Application of the interfacial indentation test for adhesion toughness determination

www.elsevier.com/locate/surfcoat

Applicationoftheinterfacialindentationtestfor

adhesiontoughnessdetermination

D.Chicota,P.Araujob,N.Hornyc,A.Tricoteauxc,T,J.LesageabLML,UMR8107,USTL,IUTAGMP,BP179,59653Villeneuved’AscqCedex,France

UniversidadeTiradentes,UNIT,ITP,Av.MuriloDantas,300Farolanda,CEP49.032-490,Aracaju,Bre´sil

cLMP/LAMAC,UVHC,IUTMP,ZIChampdel’Abbesse,59600Maubeuge,France

Availableonline5April2005

aAbstract

Interfacialindentationisusedtocreateandpropagateacrackintheinterfacebetweenacoatinganditssubstrate.Thismethodologyallowsdefininganapparentinterfacetoughness,whichcanrepresenttheadhesionofthecoatingonitssubstrate.Duringtheirlives,thermalspraycoatingsaresubjectedtovariousexternalsolicitations,whichcouldbedetrimentaltotheiradhesion:thermalshocks,fatigue,wear,corrosion...

Theobjectiveofthepresentworkistostudysomeoftheseeffectsandtheirinfluenceontheinterfaceadhesiontoughnessofvarioussubstrateandcoatingcouples.Annealingtreatment,hydrogencontaminationandthermaltreatmentsareespeciallystudiedinrelationtothemodificationoftheresidualstressesstate.D2005ElsevierB.V.Allrightsreserved.

Keywords:Coatings;Adhesion;Interfacetoughness;Residualstress

1.Introduction

Duringthepastdecadeitwasproposedtousetheinterfacialindentationtestasanalternativetootherteststocharacterizetheadhesivepropertiesofthermalspraycoat-ings.ThistestconsistsofmeasuringthelengthofcracksgeneratedattheinterfacebetweenthecoatinganditssubstrateasaconsequenceofVickersindentations.Inordertointerprettheresults,variousmethodswereproposedsearchingtoexpresseitheraninterfacialtoughnessoracrackingenergy[1].Sincetheirmodelsdidnotapplysuitablyneithertoourexperimentalresultsnortoothersavailableinliterature,wehaveproposedarepresentationthatallowsdefiningacriticalload(thelowestone)necessarytoinitiateacrack[2].Usingtheresultsobtainedonvarioussubstrate–coatingcouplesandhypothesesofthelinearfracturemechanics,wewereableafterwardsto

TCorrespondingauthor.

E-mailaddress:Arnaud.Tricoteaux@univ-valenciennes.fr(A.Tricoteaux).

0257-8972/$-seefrontmatterD2005ElsevierB.V.Allrightsreserved.doi:10.1016/j.surfcoat.2005.02.045

proposeanewrelationtoexpressanapparentinterfacetoughness,takingintoaccountthepropertiesofboththesubstrateandthecoating[3,4].Interfaceadhesiontoughnessislinkedtotheresidualstressesstateandtothemetal-lurgicalbondbetweenthecoatinganditssubstrate.Influenceofresidualstressesishighlightedwiththroughtheeffectofanannealingtreatmentonassprayedspecimenandtotheeffectofhydrogencontamination.Metallurgicalbondcharacteristicsarepointedoutafterthermaltreatmentssuchasshocksorthermalcycling.Inthispaperwepresentthestudyoftheinfluenceofresidualstresses,ofhydrogencontaminationandofthermaltreatmentsontheadhesiontoughnessofvariouscoatingsandsubstratescouples.Theinterfacialindentationtesthasbeenpreviouslydetailed[3,4]andtheapparentinterfacefracturetoughness(Kca)isdefinedas:

󰀁󰀂1=2

PCE

Kca¼0:0153=2dð1Þ

HiaCwherePCandaCarethecoordinatesofthecriticalpoint

underwhichnocrackappearsattheinterfaceand(E/H)1/2iD.Chicotetal./Surface&CoatingsTechnology200(2005)174–177175

isthesquarerootoftheelasticmodulus(E)dividedbytheVickershardness(H)attheinterface.

