Multilayer Erosion Resistant Coatings for the Protection of Aerospace Components

1. Applied Research Laboratory Advanced Coating Division Brian Borawski Multilayer Erosion Resistant Coatings for the Protection of Aerospace Components Dr. Judith Todd Dr. Douglas Wolfe Dr. Jogender Singh Dr. Albert Segall June 23rd, 2011

2. Ceramic (brittle coating) Metallic coating Base alloy (substrate) Base alloy (substrate) Challenge Statement • Hard coatings protect against glancing impacts, but fail at normal impacts. • Can a combination of hard and soft layers provide protection under a wide array of conditions? • Yes, but there is a price to pay

3. Film Deposition • Iterative and DOE based development of processing parameters. • Separately studied gas flows, substrate bias, rotation, and pressure.

4. Parameter Space Evaluation Full factorial study of power and source to substrate distance. Developed a coating with 3240 HV0.200. 3” source-to-substrate distance at 1200W sputtering power. Chosen Conditions

5. Contributions to Stress Contact Residual Flexure Residual P Ceramic (brittle coating) Flexure Base alloy (substrate) Contact

6. Impact Mechanics • Stiffer materials increase the maximum contact force by decreasing contact time • Increase compliance may allow more flexure, but significantly decreases contact force where.. Therefore: Goldsmith 1960

7. Influence of Layer Thickness Thin layers Thick layers P P

8. Super Hard Coatings • Up to a 2x increase in hardness and elastic modulus • Good for low energy (cutting) Diamond tool coating Yashar & Sproul 1999

9. Super Tough Materials Koehler Theory Caveats Equal layer thickness concentrates stress in the hard layers Excessive dislocation restriction can lead to cleavage fracture Experimental data suggest that thinning ductile layer causes increased brittle manner and reduced fracture energy • Structure of two layered materials with the elastic moduli as different as possible • Hard and soft of equal thickness • Each layer <100 atoms thick Evans and Dalgleish 1992 Hsai et al. 1994 Koehler 1970

10. [TiN/CrN]/Ti Superlattice • Multilayers create an ultrahard superlattice • Author suggest that poor performance was caused by the brittle nature of the superlattice and interfacial detachment. Yang et al. 2004

11. Hypothesis • Ductile layers must be thin to avoid excessive flexure, but not so thin to cause brittle behavior • Thicker hard layers will perform well against small and angular erodents at low speeds • Thinner layers will perform poorly against small and angular particles but show marked improvement against larger smooth particles at high speed

12. Support for Model TiN:Ti • Chai & Lawn studied the effects of: • Number of layers • Interlayer thickness • Relative Moduli Decreasing interlayer thickness Most multilayer research As the modulus mismatch increase and the number of layers increase, the interlayer thickness must decrease Chai and Lawn, 2002

13. Optimizing Layer Designs • Failure for the 1st layer can be with less load than a similar monolithic coating • However, failure for the entire coating requires far more load than similar monolithic • Multilayer coatings have built in damage tolerance • Failure load of the 1st of 10 layers is lower • Failure load of the 5th of 10 layers is higher Chai and Lawn, 2002

14. Design of Experiments • A 3x3 grid of layer designs to quantify the effect of layer design • Number of layers • Interlayer thickness • TiN/Ti • TiN/Zr • TiN/Hf • TiN/Nb Increasing layers Increasing Ti Thickness • Best result used for interlayer study

15. Erodents Glass Quartz Alumina 500X 500X 500X

16. Equipment Innovations • Dual venturi design • Atmospheric pressure erodent loading • No rubbing components • In situ feed calibration • Extremely efficient accelerating • Allow 180 m/s operation • Automated process control • Calibration

17. Erosion Rate is not a good measure for multilayer coating

18. Performance againstangular erodents Cutting and Chipping

19. Monolithic coatings protect against angular particles

20. Alumina Damage Progression 50g 75g 25g 100g Slow continuous erosion of monolithic coatings Local, layer by layer failure of the multilayer coatings 25g 75g 100g 50g

21. Performance againstGlass Beads Resistance to cracking and flexure

22. Strong benefit of the titanium interlayer 1.25 vol % Ti 5 vol % Ti 25 vol % Ti

23. Dramatic Glass Bead Performance

24. Influence of Bond Layer Another important finding was that the coatings were not too thick to transfer stress through the coating thickness the monolithic coating with the thickest bond layer offered significant performance benefits over the monolithic coating with the thinnest bond layer.

25. 32 Layer coatings were more damage tolerant

26. (a) (b) (c) Wood, 1999 Corroborating Evidence • Energy must be reflected or absorbed • Hard layers deflect glancing impacts • Crack under high energy & normal impacts • Compliant material deforms to absorb energy • Offers little direct erosion resistance • Increase coating compliance Hard (brittle coating) Base alloy (substrate) • The complaint layers in the multilayer coating blunt the crack tip • The large difference in elastic moduli creates a barrier to crack penetration and dislocation motion Base alloy (substrate) • Sketch of the mechanics of single layer and multilayer erosion. • Impact crater as a results of erosion damage. • Optical micrograph of impact crater showing depth of erosion damage

27. TiN/Nb dominated

28. Results Summary • The angularity of the erodent has a significant effect on the failure mode • Sharp, Hard  Chipping  Monolithic • Round, Friable  Cracking  Multilayer • 8L5vol% performed well in every condition • TiN/Nb coatings performed the best • High poison’s ratio gave the ability to shear, but highly incompressible

29. Conclusion • Erosion resistance is a strong function of layer design • Results suggest that an optimum design exists for each erosion condition • Testing confirms that low energy performance can be sacrificed to achieve high energy performance

30. Project Goals Completed • Create failure model • Present hypothesis • Deposit coatings • Develop testing equipment • Perform jugular experiments • Analyze and report data

31. Publications

32. Thank you Questions?

33. Regions of Erosion Response

34. Monolithic coatings were better protection against chipping

35. Sudden layer failures of Monolithic and 8 Layer coatings

36. TiN/Nb dominated

37. More Topics • Showed correlations between erosion rate and hardness was a function particle size and indention load