Aging resistance, mechanical properties and translucency of different yttria-stabilized zirconia ceramics for monolithic dental crown applications

(1)

Available

online

at

www.sciencedirect.com

ScienceDirect

j ou rn a l h o m epa ge :w w w . i n t l . e l s e v i e r h e a l t h . c o m / j o u r n a l s / d e m a

Aging

resistance,

mechanical

properties

and

translucency

of

different

yttria-stabilized

zirconia

ceramics

for

monolithic

dental

crown

applications

E. Camposilvan

a,b

_,

_R.

_Leone

c

_,

_L.

_Gremillard

a

_,

_R.

_Sorrentino

c

_,

_F.

_Zarone

c

_,

M. Ferrari

d

_,

_J.

_Chevalier

a,∗

a_Université_de_Lyon,_INSA_de_Lyon,_MATEIS_CNRS_UMR5510,₇_Av._Jean_Capelle,₆₉₆₂₁_{Villeurbanne,}_France b_Department_of_Materials_Science_and_{Metallurgical}_Engineering,_Universitat_Politècnica_de_Catalunya,_C/Eduard

Maristany,10-14,08930Barcelona,Spain

c_Department_of_{Neurosciences,}_Reproductive_and_{Odontostomatological}_Sciences,_{Prosthodontic}_Area,_University

“FedericoII”,NapoliItaly

d_Department_of_{Prosthodontics}_and_Dental_Materials,_University_of_Siena,_V.le_Bracci_1,_57100,_Italy

a

r

t

i

c

l

e

i

n

f

o

Articlehistory:

Received15July2017 Receivedinrevisedform 24February2018 Accepted12March2018 Keywords: Cubiczirconia Hydrothermaldegradation Monolithiczirconia Dentalcrown Translucentzirconia Aging

a

b

s

t

r

a

c

t

Objectives.Thedentalmarketmovestowardshigh-translucencymonolithiczirconiadental crowns,whichareusuallyplacedeitherwith–orwithout–athinglazelayer.The microstruc-turalfeaturesandthemechanicalperformancesofthesematerialsarestillcontroversial,as wellastheirsusceptibilitytoaging.Thispaperaimsatstudyingtheseaspectsinthecurrent generationofzirconiadentalcrownsshowingdifferentdegreesoftranslucency.

Methods. Fourdifferentcommercialzirconia materialswere investigated,including one standard ‘full-strength’ 3Y-TZP and three grades with improved translucency. The microstructuralfeatures(phase compositionandassemblage,grainsize)werecarefully studied,aswellasmechanicalproperties(biaxialbendingstrengthandindentation tough-ness),translucencyandagingbehavior(inautoclaveat134◦C).Agingwasconductedon crownswithandwithoutglazetobetterrepresentclinicaluses.

Results.Importantdifferencesarefoundintermsofmicrostructuresamongthematerialsin termsofcubicphasecontentandyttriainthetetragonalphase,leadingtodifferentoptical, mechanicalandagingresistanceproperties.Weshowthathighercubicphasecontentleads tobettertranslucencyandstabilityinwatersteam,butattheexpenseofstrengthand toughness.Acompromiseisalwaysinevitablebetweentranslucencyandagingresistance ononesideandmechanicalpropertiesontheotherside.

Signiﬁcance.

- Evensocalled‘hightranslucent’zirconiaceramicstestedinthisworkshouldbe consid-eredasmediumtranslucencymaterials.

- Agingoccursinstandardstate-of-the-artdentalzirconiaandglazingdoesnotfullyavoid thisissue.However,agingdidnotcompromisestrengthevenafterprolongedduration.

∗ _{Corresponding}_author.

E-mailaddress:jerome.chevalier@insa-lyon.fr(J.Chevalier). https://doi.org/10.1016/j.dental.2018.03.006

(2)

Amongceramicmaterials,polycrystallinetetragonalzirconia stabilizedwith3mol.%ofyttria(3Y-TZP)hasbecomeone ref-erenceindentistry,duetoitsexcellentmechanicalproperties andgoodaestheticsthankstoitswhitecolorandtooth-like appearance[1].Especiallyintermsofstrength,3Y-TZPisthe onlysingle-oxideceramicthatguaranteesvaluesabove1GPa inbending,allowingthedesignofthin-walledandmulti-unit restorations[2].

Nonetheless,when zirconiawas introducedin the mar-ket,its opacityobliged dental technicians toapply several layersofveneeringceramicsinordertoincreasethe super-ﬁcialtranslucency.Indeed,polycrystallinetetragonalzirconia exhibitsmoderatetranslucency:sincetherefractiveindexn

isdifferentalongthemaincrystalaxesofthetetragonal sym-metry,bothreﬂectionandrefractionoccuratgrainboundaries leadingtoareductionofthelighttransmittance[3,4]. How-ever,veneeringisasensitiveandtime-consumingprocedure, eventuallyincreasingtheriskofchipping[1–5].

Moretranslucentfull-ceramicmaterials,basedon leucite-reinforced glass-ceramics (ﬁrst generation) and lithium disilicate(secondgeneration)werethendeveloped success-fully,ascompetitivealternatives tozirconiaintheanterior sites[6,7]. Evenifthe strength ofthesematerials (approxi-mately350–400MPainthecaseoflithiumdisilicate)ismuch lowerthan3Y-TZP,theycanbeusedwithlittleorno veneer-ing,soofferinginsomecasesacomparableperformancefor the sametotal wallthickness[7].Some zirconia-reinforced lithiumsilicateglass-ceramicshavebeenrecentlyintroduced formonolithicrestorationsandshowanincrementalbut sig-niﬁcant increaseof their strength,with reportedvalues of morethan400MPa[8].

In the quest towards a higher translucency of zirconia restorations,novelzirconiagradeshavebeendevelopedand commercializedinthelastyears[9,10].Moreover,pre-sintered blocks are both available as pre-shaded in several tonali-tiesandmultilayeredwithgradientsofchromaanddifferent degreesoftranslucency[11].Thankstotheimproved aesthet-ics,therestorationsmadewithsuchmaterialscanbeplaced without any veneering ceramic(a thin,few microns thick, glazecoatinglayermaybeaddedinvisibleregions),allowinga furtherreductionoftherestorationthickness.Moreover,such so-called‘monolithic/full-contour’restorationsallow decreas-ingtherisksofchipping,theprostheticcostandproduction time[12].

