Optimization of Wet Friction Systems Based on Rheological

Home , Friction modifier

Sayali S. Satam

OPTIMIZATION OF WET FRICTION SYSTEMS BASED ON RHEOLOGICAL,

ADSORPTION, LUBRICANT AND FRICTION MATERIAL CHARACTERIZATION

A Dissertation

Presented to

The Graduate Faculty of The University of Akron

In Partial Fulfillment

of the Requirements for the Degree

Doctor of Philosophy

Sayali S. Satam

May 2018

OPTIMIZATION OF WET FRICTION SYSTEMS BASED ON RHEOLOGICAL,

ADSORPTION, LUBRICANT AND FRICTION MATERIAL CHARACTERIZATION

Sayali S. Satam

Dissertation

Approved: Accepted:

Advisor Department Chair Dr. Erol Sancaktar Dr. Sadhan C. Jana

Committee Member Dean of College Dr. Sadhan C. Jana Dr. Eric J. Amis

Committee Member Dean of the Graduate School Dr. Xiong Gong Dr. Chand K. Midha

Committee Member Date Dr. Mesfin Tsige

Committee Member Dr. Gary L. Doll

Committee Member Dr. Rashid Farahati

ABSTRACT

Improved friction characteristics and reduced wear are desired in most of the contacting surface systems. With this motivation, our work is focused on two different applications. First application involves improving friction characteristics of automobile wet clutch system by exploiting ‘lubricant additive-friction material’ interaction and material modification aspects. The second part is focused on reducing friction and wear in

boundary lubrication using multiwalled carbon nanotube (MWCNTs) as a lubricant

additive.

Wet clutch is an integral part of a transmission system in automobiles. Positive

slope of the friction coefficient versus sliding speed curve along with a high dynamic

friction coefficient value indicate ideal friction characteristics for smooth clutch

engagement between the friction material (FM – also called “friction paper”) and the

reaction plate (steel) in the presence of automatic transmission fluid (ATF). The first part

of our work involved adsorption analysis of ATF additives on friction material components

(filler and fiber) using DSC and UV/VIS techniques. Adsorption behavior was further correlated with rheological and friction phenomena. Shear stress and strain rate obtained from rheological testing were correlated with friction and sliding speed, respectively, as obtained from friction characteristics testing. It was observed that ATF causes shear thickening while base oil causes shear thinning behavior when mixed with FM filler.

Higher rate of increase of shear stress with shear rate (i.e., enhanced shear thickening) is

iii expected lead to higher friction coefficient with sliding speed. Following up on this

hypothesis, the filler component of the friction paper was found to show shear thickening

behavior and improved friction characteristics, as compared to its fiber component.

Further concentrating on the filler component, we used three different fillers

(diatomaceous earth and a proprietary clay) to top-coat the friction paper. It was observed

that filler coating increases direct contact between the steel and the filler during clutch

engagement, thus improving the friction characteristics due to the shear thickening effect.

It was further found that the filler with the highest rate of increase in shear stress with shear rate in the presence of ATF, showed the best friction characteristics as compared to other two fillers as well as the non-coated FM.

Due to our consideration of filler coating the friction paper, which may reduce its permeability, effect of oil permeability of filler coated FM surface on friction characteristics and FM durability were also studied. FM surface porosity was varied using parameters such as fiber/filler ratio in base layer FM composition, FM density and coating thickness. FM base layer composition was optimized comparing aramid and aramid/cotton compositions at different densities.

In the last part of our work, phosphonium ionic liquid was used as an additive for

MWCNT dispersion in non-polar lubricant. Ionic liquid adsorption on MWCNT walls was studied using DSC, FTIR and rheological techniques. Ionic liquid stabilized dispersion of nanotubes in lubricant was confirmed from UV/VIS and TEM. Addition of MWCNTs in lubricant resulted in decreased friction coefficient and wear in boundary lubrication for steel-steel contact as compared to base oil.

DEDICATION

This dissertation is dedicated to my parents and brother for their unconditional love, support and encouragement to pursue my dreams.

ACKNOWLEDGEMENTS

I would like to express my deep sense of thanks and gratitude to my advisor Dr.

Erol Sancaktar for all the encouragement and freedom he gave me in pursuing new ideas

and opportunities during this journey. This work could not have been possible without the

constant support and guidance from Dr. Sancaktar.

I owe my sincere gratitude to Dr. Rashid Farahati for giving me the opportunity to

work at LuK USA LLC for my dissertation work. He has been a great mentor who always

shared his tremendous knowledge in this field of study and also gave me freedom to work

in the industrial setting.

I would like to thank our other industry collaborators Dr. Timothy Newcomb and

Mr. Christopher Prengaman from Lubrizol Corporation for their support and guidance in

this work.

I would also like to thank my committee members Dr. Sadhan Jana, Dr. Xiong

Gong, Dr. Mesfin Tsige and Dr. Gary Doll for their insightful suggestions and guidance.

Special thanks to Dr. Gary Doll for guidance in the tribology experiments. I am also

grateful to Dr. Toshikazu Miyoshi for his time and help with NMR experiments.

A special thanks to all my friends in Akron for all the beautiful moments we have

shared together. Thanks to them for being with me in the ups and downs of my journey and for their constant support and love. I am also thankful to my group members and colleagues

at The University of Akron and LuK USA LLC for their help, support and enlightening

discussions.

At the end, I would like to thank my dear family and friends for their unconditional love and support. I know I always have them to count on when times are rough. I would not have made it this far without them. Thank you.

vii

TABLE OF CONTENTS

Page

LIST OF TABLES ...... xii

LIST OF FIGURES ...... xiii

CHAPTER

I. INTRODUCTION ...... 1

II. BACKGROUND ...... 5

2.1. Tribology...... 5

2.1.1. History of Tribology ...... 5

2.1.2. Lubrication Regimes and The Stribeck Curve ...... 6

2.2. Wet Clutch System ...... 8

2.2.1. Friction Materials History ...... 9

2.2.2. Friction Material Ingredients ...... 10

2.2.2.1. Fibers ...... 10

2.2.2.2. Fillers ...... 11

2.2.2.3. Friction Modifiers ...... 11

2.2.2.4. Binders ...... 12

2.2.3. Automatic Transmission Fluid (ATF) ...... 13

2.2.3.1. Friction Modifiers ...... 14

2.2.3.2. Detergents...... 15

2.2.3.3. Dispersants ...... 15

2.2.4. Wet Clutch Tribology ...... 16 viii 2.2.4.1. SAE No. 2 Friction Tester ...... 16

2.2.4.2. Friction Characteristics ...... 17

2.2.4.3. Tribo-film Formation and Additive Adsorption ...... 20

2.2.5. Factors Affecting Friction Characteristics ...... 21

2.2.5.1. Friction Material Parameters ...... 21

2.2.5.2. ATF Parameters ...... 23

2.2.5.3. Process Parameters ...... 27

2.3. Solid Nanomaterials as Lubricant Additives ...... 29

2.3.1. Carbon Nanotubes as Lubricant Additives ...... 31

2.3.2. Carbon Nanotube Dispersion ...... 34

2.3.3. Ionic Liquids as Additives for Lubricants...... 35

2.3.4. Interaction between CNTs and Ionic Liquid ...... 37

2.3.5. CNTs and Ionic Liquid Lubricant Systems ...... 39

III. CORRELATION BETWEEN ADSORPTION, RHELOGICAL AND FRICTION CHARACTERISTICS OF WET CLUTCH SYSTEM COMPONENTS ...... 40

3.1. Introduction ...... 40

3.1. Experimental ...... 42

3.1.1. Materials ...... 42

3.1.2. Characterization ...... 43

3.2. Results and Discussion ...... 45

3.2.1. Effect of ATF Additives ...... 45

3.2.1.1. Additive Adsorption ...... 45

3.2.1.2. Rheology ...... 52

3.2.1.3. Friction Characteristics ...... 58

ix 3.2.2. Filler (diatomaceous earth) versus Fiber (Aramid) ...... 61

3.2.2.1. Additive Adsorption ...... 61

3.2.2.2. Rheology ...... 62

3.2.2.3. Friction Characteristics ...... 64

3.2.3. Effect of Temperature ...... 65

3.2.3.1. Rheology ...... 65

3.2.3.2. Friction Characteristics ...... 67

3.3. Conclusions ...... 68

IV. FRICTION CHARACTERISTICS OF VARIOUS FILLER COATINGS ON FRICTION MATERIALS ...... 70

4.1. Introduction ...... 70

4.2. Experimental ...... 72

4.2.1. Materials ...... 72

4.2.2. Characterization ...... 72

4.3. Results and Discussion ...... 74

4.4. Conclusions ...... 82

V. EFFECT OF FIBER TYPE, PAPER DENSITY AND COATING THICKNESS ON FILLER-COATED FRICTION MATERIAL PERFORMANCE ...... 83

5.1. Introduction ...... 83

5.2. Experimental ...... 85

5.2.1. Materials and Procedure ...... 85

5.2.2. Characterization ...... 86

5.3. Results and Discussion ...... 88

5.3.1. Effect of Base Paper Density ...... 93

5.3.1.1. Friction Materials with ‘Cellulose + Aramid’ Fiber Content . 94

x 5.3.1.2. Friction Materials with Only ‘Aramid’ Content ...... 100

5.3.2. Effect of Coating Thickness on Friction Test ...... 108

5.4. Conclusions ...... 111

VI. CARBON NANOTUBES DISPERSION IN NON-POLAR LUBRICANT USING PHOSPHONIUM IONIC LIQUID ADSORPTION ...... 112

6.1. Introduction ...... 112

6.2. Experimental ...... 114

6.2.1. Materials and Preparation ...... 114

6.2.2. Characterization ...... 115

6.3. Results and Discussion ...... 119

6.3.1. Ionic liquid adsorption on MWCNTs ...... 119

6.3.2. MWCNT dispersion stability using ionic liquid as surfactant .... 126

6.3.3. MWCNTs as Additive for Steel-Steel Contact in Boundary Lubrication ...... 128

6.4. Conclusions ...... 132

VII. CONCLUSIONS...... 133

REFERENCES ...... 137

LIST OF TABLES

Table Page

2-1 Experimental solubility and calculated solubility parameters of ionic liquids81

(Reprinted with permission from “Ionic Liquids Composed of Phosphonium

Cations and Organophosphate, Carboxylate, and Sulfonate Anions as Lubricant

Antiwear Additives” Zhou, Y.; Graham, T. W.; Luo, H.; Leonard, D. N.; Qu, J.

4-1 Oil drop measurements for friction materials with and without filler coatings…...81

5-1 Friction material properties and composition for different ‘cellulose + aramid’ fiber

contents……………………………………………………………………………..94

5-2 Friction material properties and composition containing different ‘aramid’ fiber

contents ...... 100

6-1 Friction coefficient and wear volume measurements obtained from HFRR test….132

xii

LIST OF FIGURES

Figure Page

Figure 1.1 Breakdown of passenger car energy consumption1. (Reprinted from Tribology

International, 47, Holmberg K., Andersson P., Erdemir A., Global energy

consumption due to friction in passenger cars, 221-234, Copyright (2012),

with permission from Elsevier)...... 1

Figure 2.1 Stribeck curve showing three lubrication regimes: hydrodynamic, mixed and

boundary regime ...... 7

Figure 2.2 Schematic diagram of a torque converter23. (Republished with permission of

Tribology Letters, from “tribology of automatic transmission fluid”, Kugimiya,

T.; Yoshimura, N.; Mitsui, J., 5(1), 1998, permission conveyed through

Figure 2.3 Schematic of energy transfer from engine to transmission system when (a) a

clutch is not engaged (by ATF flow) and (b) a clutch is engaged...... 13

Figure 2.4 Schematic diagram of the SAE No. 2 test machine and detail of its test head23.

(Republished with permission of Tribology Letters, from “tribology of

automatic transmission fluid”, Kugimiya, T.; Yoshimura, N.; Mitsui, J., 5(1),

1998, permission conveyed through Copyright Clearance Center, Inc.)...... 16

Figure 2.5 Test pattern in the SAE No. 2 test machine and an example of the torque and

speed pattern obtained in the test...... 17

xiii Figure 2.6 Representations of friction coefficient versus sliding speed for SAE No. 2 test

results depicting decreasing (dμ/dν < 0), increasing (dμ/dν > 0), and

increasing/decreasing (curve C) behaviors of friction coefficient-sliding speed

curves...... 19

Figure 2.7 Friction modifier adsorption on friction surfaces...... 21

Figure 2.8 Stribeck curve of a wet clutch contact lubricated with (a) only base oil and (b) a

fully formulated ATF57. (Reprinted with permission of Tylor and Francis, from

“Friction Characteristics of a Paper-Based Facing for a Wet Clutch Under a

Variety of Sliding Conditions”, Tribology Transactions, Ito, H.; Fujimoto, K.;

Eguchi, M.; Yamamoto, T., 1993, permission conveyed through Copyright

Clearance Center, Inc.)...... 24

Figure 2.9 Friction coefficient versus sliding speed curve for various model organic friction

modifiers at 3N load and 100 °C temperature45. (Reprinted with permission of

Tylor and Francis, from “Frictional Properties of Automatic

Transmission Fluids: Part I—

Measurement of Friction–Sliding Speed Behavior”, Tribology Transactions,

Ingram, M.; Noles, J.; Watts, R.; Harris, S.; Spikes, H. A., 2010)...... 25

Figure 2.10 Molecular structure of polyisobutylene succinimide polyamines (PIBSA

PAMs) dispersant...... 26

Figure 2.11 Friction coefficient as a function of sliding speed at 50, 90, 130 and 150 °C62.

(Reprinted from “Engagement Behaviour of a Paper-Based Wet Clutch Part 2:

Influence of Temperature”, Proc. Inst. Mech. Eng. Part D, Holgerson, M.;

Lundberg, J J. Automob. Eng. 1999, 213 (5), 449–455)...... 28

xiv

Figure 2.12 Effect of lubricant additives on (a) friction coefficient and (b) wear scar

diameter66. (Reprinted from “Wear”, 261 (2), Huang, H. D.; Tu, J. P.; Gan, L.

P.; Li, C. Z.“An Investigation on Tribological Properties of Graphite

Nanosheets as Oil Additive”, 2006)…………………………………………..30

Figure 2.13 SEM image of metal ball used in four ball tester for pass load of (a) Mineral

oil; (b) Mineral oil +0.5% wt CNT; (c) Mineral oil + 0.5% Graphite.71

(Reprinted from “extreme pressure property of carbon nano tubes (CNT) based

nanolubricant”, Journal of Chemical Engineering and Materials Science, 4(8),

123-127, 2013) ...... 33

Figure 2.14 Friction (Coefficient Of Friction-COF) behavior of (a) PAO, (b) PAO-amine-

phosphate, (c) PAO-[P4444][DEHP], (d) PAO-[P66614][DEHP], (e) PAO-

[P66614][i- C7H15COO], (f) PAO-[P66614][n-C17H35COO], and (g) PAO-

[P66614][RSO3] blends; (h) wear rates of PAO and the blends. Numbers

indicate the sequence of repeat tests.81(Reprinted with permission from

“Ionic Liquids Composed of Phosphonium Cations and Organophosphate,

Carboxylate, and Sulfonate Anions as Lubricant Antiwear Additives” Zhou,

Y.; Graham, T. W.; Luo, H.; Leonard, D. N.; Qu, J. Copyright (2014) ACS)…

… … … … … … … … … … … … … … … … … … … … … … … … … 37

Figure 2.15 TEM micrographs of SWNTs (a) received from a commercial source and (b)

obtained by dropping a bucky gel of 1-butyl-3-methylimidazolium

84 tetrafluoroborate [bmim][BF4] into deionized water . (Reprinted with

permission from “Ionic Liquids for Soft Functional Materials with Carbon

Nanotubes” Fukushima, T.; Aida, T. 2007, 5048– 5058)...... 38

xv Figure 3.1 DSC measurement curve for adsorption energy calculation...... 45

Figure 3.2 Energy for different additive adsorption on friction material...... 46

Figure 3.3 UV-VIS spectra of friction modifier solutions at different concentrations. 47

Figure 3.4 UV-VIS spectra of detergent solutions at different concentrations...... 47

Figure 3.5 UV-VIS spectra of dispersant solutions at different concentrations...... 48

Figure 3.6 UV-VIS calibration curve for friction modifier...... 48

Figure 3.7 UV-VIS calibration curve for dispersant...... 49

Figure 3.8 UV-VIS calibration curve for detergent ...... 49

Figure 3.9 UV-VIS quantitative analysis of ATF additive adsorption on friction material

filler diatomaceous earth...... 51

Figure 3.10 Shear stress-shear rate curves from shear sweep experiments using cone and

plate rheometer comparing filler mixed with base oil versus ATF with (a) log

scale and (b) linear scale...... 52

Figure 3.11 Optical images of (a) filler + base oil and (b) filler + ATF dispersions 54

Figure 3.12 Schematic representation of mechanism for filler and additive interaction in

(a) base oil and (b) ATF in steady state...... 55

Figure 3.13 Schematic representation of mechanism for filler and additive interaction in

(a) base oil and (b) ATF under shear...... 56

Figure 3.14 Shear stress versus shear rate curves for filler (DE) mixed with base oil, friction

modifier, dispersant and ATF under shear sweep experiment at (a) complete

shear rate range and (b) lower shear rates ...... 57

Figure 3.15 SAE No. 2 data (friction characteristics) for friction material tested in different

lubricants...... 59

xvi Figure 3.16 Friction modifier adsorption on filler (diatomaceous earth) and fiber (aramid)

as a function of additive concentration...... 61

Figure 3.17 Shear stress versus shear rate curves for ATF mixed with aramid, filler (DE)

and aramid + filler, obtained by shear sweep experiment...... 62

Figure 3.18 SAE No. 2 data (friction characteristics) for friction material made up of 100%

aramid versus 50% aramid + 50% filler.27 (The data is collected by Dr. Murat

Bakan at LuK USA LLC)...... 64

Figure 3.19 Viscosity-temperature curve for ATF ...... 65

Figure 3.20 Shear stress versus shear rate curves for filler + ATF at 40 °C and 90 °C, under

shear sweep experiment with (a) linear y-axis and (b) log y-axis ...... 66

Figure 3.21 SAE No. 2 data (friction characteristics) of friction material tested in ATF as a

function of temperature ...... 67

Figure 4.1. Shear stress-shear rate curves from shear sweep experiments on three different

fillers mixed with ATF ...... 74

Figure 4.2 Wet clutch friction results for three different fillers in thick (60-80 μm) coats on

friction material (a) pre break-in at 40 °C, (b) pre break-in at 120 °C, (c) post

break-in at 40 °C and (d) post break-in at 120 °C ...... 77

Figure 4.3 Wet clutch friction results for three different fillers in thin (25-40 μm) coats on

friction material (a) pre break-in at 40 °C, (b) pre break-in at 120 °C, (c) post

break-in at 40 °C and (d) post break-in at 120 °C ...... 80

Figure 5.1 SAE No. 2 data (friction characteristics) for diatomaceous earth filler coated

versus non-coated friction materials (proprietary friction material composition).

...... 90

xvii Figure 5.2 Durability test samples failed after 8% test completion showing rupture of

diatomaceous earth filler-coated layer on friction material (proprietary friction

material composition)...... 92

Figure 5.3 SAE No. 2 data (friction characteristics in terms of friction coefficient versus

sliding speed) of coated versus non-coated friction materials for ‘cellulose +

aramid’ formulations at low density (a and b), medium density (c and d) and

high density (e and f), (refer to Table 5-1 for density range). 97

Figure 5.4 Durability test data for coated and non-coated friction materials containing

‘cellulose + aramid’ as fiber, at different densities...... 99

Figure 5.5 SAE No. 2 data (friction characteristics in terms of friction coefficient versus

sliding speed) of coated versus non-coated friction materials for low density (a

and b), medium density (c and d) and high density (e and f), (refer to Table 5-2

for density range)…...... 103

Figure 5.6 Durability test data for coated and non-coated friction materials containing only

aramid as fiber, at different densities...... 105

Figure 5.7 Oil drop measurement of filler-coated friction materials containing cellulose and

aramid versus only aramid...... 106

Figure 5.8 Tensile strength data for friction materials containing cellulose + aramid versus

only aramid as fiber, at different densities...... 107

Figure 5.9 SAE No. 2 data (friction characteristics in terms of friction coefficient versus

sliding speed) of filler-coated friction materials for non-coated, 25-40 μm and

60-80 μm thickness coatings...... 108

xviii Figure 5.10 Optical images of friction materials with (a) no coating, (b) 25-40 μm coating

thickness and (c) 60-80 μm coating thickness...... 110

Figure 6.1 Molecular structure of trihexyltetradecylphosphonium bis(2-ethylhexyl)

phosphate (Ph-IL) ...... 114

Figure 6.2 Schematic of high frequency reciprocating rig (HFRR) ...... 117

Figure 6.3 DSC isotherms for Ph-IL and MWCNT-IL nanofluids with different

compositions...... 119

Figure 6.4 Solid state NMR of IL and 10 wt% MWCNT-IL nanofluid (In collaboration

with Dr. Toshikazu Miyoshi, Department of Polymer Science, The University

of Akron)………………………………………...... 120

Figure 6.5 FTIR spectra of Ph-IL and MWCNT-IL nanofluids with different compositions.

...... 121

Figure 6.6 Elastic and loss moduli for 1% strain as function of angular frequency at

different MWCNT concentrations...... 123

Figure 6.7 TEM micrographs of MWCNTs dispersed in hexane (a) treated with IL and (b)

without IL ...... 125

Figure 6.8 Visual CNT dispersion after IL treatment at different molar concentrations.126

Figure 6.9 UV-VIS spectroscopic analysis for well dispersed carbon nanotubes...... 127

Figure 6.10 HFRR wear measurement on ball in (a) base oil, (b) CNT + IL + base oil, and

on disc in (c) base oil, (d) CNT + IL + base oil ...... 130

xix

CHAPTER I

INTRODUCTION

During the 21st century, global awareness about improving the fuel-efficiency of vehicles has increased tremendously due to the limited petroleum resources and environmental concerns. Around 33% of the fuel energy is lost in most of the passenger cars and heavy-duty vehicles due to friction in transmission system, engine, brakes and tires1,2. Figure 1.1 shows total fuel energy distribution in a passenger car.

Figure 1.1 Breakdown of passenger car energy consumption1. (Reprinted from Tribology International, 47, Holmberg K., Andersson P., Erdemir A., Global energy consumption due to friction in passenger cars, 221-234, Copyright (2012), with permission from Elsevier).

1 Most of the research in this field is focused on improving lubricant and additive

formulations, applying low friction coatings on the friction components, surface texturing, etc. Furthermore, smooth (shudder-free) operation of a torque conversion system between an automobile engine and its transmission depends on lubrication efficiency as revealed by friction coefficient versus sliding curve having a positive slope and by higher values of dynamic friction coefficient (i.e., friction characteristics) while avoiding lengthy and inefficient periods of wet clutch engagement.

With the motivation of reducing friction losses and having optimum friction characteristics, this dissertation focuses on two applications. The first part involves the wet clutch system used in automobiles. ‘Lubricant additive – friction material’ interactions and material modifications are considered to improve friction characteristics (as related to friction coefficient – sliding curve behavior) and to reduce friction losses created by inefficient operation of the wet clutch system. The second part of the work focuses on reducing the friction coefficient and wear in steel-steel boundary lubrication in order to reduce friction losses by lubricant modification.

Wet clutch is a part of the transmission system in automobiles. Friction material

(FM), reaction plate (steel) and automatic transmission fluid (ATF) are the major components of a wet clutch system. During clutch engagement, the friction material (also called “friction paper”) is pressed against the steel reaction plate in the presence of ATF.

