简介概要

Microstructure,wear and corrosion performance of plasma electrolytic oxidation coatings formed on D16T Al alloy

来源期刊：Rare Metals2020年第12期

论文作者：Wan-Ying Liu Ying Liu Chun-Ling Fan Mohd Talha Yuan-Hua Lin

摘要：The plasma electrolytic oxidation（PEO） coatings were produced on D16 T Al alloy in the aluminate and silicate electrolyte with and without graphene.The phase composition,microstructure and elemental distribution of the coatings were tested by X-ray diffraction（XRD）,scanning electron microscope（SEM） and energy dispersive X-ray spectroscopy（EDX）.The wear and corrosion resistance of PEO coatings were evaluated by dry sliding wear tests and electrochemical impedance spectroscopy（EIS）.The morphology feature of the wear tracks was compared and analyzed by SEM and three-dimensional microscope.The results demonstrate that the structure,wear and corrosion resistance of PEO coatings with graphene are better than that of PEO coatings without graphene.The coating fabricated in the aluminate electrolyte with graphene exhibited the lowest roughness.The coated samples formed in silicate electrolyte with graphene displayed the thickest,densest and the most compact coating.It exhibited the best wear and corrosion resistance due to the incorporation mode of graphene in the coatings.The mechanism of graphene improving the wear and corrosion resistance of PEO coating was further discussed.In summary,the comprehensive performances of PEO coatings formed in silicate electrolyte on D16 T Al alloy are superior to that produced in aluminate electrolyte.

稀有金属(英文版) 2020,39(12),1425-1439

Microstructure,wear and corrosion performance of plasma electrolytic oxidation coatings formed on D16T Al alloy

Wan-Ying Liu Ying Liu Carsten Blawert Mikhail-L.Zheludkevich Chun-Ling Fan Mohd Talha Yuan-Hua Lin

College of Materials Science and Engineering,Sichuan University

School of New Energy and Materials,Southwest Petroleum University

Helmholtz-Zentrum Geesthacht Zentrum für Material-und Küstenforschung GmbH,Institute of Materials Research

Chongqing Xinyu Pressure Vessel Manufacturing Co.,Ltd.

作者简介：*Ying Liu,e-mail:liuying5536@163.com;

收稿日期：11 August 2019

基金：financially supported by the Award of Fellowship from China Scholarship Council (No. 201608515038);the National Natural Science Foundation of China (No.51274170);the 18th College Students’ Key Open Experimental Subjects of Southwest Petroleum University (No.KSZ18503);the Plan Program about Passing a Test for the Youth Technicist worked in the Laboratory of Southwest Petroleum University (No. 201131010056);

Microstructure,wear and corrosion performance of plasma electrolytic oxidation coatings formed on D16T Al alloy

Wan-Ying Liu Ying Liu Carsten Blawert Mikhail-L.Zheludkevich Chun-Ling Fan Mohd Talha Yuan-Hua Lin

College of Materials Science and Engineering,Sichuan University

School of New Energy and Materials,Southwest Petroleum University

Helmholtz-Zentrum Geesthacht Zentrum für Material-und Küstenforschung GmbH,Institute of Materials Research

Chongqing Xinyu Pressure Vessel Manufacturing Co.,Ltd.

Abstract：

The plasma electrolytic oxidation(PEO) coatings were produced on D16 T Al alloy in the aluminate and silicate electrolyte with and without graphene.The phase composition,microstructure and elemental distribution of the coatings were tested by X-ray diffraction(XRD),scanning electron microscope(SEM) and energy dispersive X-ray spectroscopy(EDX).The wear and corrosion resistance of PEO coatings were evaluated by dry sliding wear tests and electrochemical impedance spectroscopy(EIS).The morphology feature of the wear tracks was compared and analyzed by SEM and three-dimensional microscope.The results demonstrate that the structure,wear and corrosion resistance of PEO coatings with graphene are better than that of PEO coatings without graphene.The coating fabricated in the aluminate electrolyte with graphene exhibited the lowest roughness.The coated samples formed in silicate electrolyte with graphene displayed the thickest,densest and the most compact coating.It exhibited the best wear and corrosion resistance due to the incorporation mode of graphene in the coatings.The mechanism of graphene improving the wear and corrosion resistance of PEO coating was further discussed.In summary,the comprehensive performances of PEO coatings formed in silicate electrolyte on D16 T Al alloy are superior to that produced in aluminate electrolyte.

