Novel Surface Treatments for RRR Niobium

1. Novel Surface Treatments for RRR Niobium Michael J. Kaufman Department of Materials Science & Engineering University of North Texas Denton, TX Tarek Abdel-Fattah Department of Biology, Chemistry & Environmental Engineering Christopher Newport University Newport News, VA Roy Crooks Black Laboratories, L.L.C. Newport News, VA Jeffrey Wiese Nonlinear Engineering, L.L.C. Richmond, VA funding under: DOE SBIR Grant No. DE-FG02-06ER84453

2. Applied Research Center • Jefferson Center for Research • and Technology • Newport News, Virginia • 122,000 sq. ft. lab and office • 7 Commercial Enterprises • 5 Universities • 4 Government Agencies • Jefferson Lab • Technical Library

3. Charge Document • …surface removal thickness • …alternative techniques • Surface removal of 100 mm or more is needed for optimum performance of SRF Cavities. At 1 mm/min is time consuming and costly. Why is surface removal of 100 – 300 mm needed? • surface impurities, Ta inclusions? • tooling abrasion? • damaged layer at sheet surface? • Alternative techniques to reduce thickness of undesirable surface layer, remove it at lower cost.

4. Mechanical and Chemical Surface TreatmentsProblems/Approaches Mechanical Identify Presence and source of “damaged layer” Suggestions for minimization (less polishing time) Chemical Hazardous chemicals used for etching or electropolishing to remove “damaged layer”. High cost of use and disposal. Hydride problem at 100K. Electropolishing with acid-free (and water-free) ionic/organic solutions (at much lower cost); elimination of 800°C vacuum heat treatment to remove hydrogen.

5. As-received RRR Nb Sheet Surface Roughness ~ 500 nm? Channeling Contrast (backscattered) Electron Image Weak grain contrast Rolling Plane (plan view)

6. Uniform Damage Below Surface? As-received RRR Nb Sheet, 20 mm below surface(ion milled thin foil) High dislocation density. Some ion milling damage (black spots). Rolling Plane (plan view) Bright Field transmission electron microscopy (BFTEM) image of a low-angle boundary triple junction, near the [001] zone.

7. Standard Mechanical Surface Treatment: to Combat Distortion of Rolled Product • Non-uniform plastic deformation during rolling results in buildup of residual stresses; highest adjacent to greatest plastic deformation. Near surface of sheet. • The residual stresses may cause distortion when the metal is softened during annealing. • Rolling used for metal and alloy strip products • Adds thin deformation layer to sheet metal surfaces to mitigate residual stresses

8. Finite Element, Computational Plasticity Simulation of small-roll, small-reduction (2%) pass Finite element simulation (run under LS-Dyna) of 2% reduction of 3.5 mm sheet with 1 cm diameter rolls. Strain is concentrated in the near-surface region (red). Localized strain exceeds the average by a factor of 5.

9. Surface Finishing Stress Relief Rolling Recrystallization Experiment As-received sheet metal product, in final form, re-treated by stress relief rolling; then examined by SEM and OIM.

10. Silver epoxy Distorted grains (mottled contrast) 100 Etch pits 200 300 S Cross-section, ST plane T Finished mill product re-run through final roll. Mounted in conductive epoxy, metallographically polished, BCP etched 90s Channeling Contrast (backscattered) Electron Image grain contrast

11. surface 300 m a b Mill Product Re-run Through Final Roll OIM Kernel Average Misorientationof nearest, 1m spacing Inverse Pole Figure Map, Grain directions normal to rolling plane

12. Conclusion/Mechanical • Measurable Deformation to a depth of 200 mm. • A source of the “damaged layer” • Is stress relief rolling necessary?

13. Salt solution electropolishingof RRR Nb(towards a green process) Rationale: • Standard Nb electropolish uses a mixture of HF:H2SO4. HF is extremely hazardous. • Acid electropolishing results in H charging of the Nb; hydride precipitation during cavity cooling through 100K. • Acid-free, non-aqueous solutions using organic solvents and inorganic (anhydrous) salts have been used to electropolish Nb.

14. We experimented with variants of published or reported chemistries for salt-solution electropolishing of niobium (or stainless steel*) V. R. M. Brichese, F. Stivanello and V. Palmieri, International Workshop on Thin films and New Ideas for Pushing the Llimits of RF Superconductivity. 2006: Legnaro INFN Laboratory, Padua University. *A. P. Abbott, G. Capper, B. G. Swain, and D. A. Wheeler. Institute of Metal Finishing, 2005. 83: 51–53. *A.P. Abbott, Glen Capper, Katy J. McKenzie, and Karl S. Ryder,. Electrochimica Acta, 2006. 51: 4420-4425. As possible replacements for buffered chemical polish (BCP): HF:HNO3:H3PO4 And the standard electropolish (EP): HF:H2SO4 M. L. Kinter; I. Weissman; W. W. Stein. Journal of Applied Physics, 1970. 41(2): 828. H. Diepers; O. Schmidt; H. Martens; F. S. Sun. Physics Letters A, 1971. 37(2): 139. K. Saito. in 8th Workshop on RF Superconductivity. October 4-10, 1997. Albano Terme, Italy

15. Experimental Conditions for Electropolishing Electropolishing was performed under current control, with simple two-electrode System with a stainless steel cathode. Degreased in soapy water. Temperature control (dry ice/methanol bath) was used for first two. Current density of 50 mA/cm2 corresponded to a removal rate of ~ 1 mm/min.

16. Unusual Behavior of Salt Solutions vs Acid Electropolishing: • No polishing plateau with 10 minute dwell • Thick, visible gel forms on niobium surface • Stability of gel related to flow rate • Greater electrode position sensitivity

17. AR EP BK-2 G1 ECUA ECUA Channeling Contrast (backscattered) Electron Images and Ra values 107 nm 500 nm 68 nm 1 hour Electropolish AR RRR Nb Rolling Plane (plan view) 23 nm Several cm2 electropolished

18. Recommendations: • Measure srf properties after: • Alternative stress relief method • Stretching • No stress relief (use non-flat sheet) • Acid-free electropolish • Optimum conditions • 100K dwell (no hydride precipitation) • Small scale testing would be useful (C. Reese)