Positive slope of the friction versus sliding speed curve and higher value of dynamic friction coefficient are the ideal characteristics during clutch engagement. The interaction of ATF additives with friction material components is an important factor in the boundary

2 lubrication regime which exists in a typical wet clutch system. Most of the fundamental

research in literature related to this topic has focused on additive interaction with steel plate surface. But, paper-based friction material formulations are based on trial-and-error method and vary considerably. Thus, there is a need to understand the interactions between

ATF additives and different friction material components.

Based on the above-mentioned consideration, Chapter III focuses on quantitative analysis of ATF additive adsorption on friction material components such as fillers and fibers. Interaction is further correlated with rheological and friction phenomena. Also, using rheological analysis, the shear stress and the shear strain rate are correlated with friction and sliding speed, respectively, in wet friction systems. The fundamental understanding of interactions between the ATF additives and different friction material components will help in selection of the optimal components for friction material formulation that will improve the friction performance thus reducing fuel consumption while providing a smooth operation of the automotive torque converter system.

Components providing shear thickening behavior, rheologically, are expected to increase friction with speed, as required for ideal performance.

Chapters IV and V focus on filler-coated friction material optimization. Based on the interaction study of chapter III, suitable filler is selected for coating the friction paper.

Filler coating is expected to increase friction coefficient due to higher shear thickening effect. But, simultaneously, it will also reduce ATF permeability of the surface. Hence, different approaches based on friction material density, filler/fiber selection and coating thickness are studied to affect the surface porosity.

3 The next part of the work focuses on development of additive system for lubricants in boundary lubrication regime. Carbon nanotubes (CNTs) are used as anti-wear and friction reducing additives for lubricants because of their cylindrical shape. CNTs may reduce friction by their rolling or combined rolling and sliding action during a frictional engagement. But, efficient dispersion of CNTs in lubricants is difficult due to their aggregation under strong van der Waals forces. Different chemical functionalization and mechanical mixing techniques have been used to disperse nanotubes. Chemical functionalization can hamper the electronic structure of CNTs thus, affecting their final properties. Whereas, mechanical mixing sometimes causes reduced aspect ratio under applied stresses. A non-covalent functionalization technique, such as the use of a surfactant can provide effective dispersion without affecting inherent CNT properties.

Based on the considerations described above, a new approach of Ionic Liquid (IL) use is investigated in Chapter VI as a surfactant for non-polar lubricant system.

Phosphonium ionic liquid, which is compatible with non-polar hydrocarbon oil, is used as a surfactant, and the adsorption of ionic liquid on carbon nanotubes is investigated. It is shown that, CNT-IL interaction results in overcoming the van der Waals attraction giving well dispersed nanotubes in non-polar solvent and lubricant system. Ionic liquid concentration is further optimized, as over concentration of IL can suppress the CNT dispersion. Synergistic effect of carbon nanotubes and ionic liquid for friction and wear reduction in steel-steel contact in boundary lubrication is also further investigated.

CHAPTER II

BACKGROUND

2.1. Tribology

Tribology can be defined as a science of the friction, lubrication and wear behavior of surfaces in a relative motion. ‘Tribology’ term was first introduced in the Jost Report in year 19663. The word is derived from the Greek word ‘Tribos’ which means rubbing.

Though ‘Tribology’ can be defined as a separate science, it is basically interdisciplinary

science involving material technology, physics, mechanical engineering, fluid dynamics,

etc. Tribology of the surfaces is important in different fields of study including but not

limited to automotive4–7, coatings8,9, biomedical10,11, nanotechnology12,13 fields. In all these

cases, study of friction and wear is very important to minimize energy losses and improve system efficiency.

2.1.1. History of Tribology

The concept of tribology is in use since very old times. Attempts to reduce friction

were made while using wheels from 3500 B.C. Lubricants such as water were then used to reduce the wear. It is evident from the ancient drawings that the Egyptians used some type of lubricant while building pyramids. Leonardo da Vinci (1452-1519) proposed scientific approach to friction. He discovered the law of friction showing the relationship between the friction coefficient, friction force and the normal force. He studied friction by analyzing

the static friction coefficient on an inclined plane. It is also reported that he modified the

rolling bearing with the use of balls to reduce friction. In 1699, Guillaume Amontos

showed that the frictional force is proportional to normal force and it depends not only on

adhesion but also on abrasion14. Desaguliers developed a tribology model relating friction

to the influence of cohesion and adhesion14. Newton, Euler and Coulomb also contributed to the understanding of the frictional force14. New advancements in tribology took place

over time with developing technologies14.

2.1.2. Lubrication Regimes and The Stribeck Curve

Tribology can be distinguished in two categories; ‘dry’ and ‘lubricated’ tribology.

Dry contacts are used when the use of liquid lubricants is not desired due to some

constraints such as temperature, contamination, etc. In other systems, lubricants are used

to reduce wear and to maximize efficiency. In most cases, reduction of friction is

desired15,16. Whereas, in some cases, increased and stabilized friction is required such as in

brake pads and clutch discs17–19. The interactions between lubricants and moving surfaces

must be understood to maximize efficiency of the system. Based on physical behavior,

lubrication is classified in three regimes namely the hydrodynamic regime, the elasto-

hydrodynamic (mixed) regime and the boundary regime.

Figure 2.1 Stribeck curve showing three lubrication regimes: hydrodynamic, mixed and boundary regime.

These lubrication regimes are depicted by the Stribeck curve shown in Figure 1.1.

Stribeck curve is constructed by plotting friction coefficient as a function of bearing index

‘S’. Bearing index is defined as ‘ƞV/P’, where ƞ is the viscosity of the lubricant, V is the

relative velocity and P is the pressure. Specific regimes can be recognized using the lambda ratio given as20:

0 = (Eqn. 2.1) 2 2 1ℎ+ 2

𝜆𝜆 √𝑅𝑅 𝑅𝑅 where, h0 is the minimum hydrodynamic film thickness, R1 and R2 are root mean square

roughnesses for the two surfaces involved.

In hydrodynamic regime (λ>3), there is a thick lubricant layer between the two

surfaces. Friction in this regime is a function of lubricant properties such as viscosity and

it increases with increasing speed or viscosity due to viscous drag. In boundary regime

7 (λ<1), the two surfaces are in direct contact with each other and there is no lubricant layer

between the two. Hence, friction characteristics are independent of lubricant properties.

Here, friction is a function of surface properties and surface-additive interactions. In

boundary lubrication, friction is higher as the surface asperities are sheared during motion.

The third regime is mixed lubrication regime where the value of λ is between 1 and 3. Here, there is a thin lubricant layer and surface asperities are still in contact20.

2.2. Wet Clutch System

Clutch is an integral part of the torque converter system in automatic transmission

of a vehicle as shown in Figure 2.2. Torque converter is used to transfer energy from the

engine to the transmission21. Wet clutches are used during gear change or for torque

transfer under specific conditions22. The clutch system consists of two major components

which we are focusing on in this work; The friction material which plays an important role during clutch engagement, and the Automatic Transmission Fluid (ATF) which continuously circulates through the system for torque transfer, maintaining friction characteristics while cooling the system.

Figure 2.2 Schematic diagram of a torque converter23. (Republished with permission of Tribology Letters, from “tribology of automatic transmission fluid”, Kugimiya, T.; Yoshimura, N.; Mitsui, J., 5(1), 1998, permission conveyed through Copyright Clearance Center, Inc.).

2.2.1. Friction Materials History

Friction materials are used to control friction during clutch engagement and disengagement. They should be able to provide higher and stable friction coefficient with minimum wear. Also, they should be able to sustain higher temperatures (up to 300 °C).

Before the 20th century, dry frictional pairs consisting of rubber, cloth, leather, etc. were

used as friction materials24. Asbestos was introduced in 1918 to be used as a friction

material. Majority of the friction materials till 1980s were made up of asbestos along with

other components such as cellulose, fillers and phenolic resin. But, due to the health

concerns, asbestos friction materials were replaced with other options during mid

1980s25,26. Increased percentage of cellulose, use of synthetic fibers like aramid, chemical

modification of components and the use of various resins and their blends were introduced

9 to improve the friction performance of friction materials without asbestos. Increase in

cellulose concentration showed promising results for smooth clutch engagement27, whereas

increasing aramid concentration resulted in good durability. Optimization of friction

material composition is still an ongoing area of research to improve the friction and

durability performance.

2.2.2. Friction Material Ingredients

Paper-based friction materials are used widely because of their high porosity which helps in attaining boundary lubrication. Also, they provide high dynamic friction coefficient. Fibers (organic and synthetic), fillers, friction modifiers and resins are the primary components used for paper-based friction material preparation.

2.2.2.1. Fibers

Fibers provide strong and porous skeleton to the friction material. Higher porosity

allows ATF to circulate in the system while avoiding the formation of a lubricant layer

between the friction material and the steel surface. Cellulose is a natural fiber which

provides high friction coefficient to the friction material. But, cellulose starts to carbonize

at around 200 °C and degrades completely before reaching 420 °C28. This decreases the

long-term durability of the friction material as temperature can rise to 200-300 °C during

clutch engagement. Aramid fiber is also used, to some extent, to improve durability in

heavy-duty applications. Glass fibers have also been studied in some compositions to

increase mechanical properties29.

10 The friction performance is also dependent on fiber material variables such as fiber

fibrillation, density, length, diameter and chemical treatment of the fiber. Higher

fibrillation increases fiber-fiber bonding to give higher strength, but, it can also decrease

porosity of the friction material.

2.2.2.2. Fillers

Fillers are used to increase the hardness of the friction material. They also enhance

the heat capacity of the material. Friction materials usually consist of fillers by 30-40 wt%,

and thus, fillers are expected to be inexpensive and available abundantly. Diatomaceous

earth, clay, cashew nut powder and calcium carbonate are used as fillers in majority of the

compositions19,30. Increasing filler concentration gives increased friction coefficient,

having a positive gradient with sliding speed. But, porosity decreases with increasing filler

concentration deteriorating durability. Hence, optimization of fiber-filler ratio is important

in maximizing friction material efficiency.

2.2.2.3. Friction Modifiers

Friction modifiers are used in small quantity (< 5%) to improve friction

performance. For example, graphite is used to improve friction stability, fade resistance

and for smooth engagement because of the presence of sliding planes in graphite31. But,

higher percentage of graphite can cause lower friction coefficient at higher sliding speeds.

Carbon fiber is also used in smaller quantities to improve mechanical strength and antiwear properties of the friction material32.

2.2.2.4. Binders

Thermoset polymeric resins are used to bind together all the ingredients of the

friction material. Binder covers all the ingredients and is also present at the surface of the

friction material. Hence, it should have high temperature stability and wear resistant

properties as well. Phenolic and epoxy resins are commonly used as they do not decompose up to 300 °C. Resin concentration also plays an important role on friction. Fei et. al. studied the effect of increasing resin concentration and found that mechanical properties improved with increasing resin concentration33, while the friction coefficient started to decrease with

it. Resin usually makes up 35-45% of the final dry weight of the friction material.

Resin blended or modified with other components such as Tung oil and elastomers is also used in some compositions improving the flexibility and elasticity of the friction material34 providing less damage to friction material under high pressure cycles.

12 2.2.3. Automatic Transmission Fluid (ATF)

Figure 2.3 Schematic of energy transfer from engine to transmission system when (a) a clutch is not engaged (by ATF flow) and (b) a clutch is engaged.

Automatic transmission fluid (ATF) is circulated through the torque converter

system. The torque is transferred from a pump (linked to the engine) to a turbine (linked to the transmission) by flow of the fluid as shown in Figure 2.3. When the clutch is not engaged (Figure 2.3a), torque is transferred by ATF flow across the turbine. When the clutch is engaged (Figure 2.3b), ATF additives provide the necessary friction characteristics to the system. ATF consists of approximately 80% base oil and 20% additives. These additives consist of friction modifier, detergent, dispersant, corrosion inhibitors, anti-wear and extreme pressure agents, antioxidants, antifoaming agents, viscosity modifiers, etc. The additives get adsorbed forming a layer on moving surfaces to

13 avoid stick-slip motion and wear in boundary lubrication. The three major additives studied

in this work are described below in detail.

2.2.3.1. Friction Modifiers

Friction Modifiers (FM) are very important to obtain the desired friction

characteristics. Friction modifiers are surface active and form molecular layers on the

contacting surfaces35. A typical friction modifier is a long (non-polar) hydrocarbon chain molecule with an active (polar) group which will strongly attach itself to the rubbing surfaces either by chemisorption or physisorption36.

In the previous studies, friction modifiers showed decrease in static friction

coefficient compared to base oil, providing a positive slope for the friction curve23,37. Bakan

showed that the extent to which friction changes depends on the FM molecule structure27.

He found that stearic acid provides higher positive gradient for the friction versus sliding

speed curve compared to the use of oleic and linoleic acids during SAE No. 2 test. This is

because of the effect of unsaturation on shape of the molecule. The molecule-level function of FM was also studied using different techniques and simulation38,39. Zhu et. al. showed

that 1-hexadecylamine FM adsorbs forming a monolayer on sliding surfaces and reduces

static friction force which helps to avoid stick-slip motion40. Slough et. al. studied the effect of FM concentration on friction coefficient and found that the gradient of friction versus sliding speed curve became positive with increasing concentration of FM41.

14 2.2.3.2. Detergents

Detergents are used in automatic transmission to prevent pore-clogging of friction

material by the degradation products present in the system. They also incorporate basic

ingredients which help in neutralizing the effect of acidic contaminants42. Most of the

detergents studied are magnesium or calcium sulphonate based detergents.

Detergents increase the overall value of friction coefficient at higher sliding

speeds43,44. They are adsorbed on the moving surfaces and form a film which increases the local viscosity around friction material and steel contact. This causes a rise in friction.

Molecular structure of detergents is also studied in the literature. It is well established that the branched alkyl structure in detergents provides higher friction than the linear alkyl groups45.

2.2.3.3. Dispersants

Dispersants are used in ATF to prevent agglomeration of degradation products of

the ATF and the friction material. Dispersant helps to keep the sludge particles suspended

in the system which might, otherwise, affect the clutch performance if settled down42.

Polyisobutylene succinimides are dispersant molecules largely studied in the

literature. Like detergents, dispersants also increase the value of μd. But, they do not affect

μs and keep the slope of friction versus sliding speed curve negative. Ingram studied the

effect of succinimides surfactants at different temperatures20. He showed that dispersants

cause higher friction and slightly decreasing friction with increasing sliding speed.

15 2.2.4. Wet Clutch Tribology

2.2.4.1. SAE No. 2 Friction Tester

Figure 2.4 Schematic diagram of the SAE No. 2 test machine and detail of its test head23. (Republished with permission of Tribology Letters, from “tribology of automatic transmission fluid”, Kugimiya, T.; Yoshimura, N.; Mitsui, J., 5(1), 1998, permission conveyed through Copyright Clearance Center, Inc.).

SAE No. 2 machine is used to study friction performance of a clutch system. Figure

2.4 shows a schematic of the SAE No. 2 friction tester. During testing, the flywheel is rotated at a certain rpm and then pressure is applied through the clutch pack which causes deceleration of the flywheel. The friction transmitted by the clutch system is measured. As shown in Figure 2.5, the initial friction coefficient value (μi) is measured at the beginning

of clutch engagement. The dynamic friction coefficient (μd) is measured when the speed is at half of the initial sliding speed. The end friction coefficient (μ0) is measured at the end

of the test, when the flywheel almost stops rotating20. Figure 2.5 shows the data

representation for a continuous slip test in SAE No. 2 machine. We note that, the friction 16 data represented in this dissertation is acquired from discrete engagements, where,

coefficient of friction is obtained from the torque values at specific speeds between a

certain rpm range, to get friction curves similar to one or more of those shown in Figure

2.6. During this test, the flywheel is rotated between certain rpm range and the clutch pack is slipped for 3 seconds at each specific speed. The test is performed at 40 °C, 90 °C and

120 °C temperatures and at 415 kPa, 775 kPa, 1940 kPa and 2960 kPa surface pressures.

Friction characteristic (Figure 2.6) curve is plotted based on the friction coefficients obtained in this test.

Figure 2.5 Test pattern in the SAE No. 2 test machine and an example of the torque and speed pattern obtained in the test.

2.2.4.2. Friction Characteristics

Wet clutch is engaged for generation and transmission of the torque from the engine to the transmission. When the clutch plates are pressed against each other, the shafts at

17 opposite ends are rotating at different angular velocities. During the engagement, the

relative velocity drops to zero due to higher friction between the friction material and the

steel plate. Friction characteristics during this engagement play important role in torque

generation.

During an SAE No. 2 test, the reaction plate (steel) moves towards the friction disc when the test starts. The friction transmitted from the system is monitored during the deceleration stages of the clutch plates. In the first stage, the system works in the hydrodynamic regime with a lubricant layer present between the friction disc and the reaction plate. A torque is generated by the viscous flow of this ATF film. When the friction disc and the reaction plate come closer, the ATF is squeezed through porous network of the friction material and the steel surface meets friction material asperities. As the relative speed decreases, the viscous flow torque decreases while torque contribution due to friction material roughness and asperity contact increases. Thus, the system comes into the boundary lubrication regime and the relative speed becomes almost zero. In the wet clutch system, boundary lubrication is the governing regime as it is difficult to maintain an ATF layer between the surfaces due to high porosity of the friction material.

Figure 2.6 Representations of friction coefficient versus sliding speed for SAE No. 2 test results depicting decreasing (dμ/dν < 0), increasing (dμ/dν > 0), and increasing/decreasing (curve C) behaviors of friction coefficient-sliding speed curves.

In the boundary lubrication regime, friction behavior is governed by the additive

interaction of asperity surfaces and ATF viscous flow near the surfaces. Stick-slip or

shudder behavior may be caused during clutch engagement due to torque variation. Figure

2.6 shows the friction coefficient versus sliding speed curves for wet clutch engagement.

Curves A and C represent undesired friction characteristics as friction is decreasing with

increasing sliding speed i.e. dμ/dν < 0. The system shows curve ‘A’ in presence of base

oil, without any additives. Curve ‘C’ also denotes bad friction characteristics which can be due to the improper selection of additives or friction material components. Curve ‘B’ shows monotonically increasing friction coefficient with increasing sliding speed i.e. dμ/dν > 0.

This would give smooth engagement avoiding shudder.

19 2.2.4.3. Tribo-film Formation and Additive Adsorption

There are different theories proposed in literature to explain the ideal friction

behavior of a wet clutch system. Different additives present in ATF interact with the

friction material and the steel plate to provide the necessary friction behavior. Zhao et. al

analyzed surfaces after tribo-testing with fatty amide friction modifier and calcium

detergent combinations. They found the friction modifier present in the tribo-film composition on the steel. They proposed that this adsorbed layer causes decrease in friction coefficient at low speeds due to the van der Waals interaction at friction modifier groups and solid interface36,46.

The nanotribology of the adsorbed additive films is also studied using techniques

such as surface force apparatus (SFA), X-ray photoelectron spectroscopy (XPS) and polarized neutron reflectometry40,47,48. Nano-rheological properties of the model ATF system between two mica surfaces were studied on SFA. Friction modifier molecules were found to have increased hard-wall distance reducing the adhesion force and eliminating shudder at small concentrations (0.1-0.2 weight%) of friction modifiers. Friction modifiers consist of a polar head (groups like acid or amine) and non-polar hydrocarbon tail. They form a closely packed array in multilayer assemblies as shown in Figure 2.7. The adsorbed chains are aligned parallel to each other due to van der Waals forces. The adsorbed layer prevents direct contact between the surfaces reducing wear while providing desired rheological properties.

Figure 2.7 Friction modifier adsorption on friction surfaces.

Ingram et. al. showed that the friction behavior of wet clutch system is controlled

by shear stress of the adsorbed friction modifier film present on the friction material and

the steel surfaces, where the friction coefficient increases as logarithmic function of sliding speed. Whereas, other additives like detergent and dispersant probably disrupt the adsorbed friction modifier layer thus increasing overall friction coefficient49. It is well established

from previous studies that friction modifiers help in decreasing static friction coefficient as compared to base oil. But, they also decrease the overall value of the friction coefficient20,23. On the other hand, additives like detergent and dispersant increase the overall friction coefficient while not affecting the static friction coefficient as compared to

base oil.

2.2.5. Factors Affecting Friction Characteristics

2.2.5.1. Friction Material Parameters

Paper-based friction material is made up of fiber, filler, additives and binder. The

surface of the friction material is rougher and softer than the steel reaction plate surface.

21 Surface roughness of the paper is an important factor that affects the real contact area

between the friction material and the reaction plate. Surface and bulk softness (stiffness)

plays an important role as it affects the surface deformation during shearing of the softer

friction material against the steel plate. Friction material mechanical properties vary with

paper density, fiber to filler ratio, binder concentration, etc50–52. Also, the formation of

boundary lubrication regime is dependent on the ATF penetration time through the friction material. Hence, friction material porosity affects friction behavior during clutch engagement. Elasticity of the friction material is also an important parameter as it can retain paper shape after vigorous pressure and shear cycles53. Brittle paper leads to reduced

durability as the paper domains cannot stretch back after shear deformation.

Matsumoto et. al. found that the friction coefficient increases with increasing porosity and pore diameter while keeping the ingredients constant. Mercury intrusion porosimetry was used to measure porosity percentage and diameter distribution of five friction material compositions. These five compositions were arranged in descending order of porosity, relative to each other. Increasing porosity leads to lower time for ATF penetration and expends the region where boundary regime dominates54. On the other hand, lower porosity results in hydrodynamic lubrication regime providing reduced friction.

Friction material with highest porosity showed increasing friction coefficient with increasing bearing index, ƞV/P. But, the friction coefficient of the lowest porosity friction material decreased at high speeds. Microscale texture of the friction material also plays an important role in determining friction characteristics55.

22 Friction materials with pure phenolic resin are hard and brittle in nature which can

cause crack formation in the paper after a few engagement cycles. Hence, elasticity of the

friction material is adjusted by using suitable oils like linseed or tung oil and rubbers such

as SBR or NBR. Liu et. al. incorporated nano powdered rubber in friction papers34. They found stable friction coefficient and lower wear rate of the friction material.

Resin content also has an effect on the friction behavior. Fei et. al. showed that increasing amount of phenolic resin increases the mechanical properties and hardness of the paper33. But, it also affects porosity which causes decrease in friction coefficient. The

best friction properties were obtained at the lowest concentration (35%) of phenolic resin.

Kim et. al. and Bijwe et. al. also varied phenolic resin concentration and modification to

find enhancement of friction properties by optimization30,56.

2.2.5.2. ATF Parameters

Effects of ATF additives such as friction modifier, detergent and dispersant have

been extensively studied in literature. Ito et. al. studied the effect of ATF versus base oil in

Stribeck curve representation as shown in Figure 2.8. They observed that base oil decreases friction coefficient with increasing bearing index, ƞV/P. But, the presence of additives cause increasing or stable friction at medium or higher values of the bearing index57.

Figure 2.8 Stribeck curve of a wet clutch contact lubricated with (a) only base oil and (b) a fully formulated ATF57. (Reprinted with permission of Tylor and Francis, from “Friction Characteristics of a Paper-Based Facing for a Wet Clutch Under a Variety of Sliding Conditions”, Tribology Transactions, Ito, H.; Fujimoto, K.; Eguchi, M.; Yamamoto, T., 1993, permission conveyed through Copyright Clearance Center, Inc.).

Friction modifier shows strong adsorption on both the surfaces reducing static friction coefficient and giving positive slope to the friction versus sliding speed curve.

Many theories and explanations have been studied by SAE No. 2 testing, at molecular level as well as by molecular simulation. Friction modifier molecular structure also plays an

24 important role in friction characteristics. Ingram et. al. studied the effect of stearic, oleic

and elaidic acids as friction modifiers20,45. They found that stearic acid and elaidic acid

cause decreasing friction with increasing speed as shown in Figure 2.9. Whereas, oleic acid shows lower friction at higher speeds, same as base oil. Oleic acid consists of cis- double bond causing less packing of molecules on surfaces. But, stearic acid (saturated bonds) and elaidic acid (having trans- double bond) have more linear conformation. This causes densely packed molecular layer giving increasing friction with speed58.

