One of the major causes of premature failures of wind turbine gearing systems is the formation of axial cracks in wind turbine bearings. Increased surface stress results in increased subsurface stress, which in turn leads to a phase change in steel known as white etching areas (WEA). Cracks form in the steel within or at the edge of the WEA and these cracks lead to fatigue damage. Afton Chemical has conducted research on the causes of white etching formation and the role of lubricants in controlling WEA. Afton observed that lubricants that can control surface stress by forming the proper tribofilm on a surface can control fatigue. Afton Technical Advisor Mark Devlin presented on this topic during the 2015 AWEA O&M and Safety Seminar.

1. AftonChemical.com
White Etching Formation Due to Surface Stress
2015 AWEA O&M and Safety Seminar
Drivetrain Improvements & Failure Prevention Session
Presented by
Mark Devlin
Technical Advisor
Afton Chemical Corporation

2. AftonChemical.com
Overview of Presentation
Discussion on Detection of White Etching Cracks (WEC)
 Can We See WEC in a…..
 Bearing that Failed in Field
 Bearing Tested at High Stress Conditions in FE8 Rig
Discussion of Role of Lubricant in Preventing WEC
 What Do We Already Know About Fatigue Mechanisms
• Effect of friction and tribofilm formation
 Friction and Tribofilm Properties of Commercial Lubricants
• There are different lubricants for different applications
• They may not all be suitable to prevent WEC
Summary
2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation.

3. AftonChemical.com
Introduction to White Etching Cracks (WEC)
Can Occur in High Speed, Intermediate and Planetary Bearings
 Where torque converted to electrical current
Etch Subsurface and White Cracks Appear
Leads to Axial Cracks on Surface and Premature Failure
2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation.

4. AftonChemical.com
Many Possible Reasons for White Etching Cracks
Issue is Complex and Still being Debated
 Operational Factors:
• Fluctuating Torque; Stop/Start Operation; Torque Reversals; Misalignments; Wind Gust Loading
 Environmental Drivers
• Water Contamination; Corrosion; Slip; Vibration; Bending Stress; Impact Loading
 Effects on System
• H diffusion into steel; Crack creation due to Corrosion; Traction and Impact shear stress
All of the Factors / Drivers / Effects are Interconnected
Two Major Mechanisms
 Stress induced formation of WEC
 Hydrogen induced formation of WEC
2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation.

5. AftonChemical.com
Failed Inner Race from High Speed Bearing in
Wind Turbine Gear Box
Can We Observe White Etching Crack below Axial
Crack?
2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation.

6. AftonChemical.com
Wind Turbine Bearing Race Failed in Field
Where are we looking for White Etching Cracks
2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation.

7. AftonChemical.com
Sample Preparation to Detect White Etching
Cracks
2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation.

8. AftonChemical.com
White Etching Cracks Below Macro-pit
Etched Steel has Areas that Appear White
White Areas have Undergone Phase Transition and
are Near Cracks
2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation.
2.5 mm
10 mm
50 mm
Over-Rolling Direction

9. AftonChemical.com
White Etching Cracks Which are About to Form a
Macro-pit
Phase Transition in Steel Observed which is White
When Etched
2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation.

10. AftonChemical.com
FE8 Bearing Fatigue Rig to Create WEC
Standard FE8 Bearing Wear Test Conditions
 100kN; 80 ⁰C; 7.5 RPM; 80 hrs (DIN 51819-3)
Run FE8 Bearing Rig at Higher Stress
 60 kN; 100⁰ C; 750 RPM; Until Failure
 Rougher bearing race surfaces 0.5-0.6 mm Ra
 15% sliding (roller bearing without thrust)
Fluid Tested Forms Pits Repeatable Before 20 hrs
2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation.

11. AftonChemical.com
FE8 Roller with Macro-pits
Cross-cut Surface where Surface Damage Exists on
Roller
2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation.
Cross-Cut Plane
Look for WEC
Between
Macro-pits

12. AftonChemical.com
White Etching Crack in the FE8 Roller
2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation.

13. AftonChemical.com
Generalized Stress Induced Fatigue Mechanism
Increase in Surface Stress Causes Increase in Subsurface Stress
Increase in Subsurface Stress Causes Phase Transition in Steel
 Hardness change observed in subsurface
Cracks Form at Interface Between Phases
2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation.
Rolling Fatigue
5.1 GPa
120C
4000 RPM
8 Lubricants with
Different Friction
Characteristics Run
15 times to Failure
“Fatigue Performance of Low Viscosity Energy Efficient Lubricants”, 9th International Symposium on Fuels and Lubricants, April 15-17, 2014,
New Delhi, India
“Effects of Gear Oil Properties on Pitting Life in the Rolling Four-ball Test Configuration” Tribology Transactions 57 (1), p 104-113, 2014
“Lubricant Additives for Improved Pitting Performance Through a Reduction of Thin-film Friction” Tribology International 80, p. 122-130, 2014
“Investigations into the Occurrence of Pitting in Lubricated Rolling Four-ball Tests” Lubrication Science Online 12/2013
“Pit Formation Mechanisms in Lubricated Rolling Four-Ball Tests”, Proceedings of the 15th Nordic Symposium on Tribology, Trondheim ,
June 2012.

