Mechanical and Anticorrosive Properties of TiNbTa and TiNbTaZr Films on Ti-6Al-4V Alloy

Abstract

In this study, TiNbTa and TiNbTaZr films were utilized as protective coatings on a Ti-6Al-4V alloy to inhibit corrosive attacks from NaCl aqueous solution and simulated body fluid. The structural and mechanical properties of multicomponent TiNbTa(Zr) films were investigated. The corrosion resistance of the TiNbTa(Zr)-film-modified Ti-6Al-4V alloy was evaluated using potentiodynamic polarization tests in a NaCl aqueous solution. The results indicate that the TiNbTa(Zr) films with high Ti and Zr contents exhibited inferior corrosive resistance related to the films with high Ta and Nb contents. Moreover, the TiNbTa(Zr)-coated Ti-6Al-4V plates were immersed in Ringer’s solution for eight weeks; this solution was widely used as a simulated body fluid. The formation of surficial oxide layers above the TiNbTa(Zr) films was examined using transmission electron microscopy and X-ray photoelectron spectroscopy, which prevented the elution of Al and V from the Ti-6Al-4V alloy. Ti₃₃Nb₁₉Ta₂₁Zr₂₇, Ti₁₅Nb₆₈Ta₈Zr₉, and Ti₈Nb₈Ta₇₉Zr₅ films are suggested as preferential candidates for TiNbTa(Zr)/Ti-6Al-4V assemblies applied as biocompatible materials.

1. Introduction

The development of biocompatible long-term implants without evident side effects has attracted the interest of researchers [1,2]. The literature indicates that high strength, high-wear resistance, and low Young’s modulus are the crucial requirements for orthopedic long-term implantation [3,4]. Among these material characteristics, low Young’s modulus is the essential property preventing the “stress shielding” effect [5]. In contrast to periprosthetic bone with a Young’s modulus less than 30 GPa [6], the stiffer artificial implants endure the majority of the load, resulting in bone resorption accompanied with porous constructions [7,8,9]. Ti-base alloys with a low Young’s modulus were utilized as implants [6]. Ti-6Al-4V has been widely used in medical applications due to its excellent mechanical properties, corrosion resistance, and biocompatibility [10,11,12]. However, the eluted toxic ions, aluminum and vanadium, could cause undesirable neurological disorders, Alzheimer’s disease, and local inflammation in the biological system [13], and the elution behavior needs to be inhibited. Ti-based alloys comprise two distinct crystalline structures, α-phase and β-phase, which are hexagonal close-packed (hcp) and body-centered cubic (bcc) phases [14], respectively. Nb, Ta, and Mo are recognized as β-phase stabilizers for Ti-alloys and β-phase Ti-alloys exhibit lower Young’s modulus and higher strength related to those of α-phase Ti-alloys [14,15,16]. TiNbTa [3], TiNbZrTa, and TiMoZrTa [14] alloys have been proposed as alternatives for biocompatible materials. Ti, Nb, and Ta elements, as well as Zr, Ru, Sn, and Au, exhibit exceptional biocompatibility in the human body [17]. Moreover, TiNbTa alloys have been shown to exhibit excellent corrosion resistance in contrast to the Ti-6Al-4V alloy due to the formation of a passivation oxide film consisting of Nb₂O₅ and Ta₂O₅; therefore, TiNbTa alloys without the elution effect of Al and V ions have been proposed as candidates for orthopedic implants [3].

Combining TiTaNb films and a commercial Ti-6Al-4V alloy has been recommended for application as implants [18]; however, the number of investigations into the aforementioned combination is limited. Lai et al. [19] have reported that TiNbZr and TiNbZrTa films fabricated through cathodic arc evaporation assisted the survival and growth of cells and were applied for orthopedic or dental implants. TiTaHfNbZr high-entropy alloy films prepared on Ti-6Al-4V have been explored [20,21], and these TiTaHfNbZr films exhibited an amorphous structure. This study aimed to evaluate the feasibility of TiNbTa(Zr)/Ti-6Al-4V assembly for implant materials. The low- and medium-entropy TiNbTa(Zr) films were fabricated through co-sputtering. The chemical compositions of TiNbTa(Zr) films were adjusted by distinct sputter powers applied to these targets. The phase structures and mechanical properties of the TiNbTa(Zr) films were studied. The elemental effects on the corrosion resistance of TiNbTa(Zr) films in NaCl solutions and the stability of TiNbTa(Zr) films in Ringer’s solution were explored.

