PUBLICATIONS

United States Department of Commerce

Technology Admmistration

National Institute of Standards and Technology

NIST Special Publication 876

Boundary Lubrication of Silicon Nitride

Richard S. Gates and Stephen M. Hsu

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'At Boulder, CO 80303. ^Some elements at Boulder, CO 80303.
NIST Special Publication 876

Boundary Lubrication of Silicon Nitride

Richard S. Gates

and Stephen M. Hsu

Material Science and Engineering Laboratory

National Institute of Standards and Technology

Gaithersburg, MD 20899-0001

February 1995

U.S. Department of Commerce

Ronald H. Brown, Secretary

Technology Administration

Mary L. Good, Under Secretary for Technology

National Institute of Standards and Technology

Arati Prabhakar, Director
National Institute of Standards

and Technology

Special Publication 876

Natl. Inst. Stand. Technol.

Spec. Publ. 876

387 pages (Feb. 1995)

CODEN: NSPUE2

U.S. Government Printing Office

Washington: 1995

For sale by the Superintendent

of Documents

U.S. Government Printing Office

Washington, DC 20402-9325
PREFACE

This report was bom out of a cooperative program between the Chemical Engineering

Department of The Pennsylvania State University and the Surface Properties Group in the

Ceramics Division, Materials Science and Engineering Laboratory, NIST. The intent of the

program is to assist in the education of young scientists and engineers and at the same time

fulfill the mission of NIST. The program is also supported by the Department of Energy,

Office of Transportation Technologies, Office of Transportation Materials through the

Tribology Program. Under the program, young graduate smdents, after they had finished

course work, come to NIST to conduct their thesis research under joint University and NIST

staff supervision. While at NIST, the smdents are supported through the graduate

cooperative program and receive regular salary for their work. At the conclusion of their

research, they write the thesis and defend their work in the Department. Either an abridged

or an expanded version of the thesis is published as a NIST Special Publication (SP). This is

one of the reports.

This kind of cooperative program provides additional resources to the University in

their training of scientists and engineers, and at the same time, provides a steady stream of

high caliber young people who are interested in pursuing research to work at NIST and

experience research lives first hand. NIST often not only benefits directly from the results of

their labor but also gains an advantage at recruiting the best and the brightest to come to

NIST.

I would like to acknowledge the efforts of Professors Larry Duda and Erwin Klaus in

their tireless assistance and counseling of the students in the program, Mrs. Barbara Bush in

the handing of the various administrative details at Penn State, and Carolyn Sladic and Fay

Raginski here at NIST. The financial support of DOE is gratefully acknowledged.

Stephen M. Hsu

Group Leader

Surface Properties Group

Ceramics Division

Materials Sciences and Engineering Laboratory

iii

ABSTRACT

Successful use of advanced ceramics in many tribological applications requires an

understanding of the physical, chemical, and mechanical properties of the material. Physical

and mechanical data are relatively abundant for most ceramics; however, information on the

surface chemical interactions between ceramics and lubricants is lacking.

This report describes a systematic study to investigate the effects of selected organic

chemical compounds on the friction and wear of silicon nitride under boundary lubrication

conditions. A broad range of chemistries were investigated. Results are grouped according

to predominant chemical compound class as oxygen, phosphorus, halogen, sulfur, and

nitrogen-containing chemistries.

A Ball-on-Three-Flat (BTF) wear tester was used to evaluate many different organic

compounds under boundary lubrication conditions. Tests were conducted using one weight

percent of chemical compounds in paraffin oil, and the test results were compared to a base

case of paraffin oil without additive. Results indicated that some compounds could provide

significant improvement of friction and wear characteristics. Friction coefficients varied

from + 30 % to - 45 % compared with the base case. Wear was measured as the wear

increase above the elastic contact diameter (Hertzian diameter). Relative to the base case,

wear changes ranged from + 61% to - 96 %. Successful wear reduction was obtained by

phosphorus-containing compounds, several glycol compounds, oleic acid, sulfonates, a

salicylate, an imidazoline, and a phenate and was generally associated with the formation of a film in the contact region. Additional wear tests conducted using neat compounds indicated

that acids and alcohols could provide significantly better friction and wear behavior than neat

paraffin oil. In the case of some short chain (Cg-Cio) primary linear alcohols, very low

coefficient of friction (/x=0.05) and unique wear scar morphology suggested strong chemical

interactions and a possible elastohydrodynamic lubrication contribution.

More in-depth study of reaction product films from alcohol-lubricated silicon nitride

wear tests showed that organic compounds containing alcohol functional groups can react

with the surface of silicon nitride to form high molecular weight silicon-containing organic

compound films with lubricating capability. A series of static and dynamic reactions were

conducted to elucidate the reaction sequence. Based on these results, a reaction mechanism

is proposed in which alcohols react with the silicon nitride surface to form bonded alkoxides.

Subsequent reactions in the high temperature, high shear, environment of a wearing contact

result in the formation of the high molecular weight silicon alkoxide films which can

lubricate.

V

TABLE OF CONTENTS

Page

PREFACE iii

ABSTRACT v

LIST OF TABLES xii

LIST OF FIGURES xv

Chapter 1. INTRODUCTION 1

Chapter 2. BACKGROUND AND LITERATURE SURVEY 3

2.1. Lubrication of Materials 3

2.2. Structure of Si3N4 7

2.3. Properties of Si3N4 10

2.4. General Comparison of Wear Mechanisms of Metals vs Ceramics 12

2.5. Wear of Si3N4 under Unlubricated Conditions 13

2.6. Lubricated Si3N4 Studies 16

2.7. Technical Approach 20

2.7.1. Reactivity and Bond Strength 20

Chapter 3. MATERIAL CHARACTERIZATION 27

3.L Monolithic Si3N4 27

3.1.L Manufacturers Literature 28

3.1.2. Chemical Composition 29

3.1.3. X-Ray Diffraction 32

3.1.4. Raman Spectroscopy 32

3.1.5. Ceramography/Fractography 37

3.1.6. ESCA and Auger Analysis 41

3.1.7. A Surface Model for Si3N4 45

3.2. Si3N4 Powders 51

vii
3.3. Model Chemical Compounds 52

Chapter 4. EXPERIMENTAL APPARATUS AND OPERATING CONDITIONS 53

4.1. Dynamic Tests 53

4.1.1. Wear Tester 53

4.1.2. Specimen Preparation 58

4.1.3. Operating Procedure 59

4.1.4. Friction and Wear Measurement 61

4.1.5. Selection of Wear Test Conditions 63

4.1.6. Test Repeatability and Measurement Uncertainties 66

4.2. Static Tests 69

4.2.1. Batch Reactor 70

4.2.2. Procedures 70

Chapter 5. BASELINE WEAR STUDY 75

5.1. Paraffin Oil Lubrication 75

5.2. PPO Load Capacity Study 83

5.3. Speculated PPO Lubrication Mechanism for Si3N4 92

5.4. Effect of Iron Impurities 93

5.5. Friction Trace Types Observed in this Study 94

Chapter 6. SURVEY OF OXYGEN-CONTAINING CHEMISTRIES 99

6.1. Wear Tests Resuhs of 1% Oxygenates in PPO 99

6.2. Wear Test Results of Neat Compounds 115

6.3. Wear Tests on Cg Compounds 120

Chapter 7. ALCOHOL LUBRICATION 125

7.1. Effect of Alcohol Chain Length on Wear and Friction 125

7.2. Wedge-Shaped Feature Analysis 130

7.3. Effect of Water on Lubrication of Si3N4 by Alcohols 147

7.4. Alcohol Lubricated Micro Sample Wear Tests on Si3N4 148

7.5. Analysis of Worn Si3N4 Specimens 149

viii
7.5.1. FTIR Analysis of Octanol Lubrication Reaction Products 151

7.5.2. Evidence for Organo-Silicon Reaction Product Formation as Demonstrated by GPC-GFAA 153

Chapter 8. STATIC REACTION STUDY 159

8.1. TGA Analysis Method 159

8.2. Surface Reactions of Alcohols with Si3N4 Powder 163

8.3. Evidence of the Chemical Structure of Alcohol-Si3N4 Powder Reaction

Products 165

8.4. Discussion on the Nature of the Alcohol-Si3N4 Surface Reaction 169

Chapter 9. MECHANISTIC MODEL FOR ALCOHOL LUBRICATION 177

9.1. SIMS Analysis of the 6 jwl BTF Reaction Products 177

9.2. Evatuation of Lubrication Effectiveness Using the Two-Ball Collision

Test 183

9.3. Tetra-Alkoxide Heating Experiments 186

9.4. Proposed Mechanism for Alcohol and Acid Lubricated Si3N4 188

Chapter 10. SURVEY OF PHOSPORUS CHEMISTRIES 199

10.1. 1% Compounds in PPO 199

10.2. Pure Compound BTF Tests 210

10.3. EXD Analysis of Wear Products 215

10.4. Auger Analysis of Wear Products 225

10.5. FTIR Analysis of TCP Wear Product 230

10.6. Load Capacity Plot for 1% TCP in PPO 231

10.7. Effect of [TCP] on Wear of Si3N4 238

10.8. Powder Adsorption Studies 240

10.9. Effect of Iron Impurities 242

10.10. Pin-On-Disc Wear Tests 243

10.11. Powder Reaction Studies 252

10.12. Proposed Reaction Mechanism between Si3N4 and Phosphates 253

Chapter 11. SURVEY OF HALOGEN CHEMISTRIES 263

11.1. Chlorinated Compounds 263

11.2. Chlorosilanes 268

11.3. Fluorine-Containing Compounds 276

ix
11.4. Summary 280

Chapter 12. SURVEY OF SULFUR CHEMISTRIES 283

12.1. Sulfur Containing Compounds 286

12.2. Mo-S Containing Compounds 287

12.3. Sulfur Containing Detergent Type Compounds 291

12.3.1. Load Capacity Plot for 1% Mg Sulfonate in PPO 301

12.4. BTF Wear Tests at 200°C 301

12.5. Summary and Discussion 304

Chapter 13. SURVEY OF NITROGEN CHEMISTRIES 307

13.1. Dispersant Type Compounds 310

13.2. Imidazoline 310

Chapter 14. SUMMARY AND CONCLUSIONS 315

Chapter 15. RECOMMENDATIONS FOR FUTURE WORK 321

REFERENCES 323

APPENDIX A An Estimate of the Hertzian Contact Diameter for the Four-Ball

Contact Geometry 347

APPENDIX B. An Estimate of the Hertzian Contact Diameter for the Ball-on-ThreeFlat Contact Geometry 349

APPENDIX C. An Estimate of the Hertzian Contact Diameter for the Pin-on-Disk

Contact Geometry 351

APPENDIX D. Silicon Nitride Flat Specimen Preparation Procedure 353

APPENDIX E. Cleaning Procedure for Si3N4 Wear Testing 357

X
APPENDIX F. Calculation of Wear Scar Depth and Wear Volumes for the BTF,

POD, & Four-Ball Wear Test Geometries 359

APPENDIX G. Description of Analytical Techniques 363

APPENDIX H. PPO Wear Test Data on Other Si3N4 Batches 367

APPENDIX I. Temperature Calculations for the Ball-on-Three-Flat Contact

Geometry 371

APPENDIX J. Auger Analysis Data for Alcohol-Lubricated BTF Test Flats .... 373

APPENDIX K. Hamrock and Dowson Minimum Film Thickness Calculation for

BTF EHL Contacts 377

APPENDIX L. Glossary 379

A superscript numerical format is used in this publication to denote references. The

numbered references, provided after Chapter 15, are also in alphabetical order; therefore, the

numbering sequence is not the sequence in which the references appear in the text.

NOTE: Certain commercial equipment, instruments, or materials are identified in this

report to adequately specify the experimental procedure. Such Identification

does nor imply recommendation or endorsement by the National Institute of

Standards and Technology, nor does it imply that the material or equipment

identified are necessarily the best available for the purpose.

xi
I

I

LIST OF TABLES

2.1. Lines of Defence Against Wear and Friction 6

2.2. Wear Mechanisms for Metals and Ceramics 12

2.3. Reference Summary for Unlubricated Si3N4 Wear Testing 14

2.4. Reference Sumary for Lubricated Si3N4 Wear Testing 18

2.5. Reference Summary for Si3N4 Reactivity 22

2.6. Bond Dissociation Energies for Selected Silicon Bonds (Source: Dean^^) . 24

3.1. Properties of Si3N4 Materials Used 29

3.2. Manufacturers Chemical Composition Data for Typical NCI32 and

NBDIOO Si3N4 30

3.3. Neutron Activation Analysis of Si3N4 Samples 30

3.4. Chemical Composition of Stark LC 12-N Si3N4 51

4.1. Repeatability Data on PPO Lubricated Si3N4 CER024 Batch of NC132

Flats 68

5.1. Auger Analysis of Worn Surface of PPO-Lubricated Si3N4 (Values are Atomic %) 80

5.2. Characteristic Features of the Five Wear Regions for PPO-Lubricated

Si3N4 83

6.1 Friction and Wear Data on Selected Oxygen-Containing Model

Compounds 101

6.2 Summary of Neat Oxygen-Containing Compound BTF Wear Test Data . . 116

6.3 Summary of Neat Cg Series Oxygen-Containing Compound BTF Wear

Test Data 121

7.1 Summary of Neat Octylaryl Polyether-Based Alcohol BTF Wear Test

Data 126

7.2 BTF Wear Test Results on Primary Linear Alcohols 126

xii
7.3 Effect of Water on Lubrication of Si3N4 by Octanol 148

7.4 Comparison of 1.5 ml and 6/xl BTF Wear Test Results on Primary

Alcohols 149

8.1 Solubility of Silica in Alcohols (Source: Iler^^^ p. 62) 169

9.1 SIMS Mass Spectral Data for Octanol Lubricated Si3N4 Wear Test

Products 180

9.2 SIMS Mass Spectral Data for Decanol Lubricated Si3N4 Wear Test

Products 182

10.1 Friction and Wear Test Data on Selected Phosphorus-Containing Model

Compounds 201

10.2 Summary of 200°C BTF Data on 1% Phosphorus-Containing

Compounds in PPO 208

10.3 Summary of Pure Phosphorus-Containing Compound BBTF Wear Test

Data 210

10.4 EDX Analysis Data on ZDP Reaction Product Films 219

10.5 Auger Analysis Results on 1% ZDP in PPO Lubricated Si3N4 227

10.6 Auger Analysis Results on 1% TCP Lubricated Si3N4 230

10.7 Wear Test Results for POD Tests on fused Silica 248

11.1 Friction and Wear Data on Selected Chlorine-Containing Model

Compounds 265

11.2 BTF Test Results on Neat Chlorine-Containing Compounds 267

11.3 BTF Test Results on Neat Fluorine-Containing Compounds 280

12.1 Friction and Wear Data on Selected Sulfur-Containing Model

Compounds 285

13.1 Friction and Wear Data on Selected Nitrogen-Containing Model

Compounds 309

xiii
D.l Summary Si3N4 Flat Preparation Procedure 354

H.l Repeatability Data on PPO Lubricated Si3N4 CER038 Batch of NC132

Flats 367

H.2 Repeatability Data on PPO Lubricated Si3N4 CER028 Batch of NBDIOO

Flats 369

xiv
LIST OF FIGURES

2.1 Lubrication Regimes Encountered under Different Contact Severities .... 4

2.2 Unit Layers of a and B Si3N4. Filled and unfilled symbols represent

different planes of atoms, (from Saito^'^) 7

2.3 Phase Diagram for Si-O-N System (from Somiya^^^) 9

2.4 Bond Strengths and Effective Ranges for Different Bond Types (from

Gutkowski^°^) 10

3.1 X-Ray Diffraction Pattern for NCI32 Hot Pressed Si3N4 33

3.2 X-Ray Diffraction Pattern for NBDIOO HIP'd Si3N4 34

3.3 X-Ray Diffraction Pattern for Rutgers Hot Pressed Si3N4 35

3.4 Raman Spectrum for NCI 32 Hot Pressed Si3N4 36

3.5 Raman Spectrum for NBDIOO HIP'd Si3N4 36

3.6 SEM Photomicrograph of NCI32 Hot Pressed Si3N4 Fracture Surface ... 39

3.7 SEM Photomicrograph of NCI32 Hot Pressed Si3N4 Polished/Etched

Surface 39

3.8 SEM Photomicrograph of NBDIOO HIP'd Si3N4 Fracture Surface 40

3.9 SEM Photomicrograph of NBDIOO HIP'd Si3N4 Polished/Etched

Surface 40

3.10 ESCA Survey Scan of Si3N4 Surface 42

3.11 ESCA O Is Scan for Si3N4 Surface 42

3.12 ESCA Si KLL Scan for Si3N4 Surface 43

3.13 ESCA N Is Scan for Si3N4 Surface 43

3.14 Auger Depth Profile for Si3N4 Surface 44

3.15 Surface Model for Silicon Nitride 46

XV
3.16 Structure of Crystalline Silica 47

3.17 Different States of Silica as a Function of Temperature 50

4.1 NIST-Modified Four-Ball Apparatus Design 54

4.2 Four-Ball and Ball-on-Three-Flat Contact Geometries 56

4.3 Pin-on-Disk Test 57

4.4 Three-Dimensional Surface Analysis of Polished Si3N4 60

4.5 Load Capacity Plot for PPO Lubricated Si3N4 64

4.6 Friction Traces for PPO Lubricated Si3N4 at Several Loads 65

4.7 Wear-in Data for PPO Lubricated Si3N4 67

4.8 Micro-Reactor Thermal History: a) Heating, b) Cooling 72

5.1 PPO Lubricated BTF Test at 60 kg: a) Optical Photomicrograph,

b) Friction Trace 76

5.2 SEM Photomicrographs of PPO Lubricated Si3N4 Wear Scar: a) Low

Magnification, b) High Magnification (within Scar) 78

5.3 High Magnification SEM Photomicrographs of Exit Region Outside

Wear Scar of PPO Lubricated Si3N4 Wear Test 79

5.4 Auger Depth Profile of Region within Wear Scar for PPO Lubricated

Si3N4 Wear Test at 60 kg 82

5.5 Load Capacity Plot for PPO Lubricated Si3N4 Detailing Different Wear

Regimes 84

5.6 PPO Lubricated Si3N4 BTF Test at 20 kg: a) Optical Photomicrograph,

b) Friction Trace 86

5.7 PPO Lubricated Si3N4 BTF Test at 40 kg: a) Optical Photomicrograph,

b) Friction Trace 87

5.8 PPO Lubricated Si3N4 BTF Test at 80 kg: a) Optical Photomicrograph,

b) Friction Trace 89

xvi
5.9 PPO Lubricated Si3N4 BTF Test at 210 kg: a) Optical

Photomicrograph, b) Friction Trace 90

5.10 PPO Lubricated Si3N4 BTF Test at 270 kg: a) Optical

Photomicrograph, b) Friction Trace 91

5.11 POD Test Results for Rutgers Low Fe Si3N4 Lubricated by PPO 95

5.12 Different Friction Trace Types Encountered: a) Types I, II, and IV,

b) Type III 96

6.1 Summary Friction and Wear Data for Si3N4 BTF Tests on 1% OxygenContaining Compounds in PPO 100

6.2 1% Mg Salicylate in PPO Lubricated Si3N4 BTF Test at 60 kg: a)

Optical Photomicrograph, b) Friction Trace 103

6.3 SEM Photomicrographs for 1% Mg Salicylate in PPO Lubricated Si3N4

BTF Test at 60 kg: a) Secondary Electron Image, b) EDX Image

(Mg) 104

6.4 EDX Spectrum for Surface inside Wear Scar for 1% Mg Salicylate (Ca

Overbased) in PPO Lubricated Si3N4 BTF Test at 60 kg 105

6.5 1% Polypropylene Glycol Triol [600 MW] in PPO Lubricated Si3N4

BTF Test at 60 kg: a) Optical Photomicrograph, b) Friction Trace .... 107

6.6 1% Polypropylene Glycol Diol [400 MW] in PPO Lubricated Si3N4

BTF Test at 60 kg: a) Optical Photomicrograph, b) Friction Trace .... 108

6.7 1% Mannich Product in PPO Lubricated Si3N4 BTF Test at 60 kg: a)

Optical Photomicrograph, b) Friction Trace 110

6.8 1% 3-n-Pentadecyl Phenol in PPO Lubricated Si3N4 BTF Test at 60 kg:

a) Optical Photomicrograph, b) Friction Trace 112

6.9 1% Ethyl Stearate in PPO Lubricated Si3N4 BTF Test at 60 kg: a)

Optical Photomicrograph, b) Friction Trace 113

6.10 Neat HMW Diacid Lubricated Si3N4 BTF Test at 60 kg: a) Optical

Photomicrograph, b) Friction Trace 117

6.11 Neat Oleic Acid Lubricated Si3N4 BTF Test at 60 kg: a) Optical

Photomicrograph, b) Friction Trace 119

xvii
6.12 Neat Octanol Lubricated Si3N4 BTF Test at 60 kg: a) Optical , Photomicrograph, b) Friction Trace 122

6.13 Neat Cg Compound Lubricated Si3N4 BTF Test Friction Traces: a) 2-

Octanol, b) Octanoic Acid 123

7.1 Neat Octylaryl Polyether Alcohol Lubricated Si3N4 BTF Test at 60 kg: a)

Optical Photomicrograph, b) Friction Trace 127 '

7.2 Neat Decanol Lubricated Si3N4 BTF Test at 60 kg: a) Optical

Photomicrograph, b) Friction Trace 128

7.3 Neat Hexanol Lubricated Si3N4 BTF Test at 60 kg: a) Optical

Photomicrograph, b) Friction Trace 129

7.4 Load Capacity Plots for Neat Linear Alcohols (Cg, Cg, Cio) 131

7.5 Neat Hexanol Lubricated Si3N4 BTF Test at 20 kg: a) Optical

Photomicrograph, b) Friction Trace 132

7.6 Neat Hexanol Lubricated Si3N4 BTF Test at 40 kg: a) Optical

Photomicrograph, b) Friction Trace 133

7.7 Neat Hexanol Lubricated Si3N4 BTF Test at 80 kg: a) Optical

Photomicrograph, b) Friction Trace 134

7.8 Neat Hexanol Lubricated Si3N4 BTF Test at 90 kg: a) Optical

Photomicrograph, b) Friction Trace 136

7.9 SEM Photomicrographs of Neat Decanol Lubricated Si3N4 BTF Tests:

a) 60 kg Load, b) 80 kg Load 137

7.10 SEM Photomicrographs of Neat 1-Octanol Lubricated Si3N4 BTF Test

(Direction of Sliding is from Left to Right) 139

7.11 Surface Profile of Wear Scar from Neat 1 -Octanol Lubricated Si3N4

BTF Test (Compared to Expected Surface Profile) 140

7.12 Wear Scar from Neat 1 -Octanol Lubricated Si3N4 BTF Test: a) Optical

Photomicrograph, b) Interferometric Photomicrograph 141

7.13 Optical Photomicrographs of Neat 1-Octanol Lubricated Si3N4 BTF

Tests: a) 18 Second Duration, b) 180 Second Duration 143

xviii
7.14 Surface Profile of Wear Scar from Neat 1-Octanol Lubricated Si3N4

BTF Test (Compared to Original Surface) 145

7.15 Comparison of Wear Contact Morphologies: a) Neat Oleil Alcohol

Lubricated Si3N4 Wear Scar, b) General EHL Contact Interferometry . . 146

7.16 6 )Lil Neat Octanol Lubricated Si3N4 BTF Test at 60 kg: a) Optical

Photomicrograph, b) Friction Trace 150

7.17 Optical Photomicrographs of 6 fx\ Neat Octanol Lubricated Si3N4 BTF

Test at 60 kg: a) Low Magnification, b) High Magnification 152

7.18 FTIR Spectrum of Film in Wear Scar for Octanol-Lubricated Si3N4 .... 153

7.19 GPC-GFAA Analysis of Reaction Product from 6 fi\ Neat Octanol

Lubricated Si3N4 BTF Test 155

7.20 GPC-GFAA Analysis of Reaction Product from 6 Neat Decanol

Lubricated Si3N4 BTF Test 156

7.21 GPC-GFAA Analysis of Reaction Product from 6fx\ neat Octylaryl

Polyether Alcohol Lubricated Si3N4 BTF Test 157

8.1 TGA Analysis of Si3N4 Powder 161

8.2 TGA Analysis of Neat Octanol 162

8.3 TGA Analysis of Si3N4 Powder Reacted with Octanol at lOO^C for 48

Hours Immediately After Solvent Extraction 164

8.4 FTIR Spectrum of Si3N4 Powder Reacted with Octanol at 100°C for 48

Hours 166

8.5 PDSC Analysis of Si3N4 Powder Reacted with Octanol at 100°C for 48

Hours 167

8.6 CO2 and HjO Mass Spectral Analysis of TGA Gases from Si3N4

Powder Reacted with Octanol at 100°C for 48 Hours 168

8.7 TGA Analysis of the Same Powder 1 Day After Solvent Extraction .... 171

8.8 TGA Analysis of Si3N4 Powder Exposed to Octanol Vapor 173

xix
8.9 TGA Analysis of Si3N4 Powder Reacted with Octanol at lOO^'C for 48

Hours as a Function of Time Since Solvent Extraction 174

8.10 TGA Analysis of Si3N4 Powder Reacted with Octanol at 100°C for 1,

4, and 48 Hours 176

9.1 SIMS Analysis of Residue from 6 /xL Neat Octanol Lubricated Si3N4

BTF Test 179

9.2 SIMS Analysis of Residue from 6 fxL Neat Decanol Lubricated Si3N4

BTF Test 181

9.3 Two-Ball Collision Test Geometry 183

9.4 Two-Ball Collision Test Results for Unlubricated and Lubricated Si3N4 . . 185

9.5 GPC-GFAA Analysis of Neat Tetra (C^) Si Alkoxide 187

9.6 GPC-GFAA Analysis of Neat TetraCCg) Si Alkoxide Heated at 200°C

for 5 Hours 189

9.7 Two-Ball Collision Test Results for Heated Neat Tetra(C6) Si Alkoxide

Reaction Product 190

9.8 Neat 6^i\ PPO Lubricated Si3N4 BTF Test at 60 kg: a) Optical

Photomicrograph, b) Friction Trace 195

9.9 GPC-GFAA Analysis of Reaction Product from 6 ix\ PPO Lubricated

Si3N4 BTF Test 196

10.1 Summary Friction and Wear Data for Si3N4 BTF Tests on 1%

Phosphorus-Containing Compounds in PPO 200

10.2 1% Oxymolybdenum Dithiophosphate in PPO Lubricated Si3N4 BTF

Test at 60 kg: a) Optical Photomicrograph, b) Friction Trace 203

10.3 1% 2-Ethylhexyl ZDP in PPO Lubricated Si3N4 BTF Test at 60 kg: a)

Optical Photomicrograph, b) Friction Trace 204

10.4 SEM Photomicrographs of Reaction Product Film from 1% 2-

Ethylhexyl ZDP in PPO Lubricated Si3N4 BTF Test at 60 kg: a) Film

inside Wear Scar, b) Flake Outside Entrance 205

XX
10.5 1% TCP in PPO Lubricated Si3N4 BTF Test at 60 kg: a) Optical

Photomicrograph, b) Friction Trace 207

10.6 Neat Zn Diary1 DP Lubricated Si3N4 BTF Test at 60 kg: a) Optical

Photomicrograph, b) Friction Trace 212

10.7 Neat 2-Ethylhexyl ZDP Lubricated Si3N4 BTF Test at 60 kg: a)

Optical Photomicrograph, b) Friction Trace 213

10.8 Neat TBP Lubricated Si3N4 BTF Test at 60 kg: a) Optical

Photomicrograph, b) Friction Trace 214

10.9 SEM Photomicrographs of Regions within Wear Scar for Neat TCP

Lubricated Si3N4 BTF Test at 60 kg 216

10.10 EDX Analysis of Flake within Wear Scar for Neat TCP Lubricated

Si3N4 BTF Test at 60 kg 217

10.11 EDX Analysis of Wear Particles (on Teflon Filter) from Neat TCP

Lubricated Si3N4 BTF Test at 60 kg 218

10.12 EDX Analysis of Flake outside Wear Scar for 1 % Isodecyl ZDP in

PPO Lubricated Si3N4 BTF Test at 60 kg 220

10.13 SEM Photomicrographs of Wear Scar from 1% TCP in PPO Lubricated

Si3N4 BTF Test at 60 kg: a) Zero Tih Angle, b) 75° Tih Angle 221

10.14 SEM Photomicrograph of Entrance Deposit for 1% TCP in PPO

Lubricated Si3N4 BTF Test at 60 kg: a) Bubble Formed from Electron

Beam Spot Damage, b) Same Region after Bubble has Popped 223

10.15 EDX Analysis of Entrance Deposit from 1% TCP in PPO Lubricated

Si3N4 BTF Test at 60 kg 224

10.16 EDX of FeP04 Model Compound 226

10.17 Auger Depth Profile for Region Within Wear Scar of 1% ZDP in PPO

Lubricated Si3N4 BTF Test at 60 kg 228

10.18 Auger Depth Profile for Region Within Wear Scar of 1% TCP in PPO

Lubricated Si3N4 BTF Test at 60 kg 229

10.19 FTIR Analysis of Reaction Product from 1% TCP in PPO Lubricated

Si3N4 BTF Test at 60 kg 232

xxi
10.20 Friction Traces for 1% TCP in PPO Lubricated Si3N4 BTF Tests 233

10.21 Load Capacity Plot for 1% TCP in PPO Lubricated Si3N4 BTF Tests

(Compared to Pure PPO Data) 234

10.22 Friction as a Function of Test Temperature for 1% TCP in PPO

Lubricated Si3N4 BTF Test at 60 kg 236

10.23 Wear Versus TCP Concentration for Si3N4 239

10.24 FTIR Spectra for TCP Adsorption on Si3N4 Powder Experiment 241

10.25 1% TCP + 0.1% Fe Naphthenate in PPO Lubricated Si3N4 BTF Test

at 60 kg: a) Optical Photomicrograph, b) Friction Trace 244

10.26 PPO Lubricated POD Test on Fused Silica: a) Optical

Photomicrograph, b) Friction Trace 246

10.27 SEM Photomicrograph of Wear Scar on Pin from PPO Lubricated POD

Test on Fused Silica: a) Wear Scar, b) Higher Magnification of

Radial and Tensile Cracks outside Wear Scar 247

10.28 SEM Photomicrographs of Pin Wear Scars from POD Tests on Fused

Silica: a) 1% TCP in PPO, b) 1% TCP + 0.1% Fe Naphthenate in

PPO 249

10.29 Load Capacity Plot for 1% TCP in PPO Lubricated Si3N4 (Low Iron

Impurity) POD Test 251

10.30 TGA Analysis of Reaction Product on Surface of Si3N4 Powder After

Reaction with TOP 254

10.31 TGA Analysis of Reaction Product on Surface of Si3N4 Powder After

Reaction with TCP 255

10.32 Phase Diagram for SiOj-PzOg System 258

10.33 6 jul 5% TCP in PPO Lubricated Si3N4 BTF Test at 60 kg: a) Optical

Photomicrograph, b) Friction Trace 259

10.34 GPC-GFAA Analysis of Reaction Product from 5% TCP in PPO

Lubricated Si3N4 BTF Test at 60 kg 261

xxii
10.35 GPC-GFAA of Second Extraction (with Ultrasonic) of Same Reaction

Product 262

11.1 Summary Friction and Wear Data for Si3N4 BTF Tests on 1 %

Chlorine-Containing Compounds in PPO 264

11.2 Optical Photomicrographs 1% Chlorine-Containing Compounds in PPO

Lubricated Si3N4 BTF Tests at 60 kg: a) Chlorinated Paraffin, b)

Chlorinated Fatty Acid 266

11.3 1% Dimethy-n-Octadecylchlorosilane in PPO Lubricated Si3N4 BTF

Test at 60 kg: a) Optical Photomicrograph, b) Friction Trace 271

11.4 1% Methyl-n-Octadecyldichlorosilane in PPO Lubricated Si3N4 BTF

Test at 60 kg: a) Optical Photomicrograph, b) Friction Trace 272

11.5 1% n-Octadecyltrichlorosilane in PPO Lubricated Si3N4 BTF Test at 60

kg: a) Optical Photomicrograph, b) Friction Trace 273

11.6 1% (p-t-butylphenethyl)dimethylchlorosilane in PPO Lubricated Si3N4

BTF Test at 60 kg: a) Optical Photomicrograph, b) Friction Trace .... 274

11.7 75° Tilt SEM Photomicrographs of Flat from 1% Dimethy-nOctadecylchlorosilane in PPO Lubricated Si3N4 BTF Test at 60 kg: a)

Wear Scar, b) Entrance Region 275

11.8 75° Tilt SEM Photomicrographs of Flat from 1% Dimethy-nOctadecylchlorosilane in PPO Lubricated Si3N4 BTF Test at 60 kg: a)

Exit Region, b) Exit Region (Higher Magnification) 277

11.9 0° Tilt SEM Photomicrographs of Flat from 1% Dimethy-nOctadecylchlorosilane in PPO Lubricated Si3N4 BTF Test at 60 kg: a)

Exit Region, b) Exit Region (Higher Magnification) 278

11.10 Neat n-Octadecyltrichlorosilane Lubricated Si3N4 BTF Test at 60 kg:

a) Optical Photomicrograph, b) Friction Trace 279

12.1 Summary Friction and Wear Data for Si3N4 BTF Tests on 1% SulfurContaining Compounds in PPO 284

12.2 1% Benzyl Disulfide in PPO Lubricated Si3N4 BTF Test at 60 kg: a)

Optical Photomicrograph, b) Friction Trace 288

xxiii
12.3 1% Octadecyl Disulfide in PPO Lubricated Si3N4 BTF Test at 60 kg:

a) Optical Photomicrograph, b) Friction Trace 289

12.4 1% Sulfur-Molybdenum Compound in PPO Lubricated Si3N4 BTF Test

at 60 kg: a) Optical Photomicrograph, b) Friction Trace 290

12.5 1% Calcium Phenate in PPO Lubricated Si3N4 BTF Test at 60 kg: a)

Optical Photomicrograph, b) SEM Photomicrograph at 75° Tilt 292

12.6 EDX Analysis of Film Inside Wear Scar for 1% Ca Phenate in PPO

Lubricated Si3N4 BTF Test 294

12.7 1 % Mg Sulfonate in PPO Lubricated Si3N4 BTF Test at 60 kg: a)

Optical Photomicrograph, b) Friction Trace 295

12.8 SEM Photomicrographs of 1% Mg Sulfonate in PPO Lubricated Si3N4

BTF Test at 60 kg: a) Wear Scar, b) Film Within Wear Scar 297

12.9 SEM Photomicrographs of 1% Mg Sulfonate in PPO Lubricated Si3N4

BTF Test at 60 kg: a) Entrance Region, b) Exit Region 298

12.10 EDX Analysis of Film Inside Wear Scar from 1% Mg Sulfonate in

PPO Lubricated Si3N4 BTF Test at 60 kg 299

12.11 Load Capacity Plot for 1% Mg Sulfonate in PPO Lubricated Si3N4 BTF

Tests (Compared to PPO) 302

12.12 1% Mg Sulfonate in PPO Lubricated Si3N4 BTF Test at 200°C: a)

Optical Photomicrograph, b) Friction Trace 303

12.13 1% Sulfurized Hydrocarbon in PPO Lubricated Si3N4 BTF Test at

200 °C: a) Optical Photomicrograph, b) Friction Trace 305

13.1 Summary Friction and Wear Data for Si3N4 BTF Tests on 1%

Nitrogen-Containing Compounds in PPO 308

13.2 1% Succinimide in PPO Lubricated Si3N4 BTF Test at 60 kg: a)

Optical Photomicrograph, b) Friction Trace 311

13.3 1% HMW Substituted Imidazoline in PPO Lubricated Si3N4 BTF Test

at 60 kg: a) Optical Photomicrograph, b) Friction Trace 312

F.l Ball-on-Three-Flat Wear Geometry 359

xxiv
F.2 Four-Ball Wear Geometry 360

H. 1 Graphical Comparison of PPO Lubricated Friction and Wear Data for

the Three Commercial Si3N4's used in this Study 368

H.2 Average PPO Lubricated Friction and Wear Data Used for the Wear

versus Friction Plots 370

J. 1 Auger Depth Profile of Triangular-Shaped Region within the Wear Scar

for 6 fi\ Neat Octanol Lubricated Si3N4 BTF Test 374

J. 2 Auger Depth Profile of Deposit Region Outside the Wear Scar for 6 fil Neat Octanol Lubricated Si3N4 BTF Test 375

XXV
i
Chapter 1

1

INTRODUCTION

We are in the midst of a materials revolution. Many new materials have been

developed and many materials are being modified to achieve specific properties to meet the

requirements of new technologies. Advanced ceramics is such a class of new materials.

They possess high strength, hardness, light weight, wear resistance, and corrosion resistance.

They have been considered for many applications in advanced engine designs for their

strength at high temperatures necessary for improved thermodynamic efficiency. A variety

of ceramic engine components are being considered including valve lifters, piston caps,

cylinder liners, valve seats, valve stems, and exhaust-port liners.

Si3N4 is the leading ceramic candidate for engine applications. The combination of

high strength and toughness makes it well suited for such application. However, under

engine operating conditions, wear and fatigue of the components may lead to premature

failures. The lack of proper lubrication technology to protect the ceramic components has

become a technology barrier for such application.

This study examines the surface interaction of Si3N4 with a wide variety of chemical

compounds under boundary lubrication conditions. Different chemistries are discussed by

chapter, grouped by predominant atomic composition of the active functional group (oxygen,

phosphorus, sulfur, halogen, and nitrogen). A ball-on-three-flat (BTF) wear tester is used to
2

initially evaluate the boundary lubricating ability of these different chemical compounds with

Si3N4. Surface chemistry of the tribological surfaces and the nature of the boundary

lubricating films are also studied. Where appropriate, mechanisms are proposed to describe

how the compounds function.
3

Chapter 2

BACKGROUND AND LITERATURE SURVEY

2.1. Lubrication of Materials

Lubrication can be defined as the science and technology associated with the control

of friction and wear between two surfaces, under load, in relative motion. Lubricants then

are the means to accomplish such control. A lubricant may be a gas, a liquid, or a solid.

For liquid lubrication, the lubrication process can be classified into several regimes which are

dependant on the operating conditions, as depicted in Figure 2.1.

The dependence of the lubrication mechanism on the operating conditions is complex

and is a function of many variables. These variables include not only the operating

parameters, but also material attributes such as surface morphology, and physical and

mechanical properties. This functional dependence may be generalized in terms of the

severity of the contact conditions, as suggested by the scale in Figure 2.1. The severity of

contact can be used to describe the complex interactions among several operating parameters

such as load, speed, and viscosity. For example, increasing contact severity can be caused

by either increasing load, decreasing speed, decreasing fluid viscosity, or a combination of

these parameters. There are also other indirect, or secondary parameters that can affect the

contact severity. For example, the increase in surface roughness caused by fracture of the

surface or introduction of wear debris will increase the asperity pressure, hence the contact
4

Increasing

Severity

Decreasing Fluid

Film Thichness

Hydrodynamic Elastohydrodynamic Boundary

Lubrication

Regime

Fluid Film Continuous Film Contact

Negligible Deformation Elastic Deformation Elastic & Plastic Deformation

Figure 2.1 Lubrication Regimes Encountered Under Different Contact Severities

severity. As the severity increases, the lubrication mechanism moves from one regime to the

next. Under mild contact conditions, the contact surfaces are completely separated by a

continuous fluid film and the forces that keep the surfaces apart are described by the

Reynolds Equations. This is the hydrodynamic lubrication regime. It is dominated by the

fluid properties of the lubricant, especially the viscosity.

Under more severe conditions, the surfaces in the contact region are stressed to such

a degree that they begin to elastically deform. There is still a continuous fluid film

separating the surfaces; however, the film is very thin (slightly larger than the composite
5

surface roughness). This contact condition can still be described by the Reynolds Equations

with the addition of elastic Hertzian surface deformation contributions. This region is

generally referred to as the elastohydrodynamic lubrication regime.

Under still higher severity, the asperities on the surfaces begin to deform plastically

and contact. The fluid film thickness that separates the two surfaces is often smaller than the

composite surface roughness of the surfaces. Under such conditions high temperatures are

generated at the tips of the contacting asperities, and chemical reactions take place between

the surface and the lubricant. The contact conditions now are very complicated. Fluid

mechanics still play a role but the result of the lubrication processes often is controlled by

the chemical reactions and the plastic deformation processes. This is generally referred to as

the boundary lubrication regime.

It should be recognized that in actual practice, mixtures of lubrication regimes are

often encountered. Boundary lubrication involves asperity interactions, and

elastohydrodynamic lubrication theory describes elastic deformation of a surface without

contact. The pressure distribution within a contact, coupled with the distribution in asperity

heights, produces a distribution of severity within the wearing contact. This means that some

severity conditions may provide contacting (boundary) and non-contacting

(elastohydrodynamic) conditions resulting in a "mixed" mode of lubrication.

Another convenient way to think of these different types of lubrication is as "lines of

defense "^^^ against friction and wear (Table 2-1).

