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P coating on AZ31B Mg alloy
Journal of Alloys and Compounds 635 (2015) 278–288
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Effect of ultrasonic cold forging technology as the pretreatment on the corrosion resistance of MAO Ca/P coating on AZ31B Mg alloy Lingling Chen a,b, Yanhong Gu a,⇑, Lu Liu a, Shujing Liu a, Binbin Hou c, Qi Liu a, Haiyang Ding a a
College of Mechanical Engineering, Beijing Institute of Petrochemical Technology, Beijing 102617, China College of Mechanical and Electrical Engineering, Beijing University of Chemical Technology, Beijing 100029, China c School of Engineering and Technology, China University of Geosciences, Beijing 100083, China b
a r t i c l e
i n f o
Article history: Received 17 December 2014 Received in revised form 9 February 2015 Accepted 11 February 2015 Available online 16 February 2015 Keywords: Magnesium alloy UCFT MAO Nano layer Ca/P coating
a b s t r a c t A calcium phosphate contained (Ca/P) coating was obtained on AZ31B Mg alloy by micro-arc oxidation (MAO) process under the pretreatment of ultrasonic cold forging technology (UCFT). The surface nanograins were introduced after UCFT pretreatment on AZ31B Mg alloy. Optical microscope (OM) was employed to observe the microstructures of the untreated and UCFT treated samples. Transmission electron microscopy (TEM) and atomic force microscope (AFM) were employed to observe the microstructures of nanograins and the surface roughness of the UCFT treated Mg alloys. The grain size of the UCFT treated Mg alloy is 48.67 nm and the surface roughness is 17.03 nm. The microstructures and the phase compositions of MAO samples were observed and analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The XRD results show that the coating include Ca/P phase, including hydroxyapatite (Ca10(PO4)6(OH)2), HA), tertiary calcium phosphate (Ca3(PO4)2, TCP) and calcium phosphate dehydrate (CaHPO42H2O, DCPD). The hardness of the samples was measured by the micro-hardness tester under the loads of 10 g, 25 g and 50 g. 3D topographies of hardness indenter were characterized by 3D profiler. The immersion tests and potentiodynamic polarization tests were used to evaluate the weight loss rate and corrosion current density in simulated body fluid (SBF). The results show that the corrosion resistance of Ca/P MAO coating on Mg alloy was improved greatly by the pretreatment of UCFT. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction In recent years, magnesium alloys as the innovative biodegradable metallic implant materials for bones, have attracted many researchers’ special attentions [1]. Previous studies have shown that magnesium alloys were the most applicable in the potential degradable implant materials among all the metals [2–6]. The advantages of using Mg as a biodegradable implant material are that it can degrade in the human body and it is an essential element to the human body. The biodegradation of magnesium alloys can keep the patients free from the secondary operation which can reduce the patient’s pain and medical costs [7]. The density and Young’s modulus of the magnesium alloys are much closer to the human bones than those of other implant materials, therefore, ⇑ Corresponding author at: College of Mechanical Engineering, Beijing Institute of Petrochemical Technology, 19 Qingyuan Beilu, Huangcun, Daxing District, Beijing, China. Tel.: +86 13691085981 (C), +86 10 81292290 (O); fax: +86 10 81292290. E-mail addresses: [email protected] (L. Chen), [email protected] (Y. Gu), [email protected] (L. Liu), [email protected] (S. Liu), sohu19880815@ 126.com (B. Hou), [email protected] (Q. Liu), [email protected] (H. Ding). http://dx.doi.org/10.1016/j.jallcom.2015.02.086 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.
the stress shielding can be avoided when the magnesium alloys as the implants are transplanted into the human body [8–10]. The presence of magnesium ions in the human system is beneficial to the bone healing while the excess magnesium ions can be discharged through the body fluids and urine [11]. However, the corrosion rate of magnesium alloys is so high that they might lose mechanical integrity before the bones are healed which would have limited magnesium alloys for further clinical use [4]. Hence, the simplest way to reduce the corrosion rate and improve the corrosion resistance is to prepare an efficient protective coating on the surface of magnesium alloys. Biologically relevant calcium phosphate belongs to the orthophosphate group and naturally occur in several biological structures, including teeth and bone [12]. Therefore, a calcium phosphate is perfect to the implantation. The calcium phosphate coating is known to improve the biocompatibility of magnesium alloys and to increase bone growth at the site of implantation [13–15]. The calcium phosphate coating on magnesium alloy provides corrosion protection as well as osteoconductivity [12]. Applying ceramic coating on AZ31B magnesium as a barrier can alleviate the degradation rate of pure magnesium
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alloy [12] and enhance the cell attachment through a calcium phosphate coating. It has been reported in many different studies, calcium phosphate contained (Ca/P) coating can be prepared using a variety of methods such as biomimetic approach [16,17], sol–gel coating technique [18–20], electrochemical deposition [21,22] and micro-arc oxidation [23–26]. Among those, micro-arc oxidation (MAO) is one of the most prospective surface treatment of Mg alloy [27,28] which can prepare a more compact coating. In this paper, a Ca/P coating has been prepared on the surface of AZ31B Mg alloy by MAO process under suitable electrical parameters and the appropriate compositions of electrolyte. Before MAO process, previous researchers have employed a variety of methods as the pretreatment to enhance the corrosion behavior. Wang et al. [29] have studied a pretreatment of laser surface melting (LSM) before MAO coating on AZ91D Mg alloy. The rare earth of cerium conversion coating as a pretreatment on AZ91D Mg alloy prior to MAO coating has been explored by Cai et al. [30]. And Xi et al. [31] have studied the use of immersion pretreatment with ultrasound in Al(NO3)3 before MAO coating. However, changing the structure of material surface into nanograin has become a promising development pretreatment. Among all the plastic deformation techniques applied for nanostructured materials, surface mechanical attrition treatment (SMAT) and ultrasonic cold forging technology (UCFT) are the most important and regular methods. SMAT and UCFT are basically applying severe plastic deformation on the surface layers of the samples which could lead the surface into ultrafine grains [32,33]. The SMAT method as a pretreatment before MAO process has been used on aluminum [34–36], titanium [37–39] and magnesium [40]. Masiha et al. [34] have used SMAT treatment before MAO process on aluminum to explore the corrosion behavior and wear property. The SMAT process plays a key role in the thickness of coating. However, the corrosion behavior cannot be enhanced after SMAT pretreatment. Wen et al. [35,36] have designed a bottom nanocrystalline layer and a top ceramic coating on the surface of aluminum alloy to explore the fatigue life and the corrosion resistance of the coating. The grain size of about 20–180 nm can be found at about 5 lm depth under the surface of SMAT sample. The SMAT-MAO coated samples with 10 lm thick ceramic coating exhibited better fatigue life and corrosion resistance compared with the 15 lm thickness. Zhang and Hao [37–39] have introduced the nanostructured surface layer on titanium alloy by SMAT technique before MAO process. The mechanical properties of SMAT samples were measured by the tensile test. The ceramic coating by the SMAT is more compact and less porous than that of untreated sample. Gheytani et al. [40] have applied SMAT technology to refine surface microstructure of magnesium obtaining a nanocrystalline surface with the grain size of 5–10 nm. The corrosion resistance increased 700% and the wear resistance increased 105%. Though SMAT technology can induce a nanocrystallined layer on the material surface and improve the mechanical properties, it need a vacuum environment which restricts the dimension of the sample. The surface quality of SMAT sample is lower than that of untreated sample. Additional, SMAT technology is unfriendly to the environment and would produce dust and noise pollution. However, UCFT is a new technology, easy to operate and friendly to the environment. Moreover, UCFT technology uses the ultrasonic vibration energy to induce severe plastic deformation on metal surface, which not only can improve surface hardness significantly but also reduce surface roughness to 0.08–0.5 lm. UCFT technology has only been reported on steel [41,42]. Suh et al. [41] have used UCFT technology to improve the mechanical properties of tool steel. She et al. [42] have used UCFT as a pretreatment to promote the formation and growth of nitride layer on the die steel. A thicker and harder nitride layer with smaller nitrided nanoparticles was formed after UCFT pretreatment. However, UCFT is rarely used on AZ31B Mg alloy.