Keyword：

PEO coatings; D16T Al alloy; Graphene; Wear; Corrosion;

Received： 11 August 2019

1 Introduction

D16TA1 alloy is an Al-Cu-Mg age hardening aluminum alloy with the characteristic of high strength and low density [ 1] .It is widely used in aerospace and other industrial applications.However,the shortcomings of low hardness,low wear resistance,high friction coefficient and low corrosion resistance in Clsolution largely hinder the wide applications [ 2, 3] .Various surface treatments are used to widen the application of Al alloys.Hard anodizing is a conventional method.It can increase the load-bearing performance and improve the corrosion resistance of Al alloys.But,the coatings with low hardness and thickness cannot endure the rigid working condition [ 4, 5] .So,it is necessary to take other surface modification technologies to improve the comprehensive properties of aluminum alloys.Plasma electrolytic oxidation is also called microarc oxidation.It is an environmentally friendly surface treatment method and widely applied to the light metals such as Al,Mg,Ti and their alloys [ 6, 7, 8, 9] .Al₂O₃ ceramic coating has a great potential effect in improving the hardness and wear resistance of the material surface.The conventional processes like the hard anodizing and the thermal spraying suffer from fewer loads bearing support of the underlying the material itself and insufficient adhesion for reducing the durability.Various technology methods have been used to promote the industrial applications of aluminum alloy,such as automotive,aerospace,vessels,medicine,petroleum and chemical industry.At present,plasma electrolytic oxidation (PEO) technique has been widely applied to produce ceramic coatings on aluminum alloys to improve the surface properties.Many researchers studied the effects of PEO ceramic coatings produced under different parameters of the current density and oxidation time and pretreatment of samples.Xiang et al. [ 2] researched the effects on microstructure and performances of the coatings prepared on 6063 aluminum alloy under different current densities.The results demonstrated that the pore density of the coatings decreased with the increase in the current density,and the coating formed under a current density of 15 A·dm^-2displayed the best corrosion resistant property.Wu et al. [ 10] studied the effects of different oxidation time on the structure and mechanical properties of the coatings fabricated by PEO technology on aluminized steel.The results showed that the thickness,porosity and cracks of the coating increased with longer treatment time.Wen et al. [ 11] reported that the coating with the thickness of 10μm obtained by the surface mechanical attrition treatment micro-arc oxidation exhibited prominent microstructure and corrosion resistance.

The structure and properties of PEO ceramic coatings of aluminum and its alloys are extremely affected by the compositions of electrolyte.They played an important role in the formation and growth process of PEO coatings [ 12, 13, 14] .The coatings produced in the alkaline silicate electrolyte show the crater-like morphology with a central hole and nodular structure [ 15, 16] .They owned the high microhardness [ 17] ,high wear resistance [ 18] and remarkable corrosion resistance [ 19] .Despite significant development in PEO coating technology for aluminum and its alloys in recent years,especially the PEO coating formed in silicate electrolyte,very few studies for the structure and performances of the coatings produced in aluminate electrolyte on aluminum alloys were performed [ 20, 21] .Ovundur et al. [ 22] characterized the tribological properties of PEO-coated and hard-anodized aluminum alloy.Results indicated that the ceramic coatings produced by plasma electrolytic oxidation exhibited superior tribological performance compared to the hard-anodized coating.As reported in Ref. [ 23] ,PEO coatings with the compact layer structure formed in 24 g·L^-1 NaAlO₂solution exhibited excellent wear and corrosion resistance.However,only a few studies about wear and corrosion resistance of the coatings fabricated in the alkaline silicate and aluminate electrolytes,respectively,were compared together.Furthermore,few studies about PEO coating formed in the aluminate electrolyte with graphene were reported.

Therefore,in the present work,the ceramic coatings produced on the surface of D16T aluminum alloy in four different electrolytes,i.e.,aluminate and silicate electrolytes with and without graphene addition,were systemically characterized.Their tribological behaviors under dry sliding condition and their corrosion resistance in NaCl solution with the concentration of 3.5 wt%were evaluated.This work aims to optimize the processes so as to improve the comprehensive performances of D16TAl alloy by a comparative study of microstructure,morphology,wear and corrosion resistance of PEO-coated specimens.The effect of the incorporation mode of graphene on the performances of the coated specimens was investigated,and the mechanism that graphene greatly improved the wear and corrosion resistance of PEO coatings was also discussed in detail.

2 Experimental

The substrate material used in this work was D16T Al alloy(3.85%Cu,1.29%Mg,0.65%Mn,0.068%Si,0.11%Fe,0.049%Ti,0.054%Zn and balance Al).Rectangular samples were cut from Al alloy pipe for the alumina coating deposition.The dimension of the samples was15 mm×15 mm×5 mm.Firstly,the samples were abraded with 1200 grit SiC abrasive papers,and then,they were cleaned with distilled water.During PEO process,the samples were used as the anode,and the stainless steel was used as cathode.The electrolytes for PEO experiment consisted of aluminate and silicate solutions with and without graphene addition (Table 1).Four kinds of electrolytes were noted as A1,A2,S1 and S2,respectively.Graphene with nanosheets structure used in this PEO experiment was gotten from Deyang Carbon Technology Co.,Ltd.(China).The thickness was～35 nm.Its morphology is shown in Fig.1.PEO coatings were produced on the rectangular samples by a pulsed direct current (DC)power source with a pulse ratio of forward time (t_on) to reverse time (t_off) of 1 ms:9 ms.The PEO treatment was performed under a constant current density of 5 A·dm^-2 for the treatment time of 10 min,and the electrolytes'temperature was kept below 30℃by a water-cooling system.The coated specimens were taken out from the electrolyte,and then thoroughly washed in the cold running water.Finally,they were ultrasonic ally cleaned in ethanol and dried in the warm air.A Mettler Toledo Inlab 730 probe was applied to test the conductivity of electrolytes,and a Metrohm 691 pH meter was used to measure pH value.