In this context, different means or tools are used to defend against wear. Hydrodynamics

can be used as the first line of defense. It depends mainly on the bulk fluid properties of the
6

Table 2.1

Lines of Defense Against Wear and Friction

Lubrication Regime Important Property Types

"

Examples

Hydrodynamic Fluid

Elastohydrodynamic Fluid

Material Bulk E, J' 1

Material Surface Roughness

Boundary Surface Chemical Reactiyity

lubricant, especially yiscosity (^). As the seyerity increases, the second line of defense

(elastohydrodynamics or EHD) is utilized and the macroscopic material properties become

more unportant, especially elasticity. Under still higher seyerity, contact between the

surfaces takes place and boundary lubrication is encountered. Here the chemical properties

of the surface and lubricant are important for protecting the surface. It is the chemistry in

the boundary lubrication regime that will be addressed in this study.

The significance of chemistry in boundary lubrication is demonstrated by concepts of

how lubrication may occur in this regime. One way in which chemistry may function to

reduce wear is through the formation of lubricious oxides. It is felt that the formation of

oxides of iron, for example, reduces the possibilities for bare metal-metal contact that may

lead to welding together of the surfaces, galling, scuffing, and other high deformation, high

wear processes^^^- Another important chemical reaction is the formation of

inorganic compounds such as that obseryed with tricresyl phosphate (TCP) a common
7

antiwear additive. FeP04 has been identified in the wear debris from iron-containing

surfaces lubricated with TCP^^^^. A third important chemical reaction involves the formation

of organic compounds through oxidation and degradation reactions. These reactions often

involve the metal surface as a catalyst to produce high molecular weight (HMW) compounds

that may reduce wear and friction, form sludge, or produce vamish^^^''' Metals from the

surface can also become incorporated into the HMW compounds formed^-

2.2. Structure of Si3N4

Silicon Nitride is the ceramic compound formed from Si and N in the stoichiometric

ratio of 3 silicon atoms for every 4 nitrogen atoms. The crystalographic structure and the

phase diagram relationship of Si3N4 have been extensively studied^^^' There are two

major crystalographic forms, alpha (o;), and beta (B). Both are classified crystalographically

as hexagonal; however, their unit cells are different in structure and size. Both forms are

Si

Si

A A •

o: ABCD

ft: ABAB D O

A B O

Figure 2.2. Unit layers of a and B Si3N4. Filled and unfilled symbols represent different

planes of atoms, (from Saito^^^)
8

composed of unit layers as shown in the basal plane projections of Figure 2.2. Each silicon

is bonded to four nitrogen atoms and each nitrogen atom is bonded to three silicon atoms.

Some of the bonds in Figure 2.2. (into and out of the paper) have been omitted for clarity of

presentation. The filled symbols represent atoms that exist on one layer of the structure,

while the unfilled symbols represent atoms on another, parallel, layer.

According to Figure 2.2., there are two layer types, AB and CD. Ji-Si3N4 is

composed solely of the first type of layer (AB) and repeats as ABAB. This results in

macroscopic structure that appears quite columnar - a distinguishing feature of 6-Si3N4 grains

in the monolithic material. The second layer type (CD) is actually a mirror image of the

first (AB) layer type. Q;-Si3N4 is composed of layers of ABCD. The resulting macroscopic

Q;-Si3N4 grains appear much more equiaxed than those of B-Si3N4. It is generally recognized

that the a-Si3N4 phase actually contains a small amount of oxygen, making the stoichiometry

Si23N3oO. It is not known exactly how the oxygen is incorporated into the structure.

Si3N4 does not melt, but decomposes at « 1871 °C to yield Si and Nj. For this

reason, high sintering temperatures are avoided in the fabrication of monolithic Si3N4.

Below the decomposition temperature, there are several important silicon containing phases

relevant to the Si3N4 system. The thermodynamic phase equilibria relationships between

these different forms are usually expressed in terms of partial pressures of oxygen and

nitrogen as shown in Figure 2.3. Looking at the higher temperature data (solid lines -

1800°K), at low partial pressures of nitrogen, silicon and silica (SiOj) are the stable phases,

with silica being favored at higher oxygen partial pressures. At higher nitrogen partial

pressures, the nitrogen containing phases of B-Si3N4, and Si2N20 (silicon oxynitride) become
stable, with l5-Si3N4 favored at lower oxygen

partial pressures. According to this figure, aSi3N4 is not stable at high temperatures at any

combination of partial pressures of oxygen and

nitrogen. When the temperature is lowered (to

1600°K - dotted line of Figure 2.3.) formation

of Q!-S i3N4 is possible at oxygen partial

pressures intermediate to those required of

SijNjO and B-Si^N^. Because of these

observations, Q!-Si3N4 is generally regarded as

-16

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SiOj

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1 800"K

1600"K 4-2 0

Log(P^), MPa

Figure 2.3. Phase Diagram for Si-O-N

System (from Somiya^^^)

the low temperature form of Si3N4 while fi-Si3N4 is thought of as the high temperature form.

Monolithic ceramics are produced by a thermal densification process known as

sintering. It is necessary to densify ceramics because open spaces known as pores can act as

structural defects, resulting in fracture under relatively low stress. In sintering, powders are

heated until the surfaces of the powders fuse together. This can occur through a variety of

mechanisms such as evaporation-condensation, solution-reprecipitation, and diffusion

(surface, boundary, or lattice). Because Si3N4 decomposes instead of melting, it is very

difficult to achieve densification through sintering without the use of sintering aids. Metal

oxides such as MgO, AI2O3, and Y2O3 are used, to allow lower melting liquid phases

(eutectics) to form with silicon nitride and silica and allow fully dense Si3N4 to be made.

During sintering, most of the sintering aids tend to concentrate in the glassy phase that binds
10

the individual grains of the ceramic together. This glassy phase also tends to collect the

impurities that may be present in the original fine powder.

2.3. Properties of Si3N4

The fundamental difference in the nature of interatomic bonding between atoms in

ceramics and atoms in metals leads to differences in physical and chemical properties. The

five basic types of bonds (ionic, covalent, metallic, hydrogen, and Van der Waals) are

described in detail in many references such as Kingery^^°, Gutowski^°^, or Richerson^'°.

Figure 2.4 demonstrates a graphical method used by Gutowski to compare the different bond

strengths and effective ranges.

Covalent bonds are strong bonds

that are formed when electrons are

shared between atoms of similar

electronegativity. Ionic bonds result

when complete electron transfer takes

place between atoms of different

electronegativity. Coulombic attraction

between the resulting charged ions result

in strong bonds. Bond energies are Figure 2.4 Bond Strengths and Effective

similar in magnitude to covalent bonds. Ranges for Different Bond Types

(from Gutowski^°^)

Both ionic and covalent bonds act over

relatively short intermolecular distances.
11

Metallic bonds are moderate in strength. These bonds are the result of attractive

(coulombic) forces between positive metallic atoms with commonly shared free negative

electrons. These bonds are not as directional in nature as either covalent or ionic bonds, and

are effective over a longer range as shown in Figure 2.4. Two characteristics of the high

electron mobility of metallic bonds are high electrical and thermal conductivity, properties

which are used to classify solids as having metallic character.

Hydrogen bonds are relatively weak bonds formed when hydrogen acts as a bridge

between its primary bond to an electronegative atom (such as in O-H) and another

electronegative atom such as O, F, or N.

The fifth type of bonds between atoms are Van der Waals bonds. These bonds arise

from dipole-dipole interactions between atoms and result in weak bond strengths; however.

Van der Waals bonds act over a relatively long range, as shown in Figure 2.4.

In ceramics, interatomic bonding consists of a combination of ionic, covalent. Van

Der Waals, and hydrogen bonding, with ionic and covalent being the most important^^°. One

significant feature of these types of bonding is that electrons are localized to specific atoms

or shared between pairs of atoms. As a result, bonding tends to be somewhat directional in

nature. Many ceramics contain bonding with mixed covalent and ionic character. The

degree of covalent versus ionic character for a material depends on the difference in

electronegativity between the atoms as described by Richerson^'°. For example, Si02 is

considered to be approximately 50% ionic and 50% covalent. Si^N^ is 70% covalent in

character and 30% ionic.
12

One manifestation of the difference in properties between ceramics and metals can be

demonstrated by the stress-strain relationships, as detailed in several references

Metals exhibit a linear (elastic) response to stress up to a certain point when the material

begins to plastically deform and yield. Ceramics typically exhibit a much steeper stressstrain relationship (i.e. less strain for a given stress). In addition, instead of yielding

plastically, most ceramics fail by brittle fracture. This basic difference of the ductility of

metals and the fracture of ceramics leads to fundamental differences in the wear mechanisms

for these two classes of materials.

2.4. General Comparison of Wear Mechanisms of Metals vs Ceramics

The dominant wear mechanisms for metal systems have been described by adhesion,

abrasion, corrosion, plastic deformation, fatigue, and delamination, as summarized in Table

2.2. Adhesion and abrasion are the most

common mechanisms cited. Experiments

performed in vacuum have shown that clean

metal surfaces, brought together, will adhere

strongly to each other'*^. This adhesion can

cause high wear and friction. Therefore, one

of the lubrication concepts for metals is to

provide surface barrier films to reduce or

Table 2.2

Wear Mechanisms for Metals and Ceramics

Metals: Adhesion

Abrasion

Corrosion

Fatigue

Delamination

Ceramics: Brittle Fracture

3rd body Abrasion

prevent adhesion. This can be accomplished through adsorbed and reacted surface chemical

layers.
13

The dominant wear mechanisms for ceramics are different forms of brittle fracture.

This has been seen at both the microscopic scale, and as gross fracture of the material.

Fracture has been observed to take place both inside the grains (intragranular or

intraparticle), and between the grains (intergranular or interparticle). When interparticle

fracture occurs, the entire grain may be removed from the surface due to preferential fracture

along the grain boundary. This is commonly referred to as grain pullout. Brittle fracture

can occur because of high localized (point) stresses that can originate from a variety of

sources such as asperity tips, exposed edges of grains, or trapped particles (debris).

Adsorbed layers of molecules or reacted surface chemical layers may not be sufficient to

modify the stress distribution enough to mitigate fracture.

2.5. Wear of Si3N4 under Unlubricated Conditions

Several studies have been conducted on the tribological properties of Si3N4. Some

studies focused on the unlubricated wear of Si3N4 for material evaluation purposes. Table

2.3 summarizes the unlubricated wear studies conducted on Si3N4. Most of the tests were

conducted under relatively light loads, with contact pressures in the range of 1 to 100 MPa.

Environments ranged from high vacuum to inert gas (nitrogen, argon) and dry air. A wide

range of speeds and temperatures have been covered. Despite the range of operating

conditions and different Si3N4 materials, specific wear rates for unlubricated Si3N4 are fairly

consistent at lO"' to 10"^ mmVNm. Enomoto^^ calculates wear rates of only 10"^ mm^/Nm for

his very slow speed, light load tests. Coefficients of friction of from 0.5 to 1.0 are common

for unlubricated Si3N4. In general, wear rates are highest in vacuum, and inert gas (argon.
14

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15

N2). When air with different trace levels of water is introduced, specific wear rates are

usually observed to drop by an order of magnitude. The wear rate drops further when water

vapor concentration is increased.

Observations of the worn surfaces include plastic deformation, fracture, and formation

of compacted debris. When air is present, transfer films rich in oxygen are sometimes

observed. The presence of water in the air seems to enhance the formation of these transfer

films.

Tomizawa^^^ found that the wear of Si3N4 in argon or air was dominated by fracture

over the temperature range from room temperature to 800 °C. The coefficient of friction was

0.8. In humid air, he found that the friction coefficient dropped to 0.2, and a soft film was

formed that protected the surface from gross fracture. These films were only successfiil

within a window of temperature (150°C to 650°C) and film failure within these temperatures

could be induced by increasing speed and load. Higher temperatures required higher loads

for film failure. Auger analysis of the worn surface after a test in humid argon found an

increase in surface oxygen.

Akazawa^'^ looked at rolling contact of Si3N4. Using energy dispersive x-ray analysis,

he determined that the surface had become rich in oxygen at the end of th,e test. He noted

that a glassy film had formed on the surface that contained some crystalline material.

Infrared spectrometry indicated the presence of Si02 surface groups.

Enomoto^^ looked at plane contact (sliding) of Si3N4 to find that air decreased wear

rates over dry nitrogen. He attributed the effect to oxidation of the Si3N4 surface. He also
16

noted OH absorbance peaks in the infrared spectra of wear products that he ascribed to

adsorbed water and/or silanol (SiOH). Ura^"^, Tsuya^"*^- and Sasaki^^^ also showed that the

water in the atmosphere affected the wear of Si3N4 as well as other ceramics (SiC, alumina,

and zirconia).

Boch^' et al studied the wear mechanisms of SiC and Si3N4 under a variety of (mostly

unlubricated) conditions. Their observations were described in the context of what they

termed velocity accommodation mechanisms (VAM's). Simply put, this approach classifies a

tribological interface into five regions or sites known as accommodation sites. Each interface

contains two surface sites (SI and S5) and two boundary layers (S2 and S4), and an

interlayer (S3). There are four different accommodation modes (M1-M4) of elastic, rupture,

shearing, and rolling respectively. These terms then serve as a kind of tribological shorthand

to describe the location and mode of the various wear mechanisms under different conditions.

For example, using this shorthand, a third body roll formation observed for Si3N4 (usually

associated with higher temperatures and humid air as found by Tomizawa^^^) would be

characterized as VAM S3M4.

2.6. Lubricated Si3N4 Wear Studies

Lubricated ceramic friction and wear smdies have been conducted on a wide variety

of Si3N4 materials under many different conditions. Table 2.4 summarizes the representative

studies. The lubricants used are mainly mineral oils, conventional automotive lubricants,

water, and alcohols.
Enomoto^^ in 1987 first noted a "zero wear" regime for Si3N4 lubricated by a white

oil under very light load. Habeeb'^^ examined the wear Si3N4 using a base oil, oxidized

species such acids and peroxides, and zinc dithiophosphate (ZDP) in base oil. He found that

the addition of 0.4% lauric acid or 1% t-butyl hydroperoxide increased the wear of Si3N4.

The use of 1 % ZDP in base oil decreased the wear. Jahanmir^'*^ found that both cetane and

cetane + 0.5% stearic acid gave relatively high wear. The stearic acid was shown to reduce

friction slightly (fi dropped from 0.12 to 0.10).

The effect of water on the wear of Si3N4 was pioneered by Sugita^^° who suspected

that the surface was reacting with water to produce a hydrated amorphous silica which was

removed by rubbing. This silica was soluble in the water and could be found in the water

after the test. A series of papers published between 1987 and 1990 by Tsunai,^'*^

Tomizawa,^^^ Hibi,^^° Mizutani,^^^ and Ura^'*^ expanded on the water lubricated Si3N4 work.

In these tests, wear and friction were high and the proposed explanation was the reaction

between Si3N4 and water to form a hydrated silica species. The proposed reaction was:

Si^N^ + eH^O - 3Si02 + 4NH^

Tomizawa,^^^ and Mizutani,'^^ both found extremely low friction after wear-in that they

interpreted as elastohydrodynamic lubrication achieved through tribochemical reaction of

water on the surface of Si3N4 and subsequent dissolution of the silicic acid [Si(0H)4] in

water. Essentially, the water reacts with the surface to chemically polish down the asperities

and result in mirror finish surfaces that can then support EHD lubrication. Mizutani et al.

also investigated the effect of different concentrations of sulfuric acid on the wear of Si3N4.
18

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He noted that 20% H2SO4 had the highest level of wear and attributed this to grain boundary

corrosion.

Tsunai,^'*^' and Hibi^^° conducted a series of wear tests on Si3N4 using alcohols and

found that they reduced friction and wear below that of the unlubricated case. Longer chain

alcohols (>C5) gave lowered friction and wear more than shorter chains. They proposed a

tribochemical reaction mechanism of surface esterification of the alcohols and subsequent

dissolution of the esters by alcohol. Short-chain alcohols were thought to have higher wear

than long-chain alcohols because they react with the surface at faster rate and corrosive wear

might have resulted. The authors, however, did not provide any direct chemical evidence to

support this hypothesis. Hibi^'^ also conducted tests using aqueous solutions of amine silane

coupling agents to find reduction in friction and wear below that of the water case. Very

low friction coefficients of less than 0.01 were observed; however wear rates of 10'^ were

still present. Viscous films were observed in the wear scar that he attributed to the

formation of siloxane polymers on the surface. He attributed the low friction to EHD or

mixed lubrication from the viscous surface film. Lower wear was attributed to the

prevention of hydrolysis of Si3N4 by water.

In summary, the literature provides incomplete and sometimes conflicting information

on what will lubricate Si3N4. Some key reactions were identified: surface oxidation,

hydrolysis, and lubricant polymerization. For example, oxidation of Si3N4 plays a major role

in the lubrication of Si3N4. Along with oxidation, hydrolysis by water is another key

reaction, although it is not always clear whether hydrolysis is beneficial or detrimental.

Alcohols appear promising as potential lubricant under a very limited condition, but their
20

mechanisms are not clear. Base oils seem to provide good lubrication at low loads, and

some additives seem to show lubrication improvements within certain conditions. What is

needed is a systematic approach to investigating the boundary lubrication of Si3N4 to address

the elementary questions of what works, under what conditions, and how.

2.7. Technical Approach

The approach to understand the lubrication mechanism of Si3N4 is best described

within a framework of boundary lubrication mechanism hypothesized for this material. In a

very simplistic representation, boundary lubrication of Si3N4 takes place through a chemical

reaction on the surface. This can be represented in terms of a generalized chemical reaction

as:

Si^N^ + A* ^ Lubricating Product

where "A*" represents a chemical compound which is capable of reacting with the Si3N4

surface to provide a lubricating product. This is a very simplistic "idealized" representation

of the system; however, it emphasizes the importance of the two key concepts of chemical

reaction and lubrication.

2.7.1. Reactivity and Bond Strength

As mentioned previously, the literature is relatively sparse on detailed information on

what will chemically react to boundary lubricate Si3N4. One approach to developing concepts

on what reactions might be successful involves looking at the more general notions of
reactivity of Si3N4 and bond strengths for different silicon compounds. Some of the

references for reactivity of Si3N4 are summarized in Table 2.5.

We know that Si3N4 will decompose (in inert atmosphere) at 1870°C. In the presence

of oxygen, Si3N4 will oxidize significantly at temperatures above 1000 °C. Oxidation rate

decreases with increasing oxide film thickness that is described by a parabolic rate

expression^^^.

There is also significant evidence for reaction with water^^"*' mostly derived

from observations of ammonia gas evolution, and the presence of silica. The proposed

reaction sequence is:

This reactivity with water is also consistent with wear test results on the effect of humidity

on wear of Si3N4^^^ as well as wear test results conducted in liquid water.^^'^' 237,241, m

Evidence for reaction of Si3N4 with alcohols is provided by Hattori et al.^" who

conducted reactions between short-chain alcohols (methanol to propanol) and Si3N4 and found

that ammonia and amines were produced. They proposed the following reaction took place

on Si3N4.

Si^N^ + 6ROH ^ SSiO^ + 2R^N + 2NH^

They also speculated that more complex chemical reactions were occurring to form Si-OR

groups on the surface; however, no direct evidence of these reactions were given. No wear

tests were conducted in their study.
22

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High temperature corrosion studies have indicated that Si3N4 is chemically attacked by

chlorine^^, sulfates^^"' and carbonates^^°' ^. While the temperatures used for these

corrosion studies may be too high to be directly applicable to temperatures found in a

lubricated ceramic contact, they at least point to some form of surface reactivity that may be

exploited. Sato^^° studied the corrosion behavior of Si3N4 exposed to alkali sulfates and

carbonates under nitrogen and air environments at temperatures in excess of 700 °C. He

found that the Si3N4 actually dissolved into the melts due to chemical reactions on the surface

that could be described by a shrinking core kinetic model. Si3N4 is also known to react with

molten hydroxides (NaOH, KOH) at elevated temperatures (>300°C). In fact, molten

hydroxides (NaOH, KOH) are commonly used as etchants to expose the grain boundaries of

Si3N4. The only drawback of using this method is that these hydroxides are so aggressive

that it begins to dissolve the grains themselves. Si3N4 does not appear to be readily attacked

by any organic or inorganic acids, including HF. Lay'^ ascribes HF with the ability to react

with Si3N4, but long exposure times (days) are necessary.

Bond dissociation energies can be used to estimate relative bond strengths for

different silicon bonds. Since the bond dissociation energies are related thermodynamically

through AHf, comparison of their values can give an approximation of the ease of forming

particular bonds. Bond dissociation energies for several silicon-based bonds are summarized

in Table 2.6.

The bond energy for Si-N is listed as 439 kJ/mol. The data indicates that Si-0 has a

bond energy of 770 kJ/mol, significantly higher than Si-N, suggesting that oxygen-containing

compounds are good candidates for this study. The data also suggest that sulfur and halogen
24

chemistries may also be successful. For this reason, we selected compounds based on

oxygen, sulfur and halogens (chlorine). Some fluorinated compounds were also used, but

because of solubility limitations in

paraffin oil, they were tested neat. Table 2.6

Bond Dissociation Energies for Selected Silicon

In addition to corrosion and bond Bonds (Source: Dean^*)

strength inference, there are certain Dissociation Enerev

(kJ/mol) (kcal/mol)

compounds that have the ability to Bond Mean + la Mean + la

Si-H 297 4 71 1 modify the network structure of glasses. Si-Si 318 21 76 5

Si-Br 343 50 82 12

Since the surface of Si3N4 is apparently Si-C 435 21 104 5

oxidized, it was felt that these network Si-N 439 38 105^ 9

modifiers might beneficially alter the Si-Cl 439 50 105 12

Si-F 485 50 116 12

surface oxide film to reduce friction and Si-S 619 13 148 3

Si-0 770 13 184 3

wear. Phosphorus is a known network

modifier for silica and was selected for study based on this concept.

The experimental plan for this study was formed in order to systematically address the

important issues outlined above. Different chemistries were chosen, loosely based on the

concepts of reactivity outlined previously. The basic sequence of investigation consisted of

initial wear test screening of candidate compounds to determine successful chemistries. This

was followed by an expanded study of the composition and morphology of the reaction

product films. In the case of alcohols and phosphorus-containing compounds, additional

experiments were conducted to try to reveal the nature of the surface interactions. An indepth examination of the mechanism of lubrication of Si3N4 by alcohols was launched when it
became apparent that this class of compounds had a significant and potentially identifiable

reactivity with the Si3N4 surface under boundary lubrication conditions.

Chapter 3

27

MATERIAL CHARACTERIZATION

The materials used in this study are a combination of commercially available and

experimental research materials. The monolithic silicon nitrides used for the survey of

chemistries have been commercially available for many years and represent some of the most

tested Si3N4 materials. An experimental, low iron, Si3N4 was specifically developed for use

in this study to explore a particular aspect of Si3N4 surface reactivity. This material was

made by Ajoy Zutshi at Rutgers University. The Si3N4 powder used for reactivity studies

was selected not only because of its high purity, but also because of its high surface area.

The chemical compounds studied were obtained from commercial sources in as pure a form

as possible.

3.1. Monolithic Si3N4

The monolithic Si3N4 materials used in this study were primarily commercially

available magnesia sintered materials manufactured by Norton Company and known as

NCI32 and NBDIOO. These two materials start from the same powder processing step but

differ in their heat treatment to form the final dense Si3N4. NCI 32 is hot pressed. That is to

say, it is uniaxially compressed in a graphite die as it is heated. The result is a fully dense

Si3N4 with some grain orientation due to the orientation of the pressing. The NBDIOO Si3N4
28

is Hot Isostatically Pressed (HIP), which means it is encapsulated and pressurized while it is

heated. Pressurizing the sample isostatically during heating allows the material to sinter

more uniformly since the pressing force is in all directions inward on the sample. The result

is a fully dense Si3N4 without the orientation of grains seen in the hot pressed material.

Since both Si3N4's start from the same powder specifications, they have essentially the same

bulk chemical composition; however, because of the difference in processing technique and

conditions, the microstructure and microscopic chemistry (i.e. glassy phase composition etc)

are different.

In the course of this research, it was found that the commercially available Si3N4

materials have an iron content of approximately 0.2% (2,000 ppm) Fe, which might catalyze

some reactions and affect the surface reactivity. A low-iron content Si3N4 material was

obtained from Rutgers University to study the tribochemical intercations of Si3N4 and selected

chemistries in the absence of Fe.

3.1.1. Manufacturers Literature

Typical properties of the monolithic Si3N4 used in this study are presented in Table

3.1. The values given for the commercial material were provided by the manufacturer.

There is no discemable mechanical or thermal property difference between the two

commercial materials.

The data indicate that all three of the Si3N4 materials used are similar in physical and

mechanical properties. The densities are close to the theoretical density, indicating a low

level of porosity. In the case of NC132, the porosity has been estimated to be less than 0.1%
29

Table 3.1

Properties of Si3N4 Materials Used

Property Units Commercial —

Rutgers

Density g/cm^ 3.2 3.2

Hardness kff/mm^ 2000 1700

Strength Flexural GPa 1.0

(@20°C) Compressive GPa 3.0

Hertz Compressive GPa 10.0

Fracture Toughness MPam'^ 5.4 5.5

Young's Modulus GPa 310 325

Poissons ratio 0.28 0.28

Thermal Expansion Coeff. 10-^/ °C 3.5

Thermal Conductivity W/m°K 32

a Data supplied by manufacturer for NC132 and NBDIOO Silicon Nitride.

3.1.2. Chemical Composition

Since we are primarily interested in the surface chemical reactivity of these materials,

the chemical composition is important. The manufacturers data on the typical chemical

composition of NC132 and NBDIOO Si3N4 are given in Table 3.2. Both commercial

products start with the same powders and have MgO and AI2O3 added as sintering aids. The

main impurities are oxygen, tungsten, and iron. Some of the oxygen comes from the

magnesia and alumina which are intentionally added; however, some of it comes from the

oxidized silicon layer that exists on the surface of all Si3N4 powders used as starting

materials. Tungsten (W) probably is from the tungsten carbide grinding media commonly

used to produce these powders. The values given by the manufacturers are only

approximate. The actual concentrations of elements (especially metal impurities) was
30

considered so important that Neutron

Activation Analysis (NAA) was carried out Table 3.2

Manufacturers Chemical Composition Data

on representative samples of all of the for Typical NCI32 and NBDIOO Si3N4

Si3N4 used in wear testing. The data are

-

Max.

Component Approximate Allowed

presented in Table 3.3 in order of % %

Si3N4 93.3

decreasing abundance (for CER024 hot 0 3.0

W 1.8 3.0

pressed Si3N4). Mg 0.8 1.0

Fe 0.3 0.75

The neutron activation analysis data Al 0.2 0.5

Mn 0.05

agree in large part with the manufacturers Ca . 0.01

data. For example, NAA found 2.25%

Table 3.3

Neutron Activation Analysis of Si3N4 Samples

felementl in Silicon Nitride, ppm

Element CER024' CER038' CER0282 CERO433 CER044*

W 22500 ± 1 32000 ± 1 18500 ± 1 Mg 5500 + 2 7760 + 1 6200 + 2 600 + 7 250 + 8

Fe 1900 ± 24 3790 ± 16 2000 ± 20 < 10 < 10

Al 1500 + 1 2250 + 1 1400 ± 1 24900 ± 1 21000 ± 1 Co 1400 ± 1 2000 ± 1 1300 ± 1 1 1 Ta 380 ± 1 730 ± 1 330 ± 1 Ti 221 ± 15 265 ± 12 163 ± 20 < 10 < 10

Na 220 + 19 617 ± 14 690 ± 9 39 + 25 36 ±19

Mo 100 ± 9 39 ± 32 90 ± 8

Mn 75 + 1 89 ± 1 95 ± 1 Cr 48 ± 15 39 ± 21 48 ± 12 5 : 4

All confidence limits are given as a percentage (+ Icr) of the [].

' Norton NCI 32 Hot Pressed SijN4

^ Norton NBDIOO HIPPED SijN,

^ Rutgers 10% Sintering Aid Hot Pressed Si3N4 (5% AljO,, 5% YA)

Rutgers 7% Sintering Aid Hot Pressed Si3N4 (5% AlA. 2% YjO,)

Indicates [] is below detectable limits
(22 500 ppm) W in CER024, and 3.2% (32 000 ppm) in CER038 while the manufacturers

literature estimates 1.8%. One impurity that showed up in the commercial material that was

not mentioned in their literature was cobalt. This was present at 1.4% in CER024, 2.0% in

CER038, and 1.3% in CER028. The ratio of W:Co is fairly constant for the three samples

and seems to suggest that these two metals are associated. Since cobalt is used as part of the

binder matrix in tungsten carbide materials, both tungsten and cobalt probably come from

wear of the tungsten carbide grinding media used to mill the powders. Titanium and

tantalum may also come from the tungsten carbide. There is a variation of concentration of

impurities in the commercial materials from sample to sample. For example, the

concentration of iron in NC132 varies from a low of 0.19% to a high of 0.38%, an increase

of 100%.

The experimental Rutgers Si3N4 materials were essentially free of any iron

contaminants. They also did not contain any tungsten or cobalt, since the powders from

which they were produced were not ball milled with tungsten carbide balls. Aluminum is

present in high concentration because it is one of the two sintering aids added to the Si3N4

powder to aid densification (Y2O3 was the other). The only other impurities observed at any

appreciable concentration was magnesium. The observation in the Rutgers samples that the

2% Y2O3 sample had 250 ppm Mg and the 5% Y2O3 sample had 600 ppm Mg suggests that

the Mg is an impurity in the Y2O3 added to aid densification of these samples during

sintering. The NAA technique used was not able to quantify yttrium, carbon, silicon, or

oxygen.
32

3.1.3. X-Ray Diffraction

X-ray diffraction (XRD) was used to identify the major crystalline phases present in

the Si3N4 samples used in this study. The sampling depth for this technique is approximately

100 jwm for Si3N4 therefore this can essentially be considered a bulk technique. According to

the data obtained, the hot pressed and HIP'd Si3N4 were composed of different phases.

Hot pressed Si3N4 NCI32 (Figure 3.1) had two identifiable phases. Most of the

sample consisted of i(3-Si3N4, with a small amount of silicon oxynitride (SijNjO) also

detected. The other peaks in the spectra were not identified.

HIP'd Si3N4 NBDIOO had three identifiable phases (Figure 3.2). Most of the

NBDIOO consists of a mixture of Q!-Si3N4 and i3-Si3N4, with small amounts of tungsten

carbide (WC) also present.

i3-Si3N4 was the only crystalline phase detected in the Rutgers Si3N4 sample (Figure

3.3).

3.1.4. Raman Spectroscopy

Raman spectroscopy was performed on polished samples of the commercial Si3N4's to

further characterize them. The sampling depth for the /x-Raman technique used is on the

order of 50 nm (500 A) so it is still a relatively "bulk" technique, but it is certainly more

surface sensitive than x-ray diffraction. NCI 32 hot pressed Si3N4 exhibited only one set of

raman peaks (Figure 3.4). These peaks were were associated with the j3-Si3N4 phase.

NBDIOO HIP'd Si3N4 was observed to give two sets of peaks (Figure 3.5); one set was

associated with i3-Si3N4 and the other set with Q;-Si3N4. Both of these sets of Raman peak
33

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3000

2500

2000

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1 1 1 1 1 1,1,

200 400 600 800 1000 1200 1400

Wavenumber, cm-1

Figure 3.4 Raman Spectrum for NC132 Hot Pressed Si3N4

8000

7000

6000

^ 5000 - w

4000

"S 3000

1 '1

CER028

NBDIOO Si„N^ 3 4^

200 400 600 800 1000 1200 1400

Wavenumber, cm-1

Figure 3.5 Raman Spectrum for NBDIOO HIP'd Si3N4
37

observations were consistent with the XRD data.

The samples of Si3N4 had a tendency to fluoresce. This made the analysis more

difficult, since fluorescence results in an increase in the broad background signal (noise).

The fluorescence may be associated with silicon oxide on the surface of Si3N4. This was an

early indication that silicon oxide might play an important role in the surface chemistry of

Si3N4.

3.1.5. Ceramography and Fractography

The microstructure of the two commercial Si3N4 samples used in this study were

examined using routine fractographic and ceramographic techniques. Fractography consisted

of cutting part way through a sample of the Si3N4, then using a wedge to cause a tensile

crack which fractures the specimen into two pieces. Examination of the fracture surface

revealed the grain structure since much of the fracture occured along the weaker grain

boundaries of the material. Ceramography consisted of grinding and polishing a flat crosssection of the material and etching it with molten KOH to dissolve away the grain boundaries

at the surface. Observation in the SEM revealed the grain size and morphology of the crosssection. KOH is also corrosive to the grain itself, therefore this technique tends to distort the

grain edge morphology a bit. In this work, the KOH was heated in a nickel crucible to

400°C and the Si3N4 sample was immersed for about 45 seconds. After etching, the sample

was treated with concentrated aqua-regia (4 parts HN03:1 part HCl) for 30 seconds to

remove Ni hydroxide precipitates that sometimes formed on the surface (from corrosion of

the Ni crucible).
38

The hot pressed Si3N4 (CER024) fracture surface (Figure 3.6) exhibited a mixture of

broken and whole grains at a variety of orientations. Most of the grains appear to be less

than one micron in width and vary in length up to about two or three microns. These

elongated grains are consistent with the structure associated with iS-Si3N4. Some small

(0.1 jwm - 0.5/Lim) equiaxial particles are observed that may be fragment artifacts of the

fracture process. The etched cross section of the same material (Figure 3.7) reveals more

clearly the elongated grain nature of the sample. The equiaxial-looking grains may actually

be sectioned elongated grains that were originally projecting out of the surface. This would

be consistent with the x-ray diffraction and Raman data that indicated that the hot pressed

sample was pure beta (crystalline) phase Si3N4.

The HIP'd Si3N4 (CER028) fracture surface (Figure 3.8) appears similar to the

previous sample; however, the dimensions of the fracture planes on each grain appears to be

slightly smaller. The etched cross-section (Figure 3.9) reveals some elongated (|3-Si3N4)

grains, but there are fewer than in the hot pressed sample, and they are smaller in size. The

abundance of small (O.l/xm-O.S/xm) equiaxial grains is probably partly due to the presence of

a-Si3N4 grains in this material (as detected using x-ray diffraction and Raman spectroscopy).

The combination of evidence on these two commercial Si3N4's suggests that these two

materials experienced a different temperature and/or time history during their sintering. Both

materials started with the same Q!-Si3N4 powder and sintering aids. The hot-pressed material

was subjected to high temperature during pressing that resulted in complete conversion of the

o;-Si3N4 to i3-Si3N4 and relatively large grains (growth of larger grains occurs at the expense

of smaller grains during sintering). The HIP'd material was probably subjected to higher
Figure 3.6 SEM Photomicrograph of NCI 32 Hot Pressed Si3N4 Fracture Surface

Figure 3.7 SEM Photomicrograph of NCI 32 Hot Pressed Si3N4 Polished/Etched Surface
Figure 3.9 SEM Photomicrograph of NBDIOO HIP'd Si3N4 Polished/Etched Surface
41

pressure and lower temperatures or shorter sintering times. This resulted in incomplete

conversion from a to ^ and less grain growth. This difference in microstructure may affect

macroscopic properties relevant to the tribological behavior of these materials.

3.1.6. ESCA and Auger Analysis

Two surface analytical techniques were employed to determine the composition and

structure of the surface of the Si3N4 samples. Electron Spectroscopy for Chemical Analysis

(ESCA, also known as X-ray Photoelectron Spectroscopy or XPS) was used to determine the

near surface composition and strucmre. Auger analysis was used to determine the

composition as a function of depth into the surface. Both analysis were performed at the

University of Dayton Research Institute and the data were interpreted by Dr. Pu-sen Wang of

the ceramics division at NIST.

ESCA analysis was performed on the polished surface of Si3N4 flats. The depth of

analysis was estimated at 10 nm (100 A). The analysis results from NC132 and NBDIOO

were identical, therefore only one data set is presented. The survey scan for the NCI32

sample is shown in Figure 3.10. The main constituents observed are silicon, oxygen and

nitrogen. The carbon signal is of a characteristic impurity strength and can be considered as

insignificant. The O Is scan (Figure 3.11) indicates a strong oxygen signal usually

associated with SiOj. The Si 2p peaks were not used because the Si-O and Si-N peaks differ

by only 0.7 eV and overlap too much as demonstrated by Wang.^^^ The Si KLL scan,

however, (Figure 3.12) indicates two types of bonding with silicon, one associated with the

Si-N bond in Si3N4, and another associated with the Si-O bond in silicon oxides. The
42

10000

8000

^ 6000

c

-l-> " 4000

^000

CER024

NCI 32 SigN^

0

1000 800 600 400

Binding Energy, eV

w

(n

in

>00

Figure 3.10 ESCA Survey Scan of Si3N4 Surface

nitrogen Is scan (Figure 3.13) consisted mainly of a large peak at 397.3 eV, a peak

associated with Si3N4. There is also a

small peak appearing as a shoulder to

the larger peak that is may due to

SijNjO. The suggested compounds

represented by the data are therefore

Si3N4, SiOj, and possibly SijNjO.

Sputtering the surface (removal of the

outer layers) resulted in a decrease in

the O signal, and an increase in both

silicon and nitrogen. This indicates that

6000

5000

4000

c 3000

2000

1000

0 Is Scan

CER024

NCI 32 SigN^

540 535 530

Binding Energy, eV

525

Figure 3.11 ESCA O Is Scan for Si3N4 Surface
the oxygen is concentrated at the

surface.

Auger depth profiling was

performed to understand the variation in

composition with depth into the surface.

The sampling depth for any single scan

of the surface was on the order of 10

nm (100 A). The results are shown in

Figure 3.14. The Auger data confirm a

higher concentration for oxygen on the

surface (12%) versus the bulk (3%).

3000

2500

2000

1500

1000

500

' I.I " Si KLL Scan ^^3^4

\

1

S'Os

/

\ /

1 ... 1 .... 1 : ... 1 . 1 . . . , 345 350 355 360 365

Kinetic Energy. eV

370

Figure 3.12 ESCA Si KLL Scan for Si3N4

Surface

The 3% bulk figure is in good agreement with the commercial data on bulk Si3N4. There is

a carbon contaminant on the surface

which is quickly sputtered off, but since

carbon is also an artifact of the analysis

technique interpretation of the carbon

concentration should be strictly

qualitative. One of the most striking

features in the depth profile is the

crossover of Si and N with depth.

Silicon is in higher relative abundance

on the surface, but lower relative

5000

4000 -

c 3000

C

2000

1000

N Is Scan

415 410 405 400

Rindine Enercv, eV

395 390

Figure 3.13 ESCA N Is Scan for Si3N4 Surface

) 45

abundance deeper into the sample. According to the stoichiometry of Si3N4, silicon should

always be in lower relative abundance than nitrogen. There must be another species present

in which the ratio of Si:N is greater than 1 (i.e. a silicon containing compound with little or

no Nitrogen). Si2N20 does not have the required ratio (but they are certainly closer). Si02

essentially has no nitrogens, and hence could explain the data. It would therefore appear that

there is a Si02 rich layer on the surface of Si3N4. The depth of this layer is estimated to be

o on the order of 6 nm (60 A).

3.1.7. A Surface Model for Si3N4

Other researchers have looked at the surface of Si3N4 to find oxygen containing

species. Raider^^ used ESCA to find an oxynitride surface layer on Si3N4 thin films.

Goto^°^ found similar results on amorphous Si3N4-C composite thin films using both ESCA

and Auger. Rahaman^°^ used ESCA to look at the surface of powders, discovering an

amorphous oxygen-rich surface layer 3-5 nm thick.

The analytical evidence is very clear in suggesting that the surface of Si3N4 is very

rich in oxygen. The ratios of Si:0:N as a function of depth suggest that we may be looking

at a high surface concentration of Si02, with a decreasing concentration gradient into the

surface such as depicted in Figure 3.15. The surface depicted in Figure 3.15 also suggests

that there may be an intermediate silicon oxynitride layer. The bulk oxygen concentration of

3 % suggests that there will still be oxygen containing species in the bulk, but they will be in

minor concentration, and probably concentrated as glassy phases (grain boundaries) between

Si3N4 crystallites along with other sintering aid metals (Mg, Al) and impurities.
46

Si/N Si/0

(Atmosphere) R^'tio ^atic

(Bulk)

Figure 3.15 Surface Model for Silicon Nitride

Since we are primarily concerned with chemical reactions on the surface of Si3N4, the

chemical structure of the outer layer is important. If it is indeed a stable oxide of silicon,

much like the stable "native" oxide layer that forms on silicon, it will have the chemical

properties of the stable oxide. Fortunately, the surface chemistry of silica is well

understood. In fact, it is a very well characterized material because silica and its related

compounds (i.e. silicon wafers etc.) are so widely used. It can be found in a wide variety of

applications from optics to semiconductors.

The basic structure of silica consists of silicon and oxygen in a ratio of one silicon for

every two oxygens. Silicon has a tetrahedral bond structure with four bonds per atom.
47

Oxygen forms two bonds per atom. The bulk structure can vary from amorphous to

crystalline (Figure 3.16). Among the crystalline forms, there are several primary phases

such as quartz, tridymite, and crystobalite, however there are many other phases that have

been described such as keatite, coesite,

and stishovite, to name a few.

