development of polymer resin-based wet friction sheet

DEVELOPMENT OF POLYMER RESIN-BASED WET FRICTION SHEET

MATERIALS AND UNDERSTANDING THEIR INTERACTIONS WITH

AUTOMATIC TRANSMISSION FLUIDS

A Dissertation

Presented to

The Graduate Faculty of The University of Akron

In Partial Fulfillment

of the Requirements for the Degree

Doctor of Philosophy

Murat Bakan

August, 2015

ii

DEVELOPMENT OF POLYMER RESIN-BASED WET FRICTION SHEET

MATERIALS AND UNDERSTANDING THEIR INTERACTIONS WITH

AUTOMATIC TRANSMISSION FLUIDS

Murat Bakan

Dissertation

Approved: Accepted:

______________________________ ______________________________

Advisor Department Chair

Dr. Erol Sancaktar Dr. Sadhan C. Jana

______________________________ ______________________________

Committee Member Dean of the College

Dr. Robert A. Weiss Dr. Eric J. Amis

______________________________ ______________________________

Committee Member Interim Dean of the Graduate School

Dr. Younjin Min Dr. Chand Midha

______________________________ ______________________________

Committee Member Date

Dr. Chrys Wesdemiotis

______________________________

Committee Member

Dr. Gary L. Doll

______________________________

Committee Member

Dr. Rashid Farahati

iii

ABSTRACT

As a very important component of automatic transmissions, torque converters are

one of the most complicated parts in vehicles. Wet friction materials which are present in

torque converter clutches attract attention because improvements made on these

composite materials lead to better fuel efficiency and driving comfort. It is important to

understand the possible interactions between wet friction materials and automatic

transmission fluid (ATF) as the efficiency of the system highly depends on them.

In the first part of this study, we report a novel method to measure the adsorption

energy between a liquid adsorbate, ATF, and a solid adsorbent, wet friction material,

using differential scanning calorimetry. Studies involving different adsorbents i.e. a wet

friction material and its individual ingredients and different adsorbates i.e. a commercial

ATF, base oil, and custom made oils were used for the development of the method.

Besides, the measurements were useful for understanding possible types of

intermolecular interactions occurring during adsorption.

Secondly, excimer laser treatment was performed on the wet friction material

together with its fiber components. It was shown that the adsorption energies of each

adsorbent increased as a result of the treatment. Adsorption energy measurements were

also performed on some minerals/clays which could be used as fillers in wet friction

materials. Friction performances of some of these fillers, which were tested using an

SAE#2 tester, showed a correlation with their adsorption energies.

iv

In order to provide better understanding for the performance of wet friction

materials, different friction modifiers were used to study the friction performance and

surface energies after adsorption. In case of both the friction modifier concentration and

structure, the friction performances of the materials were found to correlate with their

contact angles, contact angle hysteresis, and surface energies.

In the last part of the study, polymer resin-based, paper-type wet friction materials

were made using different fiber/filler ratios with the most commonly used ingredients in

the industry. The effect of these ratios on the mechanical properties and porosities of the

materials were characterized using different techniques. Once the tribological

performances of these materials were evaluated, it was noted that the material consisting

of 50/50 fiber/filler had the optimum composition considering the mechanical and

tribological performance of the materials.

v

ACKNOWLEDGEMENTS

I owe my deepest gratitude to my academic advisor Dr. Erol Sancaktar for the

opportunity he created for me to finish my Ph.D. degree. Besides, his support,

encouragement, and guidance throughout this study are very valuable for me.

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

work on my dissertation at LuK USA LLC, enlightening me with his knowledge and

experience, and spending countless hours to discuss about the projects we work on.

In addition, I would like to express my sincere thanks to Dr. Robert A. Weiss for

the support and guidance he showed me during my Ph.D. life and for being in my

dissertation committee.

I also would like to appreciate to my other committee members, Dr. Younjin Min,

Dr. Chrys Wesdemiotis, and Dr. Gary L. Doll.

This journey wouldn’t come to an end without the support of others. My brother

Dr. Ahmet Bakan and my friend Dr. Emre Unsal were always with me when needed. I

am also grateful to my sister, my parents, and other friends for the love and

understanding they showed to me.

I am thankful to LuK USA LLC and The Lubrizol Corporation for their financial

support on the project.

vi

TABLE OF CONTENTS

Page

LIST OF TABLES .............................................................................................................. x

LIST OF FIGURES ........................................................................................................... xi

INTRODUCTION ........................................................................................................ 1

1.1. Tribology................................................................................................................. 1

1.1.1. History of Tribology ................................................................................... 1

1.1.2. Stribeck Curve ............................................................................................ 2

1.1.3. Friction under Sliding Conditions ............................................................... 6

1.2. Wet Friction Materials ............................................................................................ 8

1.2.1. History of Wet Friction Materials ............................................................... 8

1.2.2. Wet Friction Materials .............................................................................. 10

1.2.3. Wet Friction Material Ingredients ............................................................. 11

1.2.4. Porosity ..................................................................................................... 13

1.2.5. Visco-Elasticity ......................................................................................... 15

1.3. Automatic Transmission Fluid .............................................................................. 16

1.3.1. History of Automatic Transmission Fluid ................................................ 16

1.3.2. Automatic Transmission Fluid .................................................................. 17

1.3.3. ATF Additives .......................................................................................... 18

1.3.3.1. Friction Modifiers ............................................................................ 18

1.3.3.2. Detergents ........................................................................................ 19

1.3.3.3. Dispersants ....................................................................................... 19

1.3.3.4. Viscosity Modifiers .......................................................................... 20

1.3.3.5. Antiwear (AW) - Extreme pressure (EP) agents .............................. 20

vii

1.3.3.6. Corrosion Inhibitors ......................................................................... 21

1.3.3.7. Antifoaming Agents ......................................................................... 21

1.3.3.8. Pour-point depressants ..................................................................... 22

1.3.3.9. Antioxidants ..................................................................................... 22

1.4. Adsorption............................................................................................................. 22

1.4.1. Adsorption of ATF on Wet Friction Materials ......................................... 23

1.5. Research Objectives .............................................................................................. 26

MEASUREMENT OF THE ADSORPTION ENERGY BETWEEN A SOLID

ADSORBENT AND A LIQUID ADSORBATE USING DIFFERENTIAL SCANNING

CALORIMETRY .............................................................................................................. 28

2.1. Introduction ........................................................................................................... 29

2.2. Experimental ......................................................................................................... 32

2.2.1. Materials ................................................................................................... 32

2.2.2. Characterization Techniques ..................................................................... 33

2.2.3. Procedure .................................................................................................. 33

2.3. Results and Discussion ......................................................................................... 35

2.3.1. Thermogravimetric Analysis .................................................................... 36

2.3.2. Differential Scanning Calorimetry ............................................................ 39

2.3.3. Adsorption Energy Measurements ............................................................ 39

2.4. Conclusions ........................................................................................................... 46

IMPROVING THE ADSORPTION CAPACITY OF PAPER-BASED WET

FRICTION MATERIALS USING EXCIMER LASER .................................................. 48

3.1. Introduction ........................................................................................................... 48

3.2. Experimental ......................................................................................................... 51

3.2.1. Materials ................................................................................................... 51

3.2.2. Methods..................................................................................................... 51


viii

3.4. Conclusions ........................................................................................................... 58

CORRELATION BETWEEN THE ADSORPTION ENERGIES AND FRICTION

PERFORMANCES OF VARIOUS FILLERS USED IN WET FRICTION

MATERIALS. ................................................................................................................... 59

4.1. Introduction ........................................................................................................... 59

4.2. Experimental ......................................................................................................... 61

4.2.1. Materials ................................................................................................... 61

4.2.2. Methods..................................................................................................... 61


4.4. Conclusions ........................................................................................................... 71

EFFECT OF MONOLAYER DENSITY AND STRUCTURE OF FRICTION

MODIFIERS ON WET FRICTION PERFORMANCE................................................... 72

5.1. Introduction ........................................................................................................... 72

5.2. Experimental ......................................................................................................... 75

5.2.1. Materials ................................................................................................... 75

5.2.2. Surface Treatment of Steel Plates and Friction Material .......................... 76

5.2.3. Contact Angle Measurements ................................................................... 76

5.2.4. SAE#2 Test ............................................................................................... 76

5.2.5. Fourier Transform Infrared (FT-IR) Spectroscopy ................................... 77


5.3.1. Effect of Friction Modifier Concentration ................................................ 77

5.3.2. Effect of Friction Modifier Structure ........................................................ 88

5.4. Conclusions ........................................................................................................... 95

EFFECT OF FIBER TYPE AND FIBER/FILLER RATIO ON THE

CHARACTERISTICS OF WET FRICTION MATERIALS ........................................... 97

6.1. Introduction ........................................................................................................... 98

6.2. Experimental ......................................................................................................... 99

ix

6.2.1. Handsheet Preparation ............................................................................ 100

6.2.2. Characterization Methods ....................................................................... 101

6.3. Results and Discussion ....................................................................................... 102

6.4. Conclusions ......................................................................................................... 116

CONCLUSIONS ................................................................................................... 118

BIBLIOGRAPHY ........................................................................................................... 121

APPENDIX ..................................................................................................................... 133

x

LIST OF TABLES

Table Page

1-1. Different types of wet friction materials. 37 ............................................................... 10

1-2. Examples of ingredients used in wet friction material production. 34,40 .................... 12

2-1. Adsorption energies for friction paper and its ingredients. ....................................... 42

2-2. Properties of the friction paper ingredients. .............................................................. 44

3-1. Adsorption energy results for the friction material, synthetic fibers, and organic

fibers. ................................................................................................................................ 57

4-1. Adsorption energies and particle size values for the fillers ....................................... 69

5-1. Contact angle and hysteresis results for neat and stearic acid treated friction materials

........................................................................................................................................... 80

5-2. Contact angle, contact angle hysteresis and surface free energies for steel samples

with different surface treatments using water. .................................................................. 89

6-1. Burst strength measured by Mullen tester, air permeability measured by Gurley

tester, water contact angles, and oxygen atom concentration of different handsheets. .. 104

6-2. Tensile and shear properties of raw and saturated (sat.) handsheets. ...................... 107

xi

LIST OF FIGURES

Figure Page

1-1. Stribeck curve showing different lubrication regimes. 1- Boundary lubrication, 2-

Mixed region, 3- Hydrodynamic friction. 10 ....................................................................... 3

1-2. Coefficient of friction vs. velocity curves for friction materials and automatic

transmission fluid. ............................................................................................................... 5

1-3. Representation of a SAE#2 clutch engagement curves. .............................................. 6

1-4. Development history of materials used in wet friction applications. 34....................... 9

1-5. Viscosity vs. temperature curves for two lubricants with different viscosity index

(VI) .................................................................................................................................... 20

1-6. Monolayers of friction modifiers (stearic acid) adsorbed on a surface and base oil

molecules fill the spaces between the friction modifiers. ................................................. 25

2-1. Adsorption energy contributions of ATF components based on theoretical

calculations. ...................................................................................................................... 32

2-2. DSC cell configuration before starting the adsorption energy measurements. ATF is

placed on the sample platform on the right side, and the friction paper is placed on the

reference platform on the left side. ................................................................................... 34

2-3. Mixing of adsorbate and adsorbent at -140 °C. ......................................................... 35

2-4. TGA curves of a) ATF and b) friction paper and its components. ............................ 37

2-5. DSC thermograms of a) ATF, where two different measurements show the identical

melting range, and b) friction paper and its ingredients. .................................................. 38

2-6. Adsorption energy for wet friction paper. ................................................................. 40

2-7. Adsorption energies for a) diatomaceous earth, b) synthetic fiber, c) organic fiber,

and d) resin binder. ........................................................................................................... 42

xii

2-8. Adsorption energies for friction material with different adsorbates (I would like to

thank to Sayali Satam for obtaining some of this data). ................................................... 46

3-1. Adsorption energy measurements for neat and laser treated wet friction materials.

The area between the 1st and 2nd curve corresponds to the adsorption energy between

ATF and the friction material. .......................................................................................... 53

3-2. Adsorption energy measurements for neat and laser treated synthetic fiber sheets.

The area between the 1st and 2nd curve corresponds to the adsorption energy between

ATF and the synthetic fiber. ............................................................................................. 55

3-3. Adsorption energy measurements for neat and laser treated organic fiber sheets. The

area between the 1st and 2nd curve corresponds to the adsorption energy between ATF

and the organic fibers. ....................................................................................................... 57

4-1. DSC thermograms for filler 1 showing the energy of adsorption between the first and

second heating ramps. ....................................................................................................... 62











4-7. Coefficient of friction vs. sliding speed curves for filler 2 and filler 6 at 0.4 and 3

MPa surface pressure and 40 °C fluid temperature. ......................................................... 70

4-8. Coefficient of friction vs. sliding speed curves for filler 2 and filler 6 at 0.4 and 3

MPa surface pressure and 90 °C fluid temperature. ......................................................... 70

5-1. Contact angle images for friction material a) at 0°, and b) at 90° (tilted stage), and

friction material treated with high concentration stearic acid c) at 0°, and d) at 90° (tilted

stage). ................................................................................................................................ 79

5-2. Infrared spectroscopy of neat and stearic acid treated friction materials. ................. 80

xiii

5-3. Contact angle and hysteresis results for steel samples. ............................................. 82

5-4. Contact angle and contact angle hysteresis results for heat treated steel samples. ... 83

5-5. Friction coefficient (μ) vs. sliding speed curves for 0.3 wt.% stearic acid in base oil

at different temperatures: A) 40 °C, B) 90 °C, and C) 120 °C. ........................................ 86

5-6. Friction coefficient (μ) vs. sliding speed curves for 0.5 wt. % stearic acid in base oil


5-7. The ratio of coefficient of friction at 1 rpm to 40 rpm for different lubricants at

different temperatures and pressures. ............................................................................... 88

5-8. Friction coefficient (μ) vs. sliding speed curves for neat base oil at different

temperatures: A) 40 °C, B) 90 °C, and C) 120 °C. ........................................................... 91

5-9. Friction coefficient (μ) vs. sliding speed curves for 0.5 wt.% oleic acid in base oil at

different temperatures: A) 40 °C, B) 90 °C, and C) 120 °C. ............................................ 92

5-10. Friction coefficient (μ) vs. sliding speed curves for 0.5 wt.% linoleic acid in base oil


5-11. Suggested intermolecular interactions between a) steel and friction material and b)

friction modifier adsorbed steel and friction material surfaces. ....................................... 95

6-1. Burst strength measured by Mullen tester and air permeability measured by Gurley

tester for handsheets consisting of cellulose fibers. ........................................................ 104

6-2. Tensile strength for raw and saturated handsheets consisting of cellulose fibers. .. 108

6-3. Tensile Modulus for raw and saturated handsheets consisting of cellulose fibers. 108

6-4. Water contact angle values of resin-saturated handsheets. ...................................... 112

6-5. SEM images of A) 100 % aramid fibers, B) 50 % aramid, 50 % diatomaceous earth,

C) 100 % cellulose, D) 50 % cellulose, 50 % diatomaceous earth, E) 40 % cellulose, 60

% diatomaceous earth, and F) 20 % cellulose, 80 % diatomaceous earth handsheets in

raw form (no resin saturation)......................................................................................... 114

6-6. Coefficient of friction vs. sliding speed for handsheets before break-in at 90 °C

temperature and 2960 kPa surface pressure. ................................................................... 115

6-7. Coefficient of friction vs. sliding speed for handsheets after break-in at 90 °C

temperature and 2960 kPa surface pressure. ................................................................... 115

1

INTRODUCTION

1.1. Tribology

Tribology is the study of interacting surfaces moving relative to each other.

Although it is a newly developed scientific field, the origin of tribology goes to ancient

times because friction always existed. Not only are the interactions between solid layers,

but also between solids and liquids and/or gases are studied in tribology. 1

1.1.1. History of Tribology

Friction is the resisting force against a movement and static friction is often

referred to as the force needed to start the motion. Even in very old times people did

attempts to reduce the friction because it was causing more energy consumption. The first

lubricant used in the history could be bitumen as its origin goes back to 6000 B.C. Other

lubricants were used in Egypt, later around 2500 B.C., during the construction of

pyramids. Leonardo da Vinci is the first person to define the classical rules of friction but

he couldn’t publish his work. In 1699, Guillaume Amontons rediscovered the laws of dry

2

friction confirming that friction force is directly proportional to the applied load and is

independent of the contact area. Leonhard Euler identified the difference in static and

kinetic friction forces in 1750. Charles Augustin de Coulomb later reported that kinetic

friction is independent of sliding velocity. 2,3,4

1.1.2. Stribeck Curve

Friction on lubricated surfaces differs from dry friction in the sense that

coefficient of friction is no longer independent of the load nor the speed. The

characteristics of friction materials are mainly discussed in terms of ηV/P values, where η

is the viscosity, V is the sliding speed, and P is the pressure. This product is usually

referred to as Sommerfield number. 5 Plots of μ vs. ηV/P are called Stribeck Curves

(Figure 1-1). The first region in the Stribeck curve is the boundary lubrication regime

where there is a direct contact between the asperities of each surface with the average

lubricant film thickness being less than the average surface roughness in this regime.

Therefore, the coefficient of friction is high in boundary lubrication and the oil viscosity

doesn’t play an important role. 6,7 Chemically or physically reactive components of the

lubricants interact with the contacting surfaces to form a highly resistant films which

supports the load and prevent major wear. The formation of this type of boundary film

depends strongly on its capacity to be adsorbed on friction surfaces. 8 As the surfaces

separate from each other (mixed region), friction coefficient starts to drop because the

fluid film thickness increases and provides more effective lubrication. In the third region,

there is a full fluid film on the surfaces and no surface contact. The friction is mostly

3

determined by the internal fluid friction in which fluid viscosity plays a significant role.

