Surface characteristics and in-vitro behavior of chemically treated bulk Ti6Al7Nb alloys

SCT-21723; No of Pages 11 Surface & Coatings Technology xxx (2016) xxx–xxx

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Surface characteristics and in-vitro behavior of chemically treated bulk Ti6Al7Nb alloys Ziya Esen a,⁎, Ezgi Bütev Öcal a,b a b

Çankaya University, Materials Science and Engineering Department, 06790, Ankara, Turkey Middle East Technical University, Metallurgical and Materials Engineering Department, 06800, Ankara, Turkey

a r t i c l e

i n f o

Article history: Received 30 July 2016 Revised 26 October 2016 Accepted in revised form 27 October 2016 Available online xxxx Keywords: Ti6Al7Nb alloy Sodium Calcium Simulated body fluid Apatite X-ray photoelectron spectroscopy

a b s t r a c t The effect of various treatments on surface chemical composition and structure, and bioactivity of Ti6Al7Nb bulk alloys has been investigated. The alloys were treated employing aqueous solutions of NaOH and CaCl2 separately, and also by subsequent CaCl2 treatment after NaOH treatment (NaOH-CaCl2 treatment) which were followed by heat treatment. NaOH treatment was observed to be effective in enrichment of surface layer with Na. On the other hand, Na+ ions were mostly replaced by Ca2+ ions as a result of NaOH-CaCl2 treatment, while single step CaCl2 treatment was less effective in Ca incorporation. Additionally, porous network surface structure seen in NaOH and NaOH-CaCl2 treated samples was completely different than globular morphology detected in CaCl2-treated samples in single step. Subsequent heat treatments caused coarsening of surface structure and loss of some Na+ and Ca2+ ions. NaOH and NaOH-heat treated samples did not exhibit apatite formation within 15 days immersion in simulated body fluid (SBF). On the other hand, NaOH-CaCl2 samples had the highest apatite formation; however, NaOH-CaCl2-heat treated samples did not display any mineralization. Conversely, CaCl2 treated samples allowed apatite formation after heat treatment. © 2016 Elsevier B.V. All rights reserved.

1. Introduction High biocompatibility and osseointegration as well as high corrosion and wear resistance are the primary requirements for artificial biomaterials to fulfill the desired function properly. Implants used in joint replacements such as elbow, knee and hip are also needed to possess mechanical properties similar to that of bone. High fatigue strength is another service requirement to withstand cyclic loads which makes metallic materials indispensable in load bearing applications. However, the metallic implant should also preserve its mechanical stability to perform desired mechanical function properly during service [1]. The breakdown of a material through some chemical reactions with body fluid, proteins and various cells in the biological environment [2–6] is the main concern as material loss change the mechanical stability of the implant. Unlike the bioactive materials like hydroxyapatite (HAP) and some biodegradable metals such as iron-alloys [7] and Mg-alloys [8], most of the load bearing metallic materials are classified as biotolerant materials and expected to remain as permanent fixtures during the service.

⁎ Corresponding author. E-mail addresses: [email protected] (Z. Esen), [email protected] (E.B. Öcal).

The rate of attack of corrosion or breakdown in some of metallic materials such as Co-Cr alloys, stainless steels and titanium alloys is usually low and they are self-protected as they contain inherent passive oxide films [1]. In fact, the type, structure, thickness and spontaneous regeneration in milliseconds even after damage determine the protectiveness and the rate of ion transfer through the passive film. The oxide film in Co-Cr based alloys and stainless-steels is rich in Cr2O3 [9–13]. Therefore, Cr is responsible for high passivation and it increases the resistance against localized breakdown of passivity. However, stainless steels containing nickel may cause a negative reaction and induce harmful effects when nickel ions are released [14–16]. In addition, mechanical mismatch related “stress-shielding” problem may appear due to comparatively higher elastic moduli of stainless steels like those of Co-Cr alloys. Accordingly, titanium and titanium alloys are more frequently used as they mechanically more compatible to bone and more passive due to 3–7 nm thick [17] amorphous [18] or crystalline native titanium oxide (TiO2) [3–5]. α + β titanium alloys like Ti6Al4V and Ti6Al7Nb, also contain surface oxides of the alloying elements such as Al2O3, V2O5 and Nb2O5 [19–22]. As alloying elements like vanadium are considered as potential toxic elements, which may cause allergic and adverse reactions in human body, they are replaced with more inert elements like Nb as in the case of Ti6Al7Nb. Nb2O5 is considered to be more stable, less soluble and more biocompatible oxide compared to V2O5 [1]. Ti6Al7Nb alloys, therefore, are produced as alternative α + β

