Materials Science and Engineering C 33 (2013) 2037–2045 Contents lists available at SciVerse ScienceDirect Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec Controlling the electrodeposition, morphology and structure of hydroxyapatite coating on 316L stainless steel Dinh Thi Mai Thanh a,⁎, Pham Thi Nam a, Nguyen Thu Phuong a, Le Xuan Que a, Nguyen Van Anh c, Thai Hoang a, Tran Dai Lam b,⁎⁎ a b c Institute for Tropical Technology, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet Road, Hanoi, Viet Nam Institute of Materials Science, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet Road, Hanoi, Viet Nam School of Chemical Engineering, Hanoi University of Science & Technology, Dai Co Viet Road, Hanoi, Viet Nam a r t i c l e i n f o Article history: Received 18 June 2012 Received in revised form 30 December 2012 Accepted 11 January 2013 Available online 17 January 2013 Keywords: 316L stainless steel Hydroxyapatite (HAp) Electrodeposition Simulated body fluid (SBF) Electrochemical measurements a b s t r a c t Hydroxyapatite (HAp) coatings were prepared on 316L stainless steel (316LSS) substrates by electrochemical deposition in the solutions containing Ca(NO3)2·4H2O and NH4H2PO4 at different electrolyte concentrations Along with the effect of precursor concentration, the influence of temperature and H2O2 content on the morphology, structure and composition of the coating was thoroughly discussed with the help of X-ray diffraction (XRD), Scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectra The in vitro tests in simulated body fluids (SBF) were carried out and then the morphological and structural changes were estimated by SEM and electrochemical techniques (open circuit potential, polarization curves, Nyquist and Bode spectra measurements) Being simple and cost-effective, this method is advantageous for producing HAp implant materials with good properties/characteristics, aiming towards in vivo biomedical applications © 2013 Elsevier B.V All rights reserved Introduction Stainless steel (316LSS) as well as titanium and titanium alloys are widely used as the most popular load-bearing implants due to their low cost, high corrosion resistance, excellent biocompatibility and mechanical properties [1,2] Such advantages of these metal and alloys make them to be the first choices in orthopedic and dental applications [3,4] However, these materials expose to an obvious shortage that they not form strong chemical bonds with natural bones [5–8] Many researches on coatings on bio-inert metallic prostheses are being processed, having improved biocompatibility and bioactivity that induce bone tissue growth [9,10] Among them, HAp (Ca10(PO4)6(OH)2) have received enormous considerations owing to its chemical, crystallographically structural and mineralogical compositions close to human bone and tooth minerals In addition, their strong chemical bonding makes its metal-implant coating to promote the new bone growth [11] Thus, coated metals or alloys with a HAp film are expected to overcome the disadvantages of metallic materials as well as the HAp coating shortages such as its brittleness and poor mechanical properties, enable ⁎ Corresponding author Tel.: +84 37564333x1095; fax: +84 437564696 ⁎⁎ Corresponding author Tel.: +84 37564129; fax: +84 438360705 E-mail addresses: dmthanh@itt.vast.vn (D.T.M Thanh), lamtd@ims.vast.ac.vn (T.D Lam) 0928-4931/$ – see front matter © 2013 Elsevier B.V All rights reserved http://dx.doi.org/10.1016/j.msec.2013.01.018 these materials to succeed in long term load-bearing applications [12–14] Many methods have been developed to deposit HAp onto biomedical metal surfaces including ion beam sputtering [15], sol–gel [16], electrophoretic deposition [17], pulsed laser deposition [13], plasma spraying [14,18], and electrochemical deposition [18] Alternatively, the electrochemical method has a variety of advantages for homogeneity and availability of HAp deposition