Obtaininga highertranslucency inzirconia can be pur-suedwithseveralstrategies:(a)reducingtheresidualporosity, for example by adding a glassy phase or some sintering

ablythegrainsizesotohavelessgrainboundaries[12],and/or (d)introducingsignificantamountsofcubicphase,whichis opticallyisotropicanddoesnotinducebirefringence[4].All thesestrategiescanbefoundindifferentcommercial zirco-niawithaclaimedimprovedtranslucency.Inparticular,we maynowfindnovelyttria-stabilizedgradeswithahigher con-tentofyttria,whichinturnsleadtoahighercontentofcubic grains[14].However,notonlydothesemicrostructural varia-tions implysubstantial improvementsintheoveralloptical performanceofzirconia, but theyalso inevitablyintroduce noticeablechangesinitsmechanicalpropertiesandlong-term stability,asrecentlyshownbyZhangetal.[15].Concerning long-termstability,ithastoberemindedatthisstagethat (3Y-TZP)isknowntobesusceptibletotheso-called‘hydrothermal degradation’,asuperficialagingphenomenonthatmaytake place,dependingonmicrostructure,compositionandstress state,whenthesurfaceisexposedtohumidenvironmentand moderatetemperatures,includingthosefoundinthehuman body [16]. This agingprocess consists inaslow tetragonal tomonoclinictransformationofthegrainsatthesurfacein contactwithwatermolecules[17].Thissurface transforma-tion isassociatedtotheformationofupliftsonthesurface andeventuallymicro-crackingandgrainpull-out,whichmay induceaprogressivedeteriorationofmechanicalproperties [18].

The ﬁrst objective ofthis work is to better understand hownewlydeveloped‘highlytranslucent’zirconiagradesare designedinterms ofphaseassemblage(cubicand tetrago-nalphase content)and grain sizetomeet thedemandfor monolithictranslucentzirconiarestorations.Special empha-sisisgiventonovelso-called‘cubic’zirconia,whichhavea limitedfollow-upintermsofmicrostructure-properties rela-tions,includingagingassessment.Itisalsoouraimtostudy themechanicalpropertiesandagingresistanceofthesenew zirconiagradesincomparisonwithstandard3Y-TZPgrades. Mostofagingstudiesofdentalrestorationmaterials(andthis appliestoRef.[15])aregenerallydoneonﬂatlab-scalesamples andliteratureissparseconcerningrealrestorationsfrom cur-rentzirconiagradesafterachair-sideclinically-representative preparation, whilesurface preparationis known to havea potentialeffectonagingrates[19].Inordertogoastep for-ward, aging studies were thus performed on real crowns, preparedinthesameconditionthanforclinicaluse,withor without the presenceofathin glaze thatisoftenusedfor aestheticalreasons.

(3)

Table1–Summaryofspecimens’labelingandpreparationprocedures. Label Commercial

name

Category NominalY2O3 content (mol.%)

Al2O3 content (wt%)

Heating Hold Cooling

ST AadvaST Full-strength 3 0.2 8◦C/minto

900◦C+10◦C/minto 1500◦C 2h 8◦/min toTamb EI AadvaEI Enhanced translucency 3 0.05

NT AadvaNT Hightranslucency 5.5 0.05 2h 10◦C/mintoTamb

ML KatanaUTML Hightranslucency

multi-layered

>6 Un-known 10◦C/minto1550◦C 2h 10◦C/mintoTamb

- Higher translucency in newly developed zirconia grades mightbeduetoanincreaseingrainsizeand/oranincrease ofthecubicphasecontent,

- Thischangeinmicrostructuralfeaturesshouldmodifytheir stabilityundermoistureandtheirmechanicalproperties, - Thepresenceofaglazeinmonolithicrestorationsmay

mod-ifytheagingprocessbyprovidingaprotectionlayer,ifthe glazeisabletocovertheentiresurfaceincontactwithwater.

2. Experimental

2.1. Materials

Fourdifferentcommercialzirconiamaterialswerestudied: - Oneconventional‘full-strength’3Y-TZPgrade“AadvaST”

(StandardTranslucency–STgroup),

- Onewithimprovedtranslucency“AadvaEI”(Enamel Inten-sive–EIgroup),

- Onehighlytranslucentandpartiallycubic“AadvaNT” (Nat-uralTranslucent–NTgroup),

- Onehighlytranslucent,partiallycubicandmulti-layeredin chroma“KatanaUTML”(UltraTranslucentMultiLayered).

Allthematerialswereprovidedintheformofcommercial pre-sintered98.5mmdisks.Theﬁrstthreezirconiaceramics wereprovidedbyAadva,GCTech,Leuven,Belgiumandthe fourthonebyKurarayNoritakeDentalInc.,Aichi,Japan.From thesepre-sinteredblocks,bothfull-contourmolar-crownsand diskswereprepared,accordingtotheproceduresdescribed below.

2.2. Samplepreparation:experimentalcrownsand disks

Foreachmaterial,full-contourmolar-crownspreformswere milledbyCAD-CAMtechnologyfromthepre-sintereddisks. Toobtain thesepreforms, human, sound, intact mandibu-larthirdmolarsextractedforperiodontalreasonswereﬁrst preparedaccordingtothefollowingstandardizedpreparation geometries:0.5mm circumferential chamferplaced0.5mm above the cementum-enamel junction, axial reduction of 1.2mm, total occlusal convergence of 10◦, occlusal reduc-tion of 1.5mm; all preparation angleswere rounded. Each preparedtoothwas thencoveredwithapowderfordigital scanning(CerecOptispray,SironaDental,Salzburg,Austria) andthree-dimensionally scannedbymeans ofalaboratory opticaldigitalscanner(GCAadvaLabScan,GC,Tokyo,Japan).

