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MAO composite bio-coating on AZ80 magnesium alloy for biomedical application
Accepted Manuscript LSP/MAO composite bio-coating on AZ80 magnesium alloy for biomedical application
Ying Xiong, Qiang Hu, Renguo Song, Xiaxia Hu PII: DOI: Reference:
S0928-4931(17)30805-6 doi: 10.1016/j.msec.2017.03.003 MSC 7509
To appear in:
Materials Science & Engineering C
Received date: Revised date: Accepted date:
2 July 2016 16 December 2016 1 March 2017
Please cite this article as: Ying Xiong, Qiang Hu, Renguo Song, Xiaxia Hu , LSP/MAO composite bio-coating on AZ80 magnesium alloy for biomedical application. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Msc(2017), doi: 10.1016/j.msec.2017.03.003
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ACCEPTED MANUSCRIPT Submitted to Materials Science and Engineering: C, Dec 2016
LSP/MAO composite bio-coating on AZ80 magnesium alloy for biomedical application Ying Xiong a,b,*), Qiang Hu a), Renguo Song c,d), Xiaxia Hu a,b) a) Key Laboratory of Special Purpose Equipment and Advanced Processing Technology, Ministry of Education,
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Zhejiang University of Technology, Hangzhou, Zhejiang 310014, China b) College of Mechanical Engineering, Zhejiang University of Technology, Hangzhou, Zhejiang 310014, China
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c) School of Materials Science and Engineering, Changzhou University, Changzhou 213164, China d) Jiangsu Key Laboratory of Materials Surface Science and Technology, Changzhou University, Changzhou
Correspondent: Ying Xiong, E-mail: [email protected]
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ABSTRACT
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213164, Jiangsu, China
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A composite bio-coating was fabricated on AZ80 magnesium (Mg) alloy by using micro-arc oxidation (MAO) under the pretreatment of laser shock peening (LSP) in order to improve the bio-corrosion resistance and the mechanical integrity.
LSP treatment could induce grain
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refinement and compressive residual stress field on the surface of material.
MAO bio-coating was
properties of the material.
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grown in alkaline electrolyte with hydroxyapatite (HA, Ca10(PO4)6(OH)2) to improve the biological The microstructure, element and phase composition for untreated based
material (BM) and treated samples (LSP layer, MAO bio-coating and LSP/MAO composite
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bio-coating) were investigated by transmission electron microscopy (TEM), scanning electron microscope (SEM), energy dispersion spectroscopy (EDS) and X-ray diffraction (XRD).
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Electrochemical tests and slow strain rate tensile (SSRT) tests were used to evaluate the corrosion resistance and the stress corrosion susceptibility in simulated body fluid (SBF).
The results
indicated that LSP/MAO composite bio-coating can not only improve the corrosion resistance of Mg alloy substrate evidently but also increase the mechanical properties in SBF compared to LSP layer and MAO bio-coating.
Mg alloy treated by LSP/MAO composite technique should be better
suited as biodegradable orthopedic implants. Keywords: Biomaterials; Micro-arc oxidation; Laser shock processing; Microstructure; Electrochemical; Slow strain rate tensile.
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ACCEPTED MANUSCRIPT 1. Introduction As innovative biodegradable materials, magnesium (Mg) and its alloys are very attractive for potential orthopedic implants due to their strength, density and elastic modulus are very close to those of natural bone, particularly they can help to avoid a second surgery as compared with traditional implant materials such as titanium alloy, stainless alloy and cobalt-chromium alloy [1-3].
tissues sufficiently heal in addition to its general corrosion resistance [4].
Unfortunately, the
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For orthopedic applications, the implant is essential to ensure its mechanical integrity until the
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degradation rate of Mg based implants is too rapid in the body fluid environment to keep
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mechanical integrity before the diseased or damaged bone tissue healed [4-5].
modification is generally accepted as an effective approach to delay the degradation rate of Mg
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based implants during the service period [6]. Many surface modification techniques have been In all kinds of surface modification
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developed for the corrosion protection of Mg alloys substrate.
methods, MAO is a novel and promising technique based on anodic oxidation, which could
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generate a ceramic coating on the surface of Mg based materials in a suitable electrolyte.
Some
researchers have proven that the MAO coating can not only improve the corrosion resistance of Mg alloys substrate but also enhance the fixation of the implants to bone [7-9].
Furthermore, the
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MAO bio-coating with HA addition can further enhance the biocompatibility/bioactivity and the
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corrosion resistance in physiological environments [10-13].
However, implant devices exposed to
the human body environment are often also experience considerable loadings during service period, which would lead to a sudden fracture of an implant due to stress corrosion cracking (SCC) [14,15].
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Despite MAO coating showing good corrosion resistance in electrochemical tests and also Therefore,
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performing longer elongation in SSRT tests, it could not obviate the SCC process [16].
it is necessary to develop a new surface modification technology for further improving resistance to SCC in the human body environment. A promising approach is to alter the subsurface microstructure before MAO processing in order to adjust the mechanical integrity of Mg based materials.
LSP is an alternative
non-contacting surface treatment, which utilizes laser impact wave with high power and short pulse to strike on the target surface for improving the mechanic properties of metal materials, such as fatigue, corrosion cracking and wear resistance [17].
Therefore, it may be an effective way using
the combination of LSP and MAO technique to adjust both the degradation rate and the mechanical integrity of Mg based implants in the human body environment. 2
ACCEPTED MANUSCRIPT In this work, LSP was introduced as a pretreatment and then MAO bio-coating was fabricated on AZ 80 Mg alloy.
The corrosion resistance and the stress corrosion susceptibility of LSP/MAO
composite bio-coating were investigated using electrochemical tests and SSRT tests in SBF. 2. Experimental 2.1. Coatings preparation Samples tested in the study were machined from a hot-rolled AZ80 Mg plate (Mg–7.8%
substrates for the surface modified process.
The gauge geometry of the plane-plate samples for Before LSP and MAO treatment, all the samples were
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SSRT tests was 25 mm × 8 mm × 2 mm.
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Al–0.36% Zn–0.04%Mn, in wt. %). Dimensions of 40 mm × 22 mm × 2 mm were used as
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polished from 280 to 1200 grit with silicon abrasive papers to ensure the same surface toughness. Subsequently, the samples were ultrasonically degreased in ethanol for 10 min, cleaned with
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distilled water and then dried in ambient air.
LSP experiments were performed by a Q-switched Nd:YAG laser with a wavelength of 1064 The water with a thickness of about 1 mm was used as the
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nm and a pulse width of 20 ns.
transparent overlay, and the professional aluminum foil with a thickness of 0.1 mm was used as the The other detailed LSP
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absorbing overlay to protect the sample surface from thermal effect.
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parameters were as the following: a pulse energy of 2 J, a spot diameter of 3 mm and an overlapping rate of 50%. The lapping ratio was defined as the percentage of overlap among consecutive peening diameters.
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The alkaline silicate solutions for MAO process consist of 12 g/L Na2SiO3·9H2O, 5 g/L NaF, 2g/L K2TiF6, 3 g/L HA nanoparticles (20 nm) and 10 mL/L ethylene glycol.
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pH of 12 was adjusted by NaOH solution.
The electrolyte with
In MAO process, the Mg alloy sample and the stainless
steel container were used as the anode and the cathode, respectively.
The test was carried out at a
constant voltage of 400 V, working frequency of 400 Hz and duty cycle of 5% for 15 min, and the temperature of the electrolyte was kept at 20-30°C by a cooling system.
After MAO treatment,
samples were ultrasonically cleaned in distilled water for 10 min and dried in ambient air. 2.2. Coatings characterization Potentiodynamic polarization curves were employed to evaluate the corrosion resistance of modified materials on AZ80 alloys in SBF at 36.5±0.5◦C.
