Maryam Molaei,Kazem Babaei,Arash Fattah-alhosseini

Department of Materials Engineering,Bu-Ali Sina University,Hamedan 65178-38695,Iran

Abstract As an efficien surface modificatio approach,the plasma electrolytic oxidation(PEO)technique can boost the capability of wear protection in Mg and its alloys by applying a hard and thick ceramic coating.In this procedure,more efficien protection can be acquired via adding additives (in the form of particle,powder,sheet,etc.) into solutions and producing composite coatings.These additives result in more efficien protection against wear via getting stuck in the cracks and pores of coatings and rising the thickness,hardness,and diminishing the porosity size and content.The efficien y of each additive can be changed owing to its intrinsic properties like melting point,size,participation type(reactive,partly reactive,or inert) and potential of zeta.In this review,the effects of distinct additives in nano-and micro-scale size on wear behavior of PEO coatings on Mg and its alloys is going to be reviewed.

Keywords: Plasma electrolytic oxidation (PEO);Mg alloys;Wear behavior;Nano-sized additives;Micro-sized additives.

1.Introduction

The remarkable properties including the high ratio of strength-to-weight,low density,specifi rigidity and castability,great electrical conductivity,proper electromagnetic shielding,the recycling ability,biocompatibility,biodegradability,and non-toxicity have put Mg and its alloys amongst suitable materials for automotive,aerospace,electronics,and applications of biomaterials [1-6].However,vulnerability to corrosion arising from wear and inherently high chemical reactivity is a benefi of this category of metals [1,7].So,it is necessary to utilize an approach to modify these metals surface and boost their wear and corrosion behavior.The plasma electrolytic oxidation (PEO) method is one of the best techniques that can boost the wear and corrosion behavior of metals,mainly Al [8-11],Ti [12,13],and Mg [11,14-16],by creating hard,thick,and adherent coatings [17].

The ability to use enormous and complicated shapes of samples working as an anode,the equipment simplicity,low energy consumption,and being not dangerous owing to utilizing alkaline solutions are several significan specification of the PEO processes [18,19].After appearing many transients fine and short-lived discharges on the anode surface by employing higher voltage than the voltage of dielectric breakdown and due to high local pressure (102-103MPa) and temperature (103-104K) in the discharge channels PEO coating is produced [20-26].The PEO coatings contain substrate metals or elements of solutions [27-29].

Distinct impressive parameters can affect the properties and performance of PEO coatings.These parameters can be seen in Fig.1.On account of the electrolyte elements participation in the reactions of the PEO process and changing the structure and composition subsequently the coating characteristics,the chemical composition of the solution is noticed the most significan factor [30,31].What makes the PEO process get more attractive is the capability to introducing distinct additives into the solution and alter the coating efficien y.

Fig.1.Effective factors on PEO coatings composition,structure,and properties.

Newly,it has been concluded that using nano-or microsized additives into base solutions,composite coatings can be acquired on substrates of Mg having better efficien y than simple coatings.Table 1 provides data about published papers which studied the nano-or micro-sized additives effects on distinct properties (wear,corrosion,mechanical,etc.) of PEO coatings on Mg and its alloys[5,32-103].The most published studies have been on AZ31 Mg alloys,utilizing particles of ZrO2,SiO2,and Al2O3and about corrosion behavior.One of these behaviors is wear.Although some authors have studied the PEO coatings wear performance on Mg and its alloys,none of them has reviewed the effect of adding nano-and micro-sized additives on wear behavior in detail [104-106].Thus,in the present review,we have focused on PEO coatings wear properties on Mg and its alloys before and after adding nano-or micro-sized additives into the solutions.First,the additives participation mechanism into the PEO coatings and coatings phase composition and structure are going to be discussed.Then,the effect of each type of additives on coatings wear properties will be reviewed in distinct sections.Table 2 provides data on substrates,solutions,additives and coatings phase composition and thickness in published investigations [32,33,50-52,54,58-60,62,65,67,34,69,77-80,83,85,93-95,37,96,99,102,103,107,108,38,40,42,44,45,47].

Table 1 Details of published works investigated the effects of nano-and micro-sized additives on properties of PEO coatings on Mg and its alloys.