2. Materials and Methods

TiNbTa(Zr) films were deposited on p-type (100) silicon wafers and Ti-6Al-4V coupons through co-sputtering with targets of pure Ti, Nb, Ta, and Zr. The sputtering apparatus is described in [22]. The Ti-6Al-4V coupons with dimensions of 20 × 20 × 3 mm³ were grounded and polished before deposition. The substrates were sputtered with a Ti interlayer for enhancing the adhesion between the TiNbTa(Zr) films and substrates. Then, various powers presented in

Table 1. Co-sputtering parameters and chemical compositions of TiNbTa and TiNbTaZr films.

Sample	Sputter Power (W)				Chemical Composition (at.%)					ΔSmix 1
	PTi	PNb	PTa	PZr	Ti	Nb	Ta	Zr	O	(R)
Ti83Nb10Ta7	200	20	10	—	76.3 ± 0.5	9.3 ± 0.1	6.9 ± 0.0	—	7.5 ± 0.4	0.58
Ti38Nb30Ta32	180	80	70	—	36.4 ± 0.1	28.5 ± 0.1	30.1 ± 0.1	—	5.0 ± 0.1	1.09
Ti10Nb80Ta10	30	200	20	—	9.2 ± 0.1	76.2 ± 0.3	10.0 ± 0.0	—	4.6 ± 0.4	0.64
Ti12Nb4Ta84	70	20	200	—	10.9 ± 0.2	4.2 ± 0.6	78.2 ± 1.3	—	6.7 ± 2.0	0.54
Ti61Nb13Ta11Zr15	200	30	20	30	61.3 ± 0.5	12.6 ± 0.2	11.3 ± 0.3	14.8 ± 0.0	—	1.09
Ti33Nb19Ta21Zr27	180	80	70	140	32.8 ± 0.2	18.9 ± 0.1	21.0 ± 0.3	27.3 ± 0.1	—	1.36
Ti15Nb68Ta8Zr9	70	200	20	30	14.5 ± 0.3	68.2 ± 0.3	8.1 ± 0.4	9.2 ± 0.2	—	0.96
Ti19Nb10Ta9Zr62	70	30	200	30	19.1 ± 0.2	10.5 ± 0.1	8.5 ± 0.0	61.9 ± 0.1	—	1.06
Ti8Nb8Ta79Zr5	70	30	20	200	7.6 ± 0.2	8.1 ± 0.1	78.9 ± 0.6	5.4 ± 0.6	—	0.74

¹ ΔS_mix: Mixing entropy.

were set for co-sputtering TiNbTa and TiNbTaZr films with sputtering times of 50 and 90 min, respectively. The base pressure of the sputtering chamber was below 4 ×10⁻⁵ Pa, whereas the working pressure was set at 1.6 ×10⁻¹ Pa under a 20 sccm flow of pure Ar. The substrate holder was rotated at a speed of 30 rpm during the co-sputtering process in order to homogenize the films’ compositions. These films were further immersed in a Ringer’s solution, a simulated body fluid, at 37 °C for eight weeks to evaluate the elution test.

The chemical compositions of the TiNbTa(Zr) films were examined using a field-emission electron probe microanalyzer (JXA-iHP200F, JEOL, Akishima, Japan). The phase constitutions of the films were confirmed using an X-ray diffractometer (XRD, X’Pert PRO MPD, PANalytical, Almelo, The Netherlands) under the grazing incident mode. The lattice constants, a₀, of cubic phases were evaluated according to the following equation [23]:

$$\mathit{a}=a_0+K\times \frac{{\cos }^2\theta }{\sin \theta }$$

(1)

where a is the lattice constant of the individual reflection, θ is the diffraction angle, and K is a constant. The grain sizes of the films were evaluated on the basis of a Bragg–Brentano scan (θ–2θ scan) mode by using the Scherrer formula [23]:

$$\mathit{D}=\frac{0.9\mathit{\lambda }}{\mathit{\beta }\cos {\theta }_B}$$

(2)

where D is the average grain size, β is the full width at half maximum (FWHM) of reflection, θ_B is the Bragg angle, and λ is the X-ray wavelength.

