Ting Wu ,Crsten Blwert ,Mri Serdehnov ,Polin Krlov ,Gle Dovzhenko ,D.C.Florin Wielnd ,Mikhil L Zheludkevihd

a Institute of Surface Science,Helmholtz-Zentrum Hereon,Max-Planck Strasse.1,Geesthacht 21502,Germany

b Institute of Materials Physics,Helmholtz-Zentrum Hereon,Max-Planck Strasse.1,Geesthacht 21502,Germany

c Institute of Metallic Biomaterials,Helmholtz-Zentrum Hereon,Max-Planck Strasse.1,Geesthacht 21502,Germany

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

Abstract This work reports the influence of alumina fiber reinforcement of an AZ91Nd MMC (metal matrix composite) on the PEO coating formation process in a sodium phosphate-based electrolyte.By comparison with the pure AZ91Nd,the evolution of alumina fiber during the processing and the characteristics of the resultant PEO coating were investigated.The voltage response as a function of processing time was changed.Lower voltage in the presence of alumina fiber is responsible for the lower coating thickness.The morphology and phase composition of the coatings are also influence by the incorporation of the fiber.Firstly,the fiber is embedded in the coating and interrupts the continuity of the coating.With increasing processing time,the fiber is found to be reactively incorporated in the coating.The intention to produce a MgAl2O4 containing coating is achieved and it is mainly accumulated near the coating surface.However,due to the low number of fibers the Al content is overall still low and only near to the fiber the MgAl2O4 spinel phase can form.

Keywords: Plasma electrolytic oxidation;Mg alloy;Metal matrix composite;Al2O3;MgAl2O4.

1.Introduction

Plasma electrolytic oxidation (PEO) is a promising surface engineering technology for Mg alloys to enhance the surface properties,renowned for the ease of operation and environmentally friendly process [1-8].In the case of AZ91 Mg alloys,PEO processing allows the formation of a ceramic-like coating of generally tens of microns thick [3,6,9].PEO coatings can eradicate the surface heterogeneity of AZ91 Mg alloys,which is caused by the dual-phase microstructure (α -Mg and more inertβ -Mg17Al12precipitates) [10,11].In addition,the PEO coatings on AZ91 Mg alloys are typically layered[12,13] and the inner barrier layer towards the substrate of a few hundred nanometers thick enables the mitigation of the corrosion process from delaying the penetration of corrosive media [1,11,14,15].

The formation of PEO coatings on Mg alloys involves chemical,electrochemical,plasma,and thermal activated processes in combination with the transition among solid,liquid,gas and plasma states of the species,originated from substrate and electrolyte [16-18].Thus,the process is strongly dependent on the applied voltage/current and the composition of substrate and electrolyte.Many combinations of electrical parameters and electrolytes have been studied extensively for the last decades,in order to improve the performance of PEO coating on Mg alloys [16,19-23].Some effective approaches,such as adding ceramic particles [24,25],applying bipolar electrical pulses [26,27] and post-healing process[9,28] have been proposed.However,in the case of Mg substrates,the role of alloying additions and the effect of the presence of impurities,second phases and ceramic reinforcement in the alloy microstructure during PEO processing are scarcely discussed.

In fact,different anodic behavior of α-Mg and β-Mg17Al12in AZ91 Mg alloys have been observed during PEO processing [6,29].The discrepancy is stemming from the nature of the two phases as well as the corresponding conversion products formed during initial conventional anodizing [11,30,31].At the very beginning,the oxidation and deposition starts on the α-Mg and then extends to the β-Mg17Al12intermetallics due to the electrochemically more active α-Mg.The respective conversion products of α-Mg and β-Mg17Al12present discernible local morphologies and have different dielectric properties.As a result,the discharge properties,e.g.intensity and number,which depend on the breakdown of the conversion film change at different locations [11].It was also observed that the intensity of discharges is stronger in the presence of Al in Mg alloys [3,12,32-34].Furthermore,the MgAl2O4spinel phase has been identified in the PEO coatings on AZ91 Mg alloys [6,34-36].The contribution of MgAl2O4to the corrosion resistance and the hardness of PEO coatings has been reported in some works [37-40].Nevertheless,without additional Al supply,the nominal 9.0 wt.% Al of AZ91 Mg alloys is not sufficient to synthesize MgAl2O4in a large amount during PEO processing [41].

AZ91Nd matrix reinforced with alumina fiber is a metal matrix composite(AZ91Nd/Al2O3MMC),which significantly enhance the mechanical strength of the resulting materials[42,43].Due to the introduction of alumina fibers a large number of different material phases and thus interface combinations are introduced into the AZ91Nd/Al2O3MMC.To the best of our knowledge,no published research has put emphasis on the PEO process on AZ91Nd/Al2O3MMC and the role of ca.20 vol.% alumina fiber on the coating formation.In the published studies of PEO treatment on Mg-based MMCs [5,44-48],reinforcement phases of SiC and Al2O3-SiO2were reactively incorporated in the coating as silicate and aluminate.Hence,whether or not the alumina fiber of AZ91Nd/Al2O3MMC facilitates the formation of MgAl2O4in the PEO coating is worthy of being studied.

