Abst ract
Submitted: May 16, 2016 0RGL¿FDWLRQ)HEUXDU\ Accepted: March 10, 2017
Effect s of porcelain t hickness on
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propagat ion in a bilayered zirconia
syst em
2EMHFWLYH 7KLV VWXG\ HYDOXDWHG WKH LQÀXHQFH RI SRUFHODLQ 90 9,7$ =DKQIDEULN*HUPDQ\WKLFNQHVVRQWKHÀH[XUDOVWUHQJWKDQGFUDFNSURSDJDWLRQ in bilayered zirconia syst em s ( YZ, VI TA Zahnfabrik, Germ any) . Mat erial and Met hods: Thir t y zir conia bar s ( 20.0x4.0x1.0 m m ) and six zir conia blocks ( 12.0x7.5x1.2 m m ) were prepared and veneered wit h porcelain wit h different t hickness: 1 m m , 2 m m , or 3 m m . The bars of each experim ent al group ( n= 10) ZHUHVXEMHFWHGWRIRXUSRLQWÀH[XUDOVWUHQJWKWHVWLQJ,QHDFKFHUDPLFEORFND Vickers indent at ion was creat ed under a load of 10 kgf for 10 seconds, for t he SURSDJDWLRQRIFUDFNV5HVXOWV7KHUHVXOWVRIÀH[XUDOVWUHQJWKZHUHHYDOXDWHG E\2QHZD\$129$DQG7XNH\¶VWHVWZLWKDVLJQL¿FDQFHOHYHORI7KH IDFWRU³WKLFNQHVVRIWKHSRUFHODLQ´ZDVVWDWLVWLFDOO\VLJQL¿FDQWS DQG WKHPPJURXSSUHVHQWHGWKHKLJKHVWYDOXHVRIÀH[XUDOVWUHQJWK7KHFUDFNV were predom inant am ong t he bending specim ens wit h 1 and 2 m m of porcelain, and cat ast rophic failures were found in 50% of 3- m m - t hick porcelain. Aft er t he indent at ion of blocks, t he m ost severe defect s were observed in blocks wit h 3- m m - t hick porcelain. Conclusion: The sm allest ( 1 m m ) t hickness of porcelain RQWKH]LUFRQLDLQIUDVWUXFWXUHSUHVHQWHGKLJKHUYDOXHVRIÀH[XUDOVWUHQJWK Bet t er resist ance t o defect propagat ion was observed near t he porcelain/ ]LUFRQLD LQWHUIDFH IRU DOO JURXSV +LJKHU ÀH[XUDO VWUHQJWK ZDV IRXQG IRU D t hinner porcelain layer in a bilayered zirconia syst em . The dam age caused by a Vickers indent at ion near and far t he int erface wit h t he zirconia shows t hat WKHVWUHVVSUR¿OHVDUHGLIIHUHQW
Ke yw or ds: Ceram ics. Porcelain. Mechanical st ress. Confocal m icroscopy. Viviane Maria Gonçalves de
FIGUEIREDO1
Sarina Maciel Braga PEREIRA1
Eduardo BRESSIANI2
Márcia Carneiro VALERA2
Marco Antônio BOTTINO1
Yu ZHANG3 5HQDWD0DUTXHVGH0(/21
1Univ. Estadual Paulista, Instituto de Ciência e Tecnologia de São José dos Campos, Departamento
de Materiais Odontológicos e Prótese, São Jose dos Campos, SP, Brasil.
2Univ. Estadual Paulista, Instituto de Ciência e Tecnologia de São José dos Campos, Departamento
de Odontologia Restauradora, São Jose dos Campos, SP, Brasil.
3New York University, College of Dentistry, Biomaterials and Biomimetics, New York, USA.
Corresponding address:
5HQDWD0DUTXHVGH0HOR
I nt roduct ion
Ceram ic m at erials are increasingly used in dent al
rest orat ions because of t he excellent com binat ion of
est het ic, biological, and m echanical propert ies, such as
wear resist ance and rigidit y20, m aking t hem a m at erial
of choice for oral rehabilit at ion11,12. Dent al ceram ics can
be used for fram ew ork, e.g., yt t rium - oxide part ially VWDELOL]HG ]LUFRQLD ZKLFK KDV KLJK ÀH[XUDO VWUHQJWK and fract ure t oughness, but lim it ed t ranslucency20,26.
