Si-based thin film coating on Y-TZP: Influence of deposition parameters on adhesion of resin cement

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AppliedSurfaceScience282 (2013) 245–252

ContentslistsavailableatSciVerseScienceDirect

Applied

Surface

Science

jo u r n al h om ep a g e :w w w . e l s e v i e r . c o m / l o c a t e / a p s u s c

Si-based

thin

film

coating

on

Y-TZP:

Influence

of

deposition

parameters

on

adhesion

of

resin

cement

夽

José

Renato

Cavalcanti

Queiroz

a,∗

_,

_Lafayette

_Nogueira

_Junior

b

_,

_Marcos

_Massi

c

_,

Alecssandro

de

Moura

Silva

b

_,

_Marco

_Antonio

_Bottino

b

_,

Argemiro

Soares

da

Silva

Sobrinho

d

_,

_Mutlu

_Özcan

e

a_Potiguar_University,_Department_of_{Biotechnology,}_Natal,_Brazil

b_São_Paulo_State_University,_Department_of_{Prosthodontics}_and_Dental_Materials,_São_José_dos_Campos,_Brazil c_Federal_University_of_São_Paulo,_Institute_of_Science_and_Technology,_São_José_dos_Campos,_Brazil

d_{Technological}_Institute_of_Aeronautics,_Department_of_Physics,_São_José_dos_Campos,_Brazil

e_University_of_Zurich,_Dental_Materials_Unit,_Center_for_Dental_and_Oral_Medicine,_Clinic_for_Fixed_and_Removable_{Prosthodontics}_and_Dental_Materials Science,Zurich,Switzerland

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received12November2012 Receivedinrevisedform22May2013 Accepted23May2013

Available online 30 May 2013

Keywords: Adhesion Film Plasma Roughness Surfacetreatment Zirconia

a

b

s

t

r

a

c

t

ThisstudyevaluatedtheinfluenceofdepositionparametersforSi-basedthinfilmsusingmagnetron sputteringforcoatingzirconiaandsubsequentadhesionofresincement.Zirconiaceramicblockswere randomlydividedinto8groupsandspecimenswereeithergroundfinishedandpolishedorconditioned usingair-abrasionwithaluminaparticlescoatedwithsilica.Intheremaininggroups,thepolished speci-menswerecoatedwithSi-basedfilmcoatingwithargon/oxygenmagnetrondischargeat8:1or20:1flux. Inonegroup,Si-basedfilmcoatingwasperformedonair-abradedsurfaces.Afterapplicationof bond-ingagent,resincementwasbonded.Profilometry,goniometry,EnergyDispersiveX-raySpectroscopy andRutherfordBackscatteringSpectroscopyanalysiswereperformedontheconditionedzirconia sur-faces.Adhesionofresincementtozirconiawastestedusingshearbondtestanddebondedsurfaceswere examinedusingScanningElectronMicroscopy.Si-basedfilmcoatingappliedonair-abradedrough zir-coniasurfacesincreasedtheadhesionoftheresincement(22.78±5.2MPa)comparedtothoseofother

methods(0–14.62MPa)(p=0.05).MixedtypeoffailuresweremorefrequentinSifilmcoatedgroupson eitherpolishedorair-abradedgroups.Si-basedthinfilmsincreasedwettabilitycomparedtothecontrol groupbutdidnotchangetheroughness,consideringtheparametersevaluated.Depositionparameters ofSi-basedthinfilmandafterapplicationofair-abrasioninfluencedtheinitialadhesionofresincement tozirconia.

1. Introduction

Yttrium stabilized tetragonal zirconia polycrystal (Y-TZP) ceramic (hereon: zirconia) lately gained popularity for restor-ativeapplicationsas it presentsmechanical propertiessuperior tootheravailableall-ceramicsindentistry[1].Thepossibilityof millingzirconiausingCAD/CAMdevicesalsoincreaseditsclinical indications.Unfortunately, theachievedroughnessaftermilling proceduresis notsufficient toadhereresin-basedcements that

夽_Part_of_this_study_has_been_presented_at_the_Academy_of_Dental_Materials_(ADM),

35thAnnualCongress,October13–15,2011,CostadoSauipe,Bahia,Brazil.

∗_{Corresponding}_author_at:_Potiguar_University,_Department_of_{Biotechnology,}_Av.

Gov.JuvenalLamartine,326,Tirol,ZipCode59020-280,Natal,RN,Brazil. Tel.:+558432221804;fax:+558432229975.

E-mailaddress:joserenatocq@hotmail.com(J.R.C.Queiroz).

consequentlylimittheirpotentialuseforminimalinvasive recons-tructions[2].Inaddition,traditionaladhesionprotocolsusedfor silica-basedceramicsystemssuchasacidetchingwith hydroflu-oricacidandsubsequentsilanizationisnoteffectiveforgenerating physicalandchemicalchangesonzirconiasurface[3,4].Sincegood adhesionobtainedatthecementationinterfacecouldpromote pre-vention of microleakage[4], surface modification of zirconiais essentialtoachieveastableadhesivejointwithresincements[3]. Thus,severalsurfaceconditioningmethodshavebeensuggested toconditionzirconiasurfacesphysicallyand/orchemically,suchas air-particleabrasionusingaluminaparticlesfollowedby applica-tionofceramicprimers,adhesivemonomersortribochemicalsilica coating(RocatecandCoJetSystems)followedbytheapplicationof silanecouplingagents[3,5].Chairsideair-abrasionprotocolsusing aluminaoraluminaparticlescoatedwithsilica(tribochemical coat-ing)isparticularlyfavoredtoconditionzirconiaasiteliminatesthe organiccontaminantsonthesurface,improvewettability,increase

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246 282 (2013) 245–252

Table1

Thebrands,types,chemicalcompositions,manufacturersandbatchnumbersofthematerialsusedinthisstudy.

Brand Type Chemicalcomposition Manufacturer Batchnumber

Cerconzirconia Ceramiccore Zirconiumoxide,yttriumoxide,hafnium oxide

Dentsply/Degudent,Hanau,Germany 318900-3

OxidedeAluminio Particleforair-abrasion(45␮m) Aluminumoxide Polidental,SãoPaulo,Brazil 20919 Monobond-S Silanecouplingagent EthylAlcohol,

3-methacryloxypropyltrimethoxy, methylethylketone

IvoclarVivadent,Schaan,Liechtenstein N011595

Metal/zirconiaprimer Ceramicprimeragent Methylisobutylketone,phosphonicacid acrylate,benzoylperoxide

IvoclarVivadent M68692

Multilink Resincement Benzoylperoxide,HEMA,inorganicfillers, ytterbiumtrifluoride,pigments

IvoclarVivadent N42713

bondingarea,roughness[6,7].Air-abrasionprotocolsalsopromote micromechanicalinterlockingoftheresin[8,9].Furthermore, depo-sitionofamorphoussilicalayersonthezirconiasurfaceenables silanereactionsbytribochemicalcoating[3,10].

