TiN coating on AZ31B Mg alloy

Accepted Manuscript Structure and properties of newly designed MAO/TiN coating on AZ31B Mg alloy

Xue-Jun Cui, Jing Ping, Ying-Jun Zhang, Yong-Zhong Jin, Guang-An Zhang PII: DOI: Reference:

S0257-8972(17)30862-9 doi: 10.1016/j.surfcoat.2017.08.053 SCT 22614

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

21 January 2017 24 July 2017 23 August 2017

Please cite this article as: Xue-Jun Cui, Jing Ping, Ying-Jun Zhang, Yong-Zhong Jin, Guang-An Zhang , Structure and properties of newly designed MAO/TiN coating on AZ31B Mg alloy, Surface & Coatings Technology (2017), doi: 10.1016/ j.surfcoat.2017.08.053

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ACCEPTED MANUSCRIPT Structure and properties of newly designed MAO/TiN coating on AZ31B Mg alloy Xue-Jun Cui1, 2 *, Jing Ping1, Ying-Jun Zhang1,Yong-Zhong Jin1, Guang-An Zhang3 1. School of Materials Science and Engineering, Sichuan University of Science and Engineering, Zigong 643000, China 2. Shandong Key Laboratory for High Strength Lightweight Metallic Materials, Advanced Materials

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Institute, Shandong Academy of Sciences, Jinan 250014, China

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3. State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese

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Academy of Sciences, Lanzhou 730000, China

Abstract: A duplex MAO/TiN coating was fabricated on AZ31B Mg alloy through micro-arc

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oxidation (MAO) followed by a multi-arc ion plating (M-AIP) process. The structure, composition, and corrosion resistance of these coated samples were evaluated using SEM, EDS, XRD, and

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electrochemical methods. The results showed that the porous MAO layer was effectively sealed by the M-AIP layer, indicating that the MAO layer can act as an interlayer to offer an inert rough

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surface and necessary hardness for the Mg alloy to bond with the M-AIP layer (especially for a hard

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coating such as TiN). The MAO/Ti-coated samples exhibited the worst corrosion resistance and lower hardness compared with the MAO-coated samples, while the MAO/TiN coating provided

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higher corrosion resistance and hardness. It can be speculated that unavoidable defects such as pores and cracks are responsible for the poor properties. Although the MAO layer was demonstrated to

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improve the interfacial compatibility between the hard coating and the soft substrate, a compact and dense hard coating is still essential to ensure the corrosion and wear resistance of Mg alloys. Keywords: Magnesium alloy; Coatings; Micro-arc oxidation; Multi-arc ion plating; TiN

ACCEPTED MANUSCRIPT 1. Introduction As a lightweight and high-strength material, magnesium (Mg) alloys have potential applications in the automotive industry, biomedical materials, electrical instruments, and aerospace [1–3]. However, their low hardness, wear, and corrosion resistance have restricted their widespread applications in these fields. Fortunately, there are many methods and technologies that can be

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applied to Mg alloys to solve these drawbacks, which involve purity of alloys, corrosion inhibitors,

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and coating technologies (plating, anodizing, chemical conversion coatings, physical vapour

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deposition, etc.) [4–7]. Among these approaches, the coating of Mg alloys is a possible technique to further improve their properties, especially the transition metal nitride coatings based on Ti, Cr, Al,

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and V using PVD methods [4–5, 8]. These coatings have aroused much more attention for protecting magnesium alloys against corrosion and wear owing to their high hardness and wear

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resistance, and excellent heat and oxidation stability, in the past two decades [9–24]. However, the limited bearing capacity involves the adhesive nature of bonding between the coating and the

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substrate, and the steep change of properties between the hard coatings and the soft Mg alloys is a

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significant deficiency [13–18]. Moreover, these coatings cannot ensure corrosion protection owing to intrinsic defects such as pinholes, pores, and intercolumnar crystal, which tend to provide

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corrosion channels [11, 19–21]. Producing a metallic buffer layer of Al, Ti, or Cr on Mg alloys may be a promising solution [17–18, 21–24]. However, a separation phenomenon still exists between the

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buffer layer and the substrate [9, 21–24]. It is well known that micro-arc oxidation (MAO) technology is widely applied to in-situ growth-like ceramic coating with high hardness, good corrosion, and wear resistance [25–27]. Such coatings, as demonstrated in earlier work [26–27], are diffusively bonded to the substrate and thus have the best adhesion. The typical structure of an MAO coating consists of an inner dense layer and outer porous layer [28–29]. The inner layer acts as a physical barrier to the corrosive medium, while the outer layer can form a mechanical interlocking effect for a surface coating

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ACCEPTED MANUSCRIPT [30–31]. To improve corrosion and wear resistance of Mg alloys, many hybrid coatings have been developed based on the MAO-coated substrates, which include hydrophobic coatings [32–33], electroless plating [34], diamond-like carbon coatings [35–36], and hard coatings [37–40]. These composite coatings show good structures and excellent properties. There has been little research performed to fabricate a TiN coating on MAO-coated Mg alloys. Moreover, TiN coating exhibits

