In vitro corrosion behavior of TiN layer produced on orthopedic nickel–titanium shape memory alloy by nitrogen plasma immersion ion implantation using different frequencies
Introduction
Although NiTi alloys have achieved great commercial success in medical fields due to their unique shape memory effect and super-elasticity [1], [2], their high nickel content may adversely affect the biocompatibility of the materials because nickel released into body fluids can induce toxic and allergic responses [3]. It is thus advantageous to modify the surface of NiTi to simultaneously achieve wear improvement, corrosion resistance, and biocompatibility. Investigations have shown that the corrosion resistance and biocompatibility of NiTi can be improved by the formation of a stable, uniform, and highly adherent oxide film on its surface [4], [5]. PIII is an effective surface modification technique that excels for samples and components with an irregular shape like medical implants [6]. In this present investigation, PIII was used to introduce nitrogen into the surface of NiTi to obtain oxynitride layers using different implantation frequencies. X-ray photoelectron spectroscopy (XPS) was employed to study the elemental distributions and thickness of the modified layer after immersion in SBF. The results were used to correlate with the corrosion resistance determined using electrochemical techniques such as open circuit potential-time measurements and potentiodynamic polarization studies and to determine the optimal PIII conditions.
Section snippets
Experimental details
Ni(50.8 at%)–Ti alloy bars (SE508, Nitinol Device Company, Fremont, USA) were cut to 2 mm thick and 4.8 mm in diameter. Before plasma immersion ion implantation, the specimens were mechanically polished with 200, 400, 800, and 1200 grit SiC papers sequentially, followed by ultrasonic cleaning in acetone and deionized water and then air dried. N-PIII was carried out in a custom plasma ion implanter at City University of Hong Kong [7]. The base pressure in the vacuum chamber was 4.0 × 10− 3 Pa and
Results and discussion
Fig. 1(a) shows the nitrogen depth profiles obtained from the samples implanted using frequencies of 50 Hz, 100 Hz, and 200 Hz. Surface oxygen and carbon are observed to absorb on the surface of N-implanted sample [9]. After removing about 5 nm of materials, a nitride layer is found in all the implanted samples and the nitrogen concentration increases with higher implantation frequencies. The high-resolution XPS narrow scan indicates that this layer is mainly composed of TiN (shown in Fig. 1
Conclusion
The surface of NiTi shape memory alloy has been modified by nitrogen plasma immersion ion implantation at different pulsing frequencies. The technique creates a TiN layer on the surface which effectively improves the corrosion resistance of NiTi alloy. All the implanted NiTi samples exhibit higher OCP and Eb than the untreated samples. The 50 Hz N-PIII sample exhibits better electrochemical stability at OCP while the sample implanted at higher frequencies show higher potentiodynamic stability.
Acknowledgements
The work was financially supported by Hong Kong Research Grants Council (RGC) Central Allocation Grant No. CityU 1/04C and City University of Hong Kong Applied Research Grant No. 9667002.
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