Theimpactofparticledepositiononzirconiausingair-abrasion protocolscouldpromotesurfacechanges bylocallattice distor-tionsand/or by the emergence of a new phase by ferroelastic domainswitchingunder stress[11] oryieldtocompletelateral cracks[12]. Whenalumina particlesare used,subsurface dam-agecanoccurin zirconiaandthis influencesthesurvivalinthe cyclicfatiguetesting [12–14].Instead,theuseofalumina parti-clescoatedwithsilica(CoJet)wasreportedtoreducetheimpact induced surface flaws and not affect the survival under cyclic fatigueloadingcomparedtonon-airabradedcontrolgroups[11]. Yet, tribochemical conditioning and silanization is not always indicatedforthecementationofinlay-retainedfixeddental pros-thesis(FDP)whenchemicaladhesioncouldbeobtainedwithresin cements[15].

Sincecontroversialopinionsexistregardingtosurface activa-tionofzirconiawithair-abrasion protocols,newapproaches for surface conditioningthat do not compromise thestrength and marginaladaptationofcrownsandFPDshavebeensuggestedsuch asselectiveinfiltrationetching[16],glazing[17],aluminacoating

[18],theuseofadhesivepromoterswithbifunctionalmonomers

[2,19,20],strongacidetching[21,22],chemicalvapordeposition usinghexamethyldisiloxane[23],chlorosilanegas[4],orsulphur hexafluoride[24],physicalvapordepositionusingthemagnetron sputteringtechnique[25,26],non-thermalplasmaexposure[27]

andlaserirradiation[28].Amongthesemethods,itcanbe antici-patedthatSi-basedfilmdepositionprocesscouldimproveadhesion betweenzirconiaandresincement[25,26]basedonthefactthat thesilanecouplingagentshowschemicalaffinitytosiliconoxides

[3,10,29]. Previous studies showed promising results using Si-basedfilms[25,26].However,adhesionwasnotoptimumatthe film-zirconiainterface[26].

Using magnetron sputtering technique, thin films could be depositedonzirconiasurface.Thisphysicalprocessfunctionalizes surfacesandimprovesthesurfaceenergyofthesubstrate[30].The depositionrateofcompoundsdependsonthematerialofthe sput-teredtarget,gas,electricalconductivityofthesputteredtargetand typeof powersupply usedtogrowthefilm.Whenparameters areoptimized,uniformdeposition,controlledfilmthicknessand multilayerdepositionoffilmscouldbeachieved[30].

Theobjectivesofthisstudytherefore,weretoevaluatethe influ-enceofsurfacetextureanddepositionparametersforSi-basedthin filmsusingreactivemagnetronsputteringoncoatingzirconia,to characterizethefunctionalityof thesecoatingsandassesstheir effectonadhesionofresincementcomparedtoconventional sur-faceconditioningmethods.Thetestedhypothesiswasthatplasma parametersusedfordepositionofSi-basedthinfilmswouldaffect thechemicalreactivityofzirconiaandincreaseadhesionofresin cementcomparedtoconventionalconditioningmethods.

2. Materialsandmethods

2.1. Specimenpreparationandexperimentalgroups

Thebrands,types,chemicalcompositions,manufacturersand batchnumbersof thematerialsusedin thisstudy arelistedin

Table1.

Zirconiaceramic(CerconZirconia,Dentsply/Degudent,Hanau, Germany) blocks(N=80)(6mm×6mm×3mm)wereprepared andsintered(VitaZircomatfurnace,VitaZahnfabrik,Bad Säckin-gen,Germany)accordingtothemanufacturer’sinstructions.They werethenrandomlydividedinto8groups(n=10pergroup)and conditionedasfollows:

GC:Theceramicsurfacesweregroundfinishedto1200silicon carbide(SiC)paperunderwatercoolinginapolishingmachine (DP-10,Panambra,SãoPaulo,Brazil)andcleanedinanultrasonicbath (Vitasonic,VitaZanhfabrik)indistilledwaterfor10min.

GP:ThespecimenswerepreparedasdescribedforgroupGC. Then,aceramicprimer(Metal/Zirconiaprimer,IvoclarVivadent, Schaan,Liechtenstein)wasappliedtotheceramicsurfaceusinga cleanbrushonelayerandlefttoreactwiththesurfacefor180sat roomtemperature(20◦_C),_50%_relative_humidity._Next,_the_excess

primerwasremovedbyairspray(2.8bar)freefromoil contamina-tionfor5s.

GS:ThespecimenswerepreparedasdescribedforgroupGC.The cementationsurfaceswereairabradedwith30␮malumina parti-clescoatedwithsilica(CoJetSand,3MESPEAG,Seefeld,Germany) usingachairsideairabrasiondevice(CoJet-Prep,3MESPEAG)at pressureof2.8barfor10s.Thedistancebetweentheceramic sur-faceand thenozzle wasstandardized at10mm, atan angleof 90◦_._The_specimens_were_then_{ultrasonically}_cleaned_in_distilled

waterfor10minandsilanecouplingagent(MonobondS,Ivoclar Vivadent)asdescribedingroupGP.

GR:Priortosinteringprocess,thecementationsurfaceswereair abradedwith45␮maluminaparticlesatpressureof2.8barfor10s. Thedistancebetweentheceramicsurfaceandthenozzlewas stan-dardizedat10mm,atanangleof90◦_._After_sintering,_the_specimens

wereultrasonicallycleanedindistilledwaterfor10minandthen ceramicprimer(Metal/Zirconiaprimer)wasappliedasdescribed ingroupGP.

GF1:ThespecimenswerepreparedasdescribedforgroupGC. Si-basedthinfilmcoatingwasachievedusingArgonradiofrequency magnetrondischarge.Asilicatarget(KurtJ.Lesker,Pittsburgh,USA) wassputteredonthecementationsurfaceofthezirconia. Follow-ingfilmdeposition,thespecimenswereultrasonicallycleanedin distilledwaterfor10minandsilanecouplingagent(MonobondS, IvoclarVivadent)wasappliedasdescribedinGroupGP.