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Table 1 Composition,conductivity and pH values of four electrolytes for PEO treatment

Fig.1 SEM images of graphene used in PEO process with different magnifications

Based on eddy current technique,the thickness of PEO coatings formed in four electrolytes was measured using a thickness gauge (Mini Test 2100).Twelve random points were measured on the two main surfaces of each sample.The average value was calculated to represent the thickness of PEO-coated specimen.The morphologies of the surface and the cross section of the coatings were observed by a scanning electron microscope (SEM,TESCAN Vega 3SB).The compositional distributions of the coatings were investigated using an energy dispersive X-ray system(EDX) attached to SEM by EDX mapping.The acceleration voltage was 20 kV.Gold was sputtered on the surface and cross section of PEO-coated specimens for preventing charging of the specimen before the characterization of morphology.The surface roughness Ra was tested by a profilometer (model Mitotoyo Surf Test stylus working on the standard of ISO 1997).The analysis of the crystal structure was carried out using X-ray diffractometer (XRD,X'Pert Pro) with Cu Kαradiation at 2θranging from 10°to80°with a scan rate of 0.02 (°)·min^-1 for identifying the phase composition of the coatings.

For investigating the influence of two different base electrolytes with and without graphene nanosheets on the mechanical property of PEO coatings,the dry sliding wear tests for the uncoated and PEO-treated s amples were done by Oscillating TRIBO tester.An AISI 52100 steel ball with a diameter of 6 mm was used as the friction counter material.The dry sliding wear test was performed at a load of 5 N with an oscillating amplitude of 10 mm,a sliding speed of5 mm·s^-1,and a total sliding distance of 12 m.For identifying the dominant wear mechanism,the worn surfaces of the wear tracks and the wear debris were observed by SEM and EDX test.The analysis of the wear tracks was performed using Bruker ContourGT-InMotion GTK-16-0314.

The electrochemical corrosion behaviors of the uncoated and coated aluminum alloys were investigated by a Gamry Instrument Interface 1000 potentiostat.The corrosive media was an aqueous solution of 3.5 wt%NaCl and a conventional three-electrode corrosion cell comprised out of an Ag/AgCl (Sat.KCl) was used as the reference electrode and a platinum grid was used as the counter electrode.The uncoated and coated aluminum alloy specimens were used as the working electrode.The electrochemical impedance spectroscopy (EIS) measurements were conducted with a frequency range from 100 kHz to 0.01 Hz after the samples were immersed in NaCl solution with concentration of 3.5 wt%for 2 h.Zsimpwin software was used to fill and analyze the EIS data.All of the electrochemical corrosion tests were performed at room temperature.

3 Results and discussion

3.1 Voltage-time curves of PEO treatment

Figure 2 presents voltage transient curves of different specimens during PEO process.It can be seen that two consecutive stages are visible in terms of the ramp rate of potential.The coated samples display the sameness that the voltage increases rapidly in a short time,and the increase rate of the voltage keeps slowly increasing in a long time.The results show that the samples produced in the electrolyte with graphene nanosheets have lower sparking voltage of 5-10 V throughout the whole process in comparison with that formed in the electrolyte without graphene,which are consistent with previous studies [ 24, 25] .In the first period,the voltages of A1 and A2 quickly rise up to 420 V around 80 s,and the voltages of S1 and S2rapidly increase to 375 V around 30 s.The size of sparks of A1 and A2 is smaller,which results in producing less porous coating with higher electrical resistance in the initial stage of the sparking.So,the rapid increasing voltages need to guarantee the constant current density.However,in the second stage,the voltages of all specimens keep increasing.The voltages of A1 and A2 reach 430 V,and those of S1 and S2 increase to 380 V.But they increase at a much lower rate than that in the former stage.This is attributed to more porous coating layers with less electrical resistance,so a voltage with lower increase rate is required.Meanwhile,intensive gas liberation and occasional bursting occur during this period,indicating that the oxidation behavior was dominated by spark discharge representing the characteristic of the PEO process [ 26] .Additionally,the voltage of specimens with graphene addition increased slower than that of specimens without graphene addition,hence,it has higher resistance,which is attributed to less porosity of the coating with graphene nanosheets,the lower energy required for the formation of the coating,etc.Thus,the final voltage of the coated samples without graphene is slightly higher compared to that with graphene.On the other hand,the specimens obtained from alumina electrolytes have a higher voltage response in comparison with the specimens produced in the silicate electrolyte.This phenomenon illustrates that the inert of coatings formed in alumina electrolyte is weaker than that in silicate electrolyte.Moreover,the coating with graphene additive presents worse positivity because the discharge channels are blocked by incorporated graphene.For example,the pores formed through bubbles fleeing away are embedded by graphene.Therefore,that can illustrate that the dispersing graphene involves into PEO process and has a significant influence on the voltage response and the coating formation,thus,the voltage is lower.

