Ting Wu,Crsten Blwert,Xiopeng Lu,Mri Serdechnov,Mikhil L.Zheludkevich,c

a Institute of Surface Science,Helmholtz-Zentrum Hereon,21502 Geesthacht,Germany

b Shenyang National Laboratory for Materials Science,Northeastern University,110819 Shenyang,China

c Institute for Materials Science,Faculty of Engineering,Kiel University,Kaiserstrasse 2,24143 Kiel,Germany

Abstract In order to study the substrate lattice structure(Li addition)on the growth of plasma electrolytic oxidation(PEO)coatings,MgLi alloy(11.36wt.% of Li,cubic)and pure Mg(hexagonal)were treated under a pulsed direct PEO mode in a phosphate electrolyte for different periods of time.The results revealed that the presence of Li and Li-rich phases in the cubic Mg alloy seems to be essential for the treatment result rather than the original lattice structure.A modifie discharge behavior of MgLi alloy finall led to a different microstructure of the coating.The unstable coatings of MgLi alloy tended to dissolve rapidly though shared the similar composition to that of pure Mg.Li was incorporated only in the primary conversion products at the interface of coating/MgLi.In spite of the advanced efficien y of energy input during processing,the more porous and thinner PEO coatings on the MgLi alloy were less resistant to abrasion and corrosion.? 2021 Chongqing University.Publishing services provided by Elsevier B.V.on behalf of KeAi Communications Co.Ltd.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/)Peer review under responsibility of Chongqing University

Keywords:Plasma electrolytic oxidation;Magnesium lithium alloy;Pure magnesium;Microstructure.

1.Introduction

Plasma electrolytic oxidation(PEO)is a plasma-assisted anodizing process for the surface modificatio of lightweight metals[1-8],such as Al,Mg and Ti alloys[9-12],in order to improve their surface hardness,wear and corrosion resistance.The process is characterized by numerous short-lived microdischarges under high voltage[13-15],which forces the formation of PEO coatings with intrinsic cavities and cracks[16-18].

In the framework of understanding the growth behavior of PEO coating,some researches[19-26]were focused on the migration of species,originated from the electrolyte during PEO processing.Alloying elements,such as Ti,Nb and Zr from the substrates used as tracers,were also reported and provided a solid evidence of the dielectric breakdown of the oxide layer[25,26].PEO processing of AZ magnesium alloys(eg.,AZ31,AZ61 and AZ91)showed a sequential oxidation at the initial stage starting withα-Mg and then extending to theβ-phase(Mg17Al12).A higher rate of coating formation was observed onα-Mg,deriving from the higher reactivity of Mg compared to Al[3,27-29].Also,the discharges were more intensive in the presence of Al in Mg alloys,resulting in the more porous coatings[3].Nevertheless,the PEO processing of WE43 magnesium alloy developed more rapidly and presented a reverse oxidation order compared to AZ91.The growth of the initial conversion fil started on theβ-phase(Mg14Nd2Y),possibly stemming from the higher reactivity of Y and Nd than that of Al[30,31].In binary Mg-Zn alloys the presence of Mg7Zn3phase was found to reduce the thickness,however,smoothen the surface morphology of the PEO coatings,ascribed to the hardship in passivating the Mg7Zn3intermetallics[31,32].That is to say,from manipulating the composition and microstructure of Mg alloys,the alloying elements have a significan influenc on the PEO process and hence on the properties of the PEO coatings.

Magnesium lithium(MgLi)alloys have recently been developed as potential engineering materials[33-39]due to the desirable low temperature ductility and cold formability from the body-centered cubicβ-Mg containing more slip systems[40].PEO treatment has been adopted to improve the surface stability of the vulnerable substrate materials considering the chemically active nature and ionization tendency of Li[20,41-47].Researchers have claimed that the PEO coatings of MgLi alloy doped with salt additives,such as tungstate[41],molybdate[48],cerium salts[49],or sol additives,for instance,silica[20]and titania sols[43],revealed a significan enhancement in wear and corrosion resistance[41].In fact,it is not surprising that the cooperation of different additives in the electrolytes can improve the coating performance on MgLi alloys,since it has been established already for many other magnesium alloys[2,50].However,the effect of Li in the Mg matrix during PEO processing is less taken into consideration,regarding its active nature and the difficult to trace the light element with standard techniques used for characterization of PEO coatings.Particular for high Li containing Mg alloys(>10.3 wt% Li),the mechanical strength declines considerably thus the surface protection is more necessary[51,52].Optical emission spectra of plasma discharges during the treatment of MgLi alloy evidenced the participation of Li in the plasma reactions due to the high temperature-forced excitation of Li,although the absence of Li containing phase in the PEO coating[53].

