Tribological Capabilities of Carbon Based Nano Additives in Liquid and Semi-Liquid Lubricants

Tribological Capabilities of Carbon Based Nano Additives in Liquid and Semi-liquid Lubricants

Jankhan Patel

A Thesis Presented in Partial Fulﬁllment of the

Requirements for the degree of

Master of Applied Science

Mechanical Engineering

Faculty of Engineering and Applied Science

Ontario Tech University

Oshawa, Ontario, Canada

April, 2019

c Jankhan Patel, 2019 THESIS EXAMINATION INFORMATION

Submitted by: Jankhan Patel

Master of Applied Science in Mechanical Engineering

Tribological Capabilities of Carbon Based Nano Additives in Liquid and Semi-liquid Lubricants

An oral defense of this thesis took place on April 16, 2019 in front of the following examining committee:

Examining Committee:

Chair of Examining Committee Dr. Martin Agelin-Chaab Research Supervisor Dr. Amirkianoosh Kiani Examining Committee Member Dr. Jaho Seo External Examiner Dr. Shaghayegh Bagheri

The above committee determined that the thesis is acceptable in form and content and that a satisfactory knowledge of the ﬁeld covered by the thesis was demonstrated by the candidate during an oral examination. A signed copy of the Cer- tiﬁcate of Approval is available from the School of Graduate and Postdoctoral Studies

i Abstract

The growing population, the environmental challenges and limitations of nonre- newable resources have enforced researchers to improve the energy efficiency and reduce energy wastage in all industrial sectors including manufacturing and automotive. The significant losses in the automotive and manufacturing industries are associated with frictional energy between the mating surfaces. In addition, high frictional forces accumulate material removal rate which directly affects the service life, efficiency, and maintenance cost of the components. This results into loss of functionality and efficiency depletion of the systems. One of the possible solutions includes focusing on the refinement of tribological applications by researchers and industrial sectors. Along with that, rapid growth in nanomanufacturing industries has opened an enormous range of opportunities to enhance tribological capabilities of lubricants. Improved physical and chemical characteristics of the nanomaterials allowed them to be used for many applications including production of new generation of lubricants. In this study, applications of carbon based nano- material and hybrid nanoadditives to both liquid and semi-liquid lubricants have been investigated. This thesis is divided into three phases based upon the type of nano additive into the liquid and semi-liquid lubricants. In phase 1, three forms of reduced graphene oxides (rGO) were evaluated for the tribological application using four-ball tester at 0.01% concentration. All three forms of rGO were produced using modified hummers method; however, different filtration method and synthesis time were used to vary the bulk densities and lattice structures of rGO nanoplatelets. In the second phase of this study, the best rGO nanoplatelets were further studied at three different weight concentrations to discover the optimum concentration to modify friction and wear preventive characteristics. In order to obtain precise results about friction and worn surface characterization, ball-on- disk tribometer was used in situ condition. In the last phase of this research, graphene nanoplatelets and titanium dioxide nanoparticles were used individually at three different concentrations. Furthermore, hybrid nanoadditives (graphene

ii and titanium dioxide mixed in three diﬀerent ratios) were tested in both liquid and semi-liquid lubricants. Apart from tribological evaluation, physical, chemical, and structural properties were evaluated in three phases using viscometer, RPVOT, Fourier-transform infrared spectroscopy, electron microscopy (SEM and TEM) and Raman spectroscopy.

Keywords: rGO, Graphene, TiO2, Friction, Wear, Hybrid nano additive

iii Author’s Declaration

I hereby declare that this thesis consists of original work of which I have authored. This is a true copy of the thesis, including any required ﬁnal revisions, as accepted by my examiners. I authorize the University of Ontario Institute of Technology to lend this thesis to other institutions or individuals for the purpose of scholarly research. I further authorize University of Ontario Institute of Technology to reproduce this thesis by photocopying or by other means, in total or in part, at the request of other institutions or individuals for the purpose of scholarly research. I understand that my thesis will be made electronically available to the public.

Jankhan Patel

iv Statement of Contributions

Part of work has been published in Journals and Conferences as:

• J. Patel, G. Pereira, D. Irvine, A. Kiani, Friction and wear properties of base oil enhanced by diﬀerent forms of reduced graphene, AIP Advances, 9 (4) (2019). https://doi.org/10.1063/1.5089107

• J. Patel, A. Kiani, Eﬀects of reduced graphene oxide (rgo) at diﬀerent concentrations on tribological properties of liquid base lubricants, Lubricants 7 (2) (2019). https://doi.org/10.3390/lubricants7020011

• J. Patel, A. Kiani, Tribological capabilities of graphene and titanium dioxide nano additives in solid and liquid base lubricants, Applied Sciences 9 (8) (2019). https://doi.org/10.3390/app9081629

• J.Patel, A. Kiani, Enhancement of Wear Behavior of Base Oil by Reduced Graphene Oxide Additives, STLE Tribology Frontier Conference, Chicago, USA, Oct. 2018.

v Acknowledgments

I am very grateful to my supervisor Dr. Amirkianoosh Kiani for constantly en- couraging and supporting me through out my research journey. His expertise, motivation, and dedication towards us was very essential, to explore different areas of my research. I was astonished, to be given an opportunity to attend and present my research work at society of tribologist and lubrication engineers conference at Chicago. I am very thankful to the staff at the Ontario Tech University for providing an excellent work environment. I am also thankful to Dr.Ahmad Barari for giving me an opportunity to work on friction coefficient machine in his lab. I am very grateful to Gavin Pereira (Kinectrics Inc.) and Dough Irvine (Petro-Canada Lubricants Inc.) for providing me resources. Many thanks to Nanovea Inc. for running all the tests precisely in timely manner. I thank York University life science department, University of Toronto and Sick kids for allowing me to run SEM, TEM experiments. University life not just gave me education but it allowed me to live beyond the boundaries. I am so blessed to have a great multicultural community on campus. Also, I am grateful to all my housemates who stood with me in my ups and downs. I have learned a lot from you guys Chirag (Micrometer) and Naghmeh (Mili meter) through out my time in lab. Apart from that, I am blessed to have supportive parents and my fiance, who always made me good food and accompanied me and encouraged for study. Lastly, I want to thank the Canadian government for allowing me to explore my education in Canada and NSERC (Natural Science and Engineering Research Coun- cil) for funding my research.

vi Dedication

I am dedicating my research to my parents (Nayankumar, Jagrutiben), grandpar- ents (Natvarlal, Subhadraben), brother (Sajan), and ﬁance (Shivani) for their love, support and encouragement throughout my academic journey.

vii Contents

Acknowledgments vi

Dedication vii

Table of Contents viii

List of Figures xii

List of Tables xvi

Acronyms xvii

Nomenclature xviii

1 Introduction 1 1.1 Tribology ...... 1 1.2 Friction ...... 2 1.3 Wear ...... 4 1.3.1 Adhesive wear ...... 4 1.3.2 Abrasive wear ...... 5 1.3.3 Corrosive wear ...... 5 1.3.4 Erosive & Percussion wear ...... 6 1.3.5 Fatigue wear ...... 6 1.3.6 Electric arc wear ...... 7 1.4 Lubricants ...... 7 1.5 Lubrication mechanism ...... 9

viii 1.5.1 Full film lubrication ...... 10 1.5.2 Mixed film lubrication ...... 10 1.5.3 Boundary lubrication ...... 10 1.6 Nano additives ...... 11 1.6.1 Anti wear (AW) additive ...... 11 1.6.2 Extreme pressure (EP) additive ...... 12 1.6.3 Friction modifier (FM) additive ...... 12 1.6.4 Dispersant additive ...... 12 1.6.5 Antioxidation additive ...... 13 1.6.6 Viscosity modifier ...... 13 1.6.7 Corrosion (rust) inhibitors ...... 13 1.6.8 Pour point depressants ...... 14 1.6.9 Titanium based nano additives ...... 14 1.6.10 Carbon based nano additives ...... 15

2 Experimental methods and characterisation 17 2.1 Introduction ...... 17 2.2 Material synthesis ...... 17 2.3 Mechanical mixture ...... 19 2.4 Viscometer ...... 20 2.5 Rotating pressure vessel oxidation test (RPVOT) ...... 21 2.6 Fourier-transform infrared spectroscopy ...... 22 2.7 Tribometers ...... 23 2.7.1 4 ball wear tribometer ...... 23 2.7.2 Ball on disk tribometer ...... 24 2.8 High resolution electron microscopy ...... 26 2.8.1 SEM (Scanning Electron Microscopy) ...... 26 2.8.2 TEM (Transmission Electron Microscopy) ...... 27 2.9 Raman spectroscopy ...... 28 2.10 statistical analysis ...... 29

ix 3 Tribological behaviour of three forms of rGO 30 3.1 Introduction ...... 30 3.2 Characterization of the rGO ...... 31 3.2.1 Bulk density ...... 31 3.2.2 Topological analysis of rGO ...... 32 3.2.3 Raman spectroscopy ...... 33 3.3 Kinematic viscosity and viscosity index ...... 35 3.4 RPVOT ...... 36 3.5 Resistivity test ...... 36 3.6 Four ball wear test ...... 37 3.6.1 Substrate topography investigation ...... 38 3.7 Average friction coeﬃcient ...... 39

4 Further investigation of the most eﬃcient rGO 41 4.1 Introduction ...... 41 4.2 Material characterisation ...... 42 4.2.1 Raman spectroscopy ...... 42 4.2.2 Transmission electron microscopy ...... 43 4.2.3 FTIR particle count test ...... 44 4.3 Samples characterisation ...... 45 4.4 Kinematic viscosity ...... 46 4.5 RPVOT ...... 47 4.6 Ball on disk wear and friction test ...... 47 4.7 SEM morphology study of worn surface ...... 50 4.8 Optical Microscopy ...... 51

5 Comparative study of GNP and TiNP individually and together as hybrid additive 53 5.1 Introduction ...... 53 5.2 Morphology of material ...... 54

x 5.2.1 Raman spectroscopy ...... 54 5.2.2 TEM investigation ...... 55 5.3 Samples characterisation ...... 56 5.4 Kinematic viscosity and viscosity Index ...... 58 5.5 RPVOT ...... 58 5.6 Ball on disk wear and friction coeﬃcient test ...... 59 5.7 SEM morphology investigation of worn surface ...... 61 5.8 Optical microscopy in situ ...... 63 5.9 Grease friction coeﬃcient ...... 63

6 Research summary and future work 65

Bibliography 67 6.1 Published articles ...... 78 6.1.1 Journal papers ...... 78 6.1.2 Conference papers ...... 78 6.2 Appendix ...... 79 6.2.1 Friction coeﬃcient data obtained from shaft-on-plate tribometer at 10N, 20N and 30N ...... 79 6.2.2 Optical microscopy height parameters and area hole analysis of Chapter-4 samples ...... 79 6.2.3 Optical microscopy height parameters and area hole analysis of Chapter-5 samples ...... 79

xi List of Figures

1.1 Free body diagram of sliding object under the influence of forces . .3 1.2 Schematic images of adhesive (rubbing) wear, abrasive wear (Cut- ting) and corrosive (oxidation) wear ...... 5 1.3 Stribeck curve showing friction as a function of N (RPM), η (viscosity), P (load) differentiating different lubrication mechanism along with fluid thickness ...... 9

2.1 Mechanical mixture and ultrasonicator used to prepare nano lubricants ...... 20 2.2 Working principle of viscometer ...... 21 2.3 Rotating pressure vessel oxidation tester ...... 22 2.4 Schematic view of 4 ball wear tester ...... 24 2.5 Schematic view of ball on disk tribometer [92] ...... 25 2.6 Schematic view of scanning electron microscopy ...... 28 2.7 Pictorial presentation of Raman optical laser microscopy ...... 29

3.1 Process Illustration [72] ...... 31 3.2 Morphology of the material under SEM and TEM microscopy. TEM images of Material-1,2 and 3 are shown as, in order,a,b and c. Also, SEM images of Material-1,2 and 3 are shown as, in order, A, B and C. In addition, Image-1.1 shows multi layers structure of Material-1 [72] ...... 33 3.3 A = Raman spectroscopy of three forms of rGO, B = Intensity ratio of D band and G band (Id/Ig) [72] ...... 34

xii 3.4 Worn ball scar morphology investigation under high magnification of sample-1.1 (dashed-line: scar line; arrows: accumulated rGO film) [72] ...... 38 3.5 Average friction coefficient data for pure oil and nano lubricant [72] 40 3.6 Friction coefficient curves of different samples with respect to time (seconds) [72] ...... 40