In hydrodynamic lubrication regime, friction increases with increasing speed because of

the fluid drag. Besides, lower friction coefficient can be obtained with a lubricant with

lower viscosity. 9

Figure 1-1. Stribeck curve showing different lubrication regimes. 1- Boundary

lubrication, 2- Mixed region, 3- Hydrodynamic friction. 10

Although Stribeck curve is commonly used for the study of friction in lubricated

surfaces, it may not be completely applicable to paper-based wet friction materials. In

Stribeck curve, it is commonly considered that boundary lubrication occurs at low sliding

speeds and the friction coefficient either decreases or remains constant with increasing

speed in boundary lubrication regime. 11 Because of the high porosity of wet friction

materials, boundary lubrication regime dominates the friction characteristics because a

fluid film cannot develop easily to protect surfaces from contact. Moreover, the drop of

4

friction coefficient with increasing speed is not desired and usually not observed for wet

friction materials. It is always preferred to have dynamic friction higher than static

friction in order to have good friction characteristics. 12 Therefore, the graph given in

Figure 1-2 is more commonly used for the investigation of wet friction materials. We can

split this graph into two regions as low speed region (might also be referred to as static

friction) and high speed region (might also be referred to as dynamic friction). The red

curve given with a negative slope is generally obtained by non-polar lubricants such as

base oil. The green curve given with a positive slope is due to the presence of polar

additives such as fatty acids, fatty amines, or other types of surfactants in the lubricant.

13,14 According to Figure 1-2, the chemistry of the lubricant or additive molecules mostly

affects the low speed region. However, use of different types of lubricants doesn’t affect

the dynamic friction coefficient as significantly as the static friction. Therefore, it is

important to have amphiphilic molecules in the lubricant to have low static friction. In

order to modify the dynamic friction coefficient, however, it is more important to change

the material porosity as it is already proven that fluid permeability has the most

significant effect on dynamic friction of wet friction materials. 15

5

Figure 1-2. Coefficient of friction vs. velocity curves for friction materials and automatic

transmission fluid.

The coefficient of friction obtained from wet clutch tests is calculated according

to the following formula:

𝜇 =

𝑇

𝑃𝑅𝐴𝑁

(1)

in which μ is the coefficient of friction, T is the torque, P is the surface pressure, R is the

radius of the clutch plate, A is the friction surface area, and N is the number of friction

surfaces. 16 The relative changes of some these parameters during clutch engagement are

shown in Figure 1-3. The engagement starts after the application of pressure on to the

clutch at a relatively high sliding speed. Transition occurs initially from hydrostatic to

hydrodynamic, and then to boundary lubrication regime during the engagement. For a

smooth engagement, there should not be a torque spike at the end of the torque curve,

which is often referred to as hunting or rooster tail (shown with the purple color). 17,18

When rooster tail is observed, it is often felt as shudder during the engagement. Shudder

6

is the increase of friction coefficient with decreasing sliding speed and it causes

vibrations and noise in the system. It may be caused by either the friction material or the

automatic transmission fluid. 19,20,21,22

Figure 1-3. Representation of a SAE#2 clutch engagement curves.

1.1.3. Friction under Sliding Conditions

Surface interactions, elastic contacts, and viscous drag are the factors affecting the

friction at very low loads (i.e., high Sommerfield numbers, ηV/P). Plastic deformation

will occur after the load is increased. Therefore, at low Sommerfield numbers, friction

will be determined in terms of the energy needed to deform the asperities.

Based on a general overview of the Stribeck curve, it is known that the lubricating

film thickness decreases significantly at very low speeds (boundary lubrication). As

transition occurs from boundary lubrication to mixed region, friction coefficient drops

significantly. 23,24 This region is never of interest for the wet friction material

Fri

ctio

n

Time

Sliding Speed Applied Pressure Torque High μ0

7

manufacturers, because it may be considered a failure to have friction at mixed region for

a wet friction material. Boundary lubrication regime is the main focus of the

manufacturers.

An interesting phenomenon associated with the wet friction materials is the

increase of the friction coefficient with increasing speed. This situation is opposite of

what is seen in any other systems or in nature. However, it is also observed only with the

presence of friction modifiers in a lubricant for wet friction materials. Researchers have

proposed mechanisms to explain this situation, as summarized below.

Previously it was believed that there was a balance between the construction and

destruction of monolayers of friction modifiers during boundary lubrication regime.

However, destruction rate increases at higher sliding speeds, so the friction increases. The

experiments done with limited amount of friction modifiers showed that the theory was

insufficient to explain the mechanism associated with such observations. 25,26 There were

also other theories regarding additional speed dependent drag forces which become more

effective at higher speeds or some mathematical descriptions which relate friction to the

interfacial shear stress. 27,28 However, the true mechanism is based on the change of

molecular conformations of hydrocarbon parts of the friction modifiers. The ordered

tightly packed monolayers at the surface are subjected to conformational defects due to

shear forces. The terminal gauche defects of the chain ends cause higher energy

requirements. The increasing repulsive forces at higher speeds cause molecules to have

more serious rotational, vibrational, and translational defects, so that friction increases.

28,29,30,31,32 Molecular dynamics studies with stearic acid and oleic acid revealed the

8

conformations of the molecules on a surface under sliding conditions. While the tightly

packed stearic acid monolayer conformation fits well with the explained theory, low

density of stearic acid or oleic acid shows high intermolecular interactions under static

conditions. Therefore, even if conformational defects occur during sliding, the

interactions under static condition are not low due to interdigitation of the chains, so that

friction doesn’t increase with sliding speed. In conclusion, stearic acid, when used as a

friction modifier, gives higher difference between the static and dynamic friction forces

when compared to oleic acid. 33

1.2. Wet Friction Materials

Wet friction materials, which are present in the clutch part of torque converters,

play an important role during the engagement of the clutch. Characteristics of these

materials are important for fuel efficiency and driving comfort.

1.2.1. History of Wet Friction Materials

Wet friction materials were first used in 1938 by General Motors in automatic

transmission. The sintered bronze, which was used as the friction material, was later

replaced by semi-metallic materials due to the increasing demand for better materials

during World War II. Binding of asbestos, graphite, and metal powders together with a

resin formed the semi-metallic friction materials which had low friction coefficients.

Cork as a more flexible material was used to get higher friction coefficients but it had low

9

durability. Therefore, studies led to the discovery of phenolic resin and materials with

high coefficients of friction, high strength and durability were produced subsequently. 34

Paper-based wet friction materials were first produced in 1957 using asbestos,

cellulose, various fillers, and phenolic resin binder. High friction with low static/dynamic

ratio, good heat resistance and chemical strength of these materials made them favorable.

35 After asbestos was found to be carcinogenic, it was eliminated from wet friction

materials by using other materials in combination because none of the individual

materials could replace its superior properties. Improvements on resin technology and

introduction of new synthetic fibers, i.e. aramid fibers, into the wet friction materials

were the key steps to produce materials with high performance, durability and strength. 34

The timeline on the development of wet friction materials is shown in Figure 1-4.

Asbestos Molded

Sintered Materials

Aramid Fibers

Semi-Metallic

and Cork

Materials

Other

Synthetic

Fibers

Papers with

Asbestos

Papers Without Asbestos Fillers

Bakelite

Phenolic Resin

Graphitic Papers

Papers with Synthetic

Fibers

Surface Applied

Particles to Paper and

Non-Papers

Non - Papers

1930 1940 1950 1960 1970 1980 1990 2000 2000+

Figure 1-4. Development history of materials used in wet friction applications. 34

10

1.2.2. Wet Friction Materials

After 1980 automatic transmission became more popular (as of 2015 the annual

sale of automatic transmission exceeded the annual sale of manual transmissions

worldwide 36), but it brought more stringent requirements for the manufacturers. The shift

quality, fuel efficiency, and life-time of the transmissions turned out to be the most

important criteria for the designers as well as the friction performance of the materials i.e.

positive slopes for the coefficient of friction vs. sliding speed (μ-V) curves and high

coefficients of friction. 34 Characteristics of different types of wet friction materials are

listed in Table 1-1. Not only because of their high dynamic friction coefficient, but also

because of low static/dynamic friction ratio, paper-based friction materials are superior in

terms of providing good shifting quality. However, low thermal and mechanical

resistance of these materials should be noted.

Table 1-1. Different types of wet friction materials. 37

Material type Characteristics

Paper High porosity, high dynamic friction coefficient (0.10 – 0.15), low

compressive strength and thermal resistance

Sintered Low porosity, low dynamic friction coefficient (0.05 – 0.07), high

compressive strength and thermal resistance

Fabric High porosity, medium dynamic friction coefficient (0.09 – 0.11),

medium compressive strength and thermal resistance

The lifetime of wet friction materials has to be the same as the lifetime of the

transmission because it is not practical to replace them. In general, we can focus on two

11

different time ranges considering the performance and durability of friction materials.

The short term performance is mainly determined by the porosity and surface structure

(high activity towards adsorption of friction modifiers). It is possible that drivers

experience shudder short time after driving a brand new car. This is mainly due to the bad

surface structure of the material such as high surface roughness or low surface activity

which might be due to resin rich top layer of the material. However, the material may

start to perform better after wearing off the very top layer if the existing problem is only

at the top layer of the material. The long term friction stability or durability of material

depends on the porosity and the presence of high temperature ingredients. 16 If the

problem with the friction material emerges during this period, it will be more serious and

most probably will need an action to solve the problem.

1.2.3. Wet Friction Material Ingredients

Wet friction materials are typically made of fibers, fillers, and binder. Cellulose is

a type of organic fibers that is commonly used to get desirable friction characteristics.

Although it provides high friction coefficient and low static/dynamic friction ratio, it has

lack of temperature stability. Cellulose starts decomposing around 350 °C, so that the

formation of hot spots on the friction materials will most likely degrade cellulose fibers

immediately. Therefore, the desirable characteristics and the integrity of the material will

be lost. 34 This drawback of the cellulose fibers can be eliminated by the addition of

synthetic fibers i.e. aramid fibers or carbon components such as graphite or carbon fibers.

38 Addition of too much graphite may decrease the friction coefficient because graphite is

12

considered a solid lubricant. Besides, static/dynamic friction ratio may also increase

because the shapes of the μ-V curves are mainly provided by good interactions

(adsorption) between friction materials and additives of automatic transmission fluid

(ATF). The non-polar surface of the graphite will not attract the polar parts of the

amphiphilic additives. Instead of having a well-packed monolayer of friction modifiers,

there will be molecules aligned parallel to the surface. 39 Therefore, the positive slope of

the μ-V curves will tend to decrease. Another way to improve the heat resistance of the

friction materials is by varying the pore size and overall porosity of the material. Table

1-2 presents the common ingredients used in wet friction materials. Chemicals or solid

lubricants listed last in the table are added in low concentrations because higher amounts

will take away some of the desirable characteristics of the friction materials because they

have lack of porosity and have either too high or too low Mohs’ hardness.

Table 1-2. Examples of ingredients used in wet friction material production. 34,40

Fibers Cellulose, aramid, mineral, carbon, glass

Fillers Diatomaceous earth, cashew dust, carbon, mica, calcium

carbonate, clays, activated carbon

Chemicals/Solid

lubricants

Graphite, molybdenum disulfide, acrylic rubber, alumina,

chromium oxide, zirconia

Resin Phenolic resin, silicone resin, modified-phenolic resin

The performance of the friction materials can be improved by making

multilayered materials in which the surface will consists of friction fillers which have

13

high activity towards the friction modifiers. The bottom layer may consist of mainly

fibers or high temperature ingredients. 41

Abrasive particles are also used in friction materials to increase friction.

Additionally, they may contribute in removal of the glaze on the material surface by

disturbing them. Hard particles of silicon carbide, silica, zirconia, or alumina may show

different sensitivity towards the pressure or sliding speed and must be added judiciously

depending on the application. 42

1.2.4. Porosity

Friction characteristics and temperature resistance of friction materials highly

depend on their porous structure. The higher the porosity, the more ATF flows inside the

material matrix. Therefore, better cooling is provided through the circulation of ATF.

Another advantage of higher porosity is the maintenance of desirable friction

characteristics such as high dynamic friction coefficient and low static/dynamic friction

ratio. In other words, porosity provides higher friction coefficient, but the effect is much

smaller at lower sliding speeds. These are mainly provided by the quick drainage of ATF

through the porous channels during engagement; so that asperity contact is carried by the

additives of ATF adsorbed on the surface. Therefore, having the load carried by the

friction modifiers during sliding at boundary lubrication regime will provide the desired

friction characteristics. 5,43,44,45,46

Porosity of the wet friction materials may change from 20 % to 80 % and the pore

sizes may be as big as hundreds of micron. The type of ingredients and their composition,

14

the amount of resin and the degree of curing, compression rate during paper making,

bonding temperature, and final thickness of material are the important factors which

affect porosities in friction materials. 45,47,48

Fibrillation of the fibers, which is the fluffiness of the fibers as a result of

refinement process, is another important factor influencing the material porosity. While

fibers with low fibrillation provide high porosity, increasing fibrillation decreases the

amount of voids inside the friction material thus decreasing its capacity to absorb ATF. 45

Surface geometry of the friction materials can be designed depending on the

application. Grooves on the friction materials will improve the thermal resistance of the

friction materials as they will act similar to highly porous materials, but they will also

change the friction characteristics of the materials. The coefficients of friction of grooved

samples are higher as compared to the non-grooved samples, especially at high

Sommerfield numbers, where hydrodynamic friction dominates. Therefore, having higher

friction coefficient is a similar characteristic for high porosity or grooved materials. 5,45

The wear rate of the friction materials may not be directly related to their

porosities. Even though the amount of wear decreased with increasing porosity according

to one study 45, another study showed that the wear rate increased with increasing

porosity. 49 Therefore, the wear rate may depend on other material characteristics or

application conditions as well.

Glazing of friction materials is a serious problem in relation to their permeability

or porous structure. New materials generally have good permeability but after

decomposition of the additives of ATF coupled with mechanical degradation (adhesive

15

wear) or thermal degradation (carbonization) on the friction material surface, the pores

are blocked. Therefore, negatively sloped μ-V curves are obtained because the fluid

cannot be withdrawn easily from the clutch. 50,51

There are various techniques to measure the porosity of friction materials.

Mercury porosimeter is used to measure the diameter, its distribution, and porosity

percentage in friction materials. The volume of the mercury is measured with respect to

the applied pressure and the porosity percentage is thus determined. 45 Laser thermal

investigation is another non-destructive method for measuring porosity in friction

materials. This method is based on the irradiation of the sample and measurement of the

heat produced which provides information about the presence of voids inside the

material. 43 Oil drop penetration time is also another simple method to measure the

porosity of the friction materials. While it takes only several seconds for the penetration

of oils into a highly porous material, it may take more than several minutes in the

presence of glazing. 16

1.2.5. Visco-Elasticity

Together with the porosity, visco-elasticity is an important factor affecting the

friction characteristics of wet friction materials. Changing the compression ratio during

manufacturing or the composition of the ingredients (especially resin) changes the

elasticity of the friction materials. Increasing the material density increases the

compressive strength of the materials more as compared to the shear strength. For a

material with higher porosity, compressive visco-elastic deformation increases. The

16

lower is the longitudinal modulus, the higher is the coefficient of friction for the same

material with different density. Therefore, in order to have a material with higher friction

coefficient, it is important to have softer materials. 21,44

1.3. Automatic Transmission Fluid

The lubricants used in automatic transmissions are called as automatic

transmission fluid (ATF). They have mainly red color to be distinguished from other

fluids in vehicles such as engine oil.