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excellent function as decorative [40]. Hence, the current research aimed to produce composite TiN

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layers on the MAO-coated AZ31B Mg alloy by the hybrid process, and to investigate the structure

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of the coatings and their properties. 2. Materials and methods

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The chemical composition of the AZ31B Mg alloy used in this experiment was 2.94 wt% Al, 0.9 wt% Zn, 0.23 wt% Mn, 0.01 wt% Si, 0.01 wt% Cu, 0.003 wt% Fe, ≤0.005 wt% Ni, and Mg

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balance in mass%. A 2-mm-thick AZ31B Mg alloy plate was cut into 30 mm × 30 mm samples, which were polished with SiC paper (400–1500 grit), rinsed with distilled water, and ultrasonically

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cleaned in acetone for 20 min.

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To ensure good wear and corrosion protection for Mg alloys, we attempted to create a diffusive-type metal layer on an MAO-treated Mg alloy, and then deposit a hard coating on this

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layer using PVD methods. The diffusive layer was expected to adhere to the substrate better than direct PVD coating. A schematic diagram of the hard coating fabricated by combining MAO and

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PVD methods on Mg alloys is shown in Fig. 1. In this paper, the Ti metal was used to prepare the diffusive buffer layer and then produce a TiN hard coating by the technology of multi-arc ion plating (M-AIP). Thus, an MAO/Ti/TiN composite coating should be deposited on the AZ31B Mg alloy. The MAO process was conducted with a previously reported equipment and process [32]. An AC pulse power source was employed. The positive voltage, frequency, and duty ratio were fixed at 260 V, 300 Hz, and 30%. Meanwhile, the negative voltage and duty ratio were 40 V and 5%,

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ACCEPTED MANUSCRIPT respectively. The electrolyte solution contains 15 g L-1 of Na2SiO3, 3 g L-1 of NaF, and 20 g L-1of KOH in deionized water. The AZ31B Mg alloy as an anode was discharged for 5 min with stainless cylinder as a cathode. The MAO-coated samples were rinsed for 20 min with absolute ethanol alcohol in an ultrasonic bath, and then dried in cold air. The MAO-treated samples were deposited through multi-arc ion plating (M-AIP) technology.

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First, the chamber was evacuated down to a base pressure of less than 5 × 10-3 Pa. Second, argon

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(Ar, 99.99%) with a flow rate of 500 sccm was introduced to keep the chamber pressure at 1–3 Pa.

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A glow discharge cleaning of 5 min was conducted at a substrate pulse negative bias voltage of -800 V, duty ratio of 20%, and frequency of 30 Hz. Then, decreasing the bias voltage to -80 V

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ignites the arc source. A pure Ti (99.95 wt. %) coating was deposited for 45 min. Finally, the chamber pressure was kept lower 1 Pa with a fixed flow rate for the nitrogen and argon ratio. A

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magnetic current of 10 A was added, and the Ti source was ignited to deposit a TiN layer for 60 min. The detailed parameters are listed in Table 1, and a schematic diagram of the process is

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shown in Fig. 1.

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The surface and cross-sectional morphologies, as well as the elemental composition, were determined by a scanning electron microscope (SEM, VEGA 3 SBU, Tescan, Czechia) equipped

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with energy dispersive spectroscopy (EDS, Oxford, England). X-ray diffraction (XRD, D2 PHASER, Bruker, Germany) operating with Cu-Kα radiation was used to determine the phase

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composition of the coatings.

The hardness of coatings was investigated using a nanoindentation tester (CSM Instruments) by linear loading with a maximum load of 10 mN and max deepness of 0.4 μm. The hardness values were calculated based on Oliver & Pharr methods. At least three measurements of the hardness were taken from different locations of the coatings, and an average value was regarded as the hardness of the coating. The corrosion resistances of the bare and coated samples were evaluated by a polarization

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ACCEPTED MANUSCRIPT curve test in a 3.5-wt% NaCl solution at room temperature using an electrochemical test system (CHI660E, Shanghai Chenhua, China). The test procedure and set were the same as those in [32]. The corrosion and breakdown potential (Ecorr and Eb) and cathodic Tafel slope (bc) were determined directly from the polarization curves using electrochemical software (CHI, Version 12.23, USA), while the corrosion current density (icorr) was confirmed based on the cathodic

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polarization behaviour in the Tafel linear region.

