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P ratio on the structural and corrosion properties of biomimetic CaP coatings on ZK60 magnesium alloy
Materials Science and Engineering C 72 (2017) 676–681
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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Effect of Ca/P ratio on the structural and corrosion properties of biomimetic Ca\\P coatings on ZK60 magnesium alloy Kada Xia a,1, Hui Pan b,1, Taolei Wang a, Shangjun Ma a, Junchao Niu a, Zhen Xiang a, Yiming Song a, Huawei Yang b,⁎, Xiaoshan Tang b,⁎, Wei Lu a,⁎ a b
School of Materials Science and Engineering, Shanghai Key Laboratory of D&A for Metal-Functional Materials, Tongji University, Shanghai 201804, China Shanghai Tenth People's Hospital, Tongji University, Shanghai 200072, China
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Article history: Received 19 July 2016 Received in revised form 24 October 2016 Accepted 28 November 2016 Available online 3 December 2016 Keywords: Magnesium alloy Ca-P coating Ca/P ratio Hydrothermal deposition Biodegradation
a b s t r a c t Magnesium and its alloys have attracted much attention as metallic biodegradable implants for their excellent biocompatibility and mechanical properties. However, magnesium has a poor corrosion resistance, causing its rapid degrading in vivo via an electrochemical reaction, which has become a major obstacle to their applications in implants. In this work, Ca\\P coating was successfully coated on the ZK60 magnesium alloys by a simple hydrothermal deposition method. The mechanisms of the hydrothermal reactions of Ca\\P coatings on Mg substrate are described in details. The effect of Ca/P ratio in the hydrothermal solution on the phase composition, microstructure and biodegradation properties of Ca\\P coatings on ZK60 alloys was investigated by varying the Ca/P ratio from 0.83 to 4.18. The morphology of the Ca\\P coating changed significantly with the Ca/P ratio. Biodegradation behavior of the Ca\\P coating magnesium was characterized by anodic polarization and immersion tests in a simulated body fluid. It is revealed that the corrosion resistance of ZK60 magnesium alloy was greatly improved with the biomimetic Ca\\P coatings, and the ZK60 alloy with Ca\\P coating deposited at Ca/P ratio of 1.67 has the best corrosion resistance, which indicates that the Ca\\P coatings are promising for improving the biodegradation properties of Mg-based orthopedic implants and devices. © 2016 Published by Elsevier B.V.
1. Introduction Recent years, magnesium alloys have attracted much attention as biodegradable implant materials because of their good biodegradability and biocompatibility [1–4] compared with the permanent metallic implant-materials, e.g. stainless steels, cobalt-chromium alloys and titanium alloys, which have been applied in clinical use for a long time. The density and mechanical properties of magnesium-based alloys are close to those of nature bones [5–7], reducing stress shielding effect between the implants and nature bones, which make magnesium and its alloys are suitable for implant applications including bone fracture plates, bone pegs, joint prosthesis, root implants and so on. However, magnesium has a poor corrosion resistance, causing its rapid degradation in vivo [8,9], which has become a major obstacle to their applications in implants. In order to be applied in clinics, suitable strategies are thus required to be developed to tailor the degradation rate of magnesium-based implants. On one hand, researchers studied from the perspective of the optimized manufacturing methods, including high purification, alloying, second phase refinement, grain ⁎ Corresponding authors. E-mail address: [email protected] (W. Lu). 1 These authors contribute equally to this paper.
http://dx.doi.org/10.1016/j.msec.2016.11.132 0928-4931/© 2016 Published by Elsevier B.V.
refinement, etc. [1,10–15]. However, these technologies cannot play a breakthrough role in decreasing the inhomogeneous corrosion of magnesium alloys, which makes it hard to reach a well match between degradation and rebuilding of the damaged bone tissues. On the other hand, researchers [16–19] investigated the surface modification technology which is to fabricate coatings on magnesium implants. And the results showed that surface modification, especially coating technology, is an effective way to improve the corrosion resistance, biocompatibility and bioactivity of Mg alloys based implants. As implant materials, the coatings should be non-poisonous and biocompatible, and can degrade in an appropriate rate. A board range of coating systems has been developed. As the main composition of bones, Ca\\P coatings have been paid more attentions due to their superior biocompatibility and corrosion resistance. The widely used fabricating methods of Ca\\P coatings for surface modification of Mg alloys include chemical conversion, cathodic electrodeposition, sol–gel coatings [20–26], etc. Hydrothermal deposition is a simple process and one of the most cost-effective techniques for the coating of metallic materials. It has been proved [27–29] to be a successful method in fabricating Ca\\P coatings on magnesium alloys, although it hasn't been widely studied. Sara et al. [30] reported that Ca\\P coatings with tricalcium phosphate and monetite phases have been deposited on magnesium alloy (AZ31) substrate by using hydrothermal
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process and the effect of different depositing temperatures on the structure of Ca\\P coatings is discussed. Hiromoto et al. [31] fabricated hydroxyapatite coating on AZ31 magnesium alloy using hydrothermal deposition and the growth mechanism of hydroxyapatite coating was discussed. Previously, we have reported the successful fabrication of Ca\\P coatings on Mg alloy by using hydrothermal process in aqueous solution with a low pH value [32,33]. The microstructure and properties of the resulted materials have been affected with the process parameters of fabrication significantly. In this paper, in order to optimize the fabricating procedure of hydrothermal deposition and corrosion resistance of Ca\\P coatings on magnesium alloys, the effect of Ca/P ratio of the hydrothermal solution on the phase composition, microstructure and in vitro corrosion properties of Ca\\P coatings on ZK60 alloys were investigated. ZK60 (Mg\\Zn\\Zr) magnesium alloys were chosen as the substrate for the reason that Zn and Zr elements have been shown with good biocompatibility [34,35].
2. Experimental 2.1. Sample preparation The magnesium alloy used in this study was ZK60 alloy, with the major alloying elements of approximately 5.5 wt% Zn and 0.5 wt% Zr. It was cut into rectangular samples with a size of 5 × 5 × 1 mm3. Each sample was mechanically polished with SiC papers up to 2000#, rinsed with deionized water, cleaned ultrasonically in ethanol and then dried in open air. The dried samples were etched in 90% (in volume) phosphoric acid solution at 55 °C for 15 s, followed by neutralization in 1 mol/L NaOH solution. Samples were then rinsed with distilled water and dried in air.
