Experîmental Evaluation of Sliding Friction Coefficients for Aerospace Applications

213 Pages·2002·7.63 MB·English

Experîmental Evaluation of Sliding Friction
Coefficients for Aerospace Applications
Faizal Jaffer
A thesis submitted in conforrnity with the requirements
for the degree of Mastets of Applied Science
Graduate Department of Aerospace Studies
University of Toronto
O Copyright by Faizal Jaffer 2001
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ofCamda du Canada
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Experimental Evaluation of Sliding Friction Coefficients
for Aerospace Applications
Faizal Jaffer
Master's of Applied Science, November 2001
Aerospace Studies, University of Toronto
Space tnbology is a vitat aspect of spacecraft mechanism design and survival. One of the
major challenges in the area of tribo-testing has been the ability to relate msults from one facility to
the next, and to reduce the signifieant variation observed in measurements of tribological nature. A
novel tribology facility has been designed at UTlAS in order to provide insight into these problems
and is a one of a kind facility in Canadian aerospace research. The facilii is capable of evaluating
steady state friction and static friction properties of any material pair both in air and at high vacuum
pressures of 10'901~. The facility was designed to provide insight into operations to be performed by
the Special Purpose Dextrous Manipulator (SPOM) aboard the International Space Station (ISS). In
conjunction with MD Robotics Canada, 13 material pairs were evaiuated in order to determine sliding
friction and stiction coefficients as a function of speed, Ioad, distance and test atmosphere. Further
testing was completed on Teflon polymer films in order to provide insight into the fundamental nature
of sliding frciotin in a thermal-vacuum environment.
Acknow Iedgements
I would like to thank a few people whose help and support have been invaluable over the
past 2 years. First and foremost I would Iike to thank Dr. R.C. Tennyson and Dr. W.D. Morison who
have given me constant support and guidance throughout the course of this research endeavour.
Thanks to CRESTECH, CSA, MD Robotics Canada for their initiative in establishing this facilii at the
tnstitute. Special thanks to the department of Mechanical and Industrial Engineering, particularly
Jose Garcia and to Joe Antoniazzi of MD Robotics; the administrative staff here at the Institute as
well as the many other distinguished faculty members whom t have met along the way.
Finally, I would like to thank my family for their undying support and faith, To al1 my friends
for the love, laughter and times I will alwayç remember. To Husein, Farhan, Farouk, Shahid, Nekky
and your families for always being there, and IastIy to Melissa, thank you, for everything you are and
everything you make me.
Table of Contents
........................................................................................................................................
List of Tables vii
................................... ... .............................................................................................. ...
List of Figures vil1
..................................................................................................................................
List of Appendices x
...........................................................................................................................................
Introduction 11
- ..........................................................................................
Chapter 1 Theory and Literature Review 13
. ..............................................................................
Basic Fundamentals of Tribology Friction 13
.................................................................................................................
The Engineering Surface 16
....................................................................................
Detemination of the Real Contact Area 17
.............................................................................................
The Greenwaad & Williamson Mdel 18
........................................................................................................
Friction as a Dynamic Process 22
....................................................................................................................
Thin Film Lubrication 2 3
....................................................................................................
Adhesion and its Role in Fcroiitn 24
............................................................................................................
Static Friction and Stiction 2 6
...................................................................................................................
Stick-Slip Friction 2 7
.......................................................................................................................
Solid Film Lubrication 29
............................................................................................................................
Lamellar Solids 30
.......................................................................................................................
Thin Metallic Films 32
..........................................................................
Factors Affecting Friction of Solid Lubricants 3 2
.........................................................................................................................................
bad 32
............................................................................................................................
Velocity 3 4
..........................................................................................................
Atmospheric Pressure 35
............................................................................................................................
Temperature 36
.......................................................................................................... .
Polyrner Film Lubrication 3 7
................................................................................
Factors Affecting Friction of Polymers 3 9
.........................................................................................................................................
Load 39
....................................................................................... ........
Temperature and Velocity 42
.......................................................................................................................
Filler Material 43
.................................................................................................................. .
Contact Mechanics 4 4
Contact Stresses in Static Contacî6 .............................................................................................4..4
. .
.....................................................................................
Contact Sîteçses in Sliding Conw 4 6
............................................................
Prediction of Sliding Friction Coefficients of Various Materials 47
The Ogilvy Model - Statisîical. ...,.... ,. ......................................................................................... 47
-
....................................... ....................................................
The Twa-Term Non-interactive Model SI
......-...... -. ...................................................................................
Friction as a Disipative Procesç SI
. ..........................................................................
Chapter 2 Design and Constmion of Test Facility 54
........................................................................................................................
System Components 54
.........................................................
Test chamber/AO Chamber and High-Vacuum Capability 55
......................................................................................................................
Friction Rig Assembiy 56
.................................................................................................................................
