Publication Date

Summer 2011

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Chemical and Materials Engineering

Advisor

Guna Selvaduray

Keywords

Nitinol, Nitriding, Oxidation, Surface Treatment, XPS

Subject Areas

Materials Science

Abstract

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 - 1000oC for 10 - 30 min and further nitrided in nitrogen + 5% ammonia at 500 - 675oC 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 675oC and 700oC 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.

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