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alloys exhibiting higher biocompatibility, but similar mechanical properties to those of Ti6Al4V alloys. Osseointegration, which is defined as the direct integration of living bone to artificial implant surface, is crucial as well as the biocompatibility to not to repeat the surgery operation and for using the implants safely for long period. For stable implant fixation, the bone tissues are expected to attach and bond to implant surface to ensure an interface matrix, equivalent to bone structure [23,24]. For example, in load bearing implants used in knee and hip joints, the surface portion of the implant anchored into bone is desired to exhibit high osseointegration for inducing sufficient bone growth during the healing stage. Two methods, namely, cement fixation and cementless implantation, are used for titanium implant fixation in bones. Bone cements based on polymethylmethacrylate (PMMA) are prepared and employed during surgery [25]. Although durability and bond strength of the cement affect its lifetime, the micro-movements across the metal-cement interface may induce failure and/or end up with metal ion release into body fluid [26]. Therefore, implantation with direct cementless technique through osseointegration is more promising in terms of obtaining safer fixation [27]. However, cementless implanted bioinert materials like titanium are generally encapsulated with fibrous tissue in the body thereby, eliminating direct contact with the surrounding bone. Accordingly, the surface modification technologies with the aim of enhancing osseointegration have become inevitable to increase bonding between implant surface and the bone, and to form new bones tissues at the early stages after surgery [1,28]. Mechanical methods such as machining, grinding or blasting are usually used to roughen the surface and produce high surface area [22]. On the other hand, most of the physical and chemical methods form a new layer which is compositionally different from the base and biologically more active that it induces bone like apatite formation when implanted in the living body [17]. Among these techniques, sol-gel technique is widely used to deposit thin ceramic coatings such as titanium oxide (TiO2) [29], calcium phosphate [30,31] especially hydroxyapatite (HA) coatings, [32] and TiO2–CaP composite [33]. Some silica-based coatings have also been produced using the sol–gel technique [34]. On the other hand, anodic oxidation is used to form porous titania of the anatase and/or rutile [35], titania nanotube [36] and for Ca incorporation [37]. Spray methods like thermal and plasma spraying are used to obtain HA [38,39] and calcium silicate [40] and titanium coatings [41] with porous structure on various implants, respectively. Sputtering, a physical deposition technique, is mainly utilized to produce bioactive glass–ceramic coatings on titanium based on MgO–CaO–P2O5–SiO2 [42] and calcium phosphate [30,31]. The incorporation of Ca ions by ion implantation [43] is known to increase bone conductivity as well. Successive implantation of calcium and phosphorus with subsequent annealing results formation of apatite layer [44]. Apart from the traditional coating methods, biochemical methods aim to obtain an ultrathin layer consisting of bioactive molecules [45]. A variety of techniques, such as silanized titania [46], self-assembled monolayers [47] and protein immobilization [48] have been used to obtain bioactive surface on titanium and titanium alloys. Possibility of processing at lower temperatures, applicability to complex geometries and use of simple equipments make chemical treatments advantageous over the physical deposition techniques. Enrichment of the surface layers with elements of Na [49,50], Ca [51], Mg [52] and Sr [53] using aqueous solutions and formation titanate phases of corresponding elements by subsequent heat treatment have been shown to allow growth of bone. For example, chemical treatment in aqueous solution of NaOH at 60 °C and subsequent heat treatment at 600 °C result in bioactive sodium titanate layer which forms bone-like apatite when soaked in SBF [49,54,55]. However, it has been shown that all subsequent heat treatments negatively affect the in vitro bioactivity by causing thermal decomposition of the dense sodium titanate leaving less exchangeable sodium ions [56]. Bioactivity of the sodium titanate layer is also deteriorated when kept in a humid environment for a long period of time. In contrast to sodium titanate, calcium titanate

obtained as a result of aqueous Ca solution treatment exhibits stable apatite formation even after storage in a humid environment [53]. Obtaining Ca-rich or calcium titanate layers simply by using aqueous solutions of calcium necessitates relatively higher temperatures and prolonged time. CaCl2 aqueous solution treatment after the NaOH treatment is shown to be a simpler and effective way of obtaining Ca-rich coating layer as it allows incorporation of Ca2+ ions into surface through exchange of Na+ ions. The combined chemical treatment including NaOH and CaCl2 have been used previously for surface modification of titanium [51] and titanium alloys like Ti–15Zr–4Nb–4Ta [57] and Ti– 36Nb–2Ta–3Zr–0.3O [58]. Authors have also previously shown that the method can be successfully used for surface modification of porous Ti6Al7Nb alloy foams containing porosity as high as 70% [59]. However, the response of bulk or non-porous Ti6Al7Nb alloy, an orthopedics load bearing material [60], towards various solutions is not as same as their porous counterparts due to less active and relatively smaller surface area. Therefore, in the current study, the response of bulk Ti6Al7Nb alloys to combined NaOH and CaCl2 treatment was examined in detail by considering the surface chemical composition and structure of the alloy before and after the treatment. Moreover, the effectiveness of combined chemical treatment in terms of Ca incorporation to the surface, which was not clarified before, was also checked by comparing the combined treated samples with those CaCl2 treated in single step. The samples were modified chemically using aqueous solutions of NaOH, CaCl2 and combined treatment of NaOH and CaCl2 solutions, which were followed by heat treatment in air. Response to in vitro environment and apatite formation abilities were compared using SBF by immersing the samples for different periods. Microstructural and structural changes observed before and after various surface treatments, including SBF tests, were examined using Scanning Electron Microscope (SEM) and Thin-film X-Ray Diffraction (TF-XRD). Energy Dispersion X-ray Spectroscopy (EDX) and X-Ray Photoelectron Spectroscopy (XPS) were utilized to reveal the surface chemical compositions of the samples. Atomic Force Microscope (AFM) was also used to characterize the surface morphology and roughness of the starting sample. 2. Material and methods 2.1. Raw materials and sample preparation Ti6Al7Nb alloy bars (∅ 18 mm, supplied from Acnis International, France) with chemical composition presented in Table 1, was used throughout the surface treatment studies. Alloy samples were initially heat treated to ensure homogenous and similar microstructures in all test samples. Heat treatment was carried out by heating the samples under high purity argon gas atmosphere up to 1100 °C at which they were hold for 30 min followed by cooling naturally in furnace. Next, the samples (10 mm × 10 mm × 4 mm) cut from heat treated alloy bars were ground with emery papers and grinding operation was finalized using #2000 grit SiC paper. Finally, all of the ground samples were ultrasonically washed with pure acetone, ethanol and deionized water, and dried in the hot oven at 40 °C for 2 h under vacuum. 2.2. Surface treatment Basically six groups of surface treated specimens were prepared by soaking the alloy in NaOH and CaCl2 aqueous solutions, and by applying subsequent heat treatment in air, Table 2. The first group of specimens were alkali treated only in 5.0 M NaOH aqueous solutions at 60 °C for 24 h. Some of the NaOH-treated samples were then soaked in 0.1 M aqueous CaCl2 solution at 40 °C for 24 h for production of group 3 samples. After each solution treatment operation conducted using either NaOH or combined NaOH-CaCl2, the samples were washed with