Excellent est het ic propert ies are achieved by applying
a veneering ceram ic ( feldspat hic glass or porcelain) t o
t he fram ew ork23.
The m ost com m on clinical com plicat ions associat ed
w it h t he infrast r uct ur e of zir conia r est orat ions ar e
loss of ret ent ion, t he need for endodont ic t reat m ent ,
veneering ceram ic fract ures and bleeding on probing12.
Mechanical com plicat ions such as porcelain fract ure
t end t o be m ore prevalent in veneered zirconia crow ns
t han in t hose w it h m et al coping18,24.Clinical reasons
for t he failure of t hese ceram ic rest orat ions include
m echanical st ress for high occlusal loads, which causes
crack propagat ion and lat er result s in “ chipping”20,28
and locat ion in t he dent al arch21. From a biom echanical
m at erials perspect ive, residual st resses in com pression
and t ension generat ed along t he porcelain layer play a
crit ical role in t he failure of porcelain veneered zirconia
crow ns27.
5HVLGXDOVWUHVVHVLQÀXHQFHWKHVWUHQJWKDQGIUDFWXUH behavior of ceram ic crow ns and w hen com bined w it h
funct ional st resses m ay lead t o rest orat ion failure17.
Am ong t he fact ors t hat cont rol t he am ount of residual
st resses are t he geom et ry of t he infrast ruct ure6,8,16,19,
t he t hickness of t he ceram ic overlay and fram ew ork16,
and t he cooling rat e at t em perat ures above t he glass
t ransit ion t em perat ur e10, 15. The nat ur e of st r esses,
eit her com pressive or t ensile, also affect s ceram ics
st rengt h, t he form er increases st rengt h and t he lat t er
increases crack propagat ion under occlusal loadings.
From a clinical view point , t he t hickness of zirconia
fram ework m ay vary from 0.3 t o 1.0 m m , while t hat of
t he veneering ceram ic varies from 0.3 t o 3.0 m m22,29.
This variat ion in t hickness is due t o t he dent al crown
com plex shape, w hich has regions, such as cusps and
axial walls built up w it h varying porcelain t hicknesses,
prom ot ing different st ress m agnit udes27. Few st udies
KDYHVSHFL¿FDOO\DGGUHVVHGWKHPHFKDQLFDOEHKDYLRURI different t hicknesses of porcelain- zirconia25.Therefore,
it is necessary t o know t he effect s of m echanics and
crack propagat ion at different t hicknesses of
porcelain-zirconia infrast ruct ure perform ance.
The m et hodologies used t o analyze t he cracks in
ceram ics include m icrotom ography19, t ransillum inat ion3,
and Scanning Elect ron Microscopy ( SEM) . However, t he
lit erat ure does not report crack behavior of ceram ic
bilayer syst em w it h regard t o it s dept h, volum e, and
t he ext ent of dependence on porcelain t hickness. This VWXG\ DLPHG WR HYDOXDWH WKH LQÀXHQFH RI SRUFHODLQ ]LUFRQLDWKLFNQHVVRQWKHÀH[XUDOVWUHQJWKDQGFUDFN propagat ion of a bilayered syst em by m eans of confocal
m icroscopy analysis. The hypot heses t o be t est ed were + ± 7KH VPDOOHVW WKLFNQHVV RI YHQHHULQJ FHUDPLF LQFUHDVHV ÀH[XUDO VWUHQJWK DQG + ± 7KHUH ZLOO EH differences in t he propagat ion of defect s ( indicat ive
of residual st resses) , depending on veneering ceram ic
t hicknesses.
Mat erial and Met hods
Th e m at er ials u sed in t h is st u dy ar e sh ow n in
Figure 1.