Figure 2.9 Friction coefficient versus sliding speed curve for various model organic friction modifiers at 3N load and 100 °C temperature45. (Reprinted with permission of Tylor and Francis, from “Frictional Properties of Automatic Transmission Fluids: Part I—Measurement of Friction–Sliding Speed Behavior”, Tribology Transactions, Ingram, M.; Noles, J.; Watts, R.; Harris, S.; Spikes, H. A., 2010).

The shortcoming of friction modifier is that it reduces the overall friction

coefficient. Detergent and dispersant are used to avoid aggregation of decomposition

products of the wet clutch by keeping them suspended in the system. These additives also

increase the overall friction coefficient at higher sliding speeds. Kitanaka et. al. studied the

25 effect of a series of detergents on friction behavior59. They showed that detergents increase dynamic friction coefficient, μd, and decrease static friction coefficient, μs. They also found

that the factors such as head group, extent of overbasing and metal type produce limited

effect on μd and significant effect on μs. They suggested the formation of an adsorbed layer which increases local viscosity at higher sliding speeds. Topolovec-Miklozic et. al. also proposed formation of solid-like thin film of thickness 100-150 nm on sliding surfaces60.

These films cause increasing friction due to the pad-like structure of the adsorbed layer that restrains fluid entrainment and thus delays the formation of hydrodynamic film at higher speeds. They found that the friction behavior is mostly dependent on the alkyl chain structure of sulphonate detergent molecules.

Addition of dispersant to base oil results in increase in overall friction coefficient without affecting the slope of the friction coefficient versus sliding speed curve as compared to base oil. Polyisobutylene succinimide polyamines (PIBSA PAMs) are the most commonly studied dispersants with the structure shown in Figure 2.10.

Figure 2.10 Molecular structure of polyisobutylene succinimide polyamines (PIBSA PAMs) dispersant.

Kitanaka et. al. showed that increasing the molecular weight of PIB group of

dispersant from 610 to 2350 caused decrease in the static friction coefficient, μs. The

26 increase in molecular weight of PIB causes increase in chain length and the hard-wall

distance is increased. The contact area between the friction material and the steel plate is

reduced causing decrease in friction. Increase in overall friction coefficient or μd is also

observed regardless of the PIB molecular weight, due to increase in local viscosity at higher speeds59.

2.2.5.3. Process Parameters

Process parameters such as temperature and pressure have significant effect on

friction properties of the wet clutch system61–63. When the clutch is engaged, temperature

of the system can rise to 200 to 300 °C within a few seconds of engagement. The

temperature fluctuation can alter the friction behavior by changes in ATF viscosity,

temperature dependence of additive adsorption and degradation of the friction material and

ATF. Holgerson et. al. studied the effect of rising temperature on friction behavior of paper- based wet clutch system62. They found that values of static and dynamic friction

coefficients decreased by increasing the starting and instant temperature as shown in Figure

2.11. The possible mechanisms for this behavior are decrease in ATF viscosity and more active additives at high temperatures. The quotient between static and dynamic friction also decreased with increasing temperature avoiding the risk of stick-slip behavior. Higher temperature also increased the engagement time and lowered the torque capacity.

Figure 2.11 Friction coefficient as a function of sliding speed at 50, 90, 130 and 150 °C62. (Reprinted from “Engagement Behaviour of a Paper-Based Wet Clutch Part 2: Influence of Temperature”, Proc. Inst. Mech. Eng. Part D, Holgerson, M.; Lundberg, J J. Automob. Eng. 1999, 213 (5), 449–455).

Maki et. al. also showed that rise in temperature from 40 °C to 100 °C caused decrease in friction because of increased mobility of additives along with higher activation energy64. Temperature also affects ageing and degradation of the friction material. Steel plate has higher heat conductivity and friction material has lower heat conductivity. Hence, use of additives such as graphite and carbon fiber is studied to improve the heat distribution.

Effect of normal load on friction was studied by Maki et. al. Usually, friction coefficient

decreases with increasing normal load or pressure64, probably due to increased temperature created by increasing pressure. Maki showed a temperature compensated effect on friction as a function of normal load. A small increase in friction coefficient was

28 observed for increasing normal load. They also proposed that normal load might have higher influence under low temperature and low velocity conditions.

2.3. Solid Nanomaterials as Lubricant Additives

Liquid lubricants are used to prevent wear between two surfaces for wide range of applications including vehicle engine, bearings, aerospace, biomaterials, etc. Various petroleum oils and greases have been used for years for the above mentioned applications.

In the elasto-hydrodynamic lubrication regime, there is a thick layer of lubricant between the surfaces. The friction behavior is mainly governed by lubricant properties such as viscosity. Further increase in the pressure causes formation of boundary regime, where friction is governed by the contacting surfaces. Numerous surface asperities undergo elastic deformation due to shear. To avoid wear and reduce friction in the boundary regime, use of only oils and greases without any additives is sometimes inefficient. The universally accepted solution is the use of solid additive particles in small amounts which serve as friction modifiers to avoid direct contact between the surface asperities.

These nanoparticles are added in very low concentrations, usually below 1%.

Higher concentration of the nanoparticles can lead to aggregation, unstable dispersion and high wear in the system. The working mechanism of the nanoparticles can be explained as follows65: (a) Mending effect: A physical tribolayer is formed by the nanoparticles deposited on the surface. This tribofilm compensates for the material loss during friction.

(b) Rolling effect: Spherical nanoparticles reduce wear and friction by rolling action between two surfaces under high pressure. (c) Film effect: Some nanoparticles form a surface protective tribofilm due to vigorous interaction between the particles and the

29 surfaces during friction. This tribofilm can affect friction and wear due to its rheological behavior under shear.

Figure 2.12 Effect of lubricant additives on (a) friction coefficient and (b) wear scar diameter66. (Reprinted from “Wear”, 261 (2), Huang, H. D.; Tu, J. P.; Gan, L. P.; Li, C. Z. “An Investigation on Tribological Properties of Graphite Nanosheets as Oil Additive”, 2006).

30 Use of nanoparticles such as graphite, carbon nanotubes (CNTs) and silica is widely studied and implemented in various applications. Huang et. al. studied the effect of graphite nanosheets as oil additive66. They found that addition of graphite to paraffin oil decreased

friction coefficient and increased the load-carrying capacity and anti-wear ability of the system as shown in Figure 2.12. They showed that graphite nanosheets formed a physical deposition film on the surfaces which prevented direct contact between surface asperities.

Other nanoparticles have also shown promising results as additives to reduce friction and wear.

2.3.1. Carbon Nanotubes as Lubricant Additives

Carbon nanotubes (CNTs), due to their superior physical and mechanical properties, are used in many applications. Carbon naotubes were discovered by S. Ijima in

199167,68. CNTs can have very high length to diameter ratio ranging up to 132x106:169.

CNTs are like the structure formed by rolling up graphene sheets at specific and chiral

angles. They exist in various forms, namely ‘single walled CNTs’, ‘double walled CNTs’ and ‘multi-walled CNTs’. Individual nanotubes are held together by van der Waals forces.

Carbon nanotubes are studied in literature as lubricant and composite additives.

Dassenoy et. al. studied the effect of adding CNTs to polyalphaolefin on friction. They showed that 1 to 2% of CNT concentration is effective in reducing friction coefficient and

wear. They proposed that this improved lubrication is due to the crushing of nanotubes by

shearing action. Cursaru et. al. used Single Walled Carbon Nanotubes (SWNTs) as anti-

wear and extreme pressure additive for SAE 20 base oil70. They tested tribological

31 properties of the solution on pin-on-disc tribometer and four-ball tester. They found that the friction coefficient decreases by addition of SWNTs with optimum concentration of 0.5 wt% nanotubes. They also inferred that carbon nanotubes reduce wear by filling the scars due to friction. Carbon nanotubes fill in the gaps or scars on the rubbing surfaces forming a lubricating film providing protection against wear by providing rolling effect.

Bhaumik et. al. studied the tribological effects of CNTs. They showed that at the optimum concentration of 0.5 wt%, CNTs are efficient to decrease wear as compared to base oil or other additives such as graphite. Higher wear and shear affected areas are observed as shown in Figure 2.13a and c by addition of graphite or without any additive.

But, addition of CNTs shows stable lubricant film and less wear as seen in Figure 2.13b.

Figure 2.13 SEM image of metal ball used in four ball tester for pass load of (a) Mineral oil; (b) Mineral oil +0.5% wt CNT; (c) Mineral oil + 0.5% Graphite.71 (Reprinted from “extreme pressure property of carbon nano tubes (CNT) based nanolubricant”, Journal of Chemical Engineering and Materials Science, 4(8), 123-127, 2013).

Anti-wear mechanism of carbon nanotubes at atomic level is also explored through experimental and simulation studies. Mylvaganam et. al. showed that lower friction in

CNTs is not due to spinning or rolling of nanotubes. Atomically, smooth surface of CNTs is responsible for low coefficient of friction72. On the other hand, simulation studies performed by Ni et. al. showed that the lubrication mechanism in CNTs is due to only sliding or combination of sliding and rolling depending on the orientation and interaction

of the CNTs to the rubbing surfaces.

2.3.2. Carbon Nanotube Dispersion

Theoretically, it is possible to form CNTs by rolling up a hexagonal graphene sheet. sp2 hybridization is present in graphite where each carbon atom is connected to three

carbon atoms in xy-plane and a weak п-bond is present along the z-axis. The free electrons in Pz orbital move within the electron cloud and are responsible for van der Waals interactions. High aspect ratios of CNTs along with high flexibilities, cause the entanglement of nanotubes making it difficult to disperse them in the matrix. The cohesive energy per unit length for a pair of single walled CNTs was calculated to be -0.095 eV/Å by Girifalco et. al73. Huang et. al. also calculated the theoretical cohesion energy per unit

length between a CNT pair that is parallelly aligned to be -0.09 eV/Å and perpendicularly

aligned to be -10 eV per contact. Both of these values are much greater in magnitude than

normal room temperature thermal energy which is 0.025 eV74. Hence, dispersion of carbon nanotubes is not an easy task using conventional techniques.

Different chemical modification and mechanical mixing techniques are used to disperse the entangled nanotubes75. Chemical functionalization of CNTs is used to improve chemical compatibility of the nanotubes with the matrix. Oxidation with strong acids like sulphuric or nitric acid is a commonly used technique to improve the wetting properties of

CNTs. This technique incorporates carboxylic groups on the side walls and end caps of nanotubes are opened. However, chemical treatment can introduce structural defects affecting inherent properties of CNTs. Mechanical methods include high shear mixing, ultrasonication which separate the nanotubes by applying very high shear energy to

34 overcome van der Waals forces. But, these methods can cause breakage of nanotubes

reducing the aspect ratio. Non-covalent functionalization techniques such as treatment with surfactants or polymers prior to mixing with matrix, are also widely used.

2.3.3. Ionic Liquids as Additives for Lubricants

Ionic liquids (ILs) possess unique characteristics such as negligible vapour

pressure, low volatility, non-flammability, and high thermal stability76. These are necessary characteristics for high performance lubricants. The decomposition temperatures of most of the ILs is well above 350 °C and they also possess low temperature fluidity, i.e. Tg below

-50 °C. The ionic liquids can undergo tribochemical reactions and form strongly adsorbed film due to their high polarity. This ability makes them a good candidate as an anti-wear

agent.

Otero et. al. studied ionic liquids containing phosphonium cations as neat lubricants and lubricant additives for steel-steel contact. They found the formation of protective chemical layer composed of iron phosphides and oxides imparting good antiwear and antifriction properties to the selective ionic liquids. Most of the ionic liquids studied in literature are miscible with water and have very low solubility (usually less than 1%) in non-polar solvents or hydrocarbon base oils77–79. Hence, previous studies include formation of microemulsion in ionic liquid–nonpolar lubricant system77,79 or use of polar lubricants80.

Zhou et. al. studied a series of phosphonium ionic liquids which are miscible with

hydrocarbon oils as shown in Table 2-181. They showed that phosphonium ionic liquids

decrease the wear rate as shown in Figure 2.14 by formation of a tribo-film primarily

35 contributed by phosphate anion adsorption. Here, all the ionic liquids showed decreased wear rate and friction as compared to pure polyalphaolefin as shown in Figure 2.14 h.

Table 2-1 Experimental solubility and calculated solubility parameters of ionic liquids81 (Reprinted with permission from “Ionic Liquids Composed of Phosphonium Cations and Organophosphate, Carboxylate, and Sulfonate Anions as Lubricant Antiwear Additives” Zhou, Y.; Graham, T. W.; Luo, H.; Leonard, D. N.; Qu, J. Copyright (2014) ACS).

Figure 2.14 Friction (Coefficient Of Friction-COF) behavior of (a) PAO, (b) PAO-amine- phosphate, (c) PAO-[P4444][DEHP], (d) PAO-[P66614][DEHP], (e) PAO-[P66614][i- C7H15COO], (f) PAO-[P66614][n-C17H35COO], and (g) PAO-[P66614][RSO3] blends; (h) 81 wear rates of PAO and the blends. Numbers indicate the sequence of repeat tests. (Reprinted with permission from “Ionic Liquids Composed of Phosphonium Cations and Organophosphate, Carboxylate, and Sulfonate Anions as Lubricant Antiwear Additives” Zhou, Y.; Graham, T. W.; Luo, H.; Leonard, D. N.; Qu, J. Copyright (2014) ACS).

2.3.4. Interaction between CNTs and Ionic Liquid

Carbon nanotubes have tendency to interact and disperse very well in a wide range of ionic liquids. Many researchers have studied this CNT-IL interaction and formation of

37 homogeneous ‘Bucky gel’ structure. From the experimental and simulation studies, Wang

et. al. showed that the ionic liquids interact with carbon nanotubes through van der Waals

interactions82. Whereas, Bellayer et. al. showed cation-п type interaction to be present

between carbon nanotubes and imidazolium ionic liquid using FTIR analysis83. Non-

covalent treatment of CNTs with IL can be used to disperse CNTs in different media

without hampering electronic structure. Fukushima et. al. demonstrated dispersion of

CNTs in water after they were treated with ionic liquid, as shown in Figure 2.15. Here,

carbon nanotubes are separated from each other overcoming strong van der Waals

interaction, when treated with ionic liquid (Figure 2.15b) as compared to non-treated CNTs

(Figure 2.15a).

Figure 2.15 TEM micrographs of SWNTs (a) received from a commercial source and (b) obtained by dropping a bucky gel of 1-butyl-3-methylimidazolium tetrafluoroborate 84 [bmim][BF4] into deionized water . (Reprinted with permission from “Ionic Liquids for Soft Functional Materials with Carbon Nanotubes” Fukushima, T.; Aida, T. 2007, 5048– 5058).

38 2.3.5. CNTs and Ionic Liquid Lubricant Systems

In some studies, CNTs have been used as additives for ionic liquid bulk lubricants85,86. Yu et. al. functionalized MWCNTs with 1-hydroxyethyl-3-hexyl

imidazolium chloride ionic liquid forming MWCNT-ImCl85. Transformation to other IL forms was done via anion exchange of chloride counter-anions with an excess of target

- - - anion such as BF4 , PF6 , ClO4 , etc. MWCNTs linked with 1-hydroxyethyl-3-

hexylimidazolium hexafluoro-phosphate (HEHImPF6) were used as lubricant additives.

Ionic liquid 1-methyl-3- hexylimidazolium hexafluorophosphates (P106) was used as bulk lubricant and MWCNT-HEHImPF6 were dispersed in it via ultrasonication. The resultant

lubricant was used in steel-steel friction pair. Friction coefficient decreased from 0.073 to

0.068 for CNT concentration of 0.025 wt%. Wear was also decreased significantly. It was

predicted that the IL/MWNTs composite might play the role of roller bearing during

friction process reducing friction and wear.

CHAPTER III

CORRELATION BETWEEN ADSORPTION, RHELOGICAL AND FRICTION

CHARACTERISTICS OF WET CLUTCH SYSTEM COMPONENTS

3.1. Introduction

Wet clutch is an integral part of a torque converter used in an automatic

transmission system of automobiles. Paper based friction materials are used as a sliding

member of these clutches in combination with Automatic Transmission Fluid (ATF).

Friction material (FM) is made up of fibers (cellulose, aramid), natural fillers such as

diatomaceous earth and phenolic or epoxy resin87–89. Automatic transmission fluid consists of combination of various additives such as friction modifier, dispersant and detergent

(around 20%) mixed with base oil (around 80%)90.

Friction characteristics of a clutch system are studied using friction coefficient

versus sliding speed curve. These measurements are performed on SAE No. 2 machine22,91.

Effect of each of ATF additives on friction characteristics is studied individually on SAE

No. 2 machine49. Base oil, without any additive in it, produces decreasing friction

coefficient with increasing sliding speed49,57. In other words, static friction coefficient is higher than dynamic friction coefficient for base oil. Addition of friction modifier to base oil results in decrease in static coefficient value giving positive slope for the friction curve18. But, friction modifiers also cause reduction in overall friction coefficient values

over sliding speed range20. Whereas, dispersants and detergents produce increase in overall

friction coefficient values over complete sliding speed range while maintaining negative

slope of a friction curve. Hence, each additive in ATF functions differently as demonstrated by performance tests on SAE No. 2 machine. Additive adsorption mechanisms on steel surface have been studied in detail36. But, there are very few studies concerning additive

interaction with friction material components27. The complexity of the friction paper-ATF

system led to the use of trial-and-error methods for development purposes.

When clutch assembly is engaged, friction paper contacts steel plate under high

pressure and shear. ATF additive adsorption mechanism on steel surface has been studied

in detail in the literature using nanotribology, spectroscopic techniques and molecular

dynamics simulations40,47,48,92. Using similar analogy, adsorption of ATF additives on

friction material components was studied by Sancaktar et. al. to calculate adsorption

energy90. Higher adsorption energy of ATF determined in comparison to base oil on

friction paper was in accordance with improved friction characteristics with ATF. This

shows that the adsorption of ATF additives on friction paper components can be one of the

factors responsible for friction characteristics. But, the interaction between ATF additives

and FM components has not been studied in detail. It is important to study why the addition

of additives to base oil leads to drastic change in the friction behavior of the clutch system.

In this work, we have studied the effect of additive adsorption on rheological

characteristics of FM components. During clutch engagement, very high shear force is

applied on the friction paper. Hence, we expect that rheological characteristics of the

system would affect friction behavior of the system. For example, shear thickening of the

components would lead to increasing shear stress with shear rate, and thus, expected to

41 cause increasing friction coefficient with sliding speed. This phenomenon is same as that of shear thickening body armor materials studied by Lee93 . A shear thickening fluid with low viscosity at lower shear rate, acts as a solid material at very high shear rate due to rise in viscosity. This provides higher resistance to bullet movement serving the purpose of body armor clothing.

3.1. Experimental

3.1.1. Materials

A paper-based friction material (friction paper) which is used in wet clutch application was provided by LuK USA LLC (Wooster, Ohio, USA). The raw ingredients of this paper such as cellulose, aramid fibers, fillers and phenolic resin were also provided by LuK USA LLC. The friction material was cut to the sizes as per different technique standards.

The major additives of automatic transmission fluid (ATF) additives consist of friction modifier (0.1, 0.5 and 1.0 weight%), dispersant (0.5, 5.0 and 10.0 weight%) and detergent (0.1, 0.2 and 2.0 weight%). Different concentrations of these additives mixed with base oil were provided by The Lubrizol Corporation (Cleveland, Ohio, USA).

Automatic transmission fluid (ATF) was provided by LuK USA LLC. ATF density is in the range of 0.8-0.9 g/ml at 15 °C. The exact compositions of additives are unknown. But, all the solutions were stable and shaken very well before use.

Friction papers (handsheets) were prepared at LuK USA LLC, Wooster, Ohio lab facility. The measured amounts of cellulose, aramid and filler were mixed and dispersed in

42 water using a mixer. Other ingredients such as latex and coagulant were added to the

mixture and stirred gently. This mixture was further transferred to sheet mold. Flocculant

was added to the mixture and stirred until clear water was observed. Then, the vacuum was pulled to drain water and the raw handsheet was left at the bottom of the sheet mold. The sheet was further compressed to the desired thickness using compression press and dried in the heat press. The raw handsheet was saturated with phenolic resin to desired resin uptake and cured in oven.

3.1.2. Characterization

Differential scanning calorimetry (DSC) (TA instruments DSC Q2000, New

Castle, DE) with attached nitrogen system was used to measure and calculate adsorption

energy for the system. A procedure similar to that used by Bakan was followed27. ATF

sample was placed in DSC pan and temperature was dropped to -140 °C causing ATF to

freeze. Then, the friction paper was placed on frozen ATF and the assembly was closed

with lid. This was followed by first heating cycle from -140 to 120 °C, cooling cycle to -

140 °C and a second heating cycle from -140 to 120 °C. The energy difference between

the first and second heating curves gives energy of adsorption.

Ultraviolet-visible (UV-VIS) Hewlett Packard Model 8453 spectrometer (Santa

Clara, CA) was used to measure adsorbed amount of ATF additives (friction modifier,

detergent and dispersant) on friction paper ingredients. A calibration curve for each

additive was drawn based on the UV-VIS measurements before adsorption using different

concentrations, as explained in Appendix I. Filler (diatomaceous earth, clay) and fiber

43 (aramid, cellulose) samples were mixed with additive oil samples individually and the

mixture was kept aside for 24 hours. The samples were further centrifuged to separate filler

or fiber from the oil. The supernatant oil was further analyzed for concentration using UV-

VIS calibration curves to calculate the amount of adsorbed additive.

Brunauer-Emmett-Teller (BET) surface area and pore size distribution of the filler

and fiber was measured using the Micromeritics Tristar II 3020 analyzer (Norcross, GA).

Cone and plate rheometer AR G2 (New Castle, DE) was used to study rheological

behavior of paper components added to ATF additive samples. This is a stress-controlled

type rheometer. In a cone and plate rheometer, the shear rate remains the same across the

complete sample. In the rheometer, shear stress is defined as torque multiplied by stress

constant. Shear stress was measured as a function of shear rate using shear rate sweep

mode. Samples were tested at 40 °C. Shear rate was increased from 0.001 to 1000 s-1. The

samples were allowed to attain equilibrium for 60 seconds at each interval before collecting the data.

An Olympus BX1 optical microscope (Center Valley, PA) was used to study dispersion of filler particles in base oil and ATF. 20X, 50X and 100X magnifications were used.

The friction characteristics of the clutch were studied using SAE No. 2 machine.

Coefficient of friction versus sliding speed data was obtained for fiction papers at specific engagement pressures. Detailed working principle of SAE No. 2 machine is explained in the previous chapter.

44 3.2. Results and Discussion

The effect of ATF additives (friction modifier, dispersant and detergent) adsorption on friction paper components (fillers and fibers) will be discussed in this section. The rheological properties and friction characteristics of the ‘friction paper + ATF additives’ system will be correlated to adsorption phenomena using analytical techniques.

3.2.1. Effect of ATF Additives

3.2.1.1. Additive Adsorption

Figure 3.1 DSC measurement curve for adsorption energy calculation.

DSC was used to calculate adsorption energy between ATF additives and friction

material27,90. Figure 3.1 shows DSC thermogram for ‘friction material + lubricant’ mixture, indicating energy needed for first and second heating cycles. The first heating cycle has a

45 broad exotherm due to adsorption energy as compared with the second heating cycle. Thus, the energy difference between these two heating cycles was taken as adsorption energy27,90.

Figure 3.2 Energy for different additive adsorption on friction material.

