behaviour of lubricant additives on dlc coatings

287 Pages·2011·28.64 MB·English

B O L
EHAVIOUR F UBRICANT
A O D C
DDITIVES N LC OATINGS

by
Balasubramaniam Vengudusamy

Thesis submitted to Imperial College London
for the Degree of Doctor of Philosophy
and Diploma of Imperial College (D.I.C)

March 2011

Tribology Group
Department of Mechanical Engineering
Imperial College London
A
BSTRACT

Non-ferrous surfaces such as diamond-like carbon (DLC) coatings are becoming potential
candidates for automotive engine parts because of fuel economy gains that these surfaces offer
by operating with very low friction. In recent years, a wide range of DLC coatings have been
developed and it is important to understand their film-forming, friction reduction and wear
resistance mechanisms under lubricated conditions.
This aim of the work described in this thesis is to improve our understanding of the tribological
behaviour of DLC coatings with different engine oil additives. The main focus of the thesis is to
study a wide range of available DLC coating types with currently available and widely-used
additives such as ZDDP, friction modifiers, MoDTC etc., in order to establish general rules of
their tribological behaviour that will help lubricant manufacturers produce new oil formulations.
The research shows that tribofilms are formed on all DLCs by most of the currently used
additives and that the film thickness depends on various factors such as type of DLC coating,
doping elements present in the coatings, concentration of hydrogen and tungsten present in the
coatings and the counterpart. Hydrogen-free coatings (a-C and ta-C) give lower boundary
friction compared to the other coatings whereas hydrogenated amorphous carbon (a-C:H)
coatings give better wear resistance properties. Study of a-C:H:W coatings shows that the
concentration of tungsten present in the coatings has a significant influence on wear resistance
properties but negligible influence on the friction properties when additives are present.
The steel/steel couple is known to form a thick ZDDP tribofilm. If one of the contact surfaces is
coated with DLC, the tribofilm forming properties on the steel vary and, for some cases, the low
boundary friction properties of DLCs are degraded.
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A
CKNOWLEDGEMENT

The research work of this thesis was carried out at Imperial College London, UK, during the
years 2007-2011. The project was funded by Castrol Ltd., Pangbourne, Reading, UK. This
company‟s financial support is greatly acknowledged.
This thesis was supervised by Professor Hugh Spikes, to whom I would like to express my
sincere gratitude for his constant encouragement, valuable advice, discussions, comments and
support throughout the course of the research work.
I am very grateful to Professor Andrew Olver, Dr. Richard Sayles, Dr. Daniele Dini, Dr. Janet
Wong and Dr. Philippa Cann for their comments and suggestions. I would like to thank Chrissy
Stevens, who has helped me a lot in purchasing different types of DLCs that were investigated
in this study. I would also like to thank Paul Jobson for helping me with all lab-related activities
and for providing the solvents whenever needed.
I would like to thank the following persons for their assistance, Dr. David Scurr (The University
of Nottingham) for ToF-SIMS, Dr. Xinyong Chen (The University of Nottingham) for AFM, Dr.
A.C. Ferrari (University of Cambridge) and Prof. Sergei Kazarian (Imperial College London) for
Raman and Dr. Chris Jeynes (University of Surrey) for ERD measurements.
I would like to thank my friends, Jason, Koji, Tina, Marc, Tom, Jessika, Sophie, Juliane, Jenifer,
Connor, Richard, Simon, Anant, Kazu, Mark, Ingrid, Angelos, Savy, Dani, Ales, Yewande,
Mourad, Kanik, Semanti, Maumita, Davendu, Amol, Poonam, Praveen, Ganesh and Vinu for all
their excellent help, co-operation and support during the course of this work and also for the
most enjoyable atmosphere they helped create.
Finally, I would like to express my deepest and warmest gratitude to my dear wife Madhu, my
dear daughter Deekshaa, my parents Vengudusamy and Vimala, and my grandmother Rengi.
The patience, understanding, love and care they provided during this effort were most valuable
and indispensable.
Bala
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T O C
ABLE F ONTENTS

