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Electrochemical and bioactive characteristics of the porous surface formed on Ti-xNb alloys via plasma electrolytic oxidation
Journal Pre-proof Electrochemical and bioactive characteristics of the porous surface formed on Ti-xNb alloys via plasma electrolytic oxidation Mosab Kaseem, Han-Choel Choe PII:
S0257-8972(19)31018-7
DOI:
https://doi.org/10.1016/j.surfcoat.2019.125027
Reference:
SCT 125027
To appear in:
Surface & Coatings Technology
Received Date: 20 July 2019 Revised Date:
26 September 2019
Accepted Date: 27 September 2019
Please cite this article as: M. Kaseem, H.-C. Choe, Electrochemical and bioactive characteristics of the porous surface formed on Ti-xNb alloys via plasma electrolytic oxidation, Surface & Coatings Technology (2019), doi: https://doi.org/10.1016/j.surfcoat.2019.125027. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Electrochemical and bioactive characteristics of the porous surface formed on Ti-xNb alloys via plasma electrolytic oxidation 1 1
Mosab Kaseem, 2Han-Choel Choe*
Department of Nanotechnology and Advanced Materials Engineering, Sejong University, Seoul 05006, South Korea
2
Department of Dental Materials and Research Center of Nano-Interface Activation for Biomaterials, College of Dentistry, College of Dentistry, Chosun University, Gwangju, South Korea *
Corresponding author: [email protected] (H.C. Choe)
Abstract This study examined the capability of plasma electrolytic oxidation (PEO) to fabricate an ideal implant porous material for bio-implant applications. For this purpose, the Ti-xNb binary alloys (x=10, 30, and 50 wt.%) were treated via PEO using a DC power supply at a voltage was fixed to be 280 V for 180 s using an electrolyte composed of calcium acetate and calcium glycerophosphate. The structure of Ti-xNb alloys before PEO was altered from needle-like structure to an equiaxed structure with increasing the Nb content. After PEO, the high content of Nb element in the substrate led to an increase in the size and fraction of the micropores where a highly porous coating was fabricated on the sample with 50 wt.% Nb. This was attributed to the high plasma discharges developed at high current density conditions. The coating made on Ti30Nb alloy exhibited a high ratio of Ca/P and higher corrosion resistance as compared to the counterparts formed on Ti-10Nb and Ti-30Nb alloys. The presence of anatase and Nb2O5 together with ~1.13 µm micropores in the coating made on Ti-30Nb alloy were the main factors responsible for the easy formation of hydroxyapatite during the soaking in a simulated body fluid solution.
1
Keywords: Ti-xNb alloy; Plasma electrolytic oxidation; Bio-implant applications; Corrosion; Bone formation. 1. Introduction Over past several decades, pure titanium and Ti-6Al-4V alloy have been widely used in the fields of orthopedics and dentistry on account of their high mechanical strength and excellent corrosion resistance [1, 2]. Despite these attractive properties, the leaching of Al and V elements from Ti6Al-4V alloy can lead to undesirable inflammatory and allergic reactions [3]. In addition, the high value of elastic modulus of these materials as compared to the cortical bone can lead to bone loss and implant's failure [4]. Thus, it is necessary to develop novel materials with low elastic modulus by using bio-elements in order to avoid the aforementioned problems. Indeed, new titanium materials have been developed recently by alloying the pure titanium with bioelements, such as Nb, Ta, and Zr [3-10]. For example, Kuroda et al. [8] reported that the inclusion of biocompatible Nb and Ta [9] can benefit the formation of low modulus β Ti phase without compromising the biological inertness of Cp Ti. Zr, which is also a biocompatible element was included in the alloy system as well [10]. However, the introduction of bio-elements into the Ti alloy would not solve the problem of implant’s failure in the long term [11]. Therefore, it is mandatory to improve the bioactive properties of the implant materials via surface modification methods, such as sol-gel coating [12], chemical vapor deposition [13], and plasma electrolytic oxidation (PEO) [14]. Among them, special attention has been given to PEO which lead to improve the surface bioactivity by forming a TiO2 layer on the surface of Ti alloy with inclusion of ionic species, such as Ca and P elements. The characteristics of PEO coatings including the microstructure, porosity and topography can be tailored by controlling the processing parameters, such as electrolyte, current density, frequency etc. [15-17]. For example, 2
Shin et al. [16] have reported that the surface roughness of pure titanium coated via PEO can be enhanced with the addition of ZrO2 particles into the electrolyte which can trigger the formation of hydroxyapatite (HA) on the surface of the coating immersed in simulated body fluid solution (SBF). As such, Li et al. [17] demonstrated that the bioactivity of the pure titanium can be improved upon the PEO treatment. On the other hand, due their good characteristics namely low elastic modulus, biocompatibility and good corrosion properties, several Ti alloys (binary and trinary) based on Nb element have extensively been studied in the last decade due to their high technological importance in biomedical applications [3, 4, 18-25]. For instance, Chen et al. [20] reported based the potentiodynamic polarization test in phosphate buffer saline solution that the corrosion resistance of Ti-39Nb-6Zr alloy would be improved by PEO treatment in a KOH solution which was attributed mainly the compact structure of the oxide film with the presences of stable compounds, such as anatase, rutile, Nb2O5, and ZrO2. Moreover, the hardness and wear resistance measured at the ambient temperature, 200 oC and 400 oC for the Ti-39Nb-6Zr alloy substrate exhibited significant improvements upon the formation of TiO2-Nb2O5-ZrO2 ceramic coating on the substrate surface via PE, as stated by Chen et al. in another work [21]. Chen and co-workers in another study [22] reported based on the electrochemical impedance analysis that corrosion resistance of Ti-39Nb-6Zr alloy can be enhanced by application a + 400 V/60 V during PEO treatment in a KOH electrolyte. According to Duarte et al. [23], the bioactivity of Ti-13Nb-13Zr alloy in a SBF solution can be significantly improved after the PEO treatment in a solution containing Na2HPO4 and NaH2PO4. As such, Wu et al. [24] proved the effectiveness of PEO treatment on improving biological performance of Ti-24Nb-4Zn-7.9 Sn alloy. Karbowniczek et al. [25] demonstrated the formation of rough and porous oxide layers on Ti-6Al-7Nb alloy by 3
PEO in electrolytes with Ca and P species. In addition, Mazigi et al. [6] reported that the anodization and HA deposition of Ti-35Nb-3Zr alloy would lead to improve the corrosion performance of this alloy to a level similar to that of Ti-6Al-4V alloy. According to Byeon et al. [4], the HA formation on the surface of Ti-xNb alloys subjected to anodization treatment in electrolytes containing Ca, P, and Zn ions was strongly affected by the type of the electrolyte. Although the corrosion behavior of Ti alloys has been documented earlier, few have documented the correlation between the content of Nb in the Ti-xNb alloys and the electrochemical response and biocompatibility of the coatings. Accordingly, the main aim of this study is to fabricate ideal implant porous materials by considering the role of Nb content during the PEO treatment of TixNb alloys in an electrolyte system composed of calcium acetate and calcium glycerophosphate.