Despite the progresses,the characteristics(thickness and microstructure)and performance(wear and corrosion resistance)of PEO coatings on MgLi alloys with high Li contents remain unclear.The role of Li during PEO process still requires further elucidation since the intrinsic chemical reactivity of MgLi alloy is more susceptible in aggressive environment.Thus,the aim of this work is to study the evolution of the coating growth of MgLi alloy and to understand if the different lattice structure has an effect on the coating formation and growth.Furthermore,the role of Li in the phase formation of the PEO coatings should be studied in more details.To do this,PEO coatings on a selected MgLi alloy(11.36wt.%Li),were compared at different stages of the PEO process with coatings formed on pure Mg under the same conditions.

2.Experimental

2.1.Materials and PEO treatment

Rectangular specimens of size 15mm×15mm×4mm made of MgLi alloy with the chemical composition of 11.36wt.% Li,2.39wt.% Al,0.0093wt.% Mn,0.0072wt.%Zn,0.0032wt.% Fe,0.0012wt.% Ni,0.0009wt.% Sn and Mg balance as measured by an Arc Spark OES(Spark analyze M9,Spectro Ametek,Germany)were selected.Pure Mg specimens with the same dimensions(15mm×15mm×4mm)and a purity of 99.94wt.%(0.022wt.% Mn,0.017wt.% Si,0.014wt.% Al,0018wt.% Zn,0.0017wt.% Fe,0.0002wt.%Cu,<0.0002wt.% Ni and Mg balance)were adopted as references.All of the specimens were mechanically ground up to 2500 grit,cleaned in water and ethanol and dried with compressed air at ambient temperature.A phosphate-based aqueous electrolyte was prepared by the dissolution of trisodium phosphate(Na3PO4,≥98%,Carl Roth,Germany,20g/L)and potassium hydroxide(KOH,≥85%,Th.Geyer,Germany,1g/L)in deionized water.The pH and conductivity of the electrolyte were 12.4 and 23.2 mS/cm respectively.The temperature of the electrolyte was kept at 10±2 °C by a cooling system.The specimens were screwed to a holder acting as the anode.A stainless steel tube was integrated as the cathode and part of the water-cooling system.During the PEO process,continuous stirring of the electrolyte was applied to prevent concentration and temperature gradients as much as possible.PEO treatment was carried out for six different treatment durations(15s,30s,1min,3min 10min and 20min)using a pulsed DC power supply at a constant current density of 3 A/dm2with a duty cycle of 10%(1ms:9ms)and a frequency of 100Hz.The voltage-time response was recorded using a data acquisition system(SignaSoft 6000 software package,Gantner,Germany).The optical emission spectra of the discharges during the processing was recorded using Emicon system(PLASUS,Germany)with a spectral resolution of～1.5nm in the range of 180-880nm,in combination with PLASUS SpecLine software(PLASUS,Germany).The species(Na,Mg,Li and OH)involved in the plasma reactions and the variation of emission intensity of these species as function of treatment time were monitored.

2.2.Coating characterization

Scanning electron microscopy(SEM,TESCAN Vega3 SB)combined with an energy dispersive spectrometer system(EDS,eumeX IXRFsystems)were used to characterize the surface and the cross sections of the PEO coatings in association with SE(secondary electron)and BSE(back scattering electron)modes.A lamella milled from the bulk sample of MgLi alloy after 3min of PEO treatment was obtained using focus ion beam cutting in a field-emission-scannin electron microscopy(FIB-SEM,TESCAN Lyra3).Further information about the presence of Li in the coating was obtained using a windowless Oxford Ultim Extreme EDS detector(OXFORD ULTIM EXTREME),which is capable to Li detection.The coating thickness measured by a microprocessor coating thickness gage(MiniTest 2100,ElektroPhysik)was validated and integrated with the observation of the coating cross sections.Phase composition studies were performed by X-ray diffraction(XRD,D8 Advance,Bruker AXS)with Cu Kαradiation.The following diffraction settings were selected:diffraction angle 2θbetween 20° and 80°,step size of 0.02°/s,dwell time of 1s and grazing angle of 1° Laser scanning confocal microscope(LSM 800,ZEISS)was employed to monitor the roughness profil of the coating surface.

2.3.Wear test

The dry sliding wear behavior of the thicker PEO coatings(10min and 20min)were evaluated using a ball-on-disk oscillating tribotester(TRIBOtechnic).The static friction partner was a steel ball(AISI 52,100)with a diameter of 6mm.All of the tests were performed under ambient conditions of 25±2°C and 30% r.H.Following parameters were applied for all wear tests:load of 2N,oscillating amplitude of 10mm,sliding velocity of 5mm/s and a total sliding distance of 12m.The width and depth of the wear tracks on the specimens were measured using a laser scanning confocal microscopy(LSM 800,ZEISS)and the wear tracks were observed by means of a scanning electron microscopy(SEM,TESCAN Vega3 SB).The wear rate was calculated using the wear depth.Each measurement was performed for three times in order to check reproducibility.

