Influence of ZrO2/SiO2 nanomaterial incorporation on the properties of PEO layers on Mg-Mn-Ce alloy

D.V.Mshtlyr ,I.M.Imshinetskiy ,K.V.Ndri,b ,A.S.Gnedenkov ,S.L.Sinebryukhov ,A.Yu.Ustinov ,A.V.Smokhin,S.V.Gnedenkov

aInstitute of Chemistry,Far Eastern Branch,Russian Academy of Sciences,Vladivostok 690022,Russian Federation

b Far Eastern Federal University,Vladivostok 690922,Russian Federation

c A.A.Baikov Institute of Metallurgy and Materials Science,Russian Academy of Sciences,Moscow 119991,Russian Federation

Abstract The properties of coatings formed on the MA8 magnesium alloy by the plasma electrolytic oxidation in electrolytes containing mechanical mixture of zirconia and silica nanoparticles in concentrations of 2,4 and 6 g/l have been investigated.It has been established by SEM,EDS,and XPS that ZrO2/SiO2 nanoparticles successfully were incorporated into the coatings.Micro-Raman spectroscopy showed the presence of ZrO2 in tetragonal and monoclinic forms in the PEO-coating composition as well as Mg2SiO4 in tetrahedral configuration uniformly distributed in the outer part of coatings.Obtained coatings significantly reduce corrosion current density in comparison with bare Mg alloy and base PEOlayer (from 2.4×10-7 A/cm2 for base PEO layer to 0.7×10-7 A/cm2 for coatings with nanoparticles).It has been found that the presence of solid nanoparticles in the composition of coating has a positive effect on their hardness (this parameter was increased from 2.1±0.3 GPa to 3.1±0.4 GPa) and wearproof (the wear was reduced from (4.3±0.4)×10-5 mm3/(N × m) to (3.5±0.2)×10-5 mm3/(N × m)).

Keywords: Plasma electrolytic oxidation;Composite coatings;Nanoparticles;Zirconia;Silica;Protective coatings.

1.Introduction

Magnesium alloys possessing low density and high specific strength are now actively used in the automotive industry,aviation,medical equipment and electronics.The main limiting factor of their wider usage is the high corrosion activity and low wear resistance.Moreover,traditional corrosion protection methods,such as painting and anodizing,have a limited range of use for these materials due to low adhesion and hardness of the applied layers [1-4].

Currently,the coatings formation technology by the plasma electrolytic oxidation (PEO) is being intensively developed[5-9].This way allows forming hard,wearproof layers with high anti-corrosion properties.

One of the directions of PEO’s development involves the use of nanoscale materials that have certain physicochemical properties in the electrolyte.These properties allows improving the performance characteristics of the surface layers and expand the area of practical usage of coated samples [10-15].

Currently,there are a number of works,in which coatings modified with zirconia and silica nanoparticles are studied.These studies cover a wide range of different aspects that are affected by the incorporation of nanoparticles.

Thus,in works [16-19] it has been established that incorporation of zirconia nanoparticles leads to changes in the coating morphology,reduces porosity and surface roughness.Electrochemical studies of coatings revealed a significant increase in the values of polarization resistance and impedance modulus,while the protective characteristics remained at the high level even after a week exposure in an aggressive environment;this confirm the high anti-corrosion properties of coatings,containing zirconia nanoparticles.

In several studies,materials based on zirconia are used in biomedicine to obtain implants or various coatings for implants [20-23].Zirconia is bio-neutral,and it showed results comparable to titanium without affecting the growth rate of osteoblasts [20].At the same time,ZrO2nanoparticles provide an anti-bacterial properties;after incorporation in the PEO-coatings,the bacterial growth rate decreases significantl[21].

The incorporation of silica nanoparticles in the composition of coatings allows reducing the porosity and improving corrosion properties.The silica nanoparticles reduce the coefficient of friction that has a positive effect on the wearproof of the surface layers.In addition,silica shows biocompatible properties.Therefore,they are used to create coatings and composites,which can be used in biomedicine [24-28].

In addition,there are a number of studies,in which PEO-coatings are modified simultaneously by ZrO2and SiO2nanoparticles [29,30].Such coatings make it possible to combine the positive properties of both materials for further improving the characteristics of the obtained layers.

