- Email: [email protected]
carbon composites prepared by pulsed electrodeposition
Accepted Manuscript Title: Strontium and magnesium substituted dicalcium phosphate dehydrate coating for carbon/carbon composites prepared by pulsed electrodeposition Author: Shou-jie He-jun Li Lei-lei Zhang Lei Feng Pei Yao PII: DOI: Reference:
S0169-4332(15)02553-2 http://dx.doi.org/doi:10.1016/j.apsusc.2015.10.134 APSUSC 31615
To appear in:
APSUSC
Received date: Revised date: Accepted date:
27-7-2015 18-10-2015 19-10-2015
Please cite this article as: Shou-jie, L.-l. Zhang, L. Feng, Strontium and magnesium substituted dicalcium phosphate dehydrate coating for carbon/carbon composites prepared by pulsed electrodeposition, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.10.134 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
He-jun Lei
Li*[email protected], [email protected],
Lei-lei Pei
us
Shou-jie [email protected], [email protected], [email protected]
cr
ip t
Strontium and magnesium substituted dicalcium phosphate dehydrate coatings for carbon/carbon composites were synthesized by pulsed eletrodeposition. Strontium and magnesium substituted dicalcium phosphate dehydrate coated carbon/carbon composites exhibited excellent bioactivity in the vivo. Strontium and magnesium substituted dicalcium phosphate dehydrate coated carbon/carbon composites showed lower corrosion rate with the comparison to pure carbon/carbon composites. Strontium and magnesium substituted dicalcium phosphate dehydrate coating for carbon/carbon composites prepared by pulsed electrodeposition
an
State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an 710072, China
M
Tel.+86 29 8849 5764, Fax: +86 29 8849 5764.
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Abstract Trace elements substituted apatite coatings have received a lot of interest recently as they have many benefits. In this work, strontium and magnesium substituted DCPD (SM-DCPD) coatings were deposited on carbon/carbon (C/C) composites by pulsed electrodeposition method. The morphology, microstructure, corrosion resistance and in-vitro bioactivity of the SM-DCPD coatings are analyzed. The results show that the SM-DCPD coatings exhibit a flake-like morphology with dense and uniform structure. The SM-DCPD coatings could induce the formation of apatite layers on their surface in simulated body fluid. The electrochemical test, indicates that the SM-DCPD coatings can evidently decrease the corrosion rate of the C/C composites in simulated body fluid. The SM-DCPD has potential application as the bioactive coatings.
Key words: Carbon/carbon composites; Dicalcium phosphate dehydrate; Pulsed electrodeposition; Bioactivity; Corrosion 1. Introduction Carbon/carbon (C/C) composites are primarily preferred for biomaterials applications due to their light weight, high toughness, and high specific strength [1, 2].Moreover, the Young’s modulus of C/C composites is closer to that of human bones compared with biomedical metals [3-5]. However, the lack of bioactivity and the failure to form a chemical bond with the host bone have limited their application. Applying bioactive coatings for C/C composites may address the problem. Ideally, the implant materials are replaced by newly growing bone as they are absorbed. Thus the
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solubility of the bioactive coating is a decisive factor for final clinical application. Dicalcium phosphate dehydrate (CaHPO4·2H2O, DCPD) was reported to have high solubility than other types of apatite and could improve bone formation in vivo [6]. Being a foreigner body to human tissue, there will be a bacterial infection at the interface of the implants and the tissues due to mismatch in mechanical properties between the tissues and implants [7]. Consequently, a second surgery is required to remove implants after bone healing is complete [8, 9]. Normally, the human bones not only consist of calcium and phosphorus, but also contain many trace elements, such as 1.0 wt.