Electrochemical Properties of a Novel Î²-Ta2O5 Nanoceramic Coating

Article pubs.acs.org/journal/abseba

Electrochemical Properties of a Novel β‑Ta2O5 Nanoceramic Coating Exposed to Simulated Body Solutions Jiang Xu,*,†,‡ Wei Hu,† Song Xu,§ Paul Munroe,§ and Zong-Han Xie*,‡,∥ †

Department of Material Science and Engineering, Nanjing University of Aeronautics and Astronautics, 29 Yudao Street, Nanjing 210016, P. R. China ‡ School of Mechanical & Electrical Engineering, Wuhan Institute of Technology,693 Xiongchu Avenue, Wuhan 430073, P. R. China § School of Materials Science and Engineering, University of New South Wales, Sydney, New South Wales 2052, Australia ∥ School of Mechanical Engineering, University of Adelaide, Adelaide, South Australia 5005, Australia ABSTRACT: To enhance the corrosion resistance, biocompatibility and mechanical durability of biomedical titanium alloys, a novel β-Ta2O5 nanoceramic coating was developed using a double glow discharge plasma technique. The surface morphology, phase composition and microstructure of the asdeposited coating were examined by atomic force microscopy (AFM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The coating exhibits a striated structural pattern along the growth direction, which consists of equiaxed β-Ta2O5 grains, 15−20 nm in diameter in cross-section, showing a strong (001) preferred orientation. The mechanical properties and contact damage resistance of the β-Ta2O5 coating were evaluated by nanoindentation. Additionally, scratch tests were performed to evaluate the adhesion strength between the β-Ta2O5 coating and the Ti-6Al-4V substrate. The β-Ta2O5 coating shows high hardness combined with good resistance to both indentation and scratch damage, thus favoring it for long-term load-bearing application in the human body. Electrochemical behavior of the coating was analyzed in both a 0.9 wt % NaCl solution and Ringer’s solution at 37 °C, by various electrochemical analytical techniques, including potentiodynamic polarization, electrochemical impedance spectroscopy, potential of zero charge and Mott−Schottky analysis. Compared with uncoated Ti-6Al-4V and commercially pure tantalum, the β-Ta2O5 coating possesses a more positive Ecorr and lower icorr in both aqueous solutions, which is attributed to the thicker and denser β-Ta2O5 coating that provides more effective protection against corrosive attack. In addition, the β-Ta2O5 coating shows stable impedance behavior over 5 days immersion under both simulated body solutions, corroborated by the capacitance and resistance values extracted from the EIS data. Mott−Schottky analysis reveals that the β-Ta2O5 coating shows n-type semiconductor behavior and its donor density is independent of immersion time in both aqueous solutions. Its donor density is of the order of 1 × 1019 cm−3, which is an order of magnitude less than that of the passive films formed on either commercially pure Ta or uncoated Ti-6Al-4V. Moreover, according to the differences between corrosion potential and potential of zero charge, the β-Ta2O5 coating exhibits a greater propensity to repulse chloride ions than both commercially pure Ta and uncoated Ti-6Al-4V. Therefore, the newly developed coating could be used to protect the surface of biomedical titanium alloys under harsh conditions. KEYWORDS: tantalum pentoxide, biomedical titanium alloy, corrosion behavior, simulated body solutions, EIS

1. INTRODUCTION

complicated electrolyte environment, typically having a concentration of ∼1 wt % NaCl. Hence, an important topic concerning the performance of medical devices fabricated from this alloy is degradation of the metallic implant being surrounded by physiological media.9 It is generally accepted that the excellent corrosion resistance of Ti-6Al-4V derives from the spontaneous formation of a highly stable passive film consisting mostly of amorphous

Since titanium alloys were first commercially introduced into orthopedics in the early 1960s, they have gained much popularity as one of the most commonly used materials for medical devices, including dental and orthopedic applications.1−4 This is because of their unique combination of high specific strength, good corrosion resistance and biocompatibility.5−7 Ti-6Al-4V was the first Ti alloy registered as an implant material in ASTM standards8 and remains the most frequently, and successfully used, titanium alloy in orthopedic and dental applications. In these applications, Ti-6Al-4V is often exposed to physiological fluids that present an extremely © XXXX American Chemical Society

Received: September 5, 2015 Accepted: November 30, 2015

A

DOI: 10.1021/acsbiomaterials.5b00384 ACS Biomater. Sci. Eng. XXXX, XXX, XXX−XXX

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ACS Biomaterials Science & Engineering

for applications as coatings on medical devices and dental implants by improving osseointegration,29 antibacterial properties,30 hemocompatibility,31 and corrosion resistance.32 For example, Wang et al.28 investigated the influence of Ta2O5 nanotube-based films deposited on pure tantalum on properties related with anticorrosion, protein adsorption, and associated biological function of rabbit bone mesenchymal stem cells. They found that these films can improve these properties compared to uncoated tantalum. However, the deposition methods previously employed to prepare Ta2O5 films, such as ion implantation (PIII), CVD and PVD techniques,32,33 generate very thin film thicknesses of less than 100 nm, which are not conducive to creating durable prosthetic implants. In the present work, a dense and adhesive β-Ta 2O 5 nanoceramic coating was fabricated onto a Ti-6Al-4V substrate using a double glow discharge plasma technique. The microstructure of the as-deposited coating was characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM). The corrosion behavior of the coating was evaluated by various electrochemical methods in both a 0.9 wt % aqueous solution and Ringer’s solution at 37 °C, which are common media to simulate human body fluids for corrosive evaluation. For comparative purposes, these electrochemical measurements were also performed on commercially pure tantalum and uncoated Ti-6Al-4V. By providing a detailed mechanistic understanding of the electrochemical behavior of the β-Ta2O5 nanoceramic coating, our study is expected to lay a solid basis for the biomedical application of this novel coating.

