Corrosion and tribocorrosion behaviour of AlSiCuMg alloy and itscomposites reinforced with B4C particles in 0.05 M NaCl solutionF. Toptana,b,n, A.C. Alvesb, I. Kertia, E. Arizab,c, L.A. Rochab,daYildiz Technical University, Department of Metallurgical and Materials Engineering, Faculty of Chemistry & Metallurgy, Davutpasa Campus, 34210 Esenler,Istanbul, TurkeybCentre for Mechanics and Materials Technologies (CT2M), Universidade do Minho, Azurm, 4800-058 Guimares, PortugalcUniversidade do Minho, SEMAT/UM, Azurm, 4800-058 Guimares, PortugaldUNESPUniv. Estadual Paulista, Faculdade de Cincias de Bauru, Dep. Fsica, 17033-360 Bauru, SP, Brazila rti cle in foArticle history:Received 22 November 2012Received in revised form17 June 2013Accepted 26 June 2013Available online 6 July 2013Keywords:Metalmatrix compositeCorrosionwearWear testingabstractThecorrosionbehaviourof metal matrixcomposites(MMCs) isstrictlylinkedwiththepresenceofheterogeneities such as reinforcement phase, microcrevices, porosity, secondary phase precipitates, andinteraction products. Most of the literature related to corrosion behaviour of aluminiummatrixcomposites (AMCs) is focusedonSiCreinforcedAMCs. Onthe other hand, there is verylimitedinformationavailableintheliteraturerelatedto thetribocorrosionbehaviour ofAMCs. Therefore, thepresent work aims to investigate corrosion and tribocorrosion behaviour of AlSiCuMg alloy matrixcomposites reinforced with B4C particulates. Corrosion behaviour of 15 and 19% (vol) B4C reinforced AlSiCuMg matrix composites and the base alloy was investigated in 0.05 M NaCl solution by performingimmersion tests and potentiodynamic polarisation tests. Tribocorrosion behaviour of AlSiCuMg alloyanditscompositeswerealsoinvestigatedin0.05 MNaClsolution. Thetestswerecarriedoutagainstalumina ball using a reciprocating ball-on-plate tribometer. Electrochemical measurements wereperformed before, during, and after the sliding tests together with the recording of the tangential force.Results suggest that particle addition did not affect signicantly the tendency of corrosion of AlSiCuMgalloywithout mechanical interactions. Duringthetribocorrosiontests, thecountermaterial wasfound to slide mainly on the B4C particles, which protected the matrix alloy from severe wear damage.Furthermore, theweardebriswereaccumulatedonthewornsurfacesandentrappedbetweenthereinforcing particles. Therefore, the tendency of corrosion and the corrosion rate decreased in AlSiCuMgmatrix B4C reinforced composites during the sliding in 0.05 M NaCl solution.& 2013 Elsevier B.V. All rights reserved.1. IntroductionAluminium is one of the most reactive metals with high afnityto oxygen. However, due to the inert and protective characteristicsof the aluminium oxidelm that forms on the metal surface, Al ishighly resistant to most atmospheres as well as a great variety ofchemical agents [1].AlSi casting alloys are widely used in the automotive industry,mainly due to their high castability and high mechanical proper-ties. Boththehypo-eutecticandhyper-eutecticAlSi alloysarebeingusedforseveral tribological applications, suchasinternalcombustion engines, pistons, liners, clutches, pulleys, rockers andpivots [2,3]. Mechanical strength of these alloys can be improvedby the addition of copper, where precipitation hardening leads tothe controlled precipitation of Cu-rich precipitates (usually meta-stable intermetallic Al2Cu phases) that form obstacles for disloca-tion movements [3,4]. However, due to the noble behaviour of thisphase, existenceof thegalvaniccouplingwiththesurroundingmatrixmay leadto severalconsequencesonthecorrosionresis-tance. It is well known that the Al2Cu phase acts as a preferentialcathode forthe oxygen reductionreaction and therefore acceler-ates the oxidation of aluminium. Furthermore, the OHproducedduring oxygen reduction can locally increase the pH, and may leadto