Figure 3.2 shows that the adsorption energies for ATF additives are higher than that for the base oil when mixed with the friction material. ATF additives, such as the friction modifier, are adsorbed on friction paper components by physisorption or chemisorption mechanisms causing differences in adsorption energy values. The adsorption behavior is further studied quantitatively using the UV-VIS spectroscopic technique.

All three ATF additives, friction modifier (FM), dispersant and detergent showed absorbance in ultraviolet-visible (UV-VIS) range. Figure 3.3, Figure 3.4 and Figure 3.5 show the UV-VIS absorbance spectra for friction modifier, dispersant and detergent solutions at different concentrations, respectively.

46 4

3.5 3 2.5

2 1.5

(AU) Absorbance 0.5 0

200 -0.5 300 400 500 600 Wavelength (nm)

0.1 wt% FM 0.5 wt% FM 1.0 wt% FM

Figure 3.3 UV-VIS spectra of friction modifier solutions at different concentrations.

4.5 4 3.5

3 2.5 2 1.5 1 Absorbance (AU) Absorbance 0.5

200 -0.5 300 400 500 600 Wavelength (nm)

0.5 wt% dispersant 5.0 wt% dispersant 10.0 wt% dispersant

Figure 3.4 UV-VIS spectra of detergent solutions at different concentrations.

4.5

4 3.5

2.5 2

1.5

(AU) Absorbance 1

0.5

0 -0.5 200 250 300 350 400 450 500 550 600 Wavelength (nm)

0.1 wt% detergent 0.2 wt% detergent 2.0 wt% detergent

Figure 3.5 UV-VIS spectra of dispersant solutions at different concentrations.

Absorbance calibration curves were plotted as functions of concentrations for all the additives by using absorbance at 334 nm wavelength. Figure 3.6, Figure 3.7 and Figure

3.8 show the calibration curves for friction modifier, dispersant and detergent, respectively.

0.25

y = 0.2203x + 0.0024 0.2 R² = 0.9999 0.15

0.1

rbance (AU) 0.05 Abso 0

0 0.2 0.4 0.6 0.8 1 1.2

Additive concentration (wt%)

Figure 3.6 UV-VIS calibration curve for friction modifier.

y = 0.395x + 0.0385 4 R² = 0.9994

(AU) 3

1 Absorbance 0

0 2 4 6 8 10 12 Additive concentration (wt%)

Figure 3.7 UV-VIS calibration curve for dispersant.

1.6 1.4 y = 0.7137x - 0.0199

1.2 R² = 0.9999 1 0.8 0.6 0.4

(AU) Absorbance 0.2 0 0 0.5 1 1.5 2 2.5

Additive concentration (wt%)

Figure 3.8 UV-VIS calibration curve for detergent.

These calibration curves were used to calculate additive adsorption on friction

material components. The detailed procedure and measurements for the same are explained below.

49 A fixed amount of friction material component (such as filler or fiber) was mixed

with various weight concentrations of ATF additive solutions. The initial concentration of

additive solution can be denoted as ‘Ci’. The weight ratio of component to solution was

kept as 1:20. Weight of the friction material component and additive solution were denoted as ‘Wf’ and ‘Wa’ respectively. Also, molar mass of the additive (friction modifier,

dispersant or detergent) was denoted as ‘Ma’. The solution was mixed well and kept aside for 24 hours to attain adsorption equilibrium. The samples were then centrifuged to separate filler or fiber from the solution. The supernatant was further analyzed by UV-VIS spectrometer to obtain the absorbance curve. UV-VIS absorbance curve intensity decreases due to additive adsorption on filler or fiber component. Absorbance value at 334 nm for the supernatant solution was used to obtain supernatant concentration from the calibration curves Figure 3.6, Figure 3.7 and Figure 3.8. This value corresponds to the supernatant final concentration (Cf). The total additive adsorption in terms of ‘adsorbed additive moles per weight of friction material component’ is given as follows:

( )× = (Eqn. 3.1) 100𝐶𝐶 ×𝑖𝑖− 𝐶𝐶𝑓𝑓 × 𝑊𝑊𝑎𝑎

𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 𝑊𝑊𝑓𝑓 𝑀𝑀𝑎𝑎 The above formula was used to calculate additive adsorption at various additive

concentrations.

1.2

4 - 1

0.8

0.6

0.4

0.2 filler (mol/gm) x 10 x (mol/gm) filler 0 of wt. additive/ Adsorbed 0 1 2 3 4 5 6 Initial additive concentration (wt%)

Friction Modifier Detergent Dispersant Base Oil

Figure 3.9 UV-VIS quantitative analysis of ATF additive adsorption on friction material filler diatomaceous earth.

Figure 3.9 shows the adsorbed amounts of friction modifier and dispersant on filler diatomaceous earth. BET surface area of diatomaceous earth was estimated 26.8 m2/gm.

ATF additives showed prominent adsorption on diatomaceous earth filler when base oil is

taken as a reference. Friction modifiers consist of polar head groups like carboxylic or

amine and long hydrocarbon nonpolar tail. Polar head has higher affinity for adsorption on friction paper components when compared with bulky dispersant groups. Higher number of friction modifier entities are adsorbed on filler surface because of their smaller size compared with the dispersant. Adsorption of friction modifier, dispersant and detergent attain equilibrium plateaus after specific concentrations. These adsorption curves follow

Langmuir adsorption mechanism.

51 3.2.1.2. Rheology

Filler + ATF 10000 a Filler + Base Oil

1000

(Pa) 100

Shear Stress Shear 0.1

0.01

1E-3 1E-3 0.01 0.1 1 10 100 1000 Shear rate (1/s)

Filler + ATF 600 b Filler + Base Oil

400 ss (Pa)

200

Shear Stre

20 40 60 80 100 Shear rate (1/s)

Figure 3.10 Shear stress-shear rate curves from shear sweep experiments using cone and plate rheometer comparing filler mixed with base oil versus ATF with (a) log scale and (b) linear scale

52 Figure 3.10 shows shear stress-shear rate curves from shear sweep experiments

using cone and plate rheometer comparing diatomaceous earth mixed with base oil versus

ATF. Note that the tests are performed for filler suspensions in oil samples at filler volume fraction Φ = 0.16. Data collected at lower or higher volume concentrations than Φ = 0.16 also show similar trends. Figure 3.10 a is representation of data from 0 to 1000 s-1 shear

rate. Diatomaceous earth mixed with ATF shows shear thickening or newtonian behavior

throughout this strain range. Whereas, filler mixed with base oil sample shows shear

thinning or shear stress independent of the shear rate. Figure 3.10 b focuses on lower shear rate regime. It clearly shows shear thickening behavior for the ‘filler + ATF’ sample and an initial shear thinning behavior for the ‘filler + base oil’ sample. The rate of shear stress increase is much higher with ATF than base oil, when mixed with diatomaceous earth.

Shear rate range studied is 0.001 to 1000 s-1. Shear rate can be converted to angular velocity by using the following equation:

( ) ( ) =

𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣 𝜔𝜔 𝑆𝑆ℎ𝑒𝑒𝑒𝑒𝑒𝑒 𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 𝛾𝛾 where, θ is the rheometer cone angle (2°). The slip𝑆𝑆𝑆𝑆𝑆𝑆 speed𝜃𝜃 range of the rheometer was

calculated as 0 to 350 rpm. This range matches with the SAE No. 2 test slip speed range (0

to 235 rpm) as described in next section. Increase in shear rate can be correlated to

increasing sliding speed in friction measurements. Furthermore, the shear stress can be

correlated to the friction coefficient values.

53 a

Figure 3.11 Optical images of (a) filler + base oil and (b) filler + ATF dispersions

To understand the effect of ATF additive adsorption on rheological phenomena of the system in detail, filler dispersion in oil samples was studied. Figure 3.11 a and b show dispersion of diatomaceous earth in base oil and ATF respectively. Filler particles

(diatomaceous earth) form aggregates when dispersed in base oil. Diatomaceous earth contains 80-90 wt% silica which has polar surface. Base oil consists of hydrocarbon chains making it nonpolar. Silica particles being incompatible with base oil, coagulate together

54 forming aggregates. Automatic transmission fluid is made by combining various additives in base oil. Additives like friction modifier consist of two parts: a hydrophilic head group such as acid or amine which has strong affinity for polar surfaces and a nonpolar hydrocarbon tail with less affinity for polar surfaces.

a b

Figure 3.12 Schematic representation of mechanism for filler and additive interaction in (a) base oil and (b) ATF in steady state.

Polar head groups form micellar structures through a self-assembly process with their polar head groups inside as shown in Figure 3.12. These micelles are not aggregated together and are well dispersed in the solution because of the repulsive force between the hydrocarbon chains.

55 a b

Figure 3.13 Schematic representation of mechanism for filler and additive interaction in (a) base oil and (b) ATF under shear.

When shear rate is increased on ‘ATF + filler’ sample, micelles come close to each other as shown in Figure 3.13. As hydrocarbon tails come closer, repulsive force between them increases. This increase in repulsive force causes shear thickening, and thus, increases the shear stress. This mechanism is absent in the ‘base oil + filler’ sample, as shown in

Figure 3.13.

56 Filler + Base Oil Filler + Friction Modifier Filler + Dispersant Filler + ATF 1000 a

100

0.1

0.01 Shear Stress (Pa) Stress Shear

1E-3

1E-4 1E-3 0.01 0.1 1 10 100 1000 Shear rate (1/s)

Filler + Base Oil; 40 °C Filler + Friction Modifier; 40 °C Filler + Dispersant; 40 °C Filler + ATF; 40 °C 100 b

0.1

0.01

(Pa) Stress Shear 1E-3

1E-4 0.2 0.4 0.6 0.8 1.0 Shear rate (1/s)

Figure 3.14 Shear stress versus shear rate curves for filler (DE) mixed with base oil, friction modifier, dispersant and ATF under shear sweep experiment at (a) complete shear rate range and (b) lower shear rates

57 ATF additives are further studied individually to analyze their effect on rheological properties. Figure 3.14a shows log (shear stress) versus log (shear rate) for base oil, ATF, friction modifier and dispersant individually mixed with diatomaceous earth, throughout the shear rate range 0.001 to 1000 s-1. As seen earlier, shear stress is independent of shear

rate for base oil, while ATF shows increasing shear stress with increasing shear rate as

evident from Figure 3.14a. Addition of friction modifier to base oil (concentration: 1

weight%) shows change in the slope of the curve. It reduces the shear stress values at lower shear rates as compared to base oil and maintains positive slope of the curve. Figure 3.14b shows change in shear stress in lower shear rate range at 40oC. Change of stress values at

lower shear rate from base oil to friction modifier is evident in Figure 3.14b. Addition of

dispersant (concentration: 5 weight%) also shows drastic change as compared to neat base

oil. In lower shear rate regime (Figure 3.14a), the slope is negative. Whereas, at higher shear rate values (above 0.1 s-1), the slope is much higher as compared to base oil (Figure

3.14a) and becomes almost the same as that for ATF. This shows that dispersant does not

affect shear stress at lower shear rates, but at higher rates, it provides higher value of the

slope as compared to base oil and friction modifier (Figure 3.14a). The similarity in the

rheological phenomena between the base oil and the dispersant at lower shear rates can be

seen in Figure 3.14b.

3.2.1.3. Friction Characteristics

Friction characteristics of the friction material were studied using SAE No. 2 test

machine. Using this test, we can measure and calculate friction coefficient (μ) of the system

58 at different sliding speeds. The variables which were studied during the adsorption and

rheological tests, were also considered for friction characteristics which were correlated

with the rheological characteristics.

Base Oil + Filler

0.22 ATF + Filler Friction Modifier + Filler Dispersant + Filler 0.20

0.18

0.16

0.14

0.12 Friction Coefficient

0.10 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Speed (m/s)

Figure 3.15 SAE No. 2 data (friction characteristics) for friction material tested in different lubricants.

Use of only base oil results in decreasing friction coefficient with increasing sliding speed. Static coefficient of friction (μs) is much higher than dynamic coefficient of friction

(μd) for base oil. This is evident from the negative slope of the μ-v curve, which causes stick-slip or shudder in a vehicle. ATF provides improved friction performance as compared to base oil. Static friction coefficient is lower than dynamic friction coefficient and value of μ increases with increasing speed. The slope of the curve is always positive, which gives smooth clutch engagement without shudder.

59 Friction characteristics can be correlated with rheological phenomena studied in

previous part. Shear stress is a measure of resistance to the shear flow. A shear force is

applied on friction paper during clutch engagement. Thus, rheological characteristics of

individual components of the lubricant system affect friction performance. For example,

ATF exhibits shear thickening behavior during rheological measurements. This means that the rate of increase of shear stress with shear rate is higher in ‘filler + ATF’ as compared to ‘filler + base oil’ sample. Hence, the resistance to flow will be higher when using ATF which maintains positive slope for the μ-v curve.

Effects of friction modifier and dispersant on friction characteristics are also shown in Figure 3.15. Friction modifier (1 weight%) solution mixed with filler shows positive slope for the μ-v curve. The static coefficient of friction is substantially lowered as compared to neat base oil. This data is consistent with the rheological behavior seen in

Figure 3.15. Rheological measurements also show increasing shear stress with shear rate in lower speed range. But, friction modifier lowers the overall value of the friction coefficient at higher speeds when compared to ATF. This can be explained by comparing the slopes of the shear stress versus shear rate curves in Figure 3.14a. ‘ATF + filler’ exhibits higher slope than ‘friction modifier + filler’. This means that higher resistance is offered by ATF as compared to friction modifier at higher sliding speeds. Hence, ATF provides increased friction coefficient in comparison to the friction modifier at higher speeds. On the other hand, 5 weight% dispersant solution causes overall increase in friction coefficient at higher sliding speeds (μd) as shown in Figure 3.15. But, it does not affect friction

coefficient at lower shear rates. A similar behavior was observed in rheological

60 measurements (Figure 3.14). ‘Dispersant + filler’ shows decreasing shear stress with

increasing shear rate at lower shear rate values. After a critical shear rate of 0.1 s-1, slope of the curve changes from negative to positive. The slope of the curve at higher shear rates is almost similar to ATF and higher than friction modifier (Figure 3.14a). Higher resistance to flow at high sliding speeds causes increase in friction coefficient for dispersant and eventually for ATF. In the following section, adsorption of ATF additives on fillers and rheological behavior of the system is correlated with friction characteristics.

3.2.2. Filler (diatomaceous earth) versus Fiber (Aramid)

3.2.2.1. Additive Adsorption

1.2

4 1 - 0.8 0.6

0.4 0.2

10 x (mol/gm) filler 0 of wt. additive/ Adsorbed 0 0.2 0.4 0.6 0.8 1 1.2 Initial additive concentration (wt%)

Filler (Diatomaceous Earth) Fiber (Aramid)

Figure 3.16 Friction modifier adsorption on filler (diatomaceous earth) and fiber (aramid) as a function of additive concentration.

61 Additive adsorption on diatomaceous earth and aramid was measured at different

concentrations of friction modifier by 0.05, 0.1, 0.5 and 1.0 weight%. The adsorption

increases with initial additive concentration to a certain limit after which it reaches a

plateau. Diatomaceous earth particles show higher adsorption of friction modifier as

compared to aramid fiber as shown in Figure 3.16. A possible reason could be that the polar sites of aramid are hindered because of hydrogen bonding between C=O and N-H groups.

Thus, the lack of polar sites can cause decrease in ATF additive adsorption on aramid surface.

3.2.2.2. Rheology

Aramid + ATF

Filler + ATF

Aramid + Filler + ATF 1000

100

0.1

0.01

Shear (Pa) Stress 1E-3

1E-4 1E-3 0.01 0.1 1 10 100 1000 Shear Rate (1/s)

Figure 3.17 Shear stress versus shear rate curves for ATF mixed with aramid, filler (DE) and aramid + filler, obtained by shear sweep experiment.

62 To study the effect of ATF additive adsorption on filler (DE) and fiber samples, we performed rheological measurements on the dispersions of different aramid and diatomaceous earth compositions. Figure 3.17 shows the comparison between fiber and filler samples. Shear stress for ‘Aramid + ATF’ sample showed no dependence on the shear rate similar to what was previously seen with the ‘filler + base oil’ sample. The shear stress remains constant with increasing shear rate. Aramid has lower adsorption of ATF additives as shown in Figure 3.17. Hence, there is negligible increase in repulsive force when aramid particles come closer under shear loading and shear thinning results. When 50% aramid in the mixture is replaced by diatomaceous earth, shear stress curve shows drastic change in the slope as seen in Figure 3.17. Filler (DE) helps to fill in the voids between the aramid fibers and adsorb ATF additives resulting in a transformation from shear thinning to shear thickening or newtonian behavior. In other words, resistance to flow increases with increasing shear rate by the addition of diatomaceous earth.

63 3.2.2.3. Friction Characteristics

Figure 3.18 SAE No. 2 data (friction characteristics) for friction material made up of 100% aramid versus 50% aramid + 50% filler.27 (The data is collected by Dr. Murat Bakan at LuK USA LLC)

A friction paper made up of 100% aramid fiber was tested for friction

characteristics. Figure 3.18 shows that the friction coefficient decreases with increasing

sliding speed for 100% aramid. Aramid fiber is necessary to withstand high pressure and

shear in clutch application and to improve the durability of the paper. But, the friction curve for aramid has negative slope that would result in slip-stick motion. This is caused by lower additive adsorption and shear thinning behavior of ‘ATF + Aramid’ sample as shown in previous sections.

64 3.2.3. Effect of Temperature

3.2.3.1. Rheology

ATF 0.040

0.035

0.030

0.025

0.020

0.015

Viscosity (Pa.s) 0.010

0.005

0.000 20 40 60 80 100 120 Temperature (°C)

Figure 3.19 Viscosity-temperature curve for ATF

Figure 3.19 shows viscosity variation with increasing temperature from 30 to 120

°C. ATF shows decreasing viscosity with increasing temperature.

° Filler + ATF; 40 C Filler + ATF; 90 °C 400 a

300

200

Shear Stress (Pa) 100

0 50 100 150 200 Shear rate (1/s)

Filler +ATF; 40 °C Filler +ATF; 90 °C 1000 b

100

(Pa) 10

0.1

Shear Stress Shear 0.01

1E-3

1E-4 20 40 60 80 100 Shear rate (1/s)

Figure 3.20 Shear stress versus shear rate curves for filler + ATF at 40 °C and 90 °C, under shear sweep experiment with (a) linear y-axis and (b) log y-axis

Figure 3.20 shows the shear sweep of ‘diatomaceous earth + ATF’ mixture at two different temperatures. The mixture shows shear thickening phenomena at 40 °C as well

66 as at 90 °C. But the rate of increase of shear stress is higher at 40 °C than that at 90 °C as

seen in Figure 3.20a. In other words, the shear stress required for sample deformation at any shear rate decreases with increasing temperature. The probable reason for this could be that the ATF additive adsorption decreases with increasing temperature due to higher activation energy. This leads to decreasing resistance to flow with increasing temperature.

3.2.3.2. Friction Characteristics

at 40 °C at 90 °C at 120 °C 0.150

0.125

0.100

Friction Coefficient Friction 0.075

0.0 0.4 0.8 1.2 1.6 Sliding Speed (m/s)

Figure 3.21 SAE No. 2 data (friction characteristics) of friction material tested in ATF as a function of temperature.

Figure 3.21 shows the effect of temperature on the μ-v curve. Friction paper was

studied at three different temperatures, 40 °C, 90 °C and 120 °C. It is observed that the

coefficient of friction decreases with increasing temperature. This can be attributed to

reduced additive adsorption and reduced shear stress with increasing temperature at any

67 fixed shear rate (Figure 3.20). Cai et. al.94 also found similar trend which was attributed to decreased shear strength of friction paper composites at higher temperatures.

3.3. Conclusions

Adsorption of ATF additives on friction material components and rheological properties of the system were studied in this chapter. This was further correlated with friction characteristics of friction materials. The shear thickening phenomenon caused by interaction between the ATF additives and the friction material components is a probable mechanism behind the wet clutch system operation explored in this work. Thus, we have the following major conclusions for this part of our work: a. The ATF additives were adsorbed on filler component at different extents as

illustrated by using DSC and UV/VIS spectroscopy. Filler + ATF system exhibited

shear thickening or newtonian behavior whereas filler + base oil showed shear

thinning phenomena. The observed increase in shear stress with shear rate (for filler

+ ATF) can be correlated with increasing torque and friction coefficient with sliding

speed in actual wet clutch engagement. Shear thinning to shear thickening transition

can be an effective way to increase the friction coefficient with increasing speed. b. Diatomaceous earth filler was found to have higher ATF additive adsorption in

comparison to aramid fiber based on UV/VIS analyses. In shear sweep experiments,

use of ‘fiber + ATF’ had no effect on shear stress with increasing shear rate, unlike

‘filler + ATF’. This showed that among the major components of the friction

material, filler (DE) has higher additive adsorption and shear thickening behavior.

68 c. Increasing the test temperature from 40 °C to 120 °C caused decrease in dynamic

friction coefficient. This was correlated to the decrease in shear stress with

increasing temperature at the same shear rate, for the ‘filler + ATF’ sample.

CHAPTER IV

FRICTION CHARACTERISTICS OF VARIOUS FILLER COATINGS ON FRICTION

MATERIALS

4.1. Introduction

There is increasing demand to improve the energy efficiency of automobiles. An

important part of a vehicle is its torque converter which transfers energy from its engine to its transmission. Wet clutch is an integral part of the torque converter consisting of a friction disc (friction material adhered to steel ring), a reaction plate (steel) and automatic transmission fluid (ATF)20,27,95. The friction material and the reaction plate come in contact under high pressure during clutch engagement. The friction characteristics during this phase are very important to avoid stick-slip and shudder during the operation of a vehicle.

Good friction characteristics are defined by positive gradient for friction coefficient versus sliding speed curve and sufficiently high value for the friction coefficient for proper operation of the vehicle. Friction material formulation plays an important role in friction properties of the system.

Different formulations have been studied in literature with different percentages of fibers, fillers, friction modifiers (such as graphite and carbon fiber) and resins19,34,96. Fibers

provide porous and strong network in the friction materials. Porous network is important

for ATF to pass through the friction material during engagement. This ensures operation

in boundary lubrication regime. Different types of fibers such as cellulose, aramid, carbon

fibers and glass fibers have been studied in literature97. Aramid improves strength and heat resistant properties of the friction material98. Zhang showed that 10% glass fiber addition gives highest mechanical strength of the composite29. On the other hand, Bakan showed

that friction material consisting of 100% aramid fiber exhibits negative slope for friction

coefficient versus sliding speed curve. Bakan also showed that increasing filler (DE)

concentration leads to improvement in friction characteristics27.

Fillers are used to improve heat resistance and hardness of the friction material.

Kumar et. al. showed that metallic fillers increased friction coefficient (and friction stability) along with thermal conductivity, porosity, tensile strength and thermal diffusivity99,100. Also, it is observed that the friction coefficient increases with Mohs

hardness of mineral fillers such as silica and clay101. But, the fillers with very high Mohs

hardness cannot be used as they will damage the contacting steel surface. Most of the

friction material formulations that have been studied are based on trial-and-error method.

The exact mechanism of ‘how the fillers interact with ATF additives and cause increase in friction coefficient’ have not been studied in detail. In the previous chapter, we proposed a shear thickening mechanism with the ‘filler + ATF’ system (or change from shear thinning to newtonian or shear thickening behavior as compared to ‘filler + base oil’) for this purpose based on micelle formation by ATF additives around the filler particles.

The present study investigates shear thickening behavior of three different fillers with ATF. Friction characteristics of the friction materials coated with these fillers are also studied. The friction performance is correlated with rheological properties and surface porosity. As the filler particles directly encounter the steel surface during engagement, it is

71 expected that the torque will increase with increasing speed as a result of shear thickening

effect. This effect will provide increasing friction coefficient for filler-coated friction materials. The filler which provides the best friction characteristics will be further studied in detail with different paraments in the next chapter.