titanium dioxide (TiO2). This plays a crucial role in blocking the release of ions and reducing corrosion rates in environments of various degrees of aggressiveness. Even so, the protective oxide film can suffer localized corrosion breakdown in aqueous corrosive media containing halide anions (such as chloride and fluoride ions).10 Further, vanadium oxide, present as discrete clusters embedded in the passive TiO2 layer, may act as a conduction channel, and consequently promote galvanic coupling between the α and β phases in the alloy, leading to preferential corrosion of the V-rich β phase.11The dissolution and release of V and Al ions by corrosion processes into the adjacent implant tissues may not only induce inflammation reactions around the implant resulting in mechanical failure of the implant device,12 but also cause long-term health problems, such as senile dementia, neurological disorders and allergic reactions.13 Therefore, it is necessary to improve the corrosion resistance of Ti-6Al-4V, which in turn can lead to enhancement in both durability and biocompatibility. Furthermore, load-bearing orthopedic implants made from titanium alloys are sometimes subjected to wear/abrasion by hard tissues at contact surfaces. The mechanical forces generated could compromise the integrity of passive oxide film and accelerate corrosion of implant material. Consequently, wear resistance is another critical issue for implanted materials.14 However, titanium alloys exhibit a large coefficient of friction and poor wear resistance. They are easily deformed by shear stresses, originating from their low surface hardness.15 Thus, implanted titanium alloys usually need to be replaced every 10−15 years from body fluid environments, where they may undergo harsh attack from the synergistic effects of wear and corrosion.16 For example, low wear resistance often leads to one of the potential risks of periprosthetic bone loss, and the resultant wear debris is found to cause inflammatory reactions.17 As a result, corrosion and wear resistance are principal factors determining the lifetime of implants.18 Because the surface properties of Ti-6Al-4V are responsible for its degradation processes, such properties can be tailored by surface modification techniques. In this respect, a number of surface modification methods, including hydrogen peroxide treatment,19 microarc oxidation,20 pulsed electron-beam irradiation,21 arc ion plating22 and thermo-diffusion treatment,23 have been carried out to address these challenges for titanium alloys, while preserving the favorable bulk properties of the substrate. To exploit the potential application of nanostructured coatings for surface protection, recently the double cathode glow discharge technique was successfully employed to improve the mechanical and electrochemical properties of titanium alloys through the development of nanocrystalline transition metal silicide or nitride coatings.24,25 In search of capable ingredients for making protective coatings for biomedical applications, tantalum has attracted considerable attention due to its high radiopacity and histocompatibility, as well as its ability to form bonds with bone.26 Moreover, the corrosion resistance of Ta implants is far superior to that of Ti implants.27 Its excellent corrosion resistance is attributed to the formation of a dense Ta2O5 protective film that grows spontaneously in air. This film is highly stable over the entire potential-pH range confirmed by Pourbaix diagram.28 Because the high cost and density of Ta make its direct clinical application in a bulk form difficult, tantalum nitrides or oxides are particularly suitable for use as implant coatings to improve the performance of the substrate. Recently, tantalum pentoxide, Ta2O5, has shown great potential

2. MATERIALS AND METHODS 2.1. Substrate Preparation. Disk-shaped specimens with Φ40 × 3 mm in size were cut from a medical grade Ti-6Al-4V alloy bar. The chemical composition of Ti-6Al-4V in wt % is given as Al, 6.42; V, 4.19; Fe, 0.198; O, 0.101; C, 0.011; N, 0.006; and Ti, the balance. The samples were ground with 240, 400, 800, 1000, 1500, and 2000 grit waterproof silicon carbide papers, followed by a final polishing with a diamond suspension down to 0.05 μm to achieve a smooth surface. Prior to sputter deposition, the substrates were ultrasonically cleaned in acetone, alcohol and distilled water successively and dried. 2.2. Coating Specimen Preparation. Tantalum pentoxide coating was deposited onto the polished Ti-6Al-4V substrates inside a double cathode glow discharge apparatus. One cathode acted as the target, which was a 99.99% pure Ta disk, with a diameter of 100 mm and thickness of 5 mm, and the other was the substrate. Prior to the deposition, the surfaces of the samples were further cleaned by Ar ion bombardment at −650 V for 10 min to remove residual surface contaminants. The base pressure in the chamber was evacuated to 5 × 10−4 Pa to avoid contamination during the coating process. The deposition pressure was set at 35 Pa consisting of Ar and O2 gas mixture, with an Ar:O2 flux ratio of 10:1. When voltages were applied to the two cathodes, glow discharge occurred, as described elsewhere.24 Using orthogonal test design, deposition parameters for the coating were optimized and are given as follows: target electrode bias voltage with direct current, −750 V; substrate bias voltage with impulse current, −275 V; substrate temperature, 800 °C; target/ substrate distance, 10 mm; and treatment time, 1.5 h. 2.3. Materials Characterization. The surface morphologies of the specimens before and after deposition were analyzed using an atomic force microscope (AFM, VeecoNanoscope V) equipped with NanoScope imaging software (Digital Instruments, Inc.). An area of 1 μm × 1 μm was scanned under contact mode with a NSC36 tip and scan rate 1.0 Hz for each measurement. Topographic images were collected for each of the tested samples at five different positions to B


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this study, all electrode potentials were referred to the SCE. Polarization measurements started from the moment when the open circuit potential (OCP) reached its steady state. The potential scan started at 100 mV more cathodic than open circuit potential (OCP) increasing toward the anodic direction at a constant sweep rate of 20 mVmin−1, up to 1.5 VSCE. Before starting the AC electrochemical experiments, the open-circuit potential (OCP) was measured in order to reach a stable potential. Then, EIS measurements were performed at OCP, in a frequency range between 1 × 105 and 1 × 10−2 Hz, with an acquisition of 12 points per decade of frequency and the amplitude of the exciting signal set at 10 mV. Before undertaking Mott−Schottky analysis, the reference Ti-6Al-4V alloy and commercially pure tantalum were potentiostatically polarized at +0.10 V for different times (0, 24, 48, 96, and 120 h) to form a steady-state passive layer. After that, the capacitance measurements were performed by sweeping the applied potential in the negative direction from +0.10 V to −0.80 V with potential steps of 50 mV at a fixed frequency of 1 kHz. For the measurements of the potential of zero charge (PZC), a frequency of 18 Hz and an AC disturbance signal of 10 mV were employed.34 Each type of electrochemical measurement was repeated at least three times.