The3Dshapeofeachtoothwasdigitized,soastouseitfor thefabricationofCAD–CAMmonolithiccrownsbymeansofa CADsoftware(Exocad,DentalCAD,ExocadGmbH,Darmstadt, Germany).Thefollowingconfigurationofthedigitaldieswas used:coreminimalthickness0.6mm,cementlayerspacing 0.05mm,cementlayerstarting1.3mmfromthefinishingline. Thepreformswerethensinteredaccordingtothe manu-facturer’sspecifications,assummarizedinTable1.Onehalf ofeach crownwas manuallyglazedwithaglaze thickness ranging between 10 and 100␮m, whilethe other half was leftuncoated(seeFig.1).Twodifferentglazesproposed com-merciallywere used:CeraFusion(Komet), basedonlithium disilicate, whichwasfiredat1005◦C during15, andMagic GlazeFlu,firedat900◦Cduring15(WielandZenostar,Ivoclar Vivadent).

Similarly,20disks1.5mminthicknessand12mmin diam-eterwerealsopreparedwithCAD-CAMtechnologyfromeach material.GCdiscsweremilledattheAadvaCAD-CAM Produc-tion Centre(GCEurope,Leuven,Belgium)whiletheKatana discs were milled with Roland DWX-50 milling machine (RolandDGACorporation,Irvine,CA,USA).Theyweresintered underthesameconditionsasthecrowns,soastoexhibitthe samemicrostructureandphaseassemblage.Thespecimens werepreparedandwet-ﬁnishedinagrinder/polishermachine with600gritpaperuntiltheexperimentaldimensionswere obtained.Then,thespecimenswerewet-polishedwith1200 and2400gritpaperandmirror-polishedwithdiamondpastes (downto1-␮m).Attheendoftheprocess,theroughnessRa wasmeasuredtobelessthan20nm.Finally,thespecimens wereultrasonicallycleanedindistilledwaterfor10minbefore measurementprocedure.

Thedisks wereusedformechanicaltests aswellasfor translucencymeasurement(afterreducingtheirthicknessto 1mminthatcase,andthenwiththesamepolishing proce-dureasdescribedabove).Fig.1showstheappearanceunder transmittedlightofthe4typesofspecimens,obtainedwith astereoscopeOlympusSZX10underthesameillumination andbackgroundconditions.

2.3. Methods

The experimentalcrowns and disks underwent a seriesof characterization,assummarizedinTable2.Diskswereused to characterizethe grainsize, phaseassemblage and com-positionsincegrainsizemeasurementandX-raydiffraction (XRD)analysisaremoreaccurateonﬂatsamples.Theywere alsousedtomeasurethemechanicalproperties(strengthand indentation fracture toughness).Onthe other side, crowns wereusedtomeasureartiﬁcialagingkineticsinautoclaveand

(4)

Fig.1–(a)Appearanceofthe1mmthicksamplesafterpolishingundertransmittedlight,obtainedwithastereoscope OlympusSZX10underthesameillumination,captureandbackgroundconditions.Fromlefttoright:ST,EI,NT,ML.(b) Imageofafull-contourmolar-crown,withonesideglazedandtheothernot.

Table2–Summaryofcharacterizationmethodsconductedonexperimentalcrownsordisks. Sample

geometry

Grainsize Rietveldanalysis

Translucency Indentationfracture Biaxialstrength

Agingkinetics inautoclave

FocusedIon Beamafteraging

Crowns X X

Disks X X X

theywerealsoinspectedbyFocusedIonBeam(FIB)afteraging, asdescribedbelow.

2.3.1. Microstructure,phaseassemblageandcomposition The microstructures were analyzed by scanning electron microscopy (SEM)on the disks surface after polishing and thermaletchingat1200◦C,1h.

XRD patterns were collected on disks and analyzed for different incident angle ranges with a Bruker D8 Advance equipment,usingaCuK␣radiation.10–75◦2patternswere collected from polished disks to quantify the respective amountoftetragonaland cubicphasesintheas-processed crownsbyaRietveldanalysisemployingthesoftwareTopas 4.0.TheXRDdiagramswerecalculatedusingbothcubicand tetragonalphasesofzirconia.Aﬁrstcalibrationoftheir lat-ticeparametersversus Yttriumconcentration[YO1.5],using datafromScott[20,21]andfromPDFﬁles[22],showedthat thelatticeparametersvaryas:

2at.√2=5.078495599+0.003767349·[YO1.5]

ct=5.192521486−0.002903167·[YO1.5]

ac=5.113195432+0.001731149·[YO1.5]

Thus,inthetetragonalphase,atandctlatticeparameters

whereconstrainedusingthefollowingequation: ct=9.10607452−1.08981088.at

Rietveld calculationofthe latticeparametersconducted usingthismethodgaveaccesstotheyttriaconcentrationin eachphase.Italsoenabledtocalculatetheconcentrationof bothcubicandtetragonalphases.

2.3.2. Agingkinetics

26–33◦2XRDscanswereperformedoncrowns(bothglazed and non-glazed)atleveloftheoutersurfaceattime0and after2,6,18,54hofartificialaginginautoclave(134◦C,2bar steampressure).Suchacceleratedagingtestinautoclaveis proposedintheISO13356standardonflatsamples,upto5h duration[23].Inthiswork,ourgoalwastoassessagingkinetics ontherealfinalproduct(asagingmaybesensitivetosurface preparation)anduptolongerduration,keepinginmindthat onehourofsuchanacceleratedtreatmentat134◦Cisroughly equivalentto2–4yearsat37◦C[17].Thesurfacemonoclinic content was quantifiedbymeasuringthe areas ofselected peaksandapplyingtheformulasofGarvieandNicholson[22] andTorayaetal.[24].Between26and33◦,thevolumefraction ofmonoclinicphaseisgivenby:

Vm= 1.311Xm

(5)

Fig.2–SEMimagesshowingthemicrostructureofthe4materialsstudied:ST,EI,NT,ML. where Xm= I ¯111 m +I111m I¯111 m +I111m +I101t withIhkl

p istheareaofthediffractionpeakofthephasep

relatedtotheplane(hkl).

Whenanalyzingtheglazedsurface,adeeperpenetration oftheX-raybeamisneeded.ThusXRDexperimentswerealso performedathigherangles,between53◦and65◦2.

