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Electrochemical property and in vitro degradation of DCPD–PCL composite coating on the biodegradable Mg–Zn alloy
Materials Letters 68 (2012) 435–438
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Electrochemical property and in vitro degradation of DCPD–PCL composite coating on the biodegradable Mg–Zn alloy Hongju Wang, Changli Zhao, Ying Chen, Jianan Li, Xiaonong Zhang ⁎ State Key Laboratory of Metal Matrix Composites, Institute of Biomedical Materials, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China
a r t i c l e
i n f o
Article history: Received 27 August 2011 Accepted 5 November 2011 Available online 13 November 2011 Keywords: DCPD PCL Coating Mg Corrosion Biomaterials
a b s t r a c t In this study, dicalcium phosphate dihydrate (DCPD) and polycaprolactone (PCL) were successively prepared on a Mg–Zn alloy to obtain a composite coating. Electrochemical measurement showed that the corrosion potential (Ecorr) of the DCPD–PCL coated alloy increased by 0.14 V compared to that of the DCPD coated alloy. The corrosion current (icorr) was about one third of the DCPD coated alloy. The real impedance (Zre) of the DCPD–PCL coated alloy was approximately 4 times as large as that of the DCPD coated one. The immersion test implied that the integrity of the DCPD–PCL composite coating kept better in the m-SBF after 260 h immersion than the DCPD coating. The less released hydrogen indicated that the degradation rate was reduced compared to the DCPD coated alloy. The degradation rate rapidly increased after 150 h for the DCPD coated alloy while 225 h for the DCPD–PCL coated case which meant the composite coating could retard the corrosion for a longer time. Therefore, the composite coating can protect the Mg alloy more efficiently. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Magnesium and its alloys have drawn much attention as promising biodegradable orthopedic implant materials [1,2]. However, they degrade so fast that they will lose the mechanical integrity before the healing of tissues [3]. Surface modification is regarded as an effective way to reduce the degradation rate of these Mg alloys. Ca–P coatings cannot only retard the degradation of Mg alloys in physiological conditions but also stimulate new bone formation [4–8]. However, our previous study [8] also shows that most of the Ca–P coatings are so fragile that they tend to peel off in the SBF. Moreover, the structure of the Ca–P coatings is heterogeneous and the rate of peeling off at some weakly adhered places is much larger than the average. The solution penetrates the coating at these places and corrodes the matrix even though most of the coating hasn't peeled off. So it is the fragility and the structural heterogeneity that makes the Ca–P coating lose its integrity and thus a lot of corrosion resistance. Some polymer coatings can raise the resistance and the corrosion potential of the alloy in m-SBF [9,10]. The polymer coating could reduce the corrosion rate of the Mg alloys in the initial stage of the degradation test. However, in the later stage, there was an interaction between the polymer coating and the magnesium matrix which undermined the corrosion resistance [10]. In this paper, a ceramic–polymer composite coating was prepared on the Mg–Zn alloy, expecting to avoid the disadvantages of a single
