Corrosion behaviour and microstructure of tantalum ...

Thin Solid Films 636 (2017) 54–62

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Corrosion behaviour and microstructure of tantalum film on Ti6Al4V substrate by filtered cathodic vacuum arc deposition Ay Ching Hee a, Yue Zhao a,b,⁎, Sina S. Jamali a, Philip J. Martin c, Avi Bendavid c, Hui Peng d, Xiawei Cheng a a

School of Mechanical, Materials, Mechatronic and Biomedical Engineering, University of Wollongong, NSW 2522, Australia School of Materials Science and Engineering, Tianjin University of Technology, Tianjin 300384, China CSIRO Manufacturing Business Unit, PO Box 218, Lindfield, NSW 2070, Australia d School of Materials Science and Engineering, Beihang University, Beijing 100191, China b c

a r t i c l e

i n f o

Article history: Received 18 January 2017 Received in revised form 12 May 2017 Accepted 21 May 2017 Available online 23 May 2017 Keywords: Tantalum films Filtered cathodic vacuum arc deposition X-ray photoelectron spectroscopy Electrochemical corrosion testing

a b s t r a c t Tantalum (Ta) films of about 100 nm in thickness were fabricated onto Ti6Al4V substrate using a filtered cathodic vacuum arc deposition system with and without a −100 V bias between the substrate and the chamber wall. The structure of the deposited film at zero substrate bias exhibits pure tetragonal phase (beta-Ta), and body-centred cubic structure (alpha-Ta) is found as the substrate bias increased to − 100 V. The film deposited at −100 V bias shows a significant improvement in cohesion as compared to the film deposited without substrate bias. Potentiodynamic polarization and electrochemical impedance tests in phosphate buffered saline solution revealed that the Ta film at −100 V bias shows good corrosion resistance with a low corrosion current density of 0.009 μA·cm−2. The film shows hydrophobic characteristics, low surface free energy and an inert surface presenting a uniform oxide layer, which is promising for potential biomedical implant applications. © 2017 Published by Elsevier B.V.

1. Introduction Ti6Al4V alloy has been widely utilized in biomedical implants owning to its high strength-to-weight ratio, corrosion resistance and good biocompatibility [1,2]. However, the alloy has insufficient surface wear properties and tends to degrade in sliding contact with similar or other metals. Also, the release of vanadium and aluminium at elevated levels could result in cytotoxicity [3]. Further research is needed to improve resistance against wear and corrosion of this implant material in order to achieve better bone tissue integration. Although ceramic (alumina/zirconia) and diamond-like-coatings have a low wear rate, their high elastic modulus, brittle nature and potential possibility of catastrophic fracture associated with ceramics restrict their clinical use [4]. Similarly, diamond-like-coatings possess high internal compressive stress reaching a few gigapascals [5]. This high internal stress can result in weak adhesion of the coating and exposure of the metal substrate to the aggressive media. To overcome these challenges, tantalum (Ta) has been suggested as coating material to impart high hardness, moderate elastic modulus and high wear resistance as an alternative option in orthopaedic implants [6]. The enhanced corrosion performance and adhesion properties of sputtered Ta coating on Ti6Al4V substrate has been reported in our previous work [7]. Tantalum oxide thin film prepared ⁎ Corresponding author at: School of Mechanical, Materials, Mechatronic and Biomedical Engineering, University of Wollongong, NSW 2522, Australia. E-mail address: [email protected] (Y. Zhao).

http://dx.doi.org/10.1016/j.tsf.2017.05.030 0040-6090/© 2017 Published by Elsevier B.V.

by magnetron sputtering improved the mechanical properties and corrosion resistance of the Ti6Al4V substrate, as reported by Rahmati et al. [8,9]. Ta appears to be a promising material for biomedical applications because of its superior mechanical properties and outstanding in vitro and in vivo biocompatibility [10–12]. Ta exhibits strong bonding with carbon, nitrogen and oxygen atoms and readily forms a stable protective layer. The stable pentoxide Ta2O5 protective film formed on the surface of Ta films demonstrates an outstanding ability to protect against corrosion [13]. In spite of the fact that tantalum has already been utilized for the fabrication of medical devices such as endovascular stents and neurosurgical implants [14], the utilization of tantalum implants in orthopaedics and dentistry has been limited because of the difficulty of machining due to high hardness and also the high costs of Ta metal. Alloying Ta with Ti (titanium) is a possible solution to this problem. A number of researchers have demonstrated the outstanding corrosion resistance of Ti-Ta alloys [15,16]. However, fabrication of Ti-Ta alloys by conventional arc melting can be difficult due to the large difference in the melting point (Ti: 1953 K, Ta 3273 K) and density (Ti: 4.51 g/cm3, Ta: 16.6 g/cm3) [17]. Alternatively, applying a layer of Ta film as surface coating on Ti6Al4V substrate may provide the desirable combination of properties including corrosion and wear resistance. A number of different fabrication techniques for Ta films have been reported, including magnetron sputtering [18–20], chemical vapour deposition [21,22], double glow plasma [23,24], and ion beam assisted deposition [25]. There have been a few reports on the deposition of Ta film

A.C. Hee et al. / Thin Solid Films 636 (2017) 54–62

by filtered cathodic vacuum arc deposition (FCVAD) but as a commonly adopted industrial coating method, this preparation of Ta film by FCVAD has not been sufficiently explored [26]. The effect of substrate bias on the deposition of a range of metals, alloys and compounds such as TiN, TiC, TiO2 and ZrO2 by filtered cathodic arc were reported in Ref. [27–30]. In present study, we investigate the fabrication of Ta films using FCVAD. Modifications to the deposition system were made to accommodate the high melting point and low plasma stability of the Ta target. Two deposition conditions, i.e., Ta/0 V (zero substrate voltage bias) and Ta/−100 V (with a negative substrate voltage bias of 100 V) were employed to compare the structure and mechanical properties of the Ta films. The passivity and stability of the surface films were examined using potentiodynamic polarization and electrochemical impedance spectroscopy in a phosphate buffered media to simulate the corrosivity in human body. The surface chemistry status after the corrosion testing was further accessed using X-ray photoelectron spectroscopy (XPS) to understand the corrosion protection mechanism.