2.4.Electrochemical measurements

To evaluate the corrosion performance of the PEO coatings(1min,3min,10min and 20min)on MgLi alloy and pure Mg,electrochemical impedance spectroscopy(EIS)tests were carried out using a Gamry Potentiostat(Gamry,United States)in conjunction with a classical three-electrode cell system.The coated specimens were adopted as working electrode with an exposed surface area of 0.5 cm2.The counter and the reference electrode were a platinum wire and a saturated Ag/AgCl electrode,respectively.Tests were performed in 0.5wt.%NaCl solution at ambient temperature,over a frequency range from 100kHz to 0.1Hz at open circuit potential(OCP),with 10mV RMS sinusoidal potential perturbations on specimens exposed to the solution for different durations:5min,1h,3h,6h,12h,24h and 48h respectively.The measurement was reproduced on three samples of each PEO processing.

3.Results

3.1.Microstructure of the substrate materials

The grain boundaries of selected MgLi alloy with an average grain size of 230μm are visible in the BSE mode,because they are decorated with small precipitates(Fig.1(a)).The bright and fin particles scattered also within the grains are identifie by X-ray diffraction(Fig.1(c))as MgLi2Al intermetallics,whereas the darker micro-pores seem to be the result of the mechanical removal of MgLi2Al intermetallics during the grinding preparation.Beside the fin MgLi2Al intermetallics,XRD pattern of MgLi alloy only presents intensive peak signals of cubicβ-Mg phases in comparison to the hexagonalα-Mg of pure Mg.This is in accordance with the common knowledge of cubic structure of MgLi alloy when the content of Li exceeds 10.3wt.%[54].Without precipitation at the grain boundary,the microstructure of as-cast pure Mg can be only recognized under SE mode(Fig.1(b))displaying the irregular grain boundaries due to the different sizes of grains.

Fig 1.(a)back scattered electron(BSE)micrograph of MgLi alloy,(b)secondary electron(SE)micrograph of pure Mg and(c)the XRD patterns of MgLi alloy and pure Mg.

3.2.Characterization of PEO process

The voltage-time responses of the PEO process of MgLi alloy and of pure Mg exhibit the initial fast ramps of voltage(Fig.2(a)),suggesting a rapid electrochemical formation of barrier layers of similar thickness(Fig.2(b))on both substrates.As soon as the voltage reaches 110V for MgLi alloy and 150V for pure Mg,the breakdown of the initial barrier layers occurs,being accompanied with visible sparks.This period of development is shorter than 30s.The discrepancy in breakdown voltage can be explained via the electrical resistance of the initial layer,which seems to be lower for MgLi alloy and therefore needs a lower voltage to ignite the discharges.After 1min of treatment,frequent fast moving white sparks cover the entire surface of both substrates;however,the voltage increase for MgLi specimen slows down earlier and more rapidly compared to pure Mg.From 3min to 10min of PEO processing,there is a sluggish increase of voltage in both cases,probably because of a competition between coating formation and dissolution at this stage.Nevertheless,for MgLi alloy this stage takes longer until the voltage continues to rise faster again.This stage is characterized by less numerous but larger and slowly moving orange discharges on the surface of both substrates.Voltage fluctuatio are observed after 13min and are associated with the evolution of the plasma discharges[55],which are also recognized from the sporadic long-lasting large sparks before the end of the processing.The fina voltage of MgLi alloy and pure Mg is 340V and 480V respectively.It is obvious that the electrical resistance of the coating on the MgLi alloy is always lower than the one on pure Mg as the same current density requires less voltage to drive the current through the layer on the MgLi alloy.

Fig 2.(a)Voltage-time curves of PEO processing on MgLi alloy and pure Mg and(b)PEO coating thickness of MgLi alloy and pure Mg at different treatment time.