List of Figures 10
List of Tables 21
Chapter 1: Introduction
1.1 General Introduction………………………………………………………………… 24
1.2 Introduction to the problem…………………………………………………........... 25
1.3 Research Objectives………………………………………………………………... 25
1.4 Outline of thesis……………………………………………………………….......... 26
Chapter 2: Background
2.1 Background to the current study…………………………………………………... 28
2.2 Lubrication regimes…………………………………………………………………. 29
2.3 Lubricant additives………………………………………………………………….. 31
2.4 DLC coatings and their classification……………………………………………… 32
2.5 Preparation of DLC coatings……………………………………………………….. 35
2.5.1 PVD and Sputtering………………………………………………………… 36
2.5.2 CVD and PECVD……………………………………………………………. 37
2.5.3 FCVA………………………………………………………………………… 38
2.6 Applications………………………………………………………………………….. 38
Chapter 3: Materials and test methods
3.1 Test materials……………………………………………………………………….. 41
3.1.1 Characterisation of DLC Coatings………………………………………. 42
3.1.1.1 Surface roughness……………………………………………… 42
3.1.1.2 Hardness and elastic modulus measurements……………… 46
3.1.1.3 Chemical composition measurements…………………………… 47
3.1.1.4 Hydrogen and tungsten content measurements…………… 48
3.1.1.5 sp3 content measurements…………………………………… 50
4
3.1.1.6 Morphology of DLC coatings…………………………………... 53
3.1.2 Properties of DLC Coatings………………………………………………….. 57
3.1.2.1 Significance of material selection…………………………………. 58
3.2 Test lubricants and additives……………………………………………………… 58
3.3 Test methods……………………………………………………………………….. 59
3.3.1 MTM friction testing…………………………………………………………… 59
3.3.1.1 Test strategy in friction tests………………………………………. 63
3.3.1.2 Surface characterisation after friction testing……………………. 65
3.3.1.2.1 Optical microscope…………………………………….. 65
3.3.1.2.2 SEM-EDX……………………………………………….. 66
3.3.1.2.3 ToF-SIMS……………………………………………….. 67
3.3.1.2.4 Raman spectroscopy………………………………….. 67
3.3.1.2.5 AFM……………………………………………………… 68
3.3.2 Wear testing…………………………………………………………………… 68
3.3.2.1 Volumetric wear measurements………………………………….. 70
3.3.2.2 Calculation of wear coefficients………………………………….. 75
Chapter 4: Friction and wear behaviour of DLC contacts in base oil
4.1 Introduction………………………………………………………………………… 77
4.2 Tribological properties of DLC coatings – a review…………………………….. 77
4.2.1 Influence of roughness……………………………………………………. 78
4.2.2 Influence of sp3 and hydrogen content – a combined effect………….. 79
4.2.3 Thermal effects on the friction and wear properties of DLC coatings... 81
4.2.4 Tribology of lubricated contacts………………………………………….. 82
4.2.5 Effect of transfer layer…………………………………………………… 83
4.2.6 Effect of lubricants………………………………………………………… 83
4.3 Friction and wear results in base oil……………………………………………... 85
4.4 Results using DLC/DLC tribopair in base oil……………………………………. 86
4.4.1 Rolling-sliding friction results……………………………………………… 86
4.4.2 Sliding wear results………………………………………………………… 94
4.4.3 Influence of hydrogen concentration in a-C:H coatings………………... 96
4.4.4 Influence of tungsten concentration in a-C:H:W coatings……………… 100
4.5 Results using DLC/steel tribopair in base oil…………………………………... 103
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4.5.1 Friction results……………………………………………………………… 103
4.5.2 Wear results………………………………………………………………… 104
4.6 General discussion……………………………………………………………….. 1 0 5
4.7 Summary…………………………………………………………………………… 112
Chapter 5: Friction and wear behaviour of DLC contacts with ZDDP and
dispersant containing ZDDP solutions
5.1 Introduction………………………………………………………………………….. 115
5.2 Review of ZDDP behaviour with DLC……………………………………………. 115
5.3 Comments based on the literature……………………………………………….. 119
5.4 Friction and wear tests with ZDDP……………………………………………….. 121
5.5 ZDDP solution results……………………………………………………………… 121
5.5.1 DLC/DLC tribopair in ZDDP solution……………………………………... 121
5.5.1.1 Influence of DLC type…………………………………………… 121
5.5.1.2 Influence of hydrogen concentration in a-C:Hs with ZDDP…. 131
5.5.1.3 Influence of tungsten concentration in a-C:H:Ws with ZDDP. 136
5.5.2 DLC/DLC tribopair in ZDDP solution………………………………………. 141
5.5.2.1 Friction and wear behaviour for range of DLC types…………... 141
5.5.2.2 Influence of ZDDP in the presence of steel counterpart………. 146
5.5.2.3 Influence of steel counterpart in ZDDP solution………………… 147
5.5.2.4 Summary of DLC friction and wear behaviour in ZDDP
solution……………………………………………………………… 150
5.6 Dispersant containing ZDDP solution……………………………………………. 153
5.6.1 DLC/DLC tribopair in ZDDP + dispersant solution……………………... 153
5.6.2 DLC/steel tribopair in ZDDP + dispersant solution…………………….. 158
5.6.3 Influence of steel counterpart in dispersant-containing ZDDP solution
on friction and wear………………………………………………………… 158
5.6.4 Influence of dispersant on DLC/steel contacts…………………………. 158
5.7 Durability of ZDDP tribofilms formed on DLC surfaces………………………… 165
5.8 Summary…………………………………………………………………………….. 168
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Chapter 6: Friction and wear behaviour of DLC coatings with GMO and oleic
acid
6.1 Introduction…………………………………………………………………………. 172
6.2 Review of the behaviour of DLCs with GMO…………………………………… 172
6.2.1 GMO………………………………………………………………………… 172
6.2.2 Comments based on the literature………………………………………… 174
6.3 Friction and wear results………………………………………………………….. 174
6.3.1 Results for GMO solution………………………………………………… 175
6.3.1.1 DLC/DLC tribopair in GMO solution…………………………… 175
6.3.1.1.1 Contribution of GMO to friction and wear………... 177
6.3.1.2 DLC/steel tribopair in GMO solution…………………………... 186
6.3.1.2.1 Contribution of GMO to friction and wear in the
presence of steel counter-surface……………….. 187
6.3.1.2.2 Influence of steel counterface on friction and
wear………………………………………………….. 187
6.3.2 Oleic acid solution…………………………………………………………. 195
6.3.2.1 DLC/DLC tribopair in oleic acid solution……………………… 195
6.3.2.2 DLC/steel tribopair in oleic acid solution……………………… 197
6.4 Mechanism of GMO with DLCs…………………………………………………... 199
6.5 Summary…………………………………………………………………………… 199
Chapter 7: Friction and wear behaviour of DLC coatings with MoDTC
7.1 Introduction…………………………………………………………………………. 203
7.2 Review of the behaviour of DLCs with MoDTC………………………………… 203
7.2.1 Comments based on the literature………………………………………. 204
7.3 Friction and wear results………………………………………………………….. 205
7.3.1 DLC/DLC tribopair in MoDTC solution…………………………………... 205
7.3.1.1 Influence of MoDTC in DLC/DLC tribopair…………………… 206
7.3.2 DLC/steel tribopair in MoDTC solution………………………………….. 211
7.3.2.1 Influence of MoDTC in DLC/steel tribopair…………………… 213
7.3.2.2 Influence of steel counterpart in MoDTC solution…………… 213

