化学镀镍磷合金英文文献

化学镀镍磷合金的英文文献

MaterialsandDesign31(2010)

3174–3179

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AninvestigationoneffectsofheattreatmentoncorrosionpropertiesofNi–Pelectrolessnano-coatings

TaherRabizadeh,SaeedRezaAllahkaram*,ArmanZarebidaki

SchoolofMetallurgyandMaterialsEngineering,UniversityCollegeofEngineering,UniversityofTehran,P.O.Box11155-4563,Tehran,Iran

articleinfoabstract

ElectrolessNi–Pcoatingsarerecognizedfortheirexcellentproperties.Inthepresentinvestigationelec-trolessNi–Pnano-crystallinecoatingswereprepared.X-raydiffractiontechnique(XRD),scanningelec-tronmicroscopy(SEM),potentiodynamicpolarizationandelectrochemicalimpedancespectroscopy(EIS)wereutilizedtostudypriorandpost-depositionvacuumheattreatmenteffectsoncorrosionresis-tancetogetherwiththephysicalpropertiesoftheappliedcoatings.

X-raydiffraction(XRD)resultsindicatedthattheAs-platedhadnano-crystallinestructure.Heattreat-mentofthecoatingsproducedamixtureofpolycrystallinephases.Thehighestmicro-hardnesswasachievedforthesamplesannealedat600°Cfor15minduetotheformationofaninter-diffusionallayeratthesubstrate/coatinginterface.

Lowercorrosioncurrentdensityvalueswereobtainedforthecoatingsheattreatedat400°Cfor1h.EISresultsshowedthatproperheattreatmentsalsoenhancedthecorrosionresistance,whichwasattributedtothecoatings’structureimprovement.

Ó2010ElsevierLtd.Allrightsreserved.

Articlehistory:

Received19January2010Accepted15February2010

Availableonline17February2010Keywords:C.Coating

C.HeattreatmentE.Corrosion

1.Introduction

Sincetheinventionofelectrolessplatingtechnologyin1946byA.BrennerandG.Riddell,electrolessnickel(EN)coatingshavebeenactivelyandwidelystudied[1,2].

Nano-crystallineNi–Palloysshowahighdegreeofhardness,wearresistance,lowfrictioncoef cient,non-magneticbehaviorandhighelectro-catalyticactivity.TodaysuchNi–Palloysarewidelyusedintheelectronicindustryasunder-layerinthin lmmemorydisksandinabroadrangeofotherevolvingtechnologicalapplications.Itisgenerallyacceptedthatonlynano-crystallineal-loys–irrespectiveofthewayofproduction–showhighcorrosionresistance.Indeed,electrodepositedNi–Palloyswithcrystallinestructure(6–11at.%P)showedanodicdissolutionin0.1MNaCl.Onnano-crystallinesamples(17–28at.%P)acurrentarrestwasfoundinstead[3–5].

ToexplainhighcorrosionresistanceofNi–Pelectrolesscoatingsdifferentmodelshavebeenproposed,buttheissueisstillunderdiscussion:aprotectivenickelphosphate lm,thebarrieractionofhypophosphites(called‘‘chemicalpassivity”),thepresenceofphosphides,astableP-richamorphousphaseorthephosphorusenrichmentoftheinterfacealloy-solutionwereproposed.Notethatsuchphosphorusenrichmentattheinterfacewasreported

*Correspondingauthor.Tel./fax:+982161114108.E-mailaddress:akaram@ut.ac.ir(S.R.Allahkaram).

0261-3069/$-seefrontmatterÓ2010ElsevierLtd.Allrightsreserved.bysomeoftheauthorstoexplaintheoutstandingcorrosionresis-tanceofFe70Cr10P13C7amorphousalloys[5].