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3. Results and discussion

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3.1. Surface morphology

Figure 2 shows the SEM images of the coated AZ31B Mg alloy. As seen in Fig. 2(a), pores,

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cracks, and volcano-like structures are obviously observed, and are considered as typical features of the MAO coating [25–27, 41]. The sizes of the pores are 1–5 μm, which can form a direct path

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between the substrate and a corrosive environment, and accelerate deterioration of the substrate. Figure 2(b–c) displays the typical surface morphology of the Ti and TiN layer deposited by the

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M-AIP method [10, 12, 42–43]. There are unseen pores on the surface, but there are plenty of

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microparticles with diameters up to 3 μm on the MAO-coated Mg alloy, indicating that the MAO coating is completely covered by the Ti and TiN layers. These solid particles arise from the

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droplets emitted by arc spots on the Ti target, and are generally considered as coating defects because they diminish the coating properties [42–43]. By comparison, the TiN layer appears to be

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more compact and smoother than the Ti layer. This result is attributed to their different compositions and formation processes. For example, the nitrogen partial pressure will be changed once the nitrogen inflows into the chamber during the depositing of TiN, which can affect the crystallographic structure and composition of the coatings [42]. 3.2 Cross-sectional topography and chemical composition Figure 3 presents the cross-sectional topographies of the coatings. A clear interface can be observed between the Ti or TiN layer and the MAO layer, indicating that the M-AIP layers are

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ACCEPTED MANUSCRIPT integrated with the MAO layer by physical interlocking. This should be attributed to the penetration and diffusion of Ti particles into the outer pores of the MAO layer (indicated by circles in Fig. 3). Moreover, there is no pronounced delamination in the interfaces from the MAO/Ti and MAO/TiN coatings, although the samples were thoroughly ground and polished before SEM observations. This indicates that the adhesion between the M-AIP layers and MAO layer is

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sufficient [18, 38]. Conversely, if the Ti layer or TiN layer is deposited directly on the AZ31B Mg

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alloy, the layers will drop off the substrate. These phenomena were observed during the

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experiments. The surface can be remelted when M-AIP layers are directly deposited on the bare Mg alloy, while the MAO layer can supress the surface temperature and the formation of the

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heat-affected layer [38, 44]. Thus, the MAO layer will contribute to enhancing the adhesion of the M-AIP layers and Mg alloy. However, some microcracks caused by residual stress can be observed

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in these interfaces (indicated by arrows), which will reduce the properties of the coating. Additionally, the thickness of the TiN layer should be greater than that of the Ti layer according to

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the deposition time, while a reverse result was gained from Fig. 3. This implies that the Ti layer

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should be denser during the deposition of TiN. Figure 4 displays the SEM-EDS line scan curves corresponding to the MAO/Ti coating

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shown in Fig. 3(a). The elements Mg and Al are derived from the Mg alloy, and the elements O and Si originate from the electrolyte, which formed the MAO layer. The M-AIP layer is composed

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of the element Ti and a very small amount of N. As seen in the curves, it is obvious that the interdiffusion of the elements Ti and Mg can be confirmed between the MAO layer and Ti layer. Thus, it can be speculated that the Ti layer penetrated and diffused into the porous outer layer of the MAO coating. To further prove the diffusion effect between the interlayers, the cross-sectional elemental mapping images of the MAO/TiN coating shown in Fig. 3(b) were investigated. This is demonstrated in Fig. 5. Generally, the brighter the colour of the element, the higher the content of

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ACCEPTED MANUSCRIPT that element. It seems that the content of Al element in the substrate was less than that in the MAO coating in Fig. 5(a), which should be attributed to the Al2O3 polishing powder adhered to the sample before observation. As illustrated in Fig. 5, it is clear that the interdiffusion of the elements Al, O, and N can be confirmed, but that of the elements Mg, Si, and Ti is not evident. In our opinion, it is easy for the Ti or TiN layer to diffuse into the outer porous layer of the MAO coating,

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resulting in an inconspicuous diffusion interface. Additionally, we have not yet researched the

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diffusion process intentionally. Thus, the above results prove that Ti penetrates and diffuses into

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the MAO layer by an appropriate process, and forms TiN when nitrogen gas is added. 3.3 XRD

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Figure 6 shows the XRD patterns of the coated Mg alloy. The MAO-coated sample consists of Mg, MgO, and MgSiO3. The peak of Mg is derived from the substrate, but the peaks of MgO and

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MgSiO3 are from the MAO layer. The peak of the Ti phase is obvious from the MAO/Ti-coated sample, but it was not detected on the MAO/TiN-coated sample except in the TiN, Mg, MgO, and

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MgSiO3 phases. This suggests that the Ti phase transformed throughly TiN during the addition of

MAO layer.