Sam ple preparat ion
I n t his st udy, zirconia blocks pre- synt hesized wit h
dim ensions 14x15x20 m m ( VI TA I n- Ceram® YZ Cubes,
Vit a Zahnfabrik, Bad Säckingen, Germ any) w ere used
t o m ake bar and block specim ens. Thirt y bar- shaped
zirconia specim ens, wit h dim ensions of 20.0 m m lengt h
x 4.0 m m w idt h x 1.0 m m t hickness, and six
block-shaped zirconia specim ens, w it h dim ensions of 12.0
m m lengt h x 7.50 w idt h x 1.2 m mt hickness, w ere
cut w it h t he aid of a cut t ing m achine ( 1000 I SOMET,
Buehler Lt d., Lake Bluff, I L, USA) . Specim en dim ensions
Material Type Brand Name Manufacturers Lot
Zirconia VITA In-Ceram Zirconia
YZ CUBES for Inlab
38320
Porcelain VITA VM9
Base Dentine
VITA Zahnfabrik, Bad Sackingen, Germany
42080 11620 0RGHOOLQJ/LTXLG 900RGHOOLQJ/LTXLG
w ere obt ained aft er a sint ering process in a Zyrcom at
furnace ( Vit a Zahnfabrik, Bad Sackingen, Germ any) .
Specim en s w er e clean ed in an u lt r ason ic bat h
( Vit asonic, Vit a Zahnfabrik, Bad Sackingen, Germ any)
for 5 m in in 10% isopropyl alcohol to rem ove any residue
from previous st eps and dried at room t em perat ure.
They w ere t hen random ized int o experim ent al groups,
w it h 10 bars and 2 blocks for each group. The groups
follow ed t he porcelain t hickness, accordingly:
Group 1: Bars and blocks w it h 1 m m of zirconia
and 1 m m of porcelain.
Group 2: Bars and blocks w it h 1 m m of zirconia
and 2 m m of porcelain.
Group 3: Bars and blocks w it h 1 m m of zirconia
and 3 m m of porcelain.
Applicat ion of veneering ceram ic
The applicat ion of porcelain ( VM9; Vit a Zahnfabrik,
Bad Sackingen, Germ any) was perform ed m anually.
The zirconia specim ens w ere insert ed int o a silicone
m old for t he applicat ion of porcelain layers. Thus, t he
ceram ic powder was m ixed w it h t he m odeling liquid,
applied, and condensed ont o zirconia bars. The powder
used was Base Dent in ( VM9; Vit a Zahnfabr ik , Bad
Sackingen, Germ any) , w hich was applied w it h a brush
under vibrat ion.
7KH¿ULQJSURFHVVZDVWKHEDVLFF\FOHIRU9,7$90 ( recom m ended by t he m anufact urer) . Base Dent in was ¿UVWDSSOLHGXQWLOFRPSOHWLRQRIWKHGHVLUHGWKLFNQHVV DQG ¿UHG LQ D 9DFXPDW 9LWD =DKQIDEULN %DG 6DFNLQJHQ*HUPDQ\IXUQDFHLQWZR¿ULQJF\FOHVIRU all groups ( Table 1) . Bar and block shaped specim ens ZHUH WKHQ ¿QLVKHG ZLWK VLOLFRQ FDUELGH VDQGSDSHU # 600, 800, 1000, and 1200 ( 3M, Sum aré, SP, Brazil) ,
under const ant wat er ir r igat ion ( Aut om et , Buehler,
Lake Bluff, I L, USA) .
)RXUSRLQWÀH[XUDOVWUHQJWKWHVW
The st rengt h of t he bilayer syst em was m easured
by t he four- point bending t est . Specim ens were placed LQDIRXUSRLQWEHQGLQJ¿[WXUHPPGLDPHWHUUROOHUV
support s spaced 16 m m apart , and rollers 8 m m apart ) ,
w here t he porcelain was t est ed under t ensile st ress. 7KHÀH[XUDOVWUHQJWKWHVWZDVSHUIRUPHGDWDVSHHGRI 0.5 m m / m in, w it h a 1000- kgf load cell, in a universal
t est ing m achine, EMI C DL 1000 ( São José dos Pinhais, 35%UD]LO0D[LPXPORDG3ZDVUHFRUGHGDWWKH¿UVW VLJQRIIUDFWXUHYHUL¿HGE\WKH¿UVWFUDFNLQJVRXQG DQGFKDQJHVLQWKHGHÀHFWLRQFXUYH6WUHQJWKYDOXHV ıZHUHFDOFXODWHGIRUÀH[XUDOVWUHQJWKRIWKHELOD\HU
according t o t he follow ing equat ion9:
w here P is t he applied load in New t ons ( N) , L is
t he dist ance bet ween t he support rollers in m illim et ers
( m m ) , Y’ is t he dist ance in m m from t he neut ral axis RIWKH¿EHUDQGRXWHUITOT is t he m om ent of inert ial cross- sect ion around t he cent ral axis.
