Preparation And Characterization Of Bioactivity Coatings On NiTi Alloy

NiTi alloy is an intermetallic compound that possesses unique physical and mechanical properties, such as shape memory effect, superelasticity, sufficient strength and biocompatibility, which attracts much attention. NiTi alloy has been widely used as biomaterials for medical application, but because of NiTi alloy containing about 50at.% of the nickel, when it is implanted into the human body, NiTi alloy has a tendency to corrode in the biological environment, releasing Ni²⁺, and Ni²⁺ can cause cell distortion and the occurrence of cancer, which is the largest defect of NiTi alloy as the biological materials. In view of the defect, the research on the surface modification of medical NiTi alloy mainly concentrates in reduing the suface content of the Ni, suppressing the relase of Ni ions and improving the corrosion resistance of NiTi alloy and so on.In the paper, hydroxyapatite (HAP) coating and HAP/ZrO₂, HAP/TiO₂ composite coatings were directly prepared on NiTi alloy by electrochemical deposition. The coatings enhanced the corrosion resistance of NiTi alloy, inhibited the precipitation of Ni²⁺ and improved the biocompatibility. Moreover, TiO₂ coatings were prepared on the surface of NiTi alloy by thermal oxidation in situ and anodic oxidation and the growth mechanism of TiO₂ coatings were discussed.During electrodeposition of HAP coating, the electrolyte solution contained 0.0167 M Ca (NO₃)₂, 0.01 M NH₄H₂PO₄, and the pH of the electrolyte was adjusted to 6.0. For fabricating HAP/ZrO₂, HAP/TiO₂ composite coatings, ZrO₂ and TiO₂ were added into the electrolyte respectively, forming suspension. The structures and properties of the HAP coating and HAP/ZrO₂, HAP/TiO₂ composite coatings on NiTi alloy were characterized using X-ray diffraction, scanning electron microscopy, infrared spectroscopy, bonding strength test, polarization and electrochemical impedance spectroscopy , and the growth mechanism of HAP coating and composite coatings was studied. The results indicated that addition of ZrO₂ and TiO₂ to the electrolyte changed the morphology of hydroxyapatite from thin flake-flower-like crystals to needle-flower-like crystals, and the coatings were much denser. Hydroxyapatite crystal grains in the coatings were preferentially arranged in the [0 0 1] direction, i.e. c-axis which was perpendicular to the surface of NiTi alloy. Besides, addition of ZrO₂ and TiO₂ enhanced the bonding strength between the coatings and the substrate. Corrosion resistance of HAP/ZrO₂ and HAP/TiO₂ composite coatings on NiTi alloy was significantly improved by alomost 60 and 50 times respectively by electrochemical corrosion test. The composite coatings could both effectively protect the NiTi alloy substrate from corrosion. So, HAP/ZrO₂ and HAP/TiO₂ composite coatings could both improve the biocompatibility of NiTi alloy effectively.In the process of preparation of TiO₂ coatings on NiTi alloy by thermal oxidation in situ, NiTi alloy samples were heat-treated for 1h at the the temperature range 500⁸00°C; during the preparation of TiO₂ coatings by anodic oxidation, the electrolyte was ethylene glycol and distilled water. The structures and properties of the TiO₂ coatings on NiTi alloy were characterized using X-ray diffraction, scanning electron microscopy and electrochemical impedance spectroscopy, and the growth mechanism of TiO₂ coatings was studied. The research indicated that selective suface oxidation of NiTi alloy occurred, forming TiO₂ and almost having no NiO. With increasing the temperature, TiO₂ crystal changed from anatase at 500°C, the mixture of anatase and rutile at 600°C to rutile at 700°C. When the temperature was over 500°C, Ni3Ti phase was detected. However, the corrosion resistance of the coatings formed at 500, 600°C decreased sharply, which limited the application of NiTi alloy. TiO₂ formed by anodic oxidation was amorphous which was transformed into anatase by hydrothermal treatment for 4h at 190°C. Corrosion resistance of NiTi alloy by anodic oxidation for 2h was improved by 25 times. It could protect the NiTi alloy from corrosion and improve the biocompatibility of NiTi alloys effectively. As NiTi alloy is very sensitive to the high temperature, so andic oxidation is a promising method to prepare TiO₂ coating on NiTi alloy.