1. FINISHING, FLUID COMPATIBILITY AND NDT INSPECTION OF HVOF COATING
Héroux Devtek Inc.- Landing Gear Division
ENG/R&D - Longueuil, Québec (Canada)
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2. Introduction Progress
Producibility: Finishing, fluid compatibility and NDT on WC-10Co-4Cr HVOF coating
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3. Progress/ Producibility Testing
Chemical stripping: 100% achieved (see previous presentations). Final report about to be released
Finishing and superfinishing – 80% achieved, Narrowing the optimum grinding parameters window for a better productivity
Fluid compatibility: Corrosion tests achieved – Hydrogen embrittlement in progress
NDT : MPI, FPI, Barkhausen Noise Inspection on finished parts
All tests should be finished by March 2003

4. Finishing – ”Rough” previous Results
Samples: 4inch OD x 12inch L
Higher wheel speed improves the finish (5000 to 7000 SFM)
“High work speed with low crossfeed improves the finish and decreases the total time of grinding” BUT…
Optimum infeed seems to be around in. Higher infeed decreases the finish and damage the coating.
Lower grit (bigger abrasive particles) decreases productivity. The required finish is obtained after a longer time of grinding. For the same time, finish is better for higher grit, BUT…

5. Finishing –New results & Interpretation
A good finish does not mean a good coating (FPI results)
Most of the used grinding parameters did not damage the steel substrate (100% BNI). Concerns are raised when using higher grit wheel with inappropriate wheel speed and infeed.
Higher wheel speed improves the finish but affect the coating integrity.
When using higher grit, thickness control is better
For a better control of the integrity of the coating (quality and thickness) it is safer to target an average finish after grinding and improve it by superfinishing

6. Superfinishing -Results
Abrasive used : Diamond (paste, stone, ribbon)
For these tests, 2µinch Ra and 95-99% Bearing Ratio were typical super-finish characteristics of WC-Co-Cr reached.
The stone gives a fuzzy and “less cosmetic” finish
The ribbon gives the best cosmetic finish
Pressure and work speed shall be higher when using paste and ribbon, and the minimum when using the stone
To have a better cosmetic finish, the two last passes should be done without vibration.
No need to have very high finish as starting point for superfinishing

7. Abusive Grinding
Grinding:*Small samples of Aermet 100, OD 0.625inch, ID0.440 inch
*No cooling fluid
Tests: *Metallographic preparation per ASTM E1920 and Buehler Tech-note
*Micro-hardness profile
*Cross section SEM observations
Coating: Spalling, delamination, cracks, affected microstructure
Substrate : Heat affected thickness of the substrate varies from 12 to 40 mils. Overheating could be high enough to decrease Aermet 100 hardness to 50 HRC.
DO NOT UNDERESTIMATE ABUSIVE GRINDING

8. Abusive Grinding/SEM Observations
Crack
Inequal thickness due to abusive grinding

9. Abusive Grinding/SEM Observations
Bad Surface finish
Abusive grinding seems to have affected the microstructure of the coating

10. Abusive Grinding/SEM Observations
Crack
Delamination
Crack preceding spalling in Area (A)
Delamination observed in Area (B)

11. Abusive Grinding/Micro-hardness profile

12. Abusive Grinding/Micro-hardness profile

13. NDT- Barkhausen Noise Set-up for Barkhausen Noise Inspection (BNI)
Calibration for maximum response of BNI with HVOF WC-Co-Cr coating and different substrates (4340, 300M and Aermet 100)
Calibration for burns detection OK

14. NDT- Barkhausen Noise-Set up (1)
Axial sensor
MP decreases when HVOF coating or Cr plating thickness increases
MP decreases less drastically with HVOF coating thickness than with Chrome

15. NDT- Barkhausen Noise-Set up (2)
Different calibration is needed for different substrates.
BN signal is the lowest for 300M
Circumferential sensor