However, the chemical surface structure

of both amorphous and crystalline silica

is similar. They are considered to

consist of three basic components:

1) silicon oxides

2) silicon hydroxides

3) adsorbed water

Silicon oxides are the highest Figure 3.16 Structure of Crystalline

Silica

temperature surface form found on

silica. The basic structure is depicted in the small figure below. Each silicon is bonded to

four different oxygen atoms, and each oxygen atom is bonded

I 1 to two silicon atoms. Please note that some of the subsurface

oxygen atoms bonded to the silicon atoms have been omitted

for clarity of presentation.

In the presence of water, the silica surface is easily

hydroxylated to give a silicon hydroxide surface structure.

0 0 0 0
48

A silicon hydroxide is also termed a silanol - as in a silicon

H H alcohol. The structure to the left depicts the partial

o 0

hydroxylation of the silica surface. When enough water is

present, the surface of silica

can become fully hydroxylated as shown to the right. H H H H

O O O O

There is usually one surface hydroxide per surface silicon, but

o o

mixtures of surface oxide and hydroxide are possible. There

has been some debate about the existence of silicon surface atoms with two hydroxides

(geminal hydroxides) however this postulated structure is generally discounted'^^. It should

be noted however, that free (non-surface) chemical structures containing two, three, and even

four hydroxides are found. The special case of four hydroxides per silicon is called "silicic

acid monomer. " It is termed a "monomer" because of the tendency of the Si-OH groups

from different molecules to combine into longer chains (polymers), gels, and particles. Since

the surface of silica is easily hydroxylated, the tendency of silicon hydroxides to "condense"

may explain some of the high adhesion tendencies of glasses'*^

In the case of a mixed surface which contains both silicon oxide and silicon

hydroxide, the relative location of the hydroxides makes a difference in the adsorption and

reaction mechanisms for the surface. The term "vicinal" hydroxides describes hydroxides

that are nearest neighbors. A silicon hydroxide without any other nearby hydroxide is

termed an "isolated" hydroxide. Silicon hydroxides are also known as silanol groups. These

different silanol groups (isolated versus vicinal) have different chemical reactivities.
Silanol groups can hydrogen bond with water

molecules. A fully hydroxylated silica surface will

H H H H H H O O O

easily adsorb water as depicted to the right. H H H H O O O O

In fact, high surface area hydroxylated silica (silica

o o

gel) is used as a water adsorbent to remove water from hiH^^^H^^^^^^MHBBH

the atmosphere of closed containers with water sensitive contents.

The exact form of surface silica described above is a function of thermodynamic

favorability, which is governed mainly by temperature. The relationship between

temperature and the surface forms of silica are illustrated in Figure 3.17. At low

temperature, adsorbed water on silanol groups are found. As the temperature is raised above

the boiling point of water (373 °K or 100°C), water desorbs, and the surface is fully

hydroxylated silica. As the temperature is raised, vicinal surface silanol groups condense to

give a silicon oxide surface with bridging oxygens. This structure is also termed a siloxane.

Water is a byproduct of the condensation of vicinal silanol groups and leaves as a gas. This

occurs from 100°C (373 °K) to 200°C (473 °K). Some of the hydroxyl groups are stranded

as isolated silanols and remain stable to temperatures up to 400°C (673 °K). Above this

temperature, diffusion of silanol groups becomes possible, with the result that the isolated

silanol groups find and condense with other isolated silanol groups, and the surface becomes

a complete oxide.

The two basic forms of silica described above (siloxane and silicon hydroxide or

silanol) constitute two of the main hypothesized reactants on the surface of Si3N4. Many of

the analytical procedures used to study the boundary lubrication of Si3N4 were developed
50

T,

1000

800

600

400

200

0

o

./ \

o o

H H H H

I I I I o o o o

Si

\_

H H H HH H

¥ ^ ¥

H H H H

6

1 O1 O 6

Si Si V V V \)

Figure 3.17 Different States of Silica as a Function of Temperature
51

with this surface model in mind.

3.2. Si3N4 Powders

Si3N4 powders were used to study the reactions between selected chemistries and

Si3N4 surfaces in an observable and controlled way. One of the biggest problems in studying

surface reactivity is the limitation on the quantity of reaction product available, which is

usually a function of the surface area available for reaction. Because of this, most of the

powders used had high surface areas. Another concern with studying the reactivity of Si3N4

in this way was the possible interference by minor impurities in the samples. As a result,

only very high purity powders were used.

The Si3N4 powder used for studying surface reactivity was a commercially available

powder manufactured by the Hermann C. Stark Company, and known as LC 12-N. The

chemical composition of this powder is given in Table 3.4.

This powder has extremely low concentrations Table 3.4

Chemical Composition of

of iron (and other metal) impurities. The only Stark LC 12-N Si3N4

contaminant found in any appreciable quantity Trace

Element %

is oxygen, which is found on the surface of all N 38.71

o 1.58

Si3N4 materials in much the same way as c 0.15

Al 0.04

silicon has a "native" oxide layer. This Ca 0.006

Fe 0.006

powder is composed of 0.44 /xm sized

Source: Manufacturers Data

particles of 96.2% o;-Si3N4. The surface area

of this powder is 18.3 m^/g. This surface area is substantially higher than the surface areas
52

for other Si3N4 powders ( ~ 3 m^/g) and is the principal reason that this powder was selected

for chemical reactivity studies.

The powder reactivity study was intended to investigate the surface reactivity of

Si3N4. There are also potential surface reactivities due to sintering aids and contaminants

that may play a role in the monolithic finished material that are not accounted for with the

use of these high purity powders. The investigation of such effects is beyond the scope of

this study.

3.3. Model Chemical Compounds

Model chemical compounds were obtained in as pure a form as possible and tested as

received. The purity varied from compound to compound; however, most compounds were

better than 98% pure. A few lubricant additives were supplied in diluent oils. In this study,

only the global effects are observed. The purification and identification of specific active

ingredients in some cases is beyond the scope of this study.

The compounds were selected based on bond energies for silicon compounds, silicon

chemistry, and additive literature. In general, mixtures of compounds were avoided because

of the possibility of synergistic or antagonistic interactions. No attempt was made to correct

for inequalities in absolute concentration of compounds (due to its concentration in the

commercial product) because the focus of this study was to detect gross effects.

The base oil used in this study was a 2.7 x 10"^ mVs (27 Cst) (@ 37.7°C or 100°F)

paraffinic oil purified by percolating it through 200 mesh activated alumina. This resulted in

a purified paraffin oil (PPO) free of polar impurities that might affect the wear results.
Chapter 4

53

EXPERIMENTAL APPARATUS AND OPERATING CONDITIONS

4.1. Dynamic Tests

The criteria used to evaluate the boundary lubrication effectiveness of various

chemistries are friction and wear characteristics of the chemical compounds. Two different

devices were used to measure the friction and wear properties of the different Si3N4/lubricant

combinations. The majority of the testing was performed using a ball-on-three-flat (BTF)

modification of a four-ball (FB) wear tester. Special tests were also conducted on a pin-ondisc (POD) tester, and a new experimental two-ball collision tester. Consistency of results

was ensured by developing and using specific procedures for the preparation and cleaning of

specimens, operational procedures, and measurement of friction and wear.

4.1.1. Wear Tester

Most of the initial screening wear tests were conducted using a BTF tester. This

equipment, manufactured by the Roxana Machine Works is an adaptation of the standard

"Shell/GE/Brown" four-ball wear tester and allows for friction and wear testing of

material/lubricant combinations in the boundary lubrication regime. A schematic of the

general apparatus is shown in Figure 4.1, and demonstrates the salient features of this device

including:
54

Drawbar

Bearing* •

Top Ball ChuckFriction Pickup

Driva Pullay

Taat Cup

Splndia

Envlronmantal Qaa Inlat

Locking Nut

Tharmocoupla

Haating Block

Pneumatic Load Piaton

Air Baaring

Figure 4. 1 NIST-Modified Four-Ball Apparatus Design
55

• Pneumatic loading for infinitely adjustable loads

• Air bearing for more accurate friction measurement

• Heater block for controlled temperatures

• Test cups for either FB of BTF configuration

• Gas inlet for atmosphere control

• Drive spindle connected to variable speed motor

The geometry of the four-ball and ball-on-three-flat are similar and consist of a tetrahedral

arrangement (see Figure 4.2) in which a ball is nested on top of a set of three specimens

(either spheres or flats, locked in place). A load is applied axially, and the upper specimen

rotates to provide sliding at the three contact points. The main difference between the two

wear test configurations is that the FB contact is convex on convex while the BTF is convex

on flat. This has some influence on the test severity at the beginning of the test. Hertzian

elastic contact analysis on the two contact geometries showed that the contact pressures for

the FB arrangement are 59% higher than the BTF for the same applied load (see appendices

A and B). The BTF configuration has several advantages including less lubricant required

(1.5 ml vs 10 ml), easier to optically measure the wear scars because the surface is flat, and

post-test handling and analysis is much easier because the wear scar is centered on the flat

and the flat is easy to mount on any flat surface. Both configurations use 12.7 mm (0.5

inch) diameter (minimum grade 25) balls, but the FB test requires four of them, while the

BTF test only needs one. The BTF test additionally requires 6.35 mm (0.25 inch) diameter,
56

A

Four-Ball

Configuration

Ball-on-Three-Flat

Configuration

Applied Load Applied Load

Lower Stationary Balls Lower Stationary Rats

Figure 4.2 Four-Ball and Ball-on-Three-Flat Contact Geometries
1.59 mm (1/16 inch) thick flats which are polished on one side. A second type of

wear tester known as the pin-on-disc (POD) wear tester was used for some of the wear tests

on low iron impurity Si3N4 from Rutgers University. It was used due to the difficulty in

fabricating bearing quality balls from experimental materials. The contact geometry for this

tester is shown in Figure 4.3 and consists of a hemispherically tipped pin riding on a rotating

flat disc. The contact geometry for this apparatus is similar to BTF in that it is convex on

flat; however, in the case of the POD

there is only a single point contact

instead of three point contacts. The

radius of the point of contact with

respect to the center of rotation of the

lower specimen is adjustable which

allows for several wear tracks per flat

disc. In addition, since the pin is

inclined, it can be rotated between Figure 4.3 Pin-On-Disk Test

tests to allow for several tests per specimen. The ability to get several tests from a single set

of specimens is very desirable since we are limited in our ability to generate large numbers

of specimens. The POD apparatus was chosen for several reasons.

i) The test geometry of the contact is exactly the same as the BTF.

ii) With a slight modification of the loading the POD can give the same load and

speed as the BTF conditions used.

iii) The specimens can be fabricated from experimental materials.

iv) Several tests can be conducted using a single set of samples avoiding potential

differences from variable materials batches.
58

The POD tester can accommodate a range of pin (and optionally, ball) sizes, and a range of

flat discs up to approximately 60 mm (2 !4 inch) diameter. The tests described in this study

used pins of 6.35 mm (0.25 inch) diameter, ~ 19 nrni (0.75 inch) long, with hemispherical

6.35 mm (0.25 inch) tips. The flats were 6.35 mm (0.25 inch) thick and 50.8 mm (2 inch)

diameter. Grinding and polishing steps were similar to that used for the BTF flats and

resulted in mirror finishes on the specunens. Since the curvature of the pin in POD tests is

greater than that of the similar geometry BTF test, a specific normal load will result in much

higher Hertzian contact stresses at the beginning of the test. The derivation of the contact

stresses as a function of load for the POD apparatus is supplied in appendix C along with

calculated contact diameters and pressures.

The second wear test configuration used, the 2-ball collision tester, is a new

experimental apparatus designed and developed by Ying and Hsu^^^ to study films on wearing

surfaces. This device uses two 3.2 mm (1/8 inch) diameter balls to simulate single and

multiple contacts between surface projections (such as asperities). Using this device it is

possible to smdy the fundamental properties of films reacted onto surfaces. The speed, load

and materials can all be varied to obtain data under a variety of conditions. All 2-ball

collision tests were conducted with the help of Tsi-Neng Ying.

4.1.2, Specimen Preparation

Some of the specimens used for testing were purchased commercially and used

without further modification. Balls were obtained from Norton Company in both 12.7 mm

(0.5 inch) diameter (for BTF tests) and 3.2 mm (1/8 inch) diameter (for 2-ball collision tests)
59

with a uniformity of 0.000127 mm (0.000005 inch or grade 5). The rest of the specimens

required some kind of fabrication and surface finishing before they could be used in wear

tests. Flats for the BTF test were fabricated from 6.35 mm (0.25 inch) diameter rod stocks.

Flats were sliced from these rods, then surface ground and polished using a combination of

diamond grinding, SiC grinding, and diamond polishing. The exact procedure developed for

preparing Si3N4 flats from rod stock is provided in appendix D. The final diamond polishing

resulted in a mirror finish. Three dimensional topographic analysis of the surface using an

optical interference technique (Figure 4.4) showed that it had an RMS roughness of

approximately 5 nm. Several microscopic "pits" can also be seen. This technique also

revealed that the specimen preparation procedure actually produced a slight domed curvature

on the "flats. " The radius of curvature was over 3 m, therefore this effect was insignificant

for hertzian contact stress calculation purposes.

Monolithic Si3N4 for the disks and pins for the POD tests were made at Rutgers

University by Ajoy Zutshi^^°. The grinding and polishing steps used to finish these materials

into usable specimens was similar to the procedure described in appendix D.

4.1.3. Operating Procedure

All of the wear test procedures consisted of the sequence of specimen cleaning,

assembly, application of lubricant, then testing. Specimen pre-treatment is a critical part of

any wear testing procedure and was performed just prior to testing. The cleaning procedure

used a sequence of solvents: hexane, acetone, detergent in deionized water, and pure

deionized water. Ultrasonic agitation was used during each step. The detailed cleaning
60

CER028G_3 20. 2x

RMS: 5.4Gnm SURFRCE HVLENs G51.9nm

RR: 4.22nm TiltRemoved

P-V: G? . 1 nm

Or i entat i on

Distance (micrometers) l : - 4 9 . 7 n m

SILICON NITRIDE FLRT

WYKO

Figure 4.4 Three-Dimensional Surface Analysis of Polished Si3N4
61

procedure is provided in appendix E. The final step in the cleaning procedure was to dry the

specimens under nitrogen gas. They were then immediately assembled into the testing

apparatus for testing.

When running BTF tests, the following operating procedure was followed. The upper

specimen was locked in place in the ball chuck and place into the spindle assembly. The test

cup assembly, with lubricant in place, was put onto the heater block. A load of 1-2 kg was

slowly applied to gently bring specimens into contact. The spindle was then rotated by hand

to ensure distribution of lubricant over the surface of the top ball and to check alignment and

seating of the specimens. The friction force pickup chain was attached to the test cup and

the friction and load were both balanced to register zero load and friction. The load was

applied and the test began by starting the drive motor. At the end of the test, the drive

motor was disengaged and the speed was allowed to decrease under full load until sliding

stopped. The load was then removed and the post test analytical procedures were applied.

4.1.4. Friction and Wear Measurement

Friction was continuously measured during all of the tests. Wear scars were

measured optically at the end of the test after the excess lubricant had been removed with

hexane. The measurement was made on the specimen that was in constant contact during the

test. In the case of the BTF tests, wear was measured on the flats. In the case of the POD

test, the wear scar on the pin was measured. Wear scar measurement was accomplished

using an Olympus stereozoom microscope at 144x magnification and a calibrated graduated

eyepiece reticle. Measurements were taken to the nearest 0.004 mm. For all cases,
62

measurements were taken both parallel to and perpendicular to the direction of sliding. In

the case of the BTF test, there were a total of six wear scar measurements that were

averaged to give the final wear scar diameter measurement. The Hertzian diameter is the

contact diameter formed between the ball and the flat wear specimens due to elastic

deformation of the materials. It is theoretically possible for the material to have no

appreciable wear, but have the Hertzian contact surface roughened slightly so that optically,

a Hertzian diameter-sized wear scar can be seen. The Hertzian wear scar diameter therefore

represents a theoretical minimum wear scar diameter for optical measurements. A better

way to gauge the effectiveness of different chemistries therefore is to compare the wear scar

diameters after subtracting the Hertzian wear scar diameter. Both wear scar diameter and

diameter increase above the Hertzian contact diameter values are provided in this report.

Some researchers report wear as volume of material removed. Appendix F contains a

derivation of equations that can be used to estimate wear volume from wear scar diameters

for Four-Ball (FB), Ball-on-Three-Flat (BTF), and Pin-on-Disk (POD) tests. It has been

found that these estimates lose their accuracy in cases of very high or very low wear,

therefore they should be used with caution. High wear often results in wear of the rotating

ball, changing its curvature. This can be detected by noting the elipticity of the wear scar

diameter. A perfectly circular wear scar diameter denotes little change in curvature of the

upper ball. A wear scar in which the diameter in the direction of sliding is smaller that the

diameter perpendicular to the direction of sliding denotes significant wear of the upper

rotating specimen. The accuracy of the calculations suffers at low wear as the diameter

approaches the Hertzian contact diameter because of the roughened surface argument
63

mentioned above. Independant wear volume measurements performed using optical

interferometry confirm that the calculation equation in appendix F can overestimate the wear

volume by at least an order of magnitude. Because of the potential inaccuracy in using

calculated wear volumes, data in this study are reported as wear scar diameter, and diameter

increase above the Hertzian contact diameter. Load capacity plots use the customary log/log

plot of wear scar diameter versus load.

4.1.5. Selection of Wear Test Conditions

A rotational speed of 600 rpm in the BTF test was used to produce moderate (0.23

ms"') sliding speeds. At this speed there was a large database of wear test results from Perm

State, NIST, and other literature sources.

The seven wear tests shown in Figure 4.5 constitute a load capacity plot (wear vs.

load) for purified paraffin oil lubricated silicon nitride. According to these results, low wear

is observed for loads up to 30 kg. In this load range, pressures in the wear scar at the end

of these tests are very high: about 1.2 to 1.4 GPa (175 to 200 kpsia) indicating that paraffin

oil by itself was capable of lubricating silicon nitride under this condition. The

corresponding friction traces for these tests (Figure 4.6) are smooth. At 40 kg, a wear

transition region can be identified. At this load, the friction coefficient starts out low (0.099)

but at the three minute mark, friction rises and reaches 0.131, indicating a possible change in

wear mechanism. After the three minute mark, the friction increases and becomes quite

erratic. This region of the load capacity plot is termed the incipient wear transition region.

At 60 and 80 kg loads, transition to high friction is more immediate and higher wear is found
64

'-•ocor^toio CO cvi o o d d d o d d

mm 'ja^^amisTQ Jisog j^ajy^^
65

600 \ I \ Values in parentheses are

final coefficient of friction

BTF Test

PPO lubricated Si_N,

500 - 600 rpm

400

o

o

o• 1—1 -t-> o• I—

I

300

200

100

0

30 kg

20 kg

10 kg

0 2 4 6 8

Time, minutes

(0.127)

(0.127)

(0.099)

(0.099)

(0.092)

(0.084)

10

Figure 4.6 Friction Traces for PPO Lubricated Si3N4 at Several Loads
66

at the end of the test. Based on these results, a load of 60 kg was selected for the survey of

chemistries study. This is admittedly a severe condition (initial mean Hertzian contact

pressures are as high as 2.1 GPa (300 kpsia)). However, it offers the potential to show

significant wear reduction due to chemical effects.

A wear-in study was conducted (Figure 4.7) to ensure that adequate times were

selected to avoid wear-in repeatability problems. Figure 4.7 presents both wear scar

diameter, and mean contact pressure as a function of time. According to this data, the first

ten minutes of the test is a period of relatively high wear in which the wear scar diameter

increases rapidly above that of the Hertzian contact diameter. As a result, the mean pressure

is rapidly changing during this time. After thirty minutes, the rate of wear scar diameter

increase is diminished and relatively steady pressures are observed. Based on these results, a

duration of 30 minutes was selected for the wear testing.

4.1.6 Test Repeatability and Measurement Uncertainties

The uncertainties presented throughout this study are based on ± 1 standard

deviation. Several BTF tests were conducted at the selected conditions of 60 kg, 600 rpm

(0.23 ms''), and 30 minutes to check the repeatability of the wear test with PPO lubricant.

The test summary given in Table 4.1 represents not only the original tests used to establish

that the tests were repeatable, but also several tests conducted over a period of years as a

"calibration check" on equipment and specimen performance, and operating procedures.

These tests were run on batches of Si3N4 flats produced from different rods, and prepared at

different times. Note however, the single CER designation for these flats indicates that they
67

LO

C\2

q

c\i

ID

O

TT

LOd

q6 oo

CDO

ID
68

Table 4.1

Repeatability Data on PPO-Lubricated Si3N4

CER024 Batch of NCI32 Flats

WSD

(mm)

WSD

Incr^

(mm)

COF^ Run Polishing

Batch

Flat

0.642 0.262 0.123 2813R CER024 A

0.650 0.270 0.115 2817R CER024 A

0.647 0.267 0.111 2821R CER024 B

0.648 0.268 0.120 2835R CER024 C

0.649 0.269 0.117 2858R CER024 D

0.640 0.260 0.101 2904R CER024 D

0.652 0,272 0.105 2922R CER024 E

AVERAGE 0.647 0.267 0.113

Std Deviation 0.004 0.004 0.008

1 Wear Scar Diameter Increase above Hertian contact diameter (0.380 mm)

2 Steady state value measured at the end of the test

First five BTF Tests conducted on CER019 Si3N4 ball (NC132) at 600 rpm, 60 kg, 30 minutes, 1.5 ml PPO,

21° C. Last two tests used CER037 ball (still NCI 32) under same conditions.

all come from the same manufacturers processing batch. The high repeatability of these

results is therefore a testament to a combination of high repeatability of specimen

preparation, cleaning procedure and wear test operating procedure. The wear scar diameter

obtained at the end of the test was 0.647 ± 0.004 mm. This is an increase of 0.267 mm

above the expected Hertzian contact diameter for Si3N4 under the conditions of the wear test.

The friction coefficient at the end of the test was 0.113 ± 0.008. Note that during the

course of this investigation CER037 balls were used instead of the original CER019. The

wear test results indicate that these two processing batches of balls gave similar friction and

wear data. One might argue that the CER037 balls seem to give a slightly lower friction

coefficient at the end of the test; however, statistically, this is difficult to support.
69

These average results were used as a reference point to gauge the relative chemical

effect of the 1% compounds in PPO. Compounds that gave significantly lower wear scar

diameters ( < 0.55 mm ) than the PPO case were considered to have demonstrated antiwear

capability with Si3N4. Compounds that gave significantly higher wear scar diameters ( >

0.7 mm ) than the PPO case were considered to have demonstrated prowear capability with

Si3N4.

In the course of this study, different batches of Si3N4 were used which appeared to

have minor differences in friction and wear behavior with PPO. The PPO test data for these

materials are provided in appendix H for completeness. The minor variations in performance

were not significant enough to affect the antiwear/prowear criteria described above. The

average of all 16 tests on PPO using three different batches of Si3N4 were used as

comparison data in the tabulated and graphical data analysis as detailed in appendix H. The

average wear scar diameter was 0.645 ± 0.018 nrni. The average coefficient of friction at

the end of the test was 0.117 + 0.010.

4.2. Static Tests

Static thermal tests were conducted to study the chemical reactions of interest without

the complications of wear. Critical parameters such as temperature, duration, and

concentration of reactants (including surface area) can be controlled to a degree not possible

with a dynamic system.
I

1

i

I

I

1

70

j

4.2.1. Batch Reactor

Batch reactions were conducted between Si3N4 powders and Octanol using 1.5 ml

polypropylene centrifuge tubes for a microcentrifuge. This had the advantage of allowing for

immediate separation of the powder and liquid reactants. In addition, subsequent solvent

rinses were carried out in these tubes, thus combining both product purification (solvent

extraction) and separation (centrifugation) in a single container and eliminating transfer steps

that might have resulted in contamination and loss of product. The 1.5 ml tubes contain snap

caps that keep the samples intact during the reactions. The reactions were carried out in a

constant temperature oil bath. Since the tubes tended to be buoyant in these baths, a

bouyancy compensation device (1/4 inch nut) was attached to (screwed onto) the bottom of

each tube. The tubes could then be placed onto a rack within the oil bath to allow for

several tests to be conducted at the same time. This made it easier to conduct a series of

tests and analyses as a function of time.

4.2.2. Procedures

Samples of Powder and octanol were placed into the reaction tube and mixed

thoroughly using a spatula. The cap was snapped into place, a weight attached to the tube,

and the reactor was placed into the constant temperature bath for a specified time. Careful

temperature regulation ensured that the temperature in the bath did not deviate from the

setpoint by more that 0.5 °C at any time during the reaction. At the end of the specified

time, the reactor was removed from the bath and the exterior was rinsed with hexane to

remove the oil. The reactor was then centrifuged at 16 000 g for 10 seconds. Centrifugation
71

caused the powder to pack down to the bottom of the tube. The supernatant fluid was then

poured off. Spectral grade hexane was added to the tube and mixed gently with a spatula to

form a slurry. The slurry was then centrifuged to separate the Iquid and powder and the

supernatant fluid poured off. The hexane rinse was then repeated three more times. The

solvent was then switched to a mixture of spectral grade acetone/water (9/1) and the mixingcentrifugation step was performed two more times. A final mixing and centrifugation with

pure spectral grade acetone results in a powder sample that is free of octanol reactant. The

powder was then removed with a spamla and dried on a watch glass to remove residual

acetone solvent (—10 minutes). This dried powder was analyzed using thermogravimetric

analysis (TGA) within 2 hours. The entire purification/separation process can be performed

in 30 minutes.

When conducting kinetic experiments, the duration of the reaction must be precisely

known if quantitative results are to be obtained. Since the reactors start at room

temperature, and are heated up by immersing them in a bath, there is a time lag between

when the reactor is placed in the bath and when the reactants actually come to the proper

temperature. There is a similar lag when the reactor is removed from the bath. This must

be corrected for to get meaningful reaction duration information, especially for short reaction

times. A reactor was fitted with a thermocouple in its center, and placed in the bath to see

how long it took the reactor to heat up. The results, shown in Figure 4.8A, indicate that the

reactor has reached within 10 °C of the setpoint temperature in a bit more than a minute. On

cooling, it takes 30 seconds for the reactor temperature to drop by more than 10°C (Figure

4.8B). The data suggest that the measured reaction times should be reduced by about 1
160

140

U 120

o

oT 100

u

I 80

u

a»

9i H

60

40

20

0

(a)

Heating History

0 50 100 150 200 250 300

Time, seconds

160

140

20

0

(b)

Cooling History

Heater Bath Setpoint: :120°G

0

J L. 50 100 150 200 250 300

Time, seconds

Figure 4.8 Micro-Reactor Thermal History: a) Heating, b) Cooling
minute to acommodate the thermal lag.

Chapter 5

75

BASELINE WEAR STUDY

5.1. Paraffin Oil Lubrication

All wear test results on various chemistries will be compared to the base case where

only purified paraffinic oil (PPO) is present. This section describes the friction and wear

characteristics of Si3N4 under PPO lubricated condition.

A typical worn surface and friction trace for a PPO lubricated Si3N4 test is shown in

Figure 5.1. Friction is seen to rise to about 0.13 at the beginning of the test, and settle

down to a level of 0.12 toward the end of the test. The optical micrograph shows the wear

scar at the end of the test. The direction of sliding in this photograph (and all other

photographs in this study) is from left to right. The wear scar at the end of the test is

symmetric (~ 0.65 mm), and smooth within the scar. The small striated discolorations

within the wear scar suggest a micro-abrasion process by asperities or wear debris. The

discolorations are brownish and are reminiscent of the oxidized organic compounds often

observed in the metal systems. There are significant amounts of deposit surrounding the

wear scar, in a pattern that indicates the flow field for the lubricant during the wear test.

The deposit seen around the scar seem to be a mixture of solid materials including a plasticlooking accumulation both at the entrance and in the twin wakes at the exit of the wear scar.

The interference fringes seen at the center of the exit and near the sides of the wear scar
76

o

o

O

0)o

o

0.20

o 0.15

I ^^ I <

\

, r PPO Lubricated

600 rpm

60 kg

-1—I—I—I—I—I—I

1—I—I r

0.10

0.05

0.00

0

WSD = 0.649 mm

I , _J I I L. 10 15 20 25 30

Time, minutes

Figure 5.1 PPO Lubricated BTF Test at 60 kg: a) Optical Photomicrograph, b) Friction

Trace
77

suggest that they are thin films and the rainbow-like color pattern observed suggests a

variation in film thickness. The films seen here are soft and can easily be removed by

scratching. They are not easily soluble in hexane, but can be removed with stronger organic

solvents such as acetone and tetrahydrofuran (THF). FTIR analysis of the films confirmed

the presence of organic constituents.

SEM photomicrographs of the worn surface on the flats reveal more details. The flow

field in the low magnification SEM photograph (Figure 5.2A) is quite pronounced, even

though the interference fringes are no longer visible. The size and extent of the

accumulations are also more apparent. High magnification inside the wear scar (Figure

5.2B) shows the surface to be quite smooth with what appears to be a combination of

compacted product and small, loosely attached, debris particles. The middle of the exit

region outside the wear scar contains many particle clusters (Figure 5. 3A). The clusters are

large and appear fragile. EDX analysis of the clusters revealed the presence of silicon,

oxygen, and carbon. The thick deposits that form the twin wakes (Figure 5.3B) have the

appearance of a dried mud bed. It would appear that these thick deposits were composed of

some kind of solid-like material mixed with a liquid. The vacuum of the SEM appears to

have removed the liquid and resulted in the dried mud appearance of the deposit.

PPO lubricated wear tests seem to generate large amounts of reaction product deposits

near the wear scar. Since there are only very minute amounts of "reactive" metals in the

Si3N4, this would suggest that very high temperatures are generated within the contact during

a test. An estimate of the temperature in the contact junction, performed using Archards

method^^ (Appendix I) suggested temperatures in the region of «250°C for PPO lubricated
Figure 5.2 SEM Photomicrographs of PPO Lubricated Si3N4 Wear Scar: a) Low

Magnification, b) High Magnification (within Scar)
Figure 5,3 High Magnification SEM Photomicrographs of Exit Region Outside Wear Scar

of PPO Lubricated Si^N^ Wear Test
80

tests. This temperature is sufficient to oxidize significant amounts of paraffin oil in 30

minutes.

Auger analysis was applied to wear test samples from PPO lubricated tests to try to

determine the nature of the surface after a test. The samples had been rinsed with hexane to

remove excess lubricant.

Four regions on a worn flat were examined (Table 5.1). Two spots were located

inside the wear scar, a third spot was located just outside the wear scar, and a fourth was

Table 5.1

Auger Analysis of Worn Surface of PPO-Lubricated Si3N4

(Values are Atomic %)

Sample Location C 0 Si N

Inside Scar, Spot #1 70 17 9 2

Inside Scar, Spot #2 72 16 9 2

Outside Scar, Nearby 77 11 8 2

Outside Scar, Far Away^ 68 12 12 6

Unused Si3N4 specimen 45 11 25 19

located outside the wear scar far away from the scar. The results, presented in Table 5.1,

also contains auger analysis data from an unused specimen for reference. According to the

data, the surface of the entire flat is covered with a carbonaceous material. The data also

suggests that oxygen is more concentrated inside the wear scar than outside; however, it is

not clear whether it is associated with the carbon (as oxidized PPO), or silicon (as oxidized
81

Si3N4), or both. Since N is from Si3N4 and O can be associated with either Si or C, one can

get an estimate of the relative amount of Si3N4 vs SiOj by comparing the N-Si ratio. The

data suggests that at least some of the silicon inside the wear scar is associated with oxygen.

Auger depth profiling of the region within the wear scar (Figure 5.4) reveals some

important details about the worn surface. The outermost layer is very rich in carbon and

about 20 A thick. This is most likely some kind of organic layer. After the carbon

containing layer, an oxide layer is seen to a depth of perhaps 100 A. Since the oxygen

signal does not correspond with the carbon, it is probably associated with silicon. Silicon

oxide is suggested. This is reinforced by the reduced nitrogen concentration observed with

this region. Deeper into the surface, the characteristic bulk Si3N4 elemental intensities are

observed. A comparison of these Auger depth profiles with the unused specimen (Figure

3.14) reinforces the interpretation of an outer organic layer followed by a silicon oxide

sublayer. The fact that much more oxygen ends up in the surface of the wear scar coupled

with the abundance of carbon suggests that both of these constituents are important in the

wear and lubrication of Si3N4.

Gel permeation chromatography (GPC) confirmed that a wide range of high

molecular weight products were produced from PPO lubricated wear tests on Si3N4. Gel

permeation chromatography-graphite furnace atomic absorption (GPC-GFAA) analysis failed

to detect any appreciable amounts of silicon-containing reaction products, suggesting a lack

of reactivity with the Si3N4 surface.

The smooth wear scars and lack of fracture and cracking (severe wear) artifacts

within the wear scar suggests that PPO is a reasonable base lubricant for Si3N4. A load
oo

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o 1 z # 1 1 o o \ / 1 o o \ \ 1 o o / \ 1 o o \ / 1 z # o o 1 z 1 1 o o 1z 1 1 o o / 1 1 z • o o \ 1 1 z • o o / z u o \ / ' z\ • o o / 1 1 z 1 1 cj o / \ 1 z o o \ 1 1 z o o / \ / z o o \ 1 1 z o o 1 / 1 z o o \ 1 / z o o 1 / ( z o o \ / 1 / z • a o / 1/

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83

capacity study was conducted to understand the performance of PPO lubricated Si3N4 under

different severities.

5.2. PPO Load Capacity Study

Thirty minute PPO lubricated BTF wear tests were conducted at a variety of loads to

construct the load capacity plot shown in Figure 5.5. The behavior of the different tests can

be categorized into five different regions based on wear and friction behavior. The

information associated with each of the five wear regions are summarized in Table 5.2.

Table 5.2

Characteristic Features of the Five Wear Regions

for PPO-Lubricated Si3N4

Wear Regime

'

Parameter I II in lY V

Friction

Trace

Friction

Description

Low,

Level

Transition

/ Recovery

High, I Low, t Seizure / Low, Level

Wear Scar

Appearance

Smooth,

Indistinct

Edges

Smooth,

Clear

Edges

Smooth,

Clear

Edges

Grooved Fractured,

grooved,

roughened

Lubrication Good Poor Poor Poor Very Poor

Overall

"Category"

PPO

Lubricated

Incipient

Transition,

Recovery

Immediate

Transition

Immediate Fracture

Recovery

Accelerated

Wear w/time
mm 'J8':^9m^TQ jpog J^dj^fi
85

A typical friction trace and wear scar photomicrograph for PPO lubricated Si3N4 tests

within region I are shown in Figure 5.6. The friction trace is level and relatively smooth.

The coefficient of friction for these tests are low and steady at ~ 0.09 throughout the tests.

The wear scars are quite small and their edges are a bit hard to define. Small amounts of

deposit are evident at the exit of the wear scar. The highest load within this region (25 kg)

begins to show some evidence of additional deposit surrounding the wear scar, suggesting

that significant chemical reactions were taking place in and near the wear scar. The wear

scar diameter measurements for region I are very close to the Hertzian elastic contact

diameter. Calculated initial Hertzian mean pressures for the 25 kg load are 1.58 GPa

(229,000 psi). Overall, this region is classified as well lubricated.

Region II also parallels the Hertzian contact diameter line in Figure 5.5; however, it

is displaced away from the line significantly, and a wear transition can be seen from region I

to region II. Friction traces for tests (Figure 5.7) in this region typically have either spikes

or humps in the trace signifying some kind of wear transition / recovery behavior. The

friction spikes may be single wear events such as grain pullout within the wear scar.

Coefficient of friction at the end of the test is still near 0.09. The wear scars are still smooth

and circular, and the edges are more distinct. Significant amounts of reaction product can be

seen in and around the wear scar. This region is classified as an incipient transition region,

and indicates an unstable lubrication condition between good and poor lubrication.

Region III differs from region II by another apparent wear transition (Figure 5.5).

The line joining the data in this region does not appear to parallel the Hertzian contact

diameter line as well as the other data. The answer for the wear transition from region II to
86 (a)

100 [im

(b)

o

Em

O

CD

0.20

o 0.15

0.10

£ 0.05

cuo

o

0.00

PPO Lubricated

600 rpm

20 kg

~i 1 1

1—I

\ 1 1

1—I1 1 1 1 1 1—I r

J I I 1 I I I I I I I I I I I I i 1 1 1 1 1 L

¥SD = 0.285 mm

0 5 10 15 20 25 30

Time, minutes

Figure 5.6 PPO Lubricated Si3N4 BTF Test at 20 kg:

Friction Trace

a) Optical Photomicrograph, b)
(b)

o

O

CD

(—

I

O

CDO

o

0.20

o 0.15 -

—I—

I

1 1 1 11—I 1

1

1 1 1 1—I1 1

1

1—I

1 11 1 1

1

1 r- PPO Lubricated

600 rpm

40 kg

0.10

0.05

0.00 _l I I I L

WSD = 0.509 mm

J I I I I I I I I I L 0 5 10 15 20 25 30

Time, minutes

Figure 5.7 PPO Lubricated Si^N^ BTF Test at 40 kg: a) Optical Photomicrograph, b)

Friction Trace
88

region III seems to lie in the friction behavior. Region II friction traces indicated transition /

recovery behavior. Region III friction traces exhibit immediate transition to high friction,

and friction remains at high levels throughout the test, with only a slight decrease with time

(Figure 5.8). Coefficient of friction at the end of the test are approximately 0.12 to 0.13.

Wear scars are large, smooth, and circular. A lot of deposit is seen around the wear scars.

This region is classified as immediate transition (to high wear) and represents a relatively

poor lubrication condition.

Region IV is also a poor lubrication condition. It is distinguished by the initial

friction spike then low friction coefficient (0.06 to 0.07), and subsequent increase in friction

with time to ~ 0.11 (Figure 5.9). Wear scar diameters are very large (note the 2x scale

change of Figure 5. 9A from the preceeding optical photomicrographs) and again seem to

parallel the Hertian line of Figure 5.5. The wear scars themselves are severely grooved, yet

very polished. Very large amounts of deposit are seen around the wear scar. This suggests

that reactivity may be so high that some form of chemical polishing is taking place in the

wear scar, leading to a large wear scar.

Region V is a very high wear region characterized by an immediate high friction

"seizure" followed by a low steady coefficient of friction of 0.06 (Figure 5.10). Stresses

were so severe that the specimens completely fractured. The cracks seen emanating from the

wear scar go completely through the specimen; therefore this test result may have been

influenced by the specimen geometry (thickness of the flat). The recesses that held the flats

kept the fractured specimens in place throughout the test; however there was probably some

re-alignment of the specimens after fracture that altered the contact stress distribution.
89

100 \im.

(b)

o

o

o

0.20 1 1 1—I—I 1 1 1—I

1—I—I1—I

1

1—I 1

1

1 1 1 1—I r

o 0.15

PPO Lubricated

600 rpm

80 kg

0.10

£ 0.05 -

0.00 _l I I I L_

WSD = 0.676 mm

I I I I I I I ! I ] A

0 5 10 15 20 25 30

Time, minutes

Figure 5.8 PPO Lubricated Si3N4 BTF Test at 80 kg: a) Optical Photomicrograph, b)

Friction Trace
Figure 5.9 PPO Lubricated Si3N4 BTF Test at 210 kg: a) Optical Photomicrograph, b)

Friction Trace
(b)

0.20

o

a 0.15

• i-H

0.10 -

S 0.05 -

o

o

0.00

n 1 1 1

1—I—;

1—I

1 r \

Initial friction coefficient exceeded 0.24

-1 I 1 r0

PPO Lubricated

600 rpm

270 kg

WSD = 2.521 mm

... I ... I . J I I I I I ! I 1_ 10 15 20 25 30

* Specimens fractured

Time, minutes

Figure 5.10 PPO Lubricated Si3N4 BTF Test at 270 kg: a) Optical Photomicrograph, b)

Friction Trace
92

Severe grooving is also seen in this test. It is not clear to what degree the fracture of the

specimen contributed to the high wear seen for this test.

The load capacity plot for PPO provides a reference in which one can understand the

relative severity of the 60 kg tests (region III) and why it was chosen for the evaluation of

lubricant chemistries. Region I is too mild since PPO appears to be a good lubricant in this

region. There is little room for improvement in wear. Region II is an unstable region in

which the time of transition to high wear cannot be predicted reliably. This unpredictability

makes this region undesirable for evaluation of lubrication chemistries. Region III represents

a severity in which poor lubrication is immediately encountered at the beginning of the test

and seems to continue throughout the test as evidenced by the continued high friction. This

region would provide a stable poor lubrication condition in which lubrication chemistries

could be evaluated. Region IV appears to have an initial high friction spike, followed by a

quick recovery to low friction. The expected unpredictability of this transition / recovery

behavior would make this a less than desirable region in which to initially evaluate lubricant

chemistries. Region V is not desirable for the same reasons.

For these reasons, region III (specifically, 60 kg) was selected for initially evaluating

the effect of different chemistries on the lubrication of Si3N4.

5.3. Speculated PPO Lubrication Mechanism for Si3N4

The combination of wear test results, observations, and analyses suggests that PPO

has a definite, but limited, ability to lubricate Si3N4. The abundance of HMW organic

reaction products indicates that PPO oxidizes and polymerizes under the elevated
93

temperatures of the contact junction. This "friction polymer" should have a role in

increasing the local viscosity and forming a protective film on the surfaces of the wear scar

in a manner similar to that seen for metal lubricated systems. The lack of catalytic metals in

the case of Si3N4 may be partially compensated by the elevated temperatures due to lower

thermal diffusivity of the ceramic. In metal-lubricated systems, however, the metal is also

associated with metalo-organic HMW polymer^, suggesting good adhesion between the

polymer and the metal surface. The lack of Si-containing metalo-organic compounds

indicates that PPO may not react directly with the Si3N4 surface to any measurable degree.