This work focuses on PEO processing and coating evolution of AZ91Nd/Al2O3MMC compared with pure AZ91Nd Mg alloy in a sodium phosphate-based electrolyte.The modification of oxidation behavior in the presence of alumina fiber and the stability of the alumina fiber during the processing was studied at different treatment times.The phase composition and distribution across the thickness of the coatings on AZ91Nd/Al2O3MMC and on AZ91Nd Mg alloy were investigated via micro-focused X-ray diffraction experiments.In addition,the MgAl2O4formation in the coatings was compared for the mentioned substrates in sodium phosphate and phosphate-aluminate electrolytes to understand the role of different Al sources on phase formation.

2.Experimental

As-received AZ91Nd Mg alloy was selected as the matrix.The composition was 7.16 wt.% Al,0.57 wt.% Zn,1.14 wt.% Nd,0.21 wt.% Mn,0.0049 wt.% Fe,0.0044 wt.%Cu,0.0006 wt.% Ni and Mg balance analyzed using Arc Spark OES (Spark analyze M9,Spectro Ametek,Germany).The samples of AZ91Nd/Al2O3MMC (18.022 wt.% Al)were taken from as received squeeze cast blocks consisting of 50 vol.% Saffil alumina fiber reinforcement and 20 vol.% alloy.The short alumina fiber (95.0-97.0% δ-Al2O3,3.0-5.0% silica binder and＜0.5% trace elements)are approximately 3 μm in diameter and up to 100 μm in length.

Rectangular specimens (15 mm×15 mm×4 mm) of AZ91Nd/Al2O3MMC and as-received AZ91Nd were used as the working anodes.PEO processing was performed using a pulsed DC power source under a constant current density of 6 A/dm2with a duty cycle of 10% (1 ms:9 ms) and a frequency of 100 Hz.The basic alkaline electrolyte was prepared via dissolution of Na3PO4(20 g/L) and KOH (1 g/L) in deionized water under continuously stirring in order to keep the solution homogeneous.A cooling system,using a stainless steel tube cathode,was employed to control the temperature(10±2 °C) of the electrolyte.Before and after the PEO processing,the specimens were rinsed with deionized water and dried with compressed air.Processing times of 15,30,60,300,600,and 1200 s were selected,respectively.During the processing,the voltage as a function of processing time was recorded using the SignaSoft 6000 software package,Gantner,Germany.For understanding the role of different Al sources in the synthesis of MgAl2O4based PEO coatings on pure AZ91Nd,PEO treatment was additionally employed in a phosphate-aluminate electrolyte(Na3PO410 g/L,NaAlO215 g/L and KOH 1 g/L) for 600 s and 1200 s,respectively under the same electrical PEO parameters.

The microstructure of the substrates with and without Saffil alumina fiber reinforcement were examined applying optical microscopy (OLYMPUS BX53M),scanning electron microscopy (SEM,TESCAN Vega3 SB) and X-ray diffractometry (XRD,D8 Advance,Bruker AXS),respectively.For the XRD measurement,the following parameters were selected:diffraction angle 2 between 20° and 80°,step size of 0.02°,acquisition time of 1 s per point and grazing angle of 1°

Fig.1.Schematic scanning diagram of each point of analysis (1.5×1.5 μm,0.5 s) via synchrotron X-ray diffraction giving a total scanning area of 80×80 μm across three-phase of air/PEO coating/substrate.

For the characterization of PEO coatings,surface and cross-sectional morphologies,micro-pore size (at 500×magnification and elemental distribution for the MMC and the pure AZ91Nd alloy as a function of processing time were investigated using a SEM combined with an energy dispersive spectrometer system (EDS,eumeX IXRFsystems).The coating thickness measurements were performed with a coating thickness gage (MiniTest 2100,ElektroPhysik),which was validated from the cross-section observations of the SEM.The phase composition of the coatings was obtained from the aforementioned XRD measurement.Further investigation of the phase formation of the PEO coatings formed in the phosphate electrolyte after 600 s processing was carried out at Deutsches Elektronen-Synchrotron (DESY,Hamburg,Germany)using the nano-focus end station of the beamline P03 at PETRAIII storage ring [49].The measurement set-up is illustrated in Fig.1.An Eiger 9 M detector was used,which has a pixel size of 75 μm by 75 μm.The X-ray beam had an energy of 19.7 keV and was focused to a beam size of 1.5 μm by 1.5 μm.A mesh scan of the interface was performed with 40 steps in the direction perpendicular to the interface with 2 μm step size and 20 steps parallel to the interface with a step size of 4 μm.This resulted in a measured area of 80×80 μm.We have chosen this setting as a resolution parallel to the interface was not necessary and would have cost too much time.At each point,a diffraction pattern was acquired for 0.5 s.Data reduction was done by employing in-house developed MAtlab? code and the PyFAI [50] routine.The scattering angles of the diffraction patterns were transformed to Cu K-alpha radiation for simplicity.To access the phase distribution across the coating thickness,a lab-made script in Python programming language,operated in the “Spyder” package software environment included with Anaconda Navigator,was used for analyzing and generating the two-dimensional phase mapping graphics.

