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Growth and characterization of Mg(OH)2 film on magnesium alloy AZ31
Applied Surface Science 257 (2011) 6129–6137
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Growth and characterization of Mg(OH)2 film on magnesium alloy AZ31 Yanying Zhu a , Guangming Wu b , Yun-Hong Zhang a , Qing Zhao a,∗ a b
Key Laboratory of Cluster Science of Ministry of Education, School of Science, Beijing Institute of Technology, Beijing 100081, PR China Beijing Institute of Petrochemical Technology, Beijing 102617, PR China
a r t i c l e
i n f o
Article history: Received 29 October 2010 Received in revised form 14 January 2011 Accepted 4 February 2011 Available online 18 February 2011 Keywords: Magnesium alloy Hydrothermal Magnesium hydroxide Corrosion Biomaterial
a b s t r a c t Magnesium-based biomaterials have been proposed as potential candidates for biodegradable implant materials, such as bone screws, bone plates, intraluminal stents and so on. However, the poor corrosion resistance inhibits their applications in surgery. They collapse before the injured tissues are healed. In this paper, Mg(OH)2 nonstructural film was synthesized on the substrate of AZ31 magnesium alloy by hydrothermal method with NaOH solution as mineralizer to reduce the corrosion rate of magnesiumbased materials. The obtained films were characterized by XRD, SEM, and XPS. The results showed that a Mg(OH)2 film with nanostructure surface can be synthesized by hydrothermal method. It was observed that the thickness of film increased with the holding time. Corrosion rates of the films were studied by immersing the samples in Hank’s solution (37 ◦ C). Surface deposits of samples with films soaked in Hank’s solution for 31 days were investigated by XRD, SEM, EDS, XPS, and FTIR. It verified that the corrosion rate of the magnesium alloy with grown film was slowed down in the Hank’s solution and the behavior of corrosion was inhibited effectively. Amorphous calcium apatite precursor was observed to deposit on the surface of the film during corrosion experiments in Hank’s solution. And the tape test revealed a strong adhesion between the film and the substrate. © 2011 Elsevier B.V. All rights reserved.
1. Introduction There is considerable interest in degradable biomaterials to be used for surgical implants, which offer a temporary support during the tissue recovery process until they are no longer needed, and then are absorbed and excreted by the human body, leaving no trace. They can avoid some defects of permanent implants. For example, traditional metallic implants such as stainless steels and cobalt–chromium-based alloys may release toxic metallic ions in the body which will cause inflammatory cascades [1,2]. Magnesium exhibits a poor corrosion resistance in the solution containing chloride ions. Using this property, scientists intend to develop a biodegradable metal-based implant material in the physiological environment. Magnesium is the fourth most abundant cation in human body and an essential element for many biochemical functions in the living processes of human body [3]. Many evidences which contain animal experiments and clinical reports indicated that the degradation products of magnesium in the body are non-toxic and the excess magnesium will be excreted out via kidney. Indeed, magnesium has stimulatory effects on the growth of new bone tissue [2]. Additionally, the density and elastic
∗ Corresponding author. Tel.: +86 10 68918710; fax: +86 10 68918710. E-mail address: [email protected] (Q. Zhao). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.02.017
modulus of magnesium are much closer to human bones and can avoid the “stress shielding” effects which will reduce the stability of implants [4]. Mg-based biomaterial is a new kind of biodegradable biomaterials, which attracts wide interests in them. They consist of pure magnesium [5,6], some kinds of magnesium alloys [7–9], magnesium matrix composites [10,11], magnesium-based porous materials [12,13], and magnesium-based metal glasses [14,15]. Magnesium and its alloys were investigated as biomaterials a long time ago. In early days, they were explored for many medical applications such as wires and other designs for ligature, connectors for vessel anastomosis, neurorrhaphy, and intestinal anastomosis, wires for aneurysm treatment, orthopedic fixate pins, nails, wires, pegs, cramps, sheets and plates, and so on [16]. Nowadays, studies of Mg-based biomaterials are mainly focus on the aspects of bone tissue engineering scaffolds [11,17,18], cardiovascular stents [19–22] and fracture fixation devices [23–25]. However, the corrosion rate of magnesium at the pH level 7.4–7.6 and in the high chloride environment of physiological systems is too high. Magnesium implants will disintegrate before damaged tissues heal. Thus the key issue is to reduce the corrosion rate of magnesiumbased biomaterials so that the implants can maintain mechanical integrity at the period of tissue healing. To achieve this aim, lots of methods have been studied, e.g., alloying with rare earth elements [26], heat treatment [27], hot rolling [28], powder metallurgy [29], alkali-heat treatment [30], micro arc oxidation [31], organic coating [32], etc.