Therefore, it may be a good try to use UCFT as a pretreatment means before MAO process on AZ31B Mg alloy. In this study, UCFT was introduced as a pretreatment and then MAO coating was prepared on AZ31B Mg alloy. The grain size and the surface roughness of UCFT pretreated Mg alloys are determined. The purpose of the present study is to investigate the effect of UCFT pretreatment on the corrosion resistance of MAO Ca/P coating on Mg alloys in SBF, and further to find out an optimized UCFT parameter to the lowest corrosion rate. 2. Materials and methods 2.1. Sample preparation AZ31B Mg alloy disks with 25 mm in diameter and 3 mm in thickness were prepared. Chemical composition of AZ31B Mg alloy is given in Table 1. Prior to the surface treatment, the samples were polished with various grades SiC abrasive papers (500, 800, 1000 up to 1200 grit). Then they were ultrasonically cleaned in deionized water for 10 min, rinsed in acetone, dehydrated in ethyl alcohol, and dried immediately in flowing air. The main concept and mechanism of UCFT is as follows: a tungsten carbide ball is attached to an ultrasonic device and it strikes the surface of a work piece at a frequency of 20 kHz. These strikes cause severe plastic deformation on the surface layer and could induce a nanostructure. The UCFT parameters used in this experiment are shown in Table 2. For MAO process, AZ31B Mg alloy was used as the anode while a stainless steel container with electrolyte inside was used as the cathode. The electrolyte in the MAO process was an aqueous solution of NaAlO2 (15 g/l), EDTA-Ca (4 g/l), (NaPO3)6 (2 g/l) and NaOH (2 g/l). During the MAO treatment of each sample, the applied voltage and the duration time were fixed at a constant of 425V and 3 min, respectively. Meanwhile, working frequency and duty cycle were 50 Hz and 30%, respectively. After MAO process, the samples were cleaned in deionized water and then dried by an air stream. Table 3 shows the sample designation which used the different surface treatment methods.
2.2. Surface characterization In order to investigate the depth of nanostructure layer after UCFT, metallographic structure by a BX51M Olympus optical microscope (OM) was observed and the hardness variation along the depth using a MH-6 micro-hardness tester was also conducted. A further examination of the UCFT surface topography was carried out by atomic force microscope (AFM, SPM-9500J3, Japan) and transmission electron microscope (TEM, JEM-2010, Japan). Scanning electron microscope (SEM, JSM-6400 and XXS-550, Japan) was employed to observe surface and cross-sectional morphologies of coated samples. X-ray diffraction (XRD, Smart Lab, Japan) with Cu Ka radiation was used to analyze the phase components of the samples at a scanning speed of 0.02° over a scan range of 20–80° 2h at glancing angles of 2°. The hardness of the coated samples was measured under the different loads of 10 g, 25 g and 50 g for 10 s. Five points were chosen on one load and then average value and standard deviation had been calculated. A NanoMap-D 3D surface profilometer was used to measure the depth of hardness indentation.
2.3. Weight loss experiments The treated and untreated samples with exposed area of 12.17 cm2 were immersed in SBF for 7 days. The SBF was prepared by dissolving the reagents in 1000 ml distilled water at 36.5 ± 0.5 °C shown in Table 4. Then pH value was adjusted to 7.25 with hydrochloric acid. The samples were taken out and rinsed in deionized water and then the weights were measured every day. Corrosion weight loss rate can be calculated according to Eq. (1) as followed:
CR ðg=cm2 =hÞ ¼
ðm0 m1 Þ st
ð1Þ
where m0 and m1 are weights before and after corrosion in g, s stands for surface area in cm2, t is the corrosion time in h.
Table 1 Chemical composition of AZ31B Mg alloy (wt%). Al
Zn
Mn
Si
Ni
Fe
Mg
3.01
0.9
0.5
0.04
0.005
0.005
Bal.
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Table 2 UCFT treatment parameters. Samples
UCFT1 UCFT2
Parameters Vibration frequency (kHz)
Amplitude (lm)
Load (N)
Spindle speed (rpm)
Freed rate (mm rev1)
Tip (tungsten carbide) diameter (mm)
Number of shots per mm2
20 20
30 30
300 300
200 200
0.05 0.05
15 15
20,000 30,000
Table 3 Samples designation. Surface treatment
UCFT1 UCFT2 MAO
Samples Untreated
UCFT
MAO
UCFT1-MAO
UCFT2-MAO
– – –
U – –
– – U
U – U
– U U
2.4. Electrochemical experiments Electrochemical tests were performed using an electrochemical corrosion workstation (Zahner IM-6, Germany). The electrochemical measurements were carried out in a three-electrode corrosion cells with a saturated calomel electrode (SCE) as the reference electrode and a platinum electrode as the counter electrode. The sample with an exposed area of 0.64 cm2 to the SBF was used as the working electrode. The SBF was kept at 37 °C in the water bath pot. The potentiodynamic polarization tests were measured at a sweeping rate of 0.5 mV s1 starting from 1.8 V to 0.8 V. The data of the potentiodynamic polarization tests were recorded at 1 h, 3 h and 5 h.
3. Results and discussion 3.1. Microstructure 3.1.1. UCFT as pretreatment Fig. 1 shows the metallographic structures of the untreated and UCFT treated samples. It can be seen that plastic deformation has been caused on the UCFT sample surface in which the grain size has become smaller and the grain boundaries are rarely seen [41,42]. The thickness of the UCFT treated sample layer is about 100 lm. The grain boundaries of the deformation layer is not as clear as that of the untreated Mg alloy. Fig. 2 shows the roughness of the surface UCFT pretreatment. The average roughness of area A and B was 17.03 nm, which was due to the greatest contribution of the pretreatment UCFT.