on complex shaped implant substrates at trivial conditions [1,19–22] More interestingly, controlling the process allows varying the layer thickness on demand This process depends on numerous parameters such as concentration of calcium and phosphorus and their ratio in the electrolyte, pH of the solution, ionic strength, processing temperature, and additives The impact of these parameters on crystallization behavior needs careful investigation in order to find out optimal conditions for HAp layer deposition with desirable properties and thickness In this study, the influence of key experimental conditions (such as precursor concentration, temperature and H2O2 content) on the electrodeposition process of the HAp coatings on 316LSS substrates was investigated Then, the morphology, structure and the composition of the coating were thoroughly characterized by X-ray diffraction (XRD), Scanning electron microscopy (SEM), and Fourier transform infrared (FTIR) spectra To give an insight view of controlling this electrodeposition process, the in vitro tests in simulated body fluids (SBF) were carried out The morphological and structural changes of the coating were monitored and correlated with SEM, 2038 D.T.M Thanh et al / Materials Science and Engineering C 33 (2013) 2037–2045 open circuit potential, polarization and impedance (Nyquist and Bode plots) measurements Materials and methods 2.1 Materials Commercial 316LSS sheets (100× 10 × mm, composed of carbon 0.03%, manganese 2%, silicon 0.75%, chromium 17.98%, nickel 9.34%, molybdenum 2.15%, phosphorus 0.045%, sulfur 0.03%, nitrogen 0.1% and iron 67.575% (%wt)) were used as substrates (working electrodes) for electrodeposition Substrate surfaces were polished with SiC emery papers (sandy ranging from P320 to P1200 grit), followed by ultrasonic rinsing in distilled water and acetone for several times Epoxy resin was used to cover the substrate surface and leave a precise exposed area of 200 mm2 where λ is the wavelength of the X-ray radiation (CuKα), θ (rad) is the diffraction angle, and K is the Scherrer's constant, related to the crystalline shape (in this case, it takes 0.94) Theoretically, βm is the full width at half-maximum FWHM (rad) of the peak along (002) direction Owing to the instrumental broadening contribution, the βm was corrected as followed equation [24]: βm ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi β2obs −β2inst ð2Þ where βobs and βinst are the observed FWHM from XRD pattern and the instrumental profile width, respectively Another important parameter, extracted from XRD analysis is the degree of crystallinity (Xc) It is defined as the fractional amount of crystalline phase in a considered volume The crystallinity degree (Xc) of HAp coatings can also be estimated from the (002) reflection according to the equation [25]: 2.2 Deposition procedure All chemicals were purchased from Merck with an analytical reagent grade and used without any further purification A series of electrolytes was used for deposition (Table 1) The amount of Ca(NO3)2·4H2O and NH4H2PO4 solutions was taken so that the Ca/P molar ratios were kept constant at ca 1.67, equal to that in the stoichiometric composition of HAp NaNO3 was mainly used to improve the ionic strength of the electrolytes though the electrochemical reduction of NO3− ions also contributed to generating OH − [23] The initial pH of electrolytes was adjusted to 5.77 at 25 °C by a solution of NH4OH M Counter and reference electrodes were Pt foil and saturated Hg/Hg2Cl2/KCl calomel (SCE), respectively The electrodeposition was carried out on an AUTOLAB with scanning potentials ranging from to −1.6 V/SCE, scanning rate of mV s−1 in 26.667 (5 cycles) The reaction temperature was kept by a thermostat (model NNT-2400, Eyel) A linear polarization method with potential ranging from equilibrium potential to −2.5 V/SCE was used to determine the reduction potential of reactions occurring