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Table2

ParametersusedduringSi-filmdeposition(targetcomposition,Ar,O2gasflux,pressure,powersupplytype,power,target-substratedistance).

Film Target Arflux(sccm) O2flux(sccm) Pressure(mTorr) Powersupplytype Power(W) Distance(cm)

GF1 SiO2 20 – 7 Radiofrequency 120 8

GF2/GF2R Si 20 2.5 7 Directcurrent 120 8

GF3 Si 20 1 7 Directcurrent 120 8

GF3:ThespecimenswerepreparedasdescribedforgroupGC. Adirectcurrentmagnetrondischarge,silicontarget(KurtJ.Lesker), andargon/oxygengases(20:1influx)wereusedtopromotefilm growthonthezirconiasurface.Applicationofthesilanecoupling agentwasperformedasdescribedinGF1.

GF2R:ThespecimenswerepreparedasdescribedforgroupGR. Adirectcurrentmagnetrondischarge,silicontarget(KurtJ.Lesker), andargon/oxygengases(8:1influx)wereusedtopromotefilm growthonthezirconiasurface(asGF2).Applicationofthesilane couplingagentwasperformedasdescribedinGF1.

DepositionparametersforgroupsGF1,GF2,GF3,GF2Rare pre-sentedinTable2.

Additional15 zirconiablocks (2mm×10mm×10mm)were manufacturedfor theadhesiontestand wererandomlydivided intothreegroups(n=5):(a)GC,(b)GSand(c)GF2.

2.2. Silicafilmgrowth

ThesurfacesofzirconiaspecimenswerecoatedwithSi-based thin film by physical vapour deposition (PVD) method using a reactivemagnetronsputtering(RMP)technique.Forthisprocess, highpurity(99.99%)SiO2-andSi-target(KurtJ.Lesker)andthe zir-coniablockswerepositionedina tailormadevacuumchamber (LaboratoryofPlasmaandProcess,ITA,Brazil)andevacuatedtoa backgroundpressureof5×10−5_Torr._A_previous_Argon_discharge wasperformedfor10mintoremove surfacecontaminations on thetargets.Thedepositionswereperformedataworkingpressure of7mTorr(∼2.7Pa).Athermocoupleevaluatedthetemperature onthespecimenholderduringdepositionprocesses.Allsubstrates attainedamaximumof95◦_C.

ForGF1,asilicatargetwassputteredbyanargonradiofrequency magnetrondischargeinordertodepositthefilms.ForGF2,GF3and GF2R,SiatomssputteredfromtheSi-targetreactedwithoxygen plasma,formingsiliconoxides(SiOx)thataredepositedon sub-stratesurface,producingathinfilm.Depositionofstoichiometric andnon-stoichiometricfilms(SiOxwhere0≥x≥2)wasobtained byalteringtheoxygenconcentrationintheplasma[31].The depo-sitiontime wasmaintainedconstantat30minand thecathode voltageandelectriccurrentweremeasuredevery5minto con-troltheprocessstability. Threeadditionalsiliconplatesamples (n=3)thatwereelectrochemicallypolished(Ra<15nm)and par-tiallycoveredwithamask,weresubmittedtoeachfilmdeposition processandusedforfilmthicknessmeasurements.

Subsequently,thespecimenswereremovedfromthereactor, cleanedultrasonicallyin distilledwaterfor10minanddriedby oil-freeairsprayfor30sat2.8bar.

2.3. Bondstrengthtestandfailureanalysis

Equalamountsofthebaseandcatalystpasteofresincement (Mutlilink,IvoclarVivadent)weremixedfor20sand bondedto theconditionedzirconiaspecimensusingpolyethylenemolds(Ø: 2.4mm;thickness:3mm).Followingauto-polymerizationfor24h, thespecimens werestored in distilled water (37◦_C

±1◦; 48h). Shearbondstrengthtestwasperformedusingauniversaltesting machine(DL-1000,EMIC,SãoJosédosPinhais,Brazil)(crosshead speed:1mm/min;100kgFload-cell)andthebondstrength(MPa) wascalculated(force(inN/adheredareainmm2_).

Thedebondedsurfaceswereexaminedusinganoptical micro-scopeat60×magnification(MeasuringMicroscopeMFA,Mitutoyo, Kawasaki,Japan)andScanningElectronMicroscopes(SEM) (SSX-550, Shimadzu, Kyoto, Japan) at 35×–5000× magnification in secondaryelectron(SE)modetocharacterizethefailuremode.In ordertoverifythepresenceofSiontheceramicsurfacesafterbond test,additionalsurfaceanalysiswasperformedusingSEM/Energy DispersiveSpectrometer(EDX,Shimadzu).

Failure typeswereclassifiedasfollows: A1:adhesivefailure alongtheinterfacialregionbetweenthefilmandthecement;A2: adhesivefailurealongtheinterfacialregionbetweenthefilmand theceramic;C:cohesivefailureintheresincement;andM:mixed failure,adhesivefailurebetweenthecementandceramictogether withcohesivefailureintheresincementand/orceramic.

2.4. SurfaceroughnessandRutherfordBackscattering Spectroscopy(RBS)analysis

For roughness analysis (Ra) of the zirconia surfaces follow-ing surface conditioning, four additional specimens from each group were evaluated using an optical profilometer (Wyko NT 1100,Veeco,Plainview,USA)thatwasconnectedtoacomputer drivecontainingtheSoftware Vision32 (Veeco).Measurements were performed at 20× magnification on two random areas (301.3␮m×229.2␮m)ofeachspecimen.

Thefollowingroughnessparametersweremeasured:

Ra: Arithmeticalmean ofthe absolutevalues of thesurface departuresfromthemeanplanewithinthesamplingareain␮m.

Rz:Themeanvalue(␮m)oftheabsoluteheightsof thefive highestpeaksandtheabsolutevalueofthefivedeepestvalleys withinthesamplingarea.Thisparameterissensitivetothechanges ofpronouncedtopographyfeatures.

Sdr(surfacesarearatio):Thisratioexpressestheincrementof theinterfacialsurfacearearelatedtotheareaoftheprojected(flat) xyplane.Foratotallyflatsurface,thesurfaceareaandtheareaof thexyplanewerethesameandSdrwas0%.

Thesiliconplatesampleswereevaluatedtomeasurefilm thick-nessinordertocalculatethedepositionratebydeterminingthe ratiobetweenfilmthicknessanddepositiontime.

Inaddition,specimenswerefurthergoldsputteredina sput-teringdeviceandanalyzedunderanSEM(SSX-550,Shimadzu)at 20kVtoobservethetopographicchangesonthezirconiaceramic surfaceindifferentgroupsinsecondaryelectron(SE)mode.