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In the present work, we employed a new hydrothermal method to obtain the protective film on the surface of magnesium alloy. The film consists of large quantities of Mg(OH)2 fine grains. Mg(OH)2 is a degradation product of Mg in the human body, thus it is nontoxic and biocompatible. 2. Experimental and method
Table 1 Thickness of films after hydrothermal treatment for 1, 2, 3, and 4 h. Time of hydrothermal treatment (h)
Film thickness (m)
thickness increase of the samples
1 2 3 4
2.31 12.66 19.48 39.86
Less than 1.5% Less than 1.5% Less than 1.5% 3.5%
2.1. Preparation of AZ31 sample The specimens of wrought magnesium alloy (AZ31, 3% Al, 1% Zn) were cut into small discs with 2 mm in thickness and 10 mm in diameter. Each sample was polished with SiC papers successively up to 2000 grit, ultrasonically cleaned in de-ionized water and acetone for 5 min, respectively, and then dried in the air. 2.2. Growth of Mg(OH)2 nanostructure film The film of Mg(OH)2 was synthesized on the AZ31 alloy by hydrothermal method with 5.66% (mass fraction) NaOH solution as the mineralizer. The alkali solution was poured into a hydrothermal reaction vessel, which is made of Teflon lined stainless steel, and was filled with 75%. The substrates were put into the vessel. Then the vessel was heated to 160 ◦ C by an electric oven, and several thicknesses of films were synthesized at this temperature by keeping the vessel for 1, 2, 3, and 4 h, respectively. 2.3. Characterization of films The structure and composition of the films were investigated by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The surface morphologies of the films were observed by field emitting scanning electron microscope (FESEM). And the thickness of the films was measured through the cross-sectional SEM micrographs of the samples. The XRD analyses of all samples were performed on a SHIMADZU XRD-7000 X-ray diffraction equipped with Cu K␣ radiation, and its voltage and current were maintained at 40 kV and 30 mA, respectively. The XPS measurements of the samples were analyzed by a PHI 5300 ESCA System (PerkinElmer), which was equipped with Mg anode as the X-ray source. The binding energy of contamination carbon at 284.6 eV was used as calibration to correct the shifts of XPS spectra caused by charging effect during the process of XPS analysis. The scanning electron microscopic (SEM) images were obtained using a Hitachi S4800 field emission scanning electron microscope (FESEM). 2.4. Immersion test The degradation behavior of samples was investigated by immersion test in Hank’s solution. The Hank’s solution is composed of 8 g/L NaC1 + 0.4 g/L KC1 + 0.14 g/L CaCl2 + 0.1 g/L MgCI2 ·6H2 O + 0.1 g/L MgSO4 ·7H2 O + 0.06 g/L KH2 PO4 + 0.06 g/L Na2 HPO4 ·2H2 O + 0.35 g/L NaHCO3 + 1.0 g/L glucose. Before immersing into the solution, the dimension and mass of the sample were measured by a vernier caliper and an electric balance, respectively. The ratio of solution volume to specimen area was 0.40 mL/mm2 . The container was placed in the water bath whose temperature was kept at 37 ◦ C. The solution was renewed every 2 days. The samples were taken out after immersing 1, 3, 7, 15, and 31 days, respectively. The samples were cleaned with de-ionized water and ethanol. Then the samples were put in the dryer for 24 h. And finally the masses of the dried samples were measured. The corrosion rate was calculated as follows: m − m1 v= 0 (1) St
where v is the corrosion rate, m0 is the sample mass before immersing experiment, m1 is the mass after immersing experiment, S is the surface area of the sample, t is the corrosion time. XRD, SEM, EDS, XPS and FTIR were used to characterize the structure, morphology, element distribution, composition, and chemical bonds of the sample surface with film soaked in Hank’s solution for 31 days. The instruments of XRD, SEM, XPS have already been mentioned in Section 2.3. The analyses of element distribution of the surface deposits of the samples with films soaked in Hank’s solution for 31 days were carried out using energy dispersive X-ray spectrometer (EDS), which is attached to Hitachi S4800 Field Emission Scanning Electron Microscope. The FTIR spectrum of the surface deposits was recorded on a Nicolet 670 FTIR spectrophotometer with a spectral resolution of 2 cm−1 and accumulation of 64 scans. KBr technique was used in the FTIR measurements. 2.5. Adhesion test The adhesion strength of the film to the substrate is a very important factor to evaluate the film quality and determine whether the film is to be used in practice. In order to further assess the quality of the Mg(OH)2 film, a tape test was employed to inspect the adhesion of the Mg(OH)2 film, which was synthesized by hydrothermal method for 3 h on the AZ31 substrate. According to ASTM D 3359-02 method B [33], this kind of tape test was previously used to establish the adhesion of the hydrothermal film to the substrate [34,35]. 3. Results and discussion 3.1. Cross section morphology of the films The cross sections of films with different times of hydrothermal treatment were observed by FESEM, as shown in Fig. 1. These SEM micrograms reveal that films are dense, uniform, and close association with the AZ31 alloys. The thickness value of the film was calculated by taking the average of thickness of five places in the microgram. The results are listed in Table 1. The results show that the thickness of the films and the whole dimension of the samples are increased with the holding time. The reaction equations are as follows: H2 O(l) → H+ (aq) + OH− (aq)
(2)
2+
(3)
Mg(s) → Mg
(aq) + 2e
+
2H (aq) + 2e → H2 (g) Mg
2+
−
(aq) + 2OH (aq) → Mg(OH)2 (s)
(4) (5)
A high thick film was formed after 4 h under the hydrothermal condition. This is because the diffusion and the ionization of water are both stronger under high temperature and high pressure, and the ion product of the water is increased with the temperature and the pressure. With the increasing of production of hydrogen during the reaction, the system pressure increased, which increased the degree of ionization of water, so the reaction rate between substrate and water improved. Furthermore, the NaOH solution can more easily permeate into the Mg(OH)2 film by passing through its
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Fig. 1. Cross sections of treated samples for (a) 1 h; (b) 2 h; (c) 3 h; (d) 4 h.
interstices or pores and react with the AZ31 substrate at relatively higher pressure. Surface morphologies of samples after hydrothermal treatment for 1, 2, 3, and 4 h, respectively, are shown in Fig. 2. It reveals that the surface of the film for 1 h is made up of different size particles with irregular shape. The crystallization of the surface is poor. For 2 h treatment, the surface presents a porous nanostructure, which is composed of tiny erect flakes. The magnesium hydroxide flakes are about 30–50 nm in diameter; closely gather-together while some larger flakes attach to the surface. For 3 h or 4 h, the surface morphologies are similar as that for 2 h, but the sizes of small flake grew
bigger. For 4 h, the small flake on the surface grew up to 50–70 nm in diameter. The uniformity of the surface morphology is improved with the increasing of treatment time. 3.2. Structures and chemical compositions of the films The X-ray diffraction (XRD) patterns of samples treated by hydrothermal method in NaOH aqueous solution with different holding times of 0, 1, 2, 3, and 4 h are shown in Fig. 3. The results of XRD are consistent well with the observations of SEM. The peaks marked by square points are indexed to hexagonal structure for
Fig. 2. Surface morphologies of the films: (a) 1 h, (b) 2 h, (c) 3 h and (d) 4 h.