3.Results

3.1.Microstructure of substrates

Fig.2 shows the XRD patterns of pure AZ91Nd and AZ91Nd/Al2O3MMC.In addition to the phases of α,β,Al-Mn and Al-Mn-Nd,which are present in both materials,the additional diffraction peaks of δ-Al2O3from the reinforcement fiber are found in the MMC.Al-Mn and Al-Mn-Nd intermetallics can hardly be differentiated due to the relatively similar values of their diffraction peak positions.In the MMC,Mg2Si was recognized in LM and SEM studies and consistent with the atomic ratio of Mg/Si in EDS analysis (Fig.3(b) and (d)).However,it is not detected during XRD measurement because of the relatively low content.The residual Mg2Si is attributed to the interaction between Mg substrate and the silica binder in Saffil alumina fiber during the squeeze casting process.The metallographic images of the two substrates with and without the alumina fiber are given in Fig.3,which are consistent with the phase analysis.Larger solid precipitated at the grain boundaries of the matrix are visible,surrounded by eutectic + morphology.Particulates of Al-Nd intermetallics and aciculae Al-Mn-Nd intermetallics are dispersed within the grains,respectively.Nevertheless,for the MMC,Al-Mn-Nd intermetallics are recognized less frequently,and the alumina fiber exhibit different orientations.

Fig.2.XRD patterns of AZ91Nd and AZ91Nd/Al2O3 MMC.

3.2.PEO process

The voltage responses as a function of processing time for AZ91Nd and AZ91Nd/Al2O3MMC in phosphate electrolyte are displayed in Fig.4(a),respectively.Four stages of the PEO process can be distinguished as listed in Table 1.For the pure AZ91Nd,the voltage at the firs stage is ramping up linearly at the rate of ca.30 V·s-1,indicating the rapid formation of a dielectric passive film on AZ91Nd.The voltage is increasing to maintain the pre-set current density.The second stage is established once reaching the breakdown voltage of ca.150 V.The voltage ramp at this stage decreases to the average rate of ca.2.5 V·s-1,accompanied by short-living discharges firstly appearing on the surface of AZ91Nd.At the third stage,the voltage rise is at a lower rate of ca.0.4 V·s-1,also the size and color of the discharges are changing as the initial small white discharges become larger and orange.With further increase of treatment time,the ramp of the voltage reduces further (0.07 V·s-1) at the last stage and further evolution of the discharges is observed,as the orange-yellow sparks are getting more stationary and larger at local regions.The final voltage reaches a value of 530 V after 1200 s.

Table 1Voltage-time parameters at different stages of PEO treatment.

Table 2Average composition (at.%) of PEO coatings by EDS analysis of the coating surface.

Fig.3.Optical and SEM (BSE) micrographs of AZ91Nd:(a),(c) and AZ91Nd/Al2O3 MMC:(b),(d).

Fig.4.Evolutions of voltage (a) inserted with detailed voltage change from 0 to 25 s and coating thickness (b) as a function of PEO processing time in sodium phosphate-based electrolyte.

Obviously,the evolution of the voltage for AZ91Nd/Al2O3MMC is different from the pure alloy.Following a surge to 60 V within the firs 1 s of treatment,the voltage undergoes a short delay (ca.10 s) as seen in the inset of Fig.4(a) during the firs stage.The delay is associated with the defects in the dielectric film due to the alumina fibers presence.Afterward,the rapid voltage increase resumes,and the average rate of the voltage ramp for the firs stage is ca.6 V·s-1.It takes 20 s more to reach the breakdown voltage(150 V)compared with pure AZ91Nd.At the second stage,the average rate (2.3 V·s-1) for the voltage increase is similar to the pure AZ91Nd (2.5 V·s-1).Nevertheless,the distribution of the initial discharges is not as uniform as on the pure AZ91Nd.It can be speculated that this is attributed to the disturbance of the alumina fibers With further processing(at the third stage),the combination of fast-moving tiny sparks and long-lasted large sparks dominates the surface of the MMC.The voltage increase of the MMC at this stage declines to ca.0.25 V·s-1,which is lower than that of AZ91Nd.After reaching a maximum of 470 V (775 s),the process enters the last stage.The voltage is surprisingly dropping down with increasing fluctuations to a final value of ca.425 V after 1200 s.It is possibly associated with the instability of the coating/electrolyte interface,such as the flaking-o f of the coating.