This suggests a poor adhesion potential between a PPO film and the Si3N4 surface. This

poor adhesion potential infers that PPO and HMW products from PPO may be easily

squeezed out of the wear contact and explains why PPO cannot sustain high loads as

observed in the load capacity tests.

5.4. Effect of Iron Impurities

One important concern with ceramics is the role of metal impurities. Chen^^ has

shown that small amounts of metal impurities such as iron can influence lubrication

significantly. The Si3N4 used in this study was known to contain —2000 ppm of iron,

probably concentrated in the grain boundaries; therefore analyses were conducted to

determine whether we were merely observing effects from these iron impurities. Reaction

products from PPO and octanol lubricated wear tests were analyzed using GPC-GFAA set to

detect Fe. No iron-containing reaction products were found. Given the part-per-billion
94

sensitivity of the technique it is unlikely that iron impurities are dominating the reactions

observed in these wear tests.

Pin-on-Disk (POD) tests were conducted using neat PPO and Rutgers Si3N4 with an

iron impurity content below 10 ppm to see if the low load lubrication of Si3N4 by PPO was

related to the iron impurity. The POD test was used because it was not feasible to make ball

specimens from this experimental material. The results, given in Figure 5.11, show that the

low iron Si3N4 can be lubricated by PPO at low loads (20 N), but not at higher loads (45 N).

The transition region near 25-30 N is estimated from the BTF data and show that the data

from the two tests are consistent with one another, even though the POD data is very limited.

All of this evidence suggests that iron chemistry is not dominating the reactivity in the

Si3N4 lubricated contact.

5.5. Friction Trace Types Observed in this Study

Wear tests were conducted at the selected conditions using purified paraffin oil

blended with 1 % (wt) of each chemical compound. The performance of the chemical

compounds in this study was characterized by three parameters : i) Wear scar diameter, ii)

Coefficient of friction at the end of the test, and iii) Friction trace type. The first two of

these are self explanatory, however the third needs to be illustrated.

Four types of friction traces were observed in the course of this study and classified

as types I, II, III, and IV (Figure 5.12). The first type (type I), is exhibited by purified

paraffin oil without any additive. This friction behavior indicates a relatively poor

lubrication condition. At the beginning of the test, friction immediately rises to a high level

Time, minutes

Figure 5.12 Different Friction Trace Types Encountered: a) Types I, II, and IV, b) Type
97

(typically 0.13) and either remains high or gradually lowers during the test. The friction

trace itself is usually fairly rough and can contain friction spikes.

Some chemical compounds exhibit a different kind of friction trace. For example,

polypropylene glycol (triol) produces an immediate decrease in friction to a steady value.

This is termed type II friction behavior. This type of behavior is usually characteristic of

good lubrication. In general, the friction trace for type II behavior is smooth. However,

small smooth friction "bumps" can sometimes be observed.

A third type of friction behavior (type III) is exemplified by polypropylene glycol

(diol - 400 MW). Initially, friction is low, but at some point in the test, a transition to

higher friction occurs. Final coefficient of friction values can be low or high depending on

the complexity and severity of the transitions. This type of friction may be thought of as a

sequential composite of types II and I. Initially, good lubrication exists (type II friction);

then, at some point in the test, lubrication breaks down and a transition to high wear occurs

(type I friction). This type of behavior indicates a potentially good lubricant which has

exceeded its capability. This may be due to temperature or concentration considerations,

which means that under lower severities or at higher concentrations the lubricant may

function adequately. Alternatively, this type of behavior could be due to a time dependant

wear transition similar to fatigue. In both cases, however, the indication is that the

chemistry is initially providing good lubrication.

A fourth type of friction behavior (type IV) is exemplified by Mg salicylate. Friction

starts out low (« 0.088), then slowly climbs to a steady, moderate value (= 0.108) as the

test progresses. This friction type is associated with low wear. It is speculated that the
98

difference in the friction behavior for the two low wear friction types (II and IV) reflects a

fundamental difference in the nature of the lubricating film.

These friction type classifications provide the second-level of evaluation for

identifying potential boundary lubrication chemistries for Si3N4. The first level (wear) is

straightforeward. Low wear is successfiil lubrication, and high wear is poor lubrication. Yet,

within the high wear results are several possible reasons for failure: i) the chemistry doesn't

work (Type I), ii) the chemistry works initially, but later fails (for a variety of reasons)

(Type III). In general, the friction type interpretation is:

• Good lubrication: Type II, or Type IV

• Possible (temporary) lubrication: Type III

• Poor Lubrication: Type I

The use of friction types is especially valuable for interpreting results among chemistries

within a single class of compounds.
Chapter 6

99

SURVEY OF OXYGEN-COOTAINING CHEMISTRIES

Oxygen-containing chemical compounds were evaluated for lubrication effectiveness

with Si3N4. The compounds included alcohols, acids, esters, and epoxides and were soluble

in the base paraffinic oil used in the study. One of the reasons for selecting these oxygencontaining compounds is the high bonding energy between silicon and oxygen (higher than

most of other bonds including Si-N). This suggests that compounds that contained oxygen

have a high potential for forming a stable bond with Si3N4.

Wear tests were conducted at concentrations of 1% in PPO. Standard wear test

conditions of 0.23 m/s (600 rpm), 60 kg load, and 30 minutes were used. Some compounds

were also tested in neat form under the same test conditions.

6.1. Wear Tests Results for 1% Oxygenates in PPO

The wear tests results of oxygen-containing compounds is mixed; some compounds

reduce wear and some increases wear, as illustrated in Figure 6.1. The numerical data are

also summarized in Table 6.1. Graphically, there are two groups of compounds: one set of

seven compounds gave reduced friction and wear, and a second set of compounds that gave

little benefit or even higher wear than PPO.
100

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102

Within each of the two groups, there are different chemical compound types, and varying

degree of effectiveness. The results will be discussed by chemical compound class, starting

with the effective ones.

Compounds found to reduce wear at 1 % by weight in PPO included a metal

salicylate, oleic acid, and several alcohols including glycols. All of the antiwear oxygencontaining compounds also exhibited low friction, type II or IV friction type, and observable

fihn-like features inside the wear scar. The appearance of the film varied from compound to

compound. In most cases, the surfaces of the wear scars were smooth.

Salicylates are normally used as detergents in metal systems. They are not antiwear

agents for metals, but for Si3N4, the salicylate used exhibits significant antiwear activity at

1% in PPO. According to Figure 6.2, this compound exhibited type IV friction behavior

(initial slight increase in friction at the beginning of the test).

A solid-looking film was visible optically, and very little ^

>\

deposit could be seen surrounding the wear scar. SEM analysis ^"^^x

C—

confirmed the solid-looking nature of this coherent film (Figure // O

6.3A). The salicylate used was a magnesium salicylate, and Saliq^ilate

contained the usual calcium carbonate overbasing (Total Base Number: 346). The EDX

analysis of the wear scar (Figure 6.4) indicated that the protective film within the wear scar

contained Mg but no Ca (Ca would show up at 3.7 keV). This suggests that the active

antiwear functional group is the salicylate itself, and the overbasing calcium carbonate was

not incorporated into the protective film. The iron (Fe) and tungsten (W) seen in the EDX

analysis are impurities in the original Si3N4 material. The concentrations observed are the
(a)

103

100 ixm

(b)

PIO

0.20 1 ' ' ' I -I——I—I

1—r—1—I

1

1—I1

1

1 1 1 1—I r

I—o 0.15 h

I

1% Mg Salicylate in PPO Lubricated

600 rpm

60 kg

• 1—1 o

o

o

0.10

0.05

0.00

WSD = 0.412 mm

J I I I I I I

I I I I I I I I I I I II I i_ 0 10 15 20 25 30

Time, minutes

Figure 6.2 1% Mg Salicylate in PPO Lubricated Si3N4 BTF Test at 60 kg: a) Optical

Photomicrograph, b) Friction Trace
Figure 6.3 SEM Photomicrographs for 1 % Mg Salicylate in PPO Lubricated Si3N4 BTF Test

at 60 kg: a) Secondary Electron Image, b) EDX Image (Mg)
105

- a>

00

- CD

>

CO

- C\2

O O O O O O O

o o o o o o o O 05 CO CO lO ^

o o o o

o o o

CO cv ^

=3

PL, sq.uno3
106

same as in the parent material. A Mg x-ray elemental mapping of the surface inside the

wear scar at the same orientation and magnification as Figure 6. 3A confirms that the

distribution of Mg in the wear scar is associated with the film (Figure 6.3B).

Several glycols were tested, and within the glycol

CHj- (OCH2CH2CH2)xOH

family, there are some trends. Polypropylene glycol (triol)

| CH—(OCH2CH2CH2)xOH

[600MW] (Figure 6.5) had a wear scar and friction trace I CHf—(OCH2CH2CH2)xOH

fairly typical of the effective glycol compounds. The film Polypropylene Glycol (Triol)

observed inside the wear scar had the appearance of a solid-like

plastically deformed material. Similar film shapes were obtained

H(OCH2CH2CH2)xOH

with polypropylene glycol (diol) [1000 MW], and polyethylene Polypropylene Glycol

(Diol)

glycol [400 MW]. Polypropylene glycol (diol) [400 MW] started

with a low friction just like the 1000 MW version and the 600 MW

triol, however, twenty-two minutes into the test, a transition to high H(OCH2CH2)xOH

friction took place (Figure 6.6). As a result, wear at the end of the

Polyethylene Glycol

test was the same as the pure paraffin oil. The wear scar surface is smooth and similar to

the PPO case; however there are two small grooves running across the scar.

The polyalkylene glycol derivatives were not successful r-q —(CH2CH0)x —R"

at 1 % in PPO. This may be due to the different chemical ^"

nature of the derivatives. The R group substitutions will yield

R' = Alkyl group

no more than one hydroxy1 group per molecule (the water R" = Alkyl group or H

Polyalkylene Glycol

soluble derivatives). In the case of the water insoluble ones, Derivatives

the substitutuions result in only ether linkages (R" = alkyl group). These derivatives are
(b)

O

O

• I—

(

o

^-1cuo

u

0.20

a 0.15 •1—

(

n 1—

I

1 1 1

1—I—I

1—

I

1 1 1 —I

1 1

1

1—I1 11 1 1

1

1 r-

'_ 1% Polypropylene Glycol Triol [600 MW] in PPO ]

- 600 rpm

- 60 kg

0.10

0.05

0.00

WSD = 0.452 mm

J I I I I I I I I I I \ i_ -J I I I I I I I I L 0 10 15 20 25 30

Time, minutes

Figure 6.5 1 % Polypropylene Glycol Triol [600 MW] in PPO Lubricated ^\^\\^ BTF Test at

60 kg: a) Optical Photomicrograph, b) Friction Trace
108

(a)

(b)

0.20

a

o

0.15

u

" 0.10

-t-)

(U

• 1-4 0.05

Hand Dlgltlced Data

I I

I -I— I I

I—

I—I—I—I—I—

I—I—I—I—I—

I—I—I—I—I—

I—I

I—I—I—

I—I

I—I—I—

I

1% Polypropylene Glycol Diol [400 MW] in PPG

600 rpm

60 kg

O

U

0.00

WSD = 0.659 mm

I 'I I ' I I I I I I I I I I I I I I I I L. 0 5 10 15 20 25 30

Time, minutes

Figure 6.6 1 % Polypropylene Glycol Diol [400 MW] in PPO Lubricated Si3N4 BTF Test at

60 kg: a) Optical Photomicrograph, b) Friction Trace
109

therefore only HMW monoalcohols or ethers.

Some other mono-alcohols were not effective at 1% in PPO. Octanol blended into

PPO at 1 % gave low friction for only about one

minute before a transition to high friction occurred.

A typical hindered phenol antioxidant (2,6 di-tbutyl-para-cresol) did not affect the wear

significantly.

The Mannich reaction product is an ashless

CH3 OH CH3 11 I CH3 — ^.j^ ^C—CH3

CH3

Di-t-butyl p-cresol

dispersant that contains both an aromatic OH and an amine group. It has a polyamine chain

structure similar to a succinamide. The Mannich reaction product normally does not have any

antiwear function in metal systems

but gave low wear and friction and

type II friction behavior with

silicon nitride (Figure 6.7). The

wear scar surface at the end of the

test had a patchy surface film.

R

CH2NH( CH2CH2NH) XCH2CH2NH2

OH

Mannich Product

CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2

3-n-pentadecyl Phenol
(b)

o

0.20

B 0.15

u

' '

I

' ' ' '

I

' ' ' '

I ' ' ' '

I

' ' '

I

' ' ' '

I

' 1% Mannich Product in PPO Lubricated

600 rpm

60 kg

T—l—I—I—

T

" 0.10

0.05

03O

O

0.00

WSD = 0.402 mm

I I I I I I I fI I

I

I I I I I I I 0 5 10 15 20 25 30

Time, minutes

Figure 6.7 1 % Mannich Product in PPO Lubricated Si3N4 BTF Test at 60 kg: a) Optical

Photomicrograph, b) Friction Trace
Ill

Another phenol, 3-n-pentadecyl phenol, gave very low wear and friction when

blended into PPO at 1 % . The wear scar diameter is very close to the Hertzian wear scar

diameter. The wear scar has a somewhat roughened surface (Figure 6.8) and the friction

coefficient is low and steady throughout the test. These results may suggest that the phenol

(aromatic hydroxyl) group is active and may lubricate Si3N4 if it is not sterically hindered (as

in the case of the 2,6-ditertiarybutyl-p-cresol).

Oleic acid was found to be effective at 1 % in

PPO. Other higher MW acids such as the diacid, and the ^

II high euricic acid rapeseed oil, were not effective at this CH3(CH2)7CH=CH(CH2)7C— OH

concentration; although they did exhibit type III friction.

This suggests that they may be effective at lower test severity or higher concentration.

Esters do not seem to be effective under the conditions of this test. The monoester

(ethyl stearate) had low friction for the first 3 minutes of the

test, but then friction increased (type III friction behavior).

Oleic Acid

O

The friction trace (Figure 6.9) is fairly typical for the esters CH3(CH2) C —O —CH2CH3

used in this study. The wear scar appearance at the end of the Ethyl Stearate

test was similar to PPO; however, wear was slightly

higher. The aryl di-ester Ibis (2-ethylhexyl) phthalate] o CH2CH3

II

the test; however, the wear scar diameter and wear

also exhibit a lower friction for the first 2 minutes of

scar appearance were very similar to PPO. The alkyl

O CH2CH3

Bis (2-ethylhexyl) Phthalate

version of this ester [bis (2-ethylhexyl) sebacate] had
112

(a)

(b)

0.20

o 0.15

1—

I

t

1 \

1

1—I—I—

I—I—I—I—

I—I—I

\—

I—I—I—I

1—r- 1% 3 — n-pentadecyl Phenol in PPO Lubricated

600 rpm

60 kg

0.10 -

0.05

0.00

0

WSD = 0.402 mm

_i I I I I I 1 I I

I i_ 10 15 20 25 30

Time, minutes

Figure 6.8 1% 3-n-Pentadecyl Phenol in PPO Lubricated Si3N4 BTF Test at 60 kg: a)

Optical Photomicrograph, b) Friction Trace
(a) 113

100 [im

(b)

0.20

o

o 0.15 -

1—I—

I—I—I—I—I—

I—i—I—I—I—I—I—I—I—I—I—I—I—I—I—

I—I—I—I—I—I—I—I—I—I—

I—r- 1% Ethyl Stearate in PPO Lubricated

600 rpm

60 kg

0)

• i—t o

0)o

0.10

0.05

0.00

0

WSD = 0.696 mm

I ... I I 10 15 20 25

J I I I I I I I I I I I L 30

Time, minutes

Figure 6.9 1% Ethyl Stearate in PPO Lubricated Si3N4 BTF Test at 60 kg: a) Optical

Photomicrograph, b) Friction Trace
114

lower friction for only the first 2 minutes The wear scar

diamater was significantly higher than that of PPO. The

polyol ester had no measurable low friction region during

the test and the wear scar at the end of the test was similar

in appearance to PPO.

O CH2CH3

II I C —O—CH2CH(CH2)3CH3

(CH2)s

C-0-CH2CH(CH2)3CH3

O CH2CH3

Polydimethylsiloxane did not seem to have much of Bis (2-ethylhexyl) Sebacate

an effect on the friction and wear of Si3N4 at a

concentration of 1 % in PPO. The wear scar

diameter was only slightly smaller than PPO.

The friction type and level was similar to

PPO.

The polymeric epoxides (epoxidized

CH,—

I Si—

O

I CH,

CHo

I Si—

O

I CH,

CH,

I •Si— CH,

I CH,

n

Polydimethylsiloxane

polybutenes) did not have any appreciable

performance differences from neat PPO.

These results indicate that structure,

number, and molecular weight of the functional

groups are important. Oxygen-containing

compounds can interact with the silicon nitride

CH.—

CH^

I

" c—

I CH,

CH,

CH,—CI CH,

O

/ \ C—CH

I I CHg CH^

epoxidized polybutene

surface, but the combination of chemical structure and molecular weight determines whether

or not the interaction is beneficial. Hydroxy1 groups seem to be especially important. The

number and location of OH groups with respect to each other is probably also a factor. If

these alcohols interact with the surface to form silicate esters as hypothesized by Tsunai^*"^,
115

then it would make sense that a compound with two (or more) OH groups such as glycols

might be more tenaciously bound to the surface.

In order to study the chemistry of these oxygen-containing compounds, several tests

were conducted on neat compounds. When it became apparent that the alcohol functional

group was able to lubricate Si3N4, the study was broadened to include several different types

of alcohol functional groupings and chain lengths.

6.2. Wear Test Results for Neat Compounds

A summary of the BTF wear test results of neat oxygen-containing compounds is

shown in Table 6.2. Polypheny1 ether, bis (2-ethylhexyl) sebacate (ester), and

polydimethylsiloxane could not lubricate as effectively as PPO. Polydimethylsiloxane

actually had such poor lubrication that the test was terminated after seventeen seconds due to

excessive friction (jw > 0.6). In this short time, the wear scar diameter had grown to almost

2 mm.

Many neat alcohols and acids lubricated Si3N4 better than PPO under the test

conditions. The high molecular weight diacid had the lowest wear of any of the compounds

tested. The wear scar (Figure 6.10A) is visible mainly because of a residue present inside

and in the exit region of the wear scar. This residue is slightly soluble in hexane, since

further treatment of the flat with hexane could completely remove the residue. There is very

little reaction product visible surrounding the wear scar, in contrast to that seen in the PPO

case. Part of the reason for this may be due to the lower frictional heating observed. The

friction trace is smooth, and the coefficient of friction remained near 0.06 throughout the
116

Table 6.2

Summary of Neat Oxygen-Containing Compound BTF Wear Test Data

WSD Final

WSD Incr^ COP

Compound (mm) (mm)

PPO^ 0.645 0.265 0.117

Diacid [560 MW] 0.391 0.011 0.063

Oleic Acid 0.407 0.027 0.049

Oleil Alcohol 0.429 0.049 0.049

Polyalkylene Glycol Deriv. (WS) [260 SUS] 0.463 0.083 0.065

Polyalkyiene Glycol Deriv. (WI) [285 SUS] 0.480 0.100 0.083

Polyalkylene Glycol Deriv. (WS) [460 SUS] 0.484 0.104 0.058

Ci80H(75X) 0.495 0.115 0.046

Polyethylene Glycol (Diol) [400 MW] 0.522 0.142 0.055

Epoxidized Polybutene (E6) 0.524 0.144 0.099

Polypropylene Glycol (Diol) [1000 MW] 0.546 0.166 0.099

Glycerol 0.547 0.167 0.051

Polypropylene Glycol (Triol) [600 MW] 0.552 0.172 0.100

Polypropylene Glycol (Diol) [400 MW] 0.602 0.222 0.103

Polyphenyl Ether 0.733 0.353 0.095

Bis (2-ethylhexyl) Sebacate 0.919 0.539 0.148

Polydimethylsiloxane [350 cSt]^ 1.9^ 1.5^ 0.45^

1 Wear Scar Diameter Increase above the Hertzian Contact diameter (0.380 mm)

2 Average of 16 tests 3 Test terminated at 17 seconds due to excessive friction coefficient (>0.6). Friction coefficient was the

steady-state value prior to test termination.

WS = Water Soluble

WI s= Water Insoluble

test (Figure 6.1 OB). The dimer acid used was quite viscous, having a dynamic viscosity of

8 Pa»s (8,000 cP) at 25 °C; therefore, elastohydrodynamic lubrication may be contributing to

the low friction observed.
(b)

0.20

o

o 0.15 • I—

f

o

0)O

o

o

T—I—

I—I—I—I—I—I—I—I—I—I—I—I—1—I—I—

I—I—I—I—I—

I—I—I—I—I—

I—I—I—1

I—

I—I—r- Neat HMW Diacid Lubricated

600 rpm

60 kg

0.10

0.05

0.00 , I I

WSD = 0 391 mm

J I I I I I I I I I I U_l L_ 0 5 10 15 20 25 30

Time, minutes

Figure 6.10 Neat HMW Diacid Lubricated Si3N4 BTF Test at 60 kg: a) Optical

Photomicrograph, b) Friction Trace
118

Oleic acid produced the next lowest lowest wear and one of the lowest friction

coefficients of any of the compounds tested. The wear scar (Figure 6. 11 A) contained some

kind of deformed region that looked like it might be some kind of solid-like film. This

feature seemed to cover all of the wear scar except the trailing edge of the scar. Very little

reaction product was observed outside the wear scar. The friction trace (Figure 6.1 IB) is

low and steady at a coefficient of friction of «0.05.

Oleil alcohol had similar friction and only slightly higher wear than oleic acid.

Octadecanol (CigOH) was a solid at room temperature; therefore, it was tested at 75 °C.

Even at this higher temperature, this long chain alcohol managed to produce very low

friction and wear.

The viscous polyalcohol glycerol, with three carbons

and three OH groups, had slightly higher wear, but low

OH

I friction. The other compounds tested were a variety of HO—cHjCHCH—OH

Glycerol

higher molecular weight alcohols with different number and

locations of OH groups. They also had different polymer backbones which affected other

properties such as solubility (in water and organics), viscosity, and melting point. All of the

glycols had lower wear than PPO, whether they were polyalkylene (PA) glycol derivatives,

polyethylene (PE) glycols, or polypropylene (PP) glycols. Within a single structural type

such as polypropylene glycol (Diol), longer chain length gives lower wear.
(a) 119

(b)

0.20

o

o 0.15

n—I—

I—I—I—I—I—

I—r—I—I—I—

I—I—I—I—I—I—I—r- Neat Oleic Acid Lubricated

- 600 rpm

60 kg

o

0)o

0.10

0.05

0.00

"1—I—1—I—i—I—I—I—I—[—I—rWSD = 0 407 mm

_l I I I I L_ _l I I I I I I I I I I I I I I I I L. 0 10 15 20 25 30

Time, minutes

Figure 6.11 Neat Oleic Acid Lubricated Si3N4 BTF Test at 60 kg: a) Optical

Photomicrograph, b) Friction Trace
120

6.3 Wear Tests on Cg Compounds

A series of tests were also conducted on several oxygen-containing eight carbon (Cg)

compounds in order to systematically see the effects of functional groups on lubrication of

Si3N4 within a single chain length. This would reduce some of the chain length effects

observed previously and allow a direct comparison of chemical structural effects. Eight

carbon compounds were chosen because model compound were available in sufficient purity,

and the proper "window" of melting point/boiling point existed.

According to the data in Table 6.3, the primary alcohol (1-octanol) is the only one

that can lubricate under the standard wear test conditions. Figure 6.12 indicates that the

friction was very low throughout the test, with a coefficient of friction of 0.049. This is

very low considering that we are using an eight carbon alcohol with a room temperamre

absolute viscosity of less than 0.01 Pa«s (10 cP) and suggests that distinct chemical reactions

may take place in the Si3N4 contact in the presence of alcohols. The wear scar was very

circular. A wedge-shaped feature can be seen within the wear scar in the leading edge

portion. No deposit is visible surrounding the wear scar. Some of the other compounds did

lubricate for a few minutes before there was a transition to higher friction, suggesting that

under milder conditions, they might work adequately. The compound 2-octanol sustained

low friction (^=0.06) for a bit longer than 6 minutes before experiencing a friction transition

(Figure 6.13A). The steady-state coefficient of friction was 0.12 at the end of the test.

Octanoic acid had a lower friction coefficient of approximately 0.05 for about a minute

before rising to 0.175 for the rest of the test (Figure 6.13B). Octyl aldehyde and octyl

ketone had immediate high friction levels and large wear scars.
121

Table 6.3

Summary of Neat Cg Series Oxygen-Containing Compound BTF Wear Test Data

WSD Final

WSD Incr^ COP Frict.

Compound (mm) (mm) Tvpe^

PPO* 0.645 0.265 0.117 I

1-Octanol 0.557 0.177 0.049 n

3-Octanone 0.970 0.590 0.175 I Octanoic Acid 1.068 0.688 0.187 ni(i)

2-Octanol 1.076 0.696 0.126 HI (6)

Octyl Aldehyde 1.250 0.870 0.141 I

1 Wear Scar Diameter Increase above the Hertzian Contact diameter (0.380 mm)

2 Number in parenthesis indicates time to friction transition in minutes

* Average of 16 tests

According to these BTF wear test results, alcohols posess a unique lubricating ability

with Si3N4. Testing was expanded to measure effects of chain length, and water on

lubrication by these compounds. Various surface analyses and wear product analyses were

performed to determine whether these compounds were actually chemically reacting with the

Si3N4 surface.
122

(a)

100 \im

0.20

o

o 0.15

' \' ' ' r~ Neat Octanol

600 rpm

60 kg

«4-l o

I—

I

o

0)o

0.10

0.05

0.00

(b)

WSD = 0.557 mm

J I I I I 1 1 I I I l_ J I I I L. I I I I I 0 5 10 15 20 25 30

Time, minutes

Figure 6. 12 Neat Octanol Lubricated Si3N4 BTF Test at 60 kg: a) Optical

Photomicrograph, b) Friction Trace
(a) 123

0.20

o

o 0.15 l-H

"1 \ [

1 I I I

]

I I I I

I—1—1—I—I—

p

Neat 2— Octanol Lubricated

600 rpm

60 kg

o

o

0.10

0.05

0.00

T—I—1—

r

-1 I I L. 0

WSD - 1.08 mm

10 15 20

Time, minutes

25 30

(b)

0.20 -|—I—I—1—I—I—I—I—I—I—r—1—I—I—I—I—1—I—I—I—I—I—

[—I—I—I—I—I—I—I—I—I—I—I—

r

o

o 0.15

CD

•r-l O

0)o

o

0.10

0.05

0.00 _1 I I I I I L

Neat Octanoic Acid Lubricated

600 rpm

60 kg

WSD = 1.07 mm

I

I

I

I

I 1 I I II 1I

I I I I 1 I I L_ 0 10 15 20 25 30

Time, minutes

Figure 6. 1

3

Neat Cg Compound Lubricated Si3N4 BTF Test Friction Traces: a) 2-Octanol,

b) Octanoic Acid

Chapter 7

125

ALCOHOL LUBRICATION

7.1. Effect of Alcohol Chain Length on Wear and Friction

When it became apparent that alcohol functional groups were successful in lubricating

Si3N4, series of alcohols were tested to see the effect of chain length within a single

structural type. The first alcohols selected were a series of octylaryl polyether based

alcohols. They were selected because they had

long chain lengths, and were liquids at room

temperature. They were also available in a CH3CCH2C —^ J—(OCH2CH2)nOH

CH3 CH3

variety of molecular weights, controlled by the

Octylaryl Polyether-Based Alcohols

number (n) of ethoxy "mer" units in the

polyether chain. In these tests, compounds of n = l, 5, and 9 were used.

The data in Table 7.1 shows that within an alcohol series, longer chains provide

better lubrication because both friction and wear are seen to decrease slightly as chain length

increases. This may be attributed to the fact that longer chain may form thicker films which

lubricate better. A wear scar from the n=9 octylaryl polyether alcohol lubricated wear test

is shown in Figure 7.1 A. The scar is small and has many small scratches running in the

direction of sliding at the trailing half of the wear scar. The clear definition of the edge of

the wear scar at the trailing edge and the poor definition of the scar at the leading edge
126

suggests that more wear may be

occuring at the trailing half of the

scar. The friction is low and

steady throughout the test (Figure

7. IB), characteristic for these

well-lubricated tests.

In a second series of tests,

three linear primary alcohols were

tested to determine the effects of

chain length on friction and wear.

The alcohols were comprised of

Table 7.1

Summary of Neat Octylaryl Polyether-Based Alcohol

BTF Wear Test Data

WSD Final

WSD Incr^ COF

(mm> fmm)

0.645 0.265 0.117

0.420 0.040 0.067

0.465 0.085 0.073

0.528 0.148 0.081

Compound

PPO*

n=9

n=5

n=l

1 Wear Scar Diameter Increase above the Hertzian Contact

diameter (0.380 mm)

* Average of 16 tests n Refers to the number of ethoxy "mer" units comprising

the ether side chain.

hexanol, octanol, and decanol. The data are summarized in Table 7.2. Decanol (Figure 7.2)

had a slightly smaller wear scar diameter than octanol, and a similar low coefficient of

friction. The wedge-shaped film-like feature in the wear scar is well defined. Hexanol

(Figure 7.3) has the highest wear, but the

coefficient of friction remains low. The

wedge-shaped accumulation inside the wear

scar is thin and barely recognizable. The

level and the characteristics of the friction

suggest that the same basic lubrication

mechanism is responsible for these three linear

primary alcohols. The fact that the wedgeTable 7.2

BTF Wear Test Results on

Primary Linear Alcohols

WSD Final

WSD Incr^ COF

Alcohol fmm) (mm)

Decanol 0.503 0.123 0.045

Octanol 0.557 0.177 0.049

Hexanol 0.633 0.253 0.051

; 1 Wear Scar Diameter Increase above the

Hertzian diameter (0 38 mm)
(a)

127

(b)

O

0.20

o 0.15 •1—

-1—I—

I—I—I—I—I—

I—I—I—I—I—I—I—I—I—:—

I—I—I—I—I—

I—I—I—I—:—

I—I—I—;—i—|—i—

Neat Octylaryl Polyether (n=9) Alcohol Lubricated

600 rpm

60 kg

0)

I—

(

o• I—)

o

0.10

0.05

0.00

WSD = 0.420 mm

_1 I I I I I I I I I I 1 I I I I I I I I I I I I I I I 1 L. 0 10 15 20 25 30

Time, minutes

Figure 7.

Neat Octylaryl Polyether Alcohol Lubricated S'^N^ BTF Test at 60 kg:

a) Optical Photomicrograph, b) Friction Trace
(b)

0.20

o

o 0.15 • rH

' I

' ' ' I' ' ' I Neat Decanol

600 rpm

60 kg

n 1

1 r ~i 1 1

1 r

o(—)

0)o

0.10 -

0.05

0.00

WSD = 0.503 mm

_i ; I I I I I I I I L I I I I I 0 10 15 20 25 30

Time, minutes

Figure 7.2 Neat Decanol Lubricated Si3N4 BTF Test at 60 kg: a) Optical

Photomicrograph, b) Friction Trace
(a)

129

(b)

HiO

5h

• 1—

O

0)o

0.20

o 0.15

~i 1

1

1 1 1

1

1 1 1

1

1 1 1

1

1 1 r Neat Hexanol

- 600 rpm

- 60 kg

n—I

1 1 1 1 1 1—I r

0.10 -

0.05 -

WSD = 0.633 mm

0.00 1 I I I I I I I I I I I I I I I I I ; I I I L_ 0 5 10 15 20 25 30

Time, minutes

Figure 7.3 Neat Hexanol Lubricated S'^N^ BTF Test at 60 kg: a) Optical

Photomicrograph, b) Friction Trace
130

shaped accumulation gets smaller and thinner in appearance as chain length decreases and

wear and friction increase suggests that this feature is associated with good lubrication.

The same three linear primary alcohols were tested at different loads to compare the

load-wear relationship for these compounds (Figure 7.4). All three compounds roughly

parallel the Hertzian contact diameter line. In general, hexanol has the highest wear at all

loads. In addition, there is an increase in wear at 90 kg that defines the load capacity of this

compound under the BTF test conditions. The other alcohols exhibit behavior consistent with

the previous observation that longer chain lengths provide better lubrication.

7.2. Wedge-Shaped Feature Analysis

Previously, it had been suggested that the wedge-shaped feature observed in many of

the alcahol-lubricated Si3N4 wear tests is associated with good lubrication. The series of tests

at different loads for the linear primary alcohols supports this hypothesis. The data for

hexanol will serve as an example. At 20 kg load (Figure 7.5) friction is very smooth and

low coefficient of friction of 0.047 is observed at the end of the test. The wear scar, while

being fairly small, is very well shaped and distinct. The wedge-shaped feature is clearly

seen near the entrance region of the scar. At 40 kg load (Figure 7.6) the wedge-shaped

feature is still visible inside the scar, but it has not grown in proportion to the wear scar

diameter increase. At 60 kg load (Figure 7.3 already shown) the friction behavior is similar,

but the accumulation seems to be dissapearing. At 80 kg load, there was a distinct "grating"

sound at the end of the test as the speed dropped to zero under load. This is reflected in the

wear scar (Figure 7.7), which has a large rough region where the accumulation used to be.
O o Q

B ^ ^

03 cc cd

...X ^->. O

0) o a> K o Q

131

COo

u

oo U

u

1/3

o

I

o

1

u

^

3

bfi

CT) 00 (D odd dLnd d d d

uim 'ja^^amiQTQ J^og Jisa^
(b)

O

O

-t->

CDO. I—

I

o

u

0.20

o 0.15

0.10 -

0.05

0.00

Neat Hexanol

600 rpm

20 kg

WSD = 0.416 mm

I . . I I . . . I . 0 5 10 15 20

Time, minutes

25 30

Figure 7.5 Neat Hexanol Lubricated Si3N4 BTF Test at 20 kg: a) Optical

Photomicrograph, b) Friction Trace
(a)

133

100 nm

0.20

o 0.15 -

0.10

0.05 -

0.00

T I jI I I jI r~ Neat Hexanol

600 rpm

40 kg

(b)

"T I T" n 1 1

1 1 1 1 T"

WSD = 0.545 mm

_J I I I I I I I I I I I I I I I I I I I I I L 0 5 10 15 20 25 30

Time, minutes

Figure 7.6 Neat Hexanol Lubricated Si3N4 BTF Test at 40 kg: a) Optical

Photomicrograph, b) Friction Trace
(a)

100 urn

(b)

0.20

o

- 1—o 0.15 h

I

0)O

0)o

o

0.10

0.05

0.00

T

1

j

1 1 1

j

1 1

1 1 n Neat Hexanol

600 rpm

80 kg

~i 1

1

1 r

WSD = 0.613 mm

J I I L_ _L _I_L_ _L

0 10 15 20

Time, minutes

25 30

Figure 7.7 Neat Hexanol Lubricated Si3N4 BTF Test at 80 kg: a) Optical

Photomicrograph, b) Friction Trace
135

Apparently, as the speed dropped in the final second of deceleration, the surfaces were no

longer lubricated and the surface was worn by a combination of fracture and abrasion. This

apparent loss of lubrication indicates that hexanol at 80 kg is near a transition to higher wear.

This is supported by the observation of very high wear at 90 kg load for hexanol. The wear

scar at the end of the 90 kg test (Figure 7.8 - note change in scale for the wear scar) is

large, and very smooth, with no evidence of any wedge-shaped feature. The decrease in

lubrication effectiveness is associated with disappearance of the wedge-shaped feature within

the wear scar.

Disappearance of the wedge-shaped feature can also be associated with appearance of

fracture wear mechanisms within the Si^N^ contact. This is shown in Figure 7.9 which

compares SEM micrographs from two decanol-lubricated Si3N4 BTF tests. Both of these

photomicrographs were taken at the same tilt angle (75 ° ) and similar magnification, oriented

with the direction of sliding from left to right. The upper photomicrograph shows the

wedge-shaped feature present under good lubrication at 60 kg load. At 80 kg load, a grating

sound was heard during the final deceleration of the specimens at the end of the test and the

friction coefficient jumped to greater than 0.5. The wear scar at the end of the test is shown

in the lower photomicrograph of figure 7.9. Evidence of tensile cracks and severe fracture

are seen at the leading edge indicating that lubrication failure resulted in catastrophic failure

of the ceramic. The observation that these failures occurred during the deceleration phase of

the test suggests an EHD component of lubrication may be present, since EHD lubrication is

known to be speed dependant.
136

(a)

200

(b)

0.20

o

o 0.15 -

' '

I

' ' '

I Neat Hexanol

600 rpm

90 kg

1—I—I—I—I—

r

• I—

t

O• i—t D

OO

0.10

0.05

0.00

WSD = 1.152 mm

_J I I I I I I I I I I I I 1 I I u

~i 1 1—I r

0 10 15 20 25 30

Time, minutes

Figure 7.8 Neat Hexanol Lubricated Si3N4 BTF Test at 90 kg:

Photomicrograph, b) Friction Trace

a) Optical
137
138

More detailed analyses of the wedge-shaped feature were performed using flats from

an octanol-lubricated BTF test. Optically, the wedge-shaped feature (see Figure 6.12)

appears to be an accumulation of material inside the wear scar projecting from the surface

like a plastic film undergoing deformation. There is very little deposit outside the wear scar.

SEM photomicrographs of this wear scar (Figure 7.10) show the wear scar to be very

smooth except for the feature region. Surface profilometry was used to confirm that the film

was actually protuding from the bottom of the wear scar. The surface profile (Figure 7.11)

shows that the film appears to be =2 fim thick, relative to a wear scar that is only «4 fim

deep. A more three-dimensional view of the wear scar can be obtained by using optical

interferometry to look at the wear scar depth variation throughout the scar. Figures 7.12A

and B compare the normal optical view of the wear scar to the interferometric view of the

exact same wear scar. Because of different optical characteristics of the two instruments

used, the photographs are (vertically) mirror images of one another. The photographs

confirm that the wear scar is deeper in the horseshoe-shaped region toward the trailing half

of the wear scar. Since each fringe represents 0.27 jum vertical displacement, the wear scar

is 3.5 pim deep at its deepest point, with the wedge is projecting out of the surface

approximately 2 /xm. Initially, it was thought that the wedge shaped feature seen at the end

of the test was an artifact of the test termination - as the speed decreased under load, film

thicknesses decreased until a pressure wedge strong enough to scrape the reaction product

from the upper wear track was formed. This reaction product then compacted to form what

we see at the end of the test. To address this issue a wear test was conducted in which the

specimens were shock unloaded prior to stopping the test. The feature was still present in
139

£701 25KV 500um

(b)

2702 25KV 100um

Figure 7. 10 SEM Photomicrographs of Neat 1-Octanol Lubricated Si3N4 BTF Test (Direction

of Sliding is from Left to Right)
140

o

oeocvico-^ococvico'^o <u

(hit/) aouiB-^STQ ^P0i:;j9A S
Figure 7.12 Wear Scar from Neat 1-Octanol Lubricated Si^N^ BTF Test: a) Optical

Photomicrograph, b) Interferometric Photomicrograph
142

the same shape and quantity as before; therefore, the feature is not considered to be an

artifact of the test termination.

A series of tests was conducted as a function of time in order to see how the

feature changed during the test. In all of these tests the load was quickly decreased at

the end of the test before the speed had dropped. Optical micrographs of the wear scars

from wear tests of 18 seconds and 180 seconds are shown in Figures 7. 13A and B

respectively. According to these figures, the feature is present from the very beginning of

the test. It should be noted that the wear scar diameter increases as a function of time, while

the feature area decreases. The feature also changes shape from circular to wedge shaped,

and it moves from covering almost all of the scar to covering the entrance region more than

the other regions.

Attempts were made to dissolve this wedge-shaped feature using organic and

inorganic solvents. It would not be removed by organic solvents such as hexane, acetone,

ethanol, THF, or pyridine (both cold and hot). Acids such HCL, HNO3, and even HP were

ineffective at both low and high concentrations. Weak and strong alkalais were also

ineffective. Heating the sample 600 °C in 1.4 MPa of pure oxygen for 60 minutes had no

effect, therefore it was not merely a compacted form of carbon. A diamond scribe was used

to scratch the surface of the wedge-shaped film. The film was very hard and dificult to

scratch. It could not be chipped out and detached from the wear scar, but could be scratched

about as easily as the surrounding Si3N4 surface itself. All of these observations lead to the

conclusion that the wedge is inorganic, hard, and very adherent to the Si3N4 surface.
Figure 7.13 Optical Photomicrographs of Neat 1-Octanol Lubricated S'^N^ BTF Tests:

Second Duration, b) 180 Second Duration
144

Auger depth profiling analysis of the wedge-shaped region within the wear scar,

determined that the the feature had the same composition with depth as the Si3N4 surrounding

the wear scar. This data is provided in appendix J. This means that the wedge-shaped

feature within the wear scar is unworn Si3N4 material. This lends a new interpretation to the

surface profile originally shown in Figure 7.11. The surface profile actually represents the

surface of unworn Si3N4. Figure 7.14 shows the same surface profile in relation to the

original surface of the flat. The highest amount of wear occurs at the back (trailing edge) of

the wear scar. These results also reinforce that caution must be used in calculating wear

volumes from wear scar diameters in cases where good lubrication may be protecting the

surface. In these cases, octanol has preferentially protected the leading half of the wear scar,

resulting in a wear pattern that does not correspond to the curvature of the upper (ball)

specimen.