16. NDT- Barkhausen Noise-Set up (3)
Burns in the substrate are easily detected through HVOF coating on calibration test pieces
Gain=50
Calibration with coating is optimum for shot peening effect detection on calibration test pieces
Axial sensor

17. NDT- Barkhausen Noise-Set up (4)
Cracks in the substrate that are not linked with overheating cannot be detected by the RollScan

18. NDT Inspection of the coating
Types of imperfections observed on the HVOF coating: cracks, pull-out, pits, porosities, “spiralling”
Warning: Some defects are only detected by visual inspection of ground HVOF with Neon light - Could not be detected by FPI.
These defects are more detectable in the ground condition than in the superfinished
Method used for chrome (per ASTM E1417) type I, Method C, sensitivity level 3 is more sensitive than the method used for Aluminum Method A.
On going: FPI, method C with higher sensitivity (level 4: Ultrahigh), MPI

19. NDT- Visual
Visual Neon light observed defects: Pull out of HVOF coating
“Indents”
Pull-out 
Pull-out 
These defects are rarely detected by FPI
Pull-out 

20. NDT- FPI Worst finish after grinding Ra= 19-22 µinch, tp=0%
120R75 wheel
240R75 wheel
7000/575
6500/200
4500/100
No pull-out observed
Worst finish after grinding
Ra= µinch, tp=0%
Visual after SF
Best finish after grinding
Ra=6µinch, tp=57%
visual after SF
After SF: Ra= 2-3 µinch, tp= 90-99%

21. NDT- FPI FPI detectable defects
« Spiralling » not associated with pull-out
FPI detectable defects
Ra=7
Tp=28%
Porosities?
« Spiralling » associated with some pull-out not visible here
« pits »

22. NDT What are « defects » in the HVOF after grinding?
What should the FPI/MPI operators reject?
Acceptance and rejection criteria should be defined

23. Fluid Compatibility- Tests
Organic and semi-organic solutions (Immersion tests at room and high temperature)
Corrosion preventive products
Greases
Hydraulic fluids
Grease + hydraulic fluids
Degreasers
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24. Fluid Compatibility- Tests
Aqueous Solutions : Tafel plots for solutions used by immersion, Cyclic Voltametry for solutions used with applied potential to determine dissolution rates. Tests were done on freshly prepared and used (from the shop) solutions at IMI (NRC)
Cleaning solutions
Inspection related products (FPI, MPI, nital etch)
Plating and stripping solutions
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25. Fluid Compatibility- Tests
Aqueous Solutions : Hydrogen Embrittlement susceptibility in all previous solutions.
Notched specimens for H2 embrittlement type 1a (standard) per ASTM F-519, uncoated notch (keep the test valid per Spec. + simulate any possible galvanic corrosion), Load 75% NTS
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26. Fluid Compatibility (Results)
1- None of the organic tested fluid in the given conditions reacted with the HVOF coating:
No Thickness change
No Weight change
No Finish change
No visual modification

27. Fluid Compatibility (Results)
2- Aqueous solutions:
No relevant dissolution rate observed on any tested solution used by immersion (higher rate mils/h in the Mn phosphate solution)
Plating solutions are harmless in term of dissolution because the process is cathodic (0.003 mils/h)
Oakite cleaning solution, anodic process could be a problem, voltametic curves show a possible high dissolution rate after 5 minutes of immersion (0.5 mils/h)

28. Fluid Compatibility/Hydrogen Embrittlement
(Solutions tested so far: Cleaning solutions and nital etch)

29. Fluid Compatibility/Hydrogen Embrittlement
Possible interpretation:
HVOF coating could be a barrier for hydrogen removal
Cleaning solutions/steel/HVOF coating: Galvanic cell?
Recommandations:
Increase when possible the baking temperature or the duration
Process HVOF coating after any plating operation when possible
On going:
Tests in all plating solutions
Tests with increasing baking time
Repeat tests on cleaning solutions
SEM observations of failed specimens

30. Fluid Compatibility/Hydrogen Embrittlement
How the notch looks like when the whole specimen is HVOF coated