evaluate surface roughness, and the average value was used in this paper. The crystalline structure of the as-deposited coating was characterized by X-ray diffraction (XRD) using a D8 ADVANCE diffractometer with Cu Kα irradiation (λ = 0.154060 nm) operating at 35 kV and 40 mA. X-ray spectra were collected over scanning angles ranging from 20 to 80° with a step rate of 0.05° s−1.The cross-section of the as-deposited coating was etched with Kroll’s reagent (10 mL HNO3, 4 mL HF and 86 mL H2O) for 15−25 s to reveal the coating/ substrate interface. The cross-sectional morphology and chemical composition of the Ta2O5 coating were determined using a field emission scanning electron microscope (FESEM; Hitachi, S-4800, Japan) equipped with an energy-dispersive X-ray (EDS) analyzer (EDX-4; Philips). Transmission electron microscopy (TEM) was performed using a JEOL JEM-2010 microscope operated with an accelerating voltage of 200 kV. For TEM investigation plan-view specimens were prepared by cutting, grinding, dimpling and single-jet electropolishing from the untreated side of the substrate. Cross-sectional TEM specimens were prepared using a focused ion beam (FIB) microscope (FEI xP200, FEI Company, Hillsboro, OR). Both the elemental concentration and chemical state of the coatings were analyzed by X-ray photoelectron spectroscopy (XPS) using a Kratos AXIS Ultra ESCA System containing an Al Kα X-ray source with an energy of 1486.71 eV. The accelerating voltage and emission current of the X-ray source were kept at 12 kV and 12 mA, respectively. The base pressure of the sample analysis chamber was maintained at ∼10−10 Torr. The pass energy was selected at 80 eV, for survey scans, and 10 eV, for feature scans, to ensure both high resolution and good sensitivity. After subtracting the background signal, the spectra were fitted using both Gaussian and mixed Gaussian/Lorentzian functions. Peak positions were then calibrated with respect to the C 1s peak at 284.8 eV from hydrocarbon contamination. Peak identification was performed by reference to the NIST XPS database (V4.0). 2.4. Nanoindentation and Scratching Tests. Nanoindentation tests were conducted on the as-deposited coating using a nanoindentation tester (NHT) equipped with a Berkovich diamond tip. This system comprises two main components: a measuring head for performing indentation and an optical microscope for selecting viewing indentation sites. The system has load and displacement resolutions of 10 μN and 1 nm, respectively. Fused silica was used as a standard material for the tip calibration. The indentation was performed by driving the indenter at a constant loading rate of 40 mN/min into the material surface with the maximum applied load of 8 mN. Hardness and elastic modulus data were derived from the load− displacement curves of at least five indentations to ensure reproducibility of the experimental data. The adhesion strength between the as-deposited coating and the Ti6Al-4V substrate was measured using a scratch tester (WS-97), equipped with an acoustic emission (AE) detector. The scratch tests were performed by drawing a Rockwell C diamond indenter, 200 μm in radius, across the coating surface under a normal load increasing linearly from 0 to 100 N. The loading rate was set at 20 N/min and the scratch speed was 1 mm/min. An AE sensor was attached near the diamond indenter tip to detect the acoustic signals emitted from coating damage. The minimum load at which a sudden increase in the intensity of the acoustic signals occurs is commonly defined as the critical load (Lc) that represents the coating adhesion strength. 2.5. Electrochemical Measurements. Electrochemical measurements were evaluated in both naturally aerated 0.9 wt % NaCl and Ringer’s physiological solution (NaCl 8.61 g/L, CaCl2 0.49 g/L, KCl 0.30 g/L) to simulate the human body fluid using a CHI660C electrochemical workstation. The temperature of both solutions was controlled to be at 37 ± 0.5 °C (i.e., normal body temperature). A standard three-compartment cell was used with a working electrode, a platinum sheet as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. Each specimen was used as the working electrode and was connected to a conducting wire and then embedded with nonconducting epoxy resin leaving a square surface of approximately 1 cm2 exposed to the solution. Throughout

3. RESULTS AND DISCUSSION 3.1. Microstructure and Phase Analysis. Figure 1 shows a typical XRD pattern of the as-deposited coating obtained by

Figure 1. XRD pattern recorded from the as-deposited β-Ta2O5 coating (green line) and the standard XRD pattern for orthorhombic Ta2O5 (JCPDS card No. 08−0255 (red line)).

reactive sputtering at an Ar:O2 flux ratio of 10:1. The diffraction peaks located at 22.9, 28.3, 36.5, 46.5, 55.3, and 72.7° match well with the (001), (110), (111), (002), (021), and (122) planes of the orthorhombic β-Ta2O5 phase, according to reference powder diffraction file (PDF) data (Card 08−0255). Significant peak broadening, together with relatively low peak intensity, was observed for the as-deposited coating that is characteristic of nanocrystalline materials. Chang et al.35 observed that crystalline β-Ta2O5, synthesized by pulsed magnetron sputtering with postdeposition annealing, showed better viability of human skin fibroblast cells compared to an amorphous Ta2O5 coating. Moreover, it is evident that the diffraction peak intensity of the (001) reflection is significantly stronger than that of the corresponding powder diffraction pattern, indicating that the Ta2O5 coating is strongly textured with (001) crystallographic planes parallel to the coating surface. From the intensity data, the preferred orientation of the Ta2O5 coating was evaluated in terms of texture coefficient (TChkl) expressed through the following equation:36 TChkl = C

Im(hkl)/Io(hkl) 1 n

n

∑1 Im(hkl)/Io(hkl)

(1)


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respectively, suggesting that the coating shows a strong (001) preferred orientation. To investigate the composition and chemical states for O and Ta in the coating, XPS analysis was carried out on the surface of the β-Ta2O5 coating before and after sputtering with a 3 keV Ar+ ion beam for 600 s. From the XPS survey spectra (Figure 2a) recorded from the β-Ta2O5 coating, dominant characteristic peaks for both oxygen (O 1s) and tantalum (Ta 4f, Ta 4p, Ta 4d, and Ta 4s) were observed, together with a minor C 1s peak. The presence of C 1s peak at binding energy of 284.6 eV arises from a contaminant hydrocarbon layer covering the specimen surface and this peak disappeared after Ar+ sputtering for 600 s. High-resolution XPS spectra for both Ta 4f and O 1s for the Ta2O5 coating, obtained from the surface of the β-Ta2O5 coating before and after low energy Ar+ sputtering for 600 s, are shown in Figure 2b, c, respectively. For the Ta 4f spectra, a doublet with the Ta 4f7/2 and Ta 4f5/2 peaks located at 28.2 and 26.3 eV, respectively, with a spin orbit splitting of 1.9 eV and a theoretical Ta 4f7/2/Ta 4f5/2 area ratio of 4:3, is typical of the Ta chemical states in stoichiometric Ta2O5. The spectra for O 1s appear as a single peak located at a binding energy of 530.7 eV, which is assigned to the Ta−O bonds in tantalum oxide.38 On the basis of the quantification of the XPS data, the O/Ta ratio is 2.49 ± 0.3, which corresponds well to the atomic proportion of Ta and O in Ta2O5. Figure 3 presents three-dimensional AFM images of the surface morphologies of the polished Ti-6Al-4V substrate before and after sputter deposition. Some parallel grooves induced from mechanical polishing are clearly visible on the surface of the bare substrate, whereas the surface of the Ta2O5 coating is composed of many regularly spaced protuberances with a diameter of 25.0 ± 2 nm. There is a slight increase in average surface roughness (Ra) from 3.0 nm for the polished Ti-6Al-4V substrate to 4.6 nm after sputter deposition. Figure 4 shows a typical SEM cross-sectional image and the corresponding EDS elemental maps for Ta, O and Ti for the βTa2O5 coating deposited on a Ti-6Al-4V substrate. From the cross-sectional SEM micrograph (Figure 4a), it can be seen that the β-Ta2O5 coating, with a uniform thickness of ∼25 μm, exhibits a homogeneous and dense microstructure, and is tightly adhered to the Ti-6Al-4V substrate without any visible defects. EDS mapping analysis (Figure 4c, d) reveals that O and Ta elements are distributed evenly across the coating. In general, ceramic coatings prepared by traditional sputtering methods may peel off or develop cracks when their thickness is larger than a certain threshold value (usually less than ∼10 μm). This typically originates from large residual stresses intrinsically produced during deposition, which weakens the bonding strength between the coating and the substrate.39 It is noteworthy that the β-Ta2O5 coating is both ultrathick and of high quality, which may be attributable to the higher deposition temperature used in the present study, as compared to other deposition techniques.40,41 A higher deposition temperature is supposed to have two advantages for the coating growth: (a) The surface mobility of the sputtered atoms was governed by deposition temperature. The higher deposition temperature supplies adequate energy to deposition atoms and enhances their mobility, allowing the atoms to diffuse to more equilibrium positions and to overcome self-shadowing effect exerted by the previously deposited atoms. This can favor the fabrication of a coating with dense microstructure and improve the quality of coatings. (b) The higher deposition temperature promotes the release of the internal stresses in the growing

