Biomimetic deposition of apatite coating on surface-modified NiTi alloy

TiO2 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 TiO2 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 TiO2 on the surface of NiTi. No TiO2 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.