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Enhanced uniformity of apatite coating on a PEO film formed on AZ31 Mg alloy by an alkali pretreatment
SCT-20210; No of Pages 8 Surface & Coatings Technology xxx (2015) xxx–xxx
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Enhanced uniformity of apatite coating on a PEO film formed on AZ31 Mg alloy by an alkali pretreatment Anawati a, Hidetaka Asoh a,b, Sachiko Ono a,b,⁎ a b
Research Institute for Science and Technology, Kogakuin University, 2665-1 Nakano, Hachioji, Tokyo 192-0015, Japan Department of Applied Chemistry, Kogakuin University, 2665-1 Nakano, Hachioji, Tokyo 192-0015, Japan
a r t i c l e
i n f o
Article history: Received 10 October 2014 Accepted in revised form 3 April 2015 Available online xxxx Keywords: Magnesium Anodization Apatite Alkali treatment Corrosion
a b s t r a c t Anodization by plasma electrolytic oxidation (PEO) and subsequent apatite coating were performed on a biodegradable AZ31 magnesium alloy to enhance its corrosion resistance and bioactivity in physiological solution. The PEO film itself (~ 48 μm in thickness) exhibited low bioactivity, where only aggregated apatite particles were deposited locally on its surface as a result of the alternative immersion method (AIM) in Ca– phosphate solutions. The uniformity of apatite coating on the PEO film was markedly improved by pretreatment of the film in a dilute NaOH solution. The alkali treatment induced the formation of a nano-size platelet Mg(OH)2 layer on the film surfaces that drastically enlarged the effective surface area for the precipitation of apatite. A uniform apatite layer as thick as 1 μm was successfully deposited on the hydroxide layer after AIM treatment. The enhanced uniformity of the apatite coating on an alkali- and AIM-treated surface significantly improved the corrosion resistance in both simulated body fluid (SBF) and NaCl solution, and the bioactivity in SBF. © 2015 Published by Elsevier B.V.
1. Introduction Magnesium (Mg) and its alloys exhibit excellent biocompatibility, suitable mechanical properties and spontaneous degradation in a physiological environment which are applicable for biodegradable implant devices [1–6]. In many cases of implantation, the body needs a temporary implant or device, in which case biodegradable materials represent a better option than inert ones [2,6]. Historically, Mg and its alloys have been studied as implant material since 1878 [6]; however, commercial medical devices are still not available. The corrosion rate of Mg in a physiological environment is still considered to be too high, leading to two major consequences: the rapid generation of gas (H2) bubbles and the loss of mechanical integrity. High corrosion rate was observed on the first week of implantation, as indicated by the generation of significant gas bubbles, and normally reduced in the following weeks [2]. Therefore, improving temporary corrosion resistance of Mg implant to retard early degradation in a physiological environment is crucial. Anodization is an effective way of mitigating the corrosion of an Mg metal surface by forming an anodic oxide film to prevent direct contact of the substrate with the corrosive environment. Plasma electrolytic oxidation (PEO), also called micro-arc oxidation (MAO), is a common surface treatment used to form a ceramic-type oxide film on Mg alloys as well as on other valve metals (Al, Ti, Ta, Nb, Zr, etc.) by the application of a high anodic voltage to create intense plasma near the metal surface ⁎ Corresponding author at: Department of Applied Chemistry, Kogakuin University, 2665-1 Nakano, Hachioji, Tokyo 192-0015, Japan. E-mail address: [email protected] (S. Ono).
to induce oxidation [7]. A PEO film is often applied on Mg alloy substrates before final organic coating. Compared with the direct application of organic coating on the bare Mg alloy substrate, the corrosion resistance of Mg alloys with a PEO film formed before organic coating was much improved [8]. Apatite is frequently used as a final coating for orthopedic implant materials to provide bioactivation properties. The existence of apatite early during the implantation period enables the strong fixation of an implant to the host tissue or bone. Apatite coating can be synthesized by immersion in physiological solutions such as simulated body fluid (SBF), Hank's solution and Kokubo's solution [9–15]. However, it normally requires a long immersion time, in weeks, to fully cover the surface with apatite while the Mg specimen may severely degrade in such highly corrosive environment. The application of an alternative immersion method (AIM) in Ca- and phosphate-containing solutions has been reported to be an effective means of accelerating the deposition of apatite layer on a porous oxide film formed on Ti in a short time [16]. Compared to other coating techniques, such as plasma spray and sputtering, AIM treatment does not the involve application of high temperature, which is suitable for Mg substrate, considering its low melting temperature. In this work, the AIM treatment was applied on a PEO film formed on AZ31 magnesium alloy specimens. To enhance the apatite coating uniformity, alkali treatment is proposed. Alkali treatments are typically used to promote apatite nucleation on a Ti metal surface in simulated body fluid (SBF). The alkali-treated Ti metal surface becomes highly bioactive owing to the formation of a hydrated sodium titanium (Na2Ti5O11) hydrogel layer with a submicron porous structure [9–11]. The release of Na+ from the hydrogel layer,
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which is further transformed into a Ti–OH layer during immersion in SBF, induces HA nucleation on the titanium surface. Apart from its application on metal surfaces, the specific application of an alkali treatment on a PEO film to induce strong bonding to a final coating has not been investigated. In this study, the effect of the alkali treatment on the apatite coating uniformity of a PEO film on AZ31 alloy was investigated. The corrosion resistance and bioactivity of the coated specimen were further studied by an in vitro immersion test in SBF. The electrochemical corrosion behavior was also examined by performing a polarization test in NaCl solution. 2. Materials and methods 2.1. Formation of PEO film The specimens used were obtained from an extruded plate of AZ31 alloy, with a thickness of 1 mm cut into pieces to give a working area of 5 cm2. The alloy composition is listed in Table 1. The specimens were degreased in acetone in an ultrasonic bath for 3 min prior to further use. Before anodization, the specimens were pretreated in a mixed acid solution of 10 vol.% HNO3–2 vol.% H3PO4 for 20 s followed by dipping in 5 wt.% NaOH solution at 80 °C for 1 min. Anodization was carried out in 0.5 mol dm−3 Na3PO4 solution at a constant current of 200 A m− 2 at 25 °C for 30 min. An earlier work [17] revealed that the corrosion resistance of a PEO film formed in Na3PO4 solution is higher than that formed in a silicate-based solution or other inorganic solutions. The alkali treatment was carried out in 0.25 mol dm− 3 NaOH solution at 80 °C for 90 min after anodization. The treatment conditions used in this work were substantially milder than those used for metal surfaces (5–10 mol dm−3) [9–11]. 2.2. Evaluation of apatite forming ability AIM treatment was performed to coat the anodized specimen with apatite. The treatment was accomplished by dipping the specimens in saturated Ca(OH)2 solution (pH 13.3) and 0.02 mol dm− 3 (NH4)2HPO4 solution (pH 8.3) for 1 min in each solution followed by 10 s rinsing in ultrapure water between each immersion for 20 cycles. The AIM solutions were exposed to laboratory air at room temperature. An in vitro immersion in SBF was carried out on both the bare substrate and anodized specimens of AZ31 to evaluate the bioactivity and corrosion behavior. The specimens were soaked individually in SBF10, using the ionic concentrations shown in Table 2, with a pH of 7.4 at 37 °C. The SBF10 was prepared as previously reported [18]. The specimens were exposed to the solution with a surface-to-volume ratio of 20 ml cm−2. The solution was refreshed every 3 days. After exposure, the specimens were washed in distilled (DI) water. 2.3. Electrochemical analysis The corrosion behavior of the specimens was studied by carrying out potentiodynamic polarization measurements using a potentiostat instrument (IviumStat) in an aerated 0.9 wt.% NaCl solution at 37 °C on the basis of the ASTM standard and our previous work [19,20]. Prior to polarization measurements, the specimen was left in an open circuit potential for 60 s. The polarization test was started at a potential of −1.65 VAg/AgCl and was terminated when the current output reached 30 mA with a sweep rate of 1 mV s−1. A Pt wire was used as a counter electrode and silver chloride was used as a reference electrode. The Table 1 Chemical composition of alloying elements in AZ31 alloy in wt.%. Al