The SBF solution was prepared
according to the ISO10993-15 standard [18], which is composed of 6.8 g/L NaCl, 0.1 g/L Mg2SO4, 3
ACCEPTED MANUSCRIPT 0.2 g/L CaCl2, 2.2 g/L NaHCO3, 0.216 g/L NaH2PO4, 0.026 g/L Na2HPO4, 0.4 g/L KCl, pH 7.4. Electrochemical tests were carried out by using a standard three electrode cell with Pt as counter electrode, saturated calomel electrode (SCE) as a reference and the coated sample with exposed area of 1cm2 as the working electrode.
The ratio of the test solution volume (ml) to the specimen
surface area (cm2) was set to 300 mL/cm2.
Samples were immersed into SBF for 30 min to insure
the open circuit voltage stabilization before measurements.
The potentiodynamic polarization
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data were analyzed by CorrView software and painted by OriginLab 8.0 software. The stress corrosion susceptibility of samples was investigated using SSRT tests at a strain rate The samples were connected to the pull-rods, the load and the elongation were
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of 10−6 /s in SBF.
fracture occurred.
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monitored continuously by a load cell and a linear variable differential transformer (LVDT) until The morphology and elemental compositions of the coatings were investigated The phase
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by a JSM-6510 SEM equipped with energy dispersion spectroscopy (EDS) facilities.
composition of the coatings was identified by an XRD (Rigaku, D/max-2500 PC) with Cu Kα
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radiation source at 40 kV and 100 mA, in the 2θ range of 20-90° at a step size of 0.02°. An optical microscope (OM) and a JSM-2100 transmission electron microscopy (TEM) were used to examine the microstructure of LSP layer.
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Residual stresses in the modified layer were measured by sin2Ψ XRD method using diffraction
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peak (104) at 2θ = 151°-161°. The measurements were carried out at two different Ψ angles (0° and 30°) with a scan step of 0.01° (2θ) and 2s exposure time.
The Young’s modulus E and
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Poission’s ratio γ were 45 GPa and 0.32, respectively. 3. Results and discussion
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3.1 Morphology and phase composition of coatings Fig. 1a shows the OM image of cross-section of LSP treated sample.
It is found that a fine-
crystalline layer about 30 μm thickness is formed at the sample surface in which the grain size has become smaller and the grain boundaries are rarely seen, whereas the average grain size of matrix material is approximately 50 μm.
The fine-crystalline microstructure is attributed to the plastic
strain induced grain refinement mechanism [19].
In order to understand the influence of severe
plastic deformation on the grain size distribution, TEM is conducted and the image is shown in Fig. 1b.
It is can be seen that a nanocrystalline structure characterized by the equiaxed grains about
100-300 nm long is found at about 20 μm depth below the surface of samples. 4
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Fig. 1. Microstructure of LSP treated sample: (a) OM image of cross-section and (b) TEM image of 20 μm from surface.
are illustrated in Fig. 2a-d.
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SEM morphologies of the surface and cross section of MAO and LSP/MAO treated samples In combined with previous work，it is found that the morphology of the
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MAO bio-coating is distinctly different from that of the traditional MAO coating. The traditional MAO coating without K2TiF6 and HA additions shows that large size pores distributed uniformly
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and porosity was high [10,11]. However, the MAO bio-coating with K2TiF6 and HA additions reveals a nonuniform pores distribution and relatively lower porosity. As can be seen in Fig. 2a, some micro-pores are sealed by some compounds (as marked with arrows), and the size of many
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micro-pores is smaller than 0.5 μm, which is demonstrated that K2TiF6 and HA additives fill in the discharging pores during the MAO process.
Comparing with the MAO bio-coating, the surface of
LSP/MAO composite bio-coating displays a lower porosity due to some pores are covered by Because LSP treated sample has a rough fine-grain surface with
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discharge as show in Fig. 2b.
much more defects, where it is easier to induce discharge channels during subsequent MAO process The cross
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and cause block-like discharge on the surface of LSP/MAO composite bio-coating.
section morphologies of MAO and LSP/MAO treated samples are shown in Fig. 2c and 2d, respectively.
It can be seen clearly that the LSP/MAO bio-composite coating consists of an inner
LSP layer and an outer MAO coating, and tightly integrated with the Mg alloy substrate as compared to the MAO bio-coating. Experimental investigation shows that a fine-grain layer about 20 μm thickness is formed at the surface of LSP treated sample.
After the subsequent MAO
treatment, the fine-grain layer is partially oxidized as the outer MAO bio-coating, while the unoxidized one is remained as the inner loading layer.
The average thicknesses of both MAO
coating for MAO and LSP/MAO treated samples are approximately 12 and 18 μm, respectively. According to the laser scanning microscopy images as shown in Fig. 2e and 2f, the surface 5
ACCEPTED MANUSCRIPT roughness Ra values measured on MAO and LSP/MAO treated samples are approximately 3.032 and 3.423 μm, respectively. The surface roughness of the LSP/MAO composite bio-coating is a little higher than the MAO bio-coating. This implies that LSP treatment has a significant effect on the surface roughness of MAO coating subsequent treated.
Fig. 2g presents the EDS spectra of
area A in the MAO coating for LSP/MAO treated sample, it is indicated that the peak of O, Mg, F,
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Si, Ca, P and Ti are detected in the MAO coating.
Fig. 2. SEM morphologies of surface and cross-section of MAO sample (a, c) and LSP/MAO sample (b, d) (SEM); three dimensional laser scanning morphologies of MAO sample (e) and LSP/MAO sample (f); EDS spectra of area A in LSP/MAO sample (g).
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2-MgO
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Intensity (a.u.)
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MAO
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Intensity (a.u.)
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LSP/MAO
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Fig. 3. XRD patterns of samples surface (a) and zoomed figure in the range of 33°–36° (b). Fig. 3a shows the XRD patterns of surface of untreated and treated samples.
It is found that
only Mg is detected in the surfaces of BM and LSP samples, however, besides the peak of Mg the diffraction peaks of MgO, MgF2, Mg2SiO4, Ti3O5 and HA are also detected on the surfaces of MAO and LSP/MAO treated samples.
This demonstrates that HA (Ca10(PO4)6(OH)2)) additions 7
ACCEPTED MANUSCRIPT successfully incorporate into the coating, and the K2TiF6 takes part in the reactions during MAO process.
It is worth noticing that the crystalline Ti3O5 peaks are observed in the XRD patterns for
MAO and LSP/MAO treated samples. Dong et al [20] certified that deposition of Ti3O5 can promote MgF2 to deposit into the pores and make the films more compact.
Therefore, the K2TiF6
added to the electrolyte plays an important role for forming self-sealing MAO coating. Fig. 3b shows the intensity of the diffraction (0002) peak contacting Mg in the range of 33°–36° It is found that the position and the intensity of (0002) peak observed on
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for different samples.
treated samples (LSP, MAO and LSP/MAO samples) have a significant change comparing to BM It is worth noticing that
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sample. All the peaks of treated samples shift towards the higher angle.
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the width of the (0002) peak is significantly broadened after LSP treatment, and the peak intensity is lower than those of BM sample.
For LSP/MAO treated sample, the (0002) peak intensity is This
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weaker and the peak further shifts towards the large angle comparing to LSP treated sample.
phenomenon may be attributed to the grain refinement and the plastic deformation on the surface of Experimental observations show that the sample surface
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the sample after LSP treatment [21,22].
after LSP treatment displays a compressive residual stress (σcr) with about –123 MPa and the depth of σcr up to about 1 mm from surface.
