Al2O3 composite coating on magnesium alloy AZ31

Applied Surface Science 487 (2019) 558–568

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Fabrication and characterization of an actively protective Mg-Al LDHs/ Al2O3 composite coating on magnesium alloy AZ31

T

Liang Wua,b, , Xingxing Dinga, Zhicheng Zhenga, Yanlong Mac, Andrej Atrensd, Xiaobo Chene, Zhihui Xief, Deen Suna, Fusheng Pana,b ⁎

a

College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China National Engineering Research Center for Magnesium Alloys, Chongqing University, Chongqing 400044, China c College of Materials Science and Engineering, Chongqing University of Technology, Chongqing 400054, China d School of Mechanical and Mining Engineering, The University of Queensland, Brisbane, Qld 4072, Australia e School of Engineering, RMIT University, Carlton 3053, Australia f Synthesis and Pollution Control Key Laboratory of Sichuan Province, College of Chemistry and Chemical Engineering, China West Normal University, Nanchong 637002, PR China b

ARTICLE INFO

ABSTRACT

Keywords: Magnesium alloy LDHs coating Electrophoretic deposition Wear resistance Corrosion resistance

A magnesium-aluminum layered-double-hydroxides (MgeAl LDHs) coating was fabricated on the surface of Mg alloy AZ31, followed by electrophoretic deposition of an Al2O3 nanoparticles layer. The morphology, structure and composition of the composite coatings were investigated by field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), and energy dispersive spectrometry (EDS). Ball on disc tests were carried out for measurements of the friction coefficient, wear loss and morphology of worn surfaces. Corrosion behavior of the coatings was investigated using potentiodynamic polarization curves and electrochemical impedance spectroscopy. Results indicate that the LDHs/Al2O3 composite coatings fabricated with 2.5 g/L Al2O3 nanoparticles solution exhibited best wear resistance, while best corrosion resistance was associated with the composite coating fabricated with 0.5 g/L Al2O3 nanoparticle solution. A sound mechanism for improving wear-resistance of the resulting coatings was proposed.

1. Introduction Magnesium (Mg) alloys have a series of performance advantages, such as low density, high strength-to-weight ratio, high cast-ability and good recyclability, which implies a great number of potential applications in electronics, automation, automotive and aviation industries, and biomedical devices [1–4]. However, Mg alloys are susceptible to corrosion in aqueous solutions [3,5–14]. Furthermore, wear property is insufficient for many practical applications. Such a weakness greatly limits the wide engineering application of Mg alloys. Many surface treatments have been developed to protect Mg alloys from corrosion and wear, including chemical conversion coating, anodizing, electrochemical plating, spraying, and vapor deposition [4,15–21]. However, it remains a challenge for yielding coatings with excellent wear and corrosion resistance at the same time. Moreover, those coatings play passive resistance in terms of wear and corrosion, which indicates that the desired function will be severely deteriorated when coating integrity is damaged. As such, actively-responding coating systems to

⁎

physical/chemical damages have become the focus of research [22–24]. Layered double hydroxides (LDHs) with a typical chemical composition of [Ml-x2+ Mx3+ (OH) 2]x+ (An-) x/n·yH2O (M2+ and M3+ represent divalent and trivalent metal cations of the host layer, and An- is the interlayer anions), have exchange ability, structural adjustment, high thermal stability [25,26], and promising actively-protective properties [27,28] and are widely used in metal corrosion protection. The methods for fabricating LDHs coatings mainly consist of solvent evaporation, electrophoretic deposition, spin coating and layer-by-layer assembly [29–32]. Zeng et al. [33] prepared Mg(OH)2/Mg-Al LDHs composite coatings on Mg alloy AZ31 by a combined co-precipitation method and a hydrothermal process. And they modified the composite coatings with a superhydrophobic treatment to improve the corrosion resistant performance of the LDH coatings on the AZ31 alloy substrate. But they did not make a cross cut tape test. As we known, the corrosion resistance and wear resistance of the coated samples is closely related to the bonding strength of the coating and substrate. However, the chemical bonding generated by those methods gives rise to a poor adhesion

Corresponding author at: College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China. E-mail address: [email protected] (L. Wu).

https://doi.org/10.1016/j.apsusc.2019.05.115 Received 25 January 2019; Received in revised form 23 April 2019; Accepted 10 May 2019 Available online 11 May 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. Illustration of the fabrication process for LDHs/ Al2O3 composite coating.