Fig.2 Variation of voltage with PEO processing time

3.2 Microstructure and phase analysis

3.2.1 Morphology and composition of coatings

The surface morphologies of four PEO coatings show significant difference due to the different electrolyte systems (Fig.3).The coated S1 and S2 samples show the typical porous structure with some micropores with the roughness of 0.88 and 0.69μm,respectively (Fig.3a,b).However,the number of pores in the coating S2 obviously decreases,attributing to the fill of the incorporated graphene to the pores (Fig.4a).The red arrows demonstrate the presence of graphene which is further confirmed by EDX mapping shown in Fig.4a.In addition,some microcracks are easily visible in S1 from the magnification image (Fig.3a).It is speculated that micropores produced from the molten oxide and gas bubbles which are thrown out of discharge channels.Additionally,the microcracks result from the thermal stress,ascribed to the rapid solidification of the molten oxide in the relatively cold electrolyte [ 27, 28] .Two different kinds of regions are visible in the coatings S1 and S2.They are craters with a central hole and some areas with the nodular structure.Craters can be formed when the molten material is ejected from the interface between the coating and the substrate through the central holes because of the high temperature and the strong electric field occurrence during PEO treatment process.In contrast,the coatings produced in the aluminate electrolyte exhibit fewer pores and flatter surface morphology,and no nodular structure,which is consistent with previous study [ 29] .The flat areas were tested to be rich in aluminum,but the nodular structures presented higher concentration of Si.Although,the coatings with the roughness of 0.49 and 0.32μm produced in aluminate electrolyte were flatter and less porous than the coatings S1and S2 fabricated in the silicate electrolyte with the roughness of 0.88 and 0.69μm,respectively,many microcracks appear on the surface of the coating fabricated in the aluminate electrolyte,indicated by red arrows in the magnified images (Fig.3c,d).Figure 4 reveals the incorporation mode of graphene additive.It is clear that most of the graphene embeds in the coating S2 and filled the pores,and some graphene covered on the surface of the coating.However,the formed reactants from graphene reacting with substance in the electrolyte spread on the surface of the coating A2,which look like a pancake.Therefore,fewer graphene nanosheets with small size can enter into the pores and cracks due to the block from the pancakeshaped graphene reactants covered on the surface.That can be clearly demonstrated from EDX mapping of C in PEO coatings (Fig.4).Meanwhile,the test result of the four coated samples by EDX for the surface of the coating indicates that a certain amount of C present in A1 and S1.That is mainly ascribed to the contamination of the electron microscopy chamber.However,the C content in the coated A2 and S2 samples sharply increases,which is attributed to the addition of graphene.Figure 4 indicates that extra C generates from graphene additive and C content of A2 is less than that of S2,attributing to different incorporation modes of graphene.The roughness of S2 and A2 is smaller than that of S1 and A1,due to extensive distribution of graphene and disappearance of pores and cracks,as illustrated in Figs.3,4.

Fig.3 Surface SEM images and corresponding local enlarged images of four coated samples:a S1,b S2,c A1,and d A2

Fig.4 SEM images and EDX mappings of C in PEO coatings with addition of graphene:a S2 and b A2

3.2.2 Phase analysis of PEO coatings

The PEO coating produced in four electrolytes is crystalline,as obviously seen in XRD patterns (Fig.5).It is easily seen that the coatings are mainly composed ofα-Al₂O₃,γ-Al₂O₃and Al substrate.The appearance of the diffraction peaks of Al is because that X-ray is easy to penetrate the thin coating and reach the substrate.The emergence ofα-Al₂O₃ andγ-Al₂O₃ is the result of the reaction between the A1 substrate and electrolytes.It is well known thatα-Al₂O₃ is a stable alumina phase ascribed to the high melting point(2050℃).Furthermore,it is the corundum structure which consists of oxygen anions in the hexagonal close-packed layers.Here,the cations occupy octahedral sites.However,γ-Al₂O₃ is a metastable phase consisting of layers of cubic close-packed oxygen anions with cations in the octahedral and tetrahedral sites [ 30, 31] .Moreover,the hardness ofα-Al₂O₃ is higher than that ofγ-Al₂O₃ [ 32] .