Figure 2. (a) Typical XPS survey spectra from the β-Ta2O5 coating and the core level spectra of (b) Ta 4f and (c) O 1s before and after low energy Ar+ sputtering for 600 s. The continuous line is in b and c is the fitting of the experimental curves.

where Im(hkl) is the measured X-ray relative intensity of the (hkl) plane, I0(hkl) is the relative intensity in the powder pattern, (hkl) represents the indices of the reflection plane and n is the number of reflection planes. A TChkl value greater than 1 suggests that a preferred orientation has been developed toward a specific crystalline plane, while a TChkl value close to 1 signifies a more random orientation and a TChkl value in the range from 0 to 1 denotes a lack of grain orientation for the specific plane under consideration.37 The calculated texture coefficient values for the (001), (110), (111), (002), (021), and (122) planes are 2.33, 0.51, 0.86, 0.84, 0.53, and 0.83, D


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Figure 3. AFM surface morphology of polished Ti-6Al-4V (a) before and (b) after sputter deposition of the β-Ta2O5 coating.

Figure 4. (a) SEM cross-sectional image of the β-Ta2O5 coating and the corresponding EDS elemental maps for (b) Ti, (c) O, and (d) Ta. The inset in a shows an EDS spectrum taken from region shown in a marked by “+”.

Figure 5. (a) Bright-field TEM image of a specimen obtained from the intermediate layer of the β-Ta2O5 coating (marked by “A” in Figure 4a) and the corresponding EDS elemental maps for (b) O and (c) Ta; (d) typical cross-sectional bright-field TEM image; (e) highmagnification images of the area identified in d.

coating. Consequently, a thicker β-Ta2O5 coating would provide greater protection against mechanical and corrosion damage in biomedical applications. To reveal the coating microstructure as a function of coating depth, both cross-sectional and plan-view TEM images of the β-Ta2O5 coating were acquired. Figures 5 and 6 show TEM cross-sectional micrographs taken from the intermediate layer and coating/substrate interface marked as squares A and B in Figure 4a. As shown in Figure 5d, the intermediate layer is composed of Ta2O5 having a striated structural pattern along the growth direction and higher magnification images reveal it consists of a tight network of elongated grains (Figure 5e). Inspection of the substrate/coating interface (Figure 6) further proves the good integrity and adhesion of the coating to the Ti6Al-4V substrate. From a more magnified view (Figure 6b), it is observed that there is an apparent microstructural transition from fine grains in the coating to coarser grains in the substrate with a clean and continuous interface. Plan-view TEM brightfield/dark-field images of the intermediate layer for the β-Ta2O5 coating are shown in Figure 7a, b, together with a corresponding selected area electron diffraction (SAED)

pattern (Figure 7c). Indeed, the striated architecture is composed of equiaxed grains about 15−20 nm in diameter. An intense β-Ta2O5 (001) ring in the SAED pattern indicates that the β-Ta2O5 coating grew predominantly with the c-plane perpendicular to the substrate surface, which is in good agreement with the XRD results shown in Figure 1. From the bright-field HRTEM lattice image (Figure 7d), the fringe spacing of the equiaxed crystallites, outlined by dotted blue circles, was calculated to be 3.89 Å, which corresponds to the (001) lattice plane of orthorhombic β-Ta2O5. 3.2. Nanoindentation and Scratch Tests. Nanoindentation testing has been routinely utilized to determine the mechanical properties and deformation behavior of hard coatings. Figure 8 shows representative load−displacement curves for both the β-Ta2O5 coating and bare Ti-6A1-4V. Compared to the bare Ti-6Al-4V, the β-Ta2O5 coated sample shows a smaller maximum indent depth, indicating that the βTa2O5 coating has a greater resistance to local plastic deformation. The hardness and elastic modulus of the βE


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Figure 8. Load−displacement curves for the β-Ta2O5-coated sample and uncoated Ti-6Al-4V. Figure 6. (a) Bright-field TEM image of the β-Ta2O5 coating/ substrate interfacial region and (b) higher-magnification images of selected area (marked B shown in Figure 4a). Corresponding EDS elemental maps for (c) Ta and (d) Ti.

Figure 9. Acoustic emission signal versus normal load for the β-Ta2O5 coating together with the corresponding SEM micrographs of the scratch track (inset).

The adhesion strength between the coating and the substrate affects the integrity and durability of coating/substrate systems when used in practice. The scratch test is commonly used to evaluate the adhesion strength of coating/substrate through the measurement of the critical load (Lc). Lc represents the force at which delamination of the coating occurs, identified by combined acoustic emission signals and visual examination of the scratch. Figure 9 shows the acoustic emission signals as a function of the normal load. It can be seen that when the applied normal loads is larger than 80 N, a strong burst of acoustic emission signals is observed. The microscopic evaluation of scratch morphology (inset in Figure 9) reveals that after the applied normal load is above 80 N, large-scale flakes appeared on one side of the scratch trace, indicating that the critical load for the β-Ta2O5 coating is ∼80 N. As a general rule, a critical load of above 30 N measured with a Rockwell C diamond tip in scratch testing is believed to be sufficient for engineering applications.42 Therefore, the β-Ta2O5 coating has potential for load-bearing application in the human body. 3.3. Electrochemical Corrosion. 3.3.1. Open Circuit Potentials and Potentiodynamic Polarization Tests. Figure 10a, b show the variations of open circuit potential (EOCP) measured as a function of immersion time for the β-Ta2O5 coated and the uncoated Ti-6Al-4V alloy and commercially pure tantalum in both naturally aerated 0.9 wt % NaCl and Ringer’s simulated physiological solution at 37 °C, respectively. The OCP−time curves can reflect the electrochemical processes taking place on the samples during immersion. It is clear from all the curves that the EOCP is rapidly drifting toward

Figure 7. Plan-view (a) bright-field and (b) dark-field TEM images of the β-Ta2O5 coating; (c) the corresponding selected area electron diffraction (SAED) pattern and (d) high-resolution TEM image of the β-Ta2O5 coating.