Drive System 57
........................................................................................................................
Carriage Mechanism 57
. .
................................................................................................
User Interface and Data Acquiçitron 58
................................................................................................................................
Test Samples 58
........................................................................................................................................
Calibration 59
.................................................................................................................................
Data Collection 60
...................................................................................................................................
Data Analysis 60
Chapter 3 - Test Set-Up and Experiments ................................ ... ............................................. 62
- ...................................................................................................................
Phase 1 ORU Testing 62
................................................................................................................................
Test Pmtocol 62
-
.........................................................................................................
Phase 2 Polymer Film Tests 64
...............................................................................................................................
Test Pmtocol 64
.....................................................................................................................
Test Methodology 66
Fricîion vs .D istance (Run-in) ...................................................................................................... 66
................................................................................................................................
PTFE 1 66
....................................................................................................................................
PTFE 2 66
. ..........................................................................................................................
Friction vs Load 67
..................................................................................................................................
PTFE 1 67
....................................................................................................................................
PTFE 2 67
. ......................................................................................................................
Friction vs VelocÏty 68
....................................................................................................................................
PTFE 1 68
.............................. .... ........................................................................................
PTFE 2 6 8
Chapter 4 - Analysis of Test Resuits ......................................................................................... -. .,... 70
.................... ........................................................................................................
ORU Testing ,. 70
- ......................................................................................
Gmup 1 Dry-Film Lubricant MateriaIs 70
.....................................................................................................
Steady State Friction Data 72
..........................................................................................................................
Stiction Data 74
- -. ............................................................................ .........
Gmup 2 Teflon based Materials 7 4
Steady State Friction Data .............................................................. ... ....................... 7 5
...................... ... ........................................................................................... .
Stiction Data 76
- ............................................... -. ............................... .. .....
Group 3 Electrodepasited Materiais 76
................................................................... . ..............................
Steady State Frictian Data 76
...........................................................................................................................
Stiiion Data 78
..........................................................................................................................
Polymer Film Tests 79
.......................................................................................
Run-in and Vacuum Effects on Friction 79
.....................................................................................................
Fricüon as a Function of Load 81
.................................................................................................
Friction as a Function of Velocity 83
................................................................................
Chapter 5 -Conclusions and Recornmendations 85
.......................... ...........................................................................................................
References 87
...................................................................................................................................................
Tables 91
..................................................................................................................................................
Figures 93
.........................................................................................................................................
Appendices 149
.....................................................................................................................................
Appendix 1 150
-
........................................................................................................
Section A Part Drawings 150
Section B - Part Photographs ........................ .. .................................................................... 195
- ......................................................................................
Section C Assernbly Procedures 198
-
.................,...... ..................................................................................
Section D - Parts List ,., 200
.......................................................................................................
Section E Parts Suppliers 202
- .................................................................................
Appendix 2 Facility Operaîing Procedures 203
- ................................................................................
Appendix 3 Facility Calibration Procedures 210
- .................... ... ...................................................................
Appendix 4 Data Acquisition File 212
List of Tables
Table 1 -List of wmmonly used lubricant materials for various spacecraft components.
Table 2 -List of composite lubricants and Trade names with uses in spacecraft components.
-
Table 3 Cornparison of deformation zone properties within polymer siiding systems.
Table 4 -Test matrix for Orbital Replacement Units (ORU) materiab
List of Figures
1. Wew of the Special Purpose Dextrous Manipulator (SPDM), the centrepiece of the Mobile
Servicing System (MSS).
2. Variation in tangential force coefficient as a function of the surface parameter K and the ratio
of contact areas.
3, Single asperity penetration model for an assumed conical shaped contact.
4. Section of a typical engineering surface as described by Bowden and Tabor.
5. Section of a typical engineering surface as described by Rabinowicz
6. Three-dimensional surface profile as measured with an Atomic Force Microscope (AFM).
7. Pictorial representation of the Archard surface asperity model.
8. Cumulative height distribution for two surface finishes of an aluminum substrate.
9, Variation in surface separation as a function of normal load.
10. Variation in real contact area as a function of normal load.
11. Variation in plasticity index with increased sliding time.
12 Surface height distribution curves comparing theoretical and expenmental surfaces for an
assumed truncated distribution of surface heights.
13. Surface deformation map for separation height versus contact pressure and plasticity index
for a given area ratio.
14. Change in real contact area with time as a result of surface based creep.
15. Dynarnic frictional response following initial stick period.
16. Kinetic Phase Diagram as a function of system stiffness and sliding velocity.
17. Selected curves illustrating velocity dependence of friction.
18. Various surface sliding regimes proposed by Bowden and Tabor to explain the effectiveness
of interfacial shear layer.
