XPS analysis of a heat-treated NiTi surface for elucidating Ni segregation phenomena

Heat treatment of nickel-titanium (NiTi) alloy was employed to form a chemically stable titanium dioxide (TiO2) layer on its surface to reduce Ni release. However, heating caused Ni to concentrate just beneath the topmost surface. To elucidate the mechanism of this segregation, the formation of this surface layer was analyzed with X-ray photoelectron spectroscopy under various thermal conditions. The Ni on the surface consisted of NiO and NiTiO3, and their concentrations hardly changed when treated in a reduced oxygen atmosphere. The surface formed in reduced oxygen pressure would mimic the pathway of the layer forming process. Thus, Ni segregation is considered to be completed at a very early stage. On the other hand, increasing the temperature facilitated the inward diffusion of Ni from the surface decreasing its surface content. Segregation of Ni in a heat-treated NiTi surface was hardly detected in the case of a chemical pretreatment of immersion into 1M HNO3 for 24h. It was concluded that the Ni segregation phenomenon is dominated by the surface condition before heating, which can be controlled by preliminary surface treatment. Copyright (c) 2016 John Wiley & Sons, Ltd.