Effects
of
extreme
cooling
methods
on
mechanical
properties
and
shear
bond
strength
of
bilayered
porcelain/3Y-TZP
specimens
Antonio
A.
Almeida-Ju´nior
a,
Diogo
Longhini
a,
Nata´lia
B.
Domingues
a,
Claudinei
Santos
b,
Gelson
L.
Adabo
a,*
aUNESP–UnivEstadualPaulista,FaculdadedeOdontologiadeAraraquara,R.Humaita´,1680Sala415,Centro.Araraquara,SP14801-903,
Brazil
bUniversidadedoEstadodoRiodeJaneiro–FaculdadedeTecnologiadeResende,EtrResende-Riachuelo,S/N,MoradadaColina,Resende,RJ
27523-000,Brazil
1.
Introduction
A mismatch of the thermal expansion coefficient (CTE) between feldspathicporcelain and zirconia, aswell asthe
effect ofthermaldiffusivityandthermalconductivityupon cooling aftersintering, cangeneratetransient and residual stresses and increase the clinical failure rates of zirconia-basedrestorations.1–9Belowtheglasstransitiontemperature
(Tg),theporcelainacquiresasolidstateinwhichstructural
a
r
t
i
c
l
e
i
n
f
o
Articlehistory:
Received6November2012 Receivedinrevisedform 9January2013
Accepted11January2013
Keywords:
Dentalceramic Zirconia Bilayerceramic Cooling Residualstress
a
b
s
t
r
a
c
t
Objectives: Thisstudyinvestigatedtheeffectofextremecoolingmethodsontheflexural
strength,reliabilityandshearbondstrengthofveneerporcelainforzirconia.
Methods: VitaVM9porcelainwassinteredonzirconiabarspecimensandcooledbyoneof
thefollowingmethods:insideaswitched-offfurnace(slow),atroomtemperature(normal) or immediately by compressed air(fast). Three-pointflexural strength tests (FS) were performed on specimenswith porcelain undertension (PT,n=30) andzirconia under tension(ZT,n=30).Shearbondstrengthtests(SBS,n=15)wereperformedoncylindrical blocksofporcelain,whichwereappliedonzirconiaplates.Dataweresubmittedtoone-way ANOVAandTukey’sposthoctests(p<0.05).WeibullanalysiswasperformedonthePTand
ZTconfigurations.
Results: One-wayANOVAforthePTconfigurationwassignificant,andTukey’stestrevealed
that fastcooling leads to significantly higher values (p<0.01) than the other cooling
methods.One-wayANOVAfortheZTconfigurationwasnotsignificant(p=0.06).Weibull analysisshowedthatnormalcoolinghadslightlyhigherreliabilityforboththePTandZT configurations.StatisticaltestsshowedthatslowcoolingdecreasedtheSBSvalue(p<0.01)
andshowedlessadhesivefracturemodesthantheothercoolingmethods.
ClinicalSignificance:Slowcoolingseemstoaffecttheveneerresistanceandadhesiontothe
zirconiacore;however,thereliabilityoffastcoolingwasslightlylowerthanthatoftheother methods.
#2013ElsevierLtd.
*Correspondingauthorat:R.Humaita,1680Sala415,Centro.Araraquara,SP14801-903,Brazil.Tel.:+551633016415;fax:+551633016406. E-mailaddresses:adabo@foar.unesp.br,gl.adabo@uol.com.br(G.L.Adabo).
Available
online
at
www.sciencedirect.com
journalhomepage:www.intl.elsevierhealth.com/journals/jden
0300-5712# 2013ElsevierLtd.
http://dx.doi.org/10.1016/j.jdent.2013.01.005
Open access under the Elsevier OA license.