2. Experimental details 2.1. Substrate preparation and coating deposition Prior to the deposition, Ti6Al4V substrates (2 × 2 × 2 cm3) were purchased from Shaanxi Cxmet Technology Co. Ltd., as annealed at 700– 785 °C. Chemical composition of the Ti6Al4V alloy, provided by the supplier, is Al: 6 wt%, V: 4 wt%, Fe: 0.05 wt%, C: 0.01 wt%; N: 0.009 wt%; H: 0.001 wt%; O: 0.06 wt%, Ti: balance. The substrates were mechanically ground using SiC sandpaper #500 for 5 min. The substrates were then polished by a 15 μm diamond polycrystalline suspension and DP-lubricant red (Struers) for 2 min. It was then followed by polishing with a suspension mixture of 2.5 ml of ammonia and 1.5 ml of hydrogen peroxide for 5 min. The substrates were polished to an average surface roughness of 0.05 ± 0.01 μm, as determined by a stylus type Hommel Tester T1000 profilometer. The substrates were then washed with ethanol and distilled water to remove residual particles. The deposition was carried out using an in-house made FCVAD. Fig. 1 shows schematic diagram of the deposition system. The arc cathode (the target) was a high purity tantalum rod (purity, 99.99%) with a diameter of 58 mm. The chamber in the deposition system reached a base pressure of 1.5 × 10−5 mbar. Prior to deposition, argon gas was introduced into the chamber near the cathode to reach a deposition pressure of 3.2 × 10−3 mbar. The DC arc current was set at 200 A, which was the lowest stable arc current to maintain the plasma state during deposition. The substrate holder was electrically isolated from the chamber so that a substrate voltage bias could be applied. The substrate voltage bias was varied between 0 V and − 100 V. The deposition time was set for 5 min. The Ta ion beam current was set at 200 mA and was measured in the centre of

Fig. 1. Schematic diagram of the deposition system.

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the chamber with an isolated shutter. The deposition rate was about 20 nm min−1. 2.2. Structure and surface characterizations Grazing-incidence X-ray diffraction (XRD) inspection of the deposited films was performed using a GBC MMA diffractometer equipped with the Cu Kα incident radiation. The XRD scan was operated in the 2 theta mode in the range between 20° and 100° in steps of 0.02°. The angle of incidence was fixed at 2°. A FEI xT Nova Nanolab 200 dual-beam work station with scanning electron microscopy (SEM) and focused ion beam (FIB) navigation and milling capabilities was used to prepare the cross-section of the deposited films for transmission electron microscopy (TEM) observation. Surface roughness and topography of the films were examined using a Veeco Dimension™ 3100 atomic force microscope (AFM) operating in contact mode over a scanning area of 5 × 5 μm with a scanning rate of 1 Hz. The surface wettability of the samples was obtained by measuring the contact angle with sessile drop method at room temperature, using a Rame-Hart Instrument contact angle goniometer. Prior to the measurement, the surfaces were cleaned using isopropanol. Five measurements on different locations were performed for each sample and an average value was reported. In order to assess the compatibility and bonding behaviour of the modified surface in the human body that contains mainly water and protein, two liquids of distilled water and fetal bovine serum (12003C, Sigma Aldrich) were implemented. With the known values of liquid surface tension and the contact angle, the surface tension of the solid can be calculated [31]. Since the surface tension components, i.e. dispersive and polar, of fetal bovine serum are unknown, the measurements were also made using glycerol and the values were used in the calculation of solid surface tension. The surface free energy was calculated by using the Owens-Wendt method [32] based on the following equation. 12 1 γl ∙ð1 þ cosðθÞÞ ¼ 2 γds ∙γdl þ 2 γps ∙γpl 2

ð1Þ

where θ is the contact angle, γl is the surface tension of the liquid and γs is the surface tension of the solid. The dispersive (γdl ) and polar (γpl ) components of distilled water are 29.1 mJ m−2 and 43.7 mJ m−2, respectively. The dispersive and polar components of glycerol are 37.4 mJ m−2 and 26.0 mJ m−2, respectively [33]. The film cohesion was evaluated using a Revetest Xpress scratch tester (CSM instruments SA, Switzerland) with a 200 μm radius diamond indenter. The diamond tip was driven over the coated surface in the

Fig. 2. X-ray diffraction (XRD) patterns of the Ti6Al4V substrate and the Ta films.

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Fig. 3. Cross section image of Ta deposited at 0 V voltage bias (a) and the corresponding selected area diffraction patterns (b, c, d) across the film.

load range from 0.1 to 50 N to produce a scratch. Frictional force and acoustic emission were measured in the scratch testing. The coefficient of friction was determined by dividing the frictional force by the applied normal load. The scratch length and speed were 5 mm and 10 mm/min, respectively. The scratch images were observed using a JEOL JSM6490LV scanning electron microscope (SEM) equipped with automated montage acquisition. 2.3. Electrochemical measurements The electrochemical examination was conducted using a threeelectrode cell arrangement in phosphate buffered saline (PBS) aqueous solution (Sigma P-5368, pH 7.4). The sample, platinum and Ag/AgCl were used as working electrode, auxiliary and reference electrodes, respectively. A sample area of 0.8 cm2 was exposed to the electrolyte by confining the coated surface within a glass tube held on the surface using an O-ring (1 cm) and a pinch clamp. Samples were exposed to

electrolyte for 60 min under open circuit potential (OCP) prior to measurements. The potentiodynamic polarization tests were carried out at room temperature by sweeping the potential from −250 to +250 mV (vs. OCP) at scan rate of 0.3 mV s−1. The scan rate of 0.3 mV s−1 was selected based on the work reported by Zhang et al. [34], where minimum disturbance of the charging current occurred for corrosion system of Ti6Al4V alloy. Corrosion potential (Ecorr), corrosion current density (Icorr), and polarization resistance (Rp) were determined by extrapolating the cathodic and anodic Tafel slopes, using the Stern-Geary method. Electrochemical impedance spectroscopy (EIS) tests were conducted in open circuit potential with an AC signal perturbation in a frequency range between 0.01 Hz to 100 kHz and amplitude of 10 mV. All tests were repeated at least three times to check for reproducibility. The X-ray photoelectron spectroscopy (XPS) analysis of the samples after corrosion testing was performed using a SPECS PHOIBOS 100 analyser operating at 12 kV and power of 120 W with an Al Kα X-ray source. The XPS peak fitting was conducted using CasaXPS software.

