Nanostructured surface changes of Ti–35Ta–xZr alloys with changes in anodization factors

Abstract

The purpose of this study was to investigate the changes of the nanostructured surface of Ti–35Ta–xZr alloys for dental application resulting from changes in anodization factors. TiO₂ nanotubes were formed on Ti–35Ta–xZr alloys by anodization in H₃PO₄-containing NaF solutions. Anodization was carried out using a scanning potentiostat. Microstructures of the alloys were examined by optical microscopy (OM), field emission scanning electron microscopy (FE-SEM) and x-ray diffraction (XRD). Microstructures of the Ti–35Ta–xZr alloys were changed from α" phase to β phase, and morphologies changed from a needle-like to an equiaxed structure, with increasing Zr content. As the Zr content increased from 3 to 7 to 15 wt.%, the average thickness of the TiO₂ nanotubes increased from 4.5 μm to 6.1 μm to 9.0 μm. When the anodizing potential was increased from 3 V to 10 V, the thickness of the nanotube layers increased from about 0.5 μm to 9.5 μm. As the anodization time increased from 30 min to 180 min at 10 V, the nanotube thickness increased from 4 μm to 9.5 μm. The amorphous oxide phase in the nanotubes transformed to anatase and rutile phases of TiO₂ by heat treatment above 300 °C.

Introduction

The dental implant biomaterials used in alveolar bone tissues must possess excellent biocompatibility, superior corrosion resistance, high strength and suitable elastic modulus. In order to achieve such properties, considerable time and effort have been devoted by materials scientists and engineers. Previous investigators [1], [2], [3], [4], [5], [6] recognized the excellent biocompatibility and superior corrosion resistance of pure Ti, pure Ta and some Ti–Ta alloys. This recognition indicates that Ti–Ta alloys are expected to become promising biomedical implant materials. Zhou et al. [7] reported that the Ti–35 wt % Ta alloy with a martensite α" phase has the potential to become a new candidate for biomedical applications due to its good combination of low elastic modulus and high strength, but also pointed out that the ratio of strength to elastic modulus needs to improve. The most common way to enhance the strength of most Ti alloys is by an aging heat treatment, however this approach also inevitably increases the Young's modulus of the alloys at the same time [8], [9]. Thus, it seems unnecessary to perform an aging treatment on the quenched Ti–35 wt.% Ta alloy if it is used as a biomedical implant material, since this alloy already has an elastic modulus exceeding that for bone. In contrast to Ta, Zr belongs to the same family in the periodic table as Ti, and previous studies [10], [11] have shown that the addition of Zr to Ti results in alloy with excellent mechanical properties, good corrosion resistance, and biocompatibility [10], [11].

In recent years, the formation of self-organized pores has been achieved on various titanium alloys containing Zr [12], Hf [13], Nb [14] and Ta [15], as well as on unalloyed Ti. Typically in these experiments, pores with diameters of approximately 100 nm and thickness of approximately 500 nm were reported. Titanium oxide nanotube formation on the titanium or titanium alloy surface is expected to be important to improve cell adhesion and proliferation under clinical conditions. It should be possible to control the nanotube size and morphology for biomedical implant use by controlling the applied voltage, alloying element, current density, anodization time, and electrolyte [16].

In the present study, nanotubes have been formed on Ti–35Ta–xZr alloys with an objective of studying the variation of tube dimensions with anodizing time and voltage and with Zr content.