effect of smat preprocessing on mao fabricated nanocomposite coating · 2018-12-04 · effect of...

Effect of SMAT preprocessing on MAOfabricated nanocomposite coating

M. Gheytani, H. R. Bagheri, H. R. Masiha, M. Aliofkhazraei*,A. Sabour Rouhaghdam and T. Shahrabi

The role of fine microstructure of AZ31B magnesium alloy metallic samples on the morphology,

corrosion and wear resistance of the coatings formed by microarc oxidation technique was

investigated. Surface nanocrystallisation was carried out through surface mechanical attrition

treatment, and a nanocrystalline surface with the grain size of 5–10 nm was obtained. The

coatings were studied by optical microscope and SEM images, and corrosion and wear tests

were conducted on the samples. Comparison between the maximum and minimum results

obtained for the corrosion polarisation resistance indicated 700% increase for different coating

conditions. Similar comparison for the wear test results indicated 105% increase in wear

resistance for different coating conditions.

Keywords: Magnesium, Corrosion, MAO, SMAT, Nanocrystalline, Nanostructure, Wear

IntroductionAZ31B is a workable magnesium alloy consisting ofmain elements including 3% aluminium and 1% zinc.1

Recently, various applications have been found for thisalloy in different industries including aerospace, auto-mobile, transportation, telecommunication and compu-ter. This wide range of application is due to the excellentphysical and mechanical properties of AZ31B alloy suchas low density, relatively high electrical conductivity,high specific strength, casting capability, appropriateweldability, high thermal conductivity, dimensionalstability and electromagnetic protective properties.However, Mg and its alloys have low corrosionresistance due to their high chemical activity, negativeelectrochemical potential and low tribological proper-ties.2 Hence, applying corrosion resistant and wearresistant coating or structural treatment of the surface isnecessary for their practical use. Various techniqueshave been so far proposed to fabricate coatings on thesealloys, e.g. conversion coatings (such as phosphatising,anodising), plating and other electrolytic coating meth-ods. In conventional coating, the sample is placed in aspecific solution, and chemical reactions are performedon its surface; this results in the creation of a protectivelayer (a mix of metallic matrix) produced from thereaction occurring on the alloy’s surface.3 In anodisingmethod, a metallic sample is placed in an electrochemi-cal cell as anode, and then, through applying a potentialdifference between the anode and the cathode, a resistiveoxide layer forms on the sample. Nevertheless, all of theabovementioned reactions have their own advantages

and shortcomings about making up for the limitations inresistance to the corrosion, wear, cohesion, bonding andthermal specific strengths in Mg alloys.4 During therecent years, microarc oxidation (MAO) has found greatresearch interest among the researchers because itemploys non-acidic and environment friendly electro-lyte.5 The coating obtained through this method is ableto recover any lack of corrosion or wear resistance. Themethod is basically similar to other electrolytic methodsand forms an oxide layer on the surfaces of passive lightmetals such as Al, Ti, Mg and their alloys.6

The coating created through MAO not only involvesappropriate chemical and physical properties such ascorrosion or wear resistance, electrical isolation andhigh hardness but also is a uniform coating with veryhigh adhesion to substrates with even complicatedgeometries.7 This method is recognised from otherelectrochemical methods by two main characteristics:electrolysis of liquid environment due to applying highpotentials and formation of microarcs on the surface ofthe working electrode. Release of oxygen and/or metaloxidation occurs on the surface of the anode. The maincomponents of oxide layer on the surface of Mg alloyare Mg(OH)2 and MgO. The higher is the ratio of MgOto Mg(OH)2, the more enhanced is the corrosionresistance.8 The enhancement of corrosion resistance ofMAO treated samples has been reported in manypapers;9–12 however, in many cases, the wear resistancehas not been discussed significantly.

