Nasrinsadat Azarian,Seyed Mohammad Mousavi Khoei

Department of Mining and Metallurgical Engineering,Amirkabir University of Technology(Tehran Polytechnic),P.O.Box 15875-4413,Tehran,Iran

Abstract Plasma electrolytic oxidation(PEO)has held great potential for the advancement of biodegradable implants,as it helps in developing porous bioceramic coatings on the surface of magnesium alloys.In this research work,MgO-based bioceramic coatings containing the Si,P,Ca,Na,and F elements have been successfully fabricated on an AZ31 magnesium alloy plate utilizing the PEO method.The characteristic currentvoltage behavior of the samples during the process was surveyed in an electrolyte containing Ca(H2PO4)2,Na2SiO3·9H2O,Na3PO4·12H2O,NaF,and KOH with a pH of 12.5 and electrical conductivity of 20 mS/cm.The results revealed that applying a voltage of 350-400V(that is 50-100V higher than the breakdown limit)could greatly facilitate the synthesis of a PEO ceramic coating with fewer defects and more uniform morphology.The resulting coating was a compositionally graded bioceramic layer with a thickness in the range of 3.5±0.4 to 6.0±0.7μm,comprising the above-mentioned elements as promising bioactive agents.The synthesized ceramic features were investigated in terms of the elemental distribution of components through the thickness,which indicated a gradual rise in the Si and P contents and,conversely,a decline in the F content towards the outer surface.The growth mechanism of the PEO coating has been discussed accordingly.? 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:Az31 magnesium alloy;Plasma electrolytic oxidation;Bioceramic;Coating;MgO-based ceramic.

1.Introduction

Magnesium alloys,with their excellent biocompatibility and biodegradability,have attracted a growing interest in such applications as biodegradable orthopedic materials[1-3].However,their high degradation rate in the physiological environment makes surface treatment a prerequisite for their utilization[4,5].A number of treatments have been developed,among which plasma electrolytic oxidation(PEO)has proved to be efficaciou in controlling the degradation rate[3-6]as well as in improving the bioactivity and reconstruction of hard tissues[1,2].

PEO,also known as micro-arc oxidation(MAO),is a high voltage plasma-assisted anodizing process used to modify the surface of valve metals like magnesium and its alloys[5-8].The applied voltage is high enough to exceed the breakdown limit of the developing anodized layer,causing multiple plasma discharges to occur across the entire surface[8-10].The plasma discharges generated during the process trigger partial short-term melting of the surface layer,which develops a multi-component oxide coating made up of ionic species from both the electrolyte and the substrate[5-9].

This process allows for the formation of ceramic layers with assorted chemical compositions and high levels of porosity on magnesium alloys surfaces,by employing a proper electrolyte and processing parameters[8,9,11,12].Considering these features,PEO ceramic coatings are favorable for biomedical applications[1,13,14].This claim can be validated by the growing body of literature on fabricating bioactive PEO coatings composed of hydroxyapatite,calcium phosphate(CaP),and/or silicate compounds[13-18].These types of coatings,alternatively stated as the bioceramics,have the potential to form a bone-like apatite on their surfaces,spontaneously,in the human physiological flui[11,14,19-23].In fact,their bioactivity is believed to stem from the presence of Si,Ca,P,Na,and F ions as potential bioactive agents[11,19,20,24-27].

The introduction of the above-mentioned ions in the PEO coating is attainable by adjusting the electrolyte constituents,in view of the fact that the ions existing in the PEO electrolyte are capable of being integrated into the synthesized ceramic coating through the discharging process[12,28].Nonetheless,there have been scarce publications concerning the PEO oxide layers formed on magnesium in solutions containing the ions described above,all together,and their incorporation into the bioceramic surface layer.

In addition to the electrolyte contents,the electric parameters(voltage/current)applied during the process also have a significan influenc on the properties of the PEO coatings[29,30].The meaningful variation of the applied voltage with respect to the breakdown limit(ΔV),for example,is one of the important factors that affects the overall coating properties and worth exploring[31].Controlling such electrical parameters can help to adjust the thickness,structure,and surface morphologies of the formed ceramic coatings[32].Furthermore,the PEO coatings experience various formation processes at different applied voltage or current.Accordingly,monitoring the electrical performance of the metal sample during the PEO can assist in surveying the stages of ceramic coating formation[33].