on the 316LSS electrode Thickness of the HAp coatings could be approximately estimated from the mass difference before and after the deposition (according to the following equations: d =m/V; V =S ×h; where d is the density of HAp (3.08 g/cm3); m is the HAp mass on 316LSS surface; V is the HAp volume; S is the working area; and h is the thickness of the film) 2.3 Morphological characterizations of the coatings pffiffiffiffiffi βm : X c ¼ K A where KA is a constant, whose value is 0.24 for most of HAp deposition; βm obtains the same value as it was calculated from Eq (2) The microstructure was characterized by FE-SEM combined with EDX (S4800 of Hitachi, Japan) Fourier transform infrared (FTIR) spectra were recorded with a Nicolet 6700 spectrometer, using KBr pellet technique in the range of 4000–400 cm − 1, with a resolution of cm − All measurements were performed at room temperature 2.4 Preparation of simulated body fluid (SBF) and vitro test In order to evaluate in vitro bioactivity of HAp/316LSS, simulated body fluid (SBF) soaking was used SBF was prepared according to typical procedures [26–28] by using the following salts (g/L) NaCl: 7.996; KCl: 0.224; CaCl2·2H2O: 0.278; MgCl2·6H2O: 0.305; NaHCO3:0.350; K2HPO4·3H2O: 0.228; and Na2SO4: 0.071 The fabricated HAp/316LSS samples were immersed singly into SBF for in vitro test (37±1 °C in water bath for 0, 1, 3, 5, 8, 10, 14 and 17 days) The open circuit potential (OCP), electrochemical impedance spectra (EIS Nyquist and Bode plots) and Tafel curves were measured vs immersion time EIS studies were carried out at OCP in the frequency range of 10 to 10 − Hz with 10 mV amplitude Afterwards, the samples were rinsed with absolute alcohol and distilled water, then incubated at 80 °C for 24 h for further analyses The phase purity and crystallinity of the HAp coating on the 316LSS were analyzed by X-ray diffraction (Siemens D5000 Diffractometer, CuKα radiation (λ = 1.54056 Å)), step angle of 0.030°, scanning rate of 0.04285° s−1, and 2θ in a range of 10–70° The average crystallite size along c-direction of HAp coatings was calculated from (002) reflection in XRD pattern, using Scherrer's equation [24]: ð1Þ S1 -10 i (mA/cm2) L002 Kλ ¼ βm cosθ ð3Þ -20 -30 S2 0.0 S3 S1 -0.5 S2 -40 -1.0 Table The composition of electrolytes (used in HAp electrodeposition) and HAp mass variation -50 S3 S4 -1.5 S4 -0.5 -0.6 -0.7 -0.8 -0.9 -1.0 -1.1 -1.2 Electrolytes Ca(NO3)2·4H2O (M) NH4H2PO4 (M) S1 S2 S3 S4 1.68 × 10 2.5 × 10−2 3.0 × 10−2 4.2 × 10−2 1.0 × 10 1.5 × 10−2 1.8 × 10−2 2.5 × 10−2 −2 −2 NaNO3 (M) 0.15 0.15 0.15 0.15 H2O2 (mass fraction) 6 6 HAp mass (mg) 2.7 8.5 13.6 7.0 -60 -0.5 -1.0 -1.5 -2.0 -2.5 E (V/SCE) Fig Cathodic polarization curves of HAp/316LSS formation in different electrolyte concentrations (S1, S2, S3 and S4) The inset shows a zoomed-in view of the potential ranging from −0.5 to −1.2 V D.T.M Thanh et al / Materials Science and Engineering C 33 (2013) 2037–2045 15 0.0 -0.5 6% H2O2 -1.0 10 0% H2O2 -1.5 -2.0 i (mA/cm2) -2.5 2% H2O2 -3.0 4% H2O2 -3.5 8% H2O2 -0.5 -0.6 -0.7 -0.8 -0.9 -1.0 -1.1 -1.2 -1.3 0% H2O2 -5 2% H2O2 4% H2O2 -10 -15 8% H2O2 0.0 -0.5 -1.0 -1.5 6% H2O2 -2.0 E (V/SCE) 316LSS electrode surface (Fig 1) When the voltage was less negative than − 0.7 V, the current was mainly contributed by the reduction of oxygen on the 316LSS surface [19]: − Results and discussion 3.1 Effect of precursor concentrations In this section, the variation of electrolyte concentration is discussed Obviously, electrolyte composition and its concentration significantly affect the deposition kinetics as well as the HAP coating thickness The cathodic polarization curves of 316LSS electrode in a series of electrolyte solutions (S1 to S4) were recorded with predetermined potential scanning from the equilibrium potential to − 2.5 V/SCE The obtained plots indicate that as the applied potential becomes more negative than − 0.5 V/SCE, the cathodic current could be