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Table 1 Chemical composition of Ti6Al7Nb alloy used in the current study. Element

Al

Nb

Ta

Fe

O

C

N

H

Ti

Composition, wt.%

6.00 ± 0.02

6.88 ± 0.03

0.009 ± 0.002

0.51 ± 0.02

0.17

0.004

0.003

0.003

Balance

deionized water and dried in hot oven at 40 °C for 16 h. All the aqueous treatments were achieved using mild agititation in a bench top reactor (Parr Instrument, 5522). Additional heat treatment applied to either NaOH– (group 2) and NaOH-CaCl2 treated samples (group 4) was conducted in air at 600 °C. After keeping the samples for 1 h at heat treatment temperature, they were allowed to cool in the furnace naturally. On the other hand, last two groups of specimens, groups 5 and 6, were either CaCl2- or CaCl2-heat treated using the same aqueous solution variables, subsequent washing operations, and heat treatment process utilized for the prior surface treated groups as shown in Table 2. 2.3. In vitro tests Ti6Al7Nb alloy samples exposed to various surface treatments were soaked in SBF that had ion concentration approximately equal to those of human blood plasma at 36.5 °C as reported in Table 3. SBF was prepared by adapting the procedure used by Kokubo et al. [61] in which the reagents were dissolved in the sequence of NaCl, NaHCO3, KCl, K2HPO4·3H2O, MgCl2·6 H2O, 1.0 M HCl, CaCl2, and Na2SO4. The solution was then buffered with Tris and 1.0 M HCl was used to adjust the pH of 7.4. Apatite formation was checked by keeping as-received reference sample and six groups of surface treated samples for 5 and 15 days in SBF solution, which was refreshed in every two days in order to stabilize ion concentration in the test environment. Then, the SBF soaked samples were washed with deionized water and allowed to dry at room temperature for 24 h in desiccators. 2.4. Characterization Morphological and microstructural examinations of as-received, surface treated and SBF soaked samples were performed by field emission scanning electron microscopy (FE-SEM) (FEI 430 Nano Scanning Electron Microscope). Elemental analysis was also evaluated by EDX detector of SEM. TF-XRD patterns of the samples were taken by Rigaku Ultima IV to analyse the surface phases by continuous scanning at 40 kV between 10° and 70°, 2θ values. The angle of the incident beam was 0.5° against the specimen surface. The surface chemical compositions of the as-received and surface treated samples were analysed by XPS (PHI 5000 VersaProbe) with monochromatised Al radiation. The samples were positioned with respect to the analyser at the electron take off angle of 45° to the surface. In addition to survey spectrum taken, compositional changes at different depths of the samples were investigated by sputtering (24.5 W, 58.70 eV) 4 different layers with 3 min sputter time. The measured binding energies were corrected referring the energy position of C1s in CH2 (284.6 eV). The surface roughness and three dimensional (3D) topography of alloy surface prior to surface processing were examined with AFM (Nanomagnetics Instruments) on 10 μm × 10 μm scan area using tapping mode.