Ca–P or polymer coating. Polycaprolactone (PCL) was chosen hoping that its good plasticity and uniform structure can help maintain the integrity of dicalcium phosphate dihydrate (CaHPO4·2H2O, DCPD) coating longer. In return, the DCPD coating can avoid the direct contact of the PCL and the magnesium matrix so as to avoid their negative interaction. Furthermore, the PCL with good biocompatibility makes the composite coating be a promising drug carrier [11,12]. 2. Materials and methods 2.1. The preparation of the DCPD–PCL composite coating The Mg–6 wt.% Zn alloy described in Ref. [13] was cut into disks with a diameter of 11.3 mm and a height of 2 mm. Then they were ground with SiC paper up to 1000 grits and electropolished. The DCPD coating was synthesized by electrodeposition. The electrolyte was prepared by dissolving 0.042 mol/l Ca(NO3)2·4H2O and 0.025 mol/1 NH4H2PO4 in deionized water. 10 ml/l H2O2 was added into the electrolyte and the pH was adjusted to 4.4. Platinum was used as the anode and the Mg–Zn disk as the cathode. The DCPD coating was deposited at 30 °C and the current density was controlled at 3–4 mA/cm 2 throughout the electrodeposition. The DCPD coated samples were immersed into the 2 wt.% PCL chloroform solution for 30 s. Then they were taken out slowly and placed in the air for 24 h. 2.2. Characterization of coatings
⁎ Corresponding author. Tel. /fax: + 86 21 3420 2759. E-mail address: [email protected] (X. Zhang). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.11.029
The surface morphologies of DCPD and DCPD–PCL composite coatings were examined by scanning electronic microscope (SEM, QUANTA250,
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H. Wang et al. / Materials Letters 68 (2012) 435–438
Table 1 Compositions of different coatings according to EDS (the minor constituent Mg, Zn not listed). Chemical composition
C Wt.%
At.%
O Wt.%
At.%
P Wt.%
At.%
Ca Wt.%
At.%
DCPD coating DCPD–PCL coating DCPD coating after immersion DCPD–PCL coating after immersion
3.33 46.43 3.84
5.59 58.88 6.74
57.57 34.70 45.76
72.47 33.03 60.27
15.55 8.19 15.55
10.11 4.03 10.58
23.55 10.69 20.29
11.84 4.06 10.67
37.46
50.33
36.56
36.88
10.00
5.21
11.58
4.66
FEI). The crystalline structures of these coatings were characterized by X-ray diffraction (XRD, D/MAX255, Rigaku). The XRD data were collected from 10° to 80° (2θ) using Kα radiation at the step of 0.02° with the voltage of 35 kV. 2.3. In vitro degradation test 2.3.1. Electrochemical measurement The electrochemical characteristics of these coatings were researched on an electrochemical workstation (CHI 660D, Shanghai Chenhua Instruments Co., China) with a three electrodes system in m-SBF [14] at 37 °C in a water bath. A platinum mesh and a saturated calomel electrode were used as the counter and the reference electrodes, respectively. Specimens were embedded in epoxy resin and exposed with a surface area of 1 cm 2. Potentiodynamic polarization curves were measured at a scan rate of 10 mV/s. Electrochemical impedance spectroscopy (EIS) analysis was also performed at open circuit potential with a perturbing signal of 5 mV. The frequency varied from 1 to 10,000 Hz. 2.3.2. In vitro immersion test Specimens were immersed in 150 ml m-SBF solution in polythene vessels. The surface morphologies and compositions of the different
Fig. 2. PD (a) and EIS (b) of DCPD and DCPD–PCL coated Mg–Zn alloy.
samples after immersion were examined by SEM equipped with Energy Dispersive Spectrometer (EDS). The volume of the hydrogen in inverted graduated pipettes and the pH of the solution were monitored to evaluate the corrosion properties. All the experiments were performed on three duplicate samples.
Fig. 1. XRD results of DCPD and DCPD–PCL coatings on Mg–Zn (a), surface morphologies of DCPD (b), DCPD–PCL (c) coated Mg–Zn and the cross section of the DCPD–PCL coated Mg–Zn (d).