observed, corresponding to different reactions occurred on the O2 ỵ 2H2 O ỵ 4e 4OH : 4ị More negative potentials induce other species being reduced The following reactions can be proposed: H PO4 ỵ e HPO4 þ H2 ↑ ð5Þ 2− − 3− HPO4 þ e →PO4 þ H2 ↑: ð6Þ The water reduction occurs when the voltage is more negative than − 1.5 V/SCE, resulting in the rapid change of cathodic current: − Fig Cathodic polarization curves of HAp/316LSS formation in S3 electrolyte, H2O2 concentration is varied from to 8% (mass fraction) The inset shows a zoomed-in view of the potential ranging from −0.5 to −1.2 V 2039 − 2H2 O ỵ 2e H2 ỵ 2OH : 7ị The generated hydroxide and phosphate ions reacted with Ca 2+ ions to form HAp coating on the cathode substrate Based on the analysis of the cathodic polarization curves, we performed HAp deposition on 316LSS electrodes in different electrolyte solutions with potential scanning from to − 1.6 V, in 26.667 (5 cycles), then estimated the HAp film thickness by determining their mass The variation of HAp deposition mass with respect to precursor concentrations is given in Table In response to the increase of salt concentration (from S1 to S4), the HAp film weight first increases from 2.7 mg to 13.6 mg (from S1 to S3, respectively), and then decreases to 7.0 mg (S4) This result is well consistent with the above polarization curves From the reaction equilibrium point of view this weight variation might be explained as follows Increasing reactant concentrations from S1 to S3 will generate more OH − and PO43− ions on the electrode surface, leading to the increase of the current density Consequently, the reaction of those ions with Ca 2+ ions to Fig FE-SEM images of the HAp/316LSS coatings, electrodeposited from S3 electrolyte, H2O2 concentration was 0% (A), 2% (B), 4% (C), and 6% (D) 2040 D.T.M Thanh et al / Materials Science and Engineering C 33 (2013) 2037–2045 HAp mass (mg) 14 Table The assignments of IR spectra of the HAp coatings 12 Peak position (cm−1) Assignments Peak position (cm−1) Assignments 10 3425 964 2360 O\H stretching (OH−, H2O) CO2 in air 1635 H\O\H bending mode 602 1390 B-type CO32− vibration mode ν3 P\O asymmetric stretching mode ν3a P\O asymmetric stretching mode ν3b, 3c 563 P\O symmetric stretching mode ν1 B-type CO32− vibration mode ν2/HPO42− O\P\O asymmetric bending ν4a O\P\O asymmetric bending ν4b,ac O\P\O doublet bending ν2 1099 60 65 70 75 Temperature 80 1034 85 (oC) Fig The variation of the HAp mass vs electrodeposition temperature (HAp coatings were deposited from S3 electrolyte) form HAp on the electrode surface will be more favorable At a certain point, when the concentrations continue increasing (S4) the diffusion of those ions from the electrode surface into solution will be predominant, resulting in HAp solubility and its observed weight loss on the electrode surface For the above reasons, S3 solution was treated as the optimal one and was chosen in further investigations 3.2 Effect of H2O2 As discussed above, the electrochemical reactions (Eqs (5) to (7)) at the 316LSS surface generate hydrogen gaseous bubbles, which, in their turn may attack the surface sites afterwards, preventing HAp deposition and/or reducing adhesion of HAp coatings to the substrate surface [29] To limit this problem, one strategy is to increase the amount of OH − at the substrate surface by adding hydrogen peroxide [30]: H2 O2 ỵ 2e 2OH : 8ị Direct addition of H2O2 to the electrolyte can change the mechanism of the deposition process because it provides an alternative source of OH − The abundance of OH − ions can favor the chemical reactions to form phosphate ions Therefore, the formation of H2 is expected to be minimized, and the adhesion of HAp deposition will be certainly improved [31] To investigate the effect of H2O2 concentration on HAp deposition, cathodic polarization curves were recorded under the following experimental conditions: S3 electrolyte solution, the temperature