The Y’ value was given by equat ion 2:
w here tc and tv, correspond t o t he t hickness ( T in m m ) of t he zirconia infrast ruct ure and porcelain,
respect ively, and Ec and Ev are t he elast ic m odulus ( E) of t he ceram ic layers of infrast ruct ure and porcelain,
respect ively.
Th e ITOT var iable w as det er m in ed accor din g t o equat ion 3:
and w is t he sam ple w idt h.
Mat er i al t h i ck n esses w er e m easu r ed w i t h a
digit al caliper, and t he elast ic m oduli obt ained from
t he lit erat ure w ere 209.3 GPa for YZ and 66.5 GPa
for VM97. The glaze layer was not used in t his st udy
and, t herefore, t he values for t his m at erial w ere not
considered.
Analysis by st ereom icroscopy
The failur es of t he bar - shaped specim ens w er e
observed w it h a st ereo m icroscope (Discovery z- 20,
Firing process
Starting T (°C) Pre-drying t (min) and closing t (min)
Heating t (min)
Heating rate (°C/min)
Firing T (°C) Holding t (min)
Vacuum holding t
(min)
Wash Firing 500 2 8.11 55 950 1 8.11
1st dentine
Firing
500 6 7.27 55 910 1 7.27
2nd dentine
Firing
500 6 7.16 55 900 1 7.16
Zeiss, Jena, Germ any)DW[DQG[PDJQL¿FDWLRQ
an d classif ied accor d in g t o Lim a, et al.1 5 ( 2 0 1 3 )
DV FUDFNLQJ D ¿VVXUHFUDFN LQ WKH SRUFHODLQ OD\HU delam inat ion/ chipping of veneer ing ceram ics; t ot al
rem oval of porcelain from t he zirconia layer, leaving
YZ free of veneering ceram ics; or cat ast rophic failure.
Vickers indent at ion
A defect in t he six block- shaped specim ens was
sim ulat ed by a Vickers indent at ion, perform ed w it h a
load of 10 kg for 10 s. Prior t o t hat , t he blocks w ere
em bedded in acrylic resin and w et - ground wit h SiC
sandpaper in t he sam e way t he bending specim ens
w ere.
Each block received t w o indent at ions ( Figure 2) ,
eit her close t o ( 0.3 m m fr om t he int er face) or far
from ( 0.3 m m from t he upper surface of t he porcelain)
zirconia. Vickers indent at ion was perform ed using a
DHV- 1000 Micro Hardness Test er ( Durom et er MD- 1;
Micr ot est , Asker, Kyot o, Japan) . As a high loading
force w ould lead t o int eract ions bet w een t he cracks
and im pair t he evaluat ion, w e per for m ed only one
indent at ion on each sit e.
Confocal m icroscopy analysis
Dam age sust ained fr om t he Vicker s indent at ion
were exam ined in a 3D confocal m icroscope ( Mit ut oyo,
Suzano, SP, Brazil) , whereby t he dept h, ext ension, and
volum e of t he defect obt ained w ere m easured using
t he appropriat e soft ware ( Cyber CT Scan 8, I ngolst adt ,
Germ any) .
St at ist ical analysis
5HVXOWVRIÀH[XUDOVWUHQJWKZHUHDQDO\]HGE\2QH ZD\$129$DWDVLJQL¿FDQFHOHYHORI7XNH\¶VWHVW LGHQWL¿HGGLIIHUHQFHVDPRQJJURXSV5HVXOWVRIFUDFN propagat ion analysis w ere evaluat ed by m easures of
cent ral t endency and st andard deviat ions.