H. Wang et al. / Materials Letters 68 (2012) 435–438 Table 2 The results of electrochemical measurement. Coating
Ecorr/V
icorr/(μA × cm− 2)
DCPD DCPD–PCL
− 1.44 − 1.30
8.43 2.63
3. Results and discussion 3.1. Structures and morphologies of DCPD and DCPD–PCL coatings The EDS results (Table 1) showed that there was more C and less Ca and P on the DCPD–PCL coating than DCPD. The XRD results (Fig. 1a) of the DCPD–PCL coated Mg–Zn alloy were quite similar with the DCPD coated alloy, showing typical peaks of DCPD and magnesium, except for the one more peak at 2θ = 21.4°, which signified the existence of the PCL. Fig. 1b manifested that the DCPD coating had a flake-like structure. The flakes were about 5–10 μm in width and 40–60 μm in length. The flake-like structure of the DCPD–PCL coating was vague (Fig. 1c), which was probably caused by the fact that the DCPD flakes were covered by the PCL coating. The morphology of the cross section clearly showed the existence of the outer PCL coating (Fig. 1d). The thickness of the DCPD coating and the PCL coating was about 70 μm and 20 μm, respectively. 3.2. In vitro degradation tests 3.2.1. Electrochemical measurement Fig. 2a showed the polarization curves of the specimens and Table 2 listed the Ecorr and icorr obtained from Tafel region. The Ecorr of DCPD–PCL coated Mg–Zn increased by 0.14 V with reference to the DCPD coated case indicating that the former was less susceptible to corrosion. The icorr of the DCPD–PCL coated Mg–Zn was about one
437
third of that of the DCPD, signifying the degradation was slower. The Nyquist diagram (Fig. 2b) demonstrated that the Zre of the DCPD–PCL coated Mg–Zn alloy reached about 3000 Ω, approximately 4 times of that of the DCPD coated alloy. It can be deduced that the DCPD–PCL coating had better corrosion resistance than the DCPD. 3.2.2. Immersion experiment The surface morphologies of the alloy coated with DCPD and DCPD–PCL after 260 h immersion were illustrated in Fig. 3. It was observed that there were many cracks on the DCPD coating and the matrix had been exposed somewhere (Fig. 3a). For the composite coating, the intact and vague flakes-like structure suggested the integrity of the DCPD and PCL coating kept better after immersion (Fig. 3b). The much more C on the surface of the DCPD–PCL than that on the DCPD coated case also signified the persistence of the PCL (Table 1). The released hydrogen (Fig. 3c) implied the degradation rate of the DCPD coated Mg–Zn rapidly increased after 150 h immersion. Similar results about the protective function of the DCPD on the bare Mg–Zn were reported by Song et al. [8]. The released hydrogen of the DCPD–PCL coated alloy was about half of the DCPD coated alloy in the first 100 h when the PCL probably played a leading role in holding back the solution from the matrix effectively. The degradation rate increased a little from 100 h as more solution penetrated the defect of PCL coating that formed in the earlier stage and arrived at the DCPD coating. The degradation rate further increased after 225 h. In general, there was a pH increase in the medium that corresponded with the released hydrogen (Fig. 3d). The pH for DCPD was higher than that for DCPD–PCL, but not by a lot as a result of the buffer of CaHPO4·2H2O. The DCPD was fragile that it tended to peel off in the solution. Moreover, the rate of peeling off was quite different at different places because of its inhomogenous structure. The matrix was even
Fig. 3. Surface morphologies after immersion of the DCPD (a) and the DCPD–PCL (b) coated Mg–Zn alloy. The released hydrogen (c) and the pH of the solution (d) during immersion in m-SBF.
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exposed where the DCPD peeled off most quickly. Cracks also occurred due to the brittleness of the DCPD. The ions penetrated the coating at these defects and degraded the matrix quickly. Overall, the DCPD coating lost its integrity and then much protective function due to its fragility and the structural heterogeneity. However, the PCL coating with good toughness and uniform structure helped to maintain the integrity of the DCPD so that the composite coating can protect the matrix for a longer time. It can also reduce the degradation rate because it can make less ions arrived at the DCPD. Therefore, the DCPD–PCL composite coating performed better than the DCPD coating in protecting the matrix. 4. Conclusion A novel composite coating of DCPD–PCL was prepared successfully on Mg–Zn alloy. The DCPD–PCL coated alloy had better electrochemical performance than the DCPD coated case. The immersion tests signified that the DCPD–PCL coating maintained its integrity better in m-SBF and reduced the corrosion rate more effectively compared to the DCPD coating. It is suggested that the DCPD–PCL composite coating is a promising choice to protect the Mg alloys. Acknowledgments The authors are also grateful for the supports of the National Basic Research Program of China (973 Program) (No. 2012CB619102),
“863” High-tech Plan of China (No. 2009AA03Z424), the Natural Science Foundation of China (No. 30901422), Shanghai Jiao Tong University Interdisciplinary Research Grant (Nos. YG2010MS45 and YG2010MS46) and the Opening Project of Shanghai Key Laboratory of Orthopaedic Implant (KFKT2011002).
