Electrophoretic deposition of graphene oxide on ...

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ScienceDirect Materials Today: Proceedings 5 (2018) 15603–15612

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NTNM2017

Electrophoretic deposition of graphene oxide on plasma electrolytic oxidized-magnesium implants for bone tissue engineering applications Aidin Bordbar Khiabani*, Sina Rahimi, Benyamin Yarmand**, Masoud Mozafari Nanotechnology and Advanced Materials Department, Materials and Energy Research Center (MERC), P.O. Box 14155-4777, Tehran, Iran

Abstract Biodegradable magnesium (Mg) alloys have been widely used in fabrication of biomedical orthopedic implants owing to their similar characteristics to natural bone. In order to create a strong connection to the bone and to prevent inflammation, these implants should have a high corrosion resistance in physiological environment. In this study, Graphene oxide (GO) layer was produced on a plasma electrolytic oxidation (PEO) coating of AZ91 Mg alloy by electrophoretic deposition (EPD) process. The field emission scanning electron microscopy (FE-SEM) micrographs of the coating surfaces and cross sections showed that PEO coating had porous structure which considerably changed after EPD treatment and its micropores and microcracks were sealed by GO particles. Also, X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR) results confirmed the formation of GO layers on MgO coating after PEO/EPD process. The corrosion behavior was studied by conducting potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests in simulated body fluid (SBF) solution. Potentiodynamic tafel curve measurements demonstrated that the corrosion current density of the MgO/GO coating decreased by 56 and 708 times compared to PEO coating and bare substrate, respectively. Moreover, the EIS results indicated that the value of |Z|f→0 of the MgO/GO coating (1.64×106 Ώ.cm2) was slightly higher than PEO coating (8.21×104 Ώ.cm2), attributed to the formation of a GO film as the top layer preventing infiltration of aggressive solution into the AZ91 substrate. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of INN International Conference/Workshop on “Nanotechnology and Nanomedicine’’ NTNM2017. Keywords: Corrosin resistance; AZ91 Mg alloy; Graphene oxide; PEO/EPD process; MgO/GO coating

* Corresponding author 1. Tel.: (+98-935) 3667782. E-mail address: [email protected] ** Corresponding author 2. Tel.: (+98-912) 8068366. E-mail address: [email protected] 2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of INN International Conference/Workshop on “Nanotechnology and Nanomedicine’’ NTNM2017.

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Bordbar Khiabani et al. / Materials Today: Proceedings 5 (2018) 15603–15612

1. Introduction Permanently implanted biomaterials may cause problems to the host body associated with long term chronic inflammation which would eventually require revision surgery [1-4]. The study of innovative biodegradable implant materials is one of the most interesting research topics at the forefront in the area of biomaterials. Biodegradable implant materials in the human body can be gradually dissolved, absorbed, consumed or excreted, so there is no need for the secondary surgery to remove implants after the surgery regions have healed [5]. Magnesium (Mg) alloys are being researched intensively for use as biodegradable medical implants, in applications such as (i) cardiovascular stents, (ii) bone fixations, and (iii) sutures. It is planned to use these Mg implants in applications where the implant is required for a relatively short period of time ~ 6 months, during which the body heals [6]. Unfortunately, the application of Mg implants has been hindered by problems associated with corrosion in biological environments. Mechanical integration is deteriorated by the corrosion process causing implant failure before the healing is completed. The main aim of recent researches is to reduce or control the corrosion rate of the implants. Fortunately, a series of surface modification techniques, such as electrochemical coating, conversion coating, hydride coating, anodizing, gas phase deposition, laser surface alloying and polymer coating, have been developed to improve the corrosion resistance of magnesium alloys over the last few decades [9]. Among a number of possible coating techniques available for improving the corrosion resistance of magnesium alloys, plasma electrolytic oxidation (PEO) process is a popular one due to its high efficiency and environmental friendliness. However, Irregular pores and micro-cracks that appear in the oxide layer form the pathways for corrosive species thus impairing the total protective effect of anodization [10], so that some strategies are still being used to tailor structural and composition of coatings with better anti-corrosion properties [9]. The incorporation of micro/nano particles into the oxide layer generated by the PEO process in order to improve functionality has recently attracted considerable attention. The main objective of these particles incorporation to the base electrolyte is to seal various types of defects in the PEO coating and enhance the surface hardness, anti-wear, and anticorrosion properties of the coatings. There are also numerous reports on controlling microstructure and properties of coatings by introducing inorganic particles into the electrolyte, for example, silica, titania, zirconia, alumina, etc., aiming to improve the microstructure and corrosion resistance of the PEO coatings [10]. In recent years, the researchers have added Graphene and Graphene oxide (GO) particles, into the electrolytes, and these particles can be incorporated into the coatings and produce less defective and cracked coatings [11-16]. Graphene and GO, which possess very attractive characteristics, including excellent mechanical, electrical, thermal, optical and barrier properties, are novel carbon-based topics of study [17]. Graphene based coatings have potential to be used as anti-corrosive coatings due to remarkable properties of graphene such as 2-dimensional morphology, high surface area, low density, and resistance to ion permeation [18]. Moreover, the last 10 years literature on GO for biomedical applications revealed and confirmed the scope of its potential capabilities as biomaterial [19]. Such characteristics seem to indicate that GO may be an ideal candidate for coating implant surfaces along with the fact that GO is also chemically inert, biocompatible and durable [20]. Combined technologies that produce complex, multilayer systems for magnesium is a promising way to prevent the corrosion of magnesium in harsh service environments [9]. Qiu et al [21] formed a Graphene oxide layer on PEO coated ZK60 magnesium alloy by dip coating post treatment to evaluate its corrosion-inhabitation effect. However, in recent years significant efforts have been made to improve the properties of PEO coatings is by depositing the particles onto the coatings by electrophoretic deposition (EPD) as a post treatment method. In this method, the particles migrate under the influence of an electric field (electrophoresis) and are deposited onto the anodic oxide layer. Sreekanth et al. [22] studied the incorporation of a hydroxyapatite (HA) particle into PEO coated AZ31 Mg alloy by EPD, reported that the MgO/HA composite coating showed higher corrosion resistance in simulated body fluid (SBF) environment. Gnedenkov et al. [23] studied the corrosion and wear behavior of PEO coating on MA8 Mg alloy with incorporated SPTFE particles by EPD method and found that coating with incorporated SPTFE particles showed better anti-corrosion and anti-wear properties. In this research for the first time, with the aim of increasing of the corrosion resistance of the magnesium implants in biological environments, MgO/GO composite coating was fabricated on AZ91 magnesium alloy by combining PEO with EPD methods. The results are hoped to give some useful information to the study of a novel anti-corrosion PEO coating for biomedical implant applications.


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2. Materials and methods 2.1. Preparation of the samples The rectangular samples (20×10×1 mm) of AZ91 Mg alloy were used as the substrate for the PEO coatings deposition. Prior to PEO treatment, the specimens were ground and polished with 800, 1000, 2000 grit silicon carbide paper to achieve a smooth surface, rinsed with distilled water, and ultrasonically cleaned in ethanol, and finally dried in cool air. The alkaline phosphate based electrolyte was prepared from the solution of 5.0 g/L K3PO4 in distilled water with an addition of KOH (2.0 g/L). 2.2. Preparation of the PEO coatings The PEO processes were carried out with conventional plasma electrolytic oxidation equipment consisting of a pulsed DC source with 1000 Hz modulation, a stirring and a cooling system. The AZ91 Mg alloy was used as the anode, while the stainless steel container was used as the cathode. The PEO equipment formed anodic films on AZ91 Mg alloy substrates by micro arc-discharges, which are initiated at potentials above the breakdown voltage of the growing oxide film and move rapidly across the anode surface. Schematic diagram of the PEO process is shown in Fig. 1(a) which is similar to previously reported PEO processes [24-27]. The appropriate electrical parameters were as following: duty cycle, 55%; current density, 80mA/dm2. After PEO treatment of 20 min, the coated sample was taken out from electrolyte, rinsed thoroughly with distilled water and dried in cool air. 2.3. Preparation of EPD coating after the PEO process For EPD process, GO was first dispersed in deionized water and sonicated for 5 h at room temperature. A uniform and stable suspension in water containing 2 mg cm-3 of GO platelets was obtained. The pH value of the suspension was adjusted to 12.00 by 1M NaOH solution. The modified EPD process [28-30] was carried out with a thin foil of stainless steel 316Lwhich was shaped into a cylindrical tube with a diameter of 4 cm and a length of 9 cm placed in a 100 ml beaker. An electrophoretic cell using PEO samples as the anode at the center of the beaker and stainless steel foil as the cathode. The distance between the anode and the cathode was set to be about 1 cm. EPD process was carried out at 100 V using a DC voltage source at room temperature. Fig. 1(b) represents the schematic illustration of modified EPD processes.

Fig1. (a) Schematic diagram of the PEO process and (b) schematic diagram of the EPD process used to formation of GO films on the PEO coating

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After EPD for 90 min, the graphene deposited sample was withdrawn carefully from the EPD cell and dried horizontally in air at room temperature for 1 h, and then sintered in an oven at 120 °C for 3 h. 2.4. Characterization of the coatings and Corrosion behaviour of the coatings The surface morphologies and elemental compositions of coatings were examined by TESKAN-MIRA3 Fieldemission scanning electron microscopy equipped with EDX (C-MAX). The thickness data given were the average of three measurements made at different locations. The phase composition of coatings was analyzed by X-ray diffraction (Philips, CuKa radiation, 40 kV, 35 mA, 0.02_/s scan rate). Fourier transform infrared spectroscopy (FTIR; Perkin Elmer Spectrum 100) was used to determine the surface functional groups of the obtained films in the range of 4000-400 cm−1 with the resolution of 2 cm−1. Corrosion resistance and electrochemical impedance spectroscopy (EIS) of the samples were evaluated using the Princeton Applied Research (PAR) potentiostat/galvanostat Model 263A instrument. All electrochemical measurements were conducted using a typical three electrode cell system with the substrate as the working electrode, a platinum plate as the counter electrode and an saturated calomel electrode (SCE) as the reference electrode. The EIS were measured in a frequency range from 10 mHz to 100 kHz with AC perturbation of 10 mV. 3. Results and discussion 3.1. The suspension of GO for EPD Graphene and r-GO are not appropriate for EPD because of their hydrophobic characteristics and low reactive functionality. Instead, GO was used for EPD to improve the corrosion resistance of the substrate because its high functionality enhances electrophoretic motion in the EPD process and improves adhesion to the substrate [31]. In the present work, GO was coated onto the surface of PEO coating via EPD. In this study, Anodic EPD was carried out because GO is negatively charged due to its oxygen functional groups. As shown in Fig. 2 (a), the GO dissolved in deionized water was negatively charged and the average zeta-potential of the GO dispersion at pH=12 was −62.3 mV. Fig. 2 (b) shows negatively charged GO particles which deposited on anode (AZ91 alloy with PEO coating). 3.1. Coatings characterization To further identify the incorporation of GO sheets into the plasma electrolytic oxidation coatings and the phase composition of the coatings, XRD and FT-IR analyses were performed to analyze the coating compositions. Fig. 3 (a) displayed the XRD patterns of GO sheets, with a strong and sharp (002) peak at 2θ=11.68°, corresponding to an interlayer distance of 0.788 nm. XRD patterns of the PEO and PEO/EPD coatings are shown in Fig. 3b. Peaks of the

Fig2. (a) Zeta potential of GO as a function of pH, in aqueous dispersions at a concentration of 2 mg cm-3, (b) negatively charged GO particles which deposited on PEO coating.


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magnesium (marked as Mg) substrate are observed on all the XRD patterns. It might be due to the deep penetration of the X-ray in the coatings matrix, which resulted in the reflection of the substrates. The coatings formed by PEO of magnesium in the solution containing 5.0 g/L K3PO4 and 2.0 g/L KOH are composed of MgO and Mg3(PO4)2 (Fig. 3b, plot MgO). The peaks corresponding to GO and MgO in PEO-EPD coating suggest that the GO was incorporated into the oxide coating. Compared with the PEO monolayer coating, the peaks of Mg and MgO of the PEO-EPD coating were weak, which might be related to the high thickness of duplex coating. The FT-IR spectra of MgO and MgO/GO coatings are shown in Fig. 2c. Various oxygen configurations in the structure include the vibration modes of epoxide (C–O–C) (1230–1320 cm−1 ), sp2–hybrided C=C (1500–1600 cm−1 , in-plane vibrations), carboxyl (COOH) (1650–1750 cm−1 including C–OH vibrations at 3530 and 1080 cm−1 ), ketonic species (C=O) (1600–1650 cm−1 , 1750–1850 cm−1 ) and hydroxyl (namely phenol, C–OH) (3050–3800 cm−1 and 1070 cm−1 ) with all C–OH vibrations from COOH and H2O [10,14]. In MgO/GO coating, the absorption band, which occurs at 438.92 cm-1 is attributed to the MgO compound. The morphology of the pristine graphene oxide sheets and surface morphologies and the microstructure details of PEO coating and PEO/EPD duplex coatings are presented in Fig. 4. The surface of PEO coating prepared in an alkaline phosphate electrolyte has porous microstructures and some pancake like micropores and microcracks distribute disorderly on the surface, as seen as Fig 4 (b) and (c). The micropores formed by molten oxide and gas bubbles thrown out of discharge channels during the PEO process. The microcracks result from the thermal stress attribute to the rapid solidification of molten oxide in the cooling electrolyte [24]. This porous structure with micrcracks permits the penetration of corrosive ions to the substrate of AZ91 Mg alloy and corrosion proceeds [5]. Moreover, the PEO layer, with its rough and porous structure, offered considerable sites where the GO particles were easily accommodated and provided template for the EPD development and also could have been an intermediate layer for the next coating layer. In other words, The GO coating could effectively overlay most pores and microcracks of the PEO layer by physical interlocking. According to Fig. 4 (d) and (e) all of the micropores and microcracks that existed on the surface of the PEO coating have completely covered after EPD treatment, instead a relatively rough and uniform surface. This rough structure is helpful for the bone cells to penetrate into the implants and make a good interface area between the implant and the surrounding tissue [3]. Also, GO sheets present on the PEO/EPD coating are shown in Fig. 4 (e) and magnified in Fig. 4 (f), as it is seen, they are just like pristine GO sheet before coating (Fig 4(a)), which suggest that GO sheets are successfully seated on the PEO coating. The backscattered scanning electron micrograph of the cross section of the PEO coating and the PEO/EPD composite coating were shown in Fig. 5. The PEO treated coating is comprised of almost porous and thin layer. Sreekanth et al. [32] reported that the discharge channels are cooled by electrolyte and reaction products deposit on the channel walls in the last stage of the PEO process. The cross-section morphology of the PEO-EPD coating clearly shows the relatively dense structure with higher thickness compared with PEO (Fig. 5 (b)). The composite coating presents GO particles attached on the walls of the micropores (Fig. 5 (c)).

Fig3. (a) XRD pattern of pristine GO particles; (b) XRD patterns of MgO and MgO/GO coatings; (c) FT-IR spectra of GO and MgO/GO coatings.

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Fig4. FE-SEM images of (a) GO sheets; (b, c) MgO coating; (d, e) MgO/GO duplex coating and (f) a magnification of GO sheets in MgO/GO coating.

EDX elemental maps of the PEO/EPD coating confirm the presence of Mg and O together with C in the bulk of the PEO/EPD coating (Fig. 5 (d)). The data obtained prove that the GO sheets are simultaneously depositing on the Mg surface during the PEO/EPD process. In addition, the higher content of C in the EPD layer confirms the formation of GO layer on the surface of PEO coating. The GO particles have high chemical stability and surfaces. Therefore, they can act as a good barrier to inhibit the penetration of aggressive electrolyte (SBF) to the AZ91 alloy surface and increase the tortuosity for electrolyte diffusion pathway [17].


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Fig5. SEM cross-sectional view of the (a) PEO, (b,c) PEO/EPD coatings; (d) EDX elemental maps of PEO/EPD coating cross section.

3.2. Corrosion behavior of the coatings To investigate the protection efficiency of anti-corrosion coatings, we characterize AZ91 Mg alloy, PEO coating and PEO/EPD composite coatings with potentiodynamic polarization in SBF solution. The potentiodynamic polarization curves of uncoated and coated samples are shown in Fig. 6a. The corrosion potential (Ecorr), corrosion current density (icorr), and the anodic/cathodic slopes (βa and βc, respectively) were derived directly from the potentiodynamic polarization curves by Tafel region extrapolation. The corrosion characteristics such as icorr and Ecorr are listed in Table 1. From Table 1, it can be observed that the bare AZ91 magnesium alloy substrate has a low corrosion potential (−1.751 V) and its corrosion current density is high (1.24×10−4A/cm2). After the PEO treatment, the Ecorr of all the coated specimens is increased and the icorr is reduced. This demonstrates that the PEO treatment can highly improve the corrosion resistance of AZ91 magnesium alloy. Moreover, according to the parameter values listed in Table 1, it is clear that the βa and βc of the specimens can be ranked as: MgO/GO coating > MgO coating > Bare AZ91. Based on the Stern–Geary equation, the polarization resistance (Rp) was calculated from βa and βc as follows [32]: =

2.3

(

)

(1)

Increasing the anodic/cathodic slopes and decreasing the corrosion current density increase polarization values of the samples are presented in Table1. These results clearly showed that the corrosionresistance. The protection property of the oxide endowed by the PEO process improved by incorporation of GO particle onto the PEO coating. From Fig. 6 (a) two different anodic Tafel slopes in the anodic branches of the potentiodynamic polarization curves of coated samples are evident. This indicates that the main corrosion mechanism of these samples is pitting.

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Fig6. (a) potentiodynamic polarization curves of the specimens in SBF, (b) nyquist plots of the uncoated AZ91 and MgO coating, (c) Nyquist plots of the MgO/GO coating, (d) bode impedance plots of the specimens, (e) phase angle plots of the specimens and (f) Equivalent electrical circuits used for impedance data fitting

In this regard, it has been widely reported that the main corrosion form for magnesium alloys in Cl− containing solutions is pitting corrosion [3]. Furthermore, considering these figures it is clear that the bare sample had the lowest Tafel slope (see Figure 6 (a) and table 1) for the active resolution of the magnesium surface during polarization in SBF solution. The Tafel slope of the MgO coating is higher than that of the uncoated sample which can be attributed to the Cl− ions transfer induced by the higher resistance of the PEO coating. Additionally, it can be observed that the Tafel slope of the MgO/GO coating is more than that of MgO coating. This is mainly associated with the different microstructure and thickness of samples. In order to understand more details about the corrosion behavior of the PEO and PEO/EPD coatings in the SBF solution, AC impedance experiments were conducted. Nyquist plots, impedance modulus and phase angle plots appeared in Fig. 6 (b-e). To obtain the best fit of the experimental data with the least possible errors, and various circuit elements were checked in different series and parallel combinations. The equivalent circuit model of a substrate and coated samples are shown in Fig. 6 (f) and the corresponding values of the equivalent circuit parameters were listed in Table 2. In the equivalent circuit, the SBF solution resistance (Rs) was in series with the unit of the oxide layer system. The properties of the outer porous and the inner barrier parts of the MgO coating were described by the resistances Rp and Rb, which are in paralleled with constant phase elements CPEp and CPEb, respectively. The impedance of a CPE is described as: Table 1. Tafel analysis results of uncoated and coated specimens in SBF medium at 37°C. Specimens

Ecorr (V)

icorr (A/cm2)

βa (V)

|βc| (V)

Rp (Ώ.cm2)

Uncoated AZ91 MgO coating

−1.751 −1.543

1.24×10−4 9.86×10−6

0.175 0.194

0.074 0.103

182.35 2972.75

MgO/GO coating

−0.867

1.75×10−7

0.865

0.441

725680.81


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Table 2. Data of the equivalent circuits of uncoated and coated specimens in SBF medium at 37°C. Specimens

Rs (Ώ.cm2)

(CPE-T)p ( Ώ-1.cm-2)

(CPE-P)p

Rp (Ώ.cm2)

Uncoated AZ91

17.24

−

−

−

MgO coating

21.62

2.11×10−5

0.44

MgO/GO coating

20.09

1.98×10−7

0.72

Wo-R (Ώ.cm2)

Wo-T (S)

Wo-P

Uncoated AZ91

−

−

−

MgO coating

−

−

−

MgO/GO coating

2.68×107

85

0.88

=

1 (

)

(CPE-T)b ( Ώ-1.cm-2)

(CPE-P)b

Rb (Ώ.cm2)

1.05×10−4

0.56

3.29×102

2.06×102

5.73×10−3

0.68

8.93×103

8.82×104

2.11×10−5

0.31

6.79×105

(2)

where T is the CPE constant, which nominally equals to admittance of the system at 1 rad/s; = √−1, is the angular frequency (rad/s) and the value of P ranges between 0 and 1. The value of p is associated with the nonuniform distribution of current due to the roughness and the heterogeneity of the oxide layer. The circuit with an additional Warburg open terminus (Wo) element for the MgO/GO sample is shown in Fig. 6 (f). Wo in the circuit suggests that the corrosion process was controlled by both charge transfer anddiffusion phenomena. It can be observed from Table 2 that, inner barrier layer plays a crucial role in providing corrosion resistance. Also, a lower (CPEi-T)b denotes a lower porosity of MgO/GO coating [32]. In case of MgO/GO sample, (CPE-T)b value is low with respect to PEO sample, as reported in Table 2. It is due to some pores covered by GO particles as evidenced by the surface morphology and cross sectional analysis (Fig. 4 (d) and Fig. 5 (b)) of the PEO/EPD sample. On the otherhand, the highest corrosion resistance of the MgO/GO coating is also evident from its highest impedance module (|Z|f→0 Hz = 1.64×106 Ώ.cm2) and the maximum phase angle (-84.5°) as shown in Fig. 6 (d) and (e). 4. Conclusion A novel MgO/GO duplex coating successfully produced on the surface of AZ91 Mg Alloy using the combination of plasma electrolytic oxidation and electrophoretic deposition methods. Results of XRD, FT-IR and SEM analysis confirmed that GO layer was formed on the PEO coating. After EPD treatment, whole surface of the PEO coating were covered by GO layer and GO sheets filled its micropores and microcracks. Potentiodynamic polarization test and EIS studies in SBF solution show that the MgO/GO composite coating exhibits a superior anti-corrosion property in comparison with PEO alone. Importantly, incorporation of a GO film into the PEO coating affords a higher degree of coating compactness, giving rise to superior corrosion resistance in corrosive environments. Based on these findings, the MgO/GO coating can assist in preparing AZ91 alloy for its applications as metal biodegradable bone implants. However, for clinical applications determinations of the mechanical properties, adhesion strength, cell viability, biocompatibility and degradation rate of the MgO/GO coating have to be considered. Therefore, the mentioned items are under current investigation of this research group. Acknowledgements The authors thank Mr. F. Movasagh-alangh for his technical support with the experiments. This work was partialy supported with research grant (NO.: 247383) by Materials and Energy Research Center (MERC), Tehran, Iran

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