Structural characterization and electrochemical behavior of 45S5 bioglass coating on Ti6Al4V alloy for dental applications

Structural characterization and electrochemical behavior of 45S5 bioglass coating on Ti6Al4V alloy for dental applications

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ARTICLE IN PRESS

MSB 13791 1–9

Materials Science and Engineering B xxx (2015) xxx–xxx

Contents lists available at ScienceDirect

Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb

Structural characterization and electrochemical behavior of 45S5 bioglass coating on Ti6Al4V alloy for dental applications

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M.M. Machado López a , J. Faure b , M.I. Espitia Cabrera c , M.E. Contreras García a,∗ a

Instituto de Investigaciones Metalúrgicas, Universidad Michoacana de San Nicolás de Hidalgo, C.U. Edificio “U”, C.P, 58000 Morelia, Michoacán, México Laboratoire Ingénierie et Sciences des Matériaux (LISM EA 4695) - Université de Reims Champagne-Ardenne, 21 rue Clément Ader, Reims, BP 138 Cedex 02, 51685 France c Facultad de ingeniería Química, Universidad Michoacana de San Nicolás de Hidalgo, C.U. Edificio “D”, C.P. , 58000 Morelia, Michoacán, México

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Article history: Received 10 March 2015 Received in revised form 6 September 2015 Accepted 18 September 2015 Available online xxx

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Keywords: 45S5 Bioglass coatings of Ti6Al4 V Electrodeposition Corrosion resistance Potentiodynamic curves 45S5 bioglass nanostructured coatings for dental application.

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1. Introduction

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In the present work, 45S5 bioglass coatings were deposited on the Ti6Al4 V alloy substrate through the cathodic colloidal electrophoretic deposition process (CEDP) proposed in this work. The coatings were thermally treated at temperatures of 500, 600, 700, and 800 ◦ C for 2 h, and their structure was characterized by FESEM and DRX. Nanostructure and phase evolution of the coatings and xerogels was followed as a function of temperature. The corrosion resistance of the Ti6Al4 V alloy and the 45S5/Ti6Al4 V coating was studied by means of Tafel extrapolation in Hank’s solution, at 37 ◦ C, simulating the conditions inside the mouth. The 45S5 bioglass coatings displayed an amorphous nanostructure at lower temperatures, and partial crystallization at higher temperatures. An increase in the corrosion resistance was observed in the 45S5/Ti6l4 V coating treated at 700 ◦ C because it reduced the icorr , and there was a change in the Ecorr towards more noble values. A model of the chemical anchorage of the 45S5 bioglass coating on Ti6Al4 V was proposed. © 2015 Published by Elsevier B.V.

Titanium and its alloys are materials that are widely used for implants because of their mechanical properties and nontoxic behavior. Unfortunately, metallic implants are not biologically inert, which means they can release ions, and they can only be fixed to bone by an anchoring mechanism; this can lead to dense fibrous tissue encapsulation in the body. Bone regeneration is required in clinical situations in orthopedics and dental medicine [1,2], and thus it is necessary to coat the metallic implants with bioactive materials in order to establish good interfacial bonds between the metal substrate and the bone through the increase in bioactivity [3]. Electrophoretic deposition has recently gained considerable attention as a method of material deposition with biomedical applications due to its easy application and its capacity to produce uniform coatings on substrates in complex forms, at room temperature, and with low-cost equipment [4]. Bioactive glass, specifically the 45S5 Bioglass ceramic formulation (45% SiO2 , 24.5% Na2 O, 24.5%

∗ Corresponding author. Tel.: +52 443 3223500 Ext: 4018; fax: +52 443 3223500 Ext: 4018. E-mail addresses: [email protected] (M.M.M. López), [email protected] (M.E.C. García).

CaO, and 6% P2 O5 in weight), has proven to be a functional biomaterial due to its capacity for bone integration [5–8]. Studies by various authors have shown that 45S5 bioactive glass can be inserted into areas of large-scale bone damage as an aid in increasing its repair and providing good structural support by strongly binding to the bone [9]. The bioactive 45S5 bioglass has type III binding [10] and higher solubility in physiological media, forming a more continuous phase, which includes the cations in the silica structure. It has better mechanical properties, which are important for withstanding the demands of dental surgical procedures, and it has been reported to develop a higher percentage of interfacial bone tissue [3,9]. In addition, bioactive glass has been used in various applications, such as ossicular implants for alleviating conductive hearing loss, in dental implants for maintaining endosseous prostheses, and in its particulate form, for increasing the process of natural repair in patients with periodontal disease. Antibacterial properties of bioglass particles have also been reported by Sheng [11,12]. There is clear evidence, as well, that bioglass is the strongest predictor of osseointegration with bone tissue, with the advantage that calcium phosphate apatite forms a layer on the implanted material [13]. Recently A. S. Bakry et al. have reported the use of 45S5 bioglass for: dentine hypersensitivity treatment [14], treatment of dental enamel erosion [15], dentine lession [16], and incipient enamel demineralization [17]. The deposition method used in the