% sodium (Na), 0.07 wt.% potassium (K), 1.1 wt.% magnesium (Mg), 0.04 wt.% strontium (Sr), 0.07 wt.% fluoride (F) and others [10, 11]. Each of these trace elements serves its own unique functions, such as antimicrobial property, ability to induce new bone formation and effective in reducing fracture risk in osteoporosis [12]. Considerable related studies have been conducted on the incorporation of Sr ions into CaP because of the beneficial effects of the presence of Sr in bone. Sr2+ in calcium phosphate coatings can replace Ca2+ easily, which inhibits bone absorption and improves bone formation. Moreover, the presence of Sr can improve the cell activity in vitro and in vivo studies [13-15]. Mg also participates in important functions of body. Some studies have confirmed that Mg-substituted CaP coatings have better biocompatibility and bioactivity and during the bone repair surgery, the damage body bone will recover faster and more efficient [16, 17]. As a result, the incorporation of the trace elements in CaP to make a composite coating is an effective way to improve the bioactivity and avoid infection. Motivated by this, Sr and Mg substituted DCPD (SM-DCPD) coatings for C/C composites are prepared by pulsed electrodeposition in this work. Compared with the electrodeposition, pulsed electrodeposition can effectively suppress hydrogen generation and achieve a homogenous coating by adjusting the pulse parameters during the deposition process [12, 18, 19]. The DCPD coatings for C/C composites prepared by pulsed electrodeposition are a control experiment. And their microstructures, biological activity, corrosion behaviors in simulated body fluid (SBF) were experimentally investigated. 2. Materials and Methods The C/C composites, which were used as substrate, used in this work were prepared by chemical vapor infiltration (CVI) process and have an average density of 1.7 g/cm3. The C/C composites were cut into square slabs with a dimension of 8 × 8 × 2 mm and polished with 1000 grit SiC paper. Then they were cleaned ultrasonically firstly in acetone, then in ethanol, finally in distilled water. The electrolytes were prepared by mixing a solution of 66.8 mmol/L Ca(NO3)2·4H2O, 66.8 mmol/L Sr(NO3)2, 66.8 mmol/L Mg(NO3)2·6H2O and 40 mmol/L (NH4)2HPO4. Depositions were carried out in a constant voltage mode with a voltage of 2.5 V at 50 °C, the pulse on and off time were set as 200 ms and 300 ms. The whole deposition time was 3 h. The crystalline of the coated C/C composites were measured by X-ray diffraction (XRD, PANalytical X’Pert PRO, Netherlands) with a Cu Kα radiation (λ = 0.1542 nm) with a step size of 0.033°. The compositions of the coated C/C composites were characterized by an Axis Ultra X-ray photoelectron spectroscopy (XPS, Kratos, Manchester, UK) with an Al kα X-ray source. The surface morphology and element composition of the coated C/C composites were analyzed by scanning electron microscopy (SEM, TESCAN VEGA3, Czech) equipped with energy dispersive spectroscopy (EDS). The in vitro evaluation was performed by soaking each specimen into the simulated body fluid
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(SBF). It was prepared by dissolving regent grade chemicals of 7.996 g/L NaCl, 0.350 g/L NaHCO3, 0.224 g/L KCl,0.228 g/L K2HPO4·3H2O, 0.350 g/L MgCl2·6H2O, 0.278 g/L CaCl2, 0.071 g/L Na2SO4 and 6.057 g/L (CH2OH)3CNH2 into distilled water, and buffering at pH 7.40 with hydrochloric acid. The detailed information of the SBF test could be found elsewhere [5]. In this paper, the coated C/C composites were soaking in the centrifugal tube with 5 ml SBF solution at room temperature, and the SBF was refreshed every day. The samples were removed from the SBF after 1, 3, 7 and 14 days respectively. Finally the samples were analyzed by SEM equipped with EDS. To evaluate the corrosion resistance of coatings, polarization test in SBF was carried out on electrochemical workstation (CS310 Corrtest, China). The samples were covered by epoxy resin