4.2. Experimental

4.2.1. Materials

A paper-based friction material used in wet clutches was provided by LuK USA

LLC, Wooster. Three types of fillers: 1. Diatomaceous earth type-1; 2. Diatomaceous earth

type-2, and 3. A proprietary clay were also provided by LuK USA LLC. Other ingredients

such as phenolic resin, latex and automatic transmission fluid (ATF) were also supplied by

LuK USA LLC.

Wagner control spay max (Plymouth, MN) was used to spray coat fillers on friction

material. Several formulations (filler, water and latex) were tried to achieve the desired

thickness. The coating was applied on raw hand sheets before saturation with phenolic

resin. Two thicknesses (25-40 μm and 60-80 μm) are studied in this work.

4.2.2. Characterization

A cone and plate rheometer AR G2 (TA Instruments, New Castle, DE) was used to

study the rheological behavior of three fillers mixed individually with ATF. The shear

stress was measured as a function of shear rate in shear sweep mode. Samples were tested

72 at 40 °C while the shear rate was increased from 0.001 to 1000 s-1. The samples were allowed to attain equilibrium for 60 seconds at each interval before collecting the data.

An SAE No. 2 machine was used to measure the friction coefficient as a function

of sliding speed during clutch engagement. Working principle of this test is described in

Chapter II. Data was collected at temperatures 40 °C and 120 °C under 1940 kPa pressure.

An oil drop test was performed to calculate the time required for oil penetration on

the friction paper surface. A fixed amount of ATF was dropped on the friction material

(bonded to a steel ring) surface using a syringe. The time required for the ATF to

completely penetrate through the surface was measured using a stopwatch and was denoted as ‘oil drop value’. Lower oil drop value indicates higher porosity of the paper surface leading to the formation of boundary lubrication regime after clutch engagement.

Obviously, the ‘oil drop value’ needs to be shorter than the full clutch engagement time. If the time required for oil penetration is higher, there is probability of ATF layer formation between the friction paper and the steel surface which will cause hydrodynamic lubrication regime to form and cause the friction coefficient to decrease due to the formation of a lubricant layer. In this case, the full clutch engagement time is typically shorter than the

‘oil drop value’ leading to a layer of the lubricant to remain between the friction paper and the steel surface. Furthermore, if a coating layer has been deposited on the friction paper, the presence of a lubricant layer may lead to delamination of this layer from the paper due to lubricant wicking to the coating/paper interface from the edges (and not by pore penetration due to low porosity of the coating) which may create a swelling stress leading

73 to delamination. We also note that the “oil drop test” represents a rather different physical

state in comparison to the SAE No. 2 test due to the absence of contact pressure.

4.3. Results and Discussion

In the previous chapter, we studied the shear thickening phenomenon of fillers

when mixed with ATF. There are many fillers available in different shapes, sizes and

compositions. The extent of increase in shear stress with shear rate will vary with these

filler parameters thus affecting the torque during clutch engagement. Hence, it is necessary to analyze the effect of fillers on SAE No. 2 test results. For this purpose, we selected three fillers which are commonly used in friction material formulations.

800 Filler-1 + ATF Filler-2 + ATF Filler-3 + ATF 600

400

200 (Pa) Stress Shear

0 200 400 Shear rate (sec-1)

Figure 4.1. Shear stress-shear rate curves from shear sweep experiments on three different fillers mixed with ATF.

74 Figure 4.1 shows shear stress versus shear rate curves for the steady state shear

sweep data. All three fillers exhibit shear thickening behavior when mixed with ATF at the same volumetric concentration of Φ=0.16. The rate of increase of shear stress with shear rate is higher for filler-1 when compared to other fillers. We also noticed that the torque generated on rheometer when using filler-1 at 100 s-1 was higher than the rheometer limit.

75 a Pre Break-in No Coating Temp: 40 °C Filler-1 Coating Pressure: 1940 kPa Filler-2 Coating 0.18 Filler-3 Coating

0.17

0.16

0.15

0.14

0.13

0.12

Friction Coefficient 0.11

0.10 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Sliding Speed (m/s)

b Pre Break-in No Coating Temp: 120 °C Filler-1 Coating Pressure: 1940 kPa Filler-2 Coating 0.18 Filler-3 Coating

0.17

0.16

0.15

0.14 Coefficient

0.13

0.12 Friction 0.11

0.10 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Sliding Speed (m/s)

76 c Post Break-in No Coating Temp: 40 °C Filler-1 Coating Pressure: 1940 kPa Filler-2 Coating 0.18 Filler-3 Coating

0.17

0.16

0.15

0.14

0.13

0.12

Friction Coefficient 0.11

0.10 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Sliding Speed (m/s)

d Post Break-in No Coating Temp: 120 °C Filler-1 Coating Pressure: 1940 kPa Filler-2 Coating 0.18 Filler-3 Coating

0.17

0.16

0.15

0.14

0.13

0.12

Friction Coefficient 0.11

0.10 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Sliding Speed (m/s)

Figure 4.2 Wet clutch friction results for three different fillers in thick (60-80 μm) coats on friction material (a) pre break-in at 40 °C, (b) pre break-in at 120 °C, (c) post break-in at 40 °C and (d) post break-in at 120 °C.

77 Figure 4.2 shows the friction characteristics of friction materials after coating them with three different fillers separately in 60-80 μm thickness. The data is compared with friction characteristics of friction material without any coating. In all cases, friction material coated with filler-1 (diatomaceous earth type-1) showed higher friction coefficient as compared to friction materials coated with filler-2 (diatomaceous earth type-2), filler-3

(a proprietary clay) and without any coating. Filler-1 coated friction material also has the highest positive gradient for friction versus sliding speed curve. Both of these criteria (i.e., higher friction coefficient in absolute value, as well as highest positive gradient for friction versus sliding speed curve) are necessary for smooth engagement of the clutch.

Use of Filler-2 and Filler-3 does not result in significant improvement in friction coefficient as compared to non-coated friction material. The friction data can be correlated with the rheological trend seen in Figure 4.1; Filler-1 mixed with ATF shows higher increase in shear stress with increasing shear rate. Also, we notice that filler-2 shows negative slope in Figure 4.2- a, b and c. This can be explained by higher values of oil drop test for thick coating (60-80 μm) of filler-2 as shown in Table 4-1. It requires more time for ATF to penetrate through the thicker layer of filler-2. This can cause formation of ATF layer between the surfaces leading to hydrodynamic regime, which decreases coefficient of friction.

78 a Pre Break-in No Coating Temp: 40 °C Filler-1 Coating Pressure: 1940 kPa Filler-2 Coating 0.18 Filler-3 Coating

0.17

0.16 ent 0.15

0.14

0.13

0.12

Friction Coeffici 0.11

0.10 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Sliding Speed (m/s)

b Pre Break-in No Coating Temp: 120 °C Filler-1 Coating Pressure: 1940 kPa 0.18 Filler-2 Coating Filler-3 Coating 0.17

0.16

0.15

0.14

0.13

0.12

Friction Coefficient 0.11

0.10 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Sliding Speed (m/s)

79 c Post Break-in No Coating Temp: 40 °C Filler-1 Coating Pressure: 1940 kPa Filler-2 Coating 0.18 Filler-3 Coating 0.17

0.16

0.15

0.14

0.13

0.12

Friction Coefficient 0.11

0.10 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Sliding Speed (m/s)

Post Break-in No Coating d Temp: 120 °C Filler-1 Coating Pressure: 1940 kPa Filler-2 Coating 0.18 Filler-3 Coating

0.17

0.16

0.15

0.14

0.13

0.12

Friction Coefficient 0.11

0.10 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Sliding Speed (m/s)

Figure 4.3 Wet clutch friction results for three different fillers in thin (25-40 μm) coats on friction material (a) pre break-in at 40 °C, (b) pre break-in at 120 °C, (c) post break-in at 40 °C and (d) post break-in at 120 °C.

80 Figure 4.3 shows the friction characteristics of friction materials after coating them with three different fillers separately in 25-40 μm thickness. Here, the non-coated friction material exhibits the lowest friction coefficient values. Whereas, the friction material coated with filler-1 gives the highest friction coefficient values at both temperatures used.

Filler-1 also shows the highest positive slope for the friction coefficient versus sliding speed curve at 120 oC. We also observed that, for pre- and post- break-in data at 40 °C, only the use of filler-1 coating resulted in constant or slightly increasing friction coefficient with sliding speed. Furthermore, for the pre- break-in data at 40 °C, friction materials other than filler-1 exhibited static friction coefficients higher than their dynamic friction coefficients.

With the 25-40 μm thickness, filler-2 and filler-3 coatings provided increased friction coefficient values when compared with the non-coated friction material. But, the performance was still lower than filler-1. Hence, filler-1 shows the highest improvement in friction characteristics due to its shear thickening behavior seen in Figure 4.1.

Table 4-1 Oil drop measurements for friction materials with and without filler coatings.

4.4. Conclusions

Based on the shear thickening behavior of ‘filler + ATF’ system described in the

previous chapter, friction materials coated with fillers were developed. Three different

natural material fillers were compared based on the friction characteristics data which was

further related to their rheological behavior and surface porosity measurements. In the

shear sweep test on ‘filler + ATF’ samples, filler-1 (diatomaceous earth type-1) showed the highest rate of increase in shear stress with shear rate. The same trend was seen in the

SAE No. 2 testing. Filler-2 and filler-3 coated friction materials showed better friction performance in comparison to the non-coated friction material. But, friction material coated with filler-1 provided the highest friction coefficient and the highest positive gradient for friction coefficient versus sliding speed curves. Oil drop tests showed that the surface porosity generally decreased with filler coating, indicating that optimization of the coating parameters is required to achieve optimum clutch performance.

CHAPTER V

EFFECT OF FIBER TYPE, PAPER DENSITY AND COATING THICKNESS ON

FILLER-COATED FRICTION MATERIAL PERFORMANCE

5.1. Introduction

Friction in the wet clutch system is a combination of boundary lubrication and

friction due to ATF flow through the clutches36. In boundary lubrication, the two surfaces

(friction paper and steel plate surface) are in direct contact with each other without any

lubrication layer between them. With boundary lubrication, it is well established that the

friction characteristics of the system are dependent on surface properties, topography102

and chemical structure33 as well as additive interaction with surface groups36,103. Previous

studies have shown the adsorption and lubrication mechanisms of ATF on different components of wet clutch system43,104. Wood et. al.47 and Greenfield et. al.105 showed

packed and well-ordered layers of model friction modifiers on surfaces. Zhu et. al. showed that friction modifier molecules adsorb on surfaces causing a hard-wall effect which reduces adhesion force between the two surfaces, lowering the magnitude of limiting shear stress and eliminating stick-clip40. This nano-rheological study explains the possible

mechanism at molecular level.

Other studies showed ATF additive adsorption on friction material components

such as fibers and fillers. Nowshir et. al. studied the difference between adsorption of

contaminated and non-contaminated ATF additives on cellulose based friction discs using

techniques such as ATR-FTIR, SEM-EDS and XPS106. Bakan studied adsorption of ATF

additives on friction material components using differential scanning calorimetry (DSC).

He found that ‘ATF + friction material’ system gives higher adsorption energy than ‘base

oil + friction material’. He also showed that increasing filler content in friction material

improves the friction behavior of the system27.

As seen in the previous chapters, filler (diatomaceous earth) showed higher

adsorption of ATF additives as compared to fiber (Aramid). ‘Filler + ATF’ sample also

showed shear thickening behavior or increasing shear stress with shear rate. This shows

that filler has higher ability to increase resistance to flow than fiber in presence of ATF.

Hence, if the friction paper made up of fibers, fillers and friction modifiers is coated with

a layer of filler, the friction coefficient should increase because the resistance to flow is

increased. The surface composition of the friction paper will have major effect on friction

behavior, because the friction paper surface meets steel surface under very high torque and pressure during clutch engagement. Lam et. al. studied the effect of coating friction materials with filler particles107. The coated friction materials showed promising results with increasing or stable friction coefficient. But, the effects of other parameters (such as coating thickness, surface permeability) on different properties (such as durability) also need to be studied in detail. Filler will cause shear thickening which is expected to improve performance. But, surface permeability is expected to decrease due to the filler layer.

Surface permeability or porosity plays important role in determining friction performance and durability55. Matsumoto showed that friction coefficient increases with increasing

porosity108,109.

84 In the present work, we have studied the effect of diatomaceous earth coating in

detail. This diatomaceous earth showed best friction performance compared to other fillers

studied. We have varied coating thickness and surface permeability to achieve optimum

friction performance as well as durability.

5.2. Experimental

5.2.1. Materials and Procedure

Aramid, cellulose, fillers, friction modifiers and phenolic resin were used as

available at LuK USA LLC, Wooster, Ohio. Other chemicals such as ethanol, latex, flocculant and coagulant were also received from LuK USA LLC.

To make a friction material hand-sheet, first the aramid fibers were soaked in water

and mixed in a blender. Cellulose was then added to it and mixed. Filler and friction

modifiers were then added to the blender and stirred for few more seconds. In the end, latex

and coagulant were added to the mixture and stirred gently. The mixture was then

transferred to a sheet mold. Flocculant was added along with water for mixture to rise to

certain volume limit and then it was agitated with spatula. Vacuum was pulled to drain the

water and a wet cake of the mixture was obtained at the bottom. Extra water was soaked

using cotton sheets and the thickness of the hand sheet was adjusted to required thickness

using a compression press. The sheets were then dried using a hot press at to obtain ‘raw

hand-sheets’.

85 A phenolic resin was used for raw hand-sheet saturation. After saturation process,

hand-sheets were dried overnight to get rid of solvent and cured in oven to get the final

friction material.

Wagner control spay max (Plymouth, MN) was used to spray coat diatomaceous

earth filler on friction paper. Several formulations were tried to achieve the required

thickness. The coating was applied on raw hand sheets before saturation with phenolic

resin.

5.2.2. Characterization

SAE No. 2 testing machine was used to measure friction characteristics of the

bonded friction materials as a function of the sliding speed (μ-v). This test gives the data

for coefficient of friction as a function of sliding speed. Tests were run at two different

temperatures; 40 °C and 120 °C. Also, data was collected for two different pressures; 1940 kPa and 2960 kPa. The details about the SAE No. 2 testing machine and procedure are given in chapter II. The data collected immediately after starting the test is denoted as ‘pre break-in’. Whereas, ‘post break-in’ data is collected after running a sample few times with the following conditions on the machine.

A Gurley densometer (New York, USA) was used to measure air permeability of the raw hand sheets before and after coating. The flow of air was measured at 300 ml through the hand sheets using a timer.

An oil drop test was performed to calculate the time required for oil penetration on the friction paper surface. A fixed amount of ATF was dropped on the friction material

86 (bonded to a steel ring) surface using a syringe. The time required for the ATF to

Durability test under high pressure compression cycle was performed on all the friction papers. The friction paper was cut in a specific ring size and adhered to the steel disc using adhesive used for friction paper bonding to steel ring. The test sample underwent compression cycles. Thickness of the bonded friction paper was measured after specific number of pressure cycles and % reduction in thickness was measured to relate to friction paper failure.

87 An Instron tensile tester (Massachusetts, USA) was used to measure tensile strength of saturated friction papers. Rectangular shaped samples were cut from the hand sheet for tensile testing.

5.3. Results and Discussion

To study the effect of diatomaceous earth filler coating, raw hand-sheets available at LuK USA LLC were tested with and without coating. The coating thickness was maintained in the range of 25 to 40 μm (thin coating). The friction characteristics for coated versus non-coated friction papers after saturation are shown in Figure 5.1a and b for 40 °C and Figure 5.1c and d for 120 °C.

88 a Pre Break-in, 40 °C Non-coated, 1940 kPa Non-coated, 2960 kPa

Coated, 1940 kPa 0.160 Coated, 2960 kPa

0.155

0.150

0.145

0.140

0.135

0.130 FrictionCoefficient 0.125

0.120 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Sliding Speed (m/s)

b Post Break-in, 40 °C Non-coated, 1940 kPa Non-coated, 2960 kPa Coated, 1940 kPa 0.160 Coated, 2960 kPa

0.155

0.150

0.145

0.140

0.135 ictionCoefficient 0.130 Fr 0.125

0.120 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Sliding Speed (m/s)

89 c Pre Break-in, 120 °C Non-coated, 1940 kPa Non-coated, 2960 kPa

Coated, 1940 kPa 0.160 Coated, 2960 kPa

0.155

0.150

0.145

0.140

0.135

FrictionCoefficient 0.130

0.125

0.120 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Sliding Speed (m/s)

Post Break-in, 120 °C Non-coated, 1940 kPa d Non-coated, 2960 kPa

Coated, 1940 kPa 0.160 Coated, 2960 kPa

0.155

0.150

0.145

0.140

0.135

0.130

Friction Coefficient 0.125

0.120 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Sliding Speed (m/s)

Figure 5.1 SAE No. 2 data (friction characteristics) for diatomaceous earth filler coated versus non-coated friction materials (proprietary friction material composition).

90 The friction characteristics curve shows increase in friction coefficient value (over

complete sliding speed range) for diatomaceous earth filler-coated friction materials as

compared to non-coated papers. The slope of the curve is also higher (more positive) for

filler-coated friction materials. Non-coated friction materials show negative slope for

friction curves at 40 °C (Figure 5.1 a and b). In other words, friction coefficient is

decreasing with increasing sliding speed. This would result in stick-slip behavior during

clutch engagement. Coating the surface of the same friction paper with diatomaceous earth particles changes the slope of the friction curves substantially. This could be due to shear thickening behavior of diatomaceous earth in presence of ATF. This would lead to increasing torque and hence friction coefficient with increasing speed.

Coated and non-coated friction materials were also tested at 120 °C (Figure 5.1 c and d). It was observed that non-coated friction material showed positive slope for friction curve at 120 °C, unlike to 40 °C. This can be due to reduced viscosity at higher temperature which allows ATF to penetrate inside the friction paper in less time during clutch engagement. This helps to achieve boundary lubrication regime as there is no ATF layer present between the two surfaces. It was reported that the formation of the oil film between the surfaces depends on the oil penetration time (in friction material) during clutch engagement109. We observed that the friction coefficient value and the slope of the curve

increases over complete range of sliding speed after coating for both pre- and post- break-

in cases. Hence, filler coating over friction paper resulted in improved friction behavior

with all test conditions.

Figure 5.2 Durability test samples failed after 8% test completion showing rupture of diatomaceous earth filler-coated layer on friction material (proprietary friction material composition).

The durability of the friction papers was tested at a constant pressure. Figure 5.2

shows the effect of diatomaceous earth filler coating on durability of the friction material.

Disentanglement of the paper or more than 50% loss in thickness is considered as part

failure. It is observed that the friction paper without coating is not ruptured after first 8%

of the cycles. But, friction paper with filler coating shows delamination of the coating layer from the friction material base as shown in Figure 5.2. The durability test samples undergo high pressure cycles which includes engagement and disengagement of the test samples.

During engagement, the friction material surface and the steel surface come in contact under high pressure. Hence, ATF is enforced to penetrate through the porous surface of the friction paper. Porosity of the friction material surface is reduced due to filler coating on high density paper. Hence, ATF breaks through the weakest point of the friction paper

92 which is the interface between the coating layer and the base material. This is evident from

Figure 5.2 which shows delamination of the coating layer.

We already showed that the coating of friction material with diatomaceous earth filler improves the friction characteristics by increasing the friction coefficient and the slope when it is plotted versus the sliding speed. But, it simultaneously affects the porosity of the top surface which is an important factor for durability of the friction material. We now need to further analyze the effects of different factors such as base layer density, fiber type and coating thickness to improve the durability.

5.3.1. Effect of Base Paper Density

Optimization of friction characteristics and mechanical (durability) properties of

friction material is very important as the friction material in a vehicle is not replaced during a vehicle’s life-time. Increase in the fiber content of friction material shows improved tensile and shear strength, wear resistance and durability110–112. But, it lowers the friction

coefficient at higher sliding speeds. On the other hand, use of higher amount of filler

resulted in higher and positive slope for μ-v curve and decrease in the durability. Cellulose

and aramid are the major classes of fibers that are used in friction paper composition. We

have studied effect of coating on different compositions of cellulose and aramid.

93 5.3.1.1. Friction Materials with ‘Cellulose + Aramid’ Fiber Content

Table 5-1 Friction material properties and composition for different ‘cellulose + aramid’ fiber contents.

Formulation # 1 2 3 4 5 6 Density Range (kg/m3) 500-520 450-470 400-420 500-520 450-470 400-420 Density (notation) High Medium Low High Medium Low Coating No No No Yes Yes Yes

Composition: Fiber (wt%) 50-52 56-58 63-65 50-52 56-58 63-65 Filler + Friction Modifier (wt%) 48-50 42-44 35-37 48-50 42-44 35-37

Table 5-1 shows the friction material compositions containing cellulose and aramid fibers, with different densities. The highest density was kept in the range of 500-520 kg/m3.

Two other compositions at density ranges 450-470 and 400-420 kg/m3 were prepared with

the aim of increasing the porosity and oil permeability of the base friction material. This

was achieved by decreasing the amount of filler in the composition. It was observed that decreasing density was responsible for increasing roughness which is expected to reduce the actual contact area between the friction paper and the steel surface during clutch engagement. Increased porosity and reduced contact area affect the friction performance.

94 a Pre Break-in Non-coated, 1940 kPa 3 Density: 314 kg/m Non-coated, 2960 kPa Coated, 1940 kPa 0.16 Temp: 120 °C Coated, 2960 kPa 0.15

0.14

0.13

0.12

Friction Coefficient 0.11

0.10 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Sliding Speed (m/s)

Post Break-in Non-coated, 1940 kPa b 3 Density: 314 kg/m Non-coated, 2960 kPa Coated, 1940 kPa 0.16 Temp: 120 °C Coated, 2960 kPa

0.15

0.14

0.13

0.12

Friction Coefficient 0.11

0.10 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Sliding Speed (m/s)

95 c Pre Break-in Non-coated, 1940 kPa 3 Non-coated, 2960 kPa Density: 369 kg/m Temp: 120 °C Coated, 1940 kPa 0.16 Coated, 2960 kPa

0.15 t

0.14

0.13

0.12

Friction Coefficien 0.11

0.10 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Sliding Speed (m/s)

Post Break-in Non-coated, 1940 kPa d 3 Density: 369 kg/m Non-coated, 2960 kPa 0.16 Temp: 120 °C Coated, 1940 kPa Coated, 2960 kPa

0.15

0.14

0.13

0.12

Friction Coefficient 0.11

0.10 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Sliding Speed (m/s)

96 e Pre Break-in Non-coated, 1940 kPa Density: High Non-coated, 2960 kPa Coated, 1940 kPa 0.16 Temp: 120 °C Coated, 2960 kPa

0.15

0.14

0.13

0.12

Friction Coefficient 0.11

0.10 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Sliding Speed (m/s)

f Post Break-in Non-coated, 1940 kPa 3 Non-coated, 2960 kPa Density: 410 kg/m Coated, 1940 kPa Temp: 120 °C 0.16 Coated, 2960 kPa

0.15

0.14

0.13

0.12

Friction Coefficient 0.11

0.10 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Sliding Speed (m/s)

Figure 5.3 SAE No. 2 data (friction characteristics in terms of friction coefficient versus sliding speed) of coated versus non-coated friction materials for ‘cellulose + aramid’ formulations at low density (a and b), medium density (c and d) and high density (e and f), (refer to Table 5-1 for density range).