Ta2O5 coating were determined to be 21 ± 1.30 and 282 ± 2.43 GPa, respectively, which are markedly higher than that of the Ta2O5 films (H = 9−11 GPa; E = 165−171 GPa) prepared by rf magnetron sputtering.41 Compared with the bare Ti-6Al4V, the hardness of the Ta2O5-coated Ti-6Al-4V has been increased by a factor of ∼3.5. Generally speaking, the higher the hardness, the lower the expected wear loss. Consequently, it is expected that the β-Ta2O5 coating can substantially increase the wear resistance of Ti-6Al-4V, which is one of the most important requirements for protective coatings of load-bearing implants. F


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Figure 10. Open circuit potential (EOCP) versus immersion time curves for the β-Ta2O5 coating, commercially pure Ta and uncoated Ti-6Al-4V in (a) 0.9 wt % NaCl and (b) Ringer’s simulated physiological solution at 37 °C.

Figure 11. Typical potentiodynamic polarization curves for the as-deposited β-Ta2O5 coating, commercially pure tantalum, and uncoated Ti-6Al-4V in (a) 0.9 wt % NaCl and (b) Ringer’s simulated physiological solution at 37 °C.The corresponding mixed potential theory applied to schematically interpret the changes in Ecorr and icorr for (c) 0.9 wt % NaCl solution and (d) for Ringer’s solution. Curve a is for the anodic semireaction, and curve c is for the cathodic semireaction.

Figure 11a, b shows typical potentiodynamic polarization plots of the β-Ta2O5 coated and the uncoated Ti-6Al-4V alloy and commercially pure Ta in both naturally aerated 0.9 wt % NaCl and Ringer’s simulated physiological solution at 37 °C, respectively. The corrosion potential (Ecorr), corrosion current density (icorr), passive current density (ip), and anodic (βa) and cathodic (βc) Tafel slopes were derived from the polarization curves using Tafel analysis and are summarized in Table 1. It appears that there are close similarities in the polarization behaviors for the three tested samples in both solutions investigated, because there is no distinct variation in the shape of the polarization plots. Since the measurements were carried out in the aerated condition, the cathodic current density can be determined as the reduction of dissolved oxygen and water.43 As shown in Figure 11, the three test samples change

more anodic potentials and then gradually reaches a plateau with increasing immersion time. For both uncoated Ti-6Al-4V and commercially pure tantalum, a significant increase in EOCP after a few hundred seconds is due to the formation and growth of stable oxide films on their surfaces upon exposure to the two aqueous solutions, whereas for the β-Ta2O5 coating only a slight potential shift in the initial stage of the measurement is observed. This is because the β-Ta2O5 coating itself is an inert material and the modification of the β-Ta2O5 coating during the early stages of immersion is negligible. The β-Ta2O5 coated Ti6Al-4V has the highest steady-state EOCP value among the test samples, suggesting that the β-Ta2O5 coating is more stable than the spontaneous formation of passive oxide films on either the bare Ti-6Al-4V alloy or commercially pure tantalum in both aqueous solutions. G


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3.3.2. Electrochemical Impedance Spectroscopy (EIS). EIS is a nondestructive and sensitive technique that has been widely used to characterize the corrosion behavior of coated metallic surfaces in corrosive aqueous media.44−46 Generally, implant materials are usually in contact with body fluids for relatively long periods, and the corrosion resistance of a coating may vary with increasing immersion time due to potential damage to the coating.45,46 To gain a deeper insight into the influence of immersion time on stability of the Ta2O5 coating and to predict its corrosion protective lifetimes in simulated physiological conditions, the electrochemical behavior of the β-Ta2O5 coating was investigated over an extended immersion time by EIS. Figures 12 and 13 show the Nyquist and Bode plots for the βTa2O5 coating collected over 5 days of immersion at EOCP in both naturally aerated 0.9 wt % NaCl and Ringer’s simulated physiological solution at 37 °C, respectively. These were compared to that of both the uncoated Ti-6Al-4V alloy and commercially pure tantalum. As shown in Figures 12 and 13, all the Nyquist plots possess a similar behavior that is characterized by a single capacitive loop with a large incomplete semicircle in the entire frequency range studied, which represents the typical capacitive response of a passive film.47,48 It is apparent that the β-Ta2O5-coated Ti-6Al-4V show a much larger diameter of the capacitive loop than the uncoated Ti-6Al-4V alloy and commercially pure tantalum over 5 days of exposure, demonstrating a nobler electrochemical behavior of the β-Ta2O5-coated Ti-6Al-4V in both aqueous solutions. This is in good agreement with the results from potentiodynamic observations. Over the frequency range of measurement, all of the Bode plots are also quite similar in shape with three distinguishable characteristic regions, and the shape is maintained throughout the entire immersion time, except for uncoated Ti-6Al-4V after an immersion time of 5 days, indicating that almost no significant change in the corrosion mechanism occurred for all these test specimens. In the high frequency range, |Z| impedance magnitude is almost independent of frequency with a phase angle close to 0°, indicating that the impedance is dominated by solution resistance between the working and reference electrodes in this frequency range. In the medium frequency region, log |Z| varies linearly with the log f with a linear slope of about −1 in the Bode magnitude, whereas the Bode-phase plots exhibit plateaus of phase angles close to −90° that is independent of frequency, indicative of the characteristic response of a capacitive behavior.49 Such a capacitive behavior derives either from the typical inert oxide film with a low conductivity presented on the uncoated samples or from the coating deposited on samples, which behaves as an efficient barrier to corrosion, and increase the resistance to charge transfer at the material/electrolyte interface.50,51 In the low frequency region, the phase angle decreases slightly toward lower value because of the contribution of surface film resistance to the impedance. Usually, the corrosion rate can be estimated by the value of | Z|f→0 in the modulus Bode plots. According to the value of | Z|f→0, the corrosion rates of the test samples increase in an order of the β-Ta2O5 coating < commercially pure Ta < Ti-6Al4V in both aqueous solutions. On the other hand, in terms of the phase angle maximum and the width of plateaus corresponding to the phase angle near −90°, which reflect the protective ability of the passive layer on the test samples, the same order as that in the |Z|f→0 is observed. With increasing immersion time, there is no significant difference in the profiles of the Nyquist and Bode plots for the β-Ta2O5 coating and pure

Table 1. Electrochemical Parameters Derived from the Potentiodymaic Polarization Curve of the Investigated Specimens in 0.9 wt % NaCl and Ringer’s Simulated Physiological Solution sample solutions 0.9% NaCl solution

Ringer’s solution

parameter Ecorr (VSCE) βa(V/ decade) −βc(V/ decade) icorr(A cm−2) ip (A cm−2) Ecorr (VSCE) βa (V/ decade) −βc (V/ decade) icorr (A cm−2) ip (A cm−2)