1.3.1. History of Automatic Transmission Fluid

When the automatic-transmission cars were introduced in late 1930s, the

transmissions were lubricated using engine oils. However, it didn’t take a long time to

understand the incompatibility in using engine oils because the operational requirements

for the transmission were different than the requirements for the internal combustion

engine. After about a decade, the first automatic transmission fluid (Type A) was

produced by General Motors (GM) in 1949. Ford, GM, and Chrysler put a lot of efforts to

improve the deficiencies of the ATFs they produced. Initially oxidation resistance and

then wear and friction performance of ATFs improved as these deficiencies were noticed

during service. 52

17

1.3.2. Automatic Transmission Fluid

In order to optimize the tribological performance of materials, it is important to

understand the lubricant as well as the surfaces to be lubricated. There is a lack of

knowledge in understanding the interactions of lubricants and their additives with

advanced material surfaces. 53 Interactions happening only in a few nanometers scale

determines the friction characteristics of the whole system. Some of the additives, i.e.

friction modifiers, adsorb on the surfaces of the friction materials and steel clutch plate to

form a monolayer of protective film. Monolayers of friction modifiers are close-packed

films which dissipate the forces during motion. The polar functionality, molecular

structure, orientation, and strength of adsorption are important factors in determining

friction characteristics. 54

ATF is one of the most complex lubricants. It consists of approximately 80 %

base oil with additives making up the rest. 55 ATF has to provide high friction coefficient

during clutch engagement and low viscous drag during free-wheeling in order to improve

fuel economy. 56 In addition to provide lubrication and protection in the transmission,

ATF also transmits the torque in wet clutches and provides cooling. 57 It should have a

satisfactory viscosity at elevated temperatures (might be up to 180 °C) and fluidity at low

temperatures (down to – 40 °C). ATF is also expected to resists oxidation, prevent

corrosion, and provide the required friction characteristics. 52,58

Another important function of ATF is to prevent delamination of friction

materials. Studies showed that degradation of cellulose highly depends on the condition

of ATF. Presence of over-based calcium detergents can prevent the degradation of

18

cellulose. High temperature detergency is also important to prevent glazing in order to

extend the service life of friction materials. 15

1.3.3. ATF Additives

ATF contains over 10 different types of additives. All of these additives are able

to do their function without being affected by the presence of others. For example, the

purpose of adding friction modifiers into ATF is to have positive friction gradient over

the speed, but the overall friction is also reduced at the same time. Therefore, the total

torque capacity of the clutch decreases unless the number or the radii of friction plates are

increased because the holding capacity of the clutch decreases as a result of the low

coefficient of friction. 59 However, by using the friction modifiers together with other

additives which are already known to increase the friction coefficient without changing

the shape of the μ-V curve significantly, it is possible to obtain positive gradient without

lowering the coefficient of friction. Here, we will list the main additives of ATF and their

important characteristics. 57

1.3.3.1. Friction Modifiers

Friction modifiers may be referred to as the performance additives for the ATF,

along with detergents and dispersants, because all three of these additives have

significant effect on the friction characteristics. Because of the structural similarity, rust

inhibitors, anti-wear and extreme pressure agents are also considered in the performance

19

package for ATF. All of these molecules have adsorption capability on to surfaces as they

consist of hydrocarbon chain and a polar head. Friction modifiers may contain amine,

carboxylic acid, ester, alcohol, or amide functionality. They mainly provide the positive

gradient in μ-V curves for clutches. 52,60

1.3.3.2. Detergents

Detergent molecules in ATF are commonly over-based calcium or magnesium

sulfonates or phenates. 61,62 They contain up to 40% calcium carbonates which neutralize

the acidic products of oxidation. 63 Presence of detergents in the ATF increases the

friction coefficient to a higher level without affecting the shape of the μ-V curve

significantly. Molecular weight distribution of the detergent molecules may also cause

different effects on friction. For instance, natural sulfonates with lower molecular weight

distribution provide higher friction than the synthetic ones. 64

1.3.3.3. Dispersants

Dispersants are mainly nitrogen containing compounds. They adsorb on to oil

insoluble sludge or soot particles and prevent their agglomeration. Increasing

hydrocarbon chain length may increase the dispersancy, but it will reduce the weight

percentage of the functional groups. 63 Similar to detergents, presence of dispersants in

the ATF also increases the friction coefficient to a higher level without affecting the

shape of the curve significantly.

20

1.3.3.4. Viscosity Modifiers

Viscosity modifiers are high molecular weight polymers which improve the

viscosity index of the ATF. At low temperatures, the polymers collapse and have low

radius of gyration, so they do not contribute much to the viscosity of the lubricant.

However, at high temperatures, the polymer chains relax and increase the viscosity of the

oil. Therefore, the change in viscosity with decreasing temperature is reduced in the

presence of viscosity modifiers. Figure 1-5 shows the viscosity index of two different

lubricants.

Figure 1-5. Viscosity vs. temperature curves for two lubricants with different viscosity

index (VI)

1.3.3.5. Antiwear (AW) - Extreme pressure (EP) agents

EP agents react with metal surfaces under extreme conditions such as high load or

temperature and make a protective layer on the surface. AW agents act similarly under

Vis

cosi

ty

Temperature

High VI

Low VI

21

milder conditions. Zinc dialkyldithiophosphate (ZDDP) has been a commonly used anti-

wear agent in various lubrication systems. ZDDP films are formed as a result of thermal

degradation, oxidation, hydrolysis, and adsorption. 65 Studies showed that presence of

AW and EP agents in ATF increases the friction coefficient without depending on the

presence of viscosity modifiers in the oil. They also provide less negative friction

coefficient gradient with respect to sliding speed compared to base oil alone. 66,67

1.3.3.6. Corrosion Inhibitors

As most of the components of the transmission are made of metals, corrosion is

inevitable unless the surfaces are protected well. In some cases corrosion inhibitors make

a layer of chemical film on surfaces to prevent any oxidative reaction on the ferrous

metal surfaces. The same function may be accomplished by friction modifiers on the steel

reaction plate of the clutch as it is crucial to have layer of friction modifiers for desired

friction characteristics.

1.3.3.7. Antifoaming Agents

Foaming of ATF is not desired because it may increase the oxidation to a higher

level and may cause undesirable mechanical failure in the system. Foam inhibitors reduce

the tendency of oil to foam and prevent fluid volume growth and fluid ejection from

transmission. 52

22

1.3.3.8. Pour-point depressants

It is important for ATF to flow at very low temperatures, so the crystallization of

the additives should be prevented. The main purpose of the pour-point depressants is to

disturb the organized structures and provide fluidity at low temperatures.

1.3.3.9. Antioxidants

Both the base oil and the ATF additives are susceptible to oxidation during

service either due to elevated temperatures or other reasons. The presence of antioxidants

in the ATF prevents the oxidation and formation of acidic molecules. Synthetic base oils

have better stability against oxidation but the solubility problem associated with them

may make them unfavorable in some cases.

1.4. Adsorption

Adsorption is the change of the concentration of a compound (liquid or gas) at the

interphase as compared to the neighboring phases. Adsorption may occur by physical

interactions (physisorption) via van der Waals interactions i.e. ion-dipole, dipole-dipole,

dipole-quadripole, and hydrogen bonding or by chemical interactions (chemisorption) via

covalent or ionic bonding. Physisorption is rapid, reversible and non-specific. However,

chemisorption, which is a stronger interaction than physisorption, has selectivity and

occurs only as monolayers. As a result of adsorption, free energy and entropy of the

system decreases. Therefore, the process is mostly exothermic because of the heat

23

released to the environment. 13,68 However, there might be exceptions where adsorption is

endothermic, in case the energy required for breaking the existing bonds is higher than

the energy released after the interactions. 69

Adsorption can be used in industry for various purposes such as purification of liquid

or gas mixtures, drying of gases or liquids, removing impurities from solid and liquids,

ion-exchange resin systems, and applications involving protein adsorption. Some of the

basic industrial types of adsorbents include activated carbon, fullerenes, silica gels, metal

oxides, zeolites, clay minerals, and synthetic polymers. 68,70,71,72,73,74

Adsorption of surfactants on a material surface can be characterized by neutron

reflection, ellipsometry, fluorescence spectroscopy, atomic force microscopy, quartz

crystal microbalance 75,76, UV spectroscopy 77, temperature modulated differential

scanning calorimetry 78, FTIR spectroscopy in case of hydrogen bonding formation 79,

and titration in case of having acidic functional groups 80. 39

1.4.1. Adsorption of ATF on Wet Friction Materials

Adsorption of friction modifiers on to the surfaces of wet friction materials and

steel is very important to obtain the desired friction characteristics. 81 Adsorption of

surfactant molecules occur on to polar surfaces as a monolayer of the molecules. In order

to have effective lubrication, the surfactant molecules should have at least 12 carbon

atoms in length. 23,82 It is also important to have good pair of surface functional groups

and friction modifiers as some of the molecules may not interact strongly, so the

monolayers of the friction modifiers will not stand under shear and pressure. 83

24

So far, we discussed different ATF additives used for different purposes but they

have similar chemical structures. For example, all of the performance additives have a

hydrocarbon chain and a polar functional group. Therefore, we cannot stop thinking how

these molecules do their function perfectly well. According to our adsorption studies,

adsorption occurs between different ATF additives and friction materials. Besides, the

molecular interactions involved are quite similar to each other in each case. Therefore,

for a fully formulated ATF, there is a competition between the additives of ATF for

adsorption on to the surface of friction materials. There are many proofs for the

adsorption of different additives simultaneously adsorbed on to the friction material

surfaces. Ingram et al. 27 showed that the use of friction modifiers with different chain

lengths together did not provide good friction characteristics as compared to the

individual molecules due to the variations in dangling chain ends at the material surface.

Chains with different lengths are not able to form solid monolayer films that can carry the

load and provide positive gradient under sliding conditions. 28 Some of our studies also

support this idea. Base oil and friction modifier mixtures always provide lower friction

coefficient and lower static/dynamic ratio as compared to a fully formulated ATF. Again,

the main reason for this observation is the lower surface area provided for adhesive

interactions during sliding by the formation of the solid monolayer from only one type of

molecule. Another supportive observation is the improved strength of the adsorbed

monolayer by the use of base oil molecules and friction modifiers with similar molecular

length. It was shown that the base oil molecules fill the gaps between the friction

modifiers with cohesive interactions of the hydrocarbon chains of the friction modifiers

25

as shown in Figure 1-6. Although these interactions are weaker as compared to the

surface adsorption interactions, they are able to make the monolayer stronger. 84

Figure 1-6. Monolayers of friction modifiers (stearic acid) adsorbed on a surface and base

oil molecules fill the spaces between the friction modifiers.

Although there is a competition for adsorption, there might also be selectivity that

plays an important role on the adsorption of certain molecules. Having different

functional groups or presence of ions on these functional groups might play an important

role in being adsorbed by the surface. However, this selectivity is not only related to

having stronger interactions, but also related to the solubility of the molecules. For

example, stearic acid is not soluble in water but sodium stearate is. Replacement of

hydrogen atom with sodium changes the solubility of the molecule completely. There are

26

such differences between some molecules in ATF as friction modifiers are mostly non-

ionic whereas detergents are ionic molecules. Therefore, the molecule with lower

solubility may tend to be adsorbed by the surface first. Another situation is related to the

solubility provided by the length or saturation degree of the hydrocarbons. For the

adsorption of alkanes, it was shown that the length of the molecule makes a difference. 85

Similarly, it is known that unsaturated molecules are known to have better solubility in

base oil compared to the saturated ones. Therefore, the choice of the right molecular

structure for certain functions is important for the adsorption of the correct molecules.

Still, adsorption of different molecules together is inevitable. One last effect on the

adsorption might be the temperature as it is suggested that some molecules are only

activated at elevated temperatures. Therefore, one of the reasons for getting different

friction performance at different temperatures might be related to the temperature effect.

86 The effect of temperature will be investigated in more details in the following chapters.

1.5. Research Objectives

Both wet friction materials and ATFs have very high complexity as they consist

of so many different types of components in order to serve different functions in the

transmission. Therefore, the developments in the automotive industry are mainly based

on trial-error methods because of the lack of knowledge in the understanding of typical

interactions between wet friction materials and ATFs. In this study, our objective is to

provide better understanding to such interactions using experimental techniques which

are not commonly employed in this industry. Moreover, the experimental techniques used

27

can also be employed as cost effective methods for the developments as they require

much less amounts of samples to run as compared to the friction test rigs. We also

provide effective methods to improve the performance and durability of the wet friction

materials.

28

MEASUREMENT OF THE ADSORPTION ENERGY BETWEEN A SOLID

ADSORBENT AND A LIQUID ADSORBATE USING DIFFERENTIAL SCANNING

CALORIMETRY

We report on a novel method involving the use of differential scanning

calorimetry (DSC) in evaluation of adsorption energy between a liquid adsorbate and a

solid adsorbent. This is accomplished by measuring the exothermic heat release due to

the adsorption of automotive transmission fluid (ATF), the adsorbate, onto a paper-based

friction material used in torque converters, the adsorbent. The novelty of the

measurement technique involves initial freezing of the liquid adsorbate so that the

initiation of the adsorption process can be identified. Our experimental results and

theoretical calculations reveal that the adsorption energy of the friction paper and the

summation of adsorption energies of each friction paper ingredient are in good

agreement.

29

2.1. Introduction

Friction materials have long been used in automotive industry together with

automotive transmission fluid (ATF). However, the complexity of the interactions

between each of the components led to developments that are based on a trial-and-error

method. 87

ATF is a special fluid that consists of approximately 80% of a type of base oil.

The remaining 20% of the fluid consists of viscosity modifiers, pour point depressants,

foam inhibitors and performance additives which are mainly friction modifiers,

detergents, dispersants etc. Performance additives are amphiphilic molecules in which

nonpolar tails provide oil solubility and polar heads provide interaction with the surfaces

due to their high activity. Another advantage of performance additives is the alignment of

hydrocarbon tails perpendicular to the sliding surfaces during boundary lubrication

regime which provides effective lubrication.

Previously, it was found that adsorption of ATF additives on friction material and

steel surface in a torque converter clutch (TCC) improves the performance of an

automatic transmission while preventing shudder. 88 The desired friction versus velocity

(μ-v) curve with a positive slope for a TCC can be obtained by creating a well-developed

monolayer film of friction modifiers on the surfaces. It is provided by the smooth

transition from static to dynamic conditions. Therefore, the adsorption of ATF additives

on the sliding surfaces of a clutch plate is very important. 59 These additives prevent the

direct contact of the asperities during the boundary lubrication regime even after the

depletion of the lubricant molecules due to various clutch engagement conditions.

30

Adsorption can be classified into two groups as physisorption in which the energy

release is relatively low due to van der Waals interactions and chemisorption in which

higher energy is released due to formation of chemical bonds. Because of the presence of

so many different chemical functionalities together with possible interaction of these

functionalities with externally applied physical parameters such as temperature, pressure,

and speed, it is difficult to predict the exact adsorption mechanism between ATF and the

surfaces. Friction modifiers with carboxylic acid functionality can be chemisorbed on to

metal surfaces; whereas, a friction material surface with hydroxyl and siloxane

functionality will more likely lead to physisorption.84-89

Adsorption studies of molecules similar to ATF performance additives and

possible friction material components have been performed by various researchers using

different techniques. Ellipsometry 90, quartz crystal microbalance (QCM) study of fatty

acid adsorption from alkane solution 75, scanning tunneling microscope (STM) imaging

of fatty acids on graphite 91, titration measurements of stearic acid onto kaolinite 92, or

Celtek clay 93, adsorption energies of ATF additives onto various friction material

ingredients by flow micro calorimetry measurements 88 are some of the techniques

employed previously. Although these studies provided some information about

adsorption properties, the techniques employed are not suitable to provide an insight to

the complex medium of a wet friction material because of the simplified and limited

capabilities.

In our studies, differential scanning calorimetry was used to measure the

exothermic heat release due to the adsorption of an ATF, the adsorbate, onto a wet

31

friction paper used in torque converters, the adsorbent. A commercial ATF was procured

to be used for the adsorption measurements. The ATF used in the experiments contained

over 85 % base oil. It also contained a methacrylate polymer which is a viscosity

modifier, succinimides which are dispersants, and dialkyldithiophospates which are

antiwear agents, in the percentages presented in Figure 2-1. First, theoretical estimates on

the limits of total adsorption energies possible were calculated considering the maximum

and minimum amounts of additives in ATF. For these calculations, average molecular

weight values were used for the specific molecules involved and listed in Figure 2-1. 94

The adsorption of base oil was assumed to be due to dipole-induced dipole interactions

(2 kJ/mol) and adsorption of ATF additives was assumed to be due to either dipole-dipole

interactions or hydrogen bonding (8 - 42 kJ/mol). 95 Based on these calculations and for

cases in which the amount of ATF would be the limiting factor in the adsorption process,

depending on the amounts of the adsorbate and the adsorbent involved, the energy

measurements are expected to be in between 4 and 7 kJ/g approximately, as shown in

Figure 2-1.

32

Figure 2-1. Adsorption energy contributions of ATF components based on theoretical

calculations.

2.2. Experimental

Detailed information about the materials, equipment and experimental procedures

are as follows.

2.2.1. Materials

A paper-based friction material (friction paper) consisting of synthetic fibers,

organic fibers, cured resin, and diatomaceous earth were provided by LuK LLC USA in

both composite and individual component forms. The wet friction papers were tested in

two different forms as 3 mm-diameter solid discs and in pulverized form. The cured resin

was ground into micron size particles using a blender while all the other adsorbents were

used as received, in their natural state.

33

In addition to the commercial ATF that is used as an adsorbate, a group III base

oil (hydrocracked) and base oil with friction modifier having different concentrations

were provided by The Lubrizol Corporation. 2 wt.% stearic acid was added into the

commercial ATF for limited number of adsorption measurements.

2.2.2. Characterization Techniques

Thermogravimetric analysis (TGA) (TA-Instrument Q50) was used to determine

the operational temperatures of each component and to choose the temperature range to

be used in differential scanning calorimetry.

A TA-Instrument Q2000 with an attached liquid nitrogen system which allows

cooling down to –180 °C was used for differential scanning calorimetry (DSC).

2.2.3. Procedure

It is not possible to measure the adsorption energy between a liquid adsorbate

(ATF) and a solid adsorbent (wet friction paper or its ingredients) using DSC at room

temperature because the adsorption occurs as soon as the materials are brought into

contact with each other, with adsorption occurring spontaneously and immediately.

Therefore, by the time the DSC instrument starts the measurement, the adsorption had

already occurred. In order to eliminate this problem, the ATF was frozen separately from

the adsorbents before bringing the adsorbent into contact with ATF at –140 °C in the

DSC pan. The adsorbate was placed into the DSC sample pan while the adsorbent

34

(friction paper) was placed into the lid which was set upside-down. The sample pan was

placed onto the sample platform and the lid was placed on top of the reference pan and

lid. The temperature of the sample was increased to 120 °C followed by 10 minutes of

isothermal heating to remove the moisture. Then, the sample was cooled down to –140

°C and the adsorbent was transferred onto the frozen adsorbate. The sample was covered

with the lid before starting the energy measurements.

Figure 2-2. DSC cell configuration before starting the adsorption energy measurements.

ATF is placed on the sample platform on the right side, and the friction paper is placed on

the reference platform on the left side.

35

Figure 2-3. Mixing of adsorbate and adsorbent at -140 °C.

After 5 minutes of isothermal cooling to –140 °C, the first heating ramp to 20 °C

was performed at a rate of 3 °C / min. This rather slow heating rate was chosen so that

details in the heat flow output charts would not be obscured. Then the sample was cooled

down again to –140 °C and a second heating ramp was performed at the same heating

rate. The adsorption occurred only in the first heating ramp, and thus the second ramp can

be used as a baseline. The energy differences between the two curves allowed us to

calculate the energy of adsorption.