Despite all the research efforts conducted in the fiel of PEO,the formation and growth mechanism of PEO coatings remained controversial.The reason for this is that a combination of plasma-chemical,thermochemical,and electrochemical reactions occurs during the process at the electrolyte/coating/substrate interfaces[12,34,35].Most studies in this area,however,have tended to focus on the effects of processing parameters rather than its fundamentals.In this regard,by investigating the relationship between the applied voltage and the coating growth mechanism,a greater insight into the oxidation mechanism during the PEO can be obtained than what has been done previously[33].

This study is aimed at the in-situ production of a bioceramic PEO layer containing Si,P,Ca,Na,and F on AZ31 magnesium alloy.The incorporation mechanism of these elements into the coating is discussed with regard to the electrical behavior of the alloy surface during the process and its relationship with the coating bioactivity.In this way,we have tried to deal with the uncertainties surrounding the fundamentals of coating growth as much as possible.

2.Experimental

2.1.Materials and methods

Samples of AZ31B-O magnesium alloy(wt.%,Al 2.81,Zn 1.01,Mn 0.46,Si<0.01,and Mg balance),in accordance with the ASTM-B90 standard,with the size of 30×15×2mm3were used as the coating substrate.Prior to the oxidation process,samples were mechanically polished up to 1000 grit,degreased with ethanol,thoroughly rinsed with deionized water,and finall dried in warm air.

The electrolyte of the PEO process was prepared from the solution of 15g/L Ca(H2PO4)2,10g/L Na2SiO3·9H2O,5g/L Na3PO4·12H2O,1g/L NaF,and 5g/L KOH in deionized water with a pH level of 12.5 and electrical conductivity of 20 mS/cm.The reason for selecting this high pH level(>12)is justifie by the passivation behavior of magnesium alloys in aqueous media,so that stable PEO coatings can be achievable[36,37].All chemicals used in this study had a purity of at least 99.0%,and were supplied by Kimia Tejarat Fajr,Iran.

The PEO process was conducted with the help of a 30kW DC power supply in a 2-liter polyethylene cell with a cylindrical perforated stainless steel container(type 316)as the cathode and the AZ31 sample as the anode.A pump/heat exchanger set(model:Grundfos UPS 25-40 N 180)was put into use for circulating and cooling the PEO electrolyte.The designed PEO cell set-up is schematically shown in Fig.1 along with its technical details.

Before starting the process,the current density-voltage behavior of the AZ31 sample in the electrolyte was examined by raising the cell voltage with a speed of 1V/s and monitoring its current response via two sets of GDM-396 GW INSTEK digital multimeters simultaneously.The breakdown voltage,as the micro-discharging starting point,was obtained by implementing the above-stated procedure.The PEO experiments were accordingly carried out for 600s at six fi edΔVs(between 25 and 150 V)beyond the breakdown voltage.The corresponding coating samples were labeled based on theirΔVs.Following the PEO,the samples were rinsed with deionized water and dried in warm air.

2.2.Characterization

The surface morphology,cross-section microstructure and chemical composition analysis of the ceramic coatings were evaluated by a fiel emission scanning electron microscope(FESEM,TESCAN Mira3,Czech Republic)equipped with an energy dispersive spectrometer(EDS:Oxford instruments).The FESEM micrographs were surveyed by means of the ImageJ analyzing software.

The coating phase composition was examined through grazing incidence X-ray diffraction(GIXRD)using an EQuinox 3000(INEL,France)apparatus working at 40kV and 30mA with Cu Kαradiation(λ=1.5417 °A)in the 2θrange of 10-90° and 0.03° step size.The diffraction peaks observed on the XRD patterns were identifie using the HighScore Plus software and Crystallography Open Database(COD).Supplementary phase analysis of the ceramic coating was also carried out with the aid of the Fourier transform infrared spectroscopy(FT-IR;Thermo Nicolet,Nexus 670,USA).

The adhesion strength of the coatings to the substrate was measured by a pull-off adhesion tester(DefelskoPositest,USA)according to the ASTM D4541 standard.The measurements were repeated three times for each sample and the average of the results was reported.

Fig.1.Schematic view of the PEO processing set-up.

Fig.2.Current density variation with applied voltage during the PEO of the AZ31 alloy in the studied electrolyte.

The apatite-forming ability of sample 75(as a representative of all the coated samples)was evaluated by soaking it in simulated body flui(SBF),provided by APATECH Co.Iran,with three repetitions.After 7 days of immersion in SBF at 37.5°C,the samples were removed from the SBF solution and washed with deionized water,and then dried in warm air.The surface morphology,chemical structure,and microstructure phase analysis of the immersed samples were identifie using SEM,EDS,and FT-IR techniques.