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Fig. 3. XRD patterns of the Mg(OH)2 films synthesized on the substrates of AZ31 with different holding times, 0, 1, 2, 3, and 4 h, respectively.
magnesium hydroxide according to JCPDS 75-1527. Broadening of the significant peak indicates that the particles of magnesium hydroxide in the films have a very small grain size. The intensity ratio of (0 0 1)/(1 0 1) peak of the film varies from the powder of Mg(OH)2 . The high intensities of diffraction peaks of (1 0 1) and (1 1 0) specify that better oriented films are obtained [36,37]. The other peaks in the pattern are attributed to the AZ31 sub-
strates. The intensities of diffraction peaks of the Mg(OH)2 grow up with the increasing of treatment time. It suggests that as the time of hydrothermal treatment was prolonged, the amount of Mg(OH)2 would increase and the crystalline of the film would be improved. Fig. 4 shows the C 1s, Mg 2p and survey of XPS spectra obtained for a 3 h holding time of the films synthesized by hydrothermal technique. The signals of C, N, O, Mg, and Al are observed from the survey XPS spectrum in Fig. 4a, and Zn from the enlarged spectrum, as shown in the insert of Fig. 4a, but not all of these elements are the major constituents of the film synthesized by hydrothermal method in this paper. The porous structure of the film can absorb gases from the atmosphere, such as nitrogen and oxygen. The organic contaminants can lead to carbon and oxygen signals appeared on the surface. It can be easily identified that Mg, O, Al, and Zn are the primary components of the film according to the process of our experiment. The C 1s of XPS spectra in Fig. 4b can be fitted to four components, two big features at 284, and 286.5 eV, and two smaller contributions at 288.5 and 289.5 eV. The peak position of C–C/C–H was used for the calibration of binding energy scale. Another peak corresponding to CO3 2− is one constituent of the film. The Mg 2p XPS spectra can be fitted to two components, the big one related to Mg(OH)2 at 49.3 eV while the small one for MgCO3 at 52.5 eV, as shown in Fig. 4c. Combined with the result of C 1s and Mg 2p (Fig. 4b and c), carbon at 289.5 eV exists in the form of MgCO3 in the film [38]. It is likely due to the diffusion of CO2 from the atmosphere to the sample surface, then, reacts with the substrate of the sample. It can be seen that the surface compositions of film synthesized by hydrothermal process are mainly Mg(OH)2 and MgCO3 , and a small amount of Al and Zn.
Fig. 4. XPS spectra for a sample with film of 3 h, (a) survey; (b) XPS C 1s spectrum; (c) Mg 2p spectrum.
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3.3. Results of immersion test The mass losses of the samples were measured after immersing samples into Hank’s solution for 1, 3, 7, 15, 31 days, respectively. Corrosion rates of the samples were calculated according to equation (1), and curves about the changes of corrosion rates related with immersion time are shown in Fig. 5. The rate of corrosion decreases with the prolonging of immersion time. It is probably because the corrosion products cover on the sample surfaces at the first stage (around 3 days), which assists to defense the corrosion reaction. It also shows that the sample with film performs better corrosion resistance than the naked sample. Therefore, the samples of AZ31 magnesium alloy, with magnesium hydroxide films grown on, can effectively decrease the corrosion rate after implanting into the human body. Corrosion rates calculated according to Eq. (1) show degrees of homogeneous corrosion. The main corrosion of magnesium alloys in the aqueous solution containing chloride ions is pitting, so the pitting corrosion resistance is very important as implanted materials. Fig. 6 displays the corrosion morphologies of samples after immersing into Hank’s solution for different times. The Group (a)
Fig. 5. The changes of corrosion rate of samples with and without films as a function of immersion time after immersed into Hank’s solution.
Fig. 6. Corrosion morphologies of magnesium alloy samples with films for 3 h (a) and the naked magnesium alloy samples (b) immersion in Hank’s solution for 1, 3, 7, 15, 31 days and the section of corrosion caves after immersion for 31d.
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was samples with films of 3 h while Group (b) was naked samples without films. In Group (a), some tiny cracks appeared on the film after immersing for 7 days, and some little corrosion pits could be seen after 15 days. After 31 days, the biggest corrosion pit was about 170 m in diameter, and its depth was 198 m. In Group (b), samples suffered severe corrosion after immersing into Hanks’ solution for 7 days. Several very big corrosion holes appeared on the surface after 15 days. They grew even bigger after 31 days, and the diameter of the biggest one was about 1.57 mm, with 1.012 mm depth, which was more than half of the whole thickness of the sample. In comparison the imagines of the two group (Groups (a) and (b)), it is obvious that samples with films show stronger resistance against pitting. Although samples with films immersed in Hank’s solution for days, their films did not become thinner. And furthermore, a gray layer deposited on the outside of the samples. Therefore, we investigated samples immersed for 31 days in more detail, and series of measurements were conducted on these samples. The surface morphology of the sample immersed in Hank’s solution for 31 days is shown in Fig. 7. Its cross section with the element distribution along the line perpendicular to the surface is shown in Fig. 8, which was measured by the energy dispersive X-ray spectrometers attached to Hitachi Field-Emission S4800. The element analysis of A point is displayed in Fig. 9. We can observe that particles deposited on the surface of the film are very small, with poor crystalline in Fig. 7. According to the results of Figs. 8 and 9, the gray surface deposits on the films must contain magnesium, phos-
Fig. 7. Surface morphology of the film soaked in Hank’s solution for 31 days.
phorus, calcium, aluminum, and zinc. Both of C and O may come from the contaminations or partly from the surface deposits. Magnesium, aluminum, and zinc are originally contained in the sample. And calcium along with phosphorus comes from Hank’s solution. The content of calcium decreases along the direction to the substrate of AZ31 alloy. The percentage of P changes with Ca, and the ratio of Ca/P holds invariant. The atomic ratio for Ca/P is about 1.05 according to Fig. 9.
Fig. 8. Cross section of the film after immersion in Hank’s solution for 31 days and the element distribution along the line perpendicular to the surface.
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Fig. 9. Element content analysis of the ‘A’ point near the film surface.
Fig. 10 shows the XRD patterns of films immersing in Hank’s solution for 1, 3, 7, 15, 31 days. Peak intensity of Mg(OH)2 was decreased as the increasing of immersion time. There is a very broad diffraction peak of 2 = 31◦ in XRD patterns after immersing for 15 and 31 days, which indicates the amorphous structure. The result suggests that the crystallization of the film on the sample is decreased with the increasing of immersion time. It is probably due to the dissolution of top layer of the Mg(OH)2 film and a poor crystallinity of the precipitations deposited from the solution. Mg(OH)2 are slightly soluble in water, and reacts with Cl− to generate highly soluble MgCl2 in electrolyte aqueous solutions, where chloride ions are present [2]. So when the sample with a Mg(OH)2 film was immersed in Hank’s solution with Cl− concentration of 146 mmol/L, the Mg(OH)2 film would become soluble
Fig. 10. XRD patterns of samples with Mg(OH)2 films after immersing in Hank’s solution for 0, 1, 3, 7, 15, 31 days.