rearrangements are impossible and residual stress can developwithintheporcelainlayer.Stressintensityisdirectly proportionaltothedifferencesintheCTEofthetwomaterials from Tg to room temperature.10 Furthermore, residual stresses in zirconia/porcelain bilayer specimens may be modifiedbythermaltreatmentat theTginterval,variation incoolingratesorthetemperaturegradientoftheinternaland externallayersoftheveneer.4,5,7,11–13
In contrast to metals in metal–ceramic restorations, zirconia has thermal diffusivity lower than that of dental porcelain,andaslowcoolingofzirconiaretardsthebalance betweentheinternalandexternaltemperatures. Moreover, thethermalconductivityofzirconiaisapproximately15times smallerthan thatofaluminaand approximately 100times smallerthanthatofgoldalloys.5,12Consequently,thecooling
processafterthefiringofporcelaininzirconia-based restora-tionsisveryslow,resultinginhightransitionaltemperature differences throughout the restoration, especially at the thicker and irregular layers and upon fast cooling. The temperaturegradientbetweentheveneerandzirconiacore inthefastcoolingmethodmayreachupto1408C.14During
fast cooling, the temperature of the veneer porcelain in contactwiththe zirconia core mayremain above Tgfor a longerperiodoftime,whilethesurfaceoftheveneercoolsata fasterrate.Thiscoolingintroducesahightransientthermal gradientbetweenthe outerandinnersurface. Thisprocess resultsinahighresidualtensilestresswithintheporcelain layer andhigh tempering/compressive residual stresseson thesurface.5,11,12,14
To address theseconcerns, afterMarch 2009, manufac-turershaverecommendedslowcoolingafterporcelainfiring to reduce thermal gradients and residual stresses in the porcelain sintering5,7,14 and to decrease the risk for early
ceramicchipping.6Theliteratureprovidesnostandardization
inthemanydifferentprotocolsforcooling,3,6,7,11,13–18andthis
widevariabilityamongcoolingmethodsproducesaconfusing nomenclature.Forexample,slowcoolingforoneauthormay besimilartoregularcoolingforotherauthors.
Therefore,theaimofthisstudyistoinvestigatehowthe extremesloworfastcoolingmethodsinfluencetheflexural strengthandshearbondstrengthofdentalfeldspathicveneer porcelainforzirconia.Inaddition,thereliabilityoftheflexural strength was investigated by Weibull analysis. The null hypothesiswasthattherewouldbenosignificantdifferences in the studied properties of the porcelain when different coolingconditionswereused.
2.
Materials
and
methods
2.1. Processingandthree-pointflexuraltesting
ThematerialsusedinthisstudyaredescribedinTable1. Pre-sintered 3Y-TZPblocks werecutwith adiamond disc ina precisioncuttingmachine(Isomet1000,Buehler,LakeBluff, USA)andfinishedusing600-gritsizeSiCpapersinapolishing machine(Metaserv2000,Buehler,BuehlerUKLtd.,Coventry, England).AftersinteringinaMoSi2furnace(FE1800,Maitec,
Sa˜oCarlos,SP,Brazil)at15308Cfor2h,bar-shapedframework specimens had an average final dimension of 22mm in length4.0mminwidth0.7mminthickness.The dimen-sionsofeachspecimenweremeasuredwithadigitalcaliper (Mitutoyo Corporation, Tokyo, Japan) with precision of 0.01mmforthecalculationofflexuralstrength.
Onesideofthezirconiaframeworkbarswasveneeredwith Vita VM9. The powder and liquid (VITA Modeling Liquid, VitavZahnfabrik, BadSa¨ckingen, Germany) were mixed to obtainathinaqueousmixture(washbakelayer),whichwas appliedverythinlyonthecleanedframeworkandsinteredin theceramicoven(Aluminipress,EDG,SaoCarlos,SP,Brazil) followingthemanufacturer’srecommendations.Aslurry of porcelain(dentinelayer)waspreparedbymixingthepowder andliquidinaratioof2.5:1andinsertedintoapolyethermold (ImpregumF,3MESPE,Seefeld,Germany)thatwasadapted around the zirconia bars. Excess liquid was blotted with absorbentpaperbeforethespecimenswereremovedfromthe mold.Thespecimensweresinteredintheovenaccordingto themanufacturer’sinstructions.Attheendoffiring,cooling was performed according to the following experimental protocol:
Slow – samples were left inside the closed, turned-off
furnaceuntiltheyreachedroomtemperature(about8h);
Normal–theelevatorofthefurnacewaslowered,andwhen
the temperature inside the furnace reached 5008C, the samples wereremoved and cooled at room temperature (about20min);
Fast–sampleswereremovedfromthefurnaceimmediately
aftertheholdingtimeandblastedbycompressedair(less than10s).
Aftercooling,theporcelainsurfaceofthespecimenswas sequentiallygroundedusing120-to1200-gritwetSiCpaper
Table1–Materials.