2.2. Hydrothermal deposition of biomimetic Ca\\P coating The Ca\\P coating was fabricated on the ZK60 alloys by hydrothermal deposition method. Treatment solution with various Ca/P ratio (0.83, 1.67, 2.5, 3.34, 4.18) was prepared using Ca(NO3)2·4H2O and NaH2PO4·2H2O. All chemicals were dissolved in distilled water. The concentration of both Ca(NO3)2·4H2O and NaH2PO4·2H2O was listed in Table 1. The pH value was adjusted to 4 by adding HCl solution. The solution was stirred at room temperature for about 20 min and then transferred to a Teflon-lined stainless steel pressure vessel (50 mL internal volume) with a filling factor of 75%. The pre-treated samples were set in the vessel with the main deposition side up and the vessel was heated in an electric oven in order to prepare the Ca\\P coating. The temperature and the reaction time were 140 °C and 2 h, respectively. The change in pH value of the bulk solution after the treatment was negligible. The samples were then removed from the solution, rinsed with deionized water, and dried in air.
2.3. Characterization 2.3.1. Microstructure Crystallographic structure of the coatings was characterized by Xray Diffraction (XRD). Field-emission scanning electronic microscopy (FESEM) was employed to examine the morphology of the coated samples.
Table 1 Composition of hydrothermal deposition solution. Ca/P ratio NaH2PO4·2H2O Ca(NO3)2·4H2O
0.83 0.03 mol/L 0.025 mol/L
1.67 0.03 mol/L 0.05 mol/L
2.5 0.03 mol/L 0.075 mol/L
3.34 0.03 mol/L 0.1 mol/L
4.18 0.03 mol/L 0.125 mol/L
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2.3.2. Corrosion resistance measurement Corrosion resistance of the coated samples was studied by two methods to characterize their degradation behavior: Electrochemical tests (polarization test) and immersion test. The polarization tests were carried out with CHI660C electrochemical workstation and the experiments were performed in a simulated body fluid (SBF) solution with a pH value of ~7.4 at 37 ± 0.5 °C which mimics the human body condition. The medium to sample area ratio is required to be no b200 mL/cm2 to avoid rapid increases in the local pH at the sample surface. A conventional three electrode was set up with a saturated calomel electrode (SCE) and platinum wire working as a reference and counter electrode, respectively. The area of the samples for working electrode was kept to 10 × 10 mm2 by wax-sealing the extra sides in advance. Prior to characterization, the samples were immersed in the solution for 20 min to establish the open-circuit potential (OCP), and subsequently, Tafel plot was obtained by polarization test with the potential swept from −2 V to 0 V in anodic direction under a sweep rate of 0.1 mV/s, for the corrosion potential of ZK60 alloys is around −1.60 V. The degradation behavior of samples was investigated by immersion test in the same SBF solution. The pH value was adjusted to 7.4 ± 0.1 and the temperature was kept at 37 ± 0.5 °C using a water bath. Before soaking in the solution, the masses of samples were measured by a balance. The pH value of the solution and the samples' mass (after drying) were recorded every 24 h during 1 month, with a blank SBF solution as control group. The SEM was employed to examine the morphology of the Ca\\P coatings before and after immersion tests. 3. Results and discussion 3.1. Phase composition The phase composition of samples coated with biomimetic Ca\\P coatings fabricated in solutions with different Ca/P ratio is shown in Fig. 1(a). The XRD pattern of the uncoated ZK60 substrate is also shown as comparison in Fig. 1(a). As shown in Fig. 1(a), the XRD spectrum of all five coated samples, there is a relatively high peak at around 2θ = 34°, with metallic Mg from the substrate ZK60 identified. Moreover, when Ca/P ratio = 0.83, Ca3(PO4)2 diffraction peaks were observed according to the standard card of Ca3(PO4)2 (Whitlokite, PDF 09-0169), and no other typical peaks such as TCP or CaO were detected in these patterns. However, with the increasing of Ca/P ratio, the layer products experienced transformation that the mainly component of the coatings changes from Ca3(PO4)2 to ADCP (Monetite). It can be seen from Fig. 1(a) that in the XRD spectrum of sample coated at Ca/P ratio = 1.67, 2.5, 3.34 and 4.18, there are typical high diffraction peaks at around 2θ = 26.5°, 30.6°, 33.0° corresponding to the (2 − 2 0), (2 −1 1), (1 0 2) crystal plane, respectively, that means [2 −2 0], [2 −1 1], [1 0 2] may be the preferred orientation for crystals to grow along, which indicates that the coatings are Monetite (ADCP, PDF 09-0080). In addition, there is no obvious difference for the XRD pattern among the sample with Ca/P ratio = 1.67, 2.5, 3.34 and 4.18. Furthermore, it should be noted that the peak intensity of Monetite (ADCP) with Ca/P ratio = 1.67 is found to be sharper and stronger than the other three, indicating that Monetite (ADCP) at this Ca/P ratio was better crystallized. In Fig. 1(b), it can be detected that the main peak at around 2θ = 26.5° is an overlapping of the (0 2 0) and (2 2 0) diffraction planes and the relative intensity of the two peaks changes with different Ca/P ratio. The increasing of Ca/P ratio contributes to the predominance of (2 2 0) diffraction plane. And, from Fig. 1(b), it can be clearly found that the peak at around 2θ = 26.5° of Ca/P ratio = 1.67 is more sharper and stronger than the others, suggesting that Monetite (ADCP) was better crystallized. The mechanisms of the hydrothermal reaction can be described as following. As the fresh magnesium alloy substrates are exposed to the hydrothermal reaction solution, the formation mechanism of calcium
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Fig. 1. (a) XRD patterns of ZK60 substrate and coated samples in solutions with different Ca/P ratio of 0.83, 1.67, 2.5, 3.34 and 4.18. (b) XRD patterns of the main peak at around 2θ = 26.5°.