Taking a more three-dimensional look, the wear scar for oleil alcohol lubricated Si3N4

(Figure 7. 15 A) has a horseshoe-shaped region at the back of the wear scar that represents the

region of highest wear. This shape is reminiscent of the horseshoe-shaped region in an

interferometry image of EHD lubricated point contacts (Figure 7.15B). In BHD lubrication,

the horseshoe-shaped region represents the region of the highest pressure, minimum film

thickness, and highest temperature. This suggests that in the case of alcohol-lubricated

Si3N4 chemical reactions take place preferentially in the high temperature region of the wear

scar causing corrosive wear. Hence the horse-shoe shape formation. The reaction products

enable the existence of a EHD operating condition either through local viscosity increase or

through thick surface film formation. This explanation is supported in part by the low
145

o

OCOCVlCOr}*0«0(Ma3Ti<0 3 tJ^COCOCMCMW-^'pH W)
Figure 7.15 Comparison of Wear Contact Morphologies: a) Neat Oleil Alcohol Lubricated

Si3N4 Wear Scar, b) General EHL Contact Interferometry
147

friction coefficients (/x=0.05) observed for these compounds.

An estimate of the minimum film thickness for an octanol (EHD) lubricated Si3N4

point contact was calculated using Hamrock and Dowson's^"*^ equation (Appendix K). The

minimum film thickness was found to be about 4 nm for the octanol lubricated case. This is

a very thin film and suggests that either very smooth surfaces must be involved, or a much

higher effective viscosity of the fluid must be operating. Both of these explanations can be

attributable to tribochemical reactions which can polish the surface making it smoother or the

reaction can form high molecular weight polymers increasing the effective viscosity in the

contact.

7.3. Effect of Water on Lubrication of Si3N4 by Alcohols

Some researchers have speculated that it is the water in the alcohols that are

responsible for the chain-length effects. Water has been demonstrated to have a significant

effect on the lubrication of Si3N4. Sugita^^° and others have shown wear results in

which thereis a chemical removal of the silicon nitride surface through reaction with water.

Freiman^^ indicated that water can enhance crack propagation in ceramic materials. Both of

these mechanisms may explain enhanced wear rates in the presence of water. Smaller chain

length alcohol compounds have a higher polarity and higher ability to pick up atmospheric

water; therefore, the higher wear associated with smaller chain length alcohols could be

merely due to a higher concentration of (reactive) water. Indeed Tsunai^'*^* has shown that
148

1% water in methanol doubled the wear rate over that of neat methanol. Experiments were

therefore conducted to determine whether water was affecting wear in this study.

The neat octanol used in this study contained a nominal 100 ppm water. A test was

conducted using octanol with 1% water added. As shown in Table 7.3, addition of 1% water

to octanol gave friction and

wear results identical to the

neat octanol case. A similar

experiment conducted with a

polyalkylene glycol derivative

(460 SUS water soluble) also

showed no change in friction

or wear with the addition of

Table 7.3

Effect of Water on Lubrication

of Si3N4 by Octanol

[water] WSD Final

in WSD Incr^ COP

Alcohol (mm) (mm)

100 ppm 0.557 0.177 0.049

10,000 ppm 0.560 0.180 0.055

1 Wear Scar Diameter Increase above the

Hertzian diameter (0.38 mm)

1 % water. These results suggest that the water effect is not dominant in these tests and most

of the lubrication effect is due to some unique reactivity between alcohols and the Si3N4

surface. A series of alcohol-lubricated Si3N4 micro sample tests was conducted, and the

reaction products were analyzed to elucidate this unique reactivity.

7.4. Alcohol Lubricated Micro Sample Wear Tests on Si3N4

Micro sample wear tests conducted for this smdy were run under the standard BTF

test conditions, except 6 jwl of lubricant were used instead of the usual 1.5 ml. The benefit

of this test is that the lubricant, while circulating, is restricted to a volume of 6 fi\. As a

result, the reaction products are not diluted in a larger circulating bulk lubricant reservoir.
149

This increase in reaction product concentration enhances the ability to detect the products.

The micro sample test can also have a higher test severity due to reduced cooling effects

(smaller lubricant reservoir), and less chance of oxygen depletion (thin film). Micro sample

tests conducted using octanol, decanol, and an octylaryl polyether were compared to results

from normal (1.5 ml) tests to ensure that this increase in test severity did not fundamentally

change the lubrication mechanisms observed. This comparison (Table 7.4) indicate that the

two tests are substantially equivalent. Specimens and reaction products from the micro

sample tests were analyzed to try to isolate specific products and determine what reactions

might be occurring.

Table 7.4

Comparison of 1.5 ml and 6 jul BTF Wear Tests Results on

Primary Alcohols

1.5 ml BTF Test 6-fJL\ BTF Test

WSD Final WSD Final

WSD Incr^ COF WSD Incr^ COF

Alcohol (mm) (mm) (mm) (mm)

Octanol 0.557 0.177 0.049 0.544 0.164 0.052

Decanol 0.503 0.123 0.045 0.502 0.122 0.046

Octylaryl polyether (n=l) 0.528 0.148 0.081 0.599 0.219 0.085

1 Wear Scar Diameter Increase above the Hertzian diameter (0.38 mm)

7.5. Analysis of Worn Si^N^ Specimens

The typical boundary lubricating ability of alcohols with Si3N4 is illustrated in Figure

7.16 for a 6 jwl octanol-lubricated BTF test. The wear scar is smooth and circular, with a

wedge-shaped feature similar to that seen in the 1.5 ml BTF case. The friction coefficient is

smooth and low throughout the test. In this micro-sample test, for the FTIR and Auger
(b)

O

O

0.20

o 0.15

0.10

0.05

0.00

' I' ' ' I

' ' '

I

' ' '

I 6 yu-l Neat Octanol

600 rpm

60 kg

n 1

1 1 1

1

1 r

0

WSD = 0.544 mm

10 15 20 25 30

Time, minutes

Figure 7. 16 6 ^1 Neat Octanol Lubricated Si3N4 BTF Test at 60 kg: a) Optical

Photomicrograph, b) Friction Trace
151

analysis, residual lubricant was allowed to evaporate from the surface under argon after the

wear scars were measured. In the case of octanol, this only took a few hours. The resulting

concentration of reaction products show up as residue in and around the wear scar (Figure

7.17). The interference fringes observed at higher magnification in some regions indicate

some kind of non-volatile liquid. Since octanol itself is volatile, some kind of reaction

product must have been formed that is less volatile.

Auger analysis of the worn flats after evaporating the excess octanol revealed a

carbon-containing material covered the entire surface. This material was present in a layer

260 A thick in the wear scar and 500 A outside the wear scar. The observation of high

carbon concentration suggests that the reaction product formed is probably organic in nature.

The data are provided in Appendix J.

7.5.1. FTIR Analysis of Octanol Lubrication Reaction Products

FTIR analysis of the of the film left on the surface of the wear scar after the excess

lubricant (octanol) had evaporated was conducted using FTIR (Figure 7.18). C-H stretches

are evident, indicating organic compounds are present. Octanol is the only source of organic

material in this test, therefore the reaction product observed must come from some kind of

reaction with the octanol that retains (at least some) C-H groups from octanol. The strong,

broad OH peak normally see for octanol is missing. This functional group is either missing

or has been modified, suggesting octanol has reacted with something, resulting in the loss of

the OH functional group. A strong relatively broad peak at 1050 cm"^ is indicative of Si-0-

Si. Si-O-C will also show up in this region, so it is difficult to distinguish the two
Figure 7. 17 Optical Photomicrographs of 6 /xl Neat Octanol Lubricated Si3N4 BTF Test at 60

kg: a) Low Magnification, b) High Magnification
153

0.010

0.008

o

§ 0.005

o

< 0.003

0.000

-1—I—,—1—.—I—I—I—1—I—I

,—^—lCH

S]-0-Si

Si-O-C

4000 3600 3200 2800 2400 2000 1600 1200 800

1 Wavenumber, cm

Figure 7.18 FTIR Spectrum of Film in Wear Scar for

Octanol-Lubricated Si3N4

structures, especially if the Si-O-Si peak is pronounced. The data indicate that the reaction

product is at least partially organic in nature and is formed from octanol.

7.5.2. Evidence for Organo-Silicon Reaction Product Formation as Demonstrated by

GPC-GFAA

For the detection of silicon-based organometallic polymers, Gel Permeation

Chromatography-Graphite Furnace Atomic Absorption (GPC-GFAA ) analysis was

performed without allowing the excess octanol lubricant to evaporate. Immediately after the

6-jul wear test, the top ball with the chuck was removed and soaked in one ml of

tetrahydrofuran (THF) for two minutes. The solution was then filtered through a 0.02 /xm

filter, and 100 /xl of the filtrate was injected into the GPC-GFAA for analysis.
154

The chromatogram from the octanol lubricated wear test is shown in Figure 7.19. A

large peak in the GFAA Ghromatogram indicates the presence of silicon, associated with high

molecular weight compounds in the range of 300 to 3000. The UV detector trace shows

relatively small amount of reaction products as indicated by a broad peak coinciding with

the MW range of the GFAA trace. A second, sharper peak at 250 MW indicates UV

absorbing compounds at about the same molecular weight as the parent octanol (RI trace).

Since octanol itself does not absorb in the UV, the UV peaks must be due to reaction

products.

Decanol (Figure 7.20) has chromatograms very similar to octanol but slightly

different in intensity. The alkylaryl polyether alcohol wear product was also similar (Figure

7.21), but molecular weights were shifted to higher values, reflecting the higher MW of this

compound compared to octanol and decanol. The GFAA chromatogram indicates significant

amounts of HMW silicon-containing material from 400 MW to 30 000 MW with a maximum

near 3000 MW. The RI detector observed only the base lubricant at 400 MW. The UV

chromatogram has a strong signal because the polyether alcohol contains an aromatic ring in

its structure. Aromatic rings absorb in the UV.

The GPC-GFAA data demonstrate that HMW silicon-containing compounds are

formed in wear tests lubricated by alcohols. This is the first direct evidence that alcohols are

chemically reacting with silicon nitride surfaces during rubbing. Static reactions were

carried out using Si3N4 powders to observe this phenomenon under more controlled

conditions.
155

60 80

Time, minutes

100 120

T

-i->

CO

PI

Qi

Sample injected:

Octanol Lubricated Si^N^

BTF wear test product

140

GPC-GFAA Chromatograms

0.3 ml/min flowrate

THF carrier solvent

Ultrastyrogel columns

lOOyLil injection volume

I I I I I "'II' I

I I I I I I I I I I I LU

100000 10000 1000

Approximate MW

100

Figure 7.19 GPC-GFAA Analysis of Reaction Product from 6 /xl Neat Octanol Lubricated

Si3N4 BTF Test
156

60

T

m

Time, minutes

80 100 120

r ,

.

1

, , GPC-GFAA Chromatograms

0.3 ml/min flowrate

THF carrier solvent

Ultrastyrogel columns

lOO/xl injection volume

Sample injected:

Decanol Lubricated SigN^

BTF wear test product

Ri

UV

GFAA (Si)

140

f H ' ' ' ' ' LL 100000 10000 1000

Approximate MW

100

Figure 7.20 GPC-GFAA Analysis of Reaction Product from 6 fx\ Neat Decanol Lubricated

Si3N4 BTF Test
157

•-ICO

Pi

(U

60

Time, minutes

80 100 120 140

GPC-GFAA Chromatograms

0.3 ml/min flowrate

THF carrier solvent

Ultrastyrogel columns

100/xl injection volume

Sample injected:

6 fi\ Octylaryl Polyether

Alcohol Lubricated Si_N.

BTF wear test product

GFAA (Si)

1 1 1 1 1 I I I I 1 1 1 1 1 I I I I III 100000 10000 1000 100

Approximate MW

Figure 7.21 GPC-GFAA Analysis of Reaction Product from 6/^1 neat Octylaryl

Polyether Alcohol Lubricated Si3N4 BTF Test
I
Chapter 8

159

STATIC REACTION STUDY

It has been demonstrated that some kind of surface reactions are taking place between

Si3N4 and alcohols under boundary lubrication conditions. Static reaction experiments were

conducted using Si3N4 powder and octanol in an attempt to isolate the first stage of these

surface reaction. Initial batch reactions were conducted near 100 °C. Measurement of the

organic reaction product on the surface of the Si3N4 powders after reaction was performed by

Therogravimetric Analysis (TGA). As mentioned previously, the surface of Si3N4 is

oxidized. The discussion of surface chemistry offered in this section is therefore based on

the chemistry of silicon oxide and hydroxides present on oxidized silicon nitride surfaces.

8.1. TGA Analysis Method

TGA analysis of the powders from batch reactions involved taking a small ( ~ 40 mg)

. sample of the dried powders, placing it in a gold pan, and recording the weight of the sample

as it was heated under oxygen atmosphere. Temperature ramping was controlled at

10°C/minute from 50°C to 700°C.

TGA is an extremely sensitive technique that does not discriminate among the

different potential sources of weight loss, therefore care must be taken to calibrate against
160

weight loss artifacts such as bouyancy effects and residual solvent effects. These effects

were carefully measured and accounted for.

TGA analysis of the raw Si3N4 powder under oxygen atmosphere is given in Figure

8.1 as % weight as a function of temperature. The lower trace represents the powder "as

received. " The upper trace is a repeat TGA run of the same powder. The apparent weight

gain in the second (repeat) heating is due to bouyancy effects in the TGA furnace that were

not compensated for. In the case of the procedure used, this bouyancy effect accounts for

approximately 0.06% weight gain over the course of the analysis. The difference between

the two traces actually represents the weight loss due to heating. The overall weight loss

from the raw Si3N4 is minor (0.10%) and occurs over a very wide temperature range.

Solvent extraction-centrifugation experiments on raw powders demonstrated that the

procedure did not introduce any artifacts into the TGA weight loss analysis.

Octanol was selected for the static reaction study because it had demonstrated

lubricating ability with Si3N4. Figure 8.2 is the TGA trace for pure octanol. Note that the

scale is 0-100% weight loss, and that the octanol has a maximum rate of weight loss at «

100 °C. This is far lower than the boiling point for pure octanol (196°C) because there is a

constant flow of gas around the sample pan that removes any gases generated from the

liquid. The temperature merely has to get high enough to generate sufficient vapor pressure

in the sample to provide suitable material removal rates.
161

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163

8.2 Surface Reactions of Alcohols with Si3N4 Powder

A sample of Si3N4powder was mixed with octanol and heated to 100°C for 48 hours.

The excess octanol was removed using the solvent extraction procedure already mentioned.

An example of a typical TGA weight loss curve for the reaction product between Si3N4

powder and octanol is shown in Figure 8.3. The weight loss is very distinct, with a

maximum rate of weight loss at « 270°C. This temperature is too high to be merely

adsorbed octanol, and suggests some kind of reaction product has formed on the surface.

Given the oxidized nature of the Si3N4 surface, it was thought that the product might be a

surface reaction bonded silicon alkoxide, formed from reaction with surface hydroxy1 groups

as in:

Si^-OH + R-OH ^ Si^-O-R + Hp

These reactions have been observed with silica'^*' and have been speculated to occur with

Si3N4^'^' however, no direct evidence has been provided. Brinker^^ also indicates that

reactions can take place between alcohols and siloxane functional groups to form alkoxides

and silanols (alcoholysis).

Si-O-SL + ROH - Si -OH + Si -OR s s s s

If these are indeed the reactions taking place on Si3N4, it explains how alcohols can react

with Si3N4 to form a bonded surface layer that might help in boundary lubrication. FTIR,

DSC, GPC-GFAA and TGA/Mass Spetrometry were used to understand the chemical nature

of the surface reaction products.
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8.3. Evidence of the Chemical Structure of Alcohol-Si3N4 Powder Reaction Products

FTIR was used to analyze the surface of the Si3N4 powders after reaction and

subsequent solvent extraction. Figure 8.4 compares the spectrum of the reacted powder to a

powder that has been exposed to octanol (at room temperature) and has undergone the same

solvent extraction procedure. The reacted powder conains three peaks at 2855, 2924, and

2955 cm^ that are not present in the "exposed" sample. These three peaks are associated

with C-H stretch frequencies and indicate the presence of an organic compound on the Si3N4

surface containing C-H bonds. The presence of reaction bonded octanol is consistent with

this observation.

DSC analysis of a sample of Si3N4 powder (Figure 8.5) under oxygen indicate a

strong exothermic peak with a maximum at 250 °C consistent with the oxidation of a surface

organic compound such as reaction bonded octanol (Si alkoxide).

Another TGA analysis was performed on one of the reacted Si3N4 powders in which

the evolved gases from the system were monitored using a mass spectrometer. The mass

peaks associated with HjO and COj (Figure 8.6) go through a maxima at approximately

300°C, demonstrating that the species on the surface of Si3N4 reacts with oxygen to form

H2O and CO2, i.e., it is a carbon and hydrogen containing organic compound. Again, this is

consistent with the hypothesized silicon alkoxide surface bonded species.

GPC-GFAA analysis of the supernatant fluid left after reacting the octanol with the

Si3N4 powder was analyzed using GPC-GFAA to try to detect dissolved silicon-containing

compounds. None could be found, therefore it was concluded that the pathway from surface

alkoxide to soluble silicon-containing compound is not purely thermal in nature. Iler^^^
166

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sou^qjosqv

168

aouigpunqy
describes the solubility of silica gel in three alcohols at 500°C as shown in Table 8.1. Given

the trend of solubility, it is not surprising that Cg alcohols would not form any detectable

octanol soluble silicon compounds.

Table 8.1

Apparently there is some special factor Solubility of Silica in Alcohols

(Source: Iler^^^ p. 62)

introduced by rubbing (tribochemistry) that

makes it possible to produce these

compounds. This may be either dangling

bonds, or perhaps some element of bond

strain introduced through the surface shear stress that influences reactivity. Indeed there is

some evidence in the literature^^ that grinding of silicates in the presence of alcohol resulted

in chemical reactions between the alcohol and the surface.

8.4. Discussion on the Nature of the Alcohol-Si3N4 Surface Reaction

The analytical evidence suggests that the reaction product from the batch reaction of

octanol with Si3N4 powders is a surface bonded silicon alkoxide. The weight loss shown in

Figure 8.3; therefore represents the oxidative decomposition of the organic surface alkoxide

Si-0-C8Hi7. The weight loss seems to be level at 700°C indicating that all of the organic

material has been removed from the surface by this temperature. This is very convenient

because it means that the weight loss between 200°C and 700 °C can be used to estimate the

number of moles of octanol bonded to the surface assuming the decomposition follows a

pathway similar to that seen on silica^'^:

Alcohol Solubilitv (ppm>

CH3OH 1890

C2H5OH 164

n-CjH^OH 8
170

Si^-OR + Si^-OR + O2 - Si^-O-Si^ + xHfi ^yCO^

If the TGA analysis is carried out under argon instead of oxygen, the weight loss

curve is different in shape (not as sharp a drop), but it reaches the same final end point at

700 °C. This is because the oxidative decomposition pathway for the surface bonded organic

compound is faster than the thermal decomposition one. All of the experimental data used to

quantify the reaction product on the surface of Si3N4 was generated using an oxygen

atmosphere.

In the course of this study it was found that some TGA curves exhibited two weight

losses as exemplified in Figure 8.7. The initial weight loss at « 150 °C is at too high a

temperature to be due to either water, or residual liquid octanol, therefore it was suspected

that it represented octanol that was adsorbed on the surface of the Si3N4 (probably through

hydrogen bonding). The hypothesized structures associated with each weight loss are

superimposed on Figure 8.7. This additional initial weight loss was observed if : i) the

residual octanol reactant was not adequately extracted from the reacted powder, or ii) the

extracted (and dried) reaction product powder was not immediatelv analyzed.

Improperly extracted powders had the extra initial weight loss shown in Figure 8.7,

but they also suffered from poor repeatability in quantifying the weight loss due to silicon

alkoxide (the higher temperature weight loss). It became apparent that the presence of

adsorbed octanol on Si3N4 somehow contributed to an increase in the amount of reacted

octanol (silicon alkoxide). An experiment was conducted in which a dish of powder was

suspended above a beaker of octanol in a sealed container. The octanol was allowed to

saturate the chamber and adsorb onto the Si3N4 powder. TGA analysis of the resulting
171

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172

powder (no solvent extraction) indicated both adsorbed and bonded were present (Figure 8.8

- lower trace). When the vapor exposed powder was solvent extracted, the weight loss curve

changed back to the baseline case (upper trace). Apparently, the heat supplied to the sample

during heating (and prior to loss of the adsorbed octanol) is sufficient to promote reaction

and formation of the surface alkoxide as shown in the reaction sequence superimposed on

Figure 8.8. These results indicated that the it was very important to accurate quantitation of

the results to remove any adsorbed octanol prior to TGA analysis. The ability to measure

the adsorbed octanol weight loss was used to optimize the extraction procedure to eliminate

this problem.

The second observation that powders, on sitting, can develop the initial weight loss is

exemplified in Figure 8.9. The uppermost weight loss curve is for the reaction product

powder analyzed immediately after solvent extraction. The single peak is due only to

bonded octanol. The other curves represent TGA analyses on the exact same powder, but

the analyses were performed 1, 4, 11, and 35 days after the solvent extraction. The growth

of the initial weight loss with time indicates that the quantity of adsorbed octanol on the

surface of the Si3N4 increases with time. The overall weight loss increases with time, but at

a much smaller rate than the growth in the adsorbed octanol weight loss. The data suggest

that bonded octanol is transformed to adsorbed octanol on the surface, probably due to the

presence of water vapor. The overall weight gain can more than accomodate the water

necessary to carry out this reaction. This information means that the reaction to form

surface alkoxides of octanol on the surface of Si3N4 is reversible, and apparently the reverse

reaction is favored at room temperature. The observation that this reversible reaction seems
173

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175

to reach an equilibrium at finite values of bonded octanol seems to indicate that there may be

more than one type of surface site on Si3N4, one that can easily revert back to the adsorbed

octanol, and one that cannot. The data also underscores the importance of analyzing the

reacted powders immediately after solvent extraction.

TGA analysis of Si3N4 powders reacted for different times at 100°C (Figure 8.10)

indicated that the reaction qualitatively exhibited expected kinetic behavior of increased

coverage with time, approaching a limiting value. Attempts to fit the data to a simple

pseudo first order kinetic expression (as used by Rossi^'^ on silica) were not successful. This

suggests that while silica and silicon nitride may have qualitatively similar silicon oxide

surfaces, they appear to have different kinetic behavior. One explanation, suggested by the

observation of partial reversibility seen with the Si3N4 powder, is that there may be multiple

sites on the Si3N4 that have different kinetic behavior, resulting in non pseudo first order

(lumped) kinetics. This is supported by Brinkers^^ contention that alcohols can react with

both silanol (esterification) and siloxane (alcoholysis) groups in silica.

Qualitatively, the results obtained on Si3N4 are quite similar with the results obtained

by the other researchers on silica. Additional static reactions were carried out between

octanol and Si3N4 at higher temperatures. At 200 °C and 245 °C similar bonded surface

products were observed. An estimate of flash temperatures in the octanol lubricated Si3N4

contact, perfomed using Archard's" method (see appendix I), indicated temperatures of only

120°C. This temperature is comfortably bracketed by the temperatures of the "static"

reactions (100°C to 245 °C) and reinforces the notion that these reactions may be taking

place in the contact juction.

177

Chapter 9

MECHANISTIC MODEL FOR ALCOHOL LUBRICATION

Analysis of the reaction products from an alcohol-lubricated wear test indicated that

direct chemical reactions were occurring that incorporate silicon from the Si3N4 surface.

Static reactions have demonstrated that alcohols can adsorb and react with the surface of

Si3N4 to form a surface bonded phase that appears to be a silicon alkoxide. Secondary Ion

Mass Spectrometry (SIMS) analysis was then performed on the wear test reaction product to

clarify the chemical structure of these products.

9.1. SIMS Analysis of the 6 fi\ BTF Reaction Products

In order to try to determine the structure of the reaction products from these wear

tests, "Static" Secondary Ion Mass Spectrometry (SIMS) was employed. This is a high

vacuum microanalysis technique that used heavy ions (in this case gallium) to bombard the

surface of the sample. This process causes material to be energetically kicked off the surface

so that they can be subsequently analyzed with a time of flight mass spectrometer. The term

"static" applied to this technique describes the fact that bombardment is very gentle and does

not result in appreciable damage to the surface such as would be apparent in the more severe

"dynamic" SIMS in which the surface is actually sputtered away during the analysis. The

static SIMS technique allows very high molecular weight organic compounds to be removed
178

from the surface relatively intact. It also has the capability to focus down to very small

regions (« 100 nm) which allows for good spacial resolution among features if neccessary.

SIMS analysis of the residue from the 6 ix\ BTF test resulted in the mass spectrum

seen in Figure 9.1, which was taken while focussed on a region of the flat away from the

wear scar. This region had several condensed residue "islands" that were formed when the

excess octanol lubricant was allowed to evaporate. Immediately apparent are several peaks at

mass/charge (m/e) ratios of 15, 29, 43, 57, and 71. These peaks represent the hydrocarbon

compounds CH3 + , C2H5 + , C3H7 + , C4H9+, and C5H11+ respectively. They are indicative

of the fragmentation seen for a linear hydrocarbon, and only differ from one another by m/e

14 (CH2). The smaller, lower m/e peaks near each of these peaks represent fragments of

the parent ion that have been stripped of additional pairs of hydrogens (hence they differ by 2

mass units). There are also less intense peaks that differ by only one hydrogen (1 mass

unit). Additional peaks present that overlap some of the hydrocarbon peaks are silicon (m/e

= 28), and gallium (m/e = 69), which is an artifact of the ion bombardment. The

fragmentation pattern seen in this spectrum indicates a linear hydrocarbon of at least C5.

This makes sense since we started with Cg in the original octanol lubricant. Since we

allowed the octanol to evaporate from the surface, the data suggests that there are Cg linear

ligands attached to some kind of parent molecule which is non-volatile.

Careful analysis of the higher m/e region above 100 indicated the presence of several

peaks at m/e 157, 285, 415, 543, and 718. These peaks are very close to what would be

expected for several silicon alkoxides of the series shown in Table 9.1. It is not apparent

why the observed peaks at 285 and 543 are off by one unit from the calculated peak.
179

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180

Several attempts were made to

calculate m/e ratios for Table 9.1

SIMS Mass Spectral Data for Octanol Lubricated

compounds that might be Si3N4 Wear Test Product

expected from oxidation / m/e

Compound Calculated Observed

polymerization of pure octanol,

+

however these products were not Si-O-Cg 157 157

consistent with the observations.

Tt ^HniilH hp nntpH that thp tptra-

+

Sl-(0-C8)2

+

Si-(G-C8)3

286 285

aolkoxy compound (@ m/e 543) 415 415

and the disiloxane compound (@

m/e 718) are both present in the

+

Si-(0-C8)4

+

(Cg-0)2-Si-0-Si-(0-C8)3

544 543

reaction product since 718 718

fragmentation in the mass

spectrum does not occur through breakdown of the siloxane skeleton as noted by

Polivanov^^^. The lower alkoxides may be due (at least partially) to fragmentation of the

parent tetra-alkoxide.

Repeating the SIMS analysis on a flat from decanol lubricated Si3N4 resulted in the

mass spectrum shown in Figure 9.2. This spectrum contains the same hydrocarbon fragment

series seen in the octanol case; however it is apparent that the decanol lubricated case

extends to higher m/e ratio compounds. This is consistent with the fact that decanol has two

more carbons than octanol and would be expected to generate higher m/e fragments because

of the larger parent compound.

182

The m/e peaks seen for the mass range above 100 are at m/e 157, 341, 499, 655,

and 858. These m/e ratios are consistent with the the assignments shown in Table 9.2. The

peak at 341 is consistent with the dialkoxide, similar to the one seen with octanol, but

displaced to higher m/e to reflect

the Cio ligands instead of Cg. The

Table 9.2

SIMS Mass Spectral Data for Decanol Lubricated

peaks at 499 and 655 can be

Si3N4 Wear Test Product

assigned to the tri and tetram/e

Compound Calculated Observed

alkoxide respectively. The 858

+

peak is quite weak, but is consistent Si-0-Cio 185 None

with the Si-O-Si linked alkoxide

+

Si-(0-C,o)2 342 341

indicated. Again, all of these peaks

+

are displaced to higher m/e values Si-(0-Cio)3 499 499

that reflect the larger organic ligand

+

Si-(0-Cio)4 656 655

expected (Cjo versus Cg). The peak

+

for the monoalkoxide expected at

(Cio-0)2-Si-0-Si-(0-Cio)3 858 858

185 was not detected. The fact that

there was a peak detected at 157 (same peak m/e as for octanol) indicates that the original

assignment of 157 for the monoalkoxide of Si with octanol was incorrect. The 157 peak is

probably a product not related to any organic ligand since it is present in both the octanol

and decanol lubricated Si3N4 cases. The source of this peak has not been identified.

GPC-GFAA analysis of the reaction products from octanol-lubricated Si3N4 indicated

HMW silicon-containing organic compounds were formed. SIMS analysis identified
183

compounds in the reaction product consistent with silicon alkoxide structures. This is further

reinforced by the static test data which suggests that alcohols can adsorb and bond with the

Si3N4 surface to form silicon alkoxides. One question that remains is which reaction product

can lubricate? This question was addressed using two-ball collision tests.

9.2. Evaluation of Lubrication Effectiveness Using the Two-Ball Collision Test

The Two-Ball collision apparatus was designed and built at NIST to study single and

multiple contacts in a controlled and repeatible manner. It is described in detail in the

introductory paper by Ying and Hsu^^^ The basic geometry consists of two spheres that

move past one another (Figure 9.3). The force of the contact is dictated by the overlap (h)

and the mechanical and elastic response of the apparatus and specimen materials. A triaxial

force transducer, mounted in a

plates supporting one of the balls

allows measurement of normal

and friction forces throughout the

test. Initially, it was used to

study surface and subsurface

deformation due to contact in

metals and ceramics, but for the

purposes of this study, it was

Figure 9.3 Two-Ball Collision Test Geometry

applied to measuring the ability of

films to lubricate.
184

A series of tests conducted using NCI32 hot pressed Si3N4 indicated the ability of

different films to lubricate (Figure 9.4). Unlubricated Si3N4 starts and ends with a high

coefficient of 0.5 to 0.7, which is consistent with values obtained in the literature. Octanol

gave low friction only during the beginning of the test. This may indicate some initial

lubricating ability, but the film lacks the tenacity or "film strength" to maintain low friction.

Since reaction with the surface to form the bonded octanol may be important, a second test

was conducted using balls that had been heated to 150°C for 30 minutes, immersed in

octanol. This test was also run with liquid octanol present. It did not lubricate effectively

throughout the test. A fourth test was conducted using a tetra-alkoxide model compound

that, also, did not lubricate.

A test was conducted using the residue from a six microliter BTF test (after the

excess octanol had evaporated). The material was transferred by direct contact between one

of the flats from a BTF test with one of the balls from the Two-Ball test. Low friction was

observed throughout this test. This result indicates that there is some ingredient present in

the residue that is capable of lubricating throughout this test that has not been duplicated by

the chemistries used (octanol, bonded octanol, and C(, silicon tetra-alkoxide).

GPC-GFAA analysis of the residue from the BTF test indicated that high molecular

weight silicon containing organic compounds were present. Approximate molecular weights

of over 1000 were observed. SIMS analysis confirms the high molecular weight nature of

the residue and even indicates silicon alkoxides. The highest molecular weight species

detected with SIMS were thought to be silicon "polyalkoxide" with a siloxane (Si-O-Si)

backbone. A series of experiments were designed to determine if these "polyalkoxides"
185

Oaieot^«Oif3'*COCM'-iO ^dodoodoo ooo
186

could be generated by heating silicon tetra-alkoxide.

9.3 Tetra-Alkoxide Heating Experiments

According to several sources^^- silica can polymerize through condensation

reactions involving silanol groups (water condensation reaction).

R^Si-OH + HO-SiRj - R^Si-O-SiR^ + H^O

Polymerization reactions can also take place between a silanol and an alkoxide group in a

process Brinker'^ terms alcohol condensation.

R^Si-OH + RO-SiR^ - R^Si-OSiR^ + ROH

Given the ability to hydrolyze silicon alkoxides (reverse reaction of esterification):

Si -OR + H^O - Si -OH + ROH

it may be possible for for trace water (atmospheric etc.) to hydrolyze silicon tetra-alkoxide

and form a silanol, which can then react with another tetra-alkoxide to form a silicon

alkoxide with a siloxane backbone. In this way, high molecular weight silicon alkoxide

"polymers" might be formed. Batch reactions were carried out to verify this reaction

pathway.

Silicon tetra-alkoxide (Cg) was heated to «200°C for 5 hours. At the end of this

time, the fluid had thickened noticably and contained a second, thicker, semi-solid phase.

Figure 9.5 shows the GPC-GFAA analysis for the original Cg tetra-alkoxide model

compound. A single large peak is observed at «600 MW in both the GFAA and RI traces

Analysis of the liquid phase from the tetra-alkoxide sample that was heated for 5 hours at
187

Time, minutes

40 50 60 70 80 90

T

m

PI

-1-)

P!

GPC-GFAA Chromatograms

0.5 ml/min flowrate

THF carrier solvent

Ultrastyrogel columns

100/xl injection volume

Sample injected:

Tetra (Cg) Si Alkoxide

J

GFAA (Si)

I I I I I I I I L I'll' I ' I 100000 10000 1000 100

Approximate MW

Figure 9.5 GPC-GFAA Analysis of Neat TetraCC^) Si Alkoxide
188

200°C resulted in the chromatogram shown in Figure 9.6. This chromatogram indicates

that the HMW silicon-containing reaction products have been formed. The GFAA peak

maxima coincides with the HMW bump on the RI trace.

A two-ball collision test conducted using the liquid phase from the heated tetraalkoxide sample (Figure 9.7) failed to give low friction by the end of the test. A similar test

conducted using the semi-solid higher molecular weight reaction product gave low friction

throughout the test. This suggests that the residue from the BTF test that can give low

friction in the two-ball collision test is a very high molecular weight polysiloxanepolyalkoxide. It is suggested however, that other, lower molecular weight compounds, may

also lubricate under the higher sliding speeds found in the BTF test.

9.4 Proposed Mechanism for Alcohol and Acid Lubricated Si3N4

The combination of performance and analytical data coupled with information

available in the literature suggests a sequence of reactions that can be developed into a

lubrication mechanism for silicon nitride. The first detail is that the surface of silicon nitride

is easily oxidized and hydrolyzed to form silicon oxides and hydroxides. This surface can

react with alcohols in the series of "static" and "dynamic" reactions proposed below. Static

reactions are inducible purely by thermal means. Dynamic reactions require an element of

rubbing in order for the reaction to proceed.

First, alcohol groups adsorb onto and react with the surface of Si3N4 to provide a

bonded, tenaceous surface layer. Evidence of this reaction has been observed using static

reactions on Si3N4 powders and TGA. Further reactions in the presence of the temperatures
189

Time, minutes

40 50 60 70 80 90

T

GO

p!

GPC-GFAA Chromatograms

0.5 ml/min flowrate

THF carrier solvent

Ultrastyrogel columns

100/i,l injection volume

Sample injected:

Tetra (Cg) Si Alkoxide

Heated: SOO^C, 5 hours

(signal attenuated by factor of 3)

GFAA (Si)

I I I I I ' L I I I I I

I L 100000 10000 1000 100

Approximate MW

Figure 9.6 GPC-GFAA Analysis of Neat TetraCC^) Si Alkoxide Heated at 200°C for 5 Hours
190

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191

Reaction Category

Detection

Method

R

I HO

OH

I Si /|\

A

H2O

H2O

R

I O

I Si /|\

Surface

Alkoxylation

[Static Reaction]

TGA

R

I o

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A , rubbing

ROH

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"Free Alkoxylation"

[Dynamic Reaction]

SIMS

GPC-GFAA

OR

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I OR

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"Polymerization"

[Static Reaction]

"Polymerization"

[Static Reaction]

SIMS

GPC-GFAA

GPC-GFAA

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\ ^o^ /

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/ \

O SiO, O

\

^ /

RO—Si^ ^Si —OR

/ O^ \

RO OR

"Particulate

Formation"

[Static Reaction]

(Ostwald ripening

mechanism)

GPC-GFAA
192

and shear stresses of the contact result in the formation of free (non-surface or "free") silicon

alkoxides (detected using SIMS). These alkoxides can react with each other (and the surface)

through condensation reactions to form HMW polyalkoxides with siloxane backbones, and

thicker surface films. These products were detected in a wear test using SIMS and GPCGFAA. A static simulation of this reaction was confirmed to produce HMW siliconcontaining organic compounds with demonstrated (2-Ball collision test) lubricating ability. It

is also possible that this reaction sequence can take place on the surface of small wear

particles that may be generated in wear tests. This would produce organic coatings on the

particles and reduce possible particle aggregation and growth in the contact. These coated

particles may have a gel-thickening or grease-forming type effect within the contact. The

formation of high molecular weight silicon-containing organic compounds has two

implications with respect lubrication observed with these compounds. First, reaction and

removal of silicon from the surface of Si3N4 may take place on the tips of asperities,

reducing stress concentrations. The smooth regions observed in the high temperature zones

of the wear scar may be indications of this chemical polishing mechanism. Second, the

formation of higher molecular weight, higher viscosity, products will result in thicker EHD

films in the contact. This is consistent with the low friction coefficients observed in these

tests.

Marshall^^-' studied the adsorption of oleic and linoleic acid on silica. He found that

when heated, the adsorbed acid reacted with the surface to form a bonded species. The

combination of spectroscopic information coupled with the observation of water loss during

the reaction prompted them to propose the surface reaction shown below. This is analagous
R

\

c=o

heat

^

~^

/|\

to the reaction that occurs with alcohols and explains why the acid functional groups also

lubricate Si3N4. This reaction was also cited by Hair^°'' who indicated that the reaction

needed the presence of other ions on the silica surface.

It should be mentioned that the proposed mechanism for alcohol (and acid) lubricated

Si3N4 has several possible variations that depend on the actual nature of the surface during

rubbing. It has been mentioned that dangling bonds may play a role in the reaction

sequence. It is also possible that reactions may take place with the (unoxidized) subsurface

of Si3N4 if the oxide surface layer is removed faster that it can be produced. In this case,

direct reaction may be possible as hypothesized by Hattori"':

In both Hattori's static case, and (dynamic) grinding experiments by Tsunai^'*^*, "amines and

ammonia" were detected by ion chromatography. This reaction does not account for the

observation that silicon is associated with the high molecular weight reaction products, but

perhaps Si-O-R and Six-N-R3.^ species can be formed from subsequent reactions. Indeed
194

Tsunai admitted that some nitrogen compounds different from R3N were detected but their

structure was unknown.

It was surprising that no appreciable siHcon-containing compounds were detected for

PPO lubricated Si3N4, since frictional heating should have produced acid and alcohol

oxidation products^^ that can react with the Si3N4 surface as per the proposed mechanism.

The observation of HMW products indicated that some oxidation was indeed taking place,

but perhaps not in sufficient quantity. A 6 /xl BTF test was conducted using PPO in an

attempt to provide a more oxidatively severe condition and produce more oxidation products.

The wear scar diameter at the end of this test was 0.638 mm (0.258 mm greater than the

Hertzian contact diameter), with a final coefficient of friction of 0.113. The wear scar

(Figure 9.8A) is similar in appearance and size to the 1.5 ml lubricated case. Significant

amounts of reaction product were observed surrounding the wear scar. The friction trace

(Figure 9.8B) has an unusual dip to lower friction near end of the test. After the test was

terminated, it was allowed to cool to room temperature (under load). When the ball pot was

lowered, it was discovered that so much sludge-like oxidized products (a grease?) were

produced that the flats stuck to the ball. The viscous deposit may explain the dip in friction

at the end of the test.