between coating and substrate. Wang et al. [34] in-situ produced MgAlLDHs coatings with improved corrosion resistance by insertion of vanadate ions through dipping the coatings in an 8-quinolinol (8HQ) solution. Furthermore, Chen and colleagues [35,36] proposed the mechanism of the growth of the MgAl-LDHs on the AZ31 alloy substrate. They concluded that MgeAl hydrotalcite conversion coating formation involved dissolution of Mg alloy AZ31 substrate, adsorption of ions from the solution, nucleation of the precursor, followed by dissolution of Al3+, and exchange CO32−. Compared with existing preparation methods, LDHs coatings, prepared on the anodized alloy surface by insitu growth, has a series of performance advantages: the coating is uniform and compact, the bonding is good to the substrate, and the structure is controllable. Our group [25,37–41] fabricated a Mg-M LDHs coating on the surface of an anodized Mg alloy, and showed that the LDHs coating could effectively enclose pores of the anodized coating and significantly improve the corrosion resistance of the substrate. In contrast, LDHs coatings formed directly on Mg alloy surface are discontinuous, which provides limited protection to Mg alloy substrate against the attacks from aggressive media. The composite coating based on the LDHs on an anodized surface can greatly improve the corrosion resistance of the substrate [42–45]. In addition, tribological properties of protective coatings can be improved by uniform addition of nanoparticles [46–52]. Nanoparticles provide rolling friction and reduce the coefficient of friction to the coating during friction and wear. Gu et al. [46] reported a polytetrafluoroethylene (PTFE)/Cu/Al2O3 filled poly(methyl methacrylate) (PMMA) based composite coating with low friction and high wear resistance. Furthermore, research on LDHs coating is mainly focused corrosion resistance. Friction and wear performance are rarely studied in this regard, which imposes great concerns over practical engineering applications of Mg alloys. As a result, it is of great significance to prepare coatings with both superior wear-resistant and corrosion-resistant

through a simple and cost-effective technique to satisfy the requirements in harsh service conditions. In this work, LDHs coatings are prepared through an in-situ strategy on the surface of Mg alloy AZ31 as a solution to address the above mentioned challenges. The vertical growth of LDHs coating in a nanosheet form through in-situ growth approach perpendicular to the surface of substrate results in enormous voids that may facilitate diffusion of corrosive media. To tackle such drawbacks, such voids are employed as “containers” to accommodate wear-resistant Al2O3nanoparticles by means of a simple electrophoretic deposition technique. The fully filledup composite LDHs-Al2O3 coatings are expected to exhibit a multifunctional role in wear resistance and corrosion resistance. Furthermore, the wear-resistance and corrosion-resistance mechanism of the composite coatings is elucidated. 2. Experimental 2.1. Materials Commercial cast magnesium alloy AZ31 (with nominal composition, wt%: Al 2.5–3.5, Zn 0.6–1.3, Mn 0.2–1, Ca 0.04, Si 0.1, Cu 0.05, and balance Mg) specimens (20 mm × 20 mm × 5 mm and 10 mm × 10 mm × 5 mm) were progressively ground using SiC waterproof sandpapers down to 2000 grit. Al2O3 nanoparticles (obtained from Emperor Nano Material Co. Ltd. Nanjing, China) used in the electrophoretic deposition have an average particle size of 40 nm. 2.2. Coating preparation 2.2.1. Mg-Al LDHs coatings 0.3 M NaNO3 and 0.05 M Al (NO3)3 were sequentially dissolved into 100 mL deionized water with continuous magnetic stirring. The 559

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changing the electrode potential from −2 V to 0.5 V with reference to the open circuit potential (OCP) at a sweep rate of 2 mV s−1. Impedance measurements were obtained using a 10 mV rms sinusoidal perturbation and performed at OCP from 10 mHz to 100 kHz. The experimental impedance plots were fitted using different equivalent circuits by means of the Zview software. All polarization and EIS tests were carried out at room temperature. In order to ensure the reproducibility, there were three parallel samples in each system. Tribological tests were conducted on a ball-on-disc rotating wear tester (HT-2001 POD-1, China) for 30 min using a normal load of 2 N, a rotation radius of 4 mm, a rotating velocity of 200 rpm, and ambient conditions. A Si3N4 ceramic ball with a diameter of 5 mm and surface roughness of about 0.01 μm was counterpart. The friction coefficient was continuously recorded as a function of sliding time during the test. After the experiments, worn surface morphology of each sample was examined using FE-SEM (Tescan VEGA 3 LMH SEM, Chech) and the composition of the worn tracks was investigated using EDS equipped with FE-SEM. The morphologies and depth profiles of wear tracks were measured using profilometer (Dektak 150, Veeco, America). The crosssectional areas of the wear tracks were evaluated using the Origin software. The wear volume was evaluated as the wear area multiplied by the rotation circumference. The specific wear rate of the samples was calculated from:

Fig. 2. XRD spectra of LDHs coating and LDHs/Al2O3 composite coatings prepared at different concentrations of Al2O3 nanoparticles.

solution was adjusted to alkaline pH (10.8) using 3.0 M NaOH solution. The mixture was transferred to Teflon-lined autoclaves with Mg alloy AZ31 specimens. The Teflon-lined autoclaves were heated up to 398 K and hold for 12 h. Finally, the as-prepared specimens were removed with forceps and rinsed with running deionized water and dried with a cold air flow.

= V /(F × L)

(1)

where ω is wear rate, V is wear volume, F is load, and L is sliding distance. 3. Results and discussion

2.2.2. LDHs/Al2O3 composite coatings 5 g/L Al(NO3)3 and as-received Al2O3 nanoparticles with varying concentrations (0.25, 2.5, and 5.0 g/L, respectively) were dispersed in 200 mL of ethyl alcohol and sonicated at room temperature for 10 min. Then, two vertically aligned plates (an Mg alloy AZ31 plate with LDHs coating and a bare stainless steel control), separated by a distance of 1 cm, were immersed in the dispersion and a constant voltage of 150 V was applied for 10 min (Fig. 1).

3.1 Physical and chemical characteristics

3.1.1. XRD analysis Fig. 2 depicts the XRD patterns of MgAl-LDHs and LDHs/Al2O3 composite coatings fabricated by electrophoretic deposition with different concentrations of Al2O3 nanoparticles. The pattern of MgeAl LDHs coating exhibits two peaks locating at 11.7°and 23.3°, corresponding to (003) and (006) planes respectively. This result is consistent with results of the previous works on Mg-Al-LDHs [25], indicating successful synthesis of LDHs with intercalated NO3− anions. The appearance of the characteristic peak of Mg(OH)2 in all samples suggests that the coatings consist of LDHs and Mg(OH)2. With increasing concentrations of Al2O3 nanoparticles in the solution, there was an obvious increase in the intensity of the Al2O3 phases, indicating increasing concentrations of nanoparticles contained in the deposited coating layer. In addition, with increasing the concentration of nanoparticles, there was a gradual decrease in the peak heights for Mg-AlNO3-LDHs and Mg(OH)2. When the concentration of nanoparticles reached 5 g/L, the characteristic peak of (006) almost completely disappear.

2.2.3. Immersion test Bare Mg alloy AZ31 specimens, MgeAl LDH coating and LDH/Al2O3 composite coating were immersed in 3.5 wt% NaCl solution in closed containers at 25 °C for 7 days. Afterwards, specimens were removed from the solution and clean with running deionized water to get rid of corrosion products per the instructions of ASTM NACE/ASTMG312012a standards. 2.3. Characterization The morphologies of the surface and cross-section of LDHs coatings were observed with a field-emission scanning electronic microscope (FE-SEM, JSM e7800F, JEOL, Japan) and the chemical composition was investigated using energy dispersive spectrometry (EDS, INCA Energy 350 Oxford, UK). For cross-sectional examination, sections of the specimens were generated by ultramicrotomy (UC; Leica EM UC7, Germany) using a diamond knife. The compounds present in the LDHs coatings were determined using an X-ray diffractometer (XRD, D/Max 2500×, Rigaku, Japan) at a glancing angle of 1.5° using Cu target (λ = 0.154 nm) with an accelerating voltage of 40 kV and a current of 150 mA. Corrosion resistance was investigated in 3.5 wt% NaCl solution by potentiodynamic polarization curves and electrochemical impedance spectra using the electrochemical workstation (Princeton Parstat 4000A, USA). A three-electrode system was used with the sample as working electrode (1 cm2), a saturated calomel reference electrode (SCE) as reference electrode, and a platinum plate as counter electrode. Polarization curves for all samples were obtained by automatically

3.1.2. Surface morphology and EDS analysis Fig. 3 presents SEM and EDS appearance of the MgAl-LDHs and LDHs/Al2O3 composite coatings. Fig. 3 shows that the surfaces of the pure LDHs coatings were continuous and complete. There were no clear defects, such as holes and local uplifts, on the surface. The higher magnification image reveals that the microstructure of the LDHs coating was composed of nanosheets grown vertically on the substrate, which is consistent with the morphology of the LDHs coatings obtained in other studies [41,53,54]. This leads to inevitable voids between the nanosheets, which provide channels for the aggressive medium [55]. Fig. 3c shows that the nanoparticle layer completely covered the LDHs coating. Moreover, at a concentration of 0.25 g/L, there were clear cracks in the LDHs/Al2O3 composite coatings. However, increasing the 560