References [1] Witte F, Kaese V, Haferkamp H, Switzer E, Meyer A, Wirth CJ, et al. Biomaterials 2005;26(17):3557–63. [2] Zhang EL, Xu LP, Yu GN, Pan F, Yang K. J Biomed Mater Res A 2009;90A(3):882–93. [3] Mani G, Feldman MD, Patel D, Agrawal CM. Biomaterials 2007;28(9):1689–710. [4] Xin YC, Huo KF, Tao H, Tang GY, Chu PK. Acta Biomater 2008;4(6):2008–15. [5] Song YW, Shan DY, Han EH. Mater Lett 2008;62(17–18):3276–9. [6] Klein CPAT, Patka Pv, Wolke JGC, Groot Kd. J Biomed Mater Res 1991;25:53–65. [7] Zhang SX, Li JN, Song Y, Zhao CL, Zhang XN. Front Mater Sci Chin 2010;4(2):116–9. [8] Song Y, Zhang SX, Li JN, Zhao CL, Zhang XN. Acta Biomater 2010;6(5):1736–42. [9] Wong HM, Yeung KWK, Lam KO, Tam V, Chu PK, Luk KDK, et al. Biomaterials 2010;31(8):2084–96. [10] Chen Y, Song Y, Zhang SX, Li JN, Zhao CL, Zhang XN. Biomed Mater 2011;6:025005. [11] Sawyer AA, Song SJ, Susanto E, Chuan P, Lam CXF, Woordruff MA, et al. Biomaterials 2009;30(13):2479–88. [12] Kim HW, Knowles JC, Kim HE. Biomaterials 2004;25(7–8):1279–87. [13] Zhang SX, Zhang XN, Zhao CL, Li JN, Song Y, Xie CY, et al. Acta Biomater 2010;6(2): 626–40. [14] Oyane A, Kim HM, Furuya T, Kokubo T, Miyazaki T, Nakamura T. J Biomed Mater Res A 2003;65A(2):188–95.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Electrochemical property and in vitro degradation of DCPD–PCL composite coating on the biodegradable Mg–Zn alloy Hongju Wang, Changli Zhao, Ying Chen, Jianan Li, Xiaonong Zhang ⁎ State Key Laboratory of Metal Matrix Composites, Institute of Biomedical Materials, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China
a r t i c l e
i n f o
Article history: Received 27 August 2011 Accepted 5 November 2011 Available online 13 November 2011 Keywords: DCPD PCL Coating Mg Corrosion Biomaterials
a b s t r a c t In this study, dicalcium phosphate dihydrate (DCPD) and polycaprolactone (PCL) were successively prepared on a Mg–Zn alloy to obtain a composite coating. Electrochemical measurement showed that the corrosion potential (Ecorr) of the DCPD–PCL coated alloy increased by 0.14 V compared to that of the DCPD coated alloy. The corrosion current (icorr) was about one third of the DCPD coated alloy. The real impedance (Zre) of the DCPD–PCL coated alloy was approximately 4 times as large as that of the DCPD coated one. The immersion test implied that the integrity of the DCPD–PCL composite coating kept better in the m-SBF after 260 h immersion than the DCPD coating. The less released hydrogen indicated that the degradation rate was reduced compared to the DCPD coated alloy. The degradation rate rapidly increased after 150 h for the DCPD coated alloy while 225 h for the DCPD–PCL coated case which meant the composite coating could retard the corrosion for a longer time. Therefore, the composite coating can protect the Mg alloy more efficiently. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Magnesium and its alloys have drawn much attention as promising biodegradable orthopedic implant materials [1,2]. However, they degrade so fast that they will lose the mechanical integrity before the healing of tissues [3]. Surface modification is regarded as an effective way to reduce the degradation rate of these Mg alloys. Ca–P coatings cannot only retard the degradation of Mg alloys in physiological conditions but also stimulate new bone formation [4–8]. However, our previous study [8] also shows that most of the Ca–P coatings are so fragile that they tend to peel off in the SBF. Moreover, the structure of the Ca–P coatings is heterogeneous and the rate of peeling off at some weakly adhered places is much larger than the average. The solution penetrates the coating at these places and corrodes the matrix even though most of the coating hasn't peeled off. So it is the fragility and the structural heterogeneity that makes the Ca–P coating lose its integrity and thus a lot of corrosion resistance. Some polymer coatings can raise the resistance and the corrosion potential of the alloy in m-SBF [9,10]. The polymer coating could reduce the corrosion rate of the Mg alloys in the initial stage of the degradation test. However, in the later stage, there was an interaction between the polymer coating and the magnesium matrix which undermined the corrosion resistance [10]. In this paper, a ceramic–polymer composite coating was prepared on the Mg–Zn alloy, expecting to avoid the disadvantages of a single