The coating thickness of AZ91Nd and AZ91Nd/Al2O3MMC in phosphate electrolyte increases with processing time,as indicated by Fig.4(b).The coating on the MMC is growing faster than that of AZ91Nd for the firs 600 s of processing after the occurrence of discharges.However,after 1200 s of treatment,the coating thickness of the MMC is lower than that of the pure AZ91Nd and even no further increase is observed for the last 600 s.It could be assigned to a local detachment of the outermost layer on the MMC due to thermal stress and/or poorer interface,larger pore bands,which also leads to the oscillation of the voltage at the last stage.

3.3.Surface morphologies

Fig.5(a) reveals the micro-pores on the coating surface of AZ91Nd alloy,indicating the occurrence of discharges already after 15 s of processing,which is in agreement with the voltage exceeding the threshold for breakdown.In addition,discernible local morphologies of the coating surface are present correlating with the presence of the intermetallics.The boundaries of the intermetallics are still visible via different contrast of the elements from the substrate (Al,Mn,Zn and Nd),due to the penetration of electrons through the relatively thin coating.A more intense sintering morphology and larger micro-pores of the coating are found on the β phases,confirming the stronger discharges at the regions of abundant Al.Furthermore,the conversion products are preferentially accumulated around the Al-Mn-Nd particulates and seem to be rich in O and P.The strong cathodic effect of Al-Mn-Nd particulates relative to Mg matrix possibly accelerates the dissolution and oxidation of Mg during conventional anodization [11].The heterogeneity of the local morphology is disappearing after 30 s of processing,and the size of micropores is in the range of ca.0.5-3.6 μm (Fig.5(b)).With the increasing treatment time (60 and 300 s),the elemental distribution on the coating surface also becomes uniform(Fig.5(c) and (d)).The size of micro-pores on the coating surface increases further,in the range of ca.0.6-6.1 μm for 60 s and ca.0.7-24.5 μm for 300 s,respectively,and the number of micro-pores decreases correspondingly.After 600 and 1200 s of treatment,the size range of micro-pores on the coatings are of ca.0.8-27.8 μm and ca.0.8-34.5 μm,respectively.It should be noted that there are always two discernible types of micro-pores on the coating surface on different substrates due to the visible difference of their sizes.This is the reason for the wider range of the micro-pore size especially for the coatings processed for longer treatment.A few radial-shaped micro-pores appear on the coating surface highlighted in Fig.5(e),which are possibly stemming from strong gas pressure release pushing out melted coating material.

Fig.5.Surface morphologies (BSE) and elemental distribution (EDS) of PEO coatings on AZ91Nd in sodium phosphate-based electrolyte after different processing time:(a) 15 s,(b) 30 s,(c) 60 s and (d) 300 s,(e) 600 s and (f) 1200 s.

As seen in Fig.6(a),after 15 s,dissolution and deposition processes have started on the matrix as well as on the intermetallics of the MMC.The elements from the electrolyte(P and Na) are indicators of deposition on the surface.However,different to the rapid development of the coating on pure AZ91Nd,the continuity of the coating on the MMC is interrupted by the presence of alumina fibers and the discharge formation is retarded.This is consistent with the delayed voltage ramp during the initial conventional anodization.Micropores (ca.0.3-2.3 μm) are firstly appearing on the surface of the MMC after 30 s of processing,as shown in Fig.6(b).Different local morphology on the different intermetallics is also found,which is similar to that of AZ91Nd.Furthermore,the boundaries of alumina fiber are still visible,and the alumina fiber are embedded in the coating.The fiber are still more or less inert at this stage,and no continuous coating is forming.

After 60 s of treatment (Fig.6(c)),micro-pores with an increased size in the range of ca.0.3-6.9 μm dominate the coating surface on the MMC.Few micro-pores are firstly recognized on the alumina fiber,and only the accumulation of Al at some regions indicates traces of the fiber underneath the coating.As depicted in Fig.6(d),a uniform coating is finally formed on the MMC after 300 s of processing,with homogeneous elemental distribution and a further increased micropores size of ca.0.6-28.4 μm.Compared with the coatings on AZ91Nd after 600 s and 1200 s of processing (Fig.5(e)and (f)),the coatings on the MMC are more porous (Fig.6(e)and (f)),with the micro-pore sizes in wider ranges of ca.0.6-30.7 μm and ca.0.6-42.5 μm,respectively.Similarly,radialshaped micro-pores are also discovered even more frequently on the coating of the MMC.This may indicate that the discharges generate more melt volume due to a change of the main coating composition by the reaction of the fiber with the coating phase.

