Atomic Layer Deposition as an Alternative Surface Finishing Technique: Characterisation of Oxide Layer Composition and Corrosion Behaviour.

The formation of titanium dioxide layers on the surface of nitinol implants is beneficial to ensure biocompatibility and to increase the corrosion resistance. Even though nitinol undergoes a natural passivation when exposed to air, various thermal, chemical, and electrochemical surface finishing techniques are commonly applied during device manufacturing, to replace the native oxide layer by uniform titanium dioxide layers of controlled thickness. The properties of the oxide layers depend on the technique used and the process parameters.

Atomic layer deposition is especially interesting for the application on nitinol wires to produce complex structures like braids, that undergo the final shape setting after electropolishing. Therefore, the scope of this study is to gain deeper insights in the composition and corrosion properties of the titanium dioxide layer, to assess the potential usage of this surface finishing technique for medical device manufacturing.

In this study the passivation process of nitinol is performed by combining two techniques: Initially the samples undergo electropolishing, which also removes contaminations and material defects in the outermost layers of the workpiece and ensures surface levelling. Subsequently, further oxide growth is initiated by either thermal treatment or atomic layer deposition. Characterisation of the composition and structure of the oxide layers is performed by various analytical methods like scanning electron microscopy, transmission electron microscopy and auger spectroscopy. In addition, the corrosion resistance and stability of the oxide layers is studied by means of cyclic potentiodynamic polarization and long-term immersion in phosphate buffered saline.