Table 1 (continued)

2.Additives participation mechanism into PEO coatings

In general,particles could be participated either via partly reactive,reactive or inert mode in coatings within the PEO procedure.The most significan parameters specifying the mode of incorporation are the particle size and the melting point in addition to electrolytic status and solution composition.Particles having a high melting point are mostly embedded partly reactive or inertly,ignoring their size.However,particles having quite low melting points (from 1000 to 1200°C) are reactively embedded.Also,the particle size affects the incorporation mode of the particle,and the particles having a small size and low melting point are probably to be participated reactively comparing with large-sized and high melting point particles [36,54].The nanoparticles (NPs) that are smaller than the channels of discharge are able to be incorporated deeper into the coating compared to the agglomerates of micro-sized.The quite large particles or their agglomerates can be deeply incorporated into the hetero-oxide fil mostly via very large channels of discharge found at the end of the PEO procedure.These particles are mostly embedded in the area of subsurface [85].

Conductivity and Zeta potential are major indicators that show the stability of the colloidal particles [109].The particle’s zeta potential magnitude and their dispersion stability within the solution seem so significan for their participation in coating within the PEO procedure.Zeta potential of most particles is negative within the typically utilized alkaline solutions for PEO.So,particles can be negatively charged.The zeta potential high absolute value is pleasant for particle electrophoretic participation in PEO coatings under the status of DC [36,81,110].In electrophoretic incorporation,the suspended charged particles in a liquid environment immigrate and deposit on top of a conductive substrate of the opposite charge.Particles of negatively charged will be deposited on the anode,while particles of positively charged will be deposited on the cathode.In the PEO procedure,particles having negative charge migrate in the solution to the substrate of anode [110-115].The particle’s participation mechanism in PEO coatings is illustrated in Fig.2.

Fig.2.The schematic of the participation mechanism of particles into PEO coatings.

3.Voltage-time response of coatings

Although some variations were seen in voltage-time behavior of coatings after adding additives into the base solutions,in some cases no distinction was between voltagetime responses of solutions with and without additives[45,47,54,69,103].In spite of the same trend in voltagetime behavior,the voltage response of solution having NPs of Ta2O5in the second and third areas of the PEO procedure was considerably higher than that for the solution with no Ta2O5NPs [45].On the other hand and in other cases,the voltage response of solutions having additives in the second and third areas was a bit lower than that for the solutions with no additives.This can be associated with the variation in the solution conductivity having additives [42,50].

The solution with no additive indicated higher sparking voltage compared with the solution having additive.Due to the similar current density for both sorts of solutions,the content of the required energy to create the coating,in case of the existence of additive was less than that with no additive.This could be ascribed to less porosity arisen from the existence of nanopowders within the coating and as a result less resistivity of the coating[34,37,44].The diagrams of voltagetime in the solution with nanopowders were much more steady and having fewer fluctuations indicating the more uniform production of the coating within the PEO procedure[34].

The needed time to get the solution breakdown voltage having graphene was a little more than that of a solution with no graphene.The production of the barrier fil can be postponed by the suspended graphene within the solution[50].This agreed with another investigation [32].As Fig.3 depicts,both the time and the breakdown voltage in order to reach breakdown voltage declined as the concentration of MoS2augmented from 0 to 10g/l.The declining breakdown voltage was made by the rising electrical conductivity of solutions [51].

Fig.3.The voltage-time curves of PEO treatments for electrolytes with different MoS2 NPs concentrations (M0:0,M1:2.5,M2:5,M3:7.5,and M4:10g/l)[51].(With permission from Ref.[51];License Number:4862521398508,Jul 05,2020).

Adding SiC NPs in the solution slowed down the rise in the rate of the voltage.These colloidal particles transferred toward the anode and got gathered on its surface by the electric fiel effect.So,it hindered the discharge breakdown procedure of the anode,as a result slowing down the rate of fil growth.After SiC NPs addition in the solution,both the breakdown and fina voltages declined [69].The solutions with adding particle registered higher fina voltage,showing less resistance in the coating with no particles [32].The breakdown voltage declined by rising the content of Si3N4NPs.Also,the time to get breakdown voltage declined by adding Si3N4NPs [33].Adding SiC NPs indicated less effect on the positive voltage-time responses under the regime of lower current densities.Nevertheless,higher voltage values were discovered for the coating produced in the solution having the SiC NPs suspension under the regime of higher current densities [102].

