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Multiphase ceramic coatings with high hardness and wear resistance on the 5052 aluminium alloy by a micro-arc oxidation method Decai Qin, Guiyin Xu, Yang Yang, and Song Chen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03883 • Publication Date (Web): 19 Dec 2017 Downloaded from http://pubs.acs.org on December 22, 2017

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ACS Sustainable Chemistry & Engineering

Multiphase ceramic coatings with high hardness and wear resistance on the 5052 aluminium alloy by a micro-arc oxidation method Decai Qin1,2, Guiyin Xu2,*, Yang Yang3, Song Chen1,*

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School of Chemistry & Chemical Engineering, Yancheng Institute of Technology,

Yancheng, Jiangsu, 224051, PR China 2

College of Materials Science and Technology, Key Laboratory of Materials and

Technologies for Energy Conversion, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, PR China 3

Special and Key Laboratory of Guizhou Provincial Higher Education for Green

Energy-Saving Materials, College of information Engineering, Guizhou Minzu University, Guiyang 550025, P.R. China

Corresponding Author: Guiyin Xu, E-mail: [email protected] Song Chen, E-mail: [email protected]

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ABSTRACT: The high hardness and wear resistance ceramic coatings were obtained on the 5052 aluminum alloy by the micro-arc oxidation (MAO) process in silicate electrolytes with different nano-additives (TiO2, Si3N4). And the effects of different nano-additives on the microstructural and mechanical properties of the ceramic coatings were systematically studied. The microstructure results revealed that the nano-additives could improve the thickness and compactness of the ceramic coatings. The X-ray diffraction result demonstrated that the nano-additives were successfully incorporated into the MAO coatings and some new phases of Si2N2O and TiN were formed, enhancing the comprehensive performance of the ceramic coatings. Furthermore, the energy dispersive spectrum (EDS) elements and cross-sectional composition elements results displayed a good homogeneity distribution to support the excellent mechanical properties of the ceramic coatings. Therefore, the average microhardness, the full indentation force-depth curves, the hardness and the elastic modulus, and H/E and H3/E2 ratios of the ceramic coatings with the TiO2 and TiO2+Si3N4 nano-additives delivered a very high hardness, implying good anti-friction properties. Moreover, the friction coefficients of the ceramic coatings also proved their outstanding wear resistance. Finally, the corrosion resistance and electrochemical impedance spectroscopy further revealed the compactness of the ceramic coatings, indicating a high hardness and abrasion resistance.

KEYWORDS: Aluminum alloy, Micro-arc oxidation, Hardness, Wear resistance, Corrosion resistance

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INTRODUCTION Aluminum (Al) and its alloys (AAs) have been widely used in electronic equipment, automotive and aerospace fields due to their low density, high corrosion resistance, high specific strength, and good weld ability properties.1 However, the poor mechanical properties, such as the low surface hardness, low corrosion resistance, poor wear resistance and high friction coefficient, have restricted their design requirements and wide technical applications.2,3 Therefore, the mechanical properties of AAs need to be further improved by the suitable surface treatment technologies. Based on the traditional anodic oxidation technology, micro-arc oxidation (MAO) as reliability and environmental-friendly advantages technique has attracted extensively attention to improve the surface hardness and wear resistance of light metals, such as aluminum (Al), titanium (Ti), magnesium (Mg) and their alloys, via tailoring relatively thick, dense and hard ceramic coatings on them through plasma micro-discharges under high voltage.4-6 In general, the MAO ceramic coatings prepared on AAs without other additives mainly consist of α-Al2O3 and γ-Al2O3 in their whole coatings. And the components can exhibit high hardness and excellent adhesion to the matrix.7,8 However, the MAO ceramic coatings are in general composed of a diffusion layer, a relatively dense inner layer and a porous outer layer, which forms a porous and micro-cracked heterogeneous microstructure8. The foam-like microstructure with rich porosity leads to relatively poor mechanical properties of the MAO ceramic coatings.9,10 Until now, many researchers have in detail studied the effects of the MAO parameters, such as the current density, pulse 3