Inthiscase,themonoclinicfractioniscalculatedas: Vm=_R₊R₀_.26, with R= 0.448A4 A5−A6, with: A4=Im(003)+Im(221)+Im(122)+Im(310)+Im (−311)+Im(031)+Im(−113)+Im(−131) A5=Im(−222)+Im(131)+Im(−203)+It(103)+It(211) A6=Im(311)+Im(−312)+Im(113)+It(202)

These areas cover the respective angular range: 54◦≤A4<58◦; 58◦≤A5<61◦; 61◦≤A6<64◦. This method isbasedon therelative intensitiesoftheabove diffraction linescalculatedintheoreticaldiffractiondiagrams.

Inordertostudyindetailsthelocaleffectofaglazingon aging, FocusedIon Beam(FIB,nVision40dualbeamstation –CarlZeissAG,Germany)wasemployedtocutandpolisha trenchbetweenaglazedareaandanon-glazedspotofselected aged crowns,after depositing a stripof protectiveCarbon. Thesub-surfacemicrostructure wasthen observedby Sec-ondaryElectron(SE) andbackscattered(BSE)images,which canrevealthepresenceofphasetransformationafteraslight imagetreatment.

Measuringmechanicalpropertiesofcrownsispossible,but islessreproduciblethanmaking simplebiaxialtesting and needsspecialcautionintheanalysis,especiallywhenweare willing toobtain astrength data. Therefore, aging kinetics weremeasuredoncrownsbuttheeffectofprolongedagingon themechanicalintegrityofthematerialswasdoneonsimple geometries.10STand10EIdisksweresubjectedtoartiﬁcial degradationfor18hinordertoassesspotentialeffectofa pro-longedagingonbiaxialbendingstrength,obtainingsamples that werelabeled, respectively,ST-A andEI-A. Indeed,only thesematerialswereshowingsigniﬁcanttetragonalto mon-oclinictransformationonexperimentalcrowns(seeSection 3).

2.3.3. Mechanicalandopticalproperties

Biaxialstrengthwasmeasuredondisksbymeansof piston-on-three-ballsstrengthtests,accordingtoISO6872standard using10diskspergroup.Thefracturesurfaceofaged materi-alswasobservedbySEM.Indentationfracture(IF)toughness andhardnesstestswereperformedonpolisheddisksurfaces

(6)

Fig.3–XRDrepresentativepatternsofthematerials studied.

byusingaVickersindenterandanappliedloadof10kgfor 10s.TheIFtoughnessvalueswerecalculatedfromthe sur-facelengthofthecracksdevelopingatthecornersofVickers indentusingtheformulaproposedbyNiihara[25].These val-uesaremeanttohaveonlyacomparativesigniﬁcance,since thispracticalmethodpresentsseverallimitations,especially whenappliedtophase-transformingmaterials[26].

Total transmittance (Tt%) and Contrast Ratio (CR) were measuredbya spectrophotometer(JascoY-670, France),on thedisksafterhavingreducedtheirthicknessto1mmand finepolishing(1-␮mdiamondpaste).TheCRgivesan indi-cationoftheopacityandistheratiobetweentheluminous reflectanceofa specimenover a blackbackground tothat overawhite backgroundofaknownreflectance.CRranges from0(totaltranslucency)to1(totalopacity)[27].Thevalues measuredat555nmwavelengthwerechosentocomparethe differentmaterials,accordingtothedefinitionofthe Interna-tionalCommissiononIllumination[28].

Theresultsweretreatedstatisticallywithone-wayanalysis ofvariance(ANOVA)withaTukeypost-hoctest,employingthe softwareMinitab.p-valuewassetat0.05.Meanvaluesplus standarddeviationsarepresentedintheresults,whileletters onthechartsindicatethestatisticallysigniﬁcantdifferences.

3. Results

SEM images of the samples microstructure are shown in Fig. 2. Thegrain size, measured by the intercept method, is reported in Table 3. ST shows a classical grain size of 0.43␮mfora3Y-TZP,whilethisvalueisreducedto0.33␮m inEI. Both microstructuresare quitehomogenous, whilea bimodalappearanceisfoundinthehighlytranslucent mate-rials,wherethegrainsizeismuchbigger,especiallyinML.

TheresultsofContrastRatio(CR)andtotaltransmittance (Tt%)measurementsarereportedinthesameTable3,showing signiﬁcantdifferencesinbothparameters.Inorderof increas-ingtranslucency,weﬁnd:ST,EI,ML,NT.

Fig.3showsXRD patternstakenfrom polished surfaces ofthefourmaterialsbeforeagingtreatments.STanEIhave similarpatternsrepresentativeofthetetragonalphase (dou-bletsaround 2=35◦ and 2=60◦),whileinNTand MLthe tetragonaldoubletsalmostfusetogetherinsingle,largepeaks.

Fig.4–Evolutionofthemonocliniccontentmeasuredby XRDafterartiﬁcialaging.

Theselargepeaksareinfacttriplets,constitutedbythesum of tetragonal doublets and a single, dominant cubic peak betweenthedoublets,representativeofalargevolume frac-tionofcubicphase.Duetothelargeryttriacontentpresentin NTandMLtetragonalphase(thusasmallertetragonality),the tetragonalpeaksarelessseparatedthaninSTandEI.Rietveld analysisshowedthatallthematerialsarecomposedbya mix-tureofcubicandtetragonalphases,whereapproximately30% of cubic phaseis found inhighly translucent crowns, and about20%isfoundbothinSTandEImaterials,asindicated inTable3.

The aging behavior of the four classes of materials is reportedinFig.4,intermsofmonoclinicvolumefractionat thesurfaceofthenon-glazedsideofthecrownversustime.

NTandMLarefullyresistanttophasetransformationfor theagingtimesappliedhereandnomonocliniccontentwas observedevenafterprolongedduration.Ontheotherside,ST andEImaterialsshowsigniﬁcantphasetransformation, start-ingfromveryshortagingtimes(2h).EIisagingfasterthanST until18hofexposure,whiletheyreachsimilarvaluesat54h. Ithastobeconsideredatthatstagethatthepenetrationof theX-raysisabout5␮minzirconia.Moreover,themonoclinic contentobtainedbyXRDisaconvolutedvalueforwhichthe surfacehasahigher‘weight’.Inotherwords,XRDcapturesthe t-mtransformationmostlyfromtheﬁrst1–2␮m.After54hof aging,itislogicaltoseeasaturationofthemonoclinic con-tent,becausethismeansthatallthetetragonalgrainshave transformedonthis5␮m-thicklayer.

