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Carolyn Lahti, R&D Materials Engineer
Surface Quality of Incoming Nitinol Wire Used to Manufacture Braided Implants
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Carolyn Lahti, R&D Metallurgical Engineer at Boston Scientific Materials Testing, Analysis, and Characterization (MTAC)
B.S. in Materials Science and Engineering Michigan Technological University
MTAC • In-house service lab • Analyze a wide variety of products
– Interventional Cardiology – Peripheral Interventions
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Nitinol Manufacturing Process
The Methods of Preparation of Ti-Ni-X Alloys and Their Forming Radim Kocich, Ivo Szurman and Miroslav Kursa (http://cdn.intechopen.com/pdfs-wm/44019.pdf)
Surface finishing: chemical etch,
EP, passivation, coating
Sent to customer
Supplier surface finishing techniques: • Mechanical finish
• Diamond draw (shiny polish) • Sandblast (matte surface)
• Chemical finish • Etch/pickle • Electropolish (EP)
• Oxide surface • Coating • Plating
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Importance of Surface Quality Surface quality influences performance
From Noll and Lipson, "Allowable Working Stresses", Society for Experimental Stress Analysis, Vol. III, no. 2, 1949
http://www.andersonmaterials.com/medical-devices.html
http://www.snipview.com/q/Dimensional_instruments http://sme.org/MEMagazine/Article.aspx?id=49609&taxid=1418!
Compromising factors
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Goal of Study
Analyze the surface quality of incoming wire and EP material of nitinol wire • Characterize and optimize EP
http://www.imaging-git.com/products/3d-laser-confocal-microscope-simplifies-surface-analysis
http://www.witec.de/products/raman/rise-raman-imaging-scanning-electron-microscopy/
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Surface Roughness Measurements
Ra: arithmetic average of absolute surface roughness values Rp: maximum peak height Rv: maximum valley depth Rz: average distance between the highest peak and lowest valley in each sampling length
https://upload.wikimedia.org/wikipedia/commons/3/39/Mechanical_filtering_of_surface_finish_trace.svg
http://www.wisetool.com/surfacefinish.htm
Rz Rv
Rp
Ra
http://www.olympus-ims.com/en/metrology/ols4000/
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Experimental Setup
Mechanical Finish
Chemical Etch Finish
Proprietary Finish
Supplier A Supplier C Supplier B
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3 4
2
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High purity nitinol wire
Nitinol wire
356 micron wire
25 micron material removal (radial)
50 micron material removal (radial)
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SEM Images – Incoming Material
Mechanical Finish
Chemical Etch Finish
Proprietary Finish
Supplier A Supplier C Supplier B
356 micron wire
25 micron material removal (radial)
50 micron material removal (radial)
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Material Incoming Surface Condi4on Observa4ons Peak
Rp (μm) Valley Rv (μm)
Range Rz (μm)
Mean Ra (μm)
1 Mechanically polished Heavy surface scratching 11.89 8.70 20.59 2.16
2 Mechanically polished Mostly smooth 2.89 3.56 6.46 0.15
3 Chemically etched Smooth with small draw lines 10.83 11.97 22.79 1.65
4 Chemically etched Smooth with small defects 8.57 10.08 18.65 1.59
5 Proprietary finish Smooth with small draw lines 4.63 4.32 8.95 0.97
Peak (Rp)
Valley (Rp)
Range (Rz)
Mean (Ra)
Surface Roughness – Incoming Material
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SEM Images – 25 µm Material Removal
Mechanical Finish
Chemical Etch Finish
Proprietary Finish
Supplier A Supplier C Supplier B
356 micron wire
25 micron material removal (radial)
50 micron material removal (radial)
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Surface Roughness – 25 µm Material Removal
Material Incoming Surface Condi4on
Incoming Material Observa4ons
Peak Rp (μm)
Valley Rv (μm)
Range Rz (μm)
Mean Ra (μm)
1 Mechanically polished Heavy surface scratching
2 Mechanically polished Mostly smooth 0.60 0.54 1.15 0.24
3 Chemically etched Smooth with small draw lines 0.54 0.46 1.01 0.17
4 Chemically etched Smooth with small defects 1.98 2.47 4.45 0.29
5 Proprietary finish Smooth with small draw lines 0.73 0.53 1.27 0.20
Peak (Rp)
Valley (Rp)
Range (Rz)
Mean (Ra)
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SEM Images – 50 µm Material Removal
Mechanical Finish
Chemical Etch Finish
Proprietary Finish
Supplier A Supplier C Supplier B
356 micron wire
25 micron material removal (radial)
50 micron material removal (radial)
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Surface Roughness – 50 µm Material Removal
Material Incoming Surface Condi4on
Incoming Material Observa4ons
Peak Rp (μm)
Valley Rv (μm)
Range Rz (μm)
Mean Ra (μm)
1 Mechanically polished Heavy surface scratching 3.27 3.37 6.64 0.92
2 Mechanically polished Mostly smooth 1.10 0.88 1.98 0.26
3 Chemically etched Smooth with small draw lines 0.69 1.12 1.81 0.21
4 Chemically etched Smooth with small defects 2.65 3.28 5.93 0.28
5 Proprietary finish Smooth with small draw lines 2.45 1.70 4.15 0.21
Peak (Rp)
Valley (Rp)
Range (Rz)
Mean (Ra)
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Max Peak to Valley Distance
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Mean Surface Roughness
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Summary
• Quantification of surface finish variation throughout a collection of suppliers was assessed via SEM and roughness measurements
• Large variation in incoming surface quality observed across the different suppliers and incoming surface finishes
• The same electropolishing process works for a broad range of nitinol materials and results in a smoother surface
• Overall, incoming condition was less critical to the final EP condition • However, incoming condition may be important to processing wire prior to
EP (i.e. winding wire over fixtures) • Smoother surfaces from suppliers may allow for a reduction in material
removed during EP