2. Experimental Section The binary Ti-xNb alloys (where x= 10, 30, and 50 wt.%) were fabricated by using arc melting and underwent to heat treatment for 2 h at 1000 oC with details reported elsewhere [4]. Prior to the PEO treatment, all the polished alloys were examined by optical microscopy (OM, Olympus BM60M, Japan) using an etching solution composed of 2 mL HF, 3 mL HCl, 5 mL HNO3, and 190 mL H2O. To examine the effects of Nb contents on the properties of PEO coatings, Ti-xNb alloys were coated using an electrolyte containing 0.15 M calcium acetate and 0.02 M calcium glycerophosphate at a voltage was fixed to be 280 V for 3min. Hereafter, 10Nb, 30Nb, and 50Nb indicate the bulk samples before PEO treatment (uncoated samples) while PEO-10Nb, PEO30Nb, and PEO-50Nb denote the corresponding PEO coatings of these alloys. The morphology of the PEO coating samples was observed using field-emission scanning electron microscopy (FE-SEM, HITACHI 4800, Japan) with energy dispersive X-ray spectroscopy (EDS) attachment. 4
The SEM images were analyzed via Image J software in order to calculate the micropore size and the porosity of the coatings. The thickness of the PEO coatings was measured using an eddycurrent meter tester (Minitest 2100, Electrophysik, Germany). In addition, X-ray Fluorescence (XRF, Analyzer Mde-Alloy, Analyzer Serial number-581331, Olympus, Japan) and X-ray diffraction (XRD, X'pert Pro Diffractometer, Philips, Netherlands) were used to examine the elemental and phase composition of the samples. The corrosion behavior of the samples was examined via potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) tests in 0.9% NaCl solution using three-electrodes system consist of a high-density graphite as a counter electrode, saturated calomel electrode (SCE) as a reference, and the coated/uncoated sample as a working electrode. The PDP tests were performed from -1500 to 2000 mV at a scan rate of 1.667 mV/s while EIS tests were carried out at a frequency was ranged from 0.01 Hz to 100 kHz at ambient temperature. The bioactivity of samples was examined by immersion of the PEO coatings in a simulated body fluid (SBF) solution at 37 oC for 24 h. The ion composition of the SBF solution is shown in Table 1. The surface wettability tests of the uncoated and coated samples were carried out using a water contact angle goniometer (KSA100, Kruss, Germany). In addition, all measurements were repeated at least five times to ensure the reliability. 3. Results 3.1. Morphology and composition of Ti-xNb alloys Fig. 1 presents OM images of Ti-xNb alloy as a function of Nb content. It can be observed that the microstructure of 10Nb sample exhibited needle-like structures which are typical of martensite α′ - phase. In the case of 30Nb sample, it is observed that the microstructure was composed of small needles characteristics of martensitic α′ and α″ - phases as well as β-phase 5
structure [3, 4]. In contrast, 50Nb sample was characterized by equiaxed structures with clear grain boundaries which would be related to the complete formation of β-phase. The results of XRF analysis shown in Fig. 1d clearly suggested that the fabricated alloys were comprised of alloying elements of Ti and Nb. Fig. 2 shows a comparison of the XRD patterns for Ti alloys with different Nb content. As shown in Fig. 2, 10Nb sample presented only the peaks related to the martensitic α′-phase (JCPDS: 01-089-3725), with distorted hexagonal structure. The XRD patterns of 30Nb sample showed peaks of orthorhombic martensitic α″-phase and traces of β- phase (JCPDS: 01-0894913), with body-centered cubic crystalline structure. On the other hand, the 50Nb sample presented only peaks that characterize the β-phase. The formation of β -phase at the high content of Nb suggested that Nb can play as a β -phase stabilizer in Ti-xNb alloys [3, 19]. 3.2. Effect of Nb content on the coating formation To figure out effect of Nb content on the coating formation, the current density evolution upon the coating time during the potentiostatic PEO is presented in Fig. 3. Regardless of the Nb content, the voltage is increased rapidly until reaching a constant value of 280 V after the about first 10 seconds. At potentiostatic condition, the total current density on the substrate exponentially decays with time to a steady value that is verified based on the Nb content in the alloys. As a result of high current densities of 30Nb (~5.6) and 50Nb samples (~6 A), acoustic emission during PEO treatment of these samples was stronger than that in 10Nb sample. This result indicated that the increasing of Nb content in the alloys caused PEO to progress at higher current densities levels. The high current densities levels observed in the cases of 30Nb and 50Nb samples (Fig.3d) were related to the sample passivation which is expected to induce the
6
formation of hydroxyapatite (HA), as reported by Muhaffel et al. [26]. In addition, the maximum currents density would correspond to the film breaking down and discharge initiation on the sample surface [27]. In general, the intensity of plasma discharge would reflect the morphology of the coating. The stronger plasma discharge, the bigger and more microdefects can be formed. Since the overall trends of current density-time curves was observed to be different with Nb contents, the amount of Nb in the alloy can lead to the development of different sizes of micropores on the surface of the coatings formed on Ti-xNb alloys. 3.3. Structure and composition of the coatings with respect to Nb content Fig. 4 shows the discharges phenomena of samples with different Nb content during PEO. Regardless of Nb content, the appearances of the plasma discharges of three different samples was observed at ~ 10s which indicated that all samples had similar values of breakdown voltage. This would be related to the fact that PEO treatment of three samples was conducted in the same electrolyte solution. As compared to 10 and 30 Nb samples, the PEO treatment of 50Nb sample led to develop strong plasma discharges which was characterized by the presence of strong arcs on the sides of substrate at 10 s from the onset of PEO treatment. With respect to coating time, the number of plasma discharges tended to decrease with increasing of coating time, regardless of the Nb content. Paying more careful attention to each sample, 10 Nb and 30Nb samples would show mild intensity of discharges (fine sparks) through the whole process, meanwhile 50Nb sample was likely to generate high intensity discharges (large sparks). These phenomena might result in different coating structures. Fig. 5 (a-c) presents the changes in surface morphology of the PEO coatings as a function of Nb content. The corresponding results of Image J analysis also presented in Fig. 5 (a1-c1). Regardless of the Nb content, the surface of the coatings was composed of numerous micropores 7
which are typical for the samples coated via PEO [28]. The micropores were attributed to the occurrence of gas bubbles and plasma discharges [29]. Based on the Image J analysis of the coatings, however, two primary points can be drawn from Fig.5 (a1-c1) and Table 2. Firstly, the average size of micro-pores and their distribution exhibited significant variation with the content of Nb, suggesting that the content of Nb in the substrate can greatly affect the surface structures of the coatings. Secondly, when the content of Nb in the alloy exceeds 30 wt.%, large-sized crater with grooved structure are observed (Fig. 5c). Such differences in the coating structure were mainly attributed to variations in the size and intensity of plasma discharges associated with Nb content. The higher discharge energy generated by larger spark discharges would lead to larger pores in the PEO coatings [30, 31]. In addition, the groove structure obtained in 50Nb sample was connected to the fusion of pores due to the high temperature associated with intense and large plasma discharges, as presented in Fig.4 [32, 33]. On the other hand, the thickness of the coatings was found to be 1.82±0.1µm, 1.85 ±0.3µm, and 1.86 ±0.6 µm for PEO-10Nb, PEO30Nb, and PEO-50Nb samples, respectively, indicating that the effect of Nb content on the thickness of PEO coating was insignificant. From EDS results shown in Fig.6, the Ti, O, Nb, P, and Ca elements were detected in all coatings. Ti and Nb elements generated from substrate while other elements came from electrolyte which containing P and Ca species. Interestingly, Ca/P molar ratios were found to be ~0.94, ~1.6, and ~1.36, for PEO-10Nb, PEO-30Nb, and PEO-50Nb samples, respectively, indicating that the 30 wt.% Nb can influence the deposition of HA on the surface of the coatings since the Ca/P ratio in the PEO-30Nb sample was almost identical to that of HA (HA Ca/P= 1.667) [34]. In other studies, Tsutsumi et al. [35] reported that Ca/P ratio reached ~ 0.643 during the PEO treatment of Ti-29Nb-13Ta-4.6Zr alloy in 0.1 M calcium glycerophosphate mixed with 0.15 M magnesium 8