Diffractionpatternstakenfromtheglazedsideofdental crowns were readable as well and allowedtomeasure the amount ofmonoclinic phase, since X-rays could generally penetratethroughtheglazelayerandreachzirconiasurface. Insomethickerglazesections,26–33◦2readingsdidnotshow zirconiapeaks,meaningthattheXRDbeamdidnotreachthe zirconiasubstrateforthoseangles.53–65◦2wereusedinthe lattercase.

(7)

Table3–Grainsize,phaseanalysisafterRietveldreﬁnementandopticalproperties(CRandTt%)forthefourmaterials studied.

Grade Grainsize(nm) Group Tetrag.(%) Cubic(%) %Y2O3intetrag. CR–group Tt%–group

ST 432±40 a 76.8 23.2 2.4 0.74±0.01a 36.9±0.15a

EI 333±27 b 80.3 19.7 2.5 0.70±0.01b 38.4±0.07b

NT 770±93 c 67.8 32.2 6.1 0.62±0.01c 43.4±0.13c

ML 1718±327 d 68.9 31.1 5.9 0.69±0.01d 36.0±0.07d

Fig.5–FIBstudyofanunglazedspotlocalizedinthehighlightedareaof(a),inthemesialsurfaceofanEImonolithic crown.Thespotisobservableathighermagniﬁcationin(b),wherethelineindicatethelocationofthetrench,andthe arrowsthedirectionofFIBmilling.In(c),theunglazedspotidvisibletotheleftwherethearrowsindicatethedirectionof spreadingofthedegradationfromthespot.Thepresenceofglazeisalsorecognizablebythedarkercolorwithrespectto thezirconiasurface.ByimagingtheFIBtrenchwithbackscatteredelectrons,thetransformedvolumearoundthespot becomesclearlyvisiblein(d).Unaffectedmicrostructureisobservableontherightsideofthetrench,wellbelowtheglazed region.Thedashedlineseparatestheagedandnon-agedzirconia.Microstructuraldifferencesareillustratedinthedetail pictures(e)and(f).

No monoclinic phase was found in any of the glazed zirconiaceramics,whatever thetypeofglaze (‘CeraFusion’ or ‘MagicGlaze Flu’), even after prolonged aging exposure, exceptforsomeisolatedcases.Opticalmicroscopy observa-tions revealed that in these cases a signiﬁcant portion of thesurfacewasleftunglazed.Thesubsurfacearoundsome unglazedspotsofanEIcrownwasthereforestudiedbyFIB (Fig.5)after54hartiﬁcialaging,whereSEManalysisrevealed thataging progressedfrom the exposedspot bothtowards thebulkandthesides,inahemisphericalvolumeofradius ∼16␮m.Atthesametime,noagingwasfoundattheinterface zirconia-glazebeyondthisdistance.

Fig.6summarizesthemechanicalpropertiesobtainedfor thedifferentmaterials.STexhibitstrengthvalueswellabove 1000MPa,whileslightlylowervaluesarereportedforEI.For NTandMLthestrengthissimilar,around450MPa.

Hardnesswassimilarforthefourmaterialstested, observ-ing minor differences of less than 0.5GPa. In order of increasinghardness,weﬁndST,EI,ML,NT.IFtoughnesswas similar between ST and EI (∼4.7MPa√m) and signiﬁcantly lowerforbothNTandML(∼3.8MPa√m).

Thebiaxialstrengthdidnotshowanyvariationafter18hof artiﬁcialagingforST-Amaterials,whileresultssuggestaslight increase(evenifnotstatisticallysigniﬁcant)forEI-Asamples. InthecaseofNTandMLmaterials,strengthtestswerenot performedafteragingsinceitwasshownthatthesedonot undergoanyphasetransformationfortheapplied hydrother-malexposure.

SEM observations performed on the fracture surface of 18hours-ageddisks showed amixed inter/transgranular appearanceinthebulkregionofbothofthem.Closetothe surfacethatwasputintensionduringthestrengthtest,the markedintergranularappearanceproducedbymicrocracking

(8)

Fig.6–Biaxialstrengthbeforeandafter18hartificialagingforthefourmaterialsstudiedarereportedin(a),wherethe suffix“−A”indicatesagedmaterials.HardnessandIFtoughnessvaluesareshownin(b)and(c),respectively.Letters indicatestatisticallysignificantdifferences.

(9)

Fig.7–FracturesurfaceclosetothetensileedgeforST-A(a,b)andanEI-A(c,d)representativesamples,respectively. Dashedlinesindicatetheidentiﬁedborderbetweentheintergranularappearanceinthesuperﬁcialagedlayerandthe mixedinter/transgranularappearanceinthebulk.

intheagedlayerwasrevealed,allowingtoestimateits thick-ness,asshowninFig.7.Thisthicknesswasof∼2␮mforST-A andof5–6␮mforEI-A,respectively,whichconfirmsprevious XRDfindingsonthecrowns:agingissignificantlyfasterforEI samplesthanforSTsampleseitherintheformof experimen-talcrownsordisks.

4. Discussion

4.1. Microstructure,composition&translucency

Important differences are found in terms of microstruc-tures among the materials studied. The ﬁrst reason is indeed the yttria content, which is higher in NT and ML (at least ∼5.5mol.%)with respect toST and EI(∼3mol.%). Theseadditionsallowobtainingacombinedtetragonal-cubic microstructure(inagreementwiththezirconia–yttriaphase diagram[20,21])wherethetranslucencyimprovementshould betwofold:cubicgrainsdonotexhibitbirefringencesincethey areopticallyisotropic;moreover,beingthetetragonalgrains

supersaturatedinyttria,theirtetragonalityisreducedandso aretherefractiveindexandtheamountofbirefringence[4].