4.The phase composition of coatings

According to investigations,the PEO coatings applied on Mg and its alloys mostly contain MgO[32,33,62,67,69,77,89,93-95,99,102,34,103,107,108,116,38,42,44,45,50,51,60],Mg2SiO4[32,33,99,102,103,107,60,67,69,77,93-96],MgSiO4[34,44],MgAl2O4[33,44,51,54,62,69,103,116],Mg3(PO4)2[38,42,45,50,67,93,99,103],Mg(OH)2[34,96],and SiO2[94,95] phases (see Table 2).Within the firs steps of the PEO procedure,Mg movements into Mg2+ion and then will have a reaction with other ions that are present within the channels of discharge like O2-,OH-,andthat are gained from the constituents of the solution [47].After particles addition into the base solutions,diffraction peaks associated with particles like WC [42],SiC[102],WS2[96],MoS2[51],Al2O3[38,58],Ta2O5[45],CeO2[54,89],and SiO2[32,65] were revealed in the coatings XRD patterns.This approved the accomplished particles incorporation into the PEO coatings [54,58,65,89,96].

Table 3 The details of wear tests.

Table 3 (continued)

Despite the fact that SiO2,Si3N4,and SiC particles were discovered within the coatings,clay particles were not observed in XRD pattern of the related composite coating.The particles (clay) low melting point showed totally reactive participation and particles having the quite high melting point were mostly inertly participated in the coatings,showing that the particle melting point affected its incorporation mode under a given energy input.The particles SiO2,Si3N4,and SiC were inertly embedded into the coatings,while particles of clay had been melted resulting in an amorphous coating.The particles of clay reactive incorporation verifie that low particles having melting point can be molten through the short-lived discharges and consequently react with other constituents,while particles with a high melting point are difficul to be melted and transferred to new phases [108].The Ta2O5peaks intensity augmented by rising the Ta2O5NPs concentration in the solution that further approved the Ta2O5NPs presence in the coatings[45].In some cases,no particle peaks were seen in the XRD diffraction patterns of the composite coatings [33,60,62,67,77,94-96,107].This was perhaps due to the low amount of particles in solutions [60,67,77,96] or uneven distribution of sheets inside composite coating [95].

5.Microstructure and thickness of coatings

As an unavoidable trait,surface of PEO coatings had a porous morphology such as pores with distinct shapes and sizes accidentally spread over the surface owing to the molten oxide and ejection of gas emission out of plasma discharge channels [32,33,60,67,69,78,89,94-96,99,102,38,103,107,40,45,47,50-52,54].In some conditions,some cracks were produced via releasing thermal stress in addition to pores in the channels of plasma discharge and quick cooling of the molten oxide through the neighboring solution within the growth of the coating procedure that emerged on the coatings surface [21,33,89,94-96,99,103,45,47,50,51,58,60,62,69].

Fig.4.SEM images of coated samples at different Al2O3 concentration and different stirring rates [38].(With permission from Ref.[38];License Number:4862530056873,Jul 05,2020).

Production of composite coatings after particles addition into the base solutions resulted in less porosity of surface as well as number and size of pore in comparison to simple coatings [32,34,77-80,85,94,95,99,38,40,44,50,54,58,60,69].This was due to particles adsorption and trapping within the pores and so,closing the pores outfall[32,34,67,69,77,79,85,89,94,95,37,38,40,42,50,54,59,60].

These particles were incorporated within the coatings micropores via the diffusion process and electrophoresis [117].Based on the images of SEM in Fig.4,the reason for the high porosity of the produced coating at 50rpm was the low absorption of Al2O3NPs in this stirring rate because of low stirring strength.Another cause of the high porosity of surface in the coating at 500 and 700rpm and 40g/l concentration of Al2O3NPs was the synergic effect of the agglomeration possibility in high NPs concentrations in the solution and high tolerance fl w of the solution in high rates of stirring.In cases like this,the NPs absorption into the pores declined and surface porosity augmented remarkably [38].