ElectrolessNi–Palloysarethermodynamicallyunstableandeventuallyformstablestructuresofface-centeredcubic(fcc)Nicrystalandbody-centeredtetragonal(bct)nickelphosphide(Ni3P)compounds.DifferentresultshavebeenreportedregardingthemicrostructuresintheAs-depositedconditionandthestablephasesafterheattreatments.ForlowPandmediumPalloys,nickelcrystalprecipitated rstlyandNi3Pfollowed;however,Ni3Pand(or)NixPycompoundssuchasNi2P,Ni5P2,Ni12P5,andNi7P3occur rstlyinhighPalloys[6–8].

Ingeneral,thehardnessoftheelectrolessNi–Pcoatingscanbeimprovedbyappropriateheattreatment,whichcanbeattributedto neNicrystallitesandhardinter-metallicNi3Pparticlesprecip-itatedduringcrystallizationoftheamorphousphase[8–10].

Themainreasonsforheattreatmentare:(1)toeliminateanyhydrogenembrittlementinthebasicmetal,(2)toincreasedeposithardnessorabrasionresistance,(3)toincreasedepositadhesioninthecaseofcertainsubstrateand(4)toincreasetemporarycorro-sionresistanceortarnishresistance[11].

Thecrystallizationandphasetransformationbehaviorofelec-troless-platedNi–Pdepositsduringthermalprocessinghasalsobeenthesubjectofvariousinvestigations;ithasbeenshownthatdifferentalloycompositionsandheattreatmentconditionscouldaffectboththecorrosionresistanceandcrystallizationbehaviorofthedeposit[8].

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Theaimofthisworkistostudythepost-depositionheattreat-menteffectsandcorrosionbehaviorofelectrolessdepositednano-crystallineNi–Palloys.Thetemperaturedependenceofthecoatingstructuresandcompositionswerealsoevaluatedanddiscussed.2.Experimentalprocedures

2.1.DepositionofelectrolessNi–Pcoating

ThedepositionwasperformedonAPI-5LX65steelsubstrates(30Â25Â15mm)withacompositionof(Fe:base,Mn:1.42,Si:0.199,Cu:0.144,Mo:0.132,C:0.061,Nb:0.0538,Al:0.0417,Sn:0.0167,Ti:0.0142,Cr:0.0126,P:0.01).ThesubstratesurfacewascarefullypolishedwithSiCemerypapers(fromgrades#100to#400).Allthespecimensweresubjectedtothefollowingpre-treat-mentandplatingprocedure:

1.Ultrasoniccleaninginacetone.

2.Rinsingbyimmersionindistilledwateratroomtempera-ture(RT)for2min.

3.Cleaningin20vol.%H2SO4atRTfor30s.

4.RinsingbyimmersionindistilledwateratRTfor2min.5.Cleaningin5vol.%H2SO4atRTfor30s.

6.RinsingbyimmersionindistilledwateratRTfor2min.7.Electrocleaninginsolutioncontaining75g/lsodium

hydroxide(NaOH),25g/lsodiumsulfate(Na2SO4),75g/lsodiumcarbonate(Na2CO3),atroomtemperaturefor20min.Thecurrentdensityappliedwas10mA/cm2inaccordancetoASTMG1.

8.RinsingbyimmersionindistilledwateratRTfor30s.Fordeposition,thesubstratesweredippedintocommercialelectrolessnickelbath(SLOTONIP70AfromSchlotter)withsodiumhypophosphiteasreducingagentfor2h.ThisbathprovidedNi–Pdepositswithamediumphosphorouscontent,9–10%P.Tempera-turechangedwithin88–93°CandpHchangedwithin4.5–4.7range,duringcoatingprocess.

2.2.HeattreatmentandhardnessmeasurementofNi–PcoatingsInordertostudythe lmproperties,coatedsampleswerether-mallytreatedinavacuumenvironment.Thecoatingswereisother-mallyheattreatedatdifferentconditionsthatgavemaximumhardness,i.e.200°C(for2h),400°C(for1h),600°C(for15min).Duringheatingprocess,thetotalpressureinthechamberwasmaintainedbelow1mbar.Thenthesampleswereallowedtocooldown,foratleast15mininthehighvacuumenvironmentpriortotheirexposuretotheatmosphere.