Result s
The effect of por celain t hick ness on t he bilayer ]LUFRQLDV\VWHPZDVVWDWLVWLFDOO\VLJQL¿FDQWp= 0.001) , acco r d i n g t o On e- w ay ANOVA. Th e cer am i c b ar ZLWK PP SRUFHODLQ KDG KLJKHU ÀH[XUDO VWUHQJWK 7XNH\¶VWHVWGLGQRWLGHQWLI\DVWDWLVWLFDOO\VLJQL¿FDQW difference bet w een t he bilayers ( Table 2) . Cracking
predom inat ed am ong groups w it h 1 m m and 2 m m
porcelain t hickness, cat ast rophic failures were found in
50% of specim ens wit h 3 m m veneering ceram ics, and
delam inat ion/ chipping was present in 20% of bars wit h
2 m m of porcelain. The origin of failures occurred at
t he porcelain surface w it h propagat ion at t he int erface
bet ween zirconia and porcelain in all groups. Failures
ran from t he cent er t o t he edges in all groups ( Figures
3 and 4) . The proport ion of porcelain det ached from
zirconia in cat ast rophic failures was larger in t he 3 m m
t han in t he 2 m m group ( Figure 4) .
Groups Flexural
strength means
Type of failure p-value Grouping**
(SD) Cracking Delamination/
chipping
Catastrophic
1 mm 51.5 (9.84) 10 (100%) 0 (0%) 0 (0%) 0.001 A
2 mm 35.5 (9.03) 7 (70%) 2 (20%) 1 (10%) B
3 mm 35.8 (6.01) 5 (50%) 0 (0%) 5 (50%) B
Table 2- Flexural strengths (MPa) and types of failures
*URXSVWKDWVKDUHDOHWWHUZHUHQRWVWDWLVWLFDOO\VLJQL¿FDQWO\GLIIHUHQW7XNH\¶VWHVWD IRUÀH[XUDOVWUHQJWKGDWD
Defect variables
Indented region Groups Depth
ȝP ExtensionȝP VolumeȝP3)
1 mm 37.3 465.5 2.35
Close 2 mm 39.2 425.5 3.01
3 mm 52.7 610.5 3.01
1 mm 38 288.5 0.88
Far 2 mm 33.3 324.5 1.22
3 mm 30.3 386 0.77
Table 3- Mean value of depth, extension, and volume close to and far from zirconia
Figure 3- 1-mm-thick porcelain bar. Porcelain cracking (A) (10x). Origin of the crack in the porcelain surface and propagation toward the interface, from center to ends (B) (40x)
Figure 4-PPWKLFN$%DQGPPWKLFN'&(SRUFHODLQEDU'HIHFWSURSDJDWLRQIURPSRUFHODLQVXUIDFHWRLQWHUIDFH[DQG
Figure 5-5HSUHVHQWDWLYHLPDJHVIURPFRQIRFDOPLFURVFRS\RIWKHH[SHULPHQWDOJURXSVWRSSUR¿OHGHSWKDQGYROXPHH[WHQVLRQRIGHIHFWV
generated in the block specimens close to the interface
Figure 6-5HSUHVHQWDWLYHLPDJHVIURPFRQIRFDOPLFURVFRS\RIWKHH[SHULPHQWDOJURXSVWRSSUR¿OHGHSWKDQGYROXPHH[WHQVLRQRIGHIHFWV
Average dept h, ext ension, and volum e of near and
far indent at ions are show n in Table 3. Propagat ion of
defect s show ed higher dept h, ext ension, and volum e
close t o zirconia, while all t hese param et ers were lower
w hen far from zirconia, except for dept h in t he 1 m m
group. Figures 5 and 6 show defect s generat ed in t he
block specim ens.