frequency, duty ratio, oxidation time, etc. to optimize the pore microstructure of ceramic coatings. Liu et al. obtained the best corrosion resistance of the MAO coating at 7 A·dm-2 current density.11 Wang et al. thought that a moderate sandblasting micromachines the substrate can effectively combine roughness and residual stress, which is attributed to an optimized composite technique.12 Recently, various studies had been conducted to fabricate co-deposit composite ceramic coatings on AAs via adding the hard ceramic nano-additive in the electrolyte and obtained some good performances. Therefore, various nano-additives such as Al2O3, TiO2, SiC, etc. have been studied to improve the properties of the ceramic coatings. Li et al.13 analyzed the effects of the TiO2 nano-additive concentration on the microstructural and mechanical properties of the MAO ceramic coatings on 6063 AAs and obtained ~ 1620 HV of the maximum micro-hardness at 3.2 g·L-1 nano-additive TiO2 in the electrolyte. Wang et al. introduced different content of Al2O3 micro-powder additives, which obtained ~ 500 HV of the maximum micro-hardness.14 Ao et al. showed the friction coefficient of TiO2/hBN composite ceramic coating by MAO under the best condition was ~ 0.4.15 Jin et al. prepared Ti6Al4V alloy by the hybrid microarc oxidation/enameling treatments, the results showed the micro-hardness of MAO-coated sample was ~ (486 ± 12) HV and the friction coefficient was >0.8, while the duplex coating was ~ (650 ± 13) HV and ~ 0.4.16 Wang et al. tested TiO2/Al2O3 composite ceramic coating under 300 oC temperature tribological behavior on Ti6Al4V alloy, which delivers ~ 0.25 of the friction coefficient.17 However, the mechanical properties of the multi-phase

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ceramic coatings prepared under similar conditions in this study are superior to those reported above. Herein, Silicon Nitride (Si3N4) is one of the most important hard ceramic materials for its combination of mechanical properties at room and high temperatures, such as high wear resistance, high hardness, low coefficient of thermal expansion, and low density. Nonetheless, there is no detailed research to assess the effect of nano-additive Si3N4 into the electrolyte.18 It had been revealed that the nano-additive Si3N4 as reinforcement enhanced the mechanical properties, such as hardness and tensile strength.19 Besides, the addition of titanium nitride (TiN) to ceramic matrix composites has been widely studied to improve the mechanical properties as well as a reduction of the electrical resistivity.20 Besides, the standard process to synthesis TiN-Si3N4 nanocomposites is usually based on sintering of mechanically mixed nanoparticles, but this method does not form a good combination into the matrix.21 Herein, MAO process can produce electrical sparks inside the discharge channels with the super-high pressure and temperature, where the plasma thermochemical interactions are enough between the matrix and the electrolyte. So the melt-quenched high-temperature oxides and complex nano-compounds,22-24 which are composed from both the matrix material and electrolyte-addition materials, can be prepared and were filled into the pore microstructure to obtain the dense ceramic coatings. Based on the high Moh's hardness of TiN and the super-high pressure and temperature of MAO process, we introduce the nano-additives Si3N4 and TiO2 and the content of Si3N4 is slightly higher than that of TiO2. On the one hand, it can effectively improve 5



the mechanical properties of the coatings, especially hardness and wear resistance, at the same time in order to guarantee adequate reaction of Si3N4 and TiO2 to generate a certain amount of TiN. On the other hand, we utilize the interaction of the two nano-additives to form a larger co-melting zone, further improving the overall mechanical properties of the coatings.25 To the best of our knowledge, this is the first time that the Si3N4+TiO2 nanocomposite coatings with some new phases of Si2N2O and TiN has been prepared by the MAO process, although many researchers has already synthesized the nanocomposite coatings through the different technologies. In this work, the multiphase ceramic coatings on the surface of AAs could significantly improve the microstructure, corrosion and mechanical properties of the ceramic coatings. Herein, a detailed research of this critical challenges and comparing the possible effects of nano-additives Si3N4 and TiO2 content on the properties of the ceramic coatings had been studied. And we displayed the relatively excellent results on the nano-additives Si3N4 and TiO2 content and a comprehensive properties of the ceramic coatings. Furthermore, the mechanism was also analyzed to a certain extent. The main aim of this work is to explore the effect of synthesis nanocomposite ceramic coatings by adding nano-additives in the electrolyte and enhance the hardness levels, wear resistance and corrosion behaviors of the ceramic coatings.

