A. W. Hassel, L. Neelakantan, Max-Planck-Institut für Eisenforschung, Düsseldorf, Germany; J. Kawakita, National Institute for Materials Science, Ibaraki, Japan
Surface reactions of NiTi have been an issue since its early days of application. Corrosion is not only of interest in terms of the stability of a device but also with respect to the corrosion products. In particular the nickel release is discussed when evaluating the biocompatibility.
In this work we discuss 3 different approaches for a smart surface processing of NiTi.
Gradient oxidation - It is known, that the oxide that forms on NiTi is mainly composed of titanium oxide. This is a direct consequence of the lower nobility of the Ti. This behavior can be further promoted kinetically to yield a steeper metal gradient with a pure titanium oxide layer. The product is a sandwich layer in which the valence state of the titanium increases stepwise from 0 to 4.
Oxychlorination – Annealing in a complex atmosphere of O2 and Cl2 at temperatures between 600°C and 800°C allows a selective formation of volatile NiCl2 and a concurrent oxidation of Ti. This procedure yields a significant change of the surface composition in the metal layer underneath the titanium oxide formed.
Spark anodisation – Pulse anodisation was performed in alkaline, neutral or acidic solutions at high voltages of up to 150 V. Depending on the temperature, the pulse duration, and the duty cycle; thick layers of titanium oxide were formed. They are amorphous; tile structured and will be discussed in terms of their good corrosion protection and biocompatibility.
3 different approaches for a smart surface processing of NiTi will be introduced, all aiming at producing corrosion protecting and biocompatible surfaces. These methods are: Gradient oxidation, Oxychlorination and Spark anodisation.