On contrary, after MAO treatment the sample surface has a After LSP/MAO composite treatment, the compressive
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tensile residual stress (σsr) about 14 MPa.
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residual stress also remained on the surface of AZ80 alloy, σcr is about –80 MPa. 3.2 Corrosion resistance in SBF
samples in SBF.
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Fig. 4 shows potentiodynamic polarization curves of the Mg alloy substrate and the treated The corrosion potential (Ecorr), corrosion current density (icorr), and the
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anodic/cathodic Tafel constants (βa, βc) were extracted directly from the potentiodynamic polarization curves using Tafel extrapolation and linear polarization methods [23].
The
polarization resistance (Rp) is calculated by Stern-Geary equation (Eq. (1)) [24].
Rp
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All the electrochemical parameters calculated from Tafel plots are listed in Table 1.
(1) It is shown
that the treated samples with a lower icorr, positive Ecorr and higher Rp have a good corrosion resistance.
The corrosion resistance of samples is improved after surface treatment by comparing
the data. Ecorr and icorr of BM sample are –1482mV and 1.543×10-5 A/cm2, respectively.
Ecorr
(–1517 mV) of LSP treated sample is slightly less than that of BM sample, while the icorr 8
ACCEPTED MANUSCRIPT (2.133×10-6 A/cm2) is reduced by one order of magnitude.
In addition, Rp (7.79×103 Ω·cm2) of
LSP treated sample is slightly higher than that of BM sample (1.34×103 Ω·cm2).
It is indicated
that LSP treated sample has a higher corrosion resistance than that of MB sample in SBF. mentioned above, LSP treatment could result in refinement of grains.
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The refinement of the alloy
microstructure is good for improving the corrosion resistance of the material [25].
For MAO
treated sample, Ecorr is positively shifted to –1431 mV and icorr is reduced by 1-2 order of magnitude It is
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than those of the BM and LSP treated samples, registering value of 7.354×10−7 A/cm2.
indicated that MAO bio-coating with HA and K2TiF6 additions can improve the corrosion resistance For LSP/MAO treated sample, Ecorr (–1347 mV) shows a
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of AZ80 Mg alloy substrate in SBF.
significant shift to positive direction, while icorr (2.733×10−7 A/cm2) is close to that of MAO treated However, Rp of LSP/MAO treated sample (1.19×105 Ω·cm2) is about one order of
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sample.
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magnitude higher than that of MAO treated sample (2.63×104 Ω·cm2). This result reveals that LSP/MAO composite modification technology can further improve the corrosion resistance of
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AZ80 Mg alloy compared to MAO treatment method.
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BM LSP MAO LSP/MAO
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Fig. 4. Potentiodynamic polarization curves of samples tested in SBF.
Because the vulnerability of a coating against aggressive environment is directly in connection with the percent of opened pores in the MAO coating [26, 27], thus the porosity of MAO coating can be estimated by an electrochemical data based empirical equation (Eq. (2)) [28].
R pm 10 Ec o /r ra F R p
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(2)
ACCEPTED MANUSCRIPT where F is the coating porosity, Rpm is the polarization resistance of the uncoated sample.
ΔEcorr is
the difference of corrosion potential between the coated and the uncoated samples and βa is the anodic Tafel slope of the uncoated sample.
Using the data in the Table 1, F of the MAO
bio-coating and the LSP/MAO bio-composite coating are 9.06×10-3 and 1.16×10-4, respectively. According to the results, the porosity of the MAO bio-coating is about 78 times than that with the LSP/MAO bio-composite.
It is suggested that a compact structure is formed in the LSP/MAO
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bio-composite after LSP treatment process.
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icorr (A/cm2) βa (mV) βc (mV) Rp (Ω·cm2) F -5 3 (1.543±0.34)×10 68±5 156±12 1.34×10 (2.133±0.29)×10-6 61±9 113±18 7.79×103 1.55×10-1 -7 (7.354±0.41)×10 54±12 250±11 2.63×104 9.06×10-3 (2.733±0.35)×10-7 77±8 2362±29 1.19×105 1.16×10-4
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Sample Ecorr (mV) BM -1482±10 LSP -1517±20 MAO -1431±20 LSP-MAO -1347±15
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Table 1 Electrochemical data for different sample from potentiodynamic polarization test.
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In addition, the observation after electrochemical tests confirms that the surface of AZ80 Mg alloy exhibits large pitting, whereas pitting corrosion is not evident on the surface of LSP/MAO As compared to MAO bio-coating, the improved corrosion resistance in
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treated samples.
inter LSP fine-grain layer.
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LSP/MAO composite bio-coating is possibly attributed to the self-repairing property contacting the The formation of a dense passive film at the damaged region caused by
the inter fine-grain plays an important role in improving the corrosion resistance [29].
On one
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hand, the fine-grain layer has a high density of nucleation sites for passive films, results in a high fraction of passive layers and low corrosion rate [30]. On the other hand, the fine grain size
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decreased the amount of corrosive ions (Cl−) absorbed on the surface, which promoted the formation of compact passive film and improved the corrosion resistance significantly [31].
Once
the outer MAO coating is attacked by SBF, a dense passive film could form instantly at the damaged position near to the exposed surface of LSP fine-gain layer, which served as a part of barrier layer of the outer MAO coating against the corrosive inter medium penetrate the coating.
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ACCEPTED MANUSCRIPT 3.3 Stress corrosion susceptibility in SBF
300
200 150 BM LSP MAO LSP/MAO
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Stress, MPa
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Fig. 5. Stress-strain curves of samples tested in SBF at a strain rate 1×10-6/s. Table 2
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Mechanical properties of untreated and treated samples in SBF. Strain rate Elongation to failure Ultimate tensile strength Time to fracture (s-1) εf (%) UTS (MPa) (h) BM 10-6 4.8 222.5 13 -6 LSP 10 8.1 260.3 19 MAO 10-6 10.9 252.1 21 -6 LSP/MAO 10 9.9 267.3 22
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Sample
The stress-strain curves of the SSRT tested at a strain rate 10-6/s in SBF are illustrated in Fig. 5,
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and the mechanical properties of untreated and treated samples are shown in Table 2.
It can be
seen that untreated BM sample endured the SSRT test only about 13 h, and failed at a stress level of For all the treated samples, the SSRT tested at
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222.5 MPa (UTS) with a strain value of 4.8%.
10-6/s show longer durations (19-22 h) and better mechanical properties as compared to BM sample. MAO treated sample fail at ultimate tensile stress (UTS) of about 252.1 MPa and elongation (εf) of about 10.9%.
As compare to BM sample, the UTS value and the εf values of MAO treated sample
increased by 13.3% and 127%, respectively.
Comparing with the MAO treated sample, the εf
values of LSP treated and LSP/MAO treated samples only reduced to 8.1% and 9.9%, respectively, whereas the UTS values increased to 260.3 MPa and 267.3 MPa, respectively.
Those results
suggest that the AZ80 Mg alloy has a higher susceptibility to SCC than that of other three treated samples, and LSP/MAO treated sample is the most resistant to SCC in SBF. 11
Compared with
ACCEPTED MANUSCRIPT MAO treated and LSP treated samples, it can be concluded that MAO treatment is beneficial to improve elongation of material, and LSP treatment can improve the tensile strength in SBF. As mentioned above, LSP treatment can induce a compressive residual stress filed on the sample surface, whereas a tensile residual stress remained on the surface of MAO treated sample.
For
samples loaded in tension, compressive residual stress field can retard the initiation of crack in a corrosion environment, whereas tensile residual stress is detrimental to SCC resistance [21].
alloys [32].