The ratio of adsorbate to adsorbent was 2 to 1 by weight for all measurements at

this stage of experimentation.

2.3. Results and Discussion

The results of the experiments are discussed in this part separately as in the

following sections.

Reference

Pan and Lid

Sample Pan

and Lid

36

2.3.1. Thermogravimetric Analysis

TGA was performed in order to identify the decomposition temperatures and

moisture absorption capabilities of each component. All of the components tested are

already commercially used so that they are known to be thermally stable within the

experimental conditions used for adsorption energy measurements. As we see from

Figure 2-4a, ATF does not absorb any water and it is stable up to 200 °C. Starting from

~200 °C, the low molecular weight components and polar heads of the ATF additives

decompose. Then the higher molecular weight hydrocarbons continue to decompose at

higher temperatures. 96–98 Based on this consideration, we can conclude that the thermal

stability of ATF will not interfere with the adsorption energy measurements within the

temperature range used in our experiments.

Figure 2-4b shows TGA curves for the friction paper and its components.

Although all of the components are thermally more stable than ATF, they tend to absorb

moisture in different quantities as indicated by weight loss around 100 °C (Figure 2-5).

Therefore, the samples were heated up to 120 °C before the adsorption energy

measurements to make sure that there is no contribution to the adsorption energy due to

water during the measurements.

37

Figure 2-4. TGA curves of a) ATF and b) friction paper and its components.

38

Figure 2-5. DSC thermograms of a) ATF, where two different measurements show the

identical melting range, and b) friction paper and its ingredients.

39

2.3.2. Differential Scanning Calorimetry

Typical DSC thermograms for the ATF are given in Figure 2-5a. The broad

endotherm in the graph shows that melting of ATF occurs from – 90 °C to -10 °C. This is

also the region where adsorption takes place when the ATF and the friction paper are

mixed at temperatures below -130 °C in solid state. Repeated measurements resulted in

identical melting ranges. DSC thermograms for the friction paper and its components are

given in Figure 2-5b. There is no thermal transition for any material in the temperature

range used for adsorption energy measurement. One noticeable transition is the melting

and evaporation of water in some of the components. However, the first heating cycle

removes it, thus, it does not interfere with the adsorption measurements.

2.3.3. Adsorption Energy Measurements

Adsorption exotherm examples for friction paper and its components,

diatomaceous earth, synthetic fiber, organic fiber, and resin binder are given in Figure 2-6

and Figure 2-7. Experimentally, it was determined that the most appropriate ratio of

adsorbate to adsorbent was 2 to 1 because it provided a complete wetting of the adsorbent

sample. A third heating cycle was also used in limited number of experiments to make

sure that no adsorption occurs after the first heating cycle. The third heating ramp curve

was always identical to the second one. Therefore, no transition occurred in the second

heating ramp and we can conclude that adsorption takes place only at the initial heating

ramp.

40

For the adsorption energy calculations, the end points were selected outside the

melting range of ATF where the first and the second heating curves overlap. The end

points are commonly located around -95°C and -10°C. The adsorption energy (the

exotherm in the second heating curve) was then calculated from the difference in the

melting endotherms of the both curves. The integrations, where the regions were defined

by the red lines, were performed by TA Universal Analysis software.

Figure 2-6. Adsorption energy for wet friction paper.

At least three adsorption measurements were performed for each material and the

results are summarized in Table 2-1. Adsorption energy was 6.62 ± 0.57 J/g for the

friction paper, 8.75 ± 0.77 J/g for diatomaceous earth, 3.90 ± 0.21 J/g for synthetic fiber

3.70 ± 0.54 J/g for organic fiber, and 2.00 ± 0.23 J/g for resin binder. Thus, the

41

summation of the adsorption energies for the friction paper ingredients, which was

calculated using their proportions in the friction paper, was 5.30 ± 0.61 J/g. There is a

1.32 J/g difference between the adsorption energy of the friction paper and the

summation of the adsorption energy of its components. This difference is attributed to

other components used during manufacturing of the friction paper, such as coagulants

and/or flocculants and to other kinetic/boundary condition effects, such as size and shape

of the ingredients in particulate form in comparison to the friction paper specimens in

monolithic disc form. There are also other treatments involved during the manufacture of

the friction paper, which may have an effect on the chemistry of the components, such as

thermal treatments or change of pH during manufacturing.99 Additionally, the friction

paper is a very porous material that allows the ATF to penetrate and flow through its

structure and thus forming the layer of fluid needed to have a boundary lubrication

regime in a wet clutch. The porosity of the material is provided by the combination of

fibers and fillers which breaks the cohesive forces between the ingredients. Therefore,

possible interactions between the ingredients may also increase the active surface area of

the friction paper and thus yielding higher adsorption energy as compared to the

summation of the adsorption energies of the individual ingredients.

42

Table 2-1. Adsorption energies for friction paper and its ingredients.

Material Adsorption

Energy [J/g]

Total Adsorption

Energy [J/g]

Wet Friction Paper 6.62 ± 0.57 6.62 ± 0.57

Ingredients for the Friction Paper

Diatomaceous Earth 8.75 ± 0.77

5.30 ± 0.61 Synthetic Fiber 3.90 ± 0.21

Organic Fiber 3.70 ± 0.54

Resin Binder 2.00 ± 0.23

Figure 2-7. Adsorption energies for a) diatomaceous earth, b) synthetic fiber, c) organic

fiber, and d) resin binder.

43

Details on the adsorbent materials are given in Table 2-2. All of the materials

have polar groups, and thus, they are able to interact mainly with the ATF additives.

Diatomaceous earth has the largest adsorption energy due to its smaller particle size and

larger surface of all of the components. If we compare synthetic and organic fibers, we

see that synthetic fibers have slightly higher adsorption energy. Although it is difficult to

compare their chemistries and density of their functional groups, we may expect higher

adsorption energy for the synthetic fibers due to their smaller fiber diameter, which again

leads to higher surface area. Based on our microscopic analyses, we know that the

synthetic fibers possess more fibrillated structure as compared to the organic fibers. As

for the resin, it loses most of its functional groups after curing, and thus, expected to

generate the lowest adsorption energy.

One interesting finding is related to the effects of porosity of some of the

ingredients which can be seen in the shape of the adsorption exotherms. Figure 2-7

shows that diatomaceous earth has two sharp adsorption exotherms. The second exotherm

which occurs at higher temperature is due to the inner pores of the material which are

more difficult to access. Second adsorption exotherms are also noticeable for the fibers

although they are not as intense as for diatomaceous earth. Cured and ground resin, that

has no inner pores, has no sharp adsorption peak like the other ingredients. Adsorption

exotherms for the friction paper show one less obvious peak. This could be due to the

strong adhesive interactions between the fillers and the fibers, which made the inner

pores mostly inaccessible for ATF.

44

Table 2-2. Properties of the friction paper ingredients.

Material Particle Size or Fiber

Diameter (μm) Molecular Structure

Diatomaceous

Earth 5 - 50

Siloxane (Si-O-Si) and

silanol (Si-OH) groups

Synthetic Fiber 10 - 50 Benzene rings and amide

groups

Organic Fiber ≈ 20 Alcohol (-OH) and ether

(C-O-C) groups

Resin Binder up to 200 Benzene rings and phenol

(Ar-OH) groups

In the second part of the study, different adsorbates were used to support the

theoretical calculations made and the adsorption measurements performed in the first

part. The friction modifiers are the most important additives present in ATF to be

adsorbed by the wet friction paper. Therefore, the friction paper should be selective to the

specific functionalities of such molecules. For this reason, 2 wt.% stearic acid, which was

previously shown to be a very effective friction modifier for the lubrication of wet

friction materials by. Kugimiya et. al. 100, was added into the ATF and the adsorption

measurements were repeated only for the wet friction paper. The adsorption energy for

the friction paper increased from 6.62 ± 0.57 J/g to 7.28 ± 0.39 J/g. The improvement

provided by the addition of stearic acid shows the selectivity of the material towards the

friction modifier in the oil.

In order to see the effect of friction modifiers more clearly, neat base oil and base

oil consisting of a type of friction modifier with three different concentrations were used.

45

Increasing the concentration of the friction modifier didn’t affect the adsorption energy

because the surface of the friction paper is already saturated with the lowest

concentration. Therefore, all of the results are summarized together for the base oil

consisting of friction modifiers (Figure 2-8) (I would like to thank to Sayali Satam for

obtaining some of this data). The adsorption energy for the base oil is 5.02 ± 0.69 J/g and

for the base oil with friction modifier, it is 8.96 ± 1.99 J/g. The change of the adsorption

energy is a good indicator for the capability of the experimental technique we are

reporting.

The adsorption energies we obtained for different adsorbates have met our

expectations. The lowest energy is expected for the base oil because it is made of

hydrocarbons, so that it can only interact by dipole-induced dipole forces which have

relatively lower energy compared to polar molecules. ATF should have higher adsorption

energy than the base oil due to the intermolecular interactions provided by the polar

additives. However, the competition by different additives may make the surface

inaccessible for the friction modifiers. Especially, high molecular weight molecules such

as viscosity modifiers may cover large portion of the material surface. The addition of

stearic acid increased the adsorption energy of the ATF as expected because the

concentration of the molecule was well above the suggested concentration of friction

modifiers. Besides, it has a good capability of making highly dense monolayer at the

surface. Finally, the base oil consisting of the friction modifier provided the highest

energy because the adsorption of the friction modifier occurred without competing with

other molecules.

46

Figure 2-8. Adsorption energies for friction material with different adsorbates (I would

like to thank to Sayali Satam for obtaining some of this data).

2.4. Conclusions

A new technique was developed for measuring the adsorption energy of ATF and

friction paper used in torque converters, or with any other adsorbate/adsorbent

combinations, using the DSC instrument. The adsorption energy of the friction paper and

the summation of adsorption energies of each ingredient are in good agreement. We can

summarize our findings as follows:

TGA shows that friction material is more resistant to elevated temperatures than

ATF, so ATF is more susceptible to thermal degradation first in a wet clutch.

0 2 4 6 8 10 12

Base Oil

Base Oil + Friction Modifier

ATF

ATF + Stearic Acid

Adsorption Energy (J/g)

47

Adsorption energy measurements show that diatomaceous earth which is the

main filler used in friction materials has the highest adsorption capability

among the friction paper ingredients.

More fibrillated or thinner fibers attract more ATF additives.

Based on the shapes of the adsorption exotherms, inner pores of diatomaceous

earth in a friction paper is less accessible by ATF probably due to the formation

of strong adhesive interactions with the fibers or resin during paper making.

According to our theoretical calculations, adsorption energy increases with

increasing amount of performance additives in ATF.

Friction modifiers are shown to have a good selectivity on the friction material

surface and they provide higher adsorption energy.

The use of different adsorbates showed that the experimental technique we are

reporting is able to distinguish the different types of molecular interactions

provided by different types of molecules

48

IMPROVING THE ADSORPTION CAPACITY OF PAPER-BASED WET FRICTION

MATERIALS USING EXCIMER LASER

Adsorption of the performance additives of automatic transmission fluid (ATF) on

to wet friction material surfaces in torque converters is essential for obtaining the desired

friction characteristics. Moreover, stronger intermolecular interactions and higher

monolayer density on the material surface is preferred for positively sloped friction-speed

curves. We performed KrF excimer laser treatment on a wet friction material, a synthetic

fiber, and an organic fiber in order to improve their adsorptive capacity for wet friction

materials, specifically for their fiber components. Adsorption energies of a commercial

ATF on each adsorbent were quantified using differential scanning calorimetry. While

the synthetic and the organic fibers showed more than 100 % increase on the adsorption

energies, the wet friction material showed 26 % improvement after laser treatment.

3.1. Introduction

As described earlier, paper-based wet friction materials are mainly composed of

cellulose, aramid, carbon, and glass fibers; fillers as diatomaceous earth, graphite, cashew

49

dust, and activated carbon; and binders as phenolic and epoxy resins. 40 The performance

of these materials in TCC depends on their interaction with the automatic transmission

fluid (ATF) which is used as a lubricant and a coolant. Mainly, friction modifiers that are

present in ATF have to adsorb on to friction material surface to provide good sliding and

engagement characteristics of torque converters. 81,88

Surface treatment of polymeric materials is very important because most of these

materials have inert surfaces which exhibit lack of adhesion during the preparation of

composite materials. In addition to chemical treatment of polymers, there are many

physical methods available such as UV irradiation, plasma treatment, corona discharge,

and flame treatment in order to improve their surface energies. For porous materials like

wet friction materials, the chemical treatments may have an advantage over the physical

treatments because the solution is able to penetrate throughout the material. Thus, instead

of modifying only the top layer, deeper layers of the material can be modified. However,

the requirement of large amount of solvents and their disposal and high energy

requirement for drying and curing make chemical methods less preferable for industrial

materials. 101,102

Ingram et. al. studied the effect of plasma treatment on wet friction materials. He

suggested that the oxidation provided by oxygen plasma may remove the phenolic resin

from the top layer and more hydrophilic cellulose layer is then exposed at the surface.

Therefore, the adsorption capability of the friction material, which is essential for good

friction performance, is improved. Water droplets, which had over 90° contact angle

before the plasma treatment, penetrated into the friction material immediately after the

50

treatment. However, subsequent friction studies did not show much improvement. 18 One

of the reasons for this is the ageing of the surfaces quickly after plasma treatment.

Because of the higher polymer chain mobility at the material surface, it is suggested that

neat surface properties are recovered by the reorientation of the chains or diffusion of

smaller molecules. 103 A study on poly(dimethylsiloxane) (PDMS) samples showed that

the effect of plasma treatment diminished significantly after 3 hours of plasma treatment

and the surface recovered its hydrophobicity. 104

Excimer laser is one of the physical techniques that are used to change the

chemical structure, morphology, and wetting characteristics of surfaces. It can provide

extremely high energy densities because the laser can be focused on micron-size, and

even smaller dimensions. The photon energy that is provided by laser ablation is

comparable to the bonding energies of polymers. Therefore, excimer laser radiation

provides clean ablation without having thermal problems. 105

In our previous study, we reported a method to measure the adsorption energy

between automatic transmission fluid (ATF) and solid adsorbents using differential

scanning calorimetry. We are now reporting on the improvement of the adsorption

energies after laser treatment using the same experimental technique. In addition to the

wet friction material, we also performed the same study for synthetic and organic fibers

which are the typical ingredients used in paper-based friction materials.

51

3.2. Experimental


are as follows.

3.2.1. Materials

A commercial ATF was procured to be used for the adsorption measurements. A

paper-based wet friction material was provided together with its individual components,

synthetic fibers and organic fibers by LuK USA LLC. The synthetic and organic fibers,

which were in the sheet form, were dried in a vacuum oven overnight before using. No

further treatment was performed to any material.

3.2.2. Methods

A multi-gas excimer laser (Lambda Physik LPX 240I, Gottingen, Germany) was

employed using krypton fluoride (KrF) gas to modify the surfaces of the friction material,

the synthetic fiber, and the organic fiber. 300 laser pulses (20 ns duration each) were

applied with 300 mJ/cm2 laser fluence and at a repetition rate of 1 Hz.

Adsorption energies between the ATF and the adsorbents were measured

according to a previously described method using TA-Instrument Q2000 differential

scanning calorimetry (DSC). Briefly, the liquid adsorbate, ATF, was frozen inside the

DSC cell before bringing the adsorbent into contact. After mixing both components in

solid form inside the chamber, two consecutive heating ramps were run and the

52

adsorption energy was measured from the exothermic heat release which occurred within

the broad melting range of ATF in the first heating ramp.


Adsorption of friction modifiers that are present in ATF is very important for

obtaining the desired friction – speed (μ-v) characteristics in torque converters. Friction

modifiers are amphiphilic molecules where the hydrocarbon tail provides oil solubility

and the polar head provides adsorption to the surfaces of wet friction materials or steel

reaction plates. Monolayers of friction modifiers adsorbed on these surfaces provide

positively sloped μ-v curve which is essential for preventing shudder and for having

smooth clutch engagement characteristics. The type of functional group on the friction

modifiers and the concentration of the friction modifiers dissolved in the ATF are

important because the strength of the interaction and the density of the monolayers

depend on these characteristics. Studies with friction modifiers having different

functionalities such as amine, amide, alcohol, or carboxylic acids showed very different

friction characteristics. The selectivity of specific functionalities in the lubricant and the

strength of their intermolecular interactions with the surface led to negatively or

positively slope μ-v curves. 100

53

Figure 3-1. Adsorption energy measurements for neat and laser treated wet friction

materials. The area between the 1st and 2nd curve corresponds to the adsorption energy

between ATF and the friction material.

One way to increase the adhesive forces between the friction modifiers and wet

friction materials is to modify the surface of the friction material. We performed KrF

laser treatment on friction materials to improve their surface energies. We applied three

different laser fluences of 100, 200, and 300 mJ/cm2, using 100, 200, and 300 pulses for

each of these fluences. As a result of nine different conditions, we only observed changes

on the morphology of the friction material at 300 mJ/cm2 and 300 pulses by using an

optical microscope. Concentration of synthetic and organic fibers on the top layer

decreased significantly after the treatment. However, this situation shouldn’t create any

54

problem with the durability of the friction material because the strength of the material

mainly comes from the phenolic resin as it is shown in the following chapters. According

to the optical microscope images, we decided to employ a laser fluence of 300 mJ/cm2

delivered by each of 300 pulses for the adsorption study.

We used 2/1 ATF/adsorbent weight ratio for the adsorption measurements as we

experimentally determined that it was the best ratio for wetting the samples completely.