acetate at a current density of 312 A/m2 for 8 min. Sowa et al. [11] found that Ca/P ratio reached ~ 1.44 for the PEO-treated Ti-36Nb-2Ta-3Zr alloy from an electrolyte composed of 0.1 M Ca(H2PO2)2 and 0.23 M Ca(HCOO)2 at 438 V for 10 min. As such, Sharma et al. [36] reported a value of Ca/P ratio ~ 1.50 for the PEO Ti-Zr alloys at 300 V. The present results implied that the PEO treatment of Ti-30Nb alloy can lead to a higher ratio of Ca/P in spite of low current density/voltage and short coating period. The variation in the values of Ca/P ratio is believed to be related to the differences in the characterizes of plasma discharges and the electrophoretic mobility of the ionic species during PEO [37]. In addition, Khalil and Leach [38] demonstrated that ionic transport numbers during PEO process would rely on the substrate composition, electrolyte chemistry and current density. It can be deduced, therefore, that the PEO treatment of binary Ti-30Nb alloy would be more desirable in terms of HA-forming ability as compared to the other alloys, such as Ti-29Nb-13Ta-4.6Zr, Ti-36Nb-2Ta-3Zr, and Ti-Zr alloys. Fig. 7 illustrates XRD patterns of the PEO coatings formed on Ti-xNb alloy samples. The PEO10Nb sample was composed mainly of anatase (JCPDS: 01-071-1168), brookite (JCPDS: 01072-0100), and HA (JCPDS: 00-024-0033). Additionally, Ti (JCPDS: 03-065-9622) was detected in the patterns due to the infiltration of X-ray into the Ti-xNb alloy substrate. As for the PEO-30Nb sample, the peaks related to the rutile (JCPDS: 01-084-1283), anatase, HA, Nb2O5 (JCPDS:01-070-2679), and Ti substrate were observed. In the case of the PEO-50Nb sample, peaks corresponded to anatase, rutile, Nb2O5, and CaTiO3 (JCPDS: 00-039-0145) are detected. The presence of rutile would indicate that the partial phase transformation of metastable anatase into the stable rutile was occurred at PEO-30Nb and PEO-50Nb samples [39]. Considering the fact that a hydrolysis process of CaTiO3 can be occurred during the immersion in SBF solution (Eq.1), the PEO-50Nb sample is expected to possess a good bioactivity since this reaction can 9
facilitate the HA nucleation by formation of a Ti–OH matrix and increasing ion concentration of OH− and Ca2+ at the surface [40, 41] +2
→2
+
(
) (1)
However, as shown in Fig.7, the amounts of CaTiO3 was very small, indicating that its contribution to the HA formation would be insignificant. Interestingly, on the other hand, the peaks related to HA in the case of PEO-30Nb sample was somewhat higher than other cases which was consistent with Ca/P ratio calculated from EDS analysis. 3.4. Effect of Nb content on the corrosion behavior The PDP curves for Ti-xNb alloy samples before and after PEO treatments as well as the electrochemical parameters namely corrosion potential (Ecorr), corrosion current density (icorr), and current density at passive region (ipass) alloys are presented in Fig. 8 and Table 3, respectively. As the amount of Nb increased, the corrosion resistance of the uncoated alloys tended to be increased since the 50Nb sample displayed the lowest values of icorr and ipass as compared to 10Nb and 30Nb samples. It is thought that the decrease in the value of icorr with increasing Nb content is attributed to the fact the corrosion resistance of the Ti-xNb alloy samples before PEO treatment was strongly relied on the microstructure of the samples [42]. As shown in Fig. 1, the microstructure was changed from α′ structure in the Ti-10Nb alloy into α′+ α″+ β in the Ti-30Nb alloy and into only β in the Ti-50Nb alloy. As reported by Atapour et al. [42] and Byeon et al. [4], the metastable β structure had superior corrosion resistance as compared to the α′ or α′+ α″+ β. On the other hand, a remarkable improvement in the corrosion resistance was found in the case of PEO samples as compared to the uncoated samples. Regardless of the Nb content, a passive behavior in the anodic branch was observed in the polarization curves of PEO coatings. However, no breakdown of the passive film was observed 10
in the case of PEO-30Nb and PEO-50Nb samples while a breakdown of the passive film was occurred at a potential ~1.28 V in the case of the PEO-10 Nb sample. This result indicated that the PEO-30Nb and PEO-50Nb samples have higher breakdown potentials than that in the PEO10Nb sample. In general, the breakdown potential of the passive layer would be affected by several factors related to the thickness and composition of the coatings. As the measured thickness of the coatings was approximately identical (~1.8 µm), the composition of the coatings would be the main factor affecting the values of breakdown potential of the PEO coatings in polarization curves shown in Fig. 8. Considering the fact that the icorr and ipass values of PEO30Nb sample were lower than their counterparts of PEO-10Nb and PEO-50Nb samples, an excellent corrosion performance can be obtained in the PEO-30Nb sample. To discover the correlation between Nb content and the corrosion behavior of the coatings, further analysis by EIS was conducted on the PEO coatings samples in a 0.9 wt.% NaCl solution. Thus, EIS results in terms of Nyquist and Bode plots are presented in Fig. 9. Nyquist plots in Fig.9a showed that the diameters of capacitive loops of PEO-30Nb sample were much larger than that of other samples. In addition, it was found from Bode impedance plots shown in Fig. 9b that the PEO-30Nb sample exhibited a higher value of impedance (~8.5 x 104 Ω.cm2) at the low frequency region (0.01Hz) as compared to PEO-10Nb and PEO-50Nb samples which showed lower values of ~6.1 x 104 and ~5.0 x 104 Ω.cm2, respectively. The two-time constants found in the Bode phase plots shown in Fig.9c can be related to the dual-layer structure of the coating formed via PEO [48]. As it can be seen from Fig.9c, the phase angle at the high frequency region (105 Hz) for PEO-30Nb sample was higher than those for PEO-10Nb and PEO-50Nb samples which was attributed to the formation of anatase, rutile, Nb2O5 together with high amounts of HA. 11
The EIS results of the PEO-10Nb, PEO-30Nb, and PEO-50Nb samples were analyzed using the equivalent circuit model shown in Fig.9c. In the proposed circuit model, Rs represents the solution resistance while Rl and Rb describe the resistances of the loose and compact layers, respectively. CPEl and CPEb are the corresponding constant phase elements. The corresponding values of the equivalent elements are listed in Table 4. Regardless of Nb content, the Rb was obviously higher than that of Rl due to the fact that the compact layer exhibits better corrosion protection properties than the loose layer [43-45]. In addition, Rl and Rb values of the PEO-30Nb sample were measured to be 1.96 x 105 and 2.39 x 106 Ω.cm2, respectively, which were higher than the counterparts for PEO-10Nb and PEO-50Nb samples. The above PDP and EIS tests proved that the corrosion resistance of PEO-30Nb was higher than the other samples which was attributed not only to the compact layer but also due to the phase composition considering the fact that anatase, rutile, Nb2O5, and HA have good anticorrosion properties. 3.5. Effect of Nb on the bioactivity and wettability of the coatings As reported earlier [46], HA formation on the surface of materials is demonstrated to be the primary indication for bioactivity. Fig. 10 (a-c) displays the morphologies of HA deposited on the PEO coatings of Ti-xNb alloy after the immersion in the SBF solution for 24 h. As it can be seen from Fig. 10b that the amount of HA in the PEO-30Nb sample was higher than the other samples. EDS area analyses were carried out on the white areas presented as insets in Fig.10 (a-c) and the results are given in Table 5. In all cases, Ca, P, O, Ti, and Nb elements were detected. However, the Ca/P ratio calculated for three samples was ~ 1.51, ~ 1.66, and ~ 1.48, for PEO10Nb, PEO-30Nb, PEO-50Nb samples, respectively, indicating that the PEO treatment of 30Nb sample can be useful to accelerate the growth of HA in the SBF solution. To affirm this result, however, the XRD patterns of the coatings after the immersion in SBF solution were presented in 12
Fig.10d. Interestingly, the intensities of HA in the case of PEO-30Nb sample were higher than other samples which was consistent with the SEM image in Fig.10b and EDS results listed in Table 5. The contact angle experiments for Ti-xNb alloy before and after PEO treatment are presented in Fig. 11. The low contact angle would imply a good cell adhesion, meaning that the hydrophilic surface can be used as implant bio-materials [18]. Here, two important things can be drawn from the comparison of the contact angle results shown in Fig.11. First, the contact angles of the alloys before PEO were found to decrease as the Nb content increased which was ascribed to the increase the amount of β-phase in the alloy. Second, the hydrophilic property of the Ti-xNb alloy samples was enhanced by PEO treatment where the lowest value of contact angles was found in the PEO-50Nb sample. 4. Discussion Typical SEM images, EDS, and XRD, and corrosion results revealed the surface properties TixNb alloys were significantly improved upon the PEO treatment where the PEO-30Nb sample exhibited the best properties in terms of corrosion resistance and the HA-forming ability. In terms of corrosion properties, the microstructural parameters, namely thickness, pore size and porosity together with the composition of the coatings were the main factors affecting the corrosion performance of PEO coatings [33]. As well known, the intense plasma discharges and gas evolution leads to the development of large pores and cause thermal cracking of the coatings. Accordingly, the high current densities shown in Fig. 3 during the coating formation of PEO30Nb and PEO-50Nb samples would lead to increase the size of micropores and their distributions as compared to the PEO-10Nb sample (Fig. 5). The presence of large size of
13