97 Figure 5.3 shows the friction characteristics (friction characteristics in terms of

friction coefficient versus sliding speed) before and after coating the different density

friction materials. Decreasing density from 500-520 to 400-420 kg/m3 shows drop in the

performance. μ-v curve slope becomes more negative with decreasing density which would cause stick-slip motion. Similar trends are observed for pre break-in as well as for post break-in data. The probable reason for this trend could be that the surface roughness of friction paper increases with decreasing density, due to the lack of filler. This causes reduction in actual contact area between the friction paper and the steel surface. Another reason could be that the decreased density is achieved by reducing the filler quantity. This will reduce shear thickening effect imparted by diatomaceous earth to the friction paper.

Hence, the drop in the performance of friction material with decreasing density is observed for both coated and non-coated cases.

All coated friction materials show improved friction characteristics as compared to the non-coated paper. Diatomaceous earth particle coating causes increasing shear stress with increasing sliding speed when the clutch is engaged in the presence of ATF. This is because filler particles come in direct contact with the steel surface in coated assembly.

Whereas, in case of non-coated friction paper, the surface consists of mixture of fiber and filler. This reduces the resistance to flow and the friction coefficient.

Filler coating also affects the porosity and oil permeability of the top surface of the friction material. Durability will be affected with the change in porosity. The friction material undergoes many engagements-disengagement cycles in its life time. Increasing

98 wear (thickness loss) and disentanglement are the primary symptoms of friction paper

failure.

Figure 5.4 Durability test data for coated and non-coated friction materials containing ‘cellulose + aramid’ as fiber, at different densities.

Figure 5.4 shows durability test data for formulations 1 to 6 (Table 5-1). We found

that all the formulations failed by delamination before 40% of the total pressure cycles.

The coated friction paper with highest density of 500-520 kg/m3 failed within first 6%

pressure cycles because of high density. Non-coated friction paper with density 500-520

kg/m3 failed in 26% pressure cycles. The failure by delamination of the coated layer shows that the porosity of the surface and base layer is not enough for oil penetration. Hence, ATF ruptures the interface between the coating and the base layer. Also, another reason could be the thermal degradation of cellulose which is the least heat resistant component in the

99 formulation. Yang and Lam et. al. showed that cellulose fibers start to carbonize at 200 °C and completely degrade at 420 °C. Carbonization can lead to reduce strength of the matrix28.

5.3.1.2. Friction Materials with Only ‘Aramid’ Content

Table 5-2 Friction material properties and composition containing different ‘aramid’ fiber contents

Formulation # 7 8 9 10 11 12 Density Range (kg/m3) 450-470 430-450 400-420 450-470 430-450 400-420 Density (notation) High Medium Low High Medium Low Coating No No No Yes Yes Yes

Composition: Fiber (wt%) 53-55 58-60 63-65 53-55 58-60 63-65 Filler + Friction Modifier (wt%) 45-47 40-42 35-37 45-47 40-42 35-37

Table 5-2 shows the compositions 7 to 12 where only Aramid is used as a fiber.

The density of the friction paper was reduced by decreasing the concentration of diatomaceous earth filler in the composition.

100 a Pre Break-in Non-coated, 1940 3 kPa Density: 294 kg/m Non-coated, 2960 0.16 Temp: 120 °C kPa C d 1940 kP

0.15

0.14

0.13

0.12

Friction Coefficient 0.11

0.10 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Sliding Speed (m/s)

b Post Break-in Non-coated, 1940 kPa 3 Non-coated, 2960 kPa Density: 294 kg/m Coated, 1940 kPa 0.16 Temp: 120 °C Coated, 2960 kPa

0.15

0.14

0.13

0.12

Friction Coefficient 0.11

0.10 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Sliding Speed (m/s)

101 c Pre Break-in Non-Coated, 1940 kPa 3 Non-Coated, 2960 kPa Density: 320 kg/m Coated, 1940 kPa Temp: 120 °C 0.16 Coated, 2960 kPa

0.15

0.14

0.13

0.12

Friction Coefficient 0.11

0.10 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Sliding Speed (m/s)

d Post Break-in Non-coated, 1940 3 kPa Density: 320 kg/m Non-coated, 2960 0.16 Temp: 120 °C kPa Cd 1940 kP

0.15

0.14

0.13

0.12

Friction Coefficient 0.11

0.10 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Sliding Speed (m/s)

102 e Pre Break-in Non-coated, 1940 kPa 3 Non-coated, 2960 kPa Density: 346 kg/m Coated, 1940 kPa 0.16 Temp: 120 °C Coated, 2960 kPa

0.15

0.14

0.13

0.12

Friction Coefficient 0.11

0.10 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Sliding Speed (m/s)

f Post Break-in Non-coated, 1940 kPa 3 Non-coated, 2960 kPa Density: 346 kg/m Coated, 1940 kPa 0.16 Temp: 120 °C Coated, 2960 kPa

0.15

0.14

0.13

0.12

Friction Coefficient 0.11

0.10 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Sliding Speed (m/s)

Figure 5.5 SAE No. 2 data (friction characteristics in terms of friction coefficient versus sliding speed) of coated versus non-coated friction materials for low density (a and b), medium density (c and d) and high density (e and f), (refer to Table 5-2 for density range).

103 Figure 5.5 shows friction characteristics of formulations 7 to 12 in terms of friction coefficient versus sliding speed. Here, all the non-coated friction materials show negative slope for friction characteristic curve. But, the performance is improved at all densities after application of diatomaceous earth filler coating. The μ-v curve has positive slope with higher value of friction coefficient. The best performance is observed for friction paper with medium density 430-450 kg/m3. Another observation is that the post break-in data

shows improved μ-v characteristics in comparison to that for pre break-in data for non-

coated friction papers. This can be due to the high roughness of the friction material at

lower densities and with only aramid fiber. This will reduce contact area between the

friction paper and the steel surface after first engagement giving lower friction coefficient.

But after few runs, the paper surface wears out the peaks and flattens, providing more

surface contact area. Hence, the friction coefficient increases for post break-in testing.

This study shows that the filler (diatomaceous earth) coating improves the friction

characteristics of the friction paper made up of only aramid fiber (without cellulose).

104

Figure 5.6 Durability test data for coated and non-coated friction materials containing only aramid as fiber, at different densities.

Figure 5.6 shows the durability data for formulations 7 to 12. All the samples except the coated friction paper with highest density (450-470 kg/m3) did not fail even after 80% pressure cycles. Coated fiction paper with highest density (450-470 kg/m3) failed after 48% of total pressure cycles due to delamination. This can be because of the reduced porosity after coating. But, other samples with medium and low densities showed good durability over long period. Oil drop test (Figure 5.7) showed that oil penetration times for coated friction papers with low, medium and high densities were 7.4 sec, 10.7 sec and 18.2 sec, respectively. Higher oil drop value can cause lower durability as seen with formulation# 4.

We found that friction paper with medium density is composition in terms of friction performance and durability.

105

Figure 5.7 Oil drop measurement of filler-coated friction materials containing cellulose and aramid versus only aramid.

Figure 5.7 shows the oil drop measurements for ‘cellulose + aramid’ and only

‘aramid’ fiber containing friction materials with diatomaceous earth coating. Friction

materials made with combination of cellulose and aramid show higher oil drop values than that for only aramid. This can be due to higher density and fibrillation of cellulose fiber than low fibrillated aramid. We can correlate this data with durability of coated friction materials with ‘cellulose + aramid’ and only ‘aramid’ composition in Figure 5.4 and Figure

5.6. Coated friction materials with ‘cellulose + aramid’ fiber content failed within 40% pressure cycles due to higher oil drop value or lower oil permeability. Whereas, coated friction materials with only ‘aramid’ fiber content were intact even after 80% pressure cycles.

106

Figure 5.8 Tensile strength data for friction materials containing cellulose + aramid versus only aramid as fiber, at different densities.

Figure 5.8 shows the tensile strength of formulations- 1, 2, 3, 7, 8 and 9. The coated materials also had the same tensile strengths as those of their non-coated counterparts. All the formulations show tensile strength well above 10 MPa, which is in the range of desired value for this application.

107 5.3.2. Effect of Coating Thickness on Friction Test

a Pre Break-in No coating 25-40 µm coating 0.16 60-80 µm coating

0.15

0.14

0.13

0.12

FrictionCoefficient 0.11

0.10 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Sliding Speed (m/s)

Post Break-in No coating b 25-40 µm coating 0.16 60-80 µm coating

0.15

0.14

0.13

0.12

Friction Coefficient 0.11

0.10 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 Sliding Speed (m/s)

Figure 5.9 SAE No. 2 data (friction characteristics in terms of friction coefficient versus sliding speed) of filler-coated friction materials for non-coated, 25-40 μm and 60-80 μm thickness coatings.

108 Figure 5.9 shows the effect of coating thickness on friction performance in terms

of friction coefficient versus sliding speed. The coating thicknesses were maintained in

range of 25-40 and 60-80 μm, as much thicker coatings (above 150 μm) showed reduced

surface porosity and poor durability. All the friction materials had 430-450 kg/m3 density

which was optimized in the previous section. Increase in coating improved the friction

performance. Non-coated friction material shows the worst friction performance whereas

the 60-80 μm coated friction material shows the best friction performance. The reason

could be the increased contact area between the filler and the steel plate upon engagement.

Friction material consists of very rough surface. Hence, filler will fill in the pores at lower thickness and more filler will be in plane with steel plate at higher thickness, as shown in

Figure 5.10. Oil drop test showed that the oil penetration times for non-coated paper and papers coated with thin and thick filler layer were 5.7 sec, 7.1 sec and 10.7 sec, respectively.

109 a

Figure 5.10 Optical images of friction materials with (a) no coating, (b) 25-40 μm coating thickness and (c) 60-80 μm coating thickness.

110 5.4. Conclusions

In this chapter, we studied effect of different parameters on friction performance and durability of filler-coated friction material. The overall summary is as follows: a. In the first part of this study, we showed that the filler coating increased friction

performance but reduced the durability due to reduction in porosity and oil

permeability of the friction material. Hence, in the next two parts of this chapter,

focus was to increase the surface porosity of the filler-coated friction material. b. Coated friction material containing only ‘aramid’ as fiber content showed better

performance in terms of both- friction and durability, in comparison to ‘cellulose +

aramid’. This can be due to lower density and higher oil permeability for aramid

than for cellulose. At the same time, friction performance increased with increasing

paper density in both cases. Optimum formulation obtained is filler coated friction

material containing only aramid as fiber content, with density 430-450 kg/m3. c. Increase in the coating thickness from 25-40 μm to 60-80 μm improved friction

performance. It was correlated to the increased contact area between the filler and

the steel plate upon engagement, at higher coating thickness.

111

CHAPTER VI

CARBON NANOTUBES DISPERSION IN NON-POLAR LUBRICANT USING

PHOSPHONIUM IONIC LIQUID ADSORPTION

6.1. Introduction

Over the last decade, Carbon Nanotubes (CNTs) have been recognized as

promising material for majority of applications in composites113, electrodes114, sensors115

and lubricants116 due to their excellent mechanical 117,118, electronic119, optical120 and

tribological 72,121,122 properties. CNTs are extensively studied as lubricant additives for lowering friction and wear between two moving surfaces. Cursaru et. al. studied the efficiency of cobalt based single walled CNTs as extreme pressure-anti wear additive70.

They found reduction in friction coefficient at optimum CNT concentration of 0.5 wt%.

Bhaumik et. al. also showed that 0.5 wt% MWCNT is optimum concentration for increased anti-wear and load bearing capacity of the lubricant71. But, increase in CNT concentration

showed formation of aggregates and precipitation of nanotubes caused damage to the

surface71. The fundamental cause of carbon nanotubes decreasing friction and wear was

also studied in literature. Ni et. al. proposed that carbon nanotubes reduce friction by sliding

or combination of rolling and sliding action123. Whereas, Mylvaganam et. al. showed that

atomically smooth surface of CNTs is responsible for decreasing friction and wear72.

Strong van der Waals interactions between nanotubes cause agglomeration of

CNTs in the composites. Hence, their practical applications are limited due to the poor

112

dispersion of nanotubes. To overcome CNTs agglomeration, different covalent and non-

covalent functionalization techniques are used. Covalent functionalization can create

defects in п-network of CNTs, consequently altering their inherent physical properties. The non-covalent functionalization involves adsorption of suitable molecules including surfactants, polymers or proteins by van der Waals forces or п-stacking interactions, achieving CNTs dispersion while preserving their desired properties74,124.

An effective and simple method of modifying and dispersing CNTs is achieved by

using room temperature ionic liquids (ILs)84. Ionic liquids are developing as promising

materials because of their excellent properties including very low vapor pressure, wide

temperature range with high thermal stability and non-flammability. When mechanically

mixed with CNTs, ILs form a soft material called ‘bucky gel’ which is well dispersed CNTs in IL medium. Many researchers have shown that there are van der Waals force, п-п or cation-п interactions present between the CNTs and IL82,83. Bucky gels are studied in wide range of applications such as elastomeric actuators, aqueous lubricants and CNT-polymer composites. ILs are also used as dispersants for CNTs in polymer composites and aqueous solutions. Although interaction of bucky gels with imidazolium ILs has been studied in detail 125,126, other IL groups have not been studied systematically. Phosphonium ILs

synthesized by Qu et. al. are miscible with non-polar systems and show promising

dispersant property for CNTs in non-aqueous solutions in our ongoing studies. Hence,

interaction study of CNTs with phosphonium ILs should enable understanding and

optimization of formulations in further applications.

113 In the present work, the confinement and interaction of phosphonium IL with

multiwalled carbon nanotubes (MWCNTs) are studied. Rheological measurements of

MWCNTs-IL bucky gels are also explored to understand more about the nanofluid

microstructure. The analysis reported here will be helpful in supporting further application of dispersing CNTs in non-polar hydrocarbon lubricant.

6.2. Experimental

6.2.1. Materials and Preparation

(CH2)13CH3

O OCH2CH(C2H5)CH2CH2CH2CH3 P P - (CH2)5CH3 (CH2)5CH3 O OCH2CH(C2H5)CH2CH2CH2CH3

(CH2)5CH3

Figure 6.1 Molecular structure of trihexyltetradecylphosphonium bis(2-ethylhexyl) phosphate (Ph-IL)

Unmodified MWCNTs of 10-15 nm diameter were obtained from Arkema Inc

(Philadelphia, PA). The ionic liquid, Trihexyltetradecylphosphonium bis(2-ethylhexyl)

phosphate (Ph-IL) was synthesized following the procedure described by Sun et. al127. The

molecular structure of IL is shown in Figure 6.1. The reagents tetradecyltrihexyl chloride,

bis(2-ethylhexyl) phosphate (HDEHP), sodium hydroxide (NaOH) and hexane were

obtained from Sigma Aldrich (Milwaukee, WI). In synthesis, 3 gm (5.78 mmol) of

tetradecyl trihexyl phosphonium chloride was mixed with 1.87 gm (5.0 mmol) of bis(2-

114 ethylhexyl) phosphate (HDEHP). 3.07 gm of hexane was added to the above mixture, while stirring it continuously. An aqueous solution of NaOH with molarity equal to HDEHP was then added dropwise into the reaction system and the mixture was stirred at 60 °C for 3-4 hrs. The organic phase was separated and washed with deionized water four times to ensure removal of NaCl. Water and the remaining solvent was removed under vacuum at 60 °C.

Nanofluids of MWCNT and IL were prepared at CNT concentrations of 0.025,

0.05, 0.1, 0.5, 1 and 10 wt%. The MWCNT-IL mixture was mixed in a mortar pestle for

30 minutes to prepare a nanofluid. These MWCNT-IL nanofluids were used for DSC,

FTIR, rheology and TEM studies without any further treatment to understand IL adsorption on carbon nanotubes.

MWCNT dispersion stability in hydrocarbon base oil was studied at 0, 0.2, 0.5, 5,

10 and 50 mM concentrations of ionic liquid. Ionic liquid solution in base oil was prepared first before adding the CNT. Ultrasonication probe from Misonix Sonicators (Newtown,

CT) was used to disperse the CNTs in ‘IL + base oil’ solutions using 15000 J total energy.

The resulting dispersions were then centrifuged at 9000 rpm in micro-centrifuge machine for 10 minutes to obtain stable dispersion.

6.2.2. Characterization

Differential scanning calorimetry (DSC) (TA instruments DSC Q2000, New

Castle, DE) experiments were performed to study the phase transition temperatures of bulk and confined ILs. The samples were heated to 200 °C and then cooled down to -90 °C at

10 °C/min rate. The positions of the peaks were found to be independent of the scan rate

115 up to 10 °C/min. The reported isotherms are taken from the second heating cycle. The

samples studied were pure ionic liquid and MWCNT-IL nanofluids at different

concentrations.

The solid-state experiments were done in collaboration with Dr. Toshikazu Miyoshi from the Department of Polymer Science, The University of Akron. 1H solid-state NMR one-pulse experiments were performed on Bruker Advance Ultrashield 300 NMR spectrometer with 4 mm double resonance VT CP/MAS probe. Experiments were carried out at 90° pulse.

Bruker optics alpha FT-IR spectrometer (Billerica, MA) was used to take FTIR scans. The FTIR spectra were obtained in an attenuated total reflection (ATR) mode at a resolution of 8 cm-1 of 256 scans.

Rheological measurements were carried out on AR G2 rheometer (TA Instruments,

New Castle, DE) with cone and plate geometry (25 mm diameter, 2 rad). In oscillatory

measurements, a strain sweep experiment was performed at 1 Hz frequency prior to the

frequency sweep experiments to verify linear viscoelastic range. The frequency sweep

experiments were performed in 0.1 to 100 rad/s angular frequency range to study the

development of nanofluid gel structure with different MWCNT concentrations.

Transmission electron microscopy (TEM) was used to study the exfoliation of

MWCNTs due to IL treatment. JEOL JSM- 1230 TEM (Peabody, Massachusetts) was used operating at 120 kV. IL treated and untreated MWCNTs were dispersed in hexane at concentration of 0.01 mg/ml and sonicated in a bath sonicator for 15 minutes. A drop of the above suspension was placed on the copper TEM grid and dried overnight.

116 CNT dispersion in ‘IL + base oil’ solutions was visually examined. The dispersions were kept in vials untouched for 30 days. The samples were then visually compared for dispersion stability.

UV-VIS Hewlett Packard Model 8453 spectrometer (Santa Clara, CA) was used to measure CNT dispersion as a function of ionic liquid concentration in the 200-1100 nm range. For each solution, equal amount of IL mixed with base oil without CNTs was used as baseline to subtract the absorbance effect of ionic liquid.

Figure 6.2 Schematic of high frequency reciprocating rig (HFRR)

High Frequency Reciprocating Rig (HFRR) (PCS Instruments, London, UK) was

used to study tribological or friction properties of CNT dispersion in base oil. Figure 6.2

shows schematic of high frequency reciprocating rig (HFRR). In this test, lubricant is

placed in a sample holder which has 52100 steel disc at the bottom. A ball of the same

material (6 mm diameter) is rubbed against disc at specific conditions. All the tests are

117 done in boundary lubrication regime. The ball and disc are removed after the test, washed

with hexane and air dried128.

Boundary layer conditions were maintained the same in all the experiments. 52100 steel ball (6 mm diameter) and disc were used in the experiment. Test conditions were maintained as: 20 Hz frequency, 10N load, 60-minute time and 25 °C temperature. These parameters were derived from Hamrock-Dowson equation governing boundary lubrication as shown below129.

Hamrock-Dowson equation for point contact:

= 2.69 0.67 0.53 0.067 0.73 )

ℎ𝑐𝑐 − − 𝑘𝑘 𝑈𝑈 𝐺𝐺 𝑊𝑊 (1 − 0.61𝑒𝑒 where, 𝑅𝑅′ 0.636 = 1.0339( / )

𝑦𝑦 𝑥𝑥 𝑘𝑘 = /( 𝑅𝑅2) 𝑅𝑅

𝑥𝑥 𝑊𝑊 = 𝐹𝐹0 0𝐸𝐸/′𝑅𝑅

𝑥𝑥 𝑈𝑈 = 𝜇𝜇 𝑢𝑢 𝐸𝐸′𝑅𝑅

hc is lubrication𝐺𝐺 𝛼𝛼𝛼𝛼′ layer thickness, Rx and Ry (3 mm) are equivalent radii in x and y directions

(m), F is applied load in point contact (10 N), E’ is effective elastic modulus (219 GPa), μ0

is viscosity of lubricant (0.02 Pa.s), u0 is velocity of the ball (to be calculated) and α is

piezo-viscous coefficient (25 GPa-1).

Also, for boundary lubrication regime, where = 𝑐𝑐 2 2 ℎ 1 + 2

𝜆𝜆 ≥ 1 𝜆𝜆 √𝑅𝑅𝑞𝑞 𝑅𝑅𝑞𝑞 Here, 1 2 (0.02 μm) are root mean square roughnesses of the two surfaces and hc

𝑞𝑞 𝑞𝑞 is lubrication𝑅𝑅 𝑎𝑎𝑎𝑎𝑎𝑎 layer𝑅𝑅 thickness.

118 Zygo optical profiler (Berwyn, PA) was used to analyze surface HFRR disc wear after test was done. Optical microscope was also used to analyze scar on the HFRR test ball. Wear scar dimensions and total wear volume were calculated from the analysis.

6.3. Results and Discussion

6.3.1. Ionic liquid adsorption on MWCNTs

Figure 6.3 DSC isotherms for Ph-IL and MWCNT-IL nanofluids with different compositions.

The adsorption of ionic liquid by carbon nanotubes was characterized by DSC technique. Three types of samples with varying concentrations of MWCNTs were subjected to DSC analysis under non-isothermal conditions. The samples were first heated to 200 °C and cooled down to -140 °C. The same procedure was repeated for second heating and cooling cycles. The data reported here is from the second heating cycle as the first heating cycle includes ionic liquid adsorption inside MWCNTs with decreasing 119 viscosity as temperature increases. In the second heating cycle, pure ionic liquid did not

show any crystallization or melting peak in the temperature range studied. This could be

explained as a difficulty in crystallization because of steric problems arising due to longer

alkyl side (n = 14) chains in cation of IL 130,131. MWCNT-IL nanofluids at 1 wt% and 10

wt% MWCNT concentrations showed crystallization peak at -62 °C and melting peak at -

43 °C in the heating cycle as shown in Figure 6.3. Similar results have been found for other polymer-carbon nanotube systems125,132,133. This means that, MWCNT walls provide crystallization sites for ionic liquid even at very low concentration of nanotubes in nanofluids. DSC results show that ionic liquid is getting adsorbed on CNT walls. This confinement effect of ionic liquid in nanotubes was further analyzed using FTIR and rheological tests.

Figure 6.4 Solid state NMR of IL and 10 wt% MWCNT-IL nanofluid (In collaboration with Dr. Toshikazu Miyoshi, Department of Polymer Science, The University of Akron).

120 Ionic liquid immobilization in nanofluids was also characterized by solid state

NMR technique as shown in Figure 6.4. The peak at 0 ppm corresponds to isotropic

chemical shift which is present in pure ionic liquid and MWCNT-IL nanofluid spectra. The

MWCNT-IL nanofluid spectrum also shows spinning side bonds at 4, 8 and 12 kHz which is absent in pure ionic liquid. These peaks correspond to the partial anisotropic interactions present in the nanofluid which exist between the carbon nanotubes and the ionic liquid.

This result reveals adsorption of ionic liquid on CNT walls as seen in Figure 6.3.

Figure 6.5 FTIR spectra of Ph-IL and MWCNT-IL nanofluids with different compositions.

The FTIR spectra of unmodified MWCNTs and MWCNT-IL nanofluids are shown in Figure 6.5. The peak at 810 cm-1 corresponds to stretching vibration of P-O bond of anion with attraction from cation which is suppressed in case of nanofluids134. The probable

interaction between the phosphonium cation and the MWCNTs restrains the interaction

121 between the cation and the anion of IL. The increase of negative charge on O (in P-O bond) weakens the P-O bond strength which causes lowering of the peak intensity at 810 cm-1.