MgCl2 and dissolved into the solution gradually. At the same time, calcium and phosphorus in the Hank’s solution formed precipitations deposited on the surface of the sample, whose crystallinity was poor according to Fig. 7. The survey, C 1s and P 2p of the XPS spectra of the surface of 31d corrosion sample are shown in Fig. 11. The signals of C, N, O, Mg,
Fig. 11. The XPS spectra of the sample surface with Mg(OH)2 film after immersing in Hank’s solution for 31 days (a) XPS survey; (b) C 1s; and (c) P 2p.
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Fig. 13. Optical micrographs of hydrothermal Mg(OH)2 films tested for adhesion by ASTM D 3359-02 Method B.
Fig. 12. FTIR spectrum of the deposits on sample surface with films soaked in Hank’s solution for 31 days.
Al, P, and Ca can be observed from the survey of XPS (Fig. 11a), and the existence of Zn can be found from the narrow scan of Zn 2p3/2 (the inset in Fig. 11a). The nitrogen is not the component of the corrosion sample. It is due to the porous structure of the film, which adsorbs gases from the air. The C 1s spectrum can be fitted to four components, as shown in Fig. 11b. The peak of C–C/C–H was still used for the calibration of binding energy scale. And another peak corresponding to CO3 2− is one constituent of the surface deposits [38]. According to the result of sub-peak fitting of P 2p as shown in Fig. 11c, the binding energy of P 2p3/2 was 133.7 eV, which does not correspond to a unique phosphate of calcium. For this reason, a further analysis was carried out by FTIR to investigate the compositions of the surface. In the FTIR measurement, the deposits were scratched from the substrates carefully, mixed with potassium bromide (KBr), and then compressed into transparent round disk (10 mm in diameter, and 0.1 mm in thickness). The FTIR spectrum of deposits is shown in Fig. 12. The result is consistent with FTIR spectrum of calcium apatite precursor (ACAP) in amorphous state, which had been reported in the literature [39]. ACAP can be converted to calcium apatite, and they are both constituted by the same building block—Posner’s cluster (Ca9 (PO4 )6 ). The smooth spectrum of deposits in the 4 phosphate vibration region about 530–630 cm−1 , is evidence of an amorphous state while apatite crystal has a characteristic splitting in this region. The carbonate content in deposits is demonstrated by the peaks at 1490, 1430, and 872 cm−1 . The broad absorption peaks at 3400 and at 1650 cm−1 , are indicative of bound water or a hydration layer associated with apatite [39,40]. The peak at 3700 cm−1 is quite sharp and strong, which can be attributed to the O–H stretching in the crystal structure of Mg(OH)2 . The big band around 441 cm−1 is assigned to the Mg–O stretching vibration in Mg(OH)2 [38]. Mg(OH)2 mixed in deposits may come from the residual magnesium hydroxide film under the deposits. Since the O–H stretching peak in hydroxyapatite is at 3569 cm−1 , so that the deposit of ACAP is by lack of hydroxyl radical [39]. The results of XRD, EDS, XPS, and FTIR reveal that the deposits on the surface of sample with Mg(OH)2 film are amorphous carbonate apatite precursor(ACAP) lack of hydroxyl. Because a large number of metal cations, such as K+ , Na+ , Mn2+ , Cu2+ , Zn2+ and Y3+ can substitute for Ca2+ in apatite, and the XPS results reveal that the surface deposits contain magnesium, aluminum and zinc. It can be considered that part of Ca2+ in ACAP
is substituted by Mg2+ , Al3+ and Zn2+ [41]. According to Fig. 9 ((Ca/P)adjusted = (Nca + NMg + 3/2 NAl + NZn )), the equivalent atomic ratio for Ca/P is about 1.45. 3.4. Result of adhesion test The surface of the sample after adhesion test was observed by a high resolution Olympus CK40 inverted microscope to provide a detailed observation on the adhesion of Mg(OH)2 film on the AZ31 substrate. No delaminating or peeling occurred on the crosscutting surface of the sample as shown in Fig. 13, which indicates that strong adhesion of the Mg(OH)2 film to the AZ31 substrate (classification 4B according to ASTM D 3359-02 method B) is set up. 4. Conclusions The magnesium hydroxide film was successfully grown on the substrate of AZ31 magnesium alloy by hydrothermal method. The corrosion resistance of the magnesium alloy with films grown on was effectively improved in comparison with the control sample. The amorphous carbonate apatite precursor (ACAP) lack of hydroxyl was deposited on the surface of magnesium hydroxide film in the Hank’s solution. The results suggest that formation of the magnesium hydroxide film on the Mg alloy substrate can provide a potential material for appropriate orthopedic surgery. Further studies will focus on the study of biocompatibility of the magnesium hydroxide films. Mg alloy with Mg(OH)2 film may be expected to be a novel and promising as orthopedic implantation materials. Acknowledgments This work was supported by the Ministry of Science and Technology of China (2009IM033000). Additional support was provided by the National Natural Science Foundation of China (50935001). References [1] G.O. Hofmann, Biodegradable implants in orthopaedic surgery—a review on the state-of-the-art, Clinical Materials 10 (1992) 75–80. [2] M.P. Staiger, A.M. Pietak, J. Huadmai, G. Dias, Magnesium and its alloys as orthopedic biomaterials: a review, Biomaterials 27 (2006) 1728–1734. [3] J.A. Cowan, The Biological Chemistry of Magnesium, VCH, New York, 1995. [4] J. Nagels, M. Stokdijk, P.M. Rozing, Stress shielding and bone resorption in shoulder arthroplasty, Journal of Shoulder and Elbow Surgery 12 (2003) 35–39. [5] Y. Ren, J. Huang, K. Yang, B. Zhang, Z. Yao, H. Wang, Study of bio-corrosion of pure magnesium, Acta Metallurgica Sinica (Jinshu Xuebao) 41 (2005) 1228–1232.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Growth and characterization of Mg(OH)2 film on magnesium alloy AZ31 Yanying Zhu a , Guangming Wu b , Yun-Hong Zhang a , Qing Zhao a,∗ a b