phosphate coatings can be suggested as shown in Fig. 2. Anodic dissolution of magnesium occurs because of its instability at pH values b11 [36], which is accompanied with the cathode reaction of the hydrogen reduction and OH– release. As the pH value of the treatment solution is about 4, Mg ions are released and the local pH on the magnesium surface rapidly increases during the initial stage of the treatment. Meanwhile several hydrogen bubbles can be observed at the surface of the substrate samples. With the increasing of Ca/P ratio, comparatively more Ca ions are created for reaction, thus accelerating the deposition rate of Ca\\P coating and resulting in the increasing thickness of coatings. When Ca/P ratio is comparatively low (0.83), the coating obtained is quite thin, so the deposition area could be regarded as near-surface layer. The quite high pH value of the near-surface layer accelerates the conversion of PO23 −, and with the increasing of thickness of coatings due to the increasing of Ca/P ratio, the deposition area expands to a
Fig. 2. Formation mechanism of the Ca\ \P coating in hydrothermal deposition process.
far area from the magnesium surface, which has a lower pH value and contributes to the compounds of PO23 − and HPO24 −. On this trend, P ion exists as a form of completely HPO2− 4 (pH = 9) at a far area. Therefore, it can be suggested that Ca3(PO4)2 formed at the near-surface area while CaPO3OH formed at a area far from the surface, and the Ca/P ratio has a significant effect on the formation of calcium phosphates because of its active role in controlling the pH and producing hydroxyl ions. 3.2. Surface morphology of the Ca\\P coatings Fig. 3 presents the surface morphology of the coatings deposited from the hydrothermal solution with different Ca/P ratio of 0.83, 1.67, 2.5, 3.34 and 4.18. Ca\\P coatings are successfully fabricated on ZK60 magnesium alloys by hydrothermal deposition process. SEM characterization reveals the different morphology of the coated samples with the increasing Ca/P ratio. The left column shows the morphology with the low magnification (200 times) while the right shows the morphology of high magnification (1000 times). The coatings cover the magnesium substrates uniformly on the macro scale, while cracks and defects are hardly observed, indicating that the coatings are of high quality. The figures point to crack-free and uniform coatings with no observable defects. From Fig. 3(a), a dense film can be observed near the substrates and new bulk crystals continue to grow on the film. Fig. 3(b) presents the morphology of the coating deposited at solution with Ca/P ratio = 0.83, consisting of ball-like particles with a diameter of about 3– 5 μm, and a lot of ball-like particles closely stack to form a cluster structure with small pores and cracks on the surface. When the Ca/P ratio increases to 1.67, the coating obtained remains a similar ball-like structure but with no pores or cracks due to its dense cluster structure compared with the coatings fabricated at Ca/P ratio = 0.83. It can be also observed from Fig. 3(c) that some bulk crystals randomly grow on the ball-like coating. As the Ca/P ratio continues to increase, it can be seen from the SEM images of Fig. 3(f) and (h) that the morphology of the coatings changes greatly. When the Ca/P ratio = 2.5, the morphology of the coating changes from the ball-like structure to homogeneous nano whiskerlike structure in staggered growth direction. And the deposited coating is more uniform with small pores. Such kind of structure is beneficial for the biocompatibility of samples due to its unique porous structure. As
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Fig. 5. Tafel plots of the samples in SBF solution.
composed of different sizes of crystals and shows a mean thickness of about 20 μm. In addition, the coating is properly bonded with the substrate, which suggested that the bonding between coating and the substrate is strong enough to prevent from the body fluid during the application. 3.3. Electrochemical test in SBF
Fig. 3. SEM morphology of coated samples fabricated in solution with different Ca/P ratio: (a, c, e, g, i) 0.83, 1.67, 2.5, 3.34, 4.18 (×200); (b, d, f, h, j) 0.83, 1.67, 2.5, 3.34, 4.18 (×1000).
the Ca/P ratio increases to 3.34, the coating remains the similar staggered whisker-like structure but the length of the whisker-like crystal becomes shorter and clusters are formed at the joint of the crystals. This phenomenon reveals the breakage and restruction of the crystal as the Ca/P ratio increased from 2.5 to 3.34. As can be observed in Fig. 3(i), the surface morphology of the samples fabricated at the Ca/P ratio of 4.18 consists of ball-like crystals and form a regular dendritic structure. This structure abides by the rule of crystal growth, which proves the complete facture of the crystal whiskers. The ball-like structure is regarded to be the result of the regrowth of the broken whiskers. SEM photo micrograph of cross-sectional view of the sample coated with Ca/P ratio of 1.67 is presented in Fig. 4. No significant voids and cracks observed in the cross section. As it can be seen, the coating is
As is shown in Fig. 5, the Tafel curves of the samples with Ca\\P coatings at different Ca/P ratio in SBF solution at 37 °C are plotted based on the electrochemical characterizations. The corrosion potential (Ecorr) and corrosion current density (icorr) are derived from potentiodynamic polarization curves according to Tafel extrapolation and shown in Table 2. Generally, the lower the corrosion current density is, the higher the corrosion resistance. Result shows that the order of the corrosion resistance of the coatings is Ca/P ratio = 1.67 N 0.83 N 4.18 N 2.5 N 3.34 according to the data of corrosion current density. Ca\\P coating fabricated at Ca/P ratio of 1.67 has the best corrosion resistance. The results can also be interpreted by the morphology of the coatings. As is seen from the SEM images (left column), the base morphology of the coatings fabricated at Ca/P ratio = 0.83, 1.67 and 4.18 are all small dense ball-like structures. Among them, the coatings at Ca/P ratio = 1.67 have the densest structure with no pores or cracks which contributes to the best corrosion resistance. Coatings at Ca/P ratio = 0.83 take the second place due to the more portion of exposed area to the solution caused by its small cracks. The corrosion resistance of the coatings at Ca/ P ratio of 4.18 is poorer than samples at Ca/P ratio of 0.83 and 1.67, because of its morphology of dendritic structures linking with ball-like crystals that results in more pores in the coating. Compared to samples at Ca/P ratio = 0.83, 1.67 and 4.18, the coatings at Ca/P ratio of 2.5 and 3.34 exhibit higher degradation rate due to their porous whisker-like structures. The sample fabricated at Ca/P ratio of 2.5 with a homogeneous and dense stacked whisker-like structure, has a little better corrosion resistance than the samples at Ca/P ratio of 3.34 with whiskercluster structure which is the result of the regrowth of crystals. These
Table 2 Corrosion parameters obtained from electrochemical analysis.