The produced composite coatings in solutions comprising particles of ZrO2possessed higher porosity than that of SiO2particles [78].The particle uptake was associated with the pores’ size and number,as coating with particles of SiC was more porous in comparison to other coatings [108].By keeping on the improvement in the concentration of GO from 20 to 40ml/l within the solution,the coating porosity percentage was unexpectedly augmented.Moreover,the quick solidifi cation and accumulation of ejected molten oxide from the channels led to the production of many independent triangular islands on the surface of the sample.The number of large-sized triangular islands of coating with 40ml/l GO was considerably more than those of coating having 20ml/l that can be made by the extreme content of GO absorbed on the coating surface [60].

As can be seen in images of SEM in Fig.5,more white particles of nanoscale existed on the surface for the A0S2,A0S3,and A0S4 coatings (having 2,3,and 4g/l Si3N4,respectively) that had higher Si3N4concentrations.The A0 coating (without Si3N4NPs) had the least porosity.Adding Si3N4NPs greater than 3g/l significantl raised the porosity of the coatings [33].

Fig.5.The surface morphologies of coatings with (a) 0,(b) 1,(c) 2,(d) 3,(e) 4g/l Si3N4 NPs and high magnificatio images of surface white particles for coatings with (f) 3 and (g) 4g/l Si3N4 NPs [33].(With permission from Ref.[33];License Number:4862530300165,Jul 05,2020).

Rising the addition of Ta2O5NPs into the solution led to more intensive sparks of discharging at the surface of the specimen within the PEO procedure that resulted in the cracks and bumps development which made the coating uneven and rough.Specificall,the solution having 10mg/l Ta2O5had many big and prolonged sparks spread among many small sparks that caused it to indicate higher roughness of surface having distinct morphology of surface among the coated specimens [45].

The aluminate-based solution having Al2O3NPs presented that both the size and number of the bump on the surface augmented significantl in comparison to simple coating.The NPs attachment to the coating led to the rise in bumps.The solution resistance augmented after adding NPs,a rising voltage of feedback that more accelerated the growth of several preferentially grown areas.This resulted in a big bump[58].

Based on the coatings SEM images in Fig.6,the composite coatings comprised 15 and 25g/l graphite showed a bigger multiscale pore structure having some holes of discharge and numerous micropores than those inside the solution with no addition of graphite.The cracks were present on the surface more obviously,the micropores size rose gradually and the eruption of materials was bigger than those of the specimens with no addition of graphite by rising graphite concentration in the solutions [107].There was a total rise in mean porosity for coatings of 5,7.5,and 10g/l MoS2NPs.The mean size of pore declined primarily with a rise in the content of MoS2NPs and after that augmented as the 10g/l MoS2NPs were added [51].The number of crater-like pores augmented differently by rising the GO concentrations in the solution,whereas the mean diameter of the crater-like pores and panlike structures were significantl diminished.However,when the GO concentration was frequently augmented to 40ml/l,the size of the crater-like pores and pan-like structures unexpectedly augmented [60].There was no difference between coatings having and with no microparticles of SiO2(MPs),apart from a lot of small particles adhering to the composite coating surface [32].

The WC NPs incorporation into coatings possessed no considerable influenc on the diameter of pores[42].Also,after the addition of the additives,the improvement in smoothness and uniformity of coatings were seen [32,78,79,94,95].Adding additives into the base solutions led to an increase in the thickness and/or density of coatings [33,34,60,67,69,77,78,80,94-96,99,37,102,108,38,40,42,44,45,50,51].Several graphene were placed during inner pores in addition to the cracks and several were wrapped in the surface of coating within the plasma discharge.The synergistic affection between the two aspects enhanced the density and thickness of the coating [94].The solution lower conductivity with NPs and consequently higher voltage of breakdown for this solution made the coating growth rate increase [45].

It was concluded that adding GO and graphite improved the solutions conductivity and then strengthened the plasma discharges that was useful for the molten metal oxides accumulation and raised the coatings thickness [95,107].The existence of graphite NPs raised the electrical conductibility of the solution.Thus,a fi ed current density resulted in an increase in the number and homogeneous organization of the microdischarges on the sample surface.So,making a rise in the coating growth rate [99].The electrical conductivity of the graphite particles had an effect on the discharge procedures within PEO process and resulted in a rise in thickness[67].