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present study for obtaining the 45S5/Ti6Al4 V coating for dental applications was the electrodeposition method, using a colloidal suspension obtained through the sol–gel synthesis for the purpose of getting nanostructured thin films of 45S5 bioglass, characterizing them, and evaluating their corrosion resistance under physiologic conditions. The Ti6Al4 V alloy was selected as the substrate material because it is the most widely used alloy in the fabrication of dental implants and membranes. In addition, different authors have reported on its excellent fatigue resistance, with 707 Mpa in 107 cycles, Young’s modulus of 110 GPa, yield strength of 860 Mpa and tensile strength of 930 Mpa [18–23]. Alagic et al. [24] carried out abrasion tests on the Ti13Nb13Zr and Ti6Al4 V alloys and obtained wear volume values of 5.19 and 0.0751 mm3 , as well as hardness values of 285 and 394 HV, respectively. The synthesis of 45S5 bioglass through the sol–gel synthesis method and the study of the thermal transformations of the xerogels obtained, were analyzed by L. Lefebvre et al. [25], who proposed the transformation sequence of bioglass shown in Fig. 1. 2. Experimental procedure 2.1. Ti6Al4 V substrate preparation The substrates of the Ti6Al4 V alloy were prepared in the form of plates, measuring 1 × 1 cm2 , in order to maintain a constant area in the samples and to facilitate their adaptation to the requirements of the different characterization tests that were done. The substrates were ground using SiC sandpaper with grits of 100, 180, 240, 320, 400, and 600. The substrates were then cleaned by means of ultrasound with acetone and distilled water, and dried with alcohol. 2.2. Preparation of the 45S5 bioglass coatings The bioglass precursor suspension was prepared as follows: starting from an aqueous solution of HNO3 1 M, 45% tetraethyl orthosilicate ([C8 H20 O4 Si], 99.9%,), 6% triethyl phosphate ([C6 H15 O4 P], 99.8%), 24.5% sodium nitrate ([NaNO3 ], 99%,), and 24.5% calcium nitrate ([Ca(NO3 )2 .4H2 O], 99%,), all reactives from Sigma-Aldrich, US were added to the solution one by one with 1 h intervals between each addition in the previous order. After hydrolysis and condensation reactions, the formation of oxo-hydroxo complexes led to a colloidal suspension with a pH of 2.5. One part of the suspension was dried at 100 ◦ C for 24 h to obtain the xerogels, which were then thermally treated at four different temperatures: 500, 700, 800, and 1000 ◦ C, for 2 h, in order

Fig. 2. Thermal treatment design for the 45S5 bioglass coatings.

to follow the morphology and phase transformation by FESEM and XRD in these three xerogel samples, which were labeled as: XG500, XG700, XG800, and XG1000, respectively. For the colloidal electrophoretic deposition process (CEDP), another part of the obtained colloidal suspension was transferred into the cell to electro synthesize colloidal aggregates that migrate to the electrode (cathode), where silicon, phosphorus, calcium, and sodium oxo-hydroxo complex precursors (hydrogel) were electrochemically condensed and deposited on the Ti6Al4 V alloy substrate. The colloidal electrophoretic deposition conditions used were 5 V with an electrodeposition time of 2 min, and at room temperature. The deposition was carried out in an electrophoretic cell in which both the cathode and anode electrodes were of the alloy Ti6Al4 V, connected to the power source. The deposition was performed with a LAMBDA LA-300 power supply, which has a capacity of 0–20 V. Once the samples were deposited, they were thermally treated at 500, 600, 700, and 800 ◦ C, for 2 h as shown in the thermal route presented in Fig. 2. The thermal treatment of the coatings was carried out in a covered crucible. Nevertheless, the temperatures used were lower than the sintering temperatures used by other authors describing thicker coatings. The deposited hydrogels covered the substrate and helped to avoid the oxygen transportation to the substrate surface, which had a spontaneous passive and thin oxide layer. Once the bioglass precursor coatings were carried out, the samples were thermally treated at 500, 600, 700, and 800 ◦ C, for 2 h as shown in the thermal route presented in Fig. 2; in this way, four different samples were obtained: BG500, BG600, BG700, and BG800. The heating rate, residence time, and cooling rate were constant for all the samples. X-ray diffraction (XRD) was employed for characterizing the obtained 45S5 bioglass coatings, using a SIEMENS D-5000 diffractometer with Cu K␣ irradiation. The coating morphology was determined using a field emission scanning electron microscope (JEOL JSM.7600F) and the chemical composition of all the samples was determined through EDS microanalysis. 2.3. Electrochemical tests

Fig. 1. Bioglass structural transformations as a result of the effect of temperature.