micropores at high levels on the surface of PEO coatings can increase the exposed surface area and thus the tendency of chloride ions to attack the substrate through these defects [29]. Although the PEO-50Nb sample exhibited higher fraction of rutile phase whose higher chemical stability than anatase phase, this sample showed lower corrosion properties as compared to the PEO-30Nb sample. This was attributed to the high porosity (~ 40 %) in this sample which can facilitate the infiltration of chloride ions into the Ti-50Nb substrate during the corrosion test. However, the high corrosion resistance obtained in the case of PEO-30Nb sample can be ascribed to the combined effects of the lower porosity, smaller size of micropores, and phase composition. Despite the average size of micropores, their distributions and the coating thickness in the PEO-10Nb sample were comparable to that in the PEO-30Nb sample, this sample exhibited lower corrosion properties as compared to that in the PEO-30Nb sample. This result was related, therefore, to the formation of higher amounts of anatase, rutile, Nb2O5, and HA in the PEO-30Nb sample which were responsible on such improvements. This result would be reasonably consistent with recent investigation on Ti-6Al-4V alloy treated via PEO for 3 min using electrolytes containing bioactive elements, such as Ca, P, Mg, Mn, and Si where the composition of the coating played a significant role in corrosion protection properties [47]. On the other hand, it was reported that the wettability, pore morphology, and the phase composition of the coatings were the main factors affecting the HA-forming ability [48]. As for the wettability, it is well known that the porous surface of PEO coating can help to reduce the contact angle (θ) between the crystal nucleus and the interface. The change of Gibbs free energy (∆G*) of HA nucleation can be defined as follows, [49]:
∆
∗
=
( ) [
" !(#)]
14
(2)
% (&) =
( '()* )(
()* )
+
(3)
where σ is surface tension, ν is molecular volume of precipitate, k is Boltzmann constant, I is the ion activity coefficient product of HA, and K is the solubility product of apatite. From Eq. 2, the ∆G* of HA can be reduced if the contact angle (θ) is decreased due to the direct relationship between ∆G* and θ. As shown from Fig.11, the PEO-50Nb sample exhibited the lowest value of contact angle. This result can be ascribed to the differences in the morphologies of the PEO coatings where the high porosity together with the large size of micropores in the PEO-50Nb sample would be responsible for the low value of contact angle of this sample. Accordingly, the wettability was not adequate factor for explaining the high-forming ability of HA in PEO-30Nb sample. This explanation was in accord with the findings reported by Santos-Coquillat et al. [48]. While it might be expected that the highly porous structure observed in the PEO-50Nb sample can facilitate the HA formation, the large micropores in the sample ~6.43 µm were unfavorable sites for the nucleation of HA as compared to the PEO-10Nb sample. Shin et al [50] postulated that the presence of micropores with an average size of ~5 µm can result in a less bioactive performance as compared to ~ 2 µm micropores. In addition, Nashimoto et al. [51] demonstrated that the micropore size as low as 1.2 µm was the most desirable for cell anchoring. Similar findings were reported also by Pafenov et al [52] and Shin et al. [53]. As to the phase composition, it was found that the existence of anatase in the coatings can significantly induce the HA-forming ability due to the crystallographic compatibility between anatase and HA [48, 54, 55]. Thus, the anatase phase is supposed to display a higher ability for HA formation than what the rutile phase did. Indeed, the amounts of anatase in the PEO-30Nb sample were higher than other samples. The formation of Nb2O5 in this coating would help also 15
to induce the HA formation in this sample [56]. The HA particles formed during PEO would also trigger the nucleation of HA during the immersion in SBF solution which increase the ease of HA growth. Finally, due to the high corrosion resistance together with the high HA-forming ability observed in the case of PEO-30Nb sample, the present sample can possess extraordinary potential to be used as an ideal bio-implant material in orthopedic and dental applications.
5. Conclusions This work looked into the surface properties of the Ti-xNbs alloy coated via PEO process by considering the role of Nb content. The structure of Ti-xNb alloy was modified to structure rich in β-phase with increasing Nb content from 10 to 50 wt%. The surface structure observations of the coatings formed via PEO on these alloys revealed that the mean size of the micropores and their porosity tended to increase with increasing Nb content. A good combination of high corrosion resistance with a good bioactivity were attained in the coating made on the Ti-30Nb alloy. The present findings strongly suggest that the processing of Ti-30Nb alloy by PEO is a promising technique for improving its performance in order to be used as a safe implant material in biomedical applications.
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Table 1 Ion concentrations of human blood plasma and SBF solution utilized in this study.
SBF Blood plasma
Ion concentration (mM) Na+ 142.0 142.0
K+ 5.0 5.0
Mg+2 1.5 1.5
Ca+2 2.5 2.5
Cl103.0 103.0
HCO3-2 10.0 27.0
HPO4-2 1.0 1.0
SO4-2 0.5 0.5
Table 2 Details of the micropores determined from Image J analysis of SEM images of PEO coatings with respect to the Nb content.
Porosity (%) Pore size (µm)
Sample PEO-10Nb
PEO- 30Nb
PEO-50Nb
10.13 ±1.3
12.53±1.2
40.83±1.5
1.05±0.9
1.13±0.8
6.43±1.1
Table 3 Results of the PDP tests in 0.9 wt.% NaCl solution for the Ti-xNb alloys samples before and after PEO treatment. 25
Sample 10Nb 30Nb 50Nb PEO-10Nb PEO-30Nb PEO-50Nb
icorr (A/cm2) 2.26 X 10-4 4.68 X 10-5 8.31 X 10-6 6.95 X 10-6 6.00 X 10-9 2.53 X 10-7
Ecorr (V) -0.487 -0.469 -0.398 -0.645 -0.353 -0.406
ipass (A/cm2) 2.20 X 10-6 4.73 X 10-8 1.94 X 10-7
Table 4 Results of the EIS tests in 0.9 wt.% NaCl solution for the Ti-xNb alloys coated via PEO. Sample
Rs (Ωcm2)
Rl (Ωcm2)
CPE-nl
PEO-10Nb PEO-30Nb PEO-50Nb
19.41 22.35 21.47
4.01 X 103 1.96 X 105 2.36 X 104
0.77 0.93 0.89
CPE-Yl (S.sn.cm-2) 1.81 X 10-7 3.72 X 10-9 1.17 X 10-8
Rb (Ωcm2)
CPE-nb
1.51 X 105 2.39X 106 5.06 X 105
0.69 0.81 0.75
CPE-Yb (S.sn.cm-2) 7.89 X 10-6 7.85 X 10-7 2.90 X 10-7
Table 5 Results of the EDS area analysis for the Ti-xNb alloys coated via PEO after immersion in a SBF solution for 24 h. sample PEO-10Nb PEO-30Nb PEO-50Nb
Ti (wt.%) 21.06 13.83 24.17
O (wt.%) 62.98 57.88 55.03
Ca (wt.%) 9.60 16.15 8.20
P (wt.%) 6.36 9.73 5.54
Nb (wt.%) 2.41 7.06
Ca/P ~1.51 ~1.66 ~1.48
Figures captions Fig.1 (a-c) the optical micrographs obtained for Ti-xNb alloys samples (a) 10Nb, (b) 30Nb, and (c) 50Nb (scale bare in images (a-c) is 100 µm), and (d) the results of XRF analysis. The 50Nb sample was mainly composed of β-phase. Fig.2 XRD patterns of Ti-xNb alloys showing the presence of α and β-phases. Fig.3 (a-c) Voltage and current density vs. time relationships recorded during plasma electrolytic oxidation of (a) 10Nb, (b) 30Nb, and (c) 50Nb, and (d) the variation of maximum and minimum current density with respect to Nb content.
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Fig.4 Optical images showing the micro-discharges appearance with respect to coating time during PEO treatments of 10Nb, 30Nb and 50Nb samples.
Fig.5 SEM images and the analyzed images for the PEO coating formed on (a, a1) 10Nb, (b, b1) 30Nb, and (c and c1) 50Nb samples. Insets in Fig. 4 (a-c) are the high-magnification images of the surface morphology of the PEO-10Nb, PEO-30Nb, and PEO-50Nb samples. Fig.6 (a-c) EDS analysis showing Ca/P ratio on the surface of the coatings formed on 10Nb, 30Nb, and (c) 50Nb samples, respectively. Fig.7 XRD patterns for the PEO-10Nb, PEO-30Nb, PEO-50Nb samples. Fig.8 (a, b) Potentiodynamic polarization curves of the Ti-xNb alloys before and after PEO treatment, respectively. Passive-regions were observed in the case of the PEO-coated Ti-xNb alloys. Fig.9 Nyquist and Bode plots for the PEO coatings formed on Ti-xNb alloys (a) Nyquist plots, (b) Bode impedance plots, and (c) Bode phase plots, and (d) Equivalent circuit model utilized for fitting the EIS results. Fig.10 (a-c) SEM images showing the HA morphology formed on the PEO-10Nb, PEO-30Nb, and PEO-50Nb samples, respectively after a 24 h soaking in SBF solution and (d) the corresponding XRD patterns. Insets in Fig. 9 (a-c) are the high-magnification images of the surface morphology of the PEO-10Nb, PEO-30Nb, and PEO-50Nb samples immersed in SBF solution for 24 h. Fig.11 Comparison of contact angle of Ti-xNb alloys before and after PEO treatment.
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Fig. 1
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Fig. 2
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Fig. 3
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Fig. 4
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Fig. 5
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Fig. 6
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Fig. 7
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Fig. 8
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Fig. 9
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Fig. 10
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Fig. 11
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Highlights An ideal implant porous material is successfully fabricated from Ti-30Nb alloy via PEO. Effect of Nb content on the surface properties of Ti-xNb alloy coated via PEO is significant. A protective bioactive coating is successfully obtained by PEO process of Ti-30Nb alloy. Wettability of the PEO coatings was improved as compared to the bulk alloy surface.