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Additional, the pitting corrosion is easier to form the initiation of stress corrosion cracking in Mg Therefore, it can be speculated both the high pitting corrosion resistance and the
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compressive residual stress of LSP/MAO composite bio-coating play a major role in SCC behavior.
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In vitro corrosion studies show that LSP/MAO modified AZ80 Mg alloy greatly improved the general mechanical properties and corrosion resistance of material.
It is suggested that LSP/MAO
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composite bio-coating can significantly reduce corrosion rate of implants and maintain the implant’s mechanical integrity in the initial bony reunion period.
Moreover, previous work
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indicated that the MAO bio-coating with HA has excellent biocompatibility and bioactivity [11-13]. HA is a major component of natural bone, which can promote the bone growth and can reduce the degradation rate of the substrate [33].
If the LSP/MAO modified Mg alloy is implanted to the
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human body, which will be helpful in supporting the fracture bone for a longer time.
Therefore,
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the LSP/MAO surface modification can be a promising strategy for applications in degradable orthopedic implants.
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4. Conclusions
LSP/MAO composite bio-coating was successfully fabricated on the AZ80 Mg alloy through
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the combination of the preceding LSP and MAO process.
The electrochemical and SSRT tests
indicated that LSP/MAO composite bio-coating could effectively improve bio-corrosion resistance and mechanical integrity of AZ80 Mg alloy substrate in SBF. The present study suggests that the LAP/MAO composite modification could be a promising method for applications in degradable orthopedic implants, and it is worthwhile to further investigate the in vivo behavior. Acknowledgements The authors gratefully acknowledge the project sponsored by National Natural Science Foundation of China (No. 51275472), Public Technology Application Research Project of Science Technology Department of Zhejiang Province (No. 2012C21101) and Natural Science Foundation 12
ACCEPTED MANUSCRIPT of Zhejiang Province (No. LY12E05024).
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References [1] L.P. Xu, F. Pan, G.N. Yu, L. Yang, E.L. Zhang, K. Yang, In vitro and in vivo evaluation of the surface bioactivity of a calcium phosphate coated magnesium alloy. Biomaterials 30(2009) 1512–1523. [2] Y.F. Zheng, X.N. Gu, Y.L. Xi, D.L. Chai. In vitro degradation and cytotoxicity of Mg/Ca composites produced by powder metallurgy. Acta Biomater. 6(2010) 1783–1791. [3] F. Witte, V. Kaese, H. Haferkamp, E. Switzer, A. Meyer-Lindenberg, C.J. Wirth, et al. In vivo corrosion of four magnesium alloys and the associated bone response. Biomaterials 26(2005) 3557–3563. [4] P.M. Staiger, A.M. Pietak, J. Huadmai, G. Dias, Magnesium and its alloys as orthopedic biomaterials: a review. Biomaterials 27(2006) 1728–1734. [5] X.H. Guo, M.Z. An. Experimental study of electrochemical corrosion behaviour of bilayer on AZ31B Mg alloy. Corros. Sci. 52(2010) 4017–4027. [6] J.E. Gray, B. Luan, Protective coatings on magnesium and its alloys — a critical review. J. Alloys Comp. 336( 2002) 88–113. [7] X.N. Gu, N. Li, W.R. Zhou, Y.F. Zheng, X. Zhao, Q.Z. Cai, et al. Corrosion resistance and surface biocompatibility of a microarc oxidation coating on a Mg–Ca alloy. Acta Biomater. 7(2011) 1880–1889. [8] X. Lin, L. Tan, Q. Zhang, K. Yang, Z. Hu, J. Qiu, et al. The in vitro degradation process and biocompatibility of a ZK60 magnesium alloy with a forsterite-containing micro-arc oxidation coating. Acta Biomater. 9(2013) 8631–8642. [9] S.F. Fischerauer, T. Kraus, X. Wu, S. Tangl, E. Sorantin, A.C. Hanzi, et al. In vivo degradation performance of micro-arc-oxidized magnesium implants: a micro-CT study in rats. Acta Biomater. 9(2013) 5411–5420. [10] Y. Xiong, C. Lu, C. Wang, R.G. Song, The n-MAO/EPD bio-ceramoc composite coating fabricated on ZK60 magnesium alloy using combined micro-arc oxidation with electrophoretic deposition, Appl. Surf, Sci. 322(2014) 230–235. [11] Y. Xiong, C. Lu, C. Wang, R.G. Song, Degradation behavior of n-MAO/EPD bio-ceramic composite coatings on magnesium alloy in simulated body fluid. J. Alloys Compd. 625(2015) 258–265. [12] Y. Song, S.X. Zhang, J.N. Li, C.L. Zhao, X.N. Zhang, Electrodeposition of Ca–P coatings on biodegradable Mg alloy: In vitro biomineralization behavior. Acta Biomater. 6(2010) 1736–1742. [13] D. Sreekanth, N. Rameshbabu, Development and characterization of MgO/hydroxyapatite composite coating on AZ31 magnesium alloy by plasma electrolytic oxidation coupled with electrophoretic deposition. Mater. Lett. 68(2012) 439–442. [14] B.A.O. McCormack, C.D. Walsh, S.P. Wilson, P.J. Prendergast, A statistical analysis of microcrack accumulation in PMMA under fatigue loading: applications to orthopaedic implant fixation. Int. J. Fatigue 20(1998) 581–593. [15] X.N. Gu, W.R. Zhou, Y.F. Zheng, Y. Cheng, S.C. Wei, S.P. Zhong, et al. Corrosion fatigue behaviors of two biomedical Mg alloys – AZ91D and WE43 – in simulated body fluid. Acta Biomater. 6(2010) 4605–4613. [16] P.B. Srinivasan, C. Blawert, W. Dietzel, K.U. Kainer, Stress corrosion cracking behaviour of a surface-modified magnesium alloy. Scripta Mater. 59(2008) 43–46. 13
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[17] Y.B. Guo, M.P. Sealy, C.S. Guo, Significant improvement of corrosion resistance of biodegradable metallic implants processed by laser shock peening, CIRP Annals – Manufacturing Technology 61(2012) 583–586. [18] ISO10993.15, Biological Evaluation of Medical Devices – Part 15: Identificationand Quantification of Degradation Products from Metals and Alloys, The peo-ple’s Republic of China State Administration of Quality Supervision Inspectionand quarantine, 2000. [19] H.Q. Sun, Y.N. Shi, M.X. Zhang, K. Lu. Plastic strain-induced grain refinement in the nanometer scale in a Mg alloy. Acta Mater. 55 (2007) 975–982 [20] K.H. Dong, Y.W. Song, D.Y. Shan, E.H. Han, Formation mechanism of a self-sealing pore micro-arc oxidation film on AM60 magnesium alloy. Surf. Coat. Technol. 266(2015) 188–196. [21] Y.K. Zhang, J. You, J.Z. Lu, C.Y. Cui, Y.F. Jiang, X.D. Ren, Effects of laser shock processing on stress corrosion cracking susceptibility of AZ31B magnesium alloy. Surf. Coat. Technol. 204(2010) 3947–3953. [22] Y.L. Liao, G.J. Cheng, Controlled precipitation by thermal engineered laser shock peening and its effect on dislocation pinning: Multiscale dislocation dynamics simulation and experiments. Acta Mate. 61(2013) 1957–1967. [23] M. Fathi, M. Salehi, A. Saatchi, V. Mortazavi, S. Moosavi, In vitro corrosionbehavior of bioceramic, metallic, and bioceramic-metallic coated stainless steeldental implants, Dental Mater. 19 (2003) 188–198. [24] H.P. Duan, C.W. Yan, F.H. Wang, Effect of electrolyte additives on performanceof plasma electrolytic oxidation films formed on magnesium alloy AZ91D. Electrochim. Acta 52(2007) 3785–3793. [25] B.G. Hamua, D. Eliezer, L. Wagner, The relation between severe plastic deformation microstructure and corrosion behavior of AZ31 magnesium alloy. J. Alloys Comp. 468(2009) 222–229. [26] C. Blawert, V. Heitmann, W. Dietzel, H.M. Nykyforchyn, M.D. Klapkiv, Influence of electrolyte on corrosion properties of plasma electrolytic conversion coated magnesium alloys, Sur. Coat. Technol. 201 (2007) 8709–8714. [27] W. Dietzel, M. Klapkiv, H. Nykyforchyn, V. Posuvailo, C. Blawert, Porosity and corrosion properties of electrolyte plasma coatings on magnesium alloys. Mater. Sci. 40 (2004) 585–590. [28] C. Liu, Q. Bi, A. Leyland, A. Matthews, An electrochemical impedance spectroscopy study of the corrosion behaviour of PVD coated steels in 0.5 N NaCl aqueous solution: Part II. EIS interpretation of corrosion behaviour, Corros. Sci. 45(2003) 1257–1273. [29] L. Wen, Y.M. Wang, Y. Liu, Y. Zhou, L.X. Guo, J.H. Ouyang, D.C. Jia, EIS study of a self-repairing microarc oxidation coating. Corros. Sci. 53(2011) 618–623. [30] V. Afshari, C. Dehghanian, Effects of grain size on the electrochemical corrosion behavior of electrodeposited nanocrystalline Fe coatings in alkaline solution. Corros. Sci. 51(2009) 1844–1849. [31] L. Liu, Y. Li, F.H. Wang, Influence of grain size on the corrosion behavior of a Ni-based superalloy nanocrystalline coating in NaCl acidic solution. Electrochim. Acta 53(2008) 2453–2462. [32] M.B. Kannan, R.K.S. Raman, In vitro degradation and mechanical integrity of calcium-containing magnesium alloys in modified-simulated body fluid. Biomaterials 29(2008) 2306–2314. [33] K.J.L. Burg, S. Porter, J.F. Kellam, Biomaterial developments for bone tissue engineering, Biomaterials 21(2000) 2347–2359. 14