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3.4 Polarization curves

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nitrogen gas. There is no appearance of other phases, although the nitrogen was diffused into the

Figure 7 shows the polarization curves of the bare and coated AZ31B Mg alloys in 3.5% NaCl

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solutions. Compared with bare Mg alloy, although Ecorr of the MAO-coated sample is hardly changed, its icorr value is considerably lower, indicating that the MAO coating can improve the corrosion resistance of the Mg alloy [26–28]. Moreover, compared with the MAO-coated Mg alloy, Ecorr of the MAO/Ti-coated sample is slightly positively shifted, but its icorr value increases, implying that the Ti layer can accelerate the corrosion rate of the MAO-coated Mg alloy. The reason should be attributed to the low compactness of the Ti layer and the large galvanic potential between the metals Mg and Ti [18]. Nevertheless, the icorr value of the MAO/TiN-coated sample is

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ACCEPTED MANUSCRIPT lower, and shows more positive Ecorr than that of the MAO-coated Mg alloy. This proves that the TiN layer, which is denser than the Ti layer (Fig. 3), further enhances the corrosion protection ability of the MAO layer against the Mg alloy. Additionally, the coated samples show obviously passive behaviour compared with the bare Mg alloy. Thus, the icorr value was confirmed based on the cathodic polarization behaviour in the

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Tafel linear region. The related fitting results are summarized in Table 2. The icorr values of the

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coated samples are similar and are about two orders of magnitude lower compared with those of

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the bare substrate. It can also be determined that the Ecorr values of the coated samples are more positive than those of the uncoated Mg alloy. Further, the Ecorr value of the MAO/TiN-coated

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sample shifts positively about 196 mV than that of the MAO-coated substrate. The results are in agreement with the analysis from Fig. 7, and show that the TiN layer improves the corrosion

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protection ability of the MAO layer for the AZ31B Mg alloy. 3.5 Hardness

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Figure 8 shows the indentation hardness (HIT) and modulus (EIT) values of the MAO-,

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MAO/Ti-, and MAO/TiN-coated AZ31B Mg alloy. The average HIT values are 1760, 1465, and 11508 MPa, respectively. All EIT values exhibit a trend that is similar to that of the hardness values.

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The HIT value of the MAO-coated sample is lower than those reported in the literature [45–46]. This can be ascribed to the different preparation parameters such as the constant and lower voltage,

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and shorter oxidation time carried out in this experiment. Nevertheless, we can judge the properties of coatings by the change tendency of the hardness values in Fig. 8. The HIT value of the MAO/Ti coating is lower than that of the MAO coating, which can be ascribed to the good ductility of metal Ti. Thus, it is often used for an interlayer in multilayer hard coatings [14, 16–17]. In addition, poor compactness resulted from coating defects. However, the compactness of the Ti layer was increased when nitrogen gas was added (Fig. 3), and its phase structure was transformed into TiN (Fig. 6). Thus, the MAO/TiN coating exhibits the highest HIT value.

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ACCEPTED MANUSCRIPT 3.6 Discussion An MAO/Ti/TiN composite coating should have been gained on the AZ31B Mg alloy by the presented methods in Fig. 1. However, an MAO/TiN coating was identified by the XRD pattern. Meanwhile, the results of the structure, composition, and corrosion and hardness tests proved that the mentioned method is practical, feasible, and meaningful.

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These coatings prepared by the M-AIP method are always accompanied by defects such as

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solid droplets, liquid droplets, pinholes, or shallow craters, which seem to be difficult to avoid [10,

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12, 42–43, 47] because they originate from the macro defects (droplet peripheries and craters) and inherent porosity of the coating itself [19, 48]. These defects can be also observed in Fig. 2(b–c).

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The size of these solid droplets is large, indicating that there could be some direct paths between the Ti or TiN layer and the MAO layer. These paths provide channels for corrosive media in an

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aggressive environment. Moreover, some microcracks are visible along the interfaces between the Ti or TiN layer and the MAO layer in Fig. 3. These cracks can be ascribed to the residual stress

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resulting from the structural stress (extrinsic stresses) and the thermal stress (intrinsic stresses).

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The structural stress is mainly caused by lattice mismatch and growth, but the thermal stress is involved by different thermal expansion coefficients between the layers [14]. The MAO layer

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mainly consists of MgO whose crystal structure is a face-centred cubic lattice, but the crystal structure of Ti is a body-centred cubic lattice at high temperature (≥882.5 ℃). This lattice

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mismatch causes structural stress, which leads to a lattice expansion of the MAO layer during the deposition process. In addition, the MgO is a ceramic phase where the thermal expansion coefficients are lower than those of Ti. This will generate tensile stress on the MAO layer. Further, the crystal structure of Ti is a close-packed hexagonal lattice at room temperature, which will result in volume shrinkage when the temperature decreases to room temperature. Therefore, cracks can be clearly observed in the interlayer of the MAO/Ti coating (Fig. 3a). Conversely, the crystal structures of TiN and MgO are similar, so the residual stress of the MAO/TiN coating is mainly

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ACCEPTED MANUSCRIPT thermal stress arising from different thermal expansion coefficients between the layers. Thus, small-sized microcracks can be observed in the interlayer of the MAO/TiN coating (Fig. 3b). Additionally, there are many more solid droplets observed on the surface of the M-AIP layer (Fig. 2). Thus, it can be speculated that there are fewer liquid droplets during the deposition of the M-AIP layer, resulting in an incomplete penetration of the M-AIP layer on the outer porous layer

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of the MAO coating, and partially sealed pores. Therefore, these microcracks, together with the

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unsealed pores in the MAO layer, further form channels for corrosive mediums. This will also

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form galvanic cells at the defects near the interface because the MAO layer is electrochemically more stable than the Ti, TiN layer, and Mg alloy. Once aggressive ions penetrate the coating

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through these small channels, driven by capillary forces, these exposed areas will start to experience anodic dissolution along the interfaces. Finally, the pits will be formed and link up with

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each other, resulting in the removal of the entire coating by flaking [19]. In Fig. 7, an MAO/Ti-coated sample exhibits an increasing icorr than an MAO-coated sample, indicating that the

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aggressive ions such as Cl- accelerate the corrosion of the substrate via these defects of the Ti layer.