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Fig. 3. SEM photograph and EDS data of LDHs coating and LDHs/Al2O3 composite coating layers prepared at different concentrations of Al2O3 nanoparticles at low and high magnifications: a, b, c, 1: LDHs coating; d, e, f, 2: C(Al2O3) = 0.25 g/L; g, h, I, 3: C(Al2O3) = 0.5 g/L; j, k, l, 4: C(Al2O3) = 2.5 g/L; m, n, o, 4: C(Al2O3) = 5 g/L.

concentration of the nanoparticles caused the cracks to gradually disappear and the composite coating became dense. At the same time, there were many island-like clusters on the surface of the composite coatings prepared with 0.25 g/L, 0.5 g/L and 2.5 g/L and the low-lying areas also had better coverage. But the coating prepared at a concentration of 5 g/L had no obvious island-like clusters. The coatings mostly contained magnesium, aluminum, and oxygen, wherein the concentration of nanoparticles had little effect on the content of oxygen in the entire composite coating, because all of the LDHs, Mg(OH)2 and Al2O3 contain similar amounts of oxygen. With increasing concentration of Al2O3 nanoparticles, the content of magnesium in the composite layer decreased from 33% to 2% and the aluminum element increased from 2% to 40%. The atomic ratio of each element of the coating prepared at a concentration of 5 g/L was 3: 2: 0.09, wherein the atomic

ratio of oxygen to aluminum is close to 3:2, which indicates that the coating consisted mainly of Al2O3 nanoparticles. The increasing thickness of the deposited nanoparticle layer increasingly masked the X-ray peaks of the LDHs coating. This result explains why the characteristic peak of LDHs in the above XRD spectrum gradually decreased and disappeared with increasing concentration of nanoparticles. 3.1.3. Cross-section morphology Fig. 4(a-f) shows the cross-sectional morphology of the coatings. Fig. 4a showed that there was a relatively obvious interface between the LDHs coating and the substrate, and the surface had obvious flaky LDHs nanosheets. At the same time, there was a layer of non-nano layer between the nanosheet and the substrate. The XRD results indicated that the main constituent phase of this layer was Mg (OH)2. The 561

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Fig. 4. SEM photograph of the cross-sectional morphology of LDHs coating and LDHs/Al2O3 composite coating prepared at different concentrations of Al2O3 nanoparticles; a: LDHs; b: 0.25 g/L; c: 0.5 g/L; d: 2.5 g/L; e: 5 g/L; f: 5 g/L (not completely covered).

thickness of the LDHs coating and the composite coatings prepared with nanoparticle concentrations of 0.25, 0.5, 2.5, and 5 g/L was approximately 2.04 2.31, 2.37, 3.04, and 3.66 μm, respectively. Fig. 4f is a photograph of a nano-particle layer incompletely covered in a composite coating prepared with a nanoparticle concentration of 0.5 g/L. The holes between LDHs nanosheets were sealed by nanoparticles. On the upper surface, there was such a large particle-shaped cluster of nanoparticles due to agglomeration of the nanoparticles during deposition. The surface morphology of the nanosheet was obviously different from that of the pure LDHs nanosheet. Therefore, it can be inferred that the nanoparticle layer obtained by electrophoretic deposition on the LDHs coating was divided into two layers: one layer was a finely dispersed particle layer dispersed in the gap between the LDH nanosheets; the other layer is a layer of nano-particle clusters, which was adsorbed on the surface of the LDHs coating. 3.2. Corrosion performance

Fig. 5. The potentiodynamic polarization measured in a 3.5 wt% NaCl solution of AZ31 magnesium alloy, LDHs coating and LDHs/Al2O3 composite coating samples prepared at different concentrations of Al2O3 nanoparticles after immersing for 7 days in 3.5 wt% NaCl solution.