Fig.6.The SEM images of coatings with different graphite concentration(a) 0,(b) 15 and (c) 25g/l [107].(With permission from Ref.[107];License Number:4862520936000,Jul 05,2020).

The thickness rise in the composite coatings (up to 3g/l Si3N4) was made by the production of Mg2SiO4phase due to the reaction between molten MgO and Si3N4NPs within plasma discharge [33].Even though the coatings thickness was improved by addition of MoS2NPs,it declined by rising the content of MoS2NPs within the solution [51].However,adding the additives into the base solutions occasionally declined the coatings thickness [32,34,40,42,50,108] or did not influenc them [54,60].The coating thickness having the clay particles addition was thinner than that of coatings having particles of SiO2,Si3N4,and SiC [108].

6.Wear behavior of coatings

The detail of wear tests conducted in published papers are summarized in Table 3 [32,33,50-52,54,58-60,62,65,67,34,69,77-80,83,85,93-95,37,96,99,102,103,107,108,38,40,42,44,45,47].In a lot of cases,the ball-on-disk measurement has been utilized.The loads have been in the range of 1 to 45N.Then,the coatings wear behavior will be separately reviewed for distinct additives.

Table 4 Extracted data from wear tests for simple and composite coatings.

6.1.TiN

After adding TiN NPs into the base solutions,the NPscontaining coatings wear resistance was improved comparing with the coatings with no NPs [77,79,80,85].In spite of the incomplete oxidation of TiN NPs to oxynitride and TiO2,the participation of these nanostructured materials into the PEO coatings let the solid ceramic-like coatings form having high microhardness and wear-proof functions[85].Because of lower porosity and higher microhardness,the coatings having TiN NPs tolerated more cycles of load in comparison to the simple coating [79].

Rising the TiN NPs concentration from 3 to 4g/l resulted in less wear protection [77,80,85].The 4g/l TiN NPs concentration resulted in a considerable agglomeration that in turn resulted in a decline in adhesive strength and the coatings mechanical properties.The cause of such behavior can be associated with the production of the more brittle coatings comparing with that produced in the solutions having less concentration of TiN NPs [77,80].

6.2.Si3N4

Even though the coating with 4g/l Si3N4NPs possessed the least coefficien of friction (COF),the deeper and wider wear track showed its week wear resistance.On the other hand,the hardest coating with 3g/l Si3N4NPs indicated a less COF and a narrow track of wear,both showing a proper wear resistance.Producing more phases of Mg2SiO4and MgO and related to the main microstructure of MgAl2O4may be as a result of the hardness enhancement inside coating of 3g/l Si3N4.More fla areas were discovered for the tracks of wear in coatings with 2 and 4g/l Si3N4NPs because of their lower hardness,while less fla areas were seen for the hardest coating with 3g/l Si3N4[33].The coating wear resistance was enhanced via the Si3N4MPs inert participation [108].

6.3.WC

The WC nanocomposite coatings wear resistance was superior to simple ones [34,42,44].The participation of WC NPs into the coatings can raise the coatings hardness according to the rule of the hardness in composite materials as the WC hardness is more than the PEO coatings [42,44].Thus,WC NPs can improve the PEO coatings wear resistance.

6.4.SiC

Adding SiC NPs into the base solutions led to acquiring composite coatings having less and more stable COF than those prepared in SiC-free solutions.This was because of the rolling-effect produced by SiC NPs on the coatings surface that functioned as lubricants within the sliding measurements[47,103].The SiC composite coatings wear resistance was much superior than the simple ones [37,40,47,62,69,102,103].Wear resistance of composite coatings was superior to simple coatings as the composite coatings hardness was more than simple ones [40,103].According to the wear tracks SEM images in Fig.7,the whole worn surfaces of composite coatings having SiC NPs were relatively smooth and no proof of considerable coatings detachment was discovered.These observations showed that just a little wear damage happened on the composite coatings surface within the sliding test.