S0257-8972(19)31018-7
DOI:
https://doi.org/10.1016/j.surfcoat.2019.125027
Reference:
SCT 125027
To appear in:
Surface & Coatings Technology
Received Date: 20 July 2019 Revised Date:
26 September 2019
Accepted Date: 27 September 2019
Please cite this article as: M. Kaseem, H.-C. Choe, Electrochemical and bioactive characteristics of the porous surface formed on Ti-xNb alloys via plasma electrolytic oxidation, Surface & Coatings Technology (2019), doi: https://doi.org/10.1016/j.surfcoat.2019.125027. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Electrochemical and bioactive characteristics of the porous surface formed on Ti-xNb alloys via plasma electrolytic oxidation 1 1
Mosab Kaseem, 2Han-Choel Choe*
Department of Nanotechnology and Advanced Materials Engineering, Sejong University, Seoul 05006, South Korea
2
Department of Dental Materials and Research Center of Nano-Interface Activation for Biomaterials, College of Dentistry, College of Dentistry, Chosun University, Gwangju, South Korea *
Corresponding author: [email protected] (H.C. Choe)
Abstract This study examined the capability of plasma electrolytic oxidation (PEO) to fabricate an ideal implant porous material for bio-implant applications. For this purpose, the Ti-xNb binary alloys (x=10, 30, and 50 wt.%) were treated via PEO using a DC power supply at a voltage was fixed to be 280 V for 180 s using an electrolyte composed of calcium acetate and calcium glycerophosphate. The structure of Ti-xNb alloys before PEO was altered from needle-like structure to an equiaxed structure with increasing the Nb content. After PEO, the high content of Nb element in the substrate led to an increase in the size and fraction of the micropores where a highly porous coating was fabricated on the sample with 50 wt.% Nb. This was attributed to the high plasma discharges developed at high current density conditions. The coating made on Ti30Nb alloy exhibited a high ratio of Ca/P and higher corrosion resistance as compared to the counterparts formed on Ti-10Nb and Ti-30Nb alloys. The presence of anatase and Nb2O5 together with ~1.13 µm micropores in the coating made on Ti-30Nb alloy were the main factors responsible for the easy formation of hydroxyapatite during the soaking in a simulated body fluid solution.
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Keywords: Ti-xNb alloy; Plasma electrolytic oxidation; Bio-implant applications; Corrosion; Bone formation. 1. Introduction Over past several decades, pure titanium and Ti-6Al-4V alloy have been widely used in the fields of orthopedics and dentistry on account of their high mechanical strength and excellent corrosion resistance [1, 2]. Despite these attractive properties, the leaching of Al and V elements from Ti6Al-4V alloy can lead to undesirable inflammatory and allergic reactions [3]. In addition, the high value of elastic modulus of these materials as compared to the cortical bone can lead to bone loss and implant's failure [4]. Thus, it is necessary to develop novel materials with low elastic modulus by using bio-elements in order to avoid the aforementioned problems. Indeed, new titanium materials have been developed recently by alloying the pure titanium with bioelements, such as Nb, Ta, and Zr [3-10]. For example, Kuroda et al. [8] reported that the inclusion of biocompatible Nb and Ta [9] can benefit the formation of low modulus β Ti phase without compromising the biological inertness of Cp Ti. Zr, which is also a biocompatible element was included in the alloy system as well [10]. However, the introduction of bio-elements into the Ti alloy would not solve the problem of implant’s failure in the long term [11]. Therefore, it is mandatory to improve the bioactive properties of the implant materials via surface modification methods, such as sol-gel coating [12], chemical vapor deposition [13], and plasma electrolytic oxidation (PEO) [14]. Among them, special attention has been given to PEO which lead to improve the surface bioactivity by forming a TiO2 layer on the surface of Ti alloy with inclusion of ionic species, such as Ca and P elements. The characteristics of PEO coatings including the microstructure, porosity and topography can be tailored by controlling the processing parameters, such as electrolyte, current density, frequency etc. [15-17]. For example, 2
Shin et al. [16] have reported that the surface roughness of pure titanium coated via PEO can be enhanced with the addition of ZrO2 particles into the electrolyte which can trigger the formation of hydroxyapatite (HA) on the surface of the coating immersed in simulated body fluid solution (SBF). As such, Li et al. [17] demonstrated that the bioactivity of the pure titanium can be improved upon the PEO treatment. On the other hand, due their good characteristics namely low elastic modulus, biocompatibility and good corrosion properties, several Ti alloys (binary and trinary) based on Nb element have extensively been studied in the last decade due to their high technological importance in biomedical applications [3, 4, 18-25]. For instance, Chen et al. [20] reported based the potentiodynamic polarization test in phosphate buffer saline solution that the corrosion resistance of Ti-39Nb-6Zr alloy would be improved by PEO treatment in a KOH solution which was attributed mainly the compact structure of the oxide film with the presences of stable compounds, such as anatase, rutile, Nb2O5, and ZrO2. Moreover, the hardness and wear resistance measured at the ambient temperature, 200 oC and 400 oC for the Ti-39Nb-6Zr alloy substrate exhibited significant improvements upon the formation of TiO2-Nb2O5-ZrO2 ceramic coating on the substrate surface via PE, as stated by Chen et al. in another work [21]. Chen and co-workers in another study [22] reported based on the electrochemical impedance analysis that corrosion resistance of Ti-39Nb-6Zr alloy can be enhanced by application a + 400 V/60 V during PEO treatment in a KOH electrolyte. According to Duarte et al. [23], the bioactivity of Ti-13Nb-13Zr alloy in a SBF solution can be significantly improved after the PEO treatment in a solution containing Na2HPO4 and NaH2PO4. As such, Wu et al. [24] proved the effectiveness of PEO treatment on improving biological performance of Ti-24Nb-4Zn-7.9 Sn alloy. Karbowniczek et al. [25] demonstrated the formation of rough and porous oxide layers on Ti-6Al-7Nb alloy by 3
PEO in electrolytes with Ca and P species. In addition, Mazigi et al. [6] reported that the anodization and HA deposition of Ti-35Nb-3Zr alloy would lead to improve the corrosion performance of this alloy to a level similar to that of Ti-6Al-4V alloy. According to Byeon et al. [4], the HA formation on the surface of Ti-xNb alloys subjected to anodization treatment in electrolytes containing Ca, P, and Zn ions was strongly affected by the type of the electrolyte. Although the corrosion behavior of Ti alloys has been documented earlier, few have documented the correlation between the content of Nb in the Ti-xNb alloys and the electrochemical response and biocompatibility of the coatings. Accordingly, the main aim of this study is to fabricate ideal implant porous materials by considering the role of Nb content during the PEO treatment of TixNb alloys in an electrolyte system composed of calcium acetate and calcium glycerophosphate.