Based on previously obtained results [31,32] of forming and studying of the nanostructured PEO-coatings containing separately ZrO2or SiO2nanoparticles,in this work,coatings formed by plasma electrolytic oxidation using ZrO2/SiO2nanoparticles mixture were investigated.

This paper is devoted to the study of the properties of PEO-layers formed on the MA8 magnesium alloy in silicatefluorid electrolyte with the addition of nanoparticles consisting of zirconia and silica (ZrO2/SiO2).These nanoparticles are a mechanical mixture at the nanoscale level and were obtained by plasma-chemical synthesis from ZrSiO4[33].

Modification of PEO-coatings with such nanoparticles provides the surface layers high chemical resistance and hardness and leads to a significant improvement in anti-corrosion properties and wearproof of samples.

2.Materials and methods

2.1.Samples

MA8 Mg alloy (Mn 1.30;Ce 0.15;Mg bal.(wt.%))was studied in this work.The size of specimens was 20 mm×15 mm×2 mm.Rectangular samples were mechanically ground with sanding paper of various grit (600,800,and 1200),washed with deionized water and ethanol,and then air-dried.

2.2.Electrolyte preparation

Based on the positive results of previous studies,in this work we used nanoparticles mixture of zirconia and silica,obtained by plasma chemical synthesis with a size of 100±20 nm.ZrO2/SiO2nanoparticles (70/30 wt.%) (Fig.S1)were incorporated in the electrolyte and dispersed using a Sonopulse HD 3200 ultrasonic homogenizer (Bandelin,Ger-many).The method of photon correlation spectroscopy was used to study theξ-potential and size of agglomerates of nanoparticles in suspension using Zetasizer Nano ZS analyzer(Malvern Instruments,UK).

It is well known,that the growth of PEO-coating is realized during anodic polarization of the specimen [34].Therefore,to promote the nanoparticles incorporation in the coating’s composition the anionic surfactant NaC12H25SO4(sodium dodecyl sulfate) was added to the solution.Its concentration in the prepared electrolyte was 0.5 g/l.As a base electrolyte,the solution,containing the sodium fluoride (5 g/l) and sodium silicate (15 g/l),was chosen.The conductivity of this electrolyte is 16-17 mS,and pH value is 10.7-10.8.The concentration of nanoparticles in the prepared electrolyte was 0,2,4 and 6 g/l (Table 1).The analytical grade chemicals were used in this research.

Table 1Composition of the used electrolytes.

2.3.Coatings formation

The process of coatings formation was carried out using a plasma electrolytic oxidation unit.The methodology of PEO process was described in detail elsewhere [35].During PEO,the polarizing pulse frequency was 300 Hz.All samples were processed in two-stage bipolar PEO mode.During the firs stage (200 s),the anodic and cathodic components were in potentiodynamic (from 30 V up to 300 V) and potentiostatic(-30 V) modes,respectively.For the second stage (600 s),the anodic and cathodic components were both in potentiodynamic modes (from 300 V down to 200 V,from -30 V down to -10 V).The duty cycle was 50%.

2.4.Coatings morphology and composition

The study of the surface topography was carried out by the optical laser profilomer using the OSP370 device installed on the M370 workstation(Princeton Applied Research,USA).Image analysis was performed using the Gwyddion 2.45 software.Surface topography was characterized by the most common roughness parameters,namely,Ra(arithmetical mean deviation of the profile)Rz(ten point height of irregularities);Rt(maximum height of the profile)Rv(maximum profile valley depth),as well as the ratio of the actual surface area to the area of its orthogonal projectionSreal/Spr.

Microphotographs of the samples surface were obtained using a Carl Zeiss EVO 40 scanning electron microscope(SEM) (Carl Zeiss,Germany).The elemental composition of the surface layers was determined by energy dispersive spectroscopy(EDS)using the INCA X-act EDS analyzer(Oxford Instruments,USA).The coating thickness was studied using of the SEM images of the samples’ cross-section.

The X-ray photoelectron spectroscopy (XPS) was used for the analysis of the chemical composition of the investigated coatings using the SPECS device (SPECS,Germany)with the 150 mm hemispherical electrostatic analyzer.The spectra were excited with AlKα-radiation.Calibration of the binding energy was carried out with reference to the C1s line (285.0 eV).The ion beam with the energy of Ar+ions 5000 eV was used to etch the sample.The etching time was 5 min;the average etching rate was ?10 °A/s.