Fig. 4. SEM photo micrograph of cross-sectional view of the sample coated.
Ca/P ratio
Ecorr (V)
icorr (A)
– 0.83 1.67 2.5 3.34 4.18
−1.62 V −1.43 V −1.42 V −1.46 V −1.51 V −1.50 V
2.851E-5A 2.547E-7A 1.913E-7A 1.394E-6A 2.175E-6A 1.200E-6A
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Fig. 7. Variation of the pH value of SBF solution at different immersion time. Fig. 6. Relationship of weight of the samples and immersion time.
results indicate that the protectiveness of the Ca\\P coating depends on the Ca/P ratio of the treatment solution and the Ca\\P coating formed at Ca/P ratio of 1.67 is synthetically the most protective. 3.4. Corrosion behavior under static immersion The immersion test in SBF solution is performed for a month in order to examine the long-term corrosion behavior of Ca\\P coated and uncoated ZK60 alloy samples. Fig. 6 shows the relationship between the weight of the samples and the immersion time. It can be seen that with the increasing of immersion time, the weight of coated samples does not change much and keeps almost constant except some mass waves, while that of uncoated sample decreases significantly. The waves of the weight changing indicate the dynamic process of precipitation and dissolution of Ca\\P coatings during immersion test. The precipitation process is resulted from the phase transition of the fabricated coatings and the redeposition of the new Ca\\P coatings during immersion, thus causing a weight increase. And the dissolution includes the mass loss of magnesium alloys and the corrosion of Ca\\P coatings. This complex process indicates the improved corrosion resistance of the Ca\\P coatings fabricated by simple hydrothermal deposition method. It can be observed from Fig. 6 that the volatility of the curves is significantly different owing to the varied morphology and corrosion properties of coatings. Among them, Ca\\P coating at Ca/P ratio of 3.34 has the largest volatility, which shows its poorest corrosion resistance and stability in SBF. Fig. 7 presents the variation of the pH value of SBF solution at different immersion time. The general increase of pH value results from the increase of OH– concentration caused by the release of Mg2 +. After 20 days, the pH changes for all the samples rapidly increased which could be explained as the large area exposed to the corrosion medium or the loose structure of the coatings after long-term immersion. Compared to the uncoated samples, the Ca\\P coated samples exhibit a slower pH increase (especially during the initial stage). It can be observed that the pH value of the uncoated samples reaches 10 while the pH values of the Ca\\P coated samples are all below 8.5, which is much lower than that of uncoated samples. And for the Ca\\P coated samples, it takes about one month to reach the pH value of 10, indicating the slower corrosion rate of coated samples and the greater protection of Ca\\P coatings fabricated by hydrothermal deposition method. Additionally, there is a general trend of the corrosion rate of the Ca\\P coated samples: Ca/P ratio = 1.67 N 0.83 N 4.18 N 2.5 N 3.34, which accords well with the results of the electrochemical corrosion test. Fig. 8 shows the surface morphology of the coatings after immersion with different Ca/P ratio of 0.83, 1.67, 2.5, 3.34 and 4.18. It can be
observed that on the surfaces of Ca\\P coating at Ca/P ratio of 0.83, 2.5, 3.34 and 4.18, there are several obvious corrosion products without specific shape, compared with the cluster shape of that at Ca/P ratio of 1.67. Meanwhile, except for Ca/P ratio of 1.67, the cracks resulted from corrosion can be observed obviously from the SEM images of the samples at other Ca/P ratio. As shown in Fig. 8, when the Ca/P ratio = 0.83, a few cracks and many corrosion products can be found. As the Ca/P ratio changes to 2.5 and 3.34, the cracks are clearest and the size is the largest, suggesting that the corrosion resistance is rather poor. Some cracks can be seen on the surface of the samples with Ca/P ratio of 4.18. However, no cracks can be distinguished from the surface of the samples with Ca/P ratio of 1.67, indicating its good corrosion resistance. Based on the SEM characterization of immersion test, the trend of the corrosion resistance of CaP coated ZK60 samples is listed as following: Ca/P ratio = 1.67 N 0.83 N 4.18 N 2.5 N 3.34, which is consistent with the results of the electrochemical corrosion test.
4. Conclusions The present study demonstrates the success fabrication of Ca\\P coatings on ZK60 magnesium alloys by hydrothermal deposition method and the effect of Ca/P ratio of the hydrothermal solution on the phase composition, microstructure and in vitro corrosion properties of Ca\\P coatings on ZK60 alloys were investigated systematically. With increasing Ca/P ratio, the obtained Ca\\P coatings show a phase transformation from whitlokite to monetite. The surface morphology depends strongly on the Ca/P ratio. From the results of electrochemical test and static immersion measurements, it can be clarified that Ca\\P coated ZK60 alloys exhibit a better corrosion resistance than uncoated ZK60 alloys. And the ZK60 alloy with Ca\\P coating deposited at Ca/P ratio of 1.67 has the best corrosion resistance. Therefore, optimization of Ca/P ratio in the hydrothermal solution plays an important role in getting high quality biomimetic Ca\\P coatings which are promising for controlling the biodegradation of Mg-based implants.
Acknowledgements The present work was supported by National Natural Science Foundation of China (Grant No. 51471120), Natural Science Foundation of Shanghai (Grant No. 13ZR1443700), the Fundamental Research Funds for the Central Universities, and the ‘Morning Star’ project (Grant No. 14QA1403600) supported by the Science and Technological Commission of Shanghai.
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Fig. 8. SEM morphology of coated samples with different Ca/P ratios ((a) 0.83, (b) 1.67, (c) 2.5, (d) 3.34, (e) 4.18) after immersion tests.