Fig. 4. Cross section image of Ta deposited at −100 V voltage bias (a) and the corresponding selected area diffraction patterns (b, c, d) across the film.


Fig. 5. Atomic force micrographs of the Ta films deposited at the substrate bias of (a) 0 V and (b) −100 V. ± indicates standard deviation.

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probably due to the increased energy of the depositing atoms at the substrate when a negative bias was applied. It has been reported by Bendavid et al. [28] that titanium dioxide exhibited anatase structure at zero substrate bias, and rutile diffraction peaks were deposited at −100 V bias. The high concentration of ionic species and high energy of particles impinging on the substrate were the main reasons for the formation of the rutile phase in FCVAD deposition. Similarly herein, applying a substrate bias during Ta deposition increased the ion energy and led to the formation of α-Ta. In particular, α-Ta exhibits superior chemical, thermal, and mechanical properties as well as ductility, formability and corrosion resistance [35,36], while β-Ta is hard, brittle and thermally unstable [37]. Therefore it is anticipated that the formation of α-Ta in Ta/−100 V would benefit the wear and corrosion behaviour of the surface in a biological environment. As shown in Figs. 3(a) and 4(a), the coating thickness of the deposited films was about 88 nm and 110 nm for Ta deposited at a voltage bias of 0 V and −100 V, respectively. To understand the structural characteristics of the deposited films, the corresponding selected area electron diffraction (SAED) patterns were recorded at several places along the surface of the coating. The diffraction patterns indicate that the film deposited at zero substrate bias consists of pure β-Ta (metastable tetragonal structure). The β-Ta films consist of a family of topologically close-packed structures with different SAED patterns across the surface, indicating that the crystallographic orientations of the deposited films are random. The α-Ta (body-centred cubic structure) is comprised of distinguishable rings with indexes that are in agreement with the bcc structure, as shown in Fig. 4(d). In addition, when the α phase is small grained and there is a small amount of it, there is the possibility of some β phase mixed in the diffraction pattern. The strong diffraction intensity of α-Ta phase in the SAED of Ta coating deposited at − 100 V bias, as shown in Fig. 4(d), provides evidence for the existence of α-Ta. Ta coatings revealed a pure β phase at zero substrate bias, and a mixture of α + β phase with a substrate bias of −100 V. The result is in agreement with the work reported by Alami et al. [20], where deposition of α-Ta was formed by using substrate voltage bias of more than 50 V in magnetron sputtering. 3.2. Surface roughness and wettability

The data were calibrated to the adventitious C1s peak present at 284.6 eV to minimize shifts caused by charging of the sample surface. 3. Results and discussion 3.1. Structural properties Fig. 2 shows the XRD patterns of the uncoated surface and Ta films deposited at a substrate voltage bias of 0 V and −100 V. β-Ta (JCPDS25-1280) was identified for the peaks located at 33.7° and 71.1° with the reflection of (002) and (513), respectively. Broad peaks located at 82.5° and 93.5° from the (220) and (310) planes, respectively, are attributed to α-Ta (JCPDS-04-0788) in the −100 V biased film, indicating the formation of the bcc (body-centred cubic) structure as the substrate voltage bias increased from 0 V to −100 V. The formation of the α-Ta is Table 1 The contact angle and surface energy of the uncoated substrate and the Ta films. Samples

Ti6Al4V substrate Ta/0 V Ta/−100 V

Contact angle (°)

Surface Energy (mJ m−2)

Distilled water

Fetal bovine serum

Glycerol

γds

γps

γs

70 ± 2 99 ± 2 102 ± 2

77 ± 2 79 ± 4 91 ± 3

46 ± 2 64 ± 1 72 ± 4

67 0.01 0.62

0.48 21 14

68 21 15

± indicates standard deviation; the standard deviations for the polar and dispersion surface energy are not available, because it was determined through least squares fitting of the equation proposed by Owen-Wendt.

Fig. 5 shows the atomic force micrographs of the deposited Ta films. The topography in Fig. 5(a) composed of islands protruding from the surface. The protrusions from the surface are typical feature of physical vapour deposited films as a result of ion impacts [38]. Fig. 5(b) demonstrates that the Ta/−100 V contains surface ridges which is the surface topography of the crystalline phases of the film. The shallow potholes in the Ta/−100 V film are possibly due to the shading effect of temporary surface droplets. The root mean square roughness (Rq) and average roughness (Ra) of the film deposited at −100 V substrate bias are higher than the roughness of the film of zero substrate bias. Roughness of the films increased with increasing substrate bias because of re-sputtering [39]. The re-sputtering effect is the result of competition between negative ion bombardment of an evolving thin film and the etching of the deposited material [40]. With an substrate bias of −150 V, the excessive energy of ions bombarding the film, leads to an increase of surface roughness and poor adhesion of the carbon nitride film [41]. It is recognized that wettability and surface energy are more useful than surface roughness to determine the cell adhesion and proliferation of implant materials [42]. Table 1 summarizes the values of contact angle and surface free energy of the Ti6Al4V substrate and the Ta films. The contact angle values of the Ta films in contact with the tested liquids are relatively higher than the values of the bare Ti6Al4V substrate. High contact angle values indicate a surface of low wettability (hydrophobic surface). The solid surface free energy of the Ti6Al4V substrate is approximately 68 mJ m−2, which can be separated into its two main components; the polar component with a low value of 0.48 mJ m− 2, and the dispersive component with a relatively high

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Fig. 6. (a) Coefficient of friction, acoustic emission and normal load as a function of scratch length for Ta/0 V; (b) is where the scratch started; (c) is where the scratch ended.

