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Monday, December 3, 2007 - 2:00 PM
7.1

Electropolishing and Electrochemical Micromachining of NiTi

L. Neelakantan, A. W. Hassel, Max - Planck Instiut für Eisenforschung, Düsseldorf, Germany; K. Fushimi, University of Hokkaido, Sapporo, Japan; G. Eggeler, Ruhr University Bochum, Bochum, Germany

Equiatomic Nitinol (NiTi) finds a variety of applications as stents, orthodontic arch wires and as surgical tools etc. Several surface treatments have been developed which improve the stabilitiy and biocompatibility of the alloy [1]. Among various methods, electropolishing is one of the conventional technique to achieve smooth flat surface.

 We report on the studies carried out to achieve a well defined NiTi surface. The electropolishing of NiTi in methanolic 3 M sulfuric acid solution at 263 K renders a smooth, flat and well defined surface. The influence of temperature, electrolyte concentration, etc. on resulting surface are discussed [2]. To understand the mechanism of electro-dissolution we carried out studies using the Rotating Disc Electrode (RDE). The effects due to change in rotation rate, temperature, and due to addition of Ni or Ti ions are extensively studied. RDE experiments reveal electro-dissolution is mass transport controlled and dissolution product is the mass transport limiting species (Salt Film Mechanism). Electrochemical impedance spectroscopy (EIS) confirms the salt film mechanism.

 The feasibility to work with NiTi alloys existing in single phase (either martensite or austenite) at electropolishing temperature (263 K) made possible to have an insight on the surface changes caused due to the phase transformation. We followed the evolution of surface features using temperature controlled XRD, SEM and AFM. The SEM, AFM studies revealed the psuedo martensitic relief causing changes in surface roughness and changes in surface morphology.

 References:
1.      A.W. Hassel, Min Invas Ther & Allied Technol. 13 (2004) 240
2.      K. Fushimi, et al., Electrochimica Acta 52 (2006) 1290


Summary: NiTi shape memory alloys are used in biomedical applications. Their surface stability dictates the successful usage as implants. Electropolishing is one of the promising technique. We formulated a procedure to achieve flat smooth surface,and performed studies to understand the mechanism during electropolishing of NiTi.Also experiments were done to follow the change of surface features of an electropolished surface due to phase transformation. These studies would facilitate to tailor the surface (Ni depleted and Ti enriched)and hence enhance its biocompatibility.