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Materials Science and Engineering C journal homepage: www.elsevier.com/locate/msec
Effect of Ca/P ratio on the structural and corrosion properties of biomimetic Ca\\P coatings on ZK60 magnesium alloy Kada Xia a,1, Hui Pan b,1, Taolei Wang a, Shangjun Ma a, Junchao Niu a, Zhen Xiang a, Yiming Song a, Huawei Yang b,⁎, Xiaoshan Tang b,⁎, Wei Lu a,⁎ a b
School of Materials Science and Engineering, Shanghai Key Laboratory of D&A for Metal-Functional Materials, Tongji University, Shanghai 201804, China Shanghai Tenth People's Hospital, Tongji University, Shanghai 200072, China
a r t i c l e
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Article history: Received 19 July 2016 Received in revised form 24 October 2016 Accepted 28 November 2016 Available online 3 December 2016 Keywords: Magnesium alloy Ca-P coating Ca/P ratio Hydrothermal deposition Biodegradation
a b s t r a c t Magnesium and its alloys have attracted much attention as metallic biodegradable implants for their excellent biocompatibility and mechanical properties. However, magnesium has a poor corrosion resistance, causing its rapid degrading in vivo via an electrochemical reaction, which has become a major obstacle to their applications in implants. In this work, Ca\\P coating was successfully coated on the ZK60 magnesium alloys by a simple hydrothermal deposition method. The mechanisms of the hydrothermal reactions of Ca\\P coatings on Mg substrate are described in details. The effect of Ca/P ratio in the hydrothermal solution on the phase composition, microstructure and biodegradation properties of Ca\\P coatings on ZK60 alloys was investigated by varying the Ca/P ratio from 0.83 to 4.18. The morphology of the Ca\\P coating changed significantly with the Ca/P ratio. Biodegradation behavior of the Ca\\P coating magnesium was characterized by anodic polarization and immersion tests in a simulated body fluid. It is revealed that the corrosion resistance of ZK60 magnesium alloy was greatly improved with the biomimetic Ca\\P coatings, and the ZK60 alloy with Ca\\P coating deposited at Ca/P ratio of 1.67 has the best corrosion resistance, which indicates that the Ca\\P coatings are promising for improving the biodegradation properties of Mg-based orthopedic implants and devices. © 2016 Published by Elsevier B.V.
1. Introduction Recent years, magnesium alloys have attracted much attention as biodegradable implant materials because of their good biodegradability and biocompatibility [1–4] compared with the permanent metallic implant-materials, e.g. stainless steels, cobalt-chromium alloys and titanium alloys, which have been applied in clinical use for a long time. The density and mechanical properties of magnesium-based alloys are close to those of nature bones [5–7], reducing stress shielding effect between the implants and nature bones, which make magnesium and its alloys are suitable for implant applications including bone fracture plates, bone pegs, joint prosthesis, root implants and so on. However, magnesium has a poor corrosion resistance, causing its rapid degradation in vivo [8,9], which has become a major obstacle to their applications in implants. In order to be applied in clinics, suitable strategies are thus required to be developed to tailor the degradation rate of magnesium-based implants. On one hand, researchers studied from the perspective of the optimized manufacturing methods, including high purification, alloying, second phase refinement, grain ⁎ Corresponding authors. E-mail address: [email protected] (W. Lu). 1 These authors contribute equally to this paper.
http://dx.doi.org/10.1016/j.msec.2016.11.132 0928-4931/© 2016 Published by Elsevier B.V.
refinement, etc. [1,10–15]. However, these technologies cannot play a breakthrough role in decreasing the inhomogeneous corrosion of magnesium alloys, which makes it hard to reach a well match between degradation and rebuilding of the damaged bone tissues. On the other hand, researchers [16–19] investigated the surface modification technology which is to fabricate coatings on magnesium implants. And the results showed that surface modification, especially coating technology, is an effective way to improve the corrosion resistance, biocompatibility and bioactivity of Mg alloys based implants. As implant materials, the coatings should be non-poisonous and biocompatible, and can degrade in an appropriate rate. A board range of coating systems has been developed. As the main composition of bones, Ca\\P coatings have been paid more attentions due to their superior biocompatibility and corrosion resistance. The widely used fabricating methods of Ca\\P coatings for surface modification of Mg alloys include chemical conversion, cathodic electrodeposition, sol–gel coatings [20–26], etc. Hydrothermal deposition is a simple process and one of the most cost-effective techniques for the coating of metallic materials. It has been proved [27–29] to be a successful method in fabricating Ca\\P coatings on magnesium alloys, although it hasn't been widely studied. Sara et al. [30] reported that Ca\\P coatings with tricalcium phosphate and monetite phases have been deposited on magnesium alloy (AZ31) substrate by using hydrothermal
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process and the effect of different depositing temperatures on the structure of Ca\\P coatings is discussed. Hiromoto et al. [31] fabricated hydroxyapatite coating on AZ31 magnesium alloy using hydrothermal deposition and the growth mechanism of hydroxyapatite coating was discussed. Previously, we have reported the successful fabrication of Ca\\P coatings on Mg alloy by using hydrothermal process in aqueous solution with a low pH value [32,33]. The microstructure and properties of the resulted materials have been affected with the process parameters of fabrication significantly. In this paper, in order to optimize the fabricating procedure of hydrothermal deposition and corrosion resistance of Ca\\P coatings on magnesium alloys, the effect of Ca/P ratio of the hydrothermal solution on the phase composition, microstructure and in vitro corrosion properties of Ca\\P coatings on ZK60 alloys were investigated. ZK60 (Mg\\Zn\\Zr) magnesium alloys were chosen as the substrate for the reason that Zn and Zr elements have been shown with good biocompatibility [34,35].
2. Experimental 2.1. Sample preparation The magnesium alloy used in this study was ZK60 alloy, with the major alloying elements of approximately 5.5 wt% Zn and 0.5 wt% Zr. It was cut into rectangular samples with a size of 5 × 5 × 1 mm3. Each sample was mechanically polished with SiC papers up to 2000#, rinsed with deionized water, cleaned ultrasonically in ethanol and then dried in open air. The dried samples were etched in 90% (in volume) phosphoric acid solution at 55 °C for 15 s, followed by neutralization in 1 mol/L NaOH solution. Samples were then rinsed with distilled water and dried in air.