Fig. 2. Surface roughness of the sample treated by UCFT.
Table 4 Composition of SBF (1000 ml) [47]. Order
1
2
3
4
5
6
7
8
9
Reagents Quantity
NaCl 7.996 g
NaHCO3 0.350 g
KCl 0.224 g
K2HPO43H2O 0.228 g
1 M HCl 40 ml
MgCl26H2O 0.305 g
CaCl2 0.278 g
Na2SO4 0.071 g
NH4C(CH2OH)3 6.057 g
Fig. 1. Metallographic structure of (a) untreated Mg alloy and (b) UCFT treated Mg alloy.
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Fig. 3. Grain size of the sample treated by UCFT.
Fig. 4. Typical TEM images of (a) 20 lm from the surface and (b) top surface.
Fig. 3 shows the grain size of the surface nanostructure pretreated by UCFT technology. The largest size of the nanograin is on the C–D line, which is 73.47 nm while the average size of the nanograin is 48.67 nm. This indicates that the surface characteristic of AZ31B Mg alloy was changed by UCFT pretreatment. The surface microstructure has been influenced by UCFT pretreatment which leads the grain refined effectively. In order to understand the influence of severe plastic deformation on the grain size distribution and UCFT pretreatment depth, TEM was conducted and the results are shown in Fig. 4. As reported
[43–45], at least two twinning models, ie. the {1 0 1 1}-type and the {1 0 1 2}-type compression twinning influence quite a lot in the deformation of magnesium alloys. Fig. 4(a) and (b) shows the recrystallization process of the 20 lm depth from the surface and the top surface of the UCFT treated magnesium alloys respectively. The dense dislocation structures near multiple twinning indicate that twin boundary plays an important role in the recrystallization process. It can be seen that the twin boundaries have been formed both on the surface and inner of the alloys which was the nucleation site as predicted by Galiyev et al. [46]. However, the crystal
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Fig. 5. Surface morphologies and EDS patterns of (a) MAO coating, (b) UCFT1-MAO coating and (c) UCFT2-MAO coating.
Fig. 6. Cross-sectional morphology and EDS pattern of MAO coating.
L. Chen et al. / Journal of Alloys and Compounds 635 (2015) 278–288
Fig. 7. XRD patterns of MAO coating, UCFT1-MAO and UCFT2-MAO coating: 1 – Mg, 2 – Mg3(PO4)2, 3 – TCP, 4 – DCPD, 5 – HA, 6 – MgO, 7 – MgAl2O4.
283
3.1.2. The surface morphology of MAO coated samples Fig. 5 shows SEM surface morphologies and EDS patterns of MAO coating, UCFT1-MAO coating and UCFT2-MAO coating samples. It can be seen from Fig. 5(a1)–(c1) that porous structures have been presented on the samples. The pore size of UCFT-MAO coatings (Fig. 5b1 and c1) are smaller than MAO coating (Fig. 5a1). This may attribute to the fact that the ultrasonic cold forging technology pretreatment refined the substrate surface and then the micro-arc discharge channels become smaller. The more impact shots of UCFT, the refiner the surface and the smaller the discharge channel. Therefore, the coating of the 30,000 impact shots (UCFT2MAO) shows the smallest pore size and the most compact coating among all the coated samples. This results are consistent with the metallographic structure. Fig. 5(a2)–(c2) shows the EDS patterns of different coatings, the elements composition of the coating are C, O, Na, Mg, Al, P, Ca. The detection of the Na, P, Ca means that the electrolyte ions had participated in the MAO reaction. The weight rate of P and Ca elements in the UCFT-MAO coating is more than the MAO coatings. The element of Ca in the UCFT1-MAO and UCFT2-MAO coating is twice than the MAO coating. 3.1.3. The cross-sectional morphology of MAO coated samples The SEM cross-sectional morphology and EDS pattern of MAO coating is shown in Fig. 6. It can be seen from Fig. 6(a) that the thickness of the coating was just 3.72 lm, however, it was very density. Fig. 6(b) shows the cross-sectional EDS pattern of MAO coating. It can be seen from the figure that the cross-sectional elements are the same as the surface. Mg element declines and O, Al, P, Ca elements rise with the scan line from the substrate to the surface. This suggests that MAO coating is mainly composed of Mg, O, Al, P, Ca elements. 3.2. Phase composition
Fig. 8. Micro-hardness of samples under the loads of 10 g, 25 g and 50 g.
on the surface of alloys appears small and disorganized nucleation sites, in contrast, the inner side crystal shows more stable and in-order sites.
Fig. 7 shows the X-ray diffraction (XRD) patterns of MAO coating, UCFT1-MAO coating and UCFT2-MAO coating. As can be seen in this figure, the phases of all the samples include Mg, Mg3(PO4)2, TCP (tricalcium phosphate–whitlockite, Ca3(PO4)2), DCPD (calcium phosphate dehydrate–brushite, CaHPO42H2O), HA (hydroxyapatite, Ca10(PO4)6(OH)2), MgO and MgAl2O4. Diffraction peaks of Mg are the strongest, suggesting that the coatings are relatively thin and the X-ray could penetrate into the substrate. The existence of the Mg3(PO4)2 and MgAl2O4 compounds indicates
Fig. 9. 3D profiler images of (a) surface and (b) depth of one indentation on UCFT2-MAO sample.
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L. Chen et al. / Journal of Alloys and Compounds 635 (2015) 278–288 Table 5 Corrosion weight loss rate of samples under different processes (102 mg/cm2/h). Immersion days
Untreated
UCFT
MAO
UCFT1-MAO
UCFT2-MAO
1 2 7
4.40 4.91 6.11
3.61 4.14 5.50
2.39 2.89 4.34
2.04 2.57 3.64
1.61 1.87 3.51
Table 6 Corrosion potential (Ecorr, V) of samples under different processes soaked in SBF. Time (h)
1 3 5
Fig. 10. Average indentation depth when different loads were used.
that the electrolyte ions have been an active participant in the growth process of the MAO coating.
