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

Guiyin Xu, E-mail: [email protected]. Song Chen, E-mail: [email protected] ... on the 5052 aluminum alloy by the micro-arc oxidation (MAO) process in silica...
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Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 2431−2437

Multiphase Ceramic Coatings with High Hardness and Wear Resistance on 5052 Aluminum Alloy by a Microarc Oxidation Method Decai Qin,†,‡ Guiyin Xu,*,‡ Yang Yang,§ and Song Chen*,† †

School of Chemistry & Chemical Engineering, Yancheng Institute of Technology, Yancheng, Jiangsu 224051, P. R. China College of Materials Science and Technology, Key Laboratory of Materials and Technologies for Energy Conversion, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, P. R. China § 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 ‡

S Supporting Information *

ABSTRACT: High-hardness and wear-resistant ceramic coatings were obtained on 5052 aluminum alloy by the microarc oxidation (MAO) process in silicate electrolytes with different nanoadditives (TiO2, Si3N4), and the effects of different nanoadditives on the microstructural and mechanical properties of the ceramic coatings were systematically studied. The microstructure results revealed that the nanoadditives could improve the thickness and compactness of the ceramic coatings. The X-ray diffraction results demonstrated that the nanoadditives were successfully incorporated into the MAO coatings and that some new phases of Si2N2O and TiN were formed, enhancing the comprehensive performance of the ceramic coatings. Furthermore, the distributions of elements determined from energy-dispersive X-ray (EDX) spectroscopy and cross-sectional images displayed a good homogeneity to support the excellent mechanical properties of the ceramic coatings. Therefore, the average microhardness, the full indentation force−depth curves, the hardness and elastic modulus, and the H/E and H3/E2 ratios of the ceramic coatings with TiO2 and TiO2 + Si3N4 nanoadditives delivered a very high hardness, implying good antifriction properties. Moreover, the friction coefficients of the ceramic coatings also demonstrated their outstanding wear resistance. Finally, the corrosion resistance and electrochemical impedance spectroscopy results further revealed the compactness of the ceramic coatings, indicating a high hardness and abrasion resistance. KEYWORDS: Aluminum alloy, Microarc oxidation, Hardness, Wear resistance, Corrosion resistance

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INTRODUCTION Aluminum (Al) and aluminum alloys (AAs) have been widely used in electronic equipment and in the automotive and aerospace fields because of their low density, high corrosion resistance, high specific strength, and good weldability properties.1 However, the poor mechanical properties, such as low surface hardness, low corrosion resistance, poor wear resistance, and high friction coefficients, 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, microarc oxidation (MAO), as a reliable and environmentally friendly technique, has attracted extensive attention to improve the surface hardness and wear resistance of light metals, such as aluminum (Al), titanium (Ti), magnesium (Mg), and their alloys, through the tailoring of relatively thick, dense, and hard ceramic coatings on them using plasma microdischarges under high voltage.4−6 In general, the MAO ceramic coatings © 2017 American Chemical Society

prepared on AAs without other additives mainly consist of α-Al2O3 and γ-Al2O3 throughout the coatings, and the components can exhibit high hardness and excellent adhesion to the matrix.7,8 However, MAO ceramic coatings are generally composed of a diffusion layer, a relatively dense inner layer, and a porous outer layer, forming a porous and microcracked heterogeneous microstructure.8 This foamlike microstructure with rich porosity leads to relatively poor mechanical properties of the MAO ceramic coatings.9,10 To date, many researchers have studied, in detail, the effects of the MAO parameters, such as current density, pulse frequency, duty ratio, and oxidation time, to optimize the pore microstructure of the ceramic coatings. Liu et al. obtained the best corrosion resistance of the MAO coating at a current density of 7 A· dm−2.11 Wang et al. reported that moderate sandblasting can Received: October 25, 2017 Revised: December 15, 2017 Published: December 19, 2017 2431

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zone, further improving the overall mechanical properties of the coatings.25 To the best of our knowledge, this is the first time that TiO2 + Si3N4 nanocomposite coatings with some new phases of Si2N2O and TiN have been prepared by the MAO process, although many researchers have already synthesized nanocomposite coatings using different technologies. In this work, multiphase ceramic coatings on the surface of AAs were found to significantly improve the microstructure, corrosion, and mechanical properties of the ceramic coatings. Herein, a detailed description of the critical challenges and comparison of the possible effects of the contents of the Si3N4 and TiO2 nanoadditives on the properties of the ceramic coatings is provided, and we demonstrate the relatively excellent results for the Si3N4 and TiO2 contents and a comprehensive analysis of the properties of the ceramic coatings. Furthermore, the mechanism was also analyzed to a certain extent. The main aim of this work was to explore the effects on the synthesis of nanocomposite ceramic coatings of the use of nanoadditives in the electrolyte and the enhancement of the hardness levels, wear resistance, and corrosion behaviors of the ceramic coatings.