EXPERIMENTAL SECTION Materials Preparation. The 5052 AAs samples with dimensions of 40 mm × 30 mm ×0.5 mm were used as anode in MAO process. Prior to MAO process, the 6


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prepared procedure of the samples were in detail explained in my previous study.26 The parameters of the MAO process are given as below. The electrolyte composition were 8 g L-1 Na2SiO3, 2 g L-1 NaOH, 2 g L-1 EDTA2Na. The electrolyte without nano-additives were named as the Electrolyte, with nano-additives 0.5 g L-1 TiO2 were named as TiO2, with nano-additives 0.5 g L-1 TiO2 + 1 g L-1 Si3N4 were named as TiO2+Si3N4. The electrical parameters were 8 A dm-2 of anodic current density, 5.6 A dm-2 of cathodic current density, 500 Hz of pulse frequency, 50 % of positive duty ratio and 20 % of negative duty ratio. The oxidation time was 30 min. Here the Electrolyte temperature was maintained at about 40 °C by the coolant system. The experimental principle is schematically shown in Figure 1.

Figure 1. Schematic illustration of micro-arc oxidation apparatus.

Electrochemical Characterization. The morphologies and microstructures of the ceramic coatings were taken on a scanning electron microscope (SEM, LEO 1530 VP). The component and crystallographic phase was analyzed on a Bruker D8 Advance power X-ray diffractometer (XRD) with Cu Kα radiation (λ = 0.15418 nm, 40 kV). The energy dispersive spectrum (EDS) and the composition distribution was analyzed by an energy dispersive X-ray spectrometer (EDX, JSM-5610LV/ 7



NORAN-VANTAGE). Mechanical Measurements. The average thickness of ten different positions on ceramic coatings were measured by the Coating Thickness Gauge Mini Test 600. The average micro-hardness of five different positions was carried out on a HXS-1000A micro-hardness instrument at a load of 1 kg with the retention time of 15 s. HT-500 wear test machine inspection was used to evaluate the friction and wear resistance at a load of 330 g or 550 g by using the Si3N4 ceramic ball of ɸ4 mm for 5 min or 10 min retention time at a frequency of 15 Hz (Figure s1 shows schematic diagram of experimental principle of rotary wear). Nanoindentation tests were conducted on a Keysight 9820A Nano Indenter G200 with a Berkovich (trigonal) diamond tip nanoindenter at a constant loading rate of 1.3239 mN s−1 with the maximum load of 500 mN. The surface roughness of the samples were tested on the C130 Real-Color Laser Confocal Microscopy (Japan Lasertec company). In order to ensure the accuracy of the results, three times parallel experiments were performed. Electrochemical Measurements. The potentiodynamic polarization curves and at a scan rate of 1 mV/s at 25 °C were analyzed on a CHI 660B electrical workstation via a three-electrode system to evaluate the corrosion resistances of the matrix and the ceramic coatings, where a platinum electrode as a counter electrode and a saturated calomel electrode (SCE) as reference electrode were conducted in the corrosive medium of 3.5 wt% NaCl solution. The working electrode of an available area of 1 cm2 contacted the electrolyte. The electrochemical impedance spectra measurements

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were performed on same workstation with an AC amplitude of 5 mV from 1 MHz to 0.01 Hz.