3.Results and discussion

3.1.Current density-voltage characteristics during the PEO process

Fig.2 represents the characteristic current density-voltage behavior of the AZ31 magnesium alloy in the studied electrolyte.The curve has been divided into four distinct regions that can be identifie according to the peaks of the current density transient.

Region I starts with the formation of a passive layer and then the dissolution of the layer at the corrosion potential of the material,which is around 1.5V based on the literature[36].Further increases in the applied voltage lead to repassivation and the growth of a porous oxide fil together with the oxygen evolution corresponding to general anodization[36,38].

The firs peak in the current density(U1 on Fig.2)is called‘transpassive formation peak’,and it is the starting point of Region II,which is known as the spark anodization region.The occurrence of U1 is mainly due to the impact ionization of the gas steam shell formed around the anode surface as a consequence of the constant elevation of the applied voltage.In this region,fin luminescent sparks initiate and move quickly across the sample surface,which is associated with the thickening of the passive coating[38,39].

Briefl,as Regions I & II are related to the traditional anodizing process,focusing on this part of the diagram is outside the scope of the article.Further information about these regions can be found elsewhere[33,36,38].

Region III,which is the next step in this diagram,begins right after reaching the breakdown voltage shown as U2in Fig.2.In this region,known as the micro-arcing region,the voltage is sufficien to induce the dielectric breakdown of the existing oxide layer and the gas steam shell,so an abundance of quick low-power micro-arc discharges appears.Owing to the thermal ionization and micro-arcing,the fil is fused and alloyed gradually with ions contained in the electrolyte[38].The coatings associated with this region exhibit micro-pores and micro-cracks,which are the characteristics of PEO coatings[15,16].The continuous formation and breakdown of the oxide fil in this region lead to some fluctuation in the current density[35,40,41].

When the applied voltage surpasses the critical value of U3,Region IV comes forth(which is named as the arcing region).In this region,micro-arcs turn into strong arcs and penetrate through to the substrate.The drastic sparking and gas evolution bring about the formation of large size pores and may result in destructive effects like thermal cracking of the layer[35,40].

The breakdown voltage of the AZ31 samples in the electrolyte under study is considered as 300V,according to Fig.2.In the potentiostatic PEO process,the coating process is preferably performed at a constant potential beyond the breakdown voltage.It is worth noting that the potential difference between the coating voltage and the breakdown voltage(ΔV)is a driving force affecting the overall coating properties.HigherΔVs provide stronger micro-arcs and consequently more induced energies that are required for the growth of PEO coatings[31].For that reason,the targetΔVs of 25,50,75,100,125,and 150V were applied to fabricate the PEO coatings.

Fig.3 shows the current density changes recorded during the PEO processes at different fi edΔVs.It can be seen that the current density decreases exponentially over time for all of the samples,which is accompanied by a decrease in the formation rate of sparks as the anode remained at a fi ed potential.This is a common behavior in the potentiostatic PEO processes,reflectin the increase in resistance owing to the growth of the dielectric layer on the sample surface,and causes a decline in the coating formation speed over the treatment time[33,36,42].Since the current density continues to be almost constant and close to zero after 200 s,the graph is presented up to this point.At higherΔVs,the current density levels are higher,and it takes a longer time for the current density to drop,which is also an outcome of the enhanced induced energy.Fluctuations in the current density are,to a large extent,due to continuous formation and breakdown of the coating[35,40,41].

Fig.3.The current density-time behavior during the potentiostatic PEO coating at six differentΔVs(V).

Fig.4 represents the FESEM images of the coating surfaces at differentΔVs.The surface morphology of all the samples is porous and has a pancake-like structure.This is a characteristic feature of PEO coatings regardless of the appliedΔV value,which can be ascribed to breakdown channels.The pores are generated when the molten oxide and gas bubbles are ejected out of micro-arc discharge channels.It should be taken into account that PEO surfaces normally have micro-cracks that are produced as a consequence of the thermal stress relief during the process[7,29,31,42],although they are not simply visible at the magnificatio used in this figure The PEO surface features and related formation mechanism will be discussed in detail later in this work.

A comparison of surface morphologies shown in Fig.4 reveals that the overall amount of pores decreases while the average pore diameter enlarges and the porosity size distribution expands with increasing appliedΔV.Quantitative analysis of the porosities on the surface of the ceramic coatings,derived from the FESEM micrographs by the ImageJ software,is illustrated in Fig.5,indicating consistent results.This is due to the fact that higherΔVs generate stronger but slower moving discharges on the anode surface,which in turn gives rise to bigger but fewer pores[10].Such finding have been reported in previous studies as well[2,4,15,29].It should be noted that the higher standard deviation of the average pore size for higher appliedΔVs refers to more dispersion in the porosity size distribution.

