surfactant solution has the highest coefficient of friction upto 100m and from 100200m 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. 65 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. 66 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 33: Axial load Vs sliding distance for pure water 67 Figure 34: Coefficient of friction Vs sliding distance for pure water 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 68 Figure 35: Axial load Vs sliding distance 69 Figure 36: Coefficient of friction Vs Sliding Distance 70 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. 71 Figure 37: Overview of Coefficient of friction Vs Sliding Distance 72 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) 73 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. 74 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. 75 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, G100 is having coefficient of traction values between G-350 and water. 76 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. 77 Figure 42: Wear plots for all liquids at sliding distance 100m Figure 43: Wear plots for all liquids at sliding distance 200m 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: 78 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. 79 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. 80 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 81 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. 82 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. 83 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 triborheometer. In the case of tribo-rheometer the reduction of friction is directly 84 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. 85 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. 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Tribological studies of aqueous graphene solutions in steel-steel contact. 89 APPENDIX 1:SUPPLEMENTARY INFORMATION 90 APPENDIX 2:SUPPLEMENTARY INFORMATION Figure 1:Upperflangecolumn Figure 2: Circular flexure 91 Figure 3: Flexure holder Figur 4: Disc cutout 92 Figure 5: Upper Flange 93 APPENDIX 3:SUPPLEMENTARY INFORMATION Figure 1: H2O Test A and B 94 Figure 2: H2O+surf Test A and B 95 Figure 3: G-25 Test A and B 96 Figure 4: G-50 Test A and B 97 Figure5: G-100 Test A and B 98 Figure6: G-150 Test A and B 99 Figure7: G-350 Test A and B 100