2. Experimental Section The binary Ti-xNb alloys (where x= 10, 30, and 50 wt.%) were fabricated by using arc melting and underwent to heat treatment for 2 h at 1000 oC with details reported elsewhere [4]. Prior to the PEO treatment, all the polished alloys were examined by optical microscopy (OM, Olympus BM60M, Japan) using an etching solution composed of 2 mL HF, 3 mL HCl, 5 mL HNO3, and 190 mL H2O. To examine the effects of Nb contents on the properties of PEO coatings, Ti-xNb alloys were coated using an electrolyte containing 0.15 M calcium acetate and 0.02 M calcium glycerophosphate at a voltage was fixed to be 280 V for 3min. Hereafter, 10Nb, 30Nb, and 50Nb indicate the bulk samples before PEO treatment (uncoated samples) while PEO-10Nb, PEO30Nb, and PEO-50Nb denote the corresponding PEO coatings of these alloys. The morphology of the PEO coating samples was observed using field-emission scanning electron microscopy (FE-SEM, HITACHI 4800, Japan) with energy dispersive X-ray spectroscopy (EDS) attachment. 4
The SEM images were analyzed via Image J software in order to calculate the micropore size and the porosity of the coatings. The thickness of the PEO coatings was measured using an eddycurrent meter tester (Minitest 2100, Electrophysik, Germany). In addition, X-ray Fluorescence (XRF, Analyzer Mde-Alloy, Analyzer Serial number-581331, Olympus, Japan) and X-ray diffraction (XRD, X'pert Pro Diffractometer, Philips, Netherlands) were used to examine the elemental and phase composition of the samples. The corrosion behavior of the samples was examined via potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) tests in 0.9% NaCl solution using three-electrodes system consist of a high-density graphite as a counter electrode, saturated calomel electrode (SCE) as a reference, and the coated/uncoated sample as a working electrode. The PDP tests were performed from -1500 to 2000 mV at a scan rate of 1.667 mV/s while EIS tests were carried out at a frequency was ranged from 0.01 Hz to 100 kHz at ambient temperature. The bioactivity of samples was examined by immersion of the PEO coatings in a simulated body fluid (SBF) solution at 37 oC for 24 h. The ion composition of the SBF solution is shown in Table 1. The surface wettability tests of the uncoated and coated samples were carried out using a water contact angle goniometer (KSA100, Kruss, Germany). In addition, all measurements were repeated at least five times to ensure the reliability. 3. Results 3.1. Morphology and composition of Ti-xNb alloys Fig. 1 presents OM images of Ti-xNb alloy as a function of Nb content. It can be observed that the microstructure of 10Nb sample exhibited needle-like structures which are typical of martensite α′ - phase. In the case of 30Nb sample, it is observed that the microstructure was composed of small needles characteristics of martensitic α′ and α″ - phases as well as β-phase 5
structure [3, 4]. In contrast, 50Nb sample was characterized by equiaxed structures with clear grain boundaries which would be related to the complete formation of β-phase. The results of XRF analysis shown in Fig. 1d clearly suggested that the fabricated alloys were comprised of alloying elements of Ti and Nb. Fig. 2 shows a comparison of the XRD patterns for Ti alloys with different Nb content. As shown in Fig. 2, 10Nb sample presented only the peaks related to the martensitic α′-phase (JCPDS: 01-089-3725), with distorted hexagonal structure. The XRD patterns of 30Nb sample showed peaks of orthorhombic martensitic α″-phase and traces of β- phase (JCPDS: 01-0894913), with body-centered cubic crystalline structure. On the other hand, the 50Nb sample presented only peaks that characterize the β-phase. The formation of β -phase at the high content of Nb suggested that Nb can play as a β -phase stabilizer in Ti-xNb alloys [3, 19]. 3.2. Effect of Nb content on the coating formation To figure out effect of Nb content on the coating formation, the current density evolution upon the coating time during the potentiostatic PEO is presented in Fig. 3. Regardless of the Nb content, the voltage is increased rapidly until reaching a constant value of 280 V after the about first 10 seconds. At potentiostatic condition, the total current density on the substrate exponentially decays with time to a steady value that is verified based on the Nb content in the alloys. As a result of high current densities of 30Nb (~5.6) and 50Nb samples (~6 A), acoustic emission during PEO treatment of these samples was stronger than that in 10Nb sample. This result indicated that the increasing of Nb content in the alloys caused PEO to progress at higher current densities levels. The high current densities levels observed in the cases of 30Nb and 50Nb samples (Fig.3d) were related to the sample passivation which is expected to induce the
6
formation of hydroxyapatite (HA), as reported by Muhaffel et al. [26]. In addition, the maximum currents density would correspond to the film breaking down and discharge initiation on the sample surface [27]. In general, the intensity of plasma discharge would reflect the morphology of the coating. The stronger plasma discharge, the bigger and more microdefects can be formed. Since the overall trends of current density-time curves was observed to be different with Nb contents, the amount of Nb in the alloy can lead to the development of different sizes of micropores on the surface of the coatings formed on Ti-xNb alloys. 3.3. Structure and composition of the coatings with respect to Nb content Fig. 4 shows the discharges phenomena of samples with different Nb content during PEO. Regardless of Nb content, the appearances of the plasma discharges of three different samples was observed at ~ 10s which indicated that all samples had similar values of breakdown voltage. This would be related to the fact that PEO treatment of three samples was conducted in the same electrolyte solution. As compared to 10 and 30 Nb samples, the PEO treatment of 50Nb sample led to develop strong plasma discharges which was characterized by the presence of strong arcs on the sides of substrate at 10 s from the onset of PEO treatment. With respect to coating time, the number of plasma discharges tended to decrease with increasing of coating time, regardless of the Nb content. Paying more careful attention to each sample, 10 Nb and 30Nb samples would show mild intensity of discharges (fine sparks) through the whole process, meanwhile 50Nb sample was likely to generate high intensity discharges (large sparks). These phenomena might result in different coating structures. Fig. 5 (a-c) presents the changes in surface morphology of the PEO coatings as a function of Nb content. The corresponding results of Image J analysis also presented in Fig. 5 (a1-c1). Regardless of the Nb content, the surface of the coatings was composed of numerous micropores 7
which are typical for the samples coated via PEO [28]. The micropores were attributed to the occurrence of gas bubbles and plasma discharges [29]. Based on the Image J analysis of the coatings, however, two primary points can be drawn from Fig.5 (a1-c1) and Table 2. Firstly, the average size of micro-pores and their distribution exhibited significant variation with the content of Nb, suggesting that the content of Nb in the substrate can greatly affect the surface structures of the coatings. Secondly, when the content of Nb in the alloy exceeds 30 wt.%, large-sized crater with grooved structure are observed (Fig. 5c). Such differences in the coating structure were mainly attributed to variations in the size and intensity of plasma discharges associated with Nb content. The higher discharge energy generated by larger spark discharges would lead to larger pores in the PEO coatings [30, 31]. In addition, the groove structure obtained in 50Nb sample was connected to the fusion of pores due to the high temperature associated with intense and large plasma discharges, as presented in Fig.4 [32, 33]. On the other hand, the thickness of the coatings was found to be 1.82±0.1µm, 1.85 ±0.3µm, and 1.86 ±0.6 µm for PEO-10Nb, PEO30Nb, and PEO-50Nb samples, respectively, indicating that the effect of Nb content on the thickness of PEO coating was insignificant. From EDS results shown in Fig.6, the Ti, O, Nb, P, and Ca elements were detected in all coatings. Ti and Nb elements generated from substrate while other elements came from electrolyte which containing P and Ca species. Interestingly, Ca/P molar ratios were found to be ~0.94, ~1.6, and ~1.36, for PEO-10Nb, PEO-30Nb, and PEO-50Nb samples, respectively, indicating that the 30 wt.% Nb can influence the deposition of HA on the surface of the coatings since the Ca/P ratio in the PEO-30Nb sample was almost identical to that of HA (HA Ca/P= 1.667) [34]. In other studies, Tsutsumi et al. [35] reported that Ca/P ratio reached ~ 0.643 during the PEO treatment of Ti-29Nb-13Ta-4.6Zr alloy in 0.1 M calcium glycerophosphate mixed with 0.15 M magnesium 8
acetate at a current density of 312 A/m2 for 8 min. Sowa et al. [11] found that Ca/P ratio reached ~ 1.44 for the PEO-treated Ti-36Nb-2Ta-3Zr alloy from an electrolyte composed of 0.1 M Ca(H2PO2)2 and 0.23 M Ca(HCOO)2 at 438 V for 10 min. As such, Sharma et al. [36] reported a value of Ca/P ratio ~ 1.50 for the PEO Ti-Zr alloys at 300 V. The present results implied that the PEO treatment of Ti-30Nb alloy can lead to a higher ratio of Ca/P in spite of low current density/voltage and short coating period. The variation in the values of Ca/P ratio is believed to be related to the differences in the characterizes of plasma discharges and the electrophoretic mobility of the ionic species during PEO [37]. In addition, Khalil and Leach [38] demonstrated that ionic transport numbers during PEO process would rely on the substrate composition, electrolyte chemistry and current density. It can be deduced, therefore, that the PEO treatment of binary Ti-30Nb alloy would be more desirable in terms of HA-forming ability as compared to the other alloys, such as Ti-29Nb-13Ta-4.6Zr, Ti-36Nb-2Ta-3Zr, and Ti-Zr alloys. Fig. 7 illustrates XRD patterns of the PEO coatings formed on Ti-xNb alloy samples. The PEO10Nb sample was composed mainly of anatase (JCPDS: 01-071-1168), brookite (JCPDS: 01072-0100), and HA (JCPDS: 00-024-0033). Additionally, Ti (JCPDS: 03-065-9622) was detected in the patterns due to the infiltration of X-ray into the Ti-xNb alloy substrate. As for the PEO-30Nb sample, the peaks related to the rutile (JCPDS: 01-084-1283), anatase, HA, Nb2O5 (JCPDS:01-070-2679), and Ti substrate were observed. In the case of the PEO-50Nb sample, peaks corresponded to anatase, rutile, Nb2O5, and CaTiO3 (JCPDS: 00-039-0145) are detected. The presence of rutile would indicate that the partial phase transformation of metastable anatase into the stable rutile was occurred at PEO-30Nb and PEO-50Nb samples [39]. Considering the fact that a hydrolysis process of CaTiO3 can be occurred during the immersion in SBF solution (Eq.1), the PEO-50Nb sample is expected to possess a good bioactivity since this reaction can 9
facilitate the HA nucleation by formation of a Ti–OH matrix and increasing ion concentration of OH− and Ca2+ at the surface [40, 41] +2
→2
+
(
) (1)
