On the bio-corrosion and biocompatibility of TiTaNb medium entropy alloy films

Journal Pre-proofs Full Length Article On the bio-corrosion and biocompatibility of TiTaNb medium entropy alloy films Y.H. Chen, W.S. Chuang, J.C. Huang, X. Wang, H.S. Chou, Y.J. Lai, P.H. Lin PII: DOI: Reference:

S0169-4332(20)30063-5 https://doi.org/10.1016/j.apsusc.2020.145307 APSUSC 145307

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Applied Surface Science

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21 October 2019 5 January 2020 6 January 2020

Please cite this article as: Y.H. Chen, W.S. Chuang, J.C. Huang, X. Wang, H.S. Chou, Y.J. Lai, P.H. Lin, On the bio-corrosion and biocompatibility of TiTaNb medium entropy alloy films, Applied Surface Science (2020), doi: https://doi.org/10.1016/j.apsusc.2020.145307

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On the bio-corrosion and biocompatibility of TiTaNb medium entropy alloy films Y. H. Chen 1, W. S. Chuang 2, J. C. Huang 1,2 *, X. Wang 1,3, H. S. Chou 2, Y. J. Lai 2, P. H. Lin 2 1 Department

of Materials Science & Engineering; Hong Kong Institute for Advanced Study, City University of Hong Kong, Kowloon, Hong Kong 2 Department of Materials and Optoelectronic Science, National Sun Yat-Sen University, Kaohsiung, Taiwan 804, ROC 3 School of Mechanical Engineering, Liaoning Shihua University, Fushun 113001, PRC

* Corresponding author: Electronic mail: [email protected]

Abstract Some high or medium entropy alloys (HEAs and MEAs) have been demonstrated to exhibit great mechanical and bio-corrosion properties to meet the demands as bio-implant materials. However, the difference in Young’s modulus between these alloys and human bone would induce stress shielding effects. Therefore, HEA or MEA coating films appear to be potential materials since these films can be coated on appropriate implant devices as surface modification. This study employs sputtering to produce equimolar TiTaNb MEA film and a titanium-based Ti-10Ta-6Nb film in order to compare their mechanical and bio-corrosion properties. The results indicate that the TiTaNb MEA films have higher hardness, higher wear resistance, greater biocompatibility and superior bio-corrosion resistance, likely to be a high potential bio-implant coating materials. The underlying related chemical reactions and mechanisms are also explored and discussed. Keywords: TiTaNb; magnetron sputtering; high entropy alloy; medium entropy alloy thin film; biocompatibility

1. Introduction Nowadays, Ti alloys (especially Ti-6Al-4V) have been one the most well-known materials for orthopedic implants, due to their superior mechanical properties, promising biocompatibility and high corrosion resistance [1-4]. Recently, Ti alloys with Nb and Ta addition have raised much attention [5-12]. With Nb and Ta, the modulus of the Ti alloy can 1

be reduced originated from the formation of  and ’’ phases [5, 7, 10]. Moreover, with the formation of stable oxides, Ta2O5 and Nb2O5, these Ti alloys with Nb and Ta present high corrosion resistance in comparison with the standard implant metal such as Ti-6Al-4V [6, 8]. In terms of biocompatibility, these alloys present not only no cytotoxicity but also benefits to human osteoblasts [8, 12]. Recently, a different concept of alloy design by applying the nearly equal molar multicomponent elements in an alloy, namely, the high entropy alloys (HEAs) or medium entropy alloys (MEAs) concept, also attracts much interest [13, 14]. If there are more elements in the designed alloys with minimum heat of mixing, they can classified as HEAs. And with about three elements involved, they are usually termed as MEAs. Different from conventional methods, this concept is based on mixing multiple principal elements in an equimolar or nearequimolar composition to form new alloys [15, 16]. This concept can also be applied in the biomaterial implant area with biocompatible elements, such as Ti, Zr, Nb, Ta, etc. It has been reported that the TiZrNbTaMo HEAs present better mechanical properties, corrosion resistance and biocompatibility than commercial Ti-6Al-4V (in wt%) [17, 18]. However, there are two major issues needed to be considered in applying such HEAs or MEAs. The first concern is the alloy processing. Since the melting point of Ta is above 3000oC, it makes the cast of homogenous HEAs or MEAs with Ta highly difficult. The other one is the Young’s modulus readings of these alloys (153 GPa for TiZrNbTaMo [18]) are much higher than that of human bone, causing severe stress shielding effects [19, 20]. In viewing the disadvantages mentioned above, HEA or MEA films seem to be one of the solutions. These films can be easily coated on any bulk Ti or Ta based solid substrate such as cp (commercially pure) Ti, TiAlV and TiAlNb alloys or porous implant devices. In literatures, there have been reported that some HEA or MEA films can provide high hardness [21, 22], high wear resistance [23, 24] and high corrosion resistance [25-27], suggesting promising engineering applications. With proper alloy design, a promising biocompatible HEA or MEA film can be achieved. In this study, the TiTaNb films without any non-biofriendly elements are prepared by sputtering. This system with three elements is classified as MEA in this study. The in vitro electrochemical responses in simulated body fluid (SBF) and mechanical properties are reported and discussed. 2. Experimental materials and procedures Both Ti-33Ta-33Nb (in at%, namely, equimolar TiTaNb) MEA and Ti-10Ta-6Nb films (also in at%) were deposited on the P-type (100) silicon wafer by co-sputtering pure Ti, Ta and Nb targets. The purity of these targets were all 99.99% in wt%. Ti and Ta targets were placed on the DC diode and the Nb target on the RF diode. The silicon wafer used as substrate measures 5 × 10 mm2. The operator chamber before sputtering was firstly evacuated to a level below 5 × 10-7 torr, and then received highly pure argon (Ar) at a flow rate of 23 standard cubic 2