3.4.Cross-sectional morphologies

Examinations of the cross-sections of the coatings on AZ91Nd following the processing of 60,300,600 and 1200 s in the phosphate electrolyte are displayed in Fig.7.The increase of coating thickness with treatment time agrees with the observation of the coating thicknesses shown in Fig.4(b).A horizontal pore band is already formed after 60 s of treatment,and the coating is somehow separated by it (Fig.7(a)).The strong contrast of Al-Nd and Al-Mn-Nd intermetallics in BSE mode allow deducing their location in the coating regions.The inset in Fig.7(a) illustrates that the chunks of the intermetallics are embedded in the coating near the interface of the coating/substrate.After 300 s of processing(Fig.7(b)),it is found that the coating materials converted from the Al-Nd and Al-Mn-Nd intermetallics are preferentially distributed around micro-pores.As the micro-pores result from the discharges,the local regions of the coating with elemental heterogeneity probably ease the discharging.Furthermore,it seems as if the barrier layer in or close to the intermetallics are the locations for ignition of stronger discharges,which form the larger pores,as shown in Fig.7(b).With further treatment (Fig.7(c)),the size of pore bands is surprisingly diminishing,and the element distribution across the coating thickness is becoming uniform towards the electrolyte.The highest thickness coating after 1200 s of treatment (Fig.7(d)) presents the most uniform cross-sectional morphology and elemental distribution.

The cross-sectional observations of the coatings on the MMC after different processing times (60,300,600 and 1200 s) are shown in Fig.8.The evolution of intermetallics during the processing of the MMC resembles that of AZ91Nd.However,the coatings are more porous with considerable micro-cracks and,thus,more stressed compared with the coatings on AZ91Nd.An origin for this might be the more frequent and/or intense discharges in the presence of alumina fiber.After 60 s of processing,a discharge channel close to alumina fiber shown in Fig.8(a) agrees with the observation of micro-pores in the coating surrounding an alumina fiber in Fig.6(c).However,the alumina fiber remain stable in the coating visible by the clear boundary between alumina fiber and coating.With the coating growth (Fig.8(b)),the distribution of Al,similar to Nd,is dispersing around the alumina fibers demonstrating the local dissolution or reactive incorporation of alumina fiber after 300 s of processing.It can be concluded that a voltage in the approximate range of 280-370 V (60-300 s) is necessary to start the reactive incorporation via reaction of fiber with the surrounding melt.In the phosphate electrolyte,MgAl2O4spinel is confirmed by X-ray diffraction in the coating on the MMC (Fig.9(b))and supports the reaction process stimulated by the assistance of intense discharges [17,51].However,at the interface between the coating and substrate,the passive alumina fiber interrupts the continuity of the coating.After 600 s of processing (Fig.8(c)),the elemental distribution of the coating is becoming uniform towards the coating surface,but heterogeneity remains at the interface of coating/substrates,where the alumina fiber and intermetallics are present.With further treatment (Fig.8(d)),horizontal pore bands are formed in the coating,and the heterogeneous elemental distribution is disappearing.However,a large number of micro-cracks indicate that the coating neither have good cohesion nor adhesion to the substrate.A local peel-off caused by cohesion problems could be the reason for the lower thickness of the coating compared to pure AZ91Nd after 1200 s of processing and almost no thickness increase compared to 600 s processing of the MMC.

Fig.6.Surface morphologies (BSE) and elemental distribution (EDS) of PEO coatings on AZ91Nd/Al2O3 MMC in sodium phosphate-based electrolyte after different processing time:(a) 15 s,(b) 30 s,(c) 60,(d) 300 s,(e) 600 s and (f) 1200 s.

Fig.7.Cross-section morphologies (BSE) and elemental distribution (EDS) of PEO coatings on AZ91Nd in sodium phosphate-based electrolyte after different processing time:(a) 60 s,(b) 300 s,(c) 600 s and (d) 1200 s.

Fig.8.Cross-section morphology (BSE) and elemental distribution (EDS) of PEO coatings on AZ91Nd/Al2O3 MMC in sodium phosphate-based electrolyte after different processing time:(a) 60 s,(b) 300 s,(c) 600 s and (d) 1200 s.

Fig.9.XRD patterns of PEO coatings after different treatment times:(a) AZ91Nd and (b) AZ91Nd/Al2O3 MMC in phosphate electrolyte.

3.5.Elemental and phase composition of PEO coatings

Table 2 provides the elemental composition of the coatings near the surface analyzed by EDS.In the case of the sodium phosphate-based electrolyte,the concentration of the coating forming elements Mg and Al are from the substrate decreasing with reaction time.However,the change of elements O,P and Na from the electrolyte shows a reverse trend with increasing treatment time to 60 s.Afterward,the concentration of all elements is constant.The highest Mg content after 15 s of processing is associated with the penetration of electrons into the substrates through the thin conversion layer.There is a significant discrepancy of the oxygen fraction between the coatings on the pure alloy (31.72 at.% and 51.27 at.%) and the MMC (17.98 at.% and 39.19 at.%) after 15 s and 30 s,respectively.The lower oxygen concentration observed in the coatings on the MMC confirm the delay of the oxidation process at the initial stage due to the presence of alumina fibers which reduces the initial growth rate of the coating.After 60 s of treatment,the outward growth of the coating on the MMC leads to the sharp decreasing of Al content from 12.28 at.% to 1.70 at.%,without interacting with the fibers For other treatment times,the constant Al level in coatings on the MMC is slightly higher compared to the pure alloy,suggesting the mixing with the coating materials and dilution due to coating formation of MgO and phosphate.