However, as shown in Fig.7, the amounts of CaTiO3 was very small, indicating that its contribution to the HA formation would be insignificant. Interestingly, on the other hand, the peaks related to HA in the case of PEO-30Nb sample was somewhat higher than other cases which was consistent with Ca/P ratio calculated from EDS analysis. 3.4. Effect of Nb content on the corrosion behavior The PDP curves for Ti-xNb alloy samples before and after PEO treatments as well as the electrochemical parameters namely corrosion potential (Ecorr), corrosion current density (icorr), and current density at passive region (ipass) alloys are presented in Fig. 8 and Table 3, respectively. As the amount of Nb increased, the corrosion resistance of the uncoated alloys tended to be increased since the 50Nb sample displayed the lowest values of icorr and ipass as compared to 10Nb and 30Nb samples. It is thought that the decrease in the value of icorr with increasing Nb content is attributed to the fact the corrosion resistance of the Ti-xNb alloy samples before PEO treatment was strongly relied on the microstructure of the samples [42]. As shown in Fig. 1, the microstructure was changed from α′ structure in the Ti-10Nb alloy into α′+ α″+ β in the Ti-30Nb alloy and into only β in the Ti-50Nb alloy. As reported by Atapour et al. [42] and Byeon et al. [4], the metastable β structure had superior corrosion resistance as compared to the α′ or α′+ α″+ β. On the other hand, a remarkable improvement in the corrosion resistance was found in the case of PEO samples as compared to the uncoated samples. Regardless of the Nb content, a passive behavior in the anodic branch was observed in the polarization curves of PEO coatings. However, no breakdown of the passive film was observed 10
in the case of PEO-30Nb and PEO-50Nb samples while a breakdown of the passive film was occurred at a potential ~1.28 V in the case of the PEO-10 Nb sample. This result indicated that the PEO-30Nb and PEO-50Nb samples have higher breakdown potentials than that in the PEO10Nb sample. In general, the breakdown potential of the passive layer would be affected by several factors related to the thickness and composition of the coatings. As the measured thickness of the coatings was approximately identical (~1.8 µm), the composition of the coatings would be the main factor affecting the values of breakdown potential of the PEO coatings in polarization curves shown in Fig. 8. Considering the fact that the icorr and ipass values of PEO30Nb sample were lower than their counterparts of PEO-10Nb and PEO-50Nb samples, an excellent corrosion performance can be obtained in the PEO-30Nb sample. To discover the correlation between Nb content and the corrosion behavior of the coatings, further analysis by EIS was conducted on the PEO coatings samples in a 0.9 wt.% NaCl solution. Thus, EIS results in terms of Nyquist and Bode plots are presented in Fig. 9. Nyquist plots in Fig.9a showed that the diameters of capacitive loops of PEO-30Nb sample were much larger than that of other samples. In addition, it was found from Bode impedance plots shown in Fig. 9b that the PEO-30Nb sample exhibited a higher value of impedance (~8.5 x 104 Ω.cm2) at the low frequency region (0.01Hz) as compared to PEO-10Nb and PEO-50Nb samples which showed lower values of ~6.1 x 104 and ~5.0 x 104 Ω.cm2, respectively. The two-time constants found in the Bode phase plots shown in Fig.9c can be related to the dual-layer structure of the coating formed via PEO [48]. As it can be seen from Fig.9c, the phase angle at the high frequency region (105 Hz) for PEO-30Nb sample was higher than those for PEO-10Nb and PEO-50Nb samples which was attributed to the formation of anatase, rutile, Nb2O5 together with high amounts of HA. 11
The EIS results of the PEO-10Nb, PEO-30Nb, and PEO-50Nb samples were analyzed using the equivalent circuit model shown in Fig.9c. In the proposed circuit model, Rs represents the solution resistance while Rl and Rb describe the resistances of the loose and compact layers, respectively. CPEl and CPEb are the corresponding constant phase elements. The corresponding values of the equivalent elements are listed in Table 4. Regardless of Nb content, the Rb was obviously higher than that of Rl due to the fact that the compact layer exhibits better corrosion protection properties than the loose layer [43-45]. In addition, Rl and Rb values of the PEO-30Nb sample were measured to be 1.96 x 105 and 2.39 x 106 Ω.cm2, respectively, which were higher than the counterparts for PEO-10Nb and PEO-50Nb samples. The above PDP and EIS tests proved that the corrosion resistance of PEO-30Nb was higher than the other samples which was attributed not only to the compact layer but also due to the phase composition considering the fact that anatase, rutile, Nb2O5, and HA have good anticorrosion properties. 3.5. Effect of Nb on the bioactivity and wettability of the coatings As reported earlier [46], HA formation on the surface of materials is demonstrated to be the primary indication for bioactivity. Fig. 10 (a-c) displays the morphologies of HA deposited on the PEO coatings of Ti-xNb alloy after the immersion in the SBF solution for 24 h. As it can be seen from Fig. 10b that the amount of HA in the PEO-30Nb sample was higher than the other samples. EDS area analyses were carried out on the white areas presented as insets in Fig.10 (a-c) and the results are given in Table 5. In all cases, Ca, P, O, Ti, and Nb elements were detected. However, the Ca/P ratio calculated for three samples was ~ 1.51, ~ 1.66, and ~ 1.48, for PEO10Nb, PEO-30Nb, PEO-50Nb samples, respectively, indicating that the PEO treatment of 30Nb sample can be useful to accelerate the growth of HA in the SBF solution. To affirm this result, however, the XRD patterns of the coatings after the immersion in SBF solution were presented in 12
Fig.10d. Interestingly, the intensities of HA in the case of PEO-30Nb sample were higher than other samples which was consistent with the SEM image in Fig.10b and EDS results listed in Table 5. The contact angle experiments for Ti-xNb alloy before and after PEO treatment are presented in Fig. 11. The low contact angle would imply a good cell adhesion, meaning that the hydrophilic surface can be used as implant bio-materials [18]. Here, two important things can be drawn from the comparison of the contact angle results shown in Fig.11. First, the contact angles of the alloys before PEO were found to decrease as the Nb content increased which was ascribed to the increase the amount of β-phase in the alloy. Second, the hydrophilic property of the Ti-xNb alloy samples was enhanced by PEO treatment where the lowest value of contact angles was found in the PEO-50Nb sample. 4. Discussion Typical SEM images, EDS, and XRD, and corrosion results revealed the surface properties TixNb alloys were significantly improved upon the PEO treatment where the PEO-30Nb sample exhibited the best properties in terms of corrosion resistance and the HA-forming ability. In terms of corrosion properties, the microstructural parameters, namely thickness, pore size and porosity together with the composition of the coatings were the main factors affecting the corrosion performance of PEO coatings [33]. As well known, the intense plasma discharges and gas evolution leads to the development of large pores and cause thermal cracking of the coatings. Accordingly, the high current densities shown in Fig. 3 during the coating formation of PEO30Nb and PEO-50Nb samples would lead to increase the size of micropores and their distributions as compared to the PEO-10Nb sample (Fig. 5). The presence of large size of
13
micropores at high levels on the surface of PEO coatings can increase the exposed surface area and thus the tendency of chloride ions to attack the substrate through these defects [29]. Although the PEO-50Nb sample exhibited higher fraction of rutile phase whose higher chemical stability than anatase phase, this sample showed lower corrosion properties as compared to the PEO-30Nb sample. This was attributed to the high porosity (~ 40 %) in this sample which can facilitate the infiltration of chloride ions into the Ti-50Nb substrate during the corrosion test. However, the high corrosion resistance obtained in the case of PEO-30Nb sample can be ascribed to the combined effects of the lower porosity, smaller size of micropores, and phase composition. Despite the average size of micropores, their distributions and the coating thickness in the PEO-10Nb sample were comparable to that in the PEO-30Nb sample, this sample exhibited lower corrosion properties as compared to that in the PEO-30Nb sample. This result was related, therefore, to the formation of higher amounts of anatase, rutile, Nb2O5, and HA in the PEO-30Nb sample which were responsible on such improvements. This result would be reasonably consistent with recent investigation on Ti-6Al-4V alloy treated via PEO for 3 min using electrolytes containing bioactive elements, such as Ca, P, Mg, Mn, and Si where the composition of the coating played a significant role in corrosion protection properties [47]. On the other hand, it was reported that the wettability, pore morphology, and the phase composition of the coatings were the main factors affecting the HA-forming ability [48]. As for the wettability, it is well known that the porous surface of PEO coating can help to reduce the contact angle (θ) between the crystal nucleus and the interface. The change of Gibbs free energy (∆G*) of HA nucleation can be defined as follows, [49]:
∆
∗
=
( ) [
" !(#)]
14
(2)
% (&) =
( '()* )(
()* )
+
(3)
where σ is surface tension, ν is molecular volume of precipitate, k is Boltzmann constant, I is the ion activity coefficient product of HA, and K is the solubility product of apatite. From Eq. 2, the ∆G* of HA can be reduced if the contact angle (θ) is decreased due to the direct relationship between ∆G* and θ. As shown from Fig.11, the PEO-50Nb sample exhibited the lowest value of contact angle. This result can be ascribed to the differences in the morphologies of the PEO coatings where the high porosity together with the large size of micropores in the PEO-50Nb sample would be responsible for the low value of contact angle of this sample. Accordingly, the wettability was not adequate factor for explaining the high-forming ability of HA in PEO-30Nb sample. This explanation was in accord with the findings reported by Santos-Coquillat et al. [48]. While it might be expected that the highly porous structure observed in the PEO-50Nb sample can facilitate the HA formation, the large micropores in the sample ~6.43 µm were unfavorable sites for the nucleation of HA as compared to the PEO-10Nb sample. Shin et al [50] postulated that the presence of micropores with an average size of ~5 µm can result in a less bioactive performance as compared to ~ 2 µm micropores. In addition, Nashimoto et al. [51] demonstrated that the micropore size as low as 1.2 µm was the most desirable for cell anchoring. Similar findings were reported also by Pafenov et al [52] and Shin et al. [53]. As to the phase composition, it was found that the existence of anatase in the coatings can significantly induce the HA-forming ability due to the crystallographic compatibility between anatase and HA [48, 54, 55]. Thus, the anatase phase is supposed to display a higher ability for HA formation than what the rutile phase did. Indeed, the amounts of anatase in the PEO-30Nb sample were higher than other samples. The formation of Nb2O5 in this coating would help also 15
to induce the HA formation in this sample [56]. The HA particles formed during PEO would also trigger the nucleation of HA during the immersion in SBF solution which increase the ease of HA growth. Finally, due to the high corrosion resistance together with the high HA-forming ability observed in the case of PEO-30Nb sample, the present sample can possess extraordinary potential to be used as an ideal bio-implant material in orthopedic and dental applications.