Another technique affecting the corrosion andmechanical properties of Mg alloys is surface nanocrys-tallisation (grain size smaller than 100 nm) of the surfaceof these alloys. Studies have shown that high volumefraction of grain boundary leads to considerabledecrease in their physical, chemical and mechanicalproperties.13 If the metal (such as Fe and its alloys) has

Department of Materials Science, Faculty of Engineering, Tarbiat ModaresUniversity, PO Box 14115-143, Tehran, Iran

*Corresponding author, email [email protected]

� 2014 Institute of Materials, Minerals and MiningPublished by Maney on behalf of the InstituteReceived 14 January 2014; accepted 16 January 2014DOI 10.1179/1743294414Y.0000000251 Surface Engineering 2014 VOL 30 NO 4244

active dissolution corrosion behaviour, the structurewith the grain size .35 nm does not indicate noticeablebehaviour changes; but once the grain size is ,35 nm, adecrease in the grain size leads to an increase indissolution rate.14 This can be mainly attributed to thesolubility of the produced corrosion layer on Fe alloys.Besides, formation of corrosion products on insolublesurfaces will cause a drop in the corrosion rate. In thecase of metals with passive layer such as Mg alloys,surface nanocrystallisation enhances the stability of theoxide film and improves the corrosion resistance.Surface nanocrystallisation changes the characteristicsof the passive film in many aspects including chemicalcomposition, metallurgical structure, semiconductivityand growth manner. There are various techniquesinvented for surface nanocrystallisation such as separatedeposition of nanoparticles (including physical vapourdeposition,15 chemical vapour deposition,16 electroche-mical and thermal methods and electrolytic deposition)and also mechanical methods based on severe plasticdeformation such as equal channel angular pressing,17

accumulative roll bonding18 and high pressure torsion.19

Surface mechanical attrition treatment (SMAT)20 is alsoamong the fabrication methods of nanostructured layeron metallic surfaces and competes with the techniquessuch as ball milling, sliding wear, ball drop test,ultrasonic shot peering and severe air blow shot peering.Through the SMAT process, hard balls with differentdimensions and materials hit the surface of specimen indifferent directions for a long period and lead to plasticdeformation on the surface. Owing to successivedeformations, the surface layers are miniaturised, whilegrain boundary and surface energy increase. The processcarries on until the formation of nanostructured surfacelayer. This method ultimately not only leads to theformation of a surface layer free of any cavity andimpurity but also ends with strong bonding andadhesion of the nanostructured layer to the matrix. Luet al. invented the SMAT process and showed its abilityin enhancement of different properties of surfacemechanical attrition treated samples.21–24

This research aims to study the effect of finemicrostructure of metallic matrix on the properties ofMAO coating. Since during the MAO process, in eacharc, a segment of the metallic matrix reacts with theelectrolyte, it is expected in this research that surfacenanocrystallisation of the substrate, by increasing itschemical reactivity, can be effective in acceleration ofnucleation and coating growth and reduction of theenergy required for fabrication of this type of coating. Inaddition, it is expected to form a denser and morecompact coating, which typically involves better corro-sion and wear resistance in comparison to the samplescoated with casting microstructure. Furthermore, weinvestigated the distribution manner of the hardsecondary phase particles including Al2O3 nanoparticlesin the coating. Using nanoparticles to form MAOcoatings has been previously studied, and interestingresults have been reported by fabrication of nanocom-posite coatings. Indeed, through synthesis of a nano-composite with hard particles, it is anticipated to forma stronger and harder coating for industrial applicationsin more harsh conditions. The effective parametersfollowed in this work are the effect of currentdensity variation, coating time variation and change of

the used electrolyte during the process on the coatings’properties.

ExperimentalAZ31B magnesium alloy samples were prepared in theform of rectangular cubes with given chemical composi-tion of Mg–3?5Al–0?9Zn–0?01Cu–0?64Mn–0?002Ni–0?01Si–0?001Fe (wt-%). The samples were abraded bysilicon carbide emery papers up to the mesh of 2500. Inorder to reduce edge effect on the sharp edges, it wastried to eliminate the sharp edges and corners of thesamples by slight emery. The samples were exactlydefatted, deacidified and weighed. Then, 16 sampleswere placed for 6 h in SMAT chamber containing steelballs with a diameter of 2?5 mm and a weight of,63?37 mg. Next, four samples were kept in order tocompare the initial conditions of the coating on the samecasting microstructure. An optical microscope (modelBX51M, Olympus Inc.) was used for imaging. Thesurface mechanical attrition treated samples wereweighed again after the process. Figure 1 illustrates theresults of low angle X-ray diffraction analysis from thesurface of a nanocrystalline sample.