RutherfordBackscatteringSpectroscopy(RBS)isanionbeam analysiswithhighsensitivitytodetectchemicalelemental concen-trationindepth.Theatomiccompositionofthefilmsasdeposited on zirconia specimens was analyzed by RBS (Pelletron-tanden 5SDH, National Electrostatic Corporation,Middleton, USA). RBS studieswereperformedwitha1mmdiametercollimatedbeam of2.2MeV4He+_ions._{Backscattered}_ions_were_detected_using_Si(Li) detectorsplacedat10◦_scattering_angles._The_resolution_of

detec-tionwas<15atoms/cm2_.

2.5. Contactangleandworkofadhesionanalysis(WA)

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248 282 (2013) 245–252

sessiledroptechniqueatcontrolledroomtemperature(20◦_C)_and

humidity(40%),adropofdeionizedwater(15␮L)wasappliedon thespecimensurfaceandallowedtoflowuntilitreached equilib-rium.Workofadhesion(WA)wascalculatedbytheYoung-Dupré equation,usingthemeanof themeasuredcontactanglevalues obtainedfor each group()and theinterfacialenergybetween liquidandsolid(yLS).

WA=yLS.[cos()+1]

2.6. Statisticalanalysis

Statisticalanalysiswasperformedusingone-wayANOVAand posthocmultiplecomparisonsweremadebetweengroupsbythe Tukey‘sposthoctest(SPSS11.0softwareforWindows,SPSSInc., Chicago,IL,USA).Pvalueslessthan0.05wereconsidered statisti-callysignificantforallstatisticaltests.

3. Results

TheRBSanalysisoftheSi-basedthinfilmidentifiedonly sili-con(Si)andoxygen(O)atomswithslightdifferencesamongthe films.TheSiandOconcentrationsforeachfilmwereGF1:31.25% and68.75%;GF2/GF2R:33.3%and66.7%;andGF3:39.2%and60.8%, respectively.Thefilmthicknessdata(␮m)andthedepositionrates (nm/min)foreachfilmwere:GF1:0.17and∼5;GF2/GF2R:0.23 and∼8;GF3:0.35and∼11,respectively.

SEMimagesshowedsimilarsmoothsurfacetopographyforGC, GP,GF1, GF2 and GF3. From these groups, representative SEM ofGF2is presentedinFig.1a. Othergroups presenteddifferent patternsofsurfacetopography.ForGS,sharpgroovesandpeaks werefoundwithrandomdistribution(Fig.1b).ForGR,air-abrasion beforethesinteringprocesscreatedanewretentivesurfacewith microhollowsthegrainboundarieswerevisibleinGC(Fig.1c)that disappearedafterfilmdepositiontoGF2R(Fig.1d).SEMimageto GFspecimensshowedgrainboundariesunderthefilm,suggesting ananothicknessofthefilmandmicrospotdefectsatsomeareas ontheSi-film(Fig.2aandb).

Bondstrength resultsweresignificantlyaffectedby the sur-faceconditioningmethods(p≤0.05,one-wayANOVA).Themean bondstrengthdataobtainedinGF2Rwassignificantlyhigherthan thoseofothergroups(p≤0.05)(Table3).Air-abrasiontreatment increasedtheroughnessbutthefilmdepositionprocessdidnot.

Failure analysis of the specimens under optical microscopy revealedmainlyA1typeoffailureingroupsGC,GP,GS,GR(Table4). GF2R group, where the highest bond strength was observed, demonstratedmainlyMtypeoffailures(Fig.3a).InGF2andGF2R whereMtypeoffailuresweredominant,qualitativeEDSanalysis ontheadhesivebondingareaindicatedtracesofSionthe zirco-niasurfacecorrespondingtoadhesivefailurebetweentheSifilm coatingandzirconia(Fig.3b).

Table3

Shearbondstrength(±SD)(MPa),Ra(±SD)(␮m)resultsforallgroups.

Groups Bondstrength(MPa) Tukey’stest Ra(␮m)

GC 0 D 0.12(<0.1)

GP 2.42(0.3) D 0.12(<0.1)

GS 14.62(3.2) B 0.51(0.1)

GR 8.29(1.2) C 0.62(0.2)

GF1 6.85(2.2) C 0.11(0.1)

GF2 10.51(1.5) C 0.10(0.1)

GF3 2.77(0.7) D 0.12(0.1)

GF2R 22.78(5.2) A 0.61(0.2)

*Thesamelettersinthebondstrengthcolumnindicatenosignificantdifferencesin thesamecolumn(Tukey’stest,alfa=0.05).

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Fig.2.SEMimageofaspecimenfromgroupGFshowing(a)grainboundariesunder Si-film,suggestingananometricthicknessofthefilmand(b)film(F)coatingY-TZP surface(Y)andapore(P)inducingafilmcrack.

Contactangle,WAandroughnessparameters(Ra,RzandSdr) weresignificantlyaffectedbythesurfaceconditioningevaluatedin groupsGC,GSandGF2accordingtoone-wayANOVA(p=0.0)forall results(Table5).Bothair-abrasion(GS)andfilmdeposition(GF2) improvedthewettabilitycomparedtoGC.Thebestresultswere foundintheGF2group.Themeanvaluesforsurfaceroughness, contactangleand workofadhesion,calculatedbyYoung-Dupré equation,showedsignificantdifferencesbetweengroups(Tukey‘s test)Table3.

4. Discussion

Durable adhesionofresin cementto zirconiacouldincrease theminimalinvasiveapplicationsforprostheticrehabilitation.This

Table4

Distributionandfrequencyoffailuretypesafterbondstrengthtestper experimen-talgroup:A1:adhesivefailurealongtheinterfacialregioninvolvingcement;A2: adhesivefailurealongtheinterfacialregionbetweenthefilmandtheceramic;C: cohesivefailureintheresincement;andM:mixedfailure,adhesivefailurebetween thecementandceramictogetherwithcohesivefailureintheresincementand/or ceramic.

Groups N A1 A2 C M

GC 10 10 0 0 0

GP 10 10 0 0 0

GS 10 9 0 0 1

GR 10 9 0 0 1

GF1 10 0 10 0 0

GF2 10 0 0 0 10

GF3 10 0 10 0 0

GF2R 10 0 0 1 9

Fig.3.(a)TypicalSEMimage(×35)insecondaryelectron(SE)modeofamixed typeoffailure(adhesivefailurebetweenthecementandceramictogetherwith cohesivefailureintheresincementand/orceramic)inGF2,(b)elemental compo-sitionmappingbyEDSanalysis(Zr(blue)andSi(red)mapping)ofaregionnear theresincementinamixedfailureofaspecimenfromGF2(forinterpretationof thereferencestocolorinthistext,thereaderisreferredtothewebversionofthe article).