The evolution of coating thickness with treatment time is shown in Fig.2(b).At the fina treatment time of 20min the PEO coating on MgLi alloy reaches a thickness of 43.5±4.4μm and on pure Mg it is 52.3±5.8μm with a growth rate of 2.2μm·min?1and 2.6μm·min?1,respectively.Obviously,under the same treatment conditions the response of the MgLi alloy to a PEO treatment in the sense of coating formation and resistance(lower process voltage necessary to let the predefine treatment current pass through the coating)is worse compared to pure Mg.Whether this difference is related to the different lattice structure,the lower corrosion resistance of the MgLi substrate,less stable coating phases or whether it is a combination of all possible influencin factors,still needs to be studied in more details.The optical emission spectra as depicted in Fig.3(a)and(c)confir the involvement of Na,Mg and OH in the discharges on both substrate materials after 20min of the PEO treatment.Following lines are observed during the measurements:Na I,Mg I and OH.In the case of MgLi alloy,an additional emission line of Li(Li I line at 670.8nm)is detected,indicating its participation in the plasma reactions.All the emission intensities of mentioned species,especially Na I(of pure Mg and of MgLi alloy)and Li I(of MgLi alloy)increase gradually with treatment time(Fig.3(b)and(d)),which are however related to respective trends of voltage increase.The initial white luminescence during conventional anodic oxidation,resulting from the radiative recombination of electrons at fl ws in the initial oxide fil[56],shows the lowest emission intensity of all species because the voltage is not adequate to excite them.Afterwards,the emission of Na(Na I emission line)for both pure Mg and MgLi alloy and the emission of Li(Li I line)for MgLi alloy are distinguished from ca.300s(6min)of treatment until the end of the process.The appearance of both lines takes place at initial voltages of 283V and 234V for pure Mg and MgLi alloy,respectively,latter,indicates an easier excitation of Na and Li for MgLi alloy.Nevertheless,the intensity of Na I emission line for pure Mg increases more rapidly in comparison with MgLi alloy.This can be correlated with the faster rising of the voltage in the case of pure Mg.Afterwards,the emission intensity of Na I line for pure Mg reaches the technical saturation limit of the detector once the voltage exceeds 443V.For the MgLi alloy,the emission of Li I shows a similar pattern as Na I however at a lower intensity.In regard to the emission of Mg(Mg I emission line)and OH,they show a slight increase with moderate oscillation on both substrate materials and,once appears,does not depend much on the treatment time/applied voltage.

3.3.Coating characterization

3.3.1.Surface morphology

Figs.4 and 5 exhibit the surface morphology and element distribution of the coatings on MgLi alloy and pure Mg after 15s,30s,1min and 3min of PEO processing respectively.The coverage of O over the entire surface of MgLi alloy indicates the formation of a barrier layer within 15s(Fig.4(a)).Bright fin protuberances scattered on the surface of the MgLi alloy are also observed at the earliest stage.The higher concentration of Al in these protuberances demonstrates the accumulation of initial conversion products on MgLi2Al intermetallics.The firs conversion products appear to be Mg/Na based phosphates.In comparison,larger protuberances of conversion products are found spreading randomly on the surface of pure Mg(Fig.5(a)).As the“footprints”of plasma discharges,sporadic micro-pores are distributed on the surface of MgLi alloy and more frequent on pure Mg substrates after 30s of treatment(Figs.4(b)and 5(b)),confirmin that the breakdown of the barrier layers is triggered in the range between 15 and 30s(120 to 180V).The discharge channels are considered to be the pathways for transporting the species from the substrate outwards(Mg,Mg2+,Li and Li+)and from the electrolyte inwards(OH?,H2O and PO43?)respectively.Regions of strong interactions and reactions between the different compounds refreshing the coating materials[57]can be distinguished in the figure as well.The larger micro-pores and volume of strongly melted regions on pure Mg reflec more intensive and frequent plasma discharges at a higher voltage.The higher intensity of Al on the surface of MgLi alloy after 30s of processing is from the penetration of electrons into the substrate material underneath the thin surface fil during the SEM examination.Hence,the intensity of Al from the MgLi alloy substrate is also indicative of the coating thickness.The signal of Al moderates gradually with further increase of the treatment time as the coating is thickening steadily.Hereafter,typical pancake features with centered micro-pores are observed on both coatings(Figs.4(c)and 5(c)),which become larger and more dominant after 3min treatment(Figs.4(d)and 5(d)).

Fig 3.Optical emission spectra obtained after 20min PEO treatment and evolution of the emission line intensity as a function of treatment times:(a)optical emission spectrum of pure Mg,(b)emission line intensity of Na I,Mg I and OH for pure Mg treatment,(c)optical emission spectrum of MgLi alloy,(d)emission line intensity of Na I,Li I,Mg I and OH for MgLi alloy treatment.

Fig 4.Surface morphologies and element distribution(Mg,O,Al,P and Na)of PEO coatings on MgLi alloy after the treatment time of(a)15s,(b)30s,(c)1min and(d)3min.

Fig 5.Surface morphologies and element distribution(Mg,O,Al,P and Na)of PEO coatings on pure Mg after the treatment time of(a)15s,(b)30s,(c)1min,and(d)3min.