and only the coating area 8 × 8 mm2 was exposed to the SBF solution. After a stabilization period of 15 min, Potentiodynamic polarization studies were performed in a potential range of -0.4-0.2 V at a scan rate of 0.005 V/s. 3. Results and discussion The XRD patterns of the SM-DCPD and DCPD coated C/C composites are depicted in Fig. 1a and Fig. 1b, respectively. Form the XRD analysis, at 2θ values of 26.543° was the carbon (002) peak generating from the C/C composite substrates. The main diffraction peaks for DCPD are observed at 2θ values of 11.604°, 20.786°, 29.160°, 30.485°, 31.249°, 34.066°, 35.425°. Whereas compared with DCPD coatings the 2θ values of SM-DCPD coatings experience a slight shift which may due to crystal lattice distortion that is occurred as the result of substitution of Sr and Mg in DCPD. Furthermore, the SM-DCPD coatings show the strong diffraction peaks associated with DCPD. This is because the introduction of Sr and Mg ions, leading to the molar ratio of Ca/P in the electrolyte is similar to the theoretical value of DCPD. From the Raman analysis (Fig. 2), four characteristic Raman peaks for PO43- groups are found, including υ1, υ2, υ3, υ4 of PO43- mode locating at 984 cm-1, 413 cm-1, 1053 cm-1 and 580 cm-1, respectively. The peak at 875 cm-1 is a typical feature of crystalline DCPD, which is attributed to the P-OH vibration of HPO42- mode. The Raman spectrum also shows the presence of carbon peaks, which generate from the C/C composite substrates, including D peak at 1359 cm-1 and G band at 1585 cm-1. XPS and EDS characterization are carried out on SM-DCPD coated C/C composites and DCPD coated C/C composites. For XPS spectra (Fig. 3b), peaks corresponding to O1s (531.6ev), Ca2s (428.8eV), Ca2p (351.1eV), C1s (285eV), P2s (191.1eV), P2p (133.6eV), Ca3p (25eV) are all clearly significantly. And at the location of 280.1eV Sr3p peak is found in the SM-DCPD coatings in Fig. 3a. The P2p peak is centered at 133.6 eV, as expected for phosphate groups, and Ca 2p at 351.1 eV, as expected for Ca2+. In Fig. 4a, EDS analysis show that the SM-DCPD coatings are mainly composed of O, Ca, Sr, Mg and P elements with Ca/P molar ratios 1.06. Fig. 4b shows the Ca/P molar ratios of DCPD coatings is 1.09, which is similar to the theoretical value of DCPD. Fig. 5 shows the surface micrographs of the DCPD and SM-DCPD coated C/C composites, (a) and (c) are the micrograph of the DCPD, (b) and (d) are the micrograph of the SM-DCPD. By low magnification, the two types of the coating are uniform and continuous. By high magnification, the two types of the coatings present a flake-like morphology and the crystal is approximately 3-5 µm in thickness and 10-20 µm in length. This kind of apatite is similar to the main components of the bone, which is likely to be beneficial for promoting the formation of new bone tissue and accelerating the healing of the damaged bones [20].
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The potentiodynamic polarization curve of the samples in SBF solution is shown in Fig. 6 and the corresponding electrochemical data are given in Table 1. It is obvious that the corrosion potential (Ecorr) value of the SM-DCPD coated C/C composites shift towards active direction. Meanwhile, compared with the uncoated C/C composites, the corrosion current density (Icorr) is also decreased dramatically. The Ecorr of the SM-DCPD coated C/C composites are 0.00183V, which is significantly higher than that of uncoated ones (-0.26952V). The coatings taking calcium and phosphate form the surrounding SBF for the formation of apatite on its surface is an important indication of implant osseointegration. In this respect, the SBF tests are applied to study the apatite formation ability of both DCPD and SM-DCPD coatings. After soaking in SBF for 1, 3, 7, 14 days, the surface morphology of the coatings are studied by SEM. Fig. 7 illustrates the formation of a new apatite layer on the two types of coatings. In the SBF tests, many overlapped small