Zn
Mn
Cu
Ni
Fe
Mg
2.5–3.5
0.5–1.5
N0.2
b0.1
b0.005
b0.03
Balance
Table 2 Ionic concentrations of SBF10 [14]. Ion
Na+
K+
Mg+
Ca+
Cl−
HCO− 3
HPO2− 4
SO2− 4
Concentration (mM)
142
5
1
2.5
126
10
1
1
temperature was controlled at 37 °C using an electrode heater equipped with a temperature-sensing device inside the cell. 2.4. Characterization of surface structure The surface microstructure of the anodized specimens was investigated using a JEOL JSM-6701 field-emission scanning electron microscope (FE-SEM). The elemental analysis was performed by energydispersive X-ray spectroscopy (EDX) using a JEOL EX-54175JMU spectroscope attached to an SEM (JEOL JSM-6380LA) over an area of 250 × 200 μm2 at an accelerating voltage of 15 kV. The analysis of the chemical composition and crystalline structure of the specimens was done by performing X-ray diffraction analysis in the thin-film mode (Bruker AXS, MXP-18AHF22) using an incident angle of 3°. 3. Results and discussion 3.1. Composition of PEO film Fig. 1 shows the cross-section SEM image of PEO film morphology resulting from 30 min anodization in 0.5 mol dm−3 Na3PO4 solution and the EDX maps of the film for elements Mg, O, and P. The anodization curve and surface film structure were reported previously [17]. The substrate surfaces were exposed to a uniform fine plasma discharge for 16 min followed by an intense sparking anodization for another 14 min. Micro-discharge channels were formed between the electrolyte–film interface and the metal surface (anode) due to dielectric breakdown as a result of high current application. The channels became the path for the anions PO3− from the electrolyte to penetrate through 4 the metal surface and reacted with the Mg2 + ions ejected from the metal surface. Cations were also drawn to the oxide layers and incorporated in them. The sparking discharge generates plasma similar to the ionization of oxygen, melting and oxidizing the metal surface. The molten oxide layers solidified rapidly, as in contact with the electrolyte. Vigorous gas evolution was observed when sparking occurred as a result of oxygen evolution. The resulting film was porous with a lavalike structure consisting of hills and valleys as shown in Fig. 1a. Cracks and voids were observed to appear randomly on the oxide film. Cracks were mainly caused by the release of internal stresses during rapid solidification of the molten oxide [21]. Voids of 5–10 μm in diameter were found inside the oxide, which were likely to have formed as a result of entrapment of the gas released during anodization. The average film thickness was 48 μm, as measured by coating thickness meter and cross section SEM image. Thick-film (60–100 μm) areas as a result of strong discharge during anodization were also observed by crosssectional FE-SEM image shown later in Fig. 4a. EDX maps on the PEO film, presented in Fig. 1b–d, detected O, P and Mg as the main components of the oxide film with concentrations of 56.98, 13.74, and 19.15 at.%, respectively. The concentration ratio in terms of molecular weight of O:Mg:P was 8.3:2.8:2, which is close to that of Mg3(PO4)2. Na was also detected at a concentration of 4.23 at.%. The XRD pattern obtained from the anodized specimen, shown later in Fig. 6, identified crystal phases in addition to amorphous phases in the film. The crystal phases were composed of Mg3(PO4)2 and Mg(PO3)2. Unlike the crystal phase, the amorphous phase could not be identified by XRD. During anodization, while water reduction mainly occurs in the cathode, some reactions proceed in the anode (substrate). The Mg metal surface is oxidized directly to form MgO/Mg(OH)2 or released as Mg2+ into the electrolyte. When sparking discharge occurs, metal ions which are simultaneously ejected from the metal surface
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Fig. 1. (a) Cross-section SEM image and the corresponding EDX maps for (b) Mg, (c) O, and (d) P.
further migrate outwards from the film layer through the discharge channels and react with electrolyte anions to form Mg3(PO4)2 and Mg(PO3)2 through the following reactions: 2þ
3Mg
2þ
Mg
3−
þ 2PO4 →Mg3 ðPO4 Þ2 −
þ 2PO3 →MgðPO3 Þ2 :
ð1Þ ð2Þ
The unstable PO− 3 ions consumed in reaction (2) were presumably formed as a result of a thermo-chemical reaction caused by plasma discharge where an oxygen ion was released from the phosphate ion following the reaction: 3−
−
2−
PO4 →PO3 þ O
:
ð3Þ
3.2. Apatite coating Fig. 2 shows the morphology of the PEO film surface after implementing the AIM treatment. Aggregated apatite particles, indicated by arrows, of micron-order size had formed locally on the film surface. A higher-magnification image of the apatite particles inside
Fig. 2. FE-SEM image (in-lens mode) of the film after AIM treatment showing the local growth of aggregated apatite particles, indicated by arrows, and a higher-magnification image of apatite particles inside the square area in the inset.
the square area is shown in the inset exposing the accumulated apatite particles that appeared as white areas in the image. The image also showed individual nano-size apatite grains deposited randomly on the film surface. The application of AIM treatment directly on the PEO film did not give uniform surface coverage of apatite. The uniformity of apatite nucleation on a substrate surface during immersion in Ca–phosphate-containing solutions is, in general, greatly affected by the surface roughness. Apatite is preferentially formed at a sub-micron level of an irregular structure [9–11]. The PEO film formed on AZ31 specimens exhibited a smooth surface at the sub-micron level and therefore restricted the precipitation of apatite during the AIM treatment. Similarly, in vitro immersion of the anodized AZ31 specimen in SBF for 7 days, in this work, resulted in only little deposition of apatite in the vicinity of the voids in the film as detected by EDX (not shown). Although the surface roughness increased as a result of superficial dissolution of the film which gave a corrugated structure, as observed by FESEM (Fig. 7a), the precipitation of apatite was negligible. The corrugated structure was presumed to have developed as the magnesium phosphate crystals in the PEO film emerged while the amorphous part that covered the crystals dissolved during immersion (inset Fig. 7a). Note that the crystals are less soluble than the amorphous oxide because of the larger inter-lattice bonding strength of crystalline oxide. Similarly, a PEO film that was formed on Ti substrate by anodization in Na3PO4 solution was reported to exhibit no bioactivity, as indicated by the absence of apatite deposition on its surface, even after a long immersion time of 70 days in SBF [22]. The low apatite-forming ability of the anodized AZ31 specimen in SBF gave some indication that the film was quite protective considering that the precipitation of apatite could also be induced by corrosion of the underlying substrate. This was confirmed by immersion of the bare AZ31 substrate in SBF for only 3 days, which produced a uniform corrosion product layer composed of Ca–phosphate compound with Ca and P concentrations of 3.41 and 5.73 at.%, respectively, based on EDX, on the surface. C and Mg with concentrations of 9.37 and 16.13 mass%, respectively were also detected on the surface. Amorphous, hydrated, carbonated-(Ca, Mg)-phosphate compound is the typical corrosion layer formed on an Mg alloy surface as a result of immersion in SBF [23]. Accumulation of corrosion products, in particular magnesium hydroxide, and local alkalization as a result of selective corrosion of Mg promoted precipitation of such compound. From the thermodynamic and kinetic viewpoints, both the driving force and the nucleation rate of apatite in SBF increased with increasing pH, as previously reported by Lu and Leng [24].
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Fig. 3. (a) Coarse morphology of PEO film on AZ31 specimen after alkali treatment at the step areas, marked by arrows, (b) a higher-magnification image around a step area inside the square area in image (a) showing a platelet structure in the order of 100 nm and a higher magnification image of the finer platelet layer at the flat surface in the inset.