Fig.6(a)-(d)display the typical surface morphology of PEO coatings with the open micro-pores(remains of discharge channels)on MgLi alloy and pure Mg after 10min and 20min of PEO treatment.The micro-pores cover about 21%and 11% of the surface area for the MgLi alloy and around 15% and 13% for pure Mg after 10 and 20min of treatment,respectively.Considering also the less frequent surface microcracks after the longest treatment time(20min),in particular of MgLi alloy,the coating seems to be healed to a certain degree since the discharge process always initiates in the weakest regions of the coating[58].Interestingly,an accumulation of bright fin particulates adjacent to the micro-pores are noticed on the surface of PEO coatings on the MgLi alloy,as revealed in the insertions at higher magnificatio(Fig.6(a)and(c)).It is reasonable to associate the formation of bright particulates with the presence of abundant Li since a modifie voltage response and less intensive discharges are observed.XRD patterns suggest a similar PEO layer composition based on MgO and Mg3(PO4)2for both substrates after 10min of treatment.Nevertheless,there is no Li containing phase detected after PEO processing of MgLi alloy.A bump in intensity between 2θangles of 20° to 40° especially on the MgLi alloy indicates the formation of amorphous phosphate,most likely to the fact that the discharge energy is lower,causing less crystalline phases and more phases that are amorphous.

Fig 6.Surface morphologies of PEO coatings after 10min and 20min treatment on MgLi alloy(a)10min,(c)20min;on pure Mg(b)10min,(d)20min and(e)XRD patterns of PEO coatings after 10min treatment.

Fig 7.Surface roughness profil of PEO coatings on MgLi alloy:(a)1min,(c)3min,(e)10min and(g)20min;on pure Mg:(b)1min,(d)3min,(f)10min and(h)20min.

Chemical composition and contents of the fin particulates(A and D)and the regions without particulate(B,C,E and F)are listed in Table 1.Li of light weight is beyond the detectability of current main EDS and not available here.The bright particulates(A and D)contain higher ratios of elements from the electrolyte(P,O,Na)compared to the regions of B,C,E and F without particulates.This could stem from the less intensive A-type discharges[55,59,60]occurring near the interface of coating/electrolyte,thus involving the deposition of species mainly from the electrolyte.Another explanation might be related to the fact,that those regions are further away from the discharge channels,where the material is less mixed and sintered.The surface around the discharges looks normally melted,uniform and smoother.Due to less sintering in those regions,they might be also prone to more attack from the electrolyte and those white particles are conversion products.

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Table 1Content of main elements(Mg,P,O,Na,Al)at the regions of A,B,C,D,E and F in Fig.6.

Fig.7(a)-(d)exhibit similar surface morphologies of the coatings after shorter treatment durations(1min and 3min).Due to the limited thickness of the coatings,scratches produced during grinding still present on the surface of MgLi alloy after 1min of treatment.The presence of Li or the different lattice structure of the substrate does not significantly change the surface roughness of the coatings at this stage and the respective Ra values are 0.58±0.07μm and 1.59±0.04μm for MgLi alloy and 0.45±0.03μm and 1.94±0.02μm for pure Mg after 1min and 3min treatments.However,the surface of the PEO coatings on MgLi alloy is rougher after 10min and 20min of treatment compared with pure Mg.The Ra values of the PEO coatings on MgLi alloy are 7.86±0.34μm and 5.59±1.12μm(Fig.7(e)and(g)),while the Ra values of the PEO coating on pure Mg are 6.43±2.31μm and 4.31±0.42μm(Fig.7(f)and(h))respectively.The healing effect of the discharges is herein revealed at the later stage of PEO treatment.Since the PEO coatings after 10min of treatment are thick enough to resist frequent destructive B type of discharges,relatively soft discharges of A and C types are more likely generate at the pores and fis sures within the coatings.As a result,the sequential remelting and sintering process could explain the modificatio of the surface morphologies of the coatings with increasing treatment time.

Fig 8.Cross-sectional morphologies and EDS mapping results of PEO coatings at different duration of PEO treatment on MgLi alloy:(a)1min,(c)3min,(e)10min and(g)20min;on pure Mg:(b)1min,(d)3min,(f)10min and(h)20min.

3.3.2.Cross-sectional morphology

The cross-sectional morphology and element distribution of the PEO coatings on the two substrates after different treatment times are depicted in Fig.8.The typically layered coatings are formed in both cases.A thin denser barrier layer can be differentiated from the main outer porous PEO layer by a pore bands(elongated horizontal cavity regions of agglomerated pores).The pore band starts to be formed already after 1min treatment of both substrates and expands more in volume on both substrates with the increase of treatment time.More and larger cavities are always present in the coatings on MgLi alloy compared to pure Mg,corresponding to the rough microstructure of the coating on MgLi alloy[61].Distributions of Mg,O,P and Al are quite homogeneous across the coatings regardless of the base materials and processing durations.Nevertheless,Na originating from the electrolyte concentrates around the edges of pores and large cavities and on the coating surface(Fig.8(e)-(h))due to the penetration of electrolyte.