RESULTS AND DISCUSSION

Figure 2. Scanning electron microscope images of MAO ceramic coatings prepared at (a) the Electrolyte, (b) the TiO2, (c) the TiO2+Si3N4. Figure 2 illustrates the surface morphology of MAO ceramic coatings prepared at different conditions (Figure s2 shows the corresponding low-resolution SEM images of MAO ceramic coatings). It can be seen that although the SEM images have similar porous microstructure, the morphologies of the three samples, especially their pore distribution, are quite different. Figure 2a clearly shows more and larger pores with the distribution of more large dispersed particles, Figure 2b has a significant reduction and presents a co-melting zone, while Figure 2c has a greater reduction in the size and number of pores and presents a larger co-melting zone and smaller particles stacking 9



microstructure. Figure s3 shows the voltage-time response in different electrolyte conditions of the Electrolyte, the TiO2, the TiO2+Si3N4. With the addition of different nano-additives, the voltage increased slightly at the same time, but the increase was insignificant, indicating that the nano-additives did not obviously reduce the conductivity of the electrolyte. The slight rising voltages reveal the improvement of the thickness, integrity of the ceramic coatings and the decline in the number of holes, for requiring higher voltages (energies) form new micro-arcs, which is consistent with Figure 2, Figure s2, and Figure 5a. These phenomena are due to the facts that the different nano-additives participate in the MAO process. During the rapid solidification process, the complex physical and chemical reactions between the melting of the Al2O3 from the discharge channel and the different nano-additives lead to the above phenomena.25,27 Here we also tested their roughness. Because the introduction of nano-additives will not only embed into the pores formed from the discharge channels and also form the large co-melting zone during the MAO process. Furthermore, the surface roughness with the introduction of nano-additives will be a certain decline. The surface roughness results of the Electrolyte (Figure s4), the TiO2 (Figure s5) and the TiO2+Si3N4 (Figure s6) are 3.095 µm, 2.576 µm, 1.919 µm, respectively, which are consistent with the SEM images.

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Figure 3. (a) FESEM image of the TiO2+Si3N4, the distribution of EDX elements of (b) O; (c) Al; (d) Si; (e) N and (f) Ti; (g) EDS spectrum and (h) surface chemical composition of the TiO2+Si3N4. By contrast the SEM images of the three samples, we obtained the different microstructures of the ceramic coatings. To analyze the distribution of nano-additives on the surface in three samples, we further study the element distribution and content on the surface by the EDX. Figure 3a is the FESEM image of the TiO2+Si3N4. Figure 3b-f are the EDX surface element distribution of O, Al, Si, N and Ti, respectively. They indicate that the distribution of the five elements is uniform, which benefits the stability of the hardness and other mechanical properties of the coatings. And the distribution level of O element is the most, Si, Al, Ti and N descend in turn, respectively. In addition, EDS spectrum in Figure 3g can be quantitative analysis. From the Figure 3h, the contents of Si, Ti and N are 30.01 wt%, 1.62 wt% and 1.17 11



wt%, respectively. So from above them, we can clearly know that the nano-additives have successfully gone into the ceramic coatings with well distribution, which can ensure an excellent mechanical performance (Figure s7 and s8 show the above information of the Electrolyte and the TiO2, respectively).

Figure 4. XRD spectra of MAO ceramic coatings fabricated at (a) the Electrolyte; (b) the TiO2; (c) the TiO2+Si3N4. In order to analyze the phase composition of the MAO ceramic coatings, we examine their XRD spectra fabricated at the Electrolyte, the TiO2 and the TiO2+Si3N4, as shown in Figure 4. As can be seen that the coating of the Electrolyte is mainly composed of Al, α-Al2O3 and γ-Al2O3, where the phase has been learned from our previous study.26 But the intensity of Al peak is the highest and α-Al2O3 and γ-Al2O3 are very weak, implying that the thickness is very thin. When the Electrolyte adds the TiO2 nanoparticles, we could see that the coating of the TiO2 still retains the XRD spectrum of the Electrolyte, but the intensity of 2θ ≈ 46º becomes stronger, which proves the TiO2 successfully compounded into the ceramic coating. Futhermore, when 12


