Electrophoretic deposition of bioactive glass coating on 316L stainless steel and electrochemical behavior study

Applied Surface Science 258 (2012) 9832–9839

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Electrophoretic deposition of bioactive glass coating on 316L stainless steel and electrochemical behavior study Mehrad Mehdipour a,∗ , Abdollah Afshar a , Milad Mohebali b a b

Department of Materials Science and Engineering, Sharif University of Technology, Azadi Avenue, P.O. BOX 11155-9466 Tehran, Iran Department of Material and Engineering, K.N. Toosi University of Technology, Pardis St., Vanak Sq., Tehran, Iran

a r t i c l e

i n f o

Article history: Received 11 March 2012 Received in revised form 6 May 2012 Accepted 11 June 2012 Available online 20 June 2012 Keywords: Electrophoretic Sol–gel Electrochemical behavior Bioactive glass 316L stainless

a b s t r a c t In this research, submicron bioactive glass (BG) particles were synthesized by a sol–gel process and were then coated on a 316L stainless steel substrate using an electrophoretic deposition (EPD) technique. Stable suspension of bioactive glass powders in ethanol solvent was prepared by addition of triethanol amine (TEA), which increased zeta potential from 16.5 ± 1.6 to 20.3 ± 1.4 (mv). Thickness, structure and electrochemical behavior of the coating were characterized. SEM studies showed that increasing EPD voltage leads to a coating with more agglomerated particles, augmented porosity and micro cracks. The results of Fourier transformed infrared (FTIR) spectroscopy revealed the adsorption of TEA via methyl and amid groups on bioactive glass particles. Presence of bioactive glass coating reduced corrosion current density (icorr ) and shifted corrosion potential (Ecorr ) toward more noble values in artificial saliva at room temperature. Percent porosity of the coating measured by potentiodynamic polarization technique increased as EPD voltage was raised. The results of impedance spectroscopic studies demonstrated that the coating acts as a barrier layer in artificial saliva. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The main application of metallic biomaterials includes implants and prosthesis. For their bio inert nature, they bond to their surrounding tissue through weak mechanical interlocking, which may eventually lead to interfacial loosening and failure of the implant [1]. Bioactive ceramic coatings with the capacity to promote favorable physiological interactions are therefore applied on implants surface; one popular class of which is bioactive glass (BG) [2]. BG coated metallic implants compensate for major limitations of bioactive glass – including low tensile strength, fatigue resistance and elastic modulus [3] – and metallic substrate – such as low corrosion resistance and bioactivity [4]. The composition of BG coating affects apatite formation and in this regard BG with Mg content with improved bioactivity and mechanical properties is of particular interest to researchers [5]. Synthesis and application of bioactive glass have improved significantly. In this context, Hench [6] has reviewed development of

∗ Corresponding author at: Department of Materials Science and Engineering, Sharif University of Technology, Azadi Avenue, P.O. BOX 11155-9466 Tehran, Iran. Tel.: +98 912 2470433/21 88080513. E-mail addresses: [email protected], M [email protected] (M. Mehdipour), [email protected] (A. Afshar), [email protected] (M. Mohebali). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.06.038

bioactive glass. In simulated body fluid (SBF), a biologically active hydroxyl carbonate apatite (HCA) layer forms on the surface of BG particles, the composition and structure of which is similar to those of mineral component of bone. Kokubo et al. [7] investigated the formation mechanism of this HCA layer. Of the two fundamental synthesis methods of bioactive glass, for its augmented bioactivity sol–gel derived bioactive glass is an appropriate alternative to melt processed one, because its nano porous structure increases the specific surface of the product. Various investigations regarding sol–gel derived bioactive glass have been published including synthesis of bioactive glasses [5,8,9]. The growing interest toward application of EPD in coating of bio ceramics is ascribed to its low processing temperature, simple required equipment, low cost, high production rates, ability to control the thickness of the coating and to form complex layers on substrates with intricate shapes [10,11]. EPD is a colloidal process and its mechanism involves the movement of charged particles in a suspension toward cathode or anode electrode under an applied voltage [10]. The theory and practice of EPD technique in materials science [10–12] and its specific application in biomaterials [13] have already been extensively reviewed. Hydroxyapatite (HA) was the first of biomaterials to be coated by EPD in 1986 [14] while its application in coating of bioactive glass dates back to Krause in 2006 [15]. He successfully coated 45S5 bioactive glass powders of under 3 ␮m in an aqueous suspension. Pishbin et al. [16]