Adifferenceingrainsizeanddistributionexistsbetween MLandNT, whichcanbeattributedtothedifferent sinter-ingprotocols.TheirCRandTt%isalsosigniﬁcantlydifferent, probably in relation to the presence ofa chroma gradient in ML, as it has been shown in the case of colored and non-colored 3Y-TZP [29]. At the end, the potential better translucency ofMLisnottranslatedonmeasured Contrast Ratios(beingquitesimilartoSTandEI).

ThemaindifferencesbetweenSTandEIarethegrainsize, whichissmallerforEI,andthecomposition.Eveniftheyhave the same amountofyttria, theamount ofaluminais sub-stantiallylowerforEI(0.05wt%).Aluminagrainsarerandomly presentinST,likeinotherstandard3Y-TZP,duetothefactthat tetragonalzirconiaisgenerallysupersaturatedinaluminato improveitsagingbehaviorandreinforcethegrainboundaries [15,30].Anexample isobservableinFig.1a,wheretwo alu-minagrainsappearindarkershade.Therefore,theobserved substantialimprovementinCRandTt%forEIwhencompared

(10)

andtheirapplicationasmonolithiccrownsinaestheticfrontal areasmayﬁndsomelimitations,evenforNTandMLgrades.

4.2. Agingbehavior

The ﬁrst and obvious evidence is that highly translucent zirconia with at least 5.5mol.% yttria do not suffer from hydrothermaldegradationevenafter54hofartiﬁcialaging.It canthereforebestatedthatthesematerialsarefullyresistant toagingunderinvitroconditionsandnoexternalloadapplied. Thisimpliesthatthetetragonalphasemustbesupersaturated inyttriawithrespectto3Y-TZP,andsolesstransformable,as demonstrated byourRietveld analysis.Thisis trueforthe thermal treatments appliedhere, whilea similar behavior cannotbeassuredforlongersinteringtimesorhigher tem-peratures.Infact,theenhanceddiffusionprocessinthelatter casewouldpromoteamorecompleteseparationoftetragonal andcubicphases,thecubicvolumefractionwouldincrease, pumping yttrium from the tetragonal phase, which would becomelessstable,and,eventually,vulnerableto hydrother-maldegradation[32–34].

Anotherimportantpointtounderline isthat hydrother-maldegradationwasobservedinSTandEIstartingfromquite shortagingtimes(2h).HavingEIsmallergrainsizethanST,a sloweragingprocesscouldexpectedbecauseofalower trans-formability[35].Nonetheless,theobservedkineticsisfaster forEI,whichimplies that theeffectofaluminaaddition is moreimportantthanthedifferenceingrainsize,asreported inRefs.[15,36].

Itisshownthatglazingeffectivelyprotects3Y-TZPagainst agingforthetwo glazesexamined hereand evenfor glaz-ingthicknessesoffewmicrometersand54hofhydrothermal exposure.Ontheotherhand,itwasobservedthataperfect glazing,coating100%ofthesurface,ispracticallyvery diffi-cultorevenimpossibletoobtain.Thiscanbetheconsequence of:(a)insufficientcoverageduringtheglazeprecursor appli-cation,especiallyforlithiumsilicateglazesthatareappliedby sprayinformofpowder,whichtendtoleavesomeunglazed spots inassociation with poorwetting duringthe applica-tion or de-wettingduringthermal treatment (assuggested byFig. 5)or difficulty ofglaze applicationon the marginal edge;(b)chippingoftheglaze,whichmayoccurduring cool-ingdue tothermalresidualstresses(associated tothefact thatzirconiaandtheglazedonotexhibitthesamethermal expansioncoefficient).Therefore,ifthematerialis suscepti-bletohydrothermaldegradation,afterglazingitwillinevitably undergotheagingprocessinsomelocalizedareas,asithas beendemonstratedbytheFIB-SEMobservationssummarized inFig.5.Agingwillspreadfromthenon-glazedspotinradial

isthatthisprocesscreatesdamagedlocalizedspotsinstead ofauniformdegradedlayer.Whileinthecaseof polished-and-agedzirconiathemechanicalstrengthof3Y-TZPisonly slightlyaffectedafterlongandseverehydrothermalexposure, thepresenceofthesemicro-crackedpitscouldleadtostress concentrations,whichmighthavecriticalconsequences espe-ciallyinthemarginalarea.Itcouldbeinterestingtofurther studytheimplicationofthisﬁndingbylookingattheinﬂuence ofthis‘pittingagingprocess’onthestrengthofagedsamples.

4.3. Mechanicalproperties

Theobservedslightvariationsofhardnesscanbeattributed tovariationsofmicrostructureandphasecomposition.Itis well knownfromtheHall–Petch relationthat, forthe same composition,thesmallerthegrainsize,theharderthe mate-rial,asithappensforEIwithrespecttoST.NTisevenharder duetothehighamountofcubicphase,whichisknowntobe harderthanthetetragonalone[38].MLhasalowerhardness withrespecttoNTduetothebiggergrainsize,beingthecubic phasecontentapproximatelyequalforboth.

Important differences were recorded in terms of biax-ial strength, showingthat ST and EI are atleast twice as strongasNTandML,implicatingahighdifferencebetween full-strengthandlastgenerationhighlytranslucentzirconia ceramics. Thisprovesthatthe tetragonalphase inNTand MLisover-stabilized,havingpoorornotransformabilityfor inducingthebeneﬁcialeffectsofthetransformation tough-eningprocess.Thebiaxialstrengthforthepresentlystudied highlytranslucentzirconiagradesisjustlittlebetterthanfor the best glass-ceramicsavailableon themarket,which are indeedmoretranslucent[7].Therefore,itisimportantforthe dentisttoavoidexpectingasimilarmechanicalperformance from both ‘full-strength opaque’ and ‘translucent’ zirconia ceramics.BothSTandEIfulﬁlltherequirementsfordental ceramicscapableofsupporting4andmoreunits,eventhough thebiaxialstrengthisslightlylowerforEI.Thedifferencein strengthmightbeexplainedbythefactthatsmallergrainsize meansnormallylowertransformability,soEIisless mechani-callytransformablethanST.Inaddition,amoreintergranular appearancewasreportedforfracturesurfacesof3Y-TZPwith 0.05wt%aluminawithrespecttocompositionswith0.25wt% alumina,meaningthatinthelattergrainboundariesare rein-forced[36].Thisaspectmayalsogiveacontributiontothe observedstrengthdifference.