3. Results and discussion 3.1. Structure and chemical composition Fig. 1(a) and (b) present SEM image and AFM 3D-microtopography of starting Ti6Al7Nb alloy's surface on which tiny scratches left from grinding operation can be seen clearly. Although the surface scratches were fairly seen in SEM image, they were more evident in AFM image and had roughness value of 41 nm. Most of the titanium and titanium alloys contains various native oxides on their surfaces. Accordingly, the response of the surface towards various surface treatments will mainly depend on the characteristics of the outermost surface. XPS depth profile of as-ground starting alloy as seen in Fig. 1(c) revealed enrichment of oxygen near the surface, while the concentrations of Ti, Al and Nb elements were relatively low at the surface and gradually increased with sputter time or increasing depth. The relatively higher concentration of oxygen at outermost region of the surface was evaluated as indication of layer composed of various oxides. Similar to other grades of titanium alloys, surface of the starting Ti6Al7Nb alloy was observed to be covered with oxides of alloying elements as revealed by the enrichment of oxygen. Despite various kinds of detected oxides, the oxides layer on the surface was mainly composed of TiO2, Al2O3 and Nb2O5 as shown XPS spectra of Ti, Al and Nb taken from outermost region of the surface as shown in Fig. 2. Ti 2p spectra displayed 2p3/2 peaks of Ti, TiO, Ti2O3 and TiO2 at 454.0, 455.4, 457.8 and 458.6 eV, respectively. On the other hand, Al 2p spectra exhibited two deconvoluted peaks at 71.9 and 74.4 eV corresponding to 2p3/2 peaks of metallic aluminum and Al2O3, while Nb 3d spectra was composed of Nb, NbO, NbO2 and Nb2O5 of which 3d5/2 peaks were measured to be at 202.6, 203.7, 205.2 and 207.5 eV, respectively. The underlying microstructure which determines the elemental distribution on the surface and in the alloy sample was observed to be an equilibrium lamellar microstructure consisting of α-phase rich in aluminum (7.66 wt.% Al, 5.29 wt.% Nb, balance Ti) and aluminum depleted β-phase (4.68 wt.% Al, 22.03 wt.% Nb, balance Ti) as given in Fig. 3. Since the amount of β-phase was relatively lower it couldn't be detected in TF-XRD pattern shown in Fig. 3 due to high intensity dominant αphase peaks. Surface treatment conducted at various conditions as shown in Table 2, changed the surface morphology and chemical composition of the surface (Fig. 4 and Table 4). Samples' surfaces subjected to NaOH treatment revealed a porous fine network structure having around 100 nm pore size. Additional heat treatment applied to same sample at 600 °C didn't change the morphology of the new layer; however, a slight coarsening of the struts have been observed mainly due to oxidation during heat treatment. Likewise, NaOH-CaCl2 and NaOH-CaCl2-heat treated samples' surfaces with relatively finer network structures experienced similar changes. However, the surface features had relatively finer structures compared to those of NaOH and NaOH-heat treated samples. On the other hand, CaCl2 and CaCl2- heat treatment steps

Table 2 Surface treatment applied to each group of Ti6Al7Nb alloy specimens. Group

NaOH treatment

CaCl2 treatment

Subsequent heat treatment

1 2 3 4 5 6

+ + + + − −

− − + + + +

− + − + − +

Table 3 Ion concentration of human blood plasma and SBF. Ion Concentration (mM)

Human Plasma SBF

Na+

K+

Mg2+

Ca2+

Cl−

HCO− 3

HPO2− 4

SO2− 4

142.0 142.0

5.0 5.0

1.5 1.5

2.5 2.5

103.0 147.8

27.0 4.2

1.0 1.0

0.5 0.5

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Fig. 1. Starting Ti6Al7Nb alloy surface (a) SEM image, (b) AFM 3D-microtopography, and (c) XPS profile.

induced formation of fine lump-like structure instead of porous network features observed in other samples. Heat treatment of CaCl2treated samples did not result any significant change in the surface morphology. As given in EDX analysis taken from the treated and untreated surfaces as shown in Table 4, treated surfaces were composed of additional Na and Ca elements with significantly higher oxygen concentration compared to starting alloy. Although NaOH treatment enabled enrichment of the surface layer in terms of Na, additional heat treatment caused loss of some Na element in previously Na-added to the surfaces. Na/Ti ratio in NaOH treated samples reduced to 0.1 from 0.2 after air heat treatment at 600 °C. It is known that additional heat treatment at elevated temperature helps densification and crystallization of the sodium rich phase obtained in NaOH treatment step [62]. However, sublimation and sodium loss may also appear during heat treatment especially in vacuum environment [56]. On the other hand, combined chemical treatment including NaOH and subsequent CaCl2 treatment (NaOH-CaCl2 treatment) was successful in Ca enrichment of the surface which was proposed to occur through the exchange of Na+ ions with Ca2 + ions [51]. Although very small amount of Na remained in the structure, significant amount of Ca2+ ions replaced with Na+ ions by combined NaOH-CaCl2 treatment. Subsequent heat treatment of the same samples caused reduction of Ca content of the surface similar to Na loss observed in the heat treated NaOH processed samples. On the other hand, direct single step Ca-treatment of as-received alloys in aqueous CaCl2 solution was not as effective as combined NaOH-CaCl2 treatment in enrichment of the surface layer by calcium. Accordingly, relatively lower amount of Ca was added to the surface. Similar to Ca loss in NaOH-CaCl2-heat treated samples, CaCl2-heat treated samples also exhibited Ca loss.