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investigated process parameters including pH of the suspension and voltage by Taguchi method. It was found that pH of the suspension plays an important role in deposition rate and suspension stability. To obtain thicker coatings, Wang et al. [17] studied the effect of pH on conductivity, zeta potential and stability of the suspension. Lefebvre et al. [18] observed that secondary crystallization takes place when sintering 45S5 bioactive glass in temperature range of 800–950 ◦ C. In another study, Balamurugan et al. [19] evaluated the electrochemical behavior of bioactive glass-HA composite coating on Ti6Al4V substrate. A relatively new approach in EPD involves coating bioceramic in composite with a polymer, e.g. HA/chitosan [20], bioactive glass-HA/chitosan [21] and bioactive glass/chitosan composite coating [22]. Pishbin et al. [16] and Krause et al. [15] deposited bioglass on a stainless steel substrate by EPD in water solvent. This research investigates electrophoretic deposition of BG powder on a 316L stainless steel substrate in ethanol solvent. In order to improve EPD process, triethanolamine (TEA) was added to the suspension and the effect of this additive on stability of the suspension was studied by zeta potential measurement. The influence of EPD voltage on structural and electrochemical behavior of the coating was then thoroughly evaluated.

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Table 1 Composition of artificial salivasolution at pH 6.8. Compound

Concentration mg l−1

Compound

Concentration mg l−1

NaCl KCl KSCN KH2 PO4 Urea

125.6 963.9 189.2 654.5 200

Na2 SO4 ·10H2 O NH4 Cl CaCl2 ·2H2 O NaHCO3

763.2 178 227.8 630.8

impedance experiments in artificial saliva in room temperature. Artificial saliva was prepared according to Gal et al. [24], the composition of which is given in Table 1. A three-electrode cell with a saturate calomel electrode (SCE), a working electrode (the sample) and a counter electrode (stainless steel) was employed; the working electrode having a test surface area of 1 cm2 was used for all experiments. Polarization curves were obtained by Autolab (PGSTAT 362N) device with a scan rate potential set at 1 mV/s. The impedance spectra were obtained (EG&G 273A) in the frequency range of (0.1)–(10E + 05) Hz with a ± 10 mV amplitude of open circular potential (Eocp ) sine wave. The impedance data was modeled using ZSIMS software. 3. Result and discussion