3.2.1. Potentiodynamic polarization curve Fig. 5 and Table 1 shows the results for all samples after immersion for 7 days in 3.5 wt% NaCl solution. After 7 days, the LDHs coating and LDHs/Al2O3 composite coating layer could maintain the corrosion potential higher than that of AZ31 magnesium alloy, and the corrosion current density was lower than that of AZ31 magnesium alloy, which indicates that the coatings protected the substrate. The corrosion product layer covers the substrate and thus provides protection to the substrate. However, since the corrosion product is extremely loose, the protective effect is poor [40]. But the corrosion potential of the LDHs coating after immersion was significantly increased, attributed to the fact that the volume of the thin LDHs corrosion product was larger, and the expansive volume sealed the pores of the LDHs coating to a certain extent, reducing the corrosion potential [10,11,56]. The LDHs/Al2O3 composite coating layer showed a significantly higher corrosion potential for the conditions of 0.5 g/L, 2.5 g/L and 5 g/L. The corrosion current density of the LDHs/Al2O3 composite coating prepared using the 0.5 g/L concentration was slightly lower. This indicates that the LDHs/Al2O3 composite layer could effectively protect the substrate in 3.5 wt% NaCl solution and the composite coating prepared under the condition of 0.5 g/L Al2O3 nanoparticle solution had the best corrosion resistance.

Table 1 Corrosion potential Ecorr and current density of AZ31 magnesium alloy, LDHs coating and LDHs/Al2O3 composite coating samples prepared with different concentrations of Al2O3 nanoparticles after 7 days immersing in 3.5 wt% NaCl solution.

AZ31 LDHs 0.25 g/L 0.5 g/L 2.5 g/L 5 g/L

Ecorr (V/SCE)

icorr (μA/cm2)

−1.44 −0.36 −0.38 −0.14 −0.18 −0.28

165 22 11 1.6 3.4 6.4

3.2.2. EIS In order to further study the corrosion behavior of the specimens, electrochemical impedance spectroscopy (EIS) was performed on the specimens after immersion. The test results are shown in Fig. 6(a). The 562

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and the LDHs/Al2O3 composite coatings and the fitting results are shown in Table 2. Both the LDHs coating and the LDHs/Al2O3 composite coating exhibited two relaxation processes, suggesting that the EIS test results have two time constants [57,58]. Tedim et al. [59] reported that the middle-frequency (1 × 100 to 1 × 102 Hz) relaxation process is ascribed to the capacitance of oxide film present on the alloy surface, while the low-frequency (1 × 10−2 to 1 × 10−1 Hz) time constant is related to the corrosion activity. The components in the equivalent circuit diagram correspond to the various parts of the sample surface. In order to reflect the heterogeneity of the components, a constant phase angle element (CPE) is introduced to replace the pure capacitance C. The relationship between the two is CPE = 1/C (j ω) α, where the capacitance characteristic of C is related to α. In general, when α = 1, the constant phase angle element is a pure capacitance, and when α = 0, the constant phase angle element represents a pure resistance. In the figure, Rs corresponds to the electrolyte resistance, and CPEout corresponds to the electric double layer capacitance of the LDHs coating and the LDHs/Al2O3 composite coating, respectively. Rout corresponds to the resistance of the LDHs coating and the LDHs/Al2O3 composite coating, respectively, and CPEin corresponds to the electric double layer capacitor at the metal interface, Rin corresponds to the metal matrix charge transfer resistor. Considering the Tafel polarization curve test results, the composite coating prepared at 0.5 g/L was significantly superior to LDHs coating and the composite coating prepared under other conditions and can provide effective protection for the substrate. 3.3. Wear resistance 3.3.1. Morphologies of worn surfaces Fig. 7 shows the SEM images and EDS spectra of the worn surface of the AZ31 magnesium alloy, LDHs coating and LDHs/Al2O3 composite coating. The width of the wear scar of the substrate was significantly larger than that on the LDHs coating and on the LDHs/Al2O3 composite coating. The micro-interface of the wear scar was rough and there were many frictions stripes and residual debris, which cause the friction coefficient to fluctuate greatly during the wear process in Fig. 8. Relatively speaking, the wear scar morphology of the LDHs coating samples was relatively flat, without obvious streaks and debris, and there were no obvious bumps and pits in the micrograph of the worn surface. This means that the LDHs coating was softer than the substrate, and the structure of coating was relatively uniform. As the wear progresses, it is easier to fill the cutting pit generated by the wear and also to better mesh with the substrate, thereby making the friction surface smooth gradually. Fig. 3 indicates that the content of oxygen and magnesium in the wear scar of the composite coating was 36% and 62%, respectively, while the contents of coating in Fig. 7 was 65% and 33%. This indicates that the LDHs coating was not completely penetrated during the wear process, or the coating could be fused with the substrate during the rubbing. The resulting worn surface may be formed by the mutual doping of LDHs coating and substrate deformation. The content of aluminum in the wear scar in EDS spectra in all composite coating samples was significantly less than that without wear, indicating that most of the deposited Al2O3 nanoparticles were removed during the wear process. The higher magnification photo