spheres formed calcium phosphate layer on the surface of DCPD coatings with some cracks. However, SM-DCPD coatings show the different formation process. Soaking in SBF for 1, 3, 7 days, the flake-like morphology begin to dissolve with the formation of calcium phosphate. After soaking in SBF for 14 days, the SM-DCPD coatings are fully covered by a dense calcium phosphate layer in the end. The results correspond to Sr2+and Mg2+ implantion that could promote the nucleation and growth of DCPD on C/C composites form SBF and enhance the bioactivity [21-23]. The molar ratios of Ca/P for the DCPD and SM-DCPD coatings after immersion in SBF are shown in Fig. 8. The EDS analysis show that the new layers are mainly composed of O, Ca and P elements. The Ca/P molar ratios of DCPD coatings (curve b in Fig. 8) range from 1.16 to 1.42 and the Ca/P molar ratios of SM-DCPD coatings (curve a in Fig. 8) range from 1.10 to 1.35. 4. Conclusion SM-DCPD coatings for C/C composites were synthesized by pulsed electrodeposition. The coatings exhibit a flake-like shape with a uniform and compact structure. The SBF tests show that the coatings could induce the formation of apatite layers on their surface in simulated body fluid. The incorporation of Sr and Mg in DCPD is an effective way to improve the bonding with bone. In the electrochemical test, the SM-DCPD coated C/C composites obtain a more positive potential and lower anodic current density than that of uncoated ones, which indicates that the SM-DCPD coating can evidently decrease the corrosion rate of the C/C composites in SBF. The SM-DCPD coated C/C composites could apply on the orthopedic field. Acknowledgements This work was supported by the National Natural Science Foundation of China under Grant No. 51202194, the FP7-International Research Staff Exchange Scheme-Advanced Biomaterials for Regenerative Medicine, the Global Innovation Initiative-Nanostructured Materials for the Control of Contaminants Detrimental to Health, the Fundamental Research Funds for the Central Universities under Grant No. 3102014JCQ01030 and “111” project of china (B08040). References L.L. Zhang, H.J. Li, K.Z. Li, S.Y. Zhang, J.H. Lu, W. Li, Appl. Surf. Sci. 286 (2013) 421–427. L.L. Zhang, H.J. Li, Q. Song, K.Z. Li, Surf. Interface Anal. 46 (2014) 24–29. Y.Q. Zhai, K.Z. Li, H.J. Li, Mater. Chem. Phys. 106 (2007) 22–26. L.L. Zhang, H.J. Li, K.Z. Li, S.Y. Zhang, Ceram. Int. 41 (2015) 427–435. H.J. Li, X.N. Zhao, S. Cao, K.Z. Li, Appl. Surf. Sci. 263 (2012) 163–173.
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D.L. Alge, G.S. Cruz, W.S. Goebel, Biomed. Mater.4 (2009) 016–025. Y. Liu, Z. Zheng, J. N. Zara, Biomaterials 33 (2012) 8745–8756. Y. Shi, M. Qi, Y. Chen, P. Shi, Mater. Lett. 65 (2011) 2201–2204. Z.J. Cheng, J.S. Lian, Y.X. Hui, G.Y. Li, J. Bionic Eng. 11 (2014) 610–619. G. Mabilleau, R. Filmon, P.K. Petrov, Acta Biomater. 6 (2010) 1555–1560. E. Boanini, M. Gazzano, A. Bigi, Acta Biomater. 6 (2010) 1882–1894. H.R. Bakhsheshi Rad, M.H. Idris, M.R. Abdul Kadir, Surf. Coat. Technol. 222 (2013) 79–89. Pereiro, C. Rodrguez Valencia, C. Serra, E.L. Solla, Appl. Surf. Sci. 258 (2012) 9192–9197. Y. Huang, Y.J. Yan, X.F. Pang, Q.Q. Ding, Appl. Surf. Sci. 282 (2013) 583–589. B. Bracci, P. Torricelli, S. Panzavolta, E. Boanini, R. Giardino, A. Bigi, J. Inorg. Biochem.103 (2009) 1666–1674. J.W. Park, Y.J. Kim, J.H. Jang, H.J. Song, Oral Implants Research, 40 (2010) 1278–1287. Y.J. Yan, Q.Q. Ding, Y. Huang, S.G. Han, X.F. Pang, Appl. Surf. Sci. 305 (2014) 77–85. X.N. Zhao, H.J. Li, M.D. Chen, K.Z. Li, Surf. Interface Anal. 44 (2012) 21–28. J.W. Tang, K. Azumi, Electrochimica. Acta 56 (2011) 1130–1137. K.K. Li, B. Wang, B. Yan, W. Lu, Biomater. Appl. 28(3) (2012) 375–384. D. Gopi, A. Karthika, S. Nithiya, L. Kavitha, Mater. Chem. Phys. 144 (2014) 75–85. M. Kheradmandfard, M.H. Fathi, M. Ahangarian, E.M. Zahrani, Ceram. Int. 38 (2012) 169–175. V. Aina, L. Bergandi, G. Lusvardi, G. Malavasi, F. E. Imrie, Mater. Sci. Eng. C 33 (2013) 1132–1142.