It is also apparent in Fig.4 that theΔVs between 50 and 100V deliver the most uniform morphologies among others,thanks to the regular and quasi-circular shape of the pores with narrow range of pore size distribution as measured and shown in Fig.5-c.WhileΔVs lower than 50V are inadequate because of the remaining grinding traces on the surface;likewise,ΔVs higher than 100V are less preferable on account of the destructive effects of large cracks and irregular-shaped pores.

The cross-sectional structures of the PEO bioceramic layers at differentΔVs are illustrated in Fig.6.The design of the outer porous layer beside the thin and compact inner layer(barrier layer)that adheres well to the substrate,as a characteristic of PEO coatings[10,28],is obvious in this figur for all theΔVs.It is also apparent that higherΔVs afford thicker coatings with more defects on the substrate.This phenomenon,as previously mentioned,is because of inducing higher sparking energy,which results in higher volumes of molten products to be ejected from the discharge zones and deposited on the surface[2,9,16,43].

Fig.6,furthermore,demonstrates that the coating is too thin and non-uniform atΔV=25V.This low thickness was also observed in the form of the scratches from the grinding process on the coating surface in Fig.4-a.AtΔV=125-150,lack of evenness in the coating thickness is also visible as large pores and defects that can serve as crack initiation sites and cause failure during service[2].

The variation of thickness value as a function ofΔV is presented in Fig.7,derived from measuring the average thickness of the coatings with respect to the cross-sectional FESEM micrographs.The resulting diagram indicates that the thickness of the coatings goes up gradually and relatively linearly as long asΔVs are below 125V;but above this voltage,the thickness rises dramatically.The excessive increase of thickness could be reasonable concerning the higher current densities and the resulting input energy at higherΔVs,as discussed before[2,4,15,16,43,44].

The elemental content ratio of the produced ceramic coatings detected by the EDS analysis is specifie in Table 1.It should be noted that the EDS data are used just for semiquantitative comparison.The presence of O,Si,P,Ca,Na,and F is an indication of the electrolyte constituents’participation in coating formation[3,12].High amounts of O and Mg in the coating(as main elements)refer to the oxidative nature of PEO.Coming from the substrate,small quantities of Al have also been detected in the coatings.

Fig.4.FESEM micrographs of the PEO surfaces produced by different appliedΔVs(V):(a)25,(b)50,(c)75,(d)100,(e)125,and(f)150.

Table 1EDS surface analysis at differentΔVs.

The EDS results also imply that the elemental ratio in the coating remains almost the same without significan changes,irrespective of theΔV value.This phenomenon,previously noticed by another researcher[44],could be due to invariable amounts of the ions existing in the electrolyte,since the electrolyte constituents and their ratio used throughout this study were unchanged.

The variation of the adhesive strength of the PEO coatings with the appliedΔV is shown in Fig.8.The adhesive strengths of the obtained coatings are in the range of～2-3MPa with a cohesive type failure at the coating/substrate interface for all the samples.It is also observed in the figur that the adhesion strength firs increases with the increase of the appliedΔV up to 100V,and then decreases.

The highest coating adhesions are achieved by applying a ΔV within the range of 50-100V,as a result of adequate fusion occurring between the substrate and the coating during the formation of plasma discharges[45,46].High compactness beside uniform distribution of porosities of the coating prepared in thisΔV range can also be a reason for the increase in the adhesive strength[43,46].However,whenΔVs are lower than 50V or higher than 100V,the coating adhesion declines.This may be respectively associated with the insufficien fusion during PEO deposition and the defective structure of coatings full of pores and cracks[45].

On the whole,from the visual surface uniformity,thickness,and adhesion strength points of view,our finding suggest that aΔV between 50 and 100 is likely to be a better selection for the PEO coating.The following investigation,therefore,moves on to focus on theΔV of 75V as it can provide enough driving force to yield a PEO bioceramic layer with uniform morphology,fewer defects,adequate thickness,and satisfactory adhesive strength;at the same time,thisΔV has an acceptable safe distance from Regions II & IV(Fig.2),which guarantees a fin ceramic layer.However,it can be said that this sample is just a representative of all the PEO samples in the micro-arcing region(Region III in Fig.2).

Fig.5.Pore density(a),average pore size(b),and histogram of the pore size distribution(c)for the PEO samples against appliedΔVs of 25-150V.