micromachine the substrate in a manner that effectively combines roughness and residual stress, which they attributed to an optimized composite technique.12 Recently, various studies have been conducted to fabricate co-deposited composite ceramic coatings on AAs by adding a hard ceramic nanoadditive to the electrolyte, and some good performances were obtained. Therefore, various nanoadditives such as Al2O3, TiO2, and SiC have been studied to improve the properties of the ceramic coatings. Li et al.13 analyzed the effects of the TiO2 nanoadditive concentration on the microstructural and mechanical properties of MAO ceramic coatings on 6063 AAs and obtained a maximum microhardness of ∼1620 HV at 3.2 g·L−1 nanoadditive TiO2 in the electrolyte. Wang et al. introduced different contents of Al2O3 micropowder additives, obtaining a maximum microhardness of ∼500 HV.14 Ao et al. reported that the friction coefficient of a TiO2/hexagonal boron nitride (hBN) composite ceramic coating prepared by MAO under the best conditions was ∼0.4.15 Jin et al. prepared Ti6Al4V alloy by hybrid microarc oxidation/enameling treatments; their results showed that the microhardness of MAO-coated sample was ∼(486 ± 12) HV and the friction coefficient was >0.8, whereas the values for the duplex coating were ∼(650 ± 13) HV and ∼0.4, respectively.16 Wang et al. tested the tribological behavior of a TiO2/Al2O3 composite ceramic coating at 300 °C on Ti6Al4V alloy and measured a friction coefficient of ∼0.25.17 However, the mechanical properties of the multiphase ceramic coatings prepared under similar conditions in this study are superior to those reported previously. Silicon nitride (Si3N4) is one of the most important hard ceramic materials because of 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, no detailed research assessing the effects of nanoadditive Si3N4 in the electrolyte has been reported.18 It has been revealed that nanoadditive Si3N4 as a reinforcement enhances the mechanical properties, such as hardness and tensile strength.19 Moreover, the addition of titanium nitride (TiN) to ceramic matrix composites to both improve the mechanical properties and reduce the electrical resistivity has been widely studied.20 In addition, the standard process for the synthesis of TiN/Si3N4 nanocomposites is usually based on the sintering of mechanically mixed nanoparticles, but this method does not form a good combination in the matrix.21 In contrast, the MAO process can produce electrical sparks inside the discharge channels with superhigh pressure and temperature, where the thermochemical plasma interactions between the matrix and the electrolyte are sufficient. As a result, melt-quenched hightemperature oxides and complex nanocompounds,22−24 composed of both the matrix material and additives in the electrolyte, can be prepared and filled into the pore microstructure to obtain dense ceramic coatings. Based on the high Moh’s hardness of TiN and the superhigh pressure and temperature of the MAO process, the nanoadditives Si3N4 and TiO2 can be introduced with a slightly higher content of Si3N4 than of TiO2. On one hand, this approach can effectively improve the mechanical properties of the coatings, especially hardness and wear resistance, and also guarantee an adequate reaction of Si3N4 and TiO2 to generate a certain amount of TiN. On the other hand, the interaction of the two nanoadditives can be utilized to form a larger co-melting

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EXPERIMENTAL SECTION

Materials Preparation. Samples of 5052 AA with dimensions of 40 mm × 30 mm × 0.5 mm were used as anodes in the MAO process. The procedure for preparing the samples for use in the MAO process were explained in detail in our previous study.26 The parameters of the MAO process were as follows: The electrolyte composition was 8 g L−1 Na2SiO3, 2 g L−1 NaOH, and 2 g L−1 ethylenediaminetetraacetic acid disodium salt (EDTA2Na). The electrolyte without nanoadditives is denoted as Electrolyte, that with 0.5 g L−1 TiO2 nanoadditives is denoted as TiO2, and that with 0.5 g L−1 TiO2 + 1 g L−1 Si3N4 nanoadditives is denoted as TiO2 + Si3N4. The electrical parameters were an anodic current density of 8 A dm−2, a cathodic current density of 5.6 A dm−2, a pulse frequency of 500 Hz, a positive duty ratio of 50%, and a negative duty ratio of 20%. The oxidation time was 30 min. The electrolyte temperature was maintained at about 40 °C by the coolant system. The experimental apparatus is shown schematically in Figure 1.