2. Experimental Ethanol, triethanolamine (TEA), tetra ethyl orthosilicate (TEOS), tetra ethyl phosphate (TEP), HNO3 (65%), Ca(NO3 )2 ·4H2 O and Mg (NO3 )2 ·6H2 O, were provided by Merck. Bioactive glass (SiO2 –P2 O5 –CaO–MgO) was synthesized by a sol–gel method described in Ref. [5,25]. In order to remove its nitrogen content, the as-synthesized powder was sintered at 700 ◦ C for 24 h. To obtain a submicron powder, the as-synthesized bioactive glass was grinded by a SPEX mill (Retsch Co., Germany; Model no.: PM 200) for 4 h. 316L stainless steel plates, 20 × 25 mm and 20 × 50 mm, were used as cathode and anode, respectively. Surface preparation involved washing with distilled water, rinsing and degreasing by ultrasonic cleaning in acetone for 10 min and a final drying step. Ethanol has proved to be the most suitable solvent for suspension preparation [23]. Stable suspension for EPD was prepared by adding 2.5 g/l TEA to ethanol and stirring it for 30 min. A 4 g/l bioactive glass powder was then added to the solution. Prior to EPD, the suspension was ultrasonicated for 30 min to achieve a homogeneous dispersion of bioactive glass particles. The distance between the cathode and the counter electrode was 10 mm. Deposition was performed at constant voltages of 30, 60 and 90 V cm−2 . In order to enhancement the adhesion strength of the coating, the samples were sintered at 800 ◦ C in Muffle furnace under argon atmosphere for 2 h with heating rate of 10 ◦ C/min. The zeta potential of the particles was measured by zeta sizer (Malvern, Model sizer 3000 HsA , England). FTIR was used to investigate the surface bonds of bioactive glass particles. The deposited coating was scratched to prepare a sample for FTIR analysis. FTIR was performed using PerkinElmer RX1device in the wavenumber range of 400–4000 cm−1 and a resolution of 4 cm−1 . The surface of the coatings was characterized by scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) (Philips, Holland, model: XL30). A thin section was taken from the coating and was Au sputtered about 10 nm thick before SEM analysis. The X-ray diffraction (XRD) was performed using a Philips PW1800 diffractometer operating with Cu K␣ radiation at 40 kV and 30 mA over 2 range of 10◦ –90◦ at a step size of 0.04◦ . Pull off test was performed in accordance with ASTM D4541 standard. In order to investigate the electrochemical corrosion behavior, bare and coated samples were subjected to polarization and

3.1. Characterization of bioactive glass particle Successful electrophoretic deposition of different ceramic particles in size range of up to 20 ␮m has been reported [11]. In this research the particle size lies in 0.1–2 ␮m with the maximum number of particle having a size of 0.85 ± 0.1 ␮m. The result of EDS analysis from the surface of BG coated 316L stainless steel is shown in Fig. 1. The peaks of Si, P, Mg and Ca belong to components of bioactive glass particles. It should be noted that oxygen was excluded from analysis.  potential is indicative of electric charge on the surface of particles in the suspension and depends on ion absorbance on particles [26,27].  potential of bioactive glass powder suspension in ethanol solvent at pH 7.6 was 16.5 ± 1.6 (mv), and reached 20.3 ± 1.4 (mv) upon addition of 2.54 g/lit TEA. The positive  potential indicates that the process is of cathodic electrophoretic type [10–12]. Addition of TEA increased  potential, improving stability of the suspension by increasing electrostatic repulsion between the particles. Stabilization mechanism of TEA has been reported to be similar to that of alcohols [27]. Damodaran, and Moudgil [28] investigated stabilization mechanism of alcohols and showed that alcohols donate hydrogen ions to particles and form an electric double layer on their surface, making them electrically charged. TEA with three OH bond has higher protonation capacity compared with alcohols like ethanol. Consequently, proton absorbance on particle surface increases and the double layer around particles becomes thicker and  potential increases [29]. 3.2. Effect of EPD parameters on the microstructure of the coating Fig. 2 shows SEM images of bioactive glass coating formed after 10 min in different voltages of 30, 60 and 90 (V). It can be seen that increasing process voltage leads to coarser and more agglomerated particles in the coating. A  potential distribution is present in the suspension. In a constant voltage, particles with higher  potential have higher mobility and thus move faster toward electrode with opposite electric charge. The effect of electrostatic repulsion force on particles with lower  potential is less than the effect of Van der Waals adsorption and thus these particles tend to agglomerate and sediment in the suspension [27,30]. In higher voltages,

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Fig. 1. (a) Energy dispersive spectrometry (EDS) of bioactive glass particles. (b) SEM micrograph of one representative particle.

the applied force on particles increases; consequently the probability that these agglomerated particles move toward electrode with opposite charge and form the coating [30]. Deposition of coarse agglomerate on substrates surface leads to a non-uniform porous coating with low packing density. Increasing deposition voltage leads to a more disordered arrangement of the particles in the coating. This is due to the fact that by increasing voltage, particle

velocity gets higher leaving them less time to lie in the appropriate locations [27]. Increase in porosity has a twofold effect; one is the improved tissue ingrowth upon implantation in the body and the other is the weakening of coatings adhesion strength and augmented corrosion rate. Therefore, a balance has to be made between these opposing effects.

