Elsevier

Acta Biomaterialia

Volume 3, Issue 1, January 2007, Pages 113-119
Acta Biomaterialia

Surface elastic properties of Ti alloys modified for medical implants: A force spectroscopy study

https://doi.org/10.1016/j.actbio.2006.08.009Get rights and content

Abstract

We report here the first nanoscale surface elasticity measurements on surface-modified titanium alloys using the force spectroscopy mode in scanning force microscopy. Samples of three vanadium-free titanium alloys, Ti–7Nb–6Al, Ti–13Nb–13Zr and Ti–15Zr–4Nb, were investigated. Surface modification of the three alloys was produced by thermal oxidation in air at 750 °C for different times, which resulted in the formation of protective oxide layers with different surface composition and morphology. The elastic properties of the surface layers were studied comparatively in the as-received Ti alloys and after the oxidation process using cantilevers with different stiffness to evaluate the influence of the indentation depth. In all cases, Young’s modulus of the sample surfaces was found to be lower than 65 GPa, and as low as 20 GPa for some of the oxidized samples. Variations observed for the three oxidized Ti alloys can be related to the different chemical composition of the outer layers generated for the different oxidation times.

Introduction

Their high strength-to-weight ratio and low elastic modulus combined with a satisfactory corrosion behavior make titanium and titanium-based alloys outstanding biomaterials and promising candidates for the replacement of hard tissues such as those present in artificial hip joints or dental implants. The spontaneous development of a stable and protective passive layer with a thickness of a few nanometers when exposed to an oxygen-containing atmosphere provides these materials with a high biocompatibility associated with a high corrosion resistance in aggressive biological environments. During the past 50 years the standard alloy for these purposes has been Ti–6Al–4V, but it has been recently reported that the release of vanadium ions from the alloy might cause health problems due to their toxicity [1]. For this reason, there is growing research activity aimed at developing vanadium-free titanium alloys for biomedical applications presenting a good combination of biocompatibility, corrosion resistance and mechanical properties similar to those of Ti–6Al–4V. Within this approach, three α–β Ti alloys of composition (in wt.%) Ti–7Nb–6Al, Ti–13Nb–13Zr and Ti–15Zr–4Nb, proposed as potential biomaterials, have been previously characterized [2], [3]. In those works, the in vitro corrosion behavior of the alloys, immersed in Hanks’ solution, as well as the microstructure and chemical composition of the passive layer spontaneously formed in air, were studied. The electrochemical investigations pointed to a similar corrosion resistance for the three alloys, with values lower than those obtained for Ti–6Al–4V. The microstructural study revealed the existence of two different phases, α and β, with the α/β ratio being dependent on the alloy composition. Thus, the Ti–13Nb–13Zr alloy surface is rich in the β-phase due to a high content of Nb, a β-stabilizer element. Meanwhile, as Al is an α-stabilizer, and Zr is neutral, both Ti–7Nb–6Al and Ti–15Zr–4Nb surfaces are richer in α-phase.

One way to improve corrosion prevention and wear resistance in titanium alloys would be to increase the thickness of the passive layer. In order to do that, we have used a simple thermal treatment in air at 750 °C to generate a highly protective, bioinert oxide layer on the alloys’ surface. Previously to the present study, complementary techniques had already been used to investigate some properties of these layers. In particular, characterization by soft X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), Rutherford backscattering spectroscopy (RBS) and elastic recoil detection (ERD) showed differences in composition of the oxide layer grown on the three alloys [4], [5]. The passive Ti2O3 oxide layer formed on the as-received Ti–7Nb–6Al surface evolves, after an oxidation time of 1.5 h, into an Al2TiO5 layer on top of which an Al2O3 layer grows and thickens with further oxidation time. However, for the two TiNbZr alloys a TiO2 rutile layer is formed on the alloy surface independently of the oxidation time. Cross-sectional scanning electron microscopy (SEM) and electrochemical measurements were also performed to follow the development of the layer as a function of the oxidation time and to investigate the protective character of the oxidized layers against corrosion [6]. More recently, we have conducted a scanning force microscopy (SFM) study that reveals a correlation between roughness, oxidation time and surface composition of these oxide layers [7]. In the present study we attempt a further characterization of these layers by determining their nanoscale elastic properties.

Despite the great progress that has been achieved in producing orthopedic biomaterials, one of the major reasons for implant loosening following stress shielding in bones is the mismatch between the Young’s moduli of the biomaterials employed (between 110 GPa for Ti and 230 GPa for Co–Cr alloys) and the surrounding bone (10–30 GPa). Therefore, the elastic characterization of the outermost layers, which may present different elastic properties than the bulk and are those in direct contact with the bone, is crucial when suggesting alternative alloys.

We report here the results obtained using SFM in the force spectroscopy mode to investigate the elastic modulus in the very outermost layers of the aforementioned surface-modified Ti alloys. In recent years SFM has become a powerful technique for the study of the nanoscale surface structure of materials. In addition to the topographic analysis, the sensitivity of SFM to sub-nano-Newton forces has led to its use in the characterization of surface properties such as friction, adhesion and elastic modulus. In the latter case, force vs. distance curves measured with the SFM tips lead to indentation curves and, by analogy with classical indentation techniques, the respective surface moduli can be obtained [8], [9], [10], [11], [12].

Validation of the elastic moduli obtained in the present work is discussed by comparing data obtained with cantilevers of different stiffness and by assessing the non-invasive character of the technique in the force regime employed, where neither wear nor plastic deformation occurred as confirmed by SFM imaging verification. Furthermore, by taking advantage of the imaging capability of the SFM, we have analyzed the correlation between nanoscale elasticity and surface characteristics.

Section snippets

Materials and sample preparation

The Ti-based alloys of composition (in wt.%) Ti–7Nb–6Al (T1), Ti–13Nb–13Zr (T2) and Ti–15Zr–4Nb (T3) were prepared as explained elsewhere [2]. The sample surface was abraded and polished using diamond pastes with successively finer particle size. In the final stage, colloidal silica was used to ensure a surface free of mechanical deformation. The material in this state is the as-received sample. For each composition, four different samples were investigated: the as-received one, exhibiting a

Results and discussion

A thorough investigation of the oxide layer topography as a function of the oxidation time of the three alloy materials including careful root mean square (rms) analysis has been previously reported [7]. The effect of the oxidation treatment on the surface morphology is seen in SFM images, such as those in Fig. 2(a)–(c). Since the morphological structure of some surfaces presents significant inhomogeneities on large-scale areas (>25 μm2), in order to have suitable locations for measuring force

Conclusions

The elastic properties of three different Ti alloys in their as-received state and after oxidation treatments in air at 750 °C have been studied to investigate their suitability as implant materials. By means of SFM we have obtained indentation curves and, by applying the Hertz model, we have calculated the Young’s modulus for all the samples. The values obtained with this technique are representative of the outermost layers of the alloy surfaces, which are crucial for the success of implant

Acknowledgements

We are greatly indebted to Professor G. Frommeyer from the Max Planck Institut für Eisenforschung (Düsseldorf) for kindly supplying the Ti alloy specimens. This work has been supported by Project MAT-2004-00150 of the Spanish MEC and Projects 07N-0050-1999 and 07N-0066-2002 of the Spanish “Programa de Tecnología de los Materiales de la Comunidad Autónoma de Madrid” and the European Union FEDER program. A.G. and C.O. thank the Spanish MEC for financial support through the “Ramón y Cajal” and the

Cited by (0)

View full text