The nanostructures of these films with protective C and Pt layers, prepared using a focused ion beam system (NX2000, Hatachi, Tokyo, Japan), were observed by transmission electron microscopy (TEM, JEM-2010E, JEOL, Akishima, Japan). The hardness and Young’s modulus values of the films were measured using a nanoindentation tester (TI-900 Triboindenter, Hysitron, Minneapolis, MN, USA) with a depth of 70 nm. The corrosion behavior of the TiNbTa(Zr) films and bare Ti-6Al-4V coupons were evaluated via potentiodynamic polarization tests (SP-200, BioLogic, Seyssinet-Pariset, France) in a 3.5 wt.% NaCl aqueous solution within a potential range of −2.5 V to 2.5 V. The compositional depth profiles and bonding characteristics of the films and bare Ti-6Al-4V alloy after immersing in Ringer’s solution were analyzed using X-ray photoelectron spectroscopy (XPS; PHI 1600, PHI, Kanagawa, Japan). The surface wettability of the films was evaluated by water contact angle measurements.

3. Results and Discussion

3.1. Chemical Compositions and Phase Structures

Table 1 shows the co-sputtering parameters and chemical compositions of the prepared ternary TiNbTa and quaternary TiNbTaZr films. These alloys, with mixing entropy ranging from 0.54 to 1.36 R, are considered low- and medium-entropy alloys. The ternary TiNbTa films included a near-equiatomic Ti₃₈Nb₃₀Ta₃₂, a Ti-enriched Ti₈₃Nb₁₀Ta₇, an Nb-enriched Ti₁₀Nb₈₀Ta₁₀, and a Ta-enriched Ti₁₂Nb₄Ta₈₄ film. Figure 1a displays the XRD patterns of the TiNbTa films deposited on Ti-6Al-4V substrates. The Ti₈₃Nb₁₀Ta₇ film exhibited a mixed hcp and bcc phase, whereas the Ti₃₈Nb₃₀Ta₃₂, Ti₁₀Nb₈₀Ta₁₀, and Ti₁₂Nb₄Ta₈₄ films exhibited a bcc structure. The valence electron concentration (VEC) is an indicator for forecasting the crystalline phases of solid solutions in high entropy alloys [24,25]. The VEC values were 4.17, 4.61, 4.90, and 4.88 for the Ti₈₃Nb₁₀Ta₇, Ti₃₈Nb₃₀Ta₃₂, Ti₁₀Nb₈₀Ta₁₀, and Ti₁₂Nb₄Ta₈₄ films, respectively. Guo et al. [24] reported that bcc phases were stable as the VEC of alloys was less than 6.87, whereas Yuan et al. [25] have reported that a single bcc or hcp phase was stable as VEC >4.18 and <4.09, respectively, and a mixed bcc and hcp structure formed as 4.09 ≤ VEC < 4.18. In this study, the VEC of Ti₈₃Nb₁₀Ta₇ film was 4.17, and it formed a mixed bcc and hcp phase, whereas the other three ternary films were stable in a bcc phase. Figure 2a displays the cross-sectional TEM (XTEM) image of the Ti₈₃Nb₁₀Ta₇ film, which exhibits a typical columnar structure. The selected area electron diffraction (SAED) pattern indicates the coexistence of bcc and hcp phases (Figure 2b). Figure 2c shows the high-resolution TEM (HRTEM) image around the interface between the Ti₈₃Nb₁₀Ta₇ film and C protective layer. As shown in the figure, the Ti₈₃Nb₁₀Ta₇ film is crystalline, and the selected regions exhibit lattice fringes with d-spacing values of 0.234–0.235 nm in respect of bcc (110) planes. The lattice constants of the bcc phase in Ti₈₃Nb₁₀Ta₇, Ti₃₈Nb₃₀Ta₃₂, Ti₁₀Nb₈₀Ta₁₀, and Ti₁₂Nb₄Ta₈₄ films were determined to be 0.3303, 0.3289, 0.3300, and 0.3310 nm, respectively. The near-equiatomic Ti₃₈Nb₃₀Ta₃₂ film possessed low lattice constants, whereas the Ta-enriched Ti₁₂Nb₄Ta₈₄ film exhibited large lattice constants. On the other hand, the quaternary TiNbTaZr films were categorized as a near-equiatomic Ti₃₃Nb₁₉Ta₂₁Zr₂₇ film, a Ti-enriched Ti₆₁Nb₁₃Ta₁₁Zr₁₅ film, an Nb-enriched Ti₁₅Nb₆₈Ta₈Zr₉ film, a Zr-enriched Ti₁₉Nb₁₀Ta₉Zr₆₂ film, and a Ta-enriched Ti₈Nb₈Ta₇₉Zr₅ film. The low VEC values of 4.24 and 4.19 for the Ti₆₁Nb₁₃Ta₁₁Zr₁₅ and Ti₁₉Nb₁₀Ta₉Zr₆₂ films, respectively, imply the formation of a mixed bcc and hcp structure, which agrees with their XRD patterns (Figure 1b). In contrast, the Ti₃₃Nb₁₉Ta₂₁Zr₂₇, Ti₁₅Nb₆₈Ta₈Zr₉, and Ti₈Nb₈Ta₇₉Zr₅ films exhibited VEC values of 4.40, 4.76, and 4.87, respectively, and these films displayed a single bcc phase. The lattice constants of the bcc phase in Ti₃₃Nb₁₉Ta₂₁Zr₂₇, Ti₁₅Nb₆₈Ta₈Zr₉, and Ti₈Nb₈Ta₇₉Zr₅ films were determined to be 0.3368, 0.3322, and 0.3332 nm, respectively. If Vegard’s law is obeyed [23], the lattice constants of a solid solution can be calculated using the rule of mixture. The lattice constants of bcc Ti, Nb, Ta, and Zr are 0.332 [26,27], 0.33066 (International Center for Diffraction Data (ICDD) 00-035-0789), 0.33058 (ICDD 00-004-0788), and 0.35453 (ICDD 00-034-0657) nm, respectively. Figure 3 displays the relationship between the measured and calculated lattice constants of the bcc phases of the TiNbTa(Zr) films with a single bcc phase only, which shows a linear variation tendency. The fitted line slope of the measured to the calculated values was 0.9985.