Key Laboratory of Cluster Science of Ministry of Education, School of Science, Beijing Institute of Technology, Beijing 100081, PR China Beijing Institute of Petrochemical Technology, Beijing 102617, PR China
a r t i c l e
i n f o
Article history: Received 29 October 2010 Received in revised form 14 January 2011 Accepted 4 February 2011 Available online 18 February 2011 Keywords: Magnesium alloy Hydrothermal Magnesium hydroxide Corrosion Biomaterial
a b s t r a c t Magnesium-based biomaterials have been proposed as potential candidates for biodegradable implant materials, such as bone screws, bone plates, intraluminal stents and so on. However, the poor corrosion resistance inhibits their applications in surgery. They collapse before the injured tissues are healed. In this paper, Mg(OH)2 nonstructural film was synthesized on the substrate of AZ31 magnesium alloy by hydrothermal method with NaOH solution as mineralizer to reduce the corrosion rate of magnesiumbased materials. The obtained films were characterized by XRD, SEM, and XPS. The results showed that a Mg(OH)2 film with nanostructure surface can be synthesized by hydrothermal method. It was observed that the thickness of film increased with the holding time. Corrosion rates of the films were studied by immersing the samples in Hank’s solution (37 ◦ C). Surface deposits of samples with films soaked in Hank’s solution for 31 days were investigated by XRD, SEM, EDS, XPS, and FTIR. It verified that the corrosion rate of the magnesium alloy with grown film was slowed down in the Hank’s solution and the behavior of corrosion was inhibited effectively. Amorphous calcium apatite precursor was observed to deposit on the surface of the film during corrosion experiments in Hank’s solution. And the tape test revealed a strong adhesion between the film and the substrate. © 2011 Elsevier B.V. All rights reserved.
1. Introduction There is considerable interest in degradable biomaterials to be used for surgical implants, which offer a temporary support during the tissue recovery process until they are no longer needed, and then are absorbed and excreted by the human body, leaving no trace. They can avoid some defects of permanent implants. For example, traditional metallic implants such as stainless steels and cobalt–chromium-based alloys may release toxic metallic ions in the body which will cause inflammatory cascades [1,2]. Magnesium exhibits a poor corrosion resistance in the solution containing chloride ions. Using this property, scientists intend to develop a biodegradable metal-based implant material in the physiological environment. Magnesium is the fourth most abundant cation in human body and an essential element for many biochemical functions in the living processes of human body [3]. Many evidences which contain animal experiments and clinical reports indicated that the degradation products of magnesium in the body are non-toxic and the excess magnesium will be excreted out via kidney. Indeed, magnesium has stimulatory effects on the growth of new bone tissue [2]. Additionally, the density and elastic
∗ Corresponding author. Tel.: +86 10 68918710; fax: +86 10 68918710. E-mail address: [email protected] (Q. Zhao). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.02.017
modulus of magnesium are much closer to human bones and can avoid the “stress shielding” effects which will reduce the stability of implants [4]. Mg-based biomaterial is a new kind of biodegradable biomaterials, which attracts wide interests in them. They consist of pure magnesium [5,6], some kinds of magnesium alloys [7–9], magnesium matrix composites [10,11], magnesium-based porous materials [12,13], and magnesium-based metal glasses [14,15]. Magnesium and its alloys were investigated as biomaterials a long time ago. In early days, they were explored for many medical applications such as wires and other designs for ligature, connectors for vessel anastomosis, neurorrhaphy, and intestinal anastomosis, wires for aneurysm treatment, orthopedic fixate pins, nails, wires, pegs, cramps, sheets and plates, and so on [16]. Nowadays, studies of Mg-based biomaterials are mainly focus on the aspects of bone tissue engineering scaffolds [11,17,18], cardiovascular stents [19–22] and fracture fixation devices [23–25]. However, the corrosion rate of magnesium at the pH level 7.4–7.6 and in the high chloride environment of physiological systems is too high. Magnesium implants will disintegrate before damaged tissues heal. Thus the key issue is to reduce the corrosion rate of magnesiumbased biomaterials so that the implants can maintain mechanical integrity at the period of tissue healing. To achieve this aim, lots of methods have been studied, e.g., alloying with rare earth elements [26], heat treatment [27], hot rolling [28], powder metallurgy [29], alkali-heat treatment [30], micro arc oxidation [31], organic coating [32], etc.
