An electrochemical study on self-ordered nanoporous and nanotubular oxide on Ti–35Nb–5Ta–7Zr alloy for biomedical applications

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Acta Biomaterialia 5 (2009) 2303–2310 www.elsevier.com/locate/actabiomat

An electrochemical study on self-ordered nanoporous and nanotubular oxide on Ti–35Nb–5Ta–7Zr alloy for biomedical applications Viswanathan S. Saji a, Han Cheol Choe a,*, William A. Brantley b a

Department of Dental Materials & Research Center of Nano-Interface Activation for Biomaterials, School of Dentistry, Chosun University, Gwangju 501-759, Korea b College of Dentistry, The Ohio State University, Columbus, OH 43210, USA Received 20 September 2008; received in revised form 19 January 2009; accepted 10 February 2009 Available online 20 February 2009

Abstract Highly ordered nanoporous and nanotubular oxide layers were developed on low-rigidity b Ti–35Nb–5Ta–7Zr alloy by controlled DC anodization in electrolyte containing 1 M H3PO4 and 0.5 wt.% NaF at room temperature. The as-formed and crystallized nanotubes were characterized by electron microscopy, energy-dispersive X-ray spectrometry and X-ray diffraction. The electrochemical passivation behavior of the nanoporous and nanotubular oxide surfaces were investigated in Ringer’s solution at 37 ± 1 °C employing a potentiodynamic polarization technique and impedance spectroscopy. The diameters of the as-formed nanotubes were in the range of 30–80 nm. The nanotubular surface exhibited passivation behavior similar to that of the nanoporous surface. However, the corrosion current density was considerably higher for the nanotubular alloy. The surface after nanotube formation seemed to favor an immediate and effective passivation. Electrochemical impedance spectra were simulated by equivalent circuits and the results were discussed with regard to biomedical applications. Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Nanotubes; Anodization; Ti–35Nb–5Ta–7Zr; Biomaterial; Corrosion

1. Introduction Recently it has been shown that nanoscale porous as well as tubular oxide layers on titanium alloys can increase the bioactivity of an implant material [1–3]. Such titanium oxide tubular structures find potential applications in various other fields such as catalysis, sensors, solar energy conversion, etc., due to their peculiar semiconducting and photoelectrochemical properties [4,5]. In recent years, electrochemical anodization technique in F containing electrolytes was projected as an efficient and economic approach for the production of highly ordered porous structures on valve metals [6,7]. Depending on the metal substrate and electrochemical conditions, the anodic oxide film may exhibit a compact, porous or a tubular structure. *

Corresponding author. Tel.: +82 62 230 6896; fax: +82 62 226 6876. E-mail address: [email protected] (H.C. Choe).

Such nanostructure formations have been achieved electrochemically on Ti [8,9], Zr [10], Nb [11], and Ta [12]. Anodization of Ti and Zr resulted in distinctly separated hollow cylinder shaped nanotubes; however, porous oxide layers resulted in the case of Nb and Ta [13]. Nanotube growth has been reported on binary, ternary and quaternary titanium alloys such as TiNb [14], TiZr [15], Ti–6Al–7Nb [16], Ti–30Ta–XZr [17] and Ti–29Nb–13Ta–4Zr [18]. In the anodization of Ti, the dissolution is enhanced by fluoride containing electrolytes which form soluble complexes with titanium, resulting in pore or nanotube formation [13,19]. However, selective dissolution of less stable elements or different reaction rates of different alloy phases can hinder nanotube formation. Quaternary b-titanium alloys of the system Ti–Nb–Ta– Zr are of current research interest due to their excellent mechanical properties such as very low elastic modulus, coupled with superior biocompatibility and corrosion resistance

1742-7061/$ - see front matter Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actbio.2009.02.017