3.3. Hardness analysis 3.3.1. Surface micro-hardness In order to analyze the impact of the load on the depth of indentation, the micro-hardness of the samples under the load of 10 g, 25 g and 50 g are shown in Fig. 8. The micro-hardness of UCFT sample is the highest among all the samples irrespective of the loads. This indicates that the UCFT pretreatment can largely enhance the micro-hardness of AZ31B Mg alloy. In addition, the micro-hardness of all MAO coated samples are much higher than substrate, which indicates that the MAO process also can improve the micro-hardness of Mg alloy. Fig. 9 shows the example of UCFT which used the 3D profiler to measure the depth of indentation and Fig. 10 shows the depth of UCFT2-MAO sample. From Fig. 10, it can be seen that the average depth of the UCFT2-MAO coating is 4.02 lm with the load of 50 g while the thickness of the MAO coating was just 3.72 lm (Fig. 6a), therefore, the coating may be punctured with this load. This results also can be seen from Fig. 8 that the micro-hardness
Samples Untreated
UCFT
MAO
UCFT1-MAO
UCFT2-MAO
1.488 1.493 1.505
1.436 1.447 1.454
1.415 1.421 1.441
1.342 1.373 1.389
1.308 1.335 1.350
of UCFT1-MAO and UCFT2-MAO coating is almost the same or higher than that UCFT sample under this load. 3.3.2. Cross-sectional micro-hardness Fig. 11(a) shows the cross-sectional metallographic structure of the UCFT sample with the indentations of micro-hardness. The micro-hardness variation of the untreated and UCFT treated samples along the depth are shown in Fig. 11(b). It can be seen that the micro-hardness of the UCFT sample in the top layer surface is about 87 HV and decreases sharply from the 100 lm. When the depth reaches 500 lm, the micro-hardness becomes the same as the untreated sample (about 67 HV). This indicates that the UCFT layer is about 100 lm and the nanolayer influenced the hardness of the substrate underlying the nano layer. 3.4. Immersion tests Table 5 shows that the corrosion weight loss rate of the samples under different treatment processes. The corrosion weight loss rate of UCFT sample in SBF was lower than the untreated sample, which indicates that UCFT can refine the grain size of AZ31B Mg alloy. All the coated samples showed lower corrosion weight loss rate than that of the uncoated AZ31B Mg alloy irrespective of the immersion time. The corrosion weight loss rate of the UCFT-MAO coatings had lower corrosion weight loss rate after 7-day immersion. The best corrosion resistance was produced by UCFT pretreatment with
Fig. 11. (a) Cross-sectional micro-hardness indentation location of UCFT sample, (b) micro-hardness variation along the depth.
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285
Fig. 12. Polarization curves of samples under different processes soaked in SBF for 1 h, 3 h and 5 h.
impact shots of 30,000 together with MAO process (UCFT2-MAO). It may be due to the fact that UCFT had made the surface of the AZ31B Mg alloy into the nanograin. Therefore, the corrosion ions cannot easily penetrate into the inner coating through the tiny pores. It can be proved that UCFT technology can improve the corrosion resistance through changing the nature of AZ31B Mg alloy and the surface of the MAO coating. 3.5. Electrochemical tests In order to assess the protection of the samples against corrosion, electrochemical tests were conducted on the untreated, UCFT, MAO coating, UCFT1-MAO coating and UCFT2-MAO coating. The potentiodynamic polarization curves of the samples are shown in Fig. 12. The corrosion potential (Ecorr) and corrosion current density (Icorr) of the samples have been evaluated by Tafel extrapolation as summarized in Table 6 and Fig. 13. The higher corrosion potential means that the samples are more stable. It can be seen from Table 6 that the corrosion potential of UCFT2-MAO coating was the highest (1.308 V at 1 h immersion, 1.355 V at 3 h immersion, 1.35 V at 5 h immersion) among all the samples irrespective of immersion time, which indicates that the UCFT2-MAO coating was much more stable and not easily corroded. It can be seen from Fig. 13 that Icorr of the samples with different processes increased with the increasing immersion time. The increase rate of Icorr of the coated samples was almost the same
Fig. 13. Corrosion current density of samples under different processes.
from 1 h to 3 h. Nevertheless, the increase slope of Icorr of the UCFT-MAO coatings decreased from 3 h to 5 h, which indicates that UCFT pretreatment plays a significant role on the corrosion resistance. The Icorr value of UCFT2-MAO coating increased from 19.6 lA cm2 (3 h immersion) to 21.1 lA cm2 (5 h immersion), which was the slowest among all the samples.
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Fig. 14. SEM images after immersion test: (a) untreated, (b) UCFT, (c) MAO coating, (d) UCFT1-MAO coating, and (e) UCFT2-MAO coating.
impact shots number, the better the corrosion resistance of UCFT2MAO coatings was. The more impact shots result in serious plastic deformation of AZ31B Mg alloy surface, which lead to lower roughness and smoother surface. Under the smoother pretreatment surface combined with MAO process, a denser MAO coating was formed. The denser MAO coating had the better corrosion resistance, which were consistent with the immersion tests. 3.6. Corrosion products
Fig. 15. XRD patterns of samples after immersion test: 1 – Mg, 2 – Mg3(PO4)2, 3 – TCP, 4 – DCPD, 5 – HA, 6 – Mg(OH)2.
The corrosion potential and corrosion current density clearly show that the corrosion resistance of UCFT2-MAO coating is the best. This indicates that UCFT2-MAO coatings provide a more effective layer against corrosive attack [20] compared with other samples. It would be attributed to the fact that UCFT2-MAO coatings have more dense surface morphology shown in Fig. 5(c1). The more
3.6.1. SEM morphology after immersion tests Fig. 14 shows SEM morphologies of the samples after 7-day immersion tests. The corrosion crater and micro-cracks of UCFT sample were smaller than those of the untreated sample after SBF immersion. This indicates that the ultrasonic cold forging technology can improve the corrosion resistance of the sample through making the plastic deformation and refining the nanograin of the AZ31B Mg alloy. MAO coating, UCFT1-MAO coating and UCFT2MAO coating presented corrosion crater on the surface of the samples without cracks. The UCFT-MAO coatings after SBF immersion only had some shallow and small craters compared with the MAO coating. The crater of UCFT2-MAO coating was the shallowest and smallest among all the samples. This indicates that the SBF ions did not penetrate into the coating deeply through the tiny pores which was contributed by UCFT. With increasing the number of UCFT shots, the pores on the surface were tinier, thus the SBF ions cannot penetrate into the inner layer during this testing
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period. This was why UCFT2-MAO coating only presented corrosion on the surface of the sample, which was consistent with the result of weight loss rate of immersion tests.