Fig. 2. SEM micrographs of the bioactive glass powder coated at different applied voltages of: (a) 30 V; (b) 60 V; (c) 90 V.

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Table 2 Infrared absorption spectroscopy for bioactive glass; bands and assignments.

Fig. 3. Thickness of the coating versus applied EPD voltages.

According to Zhitomirsky [31,32] deposition thickness in EPD is proportional to concentration of the suspension, electrodes surface area, the drop in particle concentration during EPD and the applied voltage: Ax =

CUt d

(1)

where C is the concentration of the particles in the suspension,  is the apparent density, A is the electrode’s surface area, x is the deposition thickness,  is the mobility of the particles, U = Uap − Udep is the applied voltage of EPD, Udep being the voltage drop during EPD, t is the deposition time, and d is the distance between the two electrodes. From Eq. (1), it could be concluded that by increasing EPD voltage higher coating thicknesses can be achieved. The results of this research (Fig. 3) comply well with this explanation.

Wavenumbers (cm−1 )

Type and group band

467 660, 1080, 793 1460 1568 1619 1732 2669 2852 2920 3435

P Si C N C C O C C O

O in PO4 O Si PEREIRA preparation H in CH3 H amide п bonds N amide п bonds O Dicarboxylic amino acids ␣-amino I bond H organo-phosphorous compounds H in CH3 H in CH2 H adsorbed water molecules

On the other hand, from Eq. (1) it could be understood that in higher voltages, the deposition thickness grows more rapidly and consequently the voltage drop (Udep ) from formation of barrier layer takes place in a shorter time and U is reduced more rapidly, reducing coating’s deposition rate. This is evident in the voltages of 60, 70 and 90 V where diagrams slope decreases. Fig. 4 shows SEM images of the samples coatings processed in different EPD voltages after sintering at 800 ◦ C. The sample prepared in EPD voltage of 90 V has more surface cracks and porosity compared to the sample prepared in EPD voltage of 60 V, while no surface crack and less porosity was observed in the sample prepared in EPD voltage of 30 V. This can be attributed to the effect of voltage on deposition of agglomerates. When large agglomerates are present in the coating, excessive strain is formed during sintering due to the difference in thermal expansion coefficients of bioactive glass coating and stainless steel substrate and eventually micro cracks develop [15]. 3.3. Structural and adhesion strength studies Fig. 5 shows FTIR spectrum of bioactive glass particles. The corresponding characteristic bands are given in Table 2 [33,34]. The

Fig. 4. Surface morphology of bioactive glass coating in different voltages of: (a) 30 V; (b) 60 V; (c) 90 V.

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Fig. 5. FTIR spectra of bioactive glass particles.