3.2. Mechanical Properties

Table 2. Mechanical properties of TiNbTa and TiNbTaZr films.

Sample	H 1 (GPa)	E 2 (GPa)	H/E	H3/E2 (GPa)
Ti-6Al-4V	4.9 ± 0.4	124 ± 8	0.040	0.008
Ti83Nb10Ta7	2.7 ± 0.3	110 ± 7	0.025	0.002
Ti38Nb30Ta32	5.0 ± 0.2	150 ± 10	0.033	0.006
Ti10Nb80Ta10	4.2 ± 0.5	142 ± 8	0.030	0.004
Ti12Nb4Ta84	7.7 ± 0.3	216 ± 6	0.036	0.010
Ti61Nb13Ta11Zr15	3.9 ± 1.0	108 ± 17	0.036	0.005
Ti33Nb19Ta21Zr27	5.9 ± 0.9	163 ± 12	0.036	0.008
Ti15Nb68Ta8Zr9	9.6 ± 1.1	165 ± 14	0.058	0.032
Ti19Nb10Ta9Zr62	7.0 ± 0.3	115 ± 4	0.061	0.026
Ti8Nb8Ta79Zr5	12.1 ± 1.0	226 ± 9	0.054	0.035

¹ H: Hardness; ² E: Young’s modulus.