Material Manufacturer Maincomponents (mass%)
Batch number
Elasticmodulus (GPa)
CTE(1068C1)
ZRHP ProtMatMateriaisAvanc¸ados, Guaratingueta´,Sa˜oPaulo,Brazil
ZrO2(94,8);Y2O3(5.2) S09-011B 210a 10,1a
VitaVM9Dentin VitaZahnfabrik,BadSackingen, Germany
SiO2(60–64);Al2O3(13–15); K2O(7–10);Na2O(4–6); B2O3(3–5)b
30830 65b 9,2b
discs(NortonAbrasivos,SaoPaulo,SP,Brazil)inapolishing machine (Metaserv 2000, Buehler) until achieving final dimensionsof40.25mminwidth,1.20.2mmin thick-nessand22mminlength.Thedimensionsofthespecimens weremeasuredwithadigitalcaliper.
Three-point flexuralstrength testingwas carried out in distilledwaterat378C(1.08C).Thesampleholderhada 15-mmspanbetweenthetworoundedsteelknife-edgebearersof 0.8-mm radius. Thespecimens were loaded on the center pointbyaroundedsteelknifeedgepiston(1.6mmradius)ina universal testing machine (DL 2000, EMIC, Sa˜o Jose´ dos Pinhais,PR,Brazil)witha5.0-kNloadcellandatacrosshead speedof1.0mm/minuntilfailure.Specimenswererandomly divided into two configurations, according to the cooling method:
1porcelainsideonthesampleholder,i.e.,porcelainunder tensile stressand zirconiaunder compressionstress(PT) (slown=30;normaln=30;fastn=30);
2zirconia side on the sample holder, i.e., zirconia under tensilestressandporcelainundercompressionstress(ZT) (slown=30;normaln=30;fastn=30).
The flexural strength (FS) was calculated according to Eq.(1):
sf¼ 6M
wt2
tK
2þtc
ttþ
Ettc
Ectc
(1)
whereMandKareobtainedbyEqs.(2)and(3).
M¼PL
4 (2)
K¼4þ6 tc
tt
2
þEc
Et
tc
tt
3
þEttt
Ectc (3)
ReplacingMandKinEq.(1),theEq.(4)isobtained forthe calculationofthebilayerspecimensFS19,20:
sf¼ 3EtLPðEct 2
cþ2EctcttþEtt2cÞ
2wðE2
ct2cþ4EcEtt3cttþ6EcEttc2t2tþ4EcEttct3tþEt2ct4tÞ
(4)
wheresfisthemaximumcentertensilestress(MPa),Pisthe
loadat fracture(N), Listhedistancebetweenthesupports (mm),Etistheelasticmodulusofthematerialundertensile stress(GPa),Ecistheelasticmodulusofthematerialunder compression(GPa),ttisthethicknessofthematerialunder
tensilestress(mm),tcisthethicknessofthematerialunder compression(mm),andwisthespecimen’swidth(mm).
2.2. Weibullanalysis
Toassessthereliabilityofthespecimensineachconfiguration testaccordingtothecoolingconditions,aWeibullregression analysis was performed on the flexural strength data to determine the Weibull modulus (m) and the characteristic strength(s0).Eq.(5)describestheWeibulldistribution:
P¼1exp s
s0
m
(5)
wherePistheprobabilityoffailure,sistheflexuralstrength,s0
is the characteristic strength at the fracture probability of
63.21%,andmistheWeibullmodulus,whichistheslopeof thelineplottedinthe‘‘ln[ln(1/(1P))]vslns’’Cartesianplane.
2.3. Shearbondstrengthtest
Square zirconia specimens, 9mm in length and 2mm in thickness,werecutandsinteredasdescribedabove.Awash bake layerwas applied and sinteredon the surface,and a cylindricalmetalmold,5mmindiameterand3mminheight, wasusedtobuildtheporcelainlayer.Dentineporcelainwas sintered according to the manufacturer’s instructions and cooledaspreviouslydescribed(n=15).