NostrengthvariationwasrecordedforSTafter18haging exposure,whileaslightincreaseintheaveragestrengthwas measuredforEI.Althoughthisdifferenceisnotsigniﬁcant,ST

(11)

andST-ApertaintothesamestatisticalgroupasEI-A,while EIdoesnot.Asdemonstrated,bothSTand EIare suscepti-bletohydrothermalt-mtransformation,with∼35Vm%and ∼45Vm%,respectively,recordedbyXRDat18h.Althoughthe monoclinicphasecontentmeasuredbyXRDinnotvery differ-ent(mainlybecauseXRDonlycapturestheveryfirstmicrons oftransformation,asdiscussedabove),thethicknessofaged andmicrocrackedlayerrevealedbythefractographicimages showninFig.7issignificantlyhigherforEI-Asamples.The differentmechanicalbehaviorcanthereforehavetwo possi-blejustifications:(a)theagedlayerinducessomedegreeof“tip blunting”forthebiggestdefects/cracksnaturallypresent in thematerial[39]:inthiscase,eithertheinitialdefectsofEIare differentinnaturethantheonesofSTandsomoreresponsive tothebluntingeffect(bigger,sharper),orthehigheramount ofmonoclinicphaseinEI-Ainducesadeeperbluntingeffect; (b)sinceaginghasprogresseddeeperinEI-A,thecompressive layerwhichnormallyaccompaniestheagingprocessinthe transitionzonebetweendegradedandnon-degradedmaterial isplacedclosertothecriticaldefectstip,inducinga reduc-tion in their stress-intensity factor. These results show at theendthat hydrothermaltransformationdoes notinduce necessarilyadecrease instrength.Theamountofstrength degradation or improvementis dependent on compressive layerthicknessversusdefectsizeratio andon theamount ofmicro-crackingintheprocesszoneandanygeneralization wouldbeun-correct.

IFtoughnessresultssupporttheabovediscussion, indicat-ingsensiblylowervaluesforNTandMLwithrespecttoSTand EI.Thisdifferencecanbeinterpretedaslower transformabil-ityforthehighlytranslucentmaterials.Ontheother hand, IFtoughnessresultsweresimilarbetweenSTandEI,sothe differenceinbiaxial strength betweenthetwo groupsmay eventuallybenotimputabletoadifferentmechanical trans-formability.Nonetheless,weprefernottospeculatefurtheron thisaspectduetotheintrinsiclimitationsoftheIFtoughness testanditsprecision,whichhavebeenextensivelydiscussed (disputed)byQuinnandBradt[26].

4.4. Clinicalimplications

Someconcernssurroundtheapplicationofzirconiain den-tistry due to the risk of hydrothermal degradation. In the presentstudy,ithasbeenshownthathydrothermal degrada-tiontakesplaceinstate-of-the-art3Y-TZPdentalprosthetic applicationspreparedforclinicaluse.Thisphenomenonwas notdetrimentalforstrengthinthestudiedmaterials,inthese invitroexperimentalconditions,evenafterlongexposures. Althoughthepossibility that cyclicfunctionalor parafunc-tionalloadscouldinducemoresevere–andmaybelocalized –agingprocessstillremainsanopenquestion,theseresults arehighlypositiveasfarastheclinicaluseofzirconia den-talrestorationsisconcerned.Giventhelimitationofin-vitro studiestofullycapturethecomplexclinicalsituation,future clinicalstudiesandanalysesofretrievedzirconiarestorations afterinvivoservice,asproposedrecentlyinRef.[40],willbe necessarytogiveadeﬁniteanswertothisquestion.

On the other hand, the use ofhighly translucent, non-transformablezirconiamaterialsshouldbecarefullyassessed. If it is true that these materials may be more

translu-cent than standard 3Y-TZPand donotsuffer fromin vitro hydrothermaldegradation,theirtoughnessandstrengthare dramaticallylower.Therefore,manipulationandcrown prepa-rationshouldbedonecarefully,avoidingthinwallsandsharp edgesasmuchaspossible.Havingsimilar(orlittlebithigher) mechanical strength than lithiumdisilicate glass-ceramics but lower translucency,the latteror noveloptions suchas zirconia-reinforcedlithiumsilicateglass-ceramicmuststillbe consideredasanalternativechoicetothesematerials.Other aspectsshouldhoweverbetakeninconsiderationasitisthe caseofdissolutionandwearresistancetofullycomparenewly developed‘cubic’translucentzirconiaandlithiumdisilicatein aglobalpicture.Anyway,theterm‘zirconia’mustnothidethe largediversityofmicrostructuresandpropertiespresentedfor thismaterial’sfamilytodayinthedentalcommunity.

Evenifglazeactsasabarrierforthehydrothermal degra-dation,locationsofun-completeglazingarealwayspresent (atleastintheconditionsofthisstudy).Therefore,theglazing proceduredoesnotrepresentarealprotectionagainstaging oftheoverallcrown,soitshouldbeconsideredonlyfor aes-theticalpurposes.

5. Conclusion

Inthe currentgeneration ofthe so-called‘tetragonal zirco-nia’monolithic crowns,hydrothermaldegradation operates andcanbeobservedforquiteshortagingtimes.Fortunately, atleast inthe invitro conditions ofthis work,aging does notmeanstrengthdegradation,evenforquitelongduration and signiﬁcantsurfacetransformation.Glazeactsasa bar-rieragainsthydrothermalagingfortheunderlyingzirconia; nonethelessglazingshouldnotbeconsideredasanabsolute protectionagainstaging,sincesomespotsoftherestoration surfacearealwaysleft unglazed,especiallyinthemarginal regions.