GPC-GFAA analysis of the wear test product (Figure 9.9) confirmed the abundance

of HMW reaction product, both with RI and UV detectors. In the case of this 6 lA test,

small amounts of silicon-containing reaction products were indeed detected, with higher

amounts at very high molecular weights. Apparently, when sufficient oxidation products are

formed, reactions can take place with the Si3N4 surface, resulting in detectable amounts of
(b)

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600 rpm

60 kg

WSD = 0.638 mm

J I I I I I I 1 I I I I I I 1 I I I 1 I I I I L 0 5 10 15 20 25 30

Time, minutes

Figure 9.8 Neat 6fi\ PPO Lubricated Si3N4 BTF Test at 60 kg: a) Optical Photomicrograph,

b) Friction Trace
196

Time, minutes

40 50 60 70 80 90

1

1

1

1 11

1 11 GPC-GFAA Chromatograms

0.5 ml/min flovrrate

THF carrier solvent

Ultrastyrogel columns

lOO/xl injection volume

Sample injected:

6 /2l PPO Lubricated SigN^

BTF wear test product

t

>%

-+j

•i-i m

d

(U

UV

GFAA (Si)

J II I I II I I II I I L. 100000 10000 1000 100

Approximate MW

Figure 9.9 GPC-GFAA Analysis of Reaction Product from 6 fil PPO Lubricated Si3N4 BTF

Test
197

silicon-containing organic products. Blaine and Savage^^ found that carboxylic acids and

alcohols are the most abundant products of oxidation of a paraffin. Since it has been

demonstrated that alcohols can lubricate Si3N4 by reacting with the surface to form siliconcontaining organic products, it is reasonable to suggest that the in-situ formation of alcohols

(and acids) in the wearing Si3N4 may contribute to the boundary lubricating ability of paraffin

oil at low loads. This implies that it may be possible to modify the reactivity of paraffin oil

and improve the boundary lubricating ability of paraffin oil to higher loads. Indeed, addition

of 1000 ppm soluble iron naphthenate to PPO decreases the wear scar diameter from 0.645

mm down to 0.583 mm. This agrees with Chen's^^ observation of better lubrication of Si3N4

in the presence of metal salts which he attributed to higher production of friction polymer

caused by the catalytic effect of the metal. In light of the results of this study, it is suggested

that there may also be a contribution from increase^ in-situ formation of alcohols and acids.
II
199

Chapter 10

SURVEY OF PHOSPHORUS CHEMISTRIES

The phosphorous-containing compounds used were all commercially available with

long chain alkyl or aryl groups attached to make them soluble in oils. Three of the

phosphates were dialkyldithiophosphates (DP) and contained sulfur as well as metals (Zn and

Sb). Some of the other compounds also contained other heteroatoms such as chlorine,

nitrogen, or boron.

10.1. 1% Phosphorus Compounds in PPO

The data from all of the 1 % phosphorus compounds in PPO are summarized in Table

10.1 and presented graphically in Figure 10.1. The first striking aspect of the data is that all

of the phosphorous-containing model compounds very effectively reduced both wear and

friction. It did not seem to matter whether they were phosphates, thiophosphates,

phosphites, or phosphonates. The best performer produced a wear scar of only 0.024 nmi

above the Hertz diameter, which is a tremendous drop from the 0.267 mm for the paraffin

oil itself. The other striking aspects of the data are that all of the phosphates had type II

friction traces, and produced some form of film in the wear scar. In most cases, the film

was easily visible optically, but a few could only be observed using SEM.
200

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202

The first data point that catches ones eye in

Figure 10.1 is the very low friction case for

sulflirized oxymolybdenum dithiophosphate (Figure

10.2). This compound contains both Mo and S and

appears to have the same characteristic low friction of

R—o s M s Mo Mo ,P

R—O S S S O—

S-N.o-R

Sulfurized Oxymolybdenum DP

the other Mo-S compounds discussed later in the sulfur chemistry chapter. The Mo-S

dithiophospate compound seems to have acquired the best attributes of both types of

chemistries: the low friction of the Mo-S chemistry and the low wear of the P chemistry.

The phosphates which contained zinc produced optically

visible, thick, fluid-like films within the wear scar. A typical

wear scar and friction trace for these compounds is shown in

Figure 10.3. The friction trace is smooth and level throughout

the test, with a coefficient of friction near 0.08. The wear scar is

Phosphate

small and circular, with a fluid-like film covering most of the

contact region. SEM analysis of the wear scar surface revealed that

the film actually covered the entire wear scar and the optically

visible film was thicker regions of film. The thick-film region Phosphite

within the wear scar (Figure 10. 4A) had a pockmarked appearance reminiscent of swiss cheese. It

is apparent that some kind of gas-entrainment has

occurred within the film, possibly a by-product gas

R—O S O—

P Zn ^P

R—O S S O—

Zinc Dithiophosphate (ZDP)

from decomposition of the ZDP (a sulfur smell is often noted during wear tests of ZDP on
203

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600 rpm

60 kg

0.10

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-I—I—I—^—I—I—I—I—I—I—1 I I

I I I I I I I I I I 0 5 10 15 20 25 30

Time, minutes

Figure 10.2 1 % Oxymolybdenum Dithiophosphate in PPO Lubricated SijN^ BTF Test at 60

kg: a) Optical Photomicrograph, b) Friction Trace
o

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^4-1 o

0.20

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600 rpm

60 kg

0.10

£ 0.05

o

o

0.00 I ... I

WSD = 0.407 mm

_l I I 1 I I I I I L_ 0 5 10 15 20 25 30

Time, minutes

Figure 10.3 1 % 2-Ethylhexyl ZDP in PPO Lubricated S'^N^ BTF Test at 60 kg: a) Optical

Photomicrograph, b) Friction Trace
Figure 10.4 SEM Photomicrographs of Reaction Product Film from 1 % 2-Ethylhexyl ZDP in

PPO Lubricated Si3N4 BTF Test at 60 kg: a) Film inside Wear Scar, b) Flake

Outside Entrance
Acid Phosphate

206

Si3N4). The lower half of the picture shows several subsurface

bubbles in which one can see through the film to the bubble

underneath. This reveals that the film is very homogeneous in

nature. Higher magnification failed to reveal any distinguishable

wear debris in the film. . The nature of this fluid-like film is further

evident by the flake observed in the entrance region (Figure

10.4B). The film is so pliant that it can wrap over on itself.

Closer inspection of this flake revealed that it was only 0. 1 /xm

thick. EDX analysis revealed the presence of Si, P, and Zn, with

small amounts of sulfur and carbon. A soft film such as this

would be a protective barrier for the harder, more brittle silicon Phosphorothionate

nitride surface and suggests that a possible mechanism of wear reduction in this system is

through the interposition of a friction reducing, stress reducing, soft film.

The other phosphorus compounds produced films with a

different, more plastic nature. These films were often not clearly

visible under an optical microscope, and required a scanning

electron microscope (SEM) in order to be seen. In addition,

significant amounts of the film were observed outside the wear

Phosphonate

scar for many of these cases. A BTF wear test on Si3N4 using 1% tricresylphosphate (TCP)

produces a very small wear scar of only 0.42 mm. The film within the wear scar is fairly

indistinct, with some grooving apparent optically (Figure 10. 5A). The friction trace is very

smooth and flat (Figure 10.5B). A large amount of reaction product/debris is visible
0.20

o

o 0.15

O

'4-1 o

0.10

0.05

0.00

; 1% TCP in PPO

1

1 1 1

1

1 1 1 1

1 1 1

1

1 1 1

1

1 1

(Hand Digitized Data) -

- 600 rpm

- 60 kg

1 1

WSD = 0.416 mm

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1 0 5 10 15 20 25 30

Time, minutes

Figure 10.5 1% TCP in PPO Lubricated Si3N4 BTF Test at 60 kg: a) Optical

Photomicrograpii, b) Friction Trace
208

surrounding the wear scar, and is concentrated mainly at the entrance - perhaps evidence that

it is scraped from the rotating ball during the test. The acid phosphate wear scar surface

appeared to be rougher than the other surfaces and may reflect more of a corrosive action

from this compound. This would be consistent in a model in which the acid phosphate was

responsible for the antiwear action as has been hypothesized for iron systems^^^.

BTF tests conducted at 200°C (Table 10.2) indicate that some compounds still work

while others do not. The metal-containing phosphorus compounds continued to work. The

non metal-containing compounds did not.

Table 10.2

Summary of 200°C BTF Data on 1% Phosphorus-Containing Compounds in PPO

WSD

WSD Incr^ COF^

Chemical Compound^ (mm) (mm)

None [024] 0.916 0.536 0130

None [038] 0.839 0.459 0.119

None [038] 0.772 0.392 0.125

2-Ethylhexyl ZDP [024] 0.421 0.041 0.075

Oxymolybdenum DP (sulfurized) [038] 0.441 0.061 0.044

Triphenyl Phosphorothionate [028] 0.831 0.451 0.144

Tricresyl Phosphate [024] 0.928 0.548 0.151

Butylated Triphenyl Phosphate [024] 0.969 0.589 0.156

1 1 wt % in PPO

2 Wear Scar Diameter Increase above the Hertzian contact diameter (0.38 mm)

3 Measured at the end of the test

Numbers in [] refer to S'l^N^ batch used for flats
209

A series of small studies were conducted on selected phosphorus containing

compounds to try to unravel the mystery of how they work. The reasoning behind these

studies are outlined below and the results are described in the following sections.

A series of pure compound tests were conducted to see the effectiveness and

chemistry of the compound could be sustained at higher concentrations, or without PPO.

SEM observation coupled with EDX analysis of the surfaces and films helped provide insight

into the role of phosphorus in the lubrication of silicon nitride.

In order to study phosphate chemistry more closely, TCP was selected as a

representative of a non metal containing phosphate for more detailed reaction product

analysis. Metals were intentionally excluded because they are known to react with TCP and

it was desirable to know if TCP had any intrinsic reactivity with Si3N4 itself. TCP also had

the advantage of having aromatic rings

which would absorb UV light and allow

detection of low concentrations. Surface

analysis was performed on wear test

samples run using 1% TCP in PPO.

A load capacity plot of 1 % TCP

Tricresylphosphate

in PPO established the load limit for

Si3N4 lubrication of this compound in the BTF test. A series of tests were conducted using

various concentrations of TCP in PPO to see the limits of effectiveness of this compound.

The finding that very low concentrations of TCP were effective suggested that adsorption

was an important consideration in the lubrication mechanism of this compound. A series of
210

adsorption experiments were then designed and performed to see if this phenomenon could be

observed for these materials. Finally, a series of experiments were performed to see if the

antiwear effects we were observing were due merely to the presence of Fe impurites in the

Si3N4, since phosphates are known to work with iron based materials, possibly through the

formation of iron phosphates. Finally a reaction was conducted to study the reaction between

the surface of Si3N4 and phosphates.

10.2. Pure Compound BTF Tests

Several phosphorus containing compounds were tested neat in order to see if they

were capable of lubricating Si3N4 in the absence of PPO. The results, summarized in Table

10.3, indicate that some of the compounds do not work without PPO.

Table 10.3

Summary of Pure Phosphorus-Containing Compound BTF Wear Test Data

1 WSD Final

WSD Incr' COF

Compound

PPO^

(mm) (mm)

0.645 0.265 0.117

Zn diaryl DP 0.398 0.018 0.086

2-Ethylhexyl ZDP (100%) 0.426 0.046 0.089

Tris-(2-ethylhexyl) P. 0.697 0.317 0.086

TBP 0.752 0.372 0.095

TCP 1.470 1.090 0.122

1 Wear Scar Diameter Increase above the Hertzian Contact Diameter (0.380 mm)

2 Average of 16 tests
211

The two metal (Zn) containing phosphates give low wear, and form visible films

inside the contact. In the case of the aryl ZDP (Figure 10.6), the film inside the wear scar

was somewhat spotty, while the film deposited outside the wear scar was very fluid and

continuous. The alkyl ZDP (Figure 10.7) had a more continuous film inside the wear scar,

but much less deposit visible outside the scar. The aryl ZDP was a commercial product and

contained some diluent mineral oil. The alkyl ZDP (2-ethylhexyl) was a special research

sample that contained no diluent oil. The phosphates without metals gave higher wear than

PPO. Pure Tributyl phosphate for example (Figure 10.8), gave a very large wear scar with

grooves visible inside the scar. The deposit outside the wear scar seemed to contain

particulate debris. Pure TCP was such a poor lubricant that there was an initial friction

spike at the beginning of the test that exceeded />i=0.5. The wear scar at the end of the test

contained many pits suggesting intergranular fracture and grain pullout may have occurred

during the test. Perhaps these compounds are too reactive at these concentrations. The other

observation of the data is that the longer alkyl chains seem to be more effective. This may

be related to a polymerization mechanism for HMW product formation. The TCP only

contains CH3 groups on each aromatic ring of the phosphate R group, and gives the highest

wear. The TBP gives intermediate wear, and contains slightly longer butyl groups. The

lowest wear (of the non metal containing phosphates) comes from the longest 2-ethylhexyl R

groups of this phosphate. Friction follows a similar trend. If the formation of beneficial

HMW products is through polymerization of these alkyl groups (and not P-O-P condensation)

as postulated by Cho and Klaus^^^ then the trend of the results makes sense.
212

0.20

o

o 0,15

Neat Zn Diaryl DP

600 rpm

60 kg

O

o

a;o

o

0. 10

0.05

0.00

WSD = 0.398 mm

0

I I I I I I 10 15 20 25 30

Time, minutes

Figure 10.6 Neat Zn Diaryl DP Lubricated Si3N4 BTF Test at 60 kg: a) Optical

Photomicrograph, b) Friction Trace
100 \itn

o

0.20

o 0.15 . rH

~l 1

1

1 1 1

1

1

1 1 1

1 1 !

1

1 1 1

1

1 T"

; Neat 2-Ethylhexyl ZDP (100%)

- 600 rpm

- 60 kg

-1 1 1

i—I r

O

a

0)

* i-H O

a;OO

0.10 -

0.05

0.00

WSD = 0.426 mm

_J I I 1 1 I I I I I I I I I I I I I I I I U I L. 0 10 15 20 25 30

Time, minutes

Figure 10.7 Neat 2-Ethylhexyl ZDP Lubricated Si3N4 BTF Test at 60 kg: a) Optical

Photomicrograph, b) Friction Trace
214

0.20

o

——^

,

I '

I

> I I

I

> Pure TBP

-1—I

1 1 1 1—I r

o 0.15

600 rpm

60 kg

o

cuo

o

0.10

0.05

0.00

WSD = 0.752 mm

0 5 10 15 20 25 30

Time, minutes

Figure 10.8 Neat TBP Lubricated Si3N4 BTF Test at 60 kg: a) Optical Photomicrograph, b)

Friction Trace
I

215

10.3. EXD Analysis of Wear Products

EDX analyses were performed on the wear scars of several phosphorus compound

lubricated tests in an attempt to isolate the Si3N4 lubrication mechanism of these compounds.

Both pure compound and 1% compound in PPO wear products were examined.

SEM had indicated that the inside of the wear scar for the Pure TCP lubricated Si3N4

had many "flakes" in addition to the pits mentioned earlier. These flakes (Figure 10.9)

appeared to be a semi-solid extruded reaction product, pressed out onto the surface of the

wear scar. EDX analysis of the flake showed it to be composed mainly of silicon,

phosphorus and carbon (Figure 10.10). A stream of hexane was used to rinse the wear scar

and physically remove wear product from the contact region. This solution was then filtered

using a teflon filter, and the resulting trapped debris analyzed using SEM/EDX. In addition

to a large amount of semi-solid reaction product containing silicon, phosphorus, and carbon

(similar to the flake seen in the wear scar), several distinct particles were observed. EDX

analysis of these particles (Figure 10.11) indicated only small amounts of phosphorus, but

higher than normal concentrations of Mg, Al, and Fe. These are impurities in the Si3N4 that

are expected to concentrate in grain boundaries. Their observation in this instance suggests

that we are looking at a particle that has been plucked out of the Si3N4 matrix along its grain

boundary. This is consistent with the idea that pure TCP is causing high wear through grain

pullout and intergranular fracture. It is still not clear whether this is due to a chemically

enhanced fracture mechanism, a preferential chemical corrosion of the grain boundary, or

merely the inability of the pure TCP reaction product to protect the surface and modify the

contact stress distribution.
216

Figure 10.9 SEM Photomicrographs of Regions within Wear Scar for Neat TCP Lubricated

Si3N4 BTF Test at 60 kg
217

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BTF tests conducted using 1% metal thiophosphates (ZDP etc.) in PPO tended to give

very small wear scars and fluid-like films within the wear scar. In addition, deposit regions

outside the wear scar were an accumulation of reaction products that were pliant and flakelike. EDX analysis of the exit flake region of a ZDP (isodecyl) wear product (Figure 10.12)

indicated the presence of Si, P, and Zn with lesser amounts of sulfur and carbon. It is not

surprising that the carbon signal is so low because the sensitivity of the technique to carbon

and oxygen (and other light elements < Na) is low. What is surprising is that sulfur is so

low. Originally the ZDP contains a ratio of Zn/P/S of 1/2/4 (Table 10.4). The wear

product in the Si3N4 case is mostly zinc and phosphorus, maintaining the same ratio as in the

original compound.. The concentration of sulfur has decreased by two orders of magnitude.

The lack of sulfur suggests that

the ZDP has undergone some . , . ^^^^^V • . EDX Analysis Data on ZDP Reaction Product Films

in which sulfur is eliminated Relative Atomic Amounts

Source of Data Zn P s

from the contact. Rounds^'^^ Initial Compound 1 2 4

Rounds (steel) 1 2.1 0.7

conducted similar analyses on Gates (SiaN^) 1 2.0 0.06

steels; however, he still found

appreciable amounts of sulfur in the product. This suggests that some of the reactions in the

Si3N4 case may be different than those of the iron (steel) case.

An SEM photomicrograph of the wear scar from a BTF wear test using 1 % TCP in

PPO is shown in Figure 10.13A and indicates clearly the shape and extent of the entrance

and exit deposits. The same wear scar at 75° tilt in the SEM (Figure 10.13B) indicates that
220
Figure 10.13 SEM Photomicrographs of Wear Scar from 1% TCP in PPO Lubricated Si3N4

BTF Test at 60 kg: a) Zero Tilt Angle, b) 75° Tilt Angle
222

the deposit is quite thick 10 /xm in some locations). The nature of this deposit is

suggested by SEM photographs (Figure 10.14A) in which the electron beam was focussed

onto a small spot on the surface of the deposit. The upper photograph shows a bubble that

has grown out of the surface because the electron beam has decomposed the deposit and

produced subsurface gasses which inflate the bubble. The second picture, taken later, shows

the bubble has popped and is slowly collapsing. A second bubble is forming behind and to

the left of the original bubble because the electron spot was moved to this position. These

photographs show that the deposit is actually quite pliant. EDX analysis on the entrance

deposit (Figure 10.15) showed it to be composed mainly of silicon and phosphorus, with

oxygen and carbon also present. It is difficult to be quantitative about the relative amounts

of oxygen and carbon because the detector is not as sensitive to these elements as it is to

others (Na and above). Also, some carbon is from the carbon coating that had to be applied

make the sample electrically conductive. Based on experience with EDX, it was concluded

that the sample does contain significant amounts of carbon and oxygen. Silicon and

phosphorus are easier to quantify. Based on the integration of the peaks from each element

(and various equipment and excitation efficiency parameters) the silicon and phosphorus are

present at about the same concentration. It is difficult to say anything about the relative

amounts of silicon versus oxygen or carbon. It can be stated; however, that phosphorus is

present at a much higher concentration than 1 % of the surface film (the original

concentration in PPO). This demonstrates that phosphorus must be concentrating in the

deposit, suggesting an adsorption/reaction mechanism involving phosphorus, preferential to

any reactions involving the PPO.
223

Figure 10.14 SEM Photomicrograph of Entrance Deposit for 1 % TCP in PPO Lubricated Si3N4

BTF Test at 60 kg: a) Bubble Formed from Electron Beam Spot Damage, b)

Same Region after Bubble has Popped
224

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Iron is not detected in any appreciable quantity in this reaction product. FePhospliorus reactions are known to lubricate in iron based systems. In order to make sure

iron chemistry is not affecting lubrication, the EDX was checked for sensitivity to iron

phosphate. Figure 10.16 is the EDX spectrum for pure FeP04 model compound.

Comparison of this spectrum to Figure 10.15 clearly shows that no iron phospate is found in

the 1 % TCP lubricated Si3N4 wear test. The very low signal for oxygen in the FeP04 model

compound also demonstrates the difficulty in detecting this light element, since we know that

four times as many oxygen atoms are present than either Fe or P.

10.4. Auger Analysis of Wear Products

Auger analyses were performed on two wear test flats from wear tests conducted

using 1% TCP and 1% 2-ethylhexyl ZDP in PPO. Each flat was examined in two locations

inside the wear scar, and one or two locations outside the wear scar.

In the case of 1 % ZDP in PPO, the spots inside the wear scar were similar in

elemental composition (Table 10.5). The ratio of Zn/P/S/0 is 1/1/0/2.6 which is quite

different from the 1/2/4/4 in the original ZDP. The lack of sulfur in the reaction products is

consistent with the same observation with EDX, and indicates a chemical reaction has taken

place in which sulfur is essentially eliminated. Auger analysis indicated a ratio of Zn:P of

1:1, whereas the EDX analysis had indicated a stronger presence of phosphorus (1:2). The

difference may be due in part to the difference in sampling depth for the two techniques: 10

nm for Auger, and 1000 nm for EDX. Some of the oxygen signal probably comes from

silicon oxides and oxidized hydrocarbons that are present. Outside the wear scar, one sees
226
227

Table 10.5

Auger Analysis Results on 1 % ZDP in PPO Lubricated Si3N4

(Results are in atomic %)

Sample Location Zn P S O C Si N

Inside Scar, Spot #1 15 13 0.6 39 26 4 3

Inside Scar, Spot #2 17 12 0.8 40 24 4 3

Outside Scar, Nearby 8 3 0.9 22 51 8 6

Outside Scar, Far

Away

2 <1 0.6 12 67 10 7

less phosphorus (and zinc) containing compounds and more carbon containing compounds.

Finally, away from the wear scar, we see mostly carbonaceous material, similar to what we

see in the pure PPO case.

Auger depth profiling of the region within the wear scar (Figure 10.17) reveals that

the outermost layer is a thin carbon rich coating 10 A deep. This could very well be only an

adsorbed hydrocarbon layer that was not removed with the hexane solvent used to prepare

the specimens for analysis. Beyond the outer hydrocarbon layer is an 0,P,Zn rich layer

about 100 A thick. Perhaps this is some kind of inorganic ZnP04 layer. The ratios of

atomic concentrations are not far from these values.

In the case of 1% TCP in PPO (Table 10.6), only a small amount of phosphorus was

seen inside the wear scar. Most of the signal seems to be due to carbon containing

compounds. Auger depth profiling (Figure 10.18) confirms the strong presence of carbon

containing material to a depth of approximately 100 A. This outer layer also appears to be
228

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Table 10.6

Auger Analysis Results on 1 % TCP Lubricated Si3N4

(Values are Atomic %)

Sample Location P 0 c Si N

Inside Scar, Spot #1 2 10 68 11 7

Inside Scar, Spot #2 2 10 68 10 8

Outside Scar, Far Away <1 10 69 11 8

slightly oxygen and phosphorus rich, although the penetration of phosphorus does not seem

to exceed 50 A.

The Auger analysis results indicate surface phosphorus concentrations inside the wear

scars far in excess of their concentration in the original lubricant. This information, coupled

with the observation in the case of ZDP, that sulfur is eliminated from the product suggests

complex reactions are taking place in the wearing contact.

Unfortunately, EDX and Auger only tells us that elements like silicon, phosphorus,

oxygen and carbon are present; they do not give us any information about bonding. To get

information about bonding, we must use a chemical structure analytical tool like FTIR.

10.5. FTIR Analysis of TCP Wear Product

FTIR analyses were performed on the reaction product from a 1 % TCP in PPO

lubricated BTF test. In order to get any chemical structural information from such a small

amount of material, a procedure was developed in which the deposit was carefully scraped
231

from the surface of the Si3N4 flat with a scalpel blade, and transferred to a KBR pellet. The

dark color of the deposit allowed for location of the transferred spot of deposit on the larger

area of the KBr pellet. The sample could then be analyzed in transmission mode to try to

determine the chemical structure of the constituents of the deposit.

The spectrum of the residue from a 1% TCP in PPO lubricated test (Figure 10.19)

contains very little information to suggest much of an organic reaction product. The few

peaks that are seen are consistent with silicate particles and some hydroxide (perhaps from

acid and alcohol groups). It is not clear from this data exactly how phosphorus may be

incorporated. The spectrum is markedly different from the spectrum of TCP, indicating that

the high concentration of phosphorus observed in Auger and EDX analyses are not merely

concentration of TCP, but a new phosphorus-containing matertial. Complex reactions must

have taken place in the contact.

10.6. Load Capacity Plot for 1% TCP in PPO

In order to get an idea of the effective load range for 1 % TCP in PPO a series of

tests were conducted at different loads. The results of this series of tests are shown in

Figures 10.20 (friction traces) and 10.21 (plot of wear scar diameters versus load). A wear

test at 20 kg produced very low wear and friction. The friction trace is very smooth and

steady, and the friction coefficient at the end of the test is 0.084. The wear scar diameter is

slightly greater than the Hertz diameter at this load and is similar in size to the pure PPO

performance. At 60 and 70 kg load, the friction traces are still smooth and level, and wear

scar diameters are still only slightly higher than the Hertzian contact diameter. Coefficient
232


235

of friction at the end of the test is 0.077-0.078. This is in contrast to the pure PPO behavior

in which friction and wear had increased at loads of 30 kg and above. At 80 kg load,

friction starts out low and steady; however, midway through the test there is a transition to

higher friction (and presumably higher wear). This transition signals a change in the wear

mechanism that results in a larger wear rates, and higher final coefficient of friction (0.142).

A wear test at 90 kg produces an almost immediate increase to a high level of friction. The

shape of the friction trace is similar to that seen for pure PPO lubricated tests at this load.

Wear at the end of the test is about as high as the pure PPO case. The severity of the test is

apparently so high that 1 % TCP in PPO is no longer effective, and the lubricant behaves as

if it were merely PPO. Since the 1% TCP in PPO test at 200°C indicated that TCP was no

longer effective at this temperature, one might hypothesize that the failure of TCP at 90 kg is

related to the higher temperatures generated in the wear contact at this higher load.

In order to get a better idea of the temperature limit of TCP lubricated Si3N4 a test

was conducted in which the temperature was increased gradually during the test. Figure

10.22 shows the friction trace for this test in which the bulk lubricant temperature has been

superimposed. According to this figure, the friction increase which signals the wear

transition occurs near 175 °C bulk oil temperature. FTIR analysis of the lubricant after the

higher temperature tests revealed that the the TCP was still present. A special wear test was

also conducted using a sequence of temperatures. First, the test was started at 200°C.

Friction was high signalling failure of the TCP to lubricat adequately. The test was then

allowed to cool down to room temperature, and the test was continued. Friction for this

second phase of the test was low indicating TCP was now effective. This information
236
237

indicates that the change in wear mechanism promoted by increasing temperature is

reversible. Of course the critical temperature that causes the wear mechanism transition is

the actual temperature inside the contact and not the bulk lubricant temperature.

Two additional tests were conducted at 200°C to determine whether this wear

transition for TCP lubricated Si3N4 is concentration dependant. This was because there may

be several reasons why 1 % TCP fails as a function of temperature. The first possibility is

that at higher temperatures the reactivity is so high that the additive gets depleted before it

goes all of the way through the scar. If this were true, then a higher concentration of TCP

in PPO should lubricate. A second possibility is that the higher temperatures cause 1 % TCP

to be too agressive and it corrodes the surface. If this were true, then lower concentrations

of TCP in PPO might be effective at higher temperatures. In order to address these two

possibilities, 200*'C BTF wear tests were conducted on Si3N4 using 0.1% TCP in PPO, and

5.0% TCP in PPO. Wear tests conducted with these two other concentrations of TCP

produced high friction and wear indicating that neither of these two proposed reasons for the

temperature dependance of TCP were correct.

Another test was performed at 200*'C using 1% TCP in PPO and 0.1% Fe

naphthenate to see if the presence of Fe would allow TCP to work at this temperature. The

addition of Fe failed to allow TCP to work at 200**C.

From the above data it was concluded that the failure of TCP to lubricate Si3N4 at

higher temperatures is not due to a kinetic barrier (i.e., insufficient or excessive reaction

rate), but to a change in the chemical mechanism of lubrication. This could be due to a lack
238

of adequate adsorption of TCP on the surface at this temperature, or a change in the actual

chemical surface reaction that results in lubrication.

10.7. Effect of [TCP] on Wear of Si^N^

Another variable that determines the effectiveness of a compound in boundary

lubrication is its concentration. The initial screening study used 1 % compounds in PPO

because the 0.5-2.0 % range is a common concentration for many antiwear additives. In

order to understand the effectiveness of TCP at other concentrations, a series of tests were

conducted using as little as 0.01% to as much as 100%. The results of thes tests are

presented in Figure 10.23. According to the data, 0.01% TCP is not effective, however

0.1% is effective. Too high a concentration (such as 100%), is also not effective. In fact,

the pure TCP gave a much higher wear scar diameter that pure PPO. This may be due to

too high a chemical corrosivity as suggested in the theory of optimum reactivity^^^.

According too this theory, too little antiwear additive causes higher than optimal (lowest)

wear because of insufficient production of beneficial reaction products. Essentially, the film

is physically removed at a faster rate than it is formed. Too much antiwear additive causes

higher than optimal wear because there is too much reaction with the surface and the

dominant wear mechanism begins to shift toward a corrosive wear mechanism.

Alternatively, wear for the pure TCP case may be due to poor viscous lubrication

contribution because of the low pressure-viscosity coefficient for TCP. It is also possible

that the reaction product of both TCP and PPO may be neccessary for adequate lubrication.
239

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10.8. Powder Adsorption Studies

In the course of investigating why phosphates in general, and TCP in particular, can

lubricate Si3N4, several literature references were found that provided important clues. Klaus

and Bieber^^^ studied the mechanism of TCP with Fe. In using silica gel thin layer

chromatography (TLC) plates to separate TCP into its constituent components, they

demonstrated that TCP had an affinity for the silica gel surface. Sinitsyn et al.^^"*^ studied

TCP in greases and found that the concentration of TCP in the oil decreased significantly

when a silica was used as a thickening agent. It was thought that these two observations

were related and suggest that phosphates can adsorb onto silica. In some cases, the

adsorption may be strong enough to reduce the concentration of "active" phosphate to affect

wear performance. Since the surface of Si3N4 is oxidized silicon, it may be very sunilar in

chemistry to the silica used in these studies. Therefore, a study was conducted to see

whether TCP could adsorb on Si3N4.

Approximately 1.5 ml of a solution of 1% TCP in hexadecane was passed through

300 mg of Stark H2 Si3N4. IR spectra were taken before and after the elution, and are

compared in Figure 10.24. According to this data, there are absorption bands at 970 cm

1145 cm \ and 1245 cm"^ (and others) that are characteristic of TCP. It is apparent that

percolation through Si3N4 has reduced the intensity of these characteristic bands

approximately 35 % . GC-MS confirmed this reduction in concentration of TCP due to

percolation through Si3N4 powder. A second experiment was performed using silica gel

instead of Si3N4 powder (surface area: 300 m^/g versus 3 m^/g for Si3N4 powder used). The
241

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242

silica gel removed all of the TCP from the hexadecane. Clearly, there is some kind of

chemical interaction between TCP and the oxidized Si3N4 surface.

10.9. Effect of Iron Impurities

One critical question that must be addressed is the possible role of iron in lubrication

of Si3N4. Many researchers have studied the antiwear behavior of TCP on iron surfaces.

Godfrey^^^ contended that iron phosphate was formed in the wearing contact as a reaction

between acid impurities in TCP and the iron in his steel specimens. This concept is

supported by data from Klaus et al.^^^ and Bieber et al.^'*^ during the same time period (1965-

1968). Later work by Klaus et al.^^^^ indicated crystalline FeP04 wear debris for steel on

silicon nitride wear tests lubricated with TCP. Clearly, iron is an important constituent in

these studies.

Jeng's PHD thesis (May 1987)''*^^ indicates that the deposition of TCP reaction

product from the vapor phase was "catalyzed" by iron. He determined that the deposition on

stainless steel followed the first order kinetic equation:

37100

Rate = 3.06jc10^^ e C

On a "non-catalytic" (Quartz) substrate the equation becomes:

55200

Rate = 2.34 x 10^^ e C
243

Unfortunately, Jeng does not make a distinction between the material formed in the two

reactions. In the case of the steel surface, Jeng suggests that the deposit consists of a

mixture of iron phosphate and organic polyphosphate. If the same reactions can apply to the

ball-on-three-flat tests, then one must be aware that iron may play a significant role in

catalyzing reactions.

A BTF wear test conducted using 1% TCP + 0.1% Fe naphthenate in PPO was

conducted to see if additions of iron would affect the test results. The wear scar was only

slightly smaller and the friction level was similar to the case of the 1% TCP in PPO. The

most obvious difference was the texture of the film in the wear scar. The test conducted

using the Fe naphthenate had a film that was much more fluid in nature (Figure 10.25).

EDX analysis of the film revealed a ratio of Fe:P of approximately 1:7, indicating that even

with Fe present the film is not purely FeP04. A wear test conducted using just 0.1% Fe

naphthenate in PPO resulted in a wear scar of 0.583 mm and final coefficient of friction of

0.105; therefore the Fe naphthenate by itself is not a great lubricant.

10.10. Pin-On-Disc Wear Tests

Since iron can play an important role in lubrication in iron based systems, and

concentrations of as little as 200 ppm may influence the chemistry of TCP, attempts were

made to conduct wear tests using low iron materials. Initially, tests were run using fused

silica, based on the concept that the surface of Si3N4 is chemically similar to silica. Fused

silica is readily available in very high purity with trace iron content below 2.5 ppm. Pin-ondisc tests were selected because pins were easier to fabricate than balls.
244

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60 kg

0.10

0.05

0.00

WSD = 0.411 mm

I JI I I I I I I I I I I I I I I I I I L 0 5 10 15 20 25 30

Time, minutes

Figure 10.25 1 % TCP + 0.1 % Fe Naphthenate in PPO Lubricated Si3N4 BTF Test at 60 kg:

a) Optical Photomicrograph, b) Friction Trace
245

Optically smooth disks of 31.75 mm diameter (1.25 in) were used as the lower

specimens. Upper specimens were made from 6.35 mm (0.25 in) diameter rods

approximately 17 mm (0.7 in) long. A hemispherical tip was fashioned using a series of

diamond and silicon carbide grinding and diamond polishing.

POD tests were conducted using a 45° pin holder. Since the contact point on the

hemisphere is midway between the tip and the equator, the pin may be rotated between tests

and several tests can be conducted on a single pin. Four tests were conducted per pin. Four

tests were conducted on each disk by varying the radius of rotation of the contact point

between runs.

Specimens were cleaned using the standard cleaning procedure (hexane, acetone,

Micro, DI water, dry) just prior to testing. Specimens were locked in place, and vertical

uniformity of the rotating disk was checked just prior to testing using a dial gauge. A

vertical uniformity of better than 125 (xm (0.005 in) could be obtained with careful

adjustment. Wear scars on the pin were measured after each test, and friction was

continuously monitored during a test. Tests were run at 10 cm/s, 20 N, 60 min, ambient T

and humidity, and 50 jwl lubricant.

A paraffin oil lubricated test gave a relatively smooth and level friction trace (Figure

10.26B). The wear scar observed after the test was relatively small and well defined with a

rough surface (Figure 10. 26A). Several dark lines can be seen emanating radially from the

scar. These are radial cracks and are visible due to the transparent nature of the fused silica.

Closer magnification using SEM (Figure 10.27) indicates that these cracks are also visible on

the surface. The direction of sliding in these photographs are indicated by the arrows. The
246

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O 0.15 -

0.10 -

S 0.05 -

0.00

0.1 m/s

20 N

WSD = 0.301 mm

0 10 20 30 40 50 60

Time, minutes

Figure 10.26 PPO Lubricated POD Test on Fused Silica: a) Optical Photomicrograph, b)

Friction Trace
247

Figure 10.27 SEM Photomicrograph of Wear Scar on Pin from PPO Lubricated POD Test on

Fused Silica: a) Wear Scar, b) Higher Magnification of Radial and Tensile

Cracks outside Wear Scar
248

surface of the wear scar is pitted and a small, severly fractured region can be seen. The

large, curved crack just outside and parallel to the wear scar near the entrance is a tensile

crack. The large dark feature outside the wear scar in the lower left portion of Figure

10. 26A is not visible on the surface of the SEM photographs (Figures 10.27), suggesting that

it represents a subsurface lateral crack. These characteristic cracks are described in detail by

many researchers^^^^. The observation of these modes of cracking and fracture suggest that

this material is undergoing a very different wear mechanism than that observed for Si3N4 and

may not be suitable as a chemically similar wear analog of the Si3N4 surface.

A wear test conducted using 1% TCP in PPO and 1% TCP + 0.1% Fe naphthenate

in PPO gave considerably higher wear than PPO by itself (Table 10.7). Radial cracking was

much reduced; however the

fracture region within the wear Table 10.7

scar was more pronounced for

Wear Test Results for POD Tests on Fused Silica

these two tests (Figures 10.28). WSD Final

The TCP lubricated test (Figure

Lubricant (mm)

PPO 0.277

COF^

0.073

10.28A) had significantly higher

PPO 0.301

PPO + 1% TCP 0.435

0.078

0.091

levels of deposit surrounding the

PPO + 1% TCP + 0.1% Fe N.2 0.476 0.110

wear scar than the PPO case.

Pin-on-disk Tester, 0.1 ms ', 20 N, 60 minutes, 50 ^1 lubricant,

21°C, 50% Relative Humidity

This deposit was rich in

' measured at the end of the test

^ Ferric Naphthenate

phosphorus. The TCP + Fe

naphthenate lubricated test (Figure 10.28B) had even more deposit. These deposits were rich

in both phosphorus and iron. The elemental composition of the deposits is similar to that
249

Figure 10.28 SEM Photomicrographs of Pin Wear Scars from POD Tests on Fused Silica- a

1 % TCP in PPO, b) 1 % TCP + 0.1% Fe Naphthenate in PPO
250

observed for the Si3N4 BTF tests; however, the wear results are not. The higher wear

observed for the TCP lubricated cases, coupled with the observation of severe fracture within

the wear scar suggests that TCP (and Fe naphthenate) may be enhancing the fracture process.

This type of behavior has been demonstrated by Freiman^^ in his study of fracture of glasses.

The lack of comparable wear mechanism between Si^N^ and fused silica indicates that fused

silica is not suitable as a model for studying the low iron lubrication of Si3N4 under bounary

lubrication conditions.

Low iron impurity Si3N4 produced at Rutgers University was used to fabricate pins

(0.25 in dia.) and disks (1.5 in diameter) for Pin-on-Disk (POD) wear tests. A 0.125 in

radius, diamond polished hemisphere was produced on one end of the pin. A 0.25 in thick,

2 in diameter disk was ground and polished using the standard procedure, and resulted in a

mirror finish on one surface. Wear tests were conducted using the CSEM POD apparatus.

Initially conditions of 23 cm/s (same as BTF) and 20 N for 60 minutes were used, but

the wear scar obtained was only 0.142 mm using PPO without any additive. This is only

0.01 mm greater than the Hertzian contact diameter for these materials and conditions. This

is too mild to allow the additive any opportunity to show improvement. Therefore,

conditions were made more severe. The second PPO lubricated test was conducted at 45 N

load and the same speed as before. The wear scar diameter at the end of the test was 0.279

mm (0.106 mm greater than the Hertzian contact diameter of 0.173 mm). A wear test

conducted under the same speed and load using 1 % TCP in PPO produced a wear scar of

only 0.195 mm (only 0.022 mm greater than the Hertzian contact diameter). The results are

summarized in Figure 10.29. The dotted line in this figure represents the transition from
251
252

low to high wear that is expected somewhere between 20 N and 45 N load for PPO. The

BTF test showed a transition for PPO at between 100 and 120 N (25-30 kg applied load).

The difference in transition is due to the smaller curvature for the POD test (0.25 in diameter

versus 0.5 in diameter for the BTF). When one calculates the hertzian contact pressures for

the BTF test at the PPO transition, one finds mean pressures of 1.58 to 1.68 GPa (229 to

243 kpsi). This corresponds to an applied load of 25 to 30 N in the POD test. Therefore,

the transition has been tentatively drawn in at this load.

The data shows that 1 % TCP in PPO is capable of lubricating Si3N4 in the absence of

iron under the conditions of this test. It is still anticipated that iron may help promote

additional reactivity and enhance lubrication in ceramic materials, as was shown using 0.1%

Fe naphthenate. This suggests that it may be possible to add small amounts of iron to

ceramics to ensure adequate reactivity at certain operating conditions.

10.11. Powder Reaction Study

Bogatyrev^^^ found that di- and triethyl phosphites could react with the surface of

silica to produce bonded organo-phosphorus films. These reactions were possible at

temperatures as low as 200°C. He also notes that the Si-O-P bond is thermally stable up to

1000°C.

Batch reactions were conducted using high surface area Si3N4 powders and pure

phosphates to try to determine whether phosphates could directly react with the Si3N4

surface. A mixture of Si3N4 powder and trioctylphosphate (TOP) was heated to 150-175 °C

for 1 hour. On cooling, the excess TOP was solvent extracted using hexane. The resulting
powder was dried, and analyzed using thermogravtmetric analysis (TGA). The TGA weight

loss curve (Figure 10.30) indicates a 1.1% weight loss due to bumoff of a surface product

that had remained after the solvent extraction.

A similar experiment was conducted using TCP with the modification that a

hexane/toluene (9/1) mixture had to be used to extract the residual TCP. The resulting

weight loss curve of the dried powder (Figure 10.31) exhibited a 1.3% weight loss. These

results are consistent with the the hypothesis that these phosphates can react directly with the

surface of Si3N4.