of 70 °C, and H2O2 concentration range from to 8% w/w (Fig 2) The morphological changes were then studied by FE-SEM images, presented Fig FTIR spectra of the HAp/316LSS coatings, synthesized at different temperatures 863 471 in Fig It was found that the presence of H2O2 could induce more homogeneous HAp coatings, probably due to the effective H2 suppression Moreover, corresponding to the variation of H2O2 concentration from 0% to 6%, it can be observed that HAp morphological changes from rod shape (with the size from 100 to 150 nm, Fig 3(A)) to plate shape (50–400 nm, Fig 3(B)) and blade shape and petiole shape (about 250 nm, Fig 3(C) and (D), respectively) 3.3 Effect of deposition temperature The influence of temperature on the mass of HAp deposition formed on the electrode surface (in electrolyte S3) was studied in the range of 60–85 °C (Fig 4) At first, HAp mass increases along with increasing temperature and reaches a maximum at 70 °C At this temperature, the HAp coating becomes more porous (image is not shown) Deposition temperature can affect HAp coatings in several ways First, it may change the reaction rate as well as the diffusion rate of ions High temperature can either promote the formation of HAp films on the surface or the bulk precipitation due to high diffusion rate Second, the decrease of HAp solubility with increasing temperature [32], will accelerate particle nucleation rate and thus favor film deposition on the substrate Finally, the rise of temperature can lower hydrogen bubble attachment on the substrate surface, making the growing HAp films and the coatings less damaged and more adherent, accordingly In summary, by adjusting the temperature, whole deposition process, the mass and thickness of HAp can be rigorously controlled It should be emphasized that beyond a certain value of temperature (70 °C in our case), the mass of HAp films on the electrode surface could be reduced, being solubilized (effectively, increasing temperature leads to a simultaneous increase in the concentration of phosphate and hydroxide ions that diffuse from the surface of the electrode into the solution to form HAp there) Fig XRD patterns of the HAp/316LSS coatings, synthesized at different temperatures 2041 D.T.M Thanh et al / Materials Science and Engineering C 33 (2013) 2037–2045 001 Eo (mV/SCE) FeKa FeKb CrKa 800 CrKb 1200 FeKesc 1600 PKa -150 CaKb 2000 SiKa PKb NaKsum SKa SKb ClKa ClKb Counts 2400 CKa ClLl FeLl FeLa OKa CrLl CrLa ClKesc NaKa 3200 2800 CaKa 3600 SiKsum 4000 400 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 -200 -250 9.00 10.00 keV Fig EDX spectra of HAp/316LSS coatings -300 -2 The effect of temperature on deposited HAp coatings is also considered in terms of FTIR and XRD results in the following sections 10 12 14 16 18 Time (days) Fig The variation of OCP vs different immersion times in SBF solution 3.4 IR analysis HAp samples prepared at different temperatures not show significant discrepancies in FTIR spectra, except for a minor difference in peak intensity ratio of CO32− (at 1390 cm−1) over PO43− (at 1033 cm−1) Namely, at 60 °C, such ratio is about 2.4 whereas these values in the other samples (70, 75, 80 and 85 °C) vary from 0.2 to 0.4 Thus, for producing HAp coating with low level carbonate, a temperature higher than 60 °C is strongly required A 1E-5 I(A) 1E-6 1E-7 1E-8 days 14 days 10 days days 17days days 1E-9 -0.30 -0.25 -0.20 -0.15 E(V/SCE) B 15 0.40 icorr 0.35 10 0.25 Rp 0.20 0.15 icorr(µA/cm2) 0.30 Rp(kΩ.cm2) Fig presents FTIR results of the HAp coatings deposited at different temperatures (60, 70, 75, 80 and 85 °C) The peak positions and their assignments are summarized in Table All the spectra show an intense peak located in the range of 900–1100 cm − 1, attributed to vibration frequencies of PO43 − ions A free phosphate ion with Td symmetry possesses fundamental vibrational modes which can be represented as A1 (ν1), E (ν2) and F2 (ν3 and