4.1 Raman spectroscopy of reduced graphene oxide [92] ...... 43 4.2 TEM images of reduced graphene oxide under 50.00 kx and 150.00 kx [92] ...... 44 4.3 Fourier transform infrared spectroscopy of rGO [92] ...... 45 4.4 Four samples of reduced graphene oxide. A: Samples after mechanical mixing and ultra-sonication, B: Samples kept in steady state condition for 45 days [92] ...... 46 4.5 Friction coefficients of pure lubricant (blue) and three different samples at 0.01% wt concentration (orange), 0.05% wt concentration (grey), 0.1% wt concentration (yellow) [92] ...... 48 4.6 Wear rate for pure oil and three nano lubricants [92] ...... 49 4.7 Schematic image showing rGO nano particles in the contact patch at different concentrations [92] ...... 50 4.8 Morphology investigation using scanning electron microscopy on specimen wear tracks at 500 µm size and 120.00 kx magnification for pure oil (A), S-1 (B), S-2 (C) and S-3 (D) [92] ...... 51 4.9 Optical microscopy using false color analysis of worn scar in situ condition,where Pureoil (A), S-1 (B), S-2 (C), S-3 (D) ...... 52

5.1 Raman spectroscopy of graphene nano particles [88] ...... 55 5.2 Raman spectroscopy of titanium nano particles [88] ...... 55

xiii 5.3 TEM images of (A) graphene (B) titanium dioxide (C) hybrid additive (graphene and titanium dioxide in equal concentration) in powder form at 25.0 kX and 1µm [88] ...... 56 5.4 Kinematic viscosity and viscosity index of liquid lubricants [88] . . . 58 5.5 Oxidation stability time in minute [88] ...... 59 5.6 Friction coeﬃcient and wear data [88] ...... 61 5.7 Worn scar evaluation in low vacuum chamber at 40µm scale using SEM for 9 disk samples [88] ...... 62 5.8 Average mean surface roughness of the disk [88] ...... 63 5.9 Average friction coeﬃcient of nano additive grease at 10N, 20N and 30N normal load [88] ...... 64

6.1 Optical microscopy height parameters and area hole analysis performed on sample-P.O (Chapter-4) substrate ...... 83 6.2 Optical microscopy height parameters and area hole analysis performed on sample-1 (Chapter-4) substrate ...... 83 6.3 Optical microscopy height parameters and area hole analysis performed on sample-2 (Chapter-4) substrate ...... 84 6.4 Optical microscopy height parameters and area hole analysis performed on sample-3 (Chapter-4) substrate ...... 84 6.5 Optical microscopy height parameters and area hole analysis performed on S1 (Chapter-5) substrate ...... 85 6.6 Optical microscopy height parameters and area hole analysis performed on S2 (Chapter-5) substrate ...... 85 6.7 Optical microscopy height parameters and area hole analysis performed on S3 (Chapter-5) substrate ...... 86 6.8 Optical microscopy height parameters and area hole analysis performed on S4 (Chapter-5) substrate ...... 86 6.9 Optical microscopy height parameters and area hole analysis performed on S5 (Chapter-5) substrate ...... 87

xiv 6.10 Optical microscopy height parameters and area hole analysis performed on S6 (Chapter-5) substrate ...... 87 6.11 Optical microscopy height parameters and area hole analysis performed on S7 (Chapter-5) substrate ...... 88 6.12 Optical microscopy height parameters and area hole analysis performed on S8 (Chapter-5) substrate ...... 88 6.13 Optical microscopy height parameters and area hole analysis performed on S9 (Chapter-5) substrate ...... 89

xv List of Tables

2.1 Ball on disk friction and wear test parameters ...... 26 2.2 Height parameters obtained from optical microscopy in situ condition 26

3.1 rGO powder characterization based on density and manufacturing method [72] ...... 32 3.2 Sample characterisation base upon the additive material and concentration [72] ...... 32 3.3 kinematic viscosity and viscosity index [72] ...... 35 3.4 Rotating pressure vessel oxidation test [72] ...... 36 3.5 Wear scar diameter [72] ...... 38

4.1 Sample characterisation based on the concentration [92] ...... 45 4.2 Kinematic viscosity and viscosity index [92] ...... 46 4.3 Rotating pressure vessel oxidation test [92] ...... 47 4.4 Height parameters of the scar after performing ball-on-disk test . . 52

5.1 Samples classiﬁcation of liquid base lubricants (liquid nano lubricants) 57 5.2 Samples characterization of semi-liquid lubricants (grease nano lubricants) ...... 57

6.1 Friction coefficient of grease sample at 10N ...... 80 6.2 Friction coefficient of grease sample at 20N ...... 81 6.3 Friction coefficient of grease sample at 30N ...... 82

xvi Acronyms

ASTM = American Society for Testing and Materials RPM = Revolution per Minute SEM = Scanning Electron Microscopy TEM = Transmission Electron Microscopy STLE = Society of Tribologist and Lubrication Engineers WC = Tungsten Carbide AISI = American Iron and Steel Institute API = American Petroleum Institute RPVOT = Rotating Pressure Vessel Oxidation Test FTIR = Fourier Transform Infrared Spectroscopy AW = Anti-wear COF = Coeﬃcient Of Friction EP = Extreme Pressure FM = Friction Modiﬁer BL = Boundary Lubrication HD = Hydrodynamic Lubrication

TiO2 = Titanium Dioxide rGO = Reduced Graphene Oxide HC = Hydro Carbon TiNP = Titanium Nano Particle GNP = Graphene Nano Particles EP = Extra Polished WSD = Wear Scar Diameter kBr = Potassium Bromide

xvii Nomenclature

Ff = Friction Force

FN = Normal Force

Fs = Static Friction Force

Fk = Kinematic Friction Force

µs = static friction coeﬃcient

µk = Kinematic friction coeﬃcient

Mt = Metric tone

Cs = Centi stroke

Id = Intensity of D band

Ig = Intensity of G band Sq = Root mean square height Sa = Arithmetical mean height Sv = Maximum pit height Sp = Maximum peak height Ssk = Skewness Sz = Maximum height Sku = Kurtosis h = Height of ﬁlm σ = Applied stress

xviii Chapter 1

Introduction

1.1 Tribology

The word Tribology was derived from the word ’Tribos’ which means the science of rubbing. It was first discovered and made public by Dr. H. Peter Jost in the year 1966 after studying high failure rate in different industries and manufacturing plants [1]. However, the importance of tribology can be seen from early 3500 BC when people used round tires to transport heavy stones which indicates that people may have concerns about friction. Main laws of friction were first shown by Leonardo Da Vinci in his experiments where he explained that frictional force is directly proportional to the weight of the object [2, 3]. In the year 1338 AD, people used water as a lubricant while transporting large devices in order to reduce friction between the exposed surfaces [4]. The rapid growth of automobiles, machineries, and manufacturing industries in the twentieth century fortified tribological applications for miscellaneous purposes. Although, it became challenging for researchers initially as the field of tribology requires knowledge about physics, chemistry, material science, lubrication, machine design, fluid dynamics, solid mechanics, reliability, heat transfer and thermody- namics. Field of tribology is divided into two main streams according to the applications as follow- Macrotribology and Nanotribology or microtribology [5]. The main dif-

1 ference between both of them is: macrotribology tests are performed under heavy load condition where wear is associated to the material properties. On the other hand, nanotribology tests are performed under light load condition in which wear is proportional to the tribological environment between the contact patch. In most of the scenarios, both types of tribology are causing damage to the world’s economy. From the global energy perspective, 23% of energy is generated from the mating surfaces from which 20% is used to overcome the frictional force and 3% is used to remanufacture worn components [6]. Due to higher consumption of energy to overcome friction and wear, many researchers are trying to modify lubrication properties by reducing surface roughness, applying surface coating and using nano lubricants to the contact surfaces. Although, before changing lubrication properties, it is essential to understand the root causes of friction and wear to control them in an eﬃcient way.

1.2 Friction

Friction is an opposing force acting between the contact surfaces of the bodies which resists them from sliding of one body over another body. As explained, laws of friction were deciphered in the year 1493 by Leonardo De Vinci in his experiments. However, the actual three laws of friction were established by Guillome Amontons (1699) and Coulomb (1785). As per Guillome Amontons law, the frictional force is directly proportional to the normal force and independent of the contact area [7]. Moreover, according to Coulomb, frictional force is independent of sliding velocity [7]. The ﬁrst law of friction can be expressed mathematically as demonstrated in equation 1.1, where Ff = Frictional force, µ = Coeﬃcient of friction and FN = Normal force

Ff = µFN (1.1)

2 Figure 1.1: Free body diagram of sliding object under the inﬂuence of forces

The ﬁrst law of friction is valid for direct metal contact where no lubricant is applied. In that case, air or polish becomes a medium of lubrication as a boundary condition. However, the ﬁrst law is not valid for low elastic modulus polymers. Furthermore, friction is characterized into two categories such as static friction and kinematic friction. Static friction is the resisting force acting on the object when it is in stationary condition. The object will be steady until the applied force is lower than the static friction force (Fs) which is dependent on mass, geom- etry, electrostatic force and chemical reaction between the contact patch. On the contrary, an object will start moving when the applied force is higher than static force, termed as kinematic friction force (Fk). The static friction force (Fs) and kinematic friction force (Fk) are exhibited in equation 1.2, 1.3

Fs = µsFN (1.2)

Fk = µkFN (1.3)

Friction is essential in our daily lives for numerous tasks such as writing, walking, driving, and ample others. But, it damages industries as far as the economic and sustainable point of views are concerned. Generation of these friction forces can be mainly focused on chemical and physical interaction of surfaces. Along

3 with these two parameters, other factors aﬀecting friction forces are operating environment, speed, temperature, humidity, and normal load [8]. Hence, it is important to control friction at micro/nano level to reduce heat generating and wear in machines.

1.3 Wear

The gradual material removal rate of the surface is deﬁned as wear. Wear is not a material property however, it is considered as the response of the system which starts at a micro level and leads to the failure of the functionality of the component. Pros and cons of wear can be justiﬁed based on its diverse applications such as high wear rate is good for machining (material removal) operations, shaving, polishing, and writing. On the other hand, it reduces the service life of the surface. Along with that, there are environmental implications due to high wear rate, leading to issues such as global warming and sustainability of industries. Proper application of tribology can save 1460 MtCo2 and can increase the GDP by 1.4% globally each year [9]. Wear can be characterized based on its generation patterns such as mechanical wear (friction) or chemical wear (heat concentration). Additionally, six fundamen- tal types of wear are adhesive, abrasive, corrosive, percussion and erosion, fatigue and electric arc-induced wear [4, 10]. Besides, new technologies such as scanning electron microscopy SEM), atomic force microscopy (AFM) allowed researchers to identify the wear debris at micro, nano levels to predict the service life of the contact surfaces and also plan pre-maintenance to stop sudden failure [11, 12].

1.3.1 Adhesive wear

Adhesive wear is caused due to strong bonding between the surfaces which causes transfer or loss of material from less wear resisting body. Additionally, sliding con-

4 Figure 1.2: Schematic images of adhesive (rubbing) wear, abrasive wear (Cutting) and corrosive (oxidation) wear tact between the friction surfaces or higher contact surface area increases chances of adhesive wear. Adhesive wear can also precipitate high friction, plastic shearing, seizure of sliding body and transfer ﬁlm between sliding substrates [13–16]. It is important to control adhesive wear as generated debris can cause abrasive wear.

1.3.2 Abrasive wear

Abrasive wear originates by sliding harder particles or surfaces between the softer material under loading conditions which results in fracture or deformation of the exposed surfaces. Abrasive wear can be found in the form of parallel furrow under microscopic vision. To add more, worn debris of harder material can also cause grain fracture or grain removal and micro pits on the substrate [17]. Abrasive wear is directly proportional to the grit hardness, size of grit, shape of grit, and brittleness and toughness of the grit.

1.3.3 Corrosive wear

Corrosive wear can be caused by electrochemical or chemical reactions between the substrate and environment. Due to excessive concentration of oxygen in the air, it becomes challenging to prevent oxide ﬁlm on the material surfaces. On the other hand, when the battery phenomena exists, material starts getting corroded. The electrochemical corrosion is caused because anode begins to loose electron towards

5 the cathode which causes rusting on the surface. Also, this reaction can migrate from one point to another which can lead to thin material ﬁlm removal from substrate. This material ﬁlms are in the form of chlorides, oxides and sulphides. As a result, it starts reducing the material’s thickness and strength of the structure. This wear is found mainly in aqueous, high humidity and high temperature envi- ronments.