the local dissolution of the Al-matrix in the surroundings of thepreferential cathodes [3,5].In MMCs, the composition of the matrix material, the reinfor-cementphase, microcracks, residualstresses, microcrevices, por-osity, secondary phase precipitates, and interaction products maysignicantly affect the corrosion behaviour [69]. The main causesof thecorrosioninMMCsarereportedas(i) galvaniccouplingbetween the matrix and the reinforcement materials, (ii) selectiveContents lists available at ScienceDirectjournal homepage: www.elsevier.com/locate/wearWear0043-1648/$ - see front matter& 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.wear.2013.06.026nCorrespondingauthor at: Centrefor Mechanics andMaterials Technologies(CT2M), Universidade do Minho, Azurm, 4800-058 Guimares, Portugal.Tel.: +351 253 510 220; fax: +351 253 516 007.E-mail address: ftoptan@dem.uminho.pt (F. Toptan).Wear 306 (2013) 2735corrosion at the matrix/reinforcement interface, (iii) chemicaldegradation ofinterphases andreinforcementmaterials,and(iv)corrosion of matrix defects [10,11].Most of the literature related to corrosion behaviour ofAMCs is focused on SiC reinforced AMCs, and pitting is reported asthe most common corrosion type in these composites [12]. On theother hand, thereis verylimitedinformationavailableonthecorrosion behaviour of AlB4C composites. In a recent work,Katkar et al.investigated the corrosion behaviour of AA6061-B4C(0, 10, 15, and20%inwt) compositesinseawater. Theauthorsstatedthat corrosiontendencyof AA6061-B4Ccomposites waslowerthantheunreinforcedalloywithinthestudiedconditions,and furthermore, corrosion tendency is decreased with theincreasing amount of B4C particles. However, with increasingamount of B4C particles, the composites exhibited lower resistanceto pitting corrosion [12].Ithasbeenreportedthat10%of thematerial degradationinengineeringpartsoccursduetocorrosion, while30%isduetoabrasion, 15% is due to adhesion, and 10% is due to tribocorrosion[13]. However, even though the tribological characteristics ofAMCs have been extensively studied [14,15], there is very limitedinformation available in the literature related to the tribocorrosionbehaviourof AMCs. TribocorrosionhasbeendenedbyLandoltet al. as an irreversible transformation of a material resulting fromsimultaneousphysico-chemical andmechanical interactionsthatoccur in a tribological contact [16]. In tribocorrosion systems, thetotal degradation rate can be different than the sumof thecorrosion rate and the wear rate that measured individually [17].Fang et al. studied the synergistic effects of wear and corrosionfor020%(vol)Al2O3particulatereinforced6061AMCs, againstAl2O3in3.5%(wt) NaCl solutionusingarevisedblock-on-ringwear tester. The authors stated that incorporation of the reinforce-mentwasdetrimental tothecorrosionresistanceofaluminium,however, particle addition improved the tribocorrosion behaviourof the matrix material [18]. Velhinho et al. investigated thetribocorrosion behaviour of AlSiCpfunctionally graded metalmatrix composites (FGMMCs) against cast iron pins in water, usinga unidirectional pin-on-disc tribometer. Two volume fractions(12.6 and 35.7%) of SiC particles were studied. The authorsreportedthat thepresenceof water facilitatedcatastrophicSiCparticlepull-out, andthereforeincreasedthematerial losssig-nicantly. Furthermore, an increase in reinforcing particles contentresultedinapoorer wear performance[19]. Gomes et al. alsostudied tribocorrosion behaviour of AlSiCp FGMMCs with volumefractions varying between 25.8 and 33.4%. Experiments performedagainst cast ironpins in3%NaCl solutionusing reciprocatingsliding. The authors reported that the wear rate of the compositeswas not signicantlyaffectedbythe presence of the aqueoussolution[20]. Besides, asopposedtoVelhinhoetal. [19], higheramounts of reinforcement lead to lower wear rates, indicating thatthe