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Characterisationandtribologicalevaluationofanickelsurfacealloyedaluminium-basedmaterialM. HeydarzadehSohiFacultyofEngineering,DepartmentofMetallurgyandMaterials,UniversityofTehran,Tehran,IranAbstractElectronbeamnickel surfacealloyingof LM13, anear eutecticAlSi-basedmaterial, wascarriedout. Thealloyingresultedintheformation of a surface layer with about 25 wt.% nickel in the form of hard nickel aluminide (NiAl3) dendrites. The overall hardness of thelayerwasabout340 HV.The sliding wear and friction properties of the treated material was then measured by means of a pin on disc wear testing machine, usinga steel ball (hardened in excess of 750 HV) as the counterface. The results showed signicant increase in the wear life of LM13 after surfacealloying.Thealloyingalsochangedthemodeofwearfromapredominantlyadhesivemode,inthecaseofunalloyedLM13,toamainlyabrasivewear mode. Nickel alloyingalsoreducedthecoefcient of friction, whichcouldotherwisebehigher becauseof thestrongadhesionofaluminiumtosteel. # 2001ElsevierScienceB.V.Allrightsreserved.Keywords: Surfacealloying;Aluminium;Tribology1. IntroductionSurface alloying of aluminium alloys has attracted a greatdeal of attentioninrecent years. This is becausesurfacealloyingcanprovidealuminiummaterialswithfairlyhard,coherent andthicksurfacelayers, andhenceconsiderableimprovementinwearlife. Manyelementshavebeenusedand investigated for surface alloying of aluminium materialssuchassilicon[1], boron[2], andalsotransitionelementslikecopper[3], iron[4], andchromium[5].Nickel is also an interesting alloying element in that it canform a number of intermetallic compounds with aluminium.Thesecompounds areAl3Ni, Al3Ni2, AlNi andAlNi3. Ifthereareasufcientnumberofnickelaluminideinterme-tallicprecipitatessuitablydispersed,therecanbeasigni-cantimprovementinthehardness.Surface alloying of aluminiumwith nickel has beeninvestigatedbyanumber of researchworkersusingbothlaser and electron beam techniques [6]. Arnberg and Lange[7] employed an electron beamfor nickel alloying ofcommerciallypurealuminium. Thehardnesswasreportedto be over 200 HV. In other work, nickel alloying ofAl10%Simaterialalsoshowedahardnessof210 HVforthe alloyed layer, where the nickel concentration in thealloyedregionwasupto26 wt.%[3]. Mordike[8]inves-tigatedthenickel surfacealloyingofaluminiummaterialsusingalaser beam. Shereportedthepresence of Al3Ni,mainly,andalsoAl3Ni2intermetallicphasesinthealloyedlayers. Themaximumhardnesswasreportedtobenearly1000 HVin a layer around 100 mmthick with 25 at.%nickel.Tribological properties of surface alloyed aluminium-based material have also been investigated and considerableimprovementinwearresistancehasbeenreported[9].Inthiswork, surfacealloyingofLM13(aneareutecticaluminiumsilicon-based material) with nickel has beeninvestigated. Optical and scanning electron microscopy,X-ray diffraction analysis, hardness testing, and pin on discweartestingwereusedinthisinvestigation.2. ExperimentalSixty millimetre disc type wear test specimens weremachinedfromanLM13cast bar. Thismaterial isanagehardeningalloyanditshardnessaftersolidsolutiontreat-mentandoptimumageingreaches140 HV.An electron beam unit was used for nickel surface alloy-ing of wear test specimens. The alloying was carried out bypre-plating of the substrate via nickel electroplating, and thethickness of the plated layer was about 50 mm. Surfacealloyingwasthencarriedoutintwostages.Therststagewas in order to fuse the pre-plated layer on the surface of thesubstrate and the second stage was to obtain a fairly deep anduniformalloyedlayer.ThebeamparametersusedforeachJournalofMaterialsProcessingTechnology118(2001)187192E-mailaddress:mhsohi@ut.ac.ir(M.HeydarzadehSohi).0924-0136/01/$seefrontmatter # 2001ElsevierScienceB.V.Allrightsreserved.PII:S0 9 2 4 - 0 1 3 6 ( 0 1 ) 0 0 9 1 2 - 8casearegiveninTable1. Thealloyedsurfaceswerethenpolishedwith1200-gritemerypaperforweartesting.The surfaceroughnessof the materials was measured bymeans of a Talysurf roughness tester. Microstructural exam-inationofthesurfacealloyedmaterial wascarriedout byusing optical and scanning electron microscopy. The nickelcontent of the alloyedlayer was determinedbyelectronprobemicroanalysis.X-raydiffraction,usingCuKaradia-tion,wasusedtoidentifythephasesinthesurfacealloyedmaterial. The hardness of the alloyed layer was measured bymeans of a Leitz microhardness machine, using a 50 g load.The wear resistances of the treated and untreated materi-alswereevaluatedusingapinondisctestrig. Fivemilli-metresteel ball bearingwithahardnessofabout 750 HVwasusedasthereferencetoslideagainst weartest speci-mens. The sliding speed and distance were xed at 10 cm/sand 50 m, respectively. The volume wear rate for each casewascalculatedonthebasisoftheareaoftheweartrack,measuredfromTalysurf roughness proles. The surface,section and debris of the worn specimens were then studied,usingoptical andscanningelectronmicroscopes. Frictionforce was also recorded during wear testing and variations incoefcientoffrictionwerethencalculatedbydividingthefrictionforcebytheappliedload.3. ResultsanddiscussionFig. 1 shows the SEM micrograph of a cross-section of thesurfacealloyedmaterial. AnX-raylinefor nickel isalsoshown in the gure. This gure also shows the formation ofnickel rich dendrites in the alloyed region. The depth of thealloyed layer after polishing was0.4 mm. The X-raydiffractionpattern of the alloyedmaterial(Fig. 2) indicatesthat these dendrites are NiAl3. This is in agreement with theEPMA analysis of these dendrites, where the nickel contentin themwas shown to be around 41 wt.%. The EPMAanalysis of the interdendritic areas around the particles,ontheother hand, showedlittlenickel content (lessthan0.5 wt.%) in these areas. The total nickel content measuredTable1ElectronbeamparametersusedinnickelsurfacealloyingofLM13Stage Voltage(kV)Current(mA)Feedrate(mm/s)Transversewidth(mm)1 100 5 13 62 100 5 5 3Fig.1. SEMmicrographofanelectronbeamnickel-alloyedLM13.Fig.2. X-raydiffractionpatterngeneratedfromtheelectronbeamnickel-alloyedLM13.188 M.HeydarzadehSohi / JournalofMaterialsProcessingTechnology118(2001)187192inthealloyedlayerwas 25 wt.%, whichcorrespondstothelevel whichmaybeestimatedfromasimplemixingrule.The hardness prole for nickel alloying is shown in Fig. 3,indicatingahardnessofupto340 HV0.05(thehardnessofthenickelaluminidedendriteswasfoundtobeashighas800 HV0.01). Thishardnessproleshowsanabrupttransi-tion between the alloyed layer and the substrate, which canbecorrelatedwiththeverysharptransitioninnickel con-centration fromthe alloyed layer to the substrate. Thevariationinthe hardness values inthe alloyedregionisdue to a small degree of heterogeneity in the alloyedstructure in terms of the dispersion of the hard nickelaluminideparticles.Fig. 4 shows a surface alloyed wear specimen after beingpolished with emery paper to grade 1200. The relevantsurfaceroughnessvaluesofthisspecimenhavebeencom-pared with the Ra and Rmax of the polished LM13 specimenin Fig. 5. As it is shown, the polished nickel surface alloyedmaterial is smoother than the polished untreated LM13. Thiscan be explained by the fact that during polishing relativelysoftLM13materialmaybeeasilyscratchedbySiCemerypaper, while the presence of the hard nickel aluminide in thealloyedlayerpreventssuchscratching.Observationoftheweartracksonthefullyheat treatedLM13,Fig.6a,anditsrelevantsteelballcounterfaceaftersliding, Fig. 6b, conrms that wear of this material iscontrolled by extensive