ThreePoint Bending Tests of Zirconia CoreVeneer Ceramics for Dental Restorations

8/11/2019 ThreePoint Bending Tests of Zirconia CoreVeneer Ceramics for Dental Restorations

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Hindawi Publishing CorporationInternational Journal o Dentistry Volume , Art icle ID , pageshttp://dx.doi.org/ . / /

Research ArticleThree-Point Bending Tests of Zirconia Core/Veneer Ceramicsfor Dental Restorations

Massimo Marrelli, 1 Carmine Maletta, 2 Francesco Inchingolo, 3

Marco Alfano, 2 and Marco Tatullo 1

Biomedical Section, ecnologica Research Institute, Via E. Fermi, Crotone, Italy Department of Mechanical Engineering, University of Calabria (UniCAL), Arcavacata, Cosenza, Italy Department of Dental Sciences and Surgery, University of Bari, Bari, Italy

Correspondence should be addressed to Marco atullo; [email protected]

Received July ; Revised October ; Accepted January

Academic Editor: Brian W. Darvell

Copyright 2013 Massimo Marrelli et al. Tis is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Introduction . Temechanical strengthandthesur ace hardnesso commercially availableyttrium-dopedzirconiawere investigated.Furthermore, a comparative study o eight different ceramic veneers, to be used or the production o two-layered all-ceramicrestorativesystems,wascarried out. Materialsand Methods. Four typeso zirconiaspecimens were analyzed, according toa standardISO procedure (ISO ). Besides, two-layered zirconia-veneer specimens were prepared or three-point bending tests. Results. Astrong effect o the sur ace roughness on the mechanical strength o zirconia specimens was observed. Finally, a comparative study o eight commercially available veneering ceramics shows different modes o ailure between the selected veneers. Conclusion. Teresults indicate that close attention should be paid to the preparation o zirconia-based crowns and bridges by CAD/CAM process,because sur ace roughness has an important effect on the mechanical strength o the material. Finally, the results o the mechanicaltests on two-layered specimens represent an important support to the choice o the veneering ceramic.

1. Introduction

Te use o advanced ceramics as restorative dental mate-rials is strongly increasing, owing to the introductiono Computer-AidedDesign/Computer-Aided Manu acturing(CAD/CAM) milling techniques which allow the abricationo large andcomplex restorationswithvery high-dimensionalaccuracy [ , ]. Te most promising production methodconsists in a sof machining o presintered blocks, whichare subsequently sintered at high temperature [ ]. As a nalstep, sintered structures are usually coated using veneeringceramics, in order to obtain two-layered all-ceramic restora-tive systems with very attractive mechanical properties, goodbiocompatibility, and excellent esthetic results [ ].

Among the ceramic materials or dental applications, thezirconia-based ones are very widespread, because o theirtrans ormation toughening capabilities [ , ].

Te aim o the present study is to analyze the mechan-ical behavior o commercially available Y- ZP ceramics

or dental applications and to estimate the effects o di -erent processing conditions, which usually occur during

production by CAD/CAM techniques. In addition to this,eight commercially available ceramic veneers, to be used orthe production o all-ceramic restorations in combinationwith Y- ZP structures, were analyzed by using three-pointbending tests on two-layered specimens. Finally, a systematiccomparative analysis o the eight selected ceramic veneerswas carried out.

2. Materials and Methods

Te mechanical strength o a commercial yttria-stabilizedzirconia, to be used as core material or the productiono crowns and bridges in combination with CAD/CAMtechniques, was analyzed by three-point bending tests onstandard specimens. Furthermore, the mechanical behavioro eight different types o veneering ceramics was analyzed by


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International Journal o Dentistry

: Nomenclature o zirconia core specimens.

Specimen type Descriptionype A Sinteredype B Colored and sinteredype C Sintered and polished

ype D Colored, sintered, and polished

exural tests o two-layered zirconia-veneer specimens andmicrohardness measurements.

. . Flexural ests of Core Specimens. Te bending testso zirconia-based core material were carried out ollowingstandard ISO procedures and recommendations (ISO ).Beam specimens, with length ( ) = 25mm, width ( ) =5 mm, and thickness ( ) = 2mm, were prepared or exuraltests; all dimensions were measured with an accuracy o . mm. Te specimens were cut rom presintered blocksby using a high-speed cutting machine, provided with a dia-mond disk ( 63mm), and their size was properly increasedin order to take into account the material shrinkage whichoccurs in the subsequent sintering process.

Te three-point bending tests were done under displace-ment control (cross-head eed rate equal to mm/min) by using a universal testing machine (Instron ), with a kNload cell, controlled by a estStar II (M S) controller.

In order to analyze the effect o sur ace nishing andcoloring process on the exural strength, our differentspecimen types were produced and tested, or example, ypeA, ype B, ype C, and ype D ( able ).

ype A specimens were tested when sintered.

ype B specimens were coloredbe ore sintering, by usingcommercial dyes.

ype C specimens were polished afer sintering; in par-ticular, sur ace polishing was done by using water-cooledcarborundum disks, with progressively ner alumina grits,ranging between , , and , respectively.

ype D specimens were colored, sintered, and polished.As the sur ace condition can have a great in uence onthe mechanical behavior o the core material, the sur aceroughness o each specimen type was measured by using acontact measuring system (MarSur III, Mahr). Te sur aceroughnesswas measuredalongthe longitudinal and transver-sal direction o the specimens. Five measurements or each

direction were carried out, with a traveling distance o mm.

. . Flexural ests of wo-Layered Core/Veneer Specimens.Flexural tests o two-layered core/veneer specimens, or each veneering ceramic analyzed, were carried out by using thesame equipment and testing parameters described in theprevious section. Te specimens, with length = 25mm,width = 5mm, and total thickness = 2.2mm, weremade by coating . mm thick zirconia core layers withidentical thickness veneering ceramics, by ollowing themanu acturers directions and instructions. It is worth notingthat zirconia substrates were coated and sintered without any preliminary sur ace treatment.

Te specimens were analyzed by three-point bendingtests,witha testspanequal to mm,andthe loadwas appliedon the veneer sur ace; that is, the veneer layer is subjectedto compressive stress. In order to carry out a comparativeanalysis between the eight selected veneering ceramics, thetotal strain energy per unit volume was calculated.

In act, this energetic parameter allows a comparativestudy between the different types o core/veneer specimens,as it describes the overall mechanical behavior o the two-layered system. Te total strain energy per unit volume wascalculated rom the experimentally measured load-de ectioncurves, by considering the volume o the specimen betweenthe test spans.

3. Results

Te results, concerning the experimental tests carried outon zirconia core specimens and on two-layered core/veneersystems, are described in this section. Inparticular, the results

o bending tests per ormed on zirconia specimens are rstly discussed; then, a comparative analysis o the eight selectedceramic veneers is given.

. . Mechanical Strength of Zirconia Core Material. wenty specimens or each type listed in able ( ypes A, B, C,and D) were analyzed, in order to measure the effects o coloring and sur ace roughness on the mechanical strength;Students -test at a % con dence level was done in orderto analyze the difference in strength between the our typeso specimens. able summarizes the results obtained orsur ace roughness and exural strength. As expected, ypesC and D (polished specimens) show similar values o sