Preparation and characterization of magnesium hydroxyapatite coatings on 316L stainless steel - Vo Thi Hanh

Vietnam Journal of Chemistry, International Edition, 55(5): 657-662, 2017 DOI: 10.15625/2525-2321.2017-00525 657 Preparation and characterization of magnesium hydroxyapatite coatings on 316L stainless steel Vo Thi Hanh 1,2* , Pham Thi Nam 3 , Nguyen Thu Phuong 3 , Nguyen Thi Thom 3 , Le Thi Phuong Thao 2 , Dinh Thi Mai Thanh 1,4 1Graduate University of Science and Technology, Vietnam Academy of Science and Technology 2Department of Chemistry, Basic Science Faculty, Hanoi University of Mining and Geology 3Institute for Tropical Technology, Vietnam Academy of Science and Technology 4University of Science and Technology of Hanoi, Vietnam Academy of Science and Technology Received 9 March 2017; Accepted for publication 20 October 2017 Abstract Magnesium hydroxyapatite coatings (MgHAp) were deposited on the surface of 316L stainless steel (316L SS) substrates by electrodeposition technique. Different concentrations of Mg2+ ion were incorporated into the apatite structure by adding Mg(NO3)2 into electrolyte solution containing 3×10 -2 M Ca(NO3)2, 1.8×10 -2 M NH4H2PO4 and 6×10-2 M NaNO3. With Mg 2+ concentration 1×10-3 M, the obtained coatings have 0.2 wt% Mg2+. The influences of scanning potential ranges, scanning times to deposit MgHAp coatings were researched. The analytical results FTIR, SEM, X-ray, EDX, thickness and adhension strength showed that MgHAp coatings were single phase of HAp, fibrous shapes, thickness 8.1 µm and adhesion strength 7.20 MPa at the scanning potential ranges of 0÷-1.7 V/SCE and scanning times of 5 scans. Keywords. 316L SS, Electrodeposition, MgHAp. 1. INTRODUCTION Hydroxyapatite (Ca10(PO4)6(OH)2, HAp) is a main component in the mineral phase of natural bone, teeth and hard tissue. HAp is widely used in medical fields because it has high biocompatibility and chemical composition as the natural bone. HAp coating is covered on implant materials such as 316L stainless steel, titanium metal,... to improve the quality and biocompatibility. HAp could stimulate the bonding between the host bone to implant materials and make bone healing ability faster [1]. HAp coatings on metals or alloys were synthesized by many methods such as sol-gel [2], plasma spraying [3], electrophoretic deposition and electrodeposition [4] .... Among them, the electrodeposition method offers much advantages, such as low temperature, controlling the coating thickness, the high purity, high adhesion strength and low cost of the equipment [5]. Sodium has been detected as an abundant trace element in natural bone, sodium can enhance cell adhesion and bone metabolism [6]. Next to the presence of sodium, magnesium is a trace element indispensable in all skeletal metabolism stages, the formation of bone tissue [7], the stimulation of the osteoblast proliferation and bone strength structure [8]. Therefore, magnesium and sodium are incorporated into HAp in order to improve further mechanical properties and biological activity. In addition, the presence of Mg2+, Na+, and NO3 - in the electrolyte solution could be increased the conductivity and the efficiency of the synthesis process by electrochemical method. This paper introduces synthesis results of MgHAp coatings on 316L SS substrate in a solution containing Ca2+, H2PO4 -, Na+ and Mg2+ by cathodic scanning potential method with different of Mg2+ concentrations, scanning potential ranges and scanning times. 2. EXPERIMENTAL 2.1. Preparation of MgHAp coatings 316L SS (size of 100×10×2 mm, composed of 0.27 % Al; 0.17 % Mn; 0.56 % Si; 17.98 % Cr; 9.34 % Ni; 2.15 % Mo; 0.045 % P; 0.035 % S and 69.45 % Fe (%wt) was designed as the substrate. It was polished with SiC papers (ranging from P320 to VJC, 55(5), 2017 Vo Thi Hanh et al. 658 P1200 grit), rinsed ultrasonic in distilled water for 15 minutes, then dried at room temperature and last limited the working area to 1 cm2 by epoxy. MgHAp coatings were synthesized on the 316L SS by cathodic scanning potential method in 80 mL solution containing Ca2+, H2PO4 -, Na+ and Mg2+. The solutions were denoted as following: SMg0: 3×10-2 M Ca(NO3)2 + 1.8×10 -2 M NH4H2PO4 + 6.0×10 -2 M NaNO3 SMg1: SMg0 + 1×10-4 M Mg(NO3)2 SMg2: SMg0 + 5×10-4 M Mg(NO3)2 SMg3: SMg0 + 1×10-3 M Mg(NO3)2 SMg4: SMg0 + 5×10-3 M Mg(NO3)2 MgHAp coatings were synthesized with the different scanning potential ranges of 0 to -1.5; 0 to - 1.7; 0 to -1.9 and 0 to -2.1 V/SCE; different scanning times of 3; 4; 5; 6; 7 and 10 scans. The electrodeposition was carried out in a three- electrode cell: 316L SS as the working electrode; platinum foil electrode acting as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode on the Autolab PGSTAT 30 equipment (Holland). 2.2. Coating characterization Mass of MgHAp coatings deposited on the surface of 316L SS was determined by the mass change of 316L SS samples before and after synthesis by a Precisa analytical balance (XR 205SM-PR, Swiss). Thickness of the coatings was measured by Alpha- Step IQ system (KLA-Tencor-USA), following the standard of ISO 4288-1998. The result is the average value of 5 measurements. The charge of synthesis process was determined by taking the integral from the start to the end point of the cathodic polarization curve. The adhesion strength of the coatings on 316L SS substrate was examined using an automatic adhesion tester (PosiTest AT-A, DeFelsko) according to ASTM D-4541 standard [9]. Fourier transform infrared (FTIR) spectra were recorded in the range of 4000-400 cm-1, with a resolution of 8 cm-1 by a Nicolet 6700 Spectrometer, using the KBr pellet technique. The spectra were the sum of 32 scans. The morphology of the coatings was characterized using scanning electron microscopy (SEM) using Hitachi S4800 equipment (Japan). JSM 6490-JED 1300 Jeol (Japan) energy- dispersive X-ray spectroscopy (EDS) was used to identify the composition of elements in MgHAp coatings. The phase structure and crystallinity of the MgHAp coatings were analyzed by X-ray diffraction (SIEMENS D5005 Bruker-Germany, CuKα radiation (λ = 1.54056 Å), with the following parameters: step angle of 0.03°, the scanning rate of 0.03 °s-1, and 2θ in a range of 10-70°. The crystallite size of HAp and MgHAp was calculated from (002) reflection in XRD pattern, using Scherrer's equation [5]. Lattice parameters (a, c) were calculated from peak (002) and (211) of XRD pattern according to equation 1. Where, d is determined from XRD, which is the distance between adjacent planes in the set of Miller indices (hkl) [10]: 2 2 2 2 2 2 4 ( ) 1 3 h kh k l d a c (1) 3. RESULTS AND DISCUSSION 3.1. Effect of Mg 2+ concentration The cathodic polarization curves of 316L SS substrate in the electrolytes SMg0, SMg1, SMg2, SMg3 and SMg4 at 50 oC, 0÷-1.7 V/SCE, 5 scanning times are shown in Fig. 1. The concentration of Mg2+ in the electrolyte solution increase leading to improve the ionic strength of the electrolyte, so the current density increases. With the potential range 0÷-0.6 V/SCE, the values of the current density are nearly unchanged and approximately zero because no reaction occurs on 316L SS substrate. With the potential is -0.6÷-1.2 V/SCE, the current density increased slightly due to the reduction of O2 to produce OH - [5]. When potential is more negative than -1.2 V/SCE, the current density increases fast because several electrochemical reactions occur, such as: the reduction: NO3 -, H2PO4 -, H2O to produce OH -; PO4 3- and H2 [5, 7, 11]. Hydroxide is generated on the cathode surface to lead the formation PO4 3- ions by acid-base reaction of H2PO4 - and OH- [5, 7]. Then MgHAp is producted on the cathode substrate according to the chemical reaction [5]: 10(Ca2+, Na+, Mg2+) + 6PO4 3− + 2OH− → (Ca,Na,Mg)10(PO4)6(OH)2 (2) Figure 2 shows the IR spectra in the wave number range from 4000 to 400 cm-1 of MgHAp coatings which were synthesized in SMg1, SMg2, SMg3 and SMg4 solutions. The results shows the characteristic peaks of HAp such as PO4 3- and OH-. The peaks of PO4 3- group are observed at 1035; 602; 565 and 437 cm-1; the vibrations of OH- are observed at 3447 and 1645 cm-1. Furthermore, the peaks of NO3 - and CO3 2- were also detected respectively at 1384 and 864 cm-1 because NO3 - ions are present in the electrolyte; CO2 from in the air can be dissolved in the electrolyte and reacts with OH- to form the CO3 2- ions [5, 7]. VJC, 55(5), 2017 Preparation and characterization of magnesium 659 -1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 -6 -5 -4 -3 -2 -1 0 SMg4 SMg3 SMg2 SMg1 SMg0 i (m A /c m 2 ) E (V/SCE) Figure 1: The cathodic polarization curve of 316L SS substrate in SMg0, SMg1, SMg2, SMg3 and SMg4 solution The element components of obtained coatings were analyzed by the EDX spectra. The results showed the presence of 5 main elements in MgHAp coatings: O, P, Ca, Mg and Na. With the concentration of Mg2+ in the solution increased from 1×10-4 to 5×10-3 M, the content of Mg in MgHAp coatings increased from 0.06 to 0.4 wt% (table 1). These results have been used to calculate the atomic ratios M/Ca, (Ca+M)/P. The ratio of (Ca+0.5Na+Mg)/P and Na/Ca in all samples are lower than the ratio of Ca/P in the natural bone (1.67 and 0.102, respectively) [12, 13]. The deposited MgHAp coatings have the ratio Mg/Ca from 0.0057 to 0.0195. However, to reach the Mg/Ca ratio ≤ 0.017, similar to natural bone [12, 13], the obtained MgHAp coatings in the electrolyte solution SMg1, SMg2 and SMg3 are suitable. Therefore, SMg3 was chosen for the next experiments. 4000 3600 3200 2800 2400 2000 1600 1200 800 400 SMg2 5 6 56 0 28 7 4 1 0 3 5 1 3 8 4 1 6 4 5 3 4 4 7 O H - T ra n m is ta n c e ( a .u ) Wave number (cm -1 ) 4 3 7 SMg1 SMg3 SMg4 H 2O P O 43 - C O 32 - P O 43 -N O 3- Figure 2: IR spectra of MgHAp coatings synthesized in the solutions: SMg1, SMg2, SMg3 and SMg4 at 50 oC, 0÷-1.7 V/SCE, 5 scans 0 2 4 6 8 10 Ca CaP Mg N a O DMg1 KeV C ou nt s DMg3 DMg2 DMg4 Figure 3: The EDX spectra of MgHAp coatings synthesized on 316L SS at 50 oC, 0÷-1.7 V/SCE, 5 scans in the electrolyte solutions: SMg1, SMg2, SMg3 and SMg4 Table 1: The component of elements of MgHAp synthesized on 316L SS in SMg1, SMg2, SMg3 and SMg4 solutions at 50 oC, 0÷-1.7 V/SCE, 5 scans Electrolyte % Mass Na/Ca Mg/Ca (0.5 Na+Sr +Ca)/ P O P Ca Na Mg SMg1 47.43 17.18 34.12 1.21 0.06 0.062 0.0029 1.59 SMg2 45.65 17.90 35.20 1.13 0.12 0.056 0.0057 1.58 SMg3 45.90 18.10 34.60 1.20 0.20 0.060 0.0096 1.54 SMg4 45.80 18.50 34.20 1.10 0.40 0.056 0.0195 1.50 3.2. Effect of the scanning potential range The influence of the scanning potential ranges on the deposition of MgHAp coatings was studied in the SMg3 solution. The charge, mass, thickness and adhesion strength of MgHAp coatings at the different potential ranges of 0÷-1.5, 0÷-1.7, 0÷ -1.9, and 0÷-2.1 V/SCE are shown in Table 2. The charge increases from 0.42 C to 6.85 C when the scanning potential range extends from 0÷-1.5 to 0÷-2.1 V/SCE. Therefore, according to Faraday law, OH- and PO4 3- ions are formed more so the mass of obtained coatings increases. However, the mass and thickness of MgHAp coatings increases and reaches the maximum value at potential range of 0÷-1.7 V/SCE (2.63 mg/cm2 and 8.1 µm). With the more negative potential range, these values decrease. The results are explained that with the negative scanning potential range leading to the increase of the charge, further increase the amount of OH- and PO4 3- ions on the electrode surface leading to diffuse into the solution to form MgHAp. Moreover, with the more VJC, 55(5), 2017 Vo Thi Hanh et al. 660 negative potential range, the adhension strength between MgHAp coatings and 316L SS substrate decreases and the obtained coatings are porous because of the formation of hydrogen bubbles on the electrode surface. Thus, the potential range 0 to -1.7 V/SCE is chosen for the next experiments. Table 2: The variation of charge, mass, thickness and adhesion strength of obtained coating at 50 oC, 5 scans in SMg3 solution with the different scanning potential ranges Potential range (V/SCE) Charge (C) MgHAp mass (mg/cm2) Thickness (µm) Adhesion strength (MPa) 0÷-1.5 0.42 1.21 5.5 7.32 0÷-1.7 3.56 2.63 8.1 7.20 0÷-1.9 4.52 1.96 6.3 7.10 0 ÷-2.1 6.85 1.41 4.5 6.95 3.3. Effect of the scanning times Results of charge, mass, thickness and adhesions strength of obtained MgHAp coatings with the scanning times from 1 to 10 scans are shown in table 3. With one scanning time, the charge is 0.76 C, the adhesion strength reached the highest value (12.97 MPa). This value is nearly with the adhesion of glue and 316L SS (15 MPa). This is explained that because the mass and thickness of deposited MgHAp are small (0.57 mg/cm 2 and 1.6 µm), not enough to cover all surface of the substrate, so the obtained adhesion strength is contributed by the substrate and glue. The charge of deposited process increases according to scanning times. However, the mass and thickness of coatings only increase with scanning times increasing from 1 to 5 scans and then decrease. The adhesion strength decrease from 12.97 MPa to 5.72 MPa with scanning times increase from 1 to 10 scans. It is explained that the charge increases leading to OH- and PO4 3- ions forming too much on the electrode surface and diffusing into the solution so MgHAp is formed in the solution without sticking the substrate. Thus, 5 scanning time is chosen for MgHAp coating electrodeposition. Table 3: The variation of the adhesion strength of MgHAp coatings to 316L SS in SMg3 at 50 oC, 0÷-1.7 V/SCE with different scanning times Scanning times (times) Charge (C) MgHAp mass (mg/cm2) Thickness (µm) Adhesion (MPa) 1 0.76 0.57 1.6 12.97 3 2.40 1.72 5.5 7.31 5 3.51 2.63 8.1 7.20 7 4.61 1.41 4.5 6.31 10 6.33 0.98 3.1 5.72 3.4. Characterization of MgHAp coating *The XRD patterns Figure 4 shows the XRD patterns of HAp and MgHAp coatings. Both XRD patterns exhibit the hydroxyapatite phase with two characteristic peaks at 2 of 32 o (211) and 26 o (002). Besides, there are some peaks of HAp with smaller intensity at 2 of 17o (101), 33o (300), 46o (222), and 54o (004). The characteristic peaks of 316L SS substrate are observed at 2 45o (Fe) and 44o, 51o (CrO.19FeO.7NiO). These results show that MgHAp coatings have crystals structure and single phase of HAp. 10 20 30 40 50 60 70 NaHAp MgNaHAp In te n s it y ( a .u ) 2 Theta (degree) 1(222) 1 (004) 2 3 2 1 (202) 1 (300) 1. HAp; 2. CrO.FeO.NiO; 3. Fe 1 (211) 1 (101) 1 (002) Figure 4: XRD patterns of HAp and MgHAp synthesized in SMg0 and SMg3 solution at 0÷-1.7 V/SCE, 50 oC, and 5 scans VJC, 55(5), 2017 Preparation and characterization of magnesium 661 The crystal diameters of HAp and MgHAp are calculated according to Scherrer formula (Equation 2). The crystal diameter of MgHAp coating is about 22.1 nm, smaller than that of HAp (44.2 nm). This can be explained that the radius of Mg2+ ion (0.65 Ǻ) is smaller than Ca2+ (0.99 Ǻ) and Na+ ( 0.95 Ǻ) so Ca2+, Na+ are replaced by Mg2+ leading to reduce the crystal diameter. Table 4 presents distance between the adjacent planes of the crystal (d) at two planes (002) and (211) and the value of the lattice parameters a, b, c of MgHAp. In comparison with NIST standard of HAp sample [10] and HAp, these values of MgHAp is lower. This result shows that Mg2+, Na+ ions incorporated into the HAp lattice structure. Table 4: Values of distance between the planes of the crystal and the lattice constant of MgHAp and HAp HAp [10] HAp MgHAp d (002) 3.44 3.438 3.420 d (211) 2.82 2.815 2.768 a = b (Ǻ) 9.4451 9.4261 9.247 c (Ǻ) 6.88 6.876 6.840 * SEM images SEM images of HAp and MgHAp coating synthesized on 316L SS were presented in Fig. 5. The results showed that with the present of Mg, the morphology changes from plate shapes of HAp to fibrous shapes of MgHAp. Figure 5: The SEM images of HAp and MgHAp coatings deposited in SMg0 and SMg3 solutions, at 50 oC, 0÷-1.7 V/SCE, 5 scans 4. 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Fluoridated hydroxyapatite coatings on titanium obtained by electrochemical deposition, Acta Biomaterialia, 5(5), 1798-1807 (2009). 12. H. J. M. Bowen. Environmental Chemistry of the Element, London: Academic Press, Inc 1979. 13. U. H. -W. Kuoa, S. -M. Kuoa, C. -H. Choub, T. -C. Leeb. Determination of 14 elements in Taiwanese bones, The Science of the Total Environment, 25, 45- 54 (2000). Corresponding author: Vo Thi Hanh Department of Chemistry, Basic Science Faculty Hanoi University of Mining and Geology E-mail: vothihanh2512@gmail.com; Telephone 0982541229.