deformation of the matrix andadhesionofaluminiumtosteel ball. Theadhesionofalu-miniumtosteel is awell-establishedfact. This has beenrelatedtothehighmutualsolubilityofthesetwometalsaswell as to the ease of formation of a solid solution ofaluminiuminsteel[10].Examination of the microsection of the LM13 weartracks, a typical example of which is shown in Fig. 7a, alsoindicates the creation of a subsurface layer beneath the wearscar. The hardness of this layer was well above 300 HV0.05,morethantwicethehardnessofthesubstrate. Thismeansthat sliding of the steel ball on LM13 results in the formationof a work hardened layer. Some other research workers havealso noticed this phenomenon [11]. They have suggested thefollowing explanation on the formation of this layer. At thecommencement of the sliding, the force acting on therubbing surface is transmitted to the subsurface region.The ductile matrix undergoes plastic deformation and cracksFig.3. ThemicrohardnessprofileofthenickelsurfacealloyedLM13.Fig.4. Apolishedelectronbeamnickel-alloyedLM13weartestdisc.Fig.5. ComparisonofRaandRmaxvaluesforpolishedLM13andnickel-alloyedspecimens.M.HeydarzadehSohi / JournalofMaterialsProcessingTechnology118(2001)187192 189developinthehardbrittleinterdendriticsiliconandotherintermetallic compounds. Eventually, these particles arefragmented and form the work hardened layer. Fig. 7b showsthatthehardenedlayerisfracturedatitsinterface withthesubstrate and it seems that, at a later stage, all or part of thislayer is separated from the wear track in the form of debris.The nucleation sites for the wear debris are probably createdbelowtheslidingsurfaceasaresultofHertzianstresses.However, ageneral lookat theSEMmicrographofthedebris, Fig. 8, indicates that the above explanation may notbe the only mechanism for the formation of the wear debris.Thepresenceofsomethin, long, debrisindicatespossibledelamination of the debris from the surface. In this case, asa result of induced Hertzian stresses the ductile matrixundergoesplasticdeformationandvoidsdevelopnearthehard brittle particles, such as silicon and some intermetalliccompoundsthatarepresentinthesubstrate.Thevoidsarethenaccumulatedandformcracks, whichextendtothesurfaceandeventuallyawearfragmentisformed[12].As showninFig. 9, nickel surface alloyingof LM13improvedthewearresistanceofthismaterialsignicantly.ByexaminingtheSEMmicrographsofthenickel-alloyedwear tracks, Fig. 10, it appearsthat themodeof wear innickelalloyingisabrasive.However,SEMexaminationofthe worn steel ball used against this material also revealed aseriesofparallelgrooves, Fig.11, indicatingthatthesteelcounterface has also been worn by an abrasive mechanism.It is postulated that the wear mechanism at the running-instage might have been adhesive, resulting in the formation ofsome debris. The wear debris, which are hard, will then rubin between the alloyed material and steel counterface,resultinginathree-bodyabrasive wear.ThisprocessleadstothecrushingofthedebristosmallerpiecesasshowninFig.12a.Ontheotherhand,agglomerationofsmalldebrisandformationofbigparticlesatsomestageofthe wearisalso possible (Fig. 12b). The abrasion of the steel ball duringthecourseofwear meansthat it islikelythat small steelparticles are among the wear debris and also in agglomerateparticles.Thechangeinthewearmechanismasaresultofnickelalloying can also be noticed in the cross-section examinationof the wear tracks. The micrographof the cross-sectionshown in Fig. 13 indicates that in nickel-alloyed material theFig.6. (a)SEMpicturesofthe weartrackproducedonLM13and(b)itsrelevantsteelcounterfaceat10 Nappliedload.Fig. 7. Microsection of worn LM13 showing: (a) the formation of ahardenedlayernearthesurface;(b)separationofthehardenedlayer.Fig.8. WeardebrisproducedfromLM13substrateunderaloadof10 