1.3.4 Erosive & Percussion wear

In high-speed machines, erosive wear is found due to projection of solid particles and liquid droplet formation on the surface. The magnitude of erosive wear is proportional to the velocity, impact angle, and hardness of the solid particles. Furthermore, ductile solid particles show the highest erosion at smaller impact angle. However, brittle material has shown the high erosion when the impact angle is normal to the surface [18]. In the case of a liquid droplet, the size and speed of droplets are important parameters as the surface pressure generation is in direct proportion. Higher the surface pressure, more likely to get erosion wear. Besides, continuous formation, growth and collapsing of bubbles generate cavitation erosion. Percussion erosion can be caused by continuous stress exertion on the body. Moreover, it is not a simple wear mechanism rather, it combines multiple wear mechanisms such as adhesive,abrasive and fatigue wear.

1.3.5 Fatigue wear

Fatigue wear generates from the micro cracks which were either generated from the machining processes or because of the abrasive or adhesive wear mechanism. In sliding and rolling contact, fatigue wear does not work until the hydrodynamic condition is maintained. However, fatigue wear takes places under high loading boundary conditions, where lubrication ﬁlms do not separate surfaces. After high

6 number of cycles, micro cracks propagate and break the surface. To add further, the material removal rate is not a concern in fatigue wear but the service life of the component surely is.

1.3.6 Electric arc wear

Electric arc induced wear is caused by potential energy stored in the thin ﬁlm which was initiated in the sliding process. As per joules law, this arc’s potential energy (Ej) can generate high heat energy for a fraction of time. This can damage the morphology of the substrate by changing its hardness and melting temperature. Additionally, this arc causes heat aﬀected zone on the substrate. Electric arc wear mechanism is found in ball bearings which are utilized in electronic applications. Besides, it does not cause more wear but this wear mechanism can encourage abrasive, corrosive and fatigue wear [19].

1.4 Lubricants

Lubricants are characterized based on the state of matter such as solid (dry lubricant), liquid, semi-liquid and gaseous lubricant. Industrial revolution increases the application of lubricant in multiple fields such as automotive, hydraulic, marine, machinery and aviation industry as a lubricator or to transfer power or to absorb shocks [20]. Solid lubricants are used when there is a higher fluctuation of load, temperature, pressure, and speeds. Additionally, solid lubricants such as graphite, molybdenum disulfide, PTFE and diamond-like carbon can be applied to the surfaces in the form of thin films or mounted on the substrate [21]. On the other hand, liquid lubricants are further characterized in biological, mineral and synthetic oil. The biological oil is obtained from animals or vegetables which can be used mainly in the food industry or cosmetic industry. Secondly, mineral oils

7 are used by a vast number of industries due to various factors such as easy avail- ability, cheaper and wear-resisting capabilities. Mineral oil is separated from crude oil by distillation and refinement process. Base oil has been characterized according to the viscosity, sulfur content and chemical formation. In addition, viscosity classification of the base oil varies from 5 [cs] to 700 [cs] at room temperature. On the other hand, sulfur concentration in the oil is important to provide better lubrication. Although, high concentration of sulfur can also accelerate corrosion rate of the seals. Based on the chemical formation, base oil can be classified in the form of paraffin, naphthalene, and aromatic base of the carbon chain. However, base oil had some challenges in working at high temperature and in order to overcome those challenges, synthetic oil came in the market. Synthetic oil is classified as synthetic HC, silicon analog HC and organohalogens to provide better lubrication under high temperature, high pressure, and worst chemical environment. Classification of this lubricant is essential to identify the application of lubricant based on different parameters. Moreover, American Petroleum Institute (API) has characterized base oil in Group-1 to Group-5 based on viscosity, sulfur content, saturation percentage and synthesizing process [22]. Most widely used semi-liquid lubricant is grease. Grease can be classified based upon the additive, thickener and base oil. Grease is classified further according to the use of thickeners such as soap type and non-soap type grease. In soap type grease, lithium, calcium and aluminum thickeners are used and in non-soap type grease inorganic micro/nanopowder is used as a thickener. The last ingredient is additive, which was used to enhance tribo characteristics of grease such as wear, oxidation, rust, thickness, and extreme pressure. Ability to improve tribocontacts of the liquid and semi-liquid lubricants is directly proportional to the lubrication mechanism.

8 1.5 Lubrication mechanism

Lubrication mechanism is a vital parameter to understand the oiliness of the substrate. Lubrication mechanism can be characterized by the parameters associated with maintaining fluid film thickness between the surfaces. When an external source of energy is used to maintain the fluid film thickness, it is called hydro- static lubrication and when the fluid film thickness is maintained without the application of any external forces, it is known as hydrodynamic lubrication (HD). HD lubrication is divided into three areas as per relative speed, viscosity, and pressure between the contact surface shown by stribeck curve [23, 24]

Figure 1.3: Stribeck curve showing friction as a function of N (RPM), η (viscosity), P (load) differentiating different lubrication mechanism along with fluid thickness [4]

9 1.5.1 Full ﬁlm lubrication

When thick fluid is pulled between the surfaces to achieve high load and velocity capabilities, it is termed as fluid film lubrication mechanism. In this mechanism, size of fluid film is 10 to 100 times thicker than the asperities which lead lubrication film parameter (h/σ) >5, where h is the height of film and σ is applied stress [4, 25]. Furthermore, this mechanism originates adhesive wear in the beginning as well as at the end of operations. Apart from that, fluid film lubrication is found most efficient as the film height is way higher than asperities.

1.5.2 Mixed ﬁlm lubrication

Mixed film lubrication is an intermediate stage between fluid film and boundary lubrication condition in which friction varies a lot. It is also called thin film lubrication as lubrication thickness (h/σ) is found between 1-5 [25]. In this mechanism, higher chances of surface contact can lead to adhesive or metal transfer wear.

1.5.3 Boundary lubrication

Boundary lubrication is also called as extreme pressure lubrication as the friction coefficient is at its peak in this condition. Additionally, lower viscosity fluid does not travel between the contact surfaces at a low speed which bring surfaces in direct contact. In this mechanism, h/σ is calculated less than 1 which discloses extremely thin film generation between the surfaces. In this scenario, chances of surface interaction is higher which leads plastic deformation to the fracture and generates a higher amount of heat [26]. Although boundary lubrication and mixed lubrication causes metal to metal contact, the presence of nano additives can prevent metal to metal contact by acting as nano bearings.

10 1.6 Nano additives

The demand for lubrication is growing due to industrialization and advancement of the automotive industry. To enhance the capabilities of lubricants, varied col- laborative studies are conducted. Out of those, combination of tribology and nan- otechnology has been proven as the most efficient combination to modify diverse characteristics. Those characteristics includes friction, wear, foaming, viscosity, oxidation, and corrosive resistance. Among the characteristics mentioned above, oxidation, corrosion and anti foaming additives are mainly applied to protect the base oil. On the other hand, viscosity modifier and pour point depressant are utilized to enhance base oil capabilities. However, dispersant, detergent, antifriction, extreme pressure, and anti wear additives can be used to protect the surface and maintain service life. To protect surfaces, nanoparticles generates protective film on the substrate and act as a nano bearing between frictional bodies [27, 28]. On the contrary, nanoparticles provide mending and polishing effect or chemical effect on the lubricant [29–31]. For various physical and chemical improvements, contrasting concentration of additives are used and classified as explained below.

1.6.1 Anti wear (AW) additive

Anti wear nano additives are used to mitigate metal to metal contact in boundary conditions and control the wear and friction under high temperature. The most widely used AW additive is zinc dithiophosphates and tricresyl phosphate [32–35]. Adding to this, better surface protection can be achieved by depositing this nano particle on the substrate and form a thin film which can be easily sheared along with the motion. However, chemical bonding between substrate and film can also affect the friction properties because of variation in shearing force of the film.

11 1.6.2 Extreme pressure (EP) additive

To control wear under high loading conditions, EP additives are added at diﬀerent concentrations. EP additives forms a triboﬁlm on the surface and mitigate abrasive chips or particles transfer on the surfaces which controls friction and seizure problems. AW additives are mainly used in automobiles or hydraulic systems where EP additives are used in the gearbox, or higher stress concentrated surfaces [36]. Commonly used EP additives are sulfur, chlorine, or phosphorous base compounds [37, 38].

1.6.3 Friction modiﬁer (FM) additive

The main purpose of FM additive is to form a triboﬁlm on the substrate to control friction coeﬃcient, scoring and micro pitting under boundary lubrication. Due to that, FM additives are also called as boundary lubrication additives. Re- cently, glyceride, oil-soluble molybdenum, Z-unsaturated tail group, acid, amid, and graphite compounds were studied as FM additives [39–41].

1.6.4 Dispersant additive

Detergent additives are applied to clean contact surfaces and neutralize acids formation in lubricant by reducing carbon sludge formation [42]. On the other hand, dispersant additive (ashless material) works more eﬃciently at a low temperature which is utilized to disperse oil insoluble products, water, fuel or any other contam- inants at low temperature [43]. Additionally, dispersant additives are applied in automobile engine oil or heavy duty diesel engine oil, but they can not be used with gas engine oil, auto transmission oil or aviation oils. Also, dispersant additives do not allow contaminant to disperse in the lubricant which prevents further damage. In addition, most of the base oil uses both dispersant and detergent additives together to suspend particle debris and clean them from the contact surfaces.

12 1.6.5 Antioxidation additive

These additives are utilized to maintain the relationship between the lubricant and oxygen particles. Ample factors aﬀect oxidation time of lubricant such as temperature, environment, water, and acids lead to greater reduction in service life and that can have a great impact on the economy of the lubrication industry. To overcome this issue, sulfur, zinc, and phosphorous compounds are widely accepted as antioxidation additives [44, 45]. The rate of oxidation reduces while moving from base oil or mineral oil to synthetic oil due to higher working temperature capabilities.

1.6.6 Viscosity modiﬁer

Viscosity is the core property of lubricant to enhance efficiency throughout temperature range. To achieve this, polymeric molecules are used as viscosity modifier additives. At low temperature, these polymers stay in normal condition but at higher temperature polymers start relaxing in the lubricant which maintains it’s thickness. Mainly these additives are applied in transmission oil, hydraulic oil, gear oil, steering oil, and various greases where greater temperature variation is encountered [46]. Based on the size of these polymers, bigger size polymers will maintain more thickness at high temperature but they are unable to shear under mechanical stresses which can affect friction characteristics. However, smaller size additives won’t maintain relatively higher thickness, but they are easily shear- able without affecting friction force. Nevertheless, more number of particles are required of smaller size FM additive [47].

1.6.7 Corrosion (rust) inhibitors

These additives protect surfaces from acidic attacks especially in water based lubricant. The rate of corrosion varies according to the quality of water, metal particles, and chemical compounds [48]. Rust inhibitors form a ﬁlm on the surface and mitigate direct environmental contacts. Corrosion inhibitors are mainly

13 used for storing parts for a long time, high and low temperature working environment, and ferrous based components. Corrosion additives are mainly classiﬁed into acidic, basic and neutral categories [49].

1.6.8 Pour point depressants

Pour point is the temperature at which lubricant will start loosing its liquidity due to wax crystallization or dispersed solid particles [50]. Furthermore, this wax crystalline separates at low temperature and removed from the mineral oil through ﬁltration process. However, removal of those crystals can manipulate viscosity of the liquid lubricant [51]. To maintain lubrication thickness at higher temperature, pour point depressants additives are added in to the oil. These nano additives are also classiﬁed based upon their compositions as explained in next section.

1.6.9 Titanium based nano additives

Researchers have tried ample organic and inorganic nano additives such as metal, metal oxide, metal composites, carbon-based materials (one-two-three dimension), boron-based material into the base oil [52]. In addition, titanium and its oxides are widely used by different industries because of high strength, corrosion resisting nature, and cost-effectiveness [53, 54]. For example, titanium powder was used by marine, aeronautical, chemical, paint, machinery industries to prevent rust and improve strength without adding more weight to the structure [53, 55]. In addition, sunscreen lotion contains the highest amount of titanium dioxide to protect skin from UVB and UVA2 lights [55, 56]. However, titanium had a bad standing in terms of sliding tribology due to unstable friction, wear and higher chances of seizing [53, 54]. As per the stability test and microscopy findings, 36% of nanoparticles should be smaller than 100nm for high dispersibility in the water [55]. In addition, the study shows that TiNP produces thick and rutile film on the substrate which can be used for rust proofing applications in tribology [57].