3.2.3 Thickness and cross section structure of coatings

The thickness test result of the coatings produced in four different electrolytes shows that the thickness of PEO coatings fabricated in the silicate electrolyte is larger than that formed in the aluminate electrolyte.Meanwhile,the thickness of PEO coatings with graphene addition is also larger than that of PEO coatings without graphene,which is consistent with the previous investigations [ 33, 34] .A comparison between the coated samples formed in the electrolytes with and without graphene shows that the presence of graphene increases the coating thickness.This phenomenon can be linked with the electrical conductivity of graphene nanosheets,influencing the discharge processes during PEO treatment [ 35] .In order to further verify the thickness and the micros true ture of the coatings,backscatter diffraction (BSD) graphs of the cross sections of PEO coatings prepared in the different electrolytes are performed (Fig.6).The thickness of the coatings varies due to different electrolytes.It is evident that the observation results of BSD micrographs of the cross section are consistent to tested result by thickness gauge.In addition,the microstructure of the coatings'cross section is in agreement with the surface morphology feature of the coatings (Fig.3).It is clearly seen that some pores and cracks are distributed in the coatings.S1 shows the highest amount of pores compared to other three coated specimens.A1 and A2 have relatively few pores and cracks.S2 shows the best microstructure,almost no cracks,and the coating is extremely compact.In fact,the presence of graphene causes the microstructure change of the coatings.Fewer pores and microcracks and a densification in the coating are observed,which are in accordance with the results in Ref. [ 36] .Cross-sectional analysis shows that the coatings have the typical microstructure,which is composed of an inner barrier layer and an outer porous layer.The inner layer is so thin that the interface between the outer layer and the inner layer is not obvious,especially for S1.However,the interface between the outer and inner layer of S2 and A2can be clearly distinguished (Fig.6a,b).

Fig.5 XRD patterns of four PEO coatings

3.3 Wear property of coatings

The evolution of friction coefficients versus the sliding distance of the coatings is depicted in Fig.7.The PEO-coated samples with graphene (A2 and S2) exhibit lower friction coefficients than the coated specimens without graphene (A1 and S1).The addition of graphene nanosheets to the electrolyte leads to the nanocomposite coatings with lower and more stable friction coefficients than that produced in the graphene-free electrolyte,which is attributed to the"rolling effect"from graphene nanosheets incorporated in the coatings.Moreover,they were acted as lubricants during the sliding test [ 37] .S1 shows the highest friction coefficient because its surface is the roughest(Figs.3a,7).The surfaces of S2,A1 and A2 are flatter than that of S1.So their friction coefficient is lower than that of S1.The results are in good agreement with the observation results of the coating's surface morphology.

Fig.6 BSD images of cross section of four PEO coatings:a S1,b S2,c Al,and d A2

Fig.7 Friction coefficients of four PEO-coated samples with respect to sliding distance

The friction behavior of PEO coatings was also further studied by SEM for the wear tracks,as illustrated in Fig.10a,c,e,g.The insert figures are corresponding macro SEM images of the overall wear track.It is obviously found that the wear track profile of S1 is the largest.The wear track of the other three coated samples is relatively small.The worn surfaces without graphene S1 and A1 are rougher than those with graphene S2 and A2.Moreover,some obvious shallow grooves (shown by red arrows in Fig.8)and the hole left by the falling debris (displayed by white arrows in Fig.8) can be clearly seen.Since graphene spreads on the surface of coating A2,tiled graphene ismore obvious after wear test (illustrated by white arrows in Fig.8g) than S2.There are almost no signs of wear track on the surface of S2.The damage of the surface of A1 is the most serious among the four coated samples,and that of S2 is the least,which displays that the wear resistance of the coating A1 is the worst due to loose and thin coating,and S2 owns the best wear resistance attributing to its coating with the most compact and dense structure.Most importantly,because of the incorporation of graphene,the performance of coatings is improved.SEM observation results of the wear tracks are consistent with the results of the friction coefficient versus the sliding distance.

Except for the micrograph of the wear track,the elemental analysis of worn surface was also tested by EDX,as illustrated in Fig.8b,d,f,h.The results reveal that the relative content of Fe and Cr from the coated samples(Fig.8d,h) is larger than that from the coated samples without graphene (Fig.8b,f).It implies that the transfer of Fe and Cr to the surface of coated A2 and S2 samples increases due to the presence of PEO coating.This is caused by the lower friction coefficient of the coated samples against AISI 52100 steel ball.It is clearly seen that iron content of S2 is the highest,but the iron content of A1is the lowest.The aluminum content of S2 is the lowest,whereas that of A1 is the highest.Furthermore,Cr is detected in the worn surface of S1,S2 and A2 through EDX analysis.Fe and Cr come from the steel ball,and Al comes from PEO coating and A1 substrate material.So,it can be concluded that the wear resistance of S2 is the best.This is due to almost no damage of the coating S2,while it causes serious damage of counterpart steel ball.Therefore,the wear volume and the friction coefficient can be decreased due to the addition of graphene.This is because the interfacial shear strength is proportional to the contactpressure [ 38] .The friction coefficient starts to decreasewhen smaller interfacial shear strength is induced by thedecrease in the internal stress.