The structure of the ZrO2/SiO2-containing PEO-coatings was investigated using micro-Raman spectroscopy using Raman spectrometer alpha 500 (WITec,Germany) and WITec Control program.Micro-Raman spectra were collected in the range from 100 to 1200 cm-1during 1 h (60 accumulated spectra) using a laser with 30 mW irradiation power and 532 nm wavelength.2D maps of ZrO2and Mg2SiO4intensity distribution on the surface of PEO-coating were obtained in the scanning mode from the area 55×45 μm,which contains 45×45 micro-Raman spectra,with 1 s integration time.

2.5.Electrochemical studies of coatings

The electrochemical tests were carried out using VersaSTAT MC (Princeton Applied Research,USA).Electrochemical impedance spectroscopy and potentiodynamic polarization were performed at room temperature using three-electrode cell Model K0235 Flat Cell (PAR,USA).The samples were studied in 3 wt.% NaCl solution.The counter electrode was a niobium mesh covered with platinum.The reference electrode was saturated calomel electrode (SCE).The area of contact of the sample with electrolyte was 1 cm2.Before electrochemical tests,the specimens were immersed in the solution for 60 min to stabilize the electrode potential.The scan rate for potentiodynamic polarization was 1 mV/s.The samples were polarized fromEC-0.15 V up toEC+0.50 V.For electrochemical impedance spectroscopy test the sinusoidal signal(10 mV,rms) was used.Spectra were recorded from 0.1 MHz down to 0.1 Hz at a logarithmic sweep rate 10 points/decade.Additionally,for more detailed evaluation of corrosion properties of studied samples,the impedance spectra were recorded after 2 and 24 h of sample exposure in 3 wt.% NaCl solution at steady state conditions in the frequency range from 0.1 MHz to 0.1 Hz with a logarithmic sweep rate of 7 points/decade.

Corrosion potential(EC),corrosion current density(IC),the slope of the cathodic and anodic polarization curve (βc,βa,respectively) were calculated using the Levenberg-Marquardt(LEV) method [36,37],Eq.(1):

The polarization resistance (RP) was calculated in separate experiment using potentiodynamic polarization test in accordance with Eq.(2).The specimens were polarized fromEC-0.02 V up toEC+0.02 V.The scan rate was 0.167 mV/s.

For a visual assessment of the behavior of the coatings during their exposure in an aggressive environment,the samples were immersed in 3 wt.% NaCl solution for 14 days.

2.6.Hydrogen evolution tests

Hydrogen evolution tests were carried out using eudiometers (art.no.2591-10-500 from Neubert-Glas,Germany) at room temperature.C0 and C4 samples were studied in 3 wt.%NaCl solution (pH=7.1).The methodic of hydrogen evolution tests was described in detail in [38].Eight specimens with a size of 10 mm×10 mm×1.5 mm with total surface area of 20.8 cm2were immersed in 500 ml of the solution for 35 days.NaCl solution was stirred at 400±100 rpm.Tests were carried out three times for C0 and C4 samples.The measurement error was about 5%.After the hydrogen evolution tests,samples were rinsed with water and air-dried.

2.7.Mechanical performance of coatings

The study of mechanical properties(the microhardness and elasticity modulus measurements),was carried out using a DUH-W201 dynamic ultra-micro hardness tester (Shimadzu,Japan).The universal microhardnessHμwas measured on sample cross-section using a Vickers indenter at the load of 100 mN.

The adhesive properties of the surface layers were investigated by the scratch testing using Revetest Scratch Tester(CSM Instruments,Switzerland).The experiments were carried out at a track length of 5 mm with a gradual increase of the applied load from 1 to 30 N with a rate of 10 N/min.Rockwell diamond indenter was used for scratch testing.The following parameters were determined for each type of coating:LC2is the load,at which the beginning of peeling of coating areas was observed,LC3is the load,at which abrasion of the coating to the substrate occurs.

Tribological properties of coatings was studied using a Tribometer TRB-S-DE device (CSM Instruments,Switzerland).The ball with a diameter of 10 mm made of silicon nitride was used as a counterbody.Dry friction condition was realized during tribological tests [39].This experiment was performed at room temperature.A sliding speed and the applied load (F) were 50 mm/s and 10 N,respectively.The experiment was stopped,when the coating was abraded to a metal substrate.To estimate the wear,the profile were studied after the tests,using a Surtronic 25 profilomete (Taylor Hobson,UK).The wear was estimated using the track profiles and,thus,the wear rate (P,mm3/(N×m)),was calculated using Eq.(3):

whereΔVis the worn volume(mm3),Nis the distance moved(m),andFis the normal load (N).