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However, the MAO/TiN-coated sample shows a decreased icorr value than MAO-coated sample, which suggests the Ti layer can be compacted by the formation of the TiN layer.

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In any case, the MAO/TiN coating simply improves the corrosion resistance of the Mg alloy slightly. The main reason for this is the contribution of the coating defects (e.g. pores and cracks

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inside the MAO, Ti, or TiN layers), which develop corrosion [11]. The MAO/TiN-coated sample shows higher HIT and EIT values in comparison with the MAO-coated sample, implying that the MAO layer plays a more important role between the hard coating and the soft substrate. The hard coating is easy to crack and exhibits poor adhesion on the Mg alloy, which was observed during our experiment. Although the MAO layer and TiN layer have excellent mechanical and tribological properties, their properties are always conditioned by the presence of structural defects such as pores, pinholes, and cracks [49–50]. Consequently, the

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ACCEPTED MANUSCRIPT TiN layer exhibits a low hardness and poor corrosion resistance to Mg alloys in comparison with reports in the literature [11, 17, and 50]. To sum, a diffusive-type metal buffer layer fabricated through the combined MAO and M-AIP method can improve the interfacial compatibility between the hard coating and soft Mg alloys. Subsequently, to ensure the corrosion and wear protection of Mg alloys, we will research

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the related process to optimize the diffusive-type buffer layer (indicated a composite coating by

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penetration and diffusion between the MAO layer and a metal layer) by increasing the thickness of

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the metal layer in the MAO outer layer and improving the compactness of the coating. Conclusions

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(1) The Ti droplets can penetrate the porous outer layer of the MAO coating, and interdiffuse

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between the M-AIP layer and MAO layer, resulting in an enhanced interfacial bonding. (2) The Ti layer reduces the corrosion resistance and hardness of the MAO-coated Mg alloy, but

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the compacted TiN layer will slightly enhance these properties. These unavoidable defects, such as

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pores and cracks, are responsible for the poor properties. (3) Combining MAO and M-AIP methods to create a diffusive-type buffer layer is a promising

Acknowledgement

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approach to improve the interfacial compatibility between the hard coating and soft Mg alloys.

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This work was supported by the Science and Technology Planning Project (2016JZ0032) of Sichuan Province, Talent Introduction Fund (2017RCL15) of the Sichuan University of Science and Engineering, and Opening Project (2016sdlsm001) from Shandong Key Laboratory for High Strength Lightweight Metallic Materials, China. References [1]. M.K. Kulekci, Magnesium and its alloys applications in automotive industry, Int. J. Adv. Manuf. Technol. (2008) 39:851-865.

11

ACCEPTED MANUSCRIPT [2]. R.C. Zeng, W. Dietzel, F. Witte, N. Hort, C. Blawert, Progress and Challenge for Magnesium Alloys as Biomaterials, Adv. Eng. Mater. 10 (2008): B3-B14. [3]. P. Saha, M.K. Datta, Oleg I. Velikokhatnyi, A. Manivannan, D. Alman, Prashant N. Kumta, Rechargeable magnesium battery: Current status and key challenges for the future, Prog. Mater. Sci. 66 (2014) 1-86.

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[4]. J.E. Gray, B. Luan, Protective coatings on magnesium and its alloys-a critical review, J. Alloys

RI

Compd. 336(2002)88-113.

SC

[5]. R.G. Hu, S. Zhang, J.F. Bu, C.J. Lin, G.L. Song, Recent progress in corrosion protection of magnesium alloys by organic coatings, Prog. Org. Coat. 73 (2012) 129-141.

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[6]. G. Song, Recent Progress in Corrosion and Protection of Magnesium Alloys. Adv. Eng. Mater. 7(2005), 563-586.

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[7]. K.W. Guo, A Review of Magnesium/Magnesium Alloys Corrosion and its Protection. Recent Patents on Corrosion Science, 2(2010)13-21.