Fig. 6. EIS plots of AZ31 magnesium alloy, LDHs coating and LDHs/Al2O3 composite coating samples prepared at different concentrations of Al2O3 nanoparticles after 7 days immersion a) and equivalent circuits used for fit EIS plots b).

impedance value in the frequency-impedance diagram gradually increased with decreasing frequency. The impedance value at 10 mHz corresponds to the impedance of the entire substrate and the coating, and its magnitude reflects the corrosion resistance of the sample. Compared with other concentration conditions, the |Z| value of the low frequency region of the composite coating prepared using 0.5 g/L concentration was significantly higher, which indicates that the composite coating layer prepared under this concentration has the best corrosion resistance. This result was consistent with the potentiodynamic polarization curve. In addition, the |Z| value of the low frequency section of the LDHs coating was significantly lower than that of the LDHs/Al2O3 composite coating layer, which indicates that the corrosion resistance of the LDHs coating was lower than that of LDHs/ Al2O3 after 7 days of immersion corrosion. The composite coating layer had a longer lasting corrosion protection capability in the aggressive media, wherein the composite coating prepared at a concentration of 0.5 g/L had the highest |Z| value at 10 mHz, close to the result at a concentration of 2.5 g/L. Fig. 6b shows the equivalent circuit diagram of the LDHs coating

Table 2 The corresponding main fitted parameters of all samples after immersing for 7 days in 3.5 wt% NaCl solution.

LDHs 0.25 0.5 2.5 5

Rs (Ω cm2)

CPEout (μF cm−2)

αout

Rout (Ω cm2)

CPEin (μF cm−2)

αin

Rin (Ω cm2)

χ2

47.5 39.7 50.6 57.1 48.5

2.81 × 10−5 1.61 × 10−5 1.39 × 10−5 2.82 × 10−6 1.54 × 10−6

0.59 0.64 0.49 0.72 0.702

112.8 987.9 4604 4984 3654

3.45 × 10−6 5.25 × 10−7 5.84 × 10−6 2.96 × 10−4 9.78 × 10−5

0.88 0.95 0.95 0.70 0.52

1004 2704 9258 1029 1812

8.8 × 10−3 3.6 × 10−3 1.7 × 10−3 4.3 × 10−3 2.4 × 10−3

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Fig. 7. SEM and EDS spectra of wear scar: a, b, c: AZ31 magnesium alloy sample; d, e, f: LDHs Coating sample; g, h, i:0.25 g/L; j, k, l: 0.5 g/L; m, n, o: 2.5 g/L; p, q, r: 5 g/L.

constant around 0.43. The curve of the friction coefficient of the sample with the LDHs coating indicates an initial sharp decline in the initial stage of wear. The initial friction was similar for the other samples with composite coatings. Then, the friction coefficient enters a gentle period in which the friction coefficient is about 0.32. For times longer than 500 s, the friction coefficient began to rise continuously, and the fluctuation also increased. This is attributed to the fact that the LDHs coating and the composite coating layer were relatively loose and rough compared with the substrate. At the beginning of the wear test, the friction coefficient was large due to the large roughness. As the wear progressed, the coating layer was quickly compacted to become dense and smooth, and the coefficient of friction dropped sharply. This was because the LDHs coating began to break. However, the residual product after the wear still lubricated the interface between the substrate and the grinding ball, so that the friction coefficient was still smaller than that of the AZ31 sample. For the friction coefficient of LDHs/Al2O3 composite coating fabricated at different concentrations of Al2O3 nanoparticles, the friction coefficient of the composite coating prepared at 0.25 g/L and 5 g/L was similar to that of the LDHs coating. But the Al2O3 nanoparticles caused rolling wear, so that the friction coefficient of the composite coating layer was less than that of the LDHs coating in the gentle period, and was about 0.28. After the wear was carried out for 500 s, the friction coefficient of the composite coating layer prepared with the concentration of 5 g/L sharply increased, indicating that the structure of the composite coating layer had changed. However, the friction coefficient of the composite coating prepared with the concentration of 0.25 g/L increased in this period, but the rising speed was slower. For the composite coatings prepared at the concentration of 0.5 g/L and 2.5 g/L, the friction coefficient of the two layers remained relatively stable after the initial sharp decline, and the whole process fluctuated less. The coefficient of friction was approximately 0.32 and 0.31, respectively. This indicates that the composite coatings prepared under these two concentrations can be well matched and have good tribological properties.