As it was assumed that a lack of sub-micron surface roughness was the reason for the low bioactivity of the PEO film, an alkali treatment was applied on the anodized AZ31 specimen to induce surface roughness. The treatment was intended to affect approximately 1 μm of the outer film layer while maintaining the protective characteristic of the bulk film. Cracks and voids may become paths for the solution to penetrate deeper, but even if the solution came into contact with the substrate, an Mg(OH)2 layer would form and passivate the surface [25]. Fig. 3 shows the PEO film morphology after immersion of the anodized specimen in a dilute NaOH solution. There was no substantial microscopic change in the surface appearance of the anodized AZ31 specimen (Fig. 3a) compared to before treatment, except for the step areas, marked by arrows, which exhibited coarser morphology. The higher-magnification image of the square area in Fig. 3a revealed a nano-size platelet structure, as shown in Fig. 3b. The inset in Fig. 3b is the higher magnification images of the flat surface. In the step areas between the valleys and hills of the PEO film, the platelet size was 10 times larger, i.e., 500 nm, than that on the flat surfaces. The flat surfaces exhibited platelets with a size in the order of 10 nm. Surface roughness in the sub-micron scale was successfully fabricated uniformly on the PEO film of AZ31 specimens by alkali treatment. A similar platelet structure as that formed on the PEO film of AZ31 substrate could also be obtained on bare Mg metal surfaces, but by different treatments, i.e., hydrothermal treatment as reported in Ref. [26]. The resulting structure enhanced the uniformity and adherence of organic coating. The elemental concentrations of the alkali-treated film on AZ31 specimens, analyzed by EDX, were significantly different from those of the as-anodized surface. The O and Mg concentrations on the alkalitreated surface increased to 61.4 and 23.7 at.%, respectively, while the P and Na concentrations decreased to 5.27 and 1.69 at.%, respectively, relative to the as-anodized surface. The surface analysis on the PEO film suggested the presence of the Mg(OH)2 layer as the result of treatment in alkaline solution. Na+ from the alkaline solution did not appear
to be incorporated in the hydroxide layer. This is in contrast to the reported results for Ti [9–11], where a large amount of Na+ from the solution was consumed for the formation of a bioactive Na2Ti5O11 layer on the surface. For biomedical implants, a large amount of Na ion released into body fluid during implantation is disadvantageous because it may cause external alkalization that triggers an inflammatory response, leading to cell death [27]. After subsequent AIM treatment, the alkali-treated surface was decorated with protruding crystal sheets, as shown in Fig. 4a. The crystals grew vertically to about 20 μm. The crystal sheets were mainly formed in the step areas where the platelet size resulting from the alkali treatment was larger than those formed on the flat surface. In addition to the crystal sheets, an apatite layer had formed uniformly on the surface. A higher-magnification cross-sectional image taken near the surface of the anodic oxide film is shown in Fig. 4b, which clearly revealed the thickness of the apatite layer to be about 1 μm. A uniform apatite coating was successfully deposited on the PEO film of AZ31 specimen by prior alkali treatment. The layer which grew following the film structure consisted of nano-size needle-shaped and spherical granulated apatite, as shown in Fig. 4b. Apatite crystals with both shapes are commonly obtained by AIM treatment [16]. The size of the apatite crystal in the nm scale was favorable for filling the micro-voids, and micro-cracks existed in the film surface. Fewer voids were noticeable on the film surface after coating. Fig. 5 shows the EDX spectra taken from an area in a crystal sheet and the apatite layer. The signal from the PEO film underneath the crystal and layer of apatite was included in the spectra, as indicated by the appearance of Mg spectra. Therefore, the Ca/P ratio of the crystal sheet, 1.62 in mass ratio or 1.26 in atomic ratio, was lower than the atomic ratio for hydroxyapatite (HA) (1.67). The analysis of the apatite layer detected concentrations of 15.3 at.% Ca and 13.3 at.% P, which gave a Ca/P ratio of 1.15. C was detected in the layer with a concentration as high as 8.13 at.%. C might have originated from the dissolved CO2 from air into the solution. The Ca/P atomic ratios from the EDX analysis
Fig. 4. Cross-section FE-SEM images of the alkali-treated film after AIM treatment showing (a) crystal sheets of apatite, indicated by white arrows, and (b) a uniform apatite layer covering a surface consisting of both platelets and needle-shaped/spherical apatite particles with thickness ~1 μm.
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Fig. 5. Plane-view image of the alkali-treated film after AIM treatment and the EDX spectra from an area in the crystal sheet, marked with a white square, and from the area covered by an apatite layer, marked with a black square.
data obtained at several different points in the apatite layer were in the range of 1.10–1.18, far below the ratio for HA, indicating that the layer corresponded to Ca-deficient type of HA (d-HA). Mg2 + and Na+ might be substituted at the Ca2 + position while CO23 − might be substituted by PO34 − or OH− position in HA [28]. Nevertheless, it has been suggested that d-HA also plays an important role in bone remodeling and bone formation, as discussed by Posner [29]. Fig. 6 shows the X-ray diffraction (XRD) patterns of the anodized surface after alkali and AIM treatments and the as-anodized surface. The XRD analysis indicated that the crystalline phases of the PEO film consisted of Mg3(PO4)2 and Mg(PO3)2 in addition to the amorphous oxide, as identified from the series of peaks appearing between 20° and 45° marked in Fig. 5. Similar peaks for Mg3(PO4)2 and Mg(PO3)2 were also detected in the alkali- and AIM-treated specimens. In addition to the oxide phases, Mg(OH)2 peaks were identified at 37.3° and 58.4°, corresponding to the platelet layer that formed as a result of alkali treatment. The peaks for HA were overlapped with the existing peaks for the oxide phases. Thermodynamically, hydroxyapatite (Ca10(OH)2(PO4)6, HA) is the most stable form of Ca–P precipitate among other phases, such as dicalcium phosphate (CaHPO4·2H2O, DCPD) and octacalcium phosphate (Ca8(HPO4)2(PO4)4, OCP), synthesized in aqueous solutions [24,29,30]. Besides pH and temperature, the type of precipitation is affected by the amount of Ca and phosphate ions in the solution. Precipitation of HA and OCP commonly occurred in SBF [12–16], while DCPD only formed in a solution containing an excess of Ca and phosphate ions relative to SBF [24]. The calcium and phosphate ion concentrations in the saturated Ca(OH)2 solution (pH 13.3) and 0.02 mol dm−3 (NH4)2HPO4 solution (pH 8.3) used for AIM treatment were in excessive amounts and, therefore, the formation of HA, OCP, and DCPD phases is possible. Considering the larger driving force for nucleation of HA, the
metastable OCP and DCPD may transform into HA with time. The type of Ca–phosphate compound formed after AIM on the anodized AZ31 surfaces could not be clearly resolved by XRD since most of the peaks for the three types of apatite described above overlapped. 3.3. Coating performance in SBF The degradation behavior of the PEO film and apatite coating in SBF was evaluated using FESEM and EDX analyses after immersion of the specimens for 7 days. Fig. 7 shows the film morphology of the
Fig. 6. X-ray diffraction patterns of (a) the PEO film on AZ31 specimen and (b) the films after alkali and AIM treatments. HA designates hydroxyapatite.
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Fig. 7. FE-SEM images of the PEO film morphology of (a) anodized, (b) anodized and AIM, and (c) anodized, alkali, and AIM treated specimens after immersion in SBF for 7 days showing superficial dissolution of the film in images (a) and (b), the undissolved crystalline oxide at the groove pointed by an arrow revealed in the inset image, while no noticeable film dissolution in image (c).
a) as-anodized, b) AIM-treated, and c) alkali- and AIM-treated specimens after immersion. The AIM-treated film had undergone superficial dissolution resulting in a corrugated structure (Fig. 7b) similar to that of the only anodized specimen (Fig. 7a). The crystalline oxide phases along the groove were exposed. A higher magnification image of the partly dissolved oxide is attached as inset in Fig. 7a, revealing the parallel crystallographic plane perpendicular to the surface. The locally grown apatite layer formed on AIM-treated oxide did not contribute to the film's resistance from being attacked by the corrosive solution. Moreover, there was no noticeable additional deposition of apatite on the corresponding surface after the following SBF immersion (Fig. 7b). The locally formed apatite crystal obtained by AIM treatment was apparently not sufficient to trigger further precipitation of apatite in SBF. Meanwhile, the alkali- and AIM-treated surfaces (Fig. 7c) did not show any noticeable film deterioration after SBF soaking. The cracks observed in the apatite coating were due to dehydration of the apatite developed during drying of the specimen and further under electron beam exposure during FE-SEM investigation. The uniform apatite coating on the alkali- and AIM-treated film surfaces restricted the dissolution of the underlying film during exposure in SBF. The apatite forming-ability of the specimens in SBF was also analyzed by performing EDX maps of the specimen surfaces after immersion. The apatite, which has chemical composition of Ca10(PO4)6(OH), is represented by Ca (green) and P (red) elements in the EDX maps shown in Fig. 8. The map for the as-anodized specimen gave a uniform red color (not shown) because the film contained a high amount of the P element. Therefore, although apatite contained both Ca and P elements, the presence of apatite in the map in Fig. 8 was indicated by
the green color. The EDX map of the alkali- and AIM-treated surfaces shown in Fig. 8a had orange and green areas. The orange areas are PEO film areas covered by a thin apatite layer, while the green areas show the existence of a thicker apatite layer. After immersion in SBF for 7 days, the green areas expanded (Fig. 8b) and the intensity of the green color was stronger which indicated the growth and thickening of the apatite layer. The results indicated that the bioactivity of the PEO film in SBF was significantly improved by the alkali- and AIMtreatments. Surface changes of the PEO film after successive alkali and AIM treatments and further soaking in SBF is schematically summarized in Fig. 9. Anodization of AZ31 specimen produced a lava-like film structure containing voids and cracks. The alkali treatment produced Mg(OH)2 platelets in sub-micron size at the PEO film surface. The platelets were also formed at cracks and voids which were exposed to the alkali solution during treatment. The resulting structure provided a substantially large contact area with solutions during AIM treatment and therefore allowed the accumulation of Ca–phosphate compound on the surface. The hydroxyl headgroups in Mg(OH)2 layer further induced nucleation of the Ca–phosphate compound into apatite crystal [31]. Eventually, a uniform apatite layer decorated by apatite-crystal sheets was formed on the PEO film as a result of alkali and AIM treatments. The high rate of apatite nucleation at the step areas which exhibited larger platelet sizes than the flat surface was probably the reason for the formation of apatite-crystal sheets. Immersion in SBF for 7 days triggered the deposition of apatite, marked as SBF-apatite in the figure, on top of the apatite coating. The apatite crystal sheets were covered by the newly grown apatite.