The peak at 1243 cm-1 in Pure IL corresponds to the P=O stretching vibrations in phosphate group of ionic liquid. It is shifted to lower energy peak of 1234 cm-1 from 1243 cm-1 for

MWCNT-IL nanofluids spectra. The suppressed interaction between the phosphonium

cation and the phosphate anion increases the negative charge on ‘O’ of the P-O- bond. This

weakens the P=O (phosphate) electron pull and weakens the bond. Hence, P=O bond IR

peak is shifted from high energy at 1243 cm-1 to low energy at 1234 cm-1. The cation-п

interaction between phosphonium cation and MWCNTs cause the stable gel formation by

MWCNT-IL.

122

Figure 6.6 Elastic and loss moduli for 1% strain as function of angular frequency at different MWCNT concentrations.

123 The rheological measurements can provide insight into the dispersion and

microstructure of the nanofluids. The oscillatory measurements were performed on the

nanofluids to measure the elastic and loss moduli. First, strain sweep was performed to

determine the linear viscoelastic region for nanofluids. MWCNT concentration effect on

rheological properties was studied through frequency sweep for 0.025, 0.05 and 0.5

weight% concentration nanofluids. As presented in Figure 6.6, elastic modulus (G’) is

smaller than viscous modulus (G”) for 0.025 weight% solution over full frequency range.

When nanotube concentration increases to 0.05 wt%, G’ becomes greater than G” at the

angular frequency lower the than crossover point (7.0 rad/s) and G’ approaches a plateau

at low frequency values. As MWCNT concentration is further increased to 0.5 wt%, the

cross over point shifts to higher frequency value (13.6 rad/s) and is independent of frequency at the lower frequency range. This frequency independence indicates strong gel structure formation which is explained by cation-п interaction by FTIR results. The phosphonium cations interact with the п-electronic surface of MWCNTs through cation-п

interactions and nanotube walls provide sites for ionic liquid crystallization. This forms a

physical network between the MWCNT bundles and the ionic liquid.

124 a b

Figure 6.7 TEM micrographs of MWCNTs dispersed in hexane (a) treated with IL and (b) without IL.

Figure 6.7 shows TEM micrographs of MWCNTs dispersed with and without IL

treatment using hexane as a solvent. MWCNT suspensions prepared in hexane contain

separated rigid rods or ropelike clusters of nanotubes and they can be observed in TEM

images. Hexane was chosen as a solvent for dispersing the system because of higher

miscibility of the chosen phosphonium IL in hexane. Figure 6.7a shows the MWCNT

bundles or agglomerates without IL. Obviously, phosphonium IL assists in CNT

exfoliation which is shown by the individual nanotubes in Figure 6.7b. TEM measurements confirm the strong interaction between the MWCNTs and the phosphonium IL. Adsorption of IL on nanotubes by strong cation-п interactions overpower the nanotube-nanotube van der Waals interactions and prevent their agglomeration.

125 6.3.2. MWCNT dispersion stability using ionic liquid as surfactant

Figure 6.8 Visual CNT dispersion after IL treatment at different molar concentrations.

Figure 6.8 shows the MWCNT dispersion in base oil at different concentrations of

phosphonium ionic liquid. All the samples were kept undisturbed for 30 days. Carbon

nanotubes showed unstable dispersion and settled down without ionic liquid as shown by

0 mM solution. A stable dispersion was observed visually for 0.2, 0.5, 5 and 10 mM solutions. Whereas, at 50 mM IL concentration, dispersion was unstable as seen by the settling nanotubes.

126

Figure 6.9 UV-VIS spectroscopic analysis for well dispersed carbon nanotubes.

The CNT dispersion was further analyzed quantitatively using UV-VIS spectroscopy. Agglomerated CNTs do not show absorbance in UV-VIS region. Stable dispersion of separated nanotubes has absorption in UV-VIS range as a function of concentration or measure of dispersion. Here, the effect of IL concentration was already subtracted by using equivalent IL concentration in base oil as a baseline. Figure 6.9a shows

UV-VIS absorbance spectra of CNT-IL-base oil solutions after the centrifugation. The

127 absorbance peak is obtained at 246 nm wavelength. To characterize CNT dispersion, the

absorbance values were recorded at 500 nm wavelength135. Absorbance intensity increased

with increasing ionic liquid concentration from 0 to 10 mM. Whereas, absorbance seemed to decrease at 50 mM concentration of the ionic liquid.

At lower concentration, the amount of ionic liquid is not enough to coat all the

CNTs giving less dispersion. As, the IL concentration is increased to certain limit, where

IL amount is just enough to coat all CNTs, the highest absorbance intensity is obtained. If the IL concentration is increased further, it can go above the Critical Micelle Concentration

(CMC) limit. Above the CMC limit, surfactant molecules self-assemble forming micelles and suppress particle dispersion. Hence, in case of 50 mM IL concentration, which is above the CMC limit, we see agglomerated and settled CNT particles at the bottom. 5 mM IL concentration was found to be optimum for CNT dispersion. Figure 6.9b shows change in absorbance values at 500 nm for all concentrations.

6.3.3. MWCNTs as Additive for Steel-Steel Contact in Boundary Lubrication HFRR

was used to study lubrication efficiency of carbon nanotube added lubricant system in steel-steel contact under boundary lubrication. It is expected that carbon nanotubes will act as an additive to reduce friction by rolling and sliding action between the ball and the disc. Ionic liquid will act as surfactant as well as friction modifier to reduce friction and wear136.

128 a

129 c

Figure 6.10 HFRR wear measurement on ball in (a) base oil, (b) CNT + IL + base oil, and on disc in (c) base oil, (d) CNT + IL + base oil.

Base oil was compared with ‘0.05wt% CNT + IL + base oil’ solution for friction and wear measurements. Figure 6.10 a and b show 3D profile of the 52100 steel ball scar and Figure 6.10 c and d show circular scar on the 52100 steel disc after the test was done.

130 Table 6-1 shows the friction coefficient and wear volume for both test conditions. Base oil

gave friction coefficient of 0.097 and ‘0.05wt% CNT + IL + base oil’ solution showed

friction coefficient of 0.084. Addition of CNT seemed to decrease friction coefficient as

compared to the base oil. This can be due to the cylindrical shape of the carbon nanotubes

which would slide and roll under high pressure in boundary lubrication regime as explained by Ni et. al123. Another reason could be atomically smooth surface of the CNTs which would decrease the friction coefficient.

Figure 6.10 a and b show steel ball wear scar for ‘base oil’ and ‘0.05wt% CNT +

IL + base oil’ solution, respectively. Test done with the base oil showed wear scar of 99.71

μm radius on steel ball. Whereas, ‘0.05wt% CNT + IL + base oil’ solution resulted in a wear scar of 62.48 μm radius on the steel ball, under the same test conditions. Higher wear is observed on the disc tested with only ‘base oil’ as compared to ‘0.05wt% CNT + IL + base oil’ solution. Wear volume build-up is observed on disc in presence of only ‘base oil’ as shown in Figure 6.10 c. This could be due to the material transfer from steel ball wear to the disc, which needs further verification by surface analysis techniques. Addition of

MWCNTs with ionic liquid surfactant caused reduction in friction coefficient and wear for both surfaces. As mentioned earlier, carbon nanotubes can reduce friction and wear because of sliding or rolling. Carbon nanotubes can also open their cylindrical structure to plane graphitic structure, under high pressure sliding. Graphite structure also causes decreasing friction and wear because of the presence of flakes or planes sliding over each other. In addition, ionic liquid can also help to decrease friction and wear in synergy with

MWCNTs.

131 Table 6-1 Friction coefficient and wear volume measurements obtained from HFRR test

CNT dispersed in base oil Base oil with IL as a surfactant

Friction Coefficient 0.097 0.084 Ball Wear Volume (μm3) 25887 3990 Disc Wear Volume (μm3) 15450 4160

6.4. Conclusions

Carbon nanotube–ionic liquid non-covalent interactions were used to disperse

CNTs in non-polar base oil. Phosphonium ionic liquid which is compatible with non-polar solvents and lubricants was used for CNT modification. Ionic liquid adsorption on CNT walls was studied using DSC, FTIR and rheological techniques. FTIR confirmed the presence of cation-п interactions present in the system. Well dispersed CNTs in hexane with ionic liquid as surfactant, are evident from the TEM images. Ionic liquid was further used as a surfactant to disperse MWCNTs in hydrocarbon base oil. Stable CNT dispersion in lubricant was obtained at 5 mM ionic liquid concentration in base oil, which is evident from the UV-VIS data. For a boundary lubrication steel-steel contact, reduction in friction coefficient and wear volume was observed by addition of CNTs and ionic liquid to the base oil.

132

CHAPTER VII

CONCLUSIONS

This work has focused on improving the lubrication efficiency and reducing the

friction losses of various contact systems. These systems include wet clutch used in the

torque converters in vehicles and steel-steel contact in boundary lubrication regime.

In the first part of the study, ideal friction characteristics of the wet clutch system

were explained. Positive slope of the friction coefficient versus sliding speed curve along

with higher dynamic friction coefficient value indicate ideal friction characteristics for

smooth engagement in the presence of ATF. The use of just base oil instead of ATF,

however, gives negative slope of the friction versus sliding speed causing stick-slip

behavior. The difference between ATF and base oil in interacting with friction material

was studied by adsorption, rheology and friction analysis. ‘ATF additives + friction

material’ system showed higher adsorption energy as compared to ‘base oil + friction

material’ system, in DSC isotherms. Also, UV-VIS results confirmed that ATF additives have higher adsorption on the friction material and its components. The effect of additive adsorption on rheological behavior was studied using shear sweep on cone and plate rheometer. Also, friction characteristics were tested on SAE No. 2 machine. These experiments and analyses provided the following conclusions for the first part of our work.

133 (a) ATF exhibited shear thickening (or newtonian) behavior while the base oil showed shear thinning behavior when mixed with the diatomaceous earth (DE) fillers used in the friction material (friction paper). Increasing shear stress with shear rate was correlated with increasing friction coefficient with sliding speed in the presence of ATF.

(b) Filler (DE) showed shear thickening and fiber (aramid) showed shear thinning

behavior when mixed with ATF. This is in correlation with friction behavior, where friction material made up of 100% fiber gives negative slope for the friction coefficient versus sliding speed curve (friction curve). Whereas, 50% fiber + 50% filler composition gives improved friction characteristics in terms of positive slope and higher dynamic friction coefficient.

same shear rate. This is in correlation with reduction of friction coefficient on the friction

curve when the temperature is increased.

This study showed that ATF additive interaction with the friction material

components plays a major role in determining friction characteristics. Also, it was observed that ATF additives interact prominently with DE filler particles.

In the second part of the study, the friction material was coated with DE filler

particles to enhance the contact between filler and steel plate during engagement, which

would promote shear thickening behavior in the system. Three different types (shapes) of

fillers; 1. Diatomaceous earth type-1; 2. Diatomaceous earth type-2 and, 3. A proprietary clay were used for comparison. All the fillers showed shear thickening phenomena when

134 mixed with ATF. But, filler-1 showed the highest rate of increase of shear stress with shear rate in comparison to the other two fillers. Friction characteristics also followed the same trend seen in rheological analysis. Diatomaceous earth type-1 particles showed the highest positive slope and the highest dynamic friction coefficient in SAE No. 2 test, as compared to other two fillers and non-coated friction material. Hence, filler-1 was further used to optimize other parameters in the subsequent part of our study.

In the third part of the study, various parameters for filler-coated friction materials were studied to obtain friction characteristics (friction coefficient and its variation with sliding speed) and for durability optimization. These experiments and analyses provided the following conclusions for the third part of our work.

d. Coated friction material containing only ‘aramid’ as fiber content showed better

performance in terms of friction characteristics and durability in comparison to the use of

‘cotton + aramid’ fibers. This can be due to lower density and higher oil permeability with

aramid than with cotton. At the same time, friction performance (coefficient and slope)

increased with increasing friction paper density in both the cases. Optimum formulation

obtained was filler coated friction material containing only aramid as fiber content, and

having 430-450 kg/m3 density.

e. Increasing the coating thickness from 25-40 μm to 60-80 μm improved the friction

performance (coefficient and slope). This behavior was attributed to increased contact area between the filler and the steel plate upon engagement at higher coating thickness.

135 The last part of the study focused on developing a lubricant system for steel-steel

boundary lubrication. Multi-walled carbon nanotubes (MWCNT)-dispersed stable

hydrocarbon oil-based lubricant was developed using phosphonium ionic liquid as a

surfactant. Ionic liquid adsorption on CNT walls was studied using DSC, FTIR, solid-state

NMR and rheological techniques. FTIR confirmed the presence of cation-п interactions in the system. Well dispersed CNTs in hexane with ionic liquid as surfactant were evident from the TEM images. Ionic liquid was further used as a surfactant to disperse MWCNTs in hydrocarbon base oil. Stable CNT dispersion in lubricant was obtained at 5 mM of ionic liquid concentration in base oil, which was evident from the UV-VIS data. For a boundary lubrication steel-steel contact, reduction in friction coefficient and wear volume was observed by the addition of CNTs and ionic liquid to the base oil.

136

REFERENCES

(1) Holmberg, K.; Andersson, P.; Erdemir, A. Global Energy Consumption due to Friction in Passenger Cars. Tribol. Int. 2012, 47, 221–234 DOI: 10.1016/j.triboint.2011.11.022.

(2) Holmberg, K.; Andersson, P.; Nylund, N.-O.; Mäkelä, K.; Erdemir, A. Global Energy Consumption due to Friction in Trucks and Buses. Tribol. Int. 2014, 78, 94– 114 DOI: 10.1016/j.triboint.2014.05.004.

(3) Jost, P. H. Lubrication (Tribology): Education and Research: A Report on the Present Position and Industry’s Needs; London, England, 1966.

(4) Tung, S. C.; McMillan, M. L. Automotive Tribology Overview of Current Advances and Challenges for the Future. Tribol. Int. 2004, 37 (7), 517–536 DOI: 10.1016/j.triboint.2004.01.013.

(5) Nakada, M. Trends in Engine Technology and Tribology. Tribol. Int. 1994, 27 (1), 3–8 DOI: 10.1016/0301-679X(94)90056-6.

(6) Taylor, C. M. Automobile Engine Tribology-Design Considerations for Efficiency and Durability. Wear 1998, 221 (1), 1–8 DOI: 10.1016/S0043-1648(98)00253-1.

(7) Wan, S.; Li, D.; Zhang, G.; Tieu, A. K.; Zhang, B. Comparison of the Scuffing Behaviour and Wear Resistance of Candidate Engineered Coatings for Automotive Piston Rings. Tribol. Int. 2017, 106 (August 2016), 10–22 DOI: 10.1016/j.triboint.2016.10.026.

(8) Pradhan, A. K.; Das, S. Dry Sliding Wear and Friction Behavior of Cu-SiC Nanocomposite Coating Prepared by Pulse Reverse Electrodeposition. Tribol. Trans. 2013, 2004 (October 2013), 130925064906003 DOI: 10.1080/10402004.2013.843739.

(9) Sahoo, P.; Das, S. K. Tribology of Electroless Nickel Coatings - A Review. Mater. Des. 2011, 32 (4), 1760–1775 DOI: 10.1016/j.matdes.2010.11.013.

(10) Díaz, C.; Lutz, J.; Mändl, S.; García, J. A.; Martínez, R.; Rodríguez, R. J. Improved Bio-Tribology of Biomedical Alloys by Ion Implantation Techniques. Nucl. Instruments Methods Phys. Res. Sect. B Beam Interact. with Mater. Atoms 2009, 267 (8–9), 1630–1633 DOI: 10.1016/j.nimb.2009.01.118.

(11) Zhou, Z. R.; Zheng, J. Tribology of Dental Materials: A Review. J. Phys. D. Appl. 137

Phys. 2008, 41 (11), 113001 DOI: 10.1088/0022-3727/41/11/113001.

(12) Reinstädtler, M.; Rabe, U.; Scherer, V.; Hartmann, U.; Goldade, A.; Bhushan, B.; Arnold, W. On the Nanoscale Measurement of Friction Using Atomic-Force Microscope Cantilever Torsional Resonances. Appl. Phys. Lett. 2003, 82 (16), 2604–2606 DOI: 10.1063/1.1565179.

(13) Gnecco, E.; Meyer, E. Fundamentals of Friction and Wear on the Nanoscale, second.; Springer, 2015.

(14) Nimi, L.; On, N.; Thus, N. Definition and History of Tribology. 1998, No. 1966.

(15) Berman, D.; Erdemir, A.; Sumant, A. V. Few Layer Graphene to Reduce Wear and Friction on Sliding Steel Surfaces. Carbon N. Y. 2012, 54, 454–459 DOI: 10.1016/j.carbon.2012.11.061.

(16) Muratore, C.; Voevodin, A. A. Chameleon Coatings: Adaptive Surfaces to Reduce Friction and Wear in Extreme Environments. Annu. Rev. Mater. Res. 2009, 39 (1), 297–324 DOI: 10.1146/annurev-matsci-082908-145259.

(17) Chan, D.; Stachowiak, G. W. Review of Automotive Brake Friction Materials. Proc. Inst. Mech. Eng. Part D J. Automob. Eng. 2004, 218 (9), 953–966 DOI: 10.1243/0954407041856773.

(18) Campen, S.; Green, J.; Lamb, G.; Atkinson, D.; Spikes, H. On the Increase in Boundary Friction with Sliding Speed. Tribol. Lett. 2012, 48 (2), 237–248 DOI: 10.1007/s11249-012-0019-4.

(19) Bijwe, J. Composites as Friction Materials: Recent Developments in Non-Asbestos Fiber Reinforced Friction Materials-A Review. Polym. Compos. 1997, 18 (3), 378– 396.

(20) Ingram, M. P. The Mechanisms of Wet Clutch Friction Behaviour. Ph.D. Dissertation; Department of Mechanical Engineering, Imperial College London, 2010.

(21) Mercure, R. A. Review of the Automotive Torque Converter. In SAE Technical Paper; 1979.

(22) Natsumeda, S.; Miyoshi, T. Numerical Simulation of Engagement of Paper Based Wet Clutch Facing. J. Tribol. 1994, 116 (2), 232 DOI: 10.1115/1.2927201.

(23) Kugimiya, T.; Yoshimura, N.; Mitsui, J. Tribology of Automatic Transmission Fluid. Tribol. Lett. 1998, 5 (1), 49–56 DOI: 10.1023/A:1019156716891.

(24) Lloyd, F. A.; DiPino, M. A. Advances in Wet Friction Materials - 75 Years of

138 Progress. SAE Paper. 1980.

(25) McDonald, A. D.; Fry, J. S.; Woolley, A. J.; McDonald, J. C. Dust Exposure and Mortality in an American Chrysotile Asbestos Friction Products Plant. Br. J. Ind. Med. 1984, 41 (2), 151–157 DOI: 10.1136/OEM.41.2.151.

(26) Berry, G.; Newhouse, M. L. Mortality of Workers Manufacturing Friction Materials Using Asbestos. Br. J. Ind. Med. 1983, 40 (1), 1–7 DOI: 10.1136/OEM.40.1.1.

(27) Bakan, M. Development of Polymer Resin-Based Wet Friction Sheet Materials and Understanding Their Interactions with Automatic Transmission Fluids. Ph.D. Dissertation; Department of Polymer Engineering, The University of Akron, 2015.

(28) Yang, Y.; Lam, R. C. Theoretical and Experimental Studies on the Interface Phenomena during the Engagement of Automatic Transmission Clutch. Tribol. Lett. 1998, 5, 57–67.

(29) Zhang, X.; Li, K. Z.; Li, H. J.; Fu, Y. W.; Fei, J. Tribological and Mechanical Properties of Glass Fiber Reinforced Paper-Based Composite Friction Material. Tribol. Int. 2014, 69, 156–167 DOI: 10.1016/j.triboint.2013.08.003.

(30) Kim, Y. C.; Cho, M. H.; Kim, S. J.; Jang, H. The Effect of Phenolic Resin, Potassium Titanate, and CNSL on the Tribological Properties of Brake Friction Materials. Wear 2008, 264 (3–4), 204–210 DOI: 10.1016/j.wear.2007.03.004.

(31) Cho, M. H.; Ju, J.; Kim, S. J.; Jang, H. Tribological Properties of Solid Lubricants (Graphite, Sb2S3, MoS2) for Automotive Brake Friction Materials. Wear 2006, 260 (7–8), 855–860 DOI: 10.1016/j.wear.2005.04.003.

(32) Gopal, P.; Dharani, L. R.; Blum, F. D. Load, Speed and Temperature Sensitivities of a Carbon-Fiber-Reinforced Phenolic Friction Material. Wear 1995, 181–183, 913–921 DOI: 10.1016/0043-1648(95)90215-5.

(33) Fei, J.; Li, H. J.; Fu, Y. W.; Qi, L. H.; Zhang, Y. L. Effect of Phenolic Resin Content on Performance of Carbon Fiber Reinforced Paper-Based Friction Material. Wear 2010, 269 (7–8), 534–540 DOI: 10.1016/j.wear.2010.05.008.

(34) Liu, Y.; Fan, Z.; Ma, H.; Tan, Y.; Qiao, J. Application of Nano Powdered Rubber in Friction Materials. Wear 2006, 261 (2), 225–229 DOI: 10.1016/j.wear.2005.10.011.

(35) Nakada, T.; Nomura, T. A Study of Additive Effects on ATF Frictional Properties Using New Test Methods. SAE Technical Paper Series. 1990, pp 1–12.

(36) Zhao, H.; Neville, A.; Morina, A.; Vickerman, R.; Durham, J. Improved Anti- Shudder Performance of ATFs-Influence of a New Friction Modifier and Surface Chemistry. Tribol. Int. 2012, 46 (1), 62–72 DOI: 10.1016/j.triboint.2011.06.012.

139 (37) Kugimiya, T. Effects of Additives of ATF and Components of Friction Material for AT on μ-v Characteristics. Toraibarojisuto/Journal of Japanese Society of Tribologists. 2000, pp 387–395.

(38) Greenfield, M. L. Packing of Simulated Friction Modifier Additives under Confinement. 2005, No. 7, 7568–7578.

(39) Koshima, H.; Kamano, H.; Hisaeda, Y.; Liu, H.; Ye, S. Analyses of the Adsorption Structures of Friction Modifiers by Means of Quantitative Structure-Property Relationship Method and Sum Frequency Generation Spectroscopy. Tribol. Online 2010, 5 (3), 165–172 DOI: 10.2474/trol.5.165.

(40) Zhu, Y.; Ohtani, H.; Greenfield, M. L.; Ruths, M.; Granick, S. Modification of Boundary Lubrication by Oil-Soluble Friction Modifier Additives. Tribol. Lett. 2003, 15 (2), 127–134 DOI: 10.1023/A:1024405115736.

(41) Slough, C. G.; Ohtani, H.; Everson, M. P.; Melotik, D. J. The Effect of Friction Modifiers on the Low-Speed Friction Characteristics of Automatic Transmission Fluids Observed with Scanning Force Microscopy.pdf. 1998, pp 253–258.

(42) Seddon, E. J.; Friend, C. L.; Roski, J. P. Detergents and Dispersants - Springer. In Chemistry and Technology of Lubricants; 2010; pp 213–236.

(43) Derevjanik, T. S. Detergent and Friction Modifier Effects on Metal / Metal and Clutch Material / Metal Frictional Performance. 2001.

(44) Kasai, M.; Ishikawa, M.; Miyaji, T. Effect of Engine Oil Additives on Motorcycle Clutch System. SAE International. 2003.

(45) Ingram, M.; Noles, J.; Watts, R.; Harris, S.; Spikes, H. A. Frictional Properties of Automatic Transmission Fluids: Part I—Measurement of Friction–Sliding Speed Behavior. Tribol. Trans. 2010, 54 (1), 145–153 DOI: 10.1080/10402004.2010.531888.