Besides, the higher ratios were difficult to deal with considering the DSC pan size. The

adsorption energies were calculated from the difference in the first and the second

heating ramps. ATF melts over a very broad temperature range which is from -90 °C to -

10 °C as seen in the DSC thermograms. After the frozen ATF and the solid adsorbent

were brought into contact, adsorption took place over a range of temperature depending

on the characteristics of the adsorbent inside the melting range of ATF in the first heating

ramp. The second heating ramp and any other temperature range didn’t indicate any type

of transition as we performed some of the experiments more than two cycles and at a

broader temperature range. Figure 3-1 - Figure 3-3 present the adsorption graphs for the

wet friction material, the synthetic fiber, and the organic fiber, respectively. Each graph

shows one result for the neat material and one result for the laser treated material. There

is an easily noticeable difference between the adsorption exotherms of the laser treated

samples in comparison to the neat ones. The adsorption energy increased for all three

adsorbents used in the experiments after the laser treatment. DSC thermograms showed

that the interactions between the melting ATF and the friction material took longer time

after laser treatment because of the higher surface polarity and possibly its higher extent

55

over the surface after the laser treatment. (Figure 3-1). Similar results can also be seen for

the synthetic and the organic fibers in Figure 3-2 and Figure 3-3, respectively.

Figure 3-2. Adsorption energy measurements for neat and laser treated synthetic fiber

sheets. The area between the 1st and 2nd curve corresponds to the adsorption energy

between ATF and the synthetic fiber.

Table 3-1 summarizes the adsorption energy data for all three adsorbents. The

adsorption energy increased from 6.62 J/g to 8.33 J/g for the wet friction material, from

3.90 J/g to 8.32 J/g for the synthetic fiber, and from 3.70 J/g to 7.57 J/g for the organic

fiber after the laser treatment. The percentages of the changes are 25.83 %, 113.33 %, and

104.59 % for the wet friction material, the synthetic fiber, and the organic fiber,

respectively. Although both fibers types showed over 100 % improvement after laser

56

treatment, the reflection of this improvement to the friction material is around 26 %. Even

though the friction material consists approximately 26 wt.% of synthetic and organic

fibers, the presence of phenolic resin binder in the friction material should also be

considered as an energy scavenger during laser irradiation. It was previously reported that

phenol formaldehyde resin is used to obtain polyacenic semiconductive films by excimer

laser ablation instead of pyrolytic treatment. Brown and/or dark brown films were

obtained after the treatment. 106 Although, we do not know about the electrical

conductivity of our materials, the same color change we observed might be an indicator

for the change in the structure of the phenolic resin. Therefore, the synthetic and the

organic fibers in the friction material are probably contributing a lower percentage of the

total adsorption energy, compared to their individual forms, for the overall friction paper,

when all the other ingredients are considered.

Another important characteristic of the laser treatment performed for the three

types of adsorbents is the resistance to ageing. Adsorption measurements were repeated

with the laser treated samples 1 year after the treatment. The improvement on the

adsorption energies were maintained at a similar level. Therefore, samples treated with

excimer laser can safely be stored at ambient conditions for a long time.

57

Figure 3-3. Adsorption energy measurements for neat and laser treated organic fiber

sheets. The area between the 1st and 2nd curve corresponds to the adsorption energy

between ATF and the organic fibers.

Table 3-1. Adsorption energy results for the friction material, synthetic fibers, and

organic fibers.

Adsorption energy

without any

treatment (J/g)

Adsorption energy

after laser

treatment (J/g)

% Change

after

treatment

Friction Material 6.62 ± 0.57 8.33 ± 0.35 25.83 %

Synthetic Fiber 3.90 ± 0.21 8.32 ± 0.32 113.33 %

Organic Fiber 3.70 ± 0.54 7.57 ± 0.86 104.59 %

58

3.4. Conclusions

We performed KrF excimer laser treatment on a wet friction material and its

components, synthetic fibers and organic fibers. 9 different conditions were used during

the laser treatment by varying the laser fluence as 100, 200, and 300 mJ/cm2 and the

number of pulses as 100, 200, and 300. We observed decrease in the amount of fiber

density on the friction material surface only at 300 pulses and 300 mJ/cm2. Therefore,

adsorption studies concentrated on these parameters.

The adsorption energies of the synthetic and organic fibers increased over 100 %

after the laser treatment. The friction material showed around 26 % improvement under

the same conditions. DSC thermograms showed that the interactions between the melting

ATF and the friction material took longer time after laser treatment because of the higher

surface polarity and possibly its higher extent over the surface due to laser treatment.

According to the results we obtained from the adsorption measurements, we believe that

application of excimer laser on wet friction materials will improve their friction

characteristics.

59

CORRELATION BETWEEN THE ADSORPTION ENERGIES AND FRICTION

PERFORMANCES OF VARIOUS FILLERS USED IN WET FRICTION MATERIALS

Paper-based wet friction materials used in torque converters are composed of

fibers, fillers, and binders. Fillers have significant effect on the characteristics of the

materials as well as other ingredients. Various types of fillers are used for different

purposes such as improving heat stability, increasing friction level or providing positive

friction-speed gradient. As a primary or secondary purpose, all of the fillers have

interactions with automatic transmission fluid (ATF). Therefore, the friction – sliding

speed (μ-v) characteristics of the system are determined accordingly. In this study, we

measured the adsorption energy of various fillers and ATF. Adsorption energies of two of

these fillers, which have similar physical properties, showed good correlation with their

μ-v performances.

4.1. Introduction

W. Hardy who is the discoverer of boundary lubrication phenomenon expressed

that the chemical nature of the sliding surfaces is as important as the characteristics of the

60

lubricant for friction. The thin lubricating film layers obtained on the surfaces are stable

at pressures up to 10 GPa. The strength of this boundary layer film changes depending on

the type of the lubricant. While paraffinic oils provide high coefficient of friction, much

lower friction coefficients are obtained with amphiphilic molecules. The decrease in the

friction coefficient is due to the adsorption of molecules through their polar groups and

their orientation on the surface. 107 Therefore, the chemistry of the material surfaces is

important to provide strong adhesive forces for the formation of a good boundary layer.

Fillers are used for filling the spaces between the fibers in wet friction materials.

Using different types of fillers will change the properties of the friction materials.

Diatomaceous earth, graphite, clays, silicone particles, cashew dust, alumina, chromium

oxide, and activated carbon are some of the fillers used in friction materials. 38,40

Incorporation of each of these fillers into friction materials imparts some unique

properties. Silica which is a relatively hard mineral provides high static friction

coefficient and alumina as a hard particle increases the friction in initial engagement

conditions. Use of disc-shaped diatomaceous earth improves the heat resistance and

provides positive gradient for μ-v curves. 108 Barium sulfates and calcium carbonates are

known as particles to improve the heat stability of the friction materials. 37 Graphite,

which is often referred to as a solid lubricant, removes the unwanted friction fluctuations

with changing speed. 109 Abrasive particles with Mohs hardnesses over 9, such as

alumina, zirconia, chromium oxide, and titanium oxide increase the coefficient of

friction. These inorganic fillers also increase the strength of the fibers after the friction

material is saturated with resin. 87

61

In this study we investigated the adsorption energy capabilities of different fillers

used in friction materials. The total adsorption energy of the fillers and the shape of

adsorption exotherms seem to correlate well with their friction performance behaviors.

4.2. Experimental


are as follows.

4.2.1. Materials

A commercial ATF was procured to be used for the adsorption measurements.

Different fillers were provided by LuK USA LLC. All the materials were used as

received. The names of the filler types are given in Table 4-1.

Friction materials to be used for SAE#2 tests were prepared at LuK, Wooster

using conventional paper making procedures. The details of the SAE#2 test machine and

the test procedure is given in Appendix.

4.2.2. Methods

Adsorption energies between the ATF and the fillers were measured according to

a previously described experimental technique. TA-Instrument Q2000 differential

scanning calorimetry (DSC) attached with a liquid nitrogen cooling system was used for

the adsorption energy measurements.

62

SAE#2 testing equipment was used to obtain coefficient of friction vs. sliding

speed curves for the specific materials using a commercial ATF.


Adsorption between wet friction materials and ATF additives is essential for

obtaining the desired friction characteristics for a wet clutch. In our previous study, we

investigated the effect of using different ingredients and different lubricating oils on the

adsorption energy using DSC while we were building the experimental technique. After

we proved that the method works with a good accuracy, our focus now is on different

types of fillers.

Figure 4-1. DSC thermograms for filler 1 showing the energy of adsorption between the

first and second heating ramps.

63



After the ATF and the fillers were mixed in DSC cell at –140 °C while both

compounds were solid, two consecutive heating ramps were applied and the energy of

adsorption was calculated from the area between two heating thermograms. Figure 4-1 -

Figure 4-6 present the DSC graphs for all six fillers. As seen in the thermograms, the

mixtures have very broad endotherms from –90 °C to –10 °C which correspond to

melting of ATF. The exotherm in the first heating ramp inside this melting range is due to

adsorption and is used for the calculation of adsorption energy. This exotherm also

provides valuable information about the physical characteristics of the adsorbents.

Particles or materials which do not have pores generally have an exotherm without sharp

changes in the heat flow. Those, which have pores, have at least a peak that causes a

64

sharp change on the heat flow. Depending on the size variation of the pores, a second

exotherm peak may be observed due to the formation of capillary bridges on the smaller

pores and delayed wetting of inside of the pores.



According to Figure 4-1, filler 1 is not expected contain many pores due to the

absence of a sharp peak and the adsorption occurs with a fairly constant gradient. Filler 2

is expected to be a very porous material based on Figure 4-2, because it has two sharp

peaks. It takes longer time for melting ATF to penetrate inside the particles since the peak

is at the very end of the melting point. On the other hand, although filler 3 has a sharp

peak (Figure 4-3), it is not a porous material. The reason for the exotherm peak is the

65

relatively low density of the particle which results in longer time for the particles to sink

into the melting ATF. Once it sinks fully, adsorption occurs on the surface resulting in

the observed peak. Filler 4 is also not a porous particle and its adsorption exotherm is

similar to that of filler 1 (Figure 4-4). Filler 5 and filler 6, adsorption behaviors shown in

Figure 4-5 and Figure 4-6, respectively, also suggest porous particles due to the presence

of two peaks. However, the smaller size of the second peaks for both probably indicates

lack of smaller pores.



Adsorption energy results are summarized in Table 4-1 together with the particle

size and filler type information. According to the data, there is no direct correlation

between the particle size and the adsorption energy. Although the particle size may

66

contribute to the total surface area of the compounds, presence of inner pores is more

important factor affecting the total adsorption energy. The other very important factor is

the surface chemistry. Presence of more polar groups on the surface will lead to higher

number of interactions which will increase the energy of adsorption. Therefore, we think

that two particles with similar physical properties but different adsorption energies should

give different friction performance in a wet clutch. The 6 particles used for the adsorption

measurements have different physical characteristics, i.e. hardness, total surface area and

particle size, except for filler 2 and filler 6. Filler 2 and 6 have similar particle size

(around 10 μm), hardness (both of them are soft particles with Mohs hardness around 1-

2), surface area, and porous structure according to DSC thermograms.



67



Friction materials, which were prepared with extreme amounts of filler 2 and

filler 6, were used to compare the friction performances of the fillers using SAE#2 test

equipment. Figure 4-7 and Figure 4-8 shows the results at 40 °C and 90 °C, respectively.

If the small fluctuations are ignored, filler 2 doesn’t have any negative gradient at 40 °C

and the increase of pressure decreases the dynamic friction coefficient (sliding speed

above 0.25 m/s) (Figure 4-7). However, filler 6 shows negative gradient at the same

temperature. Still, the effect of pressure is similar for this compound. We observe the

same trend at 90 °C (Figure 4-8). Therefore, regardless of the temperature, we observe

68

similar trend for both types of fillers. The static friction coefficient (very low sliding

speed) seems to be affected only by the surface pressure. At 0.4 MPa, both filler 2 and

filler 6 have static friction coefficient slightly above 0.16 at both 40 °C and 90 °C. At 3

MPa, the static coefficient for both fillers at both temperatures moves to 0.15. However,

the positive or negative friction gradient depends on the filler under dynamic conditions.

We can explain the negative friction gradient with two approaches. First one is

related to the material porosity. Although glazing will not be a common issue for fresh

friction materials, the negative gradient might still be due to the lack of pores on the

material surface. After the pressure is applied, ATF has to be able to drain through the

friction material in order to lead to smooth engagement where we do not observe rooster

tail (negative friction gradient). 110 This might be the case for filler 2 and filler 6 as we

related the adsorption exotherm peak around –20 °C to the porosity of the particles.

Because we observed a bigger peak for filler 2, the μ-v performance of this filler is better.

The second approach is related to the conformation defects that are happening under

sliding conditions. Studies showed that increasing disorder of the structures of molecules

that are adsorbed on the surface results in increasing friction. Kinks or gauche

deformations are the main defects happening during sliding which requires higher energy,

so that the friction increases. 31,111 Since the characteristics of the friction modifiers such

as packing density or strength of the interactions also depend on the surface it is

adsorbed, the surface chemistry will play an important role for determining the

coefficient of friction. Under the static conditions the monolayers of friction modifier are

almost free of gauche defects in case of a high packing density. With sliding, more

69

energy is needed to be imparted into the system to overcome the energy barriers due to

rotational, vibrational, and translational motions of the molecules. However, if defects are

already present on the monolayer during static conditions, higher energy may not be

required to overcome the energy barriers under dynamic conditions because the

molecular interdigitation at static condition will absorb relatively high energy. Moreover,

the molecular interactions will decrease under dynamic conditions which will decrease

the friction. 29,30,112 Therefore, the higher adsorption energy for the two fillers with

similar physical properties may mean that the monolayer of the friction modifier has

stronger interactions with the surface or it is more densely packed. Because the

adsorption energy is higher for filler 2, it maintains higher dynamic friction coefficient

compared to its static friction coefficient.

Table 4-1. Adsorption energies and particle size values for the fillers

Adsorption Energy (J/g) Particle Size Mineral/Clay

Filler 1 2.52 1 – 10 μm Alumina

Filler 2 5.86 ≈ 10 μm Boron nitride

Filler 3 0.82 5 – 30 μm Cashew dust

Filler 4 4.77 10 – 100 μm Graphite

Filler 5 3.41 ≤ 2 μm Clay Type-1

Filler 6 2.78 5 – 20 μm Talc clay

70

Figure 4-7. Coefficient of friction vs. sliding speed curves for filler 2 and filler 6 at 0.4

and 3 MPa surface pressure and 40 °C fluid temperature.

Figure 4-8. Coefficient of friction vs. sliding speed curves for filler 2 and filler 6 at 0.4

and 3 MPa surface pressure and 90 °C fluid temperature.

0.1

0.12

0.14

0.16

0.18

0 0.5 1 1.5 2

Co

eff

icie

nt

of

Fric

tio

n (μ

)

Sliding Speed (m/s)

Filler 2, 0.4 Mpa

Filler 2, 3 Mpa

Filler 6, 0.4 Mpa

Filler 6, 3 Mpa

0.1

0.12

0.14

0.16

0.18

0 0.5 1 1.5 2

Co

eff

icie

nt

of

Fric

tio

n (μ

)

Sliding Speed (m/s)

Filler 2, 0.4 Mpa

Filler 2, 3 Mpa

Filler 6, 0.4 Mpa

Filler 6, 3 Mpa

71

4.4. Conclusions

We measured the adsorption energy of six different fillers and ATF. According to

adsorption exotherms in the DSC thermograms, we get information about the porous

structures of the particles. Moreover, the total adsorption energy of the fillers is an

important consideration in choosing fillers to be used in wet friction materials in order to

improve the performance of the clutch. The μ-v performance of two fillers and their

adsorption energies seem like closely related to each other. The more porous filler with

higher total adsorption energy provided better μ-v performance according to the SAE#2

test.

72

EFFECT OF MONOLAYER DENSITY AND STRUCTURE OF FRICTION

MODIFIERS ON WET FRICTION PERFORMANCE

Wet friction materials and automatic transmission fluids have been used for

decades. However, developments are still based on a trial and error method as of today.

Therefore, there is a need for practical solutions to reduce the time and money spent on

the development of each component. Besides, it is also desirable to provide an insight

into to the complex interactions between friction materials and automatic transmission

fluid. For this reason, we make a correlation between the contact angle, contact angle

hysteresis, and the tribological performances of different fatty acids with different

concentrations.

5.1. Introduction

Boundary lubrication regime has long been studied to understand the mechanisms

of friction and factors affecting it. Although, parameters such as temperature, pressure,

viscosity, chemistry of lubricating oil and opposing surfaces are considered to have a

direct effect on friction, most of the time these factors are correlated with each other.

73

Wet friction materials are used in torque converters for the transfer of engine

power to transmission. The main difference of torque converters in automatic

transmissions from the manual systems is the presence of automatic transmission fluid

(ATF) which provides lubrication, cooling and the desired shifting conditions.

Application of pressure to the clutch rings forms boundary lubrication regime owing to

the porous nature of the friction materials. The lubricant drains through the friction

materials but the performance additives present in ATF adsorb on both the friction

material and steel reaction plate. Adsorption of these molecules prevents direct contact of

the asperities. The most important performance additives are friction modifiers which

have the most significant effect on the coefficient of friction vs. sliding speed (μ-v) curve.

Friction modifiers are amphiphilic molecules having a polar head i.e. acid, amine, ester,

amide, alcohol or urea functionality and a hydrophobic carbon tail.

There are many studies involving the friction modifiers reported in literature.