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In the present work, we employed a new hydrothermal method to obtain the protective film on the surface of magnesium alloy. The film consists of large quantities of Mg(OH)2 fine grains. Mg(OH)2 is a degradation product of Mg in the human body, thus it is nontoxic and biocompatible. 2. Experimental and method
Table 1 Thickness of films after hydrothermal treatment for 1, 2, 3, and 4 h. Time of hydrothermal treatment (h)
Film thickness (m)
thickness increase of the samples
1 2 3 4
2.31 12.66 19.48 39.86
Less than 1.5% Less than 1.5% Less than 1.5% 3.5%
2.1. Preparation of AZ31 sample The specimens of wrought magnesium alloy (AZ31, 3% Al, 1% Zn) were cut into small discs with 2 mm in thickness and 10 mm in diameter. Each sample was polished with SiC papers successively up to 2000 grit, ultrasonically cleaned in de-ionized water and acetone for 5 min, respectively, and then dried in the air. 2.2. Growth of Mg(OH)2 nanostructure film The film of Mg(OH)2 was synthesized on the AZ31 alloy by hydrothermal method with 5.66% (mass fraction) NaOH solution as the mineralizer. The alkali solution was poured into a hydrothermal reaction vessel, which is made of Teflon lined stainless steel, and was filled with 75%. The substrates were put into the vessel. Then the vessel was heated to 160 ◦ C by an electric oven, and several thicknesses of films were synthesized at this temperature by keeping the vessel for 1, 2, 3, and 4 h, respectively. 2.3. Characterization of films The structure and composition of the films were investigated by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The surface morphologies of the films were observed by field emitting scanning electron microscope (FESEM). And the thickness of the films was measured through the cross-sectional SEM micrographs of the samples. The XRD analyses of all samples were performed on a SHIMADZU XRD-7000 X-ray diffraction equipped with Cu K␣ radiation, and its voltage and current were maintained at 40 kV and 30 mA, respectively. The XPS measurements of the samples were analyzed by a PHI 5300 ESCA System (PerkinElmer), which was equipped with Mg anode as the X-ray source. The binding energy of contamination carbon at 284.6 eV was used as calibration to correct the shifts of XPS spectra caused by charging effect during the process of XPS analysis. The scanning electron microscopic (SEM) images were obtained using a Hitachi S4800 field emission scanning electron microscope (FESEM). 2.4. Immersion test The degradation behavior of samples was investigated by immersion test in Hank’s solution. The Hank’s solution is composed of 8 g/L NaC1 + 0.4 g/L KC1 + 0.14 g/L CaCl2 + 0.1 g/L MgCI2 ·6H2 O + 0.1 g/L MgSO4 ·7H2 O + 0.06 g/L KH2 PO4 + 0.06 g/L Na2 HPO4 ·2H2 O + 0.35 g/L NaHCO3 + 1.0 g/L glucose. Before immersing into the solution, the dimension and mass of the sample were measured by a vernier caliper and an electric balance, respectively. The ratio of solution volume to specimen area was 0.40 mL/mm2 . The container was placed in the water bath whose temperature was kept at 37 ◦ C. The solution was renewed every 2 days. The samples were taken out after immersing 1, 3, 7, 15, and 31 days, respectively. The samples were cleaned with de-ionized water and ethanol. Then the samples were put in the dryer for 24 h. And finally the masses of the dried samples were measured. The corrosion rate was calculated as follows: m − m1 v= 0 (1) St
where v is the corrosion rate, m0 is the sample mass before immersing experiment, m1 is the mass after immersing experiment, S is the surface area of the sample, t is the corrosion time. XRD, SEM, EDS, XPS and FTIR were used to characterize the structure, morphology, element distribution, composition, and chemical bonds of the sample surface with film soaked in Hank’s solution for 31 days. The instruments of XRD, SEM, XPS have already been mentioned in Section 2.3. The analyses of element distribution of the surface deposits of the samples with films soaked in Hank’s solution for 31 days were carried out using energy dispersive X-ray spectrometer (EDS), which is attached to Hitachi S4800 Field Emission Scanning Electron Microscope. The FTIR spectrum of the surface deposits was recorded on a Nicolet 670 FTIR spectrophotometer with a spectral resolution of 2 cm−1 and accumulation of 64 scans. KBr technique was used in the FTIR measurements. 2.5. Adhesion test The adhesion strength of the film to the substrate is a very important factor to evaluate the film quality and determine whether the film is to be used in practice. In order to further assess the quality of the Mg(OH)2 film, a tape test was employed to inspect the adhesion of the Mg(OH)2 film, which was synthesized by hydrothermal method for 3 h on the AZ31 substrate. According to ASTM D 3359-02 method B [33], this kind of tape test was previously used to establish the adhesion of the hydrothermal film to the substrate [34,35]. 3. Results and discussion 3.1. Cross section morphology of the films The cross sections of films with different times of hydrothermal treatment were observed by FESEM, as shown in Fig. 1. These SEM micrograms reveal that films are dense, uniform, and close association with the AZ31 alloys. The thickness value of the film was calculated by taking the average of thickness of five places in the microgram. The results are listed in Table 1. The results show that the thickness of the films and the whole dimension of the samples are increased with the holding time. The reaction equations are as follows: H2 O(l) → H+ (aq) + OH− (aq)
(2)
2+
(3)
Mg(s) → Mg
(aq) + 2e
+
2H (aq) + 2e → H2 (g) Mg
2+
−
(aq) + 2OH (aq) → Mg(OH)2 (s)
(4) (5)
A high thick film was formed after 4 h under the hydrothermal condition. This is because the diffusion and the ionization of water are both stronger under high temperature and high pressure, and the ion product of the water is increased with the temperature and the pressure. With the increasing of production of hydrogen during the reaction, the system pressure increased, which increased the degree of ionization of water, so the reaction rate between substrate and water improved. Furthermore, the NaOH solution can more easily permeate into the Mg(OH)2 film by passing through its
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Fig. 1. Cross sections of treated samples for (a) 1 h; (b) 2 h; (c) 3 h; (d) 4 h.
interstices or pores and react with the AZ31 substrate at relatively higher pressure. Surface morphologies of samples after hydrothermal treatment for 1, 2, 3, and 4 h, respectively, are shown in Fig. 2. It reveals that the surface of the film for 1 h is made up of different size particles with irregular shape. The crystallization of the surface is poor. For 2 h treatment, the surface presents a porous nanostructure, which is composed of tiny erect flakes. The magnesium hydroxide flakes are about 30–50 nm in diameter; closely gather-together while some larger flakes attach to the surface. For 3 h or 4 h, the surface morphologies are similar as that for 2 h, but the sizes of small flake grew
bigger. For 4 h, the small flake on the surface grew up to 50–70 nm in diameter. The uniformity of the surface morphology is improved with the increasing of treatment time. 3.2. Structures and chemical compositions of the films The X-ray diffraction (XRD) patterns of samples treated by hydrothermal method in NaOH aqueous solution with different holding times of 0, 1, 2, 3, and 4 h are shown in Fig. 3. The results of XRD are consistent well with the observations of SEM. The peaks marked by square points are indexed to hexagonal structure for
Fig. 2. Surface morphologies of the films: (a) 1 h, (b) 2 h, (c) 3 h and (d) 4 h.
