Ni-Ti surface with depressed Ni concentration prepared by plasma immersion ion implantation: superelasticity at body temperature

Plasma-immersion ion implantation (PIII) technique was employed to modify and improve the superficial region of a superelastic (at body temperature) Ni-Ti alloy (~50.4 at.% Ni). Oxygen and nitrogen PIII experiments were performed with a working pressure of 0.2 Pa, in a chamber equipped with an RF plasma source operating at a power of 350 W. High voltage pulses of either 20 or 40 kV were applied to the samples using a frequency of 400 Hz. The sample holder was not intentionally heated (T < 125°C).

The depth profiles of the elemental distribution in the alloy surface region, obtained by Auger electron spectroscopy (AES), show the formation of a Ti-rich oxide layer for the experiments carried out with oxygen. In the case of the experiments performed with nitrogen, the formation of titanium oxynitride (TiN_xO_y) is observed. Moreover, AES data show an almost Ni-free fraction for experiments performed with 40 keV. The high value of film resistance (electrochemical impedance spectroscopy analysis) suggests a very good corrosion resistance, which can be associated with the low Ni concentration at the surface of the film.

Synchrotron radiation-based X-ray diffraction data acquired in transmission mode show that the PIII technique only changes the structure of the Ni-Ti alloy top layer preserving superelastic behaviour at body temperature. Techniques like thermal oxidation and nitriding also lead to an improved corrosion resistance and Ni-depleted Ni-Ti surface but require high processing temperatures leading to modification of the phase transformation characteristics and loss of specific mechanical properties.