Ta2O5 coating

pure Ta

Ti-6Al-4V

−0.28 313.47

−0.33 277.11

−0.43 207.04

129.58

149.37

141.25

9.51 × 10−8

3.32 × 10−7

5.88 × 10−7

6.16 × 10−7 −0.24 297.09

9.33 × 10−6 −0.36 245.87

1.31 × 10−5 −0.45 189.36

118.83

127.31

127.95

1.06 × 10−7

3.85 × 10−7

5.28 × 10−7

7.08 × 10−7

9.54 × 10−6

1.32 × 10−5

directly from the “‘Tafel region”’ to a stable passivation range until the potential reaches 1.5 VSCE, with no evidence of an active-passive transition, indicating that they exhibit a typical self-passivation characterization in both aqueous solutions with a protective film on their surface at corrosion potential (Ecorr).44 Compared with the reference bare Ti-6Al-4V alloy and commercially pure tantalum, the Ecorr of the β-Ta2O5 coating shifts toward the noble direction, in the direction of what was seen from OCP vs time plots, implying a reduced probability of corrosion as a result of the broadening of the cathodic potential range, while the displacement of the anodic branches for the βTa2O5 coating move to the region of lower current density, denoting that the dense and thick β-Ta2O5 coating is more likely to hinder electrochemical anodic reactions and thus results in a lower corrosion rate in both aqueous solutions. According to the mixed potential theory and the anodic (βa) and cathodic (βc) Tafel slopes shown in Table 1, the cathodic and anodic branches shift following the order of c1 → c2 → c3 and a1 → a2 → a3, respectively, for the uncoated Ti-6Al-4V substrate, commercially pure Ta and the β-Ta2O5 coating in both aqueous solutions, as shown in Figure 11c, d. Therefore, the β-Ta2O5 coating exhibits a more positive Ecorr and lower ip and icorr than uncoated Ti-6Al-4V and pure tantalum in both solutions, suggesting that the β-Ta2O5 coating possesses better barrier properties and thus provides more effective protection against corrosive attack. It is also worth noting that the corrosion potentials obtained from the potentiodynamic curves are lower than the values of EOCP, since the two methods used to acquire the electrochemical measurements employ different experiment conditions. Prior to the potentiodynamic sweep, the specimen was polarized cathodically for 10 min at −1.0 VSCE, which removed part of the oxide layer spontaneously formed on the specimen surface. Therefore, the corresponding Ecorr obtained from the potentiodynamic curve is more negative than the potential determined under open circuit conditions (EOCP value). Moreover, all test samples have a higher corrosion resistance in 0.9% NaCl solution than they exposed to Ringer’s solution, because of the fact that a higher concentration of chlorine ion in Ringer’s solution enhances the corrosion of the test samples. H


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Figure 12. Nyquist and Bode plots of impedance spectra for the β-Ta2O5 coating, commercially pure Ta, and uncoated Ti-6Al-4V collected over 5 days of immersion time at their EOCP in 0.9 wt % NaCl solution at 37 °C. The Ta2O5coating: (a) Nyquist plot, (b) Bode plot; commercially pure Ta: (c) Nyquist plot, (d) Bode plot; uncoated Ti-6Al-4V: (e) Nyquist plot, (f) Bode plot.

Ta, implying that both of them shows consistent and stable impedance behavior as a function of exposure time. On the contrary, the impedance of bare Ti-6Al-4V gradually decreases with increasing immersion time, as evidenced by a reduction in the low-frequency limit for the impedance modulus |Z|f→0 and the phase angle maximum in the intermediate frequency region. This continuous impedance reduction for bare Ti-6Al-4V suggests a decrease in protective capability of the passive oxide films grown on its surface as immersion time increases. To provide a sound interpretation of the impedance spectra and a deep understanding of the electrochemical processes occurring at the interface between the samples and the corrosive media, the quantitative analysis of the EIS data was conducted using the appropriate electric equivalent circuit

(EEC). Based on the Bode and Nyquist plots shown in Figures 12 and 13, all of the experimental impedance data can be fitted using the equivalent circuit model with one time constant (Randles model), as shown schematically in Figure 14a, except for the uncoated Ti-6Al-4V for an immersion time of 5 days. As can be seen from Figures 12f and 13f, after exposure time of 5 days, the slight asymmetry at low frequencies in the Bode-phase plots indicates the existence of another relaxation time constant, probably associated with the formation of porous passive film. This case can be modeled using an equivalent circuit with two time constants, as shown schematically in Figure 14b. Randles model has been used in previous studies to describe the impedance spectra of Ta coatings and Ta foil in acid media.52,53 In this equivalent circuit, a constant phase I


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Figure 13. Nyquist and Bode plots of impedance spectra for the as-deposited β-Ta2O5 coating, commercially pure Ta, and uncoated Ti-6Al-4V collected over 5 days of immersion time at their EOCP in Ringer’s simulated physiological solution at 37 °C.The β-Ta2O5 coating: (a) Nyquist plot, (b) Bode plot; commercially pure Ta: (c) Nyquist plot, (d) Bode plot; uncoated Ti-6Al-4V: (e) Nyquist plot, (f) Bode plot.

inhomogeneous distribution of the electrode surface properties.54 The impedance of CPE is normally expressed as ZCPE =

1 Q (jω)n

(2)

where Q is the CPE coefficient, j is the imaginary unit, and ω is the angular frequency. The factor n, defined as a CPE exponent, is an adjustable parameter that always lies between 0.5 and 1. When n = 1, the CPE represents an ideal capacitance; for 0.5 < n < 1, the CPE describes a distribution of dielectric relaxation times in frequency space; and when n = 0.5 the CPE represents a Warburg impedance with diffusional character. The physical significance of the circuit elements can be described as follows: Rs is the solution resistance between the working electrode and

Figure 14. Equivalent circuit models used to simulate the EIS data of the tested samples: (a) Rs(QbRb) model and (b) Rs(Qpf(Rpf(QbRb))).

element (CPE) is introduced instead of an ideal capacitance element to obtain a best fit, because of the surface heterogeneity originating from surface roughness, porosity, or J


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Figure 15. Electrochemical impedance parameters for the tested samples at different immersion hours in (a, c, e, g) 0.9 wt % NaCl and (b, d, f, h) Ringer’s simulated physiological solution at 37 °C. (a, b) Resistance values Rp, (c, d) capacitance values C; (e, f) time constant τ; (g, h) CPE power (n). For Ti-6Al-4V after 5 days of immersion in both simulated human body media, R0 (R0 = Rpf + Rb), Cb, τb, and nb are listed).

the reference, and Qp and Rp are representations of the capacitive and the resistive behavior of the barrier layer, respectively. The equivalent circuit model (Rs(Qpf(Rpf(QbRb)))) presented in Figure 14b assumes that the homogeneous passive film on the uncoated Ti-6Al-4V changes to a double-layer structure, consisting of a barrier-like inner layer and a porous outer layer, when it continuously

corroded during immersion in both aqueous solutions. In this circuit, Rs is the solution resistance, Rpf and Qpf are the resistance and capacitance of the outer porous layer, Rb and Qb are the resistance and capacitance of the inner barrier layer. The chi-square (χ2) values are all on the order of 1 × 10−3 to 1 × 10−4, indicative of a satisfactory fitting with the proposed equivalent circuits. K


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Figure 16. Mott−Schottky plots for the (a) the β-Ta2O5-coated Ti6Al-4V, (b )the passive films formed on commercially pure Ta, and (c) uncoated Ti-6Al-4V after potentiostatic polarization at +0.1 V for different immersion time in naturally aerated 0.9 wt % NaCl solution at 37 °C.