Fig.1.X-ray patterns of the ZrO2/SiO2 nanoparticles obtained by plasmachemical synthesis.

The volume loss of the sample was calculated according to the Eq.(4):

whereLis the length of the of the wear track (mm),Sis the area of the track cross-section (mm2).

In all the experiments,the wear of the counterbody was not detected and not taken into account.

3.Results and discussion

3.1.Coatings morphology and composition

ZrO2/SiO2nanoparticles are a promising material for surface modification in order to improve the characteristics of the coatings formed on metals and alloys due to the high chemical stability and mechanical properties of zirconia and silica.

These nanoparticles were obtained by plasma-chemical synthesis from Zircon.XRD patterns of used nanoparticles presented on Fig.1.As result of temperature impact,zirconia is presented in two crystalline forms:monoclinic (PDF#00-037-1484) and tetragonal (PDF# 00-050-1089).

The success of the nanomaterials incorporation into composition of coatings during the PEO strongly depends on the size of the nanoparticles and their electrokinetic potential in the used electrolyte.It was established that the addition of the anionic surfactant to aqueous electrolytes and triple ultrasonic treatment (UST) give the particles a negativeξ-potential,reduce the hydrodynamic diameter (Table 2).

Table 2Parameters of the ZrO2/SiO2 nanopowder aqueous suspension with a concentration of 1 g/l at different treatment.

Analysis of the data obtained by scanning electron microscopy revealed a difference in the morphological features of the surface (Fig.2a,c,e,g) and cross-sections (Fig.2b,d,f,h) of PEO-layers formed in electrolytes without and with different concentrations of the ZrO2/SiO2nanoparticles.On the surface of the coatings containing nanoparticles the new formations (Fig.2c,e,and g) are clearly observed,which are the result of the incorporation of agglomerates into the coating during plasma electrolytic oxidation.Analysis of the EDS results confirm the presence of zirconium in the coating(Fig.3) as one of the elements of the powder presented in the electrolyte.Note the silicon incorporated into the coating not only from the solid phase but also from the electrolyte(sodium silicate,15 g/l).

Fig.2.SEM images of surfaces (a,c,e,g) and cross-sections (b,d,f,h) of coatings:C0 (a,b),C2 (c,d),C4 (e,f),C6 (g,h).

Fig.3.SEM image (a) and maps of elements distribution for the C6 sample:O (b),Mg (c),Si (d),Zr (e),F (f).

Fig.4.Surface topography of the C0 (a),C2 (b),C4 (c),C6 (d) samples.

Morphology of the surface layers obtained in electrolytes containing nanoparticles differs significantly from the morphology of PEO-coatings formed in silicate-fluorid electrolyte without nanoparticles.Fig.4 shows the 3D surface map of the studied samples.The analysis of surface topography parameters (Table 3) indicated that an increase in the concentration of ZrO2/SiO2nanopowder leads to a nonlinear change in the roughness parameters.The highest concentration was observed for C4 sample.

X-ray photoelectron spectroscopy confirmed the incorporation of the ZrO2/SiO2nanoparticles into the surface layers.The form of the survey,high-resolution spectrum obtained by XPS,and chemical composition of the C4 sample are presented in Fig.5 and Table 4,respectively.The top layer of the sample (before etching) consists mainly of elements presented in the substrate and electrolyte.The relative content of elements and the shape of the spectra (position and ratio of components) indicate the presence in the studied layer of the less oxidized state of the magnesium and silicon,possibly SiO and Mg,along with the oxides (MgO and SiO2,respectively).The zirconium content is equal to 0.2 at.%;its state,according to the binding energy of 3d5/2 electrons (about 182.5 eV),can be characterized by the oxidation state+4.From the analysis of XPS data,it was found that silicon in the top layer of the sample presented as silicon dioxide(SiO2)(Fig.5c);however,part of silicon may be in a less oxidation state(SiO)(Fig.5c).To assess the quantitative and qualitative elemental composition of coating,the top layer was removed by Ar+etching.The etching of the upper layer exposed the underlying layer,which differs from the top one by a more significant content of Mg and Si.Moreover,in accordance with the amount of oxygen,the number of the less oxidation number of Mg and Si are probably increased.This assumption is confirmed by an increase in the low-energy component of the Si 2p spectrum (Table 4).Low oxidized state of some elements may be explained by either their reduction in the hydrogen contain atmosphere,obtained during PEO,or the reduction of these elements as a result of Ar+etching [40].