2.2. Hydrothermal deposition of biomimetic Ca\\P coating The Ca\\P coating was fabricated on the ZK60 alloys by hydrothermal deposition method. Treatment solution with various Ca/P ratio (0.83, 1.67, 2.5, 3.34, 4.18) was prepared using Ca(NO3)2·4H2O and NaH2PO4·2H2O. All chemicals were dissolved in distilled water. The concentration of both Ca(NO3)2·4H2O and NaH2PO4·2H2O was listed in Table 1. The pH value was adjusted to 4 by adding HCl solution. The solution was stirred at room temperature for about 20 min and then transferred to a Teflon-lined stainless steel pressure vessel (50 mL internal volume) with a filling factor of 75%. The pre-treated samples were set in the vessel with the main deposition side up and the vessel was heated in an electric oven in order to prepare the Ca\\P coating. The temperature and the reaction time were 140 °C and 2 h, respectively. The change in pH value of the bulk solution after the treatment was negligible. The samples were then removed from the solution, rinsed with deionized water, and dried in air.
2.3. Characterization 2.3.1. Microstructure Crystallographic structure of the coatings was characterized by Xray Diffraction (XRD). Field-emission scanning electronic microscopy (FESEM) was employed to examine the morphology of the coated samples.
Table 1 Composition of hydrothermal deposition solution. Ca/P ratio NaH2PO4·2H2O Ca(NO3)2·4H2O
0.83 0.03 mol/L 0.025 mol/L
1.67 0.03 mol/L 0.05 mol/L
2.5 0.03 mol/L 0.075 mol/L
3.34 0.03 mol/L 0.1 mol/L
4.18 0.03 mol/L 0.125 mol/L
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2.3.2. Corrosion resistance measurement Corrosion resistance of the coated samples was studied by two methods to characterize their degradation behavior: Electrochemical tests (polarization test) and immersion test. The polarization tests were carried out with CHI660C electrochemical workstation and the experiments were performed in a simulated body fluid (SBF) solution with a pH value of ~7.4 at 37 ± 0.5 °C which mimics the human body condition. The medium to sample area ratio is required to be no b200 mL/cm2 to avoid rapid increases in the local pH at the sample surface. A conventional three electrode was set up with a saturated calomel electrode (SCE) and platinum wire working as a reference and counter electrode, respectively. The area of the samples for working electrode was kept to 10 × 10 mm2 by wax-sealing the extra sides in advance. Prior to characterization, the samples were immersed in the solution for 20 min to establish the open-circuit potential (OCP), and subsequently, Tafel plot was obtained by polarization test with the potential swept from −2 V to 0 V in anodic direction under a sweep rate of 0.1 mV/s, for the corrosion potential of ZK60 alloys is around −1.60 V. The degradation behavior of samples was investigated by immersion test in the same SBF solution. The pH value was adjusted to 7.4 ± 0.1 and the temperature was kept at 37 ± 0.5 °C using a water bath. Before soaking in the solution, the masses of samples were measured by a balance. The pH value of the solution and the samples' mass (after drying) were recorded every 24 h during 1 month, with a blank SBF solution as control group. The SEM was employed to examine the morphology of the Ca\\P coatings before and after immersion tests. 3. Results and discussion 3.1. Phase composition The phase composition of samples coated with biomimetic Ca\\P coatings fabricated in solutions with different Ca/P ratio is shown in Fig. 1(a). The XRD pattern of the uncoated ZK60 substrate is also shown as comparison in Fig. 1(a). As shown in Fig. 1(a), the XRD spectrum of all five coated samples, there is a relatively high peak at around 2θ = 34°, with metallic Mg from the substrate ZK60 identified. Moreover, when Ca/P ratio = 0.83, Ca3(PO4)2 diffraction peaks were observed according to the standard card of Ca3(PO4)2 (Whitlokite, PDF 09-0169), and no other typical peaks such as TCP or CaO were detected in these patterns. However, with the increasing of Ca/P ratio, the layer products experienced transformation that the mainly component of the coatings changes from Ca3(PO4)2 to ADCP (Monetite). It can be seen from Fig. 1(a) that in the XRD spectrum of sample coated at Ca/P ratio = 1.67, 2.5, 3.34 and 4.18, there are typical high diffraction peaks at around 2θ = 26.5°, 30.6°, 33.0° corresponding to the (2 − 2 0), (2 −1 1), (1 0 2) crystal plane, respectively, that means [2 −2 0], [2 −1 1], [1 0 2] may be the preferred orientation for crystals to grow along, which indicates that the coatings are Monetite (ADCP, PDF 09-0080). In addition, there is no obvious difference for the XRD pattern among the sample with Ca/P ratio = 1.67, 2.5, 3.34 and 4.18. Furthermore, it should be noted that the peak intensity of Monetite (ADCP) with Ca/P ratio = 1.67 is found to be sharper and stronger than the other three, indicating that Monetite (ADCP) at this Ca/P ratio was better crystallized. In Fig. 1(b), it can be detected that the main peak at around 2θ = 26.5° is an overlapping of the (0 2 0) and (2 2 0) diffraction planes and the relative intensity of the two peaks changes with different Ca/P ratio. The increasing of Ca/P ratio contributes to the predominance of (2 2 0) diffraction plane. And, from Fig. 1(b), it can be clearly found that the peak at around 2θ = 26.5° of Ca/P ratio = 1.67 is more sharper and stronger than the others, suggesting that Monetite (ADCP) was better crystallized. The mechanisms of the hydrothermal reaction can be described as following. As the fresh magnesium alloy substrates are exposed to the hydrothermal reaction solution, the formation mechanism of calcium
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Fig. 1. (a) XRD patterns of ZK60 substrate and coated samples in solutions with different Ca/P ratio of 0.83, 1.67, 2.5, 3.34 and 4.18. (b) XRD patterns of the main peak at around 2θ = 26.5°.