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[20,21]. The aim of developing such low modulus titanium alloys was to decrease the elastic modulus difference between the bone (10–30 GPa) and the implant material, thereby promoting load sharing between them [22]. When insufficient load sharing occurs, natural bone resorption and loosening of the joint may occur [23]. Major quaternary alloys of this kind investigated include Ti–35Nb–5Ta–7Zr [20], Ti–4Nb– 4Ta–15Zr [21], and Ti–29Nb–13Ta–4.6Zr [24]. Among these, Ti–35Nb–5Ta–7Zr alloy have the lowest elastic modulus (55 GPa) and can be considered as one of the best choices for orthopedic implants [25]. No reported information is available on nanotubular oxide formation on this alloy. Reported works on electrochemical corrosion behavior of porous oxide grown titanium alloys is limited [19]. Also, no comprehensive reported information is available on electrochemical corrosion behavior of titanium alloys after nanotubular oxide formation. Hence in the present work, with a view to study the effect of nanoporous and nanotubular oxide layer formation on the electrochemical corrosion behavior of b Ti– 35Nb–5Ta–7Zr alloy, highly ordered nanoporous and nanotubular oxide layers were produced on the alloy surface using controlled anodization in electrolyte containing 1 M H3PO4 and 0.5 wt.% NaF at room temperature. The nanotubes formed were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy/energy dispersive X-ray spectrometry (TEM/EDS). Electrochemical behavior of the nanoporous and nanotubular alloy was investigated using potentiodynamic polarization and impedance spectroscopy measurements in Ringer’s solution at 37 ± 1 °C. 2. Experimental Ti–35Nb–5Ta–7Zr alloy was fabricated by arc melting with non-consumable tungsten electrode and water-cooled copper hearth under ultra pure argon atmosphere. Commercially high-purity Ti, Nb, Ta and Zr were employed for the purpose. All the ingots were melted and inverted 10 times in order to homogenize the alloy chemical composition. To stabilize the b phase and to homogenize the microstructure, the casted alloy was heat-treated at 1000 °C for 2 h in Ar atmosphere, followed by water quenching. The phase structure and chemical composition of the heat-treated alloy were identified by X-ray diffraction (XRD, X’pert Pro, Philips, The Netherlands) using a Cu Ka radiation and EDS (JXA8900M, Jeol, Japan) respectively. Chemical etching was performed using Keller’s reagent (HF + HCl + HNO3 + H2O) and the microstructure was observed using optical microscopy (OM, Olympus BX 60MF, Japan) and SEM (FE-SEM Hitachi 4800, Japan). The chemical composition (wt.%) of the alloy as determined by EDS was Ti:Nb:Ta: Zr = 51.6:35.5:5.1:7.8. A two-electrode system consisting of platinum as the counter electrode and the working electrode as the anode was used for anodization. A DC power source (Agilent E 3641 A) was employed. The sample was mounted on a cold