The interface of the coating/MgLi substrate after 3min treatment is studied additionally in higher resolution.STEM micrographs are shown in Fig.9(a)and(b).By the different contrast a narrow inner layer not more than 0.5μm thick presents,which contains sporadic micro-pores.The distribution of Li and Al shows relative high intensity of Li at the sites of MgLi2Al intermetallic and at the interface of coating/substrate(Fig.9(c)and(d)).It seems as if Li is involved in the early coating formation via the formation of conversion products and remains enriched close to the coating/substrate interface throughout the process.

The cross-sectional morphologies of the thicker PEO coatings on MgLi alloy and on pure Mg after 10min and 20min treatments are shown in Fig.10.Since the discharges are the driving force for the coating growth,the process is impeded by the thicker coatings with increasing treatment time,especially from blocking the transport of the reaction species.Nevertheless,beside the plasma discharge channels,frequent cracks and defects are visible in the thicker outer layer on both substrates.They are the additional pathways for the species from the substrate and the electrolyte[21].Thus,the coating growth rate remains stable and constant with increase of treatment duration.In addition,next to the pore bands the inner layer allows a continuous growth due to electrolyte inward diffusion and the generation of discharges within the coating.

Fig 9.STEM micro images of interface of coating/MgLi substrate after 3min PEO treatment(a)bright field(b)dark field(c)the distribution of Li and(d)the distribution of Al.

3.4.Wear resistance of PEO coatings

The friction coefficient as function of sliding distance of the substrate materials and the different coatings running against a steel ball under a load of 2N without additional lubrication are displayed in Fig.11.The friction coefficien of pure Mg oscillates slightly around a value of 0.42±0.05 presenting an average wear rate of 4.51×10?3mm3/(N·m).However,for the MgLi alloy the wear test is interrupted already after few cycles,because the friction force exceeds the safety setting of the wear tests.The high ductility of the cubic structured MgLi substrate has a higher friction coefficien compared to the hexagonal Mg.Already after the few cycles a clear wear track has formed in the soft MgLi substrate and the material that has been worn off remains in the track as shown in Fig.12(a).PEO coatings can improve the wear performance drastically.For the PEO coatings on MgLi alloy at 10min and 20min,both friction coefficient increase to about 0.62 within the firs 2m of sliding remaining quite constant up to 7m of sliding.Until the end of the test,the friction coefficien of the thicker coating(20min)fluctuate around 0.65 showing an average wear rate of 2.26×10?3mm3/(N·m).It implies that there is no coating failure as manifested by the remaining full coverage of PEO coating(Fig.12(e)).However,the friction coefficien of the thinner coating(10min treatment)rises to a value of 0.85 within the remaining sliding distance but the coating is not failing.The worn track of the sample in Fig.12(c)unveils an average wear rate of 3.823×10?3mm3/(N·m),which is related to the lower load bearing capacity and the coating being deeper pressed into the soft substrate.

The coating on pure Mg after 10min treatment shows a rapid rise of the friction coefficien to 0.77 at the initial sliding and then some slight oscillations.Without coating failure(Fig.12(d)),the average wear rate of the coating is 3.75×10?3mm3/(N·m).For the thickest coating on pure Mg after 20min of processing,the firs increase of the friction coefficien reaches a relative low value of 0.5 at the firs 2m of sliding.After that,the friction coefficien ramps constantly to a fina value of 0.72 after 10m sliding distance.Compared to the other coatings,the thicker coating on pure Mg has the lowest wear rate of 1.15×10?3mm3/(N·m).

Fig 10.Cross-sectional morphology of PEO coatings on MgLi alloy:(a)10min and(c)20min;and on pure Mg:(b)10min and(d)20min.

Fig 11.Variation of friction coefficient versus sliding distance for pure Mg substrate and the PEO coatings on MgLi alloy and pure Mg.