ν4) However, only the triply degenerate vibration modes F2 are infrared active [33] The modification of phosphate ion configuration in HAp structure lowers the symmetry from Td to C3v, then two other fundamentals (ν1 and ν2) become apparent and the F2 mode is split to different modes [32] The symmetric P\O stretching vibration (ν1) at 964 cm − is a finger-print peak for HAp The peak at ca 471 cm − can be assigned to the doubly degenerate ν2 bending vibration of the O\P\O bonds [34] Meanwhile, the asymmetric stretching vibration ν3 of P\O bond is characterized by a band which is split in peaks: one is at 1099 cm − (ν3a) and the other is at 1034 cm − (ν3b and ν3c) [35] It was also reported for asymmetric O\P\O bending mode at 602 cm − (ν4a) and 563 cm − (ν4b and ν4c) [36] In one side, broad band at ca 3457 cm−1 can be assigned to the O\H stretching vibration of HAp coating [37] In the other side, this band is also at the position of the stretching mode of adsorbed water, which is accompanied by the peak at 1635 cm−1 for the O\H bending [30,38] It also appears that carbonate ions could exist in HAp films, taking into account the vibrations at 1390 cm−1 and 863 cm−1 The former could be attributed to a degenerate asymmetric stretching mode of C\O group whereas the latter could be assigned to the out-of-plane bending vibration of the C\O group [39] Nevertheless, there were no direct CO32− sources during the synthesis of all HAp samples The occurrence of CO32− may be due to the reaction of CO2 in air with OH− ions, hence the content of CO32− ions are expected to be negligible [40,41] These results agree well with the reported data in literature [30,42] Briefly, all data confirm that HAp films were successfully deposited onto 316LSS substrate within the considered temperature range 0.10 0.05 Table EDX results of HAp/316LSS -2 Element C O Na P Cl Ca % w/w % atom 7.24 46.11 62.63 0.73 0.69 17.20 12.07 0.69 0.42 31.27 16.96 10 12 14 16 18 Time (days) Fig A Tafel polarization curves vs different immersion times in SBF solution B The variation of corrosion current density (Icorr) and polarization resistance (Rp) 2042 D.T.M Thanh et al / Materials Science and Engineering C 33 (2013) 2037–2045 days days 14 days 17 days Fig 10 FE-SEM images of HAp/316LSS coatings vs different immersion times in SBF solution 2043 D.T.M Thanh et al / Materials Science and Engineering C 33 (2013) 2037–2045 corresponding to the (002) plane is stronger than other peaks in the diffraction patterns [44] The values of mean crystallite size along c-direction of the HAp deposition were in range of 20–35 nm (obtained from Scherrer's equation, using 2θ at (002) diffraction peak) (see the experimental part) These values are similar to those reported in literature [24] The crystallinity degree of all coatings, calculated from Eq (3), is less than 80%, similar to that of natural bone mineral [19] EDX analysis of HAp coating (presented in Fig and Table 3) reveals the presence of main elements Ca, P and O in HAp composition, corresponding to 31.27%, 17.20% and 46.11% (w/w) respectively As mentioned above, the peak of carbon can be explained as follows: throughout the deposition process, CO2 from the air reacted with OH − ions and then it was adsorbed on the surface of HAp coatings 3.5 XRD and EDX analyses XRD analyses also demonstrate a weak effect of temperature (in the above mentioned temperature range) on diffraction patterns of HAp coatings (Fig 6) The peak observation related to 316LSS substrate peaks in XRD pattern implies that the coatings are quite porous All recorded peaks can be ascribed to single hydroxyapatite phase Except for the peaks of Fe and CrO from SS L316 itself, no other phases were detected, confirming that pure apatite phase was achieved Namely, characteristic peaks at 2θ ≈ 26° and 2θ ≈ 32° correspond respectively to the (002) and (211) crystal planes [43] It is well stated that in electrochemical deposition, the growth