[12] [13]
3.6.2. Phase composition after immersion test Fig. 15 shows the XRD spectrum of three coatings after 7-day SBF immersion. The diffraction peaks of MgO and MgAl2O4 have disappeared while a very weak new diffraction peak of Mg(OH)2 has been found at about 69°. 4. Conclusion (1) The nanostructure layer of 100 lm with the grain size of 30– 80 nm were obtained by UCFT technology, which could improve the surface micro-hardness. The pore size of MAO coating pretreated by UCFT is smaller and denser than that of untreated MAO coating. (2) XRD patterns show that Mg, Mg3(PO4)2, TCP, DCPD, HA, MgO and MgAl2O4 have been found in the MAO Ca/P coating. While after the immersion into SBF, Mg(OH)2 has been found and MgAl2O4 has disappeared. The presence of HA is very significant for MAO coating in biomedical application, because HA is not only bioactive but also can induce the bone growth. (3) Immersion tests and electrochemical tests show that the UCFT2-MAO coating has the lowest weight loss and corrosion current density. Those could be due to the fact that the pretreatment UCFT has reduced surface roughness and grain size of AZ31B Mg alloy. The remained nano layer underlying the MAO coating could contribute to the better corrosion resistance of UCFT pretreated MAO Ca/P coatings.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Effect of ultrasonic cold forging technology as the pretreatment on the corrosion resistance of MAO Ca/P coating on AZ31B Mg alloy Lingling Chen a,b, Yanhong Gu a,⇑, Lu Liu a, Shujing Liu a, Binbin Hou c, Qi Liu a, Haiyang Ding a a
College of Mechanical Engineering, Beijing Institute of Petrochemical Technology, Beijing 102617, China College of Mechanical and Electrical Engineering, Beijing University of Chemical Technology, Beijing 100029, China c School of Engineering and Technology, China University of Geosciences, Beijing 100083, China b
a r t i c l e
i n f o
Article history: Received 17 December 2014 Received in revised form 9 February 2015 Accepted 11 February 2015 Available online 16 February 2015 Keywords: Magnesium alloy UCFT MAO Nano layer Ca/P coating
a b s t r a c t A calcium phosphate contained (Ca/P) coating was obtained on AZ31B Mg alloy by micro-arc oxidation (MAO) process under the pretreatment of ultrasonic cold forging technology (UCFT). The surface nanograins were introduced after UCFT pretreatment on AZ31B Mg alloy. Optical microscope (OM) was employed to observe the microstructures of the untreated and UCFT treated samples. Transmission electron microscopy (TEM) and atomic force microscope (AFM) were employed to observe the microstructures of nanograins and the surface roughness of the UCFT treated Mg alloys. The grain size of the UCFT treated Mg alloy is 48.67 nm and the surface roughness is 17.03 nm. The microstructures and the phase compositions of MAO samples were observed and analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The XRD results show that the coating include Ca/P phase, including hydroxyapatite (Ca10(PO4)6(OH)2), HA), tertiary calcium phosphate (Ca3(PO4)2, TCP) and calcium phosphate dehydrate (CaHPO42H2O, DCPD). The hardness of the samples was measured by the micro-hardness tester under the loads of 10 g, 25 g and 50 g. 3D topographies of hardness indenter were characterized by 3D profiler. The immersion tests and potentiodynamic polarization tests were used to evaluate the weight loss rate and corrosion current density in simulated body fluid (SBF). The results show that the corrosion resistance of Ca/P MAO coating on Mg alloy was improved greatly by the pretreatment of UCFT. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction In recent years, magnesium alloys as the innovative biodegradable metallic implant materials for bones, have attracted many researchers’ special attentions [1]. Previous studies have shown that magnesium alloys were the most applicable in the potential degradable implant materials among all the metals [2–6]. The advantages of using Mg as a biodegradable implant material are that it can degrade in the human body and it is an essential element to the human body. The biodegradation of magnesium alloys can keep the patients free from the secondary operation which can reduce the patient’s pain and medical costs [7]. The density and Young’s modulus of the magnesium alloys are much closer to the human bones than those of other implant materials, therefore, ⇑ Corresponding author at: College of Mechanical Engineering, Beijing Institute of Petrochemical Technology, 19 Qingyuan Beilu, Huangcun, Daxing District, Beijing, China. Tel.: +86 13691085981 (C), +86 10 81292290 (O); fax: +86 10 81292290. E-mail addresses: [email protected] (L. Chen), [email protected] (Y. Gu), [email protected] (L. Liu), [email protected] (S. Liu), sohu19880815@ 126.com (B. Hou), [email protected] (Q. Liu), [email protected] (H. Ding). http://dx.doi.org/10.1016/j.jallcom.2015.02.086 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.
the stress shielding can be avoided when the magnesium alloys as the implants are transplanted into the human body [8–10]. The presence of magnesium ions in the human system is beneficial to the bone healing while the excess magnesium ions can be discharged through the body fluids and urine [11]. However, the corrosion rate of magnesium alloys is so high that they might lose mechanical integrity before the bones are healed which would have limited magnesium alloys for further clinical use [4]. Hence, the simplest way to reduce the corrosion rate and improve the corrosion resistance is to prepare an efficient protective coating on the surface of magnesium alloys. Biologically relevant calcium phosphate belongs to the orthophosphate group and naturally occur in several biological structures, including teeth and bone [12]. Therefore, a calcium phosphate is perfect to the implantation. The calcium phosphate coating is known to improve the biocompatibility of magnesium alloys and to increase bone growth at the site of implantation [13–15]. The calcium phosphate coating on magnesium alloy provides corrosion protection as well as osteoconductivity [12]. Applying ceramic coating on AZ31B magnesium as a barrier can alleviate the degradation rate of pure magnesium
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alloy [12] and enhance the cell attachment through a calcium phosphate coating. It has been reported in many different studies, calcium phosphate contained (Ca/P) coating can be prepared using a variety of methods such as biomimetic approach [16,17], sol–gel coating technique [18–20], electrochemical deposition [21,22] and micro-arc oxidation [23–26]. Among those, micro-arc oxidation (MAO) is one of the most prospective surface treatment of Mg alloy [27,28] which can prepare a more compact coating. In this paper, a Ca/P coating has been prepared on the surface of AZ31B Mg alloy by MAO process under suitable electrical parameters and the appropriate compositions of electrolyte. Before MAO process, previous researchers have employed a variety of methods as the pretreatment to enhance the corrosion behavior. Wang et al. [29] have studied a pretreatment of laser surface melting (LSM) before MAO coating on AZ91D Mg alloy. The rare earth of cerium conversion coating as a pretreatment on AZ91D Mg alloy prior to MAO coating has been explored by Cai et al. [30]. And Xi et al. [31] have studied the use of immersion pretreatment with ultrasound in Al(NO3)3 before MAO coating. However, changing the structure of material surface into nanograin has become a promising development pretreatment. Among all the plastic deformation techniques applied for nanostructured materials, surface mechanical attrition treatment (SMAT) and ultrasonic cold forging technology (UCFT) are the most important and regular methods. SMAT and UCFT are basically applying severe plastic deformation on the surface layers of the samples which could lead the surface into ultrafine grains [32,33]. The SMAT method as a pretreatment before MAO process has been used on aluminum [34–36], titanium [37–39] and magnesium [40]. Masiha et al. [34] have used SMAT treatment before MAO process on aluminum to explore the corrosion behavior and wear property. The SMAT process plays a key role in the thickness of coating. However, the corrosion behavior cannot be enhanced after SMAT pretreatment. Wen et al. [35,36] have designed a bottom nanocrystalline layer and a top ceramic coating on the surface of aluminum alloy to explore the fatigue life and the corrosion resistance of the coating. The grain size of about 20–180 nm can be found at about 5 lm depth under the surface of SMAT sample. The SMAT-MAO coated samples with 10 lm thick ceramic coating exhibited better fatigue life and corrosion resistance compared with the 15 lm thickness. Zhang and Hao [37–39] have introduced the nanostructured surface layer on titanium alloy by SMAT technique before MAO process. The mechanical properties of SMAT samples were measured by the tensile test. The ceramic coating by the SMAT is more compact and less porous than that of untreated sample. Gheytani et al. [40] have applied SMAT technology to refine surface microstructure of magnesium obtaining a nanocrystalline surface with the grain size of 5–10 nm. The corrosion resistance increased 700% and the wear resistance increased 105%. Though SMAT technology can induce a nanocrystallined layer on the material surface and improve the mechanical properties, it need a vacuum environment which restricts the dimension of the sample. The surface quality of SMAT sample is lower than that of untreated sample. Additional, SMAT technology is unfriendly to the environment and would produce dust and noise pollution. However, UCFT is a new technology, easy to operate and friendly to the environment. Moreover, UCFT technology uses the ultrasonic vibration energy to induce severe plastic deformation on metal surface, which not only can improve surface hardness significantly but also reduce surface roughness to 0.08–0.5 lm. UCFT technology has only been reported on steel [41,42]. Suh et al. [41] have used UCFT technology to improve the mechanical properties of tool steel. She et al. [42] have used UCFT as a pretreatment to promote the formation and growth of nitride layer on the die steel. A thicker and harder nitride layer with smaller nitrided nanoparticles was formed after UCFT pretreatment. However, UCFT is rarely used on AZ31B Mg alloy.
