Processing Ti-25Ta-5Zr Bioalloy via Anodic Oxidation Procedure at High Voltage
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NTRODUCTION
TI and Ti alloys are well-known implant materials because of their performance regarding stability in bioliquids, mechanical properties, and biocompatibility.[1–4] The good biocompatibility and osteointegrability of titanium alloys are a result of the presence of a thin and adherent film, which forms spontaneously on the metal surface.[5,6] This so-called passive condition of titanium alloys, when placed in a physiological environment, gives good corrosion resistance. The native protective oxide layer spontaneously formed on the surface when exposed to media containing oxygen is responsible for stability in various environments including bioliquids.[7] The characteristics of anodic films on valve metal (such as Ti, Ta, and Zr) were shown to be strongly dependent on the film formation conditions (electrical regime of oxidation, oxidation time, composition, and temperature of electrolyte). The native oxide formed on Ti alloys consists of a mixture of oxides, mainly TiO2, and different titanium suboxides (TiO and Ti2O3), as well as other oxides of alloying elements. In fact generally speaking, pure Ti is more resistant to corrosion compared with its alloys, but alloying is a procedure for tailoring mechanical properties. In our case, the other oxides are ZrO2 and Ta2O5, because of Ta element introduced as beta stabilizer and Zr DANIELA IONITA, Assistant Professor, MIHAELA GRECU and MIRELA DILEA, PhD Students, and IOANA DEMETRESCU, Professor, are with the Faculty of Applied Chemistry and Materials Science 1-7, University Politehnica of Bucharest, 011061 Bucharest, Romania. Contact e-mail: [email protected] VASILE DANUT COJOCARU, Lecturer, is with the Faculty of Materials Science and Engineering, University Politehnica of Bucharest, 060042 Bucharest, Romania. Manuscript submitted June 16, 2011. Article published online September 27, 2011. 1352—VOLUME 42B, DECEMBER 2011
element as alfa stabilizer. Both of them are biologically inert and may be used as alloying metals for long-term implants. However, according to thermodynamic aspects,[8] Ti, Ta, and Zr are all highly passivating metals.[9] The structure and mechanical properties including Young module’s value of this alloy, which is very close to the bone module, are presented in literature.[10] In contrast, the presence of the oxide layer on the surface plays an important role in the favorable tissue response to titanium implants. Anodizing of bioalloys is being an outstanding method of improving the biocompatibility and corrosion resistance of the alloys.[11–15] Titanium oxide thin films with nanoporous structures,[16] especially nanotubes,[17] can give better cell response[18] and are desirable because of their large surface area and high compatibility as clinical implant materials. These TiO2 nanotubes enhance apatite formation in biomedical applications compared with conventional flat TiO2 layers, and thicker nanotube layers trigger apatite deposition faster than the thinner ones.[17] The biocompatibility is determined by the chemical processes occurrin
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