2.Influenceofresidualstresses

Sampleswereobtainedfromspecimenoflowcarbonsteel,graycastiron,globulargraphitecastiron,austeniticsteelandalowalloyedsteelcoatedwithachromiumcarbide(75%),nickel-chromium(25%)materialusingtheHVOFsprayingprocess.Thicknessofthecoatingvarybetween200to600Am.Fiveindentationswereperformedateachlevelofloadinordertodetermineareliablemeancracklength.Itisclearthatthestandarderroronthemeancracklengthmaybequiteimportant.Anyway,enoughdatawereobtained(severalloads)todeterminethevalueofPCwithsufficientaccuracy.Asmentionedpreviously,theuseoflogarithmiccoordinatesallowsfortheobtainingofastraightlineforthecracklength–loadrelation.Applyingthelinearregressionmethodtotheexperimentalcrackpoints(a,P)aswellastotheapparenthardnesspoints(d/2,P)itwaspossibletodeterminethecoordinatesofthecriticalpoint(PC,aC)andtocalculatetheapparentinterfacetoughnessKcausingEq.(1)(Fig.1).

OnFig.1,itisseenthatthecriticalloadstronglydependsonthecoatingthickness.Sincethecriticalloadcorrespondstothelimitforwhichnocrackisformedduringindentation,itisclearthatdifferencesinPCvaluescannotcomefromthebendingofthecoating.Ifitisassumedthatnosignificantmodificationofthemetallurgicalbondbetweenthesubstrateandthecoatingisinducedbyachangeinthickness,thenwehavetoconsiderthatonlytheresidualstressesareresponsibleforthevariationofPCwithcoatingthickness.Itisverydifficulttomeasuretheresidualstressesinthebulkofathermalspraycoatingandinparticularinthevicinityoftheinterface.Anyway,ifaparameterrelatedtothestressescouldbefound(p(r)),itwillbepossibletoevaluatetheireffectbyusingafactor1/t2,wheretisthecoating

1086thickness.Inthiscase,weobtainedalinearvariationwiththecalculatedKca(Fig.1).Thisresultisveryinterestingbecauseitcanbeextrapolatedtoaninfinitethickness.Thisextrapolatedvalue,KcaO,canbeconsideredasindependentonthecoatingthicknessandthus,themathematicalrelationassociatedtothisrepresentationhasthefollowingform:Kca¼KcaOþ

pðrÞt2

ð2Þ

WhatKcaOexactlymeansandthesecondtermoftherelationarethequestionswewilldiscussinthefollowingsentences.Thestressesinthecoatingcomefromdifferentsources,quenchingoftheimpingingsplat,restrictedcontraction,thermalmismatchbetweenthecoatingandthesubstrateforthemainones.Byanappropriateannealingtreatment,itispossibletoreducedrasticallytheresidualstresses.WewillconsidernowthehypothesisthatKcaOcorrespondstoamechanicalstateindependentofthecoatingthickness.Ifthisassumptioniscorrect,theinterfacialindentationtestsonannealedspecimenshouldgiveauniquevalueforKca(i.e.KcaO).Here,theannealingtreatment(2h,6008C)isappliedtothesamechromiumcarbidesamples.Itisremarkablethat,foragivensubstrate,allthecrackinglinesintersecttheapparenthardnesslineatasamepointindependentlyofthecoatingthickness.Thatmeanseachcoating–substratecoupleadmitsauniquevalueforKca,whichmoreoveragreesverywelltotheextrapo-latedvaluesobtainedfortheassprayedsamples[4].

3.Influenceofhydrogencontamination

Hydrogenembrittlementisknownformanyyears.Differenttheoriesexplainhowhydrogenplaystheroleofanacceleratorforfracture.Ifitisintroducedintothematerialduringitsservice,presenceofabarriertointroductionofhydrogenintothematerialmaydelayorevenimpedetheembrittlementprocess.Thepresentstudyis

(a)Kca0 = 4.04 MPa.m1/2Kca0 = 9.58 MPa.m1/2(b)Kca0 = 4.28 MPa.m1/2(c)Kca (MPa.m1/2) 420864200(d)Kca0 = 8.23 MPa.m1/2(e)Kca0 = 4.25 MPa.m1/2Kca0 = 5.80 MPa.m1/2(f)51015200510152005101520251/t2(mm-2)

Fig.1.Influenceofcoatingthickness(t)ontheapparentinterfacialtoughness(Kca)on:(a)lowcarbonsteel,(b)graycastiron,(c)globularcastiron,(d)austeniticsteel,(e)lowalloyedsteeland(f)Stellite(SeeRef.[1]).