Therefore, it may be a good try to use UCFT as a pretreatment means before MAO process on AZ31B Mg alloy. In this study, UCFT was introduced as a pretreatment and then MAO coating was prepared on AZ31B Mg alloy. The grain size and the surface roughness of UCFT pretreated Mg alloys are determined. The purpose of the present study is to investigate the effect of UCFT pretreatment on the corrosion resistance of MAO Ca/P coating on Mg alloys in SBF, and further to find out an optimized UCFT parameter to the lowest corrosion rate. 2. Materials and methods 2.1. Sample preparation AZ31B Mg alloy disks with 25 mm in diameter and 3 mm in thickness were prepared. Chemical composition of AZ31B Mg alloy is given in Table 1. Prior to the surface treatment, the samples were polished with various grades SiC abrasive papers (500, 800, 1000 up to 1200 grit). Then they were ultrasonically cleaned in deionized water for 10 min, rinsed in acetone, dehydrated in ethyl alcohol, and dried immediately in flowing air. The main concept and mechanism of UCFT is as follows: a tungsten carbide ball is attached to an ultrasonic device and it strikes the surface of a work piece at a frequency of 20 kHz. These strikes cause severe plastic deformation on the surface layer and could induce a nanostructure. The UCFT parameters used in this experiment are shown in Table 2. For MAO process, AZ31B Mg alloy was used as the anode while a stainless steel container with electrolyte inside was used as the cathode. The electrolyte in the MAO process was an aqueous solution of NaAlO2 (15 g/l), EDTA-Ca (4 g/l), (NaPO3)6 (2 g/l) and NaOH (2 g/l). During the MAO treatment of each sample, the applied voltage and the duration time were fixed at a constant of 425V and 3 min, respectively. Meanwhile, working frequency and duty cycle were 50 Hz and 30%, respectively. After MAO process, the samples were cleaned in deionized water and then dried by an air stream. Table 3 shows the sample designation which used the different surface treatment methods.
2.2. Surface characterization In order to investigate the depth of nanostructure layer after UCFT, metallographic structure by a BX51M Olympus optical microscope (OM) was observed and the hardness variation along the depth using a MH-6 micro-hardness tester was also conducted. A further examination of the UCFT surface topography was carried out by atomic force microscope (AFM, SPM-9500J3, Japan) and transmission electron microscope (TEM, JEM-2010, Japan). Scanning electron microscope (SEM, JSM-6400 and XXS-550, Japan) was employed to observe surface and cross-sectional morphologies of coated samples. X-ray diffraction (XRD, Smart Lab, Japan) with Cu Ka radiation was used to analyze the phase components of the samples at a scanning speed of 0.02° over a scan range of 20–80° 2h at glancing angles of 2°. The hardness of the coated samples was measured under the different loads of 10 g, 25 g and 50 g for 10 s. Five points were chosen on one load and then average value and standard deviation had been calculated. A NanoMap-D 3D surface profilometer was used to measure the depth of hardness indentation.
2.3. Weight loss experiments The treated and untreated samples with exposed area of 12.17 cm2 were immersed in SBF for 7 days. The SBF was prepared by dissolving the reagents in 1000 ml distilled water at 36.5 ± 0.5 °C shown in Table 4. Then pH value was adjusted to 7.25 with hydrochloric acid. The samples were taken out and rinsed in deionized water and then the weights were measured every day. Corrosion weight loss rate can be calculated according to Eq. (1) as followed:
CR ðg=cm2 =hÞ ¼
ðm0 m1 Þ st
ð1Þ
where m0 and m1 are weights before and after corrosion in g, s stands for surface area in cm2, t is the corrosion time in h.
Table 1 Chemical composition of AZ31B Mg alloy (wt%). Al
Zn
Mn
Si
Ni
Fe
Mg
3.01
0.9
0.5
0.04
0.005
0.005
Bal.
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Table 2 UCFT treatment parameters. Samples
UCFT1 UCFT2
Parameters Vibration frequency (kHz)
Amplitude (lm)
Load (N)
Spindle speed (rpm)
Freed rate (mm rev1)
Tip (tungsten carbide) diameter (mm)
Number of shots per mm2
20 20
30 30
300 300
200 200
0.05 0.05
15 15
20,000 30,000
Table 3 Samples designation. Surface treatment
UCFT1 UCFT2 MAO
Samples Untreated
UCFT
MAO
UCFT1-MAO
UCFT2-MAO
– – –
U – –
– – U
U – U
– U U
2.4. Electrochemical experiments Electrochemical tests were performed using an electrochemical corrosion workstation (Zahner IM-6, Germany). The electrochemical measurements were carried out in a three-electrode corrosion cells with a saturated calomel electrode (SCE) as the reference electrode and a platinum electrode as the counter electrode. The sample with an exposed area of 0.64 cm2 to the SBF was used as the working electrode. The SBF was kept at 37 °C in the water bath pot. The potentiodynamic polarization tests were measured at a sweeping rate of 0.5 mV s1 starting from 1.8 V to 0.8 V. The data of the potentiodynamic polarization tests were recorded at 1 h, 3 h and 5 h.
3. Results and discussion 3.1. Microstructure 3.1.1. UCFT as pretreatment Fig. 1 shows the metallographic structures of the untreated and UCFT treated samples. It can be seen that plastic deformation has been caused on the UCFT sample surface in which the grain size has become smaller and the grain boundaries are rarely seen [41,42]. The thickness of the UCFT treated sample layer is about 100 lm. The grain boundaries of the deformation layer is not as clear as that of the untreated Mg alloy. Fig. 2 shows the roughness of the surface UCFT pretreatment. The average roughness of area A and B was 17.03 nm, which was due to the greatest contribution of the pretreatment UCFT.