Two phosphate based (4 g L21) and silicate based(4 g L21) electrolytes with a given amount of NaOH (forenhancing the conductivity of solution) were used forthe coating process. In the electrolyte of a set of samples,4 g L21 of alumina nanopowder (average size of 49 nm;Plasma Chem Company, Germany) was used. Figure 2presents the TEM image of these nanopowders. Allsamples were subjected to MAO process. Tables 1 and 2illustrate the conditions of all samples in terms ofapplied electrolyte, time and current density of MAO

1 X-ray diffraction pattern of sample nanocrystallised by

SMAT preprocess

Gheytani et al. Effect of SMAT on MAO nanocomposite coating

Surface Engineering 2014 VOL 30 NO 4 245

coating treatment. Coating time began from the initia-tion of sparking.

To evaluate the effect of coating time on the finalstructure of the samples, they were coated in two coatingtimes of 5 and 10 min by two current densities of 23 and46 mA cm22. The samples were weighed before andafter the coating process using a digital microbalance(model GR202, AND Inc., Korea). The thicknesses ofobtained coatings were measured using the magneticgauge (model Qnix-8500, Paul N. Gardner). Samplepreparation was performed to observe the coatings’microstructure by SEM (model SIGMA/VP, Zeiss Inc.).Besides, energy dispersive spectroscopy (EDS) mapanalysis was performed to see elemental distribution inthe coatings’ microstructure. Figures 3 and 4 illustratetwo samples of the EDS analysis for the SS-46-5 andSSN-46-5 specimens. Additional information andrelated conclusion are mentioned in the sections on‘Results and discussion’ and ‘Conclusions’. Then,polarisation and pin on disc tests were carried out usingpotentiostat/galvanostat device (model 273A; EG&GInc.) and ASTM G99 standard on the coated samples.

Corrosion test was carried out in 3?5 wt-% solution ofpure NaCl at ambient temperature. First, after passingof 1000 s of immersion for reaching stable open circuit

potential (OCP), test initiation and final potentials wereselected as 20?2 and z0?7 V(OCP). Next, potentiody-namic polarisation test was performed for all samplesseparately using calomel reference electrode with ascanning rate of 1 mV s21. To carry out the wear testfor different samples, first, the samples were abraded byCo–WC (cermet) pin with a speed of 100 rev min21 anda diameter of 0?5 cm for 500 cycles, and the coefficientof friction variation of the coating was recorded for allsamples. In the next step, weight variation of the sampleswas recorded by applying a speed of 100 rev min21 and adiameter of 1 cm for 400 rotations.

Results and discussion

Growth of coatingTable 2 presents the weight changes of the samplesbefore and after the coating through the MAO process.As shown, all samples (except two) indicated weightincrease after the coating process. The maximum weightgain was for SP-46-10 (0?58%), while the maximumweight loss was for SSN-46-10 (0?56%). In weight lossperspective, a comparison between the samples with andwithout SMAT [e.g. (UP-23-5, SP-23-5), (UP-46-5, SP-46-5), (US-23-5, SS-23-5) and (US-46-5, SS-46-5)]indicated that the weight increase in the unsurfacemechanical attrition treated samples was less, implyingfurther (mass) dissolution of the matrix during the MAOprocess.25–27 This effect was completely anticipatedconsidering the surface nanocrystallisation of the sub-strate and an increase in its reactivity.28–30 It is worthnoting that, here, none of thickness or porosity wastaken into account. Matrix dissolution and formation ofcoating for different samples coated by the MAOmethod as well as various coating factors such ascurrent density, time and electrolyte type have beenpreviously studied, which are confirmed by thefindings of the present study.31–33 An interesting pointis the effect of nanoparticle presence. In comparison tothe samples coated in the suspension containingnanoparticles, it was observed that weight changepercentage decreased with a constant trend for the