ACCEPTED MANUSCRIPT Highlights (1) LSP/MAO modification technique is proposed and fabricated on AZ80 Mg alloy. (2) MAO bio-coating have both self sealing property and biological activity. (3) LSP/MAO bio-coating improve bio-corrosion resistance and mechanical integrity.
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(4) Effect of LSP layer on property of LAS/MAO treated alloy is discussed.
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Ying Xiong, Qiang Hu, Renguo Song, Xiaxia Hu PII: DOI: Reference:
S0928-4931(17)30805-6 doi: 10.1016/j.msec.2017.03.003 MSC 7509
To appear in:
Materials Science & Engineering C
Received date: Revised date: Accepted date:
2 July 2016 16 December 2016 1 March 2017
Please cite this article as: Ying Xiong, Qiang Hu, Renguo Song, Xiaxia Hu , LSP/MAO composite bio-coating on AZ80 magnesium alloy for biomedical application. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Msc(2017), doi: 10.1016/j.msec.2017.03.003
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ACCEPTED MANUSCRIPT Submitted to Materials Science and Engineering: C, Dec 2016
LSP/MAO composite bio-coating on AZ80 magnesium alloy for biomedical application Ying Xiong a,b,*), Qiang Hu a), Renguo Song c,d), Xiaxia Hu a,b) a) Key Laboratory of Special Purpose Equipment and Advanced Processing Technology, Ministry of Education,
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Zhejiang University of Technology, Hangzhou, Zhejiang 310014, China b) College of Mechanical Engineering, Zhejiang University of Technology, Hangzhou, Zhejiang 310014, China
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c) School of Materials Science and Engineering, Changzhou University, Changzhou 213164, China d) Jiangsu Key Laboratory of Materials Surface Science and Technology, Changzhou University, Changzhou
Correspondent: Ying Xiong, E-mail: [email protected]
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ABSTRACT
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213164, Jiangsu, China
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A composite bio-coating was fabricated on AZ80 magnesium (Mg) alloy by using micro-arc oxidation (MAO) under the pretreatment of laser shock peening (LSP) in order to improve the bio-corrosion resistance and the mechanical integrity.
LSP treatment could induce grain
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refinement and compressive residual stress field on the surface of material.
MAO bio-coating was
properties of the material.
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grown in alkaline electrolyte with hydroxyapatite (HA, Ca10(PO4)6(OH)2) to improve the biological The microstructure, element and phase composition for untreated based
material (BM) and treated samples (LSP layer, MAO bio-coating and LSP/MAO composite
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bio-coating) were investigated by transmission electron microscopy (TEM), scanning electron microscope (SEM), energy dispersion spectroscopy (EDS) and X-ray diffraction (XRD).
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Electrochemical tests and slow strain rate tensile (SSRT) tests were used to evaluate the corrosion resistance and the stress corrosion susceptibility in simulated body fluid (SBF).
The results
indicated that LSP/MAO composite bio-coating can not only improve the corrosion resistance of Mg alloy substrate evidently but also increase the mechanical properties in SBF compared to LSP layer and MAO bio-coating.
Mg alloy treated by LSP/MAO composite technique should be better
suited as biodegradable orthopedic implants. Keywords: Biomaterials; Micro-arc oxidation; Laser shock processing; Microstructure; Electrochemical; Slow strain rate tensile.
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ACCEPTED MANUSCRIPT 1. Introduction As innovative biodegradable materials, magnesium (Mg) and its alloys are very attractive for potential orthopedic implants due to their strength, density and elastic modulus are very close to those of natural bone, particularly they can help to avoid a second surgery as compared with traditional implant materials such as titanium alloy, stainless alloy and cobalt-chromium alloy [1-3].
tissues sufficiently heal in addition to its general corrosion resistance [4].
Unfortunately, the
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For orthopedic applications, the implant is essential to ensure its mechanical integrity until the
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degradation rate of Mg based implants is too rapid in the body fluid environment to keep
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mechanical integrity before the diseased or damaged bone tissue healed [4-5].
modification is generally accepted as an effective approach to delay the degradation rate of Mg
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based implants during the service period [6]. Many surface modification techniques have been In all kinds of surface modification
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developed for the corrosion protection of Mg alloys substrate.
methods, MAO is a novel and promising technique based on anodic oxidation, which could
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generate a ceramic coating on the surface of Mg based materials in a suitable electrolyte.
Some
researchers have proven that the MAO coating can not only improve the corrosion resistance of Mg alloys substrate but also enhance the fixation of the implants to bone [7-9].
Furthermore, the
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MAO bio-coating with HA addition can further enhance the biocompatibility/bioactivity and the
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corrosion resistance in physiological environments [10-13].
However, implant devices exposed to
the human body environment are often also experience considerable loadings during service period, which would lead to a sudden fracture of an implant due to stress corrosion cracking (SCC) [14,15].
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Despite MAO coating showing good corrosion resistance in electrochemical tests and also Therefore,
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performing longer elongation in SSRT tests, it could not obviate the SCC process [16].
it is necessary to develop a new surface modification technology for further improving resistance to SCC in the human body environment. A promising approach is to alter the subsurface microstructure before MAO processing in order to adjust the mechanical integrity of Mg based materials.
LSP is an alternative
non-contacting surface treatment, which utilizes laser impact wave with high power and short pulse to strike on the target surface for improving the mechanic properties of metal materials, such as fatigue, corrosion cracking and wear resistance [17].