Kugimiya et al. studied the effect of friction modifiers on the μ-v characteristics using

SAE#2 friction tester. 100 Friction modifiers having an acid or amide functionality

showed positive μ-v gradient while amine and alcohol functional modifiers had negative

slope. Although these characteristics are specific for the materials that were used in the

experiment, it shows us the importance of integrated development for friction material

and ATF pairs. Studt found that aromatic compounds are better in terms of load carrying

capabilities but straight chain molecules provide lower friction coefficient. Increasing

chain length and having the functional group of the molecule at the end of the chain are

important for providing better lubrication due to increased cohesive forces between the

74

tail groups of the friction modifiers. 113 Improvement of adhesive and cohesive

interactions between the friction surfaces and friction modifiers for the purpose of

providing better lubrication was studied by Jahanmir et al. 114 They found that stearic acid

yields lower friction coefficient than oleic and elaidic acids because of the higher packing

density of the molecules on the surface. Trans- configuration is also shown to be more

effective than cis- configuration.

Slough et al. studied the effect of friction modifiers on paper-type friction

material using a scanning force microscopy and concluded that friction reduces with the

addition of friction modifiers into the base oil and the reduction is more effective at lower

sliding speeds. Therefore, friction modifiers provide a more positive gradient for the μ-v

curves. 115

In our study we focus on the friction performance of stearic with two different

concentration, oleic, and linoleic acids as well as base oil with no friction modifier using

SAE#2 friction tester. Different types of test methods can be used with SAE#2 for the

evaluation of the performance of the system. The most common method is the

construction of μ-v curve from discrete data points. The positive slope of the curve is

then related with the desired performance which is mainly attributed with a good

interaction between the surface and the friction modifiers. Lack of friction modifiers will

cause a negative slope which is highly susceptible to shuddering. Performance of the

system can also be interpreted from a torque vs. time curve with respect to an applied

pressure. Absence of a torque spike which is often referred to as rooster tail during an

75

engagement is generally indicator of a good performance. This evaluation is based on a

continuous slipping of the clutch disc.

We carried out contact angle measurements with both the friction material and

steel plate. Besides, three friction modifiers those were used in the SAE#2 tests in order

to correlate their tribological performances with the water contact angles, contact angle

hysteresis, and the surface free energies.

5.2. Experimental


are as follows.

5.2.1. Materials

Triple pressed stearic acid tallow flakes (Acme Hardesty, Pennsylvania, USA),

lab grade oleic acid (Fisher Science Education, Pennsylvania, USA), and linoleic acid,

60% technical grade (Acros Organics, New Jersey, USA) were procured to be used as

friction modifiers. All the fatty acids were used as received. A group III base oil

according to American Petroleum Institute (API) standards were used as the main

lubricant. 1015 carbon steel and a type of paper-based friction material produced in-

house were used in the experiments.

76

5.2.2. Surface Treatment of Steel Plates and Friction Material

Friction material discs with 5 mm diameter were punched prior to surface

treatment with stearic acid. Stearic acid was dissolved in acetone with different

concentrations. The discs were immersed in stearic acid solution for 30 minutes. After the

treatment, they were washed with acetone multiple times to get rid of the excess stearic

acid. Then, they were dried in vacuum oven at room temperature.

Surface treatments for the steel samples were performed similarly using solutions

of stearic acid, oleic acid, and linoleic acid. Some of the steel samples were heat treated

to discern the effect of carbonitriding.

5.2.3. Contact Angle Measurements

A Rame-hart (New Jersey, USA) model 500 advanced goniometer equipped with

a rotating stage was used for the contact angle measurements. Advancing and receding

contact angles were measured by using the tilting base method. 3 to 5 measurements were

taken using 3 μL liquid depending on the reproducibility of the results.

5.2.4. SAE#2 Test

Different lubricating oils were prepared by dissolving 0.5 wt. % stearic, oleic,

linoleic acid, or 0.3 wt. % stearic acid in the mineral base oil. SAE#2 test machine was

used to evaluate the friction performances of the different oils on the same type of paper-

based friction material. The coefficient of friction - sliding speed (µ-v) curves were built

77

as a result of consecutive engagements that were done at a certain fluid temperature,

surface pressure, and sliding speed. Therefore, the µ-v curves were constructed using

discrete data points.

5.2.5. Fourier Transform Infrared (FT-IR) Spectroscopy

Attenuated total reflectance (ATR) mode of a Nicolet (Thermo Scientific, USA)

380 FT-IR spectrometer was used to analyze the surface structures of the friction

materials. 1 cm-1 resolution and 256 scans were used to obtain the spectra for each

sample.


Effects of friction modifier concentration and structure on the characteristics of

wet friction material are discussed separately as follows.

5.3.1. Effect of Friction Modifier Concentration

Almost 60% of friction materials are made of voids, so that they are not

commonly used for contact angle studies because of their high surface roughness.

Furthermore, the presence of air gaps under the liquid droplets makes the measurements

less reproducible. However, due to the hydrophobic nature of friction materials, it is

possible to get contact angle readings using water as the liquid. Water droplets are able to

maintain their shapes by resisting absorption. Figure 5-1 shows the water contact angle

78

images for a neat and a surface treated friction material at 0 and 90° tilted stage. Even at

90° tilted stage water stays on the friction material with a higher contact angle hysteresis

on the neat friction material, and a lower hysteresis on the surface treated friction

material. In our study for understanding the effect of friction modifier concentration,

friction materials were treated using stearic acid solutions with three different (low,

medium, and high) concentrations, noting that the low concentration has lack of

molecules to cover the surface completely considering a surface coverage of ~ 20 Å2 per

molecule. 116 Medium concentration has molecules barely enough to make a complete

monolayer and high concentration is able to make a perfect monolayer. The contact angle

and contact angle hysteresis results are given in Table 5-1. 91.0° water contact angle for

the neat friction material increased to 101.3°, 103.8°, and 108.3° after surface

modification with low, medium, and high concentrated stearic acid, respectively. The

more concentrated solution ended up with a higher angle because of the higher density of

the stearic acid molecules on the surface. The contact angle hysteresis for the friction

material decreased from 23.8° to 21.7°, 16.8°, and 17.3° after surface modification with

low, medium, and high concentrated solutions, respectively. Contact angle hysteresis was

previously related to the adhesion hysteresis, which is the difference in the energy

dissipated for bringing surfaces together and energy required for taking apart the

surfaces, and accordingly to the friction by various researchers. 117,118 Therefore, we are

expecting to correlate the contact angle hysteresis values we obtained for the different

surfaces with the wet friction performances of the lubricants having such molecules.

79

In order to get additional information about the density of the adsorbed molecules,

infrared spectroscopy was performed. Asymmetric and symmetric CH2 stretches for a

tightly packed monolayer are positioned at 2919 cm-1 and 2850 cm-1, respectively. 119 The

peak positions shift to higher wavenumbers in case of lower density monolayers because

the chains become more liquid-like. We observed the peak locations at 2925 cm-1 and

2854 cm-1 for the material treated with high concentration and 2928 cm-1 and 2857 cm-1

for the material treated with low concentration stearic acid (Figure 5-2). Although in

good agreement with the literature, these values suggest us that even the high

concentration solution didn’t provide a well-packed monolayer depending on the

wavenumber differences. 119 This might be due to the low concentration of functional

sites on the friction material which attracts the stearic acid for adsorption.

Figure 5-1. Contact angle images for friction material a) at 0°, and b) at 90° (tilted stage),

and friction material treated with high concentration stearic acid c) at 0°, and d) at 90°

(tilted stage).

80

Table 5-1. Contact angle and hysteresis results for neat and stearic acid treated friction

materials

Friction

Material (FM)

FM + Stearic

Acid

Low Conc.

FM + Stearic

Acid

Medium

Conc.

FM + Stearic

Acid

High Conc.

Water Contact

Angle (°) 91.0 ± 7.8 101.3 ± 13.1 103.8 ± 7.9 108.3 ± 4.9

Water Contact

Angle

Hysteresis (°)

23.7 ± 6.8 21.7 ± 1.2 16.8 ± 1.6 17.3 ± 1.9

Figure 5-2. Infrared spectroscopy of neat and stearic acid treated friction materials.

Contact angle studies on steel plates are also important because it is the main

component of the clutch reaction plate. Adsorption of friction modifiers occur similarly

81

on steel surfaces as well as on friction material surfaces. For this reason, we also

investigated the effect of concentration on steel surfaces.

We used two different stearic acid solutions as low concentration (LC) which has

lack of molecules to form a complete monolayer and high concentration (HC) in which

the concentration is well above the amount of monomers needed for a monolayer.

Besides, some of the steel samples treated with HC stearic acid were used in the

experiment without washing the excess molecules. We have compared the results for

steel and heat treated (HT) steel which are the two different cases that might be used as a

reaction plate. Figure 5-3 shows the results for steel samples. Increases in contact angle

from 72° to 87° and 102° with LC and HC stearic acid, respectively indicate the

formation of a higher density monolayer film on the surface with increasing

concentration. 94° for the non-washed samples show the presence of excess stearic acid

molecules on top of the monolayer. Interaction of the polar head of these molecules with

water causes a drop in the contact angle. Contact angle hysteresis for both LC and HC

modified steel are measured around 16°, while they are around 22° for a neat and non-

washed steel sample.

82

Figure 5-3. Contact angle and hysteresis results for steel samples.

Contact angle results for HT steel are shown in Figure 5-4. HT steel has 62° contact angle

which is lower than the neat steel. Formation of iron nitrides at the surface of a steel and

improvement on the tribological properties were previously reported. 120 9° decrease in

the contact angle and increase of the contact angle hysteresis from 22° to almost 40° due

to carbonitriding indicate higher surface energy for HT steel. Therefore, carbonitrided

steel would have a better lubricating film formed by friction modifier monolayer on its

surface because of the stronger intermolecular interactions. In our SAE#2 tests we

observed better performance with carbonitrided steel as a reaction plate. However, the

good performance was not durable for a long time which might be due to the depth of

modified layer that is only several microns thick. Thus, after a break in period the

effective layer is worn out. Surface treatment with LC and HC stearic acid increased the

contact angle to 85° and 72°, respectively and non-washed samples have 69° contact

0

20

40

60

80

100

120

Steel Steel LC Steel HC Steel NW

Co

nta

ct

An

gle

/ H

yst

eres

is

Contact Angle

Contact Angle

Hysteresis

83

angle. It is surprising to obtain hydrophilic water contact angles because the interaction of

stearic acid and carbonitrided steel is expected to be stronger considering the better

tribological performance obtained. The lower angles may suggest us the disorderly

alignment of the molecules on the surface. Our investigations on this subject are

continuing.

Figure 5-4. Contact angle and contact angle hysteresis results for heat treated steel

samples.

Coefficients of friction vs. sliding speed curves for base oil with stearic acid

having two different concentrations are shown in Figure 5-5 and Figure 5-6. The

following equation was used to calculate the friction coefficients:

𝜇 =𝑇

𝑃𝑅𝐴𝑁 (2)

0

10

20

30

40

50

60

70

80

90

100

HT Steel HT Steel LC HT Steel HC HT Steel NW

Con

tact

An

gle

/ H

yst

eres

is

Contact Angle

Contact Angle

Hysteresis

84

where μ is the coefficient of friction, T is the torque, P is the surface pressure, R is the

radius of the clutch plate, A is the friction surface area, and N is the number of friction

surfaces. The first point which has the lowest sliding speed is considered as the static

coefficient and the rest of the curve is considered as the dynamic coefficient of friction.

The shape of the complete curve is very important for different reasons. If the static

friction coefficient is too high, the excessive torque may cause transmission failure in

case of a slippage of the clutch. If the static friction coefficient is too low, there is going

to be extreme heat generation as a result of slippages which occurs because of the lack of

holding capability of the clutch. 121 A positive gradient for the overall curve is also

important in order to prevent shudder in case of engagement of the clutch.

For the analysis of μ-v graphs, μ1 (friction coefficient at 1 rpm) and μ50 (friction

coefficient at 50 rpm) were used by several authors. μ1 is used as an indicator for the

maximum torque capacity and μ50 is considered as the dynamic friction coefficient. A

value of lower than 1 for the μ1/ μ50 ratio is important in order to avoid shudder. 2 In our

study we replaced μ40 with μ50 because the discrete data point was gathered

accordingly. μ1/ μ40 results for Figure 5-5 and Figure 5-6 are given in Figure 5-7.

Lubricating oil with 0.3 wt.% stearic acid has higher μ1/ μ40 values than the lubricating

oil with 0.5 wt.% stearic acid; while both are being lower than 1 for all the pressures and

temperatures. Therefore, the higher concentration of stearic acid makes the slope of the μ-

v curve more positive, but it decreases the maximum torque capacity of the clutch.

0.3 and 0.5 wt.% for a friction modifier is a typical amount used in ATF. 94 One

may expect to see well-packed monolayers of friction modifiers with such concentrations

85

considering the very long service times of the ATF’s. Therefore, it wouldn’t be typical

for a fresh lubricant to cause a significant difference on the μ-v curves. However, as we

observed differences on the contact angles and infrared studies of the friction modifiers,

we see the effect of concentration on the μ-v graphs. We notice a difference of about 0.2

on the friction coefficient, especially at higher temperatures and pressures for both the

static and dynamic coefficients of friction.

One interesting observation from the μ-v graphs in Figure 5-5 and Figure 5-6 is

having a lower friction coefficient with a higher pressure. According to Chung, based on

his study with friction of nanodots, friction at higher loads depends on only shear forces

but there is a contribution from adhesion forces at lower loads. Therefore, friction is

higher at lower pressures. 122 In our study we obtained higher contact angle hysteresis for

non-washed steel plates than washed steel plates. This shows us that any residual

molecules on top of the monolayer create more adhesion forces. Therefore, it is possible

to consider that higher pressure helps to form a perfect monolayer by draining extra

molecules through the pores of friction material during an engagement in the SAE#2

tests.

86

Figure 5-5. Friction coefficient (μ) vs. sliding speed curves for 0.3 wt.% stearic acid in

base oil at different temperatures: A) 40 °C, B) 90 °C, and C) 120 °C.

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75

FR

ICT

ION

CO

EF

FIC

IEN

T (

µ)

SPEED (m/s)

A 40°C FLUID TEMPERATURE

415 kPa 775 kPa

1940 kPa 2960 kPa

0.06

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0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75

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T (

µ)

SPEED (m/s)

B 90°C FLUID TEMPERATURE

415 kPa 775 kPa

1940 kPa 2960 kPa

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EF

FIC

IEN

T (

µ)

SPEED (m/s)

C 120°C FLUID TEMPERATURE

415 kPa 775 kPa

1940 kPa 2960 kPa

87

Figure 5-6. Friction coefficient (μ) vs. sliding speed curves for 0.5 wt. % stearic acid in


0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75

FR

ICT

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EF

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µ)

SPEED (m/s)


415 kPa 775 kPa

1940 kPa 2960 kPa

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ICT

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µ)

SPEED (m/s)


415 kPa 775 kPa

1940 kPa 2960 kPa

0.06

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SPEED (m/s)


415 kPa 775 kPa 1940 kPa 2960 kPa

88

Figure 5-7. The ratio of coefficient of friction at 1 rpm to 40 rpm for different lubricants

at different temperatures and pressures.

5.3.2. Effect of Friction Modifier Structure

In the second part, different fatty acids were used to modify the surfaces of steel

plates because of the higher reproducibility of the results as compared to the friction

material. Stearic acid (saturated), oleic acid (monounsaturated), and linoleic acid (di-

unsaturated) are the molecules that are used for surface treatments. According to the

water contact angle results that are given in Table 5-2, the surface becomes more

hydrophobic with a higher degree of saturation for the fatty acid used for surface

modification. This is caused by better packing of the molecules on the surface. Because

of the cis isomerism for the unsaturated molecules, the distance between the tail groups

increase and this decreases the density of the molecules on the surface which leads to less

surface hydrophobicity. The effective monolayer coverage can also be seen from the

0.00

0.50

1.00

1.50

2.00

2.50

μ1

/ μ

40

Base Oil Stearic Acid Oleic Acid Linoleic Acid Stearic Acid LC

40 °C 120 °C90 °C

415 - 775 - 1940 - 2960

kPa

415 - 775 - 1940 - 2960

kPa

415 - 775 - 1940 - 2960kPa

89

calculated surface free energy results. The following equation was used for the

calculation of surface free energies: 123,124

𝛾𝑠 = 𝛾𝑙(𝑐𝑜𝑠𝜃𝑟 − 𝑐𝑜𝑠𝜃𝑟){ (1 + 𝑐𝑜𝑠𝜃𝑎)2/[(1 + 𝑐𝑜𝑠𝜃𝑟)2 − (1 + 𝑐𝑜𝑠𝜃𝑎)2]} (3)

where γl is the surface energy of the liquid used, θr is the receding angle and θa is the

advancing angle. Although this is a recently developed method for the calculation of

surface free energy and suggested to be a liquid dependent approach, our calculations are

quite in agreement with similar surfaces tested in the literature. 125,126 The use of other

polar solvents such as dimethylformaldehyde or non-polar solvents such as hexane or

base oil had very high dispersion on the surface and they did not provide good data.

Contact angle hysteresis results were obtained with smaller errors as compared to

the contact angle results similar to the first part of this study. 22.8° contact angle

hysteresis for untreated steel decreased to 18.3°, 17.2°, and 12.8° after linoleic acid, oleic

acid, and stearic acid treatment, respectively. Therefore, we expect to observe a better

tribological performance with increasing degree of saturation.

Table 5-2. Contact angle, contact angle hysteresis and surface free energies for steel

samples with different surface treatments using water.