The Ti6Al4 V alloy and the 45S5/Ti6Al4 V coatings were evaluated through electrochemical techniques with a Gamry series G potentiostat in Hank’s solution at 37 ◦ C, simulating conditions inside the mouth, in order to understand their behavior in relation to corrosion. The Tafel extrapolation technique was used, applying a potential scan of −250–1500 mV with a potential scan rate of 1 mV/s on an open circuit potential (OCP). An exposure area of 0.31669 cm2 was employed for all the samples and an equivalent weight of 11.44 g was used for the titanium alloy that was determined according to the ASTM G102-89 (ASTM 1999) norm [26]. Making up the electrochemical cell, a saturated silver/silver chloride (Ag/AgCl) electrode was used as the reference electrode and an ultra-high pure graphite electrode was used as the counter

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KCl

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D-Glucose

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electrode; the Ti6Al4 V alloy and the 45S5/Ti6Al4 V coating samples were used as the working electrode. The samples were tested in deaerated Hank’s solution with a pH of 7.4 and at a temperature of 37 ◦ C; the Hank’s solution composition, obtained from the SIGMA label [27], is shown in Table 1. The electrochemical tests were carried out after 1 h of immersion for stabilizing the OCP and the temperature. Polarization resistance was calculated in accordance with Eq. 1 of the Stern and Geary equation [28].

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ˇa ˇc 1 2.303(ˇa + ˇc ) RP

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3. Results and discussion

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3.1. Xerogel structural characterization

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Studies of XRD and FESEM of the 45S5 bioglass xerogels were carried out in order to establish the temperatures at which the structural and morphological changes could be expected to occur in the bioglass coatings according to the transformation temperature line of 45S5 bioglass (Fig. 1). Fig. 3 shows the diffraction patterns for the xerogels obtained and the morphologies of the XG500, XG700, XG800, and XG1000 samples resulting from the FESEM analysis; they are presented as inserts in the corresponding diffractograms. The diffraction peaks in Fig. 3a, corresponding to

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the xerogel XG500, have been indexed, the phases CaCO3 (JCPS #01-0928), Ca3 SiO5 (JCPS #01-4362), and wollastonite CaSiO3 (JCPS #84-0654), have been identified, which means that at this temperature, the salt calcium carbonate has been formed by reaction with CO groups product of the decomposition of the used organic precursors TEOS and TEP. This salt will be totally decomposed at higher temperatures so it does not appear in the diffractograms of the xerogels treated at higher temperatures. The tricalcium silicate and wollastonite are formed by condensation Q3 reactions at this temperature and they will form complex silicates found by DRX at temperatures higher than Tg1 displaying the characteristic curved pattern, signal of the amorphous phase. The insert in Fig. 3a, shows the morphology of this amorphous 45S5 bioglass sample, consisting of agglomerated elongated particles of around 0.5 microns size. Fig. 3b shows the diffractogram of the XG700 sample, displaying the characteristic peaks of the crystalline phase, Na6 Ca3 Si6 O18 (JCPS #77-2189), and the small peak at 32.43◦ of the (122) plane of the Na2 Ca4 (PO4 )2 SiO4 phase (JCPS #32-1053), a characteristic phase of 45S5 bioglass, according to the structural transformation resulting from the thermal treatment (Fig. 1). It is evident that the base line of the diffractogram is not plain, which indicates the presence of an amorphous phase. The Fig. 3b insert shows the morphology of the XG700 sample at 20000X, composed of two types of crystals: nanometric needles and smaller agglomerated round nanoparticles, both immersed in an amorphous glass phase. Fig. 3c shows the diffractogram of the XG800 sample, presenting the same phases as XG700, but with greater intensity in the peaks. Nevertheless, there is still an amorphous signal shown by the curved base line of the diffractogram; the thermal treatment of this sample was above the Tg1 , Ts , and the Na6 Ca3 Si6 O18 crystalline phase temperatures and it reached the temperature of the

Fig. 3. X-ray diffraction patterns of the sintered 45S5 bioglass xerogels. a) XG500, b) XG700, c) XG800, and d)XG1000.

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Fig. 4. General EDS microanalysis of the 45S5 bioglass xerogels. a) BG700, b) BG800, and c) BG1000.

Na2 Ca4 (PO4 )2 SiO4 crystallization phase, as observed in Fig. 1. The insert in this figure shows the morphology of the XG800 sample, at 20000X, in which there is a porous structure with acicular crystals 208 and also agglomerated nanometric particles immersed in amor209 phous glass phase, as well. Fig. 3d shows the diffractogram of the 210 XG1000 sample; compared with the XG700 and XG800 samples, 211 there is greater intensity in the peaks and, in this case, the base 212 line of the diffractogram is more curved indicating the presence of 213 a greater amount of the amorphous phase. The insert shows the 214 Q4 morphology of the xerogel XG1000 at 5000X; it is formed by an 215 amorphous porous matrix in which there are growth faceted par216 ticles, aggregated and apparently bound. This structural change is 217 attributed to the greater temperature near to melting temperature, 218 according to Fig. 1. This densification is an effect of temperature, 219 reaching temperatures higher than the Tg1 , Ts , and Tg2 , and obtain220 ing an amorphous matrix in which there is a certain degree of 221 crystallinity with the characteristic phases of the 45S5 bioglass, 222 which is a glass-ceramic. Different authors have reported these 223 same crystalline phases upon sintering bioactive glass powders, 224 specifically with the 45S5 bioglass composition [4–7,25,29,30]. 225 Fig. 4 shows the general EDS microanalyses of the 45S5 bioglass 226 xerogels, XG700, XG800, and XG1000. This test confirms the purity 227 of the 45S5 bioglass obtained by the sol–gel method. Fig. 4a shows 228 the microanalysis of XG700 in which there is the clear presence 229 of the O, Si, Ca, Na, and P elements that are characteristic of 45S5 230 bioglass. Fig. 4b shows XG800, displaying the presence of the 231 elements, characteristic of bioglass, with an increase in Si and Na, 232 as well as a reduction in Ca and P. This can be explained by the 233 diffusion that exists at thermal treatment temperatures above 234 Tg1 and Ts with Na2 Ca4 (PO4 )2 SiO4 crystalline phase formation as 235 206 207