Discussion
I nv est igat in g t h e f ailu r es in por celain - zir con ia
infrast ruct ures has becom e a m aj or focus of research
in t h e scien ce of d en t al m at er ials. I n p ar t icu lar,
underst anding t he role of residual st resses in t hese
failures rem ains a challenge for researchers. I n t he
present st udy, w e show ed t hat t hin porcelain layers PP SUHVHQWHG KLJKHU ÀH[XUDO VWUHQJWK DQG WKH indent at ion ( defect ) progression did not depend on
porcelain t hickness, but was m ore severe near t he
zirconia subst rat e t han on t he porcelain surface.
These result s agree w it h Lim a, et al.15 ( 2013)and
Whit e, et al.25 ( 2005) , in which a t hin veneer t hickness
SURYLGHGKLJKHUÀH[XUDOVWUHQJWKRIELOD\HUFHUDPLF specim ens. These st udies also present ed cracking as
t he principal failure m ode for low- t hick bilayers.
How ever, in clinics20,27, t he m ost frequent failure
is cusp fract ures of all- ceram ic rest orat ions because
t hey present great er t hicknesses of porcelain and are
affect ed by residual and m echanical st resses27. The
int erface bet w een porcelain and zirconia is sensit ive
t o t he poor heat t ransfer am ong m at erials29 and w hen
t hick layers of porcelain t hat have even low er t herm al
diffusivit y are used, t here is spalling of porcelain27.
Therefore, t hickness of porcelain increased from PP WR PP VLJQL¿FDQWO\ LQFUHDVHV WKH WKHUPDO t r an si en t st r ess g r ad i en t s b et w een t h e zi r co n i a
and veneer29, explaining t hus t he change in failure
p at t er n s b et w een 1 m m an d 2 m m g r ou p s, an d
j ust ifying t he occurrences of delam inat ion/ chipping.
Th e r esidu al st r ess scen ar io w as ev en w or se f or
t h e 3 m m sp ecim en s, also j u st if y in g t h e h ig h er
num ber of cat ast rophic ( delam inat ion) failures. The
p r ed om in an ce of cr ack s in t h e 1 m m g r ou p can
be ex plain ed by t h e com pr essiv e st r esses du e t o
t he t em per ing effect17 t hat favor ed t he m echanical
st rengt h of such bilayer syst em .
One can argue t hat bilayered disks are not t he
sam e as cr ow ns. Never t heless, t he layer ed m odel,
w hile sim plist ic in it s geom et ry, provides a physical
basis for invest igat ing t he role of m at erial propert ies
and t he effect of t hickness in t he idealizat ion of
all-ceram ic crow ns13, besides facilit at ing t he assessm ent
of t he origin and propagat ion of t he defect14. Wit h
r eg ar d t o r esid u al st r esses, t h e m ain d if f er en ce
bet w een crow ns and disks is a t endency of t he lat t er
for residual com pressive st resses t o increase w it h an
increase in t hickness from 1 t o 2 m m1. How ever, t he
m ore aggressive dam ages seen aft er t he indent at ion RIPPVSHFLPHQVGLGQRWFRQ¿UPVXFKREVHUYDWLRQ w hich is probably relat ed t o t he fact t hat w e used a
slow cooling prot ocol t hat led t o relaxat ion of st resses
w it hin t he layers.
I n t he present st udy, w e w ere not able t o quant ify
spat ially residual st resses in t he porcelain layers using
Vicker s indent at ions, alt hough t his has been done
before w it h t he sam e m et hod on curved surfaces1.
Th u s, w e on ly ob ser v ed t h e occu r r en ce of cr ack SURSDJDWLRQQHDUWKHLQWHUIDFHFRQ¿UPLQJWKHSUHVHQFH of t ensile st resses, as shown previously4,8,10,13- 17,27. Most
RIWKHFUDFNVH[WHQGHGODWHURLQZDUGO\FRQ¿UPLQJD t endency for porcelain chipping inst ead of porcelain
debonding from zirconia. Therefore, t ensile residual
st r esses occu r pr edom in an t ly par allel r at h er t h an
per pen dicu lar t o t h e por celain / zir con ia in t er face2.
Based on t he dam age ext ension, t he cracks ran m ore
rapidly in regions close t han far from zirconia. Alt hough
t he 1 m m group showed higher defect dept hs near t he
porcelain surface, crack lengt h and volum e rem ained
relat ively sm all. The higher dept h in t his group can
be explained by t he presence of a pore in t he vicinit y.