value of 67 mJ m− 2. The surface free energy of Ti6Al4V substrate is mainly contributed by the dispersive component, which is caused by the van der Waals and other non-site-specific interactions [43]. The surface free energy of the Ti6Al4V is higher than the values reported in the literature (34 mJ m−2 [44] and 51 mJ m−2 [45]). The difference of the surface free energy values may be due to surface roughness or the preparation procedure such as the liquids and protocols of cleaning [46]. Ta films presented lower surface free energy than the uncoated Ti6Al4V substrate. The surface energy of the Ta films is governed by the polar component, implying an increase of surface polarity of the metal. The lower surface energy reduces the attraction of the surface to the water particles and promotes adsorption of protein. It has been suggested that the increase in hydrophobicity of the surface enhances the absorption of β-Lg (β-lactoglobulin) on the surface [47]. In general, the competition between water and proteins adsorption is an endothermic process, the surface interaction with proteins is established by exposure either hydrophobic domain or hydrophilic domain through the surface. Therefore, the repulsion of water from the hydrophobic surface induces protein-surface interaction [33]. The adsorbed protein can also form a stable layer that can enhance the lubrication of prosthetic implants [48,49]. Ta films reduce the interaction (bonding) with protein because the contact angle is increased. It means the bonding forces (either electrostatic or van der Waals) between the protein and surface are weaker. The same trend is followed with water. Ta/−100 V film shows hydrophobic characteristic with a contact angle of ~ 100°. The hydrophobic property of Ta films is consistent with the result in our previous study [7]. In the interaction between protein and surface, other surface forces

are also contributing that they might not be present in the case of water. The result of hydrophobicity of Ta films indicates that the surface treatment has increased the compatibility with fetal bovine serum. The contact angle of Ta/−100 V film in contact with water and bovine serum is higher compared to the film of zero substrate bias because of the enhanced inertness of the film. The mechanism of the inert surface is explained in Section 3.4. 3.3. Scratch behaviour Scratch testing is a quantitative technique to evaluate the coating's adhesive and cohesive properties [50,51]. The coating failure is corresponding to the film-to-substrate critical load and their corresponding failure modes. The critical load is defined by the abrupt changes of coefficient of friction as the acoustic emission is weak and non-indicative of film failure due to the soft substrate. The film at zero substrate bias appears to have failed at the start with the evidence of continuous perforation in the magnified inset of Fig. 6(b). The fluctuation in the coefficient of friction of the load vs scratch length profile may be due to local film cracking and shifting indicating a weak interfacial cohesion. A sudden change in the coefficient of friction is observed at 5 N. This is believed to be due to the small build-up of the Ta film on the track. The Ta film was accumulated at the end of the scratch track, exposing the Ti6Al4V substrate along the track, as shown in Fig. 6(c). Fig. 7 shows the scratch track of the Ta film at −100 V and the magnified image where the scratch was initiated at the first critical load of

Fig. 7. (a) Coefficient of friction, acoustic emission and normal load as a function of scratch length for Ta/−100 V; (b) is where the scratch started; (c) is where the complete removal of the coating occurred.


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Fig. 8. Potentiodynamic polarization curves of the samples after 1 h of electrode immersion.

10 N. Buckling is observed in Fig. 7(b) and complete removal of the coating occurred at the second critical load of 44 N in Fig. 7(c). The detachment of the coating is accompanied by an abrupt increase in the coefficient of friction. Ta film deposited at − 100 V shows a greater resistance to scratch damage than the Ta/0 V due to the change in surface microstructure. Filtered arc deposition produces a highly ionized and high-density plasma [52]. When the substrate bias is increased to −100 V, the ionized particles have more energy. The energized atoms are attracted by the negatively charged substrate with sufficient energy to sputter clean the surface atoms and diffuse to low energy sites producing high density film with less contamination. The high energy of the condensing species using a substrate bias of −100 V improves the film cohesion, showing a significant increase in the critical load as compared with film deposited without a substrate bias. 3.4. Electrochemical measurements The potentiodynamic polarization curves of non-coated Ti6Al4V surface, Ta/0 V and Ta/−100 V after 1 h of electrode immersion are illustrated in Fig. 8. Table 2 shows the electrochemical parameters of the Ti6Al4V substrate and Ta coated surfaces treated in the PBS solution. Polarization resistance (Rp) is inversely proportional to the corrosion current density (Icorr) according to the Stern-Geary equation and both are measures of the protection afforded by the passive layer on the surface. The polarization resistance value of Ta/−100 V is about one order of magnitude higher than the uncoated surface, implying the improved corrosion resistance. The Ta/−100 V film exhibits a more positive electrode potential, implying a less reactive/more inert surface that is also consistent with the results of contact angle measurement. The anodic Tafel slope (βa) and cathodic Tafel slope (βc) of the Ta/−100 V shift towards the low current density region and more positive values, indicating the increased passivation effect on the anodic reaction of the Table 2 Electrochemical parameters of the Ti6Al4V substrate and the Ta films treated in the phosphate buffered saline solution after 1 h electrode immersion. Samples

Ti6Al4V substrate

Ta/0 V

Ta/−100 V

Ecorr (mV) Icorr (μA cm−2) βa (mV dec−1) −βc (mV dec−1) Rp (kΩ cm2)

−163 ± 1 0.109 ± 0.01 260 ± 12 133 ± 7 352 ± 15

−168 ± 1 0.042 ± 0.002 269 ± 13 125 ± 6 881 ± 20

−45 ± 0.1 0.009 ± 0.001 221 ± 15 188 ± 9 4570 ± 87

± indicates standard deviation.

Fig. 9. (a) Nyquist plot and (b) Bode plot of the uncoated and coated surfaces in phosphate buffer saline solution after 1 h of electrode immersion.

protective oxide film. The Ta/−100 V film shows significantly lower Icorr compared to the uncoated and Ta/0 V samples. The density of film can well be a contributing factor in reducing the corrosion current density or increasing the polarization resistance (kinetic), but unlikely to be too important in corrosion potential (thermodynamic). Thermodynamic is more influenced by the chemical nature of oxide film on the surface and the reactivity or stability of the oxide film. It is believed that the formation of a dense film of Ta2O5 with a passive nature on the surface has enhanced the corrosion resistance. The Ta coated Ti6Al4V prepared by FCVAD at the substrate bias of −100 V demonstrates improved corrosion resistance indicated by corrosion current density of about one order of magnitude lower than the uncoated surface. Similar protective Table 3 Electrical components calculated by fitting an equivalent electrical circuit on the EIS data. Electrical component

Ti6Al4V substrate

Ta/0 V

Ta/−100 V

Qoxide film (μΩ−1 sn cm−2) n1 Rpore (kΩ cm2) Qdl (μΩ−1 sn cm−2) n2 Rct (kΩ cm2)

25.6 ± 1 0.90 ± 0.01 158 ± 5 36.1 ± 1 0.90 ± 0.01 38.2 ± 2

71.7 ± 3 0.89 ± 0.01 195 ± 5 35.2 ± 1 0.89 ± 0.01 29.7 ± 1

11.2 ± 0.1 0.92 ± 0.01 2520 ± 10

± indicates standard deviation.