Table 3Thickness and roughness parameters of the studied samples with coatings obtained in silicate-fluorid electrolyte without nanoparticles and with ZrO2/SiO2 nanoparticles in various concentrations.

Table 4Binding energy and the concentration of the main elements (in parentheses) for the PEO-coatings surface layers formed in an electrolyte with a 4 g/l ZrO2/SiO2 nanoparticles concentration.

Table 5The elemental composition of the studied coatings.

Fig.5.XPS spectra of the surface of C4 coating:(a) Survey spectrum for as-prepared sample (1) and after Ar+etching for 5 min (2);(b) Zr 3d high resolution spectrum;(c) Si 2p high resolution spectrum.(For interpretation of the references to color in this figure legend,the reader is referred to the web version of this article.)

According to the elemental analysis (Table 5),with an increase in the concentration of ZrO2/SiO2nanoparticles in the electrolyte,there is a gradual increase in the content of the silicon and zirconium in the coatings.The difference in the results of elemental analysis obtained by different methods is due to the fact,that in the case of XPS only the upper part of the coating with thickness of a few °A was investigated.While results obtained by the EDS analysis due to the dipper penetration of the electrons were averaging over the more thickness of the coating.

Since Raman spectroscopy is a useful technique to study the polymorphs of ZrO2[41],it was used to investigate the ZrO2/SiO2nanopowder.Fig.5 shows the micro-Raman spectrum of the studied powder,which exhibits Raman bands of tetragonal ZrO2at 146 cm-1,272 cm-1,319 cm-1,459 cm-1,590 cm-1,and 645 cm-1[42-44],together with Raman bands at 158 cm-1,177 cm-1,190 cm-1,221cm-1,306 cm-1,333 cm-1,347 cm-1,373 cm-1,511 cm-1,537 cm-1,566 cm-1,612 cm-1,and 638 cm-1attributed to monoclinic ZrO2[42,44,45].The observed shifts in bands at 255 and 623 cm-1is typical for cubic ZrO2[46].

In this ZrO2/SiO2powder the Raman peaks around 458-471 cm-1(which overlapped witht-ZrO2) and 767-775 cm-1can be assigned as Si-O-Si rocking and bending vibrational modes,respectively[47-50].The band at 815 cm-1is responsible for Si-O stretching vibrational mode [42,51].The peak around 910 cm-1is related to the SiO-bond (O-:nonbridging oxygen) [52,53] and the band at 762 cm-1corresponds to the Si-O-Si symmetric stretching vibrational mode of tetrahedral oxygen bridging [53,54].

Fig.7.Micro-Raman spectra collected at different points of the ZrO2/SiO2-containing PEO-coating that show the presence of monoclinic and tetragonal forms of ZrO2 (a) and Mg2SiO4 (b) in the coating composition.

Two different micro-Raman spectra were recorded for the PEO-coating with ZrO2/SiO2nanopowder.The firs one shows bands attributed to the mixture of monoclinic and tetragonal ZrO2(Fig.7a).This spectrum is similar to the one obtained for ZrO2/SiO2nanopowder described previously(Fig.6).Fig.6b presents spectrum of other type,which has weak peaks corresponded to monoclinic (at 511 cm-1) and tetragonal (at 590 cm-1) forms and sharp peaks around 800-900 cm-1,which can be related to silicates in tetrahedral configuration [55,56] as a result of Mg2SiO4presence in the composition of PEO-coating [57].These two micro-Raman spectra presented in Fig.6a,6b were collected in different points of PEO-coating marked as Point 1 and Point 2 in Fig.8,respectively.

Fig.8.The optical image of the investigated area (a) and corresponded 2D maps of ZrO2 (b) and Mg2SiO4 (c) intensity distribution on the surface of PEO-layer.Points 1 and 2 show areas,where micro-Raman spectra depicted in Fig.6a and Fig.6b were collected,respectively.

Fig.9.Polarization curves for the C0 (1),C2 (2),C4 (3),C6 (4) samples.