presence of NaCl can have some inuence on the tribocorro-sionmechanisms. Theauthors attributedthis behaviour totheability of higher amount of reinforcing particles to anchor iron-richprotective tribolayers and load-supporting effect given by SiCparticles. Vieira et al. studied the tribocorrosion behaviour of AlSiCp FGMMCs against alumina ball in 0.05 M NaCl solution using aball-on-plate tribometer. After triboelectrochemical studies on 1123% (vol) composites, the authors stated that the introduction ofSiC particles did not affect the corrosion behaviour of thecomposites. The authors reported two tribocorrosion mechanismsdepending on the SiCpcontent: above 18%, wear acceleratedcorrosion by protecting effect of protruded SiC particles, and below18%, wear-accelerated corrosion without any affect of the particlesonwear[21]. Ferreiraetal. studiedtribocorrosionbehaviourofAlAl3Ti and AlAl3Zr FGMMCs against alumina ball in 0.6 M NaClsolution using a reciprocating tribometer. After tribocorrosionstudies of upto 17.3%(vol) composites, the authors reportedabettertribocorrosionbehaviour inthesampleshavinghighervolume fraction of particles [22]. Jamaati et al. investigatedtribocorrosion behaviour of AlAl2O3 composites against aluminaball in 1 wt% NaCl solution, using a ball-on-plate tribometer. Afterstudyingof 0.483.55%(vol)reinforcedcomposites, theauthorsreported that homogenous particle distribution improved thewear and corrosion resistance [23].There are several studies related to the dry sliding wear behaviourof AlB4C composites. However, to the best of our knowledge, thereis no information available in the literature related to the tribocorro-sion behaviour of these composites. It is worth to emphasize that inmany cases, the industrial components (e.g. automotive applications)are required to be operated in aqueous environments (i.e. corrosivemedia) [24]. SinceAlB4Ccompositeshavebeenconsideredasawearresistantmaterial [2527], tribocorrosionbehaviourof thesecomposites is also needed to be studied, especially before consider-ing them as an alternative material for the applications that are beingexposed to a tribocorrosion environment during their lifetime. Thus,the present study aims to investigate the corrosion and tribocorro-sionbehaviour of 15and19%(vol) B4CreinforcedAlSiCuMgmatrix composites in comparison with its base alloy in 0.05 M NaClsolution.2. Experimental procedure2.1. MaterialsB4C particles with an average particle size 32 m were used as areinforcement, andAlSiCuMgaluminiumalloywasusedasamatrixmaterial (Table1). Inorder topromotethewettabilityofboroncarbidepowdersandimprovetheirincorporationbehaviourinto aluminium melts,AlSiCuMg matrix B4C reinforced compo-siteswereproducedbythe additionofK2TiF6ux. Theprocessingprocedure and the physical properties of AlSiCuMgB4C compo-sites having two different volume fractions of 15 and 19% (nominalvalues of 16 and 22, respectively) are explained elsewhere [28].2.2. Corrosion testsTwo different corrosion tests were carried out: immersion testsandpotentiodynamic polarisationtests. Prior toeachtest, thesampleswere grindedandpolishedusingdiamondgrindersandwater baseddiamondandcolloidal silicasuspensions downto0.04 m. An amount of 0.05 M NaCl (Panreac) used as the electro-lyte, saturated calomel electrode (SCE) used as the referenceelectrode, Pt electrode used as the counter electrode, and samplesused as the working electrode in both corrosion tests.The samples were immersed to the solution for 192 h (8 days).Opencircuitpotential(OCP)wasmeasuredjustafterimmersionduring30 minandafter that, for 10 minduringthefollowingTable 1Chemical composition of AlSiCuMg matrix material.Al Si Fe Mn Cr Ni Cu Mg Pb Sn Ti Zn82.8 10.14 1.29 0.432 0.021 0. 