Stearic acid

treated Steel

Oleic acid

treated steel

Linoleic acid

treated steel Steel

Water Contact Angle (°) 102.2 ± 4.8 91.0 ± 3.2 87.7 ± 3.1 72.1 ± 3.9

Water Contact Angle

Hysteresis (°) 12.8 ± 2.2 17.2 ± 0.7 18.3 ± 1.0 22.8 ± 0.6

Surface Free Energy

(dyn/cm) 25.5 ± 4.4 30.6 ± 2.2 32.7 ± 3.9 41.0 ± 3.8

90

The SAE#2 test results for neat base oil and base oil having oleic acid, and

linoleic acid are given in Figure 5-8 - Figure 5-10, respectively. In Figure 5-8, negative

slope of the μ-v curves for base oil for all of the different pressures and temperatures are

seen. This system is susceptible to shudder for all the conditions because of the absence

of the friction modifiers. Oleic acid maintains a zero slope at low temperature but it has a

positive slope at higher temperatures. At 40 °C, μ-v curves for linoleic acid is very

similar to the base oil except the coefficient of friction is slightly lower. However,

linoleic acid has less negative slope at higher temperature. The reason for the

performance which gets better at higher temperatures was explained by the higher

reactivity of molecules at elevated temperatures. 2

μ1/μ40 ratios of all different types of lubricants are given in Figure 5-7Figure 5-7.

The ratio of coefficient of friction at 1 rpm to 40 rpm for different lubricants at different

temperatures and pressures. Base oil under all conditions and linoleic acid at 40 °C and

90 °C are very susceptible to shudder. Stearic acid and oleic acid have values well below

1 which makes them better candidate to be used as a friction modifier in an ATF. These

results are consistent with previous studies but show some variations because of the

presence of porous friction material on one side of the sliding surfaces. Fox et al.

investigated the boundary lubrication performance of same fatty acids using a ball on

plate reciprocating wear test where neither surface had porosity. They didn’t observe any

difference in friction at 50 °C nor at 100 °C. 127

In a study with adsorbed film structures of fatty acids by Lundgren et al., it was

shown that stearic and oleic acids form monolayers having adsorbed solvent molecules

91

oriented in between the fatty acid chains in an upright position. However, linoleic acid

forms a bent structure where the two sliding surfaces are more susceptible to

interdigitation like with base oil molecules. 128 This study provides a good explanation to

our system.

Figure 5-8. Friction coefficient (μ) vs. sliding speed curves for neat base oil at different

temperatures: A) 40 °C, B) 90 °C, and C) 120 °C.

0.10

0.12

0.14

0.16

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1940 kPa 2960 kPa

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92

Figure 5-9. Friction coefficient (μ) vs. sliding speed curves for 0.5 wt.% oleic acid in base

oil at different temperatures: A) 40 °C, B) 90 °C, and C) 120 °C.

0.06

0.08

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415 kPa 775 kPa

1940 kPa 2960 kPa

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1940 kPa 2960 kPa

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415 kPa 775 kPa 1940 kPa 2960 kPa

93

Figure 5-10. Friction coefficient (μ) vs. sliding speed curves for 0.5 wt.% linoleic acid in


0.06

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1940 kPa 2960 kPa

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94

It was suggested that coefficient of friction decreases at higher temperatures

because the penetration of opposing monolayers increases at higher temperatures. 27

However, we believe that change in viscosity of the oil has significant contribution on the

change of coefficient of friction. The presence of friction modifiers in the base oil

provides less energetic adhesive forces between steel and friction material surfaces

because they adsorb on both surfaces (Figure 5-11b). The strong ionic-dipole interactions

turn to weaker London dispersion forces after the adsorption. Therefore, the higher

viscosity oil will cause more chain interdigitation which will end up with more adhesive

friction forces. In this study, there was no viscosity modifier additive in the base oil, so

the viscosity index (VI) of the oil is lower as compared to the commercial ATF’s.

Therefore, the effect of temperature on friction at a given pressure is significant. We do

not observe such changes in studies involving commercial ATFs in house testings. In the

other case where there is no friction modifier in the oil (Figure 5-11a), the interaction

between the opposing surfaces is strong i.e. ionic-dipole interaction or hydrogen bonding.

Therefore, better separation of these surfaces with the oil cause lower friction coefficient.

This effect can be seen in Figure 5-8 where the higher temperature provides higher

coefficient of friction especially with the lower sliding speed. This theory is also

supported by the studies of Kalin et al. 129,130 They investigated the tribological behavior

of steel/steel and diamond like carbon (DLC)/DLC contacts using oils with different

viscosity and no additives. Under dry conditions, there is a strong interaction between

two steel surfaces. Therefore, having a more effective surface separation provides lower

friction. According to this, they obtained lower friction using higher viscosity oils

95

because the strong adhesive interactions were prevented more effectively. However, the

situation is opposite for the DLC surfaces because they already have weak intermolecular

interactions under dry conditions. Therefore, the presence of oil with higher viscosity

causes additional shear forces, so they obtained higher friction.

Figure 5-11. Suggested intermolecular interactions between a) steel and friction material

and b) friction modifier adsorbed steel and friction material surfaces.

5.4. Conclusions

Stearic acid was used to investigate the effect of solution concentration on the

monolayer density which affects the contact angle behaviors and wet friction

performance. Higher concentration of stearic acid provided lower contact angle hysteresis

for both friction material and steel, as well as higher positive slope for the μ-v curves.

However, it causes a drop on the static friction coefficient which is considered as the

maximum torque capacity of the clutch.

96

One interesting observation is to have a lower coefficient friction at higher

pressure. This is attributed to the absence of adhesion forces at higher loads which might

be due to the absence of additional stearic acid molecules that bring extra adhesive

forces. The load is supposed to be supported only by the tail groups of the friction

modifiers to have a better performance.

In the second part of the paper, the effect of chain saturation on the contact angle

behaviors and wet friction performances were investigated. Contact angle hysteresis

increases as the fatty acid molecules have more double bonds. Accordingly, linoleic acid

which has two double bonds has the highest hysteresis and the worst performance due to

shudder susceptibility. Both stearic acid and oleic acid, having μ1/μ40 values lower than

1, showed good performance. However, maximum torque capacity for the stearic acid is

lower. Therefore, considering the better solubility of unsaturated molecules, oleic acid (or

monounsaturated molecules) might be a better candidate to be used in ATF. There are

more than 20 different types of additives in ATF, so the solubility of each compound is

important.

One observation from this portion of our research was to see a better performance

at higher temperature. Although activation of friction modifiers at higher temperatures

has a contribution to this effect, the viscous effects also play a significant role.

97

EFFECT OF FIBER TYPE AND FIBER/FILLER RATIO ON THE

CHARACTERISTICS OF WET FRICTION MATERIALS

Paper-based wet friction materials are important component of torque converters

for automatic transmissions. Performance and durability of these materials are critical for

automotive industry. Wet friction materials are composed of fibers, fillers, and binders.

All of these ingredients have significant effect on the strength, performance, and

durability of the paper-based wet friction materials. Therefore, it is very important to

choose the correct ingredients and use them in proper proportions.

In this study, we investigated the effect of cellulose and aramid fibers on the

strength and the performance of wet friction materials. Mechanical tests showed that

although cellulose fibers are stronger than aramid fibers as a raw material, aramid fibers

become stronger after resin saturation due to the fiber fibrillation. Air permeability of

cellulose papers improved with the addition of diatomaceous earth, while aramid papers

showed the reversed effect might be due to the larger bulk volume of aramid fibers. The

friction performances of the materials also improved with the addition of diatomaceous

earth. We observed that, the higher the oxygen atom concentration on the surface of the

98

material or the higher is the water contact angle of the friction material, the better is the

friction performance.

6.1. Introduction

Paper-based friction materials have been used in torque converters for decades.

Researches on these composite materials have mostly focused on the effects of different

fluids or test conditions and they provided important information for the improvement of

performance, durability, and reliability of the systems. 131

Wet friction materials are composed of cellulose fibers, synthetic fibers, binders,

and various fillers such as diatomaceous earth, graphite, activated carbon etc. Each of

these ingredients has effect on the friction characteristics of the material because in

addition to having different physical characteristics such as porosity and resiliency, they

have different chemical interactions with automatic transmission fluid (ATF) which is

used as the lubricant and the coolant. Therefore, any change in the structure or the

composition of these materials lead to a significant change in frictional characteristics.

5,45,132 There are also specific purposes for using each of these ingredients; for example,

cellulose fibers impart strength and provide voids for resin saturation and fluid

permeability, aramid fibers provide heat resistance, diatomaceous earth promote fluid

flow through the friction material, and phenolic resins impart mechanical and cohesive

strength. 37

Depending on the demands, new solutions were found in the history of wet

friction materials. After asbestos found to be carcinogenic, it was replaced with cellulose

99

fibers. However, cellulose fibers as organic fibers are not heat resistant. Besides, they

were found to be not as chemical resistant as synthetic fibers. Then, aramid fibers were

used as synthetic fibers because of their high temperature resistance and mechanical

fatigue strength. In the absence of graphite, addition of synthetic fibers provided good

heat resistance. 37,38

Paper-based friction materials are known to have low static to dynamic friction

coefficient ratios to provide smooth engagement of the clutch with no feeling of vibration

or jerkiness which is often referred as shudder. The main reason for shudder is having a

negatively sloped torque vs. sliding speed curve or equivalently a negatively sloped

coefficient of friction vs. sliding speed curve. Therefore, the use of paper-based friction

materials provides comfort to the driver as well as some other benefits such as fuel

economy. 133 One of the concerns with the wet friction materials might be the low torque

capacity of the clutch because of the low static friction coefficient. However, it is

possible to increase the level of friction either by using different ingredients or by

changing the physical characteristics of the same friction material. Higher coefficient of

friction is obtained with the materials that have higher porosity. They are also thermally

more stable in durability tests because in case of a lower porosity the surface

temperatures are increased during engagement due to less lubricant movement. 45

6.2. Experimental


are as follows.

100

6.2.1. Handsheet Preparation

Cotton sheets were torn to smaller pieces and soaked in a beaker of water at least

15 minutes prior to use in order to disperse them better. Then, the wet cotton pieces were

dispersed completely using a blender. The mixture was transferred into a pulper to mix

with other ingredients. After stirring cotton fibers 10 more minutes, diatomaceous earth

was added into the pulper. Latex and coagulant were added into the mixture and stirred

for a few more seconds. After the addition of the coagulant, the mixture seems more like

a complete structure because the neutralized particles are able to come closer in water.

Then, the mixture was transferred into sheet mold. Flocculant was added inside and

stirred gently until the water becomes clear. After the drainage of water, the extra water

inside the handsheet was absorbed by additional cotton sheets using a roller. The desired

thickness was given using a compression press and then the handsheet was place into

sheet drier to remove the water completely from the raw paper. Papers containing aramid

fibers instead of cellulose were prepared similarly except using a blender for chopping

the fibers.

A resol type of phenolic resin was used for the saturation of raw handsheets. After

the saturation, handsheets were dried overnight at room temperature to remove the

solvent. Then, they were cured under same conditions.

101

6.2.2. Characterization Methods

A Gurley densometer (New York, USA) was used to measure the air permeability

of the raw papers in the z- (thickness) direction. Flow of 300 mL air was measured

through the papers using a timer manually.

A Mullen tester (Massachusetts, USA) was used to measure the burst strength of

the raw papers. The raw paper was squeezed between two clamps on top of a rubber

membrane which can inflate using hydraulic pressure. The pressure at the time of the

sample rupture was noted as the burst strength of the paper.

An Instron (Massachusetts, USA) tensile tester was used to measure the tensile

properties of raw and saturated papers and shear properties of saturated papers. For the

tensile test, rectangular strips of the handsheets were used. The sample failures occurred

in between the grips. For the shear test, friction materials were bonded on steel coupons

using an adhesive. Then, lap-shear measurements were performed as described by Chen

et al. 134

A Rame-hart model 500 (New Jersey, USA) advanced goniometer was used for

the contact angle measurements. 3 to 5 measurements were done using 3 μL deionized

water depending on the reproducibility of the results.

Aspex Explorer from FEI (Oregon, USA) was used to obtain scanning electron

microscopic images of the handsheets. Energy dispersive X-ray spectroscopy (EDS) was

used to measure presence and concentration of different elements on the surfaces of the

samples tested.

102

SAE#2 test machine was used to evaluate the friction performances of different

handsheets using a commercial ATF. The coefficient of friction - sliding speed (µ-v)

curves were constructed using discrete data points as a result of consecutive engagements

that were done at a certain fluid temperature, surface pressure, and sliding speed.


Five different formulations consisting of cellulose fibers and two different

formulations consisting of aramid fibers were prepared. The raw handsheets were tested

with Gurley and Mullen testers before resin saturation. Although rupture is not a common

mechanism of failure for wet friction materials, burst strength of the handsheets provides

good information to select the correct fiber type and ratio. Gurley tester measures air

permeability through the z-direction. ATF actually flows through the radial direction to

cool the sliding surfaces during the application, but z-direction is important for draining

the fluid in case of an engagement of the clutch. Therefore, permeability of the paper is

essential for having good boundary lubrication characteristics. Besides, it is also critical

for long term friction stability because glazing will be minimized in case of having a

paper with high porosity. 16 Otherwise, the pores of the friction material will be blocked

by the ATF additives and the friction material will not have the desired performance. 110

The results of the Gurley and Mullen tests are given in Table 6-1 for all the formulations.

Figure 6-1 shows the results for the handsheets consisting of cellulose fibers. As the

cellulose content of the handsheets decreases from 100% to 20%, the burst strength

decreases from 25 psi to 3 psi and the permeability improves from 20.5 sec / 300 mL air

103

to 6.3 sec / 300 mL air. For a wet friction material it is desirable to have higher porosity

for better friction performance, as well as for thermal durability and higher strength for

mechanical durability. 45,48 According to this information, the indicated region in Figure

6-1 is the most suitable fiber composition where we can obtain combination of both

above-mentioned properties. Based on these results, we think that incorporation of

diatomaceous earth into the wet friction material helps to disperse the tightly packed

cellulose fibers, so that the composite material has more air voids inside the friction

material matrix. However, the associated decrease in fiber concentration results in a

weaker material. Changing the aramid fiber concentration in the handsheets does not

affect the burst strength of the papers and the permeability shows a reversed effect

compared to cellulose fibers (Table 6-1). Besides, comparison of aramid and cellulose

fibers shows that handsheets with 100% and 50% cellulose fibers are stronger than their

counterparts. The main physical difference between the aramid fibers and the cellulose

fibers is the higher degree of fibrillation. Therefore, the overall weakness of aramids

compared to cellulose is probably due to the failure at the fibrillated domains. Addition of

diatomaceous earth with aramid fibers increased air permeability unlike the case for the

cellulose handsheets. This might be again related to physical structures of the fibers.

104

Table 6-1. Burst strength measured by Mullen tester, air permeability measured by

Gurley tester, water contact angles, and oxygen atom concentration of different

handsheets.

Cellulose/Diatomaceous earth

Aramid/

Diatomaceous

earth

100/0 50/50 40/60 30/70 20/80 100/0 50/50

Burst Strength (psi) 25 16 12 4 3 8 8

Air Permeability (sec) 20.5 5.4 7.3 5.0 6.3 6.7 12.2

Water Contact Angle (°) 107.6 110.6 115.0 117.9 - - 104.9

Oxygen Atom

Concentration (%) 47.8 52.7 52.6 - 54.6 14.5 34.8

Figure 6-1. Burst strength measured by Mullen tester and air permeability measured by

Gurley tester for handsheets consisting of cellulose fibers.

0

5

10

15

20

25

30

0 20 40 60 80 100 120

Cellulose %

Burst Strength (psi) Air Permeability (sec)

105

Strength of the wet friction materials is important because the life of the friction

material is supposed to be same as the life time of an automotive transmission. They are

subjected to compression and shear forces during this time. Therefore, in order to

understand more about the strength of the material we performed tensile and shear tests

for both raw and saturated materials. The data for the tensile strength of the raw and

saturated handsheets is presented in Figure 6-2. 100% cellulose paper has 3.5 MPa tensile

strength and it decreases to 0.1 MPa as the fiber concentration decreases. Tensile strength

of the resin-saturated handsheets changes from 18.5 MPa to 6.1 MPa with decreasing

fiber concentration. The increase in tensile strength after saturation clearly indicates that

the main strength of the friction materials come from the phenolic resin which is used as

a binder. One of the main reasons for having tensile strength values much less in the case

of neat cellulose fibers in comparison to resin-saturated fibers is the absence of fiber

orientation. The handsheets were made without flow, thus the fibers were distributed

randomly. Tensile modulus results which are given in Figure 6-3 have the same trend, i.e.

higher fiber concentration results in higher tensile modulus. Therefore, the strength and

stiffness of the materials are closely related to each other.

Tensile data for aramid fibers can be found in Table 6-2. The data for raw

handsheets suggest that strength of the papers with 100% or 50% aramid fibers are not

really affected with the total fiber content similar to the burst strength results. Besides,

aramid papers are weaker than the corresponding cellulose papers. However, the

handsheets with 50 % aramid fibers become stronger than the handsheets with 50%

cellulose after saturation. This has to be due to the fibrillation of aramid fibers which

106

provides more surface area for binding with phenolic resin (Fibrillation of the fibers are

not specific for aramid fibers. It just depends on the refining process. The aramid fibers

we used have fibrils and cellulose fibers have smoother surface). Therefore, the phenolic

resin is able to make more crosslinking with the fibers which provide higher strength and

stiffness. The trend we obtained for cellulose and aramid fibers is in agreement with the

literature. 100% cellulose has 255 MPa, 60% cellulose + 40 % diatomaceous earth has

206 MPa, and 30 % cellulose + 40 % diatomaceous earth + 30% synthetic fibers has 157

MPa tensile strength. The large differences in the scales might also be due to the different

percentage or type of resin used. 38 The smaller resin percentage in the material may

create domains with lack of crosslinking sites which will lead to failure under smaller

forces.