centimeters per minute (sccm) to maintain the working pressure at 3 × 10-3 torr during the sputtering process. The holder was rotated with a speed of 15.6 rpm. Besides, the pre-sputtering process was executed for 15 min before deposition by controlling the movable shutters. The thickness of sputtered film was kept to be about 700 nm. The compositions of TiTaNb and Ti-10Ta-6Nb films were examined by electron probe Xray micro analyzer (EPMA), with a working voltage set at 7 kV. The structure of both films were confirmed by D8 X-ray diffraction (XRD) with Cu-Kα radiation (λ=1.5406 Å). The working voltage was operated at 40 kV, and the electron current was set to be 40 mA at room temperature. The scanning range was from 20° to 80° and the scanning rate was 0.0125o per second. The surface morphology was characterized by JEOL JSM-6330 field-emission scanning electron microscope (SEM) in either secondary or back electronic image (SEI or BEI) mode. The detailed microstructure and the grain sizes of both TiTaNb and Ti-10Ta-6Nb films were examined under JEOL JEM-3010 analytical scanning transmission electron microscope (TEM), and the TEM samples were prepared by focus ion beam (FIB) milling. Moreover, the roughness of both films were measured by atomic force microscope (AFM) and the scanning areas were 5 × 5 μm2. The hardness and modulus of both films were measured by the Hysitron TI Premier nanoindentation equipped with diamond Berkovich indenter probe. The tests were conducted under the load control mode, with a Berkovich tip radius of ~150 nm. Each sample was received for at least 25 indents. The nano-scratch tests were also performed by the Hysitron TI Premier nanoindentation system using the scratch mode, equipped with diamond conical tip. The scratch velocity was 4 m/s with a ramping normal force to a maximum at 10 mN. To evaluate the biocompatibility response, the electrochemical properties and bio-corrosion behaviors of both films were characterized. The open circuit potential (OCP) tests, AC impendence tests and potentiodynamic polarization measurements were conducted by a CHI 614D potentiostat. The films with 5 × 5 mm2 acted as the working electrodes. Platinum plate and Ag/AgCl worked as counter electrode and reference electrode, respectively. The electrode system was immersed in simulated body fluid (SBF), with a composition of 0.137 M of NaCl, 5.4 mM of KCl, 0.25 mM of Na2HPO4, 0.44 mM of KH2PO4, 1.3 mM of CaCl2, 1.0 mM of MgSO4, and 4.2 mM of NaHCO3, pH = 7.4 at 37oC. The AC impedance tests and potentiodynamic polarization measurements were conducted after the value of OCP curve alternation became less than 2 mA per 5 min. The frequency in AC impedance test was set from 10-2 to 105 Hz and the amplitude was 10 mV. The potentiodynamic polarization measurement started from the OCP value subtracting 0.3 V up to 2 V with a scanning rate of 0.33 mV per second. The surface compositions and the depth profiles of both films after the immersion testing were measured by X-ray photoelectron spectroscopy (XPS). The target used in the

3

measurement was Mg and the working voltage and current were 10 kV and 5 mA respectively. Argon was used as etching gas and the etching rate was 0.47 nm/s. 3. Results and discussions 3.1 Basic characterization The XRD analysis of TiTaNb and Ti-10Ta-6Nb are presented in Fig. 1. It can be found that the TiTaNb MEA film exhibits a single-phase body centered cubic (BCC) structure while the Ti-10Ta-6Nb film presents the hexagonal close-packed (HCP) structure (still similar to pure Ti film). According to literatures [13, 28], there are two factors for the design of single-phase solid solution HEAs or MEAs, namely, the atomic size difference () and the mixing enthalpy (Hmix). In general, with 0% <  < 5% and -15 kJ/mol < Hmix < 5 kJ/mol, the single-phase solid solution can be obtained. Via calculation, the  and Hmix of the TiTaNb MEA film are 3.24% and 1.33 kJ/mol, respectively, well located in the region of forming single-phase solid solution. Moreover, an additional rule for determining the type of the phase in HEA or MEA has been proposed by Ye et al. [13] that HEAs or MEAs with a FCC or BCC structure would possess a valence electron concentration (VEC) around 8.5 and 5.0, respectively. In the case of TiTaNb, VEC is calculated to be 4.67, implying a BCC single phase should be obtained. The current XRD result for TiTaNb shown in Fig. 1 is consistent with the prediction. On the other hand, the crystal structure of Ti-10Ta-6Nb is HCP, consistent with the general observation for Ti based alloys or thin films. The strong (0002) texture in Fig. 1 is also frequently seen for Ti based thin films as a result of the fast deposition and cooling rate during sputtering [29, 30]. Figures 2(a) and 2(b) show the SEM micrographs of the TiTaNb MEA film at lower and enlarged magnitudes, while Figs. 2(c) and 2(d) present those of the Ti-10Ta-6Nb film. The average roughness (Ra) readings of both films measured by AFM are of very similar values, 7.6 nm for TiTaNb and 6.3 nm for Ti-10Ta-6Nb. It can be seen from Figs. 2(b) and 2(d) that the grain sizes of both films are smaller than 100 nm, but difficult to be precisely determined by SEM. The composition of both thin films are measured by EPMA, as listed in Table 1. The resulting film compositions are highly consistent with the set goals. It should be noted that there is basically no segregation of elements in both sputtered films. More detailed microstructure was revealed by cross-sectional TEM analysis, as presented in Fig. 3. The bright field images (Figs. 3(a) and 3(d)) and the corresponding dark field images (Figs. 3(b) and 3(e)) all show the columnar grains which are very common for sputtered films reported in literatures [30-32]. The grain sizes of both TiTaNb and Ti-10Ta-6Nb can be more precisely measured by TEM to be about 50 nm. The electron diffraction patterns are shown in Figs. 3(c) and 3(f), confirming the BCC crystal structure for TiTaNb and the HCP structure for Ti-10Ta-6Nb. 3.2 Mechanical properties 4