3.5.Corrosion behavior of PEO coatings

Fig.13(b)-(e)clearly illustrate the degradation processes of PEO coatings on MgLi alloy.The coating after 1min of PEO processing increases the total impedances slightly until 12h of immersion.Although the layered structure of the coating has been revealed,there is only one well-define time constant assigned to the inner layer.Afterwards,a relaxation process presented as scattering at the low frequencies reveals the failure of the coating as a result of the local breakdown of the coating.For the coating after 3min of PEO treatment,a similar behavior with only one capacitive loop in the medium frequency range can be seen after 24h of immersion.This explains the corrosion resistance of the coating,which however deteriorated after 48h immersion(corrosion pits were observed).The initial test(5min immersion)of the obvious two layered coating on MgLi alloy after 10min PEO treatment demonstrates the efficien protection of the inner layer.However,the shrink of the capacitive loop at the medium frequency with increasing immersion time afterwards indicates the gradual degradation of the coating.As for the PEO coating on MgLi alloy after 20min of PEO processing,the Bode plots at high frequencies(～6×104Hz)demonstrates the relaxation process of the outer layer during the immersion due to the remarkable thickness hindering the penetration of the solution.The full penetration of corrosive solution through the outer layer is revealed after 12h of immersion.Afterwards,the relaxation process at high frequencies is no longer differentiable and the outer layer contributes nearly nothing to the high frequency impedance.PEO coatings on MgLi alloy can effectively inhibit the penetration of corrosive solution and thus delay the corrosion process in comparison to the bare MgLi alloy.The higher total impedance|Z|of the coated MgLi specimens in Fig.15 also claims that the PEO coating can significantl improve the corrosion resistance.Furthermore,the corrosion resistance of the coating increases with treatment time from 1min to 3min.However,after 10min and 20min of processing the|Z|values decrease rapidly and show a similar trend with immersion time,which could ascribed to the high porosity of the coating after longer treatment.

Fig 12.Worn tracks of substrate materials,PEO coatings and counter parts on MgLi alloy:(a)MgLi alloy(unfinished)(c)10min and(e)20min;and on pure Mg:(b)pure Mg,(d)10min and(f)20min.

In Fig.14(b)the initial measurement(after 5min of immersion)of the coating on pure Mg after 1min of PEO treatment validates the increase of the total impedance in respective to pure Mg.However,the shrink of the capacitive loop at medium frequencies(～10Hz)correlated to the coating after 1h of immersion demonstrates the immediate deterioration of the coating.The morphology of the exposed region also reveals scattered corrosion pits at the exposed coating.For the coating obtained after 3min of PEO treatment after the immersion from 5min to 1h(Fig.14(c)),the similar behavior as the coating after 1min of PEO treatment reveals the degradation of the coating.The total impedance of PEO coatings on pure Mg after 10min and 20min of PEO treatments at initial immersion stage is two or three order of magnitude higher than that of the pure Mg substrate,ascribed to the dense inner layer.A capacitive impedance(Fig.14(d)and(e))observed at high frequencies(～105Hz)is associated with the evolution of the outer layers.The response of outer layer at high frequencies disappears gradually in course of immersion due to the penetration process through the thickness.The gradual degradation of the coatings can be inferred from the sharp shrink of the capacitive loop at medium frequency(inner layer)afterwards.PEO coatings on pure Mg show a slight improvement in corrosion resistance compared to bare Mg and the total impedance value increases with the treatment time from 1min to 20min.In summary,the evaluation of the PEO coatings on pure Mg demonstrates a better corrosion resistance in comparison to the coatings on MgLi alloy as revealed by the larger magnitude of total impedance at 0.1Hz(Fig.15).

Fig 13.EIS results of PEO coatings on MgLi alloy:(a)bare substrate,(b)1min,(c)3min,(d)10min and(e)20min.

4.Discussion

The alloying of Li(11.36wt.%)to Mg transforms the hexagonalα-Mg to a cubicβ-Mg.However,it is not possible to assign the observed changes in the PEO process directly to the lattice structure of the two materials since the presence of Li also changes the electrochemical stability of the alloy and of corresponding coatings further.Thus for the MgLi alloy a discrepancy in voltage-time response to the applied current density results in the different coating thicknesses under equal basic PEO processing conditions.The sluggish increase of the voltage for MgLi alloy does not delay the initiation of plasma discharges since the critical dielectric breakdown voltage is reached at the same treatment time around 15 to 30s,although about 40V lower than that of pure Mg.The reason for the lower breakdown voltage might be a higher conductivity of the coating due to the incorporation of Li in the initial conversion layer.Since the PEO process is following Faraday’s laws[62],a lower electrical resistivity of the barrier oxide layer is established on MgLi alloy against the fl w of current compared to pure Mg.The lower voltage of MgLi alloy also implies less energy input for the coating growth thus a thinner PEO coating in the results[63]in terms of constant current-driven process.The efficien y of the energy input(voltage)on coating growth with the duration of PEO treatment could be evaluated by the coating growth rate relative to the voltage.Hence,after 20min of PEO processing,the average efficien y of the energy input for MgLi alloy is 0.13μm/(V·min),higher than 0.11μm/(V·min)for pure Mg.There might be a positive role of Li for coating growth on MgLi alloy as well as energy conservation,since a lower voltage is sufficien to excite the species of MgLi alloy involving them in the plasma discharges during the process.