Thehardnessofcoatingswasmeasuredusingan(AMSLERD-6700)Vickersdiamondindenterataloadof100gforaloadingtimeof20s.Theaverageof verepeatedmeasurementsisreported.

2.3.MorphologyandmicrostructureofNi–Pcoatings

Themorphologyandmicrostructureofthecoatingswerestud-iedusingscanningelectronmicroscopySEM,(CAMSCANMV2300).X-raydiffraction(XRD)patternswereobtainedusingPhilip’sXpertprotypeX-raydiffractometerwithacobalttargetandanincidentbeammono-chromator(k=1.7889Å).2.4.Electrochemicalmeasurements

CorrosionbehaviorofAs-platedandheattreatedelectrolessNi–andelectrochemicalimpedancespectroscopy(EIS)in3.5wt.%NaClsolution.Thetestswerecarriedoutinastandardthree-electrodecellusinganEG&Gpotentiostat/galvanostat,model273A.Plati-numplateandAg/AgClelectrodewereusedascounterandrefer-enceelectrodes,respectively.Potentiodynamicpolarizationtestwascarriedoutbysweepingthepotentialatascanrateof1mVsÀ1withintherangeof±400mVvs.opencircuitpotential(OCP).TheEIStestswereundertakenusingaSolartronModelSI1255HFFrequencyResponseAnalyzer(FRA)coupledtoaPrincetonAppliedResearch(PAR)Model273Apotentiostat/galva-nostat.TheEISmeasurementswereobtainedat(OCP)inafrequencyrangeof0.01Hz–100kHzwithanappliedACsignalof5mV(rms)usingEISsoftwaremodel398.Theequivalentcir-cuitsimulationprogram(ZView2)wasusedfordataanalysis,syn-thesisoftheequivalentcircuitand ttingoftheexperimentaldata.3.Resultsanddiscussion3.1.Micrographandstructure

Fig.1showstheSEMimagesofthesurfacemorphologiesofNi–Pcoatingsbeforeandaftervacuumheattreatmentatdifferentcon-ditions.Asitcanbeseenheattreatmentatdifferenttemperatureshasnothadanysigni canteffectonmorphologyofthecoatings.Fig.2showsthecrosssectionimagewithlinescananalysisofNi–Pcoatingheattreatedat600°C(for15min)whichshowstheformationofaninter-diffusionallayeranditselementaldistribu-tionthataffectsthecoatingproperties.

Atomsunderlowtemperatureheattreatment(below400°C)canhaveshort-rangemovementwhichiscalledstructuralrelaxa-tionsuchasannihilationofpointdefectsanddislocationswithingrainsandgrainboundaryzonesratherthanlongrangediffusion[4].Thehigherthetemperatureis,thegreatertheatomicvibrationenergyis.Asthetemperatureofthemetalincreases,morevacan-ciesarepresentandmorethermalenergyisavailable,andsothediffusionrateishigherathighertemperatures[11].

Hence,productionofaninter-diffusionallayer,formedasare-sultofinter-diffusionofnickelandphosphorousfromthecoatingtothesubstrateandironinthereversedirectionfromthesub-stratewilldevelopuponheatingat600°C.3.2.XRDanalysesofNi–Pcoatings

Fig.3showsXRDpatternsofAs-depositedandheattreatedNi–Pcoatings.TheresultsshowthatboththephasecompositionandphasetransformationbehavioroftheelectrolessNi–Pdepositsde-pendedontheheatingtemperatures.

Therewasnosigni cantchangeinXRDpatternsobservedforthesamplestreatedat200°Candonlyasinglebroadamorphouspro lewasfound,indicatingthatnophasetransitiontookplaceatthistem-perature.Whentheheattreatmenttemperaturewasincreasedto400°C,newXRDpeakscorrespondingtocrystallinefccNiandNixPyappeared,indicatingthatthesecondphaseprecipitationwasiniti-ated.Atthistemperature,thediffractionpeakscorrespondingtothemetastableNi8P3,fccnickelandstableNi3PphasesintheXRDpro lecanbeseen.At600°CthefccNiandNi3Ppeaksintensitiesin-creasedwiththeheattreatmenttemperature.Therefore,theNi8P3metastablephasewasdecomposedcompletelyatthistemperature.SimilarbehaviorhasalsobeenobservedbyHuangsetal.[12].