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[8]. M. Fenker, M. Balzer, H. Kappl, Corrosion protection with hard coatings on steel: Past

PT E

approaches and current research efforts, Surf. Coat. Technol. 257 (2014) 182-205. [9]. G. Reiners, M. Griepentrog, Hard coatings on magnesium alloys by sputter deposition using a

CE

pulsed d.c. bias voltage, Surf. Coat. Technol. 76-77 (1995) 809-814. [10]. H. Altun, S. Sen, The effect of PVD coatings on the wear behaviour of magnesium alloys,

AC

Mater. Charact. 58 (2007) 917-921. [11]. H. Altun, H. Sinici, Corrosion behaviour of magnesium alloys coated with TiN by cathodic arc deposition in NaCl and Na2SO4 solutions, Mater. Charact. 59(2008) 266-270. [12]. L. Wang, S.H. Zhang, Z. Chen, J.L. Li, M. X. Li, Influence of deposition parameters on hard Cr-Al-N coatings deposited by multi-arc ion plating, Appl. Surf. Sci. 258 (2012) 3629-3636. [13]. M. Tacikowski, J. Morgiel, M. Banaszek, K. Cymerman, T. Wierzchon, Structure and properties of diffusive titanium nitride layers produced by hybrid method on AZ91D

12

ACCEPTED MANUSCRIPT magnesium alloy, Trans. Nonferrous Met. Soc. China 24(2014) 2767-2775. [14]. H.T. Li, Q. Wang, M.H. Zhuang, J.J. Wu, Characterization and residual stress analysis of TiN/TiCN films on AZ31 magnesium alloy by PVD, Vacuum, 112 (2015) 66-69. [15]. S. Bhowmick, R. Bhide, M. Hoffman, V. Jayaram, S.K. Biswas, Fracture mode transitions during indentation of columnar TiN coatings on metal, Philos. Mag. 85 (2005) 2927-2945.

PT

[16]. Z.H. Xie, M. Hoffman, P. Munroe, A. Bendavid, P.J. Martin, Deformation mechanisms of TiN

RI

multilayer coatings alternated by ductile or stiff interlayers, Acta Mater. 56 (2008) 852-861.

SC

[17]. Y. Sun, C. Lu, H.L. Yu, A. KietTieu, L.H. Su, Y. Zhao, H.T. Zhu, C. Kong, Nanomechanical properties of TiCN and TiCN/Ti coatings on Ti prepared by Filtered Arc Deposition, Mat. Sci.

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Eng. A, 625 (2015) 56-64.

[18]. G.S. Wu, A. Shanaghi, Y. Zhao, X.M. Zhang, R.Z. Xu, Z.W. Wu, G.Y. Li, Paul K. Chu, The

MA

effect of interlayer on corrosion resistance of ceramic coating/Mg alloy substrate in simulated physiological environment, Surf. Coat. Technol. 206 (2012) 4892-4898.

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[19]. H. Altun, S. Sen, The effect of DC magnetron sputtering AlN coatings on the corrosion

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behaviour of magnesium alloys, Surf. Coat. Technol. 197 (2005) 193-200. [20]. M. Tacikowski, M. Banaszek, J. Smolik, Corrosion-resistant composite titanium nitride layers

CE

produced on the AZ91D magnesium alloy by a hybrid method, Vacuum, 99 (2014) 298-302. [21]. S.K. Wu, S.C. Yen, T.S. Chou, A study of r.f.-sputtered Al and Ni thin films on AZ91D

AC

magnesium alloy, Surf. Coat. Technol. 200 (2006) 2769-2774. [22]. G.S. Wu, Fabrication of Al and Al/Ti coatings on magnesium alloy by sputtering, Mater. Lett. 61 (2007) 3815-3817. [23]. M. Daroonparvar, Muhamad Azizi Mat Yajid, Noordin Mohd Yusof, Hamid Reza Bakhsheshi-Rad, Esah Hamzah, Hussein Ali Kamali, Microstructural characterization and corrosion resistance evaluation of nanostructured Al and Al/AlCr coated Mg-Zn-Ce-La alloy, J. Alloy. Compd. 615 (2014) 657-671.

13

ACCEPTED MANUSCRIPT [24]. C.E. Cui, Q. Miao, J.D. Pan, Ti/Cr multi-layer coating on magnesium alloy AZ91 by arc-added glow plasma depositing technique, Surf. Coat. Technol. 201 (2007) 5400-5403. [25]. A.L. Yerokhin, X. Nie, A. Leyland, A. Matthews, S.J. Dowey, Plasma electrolysis for surface engineering, Surf. Coat. Technol. 122 (1999) 73-93. [26]. T.S.N. Sankara Narayanan, I.S. Park, M.H. Lee, Strategies to improve the corrosion resistance

PT

of microarc oxidation (MAO) coated magnesium alloys for degradable implants: Prospects and

RI

challenges, Prog. Mater. Sci. 60 (2014) 1-71.

[27]. B.V. Vladimirov, B.L. Krit, V.B. Lyudin, N.V. Morozova, A.D. Rossiiskaya, I.V. Suminov, and

SC

A.V. Epel’feld, Microarc oxidation of magnesium alloys: A Review, Surf. Eng. Appl. Elect.50

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(2014) 195-232.

[28]. K.M. Lee, K.R. Shin, S. Namgung, B Yoo, D.H. Shin, Electrochemical response of ZrO 2

Coat. Technol. 205 (2011)3779-3784.