Fig.8 SEM images and EDX spectra of wear scars:a,b S1;c,d S2;e,f A1;g,h A2

3D morphologies of the wear tracks of S1,S2,A1 and A2 after the wear tests under a load of 5 N are displayed in Fig.9a-d,respectively.Meanwhile,Fig.9d exhibits the depth of the wear track profiles of the respective specimens.It is clearly seen from Fig.11 that the wear track of the coating S1,A1 and A2 is deep,especially A1.However,the coating S2 displays the shallowest wear track among the four samples (Fig.9b).The observation results of three-dimensional (3D) morphologies are in good agreement with SEM morphologies of the worn surface(Fig.8).The depth of the wear tracks also supports the view above (Fig.9e).The depth of the wear track of S1,S2,A1 and A2 is found to be around 4.4,0.8,8.3 and4.1μm,respectively.Based on the concept of the friction coefficient,smaller contact area results in larger friction coefficient.Generally,the value of the friction coefficient will become large in the initial stage of the wear.And the surface of the coating gets smooth because the surface protrusion is ground with the wear increasing.The roughness of the coating gradually reduces,but the wear area gradually increases with the wear continuing.Finally,the friction coefficient reduces.With the wear test continuing,the contact area tends to be stable.That will lead to the stable friction coefficient.Therefore,a stable stage will appear among the whole friction stroke.This stage owns the largest proportion during the whole wear process.When the contact surface is destroyed,the real contact area decreases,but the friction coefficient rises [ 39, 40, 41] .Therefore,from the wear test result and the analysis of wear tracks,it can be concluded that PEO coating A2 and S2 with graphene displays excellent wear resistance due to the lower friction coefficient compared to the coating without graphene nanosheets A1 and S1.That indicates that graphene nanosheets significantly reduce the friction coefficient and protect D16T Al alloy from severe wear.

Fig.9 3D morphologies of the wear tracks of a S1,b S2,c A1 and d A2;e depth profiles of four PEO-coated samples

3.4 Electrochemical corrosion behavior of the coatings

In this study,an EIS test was applied to analyze the transient corrosion behavior of the coated samples produced in the silicate and aluminate electrolyte.Figure 10 shows the impedance data of four PEO-coated specimens (A1,A2,S1and S2) and the uncoated D16T Al alloy in form of Nyquist plots and Bode plots.Figure 10a,b clearly exhibits that the impedance semicircle of the coating S2 obviously magnifies.The magnified Nyquist plots of uncoated Al alloy,S1,A1 and A2 in Fig.10a are shown in Fig.10b.In the Nyquist plot of S2,the large diameter curve shows higher real impedance Z',indicating that the coating has larger capacitance and can provide much protection and better corrosion resistance [ 42, 43] .Although the impedance semicircle of S1 is smaller than that of S2,it is quite larger than those of A1 and A2.Whereas the impedance semicircle of A1 and A2 is not obvious compared to that of the coated S1 and S2 specimens,indicating that the coatings fabricated in the silicate electrolyte possess more capacitance which can provide better corrosion resistance.The impedance semicircle of A1 is the smallest among four coated samples,but it is larger than that of the uncoated Al alloy,demonstrating that PEO coating with graphene possesses excellent anticorrosion compared with the coating without graphene addition and the uncoated aluminum alloy.From Bode plots (Fig.10c,d),it is easy to be found that the coated specimens own the high impedance value compared to the uncoated D16T Al alloy.Because the impedance modulus at the low-frequency area end of the Bode plot manifests the corrosion resistance [ 44] ,the corrosion resistance of the coatings is an order of magnitude larger than that of the uncoated A1 alloy.Especially,the coating with graphene shows the best corrosion resistance due to the incorporation of graphene.The high-frequency time constant corresponds to the outer layer with porous structure.The time constant in the low-frequency area is associated with the inner layer of the dense structure.

In order to clarify and discuss these variations between the high-frequency area and the low-frequency area,furthermore,for a more quantitative explore,the electrochemical phenomena causing the impedance to improve,the impedance data were analyzed by the equivalent circuit(Fig.11) which was applied to fit Nyquist plots.It is also used by other researchers [ 45, 46, 47] ,where R_s represents the solution resistance between the reference electrode and the working electrode,R₁ refers to the coated specimen's outer layer,CPE 1 is a constant phase element of the outer porous layer,R₂ refers to the resistance of the inner dense layer of the coated specimens,and CPE2 is the constant phase element for the inner layer with dense structure.Admirable fitting results are gotten between the experimental data and fitted results,as shown in Fig.10 and Table 2.