Fig. 8. EDX maps for Ca (green) and P (red) of the surface of (a) anodized, alkali, and AIM treated specimens, and (b) the corresponding maps of the surfaces after soaking in SBF for 7 days. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 9. Schematic diagram of the PEO film surfaces resulting from anodization and after the following alkali treatment, AIM treatment, and immersion in SBF.
3.4. Electrochemical corrosion The corrosion behavior of differently treated surfaces was studied by carrying out polarization measurements in physiological 0.9% NaCl solution. The results are shown in Fig. 10. The polarization curves were replicated for at least three specimens and the results were highly reproducible. The corrosion potential and current density were determined from a Tafel plot. The substrate and anodized specimen display similar polarization curves except that the latter has higher corrosion potential and lower polarization current densities indicating a higher corrosion resistance. The substrate exhibited a corrosion potential of − 1.52 VAg/AgCl and a corrosion current density of 1.85 × 10−5 A cm− 2, while the anodized specimen showed a more noble corrosion potential at − 1.48 VAg/AgCl and a two orders lower corrosion current density of 1.98 × 10−7 A cm−2. The AIM treatment did not affect the polarization curve of the anodized specimen significantly. The corrosion potential of AIM-treated specimen was similar to that of the as-anodized specimen. The slight shift in the cathodic current region of the curve for AIM treated specimen to higher current density relative to the curve for anodized specimen increased the corrosion current density to 3.58 × 10−7 A cm− 2. The results indicated that local deposition of apatite on PEO film obtained by AIM treatment did not have a substantial effect on the improvement of corrosion resistance of the anodized specimen. Meanwhile, the polarization curve of the anodized specimen shifted to a nobler corrosion potential of −1.42 VAg/AgCl as result of alkali- and AIM-treatments. The corrosion potential of AIM- and alkali-treated specimens was 60 mV higher and the corrosion current density was appreciably lower at 5.06 × 10−9 A cm−2 relative to the as-anodized specimen. Moreover, a relatively wide passivation region existed in the anodic curve of the alkali- and AIM-treated specimens following the corrosion potential up to the pitting potential at − 1.25 VAg/AgCl.
The uniform apatite coating on the alkali- and AIM-treated films gave the characteristic of such passivation current region. The nano-size granulate apatite contributed in sealing the micro-voids and microcracks existed in the film. A stable corrosion current density is beneficial for an implant material because the material may be exposed to harsh solution conditions with various local potential changes following implantation as a result of interactions with other components of body fluid such as proteins and albumin. The polarization behavior was in good agreement with the in vitro test results (Fig. 7), where the alkali- and AIM-treated anodized specimens exhibited higher corrosion resistance. The improved bioactivity and corrosion resistance of the alkali- and AIM-treated surfaces may be beneficial for application to biomedical implant devices. 4. Conclusions Direct application of AIM treatment on an anodic film formed by the PEO technique on AZ31 specimen led to the formation of a non-uniform apatite coating due to the low apatite forming ability of the film. The lack of surface roughness at the sub-micron scale was the main reason for the low bioactivity of the film surfaces. We fabricated a sub-micron scale platelet Mg(OH)2 layer on the PEO film by the treatment in diluted NaOH solution. The apatite forming ability of the film surface after AIM treatment was markedly improved by the formation of a uniform apatite layer deposited on the alkali treated film surfaces. The layer composed of granulated apatite had a thickness of 1 μm. The alkali and AIM treatments on anodized AZ31 specimen greatly enhanced its corrosion resistance in both SBF and NaCl solutions. Combining anodization and apatite coating provides improvement in corrosion resistance and the bioactivity of the AZ31 alloy in physiological solutions. Acknowledgments This work was partially financially supported by a Grant-in-Aid for Scientific Research (A) no. 20241026 from the Japan Society for the Promotion of Science. We also acknowledge the Light Metal Educational Foundation, Inc. (2013-2014), and the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT)-Supported Program for the Strategic Research Foundation at Private Universities, 2013–2017 (S1311008). References
Fig. 10. Polarization curves of AZ31 alloy specimens treated under different conditions obtained in 0.9 wt.% NaCl solution at 37 °C.
[1] G. Song, Corros. Sci. 49 (4) (2007) 1696–1701. [2] F. Witte, N. Hort, C. Vogt, S. Cohen, K.U. Kainer, R. Willumeit, F. Feyerabend, Curr. Opin. Solid State Mater. Sci. 12 (2008) 63–72. [3] J.E. Gray-Munro, C. Seguin, M. Strong, J. Biomed. Mater. Res. 90A (2) (2009) 339–350. [4] E.P. DeGarmo, Materials and Processes in Manufacturing, 11th ed. Wiley & Sons Inc., USA, 2011. [5] S. Virtanen, Mater. Sci. Eng. B 176 (2011) 1600–1608. [6] F. Witte, Acta Biomater. 6 (2010) 1680–1692. [7] J.L. Patel, N. Saka, Interceram 50 (5) (2001) 398–400.
Please cite this article as: Anawati, et al., Surf. Coat. Technol. (2015), http://dx.doi.org/10.1016/j.surfcoat.2015.04.007
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[8] G. Song, Surf. Coat. Technol. 203 (2009) 3618–3625. [9] T. Kokubo, F. Miyaji, H.M. Kim, J. Am. Ceram. Soc. 4 (1996) 1127–1129. [10] S. Nishiguchi, H. Kato, H. Fujita, M. Oka, H.M. Kim, T. Kokubo, T. Nakamura, Biomaterials 22 (2001) 2525–2533. [11] L. Jonasova, F.A. Muller, A. Helebrant, J. Strnad, P. Greil, Biomaterials 25 (2002) 3095–3101. [12] N. Ohtsu, S. Hiromoto, M. Yamane, K. Satoh, M. Tomozawa, Surf. Coat. Technol. 218 (2013) 114–118. [13] S. Hiromoto, M. Tomozawa, Surf. Coat. Technol. 205 (2011) 4711–4719. [14] F. Barrere, P. Layrolle, C.A. van Blitterswijk, K. de Groot, Bone 25 (2) (1999) 107s–111s. [15] P.A.A.P. Marques, M.C.F. Magalhaes, R.N. Correia, Biomaterials 24 (2003) 1541–1548. [16] A. Kodama, S. Bauer, A. Komatsu, H. Asoh, S. Ono, P. Schmuki, Acta Biomater. 5 (2009) 2322–2330. [17] H. Asoh, S. Matsuoka, H. Sayama, S. Ono, J. Jpn. Inst. Light Met. 60 (2010) 608–614. [18] L. Müller, F.A. Müller, Acta Biomater. 2 (2006) 181–189. [19] 1991. 29. [20] Anawati, H. Tanigawa, H. Asoh, T. Ohno, M. Kubota, S. Ono, Corros. Sci. 70 (2013) 212–220.
[21] X. Zhou, G.E. Thompson, P. Skeldon, G.C. Wood, K. Shimizu, H. Habazaki, Corros. Sci. 41 (1999) 1599–1613. [22] Y. Han, S.H. Hong, K. Xu, Surf. Coat. Technol. 168 (2003) 249–258. [23] R. Rettig, S. Virtanen, J. Biomed. Mater. Res. 88A (2009) 359–369. [24] X. Lu, Y. Leng, Biomaterials 26 (2005) 1097–1108. [25] M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, Pergamon Press, Oxford, 1966. [26] W.F. Ng, M.H. Wong, F.T. Cheng, Surf. Coat. Technol. 204 (2010) 1823–1830. [27] I.A. Silver, J. Deas, M. Erecinska, Biomaterials 22 (2001) 175–185. [28] C. Wen, S. Guan, L. Peng, C. Ren, X. Wang, Z. Hu, Appl. Surf. Chem. 255 (2009) 6433–6438. [29] A.S. Posner, Physiol. Rev. 49 (1969) 760–792. [30] P.G. Koutsoukos, Z. Amjad, M.B. Tomson, G.H. Nancollas, J. Am. Chem. Soc. 102 (1980) 1553–1557. [31] P. Zhu, Y. Masuda, K. Koumoto, Biomaterials 25 (2004) 3915–3921.