3.2.Detailed analysis of the coating at the micro-arcing region

3.2.1.Microstructural analysis

Fig.9 illustrates a more detailed and magnifie FESEM image of sample 75 as well as the EDS analyses of its corresponding features.The morphology of the PEO ceramic layer,as pointed out earlier,is made up of pores surrounded by pancake-like areas denoted by arrows.The formation of these surface features is attributed to breakdown channels and solidifie molten products created by the plasma discharges during the PEO coating process[31,34].There are also some tiny spherical pores(<0.5μm)that are referred to gas porosities and can be formed by reason of the gas evolution during the post-discharge cooling[31].Some local micro-cracks can be spotted on the ceramic layer,which are produced as a result of the thermal stress relief during the rapid solidificatio of the coating in the cool electrolyte[7,8,29,31,32,42].

The EDS analyses presented in Fig.9 indicate the comparative elemental ratio of(b)the average analysis of the whole ceramic surface and(c)the pancake-like areas around the large pores(≥4μm),represented respectively by Points 1 and 2.Higher levels of the doped electrolyte constituents(especially Ca and P)in the synthesized ceramic layer can be attained in the proximity of the large pores where plasma discharges are more powerful.This could be owing to the intensificatio of the micro-discharges with increasing the applied ΔV.Thus,the zone for each micro-discharge expands,and a greater number of ions in the electrolyte can be absorbed into the micro-discharge neighborhood[2,47].The higher amounts of Ca and P at the pancake-like areas around the large pores are also visible in the corresponding EDS mapping results(Fig.9-e)obtained from another region of sample 75′s surface.These areas can possibly act as nucleation places for growing hydroxyapatite and promote bioactivity in physiological fluids

Fig.6.FESEM micrographs of the PEO coating cross-sections produced by different appliedΔVs(V):(a)25,(b)50,(c)75,(d)100,(e)125,and(f)150.

Fig.7.Thickness changes of the PEO coatings with appliedΔV.

Fig.8.Adhesion strength changes of the PEO coatings with appliedΔV.

It can also be made out in Fig.9-a that several tiny particles with bright contrast are randomly distributed across the entire ceramic surface(marked as Point 3).The EDS analysis of these particles(Fig.9-d)discloses even higher amounts of Ca and P compared with the pancake-like areas around the large pores.These Ca-P-rich particles,as well as the pancake-shaped structure with pores,are also discernible in the FESEM image of the ceramic coating cross-sectional view(Fig.10).Surface absorption can be inferred as the mechanism by which Ca and P get embedded into the ceramic layer as tiny particles.

3.2.2.Phase analysis

Fig.11 illustrates the X-ray diffraction pattern of the bioceramic coating grown on sample 75.Apart from the Mg peaks coming from the substrate,the principal phases of the coating are MgO[COD ID:9,013,251],MgSiO3[COD ID:9,003,428],and Mg3(PO4)2[COD ID:9,001,027],whose presence in PEO film were also reported by others[1,12,14,35,48].The formation of these phases can be put down to the outward diffusion of magnesium ions and the inward diffusion of SiO32?,PO43?,and OH?under the PEO strong electric fiel as well as the momentary high temperature and high pressure in the discharge zone[10,12,14,35,48].

Fig.9.Higher magnificatio FESEM micrograph of sample 75′s surface morphology and EDS analyses of the specifie points 1(b),2(c),3(d);and(e)EDS mapping of pancake-shaped areas.

Fig.10.FESEM image of sample 75′s ceramic coating from a cross-sectional view.

The coating formation reactions take place when the concentration of these ions at the substrate/electrolyte interface reaches a critical value.In addition,at the elevated plasma temperatures,the thermal energy supplied to the diffusing ions allows them to overcome the activation energy barrier,move and interact with the Mg2+cations more easily.Subsequently,the MgO,MgSiO3,and Mg3(PO4)2phases deposit as the PEO products as per the following equations[1,10,12,14,35,48]:

The bulge-shaped broad peak in the range 2?=20-40°signifie the presence of an amorphous phase.This broad peak that envelops high-intensity peaks of MgSiO3and Mg3(PO4)2suggests that their phases are in the form of a crystalline and amorphous mixture[32,34,44].This phenomenon can be ascribable to the local heat-ups and high cooling rates during the coating process[5].This findin is in agreement with earlier studies asserting that a combination of crystalline and/or amorphous phases is developed in the PEO coatings on magnesium alloys prepared in electrolyte solutions of silicates and/or phosphates[5,30,44].

Fig.11.XRD pattern of the PEO layer on sample 75(the XRD pattern for the substrate with COD ID:1,512,519 is shown for comparison).

Fig.12.The FT-IR spectrum of the synthesized layer on sample 75.

