Characterisation of surface-modified nickel titanium alloys

Nickel titanium shape memory alloys (NiTi) have been investigated for applications in the biomedical industry for several years. However, little is known about the influences of surface modifications on the surfaces of these alloys. The effects of shot peening, electropolishing, polishing and heat treatments were investigated using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The shot peened surfaces were found to exhibit significant surface roughness. The surfaces of all the samples were dominated by TiO2; however, the concentrations of the nickel species observed were found to vary with surface treatment. Heat treatment of polished samples was shown to oxidise surface nickel and reduce the total nickel concentration at the surface. This was explained as the result of increased outward oxide growth with elevated temperature. The results clearly show that significant differences in surface chemistry can be induced by surface modifications of nickel titanium alloys.