Figure 1. Schematic illustration of the microarc oxidation apparatus. Electrochemical Characterization. The morphologies and microstructures of the ceramic coatings were recorded by scanning electron microscopy (SEM, LEO 1530 VP). The components and crystallographic phases were analyzed on a Bruker D8 Advance powder X-ray diffractometer (XRD) with Cu Kα radiation (λ = 0.15418 nm, 40 kV). The energy-dispersive X-ray (EDX) spectrum and the composition distribution were analyzed with an energydispersive X-ray spectrometer (JSM-5610LV/Noran-Vantage). Mechanical Measurements. The average thickness of 10 different positions on ceramic coatings was measured with a MiniTest 600 coating thickness gauge. The average microhardness of five different positions was determined with a HXS-1000A microhardness instrument at a load of 1 kg and a retention time of 15 s. Inspection 2432

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ACS Sustainable Chemistry & Engineering with a HT-500 wear test machine was used to evaluate the friction and wear resistance at a load of 330 or 550 g using a Si3N4 ceramic ball of ϕ4 mm for a retention time of 5 or 10 min at a frequency of 15 Hz. (Figure s1 shows a schematic diagram of the 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 and a maximum load of 500 mN. The surface roughness of each sample was tested on the C130 Real-Color Laser Confocal Microscope (Lasertec, Yokohama, Japan). To ensure the accuracy of the results, parallel experiments were performed three times. Electrochemical Measurements. Potentiodynamic polarization curves at a scan rate of 1 mV/s at 25 °C were obtained on a CHI 660B electrical workstation using a three-electrode system to evaluate the corrosion resistances of the matrix and the ceramic coatings. A platinum electrode was used as the counter electrode, and a saturated calomel electrode (SCE) was used as the reference electrode in the corrosive medium of 3.5 wt % NaCl solution. The working electrode with an available area of 1 cm2 contacted the electrolyte. Measurements of the electrochemical impedance spectra were performed on the same workstation with an a.c. amplitude of 5 mV from 1 MHz to 0.01 Hz.

MAO process. During the rapid solidification process, the complex physical and chemical interactions between the melting Al2O3 from the discharge channel and the different nanoadditives lead to the above phenomena.25,27 We also tested the roughnesses of the samples, because the nanoadditives will not only embed in the pores formed from the discharge channels but also form a large co-melting zone during the MAO process. Furthermore, the surface roughness upon the introduction of nanoadditives will certainly decrease. The surface roughness results for Electrolyte (Figure s4), TiO2 (Figure s5), and TiO2 + Si3N4 (Figure s6) are 3.095, 2.576, and 1.919 μm, respectively, which are consistent with the SEM images. Comparison of the SEM images of the three samples shows that we obtained ceramic coatings with different microstructures. To analyze the distributions of nanoadditives on the surfaces of the three samples, we further study the elemental distributions and contents on the sample surfaces by EDX spectroscopy. Figure 3a shows a field-emission scanning

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RESULTS AND DISCUSSION Figure 2 presents the surface morphologies of MAO ceramic coatings prepared under different conditions. (Figure s2 shows

Figure 2. Scanning electron microscopy images of MAO ceramic coatings prepared in (a) Electrolyte, (b) TiO2, and (c) TiO2 + Si3N4.

Figure 3. (a) FESEM image of TiO2 + Si3N4. (b−f) EDX elemental distributions of (b) O, (c) Al, (d) Si, (e) N and (f) Ti. (g) EDX spectrum and (h) surface chemical composition of TiO2 + Si3N4.