5. Conclusions This work looked into the surface properties of the Ti-xNbs alloy coated via PEO process by considering the role of Nb content. The structure of Ti-xNb alloy was modified to structure rich in β-phase with increasing Nb content from 10 to 50 wt%. The surface structure observations of the coatings formed via PEO on these alloys revealed that the mean size of the micropores and their porosity tended to increase with increasing Nb content. A good combination of high corrosion resistance with a good bioactivity were attained in the coating made on the Ti-30Nb alloy. The present findings strongly suggest that the processing of Ti-30Nb alloy by PEO is a promising technique for improving its performance in order to be used as a safe implant material in biomedical applications.
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Table 1 Ion concentrations of human blood plasma and SBF solution utilized in this study.
SBF Blood plasma
Ion concentration (mM) Na+ 142.0 142.0
K+ 5.0 5.0
Mg+2 1.5 1.5
Ca+2 2.5 2.5
Cl103.0 103.0
HCO3-2 10.0 27.0
HPO4-2 1.0 1.0
SO4-2 0.5 0.5
Table 2 Details of the micropores determined from Image J analysis of SEM images of PEO coatings with respect to the Nb content.
Porosity (%) Pore size (µm)
Sample PEO-10Nb
PEO- 30Nb
PEO-50Nb
10.13 ±1.3
12.53±1.2
40.83±1.5
1.05±0.9
1.13±0.8
6.43±1.1
Table 3 Results of the PDP tests in 0.9 wt.% NaCl solution for the Ti-xNb alloys samples before and after PEO treatment. 25
Sample 10Nb 30Nb 50Nb PEO-10Nb PEO-30Nb PEO-50Nb
icorr (A/cm2) 2.26 X 10-4 4.68 X 10-5 8.31 X 10-6 6.95 X 10-6 6.00 X 10-9 2.53 X 10-7
Ecorr (V) -0.487 -0.469 -0.398 -0.645 -0.353 -0.406
ipass (A/cm2) 2.20 X 10-6 4.73 X 10-8 1.94 X 10-7
Table 4 Results of the EIS tests in 0.9 wt.% NaCl solution for the Ti-xNb alloys coated via PEO. Sample
Rs (Ωcm2)
Rl (Ωcm2)
CPE-nl
PEO-10Nb PEO-30Nb PEO-50Nb
19.41 22.35 21.47
4.01 X 103 1.96 X 105 2.36 X 104
0.77 0.93 0.89
CPE-Yl (S.sn.cm-2) 1.81 X 10-7 3.72 X 10-9 1.17 X 10-8
Rb (Ωcm2)
CPE-nb
1.51 X 105 2.39X 106 5.06 X 105
0.69 0.81 0.75
CPE-Yb (S.sn.cm-2) 7.89 X 10-6 7.85 X 10-7 2.90 X 10-7
Table 5 Results of the EDS area analysis for the Ti-xNb alloys coated via PEO after immersion in a SBF solution for 24 h. sample PEO-10Nb PEO-30Nb PEO-50Nb
Ti (wt.%) 21.06 13.83 24.17
O (wt.%) 62.98 57.88 55.03
Ca (wt.%) 9.60 16.15 8.20
P (wt.%) 6.36 9.73 5.54
Nb (wt.%) 2.41 7.06
Ca/P ~1.51 ~1.66 ~1.48
Figures captions Fig.1 (a-c) the optical micrographs obtained for Ti-xNb alloys samples (a) 10Nb, (b) 30Nb, and (c) 50Nb (scale bare in images (a-c) is 100 µm), and (d) the results of XRF analysis. The 50Nb sample was mainly composed of β-phase. Fig.2 XRD patterns of Ti-xNb alloys showing the presence of α and β-phases. Fig.3 (a-c) Voltage and current density vs. time relationships recorded during plasma electrolytic oxidation of (a) 10Nb, (b) 30Nb, and (c) 50Nb, and (d) the variation of maximum and minimum current density with respect to Nb content.
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Fig.4 Optical images showing the micro-discharges appearance with respect to coating time during PEO treatments of 10Nb, 30Nb and 50Nb samples.
Fig.5 SEM images and the analyzed images for the PEO coating formed on (a, a1) 10Nb, (b, b1) 30Nb, and (c and c1) 50Nb samples. Insets in Fig. 4 (a-c) are the high-magnification images of the surface morphology of the PEO-10Nb, PEO-30Nb, and PEO-50Nb samples. Fig.6 (a-c) EDS analysis showing Ca/P ratio on the surface of the coatings formed on 10Nb, 30Nb, and (c) 50Nb samples, respectively. Fig.7 XRD patterns for the PEO-10Nb, PEO-30Nb, PEO-50Nb samples. Fig.8 (a, b) Potentiodynamic polarization curves of the Ti-xNb alloys before and after PEO treatment, respectively. Passive-regions were observed in the case of the PEO-coated Ti-xNb alloys. Fig.9 Nyquist and Bode plots for the PEO coatings formed on Ti-xNb alloys (a) Nyquist plots, (b) Bode impedance plots, and (c) Bode phase plots, and (d) Equivalent circuit model utilized for fitting the EIS results. Fig.10 (a-c) SEM images showing the HA morphology formed on the PEO-10Nb, PEO-30Nb, and PEO-50Nb samples, respectively after a 24 h soaking in SBF solution and (d) the corresponding XRD patterns. Insets in Fig. 9 (a-c) are the high-magnification images of the surface morphology of the PEO-10Nb, PEO-30Nb, and PEO-50Nb samples immersed in SBF solution for 24 h. Fig.11 Comparison of contact angle of Ti-xNb alloys before and after PEO treatment.
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Fig. 1
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Fig. 2
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Fig. 3
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Fig. 4
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Fig. 5
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Fig. 6
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Fig. 7
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Fig. 8
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Fig. 9
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Fig. 10
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Fig. 11
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Highlights An ideal implant porous material is successfully fabricated from Ti-30Nb alloy via PEO. Effect of Nb content on the surface properties of Ti-xNb alloy coated via PEO is significant. A protective bioactive coating is successfully obtained by PEO process of Ti-30Nb alloy. Wettability of the PEO coatings was improved as compared to the bulk alloy surface.