lists the hardness (H) and Young’s modulus (E) of the Ti-6Al-4V substrate and TiNbTa(Zr) films. The H and E of the Ti-6Al-4V coupon were 4.9 and 124 GPa, respectively. A Young’s modulus of 110 GPa was reported for Ti-6Al-4V alloy [28]. TiNbSn [28] and TiNbTaZr [6] alloys with low Young’s modulus values of 40 and 60 GPa, respectively, were developed as alternative biocompatible materials. However, with the mechanisms of grain boundary strengthening and solid solution strengthening, metallic alloy films exhibit mechanical properties superior to those of bulk alloys [29]. The grain sizes evaluated from the FWHMs of (110) reflections in Bragg–Brentano XRD patterns were 21, 37, 24, and 32 nm for the Ti₈₃Nb₁₀Ta₇, Ti₃₈Nb₃₀Ta₃₂, Ti₁₀Nb₈₀Ta₁₀, and Ti₁₂Nb₄Ta₈₄ films, respectively, and 22, 33, 34, 33, and 38 nm for the Ti₆₁Nb₁₃Ta₁₁Zr₁₅, Ti₆₁Nb₁₃Ta₁₁Zr₁₅, Ti₁₅Nb₆₈Ta₈Zr₉, Ti₁₉Nb₁₀Ta₉Zr₆₂, and Ti₈Nb₈Ta₇₉Zr₅ films, respectively. Because these films exhibited various compositions, the relationship between hardness and grain size could not be interpreted by the normal or inverse Hall–Petch strengthening (grain boundary strengthening) mechanism. The H values were 4.0, 4.3, 11.7, and 6.9 GPa for hcp-Ti, bcc-Nb, bcc-Ta, and hcp-Zr films, respectively. The films with high Ta content should have high H values. Ti₁₂Nb₄Ta₈₄ and Ti₈Nb₈Ta₇₉Zr₅ films exhibited maximum hardness values of 7.7 and 12.1 GPa among the ternary and quaternary films, respectively, which implies that the solid solution strengthening mechanism dominated the hardness of these TiNbTa(Zr) films. As shown in Table 2, the TiNbTa(Zr) films with a single bcc phase exhibited high E values of 142–226 GPa, whereas the films with a mixture of bcc and hcp phases exhibited low E values of 108–115 GPa. This variation was not consistent with that reported for bulk Ti alloys, i.e., β(bcc)-phase Ti-alloys that exhibited low Young’s modulus. Though these TiNbTa(Zr) films exhibited high E values, these films should not cause a stress-shielding effect due to the low volume ratios of film-to-bulk Ti-6Al-4V. Moreover, selected TiNbTa(Zr) films improved the toughness and resistance to plastic deformation represented by high H/E [30] and H³/E² [31] indicators, respectively. The Ti₁₂Nb₄Ta₈₄ and Ti₈Nb₈Ta₇₉Zr₅ films exhibited favorable H/E and H³/E² values in the studied ternary and quaternary films, respectively.

3.3. Corrosion Resistance

Figure 4 depicts the potentiodynamic polarization curves of TiNbTa and TiNbTaZr films and the uncoated Ti-6Al-4V substrate as immersed in a 3.5 wt.% NaCl aqueous solution. The corrosion potential (E_corr) and corrosion current density (I_corr) values, determined by the Tafel extrapolation method, are presented in

Table 3. Corrosion characteristics of the Ti-6Al-4V substrate and TiNbTa(Zr) films.

Sample	Ecorr (mV)	Icorr (μA/cm2)	Rp (KΩcm2)	Rp Ratio
Ti-6Al-4V	−250	0.004	4.5 × 103	1.0
Ti83Nb10Ta7	−268	0.006	4.1 × 103	0.9
Ti38Nb30Ta32	−315	0.002	8.8 × 103	2.0
Ti10Nb80Ta10	−214	0.003	7.6 × 103	1.7
Ti12Nb4Ta84	−375	0.001	1.5 × 104	3.3
Ti61Nb13Ta11Zr15	−308	0.005	5.3 × 103	1.2
Ti33Nb19Ta21Zr27	−281	0.001	2.9 × 104	6.4
Ti15Nb68Ta8Zr9	−365	0.001	2.1 × 104	4.7
Ti19Nb10Ta9Zr62	−247	0.002	7.7 × 103	1.7
Ti8Nb8Ta79Zr5	−441	0.001	1.9 × 104	4.2