observed in Fig. 1. Fig. 4c shows the EDS of the XG1000 xerogel, in which the presence of the elements characteristic of bioglass are seen. However, there is an increase in Ca and a reduction in Na, due to the fact that this temperature is above those of Tg1 , Ts , and Tg2 , which are temperatures that are sufficiently high for diffusion to take place. This occurs between the transition of the second and third steps of the thermal treatment of the 45S5 bioglass. 3.2. Coating structural characterization The FESEM morphologies of the 45S5 bioglass coatings treated at different temperatures are shown in Fig. 5, at 5000X. The inserts of each micrograph show the microstructure obtained in the coating at 20000X. Fig. 5a shows BG500, in which there is a homogeneous distribution over the entire substrate surface, with no structural defects, resulting in an amorphous bioglass because it is below the Tg1 transition temperature, as seen in Fig. 1. Fig. 5b displays the morphology of BG600, with a homogeneous distribution over the entire Ti6Al4 V substrate surface. Nevertheless, the micrograph also shows that the coating is not completely densified, given that it is only 20 ◦ C above the Ts transition temperature; it is in the silicon and phosphorus-rich separation phase and, therefore is only partially sintered. The coating can be seen at a higher magnification in the Fig. 5b insert; it consists of a nanostructured material, formed by agglomerates of nanometric particles, with a broad size distribution. The line proposed by L. Lefebvre [25] (Fig. 1) established the glass-in-glass phase separation at 580 ◦ C, which is not very different from the one reported by S. Fagerlund and L. Hupa at 600 ◦ C [31]. They also performed their 45S5 bioglass synthesis by means of powder metallurgy. In the case of sol–gel

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Fig. 5. FESEM micrographs of the 45S5 bioglass coatings at different thermal treatment temperatures. a) 500 ◦ C, b) 600 ◦ C, c) 700 ◦ C, d) 800 ◦ C, e) thickness of the BG700 coating, and f) General microanalysis of the BG700 coating.

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synthesis, it is well-known that the transformation temperatures of the products can be expected to be lower than the ones reported for oxides, which is the case of the synthesis carried out in this research. We decided to use the L. Lefebvre [25] line because we did not find any reports from other authors describing the transformation line for xerogel powders. Fig. 5c shows BG700, in which the structure begins to crystallize. It has a morphologic change, presenting uniformity and being distributed over the entire substrate surface, free from structural defects that could be generated during cooling, thus, producing a compact coating with well-distributed nanometric porosity. Berbecaru et al. [30] obtained a 45S5 bioglass coating on the Ti6Al4 V alloy through the pulverization technique

by means of magnetron sputtering, and with treatment at 700 ◦ C for 2 h, they established that the coating began to crystallize according to the XRD patterns. Through Fourier transform infrared spectroscopy (FTIR), they also detected peaks corresponding to the Si–OH groups, at 24 h of immersion in SBF. In the Fig. 5c insert, at a greater magnification (20000X), the coating is seen to be formed by two types of nanometric particles, acicular and in the form of faceted plaques, that are homogeneously distributed, achieving a uniform, high-density coating. This result is in accordance with that obtained through x-ray diffraction, which determined the existence of the two crystalline phases in the coating treated at 700 ◦ C. Fig. 5d shows BG800, in which the growth of a more complex and rougher