Fig.10.XRD patterns of PEO coatings on AZ91Nd in phosphate-aluminate electrolyte after 600 s and 1200 s of processing.

The phase composition of the PEO coatings on the MMC and the pure AZ91Nd in phosphate electrolyte is illustrated in Fig.9,respectively.The diffraction signal of all the samples processed within 60 s is dominated by the scattering from the substrates since X-rays are easily penetrating the thin coatings and reach the substrate underneath.The broad bumps of amorphous phosphate [52] and crystalline MgO start to appear in the two theta range between 20° and 35° after 60 s of treatment.Afterwards,the coatings on AZ91Nd and MMC reveal the formation of an Mg3(PO4)2phase after 300 s,600 s and 1200 s of processing.Furthermore,some diffraction peaks are recognized as MgAl2O4in the coatings on the MMC,confirming the reactive incorporation of alumina fiber as the only additional source of Al.Overall SEM,EDS and XRD do suggest that the coatings are a mixture of nano-crystalline MgO,Mg3(PO4)2phases,where in the case of the MMC some of the MgO has reacted with the Al2O3of the fiber forming MgAl2O4.

In contrast to the presence of alumina fiber in the AZ91Nd matrix,the phosphate-aluminate electrolyte can provide an alternative and more effective Al source for the formation of MgAl2O4spinel phase in the PEO coating on pure AZ91Nd.The highest atomic concentrations of Al demonstrated in Table 2 are formed for the coatings on AZ91Nd in the phosphate-aluminate electrolyte after 600 s and 1200 s of treatments.Fig.10 reveals that the coatings are crystalline and composed of primary MgAl2O4,MgO and a little amount of Mg3(PO4)2,indicating that MgAl2O4-based PEO coatings are achieved with the addition of aluminate in the electrolyte much more efficiently .The cross sectional morphology and elemental distribution of the coatings are illustrated in Fig.S1.There are fewer micro-pores present within the coatings,compared to the coatings formed in sodium phosphate-based electrolyte.Additionally,the distribution of Al is uniform and shows a strong contrast in the coatings.

3.6.Synchrotron X-ray diffraction

The measurements utilizing a nano-focused X-ray beam at the P03 have been conducted to understand the phase distribution of the coatings.Fig.11 presents the high-resolution localized XRD patterns of the coatings on AZ91Nd and the MMC after 600 s of processing in phosphate electrolyte.The XRD patterns are selected from one line of the scan across the coating from the surface to the substrate,as illustrated in Fig.1.The phase composition of the coatings is consistent with the results in Fig.9,and the diffraction peaks in the two theta range of 30-40° are more discernible in Fig.S2.From the diffraction patterns of the completely scanned areas (80×80 μm),the local intensity of the primary coating phases including MgAl2O4(440),MgO (200) and Mg3(PO4)2(110) are extracted.The two-dimensional maps are plotted in Fig.12 to shows the spatial intensity of the phases across the coating thickness.The distribution of Mg (101) from the substrates is also displayed to illustrate the location of the substrate.For the coating on AZ91Nd,it is evident that MgO is the primary phase and almost uniformly spreading through the entire thickness of the coating.The coating to substrate and coating to air interface can be clearly separated by the intensity change of MgO and Mg.The coating thickness is estimated to be 60 μm,which is higher compared to the cross-sectional observation.This difference could be attributed to the wavy interfaces of the coating/substrate(Fig.7(c)).The distribution of Mg3(PO4)2is similar to that of MgO,however,with lower intensity.In addition,there is only background noise of MgAl2O4.For the coating on the MMC,a MgAl2O4rich layer of ca.25 μm is formed near the surface in Fig.12(a),demonstrating the transformation of the fiber near the coating surface in Fig.8(c).In contrast to the coating results on AZ91Nd substrate,the coating on MMC shows another phase distribution as the phases of MgO and Mg3(PO4)2appear to accumulated preferentially in the MgAl2O4rich layer.Moreover,the intensity of MgO is lower than that of Mg3(PO4)2.This confirm the reactive incorporation consuming MgO while forming MgAl2O4.The low intensity of the phases in the other region of the coating near the substrate could be ascribed to the presence of pore bands and the remaining fibers as displayed in Fig.8(d).The coating/substrate interface is quite rough since the intensity of Mg from the substrate loses its sharpness.The observation of the coating thickness,of ca.50 μm,is lower than the aforementioned measurement,possibly due to the disturbance of alumina fiber at the coating/substrate interface.

Fig.11.High-resolution XRD patterns of a selected line perpendicular to the interface (air-coating-substrate) for the PEO coatings on AZ91Nd (a) and on AZ91Nd/Al2O3 MMC (b) in phosphate electrolyte after 600 s of processing.