Presence of elements detected through EDX analysis, given in Table 4, was also verified by XPS survey spectra taken from the outermost regions of the surfaces as seen in Fig. 5. Although it was not given in Table 4, all of the surfaces also contained significant amount of carbon as seen in XPS survey spectra. It is known that titanium surface may contain physisorbed water, which is formed by bonding of chemisorbed water and hydroxide. In addition, depending on the time of exposure in air and storage conditions, hydrocarbons may be adsorbed on the outmost surface layer [17]. Likewise, carbon adsorption in the current study possibly took place due to exposure to air atmosphere during characterization studies and storage. The elements observed on the surfaces were also detected in XPS depth profiles of the samples, but with different concentration profiles as shown in Fig. 6. The surfaces of NaOH and NaOH-heat treated samples were rich in Na and O elements. Despite the fact that the oxygen concentration and its distribution were almost the same for both samples, subsequent heat treatment at 600 °C caused a slight increase of Na element near the surface. Additionally, concentration of Al was higher in the measured layers of heat treated NaOH samples, while the concentrations of Ti and Nb elements exhibited a gradual increase from surface to interior part. The most prominent change in the samples processed by combined NaOH and CaCl2 treatments was the distribution of Caelement. Both heat-treated and unheat-treated samples, Fig. 6(d) and (c), respectively, exhibited similar Ca distribution with relatively small amount of Na left from previous surface NaOH treatment step. Oxygen distributions were more or less the same found in NaOH treated samples. Ti and Nb displayed similar gradual increase, while the amount of Al was slightly higher close to surface. On the other hand, concentration profile of single step CaCl2-treated samples was similar to that of asreceived alloy given in Fig. 1(c) with relatively higher oxygen

Fig. 2. Deconvoluted XPS spectra of starting sample's surface (a) Ti 2p, (b) Al 2p, (c) Nb 3d. Experimental spectrum (••••), fitted spectrum (____).

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Table 4 EDX analysis of surface treated and untreated Ti6Al7Nb alloy. Specimen

As-received NaOH NaOH + heat NaOH + CaCl2 NaOH + CaCl2 + heat CaCl2 CaCl2 + heat

Fig. 3. Widmanstätten microstructure consisting of α-platelets and β-laths, and corresponding XRD pattern of as-received Ti6Al7Nb alloy.

concentration and lower concentrations of Ti, Al and Nb elements near the surface. Although Ca is present at outermost layer, it disappeared at deeper sections indicating that CaCl2 treatment itself is not as efficient as combined NaOH-CaCl2 treatment. CaCl2-heat treated sample was also exhibited similar elemental distribution except Al. In contrast to CaCl2-

Fig. 4. SEM micrographs of Ti6Al7Nb alloys' surfaces subjected to (a) NaOH treatment, (b) NaOH-heat treatment, (c) NaOH-CaCl2 treatment, (d) NaOH-CaCl2-heat treatment, (e) CaCl2 treatment, (f) CaCl2-heat treatment.

Element (at. %) Ti

Al

Nb

O

Na

Ca

73.5 43.3 51.5 51.5 55.3 78.3 70.8

11.7 5.4 6.7 7.1 7.2 10.6 9.8

7.9 2.3 2.7 2.9 2.9 3.9 3.7

6.9 40.2 34.1 36.7 33.1 6.8 15.4

− 8.8 5.0 0.3 0.4 − −

− − − 1.5 1.1 0.4 0.3

treated samples, subsequent heat treatment in air caused enrichment of Al in the outermost surface of the sample. Despite the presence of relatively high concentration of Na-ions in surface layer of NaOH-treated samples according to EDX and XPS analysis, identification of sodium hydrogen titanate phase (Na x H 2 – x Ti 3 O 7 ·nH 2 O), which was found in previous studies [51, 63–65], was very difficult as very weak peak broadening occurred in TF-XRD patterns between 28 and 30° 2θ values as shown in Fig. 7(a). However, sodium titanate phase (Na2Ti3O7) formation was detected with low intesity shallow peaks in NaOH-heat treated samples given in Fig. 7(b). It is known that additional heat treatment after NaOH treatment is applied for dehydration of the amorphous sodium titanate hydrogel and to transform it into more stable crystalline sodium titanate phase [17]. Although NaOH-CaCl2 treatment was effective in terms of Ca-enrichment of previously NaOH-treated samples (Table 4, Figs. 5 and 6), TF-XRD analysis revealed some CaCl2 phase as shown in Fig. 7(c). Careful SEM examination, as seen in Fig. 8, showed that samples contained some cubical shaped Ca-rich particles which were attributed to undissolved CaCl2 particle residuals. Additionally, for NaOH-CaCl2 treated samples, expected calcium hydrogen titanate, CaxH2 – 2xTi3O7 [51] was not observed. It is stated that when titanium alloy treated with CaCl2 after NaOH solution, sodium ions in previously formed sodium hydrogen titanate substitutes with the calcium ions to form a calcium hydrogen titanate [63]. Thus, as a result of heating, the formed calcium hydrogen titanate is proposed to transform into various forms of calcium titanate, such as CaTi4O9, CaTi2O4 and CaTi2O5 [51,57,58]. However, heat treated NaOH-CaCl2 samples were also revealed diffraction pattern, as seen in Fig. 7 (d), similar to that of NaOH-CaCl2 samples including only α-Ti and CaCl2 phases without calcium titanates despite

Fig. 5. XPS survey spectrum of as-received and surface treated Ti6Al7Nb alloys.