032 2.99 1.49 0.372 0.008 0.084 0.616F. Toptan et al. / Wear 306 (2013) 2735 286 days. The OCP measurements were performed using a VoltalabPGZ100potentiostat(RadiometerAnalytical, Copenhagen, Den-mark) controlled by the VoltaMaster 4 software (RadiometerAnalytical, Copenhagen, Denmark).Polarisationcurves were obtainedusinga VoltalabPGP201potentiostat (Rodiometer Analytical, Copenhagen, Denmark) con-trolledby the VoltaMaster 4 software (Radiometer Analytical,Copenhagen, Denmark). Potentiodynamic polarisationmeasure-ments started from a cathodic potential of 0.8 V up to the anodicdomain (O V vs. SCE) with a scan rate of 0.5 mV/s.2.3. Tribocorrosion testsPrior to each test, specimens were prepared following the sameprocedure for the corrosion tests. Before starting the tests, speci-mens were cleaned with propanol in ultrasonic cleaner for 10 minand kept in desicator for 1 h.For the tribocorrosion experiments, the samples were mounted ina cell containing the electrolyte (0.05 M NaCl) and electrodes (Fig. 1).Thecellwasinstalledonaball-on-platetribometer(CETR-UMT-2)with a reciprocating plate adapter with the working surface area ofthe samples facing upwards. Alumina ball (10 mmof diameter,Goodfellow)wasusedasacountermaterial anditwasmountedverticallyabovetheexposedsamplearea(2.3 cm2). Theball wasloadedandtheslidingstartedinareciprocatingsystem withtotalstroke length of 5 mm, frequency of 1 Hz, normal force of 3 N, andtotal sliding time of 600 s. The electrochemical measurements (OCPand current) were performed using the same three-electrode set-upthat used in corrosion tests. OCP and current were measured before,during, and after sliding. For the tribocorrosion tests under potentio-static conditions, the samples were stabilized in electrolyte and theEOCPvalues were appliedas the potential for eachtest. All theelectrochemical measurements were performedusing a VoltalabPGP201 potentiostat (Rodiometer Analytical, Copenhagen, Denmark)controlledbytheVoltaMaster 4software(Radiometer Analytical,Copenhagen, Denmark). The tests were performedat the roomtemperature (2571 1C).2.4. CharacterisationMetallographic samples sectioned fromthe cast bars werepreparedusingdiamondgrindersandwaterbaseddiamondandcolloidal silica suspensions down to 0.04 mgrain size. Themicrostructure of the as-cast alloy was characterised by XRD (CuKradiation, BrukerD8Discover). As-castmicrostructureswereexaminedunderLeicaDM2500opticalmicroscope(OM)andFEINova 200eld emission gun scanning electron microscope (FEG-SEM) equipped with EDAX, energy dispersive X-ray spectroscopy(EDS). Corroded surfaces after immersion, and worn surfaces aftertribocorrosion tests were also characterised using FEG-SEM/EDS.3. Results and discussion3.1. Material microstructuresAs can be observed in the XRD spectrum presented in Fig. 2, thefollowing phases were identied for as-cast AlSiCuMg alloy: -Al, Si, -Al2Cu, Q-Al4Cu2Mg8Si7and-Al8Si6Mg3Fe. Similarphaseswere also reported by Vieira et al. [3] for a similar alloy (Al10Si4.5Cu2Mg).Fig. 3ashowsthemorphologyof eachphaseasobservedbySEM. ItisknownthatceramicparticleadditioncanchangethesolidicationsequenceofMMCsfromthatofitsbasealloy. Asaresult, matrix microsegregation is reduced, crystal morphology ismodiedfromcellulardendritic(Fig. 3b)toafeaturelessstruc-ture, siliconphasenucleatedheterogeneouslyonparticles, andmatrix grains are rened (Fig. 3c and d) [29,30]. The modicationof the matrix alloy structure may inuence the mechanicalproperties and wear resistance of the composites [31].3.2. Corrosion tests3.2.1. Evolution of the open circuit potential with timeEvolution of the open circuit potential (OCP) values of the baseAlSiCuMg alloy and its composites with time is given in Fig. 4a.As can be seen on the graph, particle addition did not signicantlyaffect the tendency of corrosion of AlSiCuMg alloy along time.It has been reported that in aluminium alloys, noble phases likeAl2Cu, Mg2Si andAl3Fecancauselocalizedcorrosionduetothegalvanic coupling [3,32]. Fig. 4b shows the representative