The presence of such irregularit y inside t he layers is
a lim it at ion of using t he indent at ion m et hod, because
t he crack can easily int eract w it h t he pore in t he bulk.
On t he ot her hand, com pressive st resses are prevalent
near t he porcelain surface5 regardless of t he veneer
t hickness17, as shown by sim ilar defect sizes and crack
lengt hs. How ever, t his t ype of residual st ress is m ore
int ense in fast cooled bilayers.
The use of Vick er s indent at ions for m easur ing
residual st ress has already been proved it self useful1,2.
Though it is not appropriat e t o com pare t he m echanical
propert ies m easured by t his m et hod, t he result s of
t he present st udy w ere consist ent in show ing t hat ,
depending on t he t hickness of t he porcelain, residual VWUHVV PD\ GLI¿FXOW FRPSUHVVLYH VWUHVV RU IDYRU ( t ensile st ress) crack propagat ion. I t is im port ant t o
as in t he Vickers indent at ion t est , is not analogous
t o t he m ast icat ory load, but it is a way t o prom ot e
t he occurrence of a severe defect and t o probe t he
fract ur e r esist ance of t he por celain. We ar e awar e
of t he residual st resses im part ed by t his indent at ion
m et hod but , in t he present st udy, it helped driving t he
crack along w it h t he residual t ensile st resses inside
t he porcelain.
Ot her t echniques for t he obser vat ion of crack s
and defect s are scanning elect ron m icroscopy ( SEM) , WUDQVLOOXPLQDWLRQ DQG PLFURWRPRJUDSK\ 7KH ¿UVW m et hod is im port ant for surface assessm ent but does
not allow in- dept h observat ions. The second m et hod
allow s us t o det ect int ernal defect s, precluding t hese
m easurem ent s. Finally, m icrot om ography facilit at es
t he obser vat ion of crack pr opagat ion but r equir es
considerable specim en preparat ion, w hich can dist ort
t h e or ien t at ion of cr ack s, lead in g t o d if f icu lt y in
dam age assessm ent1. The use of confocal m icroscopy
show ed sat isfact ory result s in t he analysis of defect s
found in t he ceram ic bilayer syst em . I t proved t o be a
quick and sim ple t echnique t hat does not require cost ly
sam ple preparat ion and t hat offers crucial inform at ion
on defect s in t he sur face and beneat h it , being a
viable m et hod for observing ceram ic defect s. Thus,
we believe t hat t he associat ion of confocal m icroscopy
and t he Vickers indent at ion is an effect ive resource for
observing defect s/ crack propagat ion and it highlight s
t he presence of residual st resses in ceram ic m at erials.
The ant icipat ed hy pot heses ( H1 and H2 ) w er e
a c c e p t e d . Li m i t a t i o n s o f t h i s s t u d y w e r e t h e
absence of r ealist ic scenar ios r esem bling t he oral HQYLURQPHQWVXFKDVWKHFRPSOHWLRQRIWKHÀH[XUDO t est or t herm alcycling in wat er, w hich w ere avoided t o
m inim ize t he occurrence of subcrit ical crack grow t h.
Furt her researches should be developed using confocal
m icr oscop y t o assess t h e p at h w ay of d ef ect s in
com plex geom et ries, such as crow ns and bridges. I n
addit ion, it is im port ant t o evaluat e t he behavior of
cracks or defect s generat ed under fat igue loading or
ot her t ypes of dam age28.
Conclusions
The t hinnest ( 1 m m ) porcelain/ zirconia specim ens SUHVHQWHGKLJKHUÀH[XUDOVWUHQJWK'HIHFWVLQFHUDPLFV close t o t he porcelain/ zirconia int erface grew fast er
t han defect s close t o t he por celain sur face for all
t hicknesses, suggest ing t hat t he occurrence of residual
t ensile st resses is higher at t his sit e.
Acknow ledgem ent s
YZ acknowledges t he US Nat ional I nst it ut e of Dent al
and Craniofacial Research Grant 2R01 DE017925.
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