To thoroughly investigate the chemical composition,the FT-IR spectrum of the synthesized ceramic layer was taken into consideration(Fig.12).The band at 454 cm?1is due to the presence of Mg-O vibrations[15,16].The slight band at 520 cm?1is attributed to vibration mode of the Si-O-Si group[49,50].The strong band located around 1030-1060 can be assigned to the overlap of PO43?and Si-O-Si groups[15,16,22,49].These results,consistent with the XRD ones,can prove the formation of phosphate and silicate compounds[49,50].

The minor bands observed at 2856-2920 cm?1correspond to C-H groups[7].The band at 2960 cm?1is assigned to P-O-H group forming the hydrate of phosphate[50,51].The broad band at 3300-3600 cm?1is free water as a result of re-absorption of water molecules from the atmosphere,while the band at 1640 cm?1is the H-O-H group corresponding to crystal water[1,7,16,50-52].The CO32?absorption band is also detectable at 1421 cm?1[7,13,51]which is likely to be due to the absorption of carbonate ions from the atmosphere[31].

The FT-IR spectrum,based on what other studies claim,can be a sign for the presence of non-stoichiometric hydroxyapatite structure through those bands matching up with PO43?,O-H,CO32?,C-H and P-O-H groups[7,31].However,given that no crystalline apatite phases were detected by the XRD in this work,it is likely for these bands to be related to some amorphous Ca-P rich phases,as observed typically in Point 2 of Fig.9.Either way,they can be helpful in enhancing the bioactivity of the coating,as discussed later in this paper.

The incorporation of Ca,Na,and F in the ceramic coating as detected by the EDS analysis(Fig.9)and,on the other hand,the absence of the phases or bands related to these elements in the XRD pattern(Fig.11)and FT-IR spectrum(Fig.12)guided this study to investigate the elemental distribution of the PEO coating through its thickness.

3.2.3.Depth profilin

Fig.13 shows a cross-sectional FESEM image of the bioceramic layer and its relevant results of the elemental EDS line scans.As can be seen from the line scans,all elements are thoroughly spread along the cross-section of the coating.Magnesium appears to dwindle away towards the fil surface,and most of the oxygen is accumulated in the middle of its thickness.Silicon and phosphorus in the outer region are more intensive in comparison with the inner layer;in contrast,fluorin is at the highest level in the vicinity of the oxide layer/substrate interface.Calcium and sodium are almost unchanged across the ceramic layer.

During the PEO of magnesium,Mg2+cations transfer away from the substrate and react with the electrolyte ions to form a ceramic coating.Meanwhile,electrolyte ions transfer into the magnesium substrate(due to the high electric fiel in the discharge zone)and compete with each other to react with the Mg2+cations to create the anodic ceramic coating[10,40,52].However,after reaching the anode surface,these ions are incorporated into the coating in different ways as regards their electrical charge,ionic radius,migration rate,and concentration in the electrolyte[2,12,52,53].

Fig.13.FESEM micrograph of the cross-section of sample 75 and the corresponding EDS line-scans.

Previous studies believe that under the effect of the high electric fiel in this coating process,the anions(OH?,SiO32?,PO43?,andF?)can arrive at the anode by both diffusion and electromigration,but the cations(Ca2+and Na+)move towards the anode only through diffusion[14,52].Moreover,some authors have claimed that the presence of cations in the PEO layer is an outcome of the non-equilibrium conditions during the formation of the PEO ceramic layer[21].This can explain the relatively low amounts of calcium and sodium in the coating beside their uniform distribution across the thickness,regarding the EDS results shown in Table 1 and Fig.13,respectively.

The gradual decrease in fluorin together with the increase in silicon and phosphorus contents from the inner layer to the outer one,as clearly shown in the EDS line scans of Fig.13,implies that F-is probably able to take part in the coating formation prior to SiO32?and PO43?during the initial stage.In an opposite way,the more developed the ceramic coating,the more SiO32?and PO43?are gradually involved in the coating formation[6,48].

The higher content of fluorin in the interior layer can be justifie by the fast migration ofF?under the influenc of the high electric field This may be attributed to the smaller ionic radius and higher mobility of fluorid ions compared to that of the others[6,53].

The presence of fluorin in the proximity of the oxidemetal interface was demonstrated by the EDS cross-sectional profile whereas there was no detection of fluorine-containin crystalline phases with reference to the XRD pattern of the PEO coating.This can be explained by the relatively low amount of fluorin or its asymmetric distribution across the coating,which makes it hardly detectable.Even so,some studies attribute this phenomenon to the presence of fluoride as an amorphous phase in the coating adjacent to the base metal[54,55].