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Fig. 10. The equivalent electrical circuit model of EIS measurement.

properties have been reported for Ta coatings prepared by atmospheric pressure chemical vapour deposition [53] and double glow plasma surface tantalumising [23], with enhancing the corrosion resistance of Ti6Al4V substrate. The results of the electrochemical impedance test in the PBS solution are presented in Fig. 9 in the form of Nyquist and Bode plots. It can be seen in the Nyquist plots (Fig. 9(a)) that the real impedance of the Ta films is much higher than that of the Ti6Al4V substrate, which is in

accordance with potentiodynamic polarization results. As shown in Fig. 9(b), the maximum phase angle for Ta/−100 V film is close to π/2, implying a surface with strong capacitive response. Table 3 summarizes the quantitative analysis of EIS results by fitting the appropriate electrical equivalent circuit (EEC) on the EIS data. Fig. 10(a) represents the EEC used for modelling EIS result from Ti6Al4V and Ta/0 V and Fig. 10(b) shows the EEC used for the EIS result from Ta/−100 V. The Qoxide film and Rpore represent the constant phase element (CPE) and resistance associated with the oxide film on the surface. Qdl and Rct are the CPE and resistance associated with the electrical double layer and charge transfer resistance, respectively. The exponent n is an indication of variation from ideal capacitor/resistor where a value of 1 indicates an ideal capacitor and a value of 0 indicates the ideal resistor. The simple circuit with one loop element, i.e. Fig. 10(b), that produced a successful fit for the Ta/−100 V data indicates the presence of a uniform layer without defects. On the other hand, the need for a more complex EEC with two loop elements, i.e. Fig. 10(a), for a close fit on EIS results of Ti6Al4V and Ta/0 V suggests a heterogeneous surface with potential defects in the surface film. This is also reflected by the value of exponent n which is higher in the case of Ta/−100 V surface treatment. The value of charge transfer resistance is also consistent with the polarization resistance results and indicates the enhanced insulation and corrosion protection that is offered by the Ta/−100 V surface. In general, the high resistance of Ta against corrosion attack in sodium chloride solutions is attributed to the high stability of a passive oxide layer [54–56]. Fig. 11 shows the high resolution XPS spectra for Ta 4f and O 1s of the Ta/−100 V film before and after corrosion testing. The Ta 4f spectra is divided to four energy doublets which consist of the most stable form of pentavalent state (Ta2O5), sub-oxides (TaO2, TaO) and metallic tantalum (Ta0). It should be noted that the peak fitting of all the energy doublets of 4f7/2 – 4f5/2 should be constrained to the condition of a fixed area ratio of 4:3 and an energy separation of 1.91 eV [57,58].

Fig. 11. X-ray spectroscopy spectra of the Ta/−100 V (a) Ta 4f before corrosion test; (b) Ta 4f after corrosion test; (c) O 1s before corrosion test; (d) O 1s after corrosion test.


The Ta 4f7/2 and Ta 4f5/2 peaks located at 26.3 eV and 28.2 eV are assigned to Ta2O5 [59]. The doublets at 25.0 eV and 26.9 eV are attributed to the lower oxidation state (TaO2), peaks located at 23.5 eV and 25.4 eV are categorized to the TaO [60]. The binding energies of the low energy-doublet (21.9 eV and 23.8 eV) are attributed to metallic tantalum, Ta0 [61,62]. The amount of Ta sub-oxides and Ta metal is significant reduced after corrosion testing (Fig. 11(b)), indicating formation of the passive oxide layer on the surface. As shown in Fig. 11(c&d), the O 1s spectra resolves into two peaks. The binding energy located at 530.0 eV is assigned to Ta\\O bonds in tantalum oxides [63] and the higher binding energy peak at 531.9 eV is attributed to either oxygen in the form of hydroxide or other adsorbed oxygen species [64]. A comparison between Fig. 11(a) and (b), and between Fig. 11(c) and (d) demonstrates that after the corrosion test, the surface transforms from a mixture of valent status of Ta and hydro-oxide domination to a more corrosion inhibiting Ta2O5 domination.

[10]

[11]

[12]

[13] [14]

[15]

[16]

4. Conclusion Ta films with a thickness of ~100 nm were successfully deposited on Ti6Al4V substrate by filtered cathodic vacuum arc deposition system. The results show that Ta film deposited at zero substrate voltage bias exhibits purely tetragonal phase (β-Ta), the growth of α phase is observed as the substrate bias increased to − 100 V. The increase in the flux of depositing ions on the substrate contributes to collisional intermixing and increased film cohesion. The strong bonding between the Ta film and the Ti6Al4V substrate presents a surface with high chemical stability (inert), which is confirmed by both wettability and electrochemical measurements. The XPS results verify the shift to a Ta2O5 surface dominance on the Ta film surface during the corrosion process and therefore the fundamental mechanism of the corrosion resistance offered by a cohesive Ta/−100 V film for the Ti6Al4V substrate. The enhanced corrosion resistance by the Ta/−100 V film is promising for orthopaedic implant materials.

[17]

[18] [19]

[20]

[21]

[22]

[23]

Acknowledgement

[24]

This work was financially supported by the University Postgraduate Award (UPA), University of Wollongong. The authors acknowledge the use of facilities within the UOW Electron Microscopy Centre and the help of Dr. Madeleine Strong Cincotta in the final language editing of this paper.