Figure 17. Mott−Schottky plots for the (a) the β-Ta2O5-coated Ti6Al-4V, (b) the passive films formed on the commercially pure Ta and (c) uncoated Ti-6Al-4V after potentiostatic polarization at +0.1 V for different immersion time in Ringer’s simulated physiological solution at 37 °C.

Figure 15 show the values of Rp or R0 (R0 = Rpf + Rb), Cp, τ and n deduced from the fitting of the EIS data as a function of immersion time in both 0.9 wt % NaCl and Ringer’s simulated physiological solution. As shown in Figure 15, the resistance values (Rp) of the β-Ta2O5 coated Ti-6Al-4V are almost independent of immersion time and are of the order of 1 × 107 Ω cm2. This is 1 order of magnitude higher than that for commercially pure Ta and 2 orders of magnitude higher than that for uncoated Ti-6Al-4V, suggesting that the dense and adhesive β-Ta2O5 coating has the best corrosion resistance among the three test samples. For bare Ti-6Al-4V after 5 days of immersion in both simulated human body media, the resistances of the inner barrier layer (Rb) are significantly larger than that of the outer porous layer (Rpf), revealing that the inner barrier layer is predominantly responsible for the alloy’s anticorrosive protection. The effective capacitance value of a CPE can be calculated using the expression developed by Brug et al.,55 as described below by eq 3. C = Q P1/ n(R s−1 + R b−1)(n − 1)/ n

The effective capacitance values of test samples increase in the order of the β-Ta2O5 coating < pure Ta < Ti-6Al-4V. Higher resistance and lower capacitance endow the β-Ta2O5 coating with higher insulating and protecting properties. It is important to note that for the uncoated Ti-6Al-4V, Rp (or R0) values slightly decrease with increasing immersion time, accompanied by a gradual increase in effective capacitance values, indicating that the stability of the uncoated Ti-6Al-4V deteriorates with immersion time. This phenomenon can be explained by the fact that the presence of vanadium in the titanium oxide film increases the concentration of defects due to the differences in atomic or ionic radius for both V and Ti. The vanadium oxide formed on the surface of Ti-6Al-4V alloy is susceptible to attack by chloride ions in aqueous corrosive media, which leads to the generation and diffusion of vacancies in the passive film, aggravating the corrosion dissolution process.56 Generally, the CPE power, n, can be used as an indicator of the coating/ passive film quality and their electrochemical performance. The

(3) L


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Figure 18. Donor density of the β-Ta2O5 coating, commercially pure Ta, and uncoated Ti-6Al-4V after potentiostatic polarization at +0.1 V for different immersion time in (a) 0.9 wt % NaCl and (b) Ringer’s simulated physicological solution at 37 °C.

theory, the space charge capacitance of an n-type or a p-type semiconductor can be expressed as

closer the parameter n is to unity, the nearer the coating/ passive film is to ideal capacitance, signifying that a coating/ passive film with a smoother surface and less defects exhibits higher corrosion resistance. All the values of CPE power, n, for the β-Ta2O5 coating are larger than that of commercially pure Ta and uncoated Ti-6Al-4V throughout the exposure period, indicating that the β-Ta2O5 coating prepared on Ti-6Al-4V by reactive sputter-deposition is more homogeneous and compact than the spontaneously formed oxide films on commercially pure Ta or uncoated Ti-6Al-4V in both simulated human body media. The product R × C equivalent with the time constant (τ) was employed to evaluate the rate of relevant electrochemical process.57 When one takes a reciprocal of the obtained product values, the rate of relevant electrochemical process is provided. The time constant (τ) of the test samples reflects the total rate of the electrochemical process including the ionic migration within the passive oxide film or the β-Ta2O5 coating and charge transfer process. Clearly, throughout the 5 days exposure, the time constant (τ) for the β-Ta2O5-coated Ti6Al-4V is significantly larger than that of commercially pure Ta and uncoated Ti-6Al-4V, indicating that the β-Ta2O5 coating shows a superior retarding effect on the electrochemical corrosion process. 3.3.3. Capacitance Measurements and Mott−Schottky Analysis. From a deeper perspective, regardless of what is either fabricated by reactive sputter deposition in the present study or anodically formed at the interface Ta/electrolyte, the Ta2O5 barrier layer behaves like an n-type semiconductor containing some defects, in which donors are usually some positive point defects. It is generally accepted that the electronic properties of a semiconductor are expected to be of crucial importance in understanding its protective character against corrosion, because its breakdown involves the transport of electrons and ions.58 Capacitance measurement based on Mott−Schottky (MS) theory is a convenient technique to probe the electronic properties of a semiconducting electrode in an aqueous electrolyte.14,34 The charge distribution at the interface of a semiconductor and an electrolyte is usually determined by measuring the capacitance of the space charge layer (CSC) as a function of the electrode potential (E). According to the Mott−Schottky

1 2 ⎛ kT ⎞ = ⎟ ⎜E − Efb − 2 εrεoqN q ⎝ q ⎠ Csc

(4)

where εr is the dielectric constant of the passive film(6059 and 2560 for TiO2 and Ta2O5, respectively), ε0 is the vacuum permittivity (8.854 × 10−14 F cm−1), q is the elementary charge (+e for electrons and -e for holes), Nq is the density of charge carriers (NA for acceptors and ND for donors), E is the applied potential, Efb the flat band potential, k the Boltzmann constant (1.38 × 10−23 J K−1), and T the absolute temperature (298 K). The space charge capacitance, CSC, is obtained from C = −1/ ωZ″, where Z″ is the imaginary component of the impedance and ω = 2πf is the angular frequency. The donor density is determined from the slope of the experimental 1/CSC 2 vs E plots, and the flat band potential (Efb) by the extrapolation to 1/CSC 2 = 0. Figures 16 and 17 present Mott−Schottky plots for the βTa2O5-coated Ti-6Al-4V and the passive films formed on commercially pure Ta and uncoated Ti-6Al-4V after potentiostatic polarization at +0.1 V for different immersion times in both naturally aerated 0.9 wt % NaCl and Ringer’s simulated physiological solution at 37 °C, respectively. All the Mott− Schottky plots display a linear relationship between the Csc−2 and applied potential (E) with a positive slope in the potential range from +0.1 to about −0.6 V, denoting that the Ta2O5 coating and the passive films on commercially pure Ta and uncoated Ti-6Al-4V are characteristic of an n-type semiconductor. From the linear part of the slopes, the value of the densities of donors can be calculated based on eq 4. Figure 18 displays the donor density as a function of immersion time in both naturally aerated 0.9 wt % NaCl and Ringer’s solution at 37 °C. It is obvious that the donor densities of the β-Ta2O5 coating are independent of immersion time in both aqueous solutions, and are of the order of 1 × 1019 cm−3, which is an order of magnitude less than that of the passive films formed on commercially pure Ta and uncoated Ti-6Al-4V. The donor densities of n-type semiconductors represent the number of defects, such as oxygen vacancies or cation interstitials. Because a lower carrier density in the barrier layer generally indicates a M


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Figure 19. Double-layer capacities as a function of applied potential for the tested samples in (a−c) 0.9 wt % NaCl and (d−f) Ringer’s simulated physiological solution at 37 °C. (a, d) The β-Ta2O5 coating, (b, e) commercially pure Ta, (c, f) uncoated Ti-6Al-4V.