Fig.7.PEO coatings wear track acquired in the electrolyte of aluminate-silicate in the presence (c,d) and absence (a,b) of SiC NPs in current densities of 11.5 (a,c) and 23mA/cm2 (b,d);electrolyte of phosphate-based in the absence (e,f) and presence (g,h) of SiC NPs in current densities of 27.5 (e,g) and 55mA/cm2 (f,h) [103].(With permission from Ref.[103];License Number:4862521135992,Jul 05,2020).

Fig.8.Surface morphology of the wear tracks of the PEO coatings (a,b) without SiO2 particles,(c,d) with SiO2 NPs,(e,f) with SiO2 MPs [32].(With permission from Ref.[32];License Number:4862520017062,Jul 05,2020).

SiC is categorized in particles of ultra-hard and its hardness could be got at the last 3000 HV.As these ceramic particles of nano-sized were incorporated into coatings,they can play a protective role in improving the wear resistance even in a very low volume fraction.These SiC NPs were spread on the coatings surface and after that got involved in the wear procedure within the sliding measurement.These particles diminished considerably the frictional shear stress between steel balls and the coating and as a result led to a decline in the COF and coatings wear rate [47,103].The participation of the SiC NPs led to an increase in the thickness of the coating and the decline in the roughness of the surface that may have also helped their enhanced wear behavior [102].The incorporation of inordinate content of SiC MPs was harmful to the wear behavior of the coating [108].

6.5.WS2

The WS2composite coating exhibited an additional enhancement in anti-wear behavior in comparison to the simple coating.This considerably improved the anti-wear behavior of composite coating was because of the impressively higher degree of density,the thermodynamically stable phases,and fewer defects.These properties can efficientl enhance hardness and bonding strength and that may in turn help the great anti-wear behavior [96].

6.6.MoS2

The lowest wear rate was observed for the coating with 2.5g/l MoS2NPs and a similar amount was also discovered for coating having not any of such particles.On the other hand,rather high rates of wear were observed for the coatings having 5 and 10g/l MoS2NPs.Even though it was the hardest,the brittle nature of coating with 10g/l MoS2NPs produced poor wear and adhesion resistance.The addition of 2.5g/l MoS2NPs inside the solution prepared a quite compact microstructure having a higher hardness,proper adhesion,and high wear resistance.MoS2NPs higher concentration within solutions sounded to degrade the coating mechanical performance,likely owing to the production of a coating with more porosity,a discontinuous inner film and non-uniformly distributed NPs [51].

6.7.SiO2

Based on the worn surfaces SEM images in Fig.8,the track of wear in simple coating sounded to be wholly omitted within the test of wear that was persistent with the deep wear in the depth of profil (Fig.9).Nevertheless,the coatings with SiO2particles were revealed to be intact in the track of wear.This observation was also maintained by the low depth of wear track.The coatings hardness augmented by adding particles,so the wear rate of coatings having SiO2particles was less than that of the coating without SiO2.The SiO2NPs containing coating showed a lower wear rate than coating having SiO2MPs [32].Owing to the higher hardness of coating with SiO2NPs,it tolerated many load cycles,and wear of it was less than that for the coating with no SiO2[78].

Fig.9.Wear depth profil of the PEO coatings [32].(With permission from Ref.[32];License Number:4862520017062,Jul 05,2020).

In one condition,after the addition of SiO2NPs having a size of 35nm into the base solution,no major change in the wear intensity of coating was seen.The coating having 50nm particles of SiO2possessed the worst wear intensity [93].The improved tribological performance of composite coatings having SiO2can be attributed to the thick fil with inertly embedded SiO2MPs on the surface of the coating or higher hardness [32,65,108].

6.8.ZrO2

ZrO2NPs addition into the base solutions led to the production of composite coatings having higher hardness and less porosity and consequently higher wear resistance than simple coatings [78,79].

Fig.10.SEM images showing the appearance of the worn surfaces of coatings (a) without and (b) with CeO2 NPs [54].(With permission from Ref.[54];License Number:4862520448036,Jul 05,2020).

Fig.11.Local enlarged images of the samples and worn surface morphologies after wear experiments at (25°C):(a) Mg-Li alloy;(b) GO-free coating;(c)GO-containing coating [95].(With permission from Ref.[95];License Number:4862520751964,Jul 05,2020).

