Role of Heat Treatment on the Fabrication and Electrochemical Property of Ordered TiO2 Nanotubular Layer on the As-Cast NiTi

Fatemeh Mohammadi, Mahshid Kharaziha, Ali Ashrafi

We investigated the effect of various heat treatment processes on the formation and electrochemical properties of ordered TiO₂ nanotubes (TNTs) on NiTi. In this respect, after solution treatment of as-cast NiTi samples at 900 °C for 1 h, four different heat-treated groups were examined consisting of furnace-cooled sample, water-quenched sample, water-quenched/300 °C-aged treated (300T-NiTi) and water-quenched/500 °C-aged treated (500T-NiTi) samples. Consequently, heat-treated samples were anodized in ethylene glycol solution containing NH₄F. Results showed that the microstructure, chemical composition and grain size of the NiTi samples depended on the heat treatment process. Water-quenching and subsequent aging process provided fine precipitations distributed in the grain boundaries and reduced grain size. Furthermore, it was found that TNTs with various distributions and microstructures could be developed depending on the heat-treatment process of NiTi samples as well as anodization voltage and time. Noticeably, anodization of 500T-NiTi samples resulted in formation of well-distributed TNTs with diameters of 30 ± 5 nm. Moreover, heat-treatment process as well as TNT formation resulted in significantly enhanced corrosion resistance of as-cast NiTi substrate and reduced Ni release, depending on the treatment process. Regarding potential applications, anodization of water-quenched and 500 °C aged treated NiTi at 50 V for 10 min could provide nano-scaled biofunctional coating to promote the biological applications of NiTi implants.