Regardingtheintensitiesoffccnickeldiffractionpeaks,theyareincreasedandthefullwidthathalfmaximum(FWHM)becamenarrowerwithincreasingtheheattreatmenttemperatureto600°C.InthecaseofAs-platedcondition,usingFWHMof6.4676°andScherrerequation,thegrainsizewasestimated

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Fig.1.SEMmorphologyimagesofAs-platedandheattreatedENcoating:(a)As-plated,(b)HT-200°C(2h),(c)HT-400°C(1h)and(d)HT-600°C(15min)inavacuumenvironment.

WithregardstotheFWHMof5.496°at200°C,0.9774at400°Cand0.4027°at600°Cforfccnickelpeaks,thegrainsizeswereesti-matedas1.7,10.9and26.6nm,respectively.Thereforeitcanbededucedthatthegrainsizeofthecrystallinenickelincreasessub-stantiallywithincreasingtemperature,i.e.reachingtothemaxi-mumsizeof26.6nmat600°C.Thiseffecthasalsobeencon rmedbyotherinvestigatorsexposingNi–Pcoatingstovariousheattreatmentconditions[13,14].3.3.HardnessofelectrolessNi–Pdeposits

Hardnesswasmeasuredforthedepositsatvariousannealingtemperature.Thevariationsinmicro-hardnesswithannealingtemperaturesareshowninFig.4.Heattreatmentintheregionof200°Cdoesnotbringaboutanysigni cantchangesinthedepositproperties.Itcanbeobservedthatthereisamarginaldecreaseofand200°C.Thissmalldecreaseinthehardnessmaybeduetohydrogenembrittlementandinternalstressrelieving[11].

Signi cantincreaseofhardnessfrom625.3Hvto875.7Hvisobservedafterhaettreatmentat400°C.Thedispersionhardeningeffectmayalsobeareasonforincreaseinhardnessduetoheattreatmentabove200°C.Between200°Cand400°C,theprecipita-tionofNixPycompoundsoccurs.Theprecipitationofnickelphos-phides(Ni3P)canbeseenintheheattreatedcondition(Fig.3).Themechanisminvolvedinthesteepincreaseinhardnessmaybeduetotheprecipitationhardeningofatypicalsupersaturatedsolidsolution,forwhichtheatomsofthesolutediffusetoaspeci ccrystallographyplanewithanatomicarrangementthatresemblesthearrayofatomsonaplaneinthestructureoftheprecipitate.Astheatomsattempttoprecipitate,theyareforcedtoconformtothestructureofthesolvent.Thisforcedcoherencybetweenatomsofthesolventandatomsattemptingtoformtheprecipitatecauses

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Fig.2.Linescananalysisandelementaldistributionaroundinter-diffusionallayerofNi–Pcoatingheattreatedat600°C(for15

min).

Fig.4.MicrohardnessofAs-platedandheattreatedNi–Pelectrolesscoatings.

Fig.3.XRDspectraofelectrolessNi–Pcoatingsmeasuredbeforeandafterheattreatedatdifferent

conditions.

blefortheincreaseinhardness.Whensuf cientnumbersofatomslocalizedstressesarerelieved,sincetheforcedcoherencybetweenthematrixandtheprecipitatedatomsreachesamaximumlevel[11].

Intermediateprocessesduringtheformationofinter-metalliccompoundscanresultinprecipitationoragehardening.Particles

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locationtoformavolumeofmaterialthathasthecorrectstoichi-ometriccompositionandthatislargeenoughsothataboundarycanformaroundit.Beforethevolumeofmaterialreachesthiscrit-icalsize,itsatomicarrangementandthatofthesurroundingma-trixmustremaincontinuous.Thiscoherencybetweenregionshavingdifferentinter-atomicspacingresultsinseverestraining,whichhardensthealloy[11].