Table5

DescriptiveanalysisforgroupsGC,GSandGF2regardingtoRa(nm),Rz(␮m),Sdr (%),contactangle(CA)andworkofadhesion(WA).

GC GS GF2

Ra 155.7±30.1b 568.9±17.7a 163.4±32.6b Rz 2.6±0.5b 6.2±0.2a 2.5±0.6b Sdr 2.8±0.9b _80.4_±_4.9a _2.7_0.7b CA(o₎ _86.54_±_1.3a _73.11_±_1.6b _<20c WA 77.19±1.7a _93.95_±_2.0b _>141.21c

*Samesuperscriptlettersindicatenostatisticallysignificantdifferenceinthesame row(Tukey‘stest,p<0.05).

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Thesuccessofadhesioninordertopreventdegradationunder aggressiveenvironmentsdependsonthechemicalcompatibility anddurability oftheinterfacebetweendifferentmaterials.The surfacepropertiesofthematerialsinvolvedandthestudyof adhe-sionmechanismareimportantissuesforpredictingthebehaviorof bondedinterfacesthroughwhichbondstrengthcouldbeoptimized

[32].Inthisregard,surfacewettabilityinparticularisofimportance andinthisstudy,theSi-basedthinfilmimprovedwettability. Phys-icalandchemicalpropertiesofmaterialsinvolvedincementation processaffectthestabilityoftheinterface.Anessentialaspectinthe cementationprocessisthecementchoice.Thecementchosenin thisstudy(Multilink)isabis-GMAbasedconventionalresin com-positelutingagentthatrequiresmoisturecontrolandapplicationof asilaneandabondingagenttoachieveeffectiveadhesionto zirco-nia.Thisresincementwaschosenbecausebifunctionalmonomers arenotpresentinitschemicalcomposition.Functionalmonomers presentinsomeresincementsincreasebondstrengthtozirconia

[26,33]andcouldhavemaskedtherealeffectofnovelsurface treat-mentsproposedinthisstudy.Thecementchoiceseemedtobenot relevantaccordingtothefailuretypesobservedinthisstudy,since theweakestinterfacewasbetweentheSi-basedfilmand zirco-nia.Previousstudieshaveevaluatedthebondstrengthofbis-GMA basedresincementtozirconiaandreportedthattheuseofadhesive promoterswasnoteffectiveatpromotinganincreaseinadhesion tozirconia[3,7,26].Inagreementwiththesestudies,inthisstudy, GCgrouppresentedexclusivelypre-testfailuresandGPshowed lesschemicaladhesion.Ontheotherhand,GSpresentedbetter ini-tialbondstrengthresultscomparedtothoseofGCandGP.This indicatesthatsurfaceactivationthroughparticleabrasionandin particularusingsilica-containingabrasivesisessentialtoachieve betteradhesiontozirconiawiththiscement.Yet,theresultscould stillbeconsideredweak.

Using the plasma technique for film deposition as a condi-tioningstrategyforbetteradhesiontozirconia,theeffectofthe nanofilmdepositedonzirconiawasreportedtopromotethe chemi-caladhesionofthesilanecouplingagent[4].Thisstudyshowedthat air-abrasionwith50␮maluminaabrasivefollowedbygas-phase chlorosilanepretreatmentfordepositingultra-thinsilica-like lay-ersimprovedadhesiontozirconiausingtraditionalsilanationand bonding techniques.Thus, theresponse of an interface and its chemicalinteractionswithothermoleculesmaybeobtainedby superficialsurfacemodificationsusingadvancedtechniques.Itis wellknownthatsilanecouplingagentshowschemicalaffinitywith siliconoxides[3,10,29].Theuseofsilaneisrecommendedfor felds-pathicceramicstoformasiloxanenetworkwiththesilicainthe glassphaseoftheceramicstoimprovethebondstrength.Duetothe chemicalstabilityofzirconia,adhesionofsilanecouplingagentwas reportedtobepoorandpresentedahigherpotentialforhydrolytic degradation[10,34].In a previousstudy,it wasshown that Si-basedfilmcouldimprovebondstabilitywhencomparedtosolely air-abradedzirconia[26].Thus,thisstudywasconductedinorder togrowdifferentSi-basedthinfilmsusingthemagnetron sput-teringtechniqueatvaryingdepositionparametersonflatzirconia surfaces.

Whenthemeanbondresultsonflatsurfacesarecompared(GC, GP,GF1,GF2andGF3),itwasevidentthatchemicaladhesion(GF2 andGF1)improvedtheresultscomparedtotheremaininggroups. EveninthegroupGR,wherethemainadhesionmechanismused wasbasedonmicromechanicalretention,nostatisticaldifference wasverifiedbetweenthisgroupandthoseofGF1andGF2. More-over,GF2Rrevealedstatisticallyhigherresultscomparedtothe silicacoatingandsilanizationmethodappliedinGS.

Aroughsurfacepresentsalargersurfacearea,increasingbond sites,thateventuallyincreasesthebondstrengthby micromechan-icalretentionandimproveswettabilityforzirconiaceramics[7,8]. Despiteof this,reports intheliteratureaffirm thattheoriginal

roughnessproducedbymillingduringfabricationisnotsufficient to promote adhesion to resin cements zirconia [2]. This study showedthatroughenedsurfaceproducedbeforefiring(greenstate) usingtheair-abrasionprocedure(GR)wasnotsufficientto pro-moteinitiallyasgoodresultsasthegroupthatreceivedchairside silicacoating(GS).Thisfindingisindisagreementwitharecent study[35].Itisknownthatair-abrasionofzirconiasurfaceafter sinteringcausesphasetransformationfromtetragonalto mono-clinic[36,37].AsitisdemonstratedingroupsGSandGR,presenting higherroughnessfortheRaparameter,indicatesthatmonoclinic statemay bemore reactivethan tetragonalpolycrystals [2,38]. However,whenaSi-basedthinfilmwasusedonGR,formingthe groupGF2R,theinitialbond strengthwasimproved,increasing thefrequency of mixed failures. In addition,surface roughness remainedunchangedfollowingthefilmdepositionprocess, sug-gestingthatthechemicalmodificationpromotedinGF2andGF2R couldbethemechanismusedtoincreasebondstrengthcompare tothesameroughsurfacewithoutfilm(GPandGR).Thegrowth ofathin filmonthesurfaceusingacoldplasmatechniquecan modifysurfacechemicalandphysicalpropertieswithout chang-ingthepropertiesofthebulkmaterial[30].However,theeffect oftreatmentonthemechanicalpropertiesofzirconiaappliedin thegroupGRwasnotstudiedinthis studybut warrantsfuture research.