The Young’s modulus, hardness and friction coefficient of both TiTaNb and Ti-10Ta-6Nb films are measured by the nanoindentation system, as summarized in Table 2. The Young’s modulus of TiTaNb is about 145 GPa, which is not far from the calculated result using rule of mixture based on the respective modulus of Ti (116 GPa), Ta (186 GPa) and Nb (105 GPa). As for Ti-10Ta-6Nb, the Young’s modulus is 113 GPa, which is consistent to that of HCP Ti alloys. According to literatures [33, 34], lower Young’s modulus is required for long-time bulk implant pieces to avoid the stress shielding effect. Fortunately, the surface thin coating should not impost too much harmful effect in terms of stress shielding. It is the surface hardness or surface compatibility that is of the major concern. From Table 2, it can be seen that the hardness of TiTaNb MEA film is 8.2 GPa, which is superior to the 4.7 GPa for the Ti-10Ta-6Nb film. The much higher hardness of TiTaNb MEA film could be attributed to two factors. One appears to be the higher hardness of pure Ta (12.5 GPa) reported by Lai et al. [35], which is even higher than the Ta-based metallic glasses. The other is the solid solution strengthening caused by lattice distortion [36-38]. Note that the high hardness of 8.2 GPa is also significantly higher than that of pure Ti or Ti-6Al-6V (about 3-4 GPa). The nanoscratch results are presented in Fig. 4. It can be seen that the scratch depth increases almost linearly as a function of scratch distance with scratch distance from 0.5 to 1.3 m, as marked between the blue lines in Fig. 4(a). The linear dependence indicates that the scratch tip is pushed into the sample steadily without interference such as the peeling-off of film. The corresponding region (scratch distance from 0.5 to 1.3 m) with almost linear lateral force versus scratch distance is also marked between the blue lines in Fig. 4(b). When the scratch distance becomes greater than about 1.3 m, the slope rises in Fig. 4(b), resulting in the abrupt change of friction coefficient as shown in Fig. 4(c). Within this steady range from 0.5 to 1.3 m, the friction coefficients of TiTaNb and Ti-10Ta-6Nb films are very similar, with the value of 0.55 and 0.59, respectively. A lower friction coefficient indicates a better lubrication effect, reducing the adhesive wear to a lower wear rate [39]. Moreover, the relationship between the friction coefficient () and the wear volume loss (V) can be briefly derived from

dV =

KP H dx

,

(1)

where x is the scratch distance and H is the hardness of the sample [40]. P is the ramping load equivalent to Cx, where C is equivalent to 16.7 N/m in the test. Thus, the wear volume loss can be described as 1

C

V= 2KHx2 .

(2) 5

The calculated wear volume losses of TiTaNb MEA film and Ti-10Ta-6Nb film with scratch distance between 0.5 to 1.3 m are about 3.6x10-4 and 6.7x10-4 m3, respectively, as compared in Table 2. Overall, the mechanical properties of TiTaNb MEA films perform better than Ti10Ta-6Nb film, and much better than the commercial Ti-6Al-4V. 3.3 Bio-corrosion response Figures 5(a), 5(b), 5(c) and 5(d) present the open circuit potential (OCP) curves, the potentiodynamic polarization curves (known as the Tafel curves), the Nyquist plots, and the Bode plots, respectively, for the TiTaNb MEA and Ti-10Ta-6Nb films. It is well known that the OCP curves exhibits the relationship between corrosion potential and immersion time. As the curves gradually becomes smooth, it indicates that the passive layer (usually oxide) forms during the tests. After immersing in the SBF for 12,000 s, the OCP values of TiTaNb and Ti10Ta-6Nb are -0.279 V and -0.234 V, respectively. On the other hand, the corrosion potential (Ecorr) extracted from the Tafel curves is an important electrochemical parameter for evaluating the start of polarization reaction. The Ecorr values of TiTaNb and Ti-10Ta-6Nb are -0.266 V and -0.259 V, respectively, similar to the OCP data, as compared in Table 3. The values for TiTaNb and Ti-10Ta-6Nb are basically similar. Some data on other compatible bio-implant metals are also included for comparison. For all these Ti or Ta based alloys, their Ecorr readings are all low enough, suggesting they will all form protective oxide layers easily. More importantly, Fig. 5(b) shows that there is no pitting reaction for both for TiTaNb and Ti-10Ta6Nb films until 2 V, implying the promising of application in human body environment under about 0-0.2 V. Without pitting, the breakdown of protective passive film due to the local corrosion will not take place, sustaining the life of bio-implant [41]. Another essential parameter for bio-corrosion is the corrosion current density (Icorr), presenting the corrosion rate of the material [42]. With a lower Icorr value, the material would experience a lower corrosion rate, which is preferable in application. The Icorr of TiTaNb MEA film is 0.021 A/cm2, which is lower than that of Ti-10Ta-6Nb (0.077 A/cm2), indicating the better performance in corrosion resistance. Note from Table 3 that the Icorr reading of 0.021 A/cm2 for the TiTaNb MEA film is lower than any other Icorr values reported in literature, ranging from 0.03 to 1.1 A/cm2. The very low Icorr value directly indicates the very low biocorrosion activity of this MEA film in SBF, a very favorable indication. In addition to Icorr, AC impedance is another method to analysis the corrosion resistance. Figure 5(c) presents the Nyquist plots and the equivalent circuit model fitting for TiTaNb and Ti-10Ta-6Nb film gives the calculated polarization impedance (Rp) of 15.31×105 and 5.67× 105 cm2, respectively. The Nyquist plots for pure Ta, pure Ti and Ta75Ti10Zr8Si7 metallic glass films reported in the literature [35] are also replicated in Fig. 5(c) in order to have a clear comparison. As shown in Fig. 5(c), the diameter of semi-circle for the Ti-10Ta-6Nb film is 6

