Biomimetic deposition of apatite coating on surface-modified NiTi alloy

TiO 2 coatings were prepared on NiTi alloy by heat treatment in air at 300, 400, 600 and 800 °C. The heat-treated NiTi alloy was subsequently immersed in a simulated body fluid for the biomimetic deposition of the apatite layer onto the surface of TiO 2 coating. The apatite coatings as well as the surface oxide layer on NiTi alloy were characterized using scanning electron microscopy equipped with energy dispersive spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy. Results showed the samples heat-treated at 600 °C produced a layer of anatase and rutile TiO 2 on the surface of NiTi. No TiO 2 was detected on the surface of NiTi after heat treatment at 300 and 400 °C by X-ray diffraction, while rutile was formed on the surface of the 800 °C heat-treated sample. It was found that the 600 °C heat-treated NiTi induced a layer consisted of microcrystalline carbonate containing hydroxyapatite on its surface most effectively, while 300 and 400 °C heat-treated NiTi did not form apatite. This was due to the presence of anatase and/or rutile in the 600 and 800 °C heat-treated NiTi which could provide atomic arrangements in their crystal structures suitable for the epitaxy of apatite crystals, and anatase had better apatite-forming ability than rutile. XPS and Raman results revealed that this apatite layer was a carbonated and non-stoichiometric apatite with Ca/P ratio of 1.53, which was similar to the human bone. The formation of apatite on 600 °C heat-treated NiTi following immersion in SBF for 3 days indicated that the surface modified NiTi possessed excellent bioactivity.