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structure over the substrate can be observed, because by being above the Tg1 and Ts transition temperature, the Na6 Ca3 Si6 O18 and Na2 Ca4 (PO4 )2 SiO4 phases that are rich in silicon and phosphorus, respectively, crystallize. This causes a morphologic change due to the thermal effect, and there is also structural growth. The Fig. 5d insert shows a micrograph at 20000X, in which the preferential growth of the acicular phase in the structure is obvious. These interwoven, needle-shaped crystals remain in the nanometric range, forming a randomly ordered coating. Comparing the micrographs corresponding to the XG700 and BG700 samples, the correspondence of the morphology can be easily observed. In addition, the temperature at which the crystalline phases appeared in the bioglass coating, with the subsequent changes in the morphology of the nanoparticles forming the coating, was able to be determined through the XRD study of the xerogel powders. Fig. 5e shows the thickness of BG700 that was measured several times, obtaining a mean thickness of approximately 6.4 ␮m. The EDS microanalysis of BG700, done on the transversal area of thickness shown in Fig. 5e is presented in Fig. 5f. The elements corresponding to the substrate and the coating were detected. In accordance with the previous results, the BG500 and BG700 coatings are promising, given that they are classified as thin coatings with minimal structural defects. This does not occur when managing coatings thicker than 10 ␮m that have greater structural defects, mainly low adherence that causes detachment and microcracks [32]. Peitl et al. [33] state that the effect of volume fraction crystalline phase in the kinetics of HCA formation was small. They obtained the crystalline phase Na2 Ca2 Si3 O9 , as single phase after crystallization and established that this phase has a high rate of bioactivity. They concluded that the crystallization increases the formation time of the apatite layer, however, fully crystallized bioglasses retain their bioactivity. Chen et al. [34] concluded that crystalline phase Na2 Ca2 Si3 O9 can transform into an amorphous calcium phosphate phase after immersion in simulated body fluid for 28 days, and that the transformation kinetics can be tailored through controlling the crystallinity of the 45S5 bioglass. Plewinski et al. [5] recently determined that 45S5 bioglass crystallization provides better apatite layer formation, when compared with amorphous bioglass, and they concluded that it should be considered as coating material for dental implants. Therefore, based on these investigations we can say that having a partially crystallized bioglass favor the mechanical behavior without losing the properties of bioactivity resulting in the formation of the apatite layer to be carried out with a slower kinetic, keeping bioactivity. The uniformity of the coatings obtained at 500, 600, and 700 ◦ C, in which there were no irregularities and there was homogeneous distribution over the entire substrate, can be attributed to the colloidal electrodeposition method. The fact that the thermal expansion coefficient of the Ti6Al4 V substrate is lower than the 45S5 bioglass, with values of (10.4 × 10−6 C−1 ) and (14 × 10−6 C−1 ), respectively, Gomez et al. [36–38] established that “the glass should have a slightly higher TEC than the metal substrate so the coating is in compression”. Nevertheless, the obtained results in this work prove that difference in TEC between the substrate and the coating is not as deleterious as it would be predicted by Gomez et al. [36–38]. The obtained coatings by CEPD do not present fractures or glass recoils that could be originated by the mismatch in TEC, compared with the glass thicker coatings obtained by other authors that present this type of defects [29]. This might be due to two factors: the thickness of the coatings obtained in this paper and the nanostructured of the obtained coatings. The obtained coatings by CEDP are formed by condensation reactions forming nanolayers on the substrate to be successively adhering to previous layers. This layered nanostructure allows obtaining thin coatings in whose structure there are a large number of grain boundaries that dissipate thermal stresses due to contraction or expansion of each

individual nanoparticle representing an overall TEC average that can be less than TEC having thicker coatings formed by aggregates of particles of micron size. These grain boundaries function as a stress sink so the nanoparticles can be rearranged resulting in lower stresses in the coating. Krause et al. [29] deposited by electrophoresis 45S5 bioglass (14 × 10−6 C−1 ) on 304 stainless steel (18.7 × 10−6 C−1 ), and the TEC difference between the bioglass coating and substrate was (−4.7 × 10−6 ◦ C−1 ). His coatings were formed by microsized particles and were 10–15 microns thick; the thermal treatment at 800 ◦ C resulted in the development of microcracks in the coating. In this paper, the coating thermally treated at 800 ◦ C (Fig. 5d) did not show cracks. Although there are some reports [3,35–38] from other authors Q5 in which they make bioglass coatings by electrophoretic deposition on Ti6Al4 V and on Ti12Mo5Ta, they deposited powders obtained by sol–gel synthesis that were previously calcined and milled. The resultant deposits were thicker than the ones reported in this paper and their microstructure was composed of large particles that were not well-densified at temperatures up to 1000 ◦ C. These higher temperatures would affect the properties and phase structure of the titanium alloy employed as the substrate. Also, those coatings Q6 were neither continuous nor homogeneous and were not free from defects. 3.3. Chemical anchorage model Many authors have reported on low temperature bioglass obtained by the sol–gel method. J. Faure et al. [35] compared the characteristics of different bioglass compositions obtained by the melting and sol–gel techniques concluding that the sol–gel method gives good bioglass composition that is not altered by the electrophoretic deposition process. The anchorage of 45S5 bioglass with the Ti6Al4 V alloy occurs through chemical bonding. Hydrolysis and condensation reactions are carried out between the bioglass precursor sol–gel suspension and the Ti6Al4 V substrate. The Ti6Al4 V substrate, which is initially covered with a TiO2 film, product of the spontaneous oxidation surface reaction, reacts with water due to the attraction between the oxygen of OH and titanium, forming a hydroxyl monolayer on the substrate [39]. Similarly, the elements present in the suspension, E = Si, P, Na, and Ca are hydroxylated, according to the stage 1: reactions1, 2, 3, and 4. The hydrolysis reactions for the former elements of the 45S5 bioglass have been referenced and reported by different authors [40]. The hydrolysis of TEOS (reaction 1) and TEP (reaction 2) were catalyzed by nitric acid 1 M and also by the nitric acid product of the hydrolysis reactions of the cations, Ca and Na as nitrates. Because of the charge density difference, electrostatic attraction takes place between the hydroxyl groups linked to the E elements present in the sol–gel bioglass precursor suspension and the hydrogen of hydroxyl groups linked to the metal in the TiO2 film: M´ Ti, forming a hydrogel linked to the Ti6Al4 V electrode surface during electrodeposition (stage2). During thermal treatment (stage 3), condensation reactions via the oxolation mechanism takes place on the 45S5 bioglass precursor hydrogel over the Ti6Al4 V substrate surface (reaction 5). Oxo bridges (E O M)´ are created by water elimination, leading to the formation of bioglass that is chemically anchored to the Ti6Al4 V substrate, which results in a uniform and well-adhered bioglass coating [32,41]. 45S5 bioglass formation over the Ti6Al4 V substrate is carried out by condensation reactions via the oxolation mechanism during thermal treatment, according to the reactions: 6, 7, 8, and 9 [42], in which metal oxides corresponding to the bioglass composition are formed: SiO2 , P2 O5 , CaO, and Na2 O. The main characteristic of the bioactive glass is related to its open structure, which allows the incorporation of cations (Na+ and Ca++ ) within the matrix glass. These elements act as network