similar to those results reported in the literature [35], indicating similar corrosion resistance impedance. On the other hand, the diameter of semi-circle for TiTaNb MEA film is obviously larger than others. It means that the passive film on the TiTaNb MEA film has the highest insulating and capacitive properties, and has the largest charge transfer resistance, indicating better corrosion resistance impedance [43]. The Rp value of 15.31×105 cm2 is the highest among all data in Table 3. In addition, the Bode plots shown in Fig. 5(d) also indicate that the corrosion impedance of TiTaNb film has an advantage over Ti-10Ta-6Nb in low frequency region. The higher phase angle for TiTaNb indicates that it has higher capacitive and more protective passive film. The AC impedance results are consistent with the Icorr results, both showing the best bio-corrosion resistance for TiTaNb MEA film, in comparison with all other reported Ti and Ta based bio-implant metals. 3.4 Bio-corrosion protective layers The XPS results of the passive layers on both TiTaNb and Ti-10Ta-6Nb films after immersing in SBF for 7 days are presented in Fig. 6. Both films present the similar passive layers of TiO2, Ta2O5 and Nb2O5, as shown in Figs. 6(a), 6(b) and 6(c), respectively. It can be found that the XPS Ti 2p peak for TiTaNb film is much lower than that for Ti-10Ta-6Nb, while the Ta 4f and Nb 3d peaks are much higher. Moreover, the O 1s result shown in Fig. 6(d) indicates that both oxide and hydroxyl groups are present in the passive films. The mechanisms of the formation of passive films are schematically illustrated in Fig. 7. The metals firstly turn into metallic ions by releasing their electrons to the surface, and the hydroxyl groups would be formed from the SBF at the same time. The essential reactions are demonstrated by following equations. Ti → Ti2++2e-,

(3)

Ta → Ta5+ + 5e-,

(4)

Nb → Nb5+ + 5e-,

(5)

2H2O + 2e- → H2 + OH-.

(6)

The hydroxyl groups react firstly with those selectively dissolved metallic ions to form intermediate hydroxides, and then convert into oxides. Besides, a large number of nucleation sites result in the precipitation of Ti, Ta and Nb oxides, and the fine passive particles can be formed. The whole related chemical reactions would be Ti + 2H2O → TiO2 + 4H+ + 4e- (anodic reaction), 7

(7)

2Ta + 5H2O → Ta2O5 + 10H+ + 10e- (anodic reaction),

(8)

2Nb + 5H2O → Nb2O5 + 10H+ + 10e- (anodic reaction),

(9)

2H+ + 2e- → H2 (cathodic reaction),

(10)

in consistent with the XPS results. The depth profiles of both films are shown in Fig. 8. From the depth profiles, the thickness of oxide passive layer appears to be smaller than 6 nm. It would provide more evidence to demonstrate the formation of the oxides from pure metals. Besides, the standard Gibbs free energy of formation of the Ta and Nb oxides or sub-oxides such as NbO, NbO2, Nb2O5, TaO2 and Ta2O5 are −380, −741, −1,764, −180 and −1,908 kJ/mol, respectively. From the concept of Gibbs free energy, the formation of oxide is more stable when the standard Gibbs free energy is more negative with large magnitude, favorable for Nb2O5 (−1,746 kJ/mol) and Ta2O5 (−1,908 kJ/mol). The XPS results are consistent with this trend, showing the high-valence Nb2O5 and Ta2O5 oxides which are more stable than suboxides [44]. For the current MEA TiTaNb film, the oxides are a mixture of Ta2O5, Nb2O5, and TiO2, with more Ta2O5 followed Nb2O5 and TiO2. According to the literature, the improvement in corrosion resistance is due to the increasing of Ta and Nb, forming more stable Ta2O5 and Nb2O5 at the surface to lower the corrosion rate [8, 45, 46]. In comparison, TiO2 is the majority in Ti-10Ta-6Nb due to its higher content of Ti. The corroded surface (without any additional treatment after the bio-corrosion except drying) of both TiTaNb MEA and Ti-10Ta-6Nb films are also examined by SEM, as shown in Fig. 9. The darker streaks are the remnant of solution consisting mainly carbon. It can be found that both films present rougher surfaces after corrosion, shown in Figs. 9(a) and 9(c) as well as the enlarged images shown in Figs. 9(b) and 9(d). Moreover, there are some small cracks found on the corroded surface of Ti-10Ta-6Nb film, as pointed out by red arrows in Figs. 9(c) and 7(d). The sizes of the cracks are about 500 nm. There is no crack on the corroded surface of TiTaNb MEA film, indicating a better protection from bio-corrosion by the sound oxides of Ta2O5, Nb2O5 and TiO2 in MEA TiTaNb, as compared with the Ti-10Ta-6Nb film. 3.5