Fig.10 EIS spectra of uncoated D16T A1 alloy sample and four PEO-coated samples immersed in 3.5 wt%NaCl solution for 2 h:a,b Nyquist plots;c,d Bode plots (Z',real part of impedance;Z",imaginary part of impedance;[Z],impedance modulus)

According to the fitting results,the impedance of the inner dense layer (R₂) is much higher than that of the outer porous layer (R₁),especially the PEO-coated samples prepared in the silicate electrolyte.From Table 2,the resistance values (R₁ and R₂) of the coatings S1 and S2are much larger than those of A1 and A2.Moreover,the resistance values (R₁ and R₂) of the PEO coating with graphene addition are also larger than that of the coating without graphene.In the outer porous layer,the R₁ of S2reaches 10 times that of Al and A2.In addition,in the inner layer,the R₂ of S2 reaches nearly 1000 times that of A1 and A2,and more than 100 times that of S1,which is ascribed to the structural improvement of the coating S2.The comprehensive structure of S2 is the best due to the dense,compact and fewer cracks,and more graphene incorporated in it (Figs.3b,4a).Although the surface of PEO coatings A1 and A2 is flatter than that of S2,t A1and A2 have many microcracks,and they are osteoporosis and not dense.In addition,the thickness of their coating is smaller than that of S2.The values of dispersion index n₁ and n₂ of A2 and S2 are higher than those of A1 and S1,respectively,showing that the interface between the coating and the substrate gets much flatter and smoother,which is attributed to the incorporation of graphene with good lubrication and filling function.The values of n₁ of A1 and S1 are rather low,indicating that the outer porous layer of the coated specimens A1 and S1 owns less surface homogeneity and low capacitance.The values of CPE-Y₁ of A1 and A2 are higher than those of S1 and S2,due to that many microcracks exist in the coating;while the value of CPE-Y₁ of S2 is low,indicating that the coating with fewer microcracks and many pores are filled and embedded by graphene nanosheets (Fig.4a).The comprehensive comparative analysis shows that the corrosion resistance of the coated specimens produced in the silicate electrolyte (S1 and S2) is much better than that of the coated specimens produced in the aluminate electrolyte (A1 and A2).Furthermore,the corrosion resistance of the coated specimens with graphene nanosheets addition is better than that of the coated samples without graphene nanosheets addition.This is attributed to the coating structure and the incorporation mode of graphene.Under the same condition,the coating formed in aluminate electrolyte is thinner than that fabricated in the silicate electrolyte (Fig.8),and the incorporated graphene spreads on the surface of the coating A2.In addition,the coatings A1 and A2 are not dense,and many microcracks distribute in it.Thus,less graphene can incorporate in coating A2 than in S2.However,the thicknesses of the coatings S1 and S2 are larger than those of A1 and A2.Furthermore,the incorporated graphene distributes in the coating S2 in form of embedded and filling mode.So,many small graphene nanosheets incorporate in the coating for filling pores,which makes coating dense and compact.The incorporated graphene plays a key role in filling pores of PEO coatings and makes the coating compacter and denser as also proposed in Ref. [ 48] .Based on the EIS test and analysis results,it can be concluded that the corrosion resistance of the coating S2is significantly increased because the incorporated graphene has the function of prohibition invasion and corrosion from the corrosion medium Cl^-.In summary,the coating with graphene can significantly improve the corrosion resistance of D16T Al alloy,especially the coating formed in the silicate electrolyte with the addition of graphene.

Fig.11 Electrical equivalent circuit of four PEO-coated samples(A1,A2,S1 and S2) immersed in 3.5 wt%NaCl solution for 2 h

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Table 2 Electrochemical parameters obtained from EIS spectra of PEO-coated samples