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incorporated oxide layer on AZ91 Mg alloy processed by plasma electrolytic oxidation, Surf.

D

[29]. Y.G. Ko, S. Namgung, D.H. Shin, Correlation between KOH concentration and surface

PT E

properties of AZ91 magnesium alloy coated by plasma electrolytic oxidation, Surf. Coat. Technol. 205(2010) 2525-2531.

CE

[30]. R. Arrabal, J.M. Mota, A. Criado, A. Pardo, M. Mohedano, E. Matykina, Assessment of duplex coating combining plasma electrolytic oxidation and polymer layer on AZ31 magnesium alloy,

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Surf. Coat. Technol. 206 (2012) 4692-4703. [31]. Y. Tang, X. Zhao, K. Jiang, J. Chen, Y. Zuo, The influences of duty cycle on the bonding strength of AZ31B magnesium alloy by microarc oxidation treatment, Surf. Coat. Technol. 205 (2010), 1789-1792. [32]. X.J. Cui, X.Z. Lin, C.H. Liu, R.S. Yang, X.W. Zheng, M. Gong. Fabrication and corrosion resistance of a hydrophobic micro-arc oxidation coating on AZ31 Mg alloy, Corros. Sci. 90 (2015) 402-412.

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ACCEPTED MANUSCRIPT [33]. H. R. Bakhsheshi-Rad, E. Hamzah, R. Ebrahimi-Kahrizsangi, M. Daroonparvar, M. Medraj, Fabrication and characterization of hydrophobic microarc oxidation/poly-lactic acid duplex coating on biodegradable Mg-Ca alloy for corrosion protection, Vacuum 125 (2016) 185-188. [34]. J.M. Li, X.N. Xiao, H. Cai, B.L. Jiang, Preparation and characterization of electroless Ni coating on the surface of MgO with porous structure, Acta Metal Sin. 469 (2010) 1103-1108.

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[35]. W. Yang, P.L. Ke, Y. Fang, H. Zheng, A.Y. Wang, Microstructure and properties of duplex

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(Ti:N)-DLC/MAO coating on magnesium alloy, Appl. Surf. Sci. 270 (2013) 519-525.

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[36]. J. Liang, P. Wang, L. T. Hu, J. C. Hao, Tribological properties of duplex MAO/DLC coatings on magnesium alloy using combined microarc oxidation and filtered cathodic arc deposition, Mat.

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Sci. Eng. A, 454-455 (2007) 164-169.

[37]. H. Hoche, C. Blawert, E. Broszeit, C. Berger, General corrosion and galvanic corrosion

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properties of differently PVD treated magnesium die cast alloy AZ91, Adv. Eng. Mater. 5 (2003) 896-902.

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[38]. M. Daroonparvar, M. Azizi Mat Yajid, N. Mohd Yusof, H. Reza Bakhsheshi-Rad, E. Hamzah, T.

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Mardanikivi, Deposition of duplex MAO layer/nanostructured titanium dioxide composite coatings on Mge1%Ca alloy using a combined technique of air plasma spraying and micro arc

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oxidation, J. Alloy. Compd. 649 (2015) 591-605. [39]. F. Zhou, Y. Wang, F. Liu, Y. D. Meng, Z. D. Dai, Friction and wear properties of duplex

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MAO/CrN coatings sliding against Si3N4 ceramic balls in air, water and oil, Wear 267 (2009) 1581-1588.

[40]. X.J. Cui, J.S. Wei, C.M. Ning, Y.Z. Jin, X.Z. Lin, Effects of nitrogen volumetric flow rate on properties of MAO/TiN composite coatings on AZ31B magnesium alloy, China Surface Engineering 30 (2017) 27-34. [41]. A.L. Yerokhin, V.V. Lyubimov, R.V. Ashitkov, Phase formation in ceramic coatings during plasma electrolytic oxidation of aluminium alloys, Ceram. Int.24 (1998) 1-6.

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ACCEPTED MANUSCRIPT [42]. F. Cai, S.H. Zhang, J.L. Li, Z. Chen, M.X. Li, L. Wang, Effect of nitrogen partial pressure on Al-Ti-N films deposited by arc ion plating, Appl. Surf. Sci. 258(2011) 1819-1825. [43]. J. Zhang, H.M. Lv, G.Y. Cui, Z. Jing, C. Wang, Effects of bias voltage on the microstructure and mechanical properties of (Ti,Al,Cr)N hard films with N-gradient distributions, Thin Solid films, 519 (2011) 4818-4823.

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[44]. X.Z. Fan, Y.J. Liu, Z.H. Xu, Y. Wang, B.L. Zou, L.J. Gu, C.J. Wang, X.L. Chen, Zuhair S. Khan,

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D.W. Yang, X.Q. Cao, Preparation and characterization of 8YSZ thermal barrier coatings on

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rare earth-magnesium alloy, Therm. Spray. Technol. 20 (2011)948-957. [45]. S. Durdu, M. Usta, Characterization and mechanical properties of coatings on magnesium by

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microarc oxidation, Appl. Surf. Sci. 261 (2012) 774-782.