Therefore, it may be an effective way using
the combination of LSP and MAO technique to adjust both the degradation rate and the mechanical integrity of Mg based implants in the human body environment. 2
ACCEPTED MANUSCRIPT In this work, LSP was introduced as a pretreatment and then MAO bio-coating was fabricated on AZ 80 Mg alloy.
The corrosion resistance and the stress corrosion susceptibility of LSP/MAO
composite bio-coating were investigated using electrochemical tests and SSRT tests in SBF. 2. Experimental 2.1. Coatings preparation Samples tested in the study were machined from a hot-rolled AZ80 Mg plate (Mg–7.8%
substrates for the surface modified process.
The gauge geometry of the plane-plate samples for Before LSP and MAO treatment, all the samples were
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SSRT tests was 25 mm × 8 mm × 2 mm.
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Al–0.36% Zn–0.04%Mn, in wt. %). Dimensions of 40 mm × 22 mm × 2 mm were used as
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polished from 280 to 1200 grit with silicon abrasive papers to ensure the same surface toughness. Subsequently, the samples were ultrasonically degreased in ethanol for 10 min, cleaned with
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distilled water and then dried in ambient air.
LSP experiments were performed by a Q-switched Nd:YAG laser with a wavelength of 1064 The water with a thickness of about 1 mm was used as the
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nm and a pulse width of 20 ns.
transparent overlay, and the professional aluminum foil with a thickness of 0.1 mm was used as the The other detailed LSP
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absorbing overlay to protect the sample surface from thermal effect.
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parameters were as the following: a pulse energy of 2 J, a spot diameter of 3 mm and an overlapping rate of 50%. The lapping ratio was defined as the percentage of overlap among consecutive peening diameters.
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The alkaline silicate solutions for MAO process consist of 12 g/L Na2SiO3·9H2O, 5 g/L NaF, 2g/L K2TiF6, 3 g/L HA nanoparticles (20 nm) and 10 mL/L ethylene glycol.
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pH of 12 was adjusted by NaOH solution.
The electrolyte with
In MAO process, the Mg alloy sample and the stainless
steel container were used as the anode and the cathode, respectively.
The test was carried out at a
constant voltage of 400 V, working frequency of 400 Hz and duty cycle of 5% for 15 min, and the temperature of the electrolyte was kept at 20-30°C by a cooling system.
After MAO treatment,
samples were ultrasonically cleaned in distilled water for 10 min and dried in ambient air. 2.2. Coatings characterization Potentiodynamic polarization curves were employed to evaluate the corrosion resistance of modified materials on AZ80 alloys in SBF at 36.5±0.5◦C.
The SBF solution was prepared
according to the ISO10993-15 standard [18], which is composed of 6.8 g/L NaCl, 0.1 g/L Mg2SO4, 3
ACCEPTED MANUSCRIPT 0.2 g/L CaCl2, 2.2 g/L NaHCO3, 0.216 g/L NaH2PO4, 0.026 g/L Na2HPO4, 0.4 g/L KCl, pH 7.4. Electrochemical tests were carried out by using a standard three electrode cell with Pt as counter electrode, saturated calomel electrode (SCE) as a reference and the coated sample with exposed area of 1cm2 as the working electrode.
The ratio of the test solution volume (ml) to the specimen
surface area (cm2) was set to 300 mL/cm2.
Samples were immersed into SBF for 30 min to insure
the open circuit voltage stabilization before measurements.
The potentiodynamic polarization
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data were analyzed by CorrView software and painted by OriginLab 8.0 software. The stress corrosion susceptibility of samples was investigated using SSRT tests at a strain rate The samples were connected to the pull-rods, the load and the elongation were
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of 10−6 /s in SBF.
fracture occurred.
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monitored continuously by a load cell and a linear variable differential transformer (LVDT) until The morphology and elemental compositions of the coatings were investigated The phase
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by a JSM-6510 SEM equipped with energy dispersion spectroscopy (EDS) facilities.
composition of the coatings was identified by an XRD (Rigaku, D/max-2500 PC) with Cu Kα
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radiation source at 40 kV and 100 mA, in the 2θ range of 20-90° at a step size of 0.02°. An optical microscope (OM) and a JSM-2100 transmission electron microscopy (TEM) were used to examine the microstructure of LSP layer.
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Residual stresses in the modified layer were measured by sin2Ψ XRD method using diffraction
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peak (104) at 2θ = 151°-161°. The measurements were carried out at two different Ψ angles (0° and 30°) with a scan step of 0.01° (2θ) and 2s exposure time.
The Young’s modulus E and
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Poission’s ratio γ were 45 GPa and 0.32, respectively. 3. Results and discussion
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3.1 Morphology and phase composition of coatings Fig. 1a shows the OM image of cross-section of LSP treated sample.
It is found that a fine-
crystalline layer about 30 μm thickness is formed at the sample surface in which the grain size has become smaller and the grain boundaries are rarely seen, whereas the average grain size of matrix material is approximately 50 μm.
The fine-crystalline microstructure is attributed to the plastic
strain induced grain refinement mechanism [19].
In order to understand the influence of severe
plastic deformation on the grain size distribution, TEM is conducted and the image is shown in Fig. 1b.
It is can be seen that a nanocrystalline structure characterized by the equiaxed grains about
100-300 nm long is found at about 20 μm depth below the surface of samples. 4
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Fig. 1. Microstructure of LSP treated sample: (a) OM image of cross-section and (b) TEM image of 20 μm from surface.
are illustrated in Fig. 2a-d.
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SEM morphologies of the surface and cross section of MAO and LSP/MAO treated samples In combined with previous work，it is found that the morphology of the
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MAO bio-coating is distinctly different from that of the traditional MAO coating. The traditional MAO coating without K2TiF6 and HA additions shows that large size pores distributed uniformly
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and porosity was high [10,11]. However, the MAO bio-coating with K2TiF6 and HA additions reveals a nonuniform pores distribution and relatively lower porosity. As can be seen in Fig. 2a, some micro-pores are sealed by some compounds (as marked with arrows), and the size of many
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micro-pores is smaller than 0.5 μm, which is demonstrated that K2TiF6 and HA additives fill in the discharging pores during the MAO process.
Comparing with the MAO bio-coating, the surface of
LSP/MAO composite bio-coating displays a lower porosity due to some pores are covered by Because LSP treated sample has a rough fine-grain surface with
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discharge as show in Fig. 2b.
much more defects, where it is easier to induce discharge channels during subsequent MAO process The cross
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and cause block-like discharge on the surface of LSP/MAO composite bio-coating.
section morphologies of MAO and LSP/MAO treated samples are shown in Fig. 2c and 2d, respectively.
It can be seen clearly that the LSP/MAO bio-composite coating consists of an inner
LSP layer and an outer MAO coating, and tightly integrated with the Mg alloy substrate as compared to the MAO bio-coating. Experimental investigation shows that a fine-grain layer about 20 μm thickness is formed at the surface of LSP treated sample.
After the subsequent MAO
treatment, the fine-grain layer is partially oxidized as the outer MAO bio-coating, while the unoxidized one is remained as the inner loading layer.
The average thicknesses of both MAO
coating for MAO and LSP/MAO treated samples are approximately 12 and 18 μm, respectively. According to the laser scanning microscopy images as shown in Fig. 2e and 2f, the surface 5
ACCEPTED MANUSCRIPT roughness Ra values measured on MAO and LSP/MAO treated samples are approximately 3.032 and 3.423 μm, respectively. The surface roughness of the LSP/MAO composite bio-coating is a little higher than the MAO bio-coating. This implies that LSP treatment has a significant effect on the surface roughness of MAO coating subsequent treated.
Fig. 2g presents the EDS spectra of
area A in the MAO coating for LSP/MAO treated sample, it is indicated that the peak of O, Mg, F,
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Si, Ca, P and Ti are detected in the MAO coating.