Ca–P or polymer coating. Polycaprolactone (PCL) was chosen hoping that its good plasticity and uniform structure can help maintain the integrity of dicalcium phosphate dihydrate (CaHPO4·2H2O, DCPD) coating longer. In return, the DCPD coating can avoid the direct contact of the PCL and the magnesium matrix so as to avoid their negative interaction. Furthermore, the PCL with good biocompatibility makes the composite coating be a promising drug carrier [11,12]. 2. Materials and methods 2.1. The preparation of the DCPD–PCL composite coating The Mg–6 wt.% Zn alloy described in Ref. [13] was cut into disks with a diameter of 11.3 mm and a height of 2 mm. Then they were ground with SiC paper up to 1000 grits and electropolished. The DCPD coating was synthesized by electrodeposition. The electrolyte was prepared by dissolving 0.042 mol/l Ca(NO3)2·4H2O and 0.025 mol/1 NH4H2PO4 in deionized water. 10 ml/l H2O2 was added into the electrolyte and the pH was adjusted to 4.4. Platinum was used as the anode and the Mg–Zn disk as the cathode. The DCPD coating was deposited at 30 °C and the current density was controlled at 3–4 mA/cm 2 throughout the electrodeposition. The DCPD coated samples were immersed into the 2 wt.% PCL chloroform solution for 30 s. Then they were taken out slowly and placed in the air for 24 h. 2.2. Characterization of coatings
⁎ Corresponding author. Tel. /fax: + 86 21 3420 2759. E-mail address: [email protected] (X. Zhang). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.11.029
The surface morphologies of DCPD and DCPD–PCL composite coatings were examined by scanning electronic microscope (SEM, QUANTA250,
436
H. Wang et al. / Materials Letters 68 (2012) 435–438
Table 1 Compositions of different coatings according to EDS (the minor constituent Mg, Zn not listed). Chemical composition
C Wt.%
At.%
O Wt.%
At.%
P Wt.%
At.%
Ca Wt.%
At.%
DCPD coating DCPD–PCL coating DCPD coating after immersion DCPD–PCL coating after immersion
3.33 46.43 3.84
5.59 58.88 6.74
57.57 34.70 45.76
72.47 33.03 60.27
15.55 8.19 15.55
10.11 4.03 10.58
23.55 10.69 20.29
11.84 4.06 10.67
37.46
50.33
36.56
36.88
10.00
5.21
11.58
4.66
FEI). The crystalline structures of these coatings were characterized by X-ray diffraction (XRD, D/MAX255, Rigaku). The XRD data were collected from 10° to 80° (2θ) using Kα radiation at the step of 0.02° with the voltage of 35 kV. 2.3. In vitro degradation test 2.3.1. Electrochemical measurement The electrochemical characteristics of these coatings were researched on an electrochemical workstation (CHI 660D, Shanghai Chenhua Instruments Co., China) with a three electrodes system in m-SBF [14] at 37 °C in a water bath. A platinum mesh and a saturated calomel electrode were used as the counter and the reference electrodes, respectively. Specimens were embedded in epoxy resin and exposed with a surface area of 1 cm 2. Potentiodynamic polarization curves were measured at a scan rate of 10 mV/s. Electrochemical impedance spectroscopy (EIS) analysis was also performed at open circuit potential with a perturbing signal of 5 mV. The frequency varied from 1 to 10,000 Hz. 2.3.2. In vitro immersion test Specimens were immersed in 150 ml m-SBF solution in polythene vessels. The surface morphologies and compositions of the different
Fig. 2. PD (a) and EIS (b) of DCPD and DCPD–PCL coated Mg–Zn alloy.