. The polarization resistance (R_p) values of the samples were determined according to the Stern–Geary equation [32]. The R_p values of TiNbTa films ranged from 4.1 × 10³ to 1.5 × 10⁴ KΩcm², whereas the R_p values of TiNbTaZr films were in the range of 5.3 × 10³–2.9 × 10⁴ KΩcm². Although there are no significant pitting behaviors for Ti₈₃Nb₁₀Ta₇ and Ti₆₁Nb₁₃Ta₁₁Zr₁₅, it is noteworthy that the ternary and quaternary Ti-enriched films have the R_p values of 4.1 and 5.3 × 10³ KΩcm², respectively. These R_p values are similar to the bare Ti-6Al-4V of 4.5 × 10³ KΩcm², which confirmed that the Ti-enriched films had inferior corrosion resistance in NaCl solution in relation to the other TiNbTa(Zr) films. The enhancement of R_p values caused by adding Zr into ternary TiNbTa films is evident. For example, the R_p ratio of Ti-enriched Ti₈₃Nb₁₀Ta₇ related to the bare Ti-6Al-4V was 0.9, whereas that of the Ti-enriched Ti₆₁Nb₁₃Ta₁₁Zr₁₅ was 1.2. Moreover, the R_p ratio increased from 1.7 for the Nb-enriched Ti₁₀Nb₈₀Ta₁₀ to 4.3 for the Nb-enriched Ti₁₅Nb₆₈Ta₈Zr₉. Additionally, the R_p ratio increased from 3.3 for the Ta-enriched Ti₁₂Nb₄Ta₈₄ to 4.3 for the Ta-enriched Ti₈Nb₈Ta₇₉Zr₅. The change in corrosion resistance was associated with the fact that ZrO₂ has the more negative Gibbs free energy per mole of O₂ among the available four oxides ZrO₂, TiO₂, Ta₂O₅, and Nb₂O₅, implying that the passivation film becomes thicker with an increase in Zr content [33]. Therefore, the I_corr of quaternary films was lower than that of ternary films, resulting in higher corrosion resistance.

The Ti₃₃Nb₁₉Ta₂₁Zr₂₇ film and the bare Ti-6Al-4V exhibited E_corr values of −281 and −250 mV, respectively. The Ti₃₃Nb₁₉Ta₂₁Zr₂₇ film exhibited a more negative corrosion potential value than that of bare Ti-6Al-4V and had the highest R_p of 2.9 × 10⁴ KΩcm² or R_p ratio of 6.4 in this study. The increasing corrosion resistance of Ti₃₃Nb₁₉Ta₂₁Zr₂₇ can be attributed to the formation of amorphous films comprising Ta₂O₅, Nb₂O₅, and ZrO₂ on the free surface. These aforementioned dense and stable passive oxide layers can restrict the electrochemical reactions, leading to a decrease in corrosion current density [18]. However, the Ti₁₉Nb₁₀Ta₉Zr₆₂ film with the highest Zr content in this study exhibited current fluctuations in its potentiodynamic polarization curve similar to that observed for the uncoated Ti-6Al-4V (Figure 4), implying the conduct of pitting behavior. According to the previous literature [34,35], the possible explanation for the pitting behavior can be ascribed to the presence of chloride ions in NaCl solution. The aggressive Cl ions with high electronegativity can easily combine with oxygen vacancies in the passive layer, which results in the formation of localized pitting corrosion at the grain boundaries and induces the formation of a porous structure. Moreover, the galvanic corrosion that occurs at the interphase of bcc and hcp phase boundaries in Ti-6Al-4V alloy can accelerate the dissolving rate of the passivation film [33]. The results of the corrosion test in NaCl solution are as follows: (1) the quaternary TiNbTaZr films presented higher corrosion resistance than that of the ternary TiNbTa films, (2) the Ti-enriched Ti₆₁Nb₁₃Ta₁₁Zr₁₅ film revealed a corrosion resistance slightly higher than that of the bare Ti-6Al-4V, and (3) the Zr-enriched Ti₁₉Nb₁₀Ta₉Zr₆₂ film revealed pitting behavior. Therefore, the Ti-6Al-4V modified with Ti₃₃Nb₁₉Ta₂₁Zr₂₇, Ti₁₅Nb₆₈Ta₈Zr₉, and Ti₈Nb₈Ta₇₉Zr₅ films should be suitable assemblies for biocompatible implantation in this study, and their high oxidation resistance was attributed to the formation of the stable passivation film on the single-bcc-phase film.