Figure 16 and Figure 17, the flat band potential of the β-Ta2O5 coating is more negative than those for commercially pure Ta and uncoated Ti-6Al-4V. As pointed out by SzklarskaSmialowska64 and Schmidt et al.,65 the oxide film with a lower Efb shows dielectric behavior at a larger potential range and exhibits a higher resistance to pitting. As a result, one might anticipate that the electrochemical stability of the β-Ta2O5 coating is higher than that of commercially pure Ta and uncoated Ti-6Al-4V. 3.3.4. PZC Measurements. When exposed to two chloridecontaining media mimicking human body solutions, aggressive chloride ions can be transferred through the interface between the barrier layer and the solution, and preferentially occupy oxygen vacancies in an n-type semiconductor barrier layer, decreasing the concentration of the oxygen vacancies in accordance with the following reaction:

less conductive and thus better protective ability, this provides one reasonable explanation on why the β-Ta2O5 coating possesses a higher corrosion resistance in comparison with commercially pure Ta and uncoated Ti-6Al-4V.61,62 The flat band potential (Efb) is a critical parameter used to determine the positions of semiconductor energy bands with respect to the redox potentials of electro active ions in the electrolyte. These positions are mainly governed by the charge transfer across the semiconductor/electrolyte interface, the contact potential between semiconductor and electrolyte, and the stability of the semiconductor. According to the energyband model,63 for a sample covered with an n-type semiconducting passive layer, it behaves conductivity at potentials more negative than the passive layer’s flat band potential, as the passive layer enters an accumulation mode. In contrast, at potentials more positive than Efb, the band bending creates a barrier (space charge layer) to electron transfer and the sample exhibits dielectric characteristics. As shown in the insets in

Vö + Cl− ⇔ Cl ȯ N

(5) DOI: 10.1021/acsbiomaterials.5b00384 ACS Biomater. Sci. Eng. XXXX, XXX, XXX−XXX

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commercially pure Ta and uncoated Ti-6Al-4V, respectively. This result shows that the β-Ta2O5 coating can effectively protect the Ti-6Al-4V alloy from corrosion for long-term immersion in simulated body fluids. Mott−Schottky analysis and PZC measurement indicate that the β-Ta2O5 coating exhibits lower carrier density and a larger capability that can inhibit the adsorption of aggressive chloride ions. In so doing, the coating provides a greater corrosion resistance than two reference metals used when exposed to chloride ions containing media.

Where Vö and Clȯ represent the positively charged oxygen vacancy and the positively charged chloride ion occupying an oxygen lattice site, respectively. Hence, to preserve electroneutrality, the concentration of the negatively charged cation vacancies is increased through the release of cations from the barrier layer/solution interface, resulting in an enhanced electro-migration-dominated flux of cation vacancies from the barrier layer/solution interface toward the substrate/barrier layer interface, where they are annihilated by the emission of cations from the substrate into the barrier layer. If the rate of annihilation cannot accommodate the enhanced flux of cation vacancies, the excess vacancies will coalesce to form a cation vacancy condensate, causing the barrier layer to become unstable, which can lead to localized breakdown.66 Therefore, the corrosion resistance of the β-Ta2O5 coating in both chloride-containing simulated human body media depends not only on the densities of donors, but also on the resistance to the absorption of chloride ions. The potential of zero charge (PZC) is a fundamental property of the metal−electrolyte interface, and can be used to gain insight into double-layer phenomena, electrochemical kinetics and the adsorption behavior of charged and neutral species.67,68 Figure 19 shows the double-charge layer capacitance (Cdl) of the β-Ta2O5-coated Ti-6Al-4V and the passive films formed on commercially pure Ta and uncoated Ti-6Al-4V as a function of applied potential (E) in both naturally aerated 0.9 wt % NaCl and Ringer’s simulated physiological solution at 37 °C. As shown in Figure 19, the PZC values for the test samples are more positive than their Ecorr in both two solutions investigated, which implies that the surfaces of the test samples are negatively charged at their Ecorr. Consequently, it was expected that the chloride ions are expected to be expelled from their surfaces. Furthermore, from the potential differences between Ecorr and PZC (ΔE = Ecorr − PZC), it was deduced that the capability to repulse chloride ions for the test samples increases in the order of Ti-6Al-4V < commercially pure Ta < the β-Ta2O5 coating.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from the National Natural Science Foundation of China under Grant 51374130 and the Aeronautics Science Foundation of China under Grant 2013ZE52058. This study is also supported by the Australian Research Council Discovery Project (DP150102417).

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REFERENCES

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4. CONCLUSIONS A novel β-Ta2O5 coating was deposited onto a polished Ti-6Al4V substrate by reactive sputter-deposition using a double glow discharge plasma technique. The coating, with a thickness of ∼25 μm, exhibits an extremely dense and homogeneous microstructure, composed of an orthorhombic β-Ta2O5 phase with a strong (001) preferred orientation. The β-Ta2O5 coating exhibits a high hardness and good scratch resistance, which may be appropriate for load-bearing application in the human body. Electrochemical behavior of the β-Ta2O5 coating in both naturally aerated 0.9 wt % NaCl and Ringer’s solution at 37 °C was investigated by various electrochemical analytical techniques, and were compared with those of commercially pure Ta and uncoated Ti-6Al-4V. The following conclusions can be drawn: The β-Ta2O5 coating exhibits a more positive Ecorr and lower ip and icorr than both uncoated Ti-6Al-4V and commercially pure tantalum in both aqueous solutions, suggesting that the Ta2O5 coating possesses better barrier properties against corrosion attack. EIS spectra indicated that after immersion for 5 days in both of the two solutions investigated, no significant change in impedance behavior was observed for the β-Ta2O5 coating, and the values of resistance for the β-Ta2O5 coating are of the order of 107 Ω cm2, which is 1 order of magnitude and 2 orders of magnitude higher than that of the O


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Electrochemical Properties of a Novel Î²-Ta2O5 Nanoceramic Coating

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