2D maps were obtained using Raman spectroscopy to show the intensity distribution of ZrO2and Mg2SiO4on the PEO coating surface.For 2D map of ZrO2distribution,the micro-Raman spectra were collected in the range of its high-intensity bands from 560 up to 620 cm-1(for tetragonal zirconia)and from 490 up to 530 cm-1(for monoclinic zirconia).The micro-Raman spectra were collected in the range from 810 up to 870 cm-1to obtain the 2D map of Mg2SiO4distribution.Fig.8 presents the optical image(a)of the investigated area of PEO-coating and corresponded to 2D intensity maps of ZrO2(b) and Mg2SiO4(c) distribution acquired from this zone.Note that 2D maps for tetragonal and monoclinic zirconia were practically the same,therefore,we presented only map of tetragonal ZrO2distribution.This result indicates that both forms of zirconia connected with each other and located in the same places of PEO-layer.Analysis of the results showed that maximum zirconia concentration presented in the pores and inner layer of PEO-coating,whereas Mg2SiO4uniformly distributed in its outer part.ZrO2presented in the composition of PEO-coating in small,up to some micrometers,agglomerations,which were homogeneously distributed on the surface of PEO-coating.

3.2.Electrochemical properties of coatings

Analysis of the polarization curves (Fig.9) shows a significant effect of the ZrO2/SiO2nanoparticles on the corrosion properties of the obtained coatings.All samples containing nanoparticles demonstrated an increase in the polarization resistance and a decrease in the corrosion current density incomparison with the PEO-coating formed in base electrolyte(Table 6),with the exception of the sample C6.The best protective properties were obtained for coatings formed in the electrolyte with a 4 g/l nanoparticles concentration.The corrosion current density for these samples was decreased by more than 3 times in comparison with the PEO-coating obtained in electrolyte without the nanoparticles.One of the indirect parameters indicating an increase in the corrosion properties of coatings with nanoparticles is an increase in the potential,at which a breakdown of the coating is observed (the horizontal line of the Tafel plot in the upper part of the curve).As the concentration of particles increases,there is a consistent increase in the difference between the breakdown and corrosion potentials (Fig.9),only for sample C6 a decrease is observed compared to the base PEO layer.

Table 6Corrosion properties of the MA8 magnesium alloy samples with coatings.

One of the reasons of improving the protective properties of the layers formed in dispersed electrolytes is the incorporation of nanoparticles into the coatings (in its pores),which leads to an increase in the polarization resistance.In addition,coatings obtained using nanoparticles contain the compounds(ZrO2/SiO2),which have a significantly higher chemical resistance in comparison with the main components of the base PEO-layer (MgO and MgSiO4).

The experimental data obtained by the electrochemical impedance spectroscopy (EIS) are presented in the form of Bode plots (Fig.10),in which the dependences of the impedance modulus (|Z|) and phase angle (θ) on frequency(f) are presented.The impedance spectra obtained for the coatings under the study are significantly different,reflect ing differences in their electrochemical properties.Analysis of the impedance spectra of the PEO-coating formed in base electrolyte and coatings with nanoparticles allows distinguishing two bends on the dependence of the phase angle on frequency (Fig.10).Thereby,these impedance spectra were fit ted using equivalent electric circuit (EEC) with two time constants (twoR-CPE-chains) (Fig.11).The elementReis the resistance of the electrolyte between the sample and the reference electrode,R1describes the resistance of the electrolyte in the pores of the coating,the elementCPE1is used to describe the geometric capacitance of the whole coating.The elementsR2andCPE2are the resistance and capacity of the non-porous sublayer,respectively.In the electrochemical modeling,the constant phase element (CPE) was used instead of the ideal capacitance for the porous and non-porous layers due to unhomogeneity of studied systems.

Fig.10.Bode plots (dependences of impedance modulus |Z| and phase angle θ on frequency f) for C0 (1),C2 (2),C4 (3),C6 (4) samples.Spectra were obtained after exposure in a corrosive medium for 2 h (a) and 24 h (b).Impedance spectra contain experimental data (scatter plot marked by symbols) and theoretical fittin curves (lines),which simulate the experimental results according to equivalent electrical circuits.

Fig.11.Equivalent electrical circuit used for fittin the experimental impedance spectra.