10.12. Proposed Reaction Mechanism Between Si3N4 and Phosphates

The data suggest that phosphates can react with the Si3N4 to form a lubricious surface

compound. The first phase of the mechanism is thought to be adsorption of the phosphate

onto the Si3N4 surface. Since the surface of Si3N4 is actually a partially hydroxylated silica,

hydrogen bonding may exist between the oxygen of the phosphate and the hydrogen of the

Si-OH. Upon heating, one of the alkyl groups may be removed, resulting in a bonded

surface phosphate and possibly a surface alkoxide. This mechanism was proposed by Iler'^^

to explain the interaction between trialkyl phosphates and polysilicic acid as shown below.

O-Et

/ Et Et p-p-o-a

I CKEt \ / H H H H H H _o-P Et

I I I I -H20 OOOO 0000 i J J I

.Si. .Si. .Si. .Si. ^> Si s! ^ V V V ^o ' </ V V V
254
255

_ 1

11.3% ' ^

-

ion

of .

hour) - 111111I1TI1

linute

scan

rate; ))1 m

reacti I

TCP

j°C

for 1 -

in)>1 A wrder

fro ?T1 ° - C U - o0 gi> ' o > 1 ^)

Po

o ^ ---

Neat

TCJ - \ - 1 1 1 . { ooo

COoo

COo

LO

LOoo

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lOooo

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COo

LOoo

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C^Jo oo CD C\2

O CO CO C\2

O 6OT—1 6o

oi

05 C2 05

05

05

oi

05

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05

00

CT5 CO

CJ5 CO05

00

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uo

cuu

cua

cio

00O

<4-l Vhcoo3Oo

4—> o

13c<<OH

Ohu

COdT—(

l-i 3
256

The reaction is described in a bit more detail in Iler's patent"^ which indicates ways of

attaching organic groups to silicic acid for the purpose of separation and purification. An

additional feature of the phosphate ligand on the silicic acid mentioned in the patent, is that it

reduced the rate of the polymerization reaction that tends to form condensed silica structures.

This may offer a clue as to why phosphates lubricate.

When two asperities approach each other and they contain surface Si-OH, the pressure

and heat of the contact may cause a condensation reaction, resulting in several Si-O-Si bonds

\ 1 /

Si

1 OH

\ 1 /

Si

1 OH

Si

1 OH

1 Si / 1 ^

OH

1 Si

^

-H2O

Si

Condensation Reaction Model for Si^N^ Asperities

between the asperities. These strong bonds will lock the asperities together and cause high

friction. When the tensile and shear forces become high enough, the asperity will break off

resulting in a wear particle. This would be a very high wear process. A surface bonded

phosphate might keep these condensation reactions from occuring and result in lower friction

and wear.

A British patent issued in 1954^^'*^ offers additional insight into what reactions may be

occurring between phosphate esters and the Si3N4 surface. In this patent, "mixed condensates
257

of polymeric polyanhydric structure" are formed in reacting trimethyl phosphate and several

silicon alkoxy compounds. Since silicon alkoxides can be formed on Si3N4 in the presence of

alcohols (PPO oxidation products), this reaction may be occurring in wearing contacts. This

also explains why some PPO may be necessary for lower wear rates.

At some point, however, temperatures could become so great that organic phosphate

films can no longer lubricate. Some thermal decomposition of the surface organic phosphate

might occur. If this happens, it is hypothesized that the remaining phosphorus oxide might

form an inorganic silicophosphate glass with the surface silica as demonstrated by

Eldridge'^^^. Silicophosphate glasses can form eutectic mixtures with a lower melting point

than silica as shown in Figure 10.32^^^^ According to this figure, pure silica has a melting

point of over 1700°C, while addition of as little as 15 mole % P2O5 can reduce the melting

point to below 1000°C. In addition to lower melting point, phosphosilicate glasses also

posess different physical properties than pure silica. This lower melting glass would be a

protective soft layer on the surface of Si3N4 that would reduce point stresses at asperity

contacts and reduce wear.

GPC-GFAA analysis was performed on the reaction product from a 6 )wl BTF wear

test to see whether any silicon-containing reaction products could be detected. The wear scar

diameter (Figure 10.33) is slightly larger than that obtained from the 1.55 ml case. A large

groove runs through the center of this (and also the other two) wear scar. Significant

amounts of reaction product are seen surrounding the scar, with little evidence of any fluidlike film inside the scar. The friction trace is smooth and level throughout the test after an

initail, brief wear-in stage.
258
259

100 \xm

0.20 T—I—I—I—I—I—I—

I—I—I—I—I—I—I—I—I—I—I—I—I—I—1—I—I—1—I—I—I—I—I—I—I—I—I—

r

o

0.15

1

6 /zl 5% TCP in PPO

600 rpm

60 kg

03

•F-t o

o

u

0.10

0.05

0.00 I , I I

WSD = 0.516 mm

I I I

I I I I ] I I I I I I I I I I I i_ 0 5 10 15 20 25 30

Time, minutes

Figure 10.33 6 fx\ 5% TCP in PPO Lubricated Si3N4 BTF Test at 60 kg: a) Optical

Photomicrograph, b) Friction Trace
260

The GPC-GFAA analysis (Figure 10.34) indicates little besides the original PPO

(higher MW peak) and TCP (lower MW peak) in the RI trace, and TCP in the UV trace.

No silicon-containing species were detected. One problem noted during the analysis was that

the reaction product film observed around the wear scar was not very soluble in the THF

used for the extraction and GPC-GFAA analysis. A second extraction was performed using

THF and ultrasonic agitation to loosen and dissolve the reaction product. This second

extraction was successful in dissolving the product visible in the optical micrograph. GPCGFAA analysis of the extracted material (Figure 10.35) indicated a single peak of material

similar in MW to the original MW range of TCP (both RI and UV). A small shoulder is

seem on the HMW side of this peak in UV, probably indicating a higher MW reaction

product involving TCP. Again, no silicon-containing species were found.

The results indicate a fundamental difference in reaction product film for the PPO,

and TCP in PPO, lubricated cases. In the case of PPO, significant amounts of THF soluble

HMW reaction prducts are formed. If oxidation is enhanced (6 fA test), silicon-containing

species can be detected. In the case of TCP in PPO, reaction products are still formed,

although at lower temperatures, due to the reduced frictional heating. However, these

products are not as soluble in THF (i.e., different chemical structure), and they seem to be

more associated with TCP than the PPO. This suggests more of an organic polyphosphate

type of film than the organic-polymeric and organo-silicon types of films suggested in the

PPO lubricated cases.
261

40 50

Time, minutes

60 70 80 90

GPC-GFAA Chromatograms

0.5 ml/min flowrate

THF carrier solvent

Ultrastyrogel columns

100/xl injection volume

Sample injected:

5% TCP in PPG L

6 fil BTF wear test product

5% TCP in PPG Lubricated SigN^

Ri

UV

GFAA (Si)

100000 10000 1000 100

Approximate MW

Figure 10.34 GPC-GFAA Analysis of Reaction Product from 5% TCP in PPO Lubricated

Si3N4 BTF Test at 60 kg
262

Time, minutes

40 50 60 70 80 90

• i—i w

+->

GPC-GFAA Chromatograms

0.5 ml/min flowrate

THF carrier solvent

Ultrastyrogel columns

100/i,l injection volume

Sample injected:

5% TCP in PPO Lubricated Si.,N^

6 /xl BTF wear test product

Second THF extraction

(ultrasonic)

A

uv

GFAA (Si)

1 1 1 1

'

' 100000 10000 1000 100

Approximate MW

Figure 10.35 GPC-GFAA of Second Extraction (with Ultrasonic) of Same Reaction Product
263

Chapter 11

SURVEY OF HALOGEN CHEMISTRIES

BTF wear test data on the chlorine-containing compounds at 1 % in PPO are

summarized in Table 11.1 and presented graphically in Figure 11.1. Fluorinated compounds

were not soluble in PPO and had to be tested neat (section 11.3).

None of the chlorine-containing compounds gave significantly lower wear and/or

friction than PPO when used at 1% by weight in PPO. Two general types of chlorinecontaining compounds were tested. One type were the chlorinated compounds, which

included fatty acid and ester compounds. The second type were the chlorosilanes, which

contained chlorine attached to a silicon atom at one end of the molecule. Both types had

distictive behavior different than the pure PPO.

11.1. Chlorinated Compounds

The four high wear data points seen in Figure 11.1 are chlorinated compounds. The

wear scars were very large and very smooth, exemplified by chlorinated paraffin (Figure

11. 2A). This combination of large wear scar and very smooth finish suggests some kind of

chemical reaction and polishing action by the additives. The chlorinated paraffin oil

produced a friction trace with a similar appearance (type I) and final friction level (0.118) as

the purified paraffin oil base fluid, however, wear had increased substantially. The
264

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w 2
Figure 11.2 Optical Photomicrographs 1% Chlorine-Containing Compounds in PPO

Lubricated Si3N4 BTF Tests at 60 kg: a) Chlorinated Paraffin, b) Chlorinated

Fatty Acid
267

chlorinated fatty acid (Figure 11. 2B) also exhibited a small degree of grooving within the

wear scar. This type of artifact is usually associated with a corrosive type of wear

mechanism and can lead to surface conformity in the wearing contact^^^*. Chlorinated fatty

acid and the other two chlorinated compounds gave type III friction behavior, but in all cases

the transition to high friction occurred during the first five minutes of the wear test. No

films were visible inside the wear scar for any of the chlorine-containing compounds. The

apparent corrosion of the silicon nitride surface by chlorine-containing compounds agrees

well with literature findings that some chlorine compounds can chemically corrode silicon

nitride

A wear test conducted at 200°C using 1% chlorinated paraffin oil in PPO had similar

wear and friction to the pure PPO case. The wear scar diameter was only slightly higher at

0.923 mm. The friction type was type I.

BTF tests conducted on neat chlorinated paraffins (Table 11.2) gave low wear and

friction. In both cases, no films could be observed within the wear scar.

Table 11.2

BTF Test Results on Neat Chlorine-Containing Compounds

WSD Dia Increase COF

Chemcial Compound (mm) above Hz, mm

None 0.647 0.267 0113

Chlorinated Paraffin [389 cSt] 0.402 0.024 0.079

Chlorinated Paraffin [121 cSt] 0.402 0.024 0.076
268

It is not obvious why 1 % chlorinated paraffin would have a corrosive effect with Si3N4, yet

neat chlorinated paraffin would lubricate, but there are some possible explanations. The

behavior of the interfacial reaction product film will be dictated in part by its environment.

This environment includes the surface and the bulk fluid. The properties of the fluid, such

as surface wetting and solubility (of the film in the fluid) may therefore influence the

lubricating ability of the film. It is also possible that there is an impurity in the chlorinated

paraffin that is responsible for the low wear. In the neat fluid, it is at a high enough

concentration to be effective. At the lower dilution in PPO it may fall below its effective

concentration range.

11.2. Chlorosilanes

The chlorosilanes used in this study were silicon containing compounds with organic

groups to give them solubility in PPO, and chlorine functionalities attatched to the silicon to

allow them to react with and bond to the Si3N4 surface. The general structure of the

chlorosilanes used is:

(«,).-S'-c;(4-.,

Where the i represents different organic functional groups. In most cases, Ri is a long chain

hydrocarbon (Cig for example), and and R3 are usually methyl groups. If three R groups

are present, then only one chlorine compound can be present on the silicon (e.g., dimethyl-noctadecylchlorosilane). If only two R groups are present, two chlorine groups can be present

(e.g., methyl-n-octadecyldichlorosilane). If only one R groups are present, three chlorine

groups can be present (e.g., n-octadecyltrichlorosilane).
269

Originally, these compounds were selected for wear testing because they are known to

react with silica surfaces^"*^. In fact, these compounds are used commercially to modify the

surface chemistry of silica for chromatographic applications. The reaction is considered to

proceed through the silicon hydroxide on the surface of silica:

Where the subscript s indicates that the silicon atom is on a surface and contains bonds to

other atoms. The result is an organophilic modification of the silica surface instead of the

hydrophylic one originally present. It was thought that if this reaction could take place in the

contact junction, a protective organophilic layer could be formed "in-situ" to lubricate the

surface. This was the reason that the chlorosilanes were used in this study.

One variable in studying this class of compounds is the effect of number of chlorines

on the reactions. It is thought that if two chlorine groups are present, they can cause two

bonds to be formed with silica. It is generally considered that this does not occur with the

same silicon atom (i.e., geminal hydroxyl groups), but instead with neighboring silicon atoms

(i.e., viscinal hydroxyIgroups) such as:

Si^-OH + Cl-Si-{R)^ - Si^-0-Si-{R)^ + HCl

ci ci

R

+ OH OH O O

+ 2 HCl

Si Si Si Si O o o o o o

Possible Dichlorosilane Reaction Mechanism
270

It might also be possible that they could fonn a bridge between silicon atoms on two

different particles thus bonding the particles together. Another thought was that this

mechanism might be beneficial in bonding two slightly separated sides of a crack in silica

together, possibly arresting propogation of the crack. Since it was not known beforehand

which reaction would be most beneficial, several different chlorosilanes were used. Initially,

they were all used at 1 % in PPO.

Dimethyl-n-octadecylchlorosilane (Figure 11.3) seemed to provide lubrication for the

first 2 minutes of the test, then a transition to high friction occurred that looked like the

friction for pure PPO. The final wear scar diameter was not much smaller than that of pure

PPO. Methyl-n-octadecyldichlorosilane (Figure 11.4) and n-octadecyltrichlorosilane (Figure

11.5) provided lubrication for only the first minute of the test. Another monochlorosilane

was used to see if the long chain organic group made a difference in lubrication

performance. The compound (p-t-butylphenethyl)dimethylchlorosilane succeeded in

protecting the Si3N4 surface for only 2 minutes (Figure 11.6).

There are several reasons why these compounds might only work for a few moments

before failing. First, they might be so reactive that they are depleted in this time. Second,

they might only work in a limited temperature range, and when the surface of the Si3N4 gets

too hot, they fail. Third, they may fail because we wear and/or react through the oxide layer

neccessary for this reaction. Finally, PPO may produce (oxidation and degradation) reaction

products that compete with the chlorosilanes for the critical surface sites.

SEM analysis was performed on the wear scar from a wear test conducted using 1 %

dimethyl-n-octadecylchlorosilane in PPO. The wear scar (Figure 11. 7A) is very smooth.
0.20

o 0.15

0,10

0.05

0.00

1 1

1 1 1 1

1 1 1 1

1 1 1

1

1 1 1

1

1 1 1

1

1 1 r 1% Dimethyl-n-octadecylchlorosilane in PPO

600 rpm

60 kg

WSD = 0.629 mm

J L_ J 1 I U

0 10 15 20 25 30

(Hand Digitized Data) Time, minutes

Figure 11.3 1% Dimethy-n-Octadecylchlorosilane in PPO Lubricated Si3N4 BTF Test at 60

kg: a) Optical Photomicrograph, b) Friction Trace
o

o

o

0)o

0.20

a 0.15

T I ^ I I I

I

I I I

I

I I I jI I I

I

I I I

j 1% Methyl-n-octadecyldichlorosilane m PPO

600 rpm

60 kg

0.10

0.05

0.00

WSD = 0.647 mm

_l 1 I I I L

I

. . ^ I I ... I 0

(Hand Digitized Data)

5 10 15 20 25 30

Time, minutes

Figure 11.4 1% Methyl-n-Octadecyldichlorosilane in PPO Lubricated Si3N4 BTF Test at 60

kg: a) Optical Photomicrograph, b) Friction Trace
273

0.20

o

o 0.15

O

•r-H

o

o

0.10

0.05

0.00

'

I

' ' '

I

' ' ' I' ' ' I

' ' ' I

' 1% n — octadecyltrichlorosilane in PPO

600 rpm

60 kg

I I 0

WSD = 0.650 mm

, I I I I , I I

n 1 1 1—I r

I I I 10 15 20 25 30

(Hand Digitized Data) Time, minutes

Figure 11.5 1 % n-Octadecyltrichlorosilane in PPO Lubricated Si3N4 BTF Test at 60 kg: a)

Optical Photomicrograph, b) Friction Trace
274

100 jjim

o

I—

(

+->o

•r-t

o

CDo

o

0.20

0.15

0.10

0.05

0.00

—I

1

1 1 1 11 1 1

1

1 1 1 1

1 1 1

1

1 1 1

1

1 1 1

1

1 r-

~_ 1% (p-t-butylphenethyl)dimethylchlorosilane in PPO

"

600 rpm

60 kg

WSD = 0.617 mm

_l I I I I I I I I I I I I I I I I I L. I I 0 10 15 20 25 30

(Hand Digitized Data) Time, minutes

Figure 11.6 1 % (p-t-butylphenethyl)dimethylchlorosilane in PPO Lubricated Si3N4 BTF Test

at 60 kg: a) Optical Photomicrograph, b) Friction Trace
Figure 11.7 75° Tilt SEM Photomicrographs of Flat from 1% Dimethy-nOctadecylchlorosilane in PPO Lubricated Si,N^ BTF Test at 60 kg: a) Wear Scar,

b) Entrance Region
276

with deposits surrounding the worn surface. Most of the accumulation is in front of (Figure

11.7B), and behind (Figure 11.8), the wear scar. The exit debris consists of the usual twin

wakes seen in many tests, but it also contains small "islands" of product in the center of the

wake (Figure 11.8). These "islands" are quite unusual in shape (Figure 11.9) and seem to

reflect an oil insoluble reaction product that has condensed out as it cooled on leaving the

wear scar. EDX analysis of these products indicated they contained a higher concentration

of chlorine.

Octadecyltrichlorosilane was also tested neat to see if it would lubricate in the absence

of PPO (Figure 11.10). Initially, friction was low (ju =0.060); however, in the course of the

test, several large spikes were seen. This information, coupled with the observation of pits

in the wear scar, indicate possible grain pullout during the test. This may indicate that the

compound is too reactive and attacks the grain boundaries during the test. The oval shape of

the wear scar indicates that substantial wear has occurred on the top ball.

11.3. Fluorine-Containing Compounds

The bond strength data indicates that Si-F bonds are stronger than Si-Cl bonds. This

suggests that if the chlorine-containing compounds are not reactive enough, fluorinated

compounds might be more successful. BTF wear tests were conducted on several neat

fluorine containing compounds to investigate this possibility. Results of these tests are

summarized in Table 11.3

All of the fluids tested had type II friction traces, but in these cases, the friction levels

were relatively high. Wear scars were grooved or roughened, and none of the fluids gave
277

Figure 11.8 75° Tilt SEM Photomicrographs of Flat from 1% Dimethy-nOctadecylchlorosilane in PPO Lubricated Si3N4 BTF Test at 60 kg: a) Exit

Region, b) Exit Region (Higher Magnification)
278

Figure 11.9 0° Tilt SEM Photomicrographs of Flat from 1 % Dimethy-n-Octadecylchlorosilane

in PPO Lubricated Si3N4 BTF Test at 60 kg: a) Exit Region, b) Exit Region

(Higher Magnification)
279

0.20

o 0.15

0.10

0.05 -

0.00

0 5 10 15 20 25 30

Time, minutes

Figure 11. 10 Neat n-Octadecyltrichlorosilane Lubricated S'^N^ BTF Test at 60 kg: a) Optical

Photomicrograph, b) Friction Trace
280

Table 11.3

BTF Test Results on Neat Fluorine-Containing Compounds

WSD Dia increase COF'

Chemical Compound (mm) above Hz, mm

NONE [028] (average of 5 tests) 0.662 0.282 0.127

Perfluoropolyether [85 cSt] 0.509 0.128 0.105

Perfluoropolyether [18 cSt] 0.582 0.202 0.116

Polymethyhrifluoropropylsiloxaiie 0.634 0.254 0.088

Perfluoropolyethermonoacid 0.670 0.290 0.107

Polychlorotrifluoroethylene [27 cSt] 0.700 0.320 0.107

Perfluoropolyethermonoalcohol 0.800 0.420 0.101

All tests conducted on CER028 NBDIOO HIP'd Si3N4 at 600 rpm, 60 kg, 30 minutes, 1.5 ml neat fluid,

21°C

very low wear. The two perfluoropolyethers were identical except for chain length;

therefore , there seems to be a slight viscosity effect in these tests. No films could be

observed inside the wear scar; however, some reaction product deposits could be seen

outside the wear scars. The chlorotrifluoroethylene polymer fluid contained both chlorine

and fluorine molecules, but it too had high wear and grooving of the wear scar suggestive of

corrosion of the surface. Fluorinated monoacids and monoalcohols were tested to see if they

might work because normal (hydrogen based) monoalcohols and monoacids were successful.

The fluorinated ones did not work.

11.4. Summary

As a class, halogenated compounds were not very successful in lubricating Si3N4.

The only exceptions were the very viscous neat chlorinated compounds. In the case of 1%

chlorinated compound in PPO, many of the compounds give accelerated wear. This may be
281

due to reaction of the chlorine with the silica surface of Si3N4. For example, Iler^^^ and

Hair^°^ noted the reaction between silica and carbon tetrachloride.

2Si^-OH + CCl^ - 2Si^-Cl + CO^ + 2HCI

This reaction was noted to occur at temperatures above 350 °C (the decomposition

temperature of CCI4). Hair also proposed that CI2 dehydroxylates the silica surface

according to the following reaction.

2Si -OH + 2CL. - 2Si-Cl + 2HCI + O, s 2 s I

This is thought to actually be a two step reaction consisting of an initial (rate limiting)

decomposition of CI2 to form CI radical. These types of hemolytic decomposition reactions

to form chlorine radicals are probably rare for the particular chlorinated compounds tested,

but no literature evidence was found to suggest alternative reactions that might take place.
4i
283

Chapter 12

SURVEY OF SULFUR CHEMISTRIES

The sulfur-containing compounds gave a wide range of response depending on the

specific compound used. Individual data is summarized in Table 12.1, and presented

graphically in Figure 12.1.

The data shown in Figure 12.1 seems to naturally cluster into three distinct groups.

Each of these groups will be discussed in more detail. One cluster, near the PPO friction

and wear data represents compounds that have little beneficial affect on friction or wear. For

the sake of discussion, these are termed sulfur-containing compounds. A second cluster of

two compounds represent compounds that contain both molybdenum and sulfur and exhibit

very low friction but moderate to high levels of wear (like PPO). These are termed Mo-S

containing compounds. The third cluster represents represents sulfur containing compounds

that are classified as detergent type compounds. These compounds are normally put into

lubricant formulations to reduce deposit formation on surfaces and neutralize acids that may

be generated through lubricant oxidation. They are all able to lubricate Si^M^ at

concentration of 1% in PPO under the conditions of these tests.
284

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286

12.1. Sulfur Containing Compounds

Five compounds fall within the PPO repeatability envelope and in all respects behave

like pure PPO without any additive. These include two dithiocarbamates which are

commonly used as antioxidant/EP agents in iron-based

metal systems. Other compounds include sulfurized

hydrocarbon, octadecyl sulfide, and johoba oil (a

natural sulfur containing oil extracted from the jojoba

R S R

N —C M c—N

R s s R

Dithiocarhamate

plant and intended as a replacement for sperm whale

oil). All five of these compounds exhibit type I friction behavior (just like pure PPO), and

no visible films in the wear scars. In the same region, with slightly higher wear, we find

two compounds with type III friction behavior and

higher wear than pure PPO: sperm whale oil, and

sulfurized fatty oil. These compounds seem to

lubricate initially, but they have a transition to Octadecyl Sulfide

higher friction during the test. In the case of the sulfurized fatty oil and sperm whale oil,

this takes less than two minutes. No films are visible in the wear scars after the test.

Two compounds with slightly smaller wear scar diameters than PPO include two

disulfides. Benzyl disulfide (or dibenzyl

disulfide) provides a small amount of

antiwear effect, although the final

coefficient of friction was quite high by

the end of the test. The friction trace Benzol Disulfide
(Figure 12.2B) contains several spikes at the beginning of the test that may be evidence of

partial lubrication. By the end of the test, these spikes are gone. The wear scar (Figurre

12.2A) has a streaky partial film on both sides of the central region. The film does not

cover the central region. The other disulfide (octadecyl disulfide) has a slightly better

antiwear performance. The friction trace

(Figure 12. 3B) contains many of the same

spikes seen in the benzyl disulfide case, but

„ , , . . , Octadecyl Disulfide

the alkyl disulfide maintains these spikes

throughout the test. The film in the wear scar (Figure 12. 3 A) is also more coherent, and

covers the entire wear scar. The film has more of a spotty or blotchy appearance than the

aromatic disulfide.

12.2. Mo-S Containing Compounds

The second class of sulfur compounds is distinguished by the very low friction

coefficient, but moderately high levels of wear and type II friction behavior. These are the

only two examples of type II friction behavior giving moderate levels of wear, slightly higher

than PPO itself. Both of these compounds

are organic molybdenum-sulfur compounds.

The interior of the wear scar for these tests

(Figure 12.4) contains a somewhat blotchy

surface that could be some kind of solid

R s s s R

/ \ / s. y \

N —C Mo Mo C —N

R

II // ^c^ W II S s ^ s s

s R

Organomolybdenum Dithiocarbamate

film. EDX analysis confirmed the presence of molybdenum. The overlap of Mo and S
288

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n

" 0.15 -

1% Benzyl Disulfide in PPO

600 rpm

60 kg

0.10 -

0.05

0.00

WSD - 0.587 mm

_j I 1 I I I I I I I I I I I I I ] I I I I I I I I I I I I I 0 5 10 15 20 25 30

Time, minutes

Figure 12.2 1% Benzyl Disulfide in PPO Lubricated Si3N4 BTF Test at 60 kg: a) Optical

Photomicrograph, b) Friction Trace
289

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0.00

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I

I I I I I I I I I I I I I I I 0 5 10 15 20 25 30

Time, minutes

Figure 12.3 1% Octadecyl Disulfide in PPO Lubricated Si3N4 BTF Test at 60 kg: a) Optical

Photomicrograph, b) Friction Trace
290

1% Sulfur—Molybdenum Compound in PPO

600 rpm

60 kg

-AAA

WSD = 0.718 mm

I I I 1 I I I I I I I I I I I I I I I I ; I I I i 0 5 10 15 20 25 30

Time, minutes

o

•i-H

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•i-H

«4-4 QJo

0.20

0.15

0.10

0.05

0.00

Figure 12.4 1 % Sulfur-Molybdenum Compound in PPO Lubricated Si3N4 BTF Test at 60 kg:

a) Optical Photomicrograph, b) Friction Trace
291

peaks did not allow definative confirmation of the presence of S, however the ratio of

intensity of the Mo peak at 17.48 keV to the Mo/S peak at 2.31 keV suggested the presence

of sulfiir. It is thought that the organic Molybdenum/sulfur compounds may be decomposing

in the high temperatures of the contact to provide M0S2 solid lubricating films with low

friction. Raman spectroscopy was attempted to try to detect M0S2; however no identifiable

signal could be found due to the strong fluorescence of the silica surface layer.

12.3. Sulfur Containing Detergent Type Compounds

The third class of sulfur compounds (S containing detergent compounds) is

distinguished by low wear and somewhat lower friction than pure PPO and includes both

sulfonates and phenates. They exhibit either type II or type IV friction behavior, and films

within the wear scar. All of the sulfur containing detergents contain metals (Ca, Mg, or Zn)

incorporated through oxygen linkages. In the case of the overbased detergents, calcium or

magnesium carbonates and hydroxides have also been incorporated into the micellular^^'^

structure of the compound to neutralize acidic reaction products of lubricant degradation.

Calcium phenate exhibits low wear and type II

friction behavior. Friction is smooth and low

throughout the test. According to an optical

photograph (Figure 12. 5 A) calcium phenate has a

somewhat streaky organic-looking film, and has some

deposit surrounding the wear scar that outlines the

Phenate

lubricant flow field. An SEM photomicrograph
Figure 12.5 1% Calcium Phenate in PPO Lubricated Si3N4 BTF Test at 60 kg: a) Optical

Photomicrograph, b) SEM Photomicrograph at 75° Tilt
293

(Figure 12. 5B) indicates the film is actually very continuous, and amorphous-looking. EDX

analysis of the wear scar film (Figure 12.6) indicates a buildup of Ca in the film. Sulfur

dqes not seem to be present in any appreciable quantity. The Ca metal can come from the

overbasing component and/or the phenate molecule. The lack of sulfur suggests that if the

phenate molecule is involved, some kind of decomposition has taken place within the contact

junction resulting in deposition of the Ca (probably as an oxide).

The optical micrograph of the (overbased)

Mg sulfonate lubricated test (Figure 12.7) shows a

more solid-looking film, and contains little deposit

around the scar. The friction behavior is type IV.

0 0

II II R —SII -0 —M —O —S —

R

II 0 0

This friction behavior exhibits an initial, gradual. Sulfonate

increase in friction to steady state (/z =0.108

in this case). The wear scar is only 0.391

mm, 0.011 mm above the Hertzian contact

diameter. Optically, there appears to be a

coherent, thick coherent film covering the

0 0

II II R—S —0-

II -M —CO3--M —0—S —

R

II 0 X 0

Overbased Sulfonate

surface of the wear scar. Outside the wear

scar, there is very little evidence of any deposit usually associated with wear tests conducted

in the presence of PPO. This is not surprising since sulfonates are used in oil formulations

as detergents. The lack of deposit is a testiment to the detergent capability of these

compounds in addition to the antiwear capability with Si3N4.
294

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Time, minutes

Figure 12.7 1% Mg Sulfonate in PPO Lubricated Si3N4 BTF Test at 60 kg: a) Optical

Photomicrograph, b) Friction Trace
296

SEM photomicrographs taken at higher magnification reveal something of the nature

of the surface film. The photomicrographs were taken at 75° tilt and oriented so that the

direction of sliding is from left to right as you look at the picture. The film in the wear scar

(Figure 12. 8A) has an obvious orientation in the direction of sliding that is consistent from

the leading edge of the wear scar to the trailing edge. The film is similar in appearance to

the phenate film. A magnification of the center of the wear scar (Figure 12. 8B) shows the

film to be fairly uniform with occasional "pits" interrupting the film. The film appears very

solid in nature. Very fine debris seems to be incorporated into the film, throughout the wear

scar.

The entrance region to the wear scar (Figure 12. 9A), shows the film to have laid

down as solid "fingers" on top of the surface of the flat. Similar artifacts at the exit (Figure

12. 8B) suggests that the film is a reaction product that at one time had liquid-like properties.

EDX analysis of the region inside the wear scar indicated the presence of magnesium,

oxygen, nitrogen, and sulfur in addition to silicon and the usual metal impurities found in the

parent (Figure 12.10). Magnesium has become concentrated in the wear scar during

the test in a similar way to what was observed for Ca in the Ca phenate test. Analysis of the

low base Ca sulfonate gave similar results. This suggests that the metal (Ca in this case) can

become concentrated in the contact junction film due to the decomposition of the original

sulfonate molecule (i.e., overbasing is not necessary). This indicates a different antiwear

mechanism than the compacted overbasing mechanism suggested by Hoomaert'^^* or

Georges^ (for steels).
297

Figure 12.8 SEM Photomicrographs of 1 % Mg Sulfonate in PPO Lubricated Si3N4 BTF Test

at 60 kg: a) Wear Scar, b) Film Within Wear Scar
298

Figure 12.9 SEM Photomicrographs of 1 % Mg Sulfonate in PPO Lubricated Si3N4 BTF Test

at 60 kg: a) Entrance Region, b) Exit Region
299

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300

The observation that the concentration of metals has increased so dramatically while

the sulfur concentration is relatively low suggests that the detergent type compounds might be

providing a delivery mechanism for metals into the wear scar. It is not clear how these

metals help in lubricating Si3N4; however there are indications that the presence of metals on

silica modifies the ion exchange and adsorptive properties of the surface'^^. Perhaps they

modify the chemical interaction between Si3N4 and PPO and its reaction products. Chen^^

found that certain metals (Fe, Co, Ni) had a catalytic effect on deposition of TMPTH.

Magnesium was not tested and calcium was not found to be effective; however it may be

possible that these metals may catalyze beneficial reactions with PPO to lubricate Si3N4.

Another possibility is that the metals react with the silica surface layer to from Ca or

Mg silicate glasses. According to phase diagram information, eutectics can be formed that

are mixtures of silicon oxides, and Ca or Mg oxides. These eutectics melt at much lower

temperatures (1200-1400°C) than pure silica (1700°C). In addition, these glasses may

posess different physical and mechanical properties. The films certainly appear to have a

more liquid-like behavior. This may alter asperity stress ditributions and reduce wear.

Mg sulfonate was also tested neat, resulting in friction and wear behavior

indistinguishable from the results obtained using 1% in PPO. Additional tests were also

conducted on the 1 % Mg sulfonate in PPO over a range of loads to see the extent of its

capabilities.

The exact mechanism of lubrication of Si3N4 by these detergent type (S containing)

compounds is not entirely clear, however the following sequence of events is postulated.

First, the detergents adsorb onto the surface of Si3N4. Once on the surface, they can be
301

carried into the contact during sliding. Under the high local temperatures of the contact, the

adsorbed sulfonate molecules may decompose to give metal oxides which can react with (and

perhaps mechanically mix with) the silica surface layer of Si3N4. The combination of

mechanical and chemical properties of this new surface film then allows lower wear.

12.3.1. Load Capacity Plot for 1% Mg Sulfonate in PPO

BTF wear tests were conducted using 1% Mg sulfonate in PPO at a variety of loads,

constituting a load capacity plot. The resulting plot (Figure 12.11) indicates that this

compound is very effective at lubricating Si3N4, even at high loads. The wear scar diameter

actually follows the Hertzian contact diameter quite closely all the way to just above 200 kg

load. Even at 270 kg load, the wear scar diameter is only 0.8 mm. This translates to a

mean pressure in the contact junction of over 2.0 GPa (300 000 psi) at the end of the test.

At the beginning of the 270 kg test, the high elastic modulus of Si3N4 generates Hertzian

mean pressures of 3.5 GPa (500 000 psi) and maximum Hertzian pressures of 5.2 GPa

(750 000 psi).

12.4. BTF Wear Tests at 200°C

It was expected that Mg sulfonate would work at higher bulk lubricant temperatures,

since it had such a high load capacity. BTF tests conducted at 200°C (Figure 12.12)

confirmed this. Wear increased slightly above the room temperature case and the friction

level was relatively high in the initial stages of the test. The nature of the surface film had

also changed visibly according to Figure 12.12A. There was still no evidence of any deposit
302

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0.10

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; 1% Mg Sulfonate

1

1 1 1 11 1 1 11

1 1 m PPO1

1

1 1 1 200°C

- 600 rpm

- 60 kg

1 1

1 1 1 1 1 1

WSD = 0.490 mm

1 1 1 1 1 1 1 1 1 1 1 1 , 1 , , , 1 1 1 0 5 10 15 20 25 30

Time, minutes

Figure 12.12 1% Mg Sulfonate in PPO Lubricated Si^N^ BTF Test at 200°C: a) Optical

Photomicrograph, b) Friction Trace
304

around the wear scar. As a point of reference, PPO by itself had an increase in wear scar

diameter to 0.8-0.9 mm for the same temperature.

A 200°C BTF wear test conducted using 1 % sulfiirized hydrocarbon in PPO (Figure

12.13) also gave lower wear than PPO, but higher than the sulfonate. In this case; however,

there was a tremendous amount of reaction product deposit surrounding the wear scar. The

friction coefficient is very low initially, and rises substantially about halfway through the

test. If this higher friction is associated with a higher wear rate, it would suggest that the

sulfurized hydrocarbon was very effective initially, but perhaps became depleted during the

course of the test. The wear scar diameter at 200 °C is actually lower than the same test

conducted at room temperature. This suggests that higher temperatures are necessary to

promote the reactions necessary to lubricate Si3N4 with sulfurized hydrocarbons.

12.5. Summary and Discussion

Room temperature BTF tests on Si3N4 conducted using sulfur containing compounds

gave different results depending on the chemical structures of the compounds. In general,

compounds that contained only sulfur did not work under the conditions selected.

Compounds that contained both sulfur metals were successful. In the case of Mo-S

compounds, friction was low, but wear was high. It is thought that decomposition of the

Mo-S containing compounds in the contact junction may be forming M0S2 in-situ.

In the case of the metal-S containing detergent type compounds, low friction and wear

were achieved. The data suggests a mechanism that includes adsorption and decomposition

of the compound inside the wear scar to form surface films rich in the specific metals (Ca,
0.20

o 0.15

0.10

0.05

0.00

I 1 1I 1 1 11 1 1

j

1 1 1 1I 1 1 11 1 1% Sulfurized Hydrocarbon in PPO1

1 < . I 200°C

1 1

- 600 rpm

- 60 kg

WSD = 0.628 mm

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1 1 < 1 < 1

1

1 0 10 15 20

Time, minutes

25 30

Figure 12.13 1% Sulfurized Hydrocarbon in PPO Lubricated Si3N4 BTF Test at 200°C: a)

Optical Photomicrograph, b) Friction Trace
306

Mg, Zn). The exact role of this metal-rich film in the lubrication of Si3N4 by these

compounds is uncertain. Sulfur was not found in these films in any appreciable quantity.
307

Chapter 13

SURVEY OF NITROGEN CHEMISTRIES

Several different nitrogen-containing compounds were tested at 1% in PPO. They

were all high molecular weight compounds that have been used in lubricant formulations;

however, they are regarded as antioxidants, corrosion inhibitors, and dispersants, not

antiwear compounds. Amine compounds were specifically selected because they were known

to adsorb onto the surface of silica^^^. The results are summarized in Table 13.1 and

presented graphically in Figure 13.1.

Diphenylamine is a typical amine

type antioxidant. It had little or no effect on

friction or wear. There were no

characteristics in either the friction behavior

Dialkyldiphenylamine

or the wear scar appearance that could

distinguish it from pure PPO.

A long chain alkyl amine model compound

(octadecyl amine) gave slightly higher wear than PPO.

The friction trace and wear scar appearance were

indistinguishable from the PPO case. Octadecyl Amine
308

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310

13.1. Dispersant Type Compounds

The succinamide (typical ashless dispersant)

gave significantly lower wear but relatively high

friction than PPO as shown in Figure 13.2. The

high friction measured at the end of the test was due

to the transition (Type III friction behavior) from low

friction (« 0.09) to high friction (= 0.13) at the

Succinamide

twenty minute mark of the test. The wear scar is quite smooth, with little deposit around it,

and no visible film within the wear scar. The size of the wear scar would probably have

been even smaller if there had not been a transition before the end of the test. The transition

may have also erased any film that might have been present. The lack of deposit is probably

a testiment of the dispersency of the succinamide, and is reminiscent of the lack of deposit

seen for the detergent type compounds discussed in the sulfur chemistry section.

13.2. Imidazoline

The substituted imidazoline reduced wear and

friction (Figure 13.3); however, no visible film could be

found in the wear scar even in the SEM. Significant

amounts of smooth polymeric film were found outside the

wear scar for this compound suggesting that high

temperatures in the contact junction had promoted Imidazoline

polymerization of the additive. The film may have been present inside the wear scar, but
0.20

o

n 1—

I

1 i1—

I—I—I 1

1—I

1—I

1—I

1—I

1% Succinamide in PPO Lubricated

- 600 rpm

~i 1 1

1 ; 1 11 ro 0.15 - 60 kg

0.10

'f^. 0.05 -

CDO

o

0.00

WSD = 0.567 mm

_J i I I I I 1 I I I I I I I I I I I L_ 0 5 10 15 20 25 30

Time, minutes

Figure 13.2 1% Succinimide in PPO Lubricated Si3N4 BTF Test at 60 kg: a) Optical

Photomicrograph, b) Friction Trace
o

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1—I—

I—I—I—I—I—

I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—

1% HMW Substituted Imidazoline in PPO

600 rpm

60 kg

0.10 -

0.05

0.00

WSD = 0.536 mm

I II I I I I I I

I

I I I I I I I I I I I I I I I I I I I 0 5 10 15 20 25 30

Time, minutes

Figure 13.3 1 % HMW Substituted Imidazoline in PPO Lubricated Si3N4 BTF Test at 60 kg:

a) Optical Photomicrograph, b) Friction Trace
313

perhaps too smooth to be detected.

Nilsen'^^ studied the use of imidazolines as dispersants for Si3N4 powder in hexane

solutions. He hypothesized that imidazolines reacted with water adsorbed on the surface of

Si3N4 to form secondary amides. It is not apparent how this mechanism might explain the

high molecular weight polymeric material found around the wear scar.

315

Chapter 14

SUMMARY AND CONCLUSIONS

This study has uncovered several important characteristics of the surface chemistry of

Si3N4 related to the boundary lubrication of this material. Several classes of compounds have

been discovered that have the capability of reducing friction and wear of Si3N4 under

boundary lubrication conditions. Where possible, mechanisms have been postulated to help

explain the type of reaction sequences responsible for lubrication. A specific chemical

reaction mechanism is proposed to explain the boundary lubrication of Si3N4 by alcohols.

Oxides and hydroxides easily form on the surface of Si3N4. Because it is on the

surface, the oxide layer is very important to the surface chemistry and hence the boundary

lubrication of Si3N4. The Si3N4 surface chemistry observed in this study is consistent with

silica chemistry, suggesting that the silica chemistry literature is a rich resource for relevant

information. Many of the concepts for what might react and lubricate Si3N4 were developed

based on the notion that the Si3N4 surface may behave similar to silicon oxide chemically.