Closing remarks The overall comparison of the current bio-corrosion response with those obtained from previous investigations [18, 35, 47-49] are presented in Table 3. For example, by comparing the corrosion current density (Icorr) of many related materials, the current TiTaNb MEA film exhibits outstanding corrosion resistance, exhibiting the lowest Icorr reading. Moreover, it has been found that the more disordered lattice (such as simple solid solution) would result in the lower corrosion rates [50]. Such more disordered lattice could usually increase the surface 8

reactivity for oxide film formation, leading to higher corrosion resistance [50, 51]. Consistent with this concept, it is reasonable to see the lowest Icorr value of the current TiTaNb MEA film (0.021 A/cm2), even lower than that for pure Ta film (0.061 A/cm2) in the previous study of our group [35], demonstrating an outstanding bio-corrosion resistance. In future, this MEA TiTaNb film will be further examined in terms of the effect of film thickness, the interface adhesion with various substrates, the performance of cell attachment, adhesion and viability, etc, in order to match to the real bio-implant condition and to ensure the feasibility to be applied as a promising bio-implant coating material. 4. Conclusions In this study, the potential of TiTaNb MEA film for bio-implant material is evaluated via mechanical property tests, electrochemical property measurements and surface properties analyses. All the selected elements of Ti, Ta, and Nb are highly bio-friendly. Besides, TiTaNb MEA film owns outstanding bio-corrosion resistance, higher wear resistance and higher hardness. The surface of the TiTaNb equimolar MEA can form protective layers simultaneously. Although the Young’s modulus of TiTaNb MEA film is higher than human bones, the surface coating would not change the Young’s modulus of the coated bulk materials. In conclusion, TiTaNb MEA film is demonstrated in this study to be a high potential candidate for bio-implant applications. Acknowledgements The authors gratefully acknowledge the sponsorship from Ministry of Science and Technology of Taiwan, ROC, under the project no. MOST 105-2221-E-110-019-MY3, and from City University of Hong Kong under the grants no. 9380088, 7005078 and 9231348. References [1] R. Kumari, T. Scharnweber, W. Pfleging, H. Besser, J.D. Majumdar, Laser surface textured titanium alloy (Ti–6Al–4V)–Part II–Studies on bio-compatibility, Appl. Surf. Sci. 357 (2015) 750-758. [2] R. Narayan, ASM Handbook, Volume 23, Materials for Medical Devices, Materials Park: ASM International, 2012. [3] S. Bagherifard, Mediating bone regeneration by means of drug eluting implants: from passive to smart strategies, Mater. Sci. Eng. C 71 (2017) 1241-1252. [4] M. Niinomi, Mechanical properties of biomedical titanium alloys, Mater. Sci. Eng. A 243 (1998) 231-236. [5] 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. 9

[6] Y.L. Zhou, M. Niinomi, T. Akahori, H. Fukui, H. Toda, Corrosion resistance and biocompatibility of Ti–Ta alloys for biomedical applications, Mater. Sci. Eng. A 398 (2005) 28-36. [7] Y.L. Zhou, M. Niinomi, Ti–25Ta alloy with the best mechanical compatibility in Ti–Ta alloys for biomedical applications, Mater. Sci. Eng. C 29 (2009) 1061-1065. [8] A.H. Hussein, M.A.H. Gepreel, M.K. Gouda, A.M. Hefnawy, S.H. Kandil, Biocompatibility of new Ti–Nb–Ta base alloys, Mater. Sci. Eng. C 61 (2016) 574-578. [9] D. Kuroda, M. Niinomi, M. Morinaga, Y. Kato, T. Yashiro, Design and mechanical properties of new β type titanium alloys for implant materials, Mater. Sci. Eng. A 243 (1998) 244-249. [10] T. Wei, J. Huang, C.Y. Chao, L. Wei, M. Tsai, Y. Chen, Microstructure and elastic modulus evolution of TiTaNb alloys, J. Mech. Behav. Biomed. Mater. 86 (2018) 224-231. [11] M. Stiehler, M. Lind, T. Mygind, A. Baatrup, A. Dolatshahi‐Pirouz, H. Li, M. Foss, F. Besenbacher, M. Kassem, C. Bünger, Morphology, proliferation, and osteogenic differentiation of mesenchymal stem cells cultured on titanium, tantalum, and chromium surfaces, J. Biomed. Mater. Res. A 86 (2008) 448-458. [12] D.M. Findlay, K. Welldon, G.J. Atkins, D.W. Howie, A.C. Zannettino, D. Bobyn, The proliferation and phenotypic expression of human osteoblasts on tantalum metal, Biomaterials 25 (2004) 2215-2227. [13] Y. Ye, Q. Wang, J. Lu, C. Liu, Y. Yang, High-entropy alloy: challenges and prospects, Mater. Today 19 (2016) 349-362. [14] W. Li, P. Liu, P.K. Liaw, Microstructures and properties of high-entropy alloy films and coatings: a review, Mater. Res. Lett. 6 (2018) 199-229. [15] J.W. Yeh, S.K. Chen, S.J. Lin, J.Y. Gan, T.S. Chin, T.T. Shun, C.H. Tsau, S.Y. Chang, Nanostructured high‐entropy alloys with multiple principal elements: novel alloy design concepts and outcomes, Adv. Eng. Mater. 6 (2004) 299-303. [16] B. Cantor, I. Chang, P. Knight, A. Vincent, Microstructural development in equiatomic multicomponent alloys, Mater. Sci. Eng. A 375 (2004) 213-218. [17] M. Todai, T. Nagase, T. Hori, A. Matsugaki, A. Sekita, T. Nakano, Novel TiNbTaZrMo high-entropy alloys for metallic biomaterials, Scr. Mater. 129 (2017) 65-68. [18] S.P. Wang, J. Xu, TiZrNbTaMo high-entropy alloy designed for orthopedic implants: As-cast microstructure and mechanical properties, Mater. Sci. Eng. C 73 (2017) 80-89. [19] S. Ren, J. Huang, M. Cui, J. Pu, L. Wang, Improved adaptability of polyaryl-ether-etherketone with texture pattern and graphite-like carbon film for bio-tribological applications, Appl. Surf. Sci. 400 (2017) 24-37. [20] B.H. Moon, H.C. Choe, W.A. Brantley, Surface characteristics of TiN/ZrN coated nanotubular structure on the Ti–35Ta–xHf alloy for bio-implant applications, Appl. Surf. Sci. 258 (2012) 2088-2092. 10