19. Variation in friction coefficient with film thickness for lead film coating.
20. Cornparison of dominating adhesion theories.
21. Proposed theoretical models for variation of static friction coefficient with time.
22. Load rate dependence on static friction coefficient.
23. Transition from steady sliding 10 stick-slip friction for friction force veffius time curve.
24. Characteristic creep distance observed at the transition from stick to slip.
25. Microfriction trace of a Molybdenum Disulphide (Mo&) sputtered Silicon surface.
26. Variation in vapour pressure for a high performance liquid lubricant as a function of
temperature and vacuum exposure time.
27. Atomic structure of a Mo& ciystal.
28. Friction and endurance characteristics of various deposition methods for Mo& films.
29. Mechanism for transfer film formation of Mo& films in air.
30. Friction trace for a solid film lubricant displaying run-in.
31. Friction versus load curves for Mo& and Indium coatings.
32, Friction venus velocity curves for several thin metallic films
33. Hysteretic response curves for several material pairs on acceleration and deceleration.
34. Variation in friction for several lubricant coating materials as a function of ambient pressure.
35. Temperature dependent hysteresis in friction curve for nickel surface.
36. Molecular structure of longçhain polymer molecules.
37. Friction traces for Vespel-Spl showing effect of moisture uptake in polymer lubricant
systerns.
38. Friction versus load curves for several material pairs.
39. Variation in interfacial yield stress as a function of contact pressure for several potymers.
40. Variation in friction for pure Po~etrafluorethylene(P TFE) at various humidiil evels and in
vacuum.
41. Reduction in Wear rate for PTFE systems with inorganic fiilers
42. Reduction in Wear rate for PTFE systems with metallic powder fillers.
43. Reducîoin in friction for various systems filled with PTFE.
44. Surface stress distribution for a sliding contact.
45. Stress contour map for normalized Von Mises yield parameter with an SFC of 0.25 for a
sliding contact.
46. Normalized Von Mises yield parameter as function of SFC showing depth of first yield.
47. Equilibrium position of rnean plane of contact for statistically based friction model.
48. Predicted variation in friction wlh load for statistically based friction model.
49. Schernatic representation of deformation zones within a sliding polymer contact.
50. Schernatic diagram of the University of Toronto lnstitute for Aerospace Studies' (UTIAS)
Tribology Facilii
51. Photograph of the main chamber of the UTIAS Tribology facility.
52. Pictarial view of the friction rig sensing rnechanism.
53. Schernatic representation of friction mechanism and data acquisition signals.
54. Photograph of the load cell calibration rig.
55. Friction force and Normal force load cell calibration cum.
56. Magnified view of PTFE film coating showing stress cracks on lubricant surface.
57. Friction traces showing comparison between a typical friction signal and one affected by
mechanical vibrations or noise.
58. Measurements of stationary sample thickness at various locations where significant load
ramping was observed.
59. Friction trace showing the presence of load ramping over a full sample surface run.
60. Residual load of normal force load ce11 due to sticking of normal force adjustment assembly.
61. Orbital Replacement Unit (ORU) Samples B and C sliding friction data at 0.1 and 1. O
mmlsec.
62. Sample trace from ORU Sample A showing presence of surface Wear and debris within the
contact zone.
63. Sliding friction data for dry film lubricated ORU materials at 1.0 mdsec.
64. Stiction data for dry film Iubricated ORU materials.
65. Steady state friction data for Teflon coated ORU Samples at 5.0 mrnlsec.
66. Stiction data for Teflon coated ORU samples.
67. Magnified view of contact interface for ORU Samples J and K.
-
68. Summary of sliding friction data for ORU Sample J line and spherical contacts.
-
69. Comparison of sliding friction data for ORU Sarnple K line and spherical contacts.
70. ORU Sarnple L sliding friction data at 5.0 rnmlsec.
71. ORU Sample M sliding friction data showing incremental Wear of surface in increasing friction
with each test set.
72. Sample friction traces from ORU Sample M showing the presence of Wear as manifested in
the nature of trace variability.
73. Stiction data from ORU Samples J and K showing variation in static friction.
74. lncrease in stiction peaks with successive tests for ORU Sample M.
75. Run-in curves for both generations of PTFE coatings.
76. Normalized an-in curves for PTFE coatings showing onset of steady state behaviour.
TI, Friction-Distance curves measured at various contact loads.
78. Run-in curves for a glass fibre filled PTFE composite at various humidii levels and in
vacuum.
79. Friction-Distancec urves for PTEc omposite systems showing increase in Wear depth with
distance.
80. Friction versus load for Mo% coatings with cylindrical and sphericai contacts showing two
forms of load dependencies.
81. Friction versus nominal load measured for PTFE films on a single sarnple side.
82 Friction versus nominal load measureâ for PTFE films on indiiual sample sides.
83. Friction velocity curves measured for both generations of PTFE coatings.
84. Friction-velocityd ata taken for various PTFE tornposites.
85. Friction-velacityd ata for a meM surface at various loads.