176D.Chicotetal./Surface&CoatingsTechnology200(2005)174–177

ln a (a in µm)ln a (a in µm)8 7 6 5 PC0.53 = 48.2 N 8t1 = 0.24 mmt2 = 0.45 mmt3 = 0.56 mmt1 = 0.24 mmt2 = 0.43 mmt3 = 0.53 mm76PC0.56 = 249.1 NPC0.45 = 106.7 NPC0.24 = 44.4 N Apparent hardness Apparenthardness4 PC0.43 = 23.6 N PC0.24 = 8.2 N 53 2 (a) As sprayed4 5 6743(b) Annealed56 73 4ln P (P in N)ln P (P in N)

Fig.2.Meancracklength(a)asafunctionoftheappliedload(P)forthenormalisedsubstratefor:(a)theassprayedsampleand(b)theannealedsample.

devotedtothemechanicalaspectsofhydrogencontami-nationofaNi(80%)Cr(20%)thermalsprayedcoatingand,inparticular,totheinfluenceofhydrogenonthecoatingadhesionofalowcarbonsteelsubstrate.Asweshowedinthefirstpartthatasuitableannealingtreatmentincreasestheadhesionofthermallysprayedcoatingwewillstudytheinfluenceofhydrogenonlyontheannealedspecimens(2h,6008C)with3coatingthicknessrangingbetween200to600Am.Theconditionsofhydrogencontaminationarechoseninordertofillallthehydrogen-trappingsites[5,6].Fig.2showsthatPCand,consequentlyKca,increaseafterannealingtreatment.Inaddition,forhigherloadsappliedtoassprayedsamples,thereisobservedachangeofslope,whichisconnectedtothepresenceofacrackinthecoatingfromthetipoftheindenttowardsthesurfaceinadditiontothecrackformedattheinterface.Onannealedsamples,itwasobservedthatnocrackwasformedatthetipoftheindentinthecoatingforhigherloadsand,consequently,thechangeofslopedisappears.Contrarytoresultspresentedabove,auniquevalueofPCisnotobtainedneitheronannealedsamplesnorontheannealedwithhydrogencontaminationsamples.Thelowhardnessofthecoatingisrenderedresponsibleforsuchphenomenon[7].Ifweconsideredonlytheeffectofhydrogen,decreasingvaluesofPCwereobservedincomparisontothoseofannealedspecimens,henceitisclearthathydrogentrappinggeneratesresidualstressesinthematerial.Asaconse-quence,theKcaforthehydrogenatedsamplesarereducedcomparedtotheonlyannealedones.

4.Influenceofthermalsolicitations

Thermalsprayedcoatingsareveryoftenusedtoresistwearandtopresentimportanttribologicalandcorrosionresistanceproperties.Insuchsituations,thecontinuousorcycliccontactbetweenthepartsmaygeneratelocaltemper-aturevariations.Asaconsequence,changesintheresidualstressfieldandonthemicrostructureofthecoating–substrateinterfaceregionmaytakeplacedueto:1)differenceinthermalexpansioncoefficientsbetweenthesubstrateandthe

coating;2)thermo-mechanicalinducedmetallurgicaltrans-formationsinthecoatingand/orthesubstrate,suchassolidstatediffusion,phasetransformations,graingrowth,precip-itation,coalescenceofsecondphaseparticles,segregationanddislocationrearrangements.Sincetheseeffectsmaymodifythemetallurgicalandmechanicalfeaturesoftheinterface,theadhesionofthecoatingmaybeaffected.Therefore,onemustbeawareofappropriatemethodologiestocharacterisethecoatingadherenceand/ortheinterfacetoughness,aswellastodevelopexperimentalprocedurestosimulatethethermo-mechanicalstressingarisinginservice.Thishasmotivatedthepresentstudy,inwhichNi80%Cr20%coatingswereAirPlasmaSprayed(APS)ontolowcarbonsteelsamplesandweresubmittedtoheattreatments,followedbyadhesioncharacterisation.Twodifferentheattreatmentswereexaminedafterfirstlyanannealingtreatment(2h,6008C)wasperformed.

Wehaveobservedinsomeoccasionsthatthecoatingmaybeseparatedfromitssubstrateduetoathermalshock.Themaincauseofthisdelaminationisthemismatchbetweenthethermalexpansionofthesubstrateandthecoatingduringthefastcooling.Becauseofthat,wedecidedtoperformfivethermalshocks,whichconsistedofheatingat9108Cbycontactwithathermalresistance,maintainingitfor3minandcoolinginanoilbathat208C.Anotherlessdrastictreatmentwasalsotested.Itconsistedoffivethermalcyclings,byheatingslowlyuntil9108C,maintainingitfor3minandswitchingoffthefurnaceforthecooling.Withthesetreatments,weproducedfourmodalitiescalled1)assprayed,2)annealed,3)annealed+thermalshockand4)annealed+cycling.