14 1.6.10 Carbon based nano additives

Carbon-based nano additives have revealed great improvement in tribology because of their versatile mechanical, electrical, thermal and optical properties. These materials are characterized in four forms based on their allotropes such as zero dimension (0-D), one dimension (1-D), two-dimension (2-D) and three dimension (3-D). In case of zero dimension, fullerene nano additive were tested in the past and has shown considerable extreme pressure and anti-wear properties [58]. In addition, the zero-dimensional additive has shown more eﬃciency only at low viscosity oil [59] which constrains them from using a wide variety of lubricants. One-dimensional carbon nanomaterials such as carbon nanotubes, nanowires, nanorods are used in tribology for diverse applications. Although, chemical inertness of 1-D material causes dispersibility problem in lubricant and start conglomerating at the surface, multi-wall carbon nanotubes can solve this problem and increase dispersibility [60]. Two dimension graphene is widely utilized in industry as an additive because of innumerable reasons such as diﬀerent density, high strength, and hardness [61]. Due to the wide variety of applications, innumerable processes have been developed to manufacture graphene. Based upon manufacturing process, wide range of lattice defect and the number of layers in graphene allotropes are found which are associated with tribological characteristics. Friction in case of graphene is caused by two parameters namely; friction between the tip and substrate and friction between the layers of graphene [52]. Three-dimensional carbon-based material graphite is mostly used as a solid lubricant.

Nanoadditives in the lubricant has shown a wide range of results using distinct parameters such as diﬀerent base oils, manufacturing processes, environmental parameters and nature of the load. In this research, three forms of carbon-based material reduced graphene oxide were tested under speciﬁc laboratory conditions. In addition, the best performed material was further evaluated at discrete concen-

15 trations to obtain highest tribological improvement. Apart from that, graphene and titanium dioxide and hybrid nanoadditives were also examined to analyze effect of individual additive and hybrid composition.

The rest of the thesis is organized as follows: Chapter 2 Covers experimental parameters, principles, and machines used to evaluate tribological, physical and chemical characteristics of liquid and semi-liquid nano lubricants. Chapter 3 Reveals the study of evaluation of three forms of reduced graphene oxide for tribological application. Along with that, this study revels how lattice defect, bulk density and morphology of reduced graphene oxide can deviate tribological properties of the base lubricant. Physical and chemical evaluation test were conducted to see the effect of nano additives on the base oil lubrication proporties. Chapter 4 Addition of nano additive also increases the economical cost of the lubrication industry and to find out optimum concentration of this additive this study was conducted. In which best material of study-3 was further examined at three different concentrations to find optimum concentration of additives in the base oil. Chapter 5 Graphene and titanium nano particles were tested as an individual additive at three concentration as well as hybrid nano additives were examined at best concentration using liquid and semi-liquid lubricant. Comparative study was conducted to see how individual additive can vary with respect to the hybrid additives. In addition, this study give an idea about how customization of lubricant can be possible using these additives based upon the applications. Chapter 6 In this chapter, all three studies are concluded along with possible future work.

16 Chapter 2

Experimental methods and characterisation

2.1 Introduction

In order to evaluate properties of the nano lubricants, experimental research work was conducted with the help of tribometers, physical as well as chemical testers. Along with that, topography and morphology study was conducted using SEM, TEM and Raman spectrometer. In this chapter detailed analysis of each instru- ments, used parameters, principles and applied standards were disclosed.

2.2 Material synthesis

In this research, five types of nanomaterials are utilized at various concentrations. As explained earlier, tribological behavior of nanoadditive is dependent on different parameters such as particle size, lattice defect, particle shape and dispersibility of lubricant in the base lubricant [52]. Because of that reason, it is very important to optimize the effective concentration of each nanoadditives. Furthermore, higher concentration of additives deteriorate industries economically and sustainability as it requires more material resources. Three studies are classified based upon

17 applied additive, concentration and conducted tests. In the first phase of research, three forms of reduced graphene oxides were used to evaluate the most efficient rGO at lower concentration. In addition, all three forms were synthesized using modified hummers method. However, different filtration methods and synthesis time was deployed to obtain variety in material characteristics. All three forms of rGO were manufactured at Kennedy labs, Canada.

2D graphene nano-platelets (GNP) were synthesized from aqueous graphite in the presence of lysozyme, which was obtained from Nova graphene,USA. Lysozyme graphene is pH sensitive which can be useful to prevent conglomeration of graphene in the base oil. This GNP was used for anti-cancer activity along with diﬀerent solvents. To understand the lattice defects and morphological study Raman spectroscopy tests were conducted. Graphite was mixed with lysozym in the ratio of 1:2 and ultrasonicated for 6 hours. Right after that, collected black powder was passed through centrifugation process to remove unexfoliated graphite and only multilayered pure graphene was collected. In addition, Graphene produced by this method has been used to mitigate cancer cells at certain weight concentrations [62].

Titanium dioxide nanopowder was acquired from Sigma-Aldrich. The average particle size of this nanopowder was obtained from TEM spectroscopy as 21nm. The white powder of titanium dioxide has a surface area of 35-65 m2/g and trace metal analysis demonstrates 99.5% purity. Moreover, this nanopowder is used for many applications such as sunscreen, photo-voltaic, food, and many personal care applications. However, in this study, TiO2 is tested alone as well mixed with graphene to evaluate tribological applications of hybrid nanoadditive in the base oil.

Base oil for these tests was selected as 99.9% pure oil obtained from Petro Canada (200 Neutral medium HT). The aim of selecting pure oil was to understand the tribological eﬀect due to nano additive without any presence of pre-added addi-

18 tives. Weight density of the lubricant was found as 0.865 kg/l. In addition, pure base oil contains 20.2 ppm water according to the material data sheet provided by Petro Canada. On the basis of viscosity calculation, base oil falls under Group-2 or 3 as per API standards [63].

Semi-ﬂuid grease lubricant used in the third phase of study was purchased from Canadian tire. As explained in the introduction, there are various types of grease available in the market however, NLGI grade-2 commercial grease (without any pre-added additives) was selected for this study. This grade is mainly utilized for automotive, industrial, marine and farm applications. The working temperature range of commercial grease was found between 15◦Cto 120◦C.

2.3 Mechanical mixture

For homogeneous dispersibility of nano-additives in the base oil, dual blade mechanical mixture was deployed. Furthermore, to break rGO, GNP and TiNP nano ﬂakes, 1500RPM were used for 30 min. In addition, semi-liquid nano grease samples were prepared using magnetic mixtures. Once the nano-additives are mixed, they were ultrasonicated for 20 to 30min at room temperature by using ultrasonicator [64]. Along with that, the auto-calibrated digital weighing scale was used to precisely measure the weight of additives up to 4 decimal.

19 Figure 2.1: Mechanical mixture and ultrasonicator used to prepare nano lubricants

2.4 Viscometer

Viscosity is the internal resistance of the fluid layers, to flow at different temperature range and maintain fluid film lubrication condition. The viscosity is an important property of liquid lubricant because it also prevents the high heat concentration on the substrate. Additionally, it gives an idea about lubricant storage, handling and operating condition. Apart from that, lubricant’s viscosity at higher temperature is directly proportional to the lubrication consumption rate and viscosity at lower temperature uphold sealant characteristics of the lubricant [65, 66]. To evaluate the effect of nano-additives on the liquid lubricants, viscosity test was conducted as per American Society for Testing and Materials (ASTM-D445) at Kinnectrics, ON, Canada. Moreover, viscosity index calculations were performed as per ASTM-D2270 [67]. To perform these tests, capillary viscometer was utilized where nano lubricant was filled overnight in a controlled environment as shown in Figure - 2.2. After giving 40◦Cand 100◦Ctemperature bath, total flow time was recorded to reach at the original height of lubricant. After that, recorded time was multiplied by a calibration constant of viscometer which gives the value of

20 Kinematic viscosity [67]. After each successive test, capillary was cleaned using ethanol solvent. Besides, hot air was passed through the capillary, for at least 2-3 minute until ethanol evaporated. To calculate viscosity index, ASTM standard formula was used where correspond- ing L and H values were obtained from the table based on kinematic viscosity values. Furthermore, U will be viscosity of oil at 40◦C[67].

L − U V.I = X100 (2.1) L − H

Figure 2.2: Working principle of viscometer

2.5 Rotating pressure vessel oxidation test (RPVOT)

Anti-oxidation tests were conducted as per ASTM standard D2272 at Kinectrics, ON, Canada to evaluate the eﬀect of diﬀerent nano additives on lubricant oxidation stability [68]. As oxidation stability is directly proportional to the service life

21 of the lubricant, which can have a greater eﬀect on economic and maintenance performance of the industries. As the oxidation numbers increase, a lubricant will typically start producing acids and other insoluble oxidants which can hinder the operating capability of a component. In addition, more highly oxidized lubricant can produce varnish and sludge, which will inhibit the lubricant’s functionality of cooling and removing debris from contact surfaces [69]. To perform oxidation stability test, lubricant, water, and copper catalyst were placed in pressurized gauge. Right after, oxygen was injected into the vessel at 90psi as shown in Figure 2.3. The temperature was elevated at 150◦Cand vessel was rotated at 100 rpm. Additionally, time was recorded to drop the pressure to 14.5psi which is called as oxidation time of lubricant. Testing apparatus was cleaned by ethanol solvent and compressed clean air was passed as per the ASTM standard after each test [68].

Figure 2.3: Rotating pressure vessel oxidation tester

2.6 Fourier-transform infrared spectroscopy

FTIR spectroscopy was conducted for particle count in nano lubricant according to ASTM-D7619 and D7647. The main reason to conduct this test was to understand

22 the particle distribution in the nanolubricant after using mechanical mixture and ultrasonication. To achieve consistent agitation of nanoparticles, lubricants were shook rigorously and then 1ml of liquid lubricant was transferred on Potassium bromide (KBr) plate. Furthermore, these tests characterize number of particles based on their size and generates cleanliness codes as per ISO standard [70]. Per- formance of FTIR spectrometer was conﬁrmed by analyzing reference polystyrene material.

2.7 Tribometers

Tribometers are widely deployed in the lubrication industry to compare the tribological property of lubricants under varied physical and environmental conditions. These machines are classiﬁed as four ball test, ball/pin on disk test, reciprocat- ing/sliding test, low and high vacuum test, low and high-temperature test, biological wear and corrosion wear test based on the testing capabilities and working principles. Modern tribometers can perform test in situ conditions as well as optical microscopy can be carried out without removing samples from the chamber. In this research two types of tribometers were deployed to test liquid lubricant such as four ball wear tester and ball on disk tribotester. However, Semi-liquid lubricants were evaluated using shaft-on-plate tribometer.

2.7.1 4 ball wear tribometer

4-Ball wear preventive tests were conducted on rGO materials in study-1 according to ASTM-D4172 standard. Since it is very important for the substrate not to have any scars before test, extra polished (EP) grade-25 balls were utilized for measuring wear properties. According to ASTM, balls with 12.7mm in diameter size were compressed under 147 ± 2 N as shown in Figure 2.4 [71]. In addition, the temperature of test chamber was maintained at 75◦Cand the top ball was rotated at 1200 ± 60 r/min for 60 ± 1 min.

23 Figure 2.4: Schematic view of 4 ball wear tester

Before running each test, all the components of the equipment were cleaned to ensure that, they were dust and debris free. Adding to that, new steel balls were utilized for each study. In addition, the cup was ﬁlled with lubricant above the height of the ball surface to have enough lubricant between the contact surfaces. Furthermore, each test was performed twice and the average values were considered to obtain accurate data about wear scar diameter (WSD).

2.7.2 Ball on disk tribometer

Friction and wear tests were carried out using the ball-on-disk tribometer in situ conditions at Nanovea, CA, USA. To evaluate the eﬀect of nano-additives under loading conditions 20 N and 30 N loads were applied to the pin as shown in Figure 2.5. Each tests were conducted for 20 minutes, during which the rotating speed of the disk was increased from 0.01 rpm to 150 rpm logarithmically. Wear track diameter was chosen as 20 mm where the ball diameter was 6mm. Also, the test temperature was maintained at 24◦Cand humidity was kept at 40% for all the tests

24 to overcome the environmental eﬀects on the lubrication properties as shown in Table 2.1.

Figure 2.5: Schematic view of ball on disk tribometer [92]

To calculate wear rate and surface roughness, a 3D profilometer (T-14-0133) was used in situ condition. The self-turning motor was utilized to operate tribometer. Besides, strain gauge sensors were attached on both sides of the load arm to remove sensor errors. Disks for tribotests were manufactured using AISI-316-L stainless steel material which has 50 mm diameter and 6mm thickness. Moreover, 6 mm diameter SS4400C balls were deployed as non sacrificing material. The disk was machined on the lathe at ONTECHU facility and had been polished using 200, 400, 600, 800, 1500 and 2000 grits on Nano polisher-2000. Also, using 3D optical profilometer wear track volume of the disk was precisely calculated. Optical microscopy was conducted as per ISO-25178 standard to study different height and depth parameters on the wear scar morphology. In addition, root mean square (Sq), arithmetic mean height (Sa), maximum pit height, maximum peak height, skewness, kurtosis and maximum height were evaluated from the test

25 results as displayed in Table 2.2.