The impedance ofCPEwas calculated in accordance with the Eq.(5):

whereωis the radial frequency,jis an imaginary unit,nandQare exponential andCPEcoefficients respectively.

The results of the EEC parameters calculations carried out by fittin the experimental impedance spectra are presented in Table 7.TheRevalue,according to calculations,was constant for all the studied samples (28-30Ω×cm2).

The use of ZrO2/SiO2nanoparticles led to an increase in the impedance modulus measured at low frequencies from 1.6 to 20 times,depending on the concentration of nanoparticles in the electrolyte.The highest protective properties were obtained for coatings formed in an electrolyte with a 4 g/l nanoparticles concentration:|Z|f=0.01Hz=9.4×105Ω×cm2(for base PEO-coating |Z|f=0.01Hz=4.8×104Ω×cm2).

The incorporation of nanoparticles into the coatings during the plasma electrolytic oxidation led to a shift in the position and amplitude of the phase angle peaks.A decrease in theQ1parameter (Table 7) is observed with an increase in the concentration of nanoparticles in the electrolyte,which indicates a change in the surface state and an increase in the thickness of the heteroxide layer (Table 3).A successive increase in the values of theR1parameter for coatings can be explained by filling the pores and reducing their diameter due to the impregnation of nanoparticles.Under electric field conditions and microdischarges occurring the nanoparticles penetrated deep into the pores,attain the bottom of the pores,increasing the thickness and resistance of the non-porous sublayer,which actually determines the corrosion properties of the sample in whole.

Table 7Calculated parameters of equivalent electrical circuits (R and |Z|f=0.01Hz in Ω×cm2;Q in Ω×cm-2×sn) for sample with coatings at different immersion time in a 3 wt.% NaCl aqueous solution.

Table 8Microhardness and Young’s modulus of coatings formed on MA8 magnesium alloy.

Table 9Mechanical properties of the studied coatings,determined by the scratch and tribology tests.

Exposure of the samples in a corrosive medium for 24 h led to a significant change in the parameters of EEC elements.For coatings C2 and C4,a decrease in theQ2parameter was observed in comparison with a samples exposured two hour.This fact can be interpreted as an increase in the thickness of the non-porous sublayer,which is also indicated by an increase inR2resistance for these coatings.As a result of 24 h exposure,the pores were fille with corrosion products,which led to a decrease in their depth.As the concentration of nanoparticles in the electrolyte increased to 6 g/l,the electrochemical behavior of the coatings changed signifi cantly.Long-term exposure of this coatings in the aggressive medium led to a significant (more than 2 times) decrease of the impedance modulus and resistance of the non-porous sublayer.These changes are the result of the increased heterogeneity of the coatings due to the increased concentration of nanoparticles.As a result,during the process of coatings formation in the electrolyte with nanopartical concentration 6 g/l,a large number of micropores and cavities are formed(Fig.2),which contribute to an increase in the penetration rate of the corrosive medium to the substrate and an overall decrease in corrosion properties.The highest values of the impedance modulus were observed for a C4 coating.

For investigation of the changes in surface morphology,samples with coatings containing nanoparticles and without ZrO2/SiO2were immersed in a 3 wt.% NaCl solution for 14 days.After exposure,the surface of the samples was analyzed using scanning electron microscopy.

The analysis of SEM images confirm the data obtained by the EIS.After exposure in the aggressive environment,the appearance of microcracks on the surface of coatings is observed due to corrosion processes,occurring in the coatings.The smallest number of defects was found on C4 coatings (Fig.12),while in C0 a significant amount of cracks distributed over the entire surface was observed.

Fig.12.SEM images of coating surfaces in backscattered electrons (a,c,e,g) and secondary electrons (b,d,f,h) after 2 weeks exposure to 3 wt.% NaCl solution:C0 (a,b),C2 (c,d),C4 (e,f),C6 (g,h).

Fig.13.The appearance of C0 (a) and C4 (b) samples after 35 days of exposure in 3 wt.% NaCl solution.

Fig.14.Hydrogen evolution tests of C0 (1) and C4 (2) during 35 days exposure in 3 wt.% NaCl solution.

For a more accurate assessment of the corrosion rate,the hydrogen evolution tests were carried out for C0 and C4 samples,and Fig.13 shows the samples after 35 days of exposure.