Fig. 2. SEM morphologies of surface and cross-section of MAO sample (a, c) and LSP/MAO sample (b, d) (SEM); three dimensional laser scanning morphologies of MAO sample (e) and LSP/MAO sample (f); EDS spectra of area A in LSP/MAO sample (g).
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Fig. 3. XRD patterns of samples surface (a) and zoomed figure in the range of 33°–36° (b). Fig. 3a shows the XRD patterns of surface of untreated and treated samples.
It is found that
only Mg is detected in the surfaces of BM and LSP samples, however, besides the peak of Mg the diffraction peaks of MgO, MgF2, Mg2SiO4, Ti3O5 and HA are also detected on the surfaces of MAO and LSP/MAO treated samples.
This demonstrates that HA (Ca10(PO4)6(OH)2)) additions 7
ACCEPTED MANUSCRIPT successfully incorporate into the coating, and the K2TiF6 takes part in the reactions during MAO process.
It is worth noticing that the crystalline Ti3O5 peaks are observed in the XRD patterns for
MAO and LSP/MAO treated samples. Dong et al [20] certified that deposition of Ti3O5 can promote MgF2 to deposit into the pores and make the films more compact.
Therefore, the K2TiF6
added to the electrolyte plays an important role for forming self-sealing MAO coating. Fig. 3b shows the intensity of the diffraction (0002) peak contacting Mg in the range of 33°–36° It is found that the position and the intensity of (0002) peak observed on
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for different samples.
treated samples (LSP, MAO and LSP/MAO samples) have a significant change comparing to BM It is worth noticing that
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sample. All the peaks of treated samples shift towards the higher angle.
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the width of the (0002) peak is significantly broadened after LSP treatment, and the peak intensity is lower than those of BM sample.
For LSP/MAO treated sample, the (0002) peak intensity is This
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weaker and the peak further shifts towards the large angle comparing to LSP treated sample.
phenomenon may be attributed to the grain refinement and the plastic deformation on the surface of Experimental observations show that the sample surface
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the sample after LSP treatment [21,22].
after LSP treatment displays a compressive residual stress (σcr) with about –123 MPa and the depth of σcr up to about 1 mm from surface.
On contrary, after MAO treatment the sample surface has a After LSP/MAO composite treatment, the compressive
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residual stress also remained on the surface of AZ80 alloy, σcr is about –80 MPa. 3.2 Corrosion resistance in SBF
samples in SBF.
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Fig. 4 shows potentiodynamic polarization curves of the Mg alloy substrate and the treated The corrosion potential (Ecorr), corrosion current density (icorr), and the
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anodic/cathodic Tafel constants (βa, βc) were extracted directly from the potentiodynamic polarization curves using Tafel extrapolation and linear polarization methods [23].
The
polarization resistance (Rp) is calculated by Stern-Geary equation (Eq. (1)) [24].
Rp
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All the electrochemical parameters calculated from Tafel plots are listed in Table 1.
(1) It is shown
that the treated samples with a lower icorr, positive Ecorr and higher Rp have a good corrosion resistance.
The corrosion resistance of samples is improved after surface treatment by comparing
the data. Ecorr and icorr of BM sample are –1482mV and 1.543×10-5 A/cm2, respectively.
Ecorr
(–1517 mV) of LSP treated sample is slightly less than that of BM sample, while the icorr 8
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In addition, Rp (7.79×103 Ω·cm2) of
LSP treated sample is slightly higher than that of BM sample (1.34×103 Ω·cm2).
It is indicated
that LSP treated sample has a higher corrosion resistance than that of MB sample in SBF. mentioned above, LSP treatment could result in refinement of grains.
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The refinement of the alloy
microstructure is good for improving the corrosion resistance of the material [25].
For MAO
treated sample, Ecorr is positively shifted to –1431 mV and icorr is reduced by 1-2 order of magnitude It is
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than those of the BM and LSP treated samples, registering value of 7.354×10−7 A/cm2.
indicated that MAO bio-coating with HA and K2TiF6 additions can improve the corrosion resistance For LSP/MAO treated sample, Ecorr (–1347 mV) shows a
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of AZ80 Mg alloy substrate in SBF.
significant shift to positive direction, while icorr (2.733×10−7 A/cm2) is close to that of MAO treated However, Rp of LSP/MAO treated sample (1.19×105 Ω·cm2) is about one order of
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sample.
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magnitude higher than that of MAO treated sample (2.63×104 Ω·cm2). This result reveals that LSP/MAO composite modification technology can further improve the corrosion resistance of
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AZ80 Mg alloy compared to MAO treatment method.
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Fig. 4. Potentiodynamic polarization curves of samples tested in SBF.
Because the vulnerability of a coating against aggressive environment is directly in connection with the percent of opened pores in the MAO coating [26, 27], thus the porosity of MAO coating can be estimated by an electrochemical data based empirical equation (Eq. (2)) [28].
R pm 10 Ec o /r ra F R p
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(2)
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ΔEcorr is
the difference of corrosion potential between the coated and the uncoated samples and βa is the anodic Tafel slope of the uncoated sample.
Using the data in the Table 1, F of the MAO
bio-coating and the LSP/MAO bio-composite coating are 9.06×10-3 and 1.16×10-4, respectively. According to the results, the porosity of the MAO bio-coating is about 78 times than that with the LSP/MAO bio-composite.
It is suggested that a compact structure is formed in the LSP/MAO
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bio-composite after LSP treatment process.
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icorr (A/cm2) βa (mV) βc (mV) Rp (Ω·cm2) F -5 3 (1.543±0.34)×10 68±5 156±12 1.34×10 (2.133±0.29)×10-6 61±9 113±18 7.79×103 1.55×10-1 -7 (7.354±0.41)×10 54±12 250±11 2.63×104 9.06×10-3 (2.733±0.35)×10-7 77±8 2362±29 1.19×105 1.16×10-4
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Sample Ecorr (mV) BM -1482±10 LSP -1517±20 MAO -1431±20 LSP-MAO -1347±15
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Table 1 Electrochemical data for different sample from potentiodynamic polarization test.
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In addition, the observation after electrochemical tests confirms that the surface of AZ80 Mg alloy exhibits large pitting, whereas pitting corrosion is not evident on the surface of LSP/MAO As compared to MAO bio-coating, the improved corrosion resistance in
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inter LSP fine-grain layer.
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LSP/MAO composite bio-coating is possibly attributed to the self-repairing property contacting the The formation of a dense passive film at the damaged region caused by
the inter fine-grain plays an important role in improving the corrosion resistance [29].
On one
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hand, the fine-grain layer has a high density of nucleation sites for passive films, results in a high fraction of passive layers and low corrosion rate [30]. On the other hand, the fine grain size
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decreased the amount of corrosive ions (Cl−) absorbed on the surface, which promoted the formation of compact passive film and improved the corrosion resistance significantly [31].
Once
the outer MAO coating is attacked by SBF, a dense passive film could form instantly at the damaged position near to the exposed surface of LSP fine-gain layer, which served as a part of barrier layer of the outer MAO coating against the corrosive inter medium penetrate the coating.
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300
200 150 BM LSP MAO LSP/MAO
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Stress, MPa
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0 2
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6 Strain, %
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Fig. 5. Stress-strain curves of samples tested in SBF at a strain rate 1×10-6/s. Table 2
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Mechanical properties of untreated and treated samples in SBF. Strain rate Elongation to failure Ultimate tensile strength Time to fracture (s-1) εf (%) UTS (MPa) (h) BM 10-6 4.8 222.5 13 -6 LSP 10 8.1 260.3 19 MAO 10-6 10.9 252.1 21 -6 LSP/MAO 10 9.9 267.3 22
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Sample
The stress-strain curves of the SSRT tested at a strain rate 10-6/s in SBF are illustrated in Fig. 5,
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and the mechanical properties of untreated and treated samples are shown in Table 2.