the corresponding low-resolution SEM images of the MAO ceramic coatings.) It can be seen that, even though the SEM images exhibit similar porous microstructures, the morphologies of the three samples, especially their pore distributions, are quite different. Figure 2a clearly shows more and larger pores with a distribution of more large dispersed particles, Figure 2b shows a significant reduction in the number of pores and presents a co-melting zone, and Figure 2c shows a greater reduction in the size and number of pores and presents a larger co-melting zone and a smaller particle stacking microstructure. Figure s3 shows the voltage−time responses under different electrolyte conditions of Electrolyte, TiO2, and TiO2 + Si3N4. With the addition of different nanoadditives, the voltage increased slightly at the same time, but the increase was insignificant, indicating that the nanoadditives did not obviously reduce the conductivity of the electrolyte. The slightly rising voltages indicate the improvement of the thickness, the integrity of the ceramic coatings, and the decrease in the number of holes as higher voltages (energies) are required to form new microarcs, which is consistent with Figure 2, Figure s2, and Figure 5a. These phenomena are due to the facts that the different nanoadditives participate in the

electron microscopy (FESEM) image of the TiO2 + Si3N4 sample. Panels b−f of Figure 3 present the EDX surface element distributions of O, Al, Si, N, and Ti, respectively, demonstrating that the distributions of the five elements were uniform, which benefits the stability of the hardness and other mechanical properties of the coatings. In addition, the distribution level is the highest for O, followed by Si, Al, Ti, and N in turn. In addition, the EDX spectrum in Figure 3g allows for a quantitative analysis. According to Figure 3h, the contents of Si, Ti, and N are 30.01, 1.62, and 1.17 wt %, respectively. From these results, it is clear that the nanoadditives have successfully gone into the ceramic coatings with good distributions, which can ensure an excellent mechanical performance. (Figures s7 and s8 show the same information for Electrolyte and TiO2, respectively.) To analyze the phase compositions of the MAO ceramic coatings, we examined their X-ray diffraction (XRD) spectra for Electrolyte, TiO2, and TiO2 + Si3N4, as shown in Figure 4. As can be seen, the coating obtained using Electrolyte was mainly composed of Al, α-Al2O3, and γ-Al2O3, where the phase 2433

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the SEM images in Figure 2 and the EDX elemental distributions in Figure 3, and enhance the hardness and other mechanical properties of the coatings. So far, we have determined the microstructures and phase compositions of surface of the ceramic coatings. To further analyze the elemental composition and microstructure of the deep region of the ceramic coatings, we obtained crosssectional images and element distributions, as shown in Figure 5. Panels a and b of Figure 5 correspond to the cross-sectional image and element distribution of the Electrolyte sample, which shows a coating with a total thickness of ∼10 μm that mainly consists of Al, Si, and O elements with a homogeneous distribution. As shown in Figure 5b, a slope appears between ∼8 and ∼10 μm, which implies that the thickness of the diffusion layer is ∼2 μm (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 with a total thickness of ∼40 μm. The element composition in Figure 5d includes Al, Si, O, and Ti elements, implying the successful introduction of the TiO2 nanoparticles, as also confirmed by Figure 4b. Figure 5e shows a better dense layer than for the other two samples with a thickness of ∼60 μm. As above, Figure 5a clearly shows a less porous outer layer, which is because the matrix material in the initial electrolyte can easily form a relatively dense ceramic coating. Figure 5c presents a more porous outer layer, which is because the addition of TiO2 leads to a ceramic layer mixed with too many TiO 2 nanoparticles, and Figure 5e presents a dense porous outer layer, which is because the addition of TiO2 + Si3N4 leads to a complex reaction and generates Si2N2O nd TiN to fill pores or co-melt.25 The thickness (δ) and average microhardness (AM) values of the ceramic coatings obtained under different conditions of Matrix, Electrolyte, TiO2, and TiO2 + Si3N4 are shown in Figure 6a. The thicknesses of the ceramic coatings were found to be ∼4, ∼14, ∼33, and ∼65, respectively. Under the same conditions, the average microhardnesses were ∼45.3, ∼445.1, ∼1035.3, and ∼1848.5 HV, respectively. It can be clearly observed that the δ and AM values gradually increased upon the addition of the nanoadditives and that they reached the maximum values upon the addition of TiO2 + Si3N4 to the initial electrolyte. This is in addition to the high hardness of TiO2 and Si3N4 themselves and too many nanoparticles

Figure 4. XRD spectra of MAO ceramic coatings fabricated in (a) Electrolyte, (b) TiO2, and (c) TiO2 + Si3N4.