On the other hand,the higher amounts of silicon and phosphorus in the outer region of the coating,which is in harmony with similar studies[2,4,12,13],can be attributed to the migrating difficult of these ions towards the interior part of the coating.The ionic radius and ionic activity are especially likely to account for this migration difficult[2,12].

As for the distribution behavior of oxygen and magnesium,some researchers have reported that the small ionic radius and high activity of OH?(compared with the PO43?and SiO32?anions)make it easy for OH?to migrate and,consequently,it has priority over the other anions to enter the micro-discharge zones during the PEO process.Hence,the main physio-chemical reaction in this coating process occurs between the Mg2+ions from the substrate and the OH?ions from the electrolyte,generating MgO as the main phase in the middle and late periods[2,12].

3.2.4.Coating growth mechanism

As previously mentioned,the PEO coating expands gradually over the entire magnesium alloy surface by cascades of plasma discharges.It should also be noted that the growth rate of the PEO coating is mainly controlled by the intensity and the number of discharges[10,34].The formation of each plasma discharge,irrespective of its intensity,includes a sequence of steps,as shown schematically in Fig.14.

Fig.14.Schematic view of the PEO process.

There is a consensus in the literature that a plasma discharge in the PEO process takes place once the applied voltage reaches the breakdown limit,beyond which the previously formed passive layer breaks down,by preference,at relatively thin or defective locations[28,34].The discharge may occur anywhere through the passive layer,between the metal/oxide and the oxide/electrolyte interface,depending on its intensity[10,28].

After the breakdown,a localized melt channel is formed at the discharge zone because of the high levels of pressure and thermal energy of the discharge[28].The electrolyte and substrate components that exist at the melt zone can be ionized by the strong electric fiel induced by the appliedΔV[32,34].The ions produced at this zone,like OH?,F?,PO43?,SiO32?and Mg2±,can be integrated to the fina coating through the plasma chemical reactions.This could give rise to the production of new oxide compounds in the discharge channel at the electrolyte/coating interface[28,32,34],as discussed earlier by assessing the XRD pattern,FT-IR spectrum,and EDS results.

The produced molten complex oxide is ejected from the discharge channel onto the surface with a volcano-like movement due to the strong electric fiel and high temperature and,subsequently,the cool electrolyte helps it re-solidify rapidly.This results in the formation of porous pancake-like regions with a complex chemical composition,as illustrated previously in Fig.9[28,32,34].The PEO coating can be grown and reconstructed continually throughout its thickness by repeating the discharge formation steps[35,40].

Applying a rather highΔV within the usual range of the PEO process can make plasma sparks intensive and energetic to the extent that the slight absorption of cations like Na+and Ca2+towards the magnesium anode becomes possible[2].These cations,if absorbed,tend to be placed in the protrusive pancake areas around the large pores(≥4μm)as a result of their entrapment by the powerful ejection of molten plugs as well as the prolongation of the plug solidification both of which take place when the plasma discharges are strong enough.The presence of these cations at the aforementioned zones was earlier verifie as regards Fig.9.It is worth remarking that when a specifi value ofΔV is applied,the anions are more easily able to diffuse or penetrate through the coating thickness than cations[40],which could be the main reason for the more frequent attendance of Si,P,F,and O in the ceramic layer compared with Ca and Na.

The high energy input supplied by the appliedΔV could bring about more than one spark simultaneously at the same place,which is noticeable as the irregular shapes of some pores in the FESEM image(Fig.9).This irregularity may also come from(a)the occurrence of plasma discharge near the earlier created pores,and(b)partial fill-u of the pores by the fl w of reaction products[32,47].This high input energy,together with the fast solidificatio by the cool electrolyte,can also contribute to the formation of the amorphous and crystalline phases concurrently[5],which was examined formerly by exploring the XRD pattern.

3.2.5.Apatite-formation ability

As mentioned before,the synthesized ceramic layer in this study can exhibit bioactive abilities.It means that a bonelike apatite layer can be formed on its surface when exposed to the body environment.The apatite forming ability on the PEO bioceramic surface by immersion in SBF solution gives predictive information about its bioactivity[17,19-23].

Fig.15.FESEM micrograph of sample 75′s surface morphology and EDS analyses of the specifie points 1(b),2(c),3(d);and(e)EDS mapping of pancake-shaped areas after 7 days of immersion in SBF.