4.Discussion

The main intention of the study was to clarify the effect of the alumina fiber reinforcement in AZ91Nd matrix on the formation of PEO coatings during PEO processing in a phosphate electrolyte.From the comparison with pure AZ91Nd,it was demonstrated that the presence of alumina fiber influ ences the growth of the coating by affecting the PEO process itself as well as the phase formation,as the fiber are reactively incorporated into the coating.The delay of voltage ramp at the initial period for the MMC is found to be the firs influence on the process due to the alumina fiber interrupting the continuous formation of the dielectric film The defects at the interface between the alumina fiber and dielectric film serve as “shortcuts”,through which the electrolyte is easily penetrating the gap between the alumina fiber and the matrix,allowing current to flow without voltage increase[43,53].As a result,the formation of initial conversion coating on the surface of the MMC as well as the rapid voltage increase is suppressed at this stage.Subsequently,the layer thickness increase is driven by the polarization and,thus,the resistance of the forming film improves.The shortcuts are being closed,as the fiber are better integrated into the growing film After the occurrence of discharges,the growth of the coating in terms of thickness is accelerated in the presence of alumina fiber within the firs 600 s of processing.However,the decrease and fluctuation of the voltage at the last stage indicate further changes of coating development on the MMC.Lower coating thicknesses compared to pure AZ91Nd and actually no further increase in the last ten minutes of the treatment might be related to the high stresses and defects in the coating,which is consistent with the observation of the coating cross-sections of the MMC (Fig.8) showing extensive porosity,extended pore band and micro-cracks.This could cause a local flakin off,and rebuild of the coating,which can further impedes the thickening of the coating.It should be noted that this is also consistent with the voltage drop and fluctuation at the end of the processing.

Fig.12.2D maps of phase distribution across coating thickness (600 s) on AZ91Nd:(a) MgAl2O4,(c) Mg3(PO4)2 and (e) MgO and on AZ91Nd/Al2O3 MMC:(b) MgAl2O4,(d) Mg3(PO4)2 and (f) MgO in phosphate electrolyte.

For the PEO processing of Mg alloys,the main steps of coating formation Eqs.(1)-((3)) [32] following are taking place near the substrate while the coating is growing inwards:

Due to the high pressure in the discharges,the coating melt produced next to the substrate erupts outwards to the coating surface rapidly via the discharge channels.Directly at the interface also the reaction with phosphate occurs.

The finding of the present study suggest that the alumina fiber were involved in the coating formation at a voltage higher than 280 V.In the beginning,the fiber are stable enough that the coating can only grow around them even though the voltage was up to 280 V (breakdown voltage 150 V).Only when a higher voltage in the approximate range of 280-370 V(60-300 s)was reached,the reactive incorporation of the fiber into the coating started,as the alumina fiber close to the surface were disappearing and MgAl2O4spinel was identified The cross-sectional observation suggests the reactions initiated at the interface of fiber/coatin and similar evolution of SiC fibers/particle were reported in [5,45-48].The XRD peak intensity of MgAl2O4was rising with processing time/voltage.From the intensity development of the XRD peaks of the MgO phase,we speculate that this phase is involved in the spinel formation.Its intensity increased during a processing time of 300 s with a subsequent decrease,indicating its consumption.The MgAl2O4spinel formation was reported in the sintering process of MgO and Al2O3at temperatures of 900-1200 °C [54,55].This is much lower than the melting points of MgO (2827 °C),Al2O3(2054 °C)as well as spinel (2105 °C) [56].It can be inferred that the temperature in the discharges was sufficient for the formation of the spinel as the estimated temperature ranges from 4000-33,000 K in the PEO process [57].Correlated to the applied voltage,the formation/sintering process only occurred at a voltage exceeding 280 V.Due to the short-living discharges and the rapid quenching effect of the electrolyte,the effective temperatures needed for sintering can be reached only when the external applied voltage exceeded 280 V.Therefore,we suggest the following MgAl2O4formation reaction above 280 V:

Further increase of the voltage accelerates the formation process.A single alumina fiber located across the coating thickness displayed in Fig.8(c) indicates the continuous reconstruction of the coating via the discharges and step by step dissolution of the fiber.The reactive incorporation of the fiber commencing near the coating surface increases the composition/microstructure mismatches within the coating,which is visible by the non-uniform distribution of Al across the coating thickness.The discharges are preferred to occur at the locations of defects,which are weaker and have lower electrical resistance,like the coating interface around the fibers Thus,discharges appear close to the fibers but all Al oxides involved in the reaction and the spinel are quite temperature stable phases and concentration gradients remain much larger as melt volumes and/or diffusion in the solid is much more restricted.As a result,unlike the homogeneous coating on the alloy,the coating on the MMC consists of much more defects,eg.micro-pores,micro-cracks and wide pore bands,and concentration gradients which are disappearing much slower with processing time.