Test parameters Unit value Test duration 20 min Speed 0.01-150 RPM Ramp type Logarithmic Radius 10mm Revolution 312 Revolution Ball material SS440C Ball diameter 6mm

Table 2.1: Ball on disk friction and wear test parameters

Sq Root mean square height Standard deviation of the height of the spikes on the scar Sa Arithmetical mean height Absolute height of the spikes from the mean plane Sv Maximum pit height Minimum height of the spikes from the mean value Sp Maximum peak height Maximum height of the spikes from the mean plane Ssk Skewness Symmetry of the height distribution Sz Maximum height Diﬀerence of highest point and the lowest point of the surface Sku Kurtosis Kurtosis of height distribution

Table 2.2: Height parameters obtained from optical microscopy in situ condition

2.8 High resolution electron microscopy

2.8.1 SEM (Scanning Electron Microscopy)

SEM microscopy was employed to evaluate the morphology of the material as well as to investigate the worn scar surface at high magniﬁcation. As shown in Fig- ure 2.6 electron gun emits electron beam which is transmitted through magnetic lens via condenser lens. Using the deﬂection coil, position of the beam was controlled. Besides, it is important that material is highly conductive otherwise it causes charging problems which does not produce high-quality results. However, for non-conductive material, gold and the platinum coatings are other options to

26 run microscopy. The basic principle of SEM states that focused beam of the elec- trons will be transmitted to the surface and using the secondary electron detector, the peculiar pattern of an electron will be detected. Using the intensity of peculiar pattern SEM images are generated. Along with that, intensity of the black and white color of an image is classiﬁed based on atomic number. In this research, SEM was used at York University, Toronto, Canada to obtain magniﬁed images.

2.8.2 TEM (Transmission Electron Microscopy)

TEM spectroscopy was conducted to analyze internal structure classiﬁcation. Due to that, thickness of the substrate should be very thin. Furthermore, beam of elec- trons will be transmitted throughout the material which allows TEM to magnify the structure 50 million times more than actual size. Along with that, TEM is exe- cuted for a small size sections and 2D projection of the morphology. TEM used for this study was conducted at The Hospital for Sick Children (Sick Kids), Toronto, Ontario, Canada to understand the diﬀerence between the nano additives.

27 Figure 2.6: Schematic view of scanning electron microscopy

2.9 Raman spectroscopy

The chemical composition of reduced graphene oxide, grapehen and titanium dioxide was studied using Raman spectroscopy (Renishaw Imaging microscope system 2000, located in Raman Lab at UOIT, ON, Canada). In the first part of the research, three forms of rGO are classified based upon the difference in the raman peaks. Additionally, The ratio of intensity at the D band and G band was calculated to find the graphitic disorder and correlate with the mechanical properties of the additives. However, in last phase of study, titanium dioxide and graphene were characterized based upon number of layers using Raman spectroscopy.

28 Figure 2.7: Pictorial presentation of Raman optical laser microscopy

2.10 statistical analysis

To perform comparative analysis Microsoft Excel 2016 was utilized. Along with that, SEM images were studied using Image J software in which 5% error was considered. The presented error bars in the graphs reﬂects the standard errors obtained from ASTM standards or manufacturer data sheet. Besides, all the data used in this research shows p-value is less than α, which concludes that null hypothesis is incorrect. Rejected null hypothesis proves that statistical claims made in this research are correct.

29 Chapter 3

Tribological behaviour of three forms of rGO

3.1 Introduction

1 In the first phase of research, tribological effects of three different forms of reduced graphene oxide (rGO)-2D nano additives were investigated using base oil. All three forms of Reduced graphene oxides were manufactured using a modified Hummers method. However, various filtration techniques and synthesis time was utilized to manufacture the rGO having different density and lattice defects. After adding nano additives to the base oil at 0.01%w/w concentration, physical and chemical characterization tests such as viscosity test, four-ball wear test, rotating pressure vessel oxidation test (RPVOT), resistivity test and friction coefficient test were conducted as shown in Figure 3.1. The aim of this study was to evaluate possible tribological applications of three forms of rGO as a nanoadditive to improve wear and frictional properties of liquid lubricant. The presented results disclose that rGO with least density having concentration of 0.01% can reduce the wear of the material by 10.6% as well as friction by 6.3% with respect to the base oil. Promising effect on wear and friction resistive capabilities allows rGO to be used

1Results of Chapter-3 has been published as a journal paper titled âĂİFriction and wear properties of base oil enhanced by diﬀerent forms of reduced grapheneâĂİ in AIP Advances Journal. https://doi.org/10.1063/1.5089107

30 for commercial applications. In addition, performed physical and chemical characterization tests concluded that inclusion of rGO in a base oil does not compromise oxidation stability and viscosity of lubricant [72].

Figure 3.1: Process Illustration [72]

3.2 Characterization of the rGO

3.2.1 Bulk density

Density was measured using a 0.15 cc micro spoon and a calibrated digital scale having a least count of 0.0001 g (Cole-Parmer Symmetry PA 124E). The weight of each sample was measured four times to evaluate the average density. Visual inspection of all three rGO revealed that Material-1 (As displayed in Table 3.2 was found to be flaky compare to other materials. However, material-3 was in the form of conglomerated chunks of nano particles and material-2 was somewhere in between both of them. Adding to that, material-1 with lesser density has demonstrated more number of particles which can effectively produce nano lubrication film on the substrate. The presented data in Table 3.1 disclose that material-1 has the lowest density 2.0 mg/cm3, where material-3 has the highest density of 7.5 mg/cm3 and material-2 has 2.9 mg/cm3. The lower density of material-1 indicates that rGO particles had more porous structure and relatively lesser weight of flakes which can directly affect tribological and conglomeration properties of lubricant. Besides that, Table 3.2 represents sample characterization used in chapter-3 based

31 upon the nano additives concentration and type of additive used in nanolubricants.

Materials Density Method of fabrication Material-1 2.0 Modified mg/cm3 Hummers Material-2 2.9 Modified mg/cm3 Hummers Material-3 7.5 Modified mg/cm3 Hummers

Table 3.1: rGO powder characterization based on density and manufacturing method [72]

Additive Material Weight of Oil Additive con- Sample Number centration Pure oil 150g NA P.O. Material-1 150g 0.01%w/w 1.1 Material-2 150g 0.01%w/w 2.1 Material-3 150g 0.01%w/w 3.1

Table 3.2: Sample characterisation base upon the additive material and concentration [72]

3.2.2 Topological analysis of rGO

To evaluate topology of three rGO, Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) were employed to investigate the morphology of these materials under 30kV high vacuum mode. To avoid conglomeration of rGO particles under SEM, ethanol (90% w/w) solvent was utilized. Holey grid was employed to capture rGO particles after evaporation of ethanol solvent. In addition, internal structure of rGO was examined using TEM microscopy. As TEM works only on thin samples, it is important to capture rGO in one thin section. To accomplish that, powders were placed in to the bullet shape mold in which spur epoxy resin was ﬁlled and left for polymerization for 12 hours in an oven at 65◦C. Once the mixture coagulated, 90nm thin sections were cut using a diamond knife on a Leica UC7 micrometer. After preparing the samples, an

32 FEI Tecnai 20 transmission electron microscope was used to take images at higher magniﬁcation. Figure 3.2, A,B,C represents the SEM images of rGO material-1,2,3 where a,b,c reveals to the TEM images of material-1,2,3. Detail analysis unfold that Figure 3.2(A) has transparent nanoparticles stippled on the plate due to less conglomeration characteristics. Figure 3.2(B) evident that material-2 has shown distinctly curved structure in which some areas are highly dense and particle are less scattered compare to material-1. Moreover, Figure 3.2(C) manifest extremely aggregated and very dense compare to rest of two materials. Also, Image J thresh- old analysis of three TEM images reveals that moving from material-1 to 3 less number of particles are scattered on the section.

Figure 3.2: Morphology of the material under SEM and TEM microscopy. TEM images of Material-1,2 and 3 are shown as, in order,a,b and c. Also, SEM images of Material-1,2 and 3 are shown as, in order, A, B and C. In addition, Image-1.1 shows multi layers structure of Material-1 [72]

3.2.3 Raman spectroscopy

To further evaluate crystalline size, lattice defects, and degree of hybridization, Ra- man spectroscopy was deployed on the rGO. Renishaw Raman imaging microscope 2000 machine was used at 514nm laser excitation and 1% power for 10 accumulation to obtain Raman spectra for each sample. To diﬀerentiate rGO based on the spectra, D and G bands were studied. The D band stands for, out-of-plane breath- ing mode due to defects on the corners or sides, occurred while reduction process.

33 Adding to that, G band represents the scattering of out-of-plane E2g phonon from sp2 domain [73]. Another important parameter is the ratio of the intensity at both shifts (ID/IG) which is directly proportional to the degree of disorder [74]. As shown in Figure 3.3(A), D and G bands of material-1 were shifted at 1371 cm−1 and 1601 cm−1 respectively; for material-2, D band was shifted at 1361 cm−1 and G band was found at 1598 cm−1; and for material-3, D band and G bands were

−1 −1 broadened at 1354 cm and 1594 cm respectively. The intensity ratio (ID/IG) was calculated as 0.83, 0.85 and 0.87 for material-1,-2 and -3 as shown in Figure 3.3(B). Reduction in the ratio leads to conversion of sp3 to sp2 carbon [75]. The lowest intensity ratio of material-1 leads to higher restoration of carbon lattice compare to rest two rGO. On the other hand, material-3 which has higher ratio which leads lowest restoration of carbon lattice [75, 76]. Moreover, study reveals that, intensity of G ’ band is inversely proportional to the number of layers [77]. As shown in spectra, G ’ band was observed around 2905 cm−1. Also, intensity peaks for G ’ for all three materials illustrates that, moving from material-1 to 3 intensity increases which validates that material-1 has highest number of layers, material-2 has intermediate and material-3 has lowest number of layers [78].

Figure 3.3: A = Raman spectroscopy of three forms of rGO, B = Intensity ratio of D band and G band (Id/Ig) [72]

34 3.3 Kinematic viscosity and viscosity index

An important requirement for a lubricant is to maintain its film thickness and pro- long hydrodynamic lubrication through a wide temperature range [79, 80]. Fur- thermore, friction and wear reduction can be achieved by keeping rubbing surfaces apart at all temperature ranges. The value of the friction coefficient changes according to the Hersey number, which is a function of viscosity, load and velocity [80]. Higher concentration of nanoadditives increase the number of particles in the oil which can possibly accelerate the sludge formation. To understand the effect of additives on viscosity properties, kinematic viscosity test was performed as per ASTM-D445 and viscosity index was obtained from ASTM-D2270 standard chart [67].

P.O. 1.1 2.1 3.1 Viscosity at 41.5 41.2 41.2 41.2 40◦C(mm2/s) Viscosity at 6.3 6.4 6.4 6.4 100◦C(mm2/s) Viscosity index 98 104 104 104

Table 3.3: kinematic viscosity and viscosity index [72]

As indicated in Table 3.3, comparative results of kinematic viscosity and viscosity index (VI) for four samples were obtained which recommends that there is a minor reduction in the viscosity at 40◦C. However, the viscosity slightly increases at 100◦Cwhich cumulatively increases the viscosity index of nanolubricant compared to the base oil. Increment in the viscosity index allows lubricant to work in a broader temperature range without drastically changing kinematic viscosity. Along with that, reduction in viscosity at a lower temperature enables the reduction in friction which improves the eﬃciency of rubbing surfaces. Also, as per API standards, if the VI is between 80 and 120, then additives can be dispersed easily in to the base oil [81].

35 3.4 RPVOT

The reduction process of graphene to reduce graphene oxide leaves some particles of oxygen between the lattice. This oxygen particle may have influence on the oxidation stability of lubricant. The oxidation stability decides the service life of lubricant and that is why it is essential for lubrication industry to determine oxidation stability. The antioxidation capabilities of lubricant was measured by oxidation test in the presence of high temperature, high pressure, oxygen, and catalyst. To obtain comparative results ASTM-D2272 standards were employed for each test. In which, time to drop the pressure from 90psi to 14.5psi was measured. As shown in Table 3.4, oxidation time for pure oil is 27.1 min and for the three rGO samples, it is 26.1, 26.8 and 25.4 minutes respectively for material-1, -2 and -3. The negligible change in oxidation time concludes no significant effect on oxidation stability of the base oil [68, 80].