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7.4 Summary……………………………………………………………………………. 220
Chapter 8: Behaviour of DLC coatings with other film-forming additives
8.1 Introduction…………………………………………………………………………. 222
8.1.1 P-based additives………………………………………………………….. 222
8.1.2 Functionalised polymers………………………………………………….. 223
8.1.3 Overbased detergents…………………………………………………….. 223
8.2 Friction and wear results………………………………………………………….. 224
8.2.1 Metal-free P additive, DURAD 310 M solution…………………………. 224
8.2.1.1 DLC/DLC tribopair in DURAD solution………………………... 224
8.2.2 Polymer solutions………………………………………………………….. 230
8.2.2.1 DLC/DLC tribopair in non-dispersant polymer solution……... 230
8.2.2.2 DLC/DLC tribopair in dispersant polymer solution…………... 234
8.2.3 Overbased calcium sulphonate detergent solution……………………. 238
8.2.3.1 DLC/DLC tribopair in overbased calcium sulphonate
detergent solution………………………………………………... 238
8.3 Summary……………………………………………………………………………. 244
Chapter 9: Discussion
9.1 Introduction………………………………………………………………………….. 247
9.2 DLC/DLC tribopairs………………………………………………………………… 247
9.2.1 Effect of additives on the boundary friction properties of DLC/DLC
contacts……………………………………………………………………… 247
9.2.2 Film-forming properties in DLC/DLC contacts…………………………... 252
9.2.3 Wear resistance properties of DLC/DLC contacts……………………… 253
9.3 DLC/steel tribopair………………………………………………………………….. 255
9.3.1 Effect of additives on the boundary friction and film-forming
properties of DLC/steel contacts………………………........................... 255
9.3.2 Wear resistance properties in DLC/steel contacts……………………… 258
9.4 Comparison of ball and disc wear in DLC/DLC and DLC/steel tribopairs……. 260
9.4.1 Comparison of disc wear in DLC/DLC and DLC/steel tribopairs……… 260
9.4.2 Comparison of ball wear in DLC/DLC and DLC/steel tribopairs………. 262
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9.5 Overall summary of wear and friction of DLC system………………………… 265
Chapter 10: Conclusion and suggestions for future work
10.1 Conclusion…………………………………………………………………………. 268
10.2 Suggestions for future work……………………………………………………… 271
References…………………………………………………………………………………….. 272
Appendix 1…………………………………………………………………………………… 278
Appendix 2…………………………………………………………………………………… 281