of HAp crystals commonly occurs in the direction perpendicular to the electrode surface (c-direction) Therefore, the peak at 2θ ~ 26° A 25 10 10 mHz day 10 mHz 1.0 1.0 15 0.8 10 0.6 0.6 0.4 0.2 0.0 0.0 0 0.5 1.0 10 1.5 2.0 15 0.0 0.0 10 15 0.8 Hz 0.2 10 0.2 0.1 0.4 0.8 20 50 0.3 0.0 0.0 2.0 10 mHz 0.4 1.5 40 Hz 0.6 1.0 30 days 0.5 20 days 10 mHz 0.2 20 20 10 Hz 0.8 Hz 0.4 Z''(kΩ.cm2) day 20 1.2 1.6 30 40 0.0 0.0 0.2 0.4 0.6 10 Z'(kΩ.cm2) 30 days 10 mHz 20 20 10 days 10 mHz 15 1.2 1.0 Hz 1.0 0.8 10 0.8 0.4 0.4 0.2 - Z''(kΩ.cm2) 0.2 0.0 0.0 0 10 20 0.4 0.8 30 1.2 1.6 40 2.0 50 20 2.4 10 0.5 1.0 20 1.5 2.0 30 40 10 17 days 10 mHz 10 mHz 1.0 0.6 0.8 10 0.0 0.0 60 14 days 15 Hz 0.6 0.6 10 Hz Hz 0.6 0.4 0.4 0.0 0.0 0 10 0.2 0.2 0.5 20 1.0 1.5 30 2.0 40 0.0 0.0 0 0.2 10 0.4 0.6 0.8 15 1.0 1.2 20 Z'(kΩ.cm2) Fig 11 Nyquist plots (A) and Bode impedance plots of HAp/SS316L in the SBF solution vs immersion time in SBF: logZ vs logf (B) and angle phase (ϕ) vs logf (C) 2044 B D.T.M Thanh et al / Materials Science and Engineering C 33 (2013) 2037–2045 day day days days days 10 days 14 days 17 days log(Z, Ω) -3 -2 -1 log (f,Hz) C 70 60 φ, degree 50 40 30 20 0day 1day 3days 5days 10 8days 10days 14days 17days -10 -2 log (f,Hz) Fig 11 (continued) [24] This phenomenon certainly lowers the obtained Ca/P ratio from stoichiometric 1.67 to 1.4 The presence of Na and Cl in HAp coatings was also explicable, owing to NaNO3 presence in precursor solution and KCl diffusion from Calomel reference electrode 3.6 In vitro tests When metal and alloy materials are used as implants in the body, they can be corroded This can result in the weakness of the implants and/or release undesired and harmful corrosion products to the surrounding tissue In order to investigate the behavior of HAp/316LSS coatings in corrosion media, corrosion tests were carried out The strong fluctuation of open circuit potential (OCP) vs immersion time in SBF solution (for HAp/ 316LSS, deposited from S3 electrolyte, at 70 °C) is plotted in Fig In order to give an insight of this phenomenon, the discussion about the occurrence of two major processes (HAp dissolution and HAp precipitation) during the immersion period is necessary In the first days, the dominant process is dissolution and/or detachment of few parts of HAp specimen into SBF solution, owing to the low crystallinity of HAp [15] Meanwhile, the penetration of Ca + and PO43 − ions from SBF solution into HAp cavities makes local concentration of such ions relatively high Very few crystal nuclei are formed because of low supersaturation However, for a longer immersion period, when crystal nuclei are formed in a larger quantity the growth process is more prevalent [1] Such an observation is confirmed by the polarization tests of HAp/ 316LSS in SBF solution and SEM image of the sample immersed in 17 day-period The polarization potential in the range ±10 mV around the OCP was recorded against immersion times with potential scan rate of mV s −1 to determine the resistance of HAp/316LSS in SBF solution Tafel polarization curves are shown in Fig 9A The changes in corrosion current density (Icorr) and polarization resistance (Rp) obtained from the Tafel analyses are plotted in Fig 9B Fluctuations in the corrosion current density Icorr (as well as the polarization resistance Rp) in function of immersion time were also observed, that is completely consistent with the variation in OCP, at the same time interval These data indicate that the HAp coating was not in stationary condition within the above mentioned time period SEM images of leaf-like HAp coating after 5, 14 and 17 day-treatment in SBF solution is presented in Fig 10 In literature, leaf-like apatite formation in SBF, associated with the preferential orientation of crystal growth, occurs only in the initial stage of immersion (4–7 days), when Ca and P ion concentrations (dissolved from biocomposite surfaces into SBF) are still low (slow