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Fig. 6. XPS depth profiles of Ti6Al7Nb alloys subjected to (a) NaOH treatment, (b) NaOH-heat treatment, (c) NaOH-CaCl2 treatment, (d) NaOH-CaCl2-heat treatment, (e) CaCl2 treatment, (f) CaCl2-heat treatment.

the presence of Ca in the surface layer as detected by XPS. Lack of calcium titanate phase were attributed to thin amorphous nature of the Carich film and overlapping CaCl2 peaks. CaCl2 phase was also seen in samples CaCl2-treated in a single step as shown in Fig. 7(e) with low intensity peaks as relatively low amount of Ca was added to the surface by the single step treatment (Table 4). Although, weak CaCl2 peaks were

still detectable as a result of post heat treatment, Al2O3 phase became visible in TF-XRD pattern of CaCl2-heat treated samples probably because of the oxidation during heat treatment as given in Fig. 7(f). Enrichment of the surface layer with aluminum as a result of additional heat treatment after CaCl2 treatment was also verified by XPS survey spectra given in Fig. 5.

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Fig. 7. TF-XRD patterns of Ti6Al7Nb alloys subjected to various treatments (JCPDS numbers; α-Ti: 4412-94, Na2Ti3O7: 31-1329, CaCl2: 24-223, Al2O3: 46-1212).

3.2. Apatite formation It is known that for bonding of living bone to materials when implanted in living body, the materials should allow the formation of a biologically active bonelike apatite layer on their surfaces [61]. The formation of this layer on the surface of bioactive materials can be

Fig. 8. SEM image showing CaCl2 particles in NaOH-CaCl2 treated surfaces.

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reproduced by in vitro tests in simulated body fluid. Thus, bioactivity of a material can be evaluated by checking the apatite forming ability in SBF by in vitro tests prior to in vivo testing. All the samples soaked in SBF for 5 days preserved their starting surface morphologies with no apatite formation on their surfaces, while soaking for 15 days induced formation of new Ca and P rich particles on the surfaces of NaOH-CaCl2 and CaCl2-heat treated samples, Fig. 9(a) and (b). EDS analysis taken from newly formed particles revealed Ca/P ratio of around 1.4, which were indexed as apatite particles by TF-XRD patterns as shown in Fig. 9(c). Although, the apatite particles formed on the surfaces of CaCl2-heat treated samples were very tiny and precipitated homogenously, NaOH-CaCl2 treated samples' surfaces allowed relatively higher apatite formation by larger spherical apatite particles, each having around 5 μm diameter. Fig. 10 displays the structural changes of all the surface treated samples at higher magnifications after keeping them in SBF for 15 days. Porous fine network structure formed after chemical treatment remained unchanged in NaOH, NaOH-heat and NaOH-CaCl2-heat treated samples, Fig. 10(a), (b) and (d). Likewise, no morphology change was detected in CaCl2-treated sample after soaking in SBF such that lump-like structure preserved its starting morphology. On the other hand, as-presented in Fig. 9, NaOH-CaCl2 and CaCl2-heat treated samples surfaces were covered with apatite particles, each consisting of Ca-P rich flake or platelike features as seen in magnified images of Fig. 10(c) and (f). Morphology and size of HA precipitated on various surfaces depends on the pH of the solution and degradation of solutes after immersing the samples. For example, it has been shown that at pH 7.4, spherulites of plate-like crystallites are formed on 45S5 bioglass. On the other hand, increase in pH of the solution changes the morphology of apatite such that apatites in the form of multilayer porous structure form on borate glass, and rod-like shape is observed on CaSiO3 [66]. In the present study, pH of the solutions containing NaOH-CaCl2 and CaCl2-heat treated samples, which allowed flake or plate-like apatite formation, increased from 7.4 to 7.8 within 15 days of immersion, while the pH values of other solutions were more or less the same and varied between 7.4 and 7.5. It is known that sample surfaces containing Na and Ca releases Ca2+ and Na+ ions via exchange of H3O+ ions in the simulated body fluid to form various hydroxides in different materials such as Zr-OH, Nb-OH, and Ta-OH, Si-OH and Ti-OH groups which induce apatite nucleation. The released ions like Ca2+ and Na+ accelerate apatite nucleation by increasing the ionic activity of apatite [62]. Accordingly, when soaked in SBF, the sodium hydrogen titanate phase formed as a result of NaOHtreatment loses Na+ ions through exchange of H3O+ ions in surrounding environment to form Ti–OH groups. Thus, earlier apatite formation is predicted in NaOH-treated alloys compared to untreated alloys [67–69]. However, it has been shown that titanium surface containing Ca2 + ions displays higher apatite forming ability than that of rich in Na+ ions since released Ca2+ ions can increase ionic activity product of apatite more effectively in the surrounding SBF [70]. Therefore, in the present study, higher mineralization was expected for Ca-enriched surfaces. 15 days of soaking in SBF was not sufficient to observe apatite formation in NaOH and NaOH-heat treated samples, while some of the Ca-enriched surfaces have shown apatite formation on their surfaces after 15 days. Post heat treatment usually result in loss of apatiteforming ability due to reduced mobility of ions such as Ca2 + ions in transformed crystalline calcium titanate phase (CaTiO3) [57]. Accordingly, apatite forming ability of CaCl2-heat treated samples in the present study was decreased and no mineralization was observed. On the other hand, despite Ca incorporation, single step CaCl2 treated samples' surfaces did not allow apatite formation possibly due to amount of Ca which was insufficient to increase ionic activity product of apatite. Surprisingly, after heat treatment of CaCl2 treated samples, sample surfaces allowed apatite formation. The main difference between the surfaces before and after heat treatment was appearance of Al2O3 phase (Fig. 7) at the outermost layer in CaCl2-heat treated sample. Despite the previous works [71–74], Aguiar et al. [75] have shown that alumina possess