The hydrogen volume evolution during 35 days of immersion of coated Mg alloy samples are presented in Fig.14.Both C0 and C4 samples have a similar corrosion behavior.This is the result of corrosion activation and propagation through the pores of the PEO-layer,with the following short passivation of the material by formed corrosion products.However,samples with PEO-coating containing ZrO2/SiO2nanoparticles show lower corrosion rate(0.04±0.02 ml×cm-2·day-1) as compared to one for the C0 specimen (0.07±0.03 ml×cm-2·day-1).Analysis of the data confirm the results of electrochemical measurements and indicates the positive effect of nanoparticles addition to the electrolyte on the decrease of corrosion rate for the formed PEO-coating.

3.3.Mechanical properties of coatings

In addition to chemical resistance,silica and zirconia have a high hardness and strength;thus,they are used to modify the surface in order to improve the mechanical properties of the PEO-coatings.

Fig.15.The appearance of scratches on the C0 (a),C2 (b),C4 (c),C6 (d)coatings obtained on MA8 Mg alloy.

From the analysis of the results obtained by the dynamic microhardness measurements (Table 8),it could be conclude that the formation of coatings in electrolytes,containing 2 g/l nanoparticles,did not provide sufficient incorporation of the nanomaterial,which can affect the hardness of the formed PEO-layer.The maximum microhardness of the surface layers was obtained for C4 sample(Hμ=3.1±0.5 GPa) that is higher in comparison with the C0 sample (Hμ=2.1±0.3 GPa).The reducing the microhardness at a nanoparticles concentration above 4 g/l can be explained by an increase in the heterogeneity of the composition of PEO-layers and,as a consequence,a decrease in density and quality of the coatings.

Fig.15 shows the appearance of the scratch,made to identify the mechanism of coatings destruction during the scratch test.An increase in the concentration of nanoparticles to 4 g/l led to a slight change in the load(LC2),at which the beginning of peeling of coating areas was observed,while an increase in load,at which abrasion of the coating to the substrate occurs (LC3),was observed too (Table 9).The increase in the value of the parameterLC3is due to an increase in the thickness of the coatings formed in electrolytes with ZrO2/SiO2nanoparticles in comparison with the PEO-coating obtained in base electrolyte (Table 3).The increase in the concentration of nanoparticles in the electrolyte up to 6 g/l led to a significant decrease in adhesion strength,as evidenced by a decrease inLC2parameter.

Fig.16.Dependences of the friction coefficient on the cycles number during the tribological test for the C0 (1),C2 (2),C4 (3),C6 (4) samples.

Fig.16 presents the data of tribological tests as a friction coefficient μ,evolution with a cycles number for different samples.The increase in the nanoparticles concentration in the electrolyte led to a decrease in wear of the coatings (Table 9).The lowest values of wear were demonstrated by C6 sample (3.2×10-5mm3/(N×m)).For this sample,the decrease in wear by 1.3 times in comparison with the C0 coating(4.3×10-5mm3/(N×m)) was observed.This effect can be explained by an increase in the hardness and thickness of the coatings.

Conclusions

It has been established that PEO-coatings formed on the MA8 magnesium alloy in electrolytes containing ZrO2/SiO2nanoparticles mixture of zirconia and silica have improved electrochemical characteristics in comparison with the surface layers obtained without the use of nanoparticles.Based on obtained results it has been concluded,that the coating formed in an electrolyte with a 4 g/l nanoparticles concentration have the highest protective properties.According to the results of the electrochemical impedance spectroscopy,the impedance modulus measured at low frequencies for this coating is more than one order of magnitude higher than that for the PEO-coating formed in base electrolyte.Additionally,obtained layers showed high protection properties even after 24 h exposure in a corrosive medium.The incorporation of nanoparticles led to an increase in the microhardness of the surface layers by 1.5 times,and reduced a wear by 1.3 times.Formed coatings are perspective for industry and can be used in aviation,automotive and biomedicine as protective layers.

Declaration of Competing Interest

None.

Acknowledgment

This work was supported within the frames of the Grant of the Russian Science Foundation,project No.20-73-00280.SEM,EDS and coatings morphology were carried out within the framework of the Grant of the Russian Science Foundation,project No.20-13-00130.The results of Raman spectrum were collected under the government assignments from Ministry of Science and Higher Education of the Russian Federation (project no.0265-2019-0001).

Supplementary materials

Supplementary material associated with this article can be found,in the online version,at doi:10.1016/j.jma.2021.04.013.