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L O F
IST F IGURES
FIGURE
2.1. Stribeck curve
2.2. Stribeck curves for (a) solid-like and (b) viscous-like boundary films
2.3. Classification of DLC coatings, adapted from [3]
2.4. Modified-classification of DLC coatings
2.5. Typical cross section of a DLC coating
2.6. Schematic diagram of sputtering process, reproduced from [5]
2.7. Schematic diagram of CVD process, reproduced from [5]
2.8. Schematic diagram of PECVD process, reproduced from [5]
2.9. Schematic diagram of FCVA technique, reproduced from [3]
3.1. Typical coated (left) and uncoated (right) ball and disc specimens
3.2. Sample surfaces of DLC coatings investigated in this study
3.3. Typical EDX spectrum
3.4. Depth profiles of DLC coatings studied in this work
3.5. Raman spectra of DLC coatings studied in the current work
3.6. Schematic diagram of a typical AFM (in contact mode)
3.7. (a) F-S curve, (b) Loading force set-up using F-S curve
3.8. (a) Topography and (b) its derivative of all DLC coatings studied in the current work
3.9. Schematic diagram of MTM
3.10. Typical Stribeck curve obtained using MTM for a-C:H/a-C:H tribopair in base oil
3.11. Schematic diagram of MTM-SLIM test
3.12. Optical interference technique used to measure film thickness
3.13. Typical series of SLIM interference images obtained for an a-C:H(disc)/steel(ball)
tribopair in ZDDP solution
3.14. Schematic diagram of test strategy adopted in this study
3.15. SE image
3.16. BSE image
3.17. Wear tracks from MTM friction test and MTM wear test
3.18. (a) Wear track on disc, (b) X-profile and (c) Y-profile before free-level offsetting
3.19. (a) Mask editor, (b) histogram and (c) masked data on wear track
3.20. (a) Wear track on disc, (b) X-profile and (c) Y-profile after free-level offsetting
3.21. 3D wear track on disc (a) before and (b) after free-leve offsetting
3.22. Images for the calculation of (a) total wear volume and (b) wear volume of
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