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Fig. 3. XRD patterns of the Mg(OH)2 films synthesized on the substrates of AZ31 with different holding times, 0, 1, 2, 3, and 4 h, respectively.
magnesium hydroxide according to JCPDS 75-1527. Broadening of the significant peak indicates that the particles of magnesium hydroxide in the films have a very small grain size. The intensity ratio of (0 0 1)/(1 0 1) peak of the film varies from the powder of Mg(OH)2 . The high intensities of diffraction peaks of (1 0 1) and (1 1 0) specify that better oriented films are obtained [36,37]. The other peaks in the pattern are attributed to the AZ31 sub-
strates. The intensities of diffraction peaks of the Mg(OH)2 grow up with the increasing of treatment time. It suggests that as the time of hydrothermal treatment was prolonged, the amount of Mg(OH)2 would increase and the crystalline of the film would be improved. Fig. 4 shows the C 1s, Mg 2p and survey of XPS spectra obtained for a 3 h holding time of the films synthesized by hydrothermal technique. The signals of C, N, O, Mg, and Al are observed from the survey XPS spectrum in Fig. 4a, and Zn from the enlarged spectrum, as shown in the insert of Fig. 4a, but not all of these elements are the major constituents of the film synthesized by hydrothermal method in this paper. The porous structure of the film can absorb gases from the atmosphere, such as nitrogen and oxygen. The organic contaminants can lead to carbon and oxygen signals appeared on the surface. It can be easily identified that Mg, O, Al, and Zn are the primary components of the film according to the process of our experiment. The C 1s of XPS spectra in Fig. 4b can be fitted to four components, two big features at 284, and 286.5 eV, and two smaller contributions at 288.5 and 289.5 eV. The peak position of C–C/C–H was used for the calibration of binding energy scale. Another peak corresponding to CO3 2− is one constituent of the film. The Mg 2p XPS spectra can be fitted to two components, the big one related to Mg(OH)2 at 49.3 eV while the small one for MgCO3 at 52.5 eV, as shown in Fig. 4c. Combined with the result of C 1s and Mg 2p (Fig. 4b and c), carbon at 289.5 eV exists in the form of MgCO3 in the film [38]. It is likely due to the diffusion of CO2 from the atmosphere to the sample surface, then, reacts with the substrate of the sample. It can be seen that the surface compositions of film synthesized by hydrothermal process are mainly Mg(OH)2 and MgCO3 , and a small amount of Al and Zn.
Fig. 4. XPS spectra for a sample with film of 3 h, (a) survey; (b) XPS C 1s spectrum; (c) Mg 2p spectrum.
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3.3. Results of immersion test The mass losses of the samples were measured after immersing samples into Hank’s solution for 1, 3, 7, 15, 31 days, respectively. Corrosion rates of the samples were calculated according to equation (1), and curves about the changes of corrosion rates related with immersion time are shown in Fig. 5. The rate of corrosion decreases with the prolonging of immersion time. It is probably because the corrosion products cover on the sample surfaces at the first stage (around 3 days), which assists to defense the corrosion reaction. It also shows that the sample with film performs better corrosion resistance than the naked sample. Therefore, the samples of AZ31 magnesium alloy, with magnesium hydroxide films grown on, can effectively decrease the corrosion rate after implanting into the human body. Corrosion rates calculated according to Eq. (1) show degrees of homogeneous corrosion. The main corrosion of magnesium alloys in the aqueous solution containing chloride ions is pitting, so the pitting corrosion resistance is very important as implanted materials. Fig. 6 displays the corrosion morphologies of samples after immersing into Hank’s solution for different times. The Group (a)
Fig. 5. The changes of corrosion rate of samples with and without films as a function of immersion time after immersed into Hank’s solution.
Fig. 6. Corrosion morphologies of magnesium alloy samples with films for 3 h (a) and the naked magnesium alloy samples (b) immersion in Hank’s solution for 1, 3, 7, 15, 31 days and the section of corrosion caves after immersion for 31d.
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was samples with films of 3 h while Group (b) was naked samples without films. In Group (a), some tiny cracks appeared on the film after immersing for 7 days, and some little corrosion pits could be seen after 15 days. After 31 days, the biggest corrosion pit was about 170 m in diameter, and its depth was 198 m. In Group (b), samples suffered severe corrosion after immersing into Hanks’ solution for 7 days. Several very big corrosion holes appeared on the surface after 15 days. They grew even bigger after 31 days, and the diameter of the biggest one was about 1.57 mm, with 1.012 mm depth, which was more than half of the whole thickness of the sample. In comparison the imagines of the two group (Groups (a) and (b)), it is obvious that samples with films show stronger resistance against pitting. Although samples with films immersed in Hank’s solution for days, their films did not become thinner. And furthermore, a gray layer deposited on the outside of the samples. Therefore, we investigated samples immersed for 31 days in more detail, and series of measurements were conducted on these samples. The surface morphology of the sample immersed in Hank’s solution for 31 days is shown in Fig. 7. Its cross section with the element distribution along the line perpendicular to the surface is shown in Fig. 8, which was measured by the energy dispersive X-ray spectrometers attached to Hitachi Field-Emission S4800. The element analysis of A point is displayed in Fig. 9. We can observe that particles deposited on the surface of the film are very small, with poor crystalline in Fig. 7. According to the results of Figs. 8 and 9, the gray surface deposits on the films must contain magnesium, phos-
Fig. 7. Surface morphology of the film soaked in Hank’s solution for 31 days.
phorus, calcium, aluminum, and zinc. Both of C and O may come from the contaminations or partly from the surface deposits. Magnesium, aluminum, and zinc are originally contained in the sample. And calcium along with phosphorus comes from Hank’s solution. The content of calcium decreases along the direction to the substrate of AZ31 alloy. The percentage of P changes with Ca, and the ratio of Ca/P holds invariant. The atomic ratio for Ca/P is about 1.05 according to Fig. 9.
Fig. 8. Cross section of the film after immersion in Hank’s solution for 31 days and the element distribution along the line perpendicular to the surface.
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Fig. 9. Element content analysis of the ‘A’ point near the film surface.
Fig. 10 shows the XRD patterns of films immersing in Hank’s solution for 1, 3, 7, 15, 31 days. Peak intensity of Mg(OH)2 was decreased as the increasing of immersion time. There is a very broad diffraction peak of 2 = 31◦ in XRD patterns after immersing for 15 and 31 days, which indicates the amorphous structure. The result suggests that the crystallization of the film on the sample is decreased with the increasing of immersion time. It is probably due to the dissolution of top layer of the Mg(OH)2 film and a poor crystallinity of the precipitations deposited from the solution. Mg(OH)2 are slightly soluble in water, and reacts with Cl− to generate highly soluble MgCl2 in electrolyte aqueous solutions, where chloride ions are present [2]. So when the sample with a Mg(OH)2 film was immersed in Hank’s solution with Cl− concentration of 146 mmol/L, the Mg(OH)2 film would become soluble
Fig. 10. XRD patterns of samples with Mg(OH)2 films after immersing in Hank’s solution for 0, 1, 3, 7, 15, 31 days.