samples after immersion were examined by SEM equipped with Energy Dispersive Spectrometer (EDS). The volume of the hydrogen in inverted graduated pipettes and the pH of the solution were monitored to evaluate the corrosion properties. All the experiments were performed on three duplicate samples.
Fig. 1. XRD results of DCPD and DCPD–PCL coatings on Mg–Zn (a), surface morphologies of DCPD (b), DCPD–PCL (c) coated Mg–Zn and the cross section of the DCPD–PCL coated Mg–Zn (d).
H. Wang et al. / Materials Letters 68 (2012) 435–438 Table 2 The results of electrochemical measurement. Coating
Ecorr/V
icorr/(μA × cm− 2)
DCPD DCPD–PCL
− 1.44 − 1.30
8.43 2.63
3. Results and discussion 3.1. Structures and morphologies of DCPD and DCPD–PCL coatings The EDS results (Table 1) showed that there was more C and less Ca and P on the DCPD–PCL coating than DCPD. The XRD results (Fig. 1a) of the DCPD–PCL coated Mg–Zn alloy were quite similar with the DCPD coated alloy, showing typical peaks of DCPD and magnesium, except for the one more peak at 2θ = 21.4°, which signified the existence of the PCL. Fig. 1b manifested that the DCPD coating had a flake-like structure. The flakes were about 5–10 μm in width and 40–60 μm in length. The flake-like structure of the DCPD–PCL coating was vague (Fig. 1c), which was probably caused by the fact that the DCPD flakes were covered by the PCL coating. The morphology of the cross section clearly showed the existence of the outer PCL coating (Fig. 1d). The thickness of the DCPD coating and the PCL coating was about 70 μm and 20 μm, respectively. 3.2. In vitro degradation tests 3.2.1. Electrochemical measurement Fig. 2a showed the polarization curves of the specimens and Table 2 listed the Ecorr and icorr obtained from Tafel region. The Ecorr of DCPD–PCL coated Mg–Zn increased by 0.14 V with reference to the DCPD coated case indicating that the former was less susceptible to corrosion. The icorr of the DCPD–PCL coated Mg–Zn was about one
437
third of that of the DCPD, signifying the degradation was slower. The Nyquist diagram (Fig. 2b) demonstrated that the Zre of the DCPD–PCL coated Mg–Zn alloy reached about 3000 Ω, approximately 4 times of that of the DCPD coated alloy. It can be deduced that the DCPD–PCL coating had better corrosion resistance than the DCPD. 3.2.2. Immersion experiment The surface morphologies of the alloy coated with DCPD and DCPD–PCL after 260 h immersion were illustrated in Fig. 3. It was observed that there were many cracks on the DCPD coating and the matrix had been exposed somewhere (Fig. 3a). For the composite coating, the intact and vague flakes-like structure suggested the integrity of the DCPD and PCL coating kept better after immersion (Fig. 3b). The much more C on the surface of the DCPD–PCL than that on the DCPD coated case also signified the persistence of the PCL (Table 1). The released hydrogen (Fig. 3c) implied the degradation rate of the DCPD coated Mg–Zn rapidly increased after 150 h immersion. Similar results about the protective function of the DCPD on the bare Mg–Zn were reported by Song et al. [8]. The released hydrogen of the DCPD–PCL coated alloy was about half of the DCPD coated alloy in the first 100 h when the PCL probably played a leading role in holding back the solution from the matrix effectively. The degradation rate increased a little from 100 h as more solution penetrated the defect of PCL coating that formed in the earlier stage and arrived at the DCPD coating. The degradation rate further increased after 225 h. In general, there was a pH increase in the medium that corresponded with the released hydrogen (Fig. 3d). The pH for DCPD was higher than that for DCPD–PCL, but not by a lot as a result of the buffer of CaHPO4·2H2O. The DCPD was fragile that it tended to peel off in the solution. Moreover, the rate of peeling off was quite different at different places because of its inhomogenous structure. The matrix was even
Fig. 3. Surface morphologies after immersion of the DCPD (a) and the DCPD–PCL (b) coated Mg–Zn alloy. The released hydrogen (c) and the pH of the solution (d) during immersion in m-SBF.