Both adsorption and chemical reaction have been observed to occur on the surface of

Si3N4. Literature sources indicate that alcohols and acids can adsorb and react with silica

surfaces. These observations, coupled to the knowledge that the surface of Si3N4 is silicon

oxide, suggests that these compounds adsorb and react on Si3N4 in a similar manner.
316

The ball-on-three-flat (BTF) test can be an effective tool to study the boundary

lubricating effectiveness of a wide variety of chemical compounds on ceramics. Both large

bulk sample (1.5 ml) and micro sample (6 fi\) techniques have been developed and applied to

Si3N4 lubricated by many different chemistries. Effective boundary lubrication was

associated with low wear, low and stable friction, and the presence of films in and around

the wear contact. Using this technique, it was determined that Si3N4 can be effectively

lubricated at high loads (60 kg) by a variety of chemical compounds including the following

compounds in paraffin oil at a concentration of 1 % (wt) : phosphorus-containing organic

compounds, metal sulfonates, Ca phenate, Mg salicylate, a succinamide, a Mannich product,

an imidazoline, and several acids and alcohols. Using neat compounds, Si3N4 was effectively

lubricated by a wide variety of acids and alcohols including polyalcohols (glycols) and

polyacids (diacid). Purified paraffin oil (PPO) by itself can be effective at lower loads (20

kg).

Alcohols react with the Si3N4 surface to produce surface alkoxides. Static kinetic

experiments conducted at temperatures from 100°C to 245 °C using Si3N4 powders showed

that reactions can take place between the Si3N4 surface and alcohols to produce surface

bonded silicon alkoxides.

Analysis of reaction products from dynamic (wear) tests have for the first time

provided direct evidence of silicon containing organic species. Gel permeation

chromatography (GPC) indicated HMW material in this product. Gel permeation

chromatography-graphite fiimace atomic absorption (GPC-GFAA) analysis confirmed the

presence of silicon in the HMW reaction product, suggesting direct reaction between the
317

alcohol and the surface silicon of Si3N4. SIMS analysis of the reaction product revealed that

the wearing contact produced silicon containing alkoxides with up to four alkoxide groups

per silicon atom. A compound was also found consisting of two silicon atoms bridged by an

oxygen atom, with five alkoxy ligands. The HMW silicon-containing compounds observed

by GPC-GFAA suggest that much larger polyalkoxy molecules are also formed.

Analysis of the shape of the wear scar for octanol lubricated Si3N4 wear tests indicate

that elastohydrodynamic lubrication may be influencing the lubrication mechanism for this

material/lubricant combination. This is supported by the low coefficient of friction obtained

in this test and may be explained by a combination of chemical effects of surface reaction

(corrosion) to reduce asperities and promote surface conformity, and HMW formation to

provide viscous lubricating films. The lubricating ability of HMW silicon alkoxides was

demonstrated using two-ball collision tests.

It is proposed that Si3N4 is lubricated by alcohols through a sequence of reactions.

Adsorption and subsequent reaction of alcohol groups with surface silanol and siloxy groups

of the oxidized Si3N4 surface results in the formation of surface bonded alkoxides. Further

chemical reaction in the high shear stress environment of the wearing contact produce free

silicon alkoxides, which dissolve in the alcohol. Subsequent condensation reactions in the

high temperatures of the contact result in formation of higher molecular weight siliconcontaining compounds (silicon alkoxides). These high molecular weight silicon alkoxides

may have beneficial local boundary layer properties in the contact that enhance lubrication

within the contact. Similar reactions are thought to occur with organic acid functional

groups.
318

The observation that oxygenated compounds such as organic acids and alcohols can

lubricate Si3N4 under boundary lubrication conditions offers significant advantages for

ceramic engine technology. These compounds are non-ashing, commercially available, and

more compatible with alcohol fuels that are being considered as more environmentally

friendly alternatives to conventional fuels.

All phosphorus-containing compounds tested were found to be effective in boundary

lubricating Si^N^ under the conditions selected for this study. Detailed analysis of the

reaction products and study of the interaction between Si3N4 surfaces and model phosphoruscontaining compounds suggest that the lubrication mechanism involves a complex sequence

of organic and inorganic chemical interactions. Phosphorus compounds were found to adsorb

onto the surface of Si3N4. TGA analysis confirms a tenacious surface product that is thought

to be a bonded phosphate film similar to films observed in reactions between silicic acid and

trialkyl phosphates. It is also hypothesized that the success of phosphorus-containing

compounds is due, in part, to the modification of the properties of the glassy surface phase to

make it more pliant, especially at the elevated temperatures of the contact junction.

Phosphorus is known to be a network (glass) former. It can combine with silica and result in

glasses that have lower melting points and less brittle nature than silica itself. Thus a

network forming compound such as phosphorus may lubricate by providing a softer, less

brittle layer on the surface of Si3N4.

It is hypothesized that the metal containing compounds (Mg sulfonate, Ca phenate,

Mg salicylate etc.) are successful lubricants because i) they are surface active and readily

adsorb onto the Si3N4 surface, and ii) the metals may modify the silica network, resulting in
319

lower melting points and viscous glassy phases. They may also modify the adsorptive

properties of the surface silica and catalyze reactions.

In general, halogenated compounds were not successful in lubricating Si3N4. In many

cases, wear was increased substantially, suggesting a corrosion mechanism may be taking

place with the Si3N4 surface. Chlorosilanes were found to lubricate for a short time before

failing. Chlorine-containing reaction products with a distinctive morphology were observed

surrounding the wear scar in tests lubricated by these compounds. Fluorinated acids and

alcohols were tested neat, but were not as effective as their hydrocarbon-based analogs.

Sulfur-containing compounds offered mixed lubrication effectiveness with Si3N4.

Compounds containing molybdenum and sulfur gave low friction, but did not decrease wear.

When phosphorus was also present, both low friction and low wear were obtained. It is

thought that the friction reducing compound M0S2 may be forming in the wearing contact.

Detergent-type sulfur-containing compounds were successful in reducing wear; however, it is

thought that this is due to the presence of metals (Mg, Ca, Zn) and oxygen (sulfonate,

phenate) structures rather than the sulfur itself since sulfur was lacking in the reaction

product films. Most compounds containing only sulfur were ineffective or actually increased

wear. The exceptions were two disulfide compounds which appeared to be partially

effective.

Effective lubrication was observed for succinamide and imidazoline nitrogencontaining compounds. In the case of imidazoline, polymeric reaction products were

observed surrounding the wear scar.

CHAPTER 15

RECOMMENDATIONS FOR FUTURE WORK

In the course of this study, there were issues that could not be answered and should

be addressed by additional research.

The issue of the role of dangling bonds in tribochemistry is largely a mystery. This

area is especially important in ceramics where brittle fracture is such an important

mechanism, since the propagating crack tip is a potential source of dangling bonds.

Experiments conducted in this study have noted that "free" silicon-containing compounds

(silicon alkoxides) are formed in wear tests. The reactions to form these compounds could

not be duplictated with controlled "static" tests at temperatures up to 245 °C. One possibility

is that dangling bonds from wear provide the key mechanism to form these compounds.

Several well controlled experiments are necessary to further investigate this area.

Lubrication mechanisms have been postulated for some classes of compounds;

however these mechanisms need further investigation. For example, the mechanism of

lubrication by phosphorus containing compounds needs some work to substantiate the glass

forming hypothesis. The mechanism of lubrication by detergents (sulfonates, phenates, and

salicylates) needs more clarification to determine the exact reaction sequence and the specific

roles of the organic and metal constituents.

The results of this study has suggested that there may be an EHD lubrication

influence with alcohol lubricated Si3N4. Specifically, the morphology of the wear scar is

very similar to interferometric photographs of EHD point contacts. In addition, the
322

coefficient of friction observed (/x=0.05) is also significantly lower than usually observed in

boundary lubrication (/x=0.10). The possible EHD influence is even more surprising given

the high severity of the wear tests in which it was observed. More adequate investigation of

this system, including EHD modelling is required to verify this hypothesis.
323

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Appendix A

347

AN ESTIMATE OF THE HERTZIAN CONTACT DIAMETER FOR

THE FOUR-BALL CONTACT GEOMETRY

The Hertz"^ equation for the four-ball contact geometry can be derived from the

equation for a Hertzian elastic contact between two convex (ball) surfaces. For two balls

simply labeled 1 and 2, the Hertz equation is:

Where Pj = Normal force on surface of 1 ball (N) = 0.408*P*(9.8 N/kgf), where P is the applied load in kg^

R; = Radius of ball i (for our case both balls are the same radius = 6.35 x

lO-^m)

Kj = Material constants involving Young's Modulus (E) and Poisson's Ratio

(v) such that

Thus for the four-ball configuration with identical materials for the upper and lower

specimens the equation reduces to:

Contact Radius (mm) = a = (10 )

3nP^(K^+K^)R^Il,]-3

K = (l-v^)(lO-^GPa/Pa)

TZE

d = 0.534

P(l-V^) 3 E

-\ 1 where d

P

E

V

diameter of the contact in mm

Applied machine load in kgf

Poisson's Ratio

Young's Modulus in GPa
348

Calculated Hertzian Wear Scar Diameter and Pressure

for the Four-Ball configuration with Silicon Nitride Specimens

Applied Hertzian Estimated Hertzian Pressure

Load W.S.Dia Mean Maximum*

(mm) (MPa) Ckpsi) fMPa) (kpsi)

1 0.077 857 124.2 1,285 186.4

2 0.097 1,079 156.5 1,619 234.8

3 0.111 1,235 179.2 1,853 268.8

4 0.122 1,360 197.2 2,040 295.8

5 0.132 1,465 212.4 2,197 318.7

6 0.140 1,557 225.8 2,335 338.6

8 0.154 1,713 248.5 2,570 372.7

10 0.166 1,846 267.7 2,768 401.5

12 0.176 1,961 284.4 2,942 426.7

14 0.186 2,065 299.4 3,097 449.2

16 0.194 2,159 313.1 3,238 469.6

18 0.202 2,245 325.6 3,368 488.4

20 0.209 2,325 337.2 3,488 505.9

25 0.225 2,505 363.3 3,757 544.9

30 0.240 2,662 386.0 3,993 579.1

35 0.252 2,802 406.4 4,203 609.6

40 0.264 2,930 424.9 4,395 637.4

45 0.274 3,047 441.9 4,571 662.9

50 0.284 3,156 457.7 4,734 686.6

60 0.302 3,354 486.4 5,031 729.6

70 0.318 3,531 512.0 5,296 768.1

80 0.332 3,691 535.3 5,537 803.0

90 0.345 3,839 556.8 5,759 835.2

100 0.358 3,976 576.7 5,964 865.0

120 0.380 4,225 612.8 6,338 919.2

150 0.410 4,552 660.1 6,827 990.2

180 0.435 4,837 701.5 7,255 1,052.3

210 0.458 5,092 738.5 7,638 1,107.7

240 0.479 5,324 772.1 7,986 1,158.2

270 0.498 5,537 803.0 8,305 1,204.5

Maximum pressure = 1.5 x mean pressure for

hemispherical pressure distribution.

Poissons ratio = 0.26

Young's Modulus = 310 GPa

Data supplied for Norton Noralide Silicon Nitride at 21°C
Appendix B

349

AN ESTIMATE OF THE HERTZIAN CONTACT DIAMETER FOR

THE BALL-ON-THREE-FLAT CONTACT GEOMETRY

The Hertz equation for the ball-on-three-flat contact geometry is merely the limiting

case of the convex on convex (ie four-ball type) geometry where the radius of the lower ball

(Ri) -» 00 , which gives

Contact Radius {mm) = a = (10^)

Where Pi = Normal force on surface of 1 specimen (N) = 0.408 P (9.8 N/kgf), where P is the applied load in kg^

= Radius of ball (for our case = 6.35 x 10"^m)

Kj = Material constants (for material i) involving Young's Modulus (E) and

Poisson's Ratio (v) such that

^ ^ (l-v^)(10"^GPa/Pa)

TlE

Thus for the ball-on-three-flat configuration, with similar materials for the upper and

lower specimens, the equation reduces to

1 d = 0.673

P(l-v2) 3 E

where d = diameter of the contact in mm

P = Applied load in kgf

V = Poisson's Ratio

E = Young's Modulus in GPa
350

Calculated Hertzian Wear Scar Diameter and Pressure

for the Ball-on-Three-Flat configuration with Silicon Nitride Specimens

Applied Hertzian Estimated Hertzian Pressure

Load W.S.Dia Mean Maximum*

(kg) (mm) (MPa) (kpsi) (MPa) (kpsi)

1 0.097 539 78.2 809 117.3

2 0.122 679 98.5 1,019 147.8

3 0.140 778 112.8 1,167 169.2

4 0.154 856 124.2 1,284 186.2

5 0.166 922 133.8 1,383 200.6

6 0.177 980 142.1 1,470 213.2

8 0.194 1,079 156.4 1,618 234.7

10 0.209 1,162 168.5 1,743 252.8

12 0.222 1,235 179.1 1,852 268.6

14 0.234 1,300 188.5 1,950 282.8

16 0.245 1,359 197.1 2,039 295.7

18 0.255 1,413 205.0 2,120 307.5

20 0.264 1,464 212.3 2,196 318.5

25 0.284 1,577 228.7 2,366 343.1

30 0.302 1,676 243.0 2,514 364.6

35 0.318 1,764 255.9 2,646 383.8

40 0.332 1,844 267.5 2,767 401.3

45 0.346 1,918 278.2 2,878 , 417.3

50 0.358 1,987 288.2 2,980 432.3

60 0.380 2,111 306.2 3,167 459.3

70 0.400 2,223 322.4 3,334 483.6

80 0.419 2,324 337.0 3,486 505.6

90 0.435 2,417 350.5 3,625 525.8

100 0.451 2,503 363.1 3,755 544.6

120 0.479 2,660 385.8 3,990 578.7

150 0.516 2,866 415.6 4,298 623.4

180 0.549 3,045 441.7 4,568 662.5

210 0.577 3,206 464.9 , 4,809 697.4

240 0.604 3,352 486.1 5,028 729.2

270 0.628 3,486 505.6 5,229 758.4

* Maximum pressure = 1.5 x mean pressure for hemispherical pressure distribution.

Poissons ratio = 0.26

Young's Modulus =310 GPa

Data supplied for Norton Noralide Silicon Nitride at 21°C
351

Appendix C

AN ESTIMATE OF THE HERTZIAN CONTACT DIAMETER FOR

THE PIN-ON-DISK CONTACT GEOMETRY

The Hertz equation for the pin-on-disk contact geometry is similar to the ball-onthree-flat case since they are both convex on flat configurations. The only real difference is

the applied load is the same as the normal load; therefore, no geometric load factor need be

applied. The equation is:

Contact Radius {mm) = a = (10^)

Where P

R

Applied load (N)

Radius of pin (for our case the radius = 3.175 x lO'^m)

Material constants (for material i) involving Young's Modulus

(E) and Poisson's Ratio (v) such that

^ ^ (l-v^)(lO-^GPa/Pa)

nE

Thus for the pin-on-disk configuration, with similar materials for the upper and lower

specimens, the equation reduces to

1 d = 0.336

P(l-v2) 3 E

where d = diameter of the contact in mm

P = Applied load in Newtons

V = Poisson's Ratio

E = Young's Modulus in GPa
352

Calculated Hertzian Wear Scar Diameter and Pressure

for the Pin-on-Disk configuration with Silicon Nitride Specimens

Applied Hertzian Estimated Hertzian Pressure

Load W.S.Dia Mean Maximum*

(N) (mm) (MPa) (kpsi) (MPa) (kpsi)

0.5 0.038 429 62.3 644 93.4

1 0.049 541 78.5 812 117.7

2 0.061 682 98.9 1,023 148.3

3 0.070 780 113.2 1,171 169.8

4 0.077 859 124.6 1,289 186.9

5 0.083 925 134.2 1,388 201.3

6 0.088 983 142.6 1,475 213.9

8 0.097 1,082 157.0 1,623 235.5

10 0.104 1,166 169.1 1,749 253.6

12 0.111 1,239 179.7 1,858 269.5

14 0.117 1,304 189.2 1,956 283.7

16 0.122 1,364 197.8 2,045 296.6

18 0.127 1,418 205.7 2,127 308.5

20 0.132 1,469 213.0 2,203 319.6

25 0.142 1,582 229.5 2,373 344.2

30 0.151 1,681 243.9 2,522 365.8

35 0.159 1,770 256.7 2,655 385.1

40 0.166 1,851 268.4 2,776 402.6

45 0.173 1,925 279.2 2,887 418.7

50 0.179 1,994 289.1 2,990 433.7

55 0.184 2,058 298.5 3,087 447.7

60 0.190 2,119 307.3 3,178 460.9

70 0.200 2,230 323.5 3,345 485.2

80 0.209 2,332 338.2 3,498 507.3

90 0.217 2,425 351.7 3,638 527.6

100 0.225 2,512 364.3 3,768 546.4

* Maximum pressure = 1.5 x mean pressure for hemispherical pressure distribution.

Poissons ratio =0.26

Young's Modulus =310 GPa

Data supplied for Norton Noralide Silicon Nitride at 2rC

Diameter of pin hemisphere = 6.35 mm (0.25")
Appendix D

353

SILICON NITRIDE FLAT

SPECIMEN PREPARATION PROCEDURE

The following procedure is used to prepare flat wear test specimens for the ball-onthree-flat test. The final goal is to produce flats that are 6.35 mm (0.25 inch) diameter, 1.59

mm (1/16 inch) thick, with a mirror surface finish on one side. It is not critical that the

specimens end up exactly 1.59 mm thick. Tests have been successfully run on specimens

from 1.4 mm to 1.8 mm thick. It is important however, that the three specimens of a single

test be within 0.02 mm of each other in thickness. The grinding and polishing steps are summarized in Table D.l.

1) Check the diameter of the 6.35 mm (0.250 inch) rod stock. If it is not 6.35 ±

0.025 mm (0.250 + 0.001 inch) then it must be ground to these dimensions.

2) Cut 1.70 mm thick flats from the rod stock using a diamond saw. The extra

thickness is to accomodate material loss during the grinding stages.

3) Remove any burrs from at least one side of each flat. Diamond grinding and

water cooling is appropriate for this operation.

4) Mount the flats (deburred side down) to a grinding plate using a melt adhesive

such as AREMCO Crystalbond 509. Press the flats firmly onto the plate as it cools (the adhesive is setting) to ensure proper alignment of the specimens.

5) Grind the specimens manually until and burrs on the exposed surface are removed using a coarse fixed diamond grinding disc such as a LECO 240 grit

DIMAR disc. Speed of rotation should be approximately 200 rpm with water

lubrication/cooling

6) Transfer the grinding plate to an autopolisher and continue grinding on the

coarse diamond wheel at 200 rpm and apply a load of 0.5 Ibf per flat. Check

the flat thickness periodically to ensure that the flats are not becoming too thin

(1.60 mm minimum). Continue grinding until the surface of all of the

specimens have been ground flat and parallel. Repeat grinding at the same

conditions using a fine diamond disc such as LECO 11-14 /xm DIMAR disc.

7) Change the grinding disc to a fixed 120 grit SiC attached to the wheel with

Pressure Sensative Adhesive (PSA). Use this PSA backed disc at a holder for

other (plain backed) SiC grinding discs. Place a 120 grit plain backed SiC
354

Table D.l

Summary Si3N4 Flat Preparation Procedure

Step Media Speed

(rpm)

Duration

(min)

Rep

(#)

Grinding Coarse Diamond 200 2

*

Fine Diamond 200 2

120 grit SiC 200 6

240 grit SiC 200 6

320 grit SiC 200 6

400 grit SiC 200 6

600 grit SiC 200 6

Polishing 6 fim diamond 100 30 *

1 fxm diamond 100 30 *

* Indicates that judgement must be used to determine the number of times this step must be repeated to achieved the proper resuhs.

In all cases the desire is to have each step remove the surface damage caused by the previous step.

It is imperative that the samples be cleaned properly after the 6 nm polishing step to avoid contaminating the

1 /im polishing wheel with 6 /^m diamond.

disc on the wheel. Grind at 200 rpm and 0.5 Ibf per flat with water coolant

for between 30 seconds and 1 minute. Discard the SiC disc and repeat this

procedure five times with new discs. Change to the next smaller grit size SiC

and repeat the grinding procedure. The following grit sizes should be used in

order: 120,240,320,400,600.

8) Dry the specimens and holder thoroughly. Polish the specimens using 6 fxm

free diamond on polishing cloth such as LECO Technotron cloth. Rotating

speed of the wheel should be 100 rpm (VP 150 polisher) and lapping oil should

be used as the lubricant/coolant. Pressure on the autopolisher should be set at

0.5 Ibf per flat. Polish for 30 minutes. Specimens should be checked

occasionally to avoid overheating of the samples and possible melting of the

melt adhesive holding the flats. Check the surface to see if the grinding

damage has been removed using an optical microscope if necessary. If

damage still remains, continue polishing for another 30 minutes. Note some

small pits may remain at this stage. This type of damage will be removed by

the next polishing step.
355

9) Thoroughly clean the specimens and holder with detergent (MICRO) then

water to remove polishing contaminants (6 /wm diamond particles). A final

cleaning in the ultrasonic bath is a good assurance against contaminating the

next polishing step.

10) Repeat steps 8) and 9) using 1 ^m diamond paste on another polishing cloth.

11) Heat the plate to remove the polished specimens. Soak them in acetone to

dissolve the redisual adhesive. Pour off the acetone, rinse the specimens a few

times in fresh acetone then dry the specimens.

12) Size the specimens to the nearest 0.01 mm and package in groups of three.

The specimens in any one group should not differ in thickness by more than

0.02 mm. Label the bag with the thickness of the flats that they contain. The

specimens are now ready for solvent cleaning prior to testing.

357

Appendix E

CLEANING PROCEDURE FOR Si3N4 WEAR TESTING

The following cleaning procedure has been developed for removing contaminants and

organic impurities from the surfaces of Si3N4 prior to testing. It is based on a previous

study* conducted on another ceramic (alumina) which found the most effective cleaning used

a series of organic solvents (hexane, acetone) followed by either a detergent solution then

deionized water or a high temperature bakeout in air to remove residual organics. High

temperature (700 °C) bakeout is feasible for alumina since it is already in its most oxidized

state (AI2O3); however, Si3N4 cannot be subjected to such high temperatures since its surface

will easily oxidize to form thick layers of SiOj at these temperatures. For this reason, the

detergent/DI water cleaning procedure was adopted. The following procedure is used to

prepare both ball and flat specimens immediately prior to wear testing. Tweezers should be

used to handle all specimens to eliminate contamination by skin oils:

1) Place ball and flat specimens in a beaker and cover with hexane. Place in an

ultrasonic bath and ultrasonically agitate for ~ 5 seconds. Let soak for at

least one minute.

2) Pour off the hexane, and repeat this procedure using acetone.

3) Pour off the acetone and rinse the specimens with 18 Mfi deionized (DI)

water.

4) Cover specimens with a solution of non residue forming laboratory detergent

in DI water. A 5% solution of "Micro" has been found to be satisfactory.

Place in an ultrasonic bath and treat for = 5 seconds. Let soak for at least

one minute. Pour off the detergent solution and rinse four times with DI

water. Cover specimens with DI water, ultrasonically treat for 5 seconds,

pour off the water. Rinse two more times with DI water, transfer the

specimens to a clean dry lintless tissue and dry by blotting then with a stream

of nitrogen gas.

5) Immediately place the flats, polished side up, in their receptacles in the test

cup. Cover with 1.5 ml of the desired lubricant. Snap the ball into place in

the top ball chuck and insert and lock into machine.

6) Apply a slight load (<2 kg) until the test cup rises and the specimens contact.

At this point, both the ball and the flats will be covered in the lubricant and

will not be susceptible to picking up contamination from the laboratory

environment. The test can now be conducted.

359

APPENDIX F

CALCULATION OF WEAR SCAR DEPTH & WEAR VOLUMES FOR

THE BTF, POD, & FOUR-BALL WEAR TEST GEOMETRIES

Wear scar volumes for flats from ball-on-three-flat tests can be calculated using geometric

considerations and a few assumptions. The basic geometry for this test is sphere on a flat

surface (Figure F.l). If wear rates are relatively low, then wear of the upper specimen

(sphere) will have a negligible effect on the geometry (curvature) of the upper surface. One

can then calculate the volume of material removed from the flat surface by knowing only the

diameter of the wear scar. The following assumptions are inherent in this calculation :

1) Wear of the upper rotating specimen (ball) is low enough that negligible

•changes take place in the curvature.

2) Elastic contributions are minor (ie. elastic deformation does not change the

shape of the wear scar or the curvature of the upper specimen.

z

Figure F.l Ball-on-Three-Flat Wear Geometry

Where: h =depth of the wear scar (mm)

r =radius of the wear scar (mm)

R =radius of the upper ball (mm)

The calculation of height and volume of a spherical segment can be found in any number of

handbooks such as Perry
360

L J 2^

Where d = 2r = diameter of the wear scar D = 2R = diameter of the upper ball

The resulting volume (V) of wear on a single flat is:

nh

6 6

4

In the case of the POD test, the calculations are identical; however, the material is removed

from the curved specimen (pin) instead of the flat specimen (Disk).

In the case of the Four-Ball contact geometry, for any given wear scar diameter, the depth

and volume segment removed from the ball during wear is merely twice that of the BTF case

(see figure F.2).

Figure F.2 Four-Ball Wear Geometry
The equation for the BTF wear volume case is modified accordingly.

rU 3 4

where h (defined previously) represents 1/2 the four-ball wear scar depth.

Wear rates can be calculated from these wear volumes

Volume

Normalized Wear Rate = {Normal Load) {Linear Sliding Distance)

for the Roxana Ball-on-Three-Flat and Four-Ball configuration this becomes :

^ (0.408) (9.8) (P) (Q) (0.023) (0 (P) (Q) (r)

where : WRn = normalized wear rate (nrniVN m)

P = applied machine load (kgf)

Q = rotating speed of upper specimen (rpm)

t = test duration (minutes)

Appendix G

363

DESCRIPTION OF ANALYTICAL TECHNIQUES

A wide variety of analytical techniques were used in the course of this investigation.

The specific instruments and their capabilities are described below. Except where noted, all

measurements have been made by the author.

G.l. Optical Microscopy

Optical microscopy was performed using an Olympus SZH Stereozoom microscope

fitted with a calibrated graduated eyepiece reticle and a polaroid camera. This instument has

the capability of providing a zoom range of from 8.4x to 144x using a combination of a contiuous zoom lens and two different objective lenses. The eyepiece reticle was calibrated

at the highest zoom setting using a stage reticle. All wear measurements were taken using

this highest zoom setting since the mechanical stop of the zoom knob provided the most

repeatable magnification setting. For most cases the 1.5x objective lens (144x total

magnification at highest zoom) was used because it allowed the greatest measurement

accuracy (0.004 mm). In some cases, if the wear scar was over 0.8 mm, the 0.75 x

objective was used and the accuracy of measurement dropped to 0.008 mm. If the wear scar was over 1,6 mm, a lower, less accurate, zoom setting was used. In these rare cases the

accuracy was estimated to be on the order of 0.02 mm.

Polaroid photographs were taken of each wear scar of each test to provide a record of

the wear scar morphology and reaction film formation.

G.2. Scanning Electron Microscopy (SEM/EDX)

Scanning electron microscopy was performed using a Hitachi S-530 SEM with a

resolution of 5 nm (50 A). Si3N4 is not electrically conductive, therefore specimens were

coated with gold (15 nm), or carbon if Energy Dispersive X-ray (EDX) analysis was

required. EDX was performed within the SEM using a KEVEX detector equiped with a QUANTUMTM window capable of detecting elements as low as boron. It should be noted

however that it is very difficult to accurately detect the lighter elements of boron, carbon,

nitrogen, and oxygen, because the characteristic x-ray energies of these elements are so low

that they are easily absorbed by carbon coatings, and even trace vacuum pump oil adsorbed

on the detector window.
364

G.3. Thermogravimetric Analysis (TGA)

Thermogravimetric analysis was performed using a Perkin Elmer TGS-2 TGA

connected to a data station computer capable of storing and analysing the collected data. The

sensitivity of the balance used in this TGA was better than 0.1 jwg. In most cases, less than

50 mg of sample were used. The heating chamber was deigned to allow a gas to flow over

the sample during heating. A flowrate of 40 ml/min were used for all TGA analyses.

Temperatures ranged from 50°C to 700°C during the course of a run.

G.4. Fourier Transform Infrared (FTIR)

FTIR analysis was performed using a Nicolet 5sx optical system with integrated

minicomputer capable of storing and analysing the resulting data. Two different optical

benches were used. A micro-FTIR system was used that allowed for spectra to be taken

from « 100 fxm diameter spots in either transmission of reflectance mode. A mercurycadmium telluride detector allowed for a resolution of 4 cm"^ and was capable of taking data

from 4,000 cm'Ho 700 cm ^ A second optical bench allowed for the same resolution with a wider spectrum (4,000 cm"' to 400 cm"'), but required a larger sample size. This system was

used when diffuse reflectance infrared transmission analysis (DRIFT) was required, or it was

necessary to gather data from the low wavenumber, "fingerprint," region of the spectrum.

G.5. Gel Permeation Chromatography - Graphite Furnace Atomic Absorption (GPCGFAA)

GPC-GFAA is an analytical technique, developed at NIST, which combines gel

permeation chromatography with atomic absorption spectroscopy to selectively monitor the

concentration of metal associated with specific molecular weights of organic compounds.

The technique is described in greater detail in the paper by Gates et al.^^ For the purposes of

this study, the element of interest was silicon, which ideally can be measured down to

concentrations of 0.2 ppb. Calibration of the atomic absorption spectrometer was performed

using oil soluble standards. Calibration of the molecular weight scale was performed using a

series of polystyrene and hydrocarbon standards dissolved in tetrahydrofuran (THF). Since

the samples injected into the GPC are not pure hydrocarbons or polystyrene, the molecular

weight calibration scale is only an "approximation."

G.6. Secondary Ion Mass Spectrometry (SIMS)

Secondary Ion Mass Spectrometry was performed by Joe Bennett of the Chemical

Microanalysis group at NIST using a Kratos 401 LS time-of-flight "static" SIMS. The

"static" refers to the gentle namre of the ion bombarment, which results in little disturbance

to either the surface, or the analyte of interest. This ensures that the system is measuring
365

compounds that represent the true surface chemical composition. The bombarding ion used

was isotopically enriched (99.6%) gallium 69. This instrument was capable of unit

resolution through 500 AMU. Since Si3N4 is not electrically conductive, a pulsed low energy

electron charge compensation system was used to neutralize charge buildup during the

analysis.

G.7. X-Ray Diffraction (XRD)

X-ray diffraction was performed on samples of Si3N4 to identify specific crystalline

phases present. Data was acquired using a Noreico goniometer with 0.02° (20) step size

coupled to a Cu^^ x-ray source. Data was acquired and analyzed on a micro-Vax II minicomputer running under a Siemens Diffrac 500 x-ray diffraction software package.

Spectral search and match was performed on the computer against the Joint Committee on

Powder Diffraction Standards (JCPDS) reference library.

G.8. Electron Spectroscopy for Chemical Analysis (ESCA) and Auger Electron

Spectroscopy (AES or Auger)

ESCA and Auger analyses provide compositional information about surfaces of

samples. ESCA also provides some additional information about bonding. Auger has the

ability to focus down to small spot size. Both techniques can be coupled with argon ion

etching to provide depth profiling capabilities. Both ESCA and Auger analyses were

performed under contract by Tom Whittberg of the University of Dayton Research Institute.

Auger analysis was performed on a Varian model 981-2707 Auger spectrometer at a base

pressure of 10"'^ Torr. Data were acquired using a 3 keV electron beam of 0.1 mm

diameter. Additional interpretation of the results was provided by Pu-Sen Wang of NIST.

G.9. Raman Spectroscopy

Raman spectroscopy provides information on the (surface and near surface) chemical

structure of samples. Analysis was performed by Bruce Heggemann of NIST using a custom-made system. This system consisted of a Nd-YAG laser, Spex triple spectrometer,

and a diode-array detector capable of resolution of 4 cm Data were collected and anayzed

using a dedicated personal computer.

G.IO. Interferometry

Interferometry was performed with a Zeiss interference microscope using thallium

(X/2=0.27/>im) illumination. The EHL example interferogram photograph was kindly

provided by Zhu Dong of Alcoa.
366

G.ll. Neutron Activation Analysis (NAA)

Neutron Activation Analysis (NAA) is an analytical technique used to quantify the

concentrations of a wide range of elements in a bulk material. The analysis is performed by

activating a sample (subjecting to a neutron flux such as found in a nuclear reactor), and

analyzing the gamma ray spectrum of the resulting radionuclides over a period of time.

Since each radionuclide (element) will generate gamma rays of different energies,

approximately 40 elements can be simultaneously measured. The sensitivity will vary from

element to element, depending on the half-life of the radionuclide formed; however, part per

million (ppm) capability is common. The results in this study were provided under contract

by GSI Spectroscopy Division, Oxnard, California.
Appendix H

367

PPO WEAR TEST DATA ON OTHER Si3N4 BATCHES

In the course of this project, the major stock of NCI 32 Si3N4 (CER019 balls and

CER024 flats) became depleted and it became necessary to switch testing to a second

manufacturing batch of NCI 32 Si3N4 (CER037 balls and CER038 flats). Table H.l

summarizes the results of four PPO lubricated wear tests conducted on this second batch of

NCI32 material. It would appear that this second batch of NCI32 Si3N4 has a slightly

smaller wear scar diameter under the same conditions. Since the repeatibility of the entire

wear test procedure has already been established, the difference in performance is attributed

to a difference in Si3N4 processing by the manufacturer. Only a small number of wear tests

Table H.l

Repeatability Data on PPO Lubricated Si3N4

CER038 Batch of NCI32 Flats

WSD

WSD Incr^ COF^ Run Polishing

(mm) (mm) jL Batch

0.625 0.245 0.110 2940R A

0.623 0.243 0.122 2943R A

0.617 0.237 0.106 2946R A

0.621 0.241 0.105 2947R A

AVERAGE 0.622 0.242 0.111

Std Deviation 0.003 0.003 0.008

1 Wear Scar Diameter Increase above the Hertzian Contact Diameter (0.380 ram)

2 Measured at the end of the test

All BTF tests conducted on CER037 (Ball) on CER038 (Flat) NC132 hot pressed SijN^ at 0.23 m/s (600

rpm), 60 kg, 30 minutes, 1.5 ml PPO. 21 °C.

were conducted with this material.

Some of the wear tests were conducted on a second type of Si3N4 known as NBDIOO

(HIP'd). This Si3N4 had a slightly higher wear scar diameter and final coefficient of friction

than either of the NCI 32 Si3N4 materials, as shown in Table H.2. The repeatibility is also

poorer than for the previous materials. The PPO wear test results on all three batches of

Si3N4 flats are compared graphically in Figure H.l. The figure indicates that the wear scar
368

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369

Table H.2

Repeatability Data on PPO Lubricated Si3N4

CER028 Batch of NBDIOO Flats

1 WSD

WSD Incr^ COF^ Run Polishing

(mm) (mm) JL Batch

0.650 0.270 0.132 2869R A

0.646 0.266 0.128 2870R A

0.689 0.309 0.126 2964R A

0.658 0.278 0.125 3065R E

0.667 0.287 0.125 3110R G

AVERAGE 0.662 0.282 0.127

Std Deviation 0.017 0.017 0.003

1 Wear Scar Diameter Increase above the Hertzian contact diameter (0.380 mm)

2 Measured at the end of the test

All Tests conducted on CER030 (Ball) on CER028 (Flat) NBDIOO HIPPED Si3N4 at 600 rpm, 60 kg, 30 minutes, 1.5 ml PPO, irc.

diameter of the CER028 NBDIOO Si3N4 is statistically similar (within one standard deviation)

to the first (CER024) batch of NCI32. The friction coefficient at the end of the test is

significantly higher for CER028 (NBDIOO) than either NCI32 batches of Si3N4 material. In

any case, the variation in wear scar diameter among the different materials is not significant

enough to change the antiwear and prowear criteria stated earlier. The average of all 16

PPO lubricated wear tests on all three batches of Si3N4 material were used as the benchmark

to determine whether there was an effect of chemistry. These average values were: a wear

scar diameter of 0.645 ± 0.018 mm and coefficient of friction of 0.117 ± 0.010. This is

shown graphically in figure H.2. The tabulated data included in this report indicate which

specific material was used for each particular test in case a more accurate direct comparison

is desired.
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Appendix I

371

TEMPERATURE CALCULATIONS FOR THE

BALL-ON-THREE-FLAT CONTACT GEOMETRY

The maximum flash temperature for a rubbing contact can be roughly approximated

using Archard's^' method. Unfortunately, this method is really only applicable to metal

systems because a key assumption implicit in this method is that for the maximum flash

temperature the load is entirely supported by the asperities undergoing plastic deformation.

This requires knowing the yield or flow pressure for the material. In metals, this is a common measurement. For ceramics, there is considerable controversy about the prevelance

of plastic deformation, and the yield pressure is not known. It is approximated in this case by the hardness value for the ceramic (Si3N4). It does not include any effect of cooling from

a lubricating fluid.

For a circular point contact, the maximum flash temperature is estimated as:

where B is a correction for the effect of sliding speed. B ranges from 0.95 at L=0.1 to

approximately 0.5atL=5. Nisa proportionality constant based on coefficient of friction

and material parameters, and L is a dimensionless parameter that is essentially the ratio of

sliding velocity to the thermal diffusivity. This is a relative measure of the balance between

the movement of the heat source (the contact) and the ability of the thermal energy to

dissipate into the material, and is a measure of the depth of penetration of the heat into the

surface. The equations defining N and L are:

N 10^

L

2D{tzH)
372

where the values used for octanol lubricated Si3N4 were:

f = 0.050 Coefficient of friction

H = 310 GPa Hardness

p = 3.2 g/cm^ Density

Cp = 810J/kg°K Heat capacity

W = 240 N Normal load (60 kg applied load)

V = 0.23 m/s Sliding speed (600 rpm)

D = 0.123 cmVs Thermal Diffusivity

Using these values, the flash temperature for octanol lubricated Si3N4 is estimated at

120 °C. If a coefficient of friction of 0.080 is used (typical for well lubricated), the

temperature estimate jumps to 180°C. A poorly (PPO) lubricated system (coefficient of

friction = 0.120), results in an estimate of 250°C.
373

APPENDIX J

AUGER ANALYSIS DATA FOR ALCOHOL-LUBRICATED BTF TEST

FLATS

Auger depth profiling was performed on a flat from an octanol-lubricated BTF test. A region away from the wear scar (Figure J.l) indicated a thick carbon-containing film

covered the surface. The thickness of the film was estimated at 500 A and was thought to be

organic in namre. A second location, on the wedge-shaped feature within the wear scar (Figure J. 2), contained a similar but thinner carbon-containing film. The thickness of this

film was estimated to be 260A. Profilometry indicated that the wedge-shaped feature is 2

fxm thick when compared to the expected wear morphology. The Auger depth profiling

shows that 30 nm (15 minutes sputter time) into the sample, the surface is Si3N4. No strong

oxygen signal is detected. This demonstrates that the wedge-shaped feature is merely

unworn Si3N4 and not a silica or a compacted wear debris layer.
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Appendix K

HAMROCK AND DOWSON MINIMUM FILM THICKNESS

CALCULATION FOR BTF EHL CONTACTS

Hamrock and Dowson^°^ have developed an empirical expression to estimate the

minimum film thickness for a fully flooded EHL point contact. It assumes an ideal

(perfectly smooth) surface, and does not accomodate temperature increases due to friction.

The formula used for dimensionless minimum film thickness is:

min \ /

Where U, G, and W represent dimensionless speed, material, and load parameters

respectively.

G = OLE'

W = In 2 E'R

Where:

E' =

. E.

1-v

+

B
378

The values used for an octanol lubricated BTF test on Si3N4 under the standard test

conditions (600 rpm, 60 kg applied load) were:

3.10 X lO'i Pa Young's modulus

Va' Vb 0.26 Poisson's ratio

0.006 Pa s°^^ Viscosity at ambient T, P

R. 6.35 X 10-^ m Effective radius of curvature

a 0 74 X 10"^ Pa'' ^"^ Pressure-viscosity coefficient

U 0.23 m s,-' Sliding speed

F 240 N Normal force

k 1 Elipticity parameter

Using these values, a minimum film thickness of 4 nm was estimated for octanol lubricated

silicon nitride.
Appendix L

RTF Uall-(JIl-lIirCC lldl

cicciion spcciroscopy lor cncniicai analysis

DT UCiUIllZCU

aiiicrciiiaii scaniiing Caioriniciry

tLtlU ciasionyaroaynaniic

CUT elastohydrodynamic lubrication

rD lour-Daii

r 1 IK rourier transiomi inrrareci

(jrC gas chromatography

Lrr'AA grapniic lumace aiomic aosorption

gel permeation chromatography

not isosiaiic pressing

HMW high molecular weight

TDIK inirarea

Mo mass spectrometry

JVIW molecular weight

XT A A neutron activation analysis

pressurized differential scanning calorimetry

rKJU pin-on-aisc

rrU purified paraffin oil

T>T Kl refractive index

CCA /I scanning electron microscopy

CTA yf C secondary ion mass spectrometry

TCP tricresyl phosphate

TGA thermogravimetric analysis

THF tetrahydrofuran

TLC thin layer chromatography

UV ultraviolet

WI water insoluble

WS water soluble

XRD x-ray diffraction

ZDP zinc dithiophosphate

• U.S. GOVERNMENT PRINTING OPTICE: 1 995-386-627 / 37908



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