Aftercooling,thezirconiasegmentofthespecimenwas embeddedinapolyvinylchloride(PVC)tubewithPMMAresin, ensuringthatthezirconia/porcelaininterfacewasatthesame levelastheresinsurface.Thediameterofporcelainlayerof thespecimenswasmeasuredwithdigitalcalipertocalculate thebondarea.Thespecimenswereheldinametaldeviceina universal testing machine (DL 2000, EMIC, Sa˜o Jose´ dos Pinhais,PR,Brazil).Theloadwasappliedparalleltothelong axis of the specimen via a knife-edge shearing rodat the zirconia/porcelain interface, with a 5.0kN load cell at a crossheadspeedof1.0mm/min,untilfractureofthesample wasachieved.Themaximumloadatfailurewasrecorded,and theshearbondstrength(sSBS–MPa)wascalculatedaccording
toEq.(6):
sSBS¼ P
pr2 (6)
wherePisthemaximumfailureload(N)andristheratioof porcelaincircumference(mm2).
FailuremodesoftheSBStestwereclassifiedaccordingto: (1) adhesive failure at the veneering porcelain/substrate interface (AD); (2) cohesive failure within the veneering porcelain (CO); and (3) the combination of adhesive and cohesivefailures(AC).2
2.4. Statistics
Statistical analyses for flexural strength and shear bond strength wereperformedbyone-wayANOVA(a<0.05)and
Tukey’sposthoctestformultiplecomparisons.
3.
Results
Themeansofthethree-pointflexuralstrengthmeasures(sf),
thestandarddeviations(SD),andthecoefficientsofvariation (CV) of specimens with porcelain under tension (PT) and zirconiaundertension(ZT)accordingtocoolingmethodare shown in Table 2. One-way ANOVA for the PT specimen configuration was significant (F2,87=10.46, p<0.01), and
Tukey’s post hoc test showed that fast cooling leads to significantlyhighermeansfvaluesthantheslowandnormal
coolingmethods,whichwerenotdifferentfromeachother. On the other hand, one-wayANOVA for the ZT specimen configurationwasnotsignificant(F2,87=2.90,p=0.06).
specimen configurations.InthePT configuration,slowand fast were equal,but in the ZT configuration, slow cooling specimens exhibited a higher Weibull modulus than fast-coolingspecimens.
The mean shear bond strength of porcelain bonded to zirconia that underwent slow cooling was 15.9 (4.5) MPa; normalcoolingwas19.5(4.6)MPa;andfastcoolingwas20.9 (4.4)MPa.One-wayANOVAshowedasignificantdifferencefor theshearbondstrengthsamongthecoolingmethodstested (F2,42=5.430, p<0.01). Tukey’s post hoc test for multiple
comparisonsshowedthatthemeanSBSoftheslow-cooling methodwaslowerthanthoseofthenormal-andfast-cooling methods,whichweresimilartoeachother.AftertheSBStest, thefollowingfailuremodeswereobservedforslow(AD40%; CO33%;AC27%),normal(AD67%;AC33%),andfastcooling (AD67%;CO13%;AC20%).
4.
Discussion
Thefinaldimensionsoftheutilizedbilayerflexuralstrength specimenswerechosenbasedonthemonolayerspecimen tests(ISO6972:2008)withathicknessratioof1:1.Thisstudy conductedtwoflexuralstrengthtestconfigurations (porce-lainorzirconiaundertension),submergedinwaterat378C. Theresultsindicatedthatthematerialonthebottomsurface determinesthestrengthandfailuremodeofthesystem.19–24 According to the selected mathematic equation, flexural strength depends directly on the elastic modulus of the material under tension21–23 becausethe maximum loadis
concentratedonthebottom surfaceofthe sample. There-fore,ahigherflexuralstrengthwasachievedwhenzirconia was under tension. Nevertheless, there were two distinct failure modes according to the test configurations. When porcelain was under tension (PT), flaws initiated in the bottom surface and propagated throughout the porcelain untiltheyreachedtheinterface.Atthismoment,thetests werestoppedandthezirconialayerwasnotimpaired.When thezirconiawasundertension(ZT),the porcelain delami-natedbeforethefractureofthezirconiaduetolateralflaw
formation and the crushing of the loading area on the porcelain.20,25
Theflexuralstrengthvalues ofZTspecimensarelower compared to the published values for zirconia monolayer specimens12,20,24,26,27 because the compression stress
induces failureof the porcelainbefore the zirconia under tension. Nevertheless, the ZT test configuration was per-formed becauseitisbelievedthatfailures originateatthe internal surfacesof crowns.19,24,27 To decrease the failure rate, studies20,24havesuggested that theareas of
restora-tions or fixed partial dentures that are submitted to a highconcentrationoftensilestresswhere estheticsisnot needed (suchasthe bottomofthe ponticandconnectors) should be free of the veneered layer to increase the mechanical performance. However, zirconia issusceptible to degradation at low temperatures when exposed to a humidenvironment.28
Specialattentionshouldbepaidtothegreatvariabilityof coolingmethodsthataredescribedintheliteraturewithno standardization3,6,7,11,13–18(Table3).Thesedifferencesleadto difficultiesincomparingresultsanddonotallowaconsensual conclusion for which cooling method must be used. For example,somestudiesrefertoaspecificmethodas‘‘slow,’’ butthismethodisactuallysimilartonormalinourandother studies. Furthermore, porcelain manufacturers are unclear abouttheidealcoolingmethod,referringonlyto‘‘slow.’’Inour study, extreme cooling protocols (fast and slow) were performedtoidentifythechangesinthepropertiesofveneer and zirconia bilayer specimens. In the fast method, the specimenswerecooledbyblastingcompressedair immedi-atelyaftertheholdingtimeofsintering,thatis,from9108Cto room temperature in less than 10s. In the slow-cooling method, the specimens were left inside the switched-off furnaceformorethan8h.