At the initial stage of PEO processing the conventional anodization is dominant for both substrates.The conversion products of MgLi alloy are preferentially accumulated on the MgLi2Al intermetallics,appearing mainly as fin protuberances due to aluminum oxide,which has better adherence to the substrate and higher Pilling-Bedworth ratio of 1.28 than MgO(Pilling-Bedworth ratio 0.804).In spite of the participation of Li in the plasma reactions,it seems that Li only presents in the primary conversion products at the interface between coating/MgLi substrate,as the corresponding Li oxide/hydroxide is freely soluble in aqueous alkaline solutions.After longer PEO processing(10min and 20min),there are the large amount of bright particulates formed on the coating of MgLi alloy around the micro-pores.The main coating composition(MgO and Mg3(PO4)2)have O/Mg atomic ratios of 1.00 and 2.67,respectively.The O/Mg atomic ratios of the areas without the particulates(B,C,E and F in Fig.6)falls into the range of 1.00-2.67 exactly evidencing the main phase composition of MgO and Mg3(PO4)2.Interestingly,the O/Mg atomic ratios of the particulates(A and D in Fig.6)are far beyond the range of 1.00-2.67 and even reach the values of 3.80 and 3.28 respectively,indicating overwhelming O at the bright particulates of the PEO coating on MgLi alloy.Combined with the higher concentration of element P and Na in the particulates,the sintering of the electrolyte(Na3PO4)on the surface of the coatings is reasonable due to the local high temperature.

Fig 14.EIS results of PEO coatings on pure Mg:(a)bare substrate,(b)1min,(c)3min,(d)10min and(e)20min.

Fig 15.Total impedance of MgLi alloy and pure Mg before and after different time of PEO processing at 0.1Hz.

After equal processing time,PEO coating on MgLi alloy always exhibits higher inner porosity,which can be partly associated with the type of discharges since the plasma reactions involving Li in comparison to pure Mg.Further,the lower stability of the PEO coating on MgLi alloy could be also responsible for the more porous coating,considering the rapid dissolution of the coating under the elevated temperatures and alkaline conditions,which appear in the coating pores.Additionally,the gas generation could be also responsible for the high porosity.The excess formation of oxygen gas during PEO processing can be trapped within the coating and present as coating porosity due to the quick solidificatio of the melted coating.

After PEO treatment,the wear and corrosion resistance of the coating evolves,compared with the uncoated MgLi alloy and pure Mg.Nevertheless,they are mostly dependent on the microstructure of the coatings such as the coating thickness,uniformity,porosity and other defects.In our case,the higher porosity within the PEO coatings on MgLi alloy having lower thickness particularly after longer duration of PEO treatment(10min and 20min)results in the less resistance of the coatings in comparison to pure Mg.

5.Conclusion

(1)The presence of Li and the Li containing intermetallics modifie the behavior of the PEO process on the MgLi alloy.A lower breakdown threshold of the initial dielectric layer on MgLi alloy maintains the progress as that of pure Mg in terms of the slower increase of the voltage for the MgLi alloy.The efficien y of energy input(voltage)for coating formation on MgLi alloy is higher than that of pure Mg.

(2)PEO coatings were mainly composed of MgO and Mg3(PO4)2.Though the Li-containing phase was not found in the coatings of MgLi alloy,Li was probably incorporated in the primary conversion products at the interface of coating/MgLi substrate during the processing.

(3)Due to the less energy input(voltage),the sintered particulates from the insufficien recrystallization around the micro-pores on the surface of PEO coatings on MgLi alloy,is mainly originated from the electrolyte.

(4)The wear and corrosion resistance of the PEO coatings on MgLi alloy and pure Mg were improved compared with the uncoated substrates,but especially the corrosion resistance was quite poor for all coatings.The less stable coatings of MgLi alloy having the higher inner porosity even present the lower resistance to wear and corrosion.

Deceleration of Competing Interests

None.

Acknowledgement

Ting Wu would like to acknowledge China Scholarship Council for the award of fellowship and funding(NO.201708510113).The technical support of Mr.Volker Heitmann,Mr.Ulrich Burmester and Mr.Wiese Gert during this work is gratefully acknowledged.X.Lu would like to acknowledge the financia support from National Natural Science Foundation of China(NO.52071067 and U1737102),Mobility Programme of the Sino-German Center(M-0056)and the Fundamental Research Funds for the Central Universities(N2002009).