SEMmicrographstakenfromthebaseAlSiCuMgalloyafter26 hofimmersion. Thepreferential dissolutionaroundthe-Al2Cuphasecan be clearly seen on Fig. 4b. However, other phases in the AlSiCuMgalloydidnot leadtopreferential dissolutionor galvaniccoupling effect. The role of Al2Cu intermetallic compound in selectivecorrosion of Al alloys is well studied [5]. The results are in agreementwith Vieira et al. where preferential dissolution in the vicinity of -Al2CuphasewasreportedforasimilarAlSiCuMgalloyinthesame electrolyte (0.05 M NaCl), whereas no preferential dissolutionwas reported for any other phase in the alloy [3].Itisknownthatit isdifcult toproduceAlB4Ccompositesespeciallyatprocessingtemperaturesbelow1100 1C, duetothepoor wetting between Al and B4C [3335]. However, by theadditionof Ti, it is possible toproduce AlB4Ccomposites atrelatively lower temperatures with higher particle volumeFig. 1. Schematic view of the tribocorrosion test setup.Fig. 2. XRD spectrum of as-cast AlSiCuMg alloy.F. Toptan et al. / Wear 306 (2013) 2735 29fractions, viaformationofTiB2TiCreactionlayeratthematrix/reinforcement interface. The characterization of the TiB2TiCreactionlayersatthematrix/reinforcementinterfacesforAlSiCuMg, AlSi9Mg, AA1070, and AA6063 matrix B4C reinforcedcomposites have already been reported [28,3638]. The interfacesplayanimportantroleonMMCsandoneof themostcommoncorrosionproblems in AMCs is the selective corrosionat thematrix/reinforcement interface[10]. It is knownthat excessiveformationof largealuminiumcarbides at thematrix/reinforce-ment interface is detrimental to the corrosion resistance of AMCs[39]. The interphases reported for AlB4C system are AlB2, Al3BC,Al4BC, AlB24C4, Al3B48C2andAl4C3. Ithasbeenreportedthattheformationof Al4C3phase canbe preventedat the processingtemperaturesbelow1000 1C[4042]. Furthermore, thereactionlayer that contains TiC and TiB2 forms on the B4C particles with theaddition of Ti acts as a reaction barrier and limits the formation ofundesirableinterphasesthat can be formed at the interface [43].Al4C3formation wasnotdetectedonthemicrostructuralstudiesfor the present work. Besides, preferential dissolution was also notobserved at the matrix/reinforcement interface responsible for thereaction layer. The EDS analysis taken from the interfaceFig. 3. (a) BSE SEM image and (b) OM image of as-cast AlSiCuMg alloy, and OM images of (c) 15% and (d) 19% B4C reinforced composites.Fig. 4. (a) Evolution of OCP values with time and (b) SE SEM image of Q and phases.Fig. 5. SEandBSESEMimagesfromthematrix/reinforcement interfaceof 15%reinforcedcompositeafter 26 hof immersiontogether withtheEDSspectrumtaken from the marked area (interface).F. Toptan et al. / Wear 306 (2013) 2735 30conrmedthat thereactionlayer stayedintact at theinterfaceafter the immersion (Fig. 5).3.2.2. Potentiodynamic polarisation testsPolarisation curves of the base AlSiCuMg alloy and the compo-site samples in 0.05 M NaCl solution are given in Fig. 6. Passive plateauwasnot observedonthepolarisationcurvesprobablyduetothepreferential dissolution around the -Al2Cu phase. Corrosion potentialand current density values were also calculated by Tafel extrapolationmethod(Table2). Itcanbeseenfromthetablethatthecorrosioncurrent density (icorr) values were decreased by B4C particle addition. Ithasbeenreportedthatadditionofinertparticlesintoametal canincreasethecorrosionresistanceofthemetalbytheinertphysicalbarrierroleof theparticles[44,45]. Thus, thedecreaseontheicorrvalues may be due to the effect of the B4C particles. However, it hasalso been reported that incorporation of inert material may also shiftthe corrosion potential (E(i 0)) to more noble values by diminishingthe exposed metallic area [46,47]. However, a clear correlationbetweenparticlevolumefractionandcorrosionpotential has notbeen observed in the present study. Trzaskoma and Mccafferty studiedthe effect of