was determined in our previous study.26 However, the intensity of the Al peak was the highest, whereas the α-Al2O3 and γAl2O3 peaks were very weak, implying that the coating was very thin. Upon the addition of TiO2 nanoparticles to the electrolyte, the coating obtained using TiO2 still retained the XRD spectrum of Electrolyte, but the intensity at 2θ ≈ 46° became stronger, which confirms that TiO2 was successfully incorporated into the ceramic coating. Futhermore, when TiO2 + Si3N4 nanoparticles were added to the electrolyte, the XRD pattern changed dramatically. Analysis of the corresponding peak positions in the diffraction spectrum indicates the generation of new multiphase compounds,25,28 apart from the materials present in the TiO2 sample. Upon the addition of Si3N4, peaks due to the Si2N2O phase appear because of the reaction between Si3N4 and SiO2 (Na2SiO3 → SiO2 + NaOH + O2), namely, Si3N4 + SiO2 → Si2N2O.19 In addition, the TiN phase also appears as a result of the reaction Si3N4 + TiO2 → TiN + SiO2 + O2. These reactions occur smoothly, because the MAO process can produce electrical sparks inside the discharge channels with superhigh pressure and temperature, so that the thermochemical plasma interactions between the matrix and the electrolyte are sufficient. Consequently, the new multiphase compounds due to the recombination of the different nanoadditives could help improve the coating density and the uniform distribution of each phase, as demonstrated by

Figure 5. (a,c,e) Cross-sectional images and (b,d,f) elemental distributions of (a,b) Electrolyte, (c,d) TiO2, and (e,f) TiO2 + Si3N4. 2434

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Figure 6. (a) Thickness δ and average microhardness (AM); (b) full indentation force−depth curves; (c) hardness and elastic modulus; and (d) H/E and H3/E2 ratios of the ceramic coatings obtained under different conditions of Matrix, Electrolyte, TiO2, and TiO2 + Si3N4.

and 0.18), which indicates the improvement of the wear resistance. Panels a and b of Figure 7 show the friction coefficient curves at loads of 330 and 530 g, respectively, for the samples

introduced into the ceramic coatings, the new phases of Si2N2O and TiN produced during the reaction process can further enhance the AM. To further check the mechanical properties of the coatings, we performed a series of mechanical tests. Figure 6b presnts typical force−depth curves of the ceramic coatings obtained under the same conditions as above. To reduce the matrix contribution during the measurements, the maximum indentation depth was set sufficiently smaller than the total coating thickness. As can be seen, the ceramic coating containing TiO2 and Si3N4 displayed the smallest depth because of its higher AM. As shown in Figure 6c, the nanohardness (H) and the elastic modulus (Young′s modulus E),29,30 which can indicate the wear resistance and adhesion of the ceramic coatings, were determined using the Oliver−Pharr method from the results of the force−depth curves under different conditions of Matrix, Electrolyte, TiO2, and TiO2 + Si3N4. Compared with that of the matrix, the nanohardnesses of the ceramic coatings improved from 0.62 to 2.99, 9.28, and 30.07 GPa, respectively. The nanohardness of TiO2 + Si3N4 is nearly 50 times greater than that of Matrix and 3 times greater than that of TiO2. Furthermore, compared with the matrix, the average elastic modulus values of the ceramic coatings are improved from 61.88 to 88.27, 172.99, and 381.54 GPa, respectively. The AM, nanoindentation, nanohardness, and elastic modulus results all demonstrated that the ceramic coating of TiO2 + Si3N4 retained the best mechanical properties, which can be attributed to the compact microstructure with the new phases of Si2N2O and TiN and the high hardness of Si3N4. Although hardness has long been regarded as a primary mechanical property for assessing the wear resistance of a 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, rather than just hardness alone. As shown in Figure 6d, the H/E ratios of the prepared ceramic coatings were 0.010, 0.034, 0.053, and 0.079 for Matrix, Electrolyte, TiO2, and TiO2 + Si3N4, respectively, and the H3/E2 ratios also exhibited a similar growth trend (6.22 × 10−5, 0.0034, 0.026,

Figure 7. (a,b) Friction coefficient curves at sample loadings of (a) 330 and (b) 530 g; (c) average friction coefficients; and (d) potentiodynamic polarization curves of the samples fabricated under different conditions of Matrix, Electrolyte, TiO2, and TiO2 + Si3N4.

fabricated under different conditions of conditions of Matrix, Electrolyte, TiO2, and TiO2 + Si3N4. From Figure 7a,b, one can clearly observe that the friction coefficients of the samples gradually decreased with changing conditions, implying that the hardness of the ceramic coatings increased accordingly to provide good antifriction performance. In Figure 7a, the friction coefficient curves of the ceramic coatings at a load of 330 g are relatively smooth (except for that of the matrix), indicating that the ceramic coatings are relatively compact and have high hardness values and mainly point contact between the Si3N4 ball and ceramic coatings. However, when the load 2435