Regardingfilmadhesioninzirconiasubstrate,theresultsfor GF1,GF2andGF3occurredindependentofthemicromechanical adhesionmechanismastheywereproducedonaflatsurface.Yet, thefilmdetachedfromthezirconiasurfacetogetherwiththeresin cementafterdebonding,suggeststhatchemicalaffinityofthe Si-basedfilmstothecementfollowedbysilanizationwashigherthan tozirconiasurface.Consequently,theeffectofthisfilmonthebond strengthwithcementcannotbemeasured.Thelowestbondvalues obtainedin GF3showedthat thepresenceofSi-basedthinfilm asinterlayerbetweenzirconiaandresincementincombination withthesilaneapplication,wasnotsufficienttopromotestrong initialbondstrengthasthisfilmwasnotfoundtobeattachedto thezirconiasurfaceafterbondtest.ReducedoxygenfluxinGF3, diminishingtheoxygenconcentrationinthis filmcouldchange nucleationbehaviorofthefilmduringdepositionprocess, influ-encingtheadhesionbetweenthefilmandtheceramicsurface[30]. ForGF1,thereasonsfortheincreaseinoxygenconcentrationwere notinvestigatedinthisresearch.Althoughelemental concentra-tioncouldbeexpectedtobesimilartoGF2,sincetheplasmaused forGF1wasnonreactive(onlyargongas),probablythesputtering ofSiO2targetpreferentiallyremovedoxygen[30].However,the resultssuggestthattheincreaseofoxygenintheSi-basedfilm(as inGF1)couldpromotehigherinternalstressontheSi-basedfilm, diminishingadhesiontozirconiainthisgroup.AlthoughGF1and GF2presentedstatisticallysimilarresults,analysisofthefailure typesunderSEMandusingEDS,showedthattheresincementand thecoatingfilmremainedonthezirconiasurfaceafterbondtest onlyforthegroupGF2.

AccordingtoSEMimages,Si-basedthinfilmseemstobedense butmicrometricflawsinthefilmintegritywerestillapparent.The localizedpinholedefectsinthisfilmcouldbeinducedasa conse-quenceofimpuritiesonthezirconiasurface,intrinsicfilmstress, lowenergysurface ofzirconiaor plasmasputteringonthefilm

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The deposition rate for Si-based thin films decreased from approximately11–8nm×min−1_as_the_oxygen_flux_increased_from 1to2.5sccminthegasdischargedforGF3andGF2,respectively, influencingthefilmthickness.Thisisdue totheprocessof tar-getpoisoning(oxidation)thatreducesthesputteringyieldoffthe targetand,accordingly,thecondensingatomfluxtowardthe zir-coniasurface[25].AccordingtovanHattumandcoworkers,asthe oxygenflowrateincreases,thegasdischargeischaracterizedbya suddendecreaseinsiliconatomstowardthegrowingfilm,witha simultaneousincreaseinSiOxmolecules,indicatingcoverageofthe targetandsubstratebythecompound[31].Furthermore,theuseof asilicatargetforGF1reducedthedepositionrateto5nm×min−1 becausetheinsulatingaspectinherenttothistargetreducesthe sputteringrate.Thisisthereasonwhyaradiofrequencywas cho-sentosubstitutedirectcurrentasthepowersupplyinthisgroup (GF1).

SEMand atomic forcemicroscopy(AFM) analysishavebeen oftenused toevaluatethesurface roughnessin similarstudies

[2,40].Inthisstudy,profilometrywasusedforqualitativeand quan-titativeanalysesoftheroughnessfollowingsurfaceconditioning methods.Thisisanopticaltechniqueformeasuringsurface rough-nessusingopticalinterference,inwhichthelightintensityofthe fringes isrelated to thesurfaceheight. Profilometrypresents a nanometricverticalresolution withadynamicrange (scansize) thatgreatlyexceedsthemicroscopeprobes,providingquickimages ofthesurfacesimilartoSEMwiththeroughnessparameter sup-portedby3DimageasinAFM[29].

Adequateroughnessparameteranalysesandalargescansize toevaluatetheeffectoftopographymodificationonadhesioncan help tounderstandthebehavior of a newsurface conditioning methodonbondstrengthandadhesionproperties.Rahasbeen usedfrequentlyin ordertoexpressthetopographicchanges on thezirconiasurfaceinthedentalliteratureandconsideredasan importantparameterforgeneraluse[40].However,itisimportant tostressthattheRaparameterreducestheeffectsofoddscratches ornon-typicalirregularities.Instead,Rzpresentsadditional infor-mation,revealingthepossiblepresenceofdefectsonthesurface.If RaandRzparametersshowsimilaramplitudes,auniformstandard ofroughnessisexpectedtobepresentonthesurface.Thelarge dif-ferencebetweentheRaandRzparametersshowedthepresenceof spotflawsontheconditionedzirconia,evenontheflatsurface.

Analysisof theroughnessresultsofGC,GS andGF2showed thatRaincreasedby0.4␮mwhenthesurfaceswereair-abraded, suggestingabettermicromechanicalinterlockingbetweenthis sur-facewithresincements.TheincreaseintheRzparameterbetween GSandGC(3.5␮m)verifiedtheenergyeffectproducedby par-ticleimpactduringair-abrasionproducingmorphologicalsurface changes.TheSdrparameterindicatedanincreaseinsurfacearea ofapproximately80%whentheY-TZPceramicwasair-abraded, increasingbondingsitesavailabletoreactwithanadhesive pro-moter.Thus, thisprocedurecanimprove bondstrength results, affectingbothphysical(micromechanicalinterlockingmechanism) andchemical(adsorptionmechanism)adhesionandatthesame time increasing the wettability of the zirconia surface. How-ever,severalstudies showedthat additionalchemical adhesion promotedonly by martensitic transformation mediated by air-abrasionprocedurewithaluminaparticlesbutitwasinefficientto resisthydrolyticdegradationafteragingoftheinterface[4,8,39]. Theresultsofsurfaceroughnessforallthreeparameters(Ra,Rz andSdr)inGF2werenotstatisticallysignificantcomparedtoGC (controlgroup),suggestingthattheimprovedcontactangleandWA inGF2occurredduetochemicalmodificationonthesurface.WA playsavitalroleinadhesionandisdefinedasthereversiblework, requiredseparatingaunitareaofinterfacebetweentwodifferent materials[32].Evenmodestchangesintheirvaluescancauselarge differencesinpracticaladhesionmeasurements[32].Thestrong

implicationisthat ifspontaneousspreadingdoesnotoccur, the interfacialcontactcouldbeincomplete.Thisstudyshowedthat WAforGF2(calculationbasedon20◦)wasimprovedcomparedto theothergroups(GCandGS),implyingthatchemicalmodification onthesurfacesprovidedbytheSi-basedthinfilmwasmore effi-cientthanphysicalandchemicalchangespromotedbyair-abraded surfaceregardingthisproperty.