N.190 M.HeydarzadehSohi / JournalofMaterialsProcessingTechnology118(2001)187192Fig.9. ThevolumewearrateofLM13beforeandafterelectronbeamsurfacealloyingatdifferentloads.Fig. 10. SEM picture of the typical wear track produced on nickel-alloyedLM13.Fig. 11. SEMpictureofawornsteel ball usedasacounterfaceagainstnickel-alloyedLM13.Fig.12. (a,b)Weardebrisproducedfromnickel-alloyedLM13underaloadof10 N.M.HeydarzadehSohi / JournalofMaterialsProcessingTechnology118(2001)187192 191work hardened layer, which is formed during wear of LM13causing catastrophic wear, does not form any more. This isrelatedtotheabsenceof thesoft aluminiummatrix, andhence to the elimination of extensive plastic deformation ofthewearingsurface.Fig. 14showingtypical variationof thecoefcient offriction for aged LM13 and nickel surface alloyed material,indicates that surface alloyingof LM13results inlowerfriction value when it runs against steel counterface. On thewhole, aluminiumonsteel pairsresultsintheratherhighvalues of friction [13]. One explanation for this behaviour isthat seizure failure of aluminiumtosteel readilyoccursbecause, as indicatedbefore, thetwometals haveahighmutual solubility. This is also related to the ease of formationof solid solutions of aluminium in steel. Strong evidence forseizurefailureofaluminiumtosteel isshowninFig. 6b,which shows a steel ball which was used as the counterfaceforslidingagainstLM13material.It has been reported that surface alloying resulted inlimitedimprovement inseizure resistance of aluminium.This has beenrelatedtotheprecipitationof hardphaseslikenickel aluminideinthealuminiummatrixbysurfacealloying[14]. Improvementinseizureresistanceresultsinlowerfrictioninnickel-alloyedmaterialascomparedwiththeLM13itself.4. Conclusions1. Nickel surface alloying resulted in the formation of hardand wear-resistant surface layer on aluminium-basedmaterial. This was due to the formation of a largenumber of fine nickel aluminide (NiAl3) dendrites,whichwereuniformlydistributedinthealloyedzone.2. Nickel surface alloying changed the modeof wear froma predominantly adhesive mode, in the case of unalloyedLM13, toamainlyabrasivewearmode.3. Nickel surface alloying resulted in lower frictionbetweenthesubstrateandthesteelcounterface.References[1] A.M. Walker, W.M. Steen, D.R.F. West, in: Proceedings of theConferenceonAluminiumTechnology,London,1986,p.712.[2] F.Matsuda,K.Nakata,Trans.Weld.Res.Inst.17(2)(1988)457.[3] D. Bernet, B. Vient, in: Proceedings of the Second InternationalConferenceonSurfaceEngineering, Stratford-upon-Avon, June1619,1987,Paper27.[4] M. Pierantoni, J.D. Wagniere, E. Blank, Mater. Sci. Eng. A110(1989)L17.[5] A.Almedia,Surf.Coat.Technol.70(1995)221.[6] M.H. Sohi, in: Proceedings of the Fifth World Seminar on HeatTreatment and Surface Engineering, Isfahan, Iran, September 2629,1995,p.497.[7] L.Arnberg,J.Lange,Aluminium62(6)(1986)423.[8] S. Mordike, in:B. Mordike, A.B. Vannes(Eds.), Laser, Vol. 6, ITTInternational,France,1990,p.99.[9] S.Jobes,J.M.Pelletier,A.B.Vannes,in:ProceedingsoftheSecondInternational Seminar on Surface Engineering with High EnergyBeams Science and Technology, Lisbon, Portugal, September2527,1989,p.317.[10] P.S.Venkatesan,N.Ahmed,I.B. Goldman,Wear17(1981)245.[11] B.N. PramilaBai, E.S. Dwarakadasa, S.K. Biswas, Wear17(1981)381.[12] J.Clark,A.D.Sarkar,Wear69(1981)1.[13] C.L. Goodzeit, R.P.Hunnicutt, A.E. Roach, Trans. ASME 78 (1956)1669.[14] P.K.Rohatgi,B.C.Pai,Wear59(1980)323.Fig. 13. Microsectionofthewornsurfacealloyedmaterialafterwearingunderaloadof20 N.Fig. 14. Variation of the coefficient of friction with distance for LM13 andsurfacealloyedLM13atanappliedloadof20 N.192 M.HeydarzadehSohi / JournalofMaterialsProcessingTechnology118(2001)187192