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Fig. 1 XRD patterns of the SM-DCPD (a) and DCPD (b) coated C/C composites. Fig. 2 Raman spectrum of the SM-DCPD (a) and DCPD (b) coated C/C composites. Fig. 3 XPS spectra of the SM-DCPD (a) and DPCD (b) coated C/C composites. Fig. 4 EDS analysis of SM-DCPD (a) and DCPD (b) coated C/C composites. Fig. 5 SEM images of DCPD (a and c) and SM-DCPD (b and d) coated C/C composites. Fig. 6 Electrochemical polarization curves of the uncoated and SM-DCPD coated C/C composites in SBF. Fig. 7 SEM images of the DCPD coatings after immersion in SBF for (a) 1, (c) 3, (e) 7, (g) 14 days and SM-DCPD coatings after immersion in SBF for (b) 1, (d) 3, (f) 7, (h) 14 days. Fig. 8 The molar ratios of Ca/P for the SM-DCPD (a) and DCPD (b) coatings after immersion in SBF.
Table 1 Electrochemical parameters of the electrochemical test Samples
Corrosion potential Ecorr (mV)
Corrosion current Icorr (μA/cm2)
uncoated SM-DCPD coated
-269.5±16.2 1.8±1.3
1.83±0.05 0.07±0.04
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S0169-4332(15)02553-2 http://dx.doi.org/doi:10.1016/j.apsusc.2015.10.134 APSUSC 31615
To appear in:
APSUSC
Received date: Revised date: Accepted date:
27-7-2015 18-10-2015 19-10-2015
Please cite this article as: Shou-jie, L.-l. Zhang, L. Feng, Strontium and magnesium substituted dicalcium phosphate dehydrate coating for carbon/carbon composites prepared by pulsed electrodeposition, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.10.134 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
He-jun Lei
Li*[email protected], [email protected],
Lei-lei Pei
us
Shou-jie [email protected], [email protected], [email protected]
cr
ip t
Strontium and magnesium substituted dicalcium phosphate dehydrate coatings for carbon/carbon composites were synthesized by pulsed eletrodeposition. Strontium and magnesium substituted dicalcium phosphate dehydrate coated carbon/carbon composites exhibited excellent bioactivity in the vivo. Strontium and magnesium substituted dicalcium phosphate dehydrate coated carbon/carbon composites showed lower corrosion rate with the comparison to pure carbon/carbon composites. Strontium and magnesium substituted dicalcium phosphate dehydrate coating for carbon/carbon composites prepared by pulsed electrodeposition
an
State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an 710072, China
M
Tel.+86 29 8849 5764, Fax: +86 29 8849 5764.
Ac ce p
te
d
Abstract Trace elements substituted apatite coatings have received a lot of interest recently as they have many benefits. In this work, strontium and magnesium substituted DCPD (SM-DCPD) coatings were deposited on carbon/carbon (C/C) composites by pulsed electrodeposition method. The morphology, microstructure, corrosion resistance and in-vitro bioactivity of the SM-DCPD coatings are analyzed. The results show that the SM-DCPD coatings exhibit a flake-like morphology with dense and uniform structure. The SM-DCPD coatings could induce the formation of apatite layers on their surface in simulated body fluid. The electrochemical test, indicates that the SM-DCPD coatings can evidently decrease the corrosion rate of the C/C composites in simulated body fluid. The SM-DCPD has potential application as the bioactive coatings.