14. AftonChemical.com
Generalized Stress Induced Fatigue Mechanism
Increase in Surface Stress Causes Increase in Subsurface Stress
Increase in Subsurface Stress Causes Phase Transition in Steel
 Hardness change observed in subsurface
Cracks Form at Interface Between Phases
2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation.
Subsequent Tests
Stopped Prior to
Failure
Observed Subsurface
Phase Change with
Subsequent Change
in Material Hardness
“Fatigue Performance of Low Viscosity Energy Efficient Lubricants”, 9th International Symposium on Fuels and Lubricants, April 15-17, 2014,
New Delhi, India
“Effects of Gear Oil Properties on Pitting Life in the Rolling Four-ball Test Configuration” Tribology Transactions 57 (1), p 104-113, 2014
“Lubricant Additives for Improved Pitting Performance Through a Reduction of Thin-film Friction” Tribology International 80, p. 122-130, 2014
“Investigations into the Occurrence of Pitting in Lubricated Rolling Four-ball Tests” Lubrication Science Online 12/2013
“Pit Formation Mechanisms in Lubricated Rolling Four-Ball Tests”, Proceedings of the 15th Nordic Symposium on Tribology, Trondheim ,
June 2012.

15. AftonChemical.com
Role of Lubricant in Reducing Stress-Induced
Fatigue
Lower Friction to Lower Surface Stress
 Maintain low friction under extreme conditions
Lubricants that Form Tribofilms which Contain High Amounts of P and
Low Amounts of S Protect Surfaces
2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation.
Additional Tests
Performed
Effect of Tribofilm on Fatigue Life
Increase P and Reduce S in Tribofilm and Fatigue Life Improves
0
50000
100000
150000
200000
250000
GO-E GO-I GO-J
L50Life
Roller From Test With Fluid GO-J
Secondary Electron Image Phase Map/Image Overlay
0
10
20
30
Atomic%
C O P S
• Fe is also detected
• Normalize P and S
to amount of Fe,
since Tribofilm
thickness inversely
related to Fe
Tribofilms Formed on
Surface Before
Failure were
Characterized
Fatigue Life
Improved when Films
are High in P and
Low in S
Normalized Amount of P and S in Tribofilms
0.0
1.0
2.0
3.0
4.0
5.0
6.0
GO-E GO-I GO-J
Atomic%
P
S
With Changes in Anti-wear and Extreme Pressure Additives Can Raise
Amount of P and Lower Amount of S in Tribofilm
“Fatigue Performance of Low Viscosity Energy Efficient Lubricants”, 9th International Symposium on Fuels and Lubricants, April 15-17, 2014,
New Delhi, India
“Effects of Gear Oil Properties on Pitting Life in the Rolling Four-ball Test Configuration” Tribology Transactions 57 (1), p 104-113, 2014
“Lubricant Additives for Improved Pitting Performance Through a Reduction of Thin-film Friction” Tribology International 80, p. 122-130, 2014
“Investigations into the Occurrence of Pitting in Lubricated Rolling Four-ball Tests” Lubrication Science Online 12/2013
“Pit Formation Mechanisms in Lubricated Rolling Four-Ball Tests”, Proceedings of the 15th Nordic Symposium on Tribology, Trondheim ,
June 2012.

16. AftonChemical.com
Oils Examined
Different Lubricants for Different Applications
 Lubricants are made for different applications
 Some do not control WEC formation but are not designed to do so
Oils Tested
 Automotive Gear Oil (Oil A)
• Oil that causes WEC in FE8 test
 Industrial Gear Oils (Oils B and C)
 Wind Turbine Gear Oils (Oils D, E and F)
2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation.

17. AftonChemical.com
Friction Measurement and Tribofilm Formation
2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation.

18. AftonChemical.com
Low Friction Results in Decrease in Surface Stress
Fluid Should Keep Friction Low as Sliding Increases
2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation.

19. AftonChemical.com
Low Friction Results in Decrease in Surface Stress
Fluid Should Keep Friction Low as Load Increases
2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation.

20. AftonChemical.com
Tribofilm Composition
Lubricant Should Form Tribofilm High in P and Low in S
Past Research has Shown that P and S content of Lubricant IS NOT
Related to the Amount of P and S in the Tribofilm
 Tribofilm formation is complex process involving chemical transformation of additives and
pressure induced film formation
2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation.
0.0
1.0
2.0
3.0
4.0
5.0
6.0
Oil A Oil B Oil C Oil D Oil E Oil F
PercentPorSinTribofilm
P
S

21. AftonChemical.com
Summary of Lubricant Effects on WEC Formation
Issue is Complex and Still being Debated
 Operational Factors; Environmental Drivers; Effects on System
All of the Factors / Drivers / Effects are Interconnected
Two Major Mechanisms
 Stress induced formation of WEC
 Hydrogen induced formation of WEC
Lubricant can be Good at Several Things to Control WEC Formation
 Being “great” at one thing may not prevent WEC
 Friction Reduction : E, F better than A, D, C better than B
 Tribofilm Structure : C, D, E, F similar and better than B better than A
 Control of H induced mechanism – Still needs to be assessed but formation of
Tribofilm can control H ingress into steel
2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation.

22. AftonChemical.com
Summary
White Etching Cracks (WEC) are Detected in Bearings from Field That
Have Axial Cracks
 WEC are beneath axial cracks and in places where axial cracks have not formed
White Etching Cracks (WEC) are Detected in Bearings from FE8 Rig
Operated Under High Stress Conditions
 Tests performed with lubricant with moderate friction and tribofilm which contains
very low amount of P
Reduction in Stress-Induced Fatigue by Lubricant
 Lower friction under extreme conditions
 Form tribofilms that contain high amounts of P and low amounts of S
Lubricants Designed for Wind Turbines
 Do have low friction under extreme conditions
 Do form the proper tribofilms
2017 © Afton Chemical Corporation, All Rights Reserved. Not to be copied, shared, or reproduced in any media without the express written permission of Afton Chemical Corporation.