occurrence of C H bands of methyl group and N H bands of amid group is indicative of TEA adsorption on bioactive glass particles. XRD pattern of BG coating (Fig. 6) after sintering at 800 ◦ C reveals the presence of small amounts of crystalline diopside ((CaMg)3 (PO4 )2 ) and Wollastonite (CaSiO3 ) along with traces of Akermanite (Ca2 MgSiO2 O7 ) and Merwinite (Ca3 MgSi2 O8 ) phases. The low count number of peaks in XRD pattern shows that the sintering has led to partial crystallization of bioactive glass coating. Pull off test was performed to evaluate adhesion strength of BG coatings prepared with different EPD voltages. The adhesion strength for BG coatings prepared with EPD voltage of 30, 60 and 90 V (corresponding to thickness of 15 ± 2, 40 ± 2 and 65 ± 2 ␮m, respectively) was measured to be 13, 18 and 15 Mpa, respectively. It seemed that by increasing the amount of EPD voltage from 30 to 60 V, adhesion strength reached the maximum value of 18 MPa, but after this point adhesion strength was dropped. Thus optimum voltage for preparation of this coating would be about 60 V. 3.4. Electrochemical behavior of bioactive glass coating 3.4.1. Potentiodynamic polarization study Fig. 7 shows the effect of EPD voltage on corrosion resistance of BG coated stainless steel in artificial saliva by polarization method. Using Tafel extrapolation method, corrosion potential (Ecorr ), corrosion current density (Icorr ), and the anodic/cathodic Tafel (ˇ␣ ,

Fig. 7. Potentiodynamic polarization curves of the samples coated in different EPD voltages: (30, 60 and 90 V) in artificial saliva.

ˇc ) were obtained from polarization curves. Polarization resistance (Rp ) was calculated by Stern–Geary equation [35,36]: Rp =

ˇ␣ ˇc 2.31corr (ˇ␣ + ˇc )

(2)

The value of corrosion current density (Icorr ) and corrosion potential (Ecorr ) for uncoated stainless steel are 8.6E-05 A cm−2 and −0.549 V (vs. SCE), respectively. For samples coated in EPD voltages of 30, 60 and 90 V corrosion potential and corrosion current density values are given in Table 3. The higher value of Icorr for uncoated sample compared to the samples coated at different EPD voltages suggests the passive nature of the coatings. It could thus be concluded that presence of bioactive glass coating shifts polarization diagram to values of less corrosion current density and more noble potentials, i.e. the presence of bioactive glass coating improves corrosion resistance of stainless steel in artificial saliva solution. Furthermore, for the sample coated in EPD voltage of 60 V, Rp is higher and Icorr is lower than for the samples coated in EPD voltage of 30–90 V. Therefore, the optimum coating potential lies at about 60 V, and in good agreement with other reports [19]. 3.4.2. Measurement of percent porosity in bioactive glass coating Percent porosity of the coating directly affects coating’s durability in corrosive environments, and is assessed with aid of the equation suggested by Liu et al. [37], Laleh et al. [35] and J. Cai et al. [38]:



F=

Rp bare Rp Coat



× 10

 −E   ˇ corr  ␣

(3)

where F is the coating’s porosity, Rp bare is polarization resistance of the bare sample, Rp coat is the polarization resistance of bioactive glass coated sample, calculated by Stern–Geary equation, Ecorr is the difference of corrosion potential between the coated and bare samples and ˇ␣ is the anodic Tafel slope of the bare sample. Fig. 8 demonstrates the rising trend of percent porosity upon EPD voltage increase and in consistence with the results of SEM study. As voltage increases beyond 90 V and packing density is reduced, the physical bonding between agglomerates weakened [19] and they readily separated from each other when immersed in the electrolyte.

Fig. 6. XRD pattern of the bioactive glass coating prepared in EPD voltage of 60 V after sintering at 800 ◦ C.

3.4.3. Electrochemical impedance spectroscopic studies Further information on electrochemical behavior of coating was achieved through AC impedance measurements. Fig. 9 shows

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Table 3 Corrosion parameters from the potentiodynamic polarization tests for bare and BG coated samples in artificial saliva. Sample

Ecorr (V)

Icorr (␮A/cm2 )

ˇ␣ (V/decade)

ˇc (V/decade)

Rp ( cm2 )

Bare 316L stainless steel 30 V 60 V 90 V

−0.549 −0.401 −0.48 −0.445

8.6E + 01 5.1 2.9 4.8

0.174 0.17 0.166 0.158

0.245 0.2 0.252 0.181

5.14E + 02 7.83E + 03 1.5E + 04 7.64E + 03

The equivalent circuit for evaluation of electrochemical behavior of the system which included two time constants in series was obtained by fitting electrochemical impedance data using the model depicted in Fig. 10. Electrochemical behavior of porous ceramic coatings with nonsmooth surface does not correspond to ideal capacitance element, and it is therefore replaced by constant phase element. The equation of the impedance for the constant phase element is [41] ZCPE =

Fig. 8. Percentage porosity of BG coating formed in different EPD voltages.