SiCreinforcement on the corrosion behaviour of SiC/AlMMCs in 0.1 and 0.6 N NaCl solution and reported that the polariza-tionbehaviourof thealloysandcompositesweresimilar, andtheeffect of SiC reinforcements on the corrosion potential was not clear.Theauthorsalsostatedthat theadditionof SiCmayresult morepositive, morenegative, orunchangedcorrosionpotential valuesdepending on the alloy system and deaeration conditions [48]. Butbesides, anodic and cathodic polarisation of the unreinforced alloyand the composites presented similar character (Fig. 6). Katkar et al.investigated the inuence of B4C addition (10, 15, and 20% in wt) tothe polarisation behaviour of AA6061 alloy and reported theminimumcorrosioncurrent density (icorr) for the unreinforcedalloy. Furthermore, the authors also reported that the character ofanodic and cathodic polarisation curves for B4C reinforced compo-sites were very similar to that of its unreinforced alloy [12]. Vieiraet al. studiedthe polarisationbehaviour of Al10Si4.5Cu2Mgmatrix SiC reinforced FGMs and obtained similar behaviour for theunreinforcedalloy andtheSiCreinforcedFGMs.Furthermore, theauthorsreportedsimilar E(i 0)values bothfor theunreinforcedalloy and the FGMs (0.60 V vs SCE) [21].3.3. Tribocorrosion3.3.1. Electrochemical measurementsTheevolutionof theOCPandthecurrentdensitywithtimebefore, during, andaftertheslidingaregiveninFig. 7for theunreinforcedalloyandthe composites, together withtheCOFvalues obtainedduringthesliding(COFvalues presentedverysimilar evolutioninbothelectrochemical tests). Intheunrein-forced alloy, when sliding started, OCP values started to decreaseand current density values started to increase, and after a certainpoint, the values stayed relatively stable till the end of the sliding.This is awell knownbehaviour for thepassivemetals; whensliding starts, due to the periodically removing of the passivelmcausedbythemechanical actionfollowingbyexposureof freshactive material that becomes contact with the solution, tendencytocorrosionandcorrosionrateincrease[21,22,49]. However, anopposite behaviour was observed for the composites for both OCPand current density curves. When sliding started, instead ofdecreasing, OCPvaluesslightlyincreased. Aftersliding, theOCPvalues recoveredandbecame stable near the values recordedbeforetheslidingstarted, eventhoughtherecoveringtimewasshorter (approx. 2 min) compared to the base AlSiCuMg alloy(approx. 3 min). On the other hand, sudden drops were recordedon the current density values of the composites in the verybeginningof thesliding. Afterthat, theevolutionwasgenerallybelow the initial values during the sliding with a relatively stableevolutionduring the second half of the sliding. After sliding,thevalueswererecoveredquicklynearthestartingvaluesbothfor the unreinforced alloy and the composites. Similar behaviour oftheOCPandcurrent densityevolutionunder slidingwas alsoobservedbyGomesetal. andMathewetal., respectively. Ithasbeen reported in the aforementioned study of Gomes et al. that anincrease on the OCP values of AlSiC composites was occurred dueto a protective character of thelm which formed on the samplessurfacebyatransfer fromthecounter material andanchoredbetweenthe reinforcing particles [20]. Besides, Mathewet al.reportedthereductioninthecurrentdensityduringtheslidingaction due to the accumulation of the wear debris and corrosionproductsinthecontactzoneforthetribocorrosionbehaviourofthe TiCxOy thinlms in articial sweat solution [50].3.3.2. Microstructural analysis of the worn surfacesInordertounderstandthetribocorrosionmechanism, micro-structures of the worn surfaces were investigated by FEG-SEM. Allmicrographs are taken as parallel to the sliding direction and themicrostructuresexhibitedsimilarfeaturesafterbothelectroche-mical tests.The width of the wear tracks were microscopically measured asapprox. 