mount epoxy resin. Before anodization, the sample was polished by standard ANSI silicon carbide papers of different grades ranging from 100 to 2000 and finally alumina (1 micron) polished, ultrasonically cleaned in deionized water and dried in flowing nitrogen. The area of the sample exposed was 1 cm2. The electrolyte used was 1 M H3PO4 + 0.5 wt.% NaF. The anodization was performed by increasing the potential of the sample from 0 V to the desired potential with a scan rate of 100 mV s 1, followed by holding the sample in the potential for a desired time. The anodized samples were rinsed in de-ionized water and dried in air. FE-SEM was used for observing the surface, lateral and bottom morphology of the nanotubes. In order to study the crystallization behavior of the nanotubes, a heat treatment was carried out at 550 °C for 180 min in Ar atmosphere employing a tubular furnace. The heating rate used was 5 °C min 1. The phase structure of the as-formed and the heat-treated nanotubular alloy was identified by XRD using a Cu-Ka radiation. Transmission electron microscopy (FE-TEM/EDS, JEM-2100F, JEOL, Japan) was employed to observe the nanotube structure more precisely as well as to study the compositional homogeneity of the nanotubes. A focused ion beam miller (FIB, SMI3050SE, Seiko Instruments, Japan) was used for specimen preparation for TEM studies. Electrochemical potentiodynamic polarization and impedance spectroscopy studies were carried out in Ringer’s solution (9 g l 1 NaCl, 0.42 g l 1 KCl, 0.48 g l 1 CaCl2, 0.2 g l 1 NaHCO3) at 37 ± 1 °C using a potentiostat/galvanostat (EG&G, 263A) and an electrochemical impedance spectrometer (EIS, EG&G, 1025). The salt concentration in Ringer’s solution corresponded to that of body fluids. A conventional three-electrode system with high-density graphite as counter electrode and saturated calomel electrode (SCE) as reference was used. Preparation of the sample was as described above. The sample edges were carefully covered with epoxy to avoid the possible crevice attack. The electrolyte was de-aerated using high-purity Ar gas for 30 min before starting the experiment. De-aeration was continued at a uniform rate during the experiment. The scan rate used for potentiodynamic polarization was 1.667 mV s 1. Tafel extrapolation was followed to determine the corrosion parameters; based on a software-based approximation. EIS tests were conducted at the open circuit potential at the frequency range of 10 2 Hz to 105 Hz. A similar three electrode set-up was used for EIS studies. The amplitude of ac signal was 10 mV and 5 points per decade was used. An equivalent circuit was assigned for the acquired data and the data were curve fitted using ZSimpWin software. 3. Results and discussion 3.1. Phase and microstructure of Ti–35Nb–5Ta–7Zr alloy Fig. 1 shows representative OM and SEM images of the quaternary alloy investigated after chemical etching. The micrographs revealed equiaxed b grains. The black spots

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Fig. 1. Typical (A) OM and (B) SEM micrographs of Ti–35Nb–5Ta–7Zr alloy.

in the optical photo represent the pits developed during the rather lengthy etching time. An X-ray analysis (not shown here) revealed distinct peaks of body-centered cubic (bcc), (1 1 0), (2 0 0), (2 1 1) and (2 2 0); corresponding to single b phase. From the phase diagram [26] and the heat treatment followed in this study, it can be inferred that the b phase was retained completely. However, presence of early stage athermal x phase formation was detected through TEM studies in quaternary b titanium alloys that were solution-heat-treated at the same experimental conditions as that of this study [26]. 3.2. Morphology of porous and tubular oxide layers Ordered and firm nanoporous oxide layer formation was observed on the quaternary alloy when the applied potential was 15 V and the time of anodization was 90 min (Fig. 2). The diameters of the pores formed were in the range of 30–50 nm. When the applied potential was 20 V and the time of anodization was 180 min, highly ordered nanotubular oxide layer formation was achieved (Fig. 3). However, at lower applied potentials and time of anodization, a spongy porous layer formation resulted, whereas at potentials above 30 V, an irregular compact porous layer was the result. Zwilling et al., in their studies on a + b Ti–6Al–4 V alloy, reported that the pore diameter was independent of the applied voltage and dependent on the specific alloy phase. The diameter of the pores formed on b phases was small [27]. As Fig. 3 implies, highly ordered nanotubes were formed. The diameters of the nanotubes were in the range of 30–

80 nm. It can be seen that few tubes were grown more outwards. This may be related to the stress effect and the possible lifting up of the tube walls [28]. The lateral and bottom view images of the nanotubular oxide layer are shown in Fig. 4. The nanotubes were straight with uniform pore walls. From the bottom view image, a bimodal size distribution of nanotubes is evident. Also tubes having similar diameters seem to possess a short-range one-dimensional ordering. Optimized parameters such as anodization potential, nature of the electrolyte, concentration of the electrolyte, temperature and the potential sweep rate are critical in achieving self-ordered nanotubes. The nanotube formation is controlled by both the anodic oxide formation and the subsequent leaching of the oxide layer as water-soluble TiF26 fluoride complexes [13]. This complex formation at the metal oxide/electrolyte interface prevents the possible metal hydroxide precipitation. With increase in growth time, the diffusion of fluoride ions to the tube bottom, or transport of the formed soluble fluorine complexes, becomes rate determining. Under an optimized condition, the initial nanopores formed equally share the available current and that might have resulted in the self-ordering of nanotubes. Such a process may occur when the pore growth rate is equal to the leaching/dissolution rate [13]. 3.3. Crystallization behavior and compositional homogeneity of nanotubes Fig. 5 shows the XRD patterns recorded at normal angle for the as-formed and the heat-treated (550 °C for 180 min) nanotubular alloy. In case of the as-formed nano-