3.4. Elution Test and Formation of Surficial Oxide Layers

Figure 5 shows the images of water contact angles between the water drop and the TiNbTa(Zr) films and bare Ti-6Al-4V substrate. The contact angle was 50° for the Ti-6Al-4V substrate, which was comparable to the 55° contact angle reported in a previous study [36]. With a hydrophilic surface, the Ti-6Al-4V alloy enhanced the adsorption of human osteoblast cells and subsequent cell growth. The contact angles were 85° for the Ti₈₃Nb₁₀Ta₇, Ti₃₈Nb₃₀Ta₃₂, and Ti₁₀Nb₈₀Ta₁₀ films, whereas the Ti₁₂Nb₄Ta₈₄ film exhibited a lower contact angle of 77°. The passive oxide films formed on the metallic surfaces affected the water contact angle values by changing the surface energies [37]. Though the contact angles increased after the Ti-6Al-4V alloy was covered with TiNbTa films, these samples remained hydrophilic. The contact angles of Ti₆₁Nb₁₃Ta₁₁Zr₁₅, Ti₃₃Nb₁₉Ta₂₁Zr₂₇, Ti₁₅Nb₆₈Ta₈Zr₉, Ti₁₉Nb₁₀Ta₉Zr₆₂, and Ti₈Nb₈Ta₇₉Zr₅ film were 70°, 61°, 75°, 72°, and 70°, respectively, which were lower than those of the TiNbTa films.

Figure 6a depicts the XTEM image of the Ti₃₃Nb₁₉Ta₂₁Zr₂₇ film after immersion in Ringer’s solution for eight weeks. The Ti₃₃Nb₁₉Ta₂₁Zr₂₇ film exhibited a typical columnar structure and the SAED pattern exhibited a bcc structure (Figure 6b). Figure 6c displays the HRTEM image at the near-surface region. An amorphous oxide layer of approximately 10 nm thickness was observed at the free surface. No evident lattice fringes were observed in the amorphous oxide layer. Crystalline domains with d-spacing values of 0.234–0.235 nm were observed beneath the oxide layer, which represented the bcc (110) planes.

Figure 7 shows the compositional profiles of the TiNbTa and TiNbTaZr films examined by XPS after these samples were immersed in Ringer’s solutions for eight weeks. The near-equiatomic Ti₃₈Nb₃₀Ta₃₂ and Ti₃₃Nb₁₉Ta₂₁Zr₂₇ films exhibited inward diffusion of O and Al during the elution test (Figure 7a,b). The O contents of the Ti₃₈Nb₃₀Ta₃₂ and Ti₃₃Nb₁₉Ta₂₁Zr₂₇ films exhibited a common decreasing trend with an increase in the film depth. The presence of Al at the near-surface regions of 0–7.8 nm and 0–11.7 nm for the Ti₃₈Nb₃₀Ta₃₂ and Ti₃₃Nb₁₉Ta₂₁Zr₂₇ films, respectively, resulted from the inward diffusion of Al ions from the Ringer’s solution. Beneath the near-surface regions, no Al or V atoms were observed, which implies that the Ti₃₈Nb₃₀Ta₃₂ and Ti₃₃Nb₁₉Ta₂₁Zr₂₇ films formed protective barriers to isolate the Ti-6Al-4V substrate. Figure 7c displays the compositional profiles of the Ti₈Nb₈Ta₇₉Zr₅ film. The inward diffusion of O was restricted to a shallower depth of the Ti₈Nb₈Ta₇₉Zr₅ film related to those of the near-equiatomic Ti₃₈Nb₃₀Ta₃₂ and Ti₃₃Nb₁₉Ta₂₁Zr₂₇ films, which implies a higher oxidation resistance due to the formation of Ta₂O₅. In contrast, O diffused into the Ti-6Al-4V substrate (Figure 7d), which was attributed to the lack of a passivation oxide film such as Ta₂O₅ and Nb₂O₅ [3].