Regarding the large size and volume of alumina fibers the accumulation of MgAl2O4,mainly at the outer layer,is related to the reaction of the fiber and melt towards the surface,where the largest number of discharges were occurring considering an inward growth of the coating.Additionally,the sintering of MgAl2O4from the mixing oxides of MgO and Al2O3was reported to cause a 5-7% volume expansion [58],which could be a reason for the highly stressed coatings.

The free energy changes for the formation of MgO,Al2O3and Nd2O3at 0 K are -1138.6 KJ·mol-1,-1054.9 KJ·mol-1and -1147.4 KJ·mol-1per mole oxygen,respectively.With the increasing temperature,the rate of decrease of this energy change is still similar [59].Therefore,the lateral dispersion of Nd surrounding the Nd-containing intermetallics at the coating/substrate interface indicates higher stability of the intermetallics with respect to the -Mg.The oxidation process seems to take place at the interface between the coating and intermetallics in every single discharge event.However,not involving complete melting of the intermetallics since the melting point of Al-Nd intermetallics could be higher than 1400 °C [60].

It has been reported that the introduction of MgAl2O4spinel into the PEO coatings through the addition of Al source in the electrolytes can significantly improve the corrosion performance [38,61-63].One of the reasons is a lower porosity of MgAl2O4based PEO coatings [62].Furthermore,MgAl2O4spinel also has many other attractive properties,including a high density (3.58 g/cm3),high hardness (16 GPa),high mechanical strength,high melting point (2135 °C) and high chemical resistance [64,65].The crystal structure of MgAl2O4spinel is based on the structure of diamond,which can explain its good combination of properties and ensure the good stability in corrosion environments [64].The introduction of Al2O3to the matrix is a suitable way to increase the Al content in the alloy and stimulates the formation of spinel.A MgAl2O4spinel-rich coating was achieved for the AZ91Nd/Al2O3MMC.In the case of bare AZ91Nd alloy,it is possible to produce a spinel-based coating by introducing alternative Al source via using a phosphate-aluminate electrolyte.The presence of aluminate in the electrolyte is reported to moderate the discharges and,thus,gives rise to a low porosity of the coatings [66].The formation process of MgAl2O4in the coating of AZ91Nd alloy involving the aluminate originated from the electrolyte is different from the reinforcement alumina fiber,and the reactions are suggested as follows Eqs.(4)-((6)) [37]:

Without the disturbance of large size/volume alumina fibers there is a more rapid mixing process of MgO and Al2O3for the formation of MgAl2O4in Eq.(4).Also,there is more Al2O3available for the reaction which is also the more homogeneously distributed in the coating.All this results in the uniform distribution of MgAl2O4as the main coating phase in case of AlO2-addition from electrolyte.Summarizing,MgAl2O4can be formed from Al source from the substrate,but from the practical point,the electrolyte is the more preferential approach.

5.Conclusion

(1) The presence of alumina fiber in the AZ91Nd matrix influence the PEO process by firstly interrupting the continuous formation of the dielectric film thus,leading to a short delay of the voltage increase at the conventional anodization stage.Secondly,the large size/volume of fiber led to a non-uniform distribution of the discharges on the surface of AZ91Nd/Al2O3MMC after reaching the breakdown voltage.Finally,the voltage at the last stage declined with increasing fluctuation due to the reactive incorporation of the fiber disturbing the stability of the coating by causing high stresses.

(2) The coatings presented discernible local morphologies at the locations of intermetallics during the firs two stages,and then the discrepancy was disappearing gradually.The coatings on the MMC were rougher than the coatings on the pure alloy in the phosphate electrolyte.

(3) The alumina fiber was embedded in the coating at firs and then reactively incorporated in the coating.The large size and volume of the alumina fiber is responsible for the step by step dissolution of the fiber within the coatings preventing a fast and fully redistribution of Al in the coating.

(4) It is evident that the alumina fiber can be converted to the MgAl2O4spinel phase by the interaction of the fiber and MgO.A MgAl2O4rich layer of 25 μm at the outer layer of the coating was successfully formed on the MMC.

(5) Using an alternative Al source like a phosphatealuminate electrolyte is the more practical approach and results in a MgAl2O4based coating on the pure AZ91Nd alloy.This can be achieved due to higher Al2O3concentrations and a more rapid mixing process of MgO and Al2O3.

Acknowledgement

Ting Wu thanks China Scholarship Council for the award of fellowship and funding (No.201708510113).The authors would like to thank Dr.Anton Davydok for the experimental support during DESY beamline experiments.We also thank PETRA III (Hamburg,Germany) for accepting and granting the proposal I-20191340(P.03 end-station)for localized phase composition analysis of PEO layers.The technical support of Mr.Volker Heitmann,Mr.Ulrich Burmester during this work is gratefully acknowledged.

Supplementary materials

Supplementary material associated with this article can be found,in the online version,at doi:10.1016/j.jma.2021.09.017.