1. The Influence of Surface Roughness on Thin Film, Mixed Lubrication
Presented at the World Tribology Congress 2001, Vienna, Austria, 3 –7 September By
J. W. Choo
A.V. Olver
H.A. Spikes
Tribology Section
Department of Mechanical Engineering,
Imperial College of Science, Technology and Medicine, London

2. The Influence of Surface Roughness on Thin Film, Mixed Lubrication
Introduction
World Tribology Congress 2001, Vienna
Trend of machine elements
(i) Higher loads and temperature → progressively thinner films.
(ii) Increased operation in mixed lubrication.
In very thin film conditions,
Higher proportion of load is carried by asperity contact than by lubricant.
Calculate pressure distribution → Improve life predictions

3. The Influence of Surface Roughness on Thin Film, Mixed Lubrication
Method of Study
Employ 3D SLIM
 3D maps of EHD contact.
Study…
 model surface roughness features.
 effects of
(i) surface features
(i) asperity height
(iii) asperity slope.
 corresponding pressure distribution.
Correlate with real roughness results.

4. 3D SLIM (Spacer Layer Imaging Method)
The Influence of Surface Roughness on Thin Film, Mixed Lubrication
3D SLIM (Spacer Layer Imaging Method)
World Tribology Congress 2001, Vienna
White Light
Glass disk with thin Cr layer and SiO2 spacer layer
CCD Camera
3D Map or Profile of Separation

5. Calibration Procedure
The Influence of Surface Roughness on Thin Film, Mixed Lubrication
World Tribology Congress 2001, Vienna
Calibration Procedure
Ultra-thin film interferometry  log( hc ) vs. log( Ū )

6. The Influence of Surface Roughness on Thin Film, Mixed Lubrication
World Tribology Congress 2001, Vienna
Calibration Curve
[3G/(R+G+B)]2
[3R/(R+G+B)]2
[3B/(R+G+B)]2

7. The Influence of Surface Roughness on Thin Film, Mixed Lubrication
World Tribology Congress 2001, Vienna
Reliability of 3D SLIM

8. The Experimental Set-up
The Influence of Surface Roughness on Thin Film, Mixed Lubrication
The Experimental Set-up
World Tribology Congress 2001, Vienna
Steel Ball
With Ridge
Inlet
Outlet
Coated Glass Disc
Microscope
Motor
Loading
system
Ball Encoder
CCD Camera
White Light
Ball Carriage

9. Improvements to the 3D SLIM technique
The Influence of Surface Roughness on Thin Film, Mixed Lubrication
Improvements to the 3D SLIM technique
World Tribology Congress 2001, Vienna
Reduction of spacer layer compression.
 0.5 nm per 0.1 GPa
Spacer layer thickness mapping.
 measurement range 0 to 210 nm
 accuracy ±2 nm
Location accuracy of ball position is 15m.

10. The Influence of Surface Roughness on Thin Film, Mixed Lubrication
Test Specimens
World Tribology Congress 2001, Vienna
Ridge
Height (nm)
Width (μm)
Slope (0 ) A
163 55
0.437 B
342 37
1.002 C
145 20
1.359
Height
Slope
Width

11. The Influence of Surface Roughness on Thin Film, Mixed Lubrication
World Tribology Congress 2001, Vienna
Typical Film Profile
Steel Ball
With Ridge
Inlet
Outlet
Coated Glass Disc

12. Effects of Single Transverse Ridges on Lubricant Film Thickness
The Influence of Surface Roughness on Thin Film, Mixed Lubrication
Effects of Single Transverse Ridges on Lubricant Film Thickness
World Tribology Congress 2001, Vienna
Pure Rolling Condition

13. Minimum Film Thickness
The Influence of Surface Roughness on Thin Film, Mixed Lubrication
Minimum Film Thickness
World Tribology Congress 2001, Vienna
Pure Rolling Condition

14. Maximum Film Thickness
The Influence of Surface Roughness on Thin Film, Mixed Lubrication
Maximum Film Thickness
World Tribology Congress 2001, Vienna
Pure Rolling Condition

15. +  Pressure Calculation Film Thickness Undeformed Surface
World Tribology Congress 2001, Vienna
Pressure Calculation
Film Thickness
Undeformed Surface
Pressure Distribution + 

16. Pressure Ridge A: Slope = 0.437 deg Ridge C: Slope = 1.359 deg
World Tribology Congress 2001, Vienna
Pressure
Ridge A: Slope = deg
H = 163nm, W = 55 microns
Ridge C: Slope = deg
H = 145nm, W = 20 microns

17. The Influence of Surface Roughness on Thin Film, Mixed Lubrication
Conclusion of Results
World Tribology Congress 2001, Vienna
In pure rolling,
A lubricant film entrapment (film perturbation) is formed in front of the ridge.
Ridges with lower slopes generated thicker minimum films.
Ridge height and slope influences the amount of entrapped lubricant.
Pressure,
Film perturbation causes an increase in pressure.
Pressure at ridge increased with slope.

18. Effects of SRR on Inlet Flow Film Perturbation
The Influence of Surface Roughness on Thin Film, Mixed Lubrication
Effects of SRR on Inlet Flow Film Perturbation
World Tribology Congress 2001, Vienna
Ridge C (H = 145 nm, S = degrees), SRR 69%, Uball = 149mm/s, Udisk = 73mm/s
Inlet
Outlet
Peak of Ridge
Inlet
Outlet
Peak of Ridge

19. The Influence of Surface Roughness on Thin Film, Mixed Lubrication
World Tribology Congress 2001, Vienna
Array of Round Bumps
Average Surface Speed
Pure Rolling Conditions
Ū = mms-1
SRR = 0.8 %
Ū = mms-1
SRR = 35 %
Ū = mms-1
Ū = mms-1
Ū = mms-1
Ū = mms-1
Rolling-Sliding Conditions

20. The Influence of Surface Roughness on Thin Film, Mixed Lubrication
World Tribology Congress 2001, Vienna
Future Work
Isotropic Roughness
Starved Conditions
Dual Roughness
Acknowledgement
This work is financially supported by SKF Engineering & Research Centre B.V.