682, 602, and347 mfortheunreinforcedalloy, anditscomposites with 15 and 19% B4C, respectively. It is well known thataddition of hard particles increases the wear resistance of the basealuminiumalloy [5155]. Furthermore, it is also known thatmicrostructure modication in AlSi and AlSiCu alloys leads toimprovement onthe wear resistance [5659]. Therefore, eventhoughnostudywereperformedinordertomeasurethewearloss volume, it may be suggested from the microscope investiga-tions (i.e. widthof the wear track) that B4Cparticle additioncaused a better response to wear on the AlSiCuMg alloy matrixcompositesin0.05 MNaCl solution, probablymainlyduetotheload bearing effect of the reinforcing particles,but also might becontributedby the increasedwear resistance of the modiedmatrix alloy.Fig. 6. Polarisationcurvesof thebaseAlSiCuMgalloyanditscompositesin0.05 M NaCl solution.Table 2Corrosion potential (E(i 0))and corrosion current density (icorr) values.Sample E(i 0) (mV) icorr (A/cm2)AlSiCuMg 696.574 4.2270.35AlSiCuMg15% B4C 655.8715 4.0370.54AlSiCuMg19% B4C 694.072 2.6470.16F. Toptan et al. / Wear 306 (2013) 2735 31It is observed that the wear tracks were darker than the outerarea. SincetheimagesaretakeninBSE-mode, thispointstoacompositional changebetweenthewornandtheunwornareas.EDSanalyses takenfromthosetwoareas showedthat oxygencontentofwornareasishigherthantheunwornareasforeachspecimen. At higher magnication, the wornsurfaces showedfollowingfeatures: (i) grooves, (ii) higher oxidecontent intheworn area, (iii) cracks on the alloy due to plastic deformation, (iv)particlepull-outs(voids), (v)broken particles, and(vi)smootherparticle surfaces in worn area.In the present work, normal load is chosen as 3 N which resultshigher initial Hertzian contact pressures (473 MPa) that is higherthan the yield strength of the base alloy (approx. 193 MPa [60]). Itis considered that this relatively higher contact pressures lead toplastic deformation during the sliding. Fig. 8a shows the cracks onthe base AlSiCuMg alloy due to plastic deformation. It has beendeduced that this relatively higher contact pressure also resultedremoving some of the weakly attached particles (Fig. 8b) as well assomeparticlecracking(Fig. 8c). Eventhough, itisalsoobservedthat most of the particles stayed intact on the surface after sliding(Fig. 8d). Ontheotherhand, whenthesurfacesof theparticlesinside the worn area were investigated, it was observed that thosesurfacesweremuchsmoother(Fig. 8e) thantheonesfromtheunwornarea(Fig. 8f). Thiscanbeattributedtotheloadbearingeffectof thereinforcingparticles. Therefore, itcanbesuggestedthat duringthesliding, theloadis mainlycarriedbytheB4Cparticles, andthereforeparticlesurfaceswerepolished bythecounter material. Besides, wear debris were accumulated betweentheload-carryingparticles, andeventhoughthesampleswerecleaned after the tests, it was still possible to observe theaccumulations on the microstructural observations (Fig. 8d).3.3.3. Tribocorrosion mechanismAfter triboelectrochemical and microstructural studies, thewearmechanismundertribocorrosionconditionshasbeensug-gested as following:(i) Unreinforcedalloy: Undermechanical solicitation, duringtheapprox. 35 min of the sliding, while the OCP and the currentdensity values were increasing COF values were slightlyincreased(Fig. 7). It isdeducedthat withthebeginningofthe sliding, partially oxidised wear debris started to beaccumulatedonthe surface, andduring the sliding thosedebriswerepackedonthesurfacebythecountermaterial.Therefore, bothelectrochemical values andtheCOF valuespresented relatively stable values during the rest of the sliding.