Figure 8 depicts the XPS profiles of the Ti₃₈Nb₃₀Ta₃₂ film at depths of 0, 7.8, and 47.7 nm after immersion in Ringer’s solution for eight weeks. The splitting energies were 5.54, 2.72, and 1.91 eV for Ti 2p, Nb 3d, and Ta 4f doublets [38], respectively. The area ratios were set as 2:1, 3:2, and 4:3 for 2p_3/2:2p_1/2, 3d_5/2:3d_3/2, and 4f_7/2:4f_5/2, respectively. On the free surface, binding energies of 456.55, 205.23, 24.27, and 72.50 eV indicated the signals of Ti 2p_3/2 in a TiO₂ form, Nb 3d_5/2 in a Nb₂O₅ form, Ta 4f_7/2 in a TaO₂ form, and Al 1s in a Al₂O₃ form, respectively. The Ti⁴⁺ 2p_3/2 signal exhibited a shift of −2.25 eV related to 458.8 eV shown in the handbook [38]. The FWHM values of Ti⁴⁺ 2p_3/2 and 2p_1/2 were 1.31 and 2.20 eV, respectively. A wider FWHM value for Ti 2p_1/2 than that for the Ti 2p_3/2 that has been reported in the literature [39,40]. The aforementioned FWHM ratio was sustained for analyzing the other Ti doublets in this study. In contrast, both the FWHM ratios of 3d_5/2:3d_3/2 and 4f_7/2:4f_5/2 for Nb and Ta doublets, respectively, were set as 1:1. At a depth of 7.8 nm beneath the free surface, the Ti signal consisted of Ti⁴⁺, Ti³⁺, and Ti²⁺ doublets, the Nb signal consisted of Nb⁵⁺, Nb⁴⁺, Nb²⁺, and Nb⁰ doublets, and the Ta signal consisted of Ta⁵⁺, Ta⁴⁺, Ta²⁺, and Ta⁰ doublets, whereas the Al signal was not obvious. At a depth of 47.7 nm, only Ti²⁺, Ti⁰, Nb²⁺, Nb⁰, Ta²⁺, and Ta⁰ doublets were observed, and no Al signal was detectable. The decrease in the intensity of Al signals along the depth direction of the Ti₃₈Nb₃₀Ta₃₂ film suggested that the Al ions were contributed from the Ringer’s solution during the elution test as examined from the compositional profiles (Figure 7). The decrease in O content in the depth direction of the Ti₃₈Nb₃₀Ta₃₂ film (Figure 7a) resulted in the elements of the Ti₃₈Nb₃₀Ta₃₂ film varying from full oxidization at the free surface to a mixture with various oxidization states and to the state being dominated with metallic atoms accompanied by minor amounts of divalent ions. Figure 9 exhibits the XPS depth profiles of the Ti-6Al-4V substrate after immersion in Ringer’s solution for eight weeks. The shift of Ti⁴⁺ 2p_3/2 signal was −2.61 eV in relation to 458.8 eV. A Ti⁴⁺ doublet was observed at the free surface and depths of 7.8 and 47.7 nm, whereas a minor Ti³⁺ doublet was examined at a depth of 47.7 nm. Both the Al bondings in Al₂O₃ and Al-halides [38] were detected. These Al atoms were eluted from the bare Ti-6Al-4V substrate without protective films.

4. Conclusions

The assemblies of TiNbTa(Zr) films/Ti-6Al-4V alloys utilized as biocompatible implant materials were evaluated in this study. The chemical compositions of the TiNbTa(Zr) films were regulated through co-sputtering with four sputter sources. The phase constitutions of TiNbTa(Zr) films were correlated to their VEC values. The TiNbTa(Zr) films with a VEC value of 4.17–4.24 exhibited a mixture of hcp and bcc phases, whereas the films with a VEC value of 4.40–4.90 revealed a single bcc phase. The solid solution strengthening mechanism dominated the hardness of these TiNbTa(Zr) films. In contrast to the performance of bulk Ti alloys, β-phase TiNbTa(Zr) films exhibited Young’s modulus values of 142–226 GPa, which were higher than those (108–115 GPa) of the films with an α–β mixed phase. The corrosion test in a 3.5 wt.% NaCl aqueous solution indicated that the Ti₃₃Nb₁₉Ta₂₁Zr₂₇, Ti₁₅Nb₆₈Ta₈Zr₉, and Ti₈Nb₈Ta₇₉Zr₅ films exhibited high anticorrosive properties deriving from the stable passivation film comprising constitutions of Ta₂O₅, Nb₂O₅, and ZrO₂. The water contact angles of TiNbTaZr films were lower than those of the TiNbTa films, which implies that the TiNbTaZr films were more hydrophilic. The XPS depth profiles indicated that the elution of Al atoms from the Ti-6Al-4V alloy was inhibited by the TiNbTa(Zr) films as the TiNbTa(Zr)/Ti-6Al-4V assemblies were immersed in Ringer’s solution at 310 K for eight weeks, which was attributed to the formation of an amorphous surficial oxide layer consisting of Ta₂O₅ and Nb₂O₅. In summary, the Ti₃₃Nb₁₉Ta₂₁Zr₂₇, Ti₁₅Nb₆₈Ta₈Zr₉, and Ti₈Nb₈Ta₇₉Zr₅ films are suitable candidates for protective coatings on Ti-6Al-4V alloys when applied as biocompatible materials.