Available online at SciVerse ScienceDirect

J. Mater. Sci. Technol., 2013, 29(3), 267e272

Role of Al18B4O33 Whisker in MAO Process of Mg Matrix Composite and

Protective Properties of the Oxidation Coating

Yanqiu Wang1)*, Xiaojun Wang2), Kun Wu2), Fuhui Wang1,3)

1) Corrosion and Protection Laboratory, Education Ministry Key Laboratory of Superlight Materials and Surface Technology, HarbinEngineering University, Harbin 150001, China

2) School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China3) State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016,

China[Manuscript received May 24, 2012, in revised form October 11, 2012, Available online 4 January 2013]

* CorresE-mail a1005-03JournalLimited.http://dx

Selective growth of oxidation coating was observed on Mg matrix composite Al18B4O33w/AZ91 (a compositewith Al18B4O33 crystal whisker as reinforced phase) when this composite was treated by microarc oxidation(MAO) technique, and then the role of Al18B4O33 whisker in the process of MAO was analyzed. Theprotective properties of MAO coating also were investigated. Scanning electron microscopy (SEM) was usedto characterize the existing state of Al18B4O33 whisker in MAO process and the microstructure of MAOcoating. Corrosion resistance of the bare and coated composite was evaluated by immersion corrosion testand potentiodynamic polarizing test. Wear resistance of MAO coating was investigated by a ball-on-discfriction and wear tester. The results showed that sparking discharge did not occur on Al18B4O33 whisker andthe whisker existed in the coating as a heterogeneous phase when MAO coating grew on the composite; thenthe whisker would be covered gradually with growing thick of the coating. Corrosion current density of thecoated composite was decreased by 4 orders of magnitude compared with that of the uncoated composite;excellent corrosion resistance was closely related to the compact whisker-coating interface since Al18B4O33

whisker did not induce structural defects. The coating also exhibited high wear resistance and a slightadhesive wear tendency with bearing steel as its counterpart material.

KEY WORDS: Mg matrix composite; Whisker; Microarc oxidation (MAO); Coating; Corrosion resistance

1. Introduction

In recent years, there has been an increasing interest inresearching and developing Mg matrix composites for applica-tions in the region of aerospace because of their low density, highspecific strength, high specific stiffness and low coefficient ofthermal expansion etc.[1e4]. Many discontinuous reinforcedphases, such as short fibers, particles, or crystal whiskers, havebeen applied to fabricate Mg matrix composites[1e4]. Amongthese reinforced phases, aluminum borate whisker (Al18B4O33)shows potential applications in Mg matrix composites because ofits low cost and excellent properties[5].While at the same time, Mg matrix composites including those

reinforced by aluminum borate whisker are very susceptible to

ponding author. Ph.D.; Tel./Fax: þ86 451 82519190;ddress: qiuorwang@hrbeu.edu.cn (Y. Wang).02/$e see front matter Copyright� 2013, The editorial office ofof Materials Science & Technology. Published by ElsevierAll rights reserved..doi.org/10.1016/j.jmst.2012.12.017

corrosion due to two reasons. On the one hand, Mg matrix isusually susceptible to corrosion because of its intrinsic chemicalactivity[6]; on the other hand, structural flaws and/or galvaniccoupling within metal matrix composites could result inincreased localized corrosion of the matrix[7e10]. In addition, Mgmatrix composites generally have poor anti-wear performancedue to the poor wear resistance of Mg alloys, particularly duringsliding in oxidizing environments, even if the ceramic reinforcedphase in composites may improve their tribological properties tosome extent[11e13]. Consequently Mg matrix composites couldhardly be used without appropriate surface treatment; however,the information about surface treatment of Mg matrix compositesis very limited till now. Yue et al.[14,15] have investigated lasersurface treatment and laser cladding of Mg matrix compositereinforced by SiC particle, and corrosion resistance of thecomposite could be improved. In addition, chemical conversioncoating technology has also been applied to Mg matrixcomposite for protection[16,17].Microarc oxidation (MAO), as an effective method to prepare

protective coating on valve metals, has become an importantsurface treatment technology for Mg alloys and Mg matrix

Fig. 1 SEM image of Al18B4O33w/AZ91 composite.