Fig. 8. Variation of friction coefficient of AZ31 magnesium alloy, LDH coating and LDHs/Al2O3 composite coating prepared under different Al2O3 nanoparticle concentration during friction.

shows that there were loose layer and some micro-cracks, and many fine stripes on the worn surface of the LDHs/Al2O3 composite coating prepared at a concentration of 5 g/L. Fig. 7 indicates that the oxygen content was 17% and the magnesium content was 82%, which indicates that the composite coating has a large amount of wear. The possible reason is that a small amount of deposited agglomerated Al2O3 nanoparticles in the outer layer breaks and falls off the wear surface, thereby generating an abrasive wear process and accelerating wear. The wear scars of LDHs/Al2O3 composite coatings prepared with concentrations of 0.25 g/L, 0.5 g/L and 2.5 g/L of Al2O3 nanoparticles were smaller. The high-magnification photos indicate that the friction surfaces were relatively smooth, and there were no streaks and loose layers generated by wear. The contents of oxygen in Fig. 7 were 65%, 68% and 68%, respectively, indicating that the composite coating layer still had a considerable thickness and could protect the substrate during the wear process. At this time, the aluminum content in the three wear marks was 2%, 2% and 2%, respectively. Combined with the EDS results in Fig. 3, the LDHs film itself contained a small amount of Al close to the current three data. There were not necessarily nanoparticles. The role of nanoparticles was to provide rolling friction and reduce the coefficient of friction during wear. However, these were lost in the process of wear, and the content of Al in the wear scar was greatly reduced. The lubricating layer may be formed by the mutual doping of nanoparticles and the LDHs coating. This result was consistent with the change in the friction coefficient of the composite coatings fabricated in these three conditions in Fig. 8.

3.3.3. Wear rate Fig. 9 shows the sectional morphologies of wear tracks on the AZ31 magnesium alloy, LDHs coating and LDHs/Al2O3 composite coating. The wear rate of all the samples is presented in Fig. 10. Fig. 9 shows

3.3.2. Friction coefficient Fig. 8 shows the friction coefficient of AZ31 magnesium alloy, LDHs coating and LDHs/Al2O3 composite coating. The curve of the friction coefficient of AZ31 exhibits two stages: a rising period and stable period, during which the friction coefficient remains substantially constant around 0.43. At the beginning, the friction coefficient increases with time. Because in the initial stage, the soft substrate was ground by the micro-convex of the hard surface of the friction pair and produced furrows and fractures or debris, leaving a harder hard spot to form a protruding contact surface [60–62]. During this period, called the running-in phase, the friction coefficient continues to increase [36]. As the wear continues, the microstructure of the friction surface gradually stabilizes, so the friction coefficient remains substantially

Fig. 9. Abrasion surface profile of AZ31 magnesium alloy, LDHs coating and LDHs/Al2O3 composite coating prepared under different surface thickness of Al2O3 nanoparticles recorded by surface profiler. 565

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the metal substrate, a larger hard spot in the metal matrix is highlighted, thereby increasing the friction coefficient of the friction surface and generating abrasive grains wear and accelerating the wear rate. When the grinding ball is in contact with the composite coating, the larger nano-particle clusters are dispersed under the action of pressure and filled in the gap of the friction surface. At the same time, the Al2O3 nanoparticles dispersed in the LDHs nanosheets provide rolling friction. When the composite coating is rubbed, the lubricating layer formed under the friction surface is composed of the original LDHs coating and extremely fine Al2O3 nanoparticles. The nanoparticles make the smoothness of the deformed layer much higher. The lubricating layer formed by the simple LDHs coating has better friction and wear properties. Moreover, the friction coefficient was stable when the friction was performed for 1800s, and the composite coating layer was not damaged. But the composite coating fabricated under 2.5 g/L with good dispersion and deposition efficiency has the best wear resistance. It is due to the difference in the distribution of nanoparticles on the surface of the LDHs coatings. The LDHs coating in the composite presents a platelet like shape. Al2O3 nanoparticles used in this work has a shape of solid sphere. The nanoparticles can alleviate friction shearing by their filling in voids between the LDHs nanosheets. [51] As shown in Fig. 3, the size of the nanoparticles clusters increases with increasing the concentration. Thus, Al2O3 particles in 2.5 g/L with suitable size should be easier to enter and fill the surficial holes. The nanoparticle clusters in 5 g/L are relatively larger and fill the surficial holes difficultly. Nanoparticles in 2.5 g/L can mainly segregate the friction pair to alleviate the wear. Its main lubricating mechanism may be ascribed to the synergetic effect of the LDHs coating and Al2O3 nanoparticles. According to the results of the electrochemical test, the following protection mechanisms can be concluded. Firstly, the improvement in corrosion resistance lies in the blocking of chloride ions and the release of nitrate ions because of the ability of ion exchange [34]. Secondly, based on the ion-exchange process, the released NO3− ions concentrated on the surface of the film, leading to the formation of a diffusion boundary layer containing high concentration of NO3− ions. The dissolved Mg2+ could form Mg(OH)2 under alkaline conditions. The formation of Mg(OH)2 can inhibit the expansion and spread of pitting corrosion. Furthermore, there are inevitable voids between the nanosheets, which provide channels for the aggressive medium. When the channels are blocked by the Al2O3 nanoparticles, the corrosion resistance of the composite coating is improved. But as the concentration increases, the dispersibility of the nanoparticles decreases (large agglomerations will occur), and the deposition efficiency increases. Therefore, the film fabricated under the concentration with good dispersion and deposition efficiency has the best corrosion resistance.