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Fig. 6. Chemical anchorage model of 45S5 bioglass coating on Ti6Al4 V alloy.

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modifiers, producing a glass breaking network and the consequent formation of non-bridging oxygen (NBO). These NBOs are the two oxygen atoms that are not shared in the SiO4 tetrahedron, whereas the other two oxygen atoms, called oxygen bridges (BOs), are linked. Thus, the cations (Na+ and Ca++ ) profoundly affect the connectivity of the structure of the glass [43,44]. However, the breaking of Si O Si generates Si OH Si, which interacts with the body fluid components, increasing the dissolution of bioglass and favoring the formation of hydroxyapatite (HA). The results of the debate of how phosphorus interacts with the bioglass network had been inconclusive, but Tilocca [45] has extensively studied the behavior of the 45S5 bioglass elements through MD simulations and nuclear magnetic resonance (NMR) and concluded that phosphorus enters the 45S5 bioglass network as phosphate (PO4 −3 ), demonstrating that a fraction of phosphate groups shares an oxygen (BO) with the SiO4 tetrahedron to form a Si O P link predicted by the simulation. Due to this link, repolymerization of the silicate network can occur via Si O Si or the new Si O P. These new links contribute to a more hydrophilic surface favoring bioactivity. Therefore, the phosphate anion is bonded to the silica and the cations are outside of the network. These considerations are fundamental for proposing a chemical anchorage model. The proposed model in our work suggests that the SiO4 tetrahedron contains two non-bridging oxygen atoms (NBOs) that are electrostatically linked to Na+ and Ca++ cations and two binding oxygen atoms (BOs), one that is shared with a phosphate group and the other one (BO) that forms an oxo ´ The proposed oxo bridge model bridge with titanium (E O M). Ti O Si O P, is presented in Fig. 6. Hydrolysis reactions of 45S5 bioglass:

447

C8 H20 O4 Si + 4H2 O → Si(OH)4 +4C2 H5 OH

(1)

448

C6 H15 O4 P + 4H2 O → P(OH)5 +3C2 H5 OH

(2)

449

NaNO3 +H2 O → NaOH + HNO3

(3)

450

Ca(NO3 ).4H2 O → Ca(OH)2 +HNO3 +H2 O

(4)

418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445

451 452

E–O–Hı+ +OHı− –M’ → E–O–M’ + H2 O

455

SiOH + HOSi → Si–O–Si + H2 O

(6)

456

POH + HOP → P–O–P + H2 O

(7)

457

CaOH−HOCa → Ca–O–Ca + H2 O

(8)

458

NaOH−HONa → Na–O–Na + H2 O

(9)

460

The behavior of the potentiodynamic polarization curves with open circuit potentials when the samples were immersed in Hank’s solution at 37 ◦ C is presented in Fig. 7. Initially, the BG700 coating showed improvement in relation to the corrosive medium, compared with the Ti6Al4 V alloy and the rest of the coatings, as it had more noble values in the intensity of the corrosion current (icorr ) of 0.0067 ␮A/cm2 and in the corrosion potential (Ecorr ) of 33.71 mV. Alves et al. [46] evaluated the Ti6Al4 V alloy ground with SiC sandpaper up to the 600 grit in Hank’s solution at 25 and 37 ◦ C and found icorr values of 0.0877 and 0.0309 ␮A/cm2 , respectively. A similar value was determined in the test, in which the Ti6Al4 V alloy had a value of 0.0720 ␮A/cm2 . With these values, it can be said that the 45S5 bioglass coating inhibits the electrochemical attack,

(5)

Condensation reactions between two molecules of the same 45S5 bioglass species on the Ti6Al4 V electrode:

459

3.4. Tafel extrapolation

Condensation reaction between the 45S5 coating and Ti6Al4 V:

454

453

between the elements in the substrate alloy and the elements in the bioglass, once the condensation reactions have been carried out, forming the oxo bridges between the elements. This model is based on the state of the 45S5 bioglass that is formed on the substrate surface by condensation reactions, considering the Tilocca [45] MD simulations and NMR results in relation to the 45S5 bioglass structure, as well as the coordination numbers of each element involved. And in addition, the proposed chemical anchorage model is important in order to understand the stability of the coating because the chemical anchorage is what ensures the management feasibility in dental applications. The model also attempts to establish the interaction between the bioglass formation elements, as well as between the substrate alloy elements.