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the TiO2+Si3N4 nanoparticles are added into the Electrolyte, its XRD pattern has changed dramatically. By analyzing the diffraction spectrum of the corresponding peak positions, we find that there are new multi-phase compounds generation,25,28 apart from the existence materials in the TiO2. With the addition of Si3N4, Si2N2O phase peaks appear due to the reaction between Si3N4 and SiO2 (Na2SiO3 → SiO2 + NaOH + O2), namely Si3N4 + SiO2 → Si2N2O.19 In addition, the TiN phase also comes from the reaction: Si3N4 + TiO2 → TiN + SiO2 + O2. The above procedures take place smoothly, as a result that MAO process can produce electrical sparks inside the discharge channels with the super-high pressure and temperature, where the plasma thermochemical interactions are enough between the matrix and the electrolyte. So the new multi-phase compounds due to the different nano-additives recombination could benefit to improve the coatings density and uniform distribution of each phase, which can be demonstrated from the SEM of Figure 2 and the distribution of EDX elements in Figure 3, and enhance the hardness and other mechanical properties of the coatings.

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Figure 5. (a) and (b); (c) and (d); (e) and (f) are the Electrolyte, the TiO2 and the TiO2+Si3N4 of cross-sectional images and the elements distribution, respectively. We have learned the surface of the ceramic coatings the microstructure and phase composition, in order to further analyze the elemental composition and microstructure of the deep region of the ceramic coatings, we added cross-sectional images and the elements distribution, as shown in Figure 5. Figure 5a and 5b correspond to the cross-sectional images and the elements distribution of the Electrolyte, which show the total thickness of ~ 10 µm, mainly consisting of Al, Si, and O elements with homogeneous distribution. As shown in Figure 5b, there appears a slope between ~ 8 µm and ~ 10 µm, which implies ~ 2 µm the thickness of the diffusion layer (the cross-sectional image is not very clear). Figure 5c shows the three layers of the diffusion layer, the dense inner layer and the porous outer layer and the thickness is ~ 40 µm. The elements composition of Figure 5d are Al, Si, O and Ti elements, implying the successful introduction of the TiO2 nanoparticles and Figure 4b also proves it. Figure 5e shows the better dense layer than them and the thickness is ~ 60 µm. As above, Figure 5a clearly shows a less porous outer layer, which is because the matrix material in the initial electrolyte is easy to form a relatively dense ceramic coating. Figure 5c is a more porous outer layer, which are because the addition of TiO2 lead to the ceramic layer mixed with too much TiO2 nanoparticles, and Figure 5e is a dense porous outer layer, which is because the addition of TiO2+Si3N4 lead to a complex reaction and generate Si2N2O, TiN to fill porous or co-melt.25

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Figure 6. (a) the thickness δ and average Microhardness (AM); (b) the full indentation force-depth curves; (c) the hardness and the elastic modulus; (d) H/E and H3/E2 ratios of the ceramic coatings obtained at different conditions of the Matrix, the Electrolyte, the TiO2 and the TiO2+Si3N4. The thicknesses δ and average microhardness (AM) of ceramic coatings obtained at different conditions of the Matrix, the Electrolyte, the TiO2 and the TiO2+Si3N4 are shown in Figure 6a. The thickness of the ceramic coatings are ~ 4 µm, ~ 14 µm, ~ 33 µm, ~ 65 µm, respectively. At the same condition, the average microhardness are ~ 45.3 HV, ~ 445.1 HV, ~ 1035.3 HV, ~ 1848.5 HV, respectively. It can be clearly observed that the thickness δ and AM gradually increase with the addition of nano-additives, and they reach to the maximum value at the addition of TiO2+Si3N4 in the initial electrolyte. This is in addition to the high hardness of TiO2 and Si3N4 themselves and too much nanoparticles introduced into ceramic coatings, the new results of Si2N2O, TiN produced during the reaction process can further enhance the AM. To further check the mechanical properties of coatings, we made a series of 15



