Study on the formation of an apatite layer on NiTi shape memory alloy using a chemical treatment method

A chemical treatment method has been employed to prepare a bioactive layer on the surface of NiTi alloy. Prior to forming an apatite layer, all samples were dipped in 32.5% HNO3 aqueous solution and boiled in 1.2 M NaOH aqueous solution. Then, they were soaked in simulated body fluid (SBF) to form an apatite layer on the surface. The surfaces of chemically treated NiTi alloys were characterized by X-ray diffraction (XRD), X-ray photoemission spectroscopy (XPS) and environment scanning electron microscopy (ESEM). The results showed that a passive oxide film had been formed on NiTi treated with nitric acid, which mainly consisted of TiO2 and TiO. A sodium titanate hydrogel layer and Ni2O3 were formed on the surface treated by NaOH. After the above chemical treatment, an apatite layer was spontaneously deposited onto the surface when the samples were soaked in SBF. This revealed that a bioactive layer could be obtained by chemical treatment.