[21] X. Li, Z. Zheng, D. Dou, J. Li, Microstructure and properties of coating of FeAlCuCrCoMn high entropy alloy deposited by direct current magnetron sputtering, Mater. Res. 19 (2016) 802-806. [22] X. Feng, G. Tang, L. Gu, X. Ma, M. Sun, L. Wang, Preparation and characterization of TaNbTiW multi-element alloy films, Appl. Surf. Sci. 261 (2012) 447-453. [23] L. Jiang, W. Wu, Z. Cao, D. Deng, T. Li, Microstructure evolution and wear behavior of the laser cladded CoFeNi 2 V 0.5 Nb 0.75 and CoFeNi 2 V 0.5 Nb high-entropy alloy coatings, J. Therm. Spray Techn. 25 (2016) 806-814. [24] C. Huang, Y. Zhang, R. Vilar, J. Shen, Dry sliding wear behavior of laser clad TiVCrAlSi high entropy alloy coatings on Ti–6Al–4V substrate, Mater. Des. 41 (2012) 338343. [25] Q. Ye, K. Feng, Z. Li, F. Lu, R. Li, J. Huang, Y. Wu, Microstructure and corrosion properties of CrMnFeCoNi high entropy alloy coating, Appl. Surf. Sci. 396 (2017) 14201426. [26] Q. Li, T. Yue, Z. Guo, X. Lin, Microstructure and corrosion properties of AlCoCrFeNi high entropy alloy coatings deposited on AISI 1045 steel by the electrospark process, Metall. Mater. Trans. A 44 (2013) 1767-1778. [27] D. Dou, X. Li, Z. Zheng, J. Li, Coatings of FeAlCoCuNiV high entropy alloy, Surf. Eng. 32 (2016) 766-770. [28] Y. Zhang, Y.J. Zhou, J.P. Lin, G.L. Chen, P.K. Liaw, Solid‐solution phase formation rules for multi‐component alloys, Adv. Eng. Mater. 10 (2008) 534-538. [29] I. Polmear, D. StJohn, J.-F. Nie, M. Qian, Light alloys: metallurgy of the light metals, Butterworth-Heinemann, 2017. [30] G. Liu, Y. Yang, B. Huang, X. Luo, S. Ouyang, G. Zhao, N. Jin, P. Li, Effects of substrate temperature on the structure, residual stress and nanohardness of Ti6Al4V films prepared by magnetron sputtering, Applied Surface Science, 370 (2016) 53-58. [31] A.A. Ivanova, M.A. Surmeneva, A. Tyurin, T. Pirozhkova, I. Shuvarin, O. Prymak, M. Epple, M. Chaikina, R.A. Surmenev, Fabrication and physico-mechanical properties of thin magnetron sputter deposited silver-containing hydroxyapatite films, Appl. Surf. Sci. 360 (2016) 929-935. [32] D. Zhou, H. Zhu, X. Liang, C. Zhang, Z. Li, Y. Xu, J. Chen, L. Zhang, Y. Mai, Sputtered molybdenum thin films and the application in CIGS solar cells, Appl. Surf. Sci. 362 (2016) 202-209. [33] A. Gefen, Computational simulations of stress shielding and bone resorption around existing and computer-designed orthopaedic screws, Med. Biol. Eng. Comput. 40 (2002) 311322. [34] M. Niinomi, M. Nakai, Titanium-based biomaterials for preventing stress shielding between implant devices and bone, Int. J. Biomater. 2011 (2011) 836587. 11