Fig.3representstherelationbetweenthemeancracklengthsmeasuredattheinterfaceasafunctionoftheappliedindentationload.Thefirstobservationisthatauniquecriticalpointisobtainedfortheannealedspecimen,whichmeansthatnoresidualstressesaregeneratedbythesetreatments.Withoutresidualstresses,theapparentinterfacetoughnessrepresentsmainlythemetallurgicalbondattheinterface.Fig.3showsthatPC(cycling)ishigherthanPC(shock),thelatterbeinghigherthanPC(annealed).Thisresultseemstoshowthatthemetallurgicalbondis

D.Chicotetal./Surface&CoatingsTechnology200(2005)174–177177

ln a (a in µm)6 4 t1 = 0.11 mm t2 = 0.17 mm t3 = 0.23 mm t4 = 0.26 mm ln a (a in µm)8 765432PC = 4.2 N t1 = 0.15 mm t2 = 0.23 mm t3 = 0.26 mm PC0.26 = 0.65 N 2 0 -2 7 6 5 4 3 2 0 PC0.23 = 0.40 N PC0.17 = 0.25 N PC0.11 = 0.20 N Apparent hardness Apparent hardness (a) As sprayed0 1 2 34(b) Annealed0123 4 5-1 ln P (P in N)

ln a (a in µm)t1 = 0.14 mm t2 = 0.20 mm t3 = 0.29 mm ln P (P in N)

ln a (a in µm)765432PC = 5.7 N

t1 = 0.15 mmt2 = 0.19 mmt3 = 0.27 mmApparent hardness PC = 5 N Apparenthardness(c) Annealed + Thermal Shock1 2 3 4501(d) Annealed + Cycling23 4 5ln P (P in N)

ln P (P in N)

Fig.3.Meancracklength(a)measuredattheinterfaceasafunctionoftheappliedload(P)for:(a)theassprayedsample,(b)theannealedsample,(c)theannealedsamplewhichhasbeensubmittedtothermalshocksand(d)theannealedsamplewhichhasbeensubmittedtothermalcycling.

increased.Duringthethermaltreatments,thesamplesreachatemperaturewhichstimulatesdiffusionofatomicspecies,inparticular,throughtheinterface.Thisprocessinducesarearrangementoftheatomsinthevicinityoftheinterface.Asaconsequence,thephysicalbondsarereinforcedandtheresistancetoinitiationofcracksisincreased.Thishypoth-esisisconfirmedbythefactthat,forthethermalcyclingtreatment,thesampleismaintainedatanelevatedtemper-atureduringalongerperiod,whichallowsforamoreintensediffusion.Sincesampleswereannealed,itispossiblethatthediffusionwassufficienttoleadtoareinforcementofthemetallurgicalbond.Theimprovementofthemetallurgicalbondshouldalsoinduceabetterresistancetothepropagationofthecrack.

atermKcaOindependentonthethicknessandatermassociatedtoresidualstresses.Itwasdemonstratedherethatifaconvenientannealingtreatmentisperformedafterspraying,adhesionpropertiesaregreatlyincreased.Weconfirmthusthattheresidualstresseshaveadetrimentaleffectontheadhesivepropertiesofthermalsprayedcoat-ings.Asacorollaryoftheimprovementduetoannealing,itwasfoundthat,insteadofdeterioratingadhesion,thermalshockandthermalcyclingincreasedtheadhesionbythereinforcementofthemetallurgicalbondingbetweenthecoatingandthesubstrate.Itwasshownalso,thathydrogenplaysaroleinresidualstresses.

References

5.Conclusion

Thisstudyconfirmsgreatinterestfortheinterfaceinden-tationtestforthedeterminationoftheadhesionpropertiesofcoatings.Wehaveshownthevalidityofanapparentinterfacetoughnessformulationtorepresentadhesionofcoatings.Inparticular,thetestallowsforthediscussionoftheinitiationresistancetocrackingusingtheinterfacetoughnessandthecrackpropagationresistanceattheinterface.

Acknowledgingthefactthattoughnessvarieswiththethicknessofthecoating,itwasproposedtoseparateKcain

[1]D.Choulier,The`sededoctorat,Universite´deTechnologieCompie`gne

(1989).[2]P.De´mare´caux,D.Chicot,J.Lesage,J.Mater.Sci.Lett.15(16)(1996)

1377.

[3]D.Chicot,P.De´mare´caux,J.Lesage,ThinSolidFilms283(1-2)(1996)

151.

[4]J.Lesage,D.Chicot,Surf.Eng.15(6)(1999)447.

[5]P.E.V.deMiranda,F.D.Fassini,J.ofMaterialsScience,vol.28,

Chapman&Hall,London,p.5148.

[6]D.Chicot,O.Bartier,M.Zampronio,P.E.V.deMiranda,J.Lesage,

RevuedeMe´tallurgieCIT.ScienceetGe´niedesMate´riaux5(1998)659.[7]J.Lesage,D.Chicot,P.Araujo,M.Zampronio,P.E.V.DeMiranda,

ThinSolidFilms377–378(1)(2000)675.