268 Y. Wang et al.: J. Mater. Sci. Technol., 2013, 29(3), 267e272

composites[18e26]. For metal matrix composites, reinforcedphases within valve metals might affect sparking discharge,consequently affect formation and growth of MAO coatingsince reinforced phases generally are ceramic and cannot bemicroarc-oxidized[23e26]; therefore, the effects of reinforcedphase on MAO process of metal matrix composites aredeserved to be investigated to obtain MAO coatings withexcellent properties.In fact, the previous study of authors has shown that MAO

coating could be successfully prepared on Mg matrixcomposite reinforced by Al18B4O33 whisker and that the effectsof this whisker on microarc discharge behavior and coatingcharacteristics have been observed and analyzed roughly[27];however, further study showed that the discussion about theeffects of Al18B4O33 whisker in literature[27] was not exactbecause the possible effects of structural defects in thecomposite, such as flaws resulting from composite preparation,on voltage evolution trend and MAO process were neglected.Unfortunately, the factor in respect of composite preparationwas not realized and was not excluded when analyzing anddiscussing the effect mechanism of Al18B4O33 whisker in theprevious study. That is, study about effects of Al18B4O33

whisker on MAO process was interfered by other factors, sothat the speculation about effect mechanism of Al18B4O33

whisker in literature[27] was inexact.Metal matrix composites are a kind of artificially designed

materials; structural defects are very easy to form when thecomposites are prepared. In the present study, structural defectsin Mg matrix composite Al18B4O33w/AZ91 were excluded aspossible to ensure the effects of Al18B4O33 whisker can bestudied separately and then some different results are obtained.The role of Al18B4O33 whisker in MAO process was observed;its effect mechanism on MAO was re-discussed. At the sametime, corrosion resistance and wear resistance of the MAOcoatings prepared on this composite were evaluated.

Fig. 2 Surface morphology evolution of Al18B4O33w/AZ91 composi

2. Experimental

The material used in this study was Mg matrix compositeAl18B4O33w/AZ91 which had about 20% volume fraction ofAl18B4O33 whisker. The matrix of Mg alloy AZ91 has a nominalchemical composition of Mge(8.5e9.5)Ale(0.45e0.9)Zn inmass%. The composite was prepared by a squeeze castingmethod. The properties of Al18B4O33 whisker and the details ofcomposite preparation had been described elsewhere[28]. Theoriginal morphology of Al18B4O33w/AZ91 composite shows thatAl18B4O33 whiskers with diameter in the range of about 0.1e1.0 mm are distributed on the composite surface relativelyhomogeneously (as shown in Fig. 1).Disk specimens, 2.5 mm in thickness and 20 mm in diameter,

were polished with 1000# abrasive paper, degreased ultrasoni-cally in acetone, and then immersed in electrolyte for MAOtreatment. The electrolyte was prepared from a solution ofsodium silicate and potassium fluoride in distilled water. Thespecimen and stainless steel sheet were used as the anode and thecathode, respectively. An MAO device with AC pulse powersupply was adopted. A constant current density of 60 mA/cm2

was applied. At the control mode of constant current, a currentwas preset and kept constant during MAO; the cell voltagevaried with oxidation time and the response of voltage to timewas recorded. For comparison, Mg alloy AZ91 was alsomicroarc oxidized using the identical parameters and then itsvoltage response was recorded.Surface morphology evolution of the composite at the early

stage of MAO was observed by scanning electron microscopy(SEM, Hitachi S-570 and S-4700). Surface morphology andcross-sectional microstructure of the coating formed at the pro-longed stage of MAO were also observed by SEM.Corrosion resistance of the bare and coated composites was

evaluated by salt immersion test and electrochemical test in 3.5%NaCl solution. Potentiodynamic polarizing curves weremeasured by using a potentiostat (M273). A three-electrode cell,with sample as working electrode, saturated calomel electrode(SCE) as reference electrode and platinum sheet as counterelectrode, was employed in this test. After 10 min initial delay,scan was conducted at a rate of 0.5 mV/s from �250 mV vs opencircuit potential (OCP) toward more positive direction untilbreakdown of coating occurred.Tribological behaviors of the coated Al18B4O33w/AZ91

composite were investigated by using a ball-on-disc friction andwear tester (CJS111A, developed by Harbin Institute of Tech-nology) under conditions of low load and low sliding speed. Thetest was carried out using a GCr15 bearing steel sphere of 6 mmdiameter as the counterpart material under a load of 0.5 N in airwithout lubrication, with test conditions of 100 r/min and3.5 mm radius of wear track. The morphology and the elementalcompositions of the worn coating were observed and determinedby SEM and energy dispersive X-ray spectrometry (EDS).

te at early different stages of MAO: (a) 25 s, (b) 30 s, (c) 60 s.