Material Antioxidation time in min P.O. 27.1 1.1 26.1 2.1 26.8 3.1 25.4

Table 3.4: Rotating pressure vessel oxidation test [72]

3.5 Resistivity test

The lubricants are widely used in diﬀerent industries for many purposes out of them, application of lubricant in electric applications are widely used. Base lubricants are highly resistive however, addition of polar nanomaterials and variation in temperature can aﬀect conductivity of base oil. Moreover, higher conductivity of lubricant can escalate the chances of micro sparks in the system [82]. As rGO is conductive and hydrophobic, there is always a possibility of reduction in resistivity. The resistivity of the mixtures was measured as per ASTM D1169. Results show that the resistance value was higher than 50000 GΩ-cm for all the

36 samples including the base oil which indicates that the additives were not causing a noticeable reduction in resistivity of the lubricant [83].

3.6 Four ball wear test

When two surfaces are in a sliding, rolling, or impact motion or when they are directly exposed to the environment, it causes adhesive, abrasive, fatigue, erosion or chemical wear. These conditions can damage the contact surfaces. The wear is not a material property but it is directly proportional to the design of the tribological system. High cost of remanufacturing worn surfaces motivates designers to apply coherent lubrication system. Nanotribology gives an opportunity not to just analyze a system at the micro level however, it also entitles to understand the wear behaviour and lubrication mechanism between the surfaces at the nano level. This allows researchers to evaluate tribological properties of lubricant using diﬀerent test parameters. As explained in experimental section, all the parameters were selected based upon ASTM standard. Also, each component of the equipment was cleaned to ensure that it was dust and debris-free. Before running each test all the samples were ultrasonicated for another 20 minutes to guarantee a stable and homogeneous mixture. The average WSD of pure oil was found as 0.94 mm at 147 N normal load. As displayed in Table 3.5 Material-1,-2, and -3 had reduction of 0.10 mm, 0.07 mm and 0.03 mm respectively in WSD compare to the pure oil. It shows that adding one of three forms of rGO nanoparticles can enhance wear preventive characteristics by acting as nano bearings between the substrates. Out of all three forms of rGO, material-1 has shown relatively less wear due to higher surface area and less lattice defects. Beside, frictional force in the rGO nanolubricant can generated due to the friction between interlayers shearing and friction between the two balls. To evaluate wear mechanism of material-1 nanolubricant, substrate was analyzed under SEM microscopy.

37 Samples Wear scar diameter Reduction in WSD (WSD) compared to base oil 1.1 0.84 mm 0.10 mm 2.1 0.87 mm 0.07 mm 3.1 0.91 mm 0.03 mm

Table 3.5: Wear scar diameter [72]

3.6.1 Substrate topography investigation

To investigate the nature of topography and deposition of the rGO particles on the scar, high magnification microscopy was conducted using SEM. Figure 3.4 shows the full size of the scar at 500 µm and revels thin scar lines at higher magnification of 20 µm. Furthermore, arrows indicate the deposition of rGO material-1 on the scar lines which proves that rGO invade in between the surfaces. Aforementioned in SEM, material-1 has less conglomeration compare to other two forms and because of that, rGO platelets deposited on the contact patch as shown in higher magnification image. Additionally, SEM image of material-1 do not reveal any deep groves or direction marks on the scar.

Figure 3.4: Worn ball scar morphology investigation under high magniﬁcation of sample-1.1 (dashed-line: scar line; arrows: accumulated rGO ﬁlm) [72]

38 3.7 Average friction coeﬃcient

To evaluate the effect on the friction coefficient of the lubricant, the shaft-on-plate machine was used. All friction tests were performed at 30N load for 20 minute duration. To obtain accurate data, each test was performed three times and average results were considered for the study. Figure 3.5 demonstrates the reduction in average friction coefficient for Material-1 and Material-2 compared to the base oil. Along with that, friction coefficient vs time graph shown in Figure 3.6. In addition, the normalized friction coefficient lines were shown along with deviation graph. In the case of base oil, linear line shows that there is not much variation in friction coefficient along with the time. Nevertheless, all three nano lubricant shows higher deviation compare to the base oil due to the interference of nano-additives between the surfaces. In the case of material-1, it seems to have high friction under boundary condition. However, once the hydrodynamic condition is achieved, gradual reduction in friction was observed. On the other hand, material-2 demonstrated slight increment in friction. Contrary, it was lower than pure oil throughout the time. Apart from that, the conglomeration of bigger-sized particles at the contact surfaces for Material-3 results in a higher friction coefficient throughput the test. These results prove that Material-1 is relatively more effective in altering friction and wear properties under the laboratory conditions.

39 Figure 3.5: Average friction coeﬃcient data for pure oil and nano lubricant [72]

Figure 3.6: Friction coeﬃcient curves of diﬀerent samples with respect to time (seconds) [72]

40 Chapter 4

Further investigation of the most eﬃcient rGO

4.1 Introduction

1 The main motivation of this study is to find optimum concentration of best rGO material(Material-1) to minimize friction and wear of the material. To evaluate optimum concentration of material-1, three samples were prepared at 0.01% w/w, 0.05% w/w, and 0.1% w/w concentrations. To analyze the direct effect of rGO on tribological properties, 99.9% pure oil was used as a liquid lubricant. Comparative tribological study was done by performing a ball-on-disk wear test in situ, under harsh conditions. Furthermore, wear surface morphology was analyzed using a non-contact 3D optical profilometer. The topography of worn surface was studied using SEM microscopy. The basic lubricant properties such as viscosity and oxidation number were evaluated and compared for all four samples including pure oil (control sample) as per ASTM standards. Findings of all these tests manifest that adding rGO nano platelets at 0.05% w/w showed a significant reduction in friction and wear. This can be very promising for increasing the life span of contact surfaces in the industries. Oxidation and viscosity tests also proved that adding

1 Results of Chapter-4 has been published as a journal paper titled ”Eﬀects of Reduced Graphene Oxide (rGO) at Diﬀerent Concentrations on Tribological Properties of Liquid Base Lubricants” in MDPI journal of Lubricants. https://doi.org/10.3390/lubricants7020011

41 rGO nano platelets to all samples does not sacriﬁce the physical properties of the lubricant.

4.2 Material characterisation

4.2.1 Raman spectroscopy

Raman spectroscopy was conducted to characterize the chemical composition of reduced graphene oxide in terms of its nature, size of crystalline, and structural disorder. A 514 nm laser spectra was utilized to obtain D, G, 2D and D+D’ bands using monochromatic light. From past research, the D band indicates defects in the domain, where increasing the intensity of the D band leads to the formation of more sp2 lattices because of oxidation. The G band stands for the ﬁrst order dispersion of E2g phonon which increases C-C tangential vibration or stretching [84]. The ratio of the D band over G band is crucial to identify defects in the graphitic structure. As shown in Figure 4.1, Raman spectra show the D band at 1357 cm−1 and the G band at 1601 cm−1The intensity ratio of the D band over the

G band (ID/IG) represent the lattice size. A higher ratio leads to bigger lattice size and as the size of the lattice increases, defects of the sp2 carbon domain. However, for rGO this ratio was found to be 0.84, which is low enough to have fewer domain defects [76, 85]. Furthermore, the 2D peak is attributed to the number of layers in reduced graphene oxide. As displayed in spectra, 2D peak attributed at 1845 cm−1 and D+G peak at 3178 cm−1. Reduction also increases the intensity of the 2D peak, as demonstrated in the spectra. The intensity of 2D peak was found to be greater, which illustrates that rGO was supremely reduced [75, 86]. As described, rGO is hydrophobic and does not allow more oxygen groups between the layers, enabling it to generate gaps between the layers and friction less conduits in the micro structure. Moreover, it can accommodate liquid lubricants between the layers. It has been found that mechanical, tensile, and compressing strength are directly proportional to the degree of reduction. The rGO used in this study was highly

42 reduced, leading to pronounced mechanical, tensile and compression strength [75]. It is very important for rGO nano additives to mitigate metal-to-metal contact and enhance wear preventive properties.

Figure 4.1: Raman spectroscopy of reduced graphene oxide [92]

4.2.2 Transmission electron microscopy

TEM microscopy was evaluated at 50 kX magnification revealed that reduced graphene nano platelets aggregated randomly, as explained in Figure 4.2. The magnified view disclose that few layers of rGO were attached in the hexagonal lattice. Multiple layers of rGO were identified, as shown by the arrows in the images taken with 150 kX magnification. Due to fine nano particles, rGO was able to provide high surface area compared to graphene oxide, and this can be used to accelerate the nano platelets0 penetration into the mating surfaces to enhance tribological properties.

43 Figure 4.2: TEM images of reduced graphene oxide under 50.00 kx and 150.00 kx [92]

4.2.3 FTIR particle count test

As larger particles can damage the filtration system of hydraulic system which can lead too higher filtration cost. In order to investigate rGO particle size, FTIR test was conducted. FTIR test results show the total number of graphene particles per 1 ml of oil. As displayed in Figure 4.3, most of the nanoparticles size ranges from 4 µm to 38 µm. Besides, smaller sized particles can efficiently infiltrate between the surfaces and reduces wear as well as friction. It is necessary to consider particle size and filtering class for future applications because some larger particles would be filtered out from the nanolubricant.

44 Figure 4.3: Fourier transform infrared spectroscopy of rGO [92]

4.3 Samples characterisation

As explained in Table 4.1, three weight concentrations of rGO nanolubricant were prepared such as, 0.0%w/w (base oil), 0.01% w/w, 0.05% w/w, and 0.1% w/w without any surface modification of reduced graphene oxide. Figure 4.4 shows the difference between the samples immediately after preparation (A) and after keeping it in steady state for 45 days (B). Visual inspection revealed that S-1 and S-2 concentration does not have significant gravitational effect on the particles precipitation. However, for the higher concentration(S-3) it seems that particles started conglomerating and were deposited on the bottom surface of the plastic container. This illustrates that suspension additives are required for the 0.1% w/w or higher concentrations, for appropriate amalgamation of particles in the nano lubricant.

Additive Material Concentration of additive (%w/w) Sample Number NA NA P.O Material-1 0.01 S-1 Material-1 0.05 S-2 Material-1 0.1 S-3

Table 4.1: Sample characterisation based on the concentration [92]

45 Figure 4.4: Four samples of reduced graphene oxide. A: Samples after mechanical mixing and ultra-sonication, B: Samples kept in steady state condition for 45 days [92]

4.4 Kinematic viscosity

To evaluate the eﬀect of the temperature ﬂuctuation of the pure oil due to the rGO nano additive, kinematic viscosity test was performed at 40◦Cand 100◦C, according to ASTM D445 standards. As manifested in Table 4.2, viscosity at 40◦Cand 100◦Care 41.5 mm2/s and 6.3 mm2/s for the pure oil. In case of other three samples, viscosity is around 41.2 mm2/s and 6.4 mm2/s which indicates a negligible decrease in viscosity at lower temperatures and slight increment at higher temperatures.

P.O S-1 S-2 S-3 Viscosity at 40◦C(mm2/s) 41.5 41.2 41.2 41.2 Viscosity at 100◦C(mm2/s) 6.3 6.4 6.4 6.38 Viscosity Index 98 104 104 103

Table 4.2: Kinematic viscosity and viscosity index [92]

The viscosity index was calculated to investigate the change in viscosity with respect to temperature. As explained in table, VI of pure oil is 98, and nano lubricants has 104 at lower concentrations (S-1 and S-2). In addition, VI reduces to 103 for the higher concentration sample (S-3). To recapitulate, the rGO nano lubricantâĂŹs low viscosity and higher viscosity index enhance shear thinning at solid point contact and reduce frictional losses.

46 4.5 RPVOT

To provide fault-free machine life, estimation of anti-oxidation capability of lubricants can be investigated using numerous methods such as FTIR, RPVOT, odor and color inspection. Among these methods, RPVOT is widely used by industry to identify the antioxidant capability of a lubricant [87]. While transforming graphene oxide into its reduced form, there are always a small number of oxygen groups that remain in the lattice and to understand the eﬀect on anti-oxidation capability because of those oxygen particles, RPVOT test was conducted following ASTM D2272 standards. As expressed in Table 4.3, pressure drop time is noted as 27.1, 26.1, 25.8 and 25.7 minutes for pure oil and all three samples respectively. Comparative studies indicate that there is no noticeable change in anti-oxidation life of all three samples having nano additives compared to the pure oil.