mechanical tests. Figure 6b illustrates typical force-depth curves of ceramic coatings obtained at above same conditions. To reduce matrix contribution during the measurement, the maximum indentation depth was set sufficiently smaller than the total coating thickness. We can see that the ceramic coating of TiO2 and Si3N4 displays the smallest depth than them for its higher AM. As shown in Figure 6c, the nanohardness (H) and the elastic modulus (Young′s modulus E),29,30 which can evaluate wear resistance and adhesion of the ceramic coatings, were determined by using the Oliver-Pharr method from the results of force-depth curves at different conditions of the Matrix, the Electrolyte, the TiO2 and the TiO2+Si3N4. Compared with the matrix, the nanohardness of the ceramic coatings are improved from 0.62 GPa to 2.99 GPa, 9.28 GPa, 30.07 GPa, respectively. The nanohardness of the TiO2+Si3N4 is nearly 50 times greater than that of the Matrix, and 3 times greater than that of the TiO2. Furthermore, compared with the matrix, the average elastic modulus of the ceramic coatings are improved from 61.88 GPa to 88.27 GPa, 172.99 GPa, 381.54 GPa, respectively. The AM, the nanoindentation, the nanohardness and the elastic modulus all proved that the ceramic coating of the TiO2+Si3N4 retains the best mechanical properties, which may be attributed to the compact microstructure with the new results of Si2N2O, TiN and high hardness of Si3N4. Although hardness has long been regarded as a primary mechanical property to assess the wear resistance of the material based on classical theories of wear, the more suitable parameters of the elastic strain to failure (H/E) and the plastic deformation resistance factor (H3/E2) are used to evaluate wear resistance than hardness alone. As 16


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shown in Figure 6d, the H/E ratios of the above ceramic coatings are 0.010, 0.034, 0.053, 0.079, respectively, and the H3/E2 ratios also have similar growth trend (6.22 × 10-5, 0.0034, 0.026, 0.18), which indicates the improvement of the wear resistance.

Figure 7. The friction coefficient curves at a load of (a) 330 g and (b) 530 g of the samples; (c) the average friction coefficient; (d) Potentiodynamic polarization curves of the samples fabricated at different conditions of the Matrix, Electrolyte, the TiO2 and the TiO2+Si3N4 Figure 7a-b show the friction coefficient curves at a load of 330 g and 530 g of the samples fabricated at different conditions of the Matrix, Electrolyte, the TiO2 and the TiO2+Si3N4, respectively. As shown in Figure 7a-b, we can clearly observe that the friction coefficients of them gradually reduce accompanied by the changing conditions, implying that the hardness of ceramic coatings increases accordingly to play a good anti-friction performance. The friction coefficient curves of ceramic coatings at a load of 330 g are relatively smooth (except for the matrix) in Figure 7a, 17



indicating the relatively compact and high hardness of ceramic coatings and mainly point contact between Si3N4 ball and ceramic coatings. However, when the load increases to 530 g in Figure 7b, the friction coefficient curves occur a big fluctuation and a decrease trend. The big fluctuation may be due to that grinding area occurred serious plastic deformation and the grinding area may produce some small debris, which result in a larger curve fluctuations. Above all, the friction coefficients of the TiO2+Si3N4 are the smallest than them at both load, which indicates it the good hardness and anti-wear resistance properties. Figure 7c categorizes the whole friction coefficient and compare the difference. We can clearly see that the friction coefficient of the same coating at higher load of 550 g is lower (within the appropriate load range). From the theory of tribological materials and surface engineering,31-33 the friction coefficient involves two main influencing factors: molecular attraction and overcome the mechanical engagement. That is, there is molecular attraction in the actual contact area, which will cause the local adhesion. And the local adhesion is proportional to the actual contact area, the applied load simultaneously affects the actual contact area to influence the strength of the adhesive force. Herein, the formula can qualitatively reflect the connection between them. As follows, f = αSΦ/N + β. Among them, f is the friction coefficient; α and β of the coefficients are determined by the physical and mechanical properties of the friction surface, respectively; SΦ is the actual contact area; N is the normal positive load. It is obvious that the friction coefficient changes with the SΦ/N ratio. For this friction system in general, the actual contact area is proportional to about the normal load 2/3 square, so the friction 18