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3. Results and Discussion

Fig. 4 Observation of existing state of Al18B4O33 whisker at early stageof MAO (t ¼ 60 s)[29]. Letter W denotes Al18B4O33 whisker.

3.1. Role of Al18B4O33 whisker in MAO process

The surface morphology of Al18B4O33w/AZ91 composite afterMAO at different stages is shown in Fig. 2. Just after MAOtreatment for 25 s, some micropores with diameter of about 50e300 nm occur; while most of the areas remain free of micropores(as shown in Fig. 2(a)). This indicates that a barrier film hasformed on the composite surface since these micropores shouldresult from dielectric breakdown of barrier film and subsequentsparking discharge. It should be noted that the sparkingdischarge in this period is invisible by unaided eye due to toosmall dimension of sparks. When MAO is proceeded to 30 s, thefilm develops increasing number of micropores with diameter ofabout 100e500 nm (as shown in Fig. 2(b)). This time corre-sponded to the appearance of visible sparking discharge;although sparking discharge has resulted in a porousmorphology, a little fraction of the surface still remain free ofmicropores. With onset of overall dielectric breakdown of barrierfilm at 60 s, abundant sparks were observed. The surfacemorphology is transformed to a more uniform porousmorphology with micropores of sub-micron dimensions; occa-sionally micropores with larger diameter of about 1 mm occur (asshown in Fig. 2(c)). After this period, the sparks graduallydecrease in number, and increase in dimension and lifetime.In order to study the effect of Al18B4O33 whisker on MAO,

the voltageetime curves for AZ91 alloy and Al18B4O33w/AZ91composite during MAO treatment were recorded in Fig. 3. It canbe seen that voltage evolution trend of the composite is almostthe same as that of the alloy. Both the alloy and composite showa relatively rapid increase in voltage at the initial stage, and thenthe voltage increases slowly until a relatively steady state isapproached. That is, the same as AZ91 alloy, Al18B4O33w/AZ91composite reveals a desirable voltage evolution trend sincea rapid rise in voltage at the early stage indicates the formation ofbarrier film which is necessary to sparking discharge. Thedesirable voltage evolution trend suggests that Al18B4O33

whisker will not hinder the growth of MAO coating on thecomposite. This can be further demonstrated by microstructureobservation. Fig. 4 reveals the state of Al18B4O33 whisker at theearly stage of MAO[29]. It can be found that selective sparkingdischarge occurs when Al18B4O33w/AZ91 composite is microarcoxidized. Dielectric breakdown and sparking discharge justoccur on Mg alloy matrix in the composite, but do not occur on

Fig. 3 Variation of cell voltage with time during MAO treatment underconstant current mode.

Al18B4O33 whisker. The reason is that Al18B4O33 whisker isa ceramic phase and has no anodized characteristic which onlyvalve metals possess, and so sparking discharge cannot occur onthe whisker. Nonetheless, it can be seen that the interfacebetween the whisker and coating is very compact and no flawsexist on the interface.The surface morphology of the coating prepared at the pro-

longed stage of MAO is shown in Fig. 5. The coating presentsa homogeneous porous morphology which is a typical andinherent characteristic of MAO coating. The whisker is notobserved anymore on the coating surface. This indicates thatAl18B4O33 whisker has been covered within the coating gradu-ally with increasing oxidation time though MAO coating cannotform on Al18B4O33 whisker surface directly. Fig. 6 displays thecross-sectional microstructure of the MAO coating onAl18B4O33w/AZ91 composite. The coating is well bonded withthe substrate. It also can be seen that some whiskers near thesubstrate-coating interface are partially enfolded in the coatingwhen the MAO coating grows thick, and that no visible struc-tural defects exist in the interface between Al18B4O33 whiskerand MAO coating. This intuitively demonstrates that the excel-lent bonding state between MAO coating and substrate is notdestroyed by Al18B4O33 whisker in the interface; even thoughAl18B4O33 whisker exists in the coating as a heterogenous phase.On the contrary, Al18B4O33 whisker in the coating-substrateinterface can act as a junction piece and is advantageous to

Fig. 5 Typical surface morphology of the MAO coating onAl18B4O33w/AZ91 composite with oxidation time of 10 min.