Table 1 Conditions for performing MAO coating process on samples

Initial surfacestructure

Code ofsample

Currentdensity/mA cm22

Base ofelectrolyte

Time ofprocess/min

Presence ofnanoparticle

Surface mechanicalattrition treated

SP-23-5 23 Phosphate 5 …SP-23-10 23 Phosphate 10 …SS-23-5 23 Silicate 5 …SS-23-10 23 Silicate 10 …

SPN-23-5 23 Phosphate 5 YesSPN-23-10 23 Phosphate 10 YesSSN-23-5 23 Silicate 5 YesSSN-23-10 23 Silicate 10 Yes

SP-46-5 46 Phosphate 5 …SP-46-10 46 Phosphate 10 …SS-46-5 46 Silicate 5 …SS-46-10 46 Silicate 10 …

SPN-46-5 46 Phosphate 5 YesSPN-46-10 46 Phosphate 10 YesSSN-46-5 46 Silicate 5 YesSSN-46-10 46 Silicate 10 Yes

Unsurface mechanicalattrition treated

UP-23-5 23 Phosphate 5 …UP-46-5 46 Phosphate 5 …US-23-5 23 Silicate 5 …US-46-5 46 Silicate 5 …

2 Image (SEM) of alumina nanopowder used in MAO

coating process


246 Surface Engineering 2014 VOL 30 NO 4

samples containing nanopowder compared to thosewithout nanopowder. It seems that presence ofnanoparticles in the electrolyte and coating leads toa reduction in the effect of the final mass of coating insimilar conditions.

Morphology and elemental analysis of coatingFigure 5 exhibits the SEM images of the free surfaces ofsome samples coated by MAO process. The differencesbetween the presence of nanoparticles and the effect ofelectrolyte and matrix nanocrystallisation are clearlyobserved in these images (for the shown samples). Theresults obtained from the EDS tests are introduced inTable 3. As shown, the microstructure of free surface ofthe coating in the phosphate electrolytes involves a moreporous morphology compared to the samples coated insilicate based electrolytes. Moreover, the SEM images ofthe unsurface mechanical attrition treated samplesconfirm this result. It is worth mentioning that theporosity in the coatings formed by MAO process is not

the same for the free surface of the coatings and theirinside (i.e. cross-section of the coating); this finding hasalso been reported by other researchers.34–36 A compar-ison between the samples with and without SMATindicates that the elements’ percentage in the coatinghad no change except for aluminium. Regarding theunsurface mechanical attrition treated samples, it seemsthat the excess amount of aluminium is absorbed in thesilicate based electrolytes as compared to the phosphatebased ones. Therefore, performing SMAT process andmatrix nanocrystallisation may lead to an increase inreactivity of the metallic matrix and uniform distribu-tion of the elements in the coating.

The sources of used aluminium were substrate andnanopowder. The results showed that the samples withnanopowder contained higher Al content. Moreover, forthe samples with nanopowder, Al content was almostconstant and varied within the slight range of 1?1–2%. Itshows that for the majority of samples, the amount ofnanoparticle absorption in the silicate based electrolytes

3 Elemental map of sample SS-46-5

Table 2 Weight change of samples before and after MAO coating process

Code ofsample

Weight beforeprocess m1/g

Weight afterprocess m2/g

Weightchange/mg

Weight change percentagem2{m1ð Þ=m1½ �|100f g

SP-23-5 3.80650 3.81811 11.61 0.30SP-23-10 3.37197 3.38124 9.27 0.27SS-23-5 2.79430 2.79759 3.29 0.12SS-23-10 3.42569 3.43471 9.02 0.26SPN-23-5 4.64738 4.65587 8.49 0.18SPN-23-10 3.88500 3.88104 23.96 20.10SSN-23-5 3.86892 3.87164 2.72 0.07SSN-23-10 3.05806 3.06383 5.77 0.19SP-46-5 3.41598 3.43068 14.70 0.43SP-46-10 2.59974 2.61490 15.16 0.58SS-46-5 3.76639 3.77384 7.45 0.20SS-46-10 4.02293 4.03249 9.56 0.24SPN-46-5 2.91241 2.92347 11.06 0.38SPN-46-10 4.80239 4.82365 21.26 0.44SSN-46-5 4.54110 4.55199 10.89 0.24SSN-46-10 4.23722 4.21340 223.82 20.56UP-23-5 2.75108 2.75858 7.50 0.27UP-46-5 2.55423 2.56489 10.66 0.42US-23-5 3.90173 3.90483 3.10 0.08US-46-5 1.58247 1.58518 2.71 0.17