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Enhanced uniformity of apatite coating on a PEO film formed on AZ31 Mg alloy by an alkali pretreatment Anawati a, Hidetaka Asoh a,b, Sachiko Ono a,b,⁎ a b
Research Institute for Science and Technology, Kogakuin University, 2665-1 Nakano, Hachioji, Tokyo 192-0015, Japan Department of Applied Chemistry, Kogakuin University, 2665-1 Nakano, Hachioji, Tokyo 192-0015, Japan
a r t i c l e
i n f o
Article history: Received 10 October 2014 Accepted in revised form 3 April 2015 Available online xxxx Keywords: Magnesium Anodization Apatite Alkali treatment Corrosion
a b s t r a c t Anodization by plasma electrolytic oxidation (PEO) and subsequent apatite coating were performed on a biodegradable AZ31 magnesium alloy to enhance its corrosion resistance and bioactivity in physiological solution. The PEO film itself (~ 48 μm in thickness) exhibited low bioactivity, where only aggregated apatite particles were deposited locally on its surface as a result of the alternative immersion method (AIM) in Ca– phosphate solutions. The uniformity of apatite coating on the PEO film was markedly improved by pretreatment of the film in a dilute NaOH solution. The alkali treatment induced the formation of a nano-size platelet Mg(OH)2 layer on the film surfaces that drastically enlarged the effective surface area for the precipitation of apatite. A uniform apatite layer as thick as 1 μm was successfully deposited on the hydroxide layer after AIM treatment. The enhanced uniformity of the apatite coating on an alkali- and AIM-treated surface significantly improved the corrosion resistance in both simulated body fluid (SBF) and NaCl solution, and the bioactivity in SBF. © 2015 Published by Elsevier B.V.
1. Introduction Magnesium (Mg) and its alloys exhibit excellent biocompatibility, suitable mechanical properties and spontaneous degradation in a physiological environment which are applicable for biodegradable implant devices [1–6]. In many cases of implantation, the body needs a temporary implant or device, in which case biodegradable materials represent a better option than inert ones [2,6]. Historically, Mg and its alloys have been studied as implant material since 1878 [6]; however, commercial medical devices are still not available. The corrosion rate of Mg in a physiological environment is still considered to be too high, leading to two major consequences: the rapid generation of gas (H2) bubbles and the loss of mechanical integrity. High corrosion rate was observed on the first week of implantation, as indicated by the generation of significant gas bubbles, and normally reduced in the following weeks [2]. Therefore, improving temporary corrosion resistance of Mg implant to retard early degradation in a physiological environment is crucial. Anodization is an effective way of mitigating the corrosion of an Mg metal surface by forming an anodic oxide film to prevent direct contact of the substrate with the corrosive environment. Plasma electrolytic oxidation (PEO), also called micro-arc oxidation (MAO), is a common surface treatment used to form a ceramic-type oxide film on Mg alloys as well as on other valve metals (Al, Ti, Ta, Nb, Zr, etc.) by the application of a high anodic voltage to create intense plasma near the metal surface ⁎ Corresponding author at: Department of Applied Chemistry, Kogakuin University, 2665-1 Nakano, Hachioji, Tokyo 192-0015, Japan. E-mail address: [email protected] (S. Ono).
to induce oxidation [7]. A PEO film is often applied on Mg alloy substrates before final organic coating. Compared with the direct application of organic coating on the bare Mg alloy substrate, the corrosion resistance of Mg alloys with a PEO film formed before organic coating was much improved [8]. Apatite is frequently used as a final coating for orthopedic implant materials to provide bioactivation properties. The existence of apatite early during the implantation period enables the strong fixation of an implant to the host tissue or bone. Apatite coating can be synthesized by immersion in physiological solutions such as simulated body fluid (SBF), Hank's solution and Kokubo's solution [9–15]. However, it normally requires a long immersion time, in weeks, to fully cover the surface with apatite while the Mg specimen may severely degrade in such highly corrosive environment. The application of an alternative immersion method (AIM) in Ca- and phosphate-containing solutions has been reported to be an effective means of accelerating the deposition of apatite layer on a porous oxide film formed on Ti in a short time [16]. Compared to other coating techniques, such as plasma spray and sputtering, AIM treatment does not the involve application of high temperature, which is suitable for Mg substrate, considering its low melting temperature. In this work, the AIM treatment was applied on a PEO film formed on AZ31 magnesium alloy specimens. To enhance the apatite coating uniformity, alkali treatment is proposed. Alkali treatments are typically used to promote apatite nucleation on a Ti metal surface in simulated body fluid (SBF). The alkali-treated Ti metal surface becomes highly bioactive owing to the formation of a hydrated sodium titanium (Na2Ti5O11) hydrogel layer with a submicron porous structure [9–11]. The release of Na+ from the hydrogel layer,
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which is further transformed into a Ti–OH layer during immersion in SBF, induces HA nucleation on the titanium surface. Apart from its application on metal surfaces, the specific application of an alkali treatment on a PEO film to induce strong bonding to a final coating has not been investigated. In this study, the effect of the alkali treatment on the apatite coating uniformity of a PEO film on AZ31 alloy was investigated. The corrosion resistance and bioactivity of the coated specimen were further studied by an in vitro immersion test in SBF. The electrochemical corrosion behavior was also examined by performing a polarization test in NaCl solution. 2. Materials and methods 2.1. Formation of PEO film The specimens used were obtained from an extruded plate of AZ31 alloy, with a thickness of 1 mm cut into pieces to give a working area of 5 cm2. The alloy composition is listed in Table 1. The specimens were degreased in acetone in an ultrasonic bath for 3 min prior to further use. Before anodization, the specimens were pretreated in a mixed acid solution of 10 vol.% HNO3–2 vol.% H3PO4 for 20 s followed by dipping in 5 wt.% NaOH solution at 80 °C for 1 min. Anodization was carried out in 0.5 mol dm−3 Na3PO4 solution at a constant current of 200 A m− 2 at 25 °C for 30 min. An earlier work [17] revealed that the corrosion resistance of a PEO film formed in Na3PO4 solution is higher than that formed in a silicate-based solution or other inorganic solutions. The alkali treatment was carried out in 0.25 mol dm− 3 NaOH solution at 80 °C for 90 min after anodization. The treatment conditions used in this work were substantially milder than those used for metal surfaces (5–10 mol dm−3) [9–11]. 2.2. Evaluation of apatite forming ability AIM treatment was performed to coat the anodized specimen with apatite. The treatment was accomplished by dipping the specimens in saturated Ca(OH)2 solution (pH 13.3) and 0.02 mol dm− 3 (NH4)2HPO4 solution (pH 8.3) for 1 min in each solution followed by 10 s rinsing in ultrapure water between each immersion for 20 cycles. The AIM solutions were exposed to laboratory air at room temperature. An in vitro immersion in SBF was carried out on both the bare substrate and anodized specimens of AZ31 to evaluate the bioactivity and corrosion behavior. The specimens were soaked individually in SBF10, using the ionic concentrations shown in Table 2, with a pH of 7.4 at 37 °C. The SBF10 was prepared as previously reported [18]. The specimens were exposed to the solution with a surface-to-volume ratio of 20 ml cm−2. The solution was refreshed every 3 days. After exposure, the specimens were washed in distilled (DI) water. 2.3. Electrochemical analysis The corrosion behavior of the specimens was studied by carrying out potentiodynamic polarization measurements using a potentiostat instrument (IviumStat) in an aerated 0.9 wt.% NaCl solution at 37 °C on the basis of the ASTM standard and our previous work [19,20]. Prior to polarization measurements, the specimen was left in an open circuit potential for 60 s. The polarization test was started at a potential of −1.65 VAg/AgCl and was terminated when the current output reached 30 mA with a sweep rate of 1 mV s−1. A Pt wire was used as a counter electrode and silver chloride was used as a reference electrode. The Table 1 Chemical composition of alloying elements in AZ31 alloy in wt.%. Al