phosphate coatings can be suggested as shown in Fig. 2. Anodic dissolution of magnesium occurs because of its instability at pH values b11 [36], which is accompanied with the cathode reaction of the hydrogen reduction and OH– release. As the pH value of the treatment solution is about 4, Mg ions are released and the local pH on the magnesium surface rapidly increases during the initial stage of the treatment. Meanwhile several hydrogen bubbles can be observed at the surface of the substrate samples. With the increasing of Ca/P ratio, comparatively more Ca ions are created for reaction, thus accelerating the deposition rate of Ca\\P coating and resulting in the increasing thickness of coatings. When Ca/P ratio is comparatively low (0.83), the coating obtained is quite thin, so the deposition area could be regarded as near-surface layer. The quite high pH value of the near-surface layer accelerates the conversion of PO23 −, and with the increasing of thickness of coatings due to the increasing of Ca/P ratio, the deposition area expands to a
Fig. 2. Formation mechanism of the Ca\ \P coating in hydrothermal deposition process.
far area from the magnesium surface, which has a lower pH value and contributes to the compounds of PO23 − and HPO24 −. On this trend, P ion exists as a form of completely HPO2− 4 (pH = 9) at a far area. Therefore, it can be suggested that Ca3(PO4)2 formed at the near-surface area while CaPO3OH formed at a area far from the surface, and the Ca/P ratio has a significant effect on the formation of calcium phosphates because of its active role in controlling the pH and producing hydroxyl ions. 3.2. Surface morphology of the Ca\\P coatings Fig. 3 presents the surface morphology of the coatings deposited from the hydrothermal solution with different Ca/P ratio of 0.83, 1.67, 2.5, 3.34 and 4.18. Ca\\P coatings are successfully fabricated on ZK60 magnesium alloys by hydrothermal deposition process. SEM characterization reveals the different morphology of the coated samples with the increasing Ca/P ratio. The left column shows the morphology with the low magnification (200 times) while the right shows the morphology of high magnification (1000 times). The coatings cover the magnesium substrates uniformly on the macro scale, while cracks and defects are hardly observed, indicating that the coatings are of high quality. The figures point to crack-free and uniform coatings with no observable defects. From Fig. 3(a), a dense film can be observed near the substrates and new bulk crystals continue to grow on the film. Fig. 3(b) presents the morphology of the coating deposited at solution with Ca/P ratio = 0.83, consisting of ball-like particles with a diameter of about 3– 5 μm, and a lot of ball-like particles closely stack to form a cluster structure with small pores and cracks on the surface. When the Ca/P ratio increases to 1.67, the coating obtained remains a similar ball-like structure but with no pores or cracks due to its dense cluster structure compared with the coatings fabricated at Ca/P ratio = 0.83. It can be also observed from Fig. 3(c) that some bulk crystals randomly grow on the ball-like coating. As the Ca/P ratio continues to increase, it can be seen from the SEM images of Fig. 3(f) and (h) that the morphology of the coatings changes greatly. When the Ca/P ratio = 2.5, the morphology of the coating changes from the ball-like structure to homogeneous nano whiskerlike structure in staggered growth direction. And the deposited coating is more uniform with small pores. Such kind of structure is beneficial for the biocompatibility of samples due to its unique porous structure. As
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Fig. 5. Tafel plots of the samples in SBF solution.
composed of different sizes of crystals and shows a mean thickness of about 20 μm. In addition, the coating is properly bonded with the substrate, which suggested that the bonding between coating and the substrate is strong enough to prevent from the body fluid during the application. 3.3. Electrochemical test in SBF
Fig. 3. SEM morphology of coated samples fabricated in solution with different Ca/P ratio: (a, c, e, g, i) 0.83, 1.67, 2.5, 3.34, 4.18 (×200); (b, d, f, h, j) 0.83, 1.67, 2.5, 3.34, 4.18 (×1000).
the Ca/P ratio increases to 3.34, the coating remains the similar staggered whisker-like structure but the length of the whisker-like crystal becomes shorter and clusters are formed at the joint of the crystals. This phenomenon reveals the breakage and restruction of the crystal as the Ca/P ratio increased from 2.5 to 3.34. As can be observed in Fig. 3(i), the surface morphology of the samples fabricated at the Ca/P ratio of 4.18 consists of ball-like crystals and form a regular dendritic structure. This structure abides by the rule of crystal growth, which proves the complete facture of the crystal whiskers. The ball-like structure is regarded to be the result of the regrowth of the broken whiskers. SEM photo micrograph of cross-sectional view of the sample coated with Ca/P ratio of 1.67 is presented in Fig. 4. No significant voids and cracks observed in the cross section. As it can be seen, the coating is
As is shown in Fig. 5, the Tafel curves of the samples with Ca\\P coatings at different Ca/P ratio in SBF solution at 37 °C are plotted based on the electrochemical characterizations. The corrosion potential (Ecorr) and corrosion current density (icorr) are derived from potentiodynamic polarization curves according to Tafel extrapolation and shown in Table 2. Generally, the lower the corrosion current density is, the higher the corrosion resistance. Result shows that the order of the corrosion resistance of the coatings is Ca/P ratio = 1.67 N 0.83 N 4.18 N 2.5 N 3.34 according to the data of corrosion current density. Ca\\P coating fabricated at Ca/P ratio of 1.67 has the best corrosion resistance. The results can also be interpreted by the morphology of the coatings. As is seen from the SEM images (left column), the base morphology of the coatings fabricated at Ca/P ratio = 0.83, 1.67 and 4.18 are all small dense ball-like structures. Among them, the coatings at Ca/P ratio = 1.67 have the densest structure with no pores or cracks which contributes to the best corrosion resistance. Coatings at Ca/P ratio = 0.83 take the second place due to the more portion of exposed area to the solution caused by its small cracks. The corrosion resistance of the coatings at Ca/ P ratio of 4.18 is poorer than samples at Ca/P ratio of 0.83 and 1.67, because of its morphology of dendritic structures linking with ball-like crystals that results in more pores in the coating. Compared to samples at Ca/P ratio = 0.83, 1.67 and 4.18, the coatings at Ca/P ratio of 2.5 and 3.34 exhibit higher degradation rate due to their porous whisker-like structures. The sample fabricated at Ca/P ratio of 2.5 with a homogeneous and dense stacked whisker-like structure, has a little better corrosion resistance than the samples at Ca/P ratio of 3.34 with whiskercluster structure which is the result of the regrowth of crystals. These
Table 2 Corrosion parameters obtained from electrochemical analysis.