has been increased comparing to the phosphate basedelectrolytes. In addition, as it was expected, for thehigher times, further nanoparticles are absorbed.37–39

The maximum Al absorption in the coatings was 13?4%.

Figure 6 illustrates the SEM cross-section images ofthe same coatings. As shown, the oxide layer for thesamples coated in the phosphate based electrolytes is

more compact; this is also true for the samples withoutnanocrystallisation. Although porosity is higher in thefree surface of the coatings formed in phosphate basedelectrolytes, their cross-section indicates less compactcoating.40–42

Table 4 also shows thicknesses of the created coatingsregarding the SEM image of the samples’ surface and

4 Elemental map of sample SSN-46-5

5 Images (SEM) form free surfaces of the samples coated for 5 min: a sample SP-46-5; b sample SS-46-5; c sample

SPN-46-5; d sample SSN-46-5; e sample UP-46-5; f sample US-46-5



Table 3 Distribution of different elements in coating infree surface analysis of coating developedthrough MAO process using EDS technique

Code of Sample Si Na Al P O Mg

SP-23-5 0.2 0.9 1.2 26.0 30.4 41.3SP-23-10 0.2 0.7 1.4 27.2 26.6 43.9SS-23-5 24.1 0.4 2.0 0.6 21.3 51.6SS-23-10 26.9 0.6 1.7 0.3 28.0 42.5SPN-23-5 0.2 0.7 6.6 25.8 26.6 40.1SPN-23-10 0.3 1.5 4.5 25.0 28.0 40.7SSN-23-5 8.0 0.5 3.5 0.2 19.4 68.4SSN-23-10 21.8 0.5 8.2 0.2 25.3 43.9SP-46-5 0.3 0.9 1.1 25.1 29.2 43.4SP-46-10 0.2 1.2 1.5 21.2 29.5 46.4SS-46-5 31.0 5.6 1.5 1.1 22.5 38.3SS-46-10 27.3 0.7 1.2 0.2 28.7 41.8SPN-46-5 0.4 0.8 4.9 24.2 26.9 42.8SPN-46-10 0.3 0.3 5.6 20.2 25.1 48.6SSN-46-5 26.8 1.3 8.0 0.3 31.7 31.9SSN-46-10 0.3 0.0 13.4 0.0 23.5 62.7UP-23-5 0.1 1.2 0.8 27.7 29.9 40.3UP-46-5 0.3 1.7 1.6 21.8 28.3 46.1US-23-5 20.1 0.0 2.4 0.2 20.2 57.1US-46-5 26.7 0.2 2.9 0.2 22.1 47.9

6 Cross-section images (SEM) of samples coated for 5 min: a sample SP-46-5; b sample SS-46-5; c sample SPN-46-5; d

sample SSN-46-5; e sample UP-46-5; f sample US-46-5

Table 4 Thickness of coatings formed on samples coatedthrough MAO process

Code ofsample

Thickness ofcoating measuredby probe/mm

Thickness ofcoating measuredin SEM image/mm

SP-23-5 18.35 8.37SP-23-10 35.47 17.44SS-23-5 17.91 4.66SS-23-10 17.73 4.46SPN-23-5 19.63 4.94SPN-23-10 25.11 10.98SSN-23-5 13.71 6.02SSN-23-10 17.68 7.52SP-46-5 27.15 18.33SP-46-10 30.1 6.45SS-46-5 13.98 8.40SS-46-10 15.83 11.05SPN-46-5 24.57 8.16SPN-46-10 31.56 5.73SSN-46-5 22.11 1.34SSN-46-10 17.23 15.59UP-23-5 13.2 13.31UP-46-5 16.91 2.40US-23-5 5.42 4.63US-46-5 8.23 4.07