6.9.CeO2

The appearance of the worn surfaces in Fig.10 exhibited that the coating with no CeO2NPs suffered from more severe wear in comparison to the coating with CeO2,showing a rise in the wear depth having the load.The coating embedded with CeO2NPs stayed intact within the wear measurements for all loads.The wear rates of the coatings with CeO2NPs were less than those for the coatings having no CeO2in all loads.It exhibited that the presence of reinforcing CeO2NPs can considerably enhance the wear resistance of PEO coatings.

The value of the damage was augmented with load from 2 to 10N for both composite and simple coatings.The fla es detachment from the coating worn surface to the steel ball was declined for the coating having CeO2NPs in comparison to the coating with no CeO2.This could be ascribed to the fact that abrasive wear coped with the adhesive mechanism for the composite coating.The CeO2NPs integration into the coating resulted in the decline in the micropores and cracks.Furthermore,the following decline in roughness may be responsible for the improvement of the wear behavior.Moreover,the MgAl2O4phase formation having high hardness within the coatings exhibited a higher capacity of load bearing [54].

Fig.12.Wear tracks 3D topographies after wear measurements at distinct temperatures:(a,c,e) 25°C and (b,d,f) 200°C [95].(With permission from Ref.[95];License Number:4862520751964,Jul 05,2020).

6.10.ZrO2/SiO2

The wear of the coating with ZrO2/SiO2NPs was lower than that for the free NPs coating.This trend can be clarifie via the high hardness of the ZrO2/SiO2NPs [83].

6.11.Al2O3

It was seen that adding Al2O3NPs in solutions resulted in a lower wear rate of coatings [38,52,58,59].This was due to the fillin of the fil pores via NPs [38].At the start of the wear test,the porous fil of the coating was abraded and the NPs were pulled out from the structure due to the abrasion and played the role of lubrication [52].The quite high hardness of the Al2O3NPs turned sliding friction into rolling friction and declined volume of wear [58,59].

The least rate of wear was at the 30g/l Al2O3NPs concentration followed by 20,40,10,and 0g/l.Indeed,the rate of wear was declined by rising NPs absorption [38].Coatings containing high incorporation of Al2O3NPs had a mechanism of abrasive wear (samples coated with additive).For coatings having low participation of Al2O3NPs,the controlling wear mechanism was adhesive wear (samples coated without additive) [59].

6.12.Ta2O5

The COF of PEO coatings with and with no Ta2O5NPs was less in comparison to the uncoated substrate.The coatings wear rate declined as the Ta2O5value augmented from 1 to 10mg/l.This was due to the roughness of the surface in the coatings augmented by adding Ta2O5owing to a rise in the number of microcracks and pores.The coating having 10mg/l Ta2O5NPs showed distinct wear properties due to its distinct morphology of surface and higher roughness.This coatingwas approximately deleted from the substrate that showed its plowing treatment.For the coating of 5mg/l Ta2O5NPs,the result depicted great wear resistance due to the absence of the debris that did not peel off.Also,they showed that just the adhesive wear procedure was involved [45].

6.13.Clay

The reactive participation of clay particles helped to produce a dense fil and so enhanced the wear resistance of the coating [108].

6.14.Carbon-based materials (graphite,graphene,and graphene oxide (GO))

It was seen that the COF of the composite coatings was less and stable than that of the simple coatings [50,94,99].The presence of the C element and content of hard phases within the composite coating declined considerably the COF of the specimens[94].Nanosheets of graphene that functioned as solid lubricants inside the coatings were put between the contact surfaces within wear test and decreased COF [50].Adding graphite NPs was more efficien in declining friction for the AZ91 alloy than for AZ80.In detail,the existence of graphite allowed to acquire a denser coating and work as a solid lubricant declining the COF [67].After adding carbonbased additives into the base solutions,the resulting composite coatings exhibited enhancement in tribological behavior[50,60,95,96,99].The synergistic impacts of the decline in the COF and roughness and the rise in the hardness of surface were the main reason for improving the tribological treatment[60,94,95].