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Fig. 9. SEM images showing apatite formed on the surfaces of Ti6Al7Nb alloys soaked in SBF for 15 days, (a) NaOH-CaCl2 treated sample, (b) CaCl2-heat treated sample, and (c) TF-XRD patterns of apatite formed samples, a: NaOH-CaCl2 treated; b: CaCl2-heat treated.

an apatite-forming ability in SBF solution and Al-OH groups are assumed to serve as the site for apatite nucleation. However, the effect of Al2O3 layer on Ti6Al7Nb needs further investigation to clarify its apatite forming ability.

3.3. XPS analysis of apatite formed surfaces Ion exchange within biomaterial surface and surrounding medium, and the formation of apatite is directly related to structure and chemical composition of outermost surface layer of the starting material. Fig. 11

Fig. 10. SEM images showing surface changes of chemically treated Ti6Al7Nb alloys soaked in SBF for 15 days, (a) NaOH, (b) NaOH-heat, (c) NaOH-CaCl2, (d) NaOH-CaCl2heat, (e) CaCl2, (f) CaCl2-heat.

compares the XPS spectra of apatite formed alloys, namely, NaOHCaCl2 and CaCl2-heat, prior to SBF testing with that of as-received alloy. Only regions of Ti 2p, Al 2p, Ca 2p and O 1s, which exhibited significant changes, have been shown. Due to doublet by spin–orbital splitting, XPS spectra of Ca 2p and Ti 2p consisted of two peaks. On the other hand, the XPS spectra of the O 1s region was composed of three deconvoluted peaks, corresponding to anhydrous oxide (O2−), the hydroxide group (OH−), and adsorbed water (H2O). On the other hand, there were two peaks of 2p3/2 in Al spectra which belonged to metallic aluminum and three valence aluminum. The metallic titanium peak in as-received titanium at 453.7 eV was eliminated as a result of NaOH-CaCl2 and CaCl2-heat treatments. The binding energy of as-received Ti at 2p3/2 is 458.6 eV corresponds to Ti4+ peak. This indicates that surface oxide film (TiO2 film) on titanium was present in the starting alloy surface as seen also in Fig. 2(a). The binding energy of Ti at 2p3/2 (458.6 eV) was unaffected after CaCl2heat treatment as well indicating similar titanium oxide structure. However, as a result of NaOH-CaCl2 treatment and incorporation of high amount of calcium, the binding energy of Ti at 2p3/2 was shifted to 457.6 eV. It is better to note that full width half maximum (FWHM) value of 2p1/2 and 2p3/2 peaks of Ti displayed a small difference since other oxides of titanium have not been deconvoluted in the spectra. Although no calcium detected in as-received alloy, binding energy of the Ca 2p3/2 peaks in NaOH-CaCl2 and CaCl2-heat treated samples was observed to be 346.6 and 346.5 eV, respectively, which are close to binding energy of Ca 2p3/2 peak in CaTiO3 reported in the study of Hanawaa et al. [76]. CaTiO3 phase is known to activate osteogenesis on titanium through exchange of Ca2+ ions [77,78]. However, Ca/Ti ratios of the corresponding surfaces were lower than that of stoichiometric CaTiO3 as seen in Table 5, indicating surfaces also contain other types of oxides like TiO2. The measured binding energies of the Ca 2p3/2 in CaTiO3 has been found to be close to those of calcium hydroxide (Ca(OH)2), and calcium oxide (CaO) which are 346.7 eV and 346.8 eV, respectively, according to NIST-XPS Database [79], and as mentioned in the study of Ohtsu et al. [80] as well. Therefore, it is difficult to clearly identify the chemical state of the surfaces. O1s spectrum of the samples given in Fig. 11 also displayed differences in binding energies. Although, O2 −, OH– and H2O deconvoluted peaks had similar binding energies for asreceieved and CaCl2-heat treated samples, the peaks of NaOH-CaCl2 treated samples shifted to lower energies which was attributed to differences content of incorporated Ca, hydroxide, hydroxyl groups, and/ or adsorbed water on the surface. (See Table 6.) In addition to Ti 2p, Ca 2p and O 1s XPS spectra, apatite formed and as-received samples also exhibited significant differences in their Al 2p spectra although Nb 3d spectra didn't displayed a change. As shown in Fig. 2(b) and Fig. 11, as-received alloy samples had two peaks of 2p3/2

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Fig. 11. XPS spectra of elements for (a) Ti 2p, (b) Al 2p, (c) Ca 2p, (d) O 1s. a: as-received alloy; b: NaOH-CaCl2 treated alloy; c: CaCl2-heat treated alloy.