the results obtained from the thickness probe. Thecomparison among the samples in Table 4 shows anincrease in thickness of the coating formed on thesurface mechanical attrition treated samples, whichseems very common considering the further reactivityof the nanocrystallised magnesium matrix in equaltimes. Furthermore, it is worthy to mention that theobtained thicknesses through the probe differed fromthose observed in the SEM (cross-section) images of thesamples. This finding, which has also been previouslyreported by other researchers, is more obvious forporous coatings.43

Corrosion behaviourFigures 7–11 indicate the results of polarisation testbased on the electrolyte type used during the coatingprocess and the initial microstructure of the samples. Asshown, the variations of current and electrolyte and timeof the MAO process lead to no significant change in thecorrosion behaviour of the unsurface mechanical attri-tion treated samples (Fig. 7).

Presence of nanoparticles in the electrolyte and itseffect on the corrosion behaviour are shown in Figs. 8–10. As the figures indicate, variation of other parametersdid not considerably affect the corrosion behaviour in

8 Polarisation curves of samples with casting surface structure and then coated in phosphate based electrolyte with

nanopowder

7 Polarisation curves of samples with casting surface structure and then coated by MAO process



the presence of phosphate based electrolytes. However,in the presence of silicate based electrolytes withoutalumina nanoparticles, the effect of changes of otherparameters such as current density and coating time isobvious. This can be due to the different microstructuresof the samples coated in silicate based electrolytes in thepresence of nanoparticles.44–46 As shown in SEM cross-section images of the coatings, the coatings formed insilicate based electrolytes have lower porosity; hence,they show better corrosion behaviour compared to thosecoated in phosphate based electrolytes.47–49

Figures 9–11 indicate the corrosion behaviour ofsurface mechanical attrition treated samples coated inthe electrolytes without nanopowder. It is shown thatthe change in the coating parameters highly affects thecorrosion behaviour of the samples. For the phosphatebased electrolytes, the optimum result is obtained inhigh current density and low coating time. The curverelated to the sample SP-46-5 indicates the bestcorrosion behaviour. For silicate based electrolytes, thecurve related to the sample SS-46-5 (which is highlydifferent from other curves) indicates the best corrosion

9 Polarisation curves of samples with casting surface structure and then coated in phosphate based electrolyte without

nanopowder

10 Polarisation curves of samples with casting surface structure and then coated in silicate based electrolyte with nanopowder



results. Corrosion current density for this sample in theanodic branch is much lower compared to the othersamples shown in this figure. It can be concluded thatthe current density of 46 mA cm22 and 5 min of coatingtime yield the optimum corrosion results for bothelectrolytes in the absence of nanopowders. Moreover,as previously mentioned, the presence of nanopowdersleads to similar corrosion behaviour of the samples.

Table 5 summarises the results related to the para-meters measured through the corrosion test of thesamples. Analysis of these results reveals that almost allsamples indicate similar corrosion behaviour. However,in some samples, the more compacted layers cause adecrease in the corrosion current density and, conse-quently, an increase in the polarisation resistance. It

further indicates that SMAT process did not signifi-cantly affect the samples’ corrosion resistance. More-over, corrosion current density of the samples coated insilicate based electrolytes is higher than that of similarsamples coated in phosphate based electrolytes.