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coefficient decreases according to the formula as the load increases. And the results agree with the similar friction system in other literatures.34-36

To further verify the complete and compact coating and the corrosion behaviors, we analyses the potentiodynamic polarization curves in Figure 7d and electrochemical impedance spectroscopy (EIS) of the samples in 3.5 wt% NaCl solutions in Figure s9, and table s1 lists the relevant polarization parameters. As shown in table S1, the value of corrosion potential Ecorr gradually increases and Icorr decreases with the changing conditions, implying that the corrosion resistance of ceramic coatings is strengthened accordingly. Since the main decisive factor of coating corrosion resistant is dynamical corrosion, which focuses on the corrosion current Icorr. The ceramic coating of the TiO2+Si3N4 samples reaches the lowest Icorr density of 1.180×10-9 A·cm-2, and the corresponding corrosion potential is -0.005 V. The EIS plots show similar corrosion protection properties in the initial corrosion process, which are consistent with the results of polarization curves. According to the EIS behavior and the microstructural features of the coatings, the equivalent circuits and fitting results in Nyquist plots obtained from the Matrix (the inset a in particular is the Matrix's equivalent circuit), Electrolyte, the TiO2 and the TiO2+Si3N4 are displayed in Figure s9, and we can clearly see that their corrosion resistance gradually increased. In the equivalent circuits, Rs is the electrolyte resistance, R1 delivers the resistance of porous outer layer in parallel with constant phase element 1 (CPE1), which corresponds to the MAO coating exposed corrosion electrolyte. R2 presents the charge transfer resistance and parallels with CPE2, and the Warburg impedance W1 in the low 19



frequency due to the Cl- ion diffusion in the dense inner layer. R3 signs the resistance of corrosion products and parallels with CPE3.37

CONCLUSION We have successfully prepared the high hardness and wear resistance ceramic coatings on the 5052 Aluminum Alloy by the MAO process in silicate electrolytes with the nano-additives of TiO2 and the Si3N4. Meanwhile, we explored the effects of different nano-additives on the microstructural and mechanical properties of the ceramic coatings. The different nano-additives could improve the thickness and compactness of the ceramic coatings. And some new phases of Si2N2O and TiN were formed in the MAO coatings to enhance the mechanical properties. Thereby, the average microhardness, the full indentation force-depth curves, the hardness and the elastic modulus, H/E and H3/E2 ratios, the friction coefficients, the high Ecorr and low Icorr of the ceramic coatings prepared with the TiO2 and TiO2+Si3N4 nano-additives delivered the excellent mechanical properties to support their high hardness and wear resistance.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xx.xxxx/xxxxxxxxx-xxx.xxxxxxx. Schematic diagram of experimental principle of rotary wear; SEM images of MAO ceramic coatings; the voltage-time response in different electrolyte conditions; FESEM image of the Electrolyte and the TiO2 with 20


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the distribution of EDX elements; potentiodynamic polarization curves parameters.

AUTHOR INFORMATION Corresponding Author Song Chen, E-mail: [email protected] Guiyin Xu, E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The present work was financially supported by the Yancheng City Cooperative Innovation Fund Project (Grant No. YKA201219), the Natural Science Foundation of Jiangsu Province, China (No. BK20141261) and the Joint research project among industry and university and institute of Jiangsu Province, China (No. BY2015057-35), the Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions, the Natural Science Project of Education Department of Guizhou Province (No. [2015]424), the Joint Foundation of Science and Technology Department of Guizhou Province (No. [2015]7220), and the present work was experimented in the Institute of materials science and technology from Nanjing University of Aeronautics.

REFERENCES (1) Zhang, R. F.; Shi, H. W.; Liu, Z. L.; Zhang, S. F.; Zhang, Y. Q.; Guo, S. B. Property of anodic coatings obtained in an organic, environmental friendly electrolyte

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Multiphase ceramic coatings with high hardness and wear resistance can effectively improve the mechanical properties of 5052 aluminum alloy to support its sustainability.

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Multiphase ceramic coatings with high hardness ... - ACS Publications

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