Fig. 10. Wear rate of AZ31 magnesium alloy, LDHs coating and LDHs/Al2O3 composite coating prepared at different concentrations of Al2O3 nanoparticles.

that the AZ31 magnesium alloy had the largest wear scar with width and depth of 0.818 mm and 32.51 μm, respectively, and a wear rate of 1460 mm3/(N•m). The fluctuations in the curve indicate that the friction surface was rough and there were many gullies, which was consistent with the SEM high-magnification photograph in Fig. 7. The composite coating samples prepared at a concentration of 2.5 g/L of Al2O3 nanoparticles had minimal width and depth of the wear scar. The width and depth were 0.394 mm and 6.75 μm, respectively, which were much smaller than that of the AZ31 magnesium alloy sample. The wear rate was 175 mm3/(N•m). This indicates that the composite coating layer prepared with the concentration of 2.5 g/L Al2O3 nanoparticles had the best tribological properties. The width and depth of the wear scar of the LDHs coating samples were 0.494 mm and 7.62 μm, respectively, and the wear rate was 221 mm3/(N•m). Numerically, the LDHs coating samples has a low wear rate. However, compared with the composite coating sample, the sectional morphologies of wear tracks on the LDHs coating sample fluctuated greatly, which also indicates that the LDHs coating changed greatly after 30 min test, and penetration may occur. 3.4. Mechanisms Fig. 11 is a model diagram showing the mechanism of wear enhancement of LDHs coating and LDH/Al2O3 nanoparticle composite coating. When rubbing, the LDHs coating has a certain thickness and hardness due to the existence of the substrate. The upper friction pair (that is the grinding ball) firstly rubs the LDHs coating. Since the LDHs coating is mainly composed of LDHs nanosheets and magnesium hydroxide, there is no large hard spot, the wear is relatively stable, and the wear rate is slow. When the amount of wear reaches the limit of the LDHs coating, and the deformed layer of the friction surface contacts

4. Conclusions A MgeAl LDHs coating was fabricated on the surface of magnesium alloy, and then an Al2O3 nanoparticle layer was deposited on the

Fig. 11. Model of friction and wear enhancement mechanism of LDHs coating and LDH/Al2O3 nanoparticle composite coating. 566

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surface of LDHs coating by electrophoretic deposition in different concentration of Al2O3 nanoparticle solution. Both the LDHs coating and the LDHs/Al2O3 composite coatings protected the substrate in terms of wear resistance and corrosion resistance to different extents. LDHs coating and Al2O3 nanoparticles had a synergistic effect on antiwear and anti- corrosion performance of magnesium alloy AZ31. The LDHs/Al2O3 composite coatings prepared with 2.5 g/L Al2O3 nanoparticle solution exhibited the best wear resistance and the composite coatings prepared in 0.5 g/L exhibited the best corrosion resistance.

[21]

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Acknowledgements

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This work was supported by the National Natural Science Foundation of China (51701029, 51531002, 51474043), the National Key Research and Development Program of China (2016YFB0301100), China Postdoctoral Science Foundation Funded Project (2017M620410, 2018T110942), the Chongqing Postdoctoral Science Special Foundation (Xm2017010), the Chongqing Research Program of Basic Research and Frontier Technology (cstc2016jcyjA0388, cstc2017jcyjBX0040), the Fundamental Research Funds for the Central Universities (2018CDGFCL005).

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