Fig. 2. Surface roughness of the sample treated by UCFT.
Table 4 Composition of SBF (1000 ml) [47]. Order
1
2
3
4
5
6
7
8
9
Reagents Quantity
NaCl 7.996 g
NaHCO3 0.350 g
KCl 0.224 g
K2HPO43H2O 0.228 g
1 M HCl 40 ml
MgCl26H2O 0.305 g
CaCl2 0.278 g
Na2SO4 0.071 g
NH4C(CH2OH)3 6.057 g
Fig. 1. Metallographic structure of (a) untreated Mg alloy and (b) UCFT treated Mg alloy.
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281
Fig. 3. Grain size of the sample treated by UCFT.
Fig. 4. Typical TEM images of (a) 20 lm from the surface and (b) top surface.
Fig. 3 shows the grain size of the surface nanostructure pretreated by UCFT technology. The largest size of the nanograin is on the C–D line, which is 73.47 nm while the average size of the nanograin is 48.67 nm. This indicates that the surface characteristic of AZ31B Mg alloy was changed by UCFT pretreatment. The surface microstructure has been influenced by UCFT pretreatment which leads the grain refined effectively. In order to understand the influence of severe plastic deformation on the grain size distribution and UCFT pretreatment depth, TEM was conducted and the results are shown in Fig. 4. As reported
[43–45], at least two twinning models, ie. the {1 0 1 1}-type and the {1 0 1 2}-type compression twinning influence quite a lot in the deformation of magnesium alloys. Fig. 4(a) and (b) shows the recrystallization process of the 20 lm depth from the surface and the top surface of the UCFT treated magnesium alloys respectively. The dense dislocation structures near multiple twinning indicate that twin boundary plays an important role in the recrystallization process. It can be seen that the twin boundaries have been formed both on the surface and inner of the alloys which was the nucleation site as predicted by Galiyev et al. [46]. However, the crystal
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Fig. 5. Surface morphologies and EDS patterns of (a) MAO coating, (b) UCFT1-MAO coating and (c) UCFT2-MAO coating.
Fig. 6. Cross-sectional morphology and EDS pattern of MAO coating.
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Fig. 7. XRD patterns of MAO coating, UCFT1-MAO and UCFT2-MAO coating: 1 – Mg, 2 – Mg3(PO4)2, 3 – TCP, 4 – DCPD, 5 – HA, 6 – MgO, 7 – MgAl2O4.
283
3.1.2. The surface morphology of MAO coated samples Fig. 5 shows SEM surface morphologies and EDS patterns of MAO coating, UCFT1-MAO coating and UCFT2-MAO coating samples. It can be seen from Fig. 5(a1)–(c1) that porous structures have been presented on the samples. The pore size of UCFT-MAO coatings (Fig. 5b1 and c1) are smaller than MAO coating (Fig. 5a1). This may attribute to the fact that the ultrasonic cold forging technology pretreatment refined the substrate surface and then the micro-arc discharge channels become smaller. The more impact shots of UCFT, the refiner the surface and the smaller the discharge channel. Therefore, the coating of the 30,000 impact shots (UCFT2MAO) shows the smallest pore size and the most compact coating among all the coated samples. This results are consistent with the metallographic structure. Fig. 5(a2)–(c2) shows the EDS patterns of different coatings, the elements composition of the coating are C, O, Na, Mg, Al, P, Ca. The detection of the Na, P, Ca means that the electrolyte ions had participated in the MAO reaction. The weight rate of P and Ca elements in the UCFT-MAO coating is more than the MAO coatings. The element of Ca in the UCFT1-MAO and UCFT2-MAO coating is twice than the MAO coating. 3.1.3. The cross-sectional morphology of MAO coated samples The SEM cross-sectional morphology and EDS pattern of MAO coating is shown in Fig. 6. It can be seen from Fig. 6(a) that the thickness of the coating was just 3.72 lm, however, it was very density. Fig. 6(b) shows the cross-sectional EDS pattern of MAO coating. It can be seen from the figure that the cross-sectional elements are the same as the surface. Mg element declines and O, Al, P, Ca elements rise with the scan line from the substrate to the surface. This suggests that MAO coating is mainly composed of Mg, O, Al, P, Ca elements. 3.2. Phase composition
Fig. 8. Micro-hardness of samples under the loads of 10 g, 25 g and 50 g.
on the surface of alloys appears small and disorganized nucleation sites, in contrast, the inner side crystal shows more stable and in-order sites.
Fig. 7 shows the X-ray diffraction (XRD) patterns of MAO coating, UCFT1-MAO coating and UCFT2-MAO coating. As can be seen in this figure, the phases of all the samples include Mg, Mg3(PO4)2, TCP (tricalcium phosphate–whitlockite, Ca3(PO4)2), DCPD (calcium phosphate dehydrate–brushite, CaHPO42H2O), HA (hydroxyapatite, Ca10(PO4)6(OH)2), MgO and MgAl2O4. Diffraction peaks of Mg are the strongest, suggesting that the coatings are relatively thin and the X-ray could penetrate into the substrate. The existence of the Mg3(PO4)2 and MgAl2O4 compounds indicates
Fig. 9. 3D profiler images of (a) surface and (b) depth of one indentation on UCFT2-MAO sample.
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L. Chen et al. / Journal of Alloys and Compounds 635 (2015) 278–288 Table 5 Corrosion weight loss rate of samples under different processes (102 mg/cm2/h). Immersion days
Untreated
UCFT
MAO
UCFT1-MAO
UCFT2-MAO
1 2 7
4.40 4.91 6.11
3.61 4.14 5.50
2.39 2.89 4.34
2.04 2.57 3.64
1.61 1.87 3.51
Table 6 Corrosion potential (Ecorr, V) of samples under different processes soaked in SBF. Time (h)
1 3 5
Fig. 10. Average indentation depth when different loads were used.
that the electrolyte ions have been an active participant in the growth process of the MAO coating.