Nyquist and Bode diagrams for bare and bioactive glass coated samples. The occurrence of two time constants in EIS spectra (Fig. 9a) can be attributed to the presence of BG coating [39,40]. This phenomenon appears at frequencies ∼> 10 and ∼12,000 Hz in the impedance diagrams. From Nyquist spectra (Fig. 9c) it could be understood that the total resistance (Rt ) of the BG coated sample is higher than for the bare one.

1 −n (jω) Y0

(4)

where Y0 is the admittance constant and n is the empirical exponent of CPE, 0 < n < 1. If n = 1, CPE is the pure capacitance; if n = 0, CPE is the pure resistance [39,41]. In the circuit in Fig. 10, Rs is solution resistance, Rct and CPEdl are associated with the charge transfer process that occurs on the surface of the substrate, Rc is resistance of the coating and CPEc is the constant phase element related to the coating. The obtained impedance spectrum was modeled using a nonlinear regression method and the values of circuit parameters were estimated (Table 4). Solution resistance has not changed considerably for bare and coated samples (given in Table 4).

Fig. 9. (a) Bode-phase; (b) Bode–Bode and (c)Nyquist plots of bioactive glass coatings for bare and coated samples in artificial saliva.

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Table 4 Impedance fitting values of the coatings for bare and coated samples in artificial saliva. Sample

Rs ( cm2 )

Rct ( cm2 )

CPEdl (␮F/cm2 )

Rc ( cm2 )

CPEc (␮F/cm2 )

n-CPE

Bare 316L stainless steel BG coating

40.34 31.67

2.73E + 03 5.47E + 04

6.69E + 02 5.23E + 02

2.67E + 03

2.18E + 01

0.8 0.67

Fig. 10. Equivalent circuit for bioactive glass coatings on 316L stainless steel in artificial saliva solution.

This is due to the fact that solution resistance depends on the exposure surface and ion concentration in artificial saliva solution, none of which was changed in our experiments. An increase in Rct and a decrease in CPEdl were observed when BG coating was present, i.e. the coating serves as an effective barrier layer against corrosive ions, improving corrosion resistance of the implant during the time period under study. 4. Conclusion The present study deals with electrophoretic deposition of bioactive glass on a 316L stainless steel substrate.  potential studies showed that TEA addition improved stability of the suspension. The influence of EPD voltage on coatings properties showed that the coating thickness and morphology could be controlled and augmenting EPD voltage increases the number of agglomerates in the coating and leads to occurrence of cracks in the coating. The results of cyclic polarization test showed that the optimum EPD voltage lies at about 60 V and percent porosity increased by 48.5% and 23.6% by increasing EPD voltage from 30 to 60 V and from 60 to 90 V, respectively. Impedance spectroscopy revealed that the BG coating acts as a barrier layer in artificial saliva. References [1] D. Stojanovic, B. Jokic, D. Veljovic, R. Petrovic, P.S. Uskokovic, D. Janackovic, Bioactive glass–apatite composite coating for titanium implant synthesized by electrophoretic deposition, Journal of the European Ceramic Society 27 (2007) 1595–1599. [2] A.S. Hoffman, J.E. Lemons, F.J. Schoen, B.D. Ratner, Biomaterials Science: An Introduction to Materials in Medicine, Academic Press, San Diego; London, 1996. [3] J.D.B. Joon, B. Park, Biomaterials Principles and Applications The Biomedical Engineering Handbook, vol. 3, 2000, 210–230.

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