Key words: Carbon/carbon composites; Dicalcium phosphate dehydrate; Pulsed electrodeposition; Bioactivity; Corrosion 1. Introduction Carbon/carbon (C/C) composites are primarily preferred for biomaterials applications due to their light weight, high toughness, and high specific strength [1, 2].Moreover, the Young’s modulus of C/C composites is closer to that of human bones compared with biomedical metals [3-5]. However, the lack of bioactivity and the failure to form a chemical bond with the host bone have limited their application. Applying bioactive coatings for C/C composites may address the problem. Ideally, the implant materials are replaced by newly growing bone as they are absorbed. Thus the
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solubility of the bioactive coating is a decisive factor for final clinical application. Dicalcium phosphate dehydrate (CaHPO4·2H2O, DCPD) was reported to have high solubility than other types of apatite and could improve bone formation in vivo [6]. Being a foreigner body to human tissue, there will be a bacterial infection at the interface of the implants and the tissues due to mismatch in mechanical properties between the tissues and implants [7]. Consequently, a second surgery is required to remove implants after bone healing is complete [8, 9]. Normally, the human bones not only consist of calcium and phosphorus, but also contain many trace elements, such as 1.0 wt.% sodium (Na), 0.07 wt.% potassium (K), 1.1 wt.% magnesium (Mg), 0.04 wt.% strontium (Sr), 0.07 wt.% fluoride (F) and others [10, 11]. Each of these trace elements serves its own unique functions, such as antimicrobial property, ability to induce new bone formation and effective in reducing fracture risk in osteoporosis [12]. Considerable related studies have been conducted on the incorporation of Sr ions into CaP because of the beneficial effects of the presence of Sr in bone. Sr2+ in calcium phosphate coatings can replace Ca2+ easily, which inhibits bone absorption and improves bone formation. Moreover, the presence of Sr can improve the cell activity in vitro and in vivo studies [13-15]. Mg also participates in important functions of body. Some studies have confirmed that Mg-substituted CaP coatings have better biocompatibility and bioactivity and during the bone repair surgery, the damage body bone will recover faster and more efficient [16, 17]. As a result, the incorporation of the trace elements in CaP to make a composite coating is an effective way to improve the bioactivity and avoid infection. Motivated by this, Sr and Mg substituted DCPD (SM-DCPD) coatings for C/C composites are prepared by pulsed electrodeposition in this work. Compared with the electrodeposition, pulsed electrodeposition can effectively suppress hydrogen generation and achieve a homogenous coating by adjusting the pulse parameters during the deposition process [12, 18, 19]. The DCPD coatings for C/C composites prepared by pulsed electrodeposition are a control experiment. And their microstructures, biological activity, corrosion behaviors in simulated body fluid (SBF) were experimentally investigated. 2. Materials and Methods The C/C composites, which were used as substrate, used in this work were prepared by chemical vapor infiltration (CVI) process and have an average density of 1.7 g/cm3. The C/C composites were cut into square slabs with a dimension of 8 × 8 × 2 mm and polished with 1000 grit SiC paper. Then they were cleaned ultrasonically firstly in acetone, then in ethanol, finally in distilled water. The electrolytes were prepared by mixing a solution of 66.8 mmol/L Ca(NO3)2·4H2O, 66.8 mmol/L Sr(NO3)2, 66.8 mmol/L Mg(NO3)2·6H2O and 40 mmol/L (NH4)2HPO4. Depositions were carried out in a constant voltage mode with a voltage of 2.5 V at 50 °C, the pulse on and off time were set as 200 ms and 300 ms. The whole deposition time was 3 h. The crystalline of the coated C/C composites were measured by X-ray diffraction (XRD, PANalytical X’Pert PRO, Netherlands) with a Cu Kα radiation (λ = 0.1542 nm) with a step size of 0.033°. The compositions of the coated C/C composites were characterized by an Axis Ultra X-ray photoelectron spectroscopy (XPS, Kratos, Manchester, UK) with an Al kα X-ray source. The surface morphology and element composition of the coated C/C composites were analyzed by scanning electron microscopy (SEM, TESCAN VEGA3, Czech) equipped with energy dispersive spectroscopy (EDS). The in vitro evaluation was performed by soaking each specimen into the simulated body fluid