The null hypothesis that the flexural strength and reliability would not be influenced by the cooling method waspartiallyrejected.Thehigherflexuralstrengthobserved forPT-FASTmayberelatedtothetemperingoftheporcelain duetoahighcoolingrateandalargedifferenceinthethermal gradient.Thismayinduceanon-uniformsolidificationfrom the surface to the center, leading to residual compressive stress on the veneer surface5,6,29,30 and increasing the strength.4,6Ontheotherhand,inthisstudy,itwasobserved
that the PT-NORMAL showed a slightly higher Weibull modulus (m). The clinical success of ceramic restorations dependsonthestructuralreliabilityofthedentalceramics. Higher Weibull modulus (m) values represent a more homogenous distribution of flaws, a lower dispersion of values,andahigherreliabilityofthematerial.19,24,27Inthis
study,inspiteofthefactthatthePT-FASTgroupexhibiteda higherflexuralstrength,thisgrouppresentedhigher disper-sionvaluesand,consequently,lowerreliability.Thismightbe related to the possible increase inthe incidence of cracks whenfastcoolingisperformed.Guazzatoetal.7observedan
increase in the crack rate in porcelain cooled with a fast cooling rate,which affected adversely its strength, despite their cooling method had notbeen as extremeasthe fast methodemployedinourstudy(Table3).Moreover,Bellietal.31 performedcyclicfatiguetestonpre-molarcrownssubmitted to fastand slow cooling andobserved thatcracks on VM9
Table2–Meansofflexuralstrength(sf,MPa),standard
deviations(SD),coefficientsofvariation(CV,%),Weibull modulus(m),characteristicstrengths(s0,MPa)andR
-valuesofspecimenswithporcelainundertension(PT configuration)andspecimenswithzirconiaunder ten-sion(ZTconfiguration)accordingtocoolingmethod.
Parameters Flexuralstrength Weibullanalysis
sf(SD) CV m s0 R-value PTconfiguration
PT-SLOW 50.4(11.8)B 23.5% 4.7 54.8 0.973 PT-NORMAL 52.3(9.5)B 18.1% 6.2 56.1 0.956 PT-FAST 64.2(15.0)A 23.3% 4.7 69.9 0.987 ZTconfiguration
ZT-SLOW 611.6(127.3)ns 20.8% 5.2 661.3 0.966 ZT-NORMAL 612.8(125.6)ns 20.5% 5.4 663.9 0.974 ZT-FAST 685.2(151.7)ns 22.1% 4.7 747.5 0.989
specimensgrewlessunderslowregimenanditincreasedthe lifetimeofzirconia–porcelainprostheses.
Despite the statistical similarities among the flexural strengthvaluesoftheZTconfigurationgroupsaccordingto thedifferentmethods,ZT-NORMALexhibitedahighermvalue thanZT-SLOWandZT-FAST.Althoughthestrengthvalueis important,itmaynotbeextrapolatedtopredictthestructural performanceoftheceramic.