Around600°C,thehardnessreachestoamaximumvalueof938Hv.AsitcanbeseeninXRDpatternsofthecoatings,theinten-sitypeaksoftheprecipitationphaseshaveincreasedwithraisingheattreatmenttemperatureabove400°C.Thisimpliesthatmoreprecipitationphaseshavebeenformed.Inaddition,theformationofaninter-diffusionlayercanenhancethehardnessofcoating.Thislayerdevelopsuponheatingabove600°Candthickenswithincreasingannealingtime.3.4.Electrochemicalresults

3.4.1.Potentiodynamicpolarizationstudies

Fig.5showsthepotentiodynamicpolarizationcurvesobtainedforAs-platedandheattreatedelectrolessNi–Pcoatingin3.5wt.%NaClsolution.Table1liststheopencircuitandcorrosionpotentialEcorrtogetherwiththecorrosioncurrentdensityicorrofthesecoatings.

BycombiningFig.5andTable1,thecorrosionpotentialofNi–PdepositsispositivelyshiftedfromÀ0.376toÀ0.25Vwithincreas-ingtheannealingtemperatureto600°C.Moreover,thecorrosioncurrentdensity(icorr)ofheattreatedsampleat400°Cis1Â10À5(A/cm2)whichhasthelowestcorrosioncurrentdensityamongalltheheattreatedspecimens.

ItisevidentfromliteraturereportsonNi–Pcoatingsthatpref-erentialdissolutionofnickeloccursatopencircuitpotential,lead-ingtotheenrichmentofphosphorusonthesurfacelayer[15–20].Theenrichedphosphorussurfacereactswithwatertoformalayerofadsorbedhypophosphiteanions(H2POÀ2).Thislayerinturnwillblockthesupplyofwatertotheelectrodesurface,therebypre-ventingthehydrationofnickel[15],whichisconsideredtobethe rststeptoformeithersolubleNi2+speciesorapassivenickel lm[19,20].

3.4.2.Electrochemicalimpedancespectroscopystudies

Fig.6showstheNyquistplotsobtainedforAs-platedandheattreatedcoatingsin3.5%sodiumchloridesolutionattheirrespec-tiveopencircuitpotentials.Allthecurvesappeartobesimilar(Ny-quistplots),consistingofasinglesemi-circleinthehighfrequencyregionssignifyingthechargecontrolledreaction.However,itshouldbenotedthatthoughthesecurvesappeartobesimilarwith

Fig.5.PotentiodynamicpolarizationcurvesofAs-platedandheattreatedelectro-Table1

CorrosioncharacteristicsofAs-platedandheattreatedelectrolessNi–Pcoatingsin3.5%sodiumchloridesolutionbypotentiodynamicpolarizationtechnique.TypeofcoatingEcorr(Vvs.Ag/AgCl)icorr(A/cm2)As-plated

À0.37610Â10À5HT-200°C,2hÀ0.3556.3Â10À5HT-400°C,1hÀ0.321Â10À5HT-600°C,15min

À0.25

2.5Â10À5

respecttotheirshape,theydifferconsiderablyintheirsize.Thisindicatesthatthesamefundamentalprocessesmustbeoccurringonallthesecoatingsbutoveradifferenteffectiveareaineachcase[19,20].

ToaccountforcorrosionbehaviorofAs-platedandheattreatedelectrolessNi–Pcoatingsin3.5%sodiumchloridesolutionattheirrespectiveopencircuitpotentials,anequivalentelectricalcircuitmodelgiveninFig.7hasbeenutilizedtosimulatethemetal/solu-tioninterfaceandtoanalyzetheNyquistplot[19,20].