Fig. 2. SEM surface view images of the nanoporous oxide layer formed at 15 V. Time of anodization was 90 min.

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Fig. 3. SEM surface view images of the nanotubular oxide layer formed at 20 V. Time of anodization was 180 min.

Fig. 4. SEM (A) lateral and (B) bottom view images of the nanotubular oxide layer formed at 20 V. Time of anodization was 180 min.

Anatase Rutile β Ti

Intensity (a.u.)

tubular alloy, only the peaks corresponding to the cubic titanium were obtained, which probably indicates the amorphous nature of the nanotubes. After the heat treatment, the main crystallized fraction of the nanotubes was anatase TiO2, with smaller contributions from rutile phase. The crystallization behavior of the nanotubular oxide layer formed on the present alloy was similar to that of pure titanium [29]. The crystallization occurs in anatase phase at a temperature near 300 °C, and rutile phase starts appearing at around 500 °C, with the rutile phase dominating in nanotubes annealed at 700 °C. The splitting observed for the cubic titanium peaks may be associated with the aging effect of the titanium alloys [30]. As it was difficult to detect the presence of Zr, Nb and Ta oxides in the nanotubes through XRD, we have carried out TEM/EDS analysis of the nanotubes before and after heat treatment. Fig. 6 shows the TEM images of the as-formed and crystallized nanotubes. A bimodal size distribution with onedimensional short-range ordering of similar sized tubes is

B

A

20

30

40

50

60

70

80

90

2θ ( deg.

Fig. 5. XRD patterns of (A) as-formed nanotubular alloy and (B) crystallized nanotubular alloy.

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Fig. 6. TEM images of (A) as-formed nanotubes, (B) crystallized nanotubes, and (C) as-formed nanotubes; selected areas for EDS analysis.

evident from the TEM image. The average length of the tubes formed was in the range of 2 lm. There was gradual thinning of the wall towards the top end of the nanotubes. After the heat treatment, the tubes become slightly flattened and the distinct interface between the nanotubes and the substrate get fused to some extent. Table 1 represents the chemical composition of the 12 selected portions of the as-formed nanotubes (Fig. 6C). The four constituent elements of the alloy were detected in the nanotubes with an uneven distribution. The striations seem to serve as an interconnecter between the tubes and were suggested to have originated as a result of the different growth rate of nanotubes [13]. 3.4. Electrochemical studies Fig. 7 shows representative potentiodynamic polarization plots obtained for (a) bare alloy, (b) nanoporous alloy, and Table 1 TEM/EDS analysis of nanotubes formed on Ti–35Nb–5Ta–7Zr alloy. Spectrum

O

Ti

Zr

Nb

Ta

1 2 3 4 5 6 7 8 9 10 11 12

1.00 20.56 17.11 25.02 21.16 21.88 18.78 23.22 22.22 26.48 23.22 24.35

51.41 38.46 34.33 35.76 38.26 31.90 33.08 32.78 36.03 36.39 33.89 31.51

5.12 4.76 5.08 4.93 4.27 5.06 5.60 7.06 2.65 2.57 1.75 4.60

36.23 37.01 34.50 32.06 36.31 36.08 33.06 27.89 35.96 34.56 38.19 38.45

6.24 2.21 8.98 2.24 – 5.08 9.47 9.06 3.14 – 2.95 1.09

(All results in wt.%).