[25]

References [1] M. Long, H. Rack, Titanium alloys in total joint replacement—a materials science perspective, Biomaterials 19 (1998) 1621–1639, http://dx.doi.org/10.1016/S01429612(97)00146-4. [2] L.L. Liu, J. Xu, X. Lu, P. Munroe, Z.-H. Xie, Electrochemical corrosion behavior of nanocrystalline β-Ta coating for biomedical applications, ACS Biomater. Sci. Eng. 2 (2016) 579–594, http://dx.doi.org/10.1021/acsbiomaterials.5b00552. [3] E. Eisenbarth, D. Velten, M. Müller, R. Thull, J. Breme, Biocompatibility of β-stabilizing elements of titanium alloys, Biomaterials 25 (2004) 5705–5713, http://dx.doi. org/10.1016/j.biomaterials.2004.01.021. [4] J. Chevalier, L. Gremillard, Ceramics for medical applications: a picture for the next 20 years, J. Eur. Ceram. Soc. 29 (2009) 1245–1255, http://dx.doi.org/10.1016/j. jeurceramsoc.2008.08.025. [5] A. Grill, Diamond-like carbon: state of the art, Diam. Relat. Mater. 8 (1999) 428–434, http://dx.doi.org/10.1016/S0925-9635(98)00262-3. [6] H.A. Ching, D. Choudhury, M.J. Nine, N.A. Abu Osman, Effects of surface coating on reducing friction and wear of orthopaedic implants, Sci. Technol. Adv. Mater. 15 (2014) 14402, http://dx.doi.org/10.1088/1468-6996/15/1/014402. [7] A.C. Hee, S.S. Jamali, A. Bendavid, P.J. Martin, C. Kong, Y. Zhao, Corrosion behaviour and adhesion properties of sputtered tantalum coating on Ti6Al4V substrate, Surf. Coatings Technol. 307 (2016) 666–675, http://dx.doi.org/10.1016/j.surfcoat.2016. 09.061. [8] B. Rahmati, A.A.D. Sarhan, W.J. Basirun, W.A.B.W. Abas, Ceramic tantalum oxide thin film coating to enhance the corrosion and wear characteristics of Ti6Al4V alloy, J. Alloys Compd. 676 (2016) 369–376, http://dx.doi.org/10.1016/j.jallcom.2016.03.188. [9] B. Rahmati, A.A.D. Sarhan, E. Zalnezhad, Z. Kamiab, A. Dabbagh, D. Choudhury, W.A.B.W. Abas, Development of tantalum oxide (Ta-O) thin film coating on

[26]

[27]

[28]

[29]

[30]

[31]

[32]

[33]

[34]

[35]

61

biomedical Ti-6Al-4V alloy to enhance mechanical properties and biocompatibility, Ceram. Int. 42 (2016) 466–480, http://dx.doi.org/10.1016/j.ceramint.2015.08.133. H. Matsuno, A. Yokoyama, F. Watari, M. Uo, T. Kawasaki, Biocompatibility and osteogenesis of refractory metal implants, titanium, hafnium, niobium, tantalum and rhenium, Biomaterials 22 (2001) 1253–1262, http://dx.doi.org/10.1016/S01429612(00)00275-1. Y. Liu, C. Bao, D. Wismeijer, G. Wu, The physicochemical/biological properties of porous tantalum and the potential surface modification techniques to improve its clinical application in dental implantology, Mater. Sci. Eng. C. 49 (2015) 323–329, http://dx.doi.org/10.1016/j.msec.2015.01.007. V.K. Balla, S. Banerjee, S. Bose, A. Bandyopadhyay, Direct laser processing of a tantalum coating on titanium for bone replacement structures, Acta Biomater. 6 (2010) 2329–2334, http://dx.doi.org/10.1016/j.actbio.2009.11.021. J. Black, Biologic performance of tantalum, Clin. Mater. 16 (1994) 167–173, http:// dx.doi.org/10.1016/0267-6605(94)90113-9. R.A. Silva, M. Walls, B. Rondot, M. Da Cunha Belo, R. Guidoin, Electrochemical and microstructural studies of tantalum and its oxide films for biomedical applications in endovascular surgery, J. Mater. Sci. Mater. Med. 13 (2002) 495–500, http://dx. doi.org/10.1023/A:1014779008598. D. Mareci, R. Chelariu, D.-M. Gordin, G. Ungureanu, T. Gloriant, Comparative corrosion study of Ti–Ta alloys for dental applications, Acta Biomater. 5 (2009) 3625–3639, http://dx.doi.org/10.1016/j.actbio.2009.05.037. S.L. Sing, W.Y. Yeong, F.E. Wiria, Selective laser melting of titanium alloy with 50 wt% tantalum: microstructure and mechanical properties, J. Alloys Compd. 660 (2016) 461–470, http://dx.doi.org/10.1016/j.jallcom.2015.11.141. Y.L. Zhou, M. Niinomi, T. Akahori, Effects of Ta content on Young's modulus and tensile properties of binary Ti–Ta alloys for biomedical applications, Mater. Sci. Eng. A 371 (2004) 283–290, http://dx.doi.org/10.1016/j.msea.2003.12.011. A. Schauer, M. Roschy, R.F. sputtered β-tantalum and b.c.c. tantalum films, Thin Solid Films 12 (1972) 313–317, http://dx.doi.org/10.1016/0040–6090(72)90095-8. C. Hertl, L. Koll, T. Schmitz, E. Werner, U. Gbureck, Structural characterisation of oxygen diffusion hardened alpha-tantalum PVD-coatings on titanium, Mater. Sci. Eng. C. 41 (2014) 28–35, http://dx.doi.org/10.1016/j.msec.2014.03.018. J. Alami, P. Eklund, J.M. Andersson, M. Lattemann, E. Wallin, J. Bohlmark, P. Persson, U. Helmersson, Phase tailoring of Ta thin films by highly ionized pulsed magnetron sputtering, Thin Solid Films 515 (2007) 3434–3438, http://dx.doi.org/10.1016/j.tsf. 2006.10.013. K. Hieber, Structural and electrical properties of Ta and Ta nitrides deposited by chemical vapour deposition, Thin Solid Films 24 (1974) 157–164, http://dx.doi. org/10.1016/0040-6090(74)90261-2. S.-O. Kim, J.S. Byun, H.J. Kim, The effect of substrate temperature on the composition and growth of tantalum oxide thin films deposited by plasma-enhanced chemical vapour deposition, Thin Solid Films 206 (1991) 102–106, http://dx.doi.org/10. 1016/0040-6090(91)90400-R. X.H. Chen, P.Z. Zhang, D.B. Wei, J. Huang, W. Xuan, Surface modification of pure titanium by plasma tantalumising, Surf. Eng. 29 (2013) 228–233, http://dx.doi.org/10. 1179/1743294412Y.0000000107. Y. Wang, P.-Z. Zhang, H.-Y. Wu, X.-F. Wei, Q. Bi, J. Song, Microstructure and electrochemical properties of plasma tantalumising on low carbon steel, Surf. Eng. 31 (2015) 634–640, http://dx.doi.org/10.1179/1743294414Y.0000000374. Q.Y. Zhang, X.X. Mei, D.Z. Yang, F.X. Chen, T.C. Ma, Y.M. Wang, F.N. Teng, Preparation, structure and properties of TaN and TaC films obtained by ion beam assisted deposition, Nucl. Instruments Methods Phys. Res. Sect. B Beam Interact. with Mater. Atoms. 127 (1997) 664–668, http://dx.doi.org/10.1016/S0168583X(96)01151-2. P. Martin, A. Bendavid, Review of the filtered vacuum arc process and materials deposition, Thin Solid Films 394 (2001) 1–14, http://dx.doi.org/10.1016/S00406090(01)01169-5. P.J. Martin, R.P. Netterfield, T.J. Kinder, L. Descôtes, Deposition of TiN, TiC, and TiO2 films by filtered arc evaporation, Surf. Coatings Technol. 49 (1991) 239–243, http://dx.doi.org/10.1016/0257-8972(91)90062-2. A. Bendavid, P. Martin, Å. Jamting, H. Takikawa, Structural and optical properties of titanium oxide thin films deposited by filtered arc deposition, Thin Solid Films 355 (1999) 6–11, http://dx.doi.org/10.1016/S0040-6090(99)00436-8. P.J. Martin, A. Bendavid, Properties of zirconium oxide films prepared by filtered cathodic vacuum arc deposition and pulsed DC substrate bias, Thin Solid Films 518 (2010) 5078–5082, http://dx.doi.org/10.1016/j.tsf.2010.02.067. P.J. Martin, R.P. Netterfield, T.J. Kinder, Ion-beam-deposited films produced by filtered arcevaporation, Thin Solid Films 193 (1990) 77–83, http://dx.doi.org/10. 1016/S0040-6090(05)80014-8. D. Li, A. Neumann, A reformulation of the equation of state for interfacial tensions, J. Colloid Interface Sci. 137 (1990) 304–307, http://dx.doi.org/10.1016/00219797(90)90067-X. D.K. Owens, R.C. Wendt, Estimation of the surface free energy of polymers, J. Appl. Polym. Sci. 13 (1969) 1741–1747, http://dx.doi.org/10.1002/app.1969. 070130815. A. Escudeiro, T. Polcar, A. Cavaleiro, Adsorption of bovine serum albumin on Zr cosputtered a-C(:H) films: Implication on wear behaviour, J. Mech. Behav. Biomed. Mater. 39 (2014) 316–327, http://dx.doi.org/10.1016/j.jmbbm.2014.08.001. X.L. Zhang, Z.H. Jiang, Z.P. Yao, Y. Song, Z.D. Wu, Effects of scan rate on the potentiodynamic polarization curve obtained to determine the Tafel slopes and corrosion current density, Corros. Sci. 51 (2009) 581–587, http://dx.doi.org/10.1016/j.corsci. 2008.12.005. D.W. Matson, M.D. Merz, E.D. McClanahan, High rate sputter deposition of wear resistant tantalum coatings, J. Vac. Sci. Technol. A 10 (1992) 1791, http://dx.doi.org/ 10.1116/1.577748.