Wear behaviourFigures 12 and 13 indicate the results obtained fromseparate wear tests performed on all samples. As shownin Fig. 13, the last four samples without prenanocrys-tallisation process have less weight loss compared to theother samples without nanopowder. It is observed thatthe phosphate based electrolytes (samples UP-23-5 andUP-46-5) have less weight loss compared to the samplescoated in silicate based electrolytes, which is in agree-ment with the results reported by other researchers.50–52

11 Polarisation curves of samples with casting surface structure and then coated in silicate based electrolyte without

nanopowder

Table 5 Results obtained from polarisation tests

Code ofsample

Corrosion currentdensity/mA cm22 bc/V/decade ba/V/decade

Polarisationresistance/kV cm2

SP-23-5 23.86 1.179 2.472 14.55SP-23-10 0.022 72.0361023 59.4861023 632.06SS-23-5 49.58 7.149 358.961023 3.00SS-23-10 2.754 84.4461023 88.9061023 6.84SPN-23-5 2.895 114.461023 104.561023 8.20SPN-23-10 7.838 718.461023 96.1561023 4.70SSN-23-5 11.08 256.461023 163.861023 10.06SSN-23-10 17.83 11.53 160.061023 3.85SP-46-5 8.079 84.0061023 39.2661023 1.44SP-46-10 2.114 751.861023 239.661023 37.37SN-46-5 2.520 100.061023 100.061023 8.616SN-46-10 21.69 562.961023 573.561023 5.69SPN-46-5 2.157 216.361023 11.7261023 2.24SPN-46-10 4.158 192.961023 122.361023 7.83SSN-46-5 14.14 170.661023 220.2 5.24SSN-46-10 10.39 167.161023 21.6961023 0.8UP-23-5 1.791 207.961023 139.261023 20.24UP-46-5 3.332 199.361023 89.1461023 8.04US-23-5 4.873 149.361023 83.6261023 4.78US-46-5 1.063 75.5261023 26.0161023 7.91Bare Alloy 5.940 10061023 10061023 3.655



The phosphate based electrolytes fabricate more com-pact coatings compared to the silicate based electrolytes;this is also confirmed by the SEM cross-section images.In comparison to the nanocrystalline samples, it isobserved that addition of nanopowder leads to lessweight loss for most of the cases. This effect is obviousparticularly for the samples SS-23-5 and SSN-23-5,which confirms the positive effect of nanoparticles’presence in the reduction of the wear rate of the formedcoatings. However, the effect of SMAT process is notthat much obvious in this case, meaning that absorptionof nanoparticles leads to a decrease in wear rate.Moreover, it is seen that the majority of the coatedsamples in the suspension of nanoparticles in phosphatebased electrolytes have less weight loss compared to thesamples coated in the suspension of nanoparticles insilicate based electrolytes.

Figures 14–16 illustrate the results obtained frommeasuring the coefficient of friction for different studiedsamples. Figure 14 shows the samples coated in phos-phate based electrolytes, implying a decrease in thecoefficient of friction by addition of nanopowder formost cases. This trend is inverse only for SP-46-10 andSPN-46-10 samples. Indeed, a comparison between SP-46-5 and SPN-46-5 samples also indicates that coeffi-cient of friction for the samples containing nanopowderis higher than that of the samples coated with MAO atthe beginning; however, likewise the samples coated inthe current density of 23 mA cm22, by passing of thetime, it drops to the values less than that of the sampleswithout nanopowder. This finding implies that nano-powder content and current density have reciprocaleffects. The dual structure of the layers coated on Mgsubstrate has been reported in many articles.53–55 This

12 Wear rates of samples coated in electrolytes containing nanopowder

13 Wear rates of samples coated in electrolytes without nanopowder (comparison between samples with and without

SMAT)



structure contains a relatively thick porous layer and athinner barrier layer. At the beginning of the wear test,the porous layer is abraded, and the nanoparticles(which are reinforcing factors of the layer and probablyreducer of the coefficient of friction) are pulled out fromthe structure because of the abrasion and play lubricat-ing role. Considering the porous microstructure of thecoating, this process is performed easily. As the coatingtime proceeds (compare samples SPN-46-5 and SPN-46-10), due to an increase in the absorbed nanoparticles’content, the lubricating property increases, and, conse-quently, a decrease in the coefficient of friction isobserved within 10 min after the wear test. Figure 15indicates that an increase in the nanoparticles’ content insilicate based electrolytes, with no exception, leads to adecrease in the coefficient of friction for the samples.