(c) nanotubular alloy. The corresponding corrosion parameters are shown in Table 2. An immersion period of 30 min was given for stabilization of open circuit potential before starting the experiment. As expected formation of nanoporous anodized layer significantly improved the corrosion resistance property of the bare alloy. The nanotubular surface exhibited a nature of passivation behavior similar to that of the nanoporous surface. Two passive regions (marked as I and II) can be seen. The passive region I extended over a wide potential range for both the nanoporous and nanotubular alloy. This may be attributed to a rapid and effective surface blockage of the nanoporous surface. The bare alloy showed a steady passivation (II) due to the compact film formation at the interface. The current density corresponding to the passivation region (ipass) for both the nanoporous and nanotubular alloy was nobler than for the bare alloy. However the nanotubular alloy exhibited significantly higher corrosion current density (icorr) values. The icorr values increased in the order of nanoporous < bare < nanotubular. The higher icorr value of the nanotubular alloy compared to the bare alloy suggested a lower corrosion resistance property. Anodized porous titanium surfaces possess better corrosion resistance properties than bare alloys [19]. The pores of the nanoporous surface in this study behaved as perfectly passive pits due to the possible higher barrier oxide thickness and compact pore walls. When the surface has a nanopatterned surface, due to the increased effective surface area, the possible surface reaction will be favored [31]. In the case of the nanotubular surface, the tubes may act as more effective channels for the electrolyte to reach the interface. The lower corrosion resistance property of the nanotubular alloy may be associated with the concave

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B

0.8 0.6

C

A

II

Potential (V)

0.4 0.2 0.0 -0.2

ipass

-0.4 -0.6

I

-0.8 -1.0 1.0E-08

1.0E-07

1.0E-06

1.0E-05

1.0E-04

1.0E-03

Current density (A/cm2)

Fig. 7. Potentiodynamic polarization plots of Ti–35Nb–5Ta–7Zr alloy in deaerated Ringer’s solution at 37 ± 1 °C: (A) bare alloy, (B) nanoporous alloy, and (C) nanotubular alloy.

Table 2 Corrosion parameters from polarization plots. Sample Bare Nanoporous Nanotubular

Ecorr (V) 0.805 0.728 0.754

icorr (lA cm 2)

ipass (lA cm 2)

0.87 0.76 3.12

18.14 8.68 12.57

shaped tube bottom and the distinctly separated tube bottom/barrier oxide interface (Fig. 6A). Hence we have carried out potentiodynamic polarization studies of nanotubular alloy after the crystallization heat treatment (graph not shown here). As expected, the corrosion current density was lowered significantly, indicating improvement in corrosion resistance. From the TEM images (Fig. 6B), it can be seen that the distinct nanotubes/barrier oxide interface was disintegrated after the heat treatment. Hence the lower corrosion resistance property obtained for the nanotubular alloy is suggested to be associated with the distinctly separated barrier oxide/concave shaped tube bottom interface. The non-destructive impedance spectra recorded for the three surfaces in the electrolyte are shown in Fig. 8. Impedance measurements were carried out after 30 min of initial immersion in the electrolyte. A high impedance value of the order of 105–106 X cm2 was obtained at low and medium frequencies, suggesting good corrosion resistance for both the nanoporous and nanotubular surfaces. The phase shift plot showed two time constants in the case of both the porous and tubular surfaces. This corresponds to the presence of two interfaces: the high corrosion resistant barrier layer and the outer porous layer. The higher phase angles obtained for these surfaces compared with that of the bare surface at the high-frequency region is attributed to the oxide film thickening. The decrease in phase angles at the high-frequency region may be associated with the porous nature of the outer layer. The impedance behavior of such