Fig. 8 Potentiodynamic polarization curves of the bare and coatedAl18B4O33w/AZ91 composites in 3.5% NaCl solution (the MAOtime is 10 min).

Fig. 6 SEM image of cross-sectional microstructure of the MAOcoating on Al18B4O33w/AZ91 composite.

270 Y. Wang et al.: J. Mater. Sci. Technol., 2013, 29(3), 267e272

enhance the adhesion strength. Previous study[30] about surfaceand fracture morphology of MAO coating after tension test hasalso shown that Al18B4O33w/AZ91 composite has excellentbonding state with MAO coating.In our previous study[27], cell voltage corresponding to

Al18B4O33w/AZ91 composite was a little lower than that ofAZ91 alloy during MAO, and it was supposed that Al18B4O33

whisker might induce some flaws at the whisker-coating inter-face and then destroy the integrity of the coating; so theseimaginable flaws would restrict the voltage rise by promotingleakage of current at the flaw sites[29]. By contrast, all aboveresults in the present study show that voltage evolution trend ofAl18B4O33w/AZ91 composite is almost the same as that of Mgalloy AZ91; Al18B4O33 whisker exists in the coating as a heter-ogenous phase and does not react in MAO process, and then willbe covered gradually with the oxidation coating, so the interfacebetween the whisker and coating becomes very compact and nostructural defects can be observed at the interface. Therefore, theinconsistent results about voltage evolution trend of Al18B4O33w/AZ91 composite indicate that restricting voltage rise bypromoting current leakage is not due to Al18B4O33 whisker, butdue to other structural defects in the composite. As mentionedabove, metal matrix composites are a kind of artificially designedmaterials, and so structural defects such as flaws are very easy toform when the composites are prepared. When structural defectsresulting from composite preparation are excluded as possible toensure that the effects of Al18B4O33 whisker can be studied

Fig. 7 Weight loss of the bare and coated Al18B4O33w/AZ91 compos-ites as a function of immersion time in 3.5% NaCl solution (theMAO time is 10 min).

separately, it is found that the voltage evolution trend ofAl18B4O33w/AZ91 composite is almost the same as that of Mgalloy AZ91. Thereby, for metal matrix composites, not onlyreinforced phase but also preparation process and quality controlof the composites should be considered when MAO technologyis applied to surface treatment.It is known that the formation of an integral and insulative

barrier film on the substrate at the initial stage of MAO is thenecessary condition for dielectric breakdown and sparkingdischarge according to the growing mechanism of MAO coating.The integrity and electrical insulation of the MAO coating can beensured since Al18B4O33 whisker has an excellent electricalinsulation property and that the whisker-coating interface iscompact. Consequently, the cell voltage evolves in a desirabletrend without current leakage in MAO process of Al18B4O33w/AZ91 composite; dielectric breakdown and sparking dischargetake place normally like Mg alloy AZ91. It can be concluded thatAl18B4O33 whisker will not inhibit the discharge process andcoating growth even though this whisker itself cannot bemicroarc-oxidized. The speculation in literature[27] thatAl18B4O33 whisker may melt during microarc discharge and thenresult in a well interface with surrounding coating is not testifiedby the present study. Microscopic observation of Al18B4O33

whisker reveals that this whisker exists in the MAO coating asa heterogeneous phase and still has a well interface withsurrounding coating.

Fig. 9 Variation of friction coefficient with sliding time for the bare andcoated Al18B4O33w/AZ91 composite (the MAO time is 10 min).

Fig. 10 SEM images and elemental compositions of the wear track of the coated Al18B4O33w/AZ91 composite: (a) low magnification image of the weartrack, (b) high magnification image of the wear track in (a), (c) EDS spectrum of the worn coating.

Y. Wang et al.: J. Mater. Sci. Technol., 2013, 29(3), 267e272 271

3.2. Corrosion resistance of the MAO coating

Weight loss of the bare and coated Al18B4O33w/AZ91composites during immersing in 3.5% NaCl solution is shown inFig. 7. The large weight loss of the bare composite indicates itspoor corrosion resistance. When the bare composite wasimmersed in the solution, hydrogen bubbles appeared gradually.Corrosion rate of the bare composite almost kept constantthroughout the immersion period, which indicated that naturallyprotective passivation film could not form on the compositesurface. It is also obvious that the coated composite exhibits farbetter corrosion resistance than the bare one. MAO coating couldprotect the composite against serious corrosion in 3.5% NaClsolution within 7 days since the weight loss was very low in thisimmersion period.Potentiodynamic polarization curves of the bare and coated