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(SBF). It was prepared by dissolving regent grade chemicals of 7.996 g/L NaCl, 0.350 g/L NaHCO3, 0.224 g/L KCl,0.228 g/L K2HPO4·3H2O, 0.350 g/L MgCl2·6H2O, 0.278 g/L CaCl2, 0.071 g/L Na2SO4 and 6.057 g/L (CH2OH)3CNH2 into distilled water, and buffering at pH 7.40 with hydrochloric acid. The detailed information of the SBF test could be found elsewhere [5]. In this paper, the coated C/C composites were soaking in the centrifugal tube with 5 ml SBF solution at room temperature, and the SBF was refreshed every day. The samples were removed from the SBF after 1, 3, 7 and 14 days respectively. Finally the samples were analyzed by SEM equipped with EDS. To evaluate the corrosion resistance of coatings, polarization test in SBF was carried out on electrochemical workstation (CS310 Corrtest, China). The samples were covered by epoxy resin and only the coating area 8 × 8 mm2 was exposed to the SBF solution. After a stabilization period of 15 min, Potentiodynamic polarization studies were performed in a potential range of -0.4-0.2 V at a scan rate of 0.005 V/s. 3. Results and discussion The XRD patterns of the SM-DCPD and DCPD coated C/C composites are depicted in Fig. 1a and Fig. 1b, respectively. Form the XRD analysis, at 2θ values of 26.543° was the carbon (002) peak generating from the C/C composite substrates. The main diffraction peaks for DCPD are observed at 2θ values of 11.604°, 20.786°, 29.160°, 30.485°, 31.249°, 34.066°, 35.425°. Whereas compared with DCPD coatings the 2θ values of SM-DCPD coatings experience a slight shift which may due to crystal lattice distortion that is occurred as the result of substitution of Sr and Mg in DCPD. Furthermore, the SM-DCPD coatings show the strong diffraction peaks associated with DCPD. This is because the introduction of Sr and Mg ions, leading to the molar ratio of Ca/P in the electrolyte is similar to the theoretical value of DCPD. From the Raman analysis (Fig. 2), four characteristic Raman peaks for PO43- groups are found, including υ1, υ2, υ3, υ4 of PO43- mode locating at 984 cm-1, 413 cm-1, 1053 cm-1 and 580 cm-1, respectively. The peak at 875 cm-1 is a typical feature of crystalline DCPD, which is attributed to the P-OH vibration of HPO42- mode. The Raman spectrum also shows the presence of carbon peaks, which generate from the C/C composite substrates, including D peak at 1359 cm-1 and G band at 1585 cm-1. XPS and EDS characterization are carried out on SM-DCPD coated C/C composites and DCPD coated C/C composites. For XPS spectra (Fig. 3b), peaks corresponding to O1s (531.6ev), Ca2s (428.8eV), Ca2p (351.1eV), C1s (285eV), P2s (191.1eV), P2p (133.6eV), Ca3p (25eV) are all clearly significantly. And at the location of 280.1eV Sr3p peak is found in the SM-DCPD coatings in Fig. 3a. The P2p peak is centered at 133.6 eV, as expected for phosphate groups, and Ca 2p at 351.1 eV, as expected for Ca2+. In Fig. 4a, EDS analysis show that the SM-DCPD coatings are mainly composed of O, Ca, Sr, Mg and P elements with Ca/P molar ratios 1.06. Fig. 4b shows the Ca/P molar ratios of DCPD coatings is 1.09, which is similar to the theoretical value of DCPD. Fig. 5 shows the surface micrographs of the DCPD and SM-DCPD coated C/C composites, (a) and (c) are the micrograph of the DCPD, (b) and (d) are the micrograph of the SM-DCPD. By low magnification, the two types of the coating are uniform and continuous. By high magnification, the two types of the coatings present a flake-like morphology and the crystal is approximately 3-5 µm in thickness and 10-20 µm in length. This kind of apatite is similar to the main components of the bone, which is likely to be beneficial for promoting the formation of new bone tissue and accelerating the healing of the damaged bones [20].