Fig. 4. SEM photo micrograph of cross-sectional view of the sample coated.
Ca/P ratio
Ecorr (V)
icorr (A)
– 0.83 1.67 2.5 3.34 4.18
−1.62 V −1.43 V −1.42 V −1.46 V −1.51 V −1.50 V
2.851E-5A 2.547E-7A 1.913E-7A 1.394E-6A 2.175E-6A 1.200E-6A
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Fig. 7. Variation of the pH value of SBF solution at different immersion time. Fig. 6. Relationship of weight of the samples and immersion time.
results indicate that the protectiveness of the Ca\\P coating depends on the Ca/P ratio of the treatment solution and the Ca\\P coating formed at Ca/P ratio of 1.67 is synthetically the most protective. 3.4. Corrosion behavior under static immersion The immersion test in SBF solution is performed for a month in order to examine the long-term corrosion behavior of Ca\\P coated and uncoated ZK60 alloy samples. Fig. 6 shows the relationship between the weight of the samples and the immersion time. It can be seen that with the increasing of immersion time, the weight of coated samples does not change much and keeps almost constant except some mass waves, while that of uncoated sample decreases significantly. The waves of the weight changing indicate the dynamic process of precipitation and dissolution of Ca\\P coatings during immersion test. The precipitation process is resulted from the phase transition of the fabricated coatings and the redeposition of the new Ca\\P coatings during immersion, thus causing a weight increase. And the dissolution includes the mass loss of magnesium alloys and the corrosion of Ca\\P coatings. This complex process indicates the improved corrosion resistance of the Ca\\P coatings fabricated by simple hydrothermal deposition method. It can be observed from Fig. 6 that the volatility of the curves is significantly different owing to the varied morphology and corrosion properties of coatings. Among them, Ca\\P coating at Ca/P ratio of 3.34 has the largest volatility, which shows its poorest corrosion resistance and stability in SBF. Fig. 7 presents the variation of the pH value of SBF solution at different immersion time. The general increase of pH value results from the increase of OH– concentration caused by the release of Mg2 +. After 20 days, the pH changes for all the samples rapidly increased which could be explained as the large area exposed to the corrosion medium or the loose structure of the coatings after long-term immersion. Compared to the uncoated samples, the Ca\\P coated samples exhibit a slower pH increase (especially during the initial stage). It can be observed that the pH value of the uncoated samples reaches 10 while the pH values of the Ca\\P coated samples are all below 8.5, which is much lower than that of uncoated samples. And for the Ca\\P coated samples, it takes about one month to reach the pH value of 10, indicating the slower corrosion rate of coated samples and the greater protection of Ca\\P coatings fabricated by hydrothermal deposition method. Additionally, there is a general trend of the corrosion rate of the Ca\\P coated samples: Ca/P ratio = 1.67 N 0.83 N 4.18 N 2.5 N 3.34, which accords well with the results of the electrochemical corrosion test. Fig. 8 shows the surface morphology of the coatings after immersion with different Ca/P ratio of 0.83, 1.67, 2.5, 3.34 and 4.18. It can be
observed that on the surfaces of Ca\\P coating at Ca/P ratio of 0.83, 2.5, 3.34 and 4.18, there are several obvious corrosion products without specific shape, compared with the cluster shape of that at Ca/P ratio of 1.67. Meanwhile, except for Ca/P ratio of 1.67, the cracks resulted from corrosion can be observed obviously from the SEM images of the samples at other Ca/P ratio. As shown in Fig. 8, when the Ca/P ratio = 0.83, a few cracks and many corrosion products can be found. As the Ca/P ratio changes to 2.5 and 3.34, the cracks are clearest and the size is the largest, suggesting that the corrosion resistance is rather poor. Some cracks can be seen on the surface of the samples with Ca/P ratio of 4.18. However, no cracks can be distinguished from the surface of the samples with Ca/P ratio of 1.67, indicating its good corrosion resistance. Based on the SEM characterization of immersion test, the trend of the corrosion resistance of CaP coated ZK60 samples is listed as following: Ca/P ratio = 1.67 N 0.83 N 4.18 N 2.5 N 3.34, which is consistent with the results of the electrochemical corrosion test.
4. Conclusions The present study demonstrates the success fabrication of Ca\\P coatings on ZK60 magnesium alloys by hydrothermal deposition method and the effect of Ca/P ratio of the hydrothermal solution on the phase composition, microstructure and in vitro corrosion properties of Ca\\P coatings on ZK60 alloys were investigated systematically. With increasing Ca/P ratio, the obtained Ca\\P coatings show a phase transformation from whitlokite to monetite. The surface morphology depends strongly on the Ca/P ratio. From the results of electrochemical test and static immersion measurements, it can be clarified that Ca\\P coated ZK60 alloys exhibit a better corrosion resistance than uncoated ZK60 alloys. And the ZK60 alloy with Ca\\P coating deposited at Ca/P ratio of 1.67 has the best corrosion resistance. Therefore, optimization of Ca/P ratio in the hydrothermal solution plays an important role in getting high quality biomimetic Ca\\P coatings which are promising for controlling the biodegradation of Mg-based implants.
Acknowledgements The present work was supported by National Natural Science Foundation of China (Grant No. 51471120), Natural Science Foundation of Shanghai (Grant No. 13ZR1443700), the Fundamental Research Funds for the Central Universities, and the ‘Morning Star’ project (Grant No. 14QA1403600) supported by the Science and Technological Commission of Shanghai.
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Fig. 8. SEM morphology of coated samples with different Ca/P ratios ((a) 0.83, (b) 1.67, (c) 2.5, (d) 3.34, (e) 4.18) after immersion tests.
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