Surface Nitriding and Oxidation of Nitinol

Nitinol has been widely employed in biomedical devices due to its unique mechanical properties such as superelasticity, shape memory, and good biocompatibility.  However, nickel ion release from the surface of the Nitinol is an issue.  Surface nitriding and oxidation was performed on the Nitinol specimens to develop a nickel-free oxide layer on the surface.  Nitinol specimens were nitrided in nitrogen + 4% hydrogen at 800 - 1000^oC for 10 - 30 min and further nitrided in nitrogen + 5% ammonia at 500 - 675^oC for 0 - 30 min.  The thickness and chemical composition, specifically the nickel content of the surface layer, were determined by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), respectively.  The effect of the nitriding time and temperature on the thickness and chemical composition of the nitride layer was evaluated.  Nitriding temperature was found to be more effective than nitriding time on the thickness of TiN layer.  Titanium nitride, the dominant phase on the surface of the nitrided specimens, was nickel free.  The nitrided Nitinol specimens were then oxidized at 675^oC and 700^oC for 30 and 60 min, respectively.  The chemical composition and elemental depth profile showed that oxidizing Nitinol specimens with a 0.4 µm thick nitride layer on the surface did not provide a nickel-free oxide layer on the surface of the Nitinol.  However, oxidizing the Nitinol specimens with a surface nitride layer that was thicker than 6 µm resulted in a nickel-free oxide layer.