Materials Antioxidation time in min P.O. 27.1 S-1 26.1 S-2 25.8 S-3 25.7

Table 4.3: Rotating pressure vessel oxidation test [92]

4.6 Ball on disk wear and friction test

To understand the wear preventive characteristics of the lubricants, a ball-on- disk instrument (TRB181002-36-D at Nanovea, California, USA) was utilized to test all four samples in the same environmental condition. In addition, precise friction coeﬃcients data points were taken at 20ms (millisecond) time intervals using strain gauge sensor. To calculate wear rate and surface roughness, a 3D proﬁlometer (T-14-0133) was used in situ condition. The motor of the tribometer was integrated with self-turning and all-time calibration capabilities to generate data precisely. An indenter ball having 6mm diameter was made from SS440C. The 50mm diameter and 10mm thick disk was manufactured from AISI-316-L

47 stainless steel (C-0.023, Si- 0.40, Mn-1.59, P-0.037, S- 0.027, Cr-16.55, Mo-2.06, Ni-10.20, Co-0.20, N-0.0350, Cu-0.47) solid bar. After the disk was machined on lathe machine, it was taken to the polisher (Nano-2000) and polished to less than 0.1 µm surface roughness. Furthermore, tribology test was conducted at 30N load for 20min, where rotating speed was increase logarithamically from 0.01RPM to 150RPM.

Figure 4.5: Friction coeﬃcients of pure lubricant (blue) and three diﬀerent samples at 0.01% wt concentration (orange), 0.05% wt concentration (grey), 0.1% wt concentration (yellow) [92]

As exhibited in Figure 4.5, the friction coefficient for pure oil was lower for fewer revolutions and lower rotating speed. However, as speed increased logarithmically with the number of revolutions, friction went up and then normalized around 0.2. In the case of S-1 and S-3, the study found the same trend, where friction increased at fewer revolutions under boundary condition and then normalized around 0.3 and 0.35 respectively. The reason behind this could be that S-1 had an uneven rGO nano lubricant film at 30N load. In the case of S-3, higher conglomerations of the particles might have created a wall at the joint, which can reduce the circulation of lubricant through the rubbing surface. Although, S-2 shows a different trend

48 where friction increased a bit at low speed and less number of revolutions. Even- tually it started normalizing closer to 0.1 at higher speed and and more number of revolutions because of ﬂuid ﬁlm mechanism. As shown in Figure 4.6, the wear rate for the pure oil was calculated as 48.33 X 10−5 mm3/Nm which is comparatively higher than the other three nano lubricants. S-1 reduced wear by 22.06%, S-2 by 51.85% and S-3 by 25.64% in comparison to the pure oil, which allows the lubricant to improve service life of tribological surfaces.

Figure 4.6: Wear rate for pure oil and three nano lubricants [92]

Figure 4.7 reveals the schematic view of the interaction of ball, disk and nano additives used for these experiments to understand the behavior of nano particles under the test conditions. For S-1, only 0.01% wt nano additives were amalga- mated into the pure oil, indicates low wear compared to pure oil due to penetration of rGO platelets in the contact regime. In the case of S-2, 0.05% wt nano additives were mixed into the pure oil, shows the lowest wear among all four samples as well as a reduction in friction after a perfectly hydrodynamic condition was achieved. This allows the lubricant to mitigate metal-to-metal contact, that can be considered as optimal concentration. On the other hand, in S-3, 0.1% wt concentration was prepared, which led to reduction in wear; however, friction for this sample was highest. This can be due to a high concentration rGO nano platelets start-

49 Figure 4.7: Schematic image showing rGO nano particles in the contact patch at diﬀerent concentrations [92] ing to conglomerate around the contact patch. Conglomerated nano platelets will generate lower suspension and will inhibit the rGO nano platelets from entering into the patch; but due to high concentration and lower viscosity it will require more energy, which can result in increasing friction. Conversely, some particles will enter the tribo contact and shear the rGO layer, which helps to reduce wear with compared to the base oil.

4.7 SEM morphology study of worn surface

The SEM images reveals some deep groove marks on the worn surfaces of the pure oil (control sample) and S-1. In addition, width of the wear scars are about 890 µm and 746 µm, respectively. Control sample (pure oil) and low rGO sample (S-1) suffered severe wear and deep grooves along with the sliding direction as appeared in Figure 4.8A,B. This type of wear mechanism can be characterized as adhesive wear because at 30N load very thin film can be found between the pin and substrate. However, sample (S-2) reveals smooth wear track (605 µm) which does not have any deep scratches, significant plastic deformation, or deep

50 sliding direction marks as indicated in Figure 4.8C. The rGO nano platelets in S-2 reduce friction by preventing sliding contact interfaces from severe or more frequent metal-to-metal contacts. By increasing the rGO concentration in S-3, the rGO nano platelets agglomerated together to form abrasive particles which resulted in increasing the width of the wear scar for S-3 to 709 µm as demonstrated in Figure 4.8D.

Figure 4.8: Morphology investigation using scanning electron microscopy on specimen wear tracks at 500 µm size and 120.00 kx magniﬁcation for pure oil (A), S-1 (B), S-2 (C) and S-3 (D) [92]

4.8 Optical Microscopy

Profilometry data was obtained in situ to understand the effect of nano lubricants on surface roughness using a non-contact type optical probe in situ. Furthermore, different height parameters were obtained such as Sa, Sz ,Sq, Sv, Sp, Ssk, Sku according to ISO 25178. However, most important parameter, arithmatic mean height and maximum heigh of substrate is shown in Table 4.4. In which, pure oil has an arithmetic mean height of 1.386 µm and the maximum height of the surface is 15.19 µm, compared to that of S-2 which has an arithmetic mean height of 0.6091 µm and the maximum height of the surface is 11.89 µm, which leads us to conclude that, except for S-2, all three samples, including pure oil has high adhesive wear. Furthermore, False color analysis reveals that S-2 has no red marks (deep groove) on whole periphery of the scar which reveals smooth surface on the whole periphery. The P.O and S-1 has higher variation of color which proves that

51 Samples Sa (µm) Sz (µm) PO 1.386 ± 0.0693 15.19 ± 0.0035 S-1 2.018 ± 0.1009 25.67 ± 0.0050 S-2 0.6091 ± 0.0305 11.89 ± 0.0015 S-3 1.995 ± 0.0998 37.55 ± 0.0050

Table 4.4: Height parameters of the scar after performing ball-on-disk test uneven surfaces as disclosed in SEM images.

Figure 4.9: Optical microscopy using false color analysis of worn scar in situ condition,where Pureoil (A), S-1 (B), S-2 (C), S-3 (D)

52 Chapter 5

Comparative study of GNP and TiNP individually and together as hybrid additive

5.1 Introduction

1 In this phase of research, the tribological behavior of both liquid (oil) and semi- liquid (grease) lubricants enhanced by multilayer graphene nano platelets and titanium dioxide nano powder, and hybrid nanoadditive was evaluated using ball on disk and shaft-on-plate tribo-meters. Oil samples for both, 2D graphene nano platelets (GNP) and titanium nanopowders (TiNP) were prepared at three concentrations of 0.01%w/w, 0.05%w/w, and 0.1%w/w. In addition, 0.05%w/w hybrid- mixtures of GNP and TiNP were prepared with three diﬀerent ratios to analyze collective eﬀects of both nano additives on friction and wear properties. For semi- liquid lubricants, 0.5%w/w concentrations were prepared for both nano additives for shaft-on-plate tests. Viscosity and oxidation stability tests were conducted on the liquid-base lubricants. Nanopowders of both additive and substrate were analyzed using transmission electron microscopy (TEM) and scanning electron

1Results of Chapter-5 has been published as a journal paper titled ”Tribological Capabilities of Graphene and Titanium Dioxide Nano Additives in Solid and Liquid Base Lubricants” in MDPI journal of Applied Science. https://doi.org/10.3390/app9081629

53 microscopy (SEM). Besides, Raman spectroscopy was conducted to characterize the graphene and titanium dioxide. The study shows that adding graphene and titanium dioxide individually sacriﬁces either the wear or friction of lubricants. However, use of both additives together can enhance friction resistive and wear preventive properties of a liquid lubricant signiﬁcantly. For a semi-liquid lubricant, the use of both additives together and individually reduces friction compared to base grease [88].

5.2 Morphology of material

5.2.1 Raman spectroscopy

Raman spectroscopy allows us to determine the characteristics of the nano additives used in this research. It identifies defects in the carbon lattice by generating different intensity peaks using nondestructive method. As shown in Figure 5.1, Raman spectroscopy shows main peaks at different wavelengths of 1354 cm−1 (D band), 1581 cm−1 (G band), and 2719 cm−1 (2D band). The sp2 carbon breath- ing mode requires defects for its activation, and this is illustrated by the D band intensity. In addition, the narrow G band shows that the defects do not originate from basal plane but are from new edges. The D+D0 peak at 2959 cm−1 shows the exfoliation of graphite. The intensity ratio at the D and G bands is important to understand the transformation of sp3 to sp2 conversion of carbons; further, it is directly proportional to the defect rate. This graphene sample, ID/IG is 0.153, is very low compared to restored graphene, proving that this graphene has very few defects in the carbon lattice [89, 90]. Titanium dioxide nano powder was investigated under the Raman spectra as shown in Figure 5.2. The powder generates a primary peak at 144 cm−1 and secondary intensity peaks at 199 cm−1, 397 cm−1, 516 cm−1, and 638 cm−1, which is associated with different modes of energy such as Eg (strong), Eg (weak), B1g,

A1g+B1g,Eg (intermediate) respectively. These peaks represent the characteristics

54 Figure 5.1: Raman spectroscopy of graphene nano particles [88]

Figure 5.2: Raman spectroscopy of titanium nano particles [88] of the anatase phase of titanium dioxide nano particles [91].

5.2.2 TEM investigation

Graphene and titanium dioxide powders tend to conglomerate during imagining. Hence, epoxy was used to mold the powder. Nano sliced pieces of the epoxy were observed under transmission electron microscopy (TEM) at 25.0 kx magniﬁcation of 1 µm size. Figure 5.3(A) is a TEM image of GNP which discloses a 2D structure with multiple layers. In addition, Figure 5.3(B) demonstrate the titanium dioxide round particles conglomerated in the small chains. On the other hand, Figure

55 Figure 5.3: TEM images of (A) graphene (B) titanium dioxide (C) hybrid additive (graphene and titanium dioxide in equal concentration) in powder form at 25.0 kX and 1µm [88]

5.3(D) shows the mixed form of graphene and titanium dioxide. It is demon- strating that how titanium particles stick to the graphene surfaces.Furthermore, It motivates us to investigate the tribological properties of both additives as a hybrid additive.

5.3 Samples characterisation

Pure oil provided by Petro Canada Inc. was used to prepare the liquid base lubrication samples. Crystal clear oil has 99.9% purity (Product code: N200MHT). The pour point and ﬂash point (Method: Cleveland open cup) of the base oil were found to be -18◦Cand 240◦C. The density of base oil is 0.865 kg/l. GNP and TiNP were mixed at three diﬀerent concentrations of 0.01%, 0.05% and 0.1% into the base oil using a mechanical mixer at 1500 rpm for 30 minutes for homogeneous dispersion. TiNP and GNP were added in three ratios at 0.05%w/w concentration as shown in Table 5.1. The mechanical mixer was cleaned after each use. Samples were ultrasonicated for 30 minutes at room temperature to break the conglomeration of nano platelets. The semi-liquid lubricant (grease) used in this study was commercially available white grease (National Lubricating Grease Institute-NLGI grade-2), which can be used in automotive, marine and farm equipment. The temperature capability of this grease is between -15◦Cand 120◦C. Grease samples

56 were prepared after rigorously mixing nano additive at 0.5% concentration in to the base grease using magnetic mixer. Additionally, they were also ultrasonicated for 30 minutes at an elevated temperature. Furthermore, grease samples were charatcerised based on the individual and hbrid nano additive as shown in Table 5.2.

Table 5.1: Samples classiﬁcation of liquid base lubricants (liquid nano lubricants) [88]

Table 5.2: Samples characterization of semi-liquid lubricants (grease nano lubricants) [88]

57 5.4 Kinematic viscosity and viscosity Index

To compare the effects of different concentrations and types of nano additives in liquid base lubricants, kinematic viscosity tests were performed as per ASTM standards. Kinematic test results manifest the viscosity of nano lubricant at 40◦C and 100◦C. The resistance of lubricants to change in temperature (viscosity index) is important to improve hydrodynamic lubrication regimes at large temperature ranges. As per the base oil data sheet provided by Petro Canada, viscosity at 40◦C and 100◦C are 41.5 mm2/s and 6.3 mm2/s respectively. The calculated viscosity index for base oil is 98. However, results exhibited in Figure 5.4 prove that there is no difference between the viscosity at either temperature or in the viscosity index except S-3. Result exhibits that improvement in tribology applications does not lead to sacrifices in physical properties of the base oil.

Figure 5.4: Kinematic viscosity and viscosity index of liquid lubricants [88]

5.5 RPVOT

Anti-oxidation capabilities of lubricants were evaluated using the rotating pressure vessel oxidation test (RPVOT) as per ASTM standards. For sustainable improvement, it is important to consider service life of the lubricant, because lubricants that are not antioxidant can start causing cavitation in the system, which requires replacing parts and lubricant. This damages the economic performance of the

58 industry and generates environmental waste.