in Al spectra at binding energies of 71.9 and 74.4 eV corresponding to metallic aluminum and Al2O3, respectively [79]. Both peaks were observed to disappear as a result of NaOH-CaCl2 treatment probably because of the formation of thick Ca-rich layer. However, in CaCl2-heat treated sample, only one peak of 2p3/2 has been detected at 73.7 eV, which corresponds to Al2O3 as well [79]. Due to post heat treatment step, Al2O3 film grew, became thicker and caused disappearance of aluminum on the surface layer. Although both CaCl2 and CaCl2-heat treated samples contained some calcium on the surface (Table 4, Fig. 6), apatite formation was observed only in CaCl2-heat treated sample. The main difference between CaCl2-heat treated sample and all other surface treated samples was the presence of thick Al2O3 layer (Fig. 7 and Fig. 11), in which Al-OH groups are assumed to serve as the site for apatite nucleation [75]. However, the effect of Al2O3 layer on Ti6Al7Nb needs further investigation to clarify its apatite forming ability as mentioned previously.

15 days of soaking was not sufficient for apatite formation in none of the NaOH-treated samples although it is known that Na+ ions accelerate apatite nucleation. On the other hand, apatite formation was detected in samples processed with CaCl2-heat treatment and combined treatment of NaOH-CaCl2. As previously mentioned, released Ca2 + ions from Ca-rich surface in SBF can increase ionic activity product of apatite more effectively than Na+ ions in the surrounding SBF. XPS analysis of the surfaces before SBF tests revealed Ca peaks with energy levels close to CaTiO3 phase, which is known to activate osteogenesis on titanium through exchange of Ca2+ ions [77,78]. However, less dense apatite formation was observed in CaCl2 heat-treated samples, which may be due insufficient amount of Ca that is needed for increasing ionic activity in SBF. CaCl2 treated sample with no heat treatment did not display mineralization while CaCl2-heat treated sample allowed apatite formation. Interestingly, apatite formed CaCl2-heat treated sample's surface contained additional Al2O3, which is known to increase ionic

4. Conclusions Although single step NaOH and combined NaOH-CaCl2 treatments were successful in adding significant amount of Na and Ca ions to the surfaces, respectively, single step CaCl2 treatment was not as effective as combined treatment in obtaining Ca-rich surface layer. The CaCl2treated surface contained fine lump-like structure instead of interconnected porous surface layer observed in NaOH and NaOH-CaCl2 treated surfaces. TF-XRD patterns taken from solution treated and subsequently heat-treated samples contained either weak swallow peaks or no peaks of expected sodium titanate hydrogel, sodium titanate, calcium hydrogen titanate and calcium titanate phases mainly due to presence of thin coatings and their amorphous nature. All samples treated with CaCl2 solutions contained also solid cubical CaCl2 particles left from chemical treatments. Although CaCl2 particles were present in assolution treated and solution-heat treated samples of NaOH-CaCl2 and CaCl2, they were observed to have no influence on apatite formation.

Table 5 Comparison of starting XPS surface chemical compositions apatite formed samples with that of starting as-received alloy. Sample

Ti

Al

Nb

O

C

Ca

Na

As-received, at.% NaOH-CaCl2 treated, at.% CaCl2-heat treated, at.%

12.47 11.41 10.77

4.35 2.15 16.73

0.27 – –

41.65 41.16 40.67

41.26 35.88 31.43

– 7.45 0.40

– 1.96 –

Table 6 Peak parameters of the standard spectra used for evaluating the as-received and surface treated Ti6Al7Nb samples. Sample

Peak

Oxidation state/species

Eb (eV)

FWHM (eV)

As-received As-received NaOH-CaCl2 NaOH-CaCl2 CaCl2-heat CaCl2-heat As-received As-received NaOH-CaCl2 NaOH-CaCl2 CaCl2-heat CaCl2-heat As-received As-received NaOH-CaCl2 NaOH-CaCl2 CaCl2-heat CaCl2-heat As-received As-received As-received NaOH-CaCl2 NaOH-CaCl2 NaOH-CaCl2 CaCl2-heat CaCl2-heat CaCl2-heat

Ti 2p3/2 Ti 2p1/2 Ti 2p3/2 Ti 2p1/2 Ti 2p3/2 Ti 2p1/2 Al 2p3/2 Al 2p3/2 Al 2p3/2 Al 2p3/2 Al 2p3/2 Al 2p3/2 Ca 2p3/2 Ca 2p1/2 Ca 2p3/2 Ca 2p1/2 Ca 2p3/2 Ca 2p1/2 O 1s O 1s O 1s O 1s O 1s O 1s O 1s O 1s O 1s

IV IV IV IV IV IV metal III – – – III – – II II II II O2− OH− H2O O2− OH− H2O O2− OH− H2O

458.6 464.3 457.6 463.3 458.6 464.3 71.9 74.4 – – – 73.70 – – 346.6 350.1 346.5 350.0 530.4 531.5 532.5 529.6 531.0 532.3 530.4 531.4 532.5

1.8 1.9 1.8 1.9 1.8 1.9 1.5 1.5 – – – 1.5 – – 1.8 1.8 1.8 1.8 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4

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Please cite this article as: Z. Esen, E.B. Öcal, Surf. Coat. Technol. (2016), http://dx.doi.org/10.1016/j.surfcoat.2016.10.078