Nihad Ben Salah, M&P

1. FINISHING, FLUID COMPATIBILITY AND NDT INSPECTION OF HVOF COATING Héroux Devtek Inc.- Landing Gear Division ENG/R&D - Longueuil, Québec (Canada) Nihad Ben Salah, M&P

2. Introduction • Progress • Producibility: Finishing, fluid compatibility and NDT on WC-10Co-4Cr HVOF coating

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 0.0002 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 (B) (A) 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 • Calibration for maximum response of BNI with HVOF WC-Co-Cr coating and different substrates (4340, 300M and Aermet 100) Set-up for Barkhausen Noise Inspection (BNI) • 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 120R75 wheel 240R75 wheel 7000/575 6500/200 4500/100 No pull-out observed Worst finish after grinding Ra= 19-22 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 « 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

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

25. 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 Fluid Compatibility- Tests

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 0.008 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