MgCl2 and dissolved into the solution gradually. At the same time, calcium and phosphorus in the Hank’s solution formed precipitations deposited on the surface of the sample, whose crystallinity was poor according to Fig. 7. The survey, C 1s and P 2p of the XPS spectra of the surface of 31d corrosion sample are shown in Fig. 11. The signals of C, N, O, Mg,
Fig. 11. The XPS spectra of the sample surface with Mg(OH)2 film after immersing in Hank’s solution for 31 days (a) XPS survey; (b) C 1s; and (c) P 2p.
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Fig. 13. Optical micrographs of hydrothermal Mg(OH)2 films tested for adhesion by ASTM D 3359-02 Method B.
Fig. 12. FTIR spectrum of the deposits on sample surface with films soaked in Hank’s solution for 31 days.
Al, P, and Ca can be observed from the survey of XPS (Fig. 11a), and the existence of Zn can be found from the narrow scan of Zn 2p3/2 (the inset in Fig. 11a). The nitrogen is not the component of the corrosion sample. It is due to the porous structure of the film, which adsorbs gases from the air. The C 1s spectrum can be fitted to four components, as shown in Fig. 11b. The peak of C–C/C–H was still used for the calibration of binding energy scale. And another peak corresponding to CO3 2− is one constituent of the surface deposits [38]. According to the result of sub-peak fitting of P 2p as shown in Fig. 11c, the binding energy of P 2p3/2 was 133.7 eV, which does not correspond to a unique phosphate of calcium. For this reason, a further analysis was carried out by FTIR to investigate the compositions of the surface. In the FTIR measurement, the deposits were scratched from the substrates carefully, mixed with potassium bromide (KBr), and then compressed into transparent round disk (10 mm in diameter, and 0.1 mm in thickness). The FTIR spectrum of deposits is shown in Fig. 12. The result is consistent with FTIR spectrum of calcium apatite precursor (ACAP) in amorphous state, which had been reported in the literature [39]. ACAP can be converted to calcium apatite, and they are both constituted by the same building block—Posner’s cluster (Ca9 (PO4 )6 ). The smooth spectrum of deposits in the 4 phosphate vibration region about 530–630 cm−1 , is evidence of an amorphous state while apatite crystal has a characteristic splitting in this region. The carbonate content in deposits is demonstrated by the peaks at 1490, 1430, and 872 cm−1 . The broad absorption peaks at 3400 and at 1650 cm−1 , are indicative of bound water or a hydration layer associated with apatite [39,40]. The peak at 3700 cm−1 is quite sharp and strong, which can be attributed to the O–H stretching in the crystal structure of Mg(OH)2 . The big band around 441 cm−1 is assigned to the Mg–O stretching vibration in Mg(OH)2 [38]. Mg(OH)2 mixed in deposits may come from the residual magnesium hydroxide film under the deposits. Since the O–H stretching peak in hydroxyapatite is at 3569 cm−1 , so that the deposit of ACAP is by lack of hydroxyl radical [39]. The results of XRD, EDS, XPS, and FTIR reveal that the deposits on the surface of sample with Mg(OH)2 film are amorphous carbonate apatite precursor(ACAP) lack of hydroxyl. Because a large number of metal cations, such as K+ , Na+ , Mn2+ , Cu2+ , Zn2+ and Y3+ can substitute for Ca2+ in apatite, and the XPS results reveal that the surface deposits contain magnesium, aluminum and zinc. It can be considered that part of Ca2+ in ACAP
is substituted by Mg2+ , Al3+ and Zn2+ [41]. According to Fig. 9 ((Ca/P)adjusted = (Nca + NMg + 3/2 NAl + NZn )), the equivalent atomic ratio for Ca/P is about 1.45. 3.4. Result of adhesion test The surface of the sample after adhesion test was observed by a high resolution Olympus CK40 inverted microscope to provide a detailed observation on the adhesion of Mg(OH)2 film on the AZ31 substrate. No delaminating or peeling occurred on the crosscutting surface of the sample as shown in Fig. 13, which indicates that strong adhesion of the Mg(OH)2 film to the AZ31 substrate (classification 4B according to ASTM D 3359-02 method B) is set up. 4. Conclusions The magnesium hydroxide film was successfully grown on the substrate of AZ31 magnesium alloy by hydrothermal method. The corrosion resistance of the magnesium alloy with films grown on was effectively improved in comparison with the control sample. The amorphous carbonate apatite precursor (ACAP) lack of hydroxyl was deposited on the surface of magnesium hydroxide film in the Hank’s solution. The results suggest that formation of the magnesium hydroxide film on the Mg alloy substrate can provide a potential material for appropriate orthopedic surgery. Further studies will focus on the study of biocompatibility of the magnesium hydroxide films. Mg alloy with Mg(OH)2 film may be expected to be a novel and promising as orthopedic implantation materials. Acknowledgments This work was supported by the Ministry of Science and Technology of China (2009IM033000). Additional support was provided by the National Natural Science Foundation of China (50935001). References [1] G.O. Hofmann, Biodegradable implants in orthopaedic surgery—a review on the state-of-the-art, Clinical Materials 10 (1992) 75–80. [2] M.P. Staiger, A.M. Pietak, J. Huadmai, G. Dias, Magnesium and its alloys as orthopedic biomaterials: a review, Biomaterials 27 (2006) 1728–1734. [3] J.A. Cowan, The Biological Chemistry of Magnesium, VCH, New York, 1995. [4] J. Nagels, M. Stokdijk, P.M. Rozing, Stress shielding and bone resorption in shoulder arthroplasty, Journal of Shoulder and Elbow Surgery 12 (2003) 35–39. [5] Y. Ren, J. Huang, K. Yang, B. Zhang, Z. Yao, H. Wang, Study of bio-corrosion of pure magnesium, Acta Metallurgica Sinica (Jinshu Xuebao) 41 (2005) 1228–1232.
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