Al18B4O33w/AZ91 composites in 3.5% NaCl solution are shownin Fig. 8. It can be seen that corrosion potential (Ecorr) movestoward more positive direction (from �1.51 to �1.42 VSCE) andcorrosion current (Icorr) is decreased by 4 orders of magnitudeafter Al18B4O33w/AZ91 composite is coated by MAO. That is,the coated composite shows more positive corrosion potentialand lower corrosion current than the bare composite; this indi-cates that both anodic dissolution reaction and cathodic reactioncan be restrained greatly by MAO coating, and that the coatingcan restrain anodic reaction more effectively than cathodicreaction since Icorr is decreased while Ecorr moves toward morepositive direction. So MAO coating can provide effectivecorrosion protection for Al18B4O33w/AZ91 composite byrestraining both cathodic reaction and anodic reaction. Addi-tionally, for the coated composite, anodic current increases veryslowly from Ecorr (�1.42 VSCE) to �1.11 VSCE, and thenincreases quickly when the potential is more positivethan �1.11 VSCE. This result shows that MAO coating canprevent the composite from corroding when the potential ismore negative than �1.11 VSCE. Thus it can be concluded thatMAO coating with excellent corrosion resistance could beachieved on Mg matrix composite despite the presence ofAl18B4O33 whisker. The high corrosion resistance of the coatedcomposite should be closely related to the compact whisker-coating interface since Al18B4O33 whisker does not inducestructural defects.

3.3. Wear resistance of the MAO coating

Friction coefficient evolution with sliding time of the bareand coated Al18B4O33w/AZ91 composites is shown in Fig. 9.

The coated composite exhibits an initial lower and then higherfriction coefficient compared with the uncoated one. Fig. 10shows the surface morphology and elemental composition ofthe wear track of the coated composite. Fig. 10(a) indicates thatthe worn coating does not show galling, tearing and plastic flowtendency which generally occur on the worn composite. Thecoating exhibits high wear resistance and slight tendency ofadhesive wear as shown in Fig. 10(b). Fig. 10(c) shows that thecontent of Fe element on the wear track can reach about90 at.%, which indicates that the softer bearing steel has largelytransferred onto the harder coating during friction. The mainconstituent phase of the MAO coating is MgO which is a hardceramic phase. The ceramic coating sliding against counterpartof bearing steel will become the bearing steel sliding againstitself after much transfer of counterpart material; therefore thecoated composite shows a slight tendency of adhesive wear andits friction coefficient increases gradually with sliding time. Inaddition, Fig. 10(b) also shows that the MAO coating is notcompletely covered with the transferred counterpart materialand some micropores in the coating still could be observed onthe wear track. This phenomenon further indicates that theMAO coating on Al18B4O33w/AZ91 composite can hardly beworn by the counterpart material and exhibits good wearresistance.

4. Conclusions

(1) The MAO coating exhibits selective growth on Mg matrixcomposite Al18B4O33w/AZ91. Sparking discharge just occurson Mg matrix in the composite, while does not occur onAl18B4O33 whisker. The whisker exists in the MAO coatingas a heterogeneous phase and does not react in MAO process.

(2) Excellent electrical insulation property of Al18B4O33 whiskerand compact whisker-coating interface at the initial stageof MAO ensure that Al18B4O33 whisker does not inhibitsparking discharge and coating growth. The whisker willbe gradually covered within the coating with increasingoxidation time.

(3) The MAO coating can provide effective corrosionprotection for Al18B4O33w/AZ91 composite by greatlyrestraining both cathodic reaction and anodic reaction.Corrosion current decreases by 4 orders of magnitudeafter the composite is coated by MAO.

(4) The MAO coating on Al18B4O33w/AZ91 composite exhibitshigh wear resistance with GCr15 bearing steel as its counterpart material. A slight tendency of adhesive wear is shown

272 Y. Wang et al.: J. Mater. Sci. Technol., 2013, 29(3), 267e272

due to transfer of softer bearing steel into the harder coatingduring friction.

AcknowledgmentsThis work was supported by the National Natural Science

Foundation of China (No. 51001036) and the FundamentalResearch Funds for the Central Universities (Nos.HEUCFR1021 and HEUCF201210001).

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