[46]. S. Durdu, S. Bayramoğlu, A. Demirtaş, M. Usta, A. Hikmet Üçışık, Characterization of AZ31

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Mg Alloy coated by plasma electrolytic oxidation, Vacuum 88 (2013) 130-133. [47]. S.H. Zhang, L.Wang, Q.M. Wang, M.X. Li, A superhard CrAlSiN superlattice coating deposited

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by multi-arc ion plating: I. Microstructure and mechanical properties, Surf. Coat. Technol. 214

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(2013) 160-167.

[48]. M. Urgen, Ali Fuat caklir, The effect of heating on corrosion behavior of TiN- and CrN-coated

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steels, Surf. Coat. Technol. 96 (1997) 236-244. [49]. A. Conde, C. Navas, A.B. Cristóbal, J. Housden, J. de Damborenea, Characterisation of

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corrosion and wear behaviour of nanoscaled e-beam PVD CrN coatings, Surf. Coat. Technol. 201 (2006) 2690-2695. [50]. F. Hollstein, R. Wiedemann, J. Scholz, Characteristics of PVD-coatings on AZ31hp magnesium alloys, Surf. Coat. Technol. 162 (2003) 261-268.

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ACCEPTED MANUSCRIPT Figure captions: Fig. 1 Schematic diagram of hard coating fabricated by combining MAO and PVD methods on Mg alloys. Fig. 2 SEM images of coated AZ31 Mg alloy: (a) MAO coating, (b) MAO/Ti coating, and (c) MAO/TiN coating.

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Fig. 3 Cross-sectional topographies of (a) MAO/Ti and (b) MAO/TiN coatings.

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Fig. 4 SEM-EDS line scan curves corresponding to MAO/Ti coating shown in Fig. 3(a).

Fig. 6 XRD patterns of coated AZ31B Mg alloy.

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Fig. 5 Cross-sectional elemental mapping images of MAO/TiN coating shown in Fig. 3(b).

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Fig. 7 Polarization curves of bare and coated AZ31B Mg alloys in 3.5 wt. % NaCl aqueous solution.

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Fig. 8 HIT and EIT values of MAO-, MAO/Ti-, and MAO/TiN-coated AZ31B Mg alloy.

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ACCEPTED MANUSCRIPT Fig. 1 Schematic diagram of hard coating fabricated by combining MAO and PVD methods on Mg

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alloys.

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ACCEPTED MANUSCRIPT Fig. 2 SEM images of coated AZ31 Mg alloy: (a) MAO coating, (b) MAO/Ti coating, and (c) MAO/TiN coating. (b)

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ACCEPTED MANUSCRIPT Fig. 3 Cross-sectional topographies of (a) MAO/Ti and (b) MAO/TiN coatings.

(a)

Resin

(b) Resin

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TiN layer

MAO coating

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Substrate

ACCEPTED MANUSCRIPT Fig. 4 SEM-EDS line scan curves corresponding to MAO/Ti coating shown in Fig. 3(a).

Mass %

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Mg

O

40 60 Point number (length 32 μm)

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0

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Si

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Ti

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N

80

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(c)

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Fig. 6 XRD patterns of coated AZ31B Mg alloy.

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solution.

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Fig. 8 HIT and EIT values of MAO-, MAO/Ti-, and MAO/TiN-coated AZ31B Mg alloy.

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ACCEPTED MANUSCRIPT Table 1 Deposition parameters of the multi-arc ion plating player.

Table 1 Deposition parameters of the multi-arc ion plating player Ti

TiN

Ar flow rate (sccm)

230

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Parameters

0.5 ~ 1

- 80

- 80

49 ~ 52

49 ~ 52

0

10

396

396

1 ~ 1.5

Bias voltage (V)

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Arc current of Ti cathode (A) Magnetic current (A)

130

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Total pressure (Pa)

0

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N2 flow rate (sccm)

Distance between sample and arc source (mm)

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Deposition time (min)

130 ~ 140 180 ~ 190

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ACCEPTED MANUSCRIPT Table 2 Fitting results of the polarization curves from Fig. 7.

Table 2 Fitting results of the polarization curves from Fig. 6 -Ecorr

icorr

-Eb

(mV·dec-1)

(mV vs. SCE)

(μA·cm-2)

(mV vs. SCE)

Uncoated AZ31

111.3

1524

MAO coating

128.3

1476

MAO/Ti coating

117.2

MAO/TiN coating

108.8

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-bc

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3.194

1288

1373

9.441

1289

1280

1.151

1056

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160.6

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Highlights

► A hard coating process on Mg alloys was represented by combining MAO and PVD.

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► Ti and TiN layer was deposited on an MAO coated Mg alloy, respectively.

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► The structure and properties of the composited coating were discussed.

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