ThechargetransferresistanceRctanddoublelayercapacitanceCdlobtainedforAs-platedandheattreatedelectrolessNi–Pcoat-ingsarecompiledinTable2.Theopencircuitpotential(OCP),areliableparameterthatindicatesthetendencyofthesesystemstocorrode,isalsoincludedinthesametable,alongwithRctandCdlvaluesforeffectivecomparison.Theoccurrenceofasinglesemi-circleintheNyquistplotsindicatesthatthecorrosionpro-cessofthesecoatingsinvolvesasingletimeconstant[19].

AsimilarconclusionoftheexistenceofasingletimeconstanthasbeenreportedbyZeller[21],VanDerKouwe[22]andLoetal.[23]forthecorrosionofelectrolessNi–Pcoatingsinsodiumchlorideandsodiumhydroxidesolutionsattherespectiveopencircuitpotentials[19].

Thehighvaluesofchargetransferresistance(Rct),intherange18,172–52,903Xcm2,obtainedforthecoatingsofpresent

study

Fig.7.EquivalentelectricalcircuitmodelusedtoanalyzetheEISdataoftheAs-

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Table2

ElectrochemicalparametersfromEISdataofAs-platedandheattreatedNi–Pelectrolesscoatingsin3.5%sodiumchloridesolution.TypeofcoatingOCP(mVvs.Ag/AgCl)Rct(Xcm2)Cdl(lF/cm2)As-plated

À34313,93438.334HT-200°C,2hÀ32418,17232.952HT-400°C,1hÀ24152,90328.154HT-600°C,15min

À256

27,352

23.823

implyabettercorrosionprotectiveabilityofheattreatedcoatingsthantheAs-platedelectrolessNi–Pcoating.

ThecapacitancevalueobtainedforAs-platedandheattreatedelectrolessNi–Pcoatingsareintheorderof23–38lF/cm2.TheCdlvalueisrelatedtotheporosityofthecoating.ThelowCdlvaluescon rmthattheheattreatedelectrolessNi–Pcoatingsofpresentstudyarerelativelylessporousinnature.

BycomparingelectrochemicalparametersandfromEISdataofAs-platedandheattreatedNi–Pelectrolesscoatingsin3.5%so-diumchloridesolutionandfromXRDpatterns,itcanbeseethatbecauseAs-platedcoatingisnano-crystallineinnature,ithasmoregrainboundaries,henceitislessprotective.Byincreasingheattreatmenttemperaturesfrom200°Cto600°CcoatingsbecomedenserandlessporousthatdecreasingCdlvalueprovesthis.At200°C,thereisgraingrowththatreducesgrainboundariesanditscorrosionpropertiesarebetterthanAs-platedsample.At600°Cand400°Citisexpectedthatthealloysconsistoftwocon-stituents:crystalsofthenickel-richphaseandtheNixPyphases.Therefore,itcanbeobservedthatalloysarenotcontinuous,be-causetheyhavesurfaceinhomogeneities(grainboundariesandsecondphase),whichareactivesitesforcorrosionattack.Thus,themixturesoftwophaseswithtwodifferentcompositions,prob-ablyproducesactive–passivecorrosioncellswithinthealloy,caus-ingittosufferseverchemicalattack.

At400°CformationofsecondphasesofNixPyhavetodecreasethecorrosionresistanceofcoatingbutgraingrowthatthistemper-aturetriumphtheformationofsecondphasesandthecorrosionresistanceincreases.Afterheattreatmentat600°Cbecauseofincreasingsecondphases’quantity,thecorrosionresistancede-creasescompareto400°C.4.Conclusions

Nano-crystallineNi–PelectrolesscoatingswasdepositedonX65steelsamples.Theheattreatmenteffectsoncoatingpropertiesweresystematicallystudied.Theresultsshowthat:

(1)As-depositedNi–Pcoatinghasahomogeneousnano-crystal-linestructure.

(2)Byapplyingheattreatment,secondphaseprecipitation

occurredaround400°C.

(3)Thehighesthardnesswasobtainedafterheattreatment

temperatureat600°Cfor15minbecauseofformationofaninter-diffusionallayerandNi3Pphaseandsealingporosities.

(4)Properheattreatmentcansigni cantlyimprovethecorro-sionresistanceofthesecoatings.References

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