The proposed chemical anchorage model of 45S5 bioglass coating on the Ti6Al4 V alloy (Fig. 6), takes into account the bonding

Fig. 7. Tafel extrapolation curves of the samples immersed in Hank’s solution at 37 ◦ C.

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461 462 463 464 465 466 467 468 469 470 471 472 473

474

475 476 477 478 479 480 481 482 483 484 485 486 487

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8

Table 2 Values obtained through the Tafel extrapolation method. Sample

icorr (␮A/cm2 )

Ecorr (mV vs. AgCl)

Ba mV/decade

Bc mV/decade

B mV

Rp M

Ti6Al4V BG500 BG600 BG700 BG800

0.0720 0.0074 0.0411 0.0067 0.1330

−585.16 30.703 −456.97 33.71 −56.974

57.44 46.36 71.07 57.26 57.41

60.38 34.822 55.62 48.22 56.19

12.771 8.625 13.53 11.358 12.318

0.176 1.154 0.328 1.687 0.093

which would indicate the prevention of the detachment of the Al+ 489 and V+ ions that would directly enter the bone tissue, and conse490 quently, the blood stream [47,48]. This protection is noted by the 491 shift to the left of the anodic and cathodic curves. This shift in the 492 curves indicate a reduction in the intensity of the corrosion current 493 (icorr ), which means that there are less electrochemical reactions 494 that cause the Ti6Al4 V alloy to deteriorate. The Ti6Al4 V alloy had 495 a less noble behavior, presenting a corrosion potential (Ecorr ) of 496 −585.16 mV with greater activity. This means that it carries out 497 an activation process that promotes the formation of a passivation 498 layer [49,22]. BG800 presented the greatest intensity of corrosion 499 current (icorr ), obtaining a value of 0.133 ␮A/cm2 because there was 500 greater crystalline growth, resulting in a more porous structure, as 501 observed in Fig. 5d. 502 The BG500 coating presented good behavior when immersed in 503 Hank’s solution at 37 ◦ C, since it obtained an icorr of 0.0074 ␮A/cm2 , 504 reducing the electric flow during the reactions that were carried out 505 when applying the overpotential onto the system and increasing 506 corrosion resistance, when compared with the Ti6Al4 V alloy. The 507 BG500 coating had similar values to those of BG700, as is shown in 508 Q7 Table 3, which is attributed to the fact that these coatings have no 509 structural defects. Both the BG500 and BG700 bioglass coatings pre510 sented good corrosion behavior in Hank’s solution. Nevertheless, 511 the BG500 bioglass was still amorphous according to the trans512 formation temperature line of the 45S5 bioglass (Fig. 1). The XRD 513 pattern (Fig. 3) and the FESEM morphology of the BG700 (Fig. 5c) 514 showed a homogeneous and partially crystallized coating with fine 515 nanostructure, which would ensure better mechanical behavior 516 and, according to Plewinski et al. [5], Peitl et al. [33], and Chen 517 et al. [34], a partially crystallized 45S5 bioglass coating would have 518 better bioactivity than the amorphous BG500 coating. 519 The BG600 coating behaved similarly to the Ti6Al4 V alloy, pre520 senting values of the same order of magnitude with an icorr of 521 0.0411 ␮A/cm2 and an Ecorr of −456.97 mV. This can be explained 522 by the fact that the temperature was only 20 ◦ C above that of the 523 Ts transition. The separation from the glass phase takes place at 524 that transition temperature and there are active sites still present 525 in the coating, thus, the alloy continues to interact with the Hank’s 526 solution. 527 The behavior of the corrosion current density (icorr ) presented 528 the inverse of polarization resistance (Rp). The highest value was 529 presented by BG700 and BG500, with values of 1.687 and 1.154 M, 530 respectively, compared with Ti6Al4 V, which presented a value of 531 0.176 M, and the rest of the coatings. The samples with the high532 est temperature (BG800) had the least resistive behavior. Due to 533 the fact that the accepted levels for biomaterials that are highly 534 corrosion-resistant in biomedical applications are expressed in M 488