It can be
seen that untreated BM sample endured the SSRT test only about 13 h, and failed at a stress level of For all the treated samples, the SSRT tested at
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222.5 MPa (UTS) with a strain value of 4.8%.
10-6/s show longer durations (19-22 h) and better mechanical properties as compared to BM sample. MAO treated sample fail at ultimate tensile stress (UTS) of about 252.1 MPa and elongation (εf) of about 10.9%.
As compare to BM sample, the UTS value and the εf values of MAO treated sample
increased by 13.3% and 127%, respectively.
Comparing with the MAO treated sample, the εf
values of LSP treated and LSP/MAO treated samples only reduced to 8.1% and 9.9%, respectively, whereas the UTS values increased to 260.3 MPa and 267.3 MPa, respectively.
Those results
suggest that the AZ80 Mg alloy has a higher susceptibility to SCC than that of other three treated samples, and LSP/MAO treated sample is the most resistant to SCC in SBF. 11
Compared with
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For
samples loaded in tension, compressive residual stress field can retard the initiation of crack in a corrosion environment, whereas tensile residual stress is detrimental to SCC resistance [21].
alloys [32].
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Additional, the pitting corrosion is easier to form the initiation of stress corrosion cracking in Mg Therefore, it can be speculated both the high pitting corrosion resistance and the
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compressive residual stress of LSP/MAO composite bio-coating play a major role in SCC behavior.
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In vitro corrosion studies show that LSP/MAO modified AZ80 Mg alloy greatly improved the general mechanical properties and corrosion resistance of material.
It is suggested that LSP/MAO
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composite bio-coating can significantly reduce corrosion rate of implants and maintain the implant’s mechanical integrity in the initial bony reunion period.
Moreover, previous work
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indicated that the MAO bio-coating with HA has excellent biocompatibility and bioactivity [11-13]. HA is a major component of natural bone, which can promote the bone growth and can reduce the degradation rate of the substrate [33].
If the LSP/MAO modified Mg alloy is implanted to the
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human body, which will be helpful in supporting the fracture bone for a longer time.
Therefore,
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the LSP/MAO surface modification can be a promising strategy for applications in degradable orthopedic implants.
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4. Conclusions
LSP/MAO composite bio-coating was successfully fabricated on the AZ80 Mg alloy through
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the combination of the preceding LSP and MAO process.
The electrochemical and SSRT tests
indicated that LSP/MAO composite bio-coating could effectively improve bio-corrosion resistance and mechanical integrity of AZ80 Mg alloy substrate in SBF. The present study suggests that the LAP/MAO composite modification could be a promising method for applications in degradable orthopedic implants, and it is worthwhile to further investigate the in vivo behavior. Acknowledgements The authors gratefully acknowledge the project sponsored by National Natural Science Foundation of China (No. 51275472), Public Technology Application Research Project of Science Technology Department of Zhejiang Province (No. 2012C21101) and Natural Science Foundation 12
ACCEPTED MANUSCRIPT of Zhejiang Province (No. LY12E05024).
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References [1] L.P. Xu, F. Pan, G.N. Yu, L. Yang, E.L. Zhang, K. Yang, In vitro and in vivo evaluation of the surface bioactivity of a calcium phosphate coated magnesium alloy. Biomaterials 30(2009) 1512–1523. [2] Y.F. Zheng, X.N. Gu, Y.L. Xi, D.L. Chai. In vitro degradation and cytotoxicity of Mg/Ca composites produced by powder metallurgy. Acta Biomater. 6(2010) 1783–1791. [3] F. Witte, V. Kaese, H. Haferkamp, E. Switzer, A. Meyer-Lindenberg, C.J. Wirth, et al. In vivo corrosion of four magnesium alloys and the associated bone response. Biomaterials 26(2005) 3557–3563. [4] P.M. Staiger, A.M. Pietak, J. Huadmai, G. Dias, Magnesium and its alloys as orthopedic biomaterials: a review. Biomaterials 27(2006) 1728–1734. [5] X.H. Guo, M.Z. An. Experimental study of electrochemical corrosion behaviour of bilayer on AZ31B Mg alloy. Corros. Sci. 52(2010) 4017–4027. [6] J.E. Gray, B. Luan, Protective coatings on magnesium and its alloys — a critical review. J. Alloys Comp. 336( 2002) 88–113. [7] X.N. Gu, N. Li, W.R. Zhou, Y.F. Zheng, X. Zhao, Q.Z. Cai, et al. Corrosion resistance and surface biocompatibility of a microarc oxidation coating on a Mg–Ca alloy. Acta Biomater. 7(2011) 1880–1889. [8] X. Lin, L. Tan, Q. Zhang, K. Yang, Z. Hu, J. Qiu, et al. The in vitro degradation process and biocompatibility of a ZK60 magnesium alloy with a forsterite-containing micro-arc oxidation coating. Acta Biomater. 9(2013) 8631–8642. [9] S.F. Fischerauer, T. Kraus, X. Wu, S. Tangl, E. Sorantin, A.C. Hanzi, et al. In vivo degradation performance of micro-arc-oxidized magnesium implants: a micro-CT study in rats. Acta Biomater. 9(2013) 5411–5420. [10] Y. Xiong, C. Lu, C. Wang, R.G. Song, The n-MAO/EPD bio-ceramoc composite coating fabricated on ZK60 magnesium alloy using combined micro-arc oxidation with electrophoretic deposition, Appl. Surf, Sci. 322(2014) 230–235. [11] Y. Xiong, C. Lu, C. Wang, R.G. Song, Degradation behavior of n-MAO/EPD bio-ceramic composite coatings on magnesium alloy in simulated body fluid. J. Alloys Compd. 625(2015) 258–265. [12] Y. Song, S.X. Zhang, J.N. Li, C.L. Zhao, X.N. Zhang, Electrodeposition of Ca–P coatings on biodegradable Mg alloy: In vitro biomineralization behavior. Acta Biomater. 6(2010) 1736–1742. [13] D. Sreekanth, N. Rameshbabu, Development and characterization of MgO/hydroxyapatite composite coating on AZ31 magnesium alloy by plasma electrolytic oxidation coupled with electrophoretic deposition. Mater. Lett. 68(2012) 439–442. [14] B.A.O. McCormack, C.D. Walsh, S.P. Wilson, P.J. Prendergast, A statistical analysis of microcrack accumulation in PMMA under fatigue loading: applications to orthopaedic implant fixation. Int. J. Fatigue 20(1998) 581–593. [15] X.N. Gu, W.R. Zhou, Y.F. Zheng, Y. Cheng, S.C. Wei, S.P. Zhong, et al. Corrosion fatigue behaviors of two biomedical Mg alloys – AZ91D and WE43 – in simulated body fluid. Acta Biomater. 6(2010) 4605–4613. [16] P.B. Srinivasan, C. Blawert, W. Dietzel, K.U. Kainer, Stress corrosion cracking behaviour of a surface-modified magnesium alloy. Scripta Mater. 59(2008) 43–46. 13
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ACCEPTED MANUSCRIPT Highlights (1) LSP/MAO modification technique is proposed and fabricated on AZ80 Mg alloy. (2) MAO bio-coating have both self sealing property and biological activity. (3) LSP/MAO bio-coating improve bio-corrosion resistance and mechanical integrity.
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(4) Effect of LSP layer on property of LAS/MAO treated alloy is discussed.
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