Primarily,graphene declined its roughness through fillin the microcracks and micropores of the composite coating and as a result,a rise in the real contact zone and COF,densifie the coating and raised its thickness.In addition,dispersed graphene inside the composite coatings matrix occupied a pinning and second phase strengthening the role that enhanced the hardness of the composite coatings.Consequently,the synergistic effects of the decline in COF and improvement in hardness of surface for the composite coating declined the plowing effect that induced a decrease in the wear track depth[50,94].The effect of self-lubricating in graphene and GO resulted in the limitation of abrasive wear in composite coatings[50,60].

As the morphologies of worn surface and local big images indicate in Fig.11,the worn surface morphology of the Mg-Li alloy exhibited a common characteristic of adhesive wear,associated with considerable grooves and peelings paralleling to the direction of frictional.For the GO-free coating,the total wear trace showed hard adhesive and abrasive wear having many peelings and debris on the surface of the coating.Nevertheless,the worn trace is not as deep as and quite narrower than that of the alloy of Mg-Li that offered the coating without GO was present to possess improved the Mg-Li alloy wear resistance against the adhesive and plow wear.From the local increased images of this coating,it can be observed that some wear debris was entrapped in the micropores under the stainless steel ball stress and the GO-free coating was not deleted within the tribological measurement.The wear trace of the GO-containing coating was divulged less deep and narrower and the worn zone sounded relatively smooth.The re-modificatio and smoothening effects on the morphology of surface in the GO-containing coatings were owing to the rubbing action of the stainless steel ball.Also,the micropores showed on the surface of the coating can serve as containers for fin debris that played an antifriction role within the sliding procedure.

Both the wear depth and wear width of the coatings declined considerably in comparison to the substrate,particularly in the case of the GO-containing coating as can be seen in Fig.12.Because of the severe abrasive and adhesive wear which happened at 200°C,the depth and width of the wear traces were bigger than those at 25°C.Moreover,owing to the severe abrasive and adhesive wear that happened at 200°C,the depth and width of the wear traces were bigger than those at 25°C [95].The rates of wear in the case of the GO composite coatings declined in the consequent order:5ml/l GO>10ml/l GO>40ml/l GO>20ml/l GO [60].

7.Comparison of wear protection of different additives

The extracted wear rate of tribology measurements for coatings before and after adding additives into the base solutions is summarized in Table 4 [32,33,50-52,54,58-60,62,65,67,34,69,77-80,83,85,93-95,37,96,99,102,103,107,108,38,40,42,44,45,47].Furthermore,the estimated decrease in the rate of wear in coatings having additives related to coatings with no additives is also given.The coatings wear rates (W) can be obtained from the Eq.(1) [45,47,58,60,103]:

thatVshows wear volume,Lis sliding distance andNstands for load [45,47,58,60,103].

While nano-additives decreased the rate of wear in PEO coatings from about 13.16 to 99.8%,these numbers were about 52.86-80.27% in the case of micro-additives.The most decrease in the rate of wear was for SiO2MPs and MoS2NPs.However,as the rate of wear has not been dealt with in some investigations,these results can be varied.

8.Conclusions

Surface modificatio of Mg and its alloys after the production of hard,porous,and thick coatings using the PEO technique has led to the considerable improvement in their wear resistance.Incorporating the additives into solutions and following the participation of them into the coatings has led to a more rise in wear protection of PEO coatings that were applied on substrates of Mg.Nevertheless,the participation mode in terms of reactive,partly reactive,or inertness,size,melting point,and zeta potential of additives affect the participation degree of additives and wear improvement of coatings.In addition to the principal phases like Mg2SiO4,MgO,MgSiO4,MgAl2O4,Mg3(PO4)2,and Mg(OH)2obtained from reactions among elements of electrolytes and Mg2+ions,additives peaks also have showed in diffraction patterns of XRD for composite coatings pointing to their successful participation.

PEO coatings applied on substrates of Mg had a porous structure containing many pores arisen from ejected molten oxide and gas emission out of plasma discharge channels and cracks created via thermal stress release in the channels of plasma discharge and quick cooling of the molten oxide through the surrounding solution.Adsorption and trapping particles in the pores and as a result,closing the outfall of pores has resulted in less porosity of surface as well as size and number of the pore.Factors such as higher thickness,lower porosity,higher density,higher surface hardness,higher bonding strength,lower roughness of the surface,and solid lubricity role of additives resulted in a lower rate of wear in composite coatings in comparison to the simple ones.

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.