Fig.15 shows the FESEM micrograph and EDS analyses of the surface features of sample 75 after being immersed in SBF for 7 days.Compared with the surface before immersion(Fig.9),the number of micropores has obviously dropped(around 90%),which is accompanied by higher amounts of Ca and P,based on the EDS data on all three selected areas in the figure More precisely,the small pores of the original coating surface(≤0.5μm)have disappeared and the diameter of the large pores has decreased from 4μm to 1.5μm due to the SBF immersion.It is also worth saying that the average area size of the pores has decreased from 1.28μm2before the SBF immersion to 0.75μm2after it.These alterations indicate that the entire sample surface has been covered by a thin homogeneous CaP layer comprising some white Ca-P-rich particles.Moreover,the intrinsic porosity of the PEO layer is about to disappear by this CaP layer;its high surface bioactivity can thereby be deduced[1,17].The cracks on the CaP layer can be associated with the drying process after the removal of the sample from the SBF solution[1].

Interestingly,the pancake-like areas around the large micropores(marked as Point 2 in Fig.15-c),similar to what was seen in Fig.9-c before the SBF immersion,have still relatively higher amounts of calcium and phosphorus in comparison with the whole surface.EDS mapping of these areas can confir the local supersaturation of the Ca and P elements in these areas,as shown in Fig.15-e.There is also a number of white particles(represented by Point 3)that are rich in Ca and P.The Ca/P atomic ratio identifie by the EDS analysis ranges from 1.5 to 1.7 in these areas,which is close to that of hydroxyapatite(1.67)[2,9,11].As the Ca/P ratio is an important factor in predicting the bioactivity of implants[7,18,31],it may be concluded that these areas play an essential role in the surface bioactivity as predicted before,which requires a more detailed investigation.

The specifi phase content of the CaP layer could not be identifie by XRD since its thickness was very thin after just 7 days SBF immersion.However,by examining the FT-IR spectrum,one can reach supplementary conclusions about the phases and functional groups formed on the bioceramic layer after soaking in SBF(Fig.16).

The FT-IR results after immersion in SBF show new bands at 580,710,875,1750,1785,and 2550 cm?1,which are demonstrated by blue arrows.These bands are related to PO43?[15,19],P-O-H[7,50],CO32?[7],and 1750 H-O-H groups[50].This observation suggests that a layer of carbonated apatite with a non-stoichiometric complex structure,which is the main constituent of hard tissues such as bones,may be deposited on the immersed PEO layer surface[7,19,56].

Fig.16.The FT-IR spectrum of sample 75 after 7 days of immersion in SBF.

Although the specifi composition of the generated CaP layer is hard to be determined,all of the SEM,EDS,and FT-IR results of the immersed sample show that the synthesized PEO bioceramic layer possessed excellent bioactivity in SBF.Demonstrating remarkable capacity,this specifi area of research therefore deserves more effort and scrutiny.

4.Conclusions

Compositionally graded bioceramic coatings containing Si-P-Ca-Na-F were in-situ synthesized on the AZ31 magnesium substrate by the PEO method in an electrolyte made up of Ca(H2PO4)2,Na2SiO3·9H2O,Na3PO4·12H2O,NaF,and KOH.To sum up,the following points are listed:

?A quantity was define asΔV that is the potential difference between the coating voltage and the breakdown voltage.Functioning as the driving force,it had a direct bearing on the overall coating properties.Its higher values provided more input energies and stronger plasma discharges for the growth of the PEO coatings.

?An appliedΔV between 50 and 100 was deemed to be a proper selection in that it yielded enough driving force to form a PEO coating with uniform morphology,fewer imperfections,adequate thickness,and higher adhesive strength.

?By applying aΔV of 75V,a PEO bioceramic layer was synthesized comprising the MgO,MgSiO3,and Mg3(PO4)2phases,as a crystalline and amorphous mixture,on the AZ31 substrate.

?The distribution of elements through the coating thickness was affected by such factors as ionic radius,the electrical charge of ions,their migration rate,and concentration in the electrolyte.It was confirme that fluorin preferably accumulated in the inner side of the coating close to the substrate while the content of silicon and phosphorus piled up at the highest level in the outer side near the surface.Calcium and sodium contents,with their lower proportions,remained almost unvarying across the ceramic layer.

?Higher levels of the doped electrolyte constituents(especially Ca and P)in the synthesized ceramics were achieved in the vicinity of the large pores(≥4μm),where plasma discharges were more powerful.These areas appeared to have remarkable bioactive abilities and accordingly to favor the nucleation and growth of hydroxyapatite in SBF.

Declaration of Competing Interest

The authors declare that they have no known competing financia interests or personal relationships that could have appeared to influenc the work reported in this paper.