Shear strength data is only available for saturated papers because the handsheets

are needed to be bonded to steel coupons for testing. The raw handsheets are not suitable

for the sample preparation. 3 formulations that are consisting of cellulose fibers in Table

6-2 have shear strength and modulus values very close to each other and these values are

lower than the tensile strength results. This is due to the lower concentration of phenolic

resin at the middle layer of the friction material according to shear direction. Handsheets

are immersed into resin solution during saturation and they are supposed to intake the

resin homogenously through the material. However, resin migrates to outer layers during

the curing because of the release of water as a byproduct which is formed as a vapor.

Therefore, failure of the shear samples always occurs at the very middle layer. Having the

failure through the paper shows us the successful bonding of the papers to the steel plates

107

because of not getting delamination or de-bonding. Besides, we are able to compare

materials with different formulations. Since we know that the main strength of the

friction material comes from the phenolic resin, we can conclude that different fiber

concentrations do not have significant difference on the shear strength and modulus.

Table 6-2. Tensile and shear properties of raw and saturated (sat.) handsheets.

Cellulose/Diatomaceous Earth

(D.E.)

Aramid/

D.E.

100/0 50/50 40/60 30/70 20/80 100/0 50/50

Tensile Strength (Raw) (MPa) 3.5 1.0 0.4 0.1 0.3 0.2 0.3

Tensile Modulus (Raw) (MPa) 635 200 115 80 62 112 72

Tensile Strength (Sat.) (MPa) 18.5 9.5 8.1 6.1 - - 12.4

Tensile Modulus (Sat.) (MPa) 2328 1628 1525 1389 - - 1932

Shear Strength (Sat.) (MPa) - 3.7 3.9 - 3.9 - -

Shear Modulus (Sat.) (MPa) - 13.8 13.8 - 11.8 - -

108

Figure 6-2. Tensile strength for raw and saturated handsheets consisting of cellulose

fibers.

Figure 6-3. Tensile Modulus for raw and saturated handsheets consisting of cellulose

fibers.

0

5

10

15

20

0 20 40 60 80 100 120

MP

a

Cellulose %

Tensile Strength (Raw) Tensile Strength (Saturated)

0

500

1000

1500

2000

2500

0 20 40 60 80 100 120

MP

a

Cellulose %

Tensile Modulus (Raw) Tensile Modulus (Saturated)

109

Water contact angle measurements were performed on saturated handsheets to

make a correlation with the wet friction performance of the materials. The raw

handsheets have high surface roughness and also tend to absorb the water droplets, so

that the reproducibility of the results decreases. However, resin-saturated handsheets give

more accurate results. The data for the contact angles are presented in Figure 6-4. 100%

cellulose paper has 107.6° angle and it increases up to 117.9° with the addition of more

diatomaceous earth into the paper. Both cellulose fibers and diatomaceous earth were

found to be hydrophilic according to water contact angle measurements in the literature.

135,136 Therefore, it is surprising for us to get hydrophobic angles since the friction

material is able to make interactions with the polar head of the amphiphilic friction

modifiers. The phenolic resin which is cured alone has 73° water contact angle, so it

doesn’t cause an increase in the contact angle. Most probably, addition of chemicals such

as latex, coagulant, or flocculants during paper making changes the surface chemistry of

the friction material. However, these chemicals do not remain on the surface of friction

material during application. The heat treatment step during or after bonding decomposes

the organic molecules which even includes the cellulose fibers on the surface of the

friction materials. The application of heat on cotton fibers turns them to activated carbon

by thermally decomposing them. 137,138 Therefore, the surface ends up with higher energy

because there are so many free radicals, lone pair electrons, and oxygen containing

functional groups in the chemical structure of activated carbon which forms as a result of

oxidation and decomposition. 139 For this reason, we used EDS on saturated papers to get

more information about the surface chemistry. We analyzed the concentration of the

110

polar atom, which is oxygen, in case of cellulose and diatomaceous earth as the main

components. According to oxygen atom concentrations given in Table 6-1, it increases

with the increasing amount of diatomaceous earth. This is an expected result considering

the general chemical formula of each ingredient. Cellulose has (C6H10O5)n formula, so the

concentration of oxygen is about 45% if we ignore H atoms. Diatomaceous earth is

mostly made of SiO2, so the concentration of oxygen is about 67%. In our measurements

oxygen concentration ranged from 47.8% to 54.8% for 100% cellulose handsheets and

20/80 cellulose/diatomaceous earth handsheets, respectively. Therefore, the higher

oxygen atom concentration on the friction material attracted more of the chemicals, such

as latex or coagulant, used during the paper making process. Consequently, the contact

angle increases because of the hydrophobic part of these chemicals remaining on the

surface. According to the contact angle and EDS results, we expect to have better wet

friction performance with higher water contact angle or higher oxygen concentration on

the surface of the handsheets. EDS results of aramid fibers may not be directly

comparable with the cellulose fibers because there is also nitrogen atom. However, the

lower contact angle for 50/50 aramid/diatomaceous earth handsheet suggests worse wet

friction performance than the corresponding cellulose handsheet.

SEM images of the raw handsheets are given in Figure 6-5. Only 100X

magnification is presented to give a larger picture of the surface. Completely random

distribution of the fibers is seen in the images. Aramid fibers exhibit fibrillation in Figure

6-5a while there is no fibrillation for cellulose fibers in Figure 6-5c. Comparison of

Figure 6-5b and Figure 6-5d shows that there is more diatomaceous earth distributed on

111

the surface of cellulose paper than aramid paper. This might be related to the specific

interactions between the fibers and fillers because both of the fibers showed different

response for the addition of diatomaceous earth filler in to the handsheets. The addition

of fillers into cellulose paper improved the air permeability, so that the interactions

between the fibers and the filler are favored. Therefore, the cellulose fibers are able to

hold more filler on the surface as we see on the images. Moreover, diatomaceous earth

formed more compact structure with aramid fibers, so it has less distribution on the

surface. This difference might be due to the different functional groups of two fibers, i.e.

higher number of hydroxyl or ether functionality as compared to the amide groups in

aramid fibers. 40/60 and 50/50 cellulose/diatomaceous earth papers do not show

significant difference in terms of particle distribution which can also be noticed from the

EDS results (Table 6-1). Very high concentration of diatomaceous earth on the surface of

the handsheet is seen for 20/80 cellulose/diatomaceous earth paper in Figure 6-5f.

Wet friction performances of the handsheets were tested using SAE#2 tester. The

handsheet with 100% cellulose couldn’t be tested because the material completely burned

during bonding. Coefficient of friction vs. sliding speed data for the rest of the

handsheets are given in Figure 6-6 and Figure 6-7. Figure 6-6 presents the data that is

gathered before break in of the friction material at 90 °C and 2960 kPa. Static friction

coefficient is almost same for all materials. However, the handsheets that are made of

aramid fibers show negative slope over the dynamic friction coefficient range. The

handsheet with 100% aramid fibers is very susceptible to shudder because of sharp drop

on the friction coefficient with increasing sliding speed. Addition of 50% diatomaceous

112

earth could recover the dynamic friction to a higher level because it was previously

shown that diatomaceous earth has very high adsorption capacity among the possible

filler that can be used in wet friction materials. 81 Handsheets with cellulose fibers have

positive slope at almost all sliding speeds. Considering that the higher slope or higher

friction coefficient is important for higher torque capacity, we can order the papers as

50/50, 40/60, 30/70, and 20/80 cellulose/diatomaceous earth from good to bad. During

the break in, the SAE#2 test clutch was slipped for 30 minutes to wear out the original

surface of the friction material. The fresh surface that appears after break in shows

different characteristics than the original surface. The main change was shown by Ingram

et al. as the increase of the real contact area as a result of break in period. 40 Therefore,

the mean contact pressure is much higher before break in. The period before the break in

is only a very short time in service and the period after break in represents the

performance of the material for much longer time.

Figure 6-4. Water contact angle values of resin-saturated handsheets.

104

108

112

116

120

0 20 40 60 80 100 120

Con

tact

An

gle

(°)

Fiber %

Cellulose / Diatomaceous Earth

Aramid / Diatomaceous Earth

113

Figure 6-7 shows the data after break in period. There are slight changes

compared to before break in period. Handsheets with aramid fibers show slightly better

performance, but they still have negative slope. Among the formulations consisting of

cellulose fibers, 30/70 cellulose/diatomaceous earth shows the best performance after

break in. Still, all of them show pretty much the same performance except there is a

negative slope for the handsheet with 20/80 cellulose/diatomaceous earth. There is a

similar study done earlier to patent the use of diatomaceous earth with a unique shape. 140

They used 10%, 30%, and 50% disc-shaped diatomaceous earth with aramid fibers and

obtained better performance and higher coefficient of friction with the diatomaceous

earth composition higher than 30%.

According to the SAE#2 test results, we can conclude that friction materials

consisting of cellulose fiber shows better performance compared to those with aramid

fibers. However, the handsheets with aramid fibers show better performance after break

in unlike those with cellulose fibers. The superior performance of cellulose papers with

highly positive friction coefficient gradient before break in is due to the presence of

activated carbons on the surface which formed as a result of thermal decomposition of

cotton fibers. Nevertheless, the thickness of the activated layer is not very deep. The

average thickness loss after the SAE#2 test is around 50 μm, probably enough to wear out

most of the more activated surface.

We performed more SAE#2 tests under different temperatures and different

surface pressures. However, the results are not much different that the results presented,

and thus will not be shown here.

114

Figure 6-5. SEM images of A) 100 % aramid fibers, B) 50 % aramid, 50 % diatomaceous

earth, C) 100 % cellulose, D) 50 % cellulose, 50 % diatomaceous earth, E) 40 %

cellulose, 60 % diatomaceous earth, and F) 20 % cellulose, 80 % diatomaceous earth

handsheets in raw form (no resin saturation).

115

Figure 6-6. Coefficient of friction vs. sliding speed for handsheets before break-in at 90

°C temperature and 2960 kPa surface pressure.

Figure 6-7. Coefficient of friction vs. sliding speed for handsheets after break-in at 90 °C

temperature and 2960 kPa surface pressure.

0.10

0.11

0.12

0.13

0.14

0.15

0.16

0 0.5 1 1.5 2

Co

eff

icie

nt

of

Fric

tio

n

Sliding Speed (m/s)

PreBI, 90 °C, 2960 kPa 100 % Aramid

50 % Aramid 50 %

Diatomaceous Earth

50 % Cellulose 50 %

Diatomaceous Earth

40 % Cellulose 60 %

Diatomaceous Earth

30 % Cellulose 70 %

Diatomaceous Earth

20 % Cellulose 80 %

Diatomaceous Earth

0.10

0.11

0.12

0.13

0.14

0.15

0.16

0 0.5 1 1.5 2

Co

eff

icie

nt

of

Fric

tio

n

Sliding Speed (m/s)

PostBI, 90 °C, 2960 kPa 100 % Aramid

50 % Aramid 50 %

Diatomaceous Earth

50 % Cellulose 50 %

Diatomaceous Earth

40 % Cellulose 60 %

Diatomaceous Earth

30 % Cellulose 70 %

Diatomaceous Earth

20 % Cellulose 80 %

Diatomaceous Earth

116

6.4. Conclusions

We investigated the effect of two different fibers, cellulose and aramid, and their

concentration on paper-based friction materials used in torque converters. According to

the burst strength, tensile and shear test results, we found that cellulose fibers are stronger

than aramid fibers mainly because of high degree of the fibrillation of aramid fibers. The

mechanical strength of the wet friction materials mainly comes from the phenolic resin

which is used as the binder. Although aramid fibers are weaker than cellulose fibers in

raw handsheets, resin saturated handsheets of aramid fibers are stronger than those with

cellulose fibers because the high fibrillation density provides more crosslink sites to the

resin.

Incorporation of diatomaceous earth into cellulose handsheets improves their air

permeability, but it decreases the permeability for aramid handsheets. However, the wet

friction performance evaluated by SAE#2 tester showed improvements for both fiber

types with the addition of the diatomaceous earth. The main reason for this is the increase

of surface polarity which is supported by the EDS data. Not only because of surface

polarity, but also due to the formation of activated surface as a result of thermal

decomposition, cellulose fibers provided better performance for handsheets compared to

aramid fibers. The μ-v performances of handsheets are also correlated with their water

contact angles. The higher the contact angle, the better μ-v performance we observed.

The fiber/filler ratio did not cause a significant difference for the performances of 50/50,

40/60, and 30/70 cellulose/diatomaceous earth handsheets. However, 20/80

cellulose/diatomaceous earth handsheet showed worse performance possibly due to the

117

poor mechanical properties of the handsheet. To sum up our findings about the

tribological properties of the handsheets, we can say that rather than increasing the

amount of filler, which provides good friction properties, in a friction material, it is more

important to concentrate on the surface of the material. Having an activated surface and

an optimized amount of friction filler will give the best performance. 41

118

CONCLUSIONS

This research provides better understanding of the intermolecular interactions

between wet friction materials and automatic transmission fluid (ATF). The

characterization techniques used in this study can be employed for the development of

new generation materials and lubricants.

In the first part of the study, we developed a technique to measure the adsorption

energy between a liquid adsorbate and a solid adsorbent using differential scanning

calorimetry. Measurements done with a paper-based wet friction material and its

ingredients provided us good information about the possible interactions occurring with

ATF. The summation of the adsorption energies of ingredients with respect to their

compositions in the friction material was in agreement with the adsorption energy of the

friction material. Diatomaceous earth which is mainly used as a filler in friction materials

has the highest adsorption capability with ATF. Organic and synthetic fibers also have

reasonably high adsorption capability, but the presence of phenolic resin binder decreases

their possible interactions with ATF. In order to support the technique we performed

other measurements using the same friction material and changing the adsorbate. The

119

highest adsorption energy was obtained with base oil containing only a type of friction

modifier and the lowest adsorption energy was obtained with only base oil that doesn’t

contain any additives. ATF and ATF with extra friction modifier have adsorption energy

in between the first two cases because of the competition of additives for adsorption.

Moreover, these different measurements revealed that either the surface has more

selectivity towards friction modifiers or has stronger interactions with the friction

modifiers.

In the second part of the study we performed laser treatment on wet friction

material and organic and synthetic fibers to improve their adsorption capabilities.

Although the presence of phenolic resin decreases the amount of surface activation, the

adsorption capabilities of both the fibers and the friction material improved with KrF

excimer laser ablation.

In the third part, adsorption energy measurements were performed with various

fillers which can be used during the preparation of wet friction materials. Depending on

the physical and chemical characteristics of the fillers they provided different adsorption

energies with the same ATF. Two different fillers with similar physical properties were

used to prepare wet friction materials to be tested with SAE#2 tester to evaluate their

friction performances. Adsorption energies and the tribological performances of the two

fibers showed correlation potential due to their chemical differences.

In the fourth part, the effect of friction modifier concentration and structure on the

surface polarity and frictional performances of friction materials were studied. Lower

120

surface energy (as a result of higher degree of adsorption) was obtained for the friction

materials when using higher surface coverage with a friction modifier according to

contact angle measurements. Therefore, lower friction coefficients with lower

static/dynamic friction ratios were obtained. Saturated fatty acid is also found to be better

in terms of decreasing the surface polarity and the friction coefficient of the friction

materials as compared to mono-unsaturated and di-unsaturated fatty acids.

In the last part, different wet friction materials were made using cellulose or

aramid fibers with or without diatomaceous earth at different compositions. Cellulose

containing materials were found to be stronger in case of raw handsheets, but after

saturating with phenolic resin aramid fibers give better mechanical properties. Addition

of more diatomaceous earth increases the permeability of the handsheets in case of both

fiber types. While the handsheets made with cellulose fibers give reasonable performance

independent of the concentration, the handsheets consisting of aramid fibers failed to give

good performance because of the lack of attractive forces towards the adsorption of

friction modifiers. The combination of both mechanical and tribological properties

suggests that the best formulations are obtained with 50/50 fiber/filler ratios mainly with

cellulose fibers.

121

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APPENDIX

SAE#2 test machine is used to study engagement characteristics of a shifting

clutch. It is useful for evaluating the characteristics of both automatic transmission fluid

and wet friction materials. A representation of an SAE#2 test machine is given in Figure

1. As the pressure is applied through the clutch pack inside the test head, the flywheel

decelerates. Three different friction values are measured during the engagement

(deceleration of flywheel) of the clutch. μi, is the initial friction value where the sliding

speed (the speed difference of friction and steel plates) starts reducing after the

application of the air pressure. μd is the dynamic friction which is often recorded at the

half of the sliding speed. μ0 is the end friction which is taken at almost zero sliding speed.

1 Representation of three different friction values is shown in Figure 2.

Figure 1. Schematic representation of an SAE#2 test machine. 2

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Figure 2. Representation of the torque response during en SAE#2 clutch engagement.

The data represented in Figure 2 is obtained as a result of a continuous slipping in

which the engagement takes only a few seconds. However, the data of the μ-V graphs

presented in this dissertation was obtained as a result of discrete engagements. In such

engagements profiles, the friction coefficients were calculated from the torque responses

at certain sliding speeds to build the curves represented in Figure 1-2. During the test, the

clutch pack was slipped for about 3 seconds at each speed and the average coefficient of

friction in that time frame was used to make the μ-V plots. In the μ-V curves we use,

static friction coefficient corresponds to μ0 and dynamic friction coefficient corresponds

to μd.

1. Ingram, M. P. The Mechanisms of Wet Clutch Friction Behaviour, Imperial

College London, 2010.

2. Kugimiya, T.; Yoshimura, N. Tribology of automatic transmission fluid. Tribol.

Lett. 1998, 5, 49–56.

μi

μdμ0

development of polymer resin-based wet friction sheet

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