Zn
Mn
Cu
Ni
Fe
Mg
2.5–3.5
0.5–1.5
N0.2
b0.1
b0.005
b0.03
Balance
Table 2 Ionic concentrations of SBF10 [14]. Ion
Na+
K+
Mg+
Ca+
Cl−
HCO− 3
HPO2− 4
SO2− 4
Concentration (mM)
142
5
1
2.5
126
10
1
1
temperature was controlled at 37 °C using an electrode heater equipped with a temperature-sensing device inside the cell. 2.4. Characterization of surface structure The surface microstructure of the anodized specimens was investigated using a JEOL JSM-6701 field-emission scanning electron microscope (FE-SEM). The elemental analysis was performed by energydispersive X-ray spectroscopy (EDX) using a JEOL EX-54175JMU spectroscope attached to an SEM (JEOL JSM-6380LA) over an area of 250 × 200 μm2 at an accelerating voltage of 15 kV. The analysis of the chemical composition and crystalline structure of the specimens was done by performing X-ray diffraction analysis in the thin-film mode (Bruker AXS, MXP-18AHF22) using an incident angle of 3°. 3. Results and discussion 3.1. Composition of PEO film Fig. 1 shows the cross-section SEM image of PEO film morphology resulting from 30 min anodization in 0.5 mol dm−3 Na3PO4 solution and the EDX maps of the film for elements Mg, O, and P. The anodization curve and surface film structure were reported previously [17]. The substrate surfaces were exposed to a uniform fine plasma discharge for 16 min followed by an intense sparking anodization for another 14 min. Micro-discharge channels were formed between the electrolyte–film interface and the metal surface (anode) due to dielectric breakdown as a result of high current application. The channels became the path for the anions PO3− from the electrolyte to penetrate through 4 the metal surface and reacted with the Mg2 + ions ejected from the metal surface. Cations were also drawn to the oxide layers and incorporated in them. The sparking discharge generates plasma similar to the ionization of oxygen, melting and oxidizing the metal surface. The molten oxide layers solidified rapidly, as in contact with the electrolyte. Vigorous gas evolution was observed when sparking occurred as a result of oxygen evolution. The resulting film was porous with a lavalike structure consisting of hills and valleys as shown in Fig. 1a. Cracks and voids were observed to appear randomly on the oxide film. Cracks were mainly caused by the release of internal stresses during rapid solidification of the molten oxide [21]. Voids of 5–10 μm in diameter were found inside the oxide, which were likely to have formed as a result of entrapment of the gas released during anodization. The average film thickness was 48 μm, as measured by coating thickness meter and cross section SEM image. Thick-film (60–100 μm) areas as a result of strong discharge during anodization were also observed by crosssectional FE-SEM image shown later in Fig. 4a. EDX maps on the PEO film, presented in Fig. 1b–d, detected O, P and Mg as the main components of the oxide film with concentrations of 56.98, 13.74, and 19.15 at.%, respectively. The concentration ratio in terms of molecular weight of O:Mg:P was 8.3:2.8:2, which is close to that of Mg3(PO4)2. Na was also detected at a concentration of 4.23 at.%. The XRD pattern obtained from the anodized specimen, shown later in Fig. 6, identified crystal phases in addition to amorphous phases in the film. The crystal phases were composed of Mg3(PO4)2 and Mg(PO3)2. Unlike the crystal phase, the amorphous phase could not be identified by XRD. During anodization, while water reduction mainly occurs in the cathode, some reactions proceed in the anode (substrate). The Mg metal surface is oxidized directly to form MgO/Mg(OH)2 or released as Mg2+ into the electrolyte. When sparking discharge occurs, metal ions which are simultaneously ejected from the metal surface
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Fig. 1. (a) Cross-section SEM image and the corresponding EDX maps for (b) Mg, (c) O, and (d) P.
further migrate outwards from the film layer through the discharge channels and react with electrolyte anions to form Mg3(PO4)2 and Mg(PO3)2 through the following reactions: 2þ
3Mg
2þ
Mg
3−
þ 2PO4 →Mg3 ðPO4 Þ2 −
þ 2PO3 →MgðPO3 Þ2 :
ð1Þ ð2Þ
The unstable PO− 3 ions consumed in reaction (2) were presumably formed as a result of a thermo-chemical reaction caused by plasma discharge where an oxygen ion was released from the phosphate ion following the reaction: 3−
−
2−
PO4 →PO3 þ O
:
ð3Þ
3.2. Apatite coating Fig. 2 shows the morphology of the PEO film surface after implementing the AIM treatment. Aggregated apatite particles, indicated by arrows, of micron-order size had formed locally on the film surface. A higher-magnification image of the apatite particles inside
Fig. 2. FE-SEM image (in-lens mode) of the film after AIM treatment showing the local growth of aggregated apatite particles, indicated by arrows, and a higher-magnification image of apatite particles inside the square area in the inset.
the square area is shown in the inset exposing the accumulated apatite particles that appeared as white areas in the image. The image also showed individual nano-size apatite grains deposited randomly on the film surface. The application of AIM treatment directly on the PEO film did not give uniform surface coverage of apatite. The uniformity of apatite nucleation on a substrate surface during immersion in Ca–phosphate-containing solutions is, in general, greatly affected by the surface roughness. Apatite is preferentially formed at a sub-micron level of an irregular structure [9–11]. The PEO film formed on AZ31 specimens exhibited a smooth surface at the sub-micron level and therefore restricted the precipitation of apatite during the AIM treatment. Similarly, in vitro immersion of the anodized AZ31 specimen in SBF for 7 days, in this work, resulted in only little deposition of apatite in the vicinity of the voids in the film as detected by EDX (not shown). Although the surface roughness increased as a result of superficial dissolution of the film which gave a corrugated structure, as observed by FESEM (Fig. 7a), the precipitation of apatite was negligible. The corrugated structure was presumed to have developed as the magnesium phosphate crystals in the PEO film emerged while the amorphous part that covered the crystals dissolved during immersion (inset Fig. 7a). Note that the crystals are less soluble than the amorphous oxide because of the larger inter-lattice bonding strength of crystalline oxide. Similarly, a PEO film that was formed on Ti substrate by anodization in Na3PO4 solution was reported to exhibit no bioactivity, as indicated by the absence of apatite deposition on its surface, even after a long immersion time of 70 days in SBF [22]. The low apatite-forming ability of the anodized AZ31 specimen in SBF gave some indication that the film was quite protective considering that the precipitation of apatite could also be induced by corrosion of the underlying substrate. This was confirmed by immersion of the bare AZ31 substrate in SBF for only 3 days, which produced a uniform corrosion product layer composed of Ca–phosphate compound with Ca and P concentrations of 3.41 and 5.73 at.%, respectively, based on EDX, on the surface. C and Mg with concentrations of 9.37 and 16.13 mass%, respectively were also detected on the surface. Amorphous, hydrated, carbonated-(Ca, Mg)-phosphate compound is the typical corrosion layer formed on an Mg alloy surface as a result of immersion in SBF [23]. Accumulation of corrosion products, in particular magnesium hydroxide, and local alkalization as a result of selective corrosion of Mg promoted precipitation of such compound. From the thermodynamic and kinetic viewpoints, both the driving force and the nucleation rate of apatite in SBF increased with increasing pH, as previously reported by Lu and Leng [24].
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Fig. 3. (a) Coarse morphology of PEO film on AZ31 specimen after alkali treatment at the step areas, marked by arrows, (b) a higher-magnification image around a step area inside the square area in image (a) showing a platelet structure in the order of 100 nm and a higher magnification image of the finer platelet layer at the flat surface in the inset.