5. Conclusions

Overall,thisstudyshowedthattheair-abrasionprotocolused inGSimprovedthecontactangleagainstthepolishedsurfacein GC.Likewise,theeffectoftheSi-basedthin filmonthesurface wassignificant.Despiteusingasmoothsurfaceforthesegroups, theyexceededtheanglelimit(20◦₎_detected_by_the_goniometer.

Therearetwoprimaryreasonsforseekingmethodstominimize thecontactangleofthebondingagentagainstthezirconia sur-face:(a)minimumcontactanglecorrespondstomaximumarea andintimacyofcontactbetweenthebondingagentandceramic surfaceand (b)minimum contactanglecorrespondstoa maxi-mumthermodynamicworkofadhesion(WA).TheSi-basedthin filmsdepositedonzirconiabyreactivemagnetronsputtering tech-niquedidnotchangetheroughness,consideringtheparameters evaluated.Theadjusteddepositionparametersfor Si-basedthin films followed by silane application increased the initial bond strengthresultsbetweenresincementandzirconiaceramictested butthevariationindepositionparametersseemedtoplayaroleon improvedadhesionwithzirconiainterface.Initiallyroughzirconia surfacesmadeitpossibletoachievebetterdepositionofSi-based thin films compared to smoothersurfaces. Different sputtering parametersforSi-basedthinfilmdepositionaffectedthe chemi-calpropertiesofthezirconiasurfaceandallconditioningmethods testedimprovedwettabilityofthezirconiasurfacecomparedto thecontrolgroup.Finally,adjustmentsintheparametersusedfor a moreuniformfilmgrowthand forimprovingadhesionof the filmonthesubstrate,usingdifferentdepositiontechniques,require furtherinvestigations.Irrespectiveoffutureresearch,theplasma techniqueisnotacommonprocessindentistryandthecostofits implementationinaprostheticslaboratoryremainscurrentlyhigh.

Acknowledgements

Thisstudywasbased ona thesissubmitted tothegraduate faculty, São Paulo StateUniversity, in partialfulfillment of the requirementsfora PhDdegree.TheStateofSãoPauloResearch Foundation(FAPESP)isgreatlyacknowledgedforsupportingthis research(Grantno.09/53584-4).

References

[1]H.J.Conrad,W.J.Seong,I.J.Pesun,Currentceramicmaterialsandsystemswith clinicalrecommendations:asystematicreview,JournalofProstheticDentistry 98(2007)389–404.

[2]P.Magne,M.P.G.Paranhos,L.H.BurnettJr.,Newzirconiaprimerimprovesbond strengthofresin-basedcements,DentalMaterials26(2010)345–352. [3]M.Özcan,H.Nijhuis,L.F.Valandro,Effectofvarioussurfaceconditioning

meth-odsontheadhesionofdual-cureresincementwithMDPfunctionalmonomer tozirconiaafterthermalaging,DentalMaterialsJournal27(2008)99–104. [4]J.R.Piascik,E.J.Swift,J.Y.Thompson,S.Grego,B.R.Stoner,Surface

modifica-tionforenhancedsilanationofzirconiaceramics,DentalMaterials25(2009) 1116–1121.

[5]M.A.Bottino,L.F.Valandro,R.Scotti,L.Buso,Effectofsurfacetreatmentsonthe resinbondtozirconium-basedceramic,InternationalJournalofProsthodontics 18(2005)60–65.

[6]M.Wolfart,F.Lehmann,S.Wolfart,M.Kern,Durabilityoftheresinbond strengthtozirconiaceramicafterusingdifferentsurfaceconditioningmethods, DentalMaterials23(2007)45–50.

(8)

252 282 (2013) 245–252

[8]M.B.Blatz,G.Chiche,S.Holst,A.Sadan,Influenceofsurfacetreatmentand simulatedagingonbondstrengthsoflutingagentstozirconia,Quintessence Int.38(2007)745–753.

[9]A.DellaBona,T.A.Donassollo,F.F.Demarco,A.A.Barrett,J.J.MecholskyJr., Char-acterizationandsurfacetreatmenteffectsontopographyofaglass-infiltrated alumina/zirconia-reinforcedceramic,DentalMaterials23(2007)769–775. [10]C.Y.K.Lung,J.P.Matinlinna,Aspectsofsilanecouplingagentsandsurface

con-ditioningindentistry:anoverview,DentalMaterials28(2012)467–477. [11]M.CattaniLorente,S.S.Scherrer,J.Richard,R.Demellayer,M.Amez-Droz,H.W.

Wiskott,SurfaceroughnessandEDScharacterizationofaY-TZPdentalceramic treatedwiththeCoJetTMSand,DentalMaterials26(2010)1035–1042. [12]T.Kosmac,C.Oblak,P.Jevnikar,N.Funduk,L.Marion,Strengthandreliabilityof

surfacetreatedY-TZPdentalceramics,JournalofBiomedialMaterialsResearch B:AppliedBiomaterials53(2000)304–313.

[13]Y.Zhang,B.R.Lawn,K.A.Malament,V.P.Tompson,E.D.Rekow,Damage accu-mulationandfatiguelifeofparticle-abradedceramics,InternationalJournalof Prosthodontics19(2006)442–448.

[14]S.Karakoca,H.Yilmaz,Influenceofsurfacetreatmentsonsurfaceroughness, phasetransformation,andbiaxialflexuralstrengthofY-TZPceramics,Journal ofBiomedialMaterialsResearchB:AppliedBiomaterials91(2009)930–937. [15]B.Ohlmann,P.Rammelsberg,M.Schmitter,S.Schwarz,O.Gabbert,All-ceramic

inlay-retainedfixedpartialdentures:preliminaryresultsfromaclinicalstudy, JournalofDentistry36(2008)692–696.