Chippinganddelaminatingofporcelaininzirconia-based restorationshasoccurredclinically,andtheseprocessesare consideredto bethemostcommon clinical failures.8,9 The
bondstrengthmaybeexplainedbytheinherentfragilityofthe interfacebetweenthecoreandveneer,oritmaybesimplydue to the weak strength of the porcelain layer and can be influenced bythe materials,3,6 surfacetreatments of
zirco-nia32, cooling rate after sintering3,6,14 or bond strength
tests.3,6,14 In this study, fast and normal cooling methods
ledtosignificantlyhigherSBSvaluesandshowed predomi-nantly adhesive failure mode between VM9 porcelain and zirconia. These results may be attributed to compressive/
tempering stresses in the porcelain layer under faster cooling, which can improvethe resistance of the surface layer6andtodevelophighresidualtensiononthezirconia/
porcelaininterface,5,7,14whichledtothe predominanceof
adhesivefailuremode.Incontrast,theslowcoolingmethod hadlowerSBSvalueandhigherincidenceofcohesiveand combinationfailuremodes.However,adirectcomparisonto other studies is difficult once, tothe authors’ knowledge, thereisanypublishedbondstrengthinvestigationthatused slowcoolingmethodcomparabletothatusedinthisstudy. The cohesive failure mode is more related to porcelain strengththantoitsadhesiontothezirconiacore.1Inslower cooling regimen, the tempering does not occur because tensile stresses are generated in the porcelain due to structural relaxation of the glass above and around the Tg.5,6,13,29 Moreover, as discussed before, PT-SLOW group
in flexural strength test showed lower mean value than PT-FAST.Itispossiblethatporcelaininslowcoolingmethod had resistancebelow thanthat for the adhesionbetween veneerandzirconia.
Table3–Comparisonamongstudiesthatevaluateddifferentcoolingmethodsinporcelain/zirconiaspecimens.
Authors(year) Designofspecimens (v:cthicknessratio)
Coolingmethod Classified Comparedto ourstudy
Taskonaketal.(2008)13 Bilayerdisks(6:10) Immediate Fast Fast
Monolayerdisks Insidethefurnace Slow Slow
Zhangetal.(2009)11 Monolayerdisks
Bilayerplates(6:1)
Insidethefurnace
Inambientairatroomtemperature Temperedbyblastingcompressedair
Slow Normal Fast
Slow Normal Fast Gostemeyeretal.(2010)6 Bilayerrectangular
plate(1:1)
Insidethefurnacefor5min Immediatelyinambientair
Slow Rapid
Normal? ? Guazzatoetal.(2010)7 Bilayerspheres(1:2;1:1) Immediatelymovedtothefurnacebench Fast ?
Accordingtothemanufacturer’s recommendations
Normal ?
Komineetal.(2010)3 Bilayerdisks Outsidethefurnacefor4minuntil
reachingTg
Slow Normal
Immediatelyinambientair Rapid ?
Ruesetal.(2010)15 Centralincisorcrowns Conventionalfiring Protocol1 Normal?
Extracoolingtimeof6minafterglazefiring Protocol2 ? Extracoolingtimeof6minafterallfirings Protocol3 ? Choietal.(2011)16 Bilayerplate(1:1;2:1)
PorcelainMonolayerplate
Forcecooledwithcompressedair Removedwhenthetemperatureoffurnace dropped
Stoppedthedescentofthemuffleandwaited until100C.
Fast Normal Slow
Fast Normal Slow
Mainjotetal.(2011)17 Bilayerdisks(2:1) Openedfurnace Classic Normal
Insidetheswitched-offfurnacefrom 9008Cto6008C
Modified Slow
Atarateof28C/mininspecialfurnace Slow ? Tholeyetal.(2011)14 Crownswithuniformand
anatomicalframeworks
Insidethefurnaceuntil6008C Slow ?
Openedthechamberdirectlyandswitched offthefurnace
Fast Normal?
Tanetal.(2012)18 Beamofzirconiawith
porcelainbutton(3:1)
Removedassoonasthemufflehadfully descended
Fast ?
Leftinthemufflefor7.5minuntilreaching 5008C
Moderate Normal
Leftinthepartially(30%)openmufflefor 15minuntilreachingamuffletemperature of5008C.
Slow ?
Bellietal.31 Premolarcrowns Immediatelyinambientair Fast ?
Leftinthepartially(10%)openmuffleuntil reachingamuffletemperatureof2008C.