(ii) Composites:Both15and19%B4Creinforcedcompositespre-sented lower COF valuesatthe very beginning ofthesliding(Fig. 7). ItisdeducedthatthoselowerCOFvaluesindicateaceramicceramic contact and therefore, it is suggested that thecounter material was mainly in contact with the B4C particlesat the onset of the sliding (Fig. 9a). As sliding continued, whenthecountermaterial metaprotrudedB4Cparticle(Fig. 9b),localincrements were recordedontheCOFvalues(Fig. 7). Ifthat protruded particle was weakly attached on the matrix, itpulled-out or broke, as shown in the micrographs presented inFig. 8b and c. On the other hand, if particle pull-out happened,a fresh active metal became contact with the solution (Fig. 9c)whichledtodecreaseintheOCPvaluesandincreaseinthecurrentdensityvalues(Fig. 7). Inthesecondhalf of sliding,most of the weak protruding particles were removed from thesurface and/or broke and therefore, the COF values were morestable around 0.3 indicating the ceramicceramic contact.Moreover, since the load was mainly carried by the reinforcingparticles, metal surfacewasmainlynot incontact withthecounter material. Besides, even though there is no totalpassivationonthemetal duetothepreferential dissolutionaroundthe -Al2Cuphase, freshmetalsurfacesafterparticlepull-outspartiallyrepassivated, furthermoretheweardebriswere accumulated on the metal surface and entrappedbetweentheB4Cparticles (Fig. 9d). Similar tothesystemsstudied by Gomes et al. [20] and Mathew et al. [50], this weardebrisaccumulationledtoamoreprotectivemetal surface,thus higher OCP values and lower current density values wereobserved as compared to the un-sliding stage.Afterthis rst approachtothetribocorrosionbehaviourof theAlB4C composites, there is a need of further studies in order to have adeeper understanding on the tribocorrosion behaviour of thesecomposites. First, theaccumulatedweardebrisortribolayerthatisactedas aprotectivelayer shouldbemicrostructurally(i.e. cross-sectional microstructural and chemical analysis) and electrochemically(i.e. electrochemical impedancespectroscopy) analysedinordertounderstand the protective characteristics better. Further, tribocorrosiontests should be performed on various potentials, and individualcontributions of wear, corrosion, and their synergistic effects onmaterial degradation should be quantied.Fig. 7. The evolution of the (a) OCP and (b) current density together with the evolution of the COF values during sliding.F. Toptan et al. / Wear 306 (2013) 2735 324. ConclusionsCorrosion and tribocorrosion behaviour of the base AlSiCuMgalloy and its composites reinforced with B4C particles were investi-gated in 0.05 M NaCl solution. After electrochemical, tribological andmicrostructural studies, the followings can be concluded:(1) Particleadditiondidnotaffectsignicantlythetendencyofcorrosion of AlSiCuMg alloy.(2) Passiveplateauwasnotobservedonthepolarisationcurvesdue to the preferential dissolution around the -Al2Cu phase.(3) After immersion, preferential dissolutionwas not observedin the vicinity of the reinforcement particles, and thematrix/reinforcement reaction layer stayed intact duringthe immersion.(4) During the tribocorrosion tests, the counter material wasfoundtoslidemainlyontheB4Cparticles, whichprotectedthe matrix alloy from severe corrosion/wear damage.Fig. 8. SEM images after tribocorrosion tests under potentiostatic conditions representing (a) plastic deformation on the AlSiCuMg sample (SE), (b) particle pull-out and(c) broken particle on the AlSiCuMg15% B4C sample (SE), (d) wear track of the AlSiCuMg15% B4C sample (BSE), (e) load bearing particles on the wear track of theAlSiCuMg15% B4C sample and (f) particles on the unworn area of the AlSiCuMg19% B4C sample (SE).Fig. 9. Schematic view of the suggested tribocorrosion mechanism; (a) in the beginning of the sliding, the counter material is in contact with B4C particles, (b) the countermaterial meets with a protruded particle and (c) after particle pull-out, a fresh active metal becomes contact with the solution andnally (d) the surface of the metal iscovered by the accumulated wear debris.F. 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