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:[email protected](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]
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for LM13 andsurfacealloyedLM13atanappliedloadof20 N.192
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JournalofMaterialsProcessingTechnology118(2001)187192