As it was assumed that a lack of sub-micron surface roughness was the reason for the low bioactivity of the PEO film, an alkali treatment was applied on the anodized AZ31 specimen to induce surface roughness. The treatment was intended to affect approximately 1 μm of the outer film layer while maintaining the protective characteristic of the bulk film. Cracks and voids may become paths for the solution to penetrate deeper, but even if the solution came into contact with the substrate, an Mg(OH)2 layer would form and passivate the surface [25]. Fig. 3 shows the PEO film morphology after immersion of the anodized specimen in a dilute NaOH solution. There was no substantial microscopic change in the surface appearance of the anodized AZ31 specimen (Fig. 3a) compared to before treatment, except for the step areas, marked by arrows, which exhibited coarser morphology. The higher-magnification image of the square area in Fig. 3a revealed a nano-size platelet structure, as shown in Fig. 3b. The inset in Fig. 3b is the higher magnification images of the flat surface. In the step areas between the valleys and hills of the PEO film, the platelet size was 10 times larger, i.e., 500 nm, than that on the flat surfaces. The flat surfaces exhibited platelets with a size in the order of 10 nm. Surface roughness in the sub-micron scale was successfully fabricated uniformly on the PEO film of AZ31 specimens by alkali treatment. A similar platelet structure as that formed on the PEO film of AZ31 substrate could also be obtained on bare Mg metal surfaces, but by different treatments, i.e., hydrothermal treatment as reported in Ref. [26]. The resulting structure enhanced the uniformity and adherence of organic coating. The elemental concentrations of the alkali-treated film on AZ31 specimens, analyzed by EDX, were significantly different from those of the as-anodized surface. The O and Mg concentrations on the alkalitreated surface increased to 61.4 and 23.7 at.%, respectively, while the P and Na concentrations decreased to 5.27 and 1.69 at.%, respectively, relative to the as-anodized surface. The surface analysis on the PEO film suggested the presence of the Mg(OH)2 layer as the result of treatment in alkaline solution. Na+ from the alkaline solution did not appear
to be incorporated in the hydroxide layer. This is in contrast to the reported results for Ti [9–11], where a large amount of Na+ from the solution was consumed for the formation of a bioactive Na2Ti5O11 layer on the surface. For biomedical implants, a large amount of Na ion released into body fluid during implantation is disadvantageous because it may cause external alkalization that triggers an inflammatory response, leading to cell death [27]. After subsequent AIM treatment, the alkali-treated surface was decorated with protruding crystal sheets, as shown in Fig. 4a. The crystals grew vertically to about 20 μm. The crystal sheets were mainly formed in the step areas where the platelet size resulting from the alkali treatment was larger than those formed on the flat surface. In addition to the crystal sheets, an apatite layer had formed uniformly on the surface. A higher-magnification cross-sectional image taken near the surface of the anodic oxide film is shown in Fig. 4b, which clearly revealed the thickness of the apatite layer to be about 1 μm. A uniform apatite coating was successfully deposited on the PEO film of AZ31 specimen by prior alkali treatment. The layer which grew following the film structure consisted of nano-size needle-shaped and spherical granulated apatite, as shown in Fig. 4b. Apatite crystals with both shapes are commonly obtained by AIM treatment [16]. The size of the apatite crystal in the nm scale was favorable for filling the micro-voids, and micro-cracks existed in the film surface. Fewer voids were noticeable on the film surface after coating. Fig. 5 shows the EDX spectra taken from an area in a crystal sheet and the apatite layer. The signal from the PEO film underneath the crystal and layer of apatite was included in the spectra, as indicated by the appearance of Mg spectra. Therefore, the Ca/P ratio of the crystal sheet, 1.62 in mass ratio or 1.26 in atomic ratio, was lower than the atomic ratio for hydroxyapatite (HA) (1.67). The analysis of the apatite layer detected concentrations of 15.3 at.% Ca and 13.3 at.% P, which gave a Ca/P ratio of 1.15. C was detected in the layer with a concentration as high as 8.13 at.%. C might have originated from the dissolved CO2 from air into the solution. The Ca/P atomic ratios from the EDX analysis
Fig. 4. Cross-section FE-SEM images of the alkali-treated film after AIM treatment showing (a) crystal sheets of apatite, indicated by white arrows, and (b) a uniform apatite layer covering a surface consisting of both platelets and needle-shaped/spherical apatite particles with thickness ~1 μm.
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Fig. 5. Plane-view image of the alkali-treated film after AIM treatment and the EDX spectra from an area in the crystal sheet, marked with a white square, and from the area covered by an apatite layer, marked with a black square.
data obtained at several different points in the apatite layer were in the range of 1.10–1.18, far below the ratio for HA, indicating that the layer corresponded to Ca-deficient type of HA (d-HA). Mg2 + and Na+ might be substituted at the Ca2 + position while CO23 − might be substituted by PO34 − or OH− position in HA [28]. Nevertheless, it has been suggested that d-HA also plays an important role in bone remodeling and bone formation, as discussed by Posner [29]. Fig. 6 shows the X-ray diffraction (XRD) patterns of the anodized surface after alkali and AIM treatments and the as-anodized surface. The XRD analysis indicated that the crystalline phases of the PEO film consisted of Mg3(PO4)2 and Mg(PO3)2 in addition to the amorphous oxide, as identified from the series of peaks appearing between 20° and 45° marked in Fig. 5. Similar peaks for Mg3(PO4)2 and Mg(PO3)2 were also detected in the alkali- and AIM-treated specimens. In addition to the oxide phases, Mg(OH)2 peaks were identified at 37.3° and 58.4°, corresponding to the platelet layer that formed as a result of alkali treatment. The peaks for HA were overlapped with the existing peaks for the oxide phases. Thermodynamically, hydroxyapatite (Ca10(OH)2(PO4)6, HA) is the most stable form of Ca–P precipitate among other phases, such as dicalcium phosphate (CaHPO4·2H2O, DCPD) and octacalcium phosphate (Ca8(HPO4)2(PO4)4, OCP), synthesized in aqueous solutions [24,29,30]. Besides pH and temperature, the type of precipitation is affected by the amount of Ca and phosphate ions in the solution. Precipitation of HA and OCP commonly occurred in SBF [12–16], while DCPD only formed in a solution containing an excess of Ca and phosphate ions relative to SBF [24]. The calcium and phosphate ion concentrations in the saturated Ca(OH)2 solution (pH 13.3) and 0.02 mol dm−3 (NH4)2HPO4 solution (pH 8.3) used for AIM treatment were in excessive amounts and, therefore, the formation of HA, OCP, and DCPD phases is possible. Considering the larger driving force for nucleation of HA, the
metastable OCP and DCPD may transform into HA with time. The type of Ca–phosphate compound formed after AIM on the anodized AZ31 surfaces could not be clearly resolved by XRD since most of the peaks for the three types of apatite described above overlapped. 3.3. Coating performance in SBF The degradation behavior of the PEO film and apatite coating in SBF was evaluated using FESEM and EDX analyses after immersion of the specimens for 7 days. Fig. 7 shows the film morphology of the
Fig. 6. X-ray diffraction patterns of (a) the PEO film on AZ31 specimen and (b) the films after alkali and AIM treatments. HA designates hydroxyapatite.
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Fig. 7. FE-SEM images of the PEO film morphology of (a) anodized, (b) anodized and AIM, and (c) anodized, alkali, and AIM treated specimens after immersion in SBF for 7 days showing superficial dissolution of the film in images (a) and (b), the undissolved crystalline oxide at the groove pointed by an arrow revealed in the inset image, while no noticeable film dissolution in image (c).
a) as-anodized, b) AIM-treated, and c) alkali- and AIM-treated specimens after immersion. The AIM-treated film had undergone superficial dissolution resulting in a corrugated structure (Fig. 7b) similar to that of the only anodized specimen (Fig. 7a). The crystalline oxide phases along the groove were exposed. A higher magnification image of the partly dissolved oxide is attached as inset in Fig. 7a, revealing the parallel crystallographic plane perpendicular to the surface. The locally grown apatite layer formed on AIM-treated oxide did not contribute to the film's resistance from being attacked by the corrosive solution. Moreover, there was no noticeable additional deposition of apatite on the corresponding surface after the following SBF immersion (Fig. 7b). The locally formed apatite crystal obtained by AIM treatment was apparently not sufficient to trigger further precipitation of apatite in SBF. Meanwhile, the alkali- and AIM-treated surfaces (Fig. 7c) did not show any noticeable film deterioration after SBF soaking. The cracks observed in the apatite coating were due to dehydration of the apatite developed during drying of the specimen and further under electron beam exposure during FE-SEM investigation. The uniform apatite coating on the alkali- and AIM-treated film surfaces restricted the dissolution of the underlying film during exposure in SBF. The apatite forming-ability of the specimens in SBF was also analyzed by performing EDX maps of the specimen surfaces after immersion. The apatite, which has chemical composition of Ca10(PO4)6(OH), is represented by Ca (green) and P (red) elements in the EDX maps shown in Fig. 8. The map for the as-anodized specimen gave a uniform red color (not shown) because the film contained a high amount of the P element. Therefore, although apatite contained both Ca and P elements, the presence of apatite in the map in Fig. 8 was indicated by
the green color. The EDX map of the alkali- and AIM-treated surfaces shown in Fig. 8a had orange and green areas. The orange areas are PEO film areas covered by a thin apatite layer, while the green areas show the existence of a thicker apatite layer. After immersion in SBF for 7 days, the green areas expanded (Fig. 8b) and the intensity of the green color was stronger which indicated the growth and thickening of the apatite layer. The results indicated that the bioactivity of the PEO film in SBF was significantly improved by the alkali- and AIMtreatments. Surface changes of the PEO film after successive alkali and AIM treatments and further soaking in SBF is schematically summarized in Fig. 9. Anodization of AZ31 specimen produced a lava-like film structure containing voids and cracks. The alkali treatment produced Mg(OH)2 platelets in sub-micron size at the PEO film surface. The platelets were also formed at cracks and voids which were exposed to the alkali solution during treatment. The resulting structure provided a substantially large contact area with solutions during AIM treatment and therefore allowed the accumulation of Ca–phosphate compound on the surface. The hydroxyl headgroups in Mg(OH)2 layer further induced nucleation of the Ca–phosphate compound into apatite crystal [31]. Eventually, a uniform apatite layer decorated by apatite-crystal sheets was formed on the PEO film as a result of alkali and AIM treatments. The high rate of apatite nucleation at the step areas which exhibited larger platelet sizes than the flat surface was probably the reason for the formation of apatite-crystal sheets. Immersion in SBF for 7 days triggered the deposition of apatite, marked as SBF-apatite in the figure, on top of the apatite coating. The apatite crystal sheets were covered by the newly grown apatite.