porous layers can vary depending on the aspect ratio of the nanotubes formed [32]. The bare alloy exhibited high capacitive behavior with one time constant, indicating a single and compact passive layer at the interface. The equivalent circuits used to fit the experimental data are shown in Fig. 9. A similar equivalent circuit was used for both the nanoporous and nanotubular alloy. For the nanoporous alloy, C1 and R1 were, respectively, assigned for the capacitance and resistance of the outer porous layer and C2 and R2 to the capacitance and resistance associated with the pore walls. It was assumed that the pits formed were passive in nature. In the case of the nanotubular surface C1 and R1, respectively, represent the capacitance and resistance of the tube walls and C2 and R2 to the capacitance and resistance of the tube bottom (oxide barrier layer). Rs represents the solution resistance. A constant phase element was used instead of capacitance for curve fitting. Chi-square values of the order of 10 3 indicated excellent agreement between the experimental and model values and the simulated values obtained are shown in Table 3. A higher value of R implies higher corrosion resistance. The value of n less than one indicates a non-ideal capacitance interface. The electrochemical studies suggested that the surface after nanotubular oxide formation possesses good corrosion resistance with an enhanced passivation. Reported works showed that osteoblast proliferation was significantly greater on nanophase ceramics [33]. In vitro studies [34] indicated that nanotubular titania surfaces provide a favorable template for the growth of bone cells. The cells cultured on nanotubular surfaces showed higher adhesion, proliferation, alkaline phosphate activity and bone matrix deposition compared to those grown on flat titanium surfaces. Their in vivo biocompatibility results suggested that nanotubular titania does not cause chronic inflammation or fibrosis [34]. Increased effective surface area of the nanotubular alloy and the possible introduction of anatase phase after a proper heat treatment can improve the bio-

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1.0E+07

|Z| (Ohms.cm^2)

1.0E+06

1.0E+05

1.0E+04

C 1.0E+03

B A

1.0E+02

1.0E+01 0.01

0.1

1

10 100 Frequency (Hz)

1000

10000

100000

80 70

Phase of Z (deg)

60 50

C

40

B A

30 20 10 0 0.01

0.1

1

10

100

1000

10000

100000

Frequency (Hz)

Fig. 8. Bode plots of Ti–35Nb–5Ta–7Zr alloy in deaerated Ringer’s solution at 37 ± 1 °C: (A) bare surface, (B) nanoporous surface, and (C) nanotubular surface.

A

bular oxide formation on low rigidity quaternary titanium alloys by anodization has immense potential for biomedical applications.

C1

RS

B

C1

RS

4. Conclusions

R1

R1

C2

R2

Fig. 9. Equivalent circuits used to fit EIS spectra: (A) for bare surface, and (B) for nanoporous and nanotubular surfaces.

logical activity. The possible occurrence of phosphorous on the nanotubes from the H3PO4 based electrolyte may also help in osseointegration. Under these perspectives nanotu-

Self-ordered nanoporous and nanotubular oxide layers were developed on Ti–35Nb–5Ta–7Zr alloy using DC anodization in electrolyte containing 1 M H3PO4 and 0.5 wt.% NaF at room temperature. The as-formed nanotubes possess a bimodal size distribution with diameters in the range of 30–80 nm. There seems to be a one-dimensional short-range ordering of similar sized nanotubes. TEM/EDS analysis detected all four component elements of the alloy in the nanotubes. Potentiodynamic polarization studies showed that the corrosion resistance of the alloys was in increasing order of nanotubular < bare < nanoporous. The lower corrosion resistance property obtained for the nanotubular alloy was suggested to be associated with the distinctly separated barrier oxide/concave shaped tube bottom interface. EIS studies indicated

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Table 3 EIS fit parameters from Fig. 8. Sample

R (X cm2)

R1 (MX cm2)

C1 (lF cm 2)

n

R2 (MX cm2)

C2 (lF cm 2)

n

Bare Nanoporous Nanotubular

25.76 32.70 44.95

2.90 1.12 0.0026

10.38 4.667 9.605

0.907 0.948 0.674

– 0.0364 1.807

– 7.919 3.407

– 0.748 0.762

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