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CONCLUSIONS We have successfully prepared high-hardness and wearresistant ceramic coatings on 5052 aluminum alloy by the MAO process in silicate electrolytes with the nanoadditives TiO2 and Si3N4. At the same time, we explored the effects of different nanoadditives on the microstructural and mechanical properties of the ceramic coatings. The different nanoadditives 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. As a result, the average microhardness, the full indentation force−depth curves, the hardness, the elastic modulus, the H/E and H3/E2 ratios, the friction coefficients, and the high Ecorr and low Icorr values of the ceramic coatings prepared with the TiO2 and TiO2 + Si3N4 nanoadditives provided excellent mechanical properties to support their high hardness and wear resistance.

was increased to 530 g, as shown in Figure 7b, the friction coefficient curves exhibited large fluctuations and a decreasing trend. The large fluctuations might be due to serious plastic deformation occurring in the grinding area and possibly producing some small debris, resulting in larger curve fluctuations. Overall, the friction coefficients of TiO2 + Si3N4 are the smallest at both loads, which indicates that this coating has good hardness and antiwear resistance properties. Figure 7c summarizes the overall friction coefficients and compares the differences. One can clearly see that the friction coefficient of a given coating at the higher load of 550 g is lower (within the appropriate load range). According to the theory of tribological materials and surface engineering,31−33 the friction coefficient involves two main influencing factors: molecular attraction and overcoming mechanical engagement. That is, there is molecular attraction in the actual contact area, which will cause local adhesion. Moreover, the local adhesion is proportional to the actual contact area, and the applied load simultaneously affects the actual contact area to influence the strength of the adhesive force. An equation is available that can qualitatively reflect the connection between these parameters, namely, f = αSΦ/N + β, where f is the friction coefficient; α and β are coefficients that are determined by the physical and mechanical properties, respectively, of the friction surface; SΦ is the actual contact area; and 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 to the power of 2/3, so the friction coefficient decreases accordingly as the load increases. These results agree with those for similar friction systems reported in the literature.34−36 To further verify the formation of complete and compact coatings and the corrosion behaviors, we present the potentiodynamic polarization curves in Figure 7d and the electrochemical impedance spectra of the samples in 3.5 wt % NaCl solution in Figure s9; Table s1 lists the relevant polarization parameters. As shown in Table S1, the value of the corrosion potential Ecorr gradually increases and Icorr decreases with the changing conditions, implying that the corrosion resistance of the ceramic coatings is strengthened accordingly. The main determining factor of coating corrosion resistance 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 electrochemical impedance spectroscopy (EIS) plots show similar corrosion protection properties in the initial corrosion process, consistent with the results of the polarization curves. According to the EIS behavior and the microstructural features of the coatings, Figure s9 shows the equivalent circuits and fitting results in Nyquist plots obtained from Matrix (inset a is the equivalent circuit for Matrix), Electrolyte, TiO2, and TiO2 + Si3N4, and one can clearly see that their corrosion resistance gradually increased. In the equivalent circuits, Rs is the electrolyte resistance, R1 is the resistance of porous outer layer in parallel with constant phase element 1 (CPE1), which corresponds to the MAO coating exposed corrosion electrolyte. R2 is the charge-transfer resistance and parallels with CPE2, and the Warburg impedance W1 at low frequency due to the Cl− ion diffusion in the dense inner layer. R3 is the resistance of corrosion products and parallels with CPE3.37

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b03883.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Decai Qin: 0000-0002-4649-8575 Guiyin Xu: 0000-0002-5959-4814 Song Chen: 0000-0002-5208-2090 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present work was financially supported by the Yancheng City Cooperative Innovation Fund Project (Grant 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); and the Joint Foundation of Science and Technology Department of Guizhou Province (No. [2015]7220). The experiments reported in the present work were performed at the Institute of Materials Science and Technology from Nanjing University of Aeronautics.

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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 on aluminum alloy 2024-T3. Appl. Surf. Sci. 2014, 289 (1), 326−331. (2) Yang, Y.; Zhou, L. Improving corrosion resistance of friction stir welding joint of 7075 aluminum alloy by micro-arc oxidation. J. Mater. Sci. Technol. 2014, 30 (12), 1251.

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ACS Sustainable Chemistry & Engineering

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DOI: 10.1021/acssuschemeng.7b03883 ACS Sustainable Chem. Eng. 2018, 6, 2431−2437