Figure 16 shows that variation in the coefficient offriction is almost the same (50?04) for all sampleswithout SMAT. Besides, the coefficient of friction’scurve variation does not indicate much fluctuation,implying the uniform wear of the MAO coating. Theamount of fluctuations for silicate based electrolytes issimilar to that of the unsurface mechanical attritiontreated samples but higher for the samples coated inphosphate based electrolytes. This effect can be seen dueto different reasons such as further absorption ofnanoparticles, particularly at higher current densitiesof coating. Again, it is observed that the coefficient offriction’s curve indicates less variations with time, and

the samples coated in phosphate based electrolytes alsoreach to the values similar to those of other samples;thus, their average coefficient of frictions are almostequal. The noticeable point regarding the samplescoated in phosphate based electrolyte is the greatdifference between the samples SP-46-5 and SP-46-10,which only differ in coating time, but their percentage ofabsorbed aluminium and oxygen is almost the same.This can be attributed to the considerable difference inthe thicknesses of these two coatings. The case similar tothe sample SP-46-5 is the sample SPN-46-5, which alsohas rather high coefficient of friction compared to othersamples. Although this sample (SPN-46-5) also containsnanoparticles, its high thickness (as compared to othersamples) can be the reason for the observed behaviours.Overall, the wear behaviour of the surface mechanicalattrition treated samples implies a decrease in their wearrate and coefficient of friction (or their maintenance) incomparison to the unsurface mechanical attrition treat-ed samples. Presence of nanopowder in the coating hasbeen significantly attributed to the reduction of wearrate. The results obtained from the wear test coupled withprevious ones indicate that the applied hybrid process canimprove tribological properties of the magnesium sam-ples without affecting their other properties.

ConclusionThe results revealed that almost all samples had weightincrease after the coating process. The unsurfacemechanical attrition treated samples had more dissolu-tion in the electrolytes during the coating processcompared to the surface mechanical attrition treatedsamples. The samples coated in suspension showed lessweight change after the coating process compared to thesamples coated in the electrolyte without nanopowder,which can be attributed to the decrease in the effect offinal mass in the presence of nanoparticles. Free surfaceof the coatings formed in the phosphate based electro-lytes was more porous as compared to that of the silicatebased electrolytes, whereas this trend was inverse in thecross-section of the coatings. Different elements formedinside the coating had more unified distribution on thesamples with SMAT because of increase in the surfacereactivity of the preprocessed samples. Moreover, theprocess of alumina nanoparticles’ absorption wasgreater in the coatings with silicate based electrolytes.The surface mechanical attrition treated samples coatedin phosphate based electrolytes indicated superior

14 Coefficient of friction variations for samples coated

with initial nanocrystallised surface in phosphate

based electrolyte


with initial nanocrystalised surface structure in silicate

based electrolyte


with initial casting surface structure



corrosion behaviour as compared to their correspondingsamples in silicate based electrolytes, which can beattributed to the different cross-section microstructuresof the coating formed in phosphate based electrolytes.For both silicate and phosphate based electrolytes with-out nanopowder, the best corrosion result was at highcurrent density and less coating time. The results of weartest implied less weight loss of the samples coated withoutSMAT process. Again for these samples, the behaviour ofthe coating created in the phosphate based electrolyteswas superior compared to the samples coated in thesilicate based electrolytes. Addition of nanopowder to thenanocrystalline samples results in better lubrication and,consequently, decrease in their wear rate in comparison tothe samples without nanopowder. This effect of nano-powder is due to the dual structure of the coating createdon Mg, which consists of thick porous and thin barrierparts. Hence, an increase in the coating time leads toincreased thickness of the porous part and improvementof lubrication. Coefficient of friction was almost equal forall surface mechanical attrition treated samples. TheSMAT process represented its effect mainly in betterabsorption of nanoparticles in the coating, and itsconsequences can be seen in the properties, which dependon the presence of nanopowder in the coating, e.g.friction, corrosion and wear properties. Comparison ofthe maximum and minimum results obtained for thecorrosion polarisation resistance indicated 700% increaseamong different coating conditions. Similar comparisonfor the wear test results showed 105% increase in the wearresistance of different coating conditions.

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