[16]M.N.Aboushelib,J.P.Matinlinna,Z.Salameh,H.Ounsi,Innovationsinbonding tozirconia-basedmaterials:PartI,DentalMaterials24(2008)1268–1272. [17]P.Ntala,X.Chen,J.Niggli,M.Cattell,Developmentandtestingofmulti-phase

glazesforadhesivebondingtozirconiasubstrates,JournalofDentistry38 (2010)773–781.

[18]P.Jevnikar,K.Krnel,A.Kocjan,N.Funduk,T.Kosmac,Theeffectof nano-structuredaluminacoatingonresin-bondstrengthtozirconiaceramics,Dental Materials26(2010)688–696.

[19]J.P.Matinlinna,T.Heikkinen,M.Özcan,L.V.Lassila,P.K.Vallittu,Evaluationof resinadhesiontozirconiaceramicusingsomeorganosilanes,DentalMaterials 22(2006)824–831.

[20]M.N.Aboushelib,H.Mirmohamadi,J.P.Matinlinna,E.Kukk,H.F.Ounsi,Z. Salameh,Innovationsinbondingtozirconia-basedmaterials.PartII:focusing onchemicalinteractions,DentalMaterials25(2009)989–993.

[21]U.Lohbauer,M.Zipperle,K.Rischka,A.Petschelt,F.A.Müller,Hydroxylation ofdentalzirconiasurfaces:characterizationandbondingpotential,Journalof BiomedialMaterialsResearchB:AppliedBiomaterials87(2008)461–467. [22]C.Y.K.Lung,E.Kukk,T.Hägerth,J.P.Matinlinna,Surfacemodificationof

silica-coatedzirconiabychemicaltreatments,AppliedSurfaceScience257(2010) 1228–1235.

[23]T.Derand,M.Molin,K.Kvam,Bondstrengthofcompositelutingcementto zirconiaceramicsurfaces,DentalMaterials21(2005)1158–1162.

[24]J.R.Piascik,S.D.Wolter,B.R.Stoner,Developmentofanovelsurface modifica-tionforimprovedbondingtozirconia,DentalMaterials27(2011)e99–e105. [25]J.R.C.Queiroz,D.A.Duarte,R.O.A.Souza,S.F.Fissmer,M.Massi,M.A.Bottino,

DepositionofSiOx thinfilmsbyreactivemagnetron sputtering:influence

ofplasmaparametersontheadhesionpropertiesbetweenY-TZPandresin

cementforapplicationindentalprosthesis,MaterialsResearch14(2011) 212–216.

[26]J.R.C.Queiroz,M.Massi,L.NogueiraJr.,A.S.S.Sobrinho,M.A.Bottino,M.Özcan, Silica-basednano-coatingonzirconiasurfacesusingreactivemagnetron sput-tering:effectonchemicaladhesionofresincements,JournalofAdhesive Dentistry15(2013)151–159.

[27]N.R.Silva,P.G.Coelho,G.B.Valverde,K.Becker,R.Ihrke,A.Quade,V.P. Thomp-son,SurfacecharacterizationofTiandY-TZPfollowingnon-thermalplasma exposure,JournalofBiomedialMaterialsResearchB:AppliedBiomaterials99 (2011)199–206.

[28]A.N.Cavalcanti,R.M.Foxton,T.F.Watson,M.T.Oliveira,M.Giannini,G.M. Marchi,Bondstrengthofresincementstoazirconiaceramicwithdifferent surfacetreatments,OperativeDentistry34(2009)280–287.

[29]J.R.C.Queiroz,P.Benetti,M.Özcan,L.F.C.Oliveira,A.DellaBona,F.E.Takahashi, M.A.Bottino,SurfacecharacterizationoffeldspathicceramicusingATR FT-IRandellipsometryaftervarioussilanizationprotocols,DentalMaterials28 (2012)189–196.

[30]D.M.Mattox,Adhesionandde-adhesion,in:HandbookofPhysicalVapor Depo-sition(PVD)Process,Elsevier,Oxford,2010.

[31]E.D. vanHattum,A.Palmero, W.M.Arnoldbik, F.H.P.M.Habraken, Exper-imental characterization of thedepositionof siliconsuboxide filmsin a radiofrequencymagnetronreactivesputteringsystem,SurfaceandCoatings Technology188–189(2004)399–403.

[32]J.C.Berg,Semi-empiricalstrategiesforpredictingadhesion,in:M.Chaudhury, A.V.Pocius(Eds.),AdhesionScienceandEngineering2–SurfacesChemistry andApplications,Elsevier,Amsterdam,2002,pp.1–73.

[33]F.P.Nothdurft,P.J.Motter,P.R.Pospiech,Effectofsurfacetreatmentontheinitial bondstrengthofdifferentlutingcementstozirconiumoxideceramic,Clinical OralInvestigations13(2009)229–235.

[34]J.P.Matinlinna,L.V.J.Lassiva,M.Özcan,A.Yli-Urpo,P.K.Vallittu,Anintroduction tosilanesandtheirsclinicalapplicationsindentistry,InternationalJournalof Prosthodontics17(2004)155–164.

[35]C.Monaco,P.Cardelli,R.Scotti,L.F.Valandro,Pilotevaluationoffour experi-mentalconditioningtreatmentstoimprovethebondstrengthbetweenresin cementandy-tzpceramic,JournalofProsthodontics20(2011)97–100. [36]M.Guazzato,L.Quach,M.Albakry,M.V.Swain,InfluenceofSurfaceandheat

treatmentsontheflexuralstrengthofY-TZPdentalceramic,Journalof Den-tistry33(2005)9–18.

[37]M.Özcan,R.M.Melo,R.O.Souza,J.P.Machado,F.L.Valandro,M.A.Botttino, Effectofair-particleabrasionprotocolsonthebiaxialflexuralstrength,surface characteristicsandphasetransformationofzirconiaaftercyclicloading,Journal oftheMechanicalBehaviorofBiomedicalMaterials29(2013)19–28. [38]A.Christensen,E.A.Carter,First-principlesstudyofthesurfacezirconia,

Phys-icalReviewB58(1998)8050–8064.

[39]R.L.Smith,C.Villanueva,J.K.Rothrock,C.E.Garcia-Godoy,B.R.Stoner,J.R. Pias-cik,J.Y.Thompson,Long-termmicrotensilebondstrengthofsurfacemodified zirconia,DentalMaterials27(2011)779–785.