Komine et al.3 observed that depending on porcelain
material, slow cooling increased the SBS between veneer and zirconia, and all specimens showedcohesive fracture, howeverthey used other porcelain brand marks (CZR and e.Max Ceram), the surface of zirconia specimens was air-blastedwithAl2O3andtheslowcoolingusedwassimilarto
ournormalcooling(Table3).DespiteGostemeyeretal.6also
had observed the dependence on porcelain material, they showed that slow cooling method (similar to our normal cooling–Table3)adverselyaffectedthestrainenergyrelease in four-point bending test and prevailed the combination fracturemodeforVM9andTricerambuttherewerenoeffect ofcooling methods onZirox andLava Ceram.Tan etal.,14
using modified four-point flexural technique with VM9 porcelainbuttonunderbeamsofzirconia,observedthatslow cooling(intermediarytoournormalandslowcoolingmethod) increasedthe fractureloadand thevisualfractureanalysis owedathinlayerofporcelainonthezirconia, classifiedas cohesivemode.
Nevertheless,nodirectclinicalcorrelationshouldbemade between failures in shear bond studies with the failure mechanismsofrestorationsbecausethetestingmethodsdo notreproduceclinicalconditions.33Ideally,long-termclinical
trialsshouldbeusedtodeterminethefailuremechanismsfor all-ceramic restorations. Moreover, while a fracture bond strengthvaluehigherthan25MPaisacceptedasadequatefor metal–ceramics,theacceptablebondstrengthforall-ceramic materials has not yet been determined.34 Moreover, the
bonding mechanisms between zirconia and porcelain are unclear.2,12Surfacetreatmentsseemtodamagethestructure
ofzirconia due to its martensitic transformation fromthe tetragonaltomonoclinic phase.26,28 Someporcelainbrands
require a surface liner before porcelain application; the porcelainsamplesusedinthisstudynecessitatedawashbake layer. Thus,theeffects ofchemicalbondingand microme-chanical retention, as well as the improvement of the wettabilityofzirconia,areactiveareasofresearch.
Whileafracturebondstrengthvaluehigherthan25MPais acceptedasadequateformetal–ceramics,theacceptablebond strength for all-ceramic materials has not yet been deter-mined.34Moreover,thebondingmechanismsbetween
zirco-niaandporcelainareunclear.2,12Surfacetreatmentsseemto damage the structure of zirconia due to its martensitic transformationfromthetetragonaltomonoclinicphase.26,28
Someporcelainbrandsrequireasurfacelinerbeforeporcelain application; the porcelain samples used in this study necessitatedawashbakelayer.Thus,theeffectsofchemical bonding and micromechanical retention, as well as the improvementofthewettabilityofzirconia,areactiveareas ofresearch.
Thisstudyhassomelimitationsthatmakeit difficultto directlyrelateitsresultstoclinicalsituations.The tempera-turedistributionofspecimensiscomplexanddependsonthe thermal proprieties of the materials, the cooling rate, the gradientanddistributionoftemperature,thesupportofthe materialsinsidetheoven,andthethickness,preparationand design.5,14,30 In this study, plate specimens were used to
investigate the effect of extreme cooling methods under controlledconditionsforthemechanicaltests.35However,the
geometryofdentalcrownsandFPDsismultifaceted,witha
sphero-cylindrical form,heterogeneous contoursand varia-tions in the thickness of the veneer and core.1,2,5,10,36
Furthermore,themicrostructureofporcelain,thethickness ratio,themanufacturingprocess,theflawpopulation (num-ber,distribution,andsize),theocclusaladjustment,moisture, thermal and load cycling, and individual habits also have influences on the fracture resistance of the veneering porcelain and its clinical reliability.15,24,27,36 Therefore,
in vitro thermal, chemical and mechanical fatigue studies should be conducted usingcomplex specimens as crowns, whicharesubmittedtovariouscoolingmethodswithdifferent zirconiaceramic/veneeringporcelainsystems;thethickness ratioinhumidenvironmentsshouldalsobefurtherstudied. Moreover, morerandomizedclinicaltrials arenecessaryto evaluate the actual performance of this zirconia-based methodofrestoration.9
5.
Conclusion
Thefast-coolingmethodshowedahigherflexuralstrengthbut alowerreliabilityinporcelainundertensiontest configura-tions. The slow-cooling methoddecreased the shear bond strengthandshowedhighercohesiveandcombinationfailure modes. Theseresultssuggestthatthecooling methodmay affectthelongevityofzirconia-basedrestorations.Thus,other investigations are needed to establish the ideal cooling protocol.
r
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