62


[36] S.L. Lee, D. Windover, Phase, residual stress, and texture in triode-sputtered tantalum coatings on steel, Surf. Coatings Technol. 108 (1998) 65–72, http://dx.doi.org/ 10.1016/S0257-8972(98)00666-5. [37] S.L. Lee, M. Cipollo, D. Windover, C. Rickard, Analysis of magnetron-sputtered tantalum coatings versus electrochemically deposited tantalum from molten salt, Surf. Coatings Technol. 120 (1999) 44–52, http://dx.doi.org/10.1016/S02578972(99)00337-0. [38] R. Ritter, G. Kowarik, W. Meissl, A.S. El-Said, L. Maunoury, H. Lebius, C. Dufour, M. Toulemonde, F. Aumayr, Nanostructure formation due to impact of highly charged ions on mica, Vacuum 84 (2010) 1062–1065, http://dx.doi.org/10.1016/j.vacuum. 2009.10.018. [39] A. Bendavid, P.J. Martin, Review of thin films materials deposition by the filtered cathodic vacuum arc process at CSIRO, J. Aust. Ceram. Soc. 501 (2014) 86–101. [40] A. Bendavid, P.J. Martin, X. Wang, M. Wittling, T.J. Kinder, Deposition and modification of titanium nitride by ion assisted arc deposition, J. Vac. Sci. Technol. A Vacuum, Surfaces, Film. 13 (1995) 1658–1664, http://dx.doi.org/10.1116/1.579747. [41] J. Li, W. Zheng, Z. Jin, X. Lu, G. Gu, X. Mei, C. Dong, Influence of substrate dc bias on chemical bonding, adhesion and roughness of carbon nitride films, Appl. Surf. Sci. 191 (2002) 273–279, http://dx.doi.org/10.1016/S0169-4332(02)00221-0. [42] J.M. Schakenraad, H.J. Busscher, C.R.H. Wildevuur, J. Arends, The influence of substratum surface free energy on growth and spreading of human fibroblasts in the presence and absence of serum proteins, J. Biomed. Mater. Res. 20 (1986) 773–784, http://dx.doi.org/10.1002/jbm.820200609. [43] V. Gurau, M.J. Bluemle, E.S. De Castro, Y.-M. Tsou, J.A. Mann, T.A. Zawodzinski, Characterization of transport properties in gas diffusion layers for proton exchange membrane fuel cells: 1. Wettability (internal contact angle to water and surface energy of GDL fibers), J. Power Sources. 160 (2006) 1156–1162, http://dx.doi.org/10. 1016/j.jpowsour.2006.03.016. [44] A.M. Gallardo-Moreno, M.A. Pacha-Olivenza, L. Saldaña, C. Pérez-Giraldo, J.M. Bruque, N. Vilaboa, M.L. González-Martín, In vitro biocompatibility and bacterial adhesion of physico-chemically modified Ti6Al4V surface by means of UV irradiation, Acta Biomater. 5 (2009) 181–192, http://dx.doi.org/10.1016/j.actbio.2008.07.028. [45] L. Ponsonnet, K. Reybier, N. Jaffrezic, V. Comte, C. Lagneau, M. Lissac, C. Martelet, Relationship between surface properties (roughness, wettability) of titanium and titanium alloys and cell behaviour, Mater. Sci. Eng. C. 23 (2003) 551–560, http://dx.doi. org/10.1016/S0928-4931(03)00033-X. [46] M.A. Pacha-Olivenza, A.M. Gallardo-Moreno, A. Méndez-Vilas, J.M. Bruque, J.L. González-Carrasco, M.L. González-Martín, Effect of UV irradiation on the surface Gibbs energy of Ti6Al4V and thermally oxidized Ti6Al4V, J. Colloid Interface Sci. 320 (2008) 117–124, http://dx.doi.org/10.1016/j.jcis.2007.11.060. [47] V. Krisdhasima, J. McGuire, R. Sproull, Surface hydrophobic influences on β-lactoglobulin adsorption kinetics, J. Colloid Interface Sci. 154 (1992) 337–350, http:// dx.doi.org/10.1016/0021-9797(92)90148-F. [48] B.A. Hills, Boundary lubrication in vivo, Proc. Inst. Mech. Eng. H. 214 (2000) 83–94, http://dx.doi.org/10.1243/0954411001535264. [49] S. Ghosh, D. Choudhury, B.P. Murphy, Lubricating ability of albumin and globulin on artificial joint implants: a tribological perspective, Int. J. Surf. Sci. Eng. 10 (2016) 193, http://dx.doi.org/10.1504/IJSURFSE.2016.076516. [50] P.A. Steinmann, Y. Tardy, H.E. Hintermann, Adhesion testing by the scratch test method: The influence of intrinsic and extrinsic parameters on the critical load,