[50], BG800 would not be a good coating for the Ti6Al4 V alloy. These values can be seen in Table 2. A. Balamurugan et al. [3] reported obtaining bioglass coatings with a thickness of 15 ␮m. However, they did not include the crosssection image. The morphology shows the coating was formed by particles of micrometer size. They reported good adherence using the peel-off method, which was better for the HA-bioglass composite coating. Their coatings were sintered at 800 and 900 ◦ C. In this paper, better results were obtained at 700 ◦ C. This lower temperature did not affect the phase composition of the Ti6Al4 V alloy substrate. The potentiodynamic polarization tests in SBF done by Balamurugan et al. showed an icorr of 0.33 A/cm2 for the coating and 0.32 A/cm2 for the naked substrate. This result means that the substrate was more corrosion resistant than the coating. In the case of the HA-bioglass composite coating they reported a lower icorr of 0.22 A/cm2 . Nevertheless, in the impedance test, the three types of coatings that they reported showed better corrosion behavior than the naked substrate, and the best one was the composite coating. In conclusion, based on the different magnitude order of the structure of the coatings (micrometer size) described by Balamurugan et al. and the coatings of this paper (nanometric size), it is difficult to establish a real comparison. They deposited oxide bioglass particles and this paper reported the colloidal electrophoretic deposition of 45S5 composition hydrogels. Once they were thermally treated, they formed a 45S5 bioglass nanostructured coating homogenously distributed over the entire substrate surface. With respect to the corrosion test, the two papers employed different media, for corrosion test, SBF and Hank’s solution, respectively, and thus, the results are difficult to compare. In the case of the Ti6Al4 V alloy, in this paper, the icorr obtained was of 0.072 × 10−6 A/cm2 in Hank’s solution at 37 ◦ C. The only thing that we can confirm is that the coatings described by Balamurugan et al. protect the alloy from corrosion in SBF and the nanostructured coatings proposed by this paper protect the alloy from corrosion in Hank’s solution. It would be expected that the nanostructured bioglass coating would be more reactive to biological fluids and would easily form a hydroxyapatite layer. Krzakala et al. [23] anodized the Ti6Al4 V alloy in Ca(H2 PO2 )2 and NaOH baths, obtaining bioactive films, and found low values of icorr , as well as reduced activity in the Ecorr , with positive values and high polarization resistance, compared with the alloy. Corrosion rate and mass loss were calculated based on the ASTM G102-89 (ASTM 1999) norm [26]. BG700 and BG500 presented the lowest corrosion rate in mm/year, with values of 8.850 × 10−5 and 9.823 × 10−5 mm/year, respectively, compared with Ti6Al4 V and the rest of the coatings. They also presented the least mass loss,

Table 3 Corrosion rate, mass loss, and ion release values obtained through the Tafel method. Sample Ti6Al4V BG500 BG600 BG700 BG800

Corrosion rate (mm/year) −4

6.198 × 10 9.823 × 10−5 5.405 × 10−4 8.85 × 10−5 1.744 × 10−3

Mass loss (g/m2 ) −3

7.515 × 10 7.797 × 10−4 4.292 × 10−3 7.025 × 10−4 0.0138

Ion release (g/cm2 ) 6.297 × 10−7 0.998 × 10−7 5.491 × 10−7 0.899 × 10−7 17.71 × 10−7

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535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581

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with a value of 7.025 × 10−4 and 7.797 × 10−4 g/m2 , respectively, as shown in Table 3. These values are directly proportional to the intensity of the corrosion current (icorr ). 584 In accordance with Popa et al. [51], an estimate of the ion release 585 rate (Eq. 2), directly related to the corrosion rate, was made for each 586 Q8 of the samples and these values are shown in Table 4. BG700 and 587 BG500 presented the least release with values of 0.889 × 10−7 and 588 0.998 × 10−7 g/cm2 , respectively. 589 582 583

590

Ion release rate = 1.016(Vcorr )105

591

4. Conclusions

592 593

(2)

The following conclusions were reached in relation to the 45S5 bioglass coatings on the Ti6Al4 V alloy:

621

• By means of colloidal electrophoretic deposition, proposed in this paper, thin and nanostructured 45S5 bioglass coatings, homogeneously distributed over the entire surface of the Ti6Al4 V alloy were obtained. • The 45S5 bioglass coatings are composed of nanometric-sized particles, and had no structural defects, which are commonly present in thicker coatings, the obtained coatings in this work had a thickness of approximately 6.4 ␮m. • A model of the chemical anchorage of 45S5 bioglass on the Ti6Al4 V alloy surface was proposed in this work, in order to understand the stability of the coating because the chemical anchorage, which ensure the feasibility of the management in dental applications. • The 45S5 bioglass coatings obtained presented good protection from the ionic attack of Hank’s solution; this was particularly true for the coating treated at 700 ◦ C (Sample BG700). • In accordance with the Tafel extrapolation data, comparing BG700 45S5 bioglass coating with the naked Ti6Al4 V alloy, there was a reduction of 10 times in corrosion current density (icorr ), in the corrosion rate and mass loss and all these values are less than that obtained for the rest of the coatings. • The bioglass coating treated at 700 ◦ C (BG700) presented the most noble behavior, with a change to more positive values in the corrosion potential (Ecorr ) and the consequently reduced activity in the coatings. • In relation to the data obtained through Tafel extrapolation, the bioglass coating treated at 700 ◦ C (BG700) had the lowest value in the ion release estimate.

622

Acknowledgments

594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620

628

The authors acknowledge the financial support of CONACYT project number 83880, and the financial support of CIC UMSNH. The authors acknowledge the CONACYT doctorate scholarship for M.M. Machado-López. The authors acknowledge Gusti Gould for the paper English revision and thank Ing. Antonio Rodríguez Torres for his technical support in the FESEM analysis.

Q11 629

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630 631 632 633 634 635

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