3.3. Hardness analysis 3.3.1. Surface micro-hardness In order to analyze the impact of the load on the depth of indentation, the micro-hardness of the samples under the load of 10 g, 25 g and 50 g are shown in Fig. 8. The micro-hardness of UCFT sample is the highest among all the samples irrespective of the loads. This indicates that the UCFT pretreatment can largely enhance the micro-hardness of AZ31B Mg alloy. In addition, the micro-hardness of all MAO coated samples are much higher than substrate, which indicates that the MAO process also can improve the micro-hardness of Mg alloy. Fig. 9 shows the example of UCFT which used the 3D profiler to measure the depth of indentation and Fig. 10 shows the depth of UCFT2-MAO sample. From Fig. 10, it can be seen that the average depth of the UCFT2-MAO coating is 4.02 lm with the load of 50 g while the thickness of the MAO coating was just 3.72 lm (Fig. 6a), therefore, the coating may be punctured with this load. This results also can be seen from Fig. 8 that the micro-hardness
Samples Untreated
UCFT
MAO
UCFT1-MAO
UCFT2-MAO
1.488 1.493 1.505
1.436 1.447 1.454
1.415 1.421 1.441
1.342 1.373 1.389
1.308 1.335 1.350
of UCFT1-MAO and UCFT2-MAO coating is almost the same or higher than that UCFT sample under this load. 3.3.2. Cross-sectional micro-hardness Fig. 11(a) shows the cross-sectional metallographic structure of the UCFT sample with the indentations of micro-hardness. The micro-hardness variation of the untreated and UCFT treated samples along the depth are shown in Fig. 11(b). It can be seen that the micro-hardness of the UCFT sample in the top layer surface is about 87 HV and decreases sharply from the 100 lm. When the depth reaches 500 lm, the micro-hardness becomes the same as the untreated sample (about 67 HV). This indicates that the UCFT layer is about 100 lm and the nanolayer influenced the hardness of the substrate underlying the nano layer. 3.4. Immersion tests Table 5 shows that the corrosion weight loss rate of the samples under different treatment processes. The corrosion weight loss rate of UCFT sample in SBF was lower than the untreated sample, which indicates that UCFT can refine the grain size of AZ31B Mg alloy. All the coated samples showed lower corrosion weight loss rate than that of the uncoated AZ31B Mg alloy irrespective of the immersion time. The corrosion weight loss rate of the UCFT-MAO coatings had lower corrosion weight loss rate after 7-day immersion. The best corrosion resistance was produced by UCFT pretreatment with
Fig. 11. (a) Cross-sectional micro-hardness indentation location of UCFT sample, (b) micro-hardness variation along the depth.
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Fig. 12. Polarization curves of samples under different processes soaked in SBF for 1 h, 3 h and 5 h.
impact shots of 30,000 together with MAO process (UCFT2-MAO). It may be due to the fact that UCFT had made the surface of the AZ31B Mg alloy into the nanograin. Therefore, the corrosion ions cannot easily penetrate into the inner coating through the tiny pores. It can be proved that UCFT technology can improve the corrosion resistance through changing the nature of AZ31B Mg alloy and the surface of the MAO coating. 3.5. Electrochemical tests In order to assess the protection of the samples against corrosion, electrochemical tests were conducted on the untreated, UCFT, MAO coating, UCFT1-MAO coating and UCFT2-MAO coating. The potentiodynamic polarization curves of the samples are shown in Fig. 12. The corrosion potential (Ecorr) and corrosion current density (Icorr) of the samples have been evaluated by Tafel extrapolation as summarized in Table 6 and Fig. 13. The higher corrosion potential means that the samples are more stable. It can be seen from Table 6 that the corrosion potential of UCFT2-MAO coating was the highest (1.308 V at 1 h immersion, 1.355 V at 3 h immersion, 1.35 V at 5 h immersion) among all the samples irrespective of immersion time, which indicates that the UCFT2-MAO coating was much more stable and not easily corroded. It can be seen from Fig. 13 that Icorr of the samples with different processes increased with the increasing immersion time. The increase rate of Icorr of the coated samples was almost the same
Fig. 13. Corrosion current density of samples under different processes.
from 1 h to 3 h. Nevertheless, the increase slope of Icorr of the UCFT-MAO coatings decreased from 3 h to 5 h, which indicates that UCFT pretreatment plays a significant role on the corrosion resistance. The Icorr value of UCFT2-MAO coating increased from 19.6 lA cm2 (3 h immersion) to 21.1 lA cm2 (5 h immersion), which was the slowest among all the samples.
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Fig. 14. SEM images after immersion test: (a) untreated, (b) UCFT, (c) MAO coating, (d) UCFT1-MAO coating, and (e) UCFT2-MAO coating.
impact shots number, the better the corrosion resistance of UCFT2MAO coatings was. The more impact shots result in serious plastic deformation of AZ31B Mg alloy surface, which lead to lower roughness and smoother surface. Under the smoother pretreatment surface combined with MAO process, a denser MAO coating was formed. The denser MAO coating had the better corrosion resistance, which were consistent with the immersion tests. 3.6. Corrosion products
Fig. 15. XRD patterns of samples after immersion test: 1 – Mg, 2 – Mg3(PO4)2, 3 – TCP, 4 – DCPD, 5 – HA, 6 – Mg(OH)2.
The corrosion potential and corrosion current density clearly show that the corrosion resistance of UCFT2-MAO coating is the best. This indicates that UCFT2-MAO coatings provide a more effective layer against corrosive attack [20] compared with other samples. It would be attributed to the fact that UCFT2-MAO coatings have more dense surface morphology shown in Fig. 5(c1). The more
3.6.1. SEM morphology after immersion tests Fig. 14 shows SEM morphologies of the samples after 7-day immersion tests. The corrosion crater and micro-cracks of UCFT sample were smaller than those of the untreated sample after SBF immersion. This indicates that the ultrasonic cold forging technology can improve the corrosion resistance of the sample through making the plastic deformation and refining the nanograin of the AZ31B Mg alloy. MAO coating, UCFT1-MAO coating and UCFT2MAO coating presented corrosion crater on the surface of the samples without cracks. The UCFT-MAO coatings after SBF immersion only had some shallow and small craters compared with the MAO coating. The crater of UCFT2-MAO coating was the shallowest and smallest among all the samples. This indicates that the SBF ions did not penetrate into the coating deeply through the tiny pores which was contributed by UCFT. With increasing the number of UCFT shots, the pores on the surface were tinier, thus the SBF ions cannot penetrate into the inner layer during this testing
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period. This was why UCFT2-MAO coating only presented corrosion on the surface of the sample, which was consistent with the result of weight loss rate of immersion tests.
[12] [13]
3.6.2. Phase composition after immersion test Fig. 15 shows the XRD spectrum of three coatings after 7-day SBF immersion. The diffraction peaks of MgO and MgAl2O4 have disappeared while a very weak new diffraction peak of Mg(OH)2 has been found at about 69°. 4. Conclusion (1) The nanostructure layer of 100 lm with the grain size of 30– 80 nm were obtained by UCFT technology, which could improve the surface micro-hardness. The pore size of MAO coating pretreated by UCFT is smaller and denser than that of untreated MAO coating. (2) XRD patterns show that Mg, Mg3(PO4)2, TCP, DCPD, HA, MgO and MgAl2O4 have been found in the MAO Ca/P coating. While after the immersion into SBF, Mg(OH)2 has been found and MgAl2O4 has disappeared. The presence of HA is very significant for MAO coating in biomedical application, because HA is not only bioactive but also can induce the bone growth. (3) Immersion tests and electrochemical tests show that the UCFT2-MAO coating has the lowest weight loss and corrosion current density. Those could be due to the fact that the pretreatment UCFT has reduced surface roughness and grain size of AZ31B Mg alloy. The remained nano layer underlying the MAO coating could contribute to the better corrosion resistance of UCFT pretreated MAO Ca/P coatings.
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