[35] J. Lai, Y. Lin, C. Chang, T. Wei, J. Huang, Z. Liao, C. Lin, C. Chen, Promising Ta-TiZr-Si metallic glass coating without cytotoxic elements for bio-implant applications, Appl. Surf. Sci. 427 (2018) 485-495. [36] Z. Wang, Q. Fang, J. Li, B. Liu, Y. Liu, Effect of lattice distortion on solid solution strengthening of BCC high-entropy alloys, J. Mater. Sci. Technol. 34 (2018) 349-354. [37] M. Walbrühl, D. Linder, J. Ågren, A. Borgenstam, Modelling of solid solution strengthening in multicomponent alloys, Mater. Sci. Eng. A 700 (2017) 301-311. [38] Y. Wu, Y. Cai, X. Chen, T. Wang, J. Si, L. Wang, Y. Wang, X. Hui, Phase composition and solid solution strengthening effect in TiZrNbMoV high-entropy alloys, Mater. Des. 83 (2015) 651-660. [39] X. Sui, J. Liu, S. Zhang, J. Yang, J. Hao, Microstructure, mechanical and tribological characterization of CrN/DLC/Cr-DLC multilayer coating with improved adhesive wear resistance, Appl. Surf. Sci. 439 (2018) 24-32. [40] A. Hodge, T. Nieh, Evaluating abrasive wear of amorphous alloys using nanoscratch technique, Intermetallics 12 (2004) 741-748. [41] J.M. Kolotyrkin, Pitting corrosion of metals, Corrosion 19 (1963) 261-268. [42] C. Zhao, F. Pan, L. Zhang, H. Pan, K. Song, A. Tang, Microstructure, mechanical properties, bio-corrosion properties and cytotoxicity of as-extruded Mg-Sr alloys, Mater. Sci. Eng. C 70 (2017) 1081-1088. [43] R. Ion, S.I. Drob, M.F. Ijaz, C. Vasilescu, P. Osiceanu, D.-M. Gordin, A. Cimpean, T. Gloriant, Surface Characterization, Corrosion Resistance and in Vitro Biocompatibility of a New Ti-Hf-Mo-Sn Alloy, Materials (Basel) 9 (2016) 818. [44] H.Z. Li, X. Zhao, J. Xu, MRI-compatible Nb–60Ta–2Zr alloy for vascular stents: Electrochemical corrosion behavior in simulated plasma solution, Mater. Sci. Eng.C, 56 (2015) 205-214. [45] D.B. Wei, X.H. Chen, P.Z. Zhang, F. Ding, F.K. Li, Z.J. Yao, Plasma surface tantalum alloying on titanium and its corrosion behavior in sulfuric acid and hydrochloric acid, Appl.Surf. Sci. 441 (2018) 448-457. [46] P.P. K, D. N, K. Manikantan Syamala, R. N, Antibacterial effects, biocompatibility and electrochemical behavior of zinc incorporated niobium oxide coating on 316L SS for biomedical applications, Appl. Surf. Sci. 427 (2018) 1166-1181. [47] A. Biesiekierski, J. Lin, Y. Li, D. Ping, Y. Yamabe-Mitarai, C. Wen, Investigations into Ti–(Nb,Ta)–Fe alloys for biomedical applications, Acta Biomater. 32 (2016) 336-347. [48] J.Y. Jiang, J.L. Xu, Z.H. Liu, L. Deng, B. Sun, S.D. Liu, L. Wang, H.Y. Liu, Preparation, corrosion resistance and hemocompatibility of the superhydrophobic TiO2 coatings on biomedical Ti-6Al-4V alloys, Appl. Surf. Sci. 347 (2015) 591-595. [49] S. Tamilselvi, N. Rajendran, In vitro corrosion behaviour of Ti-5Al-2Nb-1Ta alloy in Hanks solution, Mater. Corros. 58 (2007) 285-289. 12

[50] R. Ramanauskas, R. Juškėnas, A. Kaliničenko, L.F. Garfias-Mesias, Microstructure and corrosion resistance of electrodeposited zinc alloy coatings, J. Solid State Electrochem. 8 (2004) 416-421. [51] R. Ramanauskas, Structural factor in Zn alloy electrodeposit corrosion, Appl. Surf. Sci. 153 (1999) 53-64.

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Table 1

The composition of TiTaNb and Ti-10Ta-6Nb coating.

Material

Ti

Ta

Nb

TiTaNb Ti-10Ta-6Nb

33.6±0.7 83.9±0.1

33.4±0.8 10.1±0.2

33.0±0.4 6.0±0.2

Table 2

The mechanical properties of TiTaNb and Ti-10Ta-6Nb coating.

Materials

Structure

Young’s modulus (GPa)

Hardness (GPa)

Friction coefficient

Wear volume

TiTaNb Ti-10Ta-6Nb

BCC HCP

145±13 113±11

8.2±1.2 4.7±0.9

0.55±0.03 0.59±0.02

3.6x10-4 6.7x10-4

Table 3

loss (m3)

The bio-corrosion properties of the TiTaNb and Ti-10Ta-6Nb films, in comparison with others reported in literatures.

TiTaNb film

Ecorr (V) -0.266±0.008

Epit (V) >2

Icorr Ipass Rp 2 2 5 (A/cm ) (A/cm ) (10 cm2) 0.021±0.005 1.82±0.45 15.31±2.95

Current

Ti-10Ta-6Nb film

-0.259±0.021

>2

0.077±0.016 6.29±0.62

5.67±0.92

Current

Pure Ti film

-0.252

>2

1.101

47.96

2.87

[35]

Ta57Ti17Zr15Si11 film Ta75Ti10Zr8Si7 film Pure Ta film Ti-12Nb-5Fe bulk Ti-7Ta-5Fe bulk Ti-10Ta-5Fe bulk Ti-6Al-4V bulk

-0.304 -0.336 -0.432 -0.23 -0.25 -0.24 -0.404

>2 >2 >2 >2 >2 >2 >2

0.077 0.083 0.061 0.20 0.08 0.50 0.221

3.84 5.01 3.3 ---3.16

4.61 4.64 3.47 ---1.8

[35] [35] [35] [47] [47] [47] [48]

Ti-6Al-4V with MAO

-0.221

>2

0.133

5.6

2.37

[48]

Ti-5Al-2Nb-1Ta TiZrNbTaMo

-0.435 -0.607

>2 2

0.485 0.03

12 1

5.6 --

[49] [18]

Material

14

Ref.

20

30

40

50

HCP (10-13)

HCP (0002)

HCP (10-12)

Intensity

BCC (200)

BCC (211)

BCC (110)

TiTaNb Ti-10Ta-6Nb

60

70

80

2 (degree)

Figure 1

The XRD analysis of TiTaNb and Ti-10Ta-6Nb films.

Figure 2 The SEM SEI images of (a)(b) TiTaNb film at lower and enlarged magnitudes, and (c)(d) Ti-10Ta-6Nb film at lower and enlarged magnitudes. 15

Figure 3 The cross-sectional TEM results for TiTaNb: (a) bright field, (b) dark field images and (c) diffraction pattern. The cross-sectional TEM for Ti-10Ta-6Nb: (a) bright field, (b) dark field images and (c) diffraction pattern.