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H. Wang et al. / Materials Letters 68 (2012) 435–438
exposed where the DCPD peeled off most quickly. Cracks also occurred due to the brittleness of the DCPD. The ions penetrated the coating at these defects and degraded the matrix quickly. Overall, the DCPD coating lost its integrity and then much protective function due to its fragility and the structural heterogeneity. However, the PCL coating with good toughness and uniform structure helped to maintain the integrity of the DCPD so that the composite coating can protect the matrix for a longer time. It can also reduce the degradation rate because it can make less ions arrived at the DCPD. Therefore, the DCPD–PCL composite coating performed better than the DCPD coating in protecting the matrix. 4. Conclusion A novel composite coating of DCPD–PCL was prepared successfully on Mg–Zn alloy. The DCPD–PCL coated alloy had better electrochemical performance than the DCPD coated case. The immersion tests signified that the DCPD–PCL coating maintained its integrity better in m-SBF and reduced the corrosion rate more effectively compared to the DCPD coating. It is suggested that the DCPD–PCL composite coating is a promising choice to protect the Mg alloys. Acknowledgments The authors are also grateful for the supports of the National Basic Research Program of China (973 Program) (No. 2012CB619102),
“863” High-tech Plan of China (No. 2009AA03Z424), the Natural Science Foundation of China (No. 30901422), Shanghai Jiao Tong University Interdisciplinary Research Grant (Nos. YG2010MS45 and YG2010MS46) and the Opening Project of Shanghai Key Laboratory of Orthopaedic Implant (KFKT2011002).
References [1] Witte F, Kaese V, Haferkamp H, Switzer E, Meyer A, Wirth CJ, et al. Biomaterials 2005;26(17):3557–63. [2] Zhang EL, Xu LP, Yu GN, Pan F, Yang K. J Biomed Mater Res A 2009;90A(3):882–93. [3] Mani G, Feldman MD, Patel D, Agrawal CM. Biomaterials 2007;28(9):1689–710. [4] Xin YC, Huo KF, Tao H, Tang GY, Chu PK. Acta Biomater 2008;4(6):2008–15. [5] Song YW, Shan DY, Han EH. Mater Lett 2008;62(17–18):3276–9. [6] Klein CPAT, Patka Pv, Wolke JGC, Groot Kd. J Biomed Mater Res 1991;25:53–65. [7] Zhang SX, Li JN, Song Y, Zhao CL, Zhang XN. Front Mater Sci Chin 2010;4(2):116–9. [8] Song Y, Zhang SX, Li JN, Zhao CL, Zhang XN. Acta Biomater 2010;6(5):1736–42. [9] Wong HM, Yeung KWK, Lam KO, Tam V, Chu PK, Luk KDK, et al. Biomaterials 2010;31(8):2084–96. [10] Chen Y, Song Y, Zhang SX, Li JN, Zhao CL, Zhang XN. Biomed Mater 2011;6:025005. [11] Sawyer AA, Song SJ, Susanto E, Chuan P, Lam CXF, Woordruff MA, et al. Biomaterials 2009;30(13):2479–88. [12] Kim HW, Knowles JC, Kim HE. Biomaterials 2004;25(7–8):1279–87. [13] Zhang SX, Zhang XN, Zhao CL, Li JN, Song Y, Xie CY, et al. Acta Biomater 2010;6(2): 626–40. [14] Oyane A, Kim HM, Furuya T, Kokubo T, Miyazaki T, Nakamura T. J Biomed Mater Res A 2003;65A(2):188–95.