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The potentiodynamic polarization curve of the samples in SBF solution is shown in Fig. 6 and the corresponding electrochemical data are given in Table 1. It is obvious that the corrosion potential (Ecorr) value of the SM-DCPD coated C/C composites shift towards active direction. Meanwhile, compared with the uncoated C/C composites, the corrosion current density (Icorr) is also decreased dramatically. The Ecorr of the SM-DCPD coated C/C composites are 0.00183V, which is significantly higher than that of uncoated ones (-0.26952V). The coatings taking calcium and phosphate form the surrounding SBF for the formation of apatite on its surface is an important indication of implant osseointegration. In this respect, the SBF tests are applied to study the apatite formation ability of both DCPD and SM-DCPD coatings. After soaking in SBF for 1, 3, 7, 14 days, the surface morphology of the coatings are studied by SEM. Fig. 7 illustrates the formation of a new apatite layer on the two types of coatings. In the SBF tests, many overlapped small spheres formed calcium phosphate layer on the surface of DCPD coatings with some cracks. However, SM-DCPD coatings show the different formation process. Soaking in SBF for 1, 3, 7 days, the flake-like morphology begin to dissolve with the formation of calcium phosphate. After soaking in SBF for 14 days, the SM-DCPD coatings are fully covered by a dense calcium phosphate layer in the end. The results correspond to Sr2+and Mg2+ implantion that could promote the nucleation and growth of DCPD on C/C composites form SBF and enhance the bioactivity [21-23]. The molar ratios of Ca/P for the DCPD and SM-DCPD coatings after immersion in SBF are shown in Fig. 8. The EDS analysis show that the new layers are mainly composed of O, Ca and P elements. The Ca/P molar ratios of DCPD coatings (curve b in Fig. 8) range from 1.16 to 1.42 and the Ca/P molar ratios of SM-DCPD coatings (curve a in Fig. 8) range from 1.10 to 1.35. 4. Conclusion SM-DCPD coatings for C/C composites were synthesized by pulsed electrodeposition. The coatings exhibit a flake-like shape with a uniform and compact structure. The SBF tests show that the coatings could induce the formation of apatite layers on their surface in simulated body fluid. The incorporation of Sr and Mg in DCPD is an effective way to improve the bonding with bone. In the electrochemical test, the SM-DCPD coated C/C composites obtain a more positive potential and lower anodic current density than that of uncoated ones, which indicates that the SM-DCPD coating can evidently decrease the corrosion rate of the C/C composites in SBF. The SM-DCPD coated C/C composites could apply on the orthopedic field. Acknowledgements This work was supported by the National Natural Science Foundation of China under Grant No. 51202194, the FP7-International Research Staff Exchange Scheme-Advanced Biomaterials for Regenerative Medicine, the Global Innovation Initiative-Nanostructured Materials for the Control of Contaminants Detrimental to Health, the Fundamental Research Funds for the Central Universities under Grant No. 3102014JCQ01030 and “111” project of china (B08040). References L.L. Zhang, H.J. Li, K.Z. Li, S.Y. Zhang, J.H. Lu, W. Li, Appl. Surf. Sci. 286 (2013) 421–427. L.L. Zhang, H.J. Li, Q. Song, K.Z. Li, Surf. Interface Anal. 46 (2014) 24–29. Y.Q. Zhai, K.Z. Li, H.J. Li, Mater. Chem. Phys. 106 (2007) 22–26. L.L. Zhang, H.J. Li, K.Z. Li, S.Y. Zhang, Ceram. Int. 41 (2015) 427–435. H.J. Li, X.N. Zhao, S. Cao, K.Z. Li, Appl. Surf. Sci. 263 (2012) 163–173.
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Fig. 1 XRD patterns of the SM-DCPD (a) and DCPD (b) coated C/C composites. Fig. 2 Raman spectrum of the SM-DCPD (a) and DCPD (b) coated C/C composites. Fig. 3 XPS spectra of the SM-DCPD (a) and DPCD (b) coated C/C composites. Fig. 4 EDS analysis of SM-DCPD (a) and DCPD (b) coated C/C composites. Fig. 5 SEM images of DCPD (a and c) and SM-DCPD (b and d) coated C/C composites. Fig. 6 Electrochemical polarization curves of the uncoated and SM-DCPD coated C/C composites in SBF. Fig. 7 SEM images of the DCPD coatings after immersion in SBF for (a) 1, (c) 3, (e) 7, (g) 14 days and SM-DCPD coatings after immersion in SBF for (b) 1, (d) 3, (f) 7, (h) 14 days. Fig. 8 The molar ratios of Ca/P for the SM-DCPD (a) and DCPD (b) coatings after immersion in SBF.
Table 1 Electrochemical parameters of the electrochemical test Samples
Corrosion potential Ecorr (mV)
Corrosion current Icorr (μA/cm2)
uncoated SM-DCPD coated
-269.5±16.2 1.8±1.3
1.83±0.05 0.07±0.04
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