16

20

TiTaNb Ti-10Ta-6Nb

Scratch depth (nm)

(a) 0 -20 -40 -60 -80

0

2

1

3

Scratch distance (m)

Lateral Force (μN)

TiTaNb Ti-10Ta-6Nb

(b)

600 500 400 300 200 100 0

0

2

1

3

Scratch distance (μm)

1.1

(c)

Friction coefficient

1.0

TiTaNb Ti-10Ta-6Nb

0.9 0.8 0.7 0.6 0.5 0.4 0

2

1

3

Scratch distance (μm)

Figure 4 The nano-scratch results of both TiTaNb and Ti-10Ta-6Nb films: (a) the scratch depth as a function of scratch distance, (b) the lateral force as a function of scratch distance, and (c) the friction coefficient as a function of scratch distance. 17

-4 -5

(b)

log i (A/cm2)

-6 -7 -8 -9 TiTaNb Ti-10Ta-6Nb

-10 -0.5

0.0

0.5

1.0

1.5

Potential v.s Ag/AgCl (V)

18

2.0

-6

Z'' (105 ohm cm2)

-5

(c)

-4 -3 TiTaNb (This study) Ti-10Ta-6Nb (This study) Pure Ta [35] Pure Ti [35] Ta75Ti10Zr8Si7 [35]

-2 -1 0 0.0

2.5

2.0

1.5

1.0

0.5

3.0

Z' (105 ohm cm2)

8

100

(d)

TiTaNb Ti-10Ta-6Nb

90 80

mag (dB)

7

70

6

60

5

50

4

40 30

3

20

2 1 -2

-phase (degree)

9

10

-1

0

1

2

3

4

5

0

log frequency (Hz) Figure 5 The (a) OCP curves, (b) Tafel curves, (c) Nyquist plots and (d) Bode plots for the TiTaNb and Ti-10Ta-6Nb films.

19

Figure 6 The XPS analysis of (a) Ti 2p, (b) Ta 4f, (c) Nb 3d and (d) O 1s peak on the passive layer of TiTaNb film and Ti-10Ta-6Nb film after 7-day immersion test in SBF.

20

21

Figure 7 The process of corrosion for TiTaNb film: (a) pre-reaction state, (b)(c) intermediate states and (d) final state.

22

(a) TiTaNb

Ti

468

466

464

462

460

458

456

454

452

Ta

450

28

26

Binding energy (eV)

20

Ti 2p

Nb

204

202

468

466

464

462

460

458

456

454

452

Binding energy (eV)

(f) Ti-10Ta-6Nb

Surface Etching 3nm Etching 6nm

Surface Etching 3nm Etching 6nm

Nb 3d Nb2O5

CPS

CPS

Ta 4f Ta2O5

TiO2

Ti

Binding energy (eV)

(e) Ti-10Ta-6Nb

Surface Etching 3nm Etching 6nm

CPS

CPS

Nb 3d Nb2O5

206

22

(d) Ti-10Ta-6Nb

Surface Etching 3nm Etching 6nm

208

24

Binding energy (eV)

(c) TiTaNb

210

Surface Etching 3nm Etching 6nm

Ta 4f Ta2O5

CPS

TiO2

CPS

Ti 2p

(b) TiTaNb

Surface Etching 3nm Etching 6nm

Ta

28

26

24

Nb

22

210

Binding energy (eV)

208

206

204

202

Binding energy (eV)

Figure 8 The depth profiles of (a) Ti 2p, (b) Ta 4f and (c) Nb 3d peak of the MEA TiTaNb film. The depth profiles of (d) Ti 2p, (e) Ta 4f and (f) Nb 3d peak of the Ti-10Ta-6Nb film.

23

Figure 9 The SEM SEI images of corroded surfaces of the (a)(b) TiTaNb film at lower and enlarged magnitudes, (c)(d) Ti-10Ta-6Nb film at lower and enlarged magnitudes. The small cracks measuring about 500 nm in the Ti-10Ta-6Nb film are indicated by red arrows.

24

Declaration of Interest Statement This paper reports the new exciting findings on the bio-corrosion and biocompatibility of TiTaNb medium entropy alloy films. Some high or medium entropy alloys (HEAs and MEAs) have been demonstrated to exhibit great mechanical and bio-corrosion properties to meet the demands as bio-implant materials. However, the difference in Young’s modulus between these alloys and human bone would induce stress shielding effects. Therefore, HEA or MEA coating films appear to be potential materials since these films can be coated on appropriate implant devices as surface modification. This study employs sputtering to produce equimolar TiTaNb MEA film and a titanium-based Ti-10Ta-6Nb film in order to compare their mechanical and bio-corrosion properties. The results indicate that the TiTaNb MEA films have higher hardness, higher wear resistance, greater biocompatibility and superior bio-corrosion resistance, likely to be a high potential bio-implant coating materials. The bio-corrosion and biocompatibility performance of this new alloy film is the best as compared with all related other metallic materials. The underlying related chemical reactions and mechanisms are also explored and discussed.

25

Credit Author Statement YH Chen and HS Chou are responsible for the sputter work. YH Chen, WS Chuang and PH Lin are responsible for the film microstructure characterization and mechanical testing. YH Chen and X Wang are responsible for the bio-corrosion testing and analysis. YH Chen and YJ Lai are responsible for the XPS work. YH Chen and Prof. JC Huang are responsible for the paper writing and research planning. Prof. JC Huang is the corresponding author.

26

Graphic Abstract

27

Highlights ► The TiTaNb MEA films have higher hardness and wear resistance as promising coating films. ► The TiTaNb MEA films exhibit superior bio-corrosion resistance than other Ti alloy films. ► The underlying related chemical reactions and mechanisms are explored and discussed.

28