nucleation) But as the immersion time prolongs (beyond days), due to increasing concentrations of Ca and P ions (rapid nucleation) as well as energy minimization principle, this preferential orientation disappears, leaf-like crystals cannot be observed in the late stage of immersion Theoretically, the spherical apatite formed particles should have been obtained However, in our case, the preferential orientation, leading to leaf-like HAp formation is still clearly observed after 17 days of SBF This interesting feature, stemming from dissolution to reprecipitation mechanism, can be related to the difference in pristine crystal morphology and compactness of our HAp coatings, compared to those reported in the previous studies [1] It can be noted that the above feature could be also used advantageously for producing HAp coating on predetermined and complex geometries To confirm the results, obtained from polarization tests, electrochemical impedance measurements were further recorded Fig 11 shows EIS spectra of HAp/316LSS vs different immersion time of HAp/316LSS in SBF solution It is clearly seen from Nyquist plots (Fig 11A) that only intermediate frequency (10 to 10 Hz) and low frequency regions (f b 10Hz), corresponding to capacitive behavior of the HAp films and mass transfer process (diffusion or migration) were observed Next, the obtained impedance data are in good agreement with OCP and polarization results and further demonstrate the formation of dynamic but well resisted HAp layer on the 316LSS surface Effectively, after day-immersion, imaginary Z″ increases to ca.17.5 kΩ cm 2, then decreases and bounces again after day- and day-periods respectively The same “up and down” fluctuation was recorded for the remaining periods (10, 14 and 17 days) Furthermore, it can be seen from Bode plots (Fig 11C) that instead of a typical two-component shape for 316LSS (figure not shown), only one-component plot was observed and characterized for HAP layer at the low frequency domain This degeneration confirms the successful formation of HAp on entire electrode surface The phase angle curves of Bode plot (Fig 11C) also reflect the fluctuation tendency as the Nyquist and polarization plots previously Conclusion HAp coatings have been successfully electrodeposited on 316LSS surface via a simple technique The characteristics of the HAp/316LSS films can be easily adjusted by optimizing some controlling factors The optimal conditions were determined as follows: potential range from to −1.6 V/SCE, temperature of 70 °C, with the electrolyte composition of 3.10−2 M Ca(NO3)2·4H2O+ 1.8.10−2 M NH4H2PO4 +0.15 M NaNO3 and 6% H2O2 It is evident that H2O2 addition leads to smoother HAp coatings, when intervening the reaction mechanism The in vitro tests in SBF were carried out and then the morphological and structural changes were estimated by SEM, OCP, polarization curves, and Nyquist and Bode measurements It was also found that the occurrence of two D.T.M Thanh et al / Materials Science and Engineering C 33 (2013) 2037–2045 processes (dissolution and formation of HAp on the surface) should be taken seriously into account in order to 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[44] M.H.P Da Silva, J.H.C Lima, G.A Soares, C.N Elias, M.C de Andrade, S.M Best, I.R Gibson, Surf Coat Technol 137 (2001) 270–276 ... increase in the concentration of phosphate and hydroxide ions that diffuse from the surface of the electrode into the solution to form HAp there) Fig XRD patterns of the HAp/316LSS coatings, synthesized... change the reaction rate as well as the diffusion rate of ions High temperature can either promote the formation of HAp films on the surface or the bulk precipitation due to high diffusion rate Second,... phosphate ions Therefore, the formation of H2 is expected to be minimized, and the adhesion of HAp deposition will be certainly improved [31] To investigate the effect of H2O2 concentration on HAp