[51] [52]

[53]

[54]

[55]

[56]

[57]

[58]

[59]

[60]

[61]

[62]

[63]

[64]

Thin Solid Films 154 (1987) 333–349, http://dx.doi.org/10.1016/00406090(87)90377-4. A.J. Perry, Scratch adhesion testing of hard coatings, Thin Solid Films 107 (1983) 167–180, http://dx.doi.org/10.1016/0040-6090(83)90019-6. A. Anders, S. Anders, I.G. Brown, Transport of vacuum arc plasmas through magnetic macroparticle filters, Plasma Sources Sci. Technol. 4 (1995) 1–12, http://dx.doi.org/ 10.1088/0963-0252/4/1/001. X. Yu, L. Tan, H. Yang, K. Yang, Surface characterization and preparation of ta coating on Ti6Al4V alloy, J. Alloys Compd. 644 (2015) 698–703, http://dx.doi.org/10.1016/j. jallcom.2015.04.207. F. Cardarelli, P. Taxil, A. Savall, C. Comninellis, G. Manoli, O. Leclerc, Preparation of oxygen evolving electrodes with long service life under extreme conditions, J. Appl. Electrochem. 28 (1998) 245–250, http://dx.doi.org/10.1023/A: 1003251329958. S. Zein El Abedin, U. Welz-Biermann, F. Endres, A study on the electrodeposition of tantalum on NiTi alloy in an ionic liquid and corrosion behaviour of the coated alloy, Electrochem. Commun. 7 (2005) 941–946, http://dx.doi.org/10.1016/j.elecom.2005. 06.007. M.M. Momeni, M. Mirhosseini, M. Chavoshi, Fabrication of Ta2O5 nanostructure films via electrochemical anodisation of tantalum, Surf. Eng. 160226022403003 (2015) http://dx.doi.org/10.1179/1743294415Y.0000000071. M. Khanuja, H. Sharma, B.R. Mehta, S.M. Shivaprasad, XPS depth-profile of the suboxide distribution at the native oxide/Ta interface, J. Electron Spectros. Relat. Phenomena. 169 (2009) 41–45, http://dx.doi.org/10.1016/j.elspec.2008.10.004. O. Kerrec, D. Devilliers, H. Groult, P. Marcus, Study of dry and electrogenerated Ta2O5 and Ta/Ta2O5/Pt structures by XPS, Mater. Sci. Eng. B 55 (1998) 134–142, http://dx. doi.org/10.1016/S0921-5107(98)00177-9. B. Díaz, J. Światowska, V. Maurice, A. Seyeux, E. Härkönen, M. Ritala, S. Tervakangas, J. Kolehmainen, P. Marcus, Tantalum oxide nanocoatings prepared by atomic layer and filtered cathodic arc deposition for corrosion protection of steel: comparative surface and electrochemical analysis, Electrochim. Acta 90 (2013) 232–245, http:// dx.doi.org/10.1016/j.electacta.2012.12.007. C.H. An, K. Sugimoto, Ellipsometric examination of structure and growth rate of metallorganic chemical vapor deposited Ta2O5 films on Si(100), J. Electrochem. Soc. 141 (1994) 853, http://dx.doi.org/10.1149/1.2054821. S. Kohli, P.R. McCurdy, C.D. Rithner, P.K. Dorhout, A.M. Dummer, F. Brizuela, C.S. Menoni, X-ray characterization of oriented β-tantalum films, Thin Solid Films 469 (2004) 404–409, http://dx.doi.org/10.1016/j.tsf.2004.09.001. J.G.S. Moo, Z. Awaludin, T. Okajima, T. Ohsaka, An XPS depth-profile study on electrochemically deposited TaOx, J. Solid State Electrochem. 17 (2013) 3115–3123, http://dx.doi.org/10.1007/s10008-013-2216-y. Y. Cheng, W. Cai, H.T. Li, Y.F. Zheng, L.C. Zhao, Surface characteristics and corrosion resistance properties of TiNi shape memory alloy coated with Ta, Surf. Coatings Technol. 186 (2004) 346–352, http://dx.doi.org/10.1016/j.surfcoat.2004.01.012. M. Textor, L. Ruiz, R. Hofer, A. Rossi, K. Feldman, G. Hähner, N. Spencer, Structural chemistry of self-assembled monolayers of octadecylphosphoric acid on tantalum oxide surfaces, Langmuir 16 (2000) 3257–3271, http://dx.doi.org/10.1021/ LA990941T.