(46) Zhao, H.; Morina, A.; Neville, A.; Vickerman, R. Tribochemistry on Clutch Friction Material Lubricated by Automatic Transmission Fluids and the Link to Frictional Performance. J. Tribol. 2013, 135 (4), 41801 DOI: 10.1115/1.4024375.

(47) Wood, M. H.; Welbourn, R. J. L.; Charlton, T.; Zarbakhsh, A.; Casford, M. T.; Clarke, S. M. Hexadecylamine Adsorption at the Iron Oxide − Oil Interface. 2013.

(48) Doig, M.; Warrens, C. P.; Camp, P. J. Structure and Friction of Stearic Acid and Oleic Acid Films Adsorbed on Iron Oxide Surfaces in Squalane. 2014.

(49) Ingram, M.; Noles, J.; Watts, R.; Harris, S.; Spikes, H. A. Frictional Properties of Automatic Transmission Fluids: Part II—Origins of Friction–Sliding Speed

140 Behavior. Tribol. Trans. 2010, 54 (1), 154–167 DOI: 10.1080/10402004.2010.531889.

(50) Milayzaki, T.; Matsumoto, T.; Yamamoto, T. Effect of Visco-Elastic Property on Friction Characteristics of Paper-Based Friction Materials for Oil Immersed Clutches. J. Tribol. 1998, 120 (2), 393 DOI: 10.1115/1.2834439.

(51) Hong, U. S.; Jung, S. L.; Cho, K. H.; Cho, M. H.; Kim, S. J.; Jang, H. Wear Mechanism of Multiphase Friction Materials with Different Phenolic Resin Matrices. Wear 2009, 266 (7–8), 739–744 DOI: 10.1016/j.wear.2008.08.008.

(52) Cai, P.; Wang, Y.; Wang, T.; Wang, Q. Effect of Resins on Thermal, Mechanical and Tribological Properties of Friction Materials. Tribol. Int. 2015, 87, 1–10 DOI: 10.1016/j.triboint.2015.02.007.

(53) Fei, J.; Luo, W.; Li, H.; Huang, J.; Ouyang, H.; Wang, H. Effects of NBR Particle Size on Performance of Carbon Fiber–Reinforced Paper-Based Friction Material. Tribol. Trans. 2015, 58 (6), 1012–1020 DOI: 10.1080/10402004.2015.1034906.

(54) Matsumoto, T. A Study of the Influence of Porosity and Resiliency of a Paper-Based Friction Material on the Friction Characteristics and Heat Resistance of the Material. SAE Paper. 1993.

(55) Yagi, S.; Katayama, N.; Hasegawa, H.; Matsushita, H.; Okihara, S. Effects of Microscale Texture on the Tribological Behavior of Paper-Based Friction Materials for a Wet Clutch. Tribol. Online 2015, 10 (5), 390–396 DOI: 10.2474/trol.10.390.

(56) Bijwe, J.; Nidhi; Majumdar, N.; Satapathy, B. K. Influence of Modified Phenolic Resins on the Fade and Recovery Behavior of Friction Materials. Wear 2005, 259 (7–12), 1068–1078 DOI: 10.1016/j.wear.2005.01.011.

(57) Ito, H.; Fujimoto, K.; Eguchi, M.; Yamamoto, T. Friction Characteristics of a Paper- Based Facing for a Wet Clutch Under a Variety of Sliding Conditions. Tribology Transactions. 1993, pp 134–138.

(58) Campen, S.; Green, J.; Lamb, G.; Atkinson, D.; Spikes, H. On the Increase in Boundary Friction with Sliding Speed. 2012, 237–248 DOI: 10.1007/s11249-012- 0019-4.

(59) Kitanaka, M. Friction Characteristics of Some Additives in ATFs. In Proceedings of the 8th International Colloquium on Tribology, Germany; 2001.

(60) Topolovec-Miklozic, K.; Forbus, T. R.; Spikes, H. Film Forming and Friction Properties of Overbased Calcium Sulphonate Detergents. Tribol. Lett. 2008, 29 (1), 33–44 DOI: 10.1007/s11249-007-9279-9.

141 (61) Fish, R. L.; Lloyd, F. A. Effect of Fluid Temperature on the Performance of Friction Material; 1975.

(62) Holgerson, M.; Lundberg, J. Engagement Behaviour of a Paper-Based Wet Clutch Part 2: Influence of Temperature. Proc. Inst. Mech. Eng. Part D J. Automob. Eng. 1999, 213 (5), 449–455 DOI: 10.1243/0954407991527017.

(63) Wenbin, L.; Jianfeng, H.; Jie, F.; Liyun, C.; Chunyan, Y. Simulation and Application of Temperature Field of Carbon Fabric Wet Clutch during Engagement Based on Finite Element Analysis. Int. Commun. Heat Mass Transf. 2016, 71, 180– 187 DOI: 10.1016/j.icheatmasstransfer.2015.12.026.

(64) Mäki, R. Wet Clutch Tribology - Friction Characteristics in Limited Slip Differentials. Doctoral Thesis; Department of Applied Physics and Mechanical Engineering Division of Machine Elements, Luleå University of Technology. 2005.

(65) Zhang, Z. J.; Simionesie, D.; Schaschke, C. Graphite and Hybrid Nanomaterials as Lubricant Additives. 2014, 44–65 DOI: 10.3390/lubricants2020044.

(66) Huang, H. D.; Tu, J. P.; Gan, L. P.; Li, C. Z. An Investigation on Tribological Properties of Graphite Nanosheets as Oil Additive. Wear 2006, 261 (2), 140–144 DOI: 10.1016/j.wear.2005.09.010.

(67) Iijima, S. Helical Microtubules of Graphitic Carbon. Nature 1991, 354 (6348), 56– 58 DOI: 10.1038/354056a0.

(68) Iijima, S.; Ichihashi, T. Single-Shell Carbon Nanotubes of 1-Nm Diameter. Nature 1993, 363 (6430), 603–605 DOI: 10.1038/363603a0.

(69) Kerikas, D.; Martini, F. A Study of The Mechanical Properties of Carbon Nanotubes and Their Tribological Applications. Department of Mechanical & Industrial Engineering; Northeastern University. Mech. Contact Lubr. ME5656 2011.

(70) Cursaru, D.; Andronescu, C.; Pirvu, C.; Ripeanu, R. The Efficiency of Co-Based Single-Wall Carbon Nanotubes ( SWNTs ) as an AW / EP Additive for Mineral Base Oils. 2012, 291, 133–139.

(71) Bhaumik, S.; Prabhu, S.; Uppala, N. Extreme Pressure Property of Carbon Nano Tubes ( CNT ) Based Nanolubricant. 2013, 4 (December), 123–127 DOI: 10.5897/JCEMS2013.0163.

(72) Mylvaganam, K.; Zhang, L. C.; Xiao, K. Q. Origin of Friction in Films of Horizontally Oriented Carbon Nanotubes Sliding against Diamond. 2009, 7, 4–11 DOI: 10.1016/j.carbon.2009.02.020.

(73) Girifalco, L.; Hodak, M.; Lee, R. Carbon Nanotubes, Buckyballs, Ropes, and a

142 Universal Graphitic Potential. Phys. Rev. B 2000, 62.

(74) Properties, C. Dispersion of Carbon Nanotubes: Mixing, Sonication, Stabilization, and Composite Properties. 2012, 275–295 DOI: 10.3390/polym4010275.

(75) Balasubramanian, K.; Burghard, M. Chemically Functionalized Carbon Nanotubes. Small 2005, 1 (2), 180–192 DOI: 10.1002/smll.200400118.

(76) Zhou, F.; Liang, Y.; Liu, W. Ionic Liquid Lubricants : Designed Chemistry for Engineering Applications. 2009 DOI: 10.1039/b817899m.

(77) Qu, J.; Truhan, J. J.; Dai, S.; Luo, H.; Blau, P. J. Ionic Liquids with Ammonium Cations as Lubricants or Additives. 2008, 22 (3) DOI: 10.1007/s11249-006-9081-0.

(78) Renguo, L.; Hidetaka, N.; Kimihiro, K.; Tomoo, K.; Shigeyuki, M. Effect of Lubricant Additives on Tribochemical Decomposition of Hydrocarbon Oil on Nascent Steel Surfaces. 2010, 53 (1), 55–60.

(79) Jim, A. Imidazolium Ionic Liquids as Additives of the Synthetic Ester Propylene Glycol Dioleate in Aluminium – Steel Lubrication. 2008, 265, 787–798 DOI: 10.1016/j.wear.2008.01.009.

(80) Cai, M.; Liang, Y.; Yao, M.; Xia, Y.; Zhou, F.; Liu, W. Imidazolium Ionic Liquids As Antiwear and Antioxidant Additive in Poly(ethylene Glycol) for Steel/Steel Contacts. ACS Appl. Mater. Interfaces 2010, 2 (3), 870–876 DOI: 10.1021/am900847j.

(81) Zhou, Y.; Graham, T. W.; Luo, H.; Leonard, D. N.; Qu, J. Ionic Liquids Composed of Phosphonium Cations and Organophosphate , Carboxylate , and Sulfonate Anions as Lubricant Antiwear Additives. Langmuir 2014, 30 (44), 13301–13311 DOI: 10.1021/la5032366.

(82) Wang, J.; Chu, H.; Li, Y. Why Single-Walled Carbon Nanotubes Can Be Dispersed in Imidazolium-Based Ionic Liquids. 2 (12), 2540–2546 DOI: 10.1021/nn800510g.

(83) Bellayer, B. S.; Gilman, J. W.; Eidelman, N.; Bourbigot, S.; Flambard, X.; Fox, D. M.; Long, H. C. De; Trulove, P. C. Preparation of Homogeneously Dispersed Multiwalled Carbon Nanotube / Polystyrene Nanocomposites via Melt Extrusion Using Trialkyl Imidazolium Compatibilizer. Adv. Funct. Mater. 2005, 15, 910–916 DOI: 10.1002/adfm.200400441.

(84) Fukushima, T.; Aida, T. Ionic Liquids for Soft Functional Materials with Carbon Nanotubes. 2007, 5048–5058 DOI: 10.1002/chem.200700554.

(85) Yu, B.; Liu, Z.; Zhou, F.; Liu, W.; Liang, Y. A Novel Lubricant Additive Based on Carbon Nanotubes for Ionic Liquids. 2008, 62, 2967–2969 DOI:

143 10.1016/j.matlet.2008.01.128.

(86) Wang, B.; Wang, X.; Lou, W.; Hao, J. Rheological and Tribological Properties of Ionic Liquid-Based Nanofluids Containing Functionalized Multi-Walled Carbon Nanotubes. 2010, 8749–8754.

(87) Lu, Y. A Combinatorial Approach for Automotive Friction Materials: Effects of Ingredients on Friction Performance. Compos. Sci. Technol. 2006, 66 (3–4), 591– 598 DOI: 10.1016/j.compscitech.2005.05.032.

(88) Gao, H.; Barber, G. C. Engagement of a Rough, Lubricated and Grooved Disk Clutch Ith a Porous Deformable Paper-Based Friction Material. Tribol. Trans. 2002, 45 (4), 464–470 DOI: 10.1080/10402000208982575.

(89) Zhang, X.; Li, K. Z.; Li, H. J.; Fu, Y. W.; Fei, J. Tribological and Mechanical Properties of Glass Fiber Reinforced Paper-Based Composite Friction Material. Tribol. Int. 2014, 69, 156–167 DOI: DOI 10.1016/j.triboint.2013.08.003.

(90) Sancaktar, E.; Bakan, M.; Chang, I. T.; Satam, S.; Farahati, R. Measurement of Adsorption Energy between a Solid Adsorbent and a Liquid Adsorbate Using Differential Scanning Calorimetry. Polym. Test. 2016, 56, 185–191 DOI: 10.1016/j.polymertesting.2016.10.011.

(91) Holgerson, M. Apparatus for Measurement of Engagement Characteristics of a Wet Clutch. Wear 1997, 213 (1–2), 140–147 DOI: 10.1016/S0043-1648(97)00202-0.

(92) Campana, M.; Teichert, A.; Clarke, S.; Steitz, R.; Webster, J. R. P.; Zarbakhsh, A. Surfactant Adsorption at the Metal-Oil Interface. Langmuir 2011, 27, 6085–6090.

(93) Lee, Y. S.; Wetzel, E. D.; Wagner, N. J. The Ballistic Impact Characteristics of Kevlar Woven Fabrics Impregnated with a Colloidal Shear Thickening Fluid. 2003, 8, 2825–2833.

(94) Cai, P.; Li, Z.; Wang, T.; Wang, Q. Effect of Aspect Ratios of Aramid Fiber on Mechanical and Tribological Behaviors of Friction Materials. Tribol. Int. 2015, 92, 109–116 DOI: 10.1016/j.triboint.2015.05.024.

(95) Marklund, P. Wet Clutch Tribological Performance Optimization Methods. Ph.D. Dissertation; Department of Applied Physics and Mechanical Engineering Division of Machine Elements, Luleå University of Technology, Lulea University of Technology, 2008.

(96) Bijwe, J.; Aranganathan, N.; Sharma, S.; Dureja, N.; Kumar, R. Nano-Abrasives in Friction Materials-Influence on Tribological Properties. Wear 2012, 296, 693–701 DOI: 10.1016/j.wear.2012.07.023.

144 (97) Gallagher, J.; Dougherty, P. Aramid Containing Friction Materials, 1981.

(98) Ito, K.; Barker, K. A.; Kubota, M.; Yoshida, S. Designing Paper Type Wet Friction Material for High Strenght and Durability. SAE Paper. 1998.

(99) Kumar, M.; Bijwe, J. Optimized Selection of Metallic Fillers for Best Combination of Performance Properties of Friction Materials: A Comprehensive Study. Wear 2013, 303 (1–2), 569–583 DOI: 10.1016/j.wear.2013.03.053.

(100) Kumar, M.; Bijwe, J. Composite Friction Materials Based on Metallic Fillers: Sensitivity of ?? To Operating Variables. Tribol. Int. 2011, 44 (2), 106–113 DOI: 10.1016/j.triboint.2010.09.013.

(101) Spurr, R. T. Fillers in Friction Materials. Wear 1972, 22 (3), 367–372 DOI: 10.1016/0043-1648(72)90395-X.

(102) Nyman, P.; M??ki, R.; Olsson, R.; Ganemi, B. Influence of Surface Topography on Friction Characteristics in Wet Clutch Applications. Wear 2006, 261 (1), 46–52 DOI: 10.1016/j.wear.2005.09.020.

(103) Berman, A. D.; Ducker, W. A.; Israelachvili, J. N. Origin and Characterization of Different Stick - Slip Friction Mechanisms †. 1996, 7463 (1), 4559–4563.

(104) Ruths, M.; Ohtani, H.; Greenfield, M. L.; Granick, S. Exploring the “ Friction Modifier ” Phenomenon : Nanorheology of N -Alkane Chains with Polar Terminus Dissolved in N -Alkane Solvent. 1999, 6, 207–214.

(105) Greenfield, M. L.; Ohtani, H. Packing of Simulated Friction Modifier Additives under Confinement. Langmuir 2005, 21 (16), 7568–7578 DOI: 10.1021/la046862l.

(106) Fatima, N.; Minami, I.; Holmgren, A.; Marklund, P.; Larsson, R. Adsorption of ATF Additives on Wet Clutch Friction Interfaces under Water Contaminated Lubricant Conditions.

(107) Chen, Y.-F.; Lam, R.; Dong, F.; Chavdar, B. Friction Material with Friction Modifying Layer. Patent, 2005.

(108) Matsumoto, T. Influence of Paper-Based Friction Material Visco-Elasticity on the Performance of a Wet Clutch. In SAE International; 1997.

(109) Matsumoto, T. The Influence of Paper-Based Friction Material Porosity on the Performance of a Wet Clutch.pdf. Conf. Proc. 1994 DOI: 10.4271/941032.

(110) Ho, S. C.; Lin, J. H. C.; Ju, C. P. Effect of Fiber Addition on Mechanical and Tribological Properties of a Copper/phenolic-Based Friction Material. Wear 2005, 258 (5–6), 861–869 DOI: 10.1016/j.wear.2004.09.050.

145 (111) Kim, S. J.; Cho, M. H.; Lim, D.-S.; Jang, H. Synergistic Effects of Aramid Pulp and Potassium Titanate Whiskers in the Automotive Friction Material. Wear 2001, 251 (1–12), 1484–1491 DOI: 10.1016/S0043-1648(01)00802-X.

(112) Guan, Q. F.; Li, G. Y.; Wang, H. Y.; An, J. Friction-Wear Characteristics of Carbon Fiber Reinforced Friction Material. J. Mater. Sci. 2004, 39 (2), 641–643 DOI: 10.1023/B:JMSC.0000011520.48580.fc.

(113) Yu, S.; Zheng, W.; Yu, W.; Zhang, Y.; Jiang, Q.; Zhao, Z. Formation Mechanism of β-Phase in PVDF/CNT Composite Prepared by the Sonication Method. Macromolecules 2009, 42 (22), 8870–8874 DOI: 10.1021/ma901765j.

(114) Jiang, J.; Liu, J.; Zhou, W.; Zhu, J.; Huang, X.; Qi, X.; Zhang, H.; Yu, T. CNT/Ni Hybrid Nanostructured Arrays: Synthesis and Application as High-Performance Electrode Materials for Pseudocapacitors. Energy Environ. Sci. 2011, 4 (12), 5000 DOI: 10.1039/c1ee02293h.

(115) CHEN, H. The Application of CNT/Nafion Composite Material to Low Humidity Sensing Measurement. Sensors Actuators B Chem. 2005, 104 (1), 80–84 DOI: 10.1016/j.snb.2004.04.105.

(116) Muzakkir, S. M.; Lijesh, K. P.; Hirani, H. Carbon Nanotubes as Solid Lubricant Additives for Anti- Wear Performance Enhancement under Mixed Lubrication Conditions. www.ijmer.com 2015, 5.

(117) Salvetat, J.-P.; Bonard, J.-M.; Thomson, N. H.; Kulik, A. J.; Forró, L.; Benoit, W.; Zuppiroli, L. Mechanical Properties of Carbon Nanotubes. Appl. Phys. A 69 (3), 255–260 DOI: 10.1007/s003390050999.

(118) Kim, J. A.; Seong, D. G.; Kang, T. J.; Youn, J. R. Effects of Surface Modification on Rheological and Mechanical Properties of CNT/epoxy Composites. Carbon N. Y. 2006, 44 (10), 1898–1905 DOI: 10.1016/j.carbon.2006.02.026.

(119) Li, J.; Stevens, R.; Delzeit, L.; Ng, H. T.; Cassell, A.; Han, J.; Meyyappan, M. Electronic Properties of Multiwalled Carbon Nanotubes in an Embedded Vertical Array. Appl. Phys. Lett. 2002, 81 (5), 910–912 DOI: 10.1063/1.1496494.

(120) Tsyboulski, D. A.; Bachilo, S. M.; Weisman, R. B. Versatile Visualization of Individual Single-Walled Carbon Nanotubes with Near-Infrared Fluorescence Microscopy. Nano Lett. 2005, 5 (5), 975–979 DOI: 10.1021/nl050366f.

(121) Hwang, H. J.; Jung, S. L.; Cho, K. H.; Kim, Y. J.; Jang, H. Tribological Performance of Brake Friction Materials Containing Carbon Nanotubes. 2010, 268, 519–525 DOI: 10.1016/j.wear.2009.09.003.

(122) Zhang, W.; Ma, G. J.; Wu, C. W. ANTI-FRICTION , WEAR-PROOF AND SELF- 146 LUBRICATION APPLICATION OF CARBON NANOTUBES. 2014, 36.

(123) Ni, B.; Sinnott, S. B. Tribological Properties of Carbon Nanotube Bundles Predicted from Atomistic Simulations. 2001, 487, 87–96.

(124) Vaisman, L.; Wagner, H. D.; Marom, G. The Role of Surfactants in Dispersion of Carbon Nanotubes. 2007, 130 (2006), 37–46 DOI: 10.1016/j.cis.2006.11.007.

(125) Chen, S.; Wu, G.; Sha, M.; Huang, S. Transition of Ionic Liquid [ Bmim ][ PF 6 ] from Liquid to High-Melting-Point Crystal When Confined in Multiwalled Carbon Nanotubes. 2007, 2416–2417.

(126) Hameed, N.; Church, J. S.; Salim, N. V.; Hanley, T. L.; Amini, A.; Fox, B. L. Dispersing Single-Walled Carbon Nanotubes in Ionic Liquids: A Quantitative Analysis. RSC Adv. 2013, 3 (43), 20034 DOI: 10.1039/c3ra42488j.

(127) Sun, J.; Howlett, P. C.; Macfarlane, D. R.; Lin, J.; Forsyth, M. Electrochimica Acta Synthesis and Physical Property Characterisation of Phosphonium Ionic Liquids Based on P ( O ) 2 ( OR ) 2 − and P ( O ) 2 ( R ) 2 − Anions with Potential Application for Corrosion Mitigation of Magnesium Alloys. 2008, 54, 254–260 DOI: 10.1016/j.electacta.2008.08.020.

(128) A. K. Bhatnagar, *; Savita Kaul; V. K. Chhibber, and; Gupta, A. K. HFRR Studies on Methyl Esters of Nonedible Vegetable Oils. 2006 DOI: 10.1021/EF0503818.

(129) Hamrock, B. J.; Dowson, D. Film Thickness for Different Regimes of Fluid-Film Lubrication. NASA Tech. Memo. 81700 1983.

(130) Nishida, T.; Tashiro, Y.; Yamamoto, M. Physical and Electrochemical Properties of 1-Alkyl-3-Methylimidazolium Tetrafluoroborate for Electrolyte. J. Fluor. Chem. 2003, 120 (2), 135–141 DOI: 10.1016/S0022-1139(02)00322-6.

(131) Ngo, H. L.; LeCompte, K.; Hargens, L.; McEwen, A. B. Thermal Properties of Imidazolium Ionic Liquids. Thermochim. Acta 2000, 357–358, 97–102 DOI: 10.1016/S0040-6031(00)00373-7.

(132) Grady, B. P.; Pompeo, F.; Shambaugh, R. L.; Resasco, D. E. Nucleation of Polypropylene Crystallization by Single-Walled Carbon Nanotubes. J. Phys. Chem. B 2002, 106 (23), 5852–5858 DOI: 10.1021/jp014622y.

(133) Jin, J.; Song, M.; Pan, F. A DSC Study of Effect of Carbon Nanotubes on Crystallisation Behaviour of Poly(ethylene Oxide). Thermochim. Acta 2007, 456 (1), 25–31 DOI: 10.1016/j.tca.2007.02.003.

(134) Qu, J.; Barnhill, W. C.; Luo, H.; Meyer, H. M.; Leonard, D. N.; Landauer, A. K.; Kheireddin, B.; Gao, H.; Papke, B. L.; Dai, S. Synergistic Effects Between

147 Phosphonium-Alkylphosphate Ionic Liquids and Zinc Dialkyldithiophosphate (ZDDP) as Lubricant Additives. Adv. Mater. 2015, 27 (32), 4767–4774 DOI: 10.1002/adma.201502037.

(135) Rastogi, R.; Kaushal, R.; Tripathi, S. K.; Sharma, A. L.; Kaur, I.; Bharadwaj, L. M. Journal of Colloid and Interface Science Comparative Study of Carbon Nanotube Dispersion Using Surfactants. J. Colloid Interface Sci. 2008, 328 (2), 421–428 DOI: 10.1016/j.jcis.2008.09.015.

(136) Yu, B.; Ma, C. Ionic Liquid Modified Multi-Walled Carbon Nanotubes as Lubricant Additive. Tribol. Int. 2015, 38–42 DOI: 10.1016/j.triboint.2014.07.019.

148