The result of previous studies shows that the wear resistance of the coating with graphene is superior to that of the coating without graphene addition,as reported in Refs. [ 40, 49] .The PEO coating will pile up on the both sides of the wear tracks during the process of the wear.So,it becomes thinner and thinner in the middle of the coating.More serious result of the wear that the D16T Al alloy substrate is worn will appear.The coatings A1 and S1display more microcracks and pores compared to the coating A2 and S2 due to no addition of graphene.So,the site where pores and cracks exist firstly is cracked and shed,until the large wear area appears.However,the coatings with graphene A2 and S2 display a discontinuous deformed graphene layer which can reduce the friction coefficient [ 50] .During the wear process,the deformation and transference of graphene occur under the load of wear,and then,they are worn flat and spread on the surface of the coating which is as the barrier layer for blocking.In addition,the reason that graphene improves the wear resistance of the coating can be explained by its lubricity and graphene is used as conventional lubricants [ 51] .Hua et al. [ 52] investigated the scratch and wear resistance of graphene oxide/polysiloxane nanocomposite coatings and found that the introduction of GO could efficiently alleviate the process that the coating was pushed forward during the wear.Owing to the ultrahigh specific surface area and lubricity of graphene,embedded dispersed GO sheets form the strong interface with the coating matrix by the curing reaction.In this way,the coatings with graphene exhibits obvious enhancement in wear resistance performance.Figure 12 shows that the mechanism of wear resistance of S2 is much better than that of A2.From the result of SEM and EDX,it can be easily known that the incorporation mode of the coating A2 is spread mode,however,the incorporation mode of the coating S2 is different from that of A2,which is incorporated in the coating with the embedded mode.Furthermore,most of graphene spreading on the surface of the coating A2 are in big size,and they block more small-sized graphene from incorporating into the coating.Because graphene with small size embedded in the coating S2,much more graphene can incorporate in the coating S2 than in the coating A2.They fill the microcracks and pores.Ling et al. [ 53] studied the friction and wear resistance of graphene sheets by first principles and revealed that the difference of tribological performance was ascribed to the incorporation mode.The results showed that the graphene coating in form of embedded mode had lower adhesion than the graphene incorporated in the coating with spreading mode due to less dangling bonds.Moreover,graphene sheets with perfect lamellar structure and low surface energy exhibited promising wear resistance.

The schematic of corrosion behavior and mechanism of the coatings with and without the addition of graphene is exhibited in Fig.13.It can be easily found that more pores and microcracks are in the coatings without graphene A1and S1 than that in the coatings with graphene A2 and S2,so,it is easier for corrosive particles to reach and contact the Al substrate along the path way through the pores and cracks,and then corrode it,indicated by the red dotted arrows in Fig.13a-c.Liu et al. [ 54] studied the influence of graphene on the corrosion resistance of PEO coatings produced on D16T Al alloy and found that the coating with graphene addition significantly improved the corrosion resistance of Al alloy due to the physical separation of graphene.Functionalized graphene exists in the coatings A2 and S2 in form of embedded mode because of graphene withπ-πstacking and strong van der Waals forces [ 55] ,thus,the pores and microcracks in the coating formed in PEO process are easily filled by incorporated graphene.Furthermore,graphene additive acts as a good barrier for inhibiting the penetration of corrosive medium diffusion pathway [ 48] .Therefore,it can diminish the pores and cracks of the coatings,and then decrease the channel and pathway providing for the corrosive particles.Additionally,the coatings with graphene A2 and S2 possess less pores and microcracks than A1 and S1,so,the coatings A2 and S2 display better corrosion resistance to the chloride ions.The coatings A2 and S2 show different corrosion resistances due to the different incorporation modes of graphene.Graphene with big size spreads on the surface of coating A2,which can effectively prevent the intrusion of corrosive particles.But most of areas without graphene are easily corroded by corrosive particles.That's because graphene with large size blocks more small-sized graphene to enter the coating,corrosive particles enter the coating along cracks and pores and corrode the coating (Fig.13b).However,for the coating S2,graphene disperses in the coating with the embedded mode and those modes that some graphene embeds in the pores and microcracks,and some graphene and aluminum oxide coatings form a cosolvent presenting in the coating.So,much more graphene incorporates in the coating S2 than in the coating A2.Yin et al. [ 56] studied the tribology and wear properties of the alumina composite coating with graphene.The results presented that the composite coating possessed lower friction coefficients and better anti-wear behaviors.The superior tribology and wear performances of the coating are ascribed to the synergistic effects of graphene and alumina.Therefore,based on the above discussions,the coating S2 exhibits superior corrosion resistance than the coating A2.

Fig.12 Schematic illustration of wear mechanism of PEO coatings:a A2 and b S2

Fig.13 Schematic of corrosion behavior and mechanism of PEO coatings:a A1,b A2,c S1,and d S2

4 Conclusion

The main goal of this study was to optimize one electrolyte where PEO coatings with good morphology and properties could form.SEM results indicate that the micros true ture of PEO coatings A1 and A2 produced in aluminate electrolyte is flatter,less porous,and has more microcracks than PEO coatings Sl and S2 fabricated in the silicate electrolyte.The coating S2 formed in the silicate electrolyte containing graphene is compacter and thicker than other three coatings.SEM and EDX confirm that graphene is successfully incorporated into the coating S2 in form of embedded mode,while in the coating A2 in form of spreading mode.XRD results also confirm the formation ofα-Al₂O₃,γ-Al₂O₃ and Al phase.The performances of the coating with embedded graphene are superior to those of the coating with spread graphene.The thickness of the coating S2 is the largest.The wear test results clarify that the friction coefficient of the coating S1 is the largest,and those of other three coatings are similar.Additionally,the depth of the wear track of the coating G2 is the shallowest.Then,the EIS test results indicate that the impedance of the coating G2 is the best,and its corrosion resistance performance is optimal.Furthermore,the corrosion and wear resistance of the coatings are obviously improved by the addition of graphene,especially for S2.