Fig. 8. EDX maps for Ca (green) and P (red) of the surface of (a) anodized, alkali, and AIM treated specimens, and (b) the corresponding maps of the surfaces after soaking in SBF for 7 days. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 9. Schematic diagram of the PEO film surfaces resulting from anodization and after the following alkali treatment, AIM treatment, and immersion in SBF.
3.4. Electrochemical corrosion The corrosion behavior of differently treated surfaces was studied by carrying out polarization measurements in physiological 0.9% NaCl solution. The results are shown in Fig. 10. The polarization curves were replicated for at least three specimens and the results were highly reproducible. The corrosion potential and current density were determined from a Tafel plot. The substrate and anodized specimen display similar polarization curves except that the latter has higher corrosion potential and lower polarization current densities indicating a higher corrosion resistance. The substrate exhibited a corrosion potential of − 1.52 VAg/AgCl and a corrosion current density of 1.85 × 10−5 A cm− 2, while the anodized specimen showed a more noble corrosion potential at − 1.48 VAg/AgCl and a two orders lower corrosion current density of 1.98 × 10−7 A cm−2. The AIM treatment did not affect the polarization curve of the anodized specimen significantly. The corrosion potential of AIM-treated specimen was similar to that of the as-anodized specimen. The slight shift in the cathodic current region of the curve for AIM treated specimen to higher current density relative to the curve for anodized specimen increased the corrosion current density to 3.58 × 10−7 A cm− 2. The results indicated that local deposition of apatite on PEO film obtained by AIM treatment did not have a substantial effect on the improvement of corrosion resistance of the anodized specimen. Meanwhile, the polarization curve of the anodized specimen shifted to a nobler corrosion potential of −1.42 VAg/AgCl as result of alkali- and AIM-treatments. The corrosion potential of AIM- and alkali-treated specimens was 60 mV higher and the corrosion current density was appreciably lower at 5.06 × 10−9 A cm−2 relative to the as-anodized specimen. Moreover, a relatively wide passivation region existed in the anodic curve of the alkali- and AIM-treated specimens following the corrosion potential up to the pitting potential at − 1.25 VAg/AgCl.
The uniform apatite coating on the alkali- and AIM-treated films gave the characteristic of such passivation current region. The nano-size granulate apatite contributed in sealing the micro-voids and microcracks existed in the film. A stable corrosion current density is beneficial for an implant material because the material may be exposed to harsh solution conditions with various local potential changes following implantation as a result of interactions with other components of body fluid such as proteins and albumin. The polarization behavior was in good agreement with the in vitro test results (Fig. 7), where the alkali- and AIM-treated anodized specimens exhibited higher corrosion resistance. The improved bioactivity and corrosion resistance of the alkali- and AIM-treated surfaces may be beneficial for application to biomedical implant devices. 4. Conclusions Direct application of AIM treatment on an anodic film formed by the PEO technique on AZ31 specimen led to the formation of a non-uniform apatite coating due to the low apatite forming ability of the film. The lack of surface roughness at the sub-micron scale was the main reason for the low bioactivity of the film surfaces. We fabricated a sub-micron scale platelet Mg(OH)2 layer on the PEO film by the treatment in diluted NaOH solution. The apatite forming ability of the film surface after AIM treatment was markedly improved by the formation of a uniform apatite layer deposited on the alkali treated film surfaces. The layer composed of granulated apatite had a thickness of 1 μm. The alkali and AIM treatments on anodized AZ31 specimen greatly enhanced its corrosion resistance in both SBF and NaCl solutions. Combining anodization and apatite coating provides improvement in corrosion resistance and the bioactivity of the AZ31 alloy in physiological solutions. Acknowledgments This work was partially financially supported by a Grant-in-Aid for Scientific Research (A) no. 20241026 from the Japan Society for the Promotion of Science. We also acknowledge the Light Metal Educational Foundation, Inc. (2013-2014), and the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT)-Supported Program for the Strategic Research Foundation at Private Universities, 2013–2017 (S1311008). References
Fig. 10. Polarization curves of AZ31 alloy specimens treated under different conditions obtained in 0.9 wt.% NaCl solution at 37 °C.
[1] G. Song, Corros. Sci. 49 (4) (2007) 1696–1701. [2] F. Witte, N. Hort, C. Vogt, S. Cohen, K.U. Kainer, R. Willumeit, F. Feyerabend, Curr. Opin. Solid State Mater. Sci. 12 (2008) 63–72. [3] J.E. Gray-Munro, C. Seguin, M. Strong, J. Biomed. Mater. Res. 90A (2) (2009) 339–350. [4] E.P. DeGarmo, Materials and Processes in Manufacturing, 11th ed. Wiley & Sons Inc., USA, 2011. [5] S. Virtanen, Mater. Sci. Eng. B 176 (2011) 1600–1608. [6] F. Witte, Acta Biomater. 6 (2010) 1680–1692. [7] J.L. Patel, N. Saka, Interceram 50 (5) (2001) 398–400.
Please cite this article as: Anawati, et al., Surf. Coat. Technol. (2015), http://dx.doi.org/10.1016/j.surfcoat.2015.04.007
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[8] G. Song, Surf. Coat. Technol. 203 (2009) 3618–3625. [9] T. Kokubo, F. Miyaji, H.M. Kim, J. Am. Ceram. Soc. 4 (1996) 1127–1129. [10] S. Nishiguchi, H. Kato, H. Fujita, M. Oka, H.M. Kim, T. Kokubo, T. Nakamura, Biomaterials 22 (2001) 2525–2533. [11] L. Jonasova, F.A. Muller, A. Helebrant, J. Strnad, P. Greil, Biomaterials 25 (2002) 3095–3101. [12] N. Ohtsu, S. Hiromoto, M. Yamane, K. Satoh, M. Tomozawa, Surf. Coat. Technol. 218 (2013) 114–118. [13] S. Hiromoto, M. Tomozawa, Surf. Coat. Technol. 205 (2011) 4711–4719. [14] F. Barrere, P. Layrolle, C.A. van Blitterswijk, K. de Groot, Bone 25 (2) (1999) 107s–111s. [15] P.A.A.P. Marques, M.C.F. Magalhaes, R.N. Correia, Biomaterials 24 (2003) 1541–1548. [16] A. Kodama, S. Bauer, A. Komatsu, H. Asoh, S. Ono, P. Schmuki, Acta Biomater. 5 (2009) 2322–2330. [17] H. Asoh, S. Matsuoka, H. Sayama, S. Ono, J. Jpn. Inst. Light Met. 60 (2010) 608–614. [18] L. Müller, F.A. Müller, Acta Biomater. 2 (2006) 181–189. [19] 1991. 29. [20] Anawati, H. Tanigawa, H. Asoh, T. Ohno, M. Kubota, S. Ono, Corros. Sci. 70 (2013) 212–220.
[21] X. Zhou, G.E. Thompson, P. Skeldon, G.C. Wood, K. Shimizu, H. Habazaki, Corros. Sci. 41 (1999) 1599–1613. [22] Y. Han, S.H. Hong, K. Xu, Surf. Coat. Technol. 168 (2003) 249–258. [23] R. Rettig, S. Virtanen, J. Biomed. Mater. Res. 88A (2009) 359–369. [24] X. Lu, Y. Leng, Biomaterials 26 (2005) 1097–1108. [25] M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, Pergamon Press, Oxford, 1966. [26] W.F. Ng, M.H. Wong, F.T. Cheng, Surf. Coat. Technol. 204 (2010) 1823–1830. [27] I.A. Silver, J. Deas, M. Erecinska, Biomaterials 22 (2001) 175–185. [28] C. Wen, S. Guan, L. Peng, C. Ren, X. Wang, Z. Hu, Appl. Surf. Chem. 255 (2009) 6433–6438. [29] A.S. Posner, Physiol. Rev. 49 (1969) 760–792. [30] P.G. Koutsoukos, Z. Amjad, M.B. Tomson, G.H. Nancollas, J. Am. Chem. Soc. 102 (1980) 1553–1557. [31] P. Zhu, Y. Masuda, K. Koumoto, Biomaterials 25 (2004) 3915–3921.
Please cite this article as: Anawati, et al., Surf. Coat. Technol. (2015), http://dx.doi.org/10.1016/j.surfcoat.2015.04.007