Thepresenceofcubicphaseinhighlytranslucentmaterials entailstwomainadvantages:asensibleincreasein translu-cencyandthecompleteabsenceofhydrothermaldegradation. Ontheotherhand,thelackoftransformationtougheningfor cubiczirconiaandthecoarsermicrostructurecauseasevere dropinmechanicalproperties,whichcanrepresenta limita-tionfortheirapplicationinconditionswherehighmechanical stressesareapplied.

r

e

f

e

r

e

n

c

e

s

[1] ZaroneF,RussoS,SorrentinoR.From

porcelain-fused-to-metaltozirconia:clinicaland

experimentalconsiderations.DentMater2011;27(1):83–96. [2] DenryI,KellyJR.Stateoftheartofzirconiafordental

applications.DentMater2008;24(March):299–307. [3] ZhangY.Makingyttria-stabilizedtetragonalzirconia

translucent.DentMater2014;30(10):1195–203. [4] KlimkeJ,TrunecM,KrellA.Transparenttetragonal

yttria-stabilizedzirconiaceramics:inﬂuenceofscattering causedbybirefringence.JAmCeramSoc2011;94(6):1850–8. [5] FerrariM,VichiA,ZaroneF.Zirconiaabutmentsand

restorations:fromlaboratorytoclinicalinvestigations.Dent Mater2015;31(3):e63–76.

(12)

[11] YamadaY,MatsumotoA,ItoY.US2016/0074142A1–zirconia sinteredbody,zirconiacompositionandzirconiacalcined body,productionmethodofthese,anddentalprosthesis. [12] MalkonduÖ,TinastepeN,AkanE,Kazazo ˘gluE.Anoverview

ofmonolithiczirconiaindentistry.BiotechnolBiotechnol Equip2016;30(4):644–52.

[13] Anselmi-TamburiniU,WoolmanJN,MunirZA.Transparent nanometriccubicandtetragonalzirconiaobtainedby high-pressurepulsedelectriccurrentsintering.AdvFunct Mater2007;17(16):3267–73.

[14] ShahmiriR,StandardOC,HartJN,SorrellCC.Optical propertiesofzirconiaceramicsforestheticdental restorations:asystematicreview.JProsthetDent 2018;119(1):36–46.

[15] ZhangF,InokoshiM,BatukM,VanMeerbeekB,VleugelsJ. Strength,toughnessandagingstabilityof

highly-translucentY-TZPceramicsfordentalrestorations. DentMater2016;32(12):e327–37.

[16] HaraguchiK,SuganoN,NishiiT,MikiH,OkaK,YoshikawaH. Phasetransformationofazirconiaceramicheadaftertotal hiparthroplasty.JBoneJointSurgBr2001;83(7):996–1000. [17] ChevalierJ,CalesB,DrouinJM.Low-temperatureagingof

Y-TZPceramics.JAmCeramSoc1999;82(8):2150–4. [18] ChevalierJ,GremillardL,DevilleS.Low-temperature

degradationofzirconiaandimplicationsforbiomedical implants.AnnuRevMaterRes2007;37:1–32.

[19] DevilleS,ChevalierJ,GremillardL.Inﬂuenceofsurface ﬁnishandresidualstressesontheageingsensitivityof biomedicalgradezirconia.Biomaterials2006;27(10):2186–92. [20] ScottHG.Phaserelationshipsinthezirconia–yttriasystem.J

MaterSci1975;10:1527–35.

[21] ScottHG.Phaserelationshipsintheyttira-richpartofthe yttria–zirconiasystem.JMaterSci1977;12:311–6.

[22] GarvieRC,NicholsonPS.Phaseanalysisinzirconiasystems. JAmCeramSoc1972;55(6):303–5.

[23] ISO13356:2015Implantsforsurgery–ceramicmaterials basedonyttria-stabilizedtetragonalzirconia(Y-TZP). [24] TorayaH,YoshimuraM,SomiyaS.Calibrationcurvefor

quantitativeanalysisofthemonoclinic-tetragonalZrO2

systembyX-raydiffraction.JAmCeramSoc1984;67(6). C-119-C-121.

[30] TsubakinoH,SonodaK,NozatoR.Martensite transformationbehaviourduringisothermalageingin partiallystabilizedzirconiawithandwithoutalumina addition.JMaterSciLett1993;12:196–8.

[31] VichiA,CarrabbaM,ParavinaR,FerrariM.Translucencyof ceramicmaterialsforCERECCAD/CAMsystem.JEsthet RestorDent2014;26(4):224–31.

[32] InokoshiM,ZhangF,DeMunckJ,MinakuchiS,NaertI, VleugelsJ,etal.Inﬂuenceofsinteringconditionson low-temperaturedegradationofdentalzirconia.DentMater 2014;30(June(6)):669–78.

[33] ChevalierJ,DevilleS,MünchE,JullianR,LairF.Criticaleffect ofcubicphaseonagingin3mol%yttria-stabilizedzirconia ceramicsforhipreplacementprosthesis.Biomaterials 2004;25(November(24)):5539–45.

[34] MatsuiK,HorikoshiH,OhmichiN,OhgaiM,YoshidaH, IkuharaY.Cubic-formationandgrain-growthmechanisms intetragonalzirconiapolycrystal.JAmCeramSoc 2003;86(August(8)):1401–8.

[35] BecherPF,SwainMV.Grain-size-dependenttransformation behaviorinpolycrystallinetetragonalzirconia.JAmCeram Soc1992;75(3):493–502.

[36] ZhangF,VanmeenselK,InokoshiM,BatukM,HadermannJ, VanMeerbeekB,etal.Criticalinﬂuenceofaluminacontent onthelowtemperaturedegradationof2–3mol%

yttria-stabilizedTZPfordentalrestorations.JEurCeramSoc 2015;35(2):741–50.

[37] CamposilvanE,TorrentsO,AngladaM.Small-scale mechanicalbehaviorofzirconia.ActaMater2014;80:239–49. [38] GaillardY,AngladaM,Jiménez-PiquéE.Nanoindentationof yttria-dopedzirconia:effectofcrystallographicstructureon deformationmechanisms.JMaterRes2009;24(3):719–27. [39] MarroFG,AngladaM.Strengtheningofvickersindented

3Y-TZPbyhydrothermalageing.JEurCeramSoc 2012;32:317–24.

[40] KoenigV,WulfmanCP,DerbanneMA,DupontNM,LeGoff SO,TangM-L,etal.Agingofmonolithiczirconiadental prostheses:protocolfora5-yearprospectiveclinicalstudy usingexvivoanalyses.ContempClinTrialsCommun 2016;4:25–32.