Figure 5.5: Oxidation stability time in minute [88]

As manifested in Figure 5.5, there is no signiﬁcant diﬀerence between oxidation time achieved with these additive lubricant compared to the base oil oxidation time. Base oil has shown oxidation time as 27.1 minutes under the same conditions [92]. It indicates that, except for S-1, S-3 and S-6, all other samples increase oxidation stability, which allows the lubricant to have a longer service life. Test results conclude that the additive will not damage the economic performance of industries if it is added to the lubricant.

5.6 Ball on disk wear and friction coeﬃcient test

To obtain comparative data about the tribological effects of GNP and TiNP additives to fluid lubricants, friction and wear tests were conducted using a ball- on-disk tester at 20N load . For semi-liquid samples (grease), a shaft rotatory shaft-on-plate test was used as described in the experimental setup. For liquid base lubricants, anti-scuffing and friction coefficients were calculated using in situ conditions. As shown in Figure 5.6, a higher concentration of graphene nano additives reduces the friction coefficient. However, wear preventive properties are

59 sacrificed. Comparing titanium dioxide with graphene illustrates that it can significantly lower the wear; however, a higher concentration of TiNP additive does not significantly affect wear preventive properties. As presented, 0.05% concentration exhibits less friction for the graphene sample. For titanium material, it disclose less wear, which led us to prepare the 0.05% concentration using three different weight ratios of additive. As illustrated, S-8 shows a lower friction coefficient and S-9 shows the lowest wear. The results of the liquid lubricant samples enhanced by GNP and TiNP were compared separately at different concentrations. In addition, the middle concentration of 0.05% was found to be most effective for hybrid nano additives of GNP and TiNP; thus, three ratios and nano additives at this concentration (0.05%) were prepared and tested. The results displayed in Figure 5.6 manifests that graphite has a higher wear rate compared to the coefficient of friction. However, the titanium dioxide samples show a lower wear rate and a higher friction coefficient. This motivated us to evaluate the tribological effects of these additives together. Sample-8 shows moderate wear as well as friction. Sample-9 express the lowest wear but it is compromised by friction. Sample-7 convoys the control over wear but it gains higher friction.

60 Figure 5.6: Friction coeﬃcient and wear data [88]

5.7 SEM morphology investigation of worn sur-

face

Analysis of the worn disk helps us understand the difference in the wear characteristics of different nano additives to lubricants. Disk wear track was evaluated at 40 µm resolution and 150.00 kX magnification in a low vacuum chamber using SEM as shown in Figure 5.7. The worn surface of S-1 reveals that abrasive and adhesive wear can be seen from surface damage. This can be the result of the debris penetration at the contact patch, leading to the generation of the rough surfaces. S-2 and S-3 shows higher number of pits on the surface, which can be caused by the micro crack on the substrate surface. Along with that, these pits cause an imbalance in the elastic hydrodynamic condition. In the longer term, it can cause corrosion on the surface [93]. Morphology of S-4 and S-6 disclose smooth wear tracks; however, relatively longer surface removal marks can be seen, which shows adhesive wear. In addition, for S-5 some plastic deformation can be found on the side of the surface. This kind of adhesive wear can be caused by

61 micro cracks, which enlarge under intense loading conditions. S-7 shows fatigue wear where micro cracks enlarge gradually, causing a removal of surface material that can is also known as mechanical pitting. In S-8 and S-9, smooth wear track marks are visible on the substrate as a result of adhesive wear. However, no plastic deformation nor any abrasive wear is seen on these samples.

Figure 5.7: Worn scar evaluation in low vacuum chamber at 40µm scale using SEM for 9 disk samples [88]

62 5.8 Optical microscopy in situ

Further investigation of the wear track was conducted when the surface texture was examined as per ISO standard-25178. Various data were recorded including arithmetic mean height, which is important to understand the surface waviness of the scar. As shown in Figure 5.8, S-1 and S-5 indicates the higher mean height with respect to the other samples, which shows the larger peaks and valleys were found on the substrate. S-9 discloses the lowest mean height, indicating a smooth wear track. In addition, average surface roughness data shows that hybrid additive is having comparatively lower roughness compare to the individual additive as all three concentrations except S-4 and S-6. Although, S-4 and S-6 has higher friction coeﬃcient which do not lead to be desired concentration.

Figure 5.8: Average mean surface roughness of the disk [88]

5.9 Grease friction coeﬃcient

Nano additive grease was tested using the prototyped rotatory plate-on-shaft friction system at the University of Ontario Institute of Technology. Semi-liquid lubricants (white grease) enhanced by GNP, TiNP and a combination of both were tested at 10 N, 20 N and 30 N load conditions to evaluate the frictional properties of the grease. Higher concentrations of nano additives increase dispersibility of

63 Figure 5.9: Average friction coefficient of nano additive grease at 10N, 20N and 30N normal load [88] particles in the grease. Initially two samples were prepared where 0.5% w/w nano particles were added. As shown in Figure 5.9, GNP reduces friction drastically at 20 N load, similar to TiNP. However, at 30 N it was found that the thickness of hydrodynamic films becomes thinner, showing minimal difference between white grease and nano particle enhanced grease. An equal concentration of graphene and titanium dioxide (ratio of GNP/TiNP:1) results in increased friction. However, a lower graphene concentration (25%) and a higher titanium dioxide concentration (75%) proclaim the highest reduction in friction at 20 N load. A higher concentration of graphene (75%) and a lower concentration of titanium dioxide (25%) manifests the slight increment in average friction coefficient at 20 N load. Each test was conducted at least 3 times and average data were considered to mitigate testing errors. To sum up, all 5 nano grease samples enhance the antifriction properties of the semi-liquid lubricant.

64 Chapter 6

Research summary and future work

This research was divided in three sections based upon the concentration of nano additives and types of additives. Overall findings of this study reveals that adding nano additives in to the lubricants opens the doors of sustainable development. In the first study, three forms of rGO were tested under high load condition using four ball wear tribometer and results demonstrate that material-1 can reduce wear by 10.63% at 147 N load. In addition, it can reduce average friction coefficient by 6.3% with respect to the base oil under laboratory conditions. To add further, resistivity test results proves that rGO additives are not affecting polarity or conductivity of the lubricant. The physical properties (viscosity) and chemical property (oxidation stability) test shows no noticeable difference of the nano lubricant compare to the base oil which concludes that, nano additives won’t cost extra to the lubrication industry economically. Along with that, four ball SEM analysis tests reveals the accumulation of rGO particles on the scar which proves that rGO particles are interfering in reduction of wear by mitigating metal to metal contact. Furthermore, positive effect of Material-1, as a wear and friction modifier at 0.01% concentration leads to find out the optimum concentration. In second part of research, three different forms of rGO were tested in situ condition using ball on disk

65 tribometer. Evaluation of anti-oxidation capabilities disclose that the service life of nanolubricant is not being compromised. In addition, FTIR particle counting results found that most of the additives of rGO material-1 were smaller than 38 µm, which allows this lubricant to be used in diverse applications without damag- ing filters. S-1 (0.01% w/w), S-2 (0.05% w/w) and S-3 (0.1% w/w) reduced wear rate by 22.06%, 51.85% and 25.64% respectively compared to pure oil. This is very promising for prolonging the life of contact surfaces. In addition, less wear allows industries to use the same contact surfaces longer than their original life cycles. As carbon is easily available in the market and manufacturing of reduced graphene oxide can be easily scaled for industrial demand, it can be efficiently introduced to the next generation of lubricants. In last phase of the research, tribological and physical properties of graphene, titanium dioxide and hybrid additive were evaluated. Results reveale positive effects on base lubricants. Both nano additives showed no negative effect on oxidation life and viscosity; however, both additives manifest modification in friction and wear properties. GNPs exhibited a great control over friction at 0.05% w/w concentration; however, GNP sacrifices its wear preventive properties compared to titanium dioxide. On the other hand, titanium dioxide nano particles convoy the highest wear resistivity at 0.1%w/w concentration along with increase friction compared to the graphene nano particles. However, for hybrid nano additives of GNP and TiNP, it was found that both wear and coefficient of friction (COF) can be controlled as per application requirements as S-9, which has higher concentration of graphene and lower concentration of titanium dioxide indicated the least wear. A higher concentration of titanium dioxide and a lower concentration of graphene divulged more control on friction than wear. For semi-liquid lubricants (white grease), both GNP and TiNP additives reveal lower friction coefficients (COF). In addition, hybrid nano additives has showed lower friction compared to pure grease at 20 N load. There is not much change in friction coefficient at 30 N high load because of the extremely thin film of grease on

66 the shaft which does not allow additives to enter between the contacts. However, at 20 N load, a higher concentration of GNP and a lower concentration of TiNP shows most signiﬁcant reduction in friction compared to the pure grease.

For future work, these additives can be tried with different tribological modifiers to determine the effects of hybrid nano additive compare to individual additive. In addition, this nano additive can be tested with extreme pressure additive or anti corrosion additive as well. In addition, Using laser nano manufacturing facility different titanium or silicon nano fibre can be generated, which can be used as a nano additive to perform comparative study with commercial nano particles. Furthermore, tribological effect of laser nano additive and additive manufactured by different process can be checked for any tribological improvement. These three studies were conducted using base oil. Further finding discloses that at lower concentration, rGO material do not need suspension additives. Along with that, hybrid nano additives have shown better dispersibility. For future work, the same nano additive can be tested with same concentrations but with different base oil(in the presence of suspension particle) to analyze the effect of different base oil on nanotribology.

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77 6.1 Published articles

6.1.1 Journal papers

Patel, J., Pereira, G., Irvine, D., Kiani, A., 2019. Friction and wear properties of base oil enhanced by diﬀerent forms of reduced graphene. AIP Advances, 9(4), p.13.

Patel, J. and Kiani, A., 2019. Eﬀects of Reduced Graphene Oxide (rGO) at Diﬀerent Concentrations on Tribological Properties of Liquid Base Lubricants. Lubricants, 7(2), p.11.

Patel, J. and Kiani, A., 2019. Tribological capabilities of graphene and titanium dioxide nano additives in solid and liquid base lubricants. Applied Sciences, 9(8), p.12.

6.1.2 Conference papers

Patel, J. and Kiani, A., 2018. Enhancement of Wear Behavior of Base Oil by Reduced Graphene Oxide Additives, presented at STLE Tribology Frontier Con- ference, Chicago, USA

Patel, J. and Kiani, A., 2019. Comparative study of tribological behaviours of diﬀerent base greases enhanced by graphene nano platelets, accepted at TANN’19 - 3rd International Conference of Theoretical and Applied Nanoscience and Nan- otechnology Conference, Ottawa, Canada

78 6.2 Appendix

6.2.1 Friction coeﬃcient data obtained from shaft-on-plate

tribometer at 10N, 20N and 30N

6.2.2 Optical microscopy height parameters and area hole

analysis of Chapter-4 samples

6.2.3 Optical microscopy height parameters and area hole

analysis of Chapter-5 samples

79 Table 6.1: Friction coeﬃcient of grease sample at 10N

80 Table 6.2: Friction coeﬃcient of grease sample at 20N

81 Table 6.3: Friction coeﬃcient of grease sample at 30N

82 Figure 6.1: Optical microscopy height parameters and area hole analysis performed on sample-P.O (Chapter-4) substrate

Figure 6.2: Optical microscopy height parameters and area hole analysis performed on sample-1 (Chapter-4) substrate

83 Figure 6.3: Optical microscopy height parameters and area hole analysis performed on sample-2 (Chapter-4) substrate

Figure 6.4: Optical microscopy height parameters and area hole analysis performed on sample-3 (Chapter-4) substrate

84 Figure 6.5: Optical microscopy height parameters and area hole analysis performed on S1 (Chapter-5) substrate

Figure 6.6: Optical microscopy height parameters and area hole analysis performed on S2 (Chapter-5) substrate

85 Figure 6.7: Optical microscopy height parameters and area hole analysis performed on S3 (Chapter-5) substrate

Figure 6.8: Optical microscopy height parameters and area hole analysis performed on S4 (Chapter-5) substrate

86 Figure 6.9: Optical microscopy height parameters and area hole analysis performed on S5 (Chapter-5) substrate

Figure 6.10: Optical microscopy height parameters and area hole analysis performed on S6 (Chapter-5) substrate

87 Figure 6.11: Optical microscopy height parameters and area hole analysis performed on S7 (Chapter-5) substrate

Figure 6.12: Optical microscopy height parameters and area hole analysis performed on S8 (Chapter-5) substrate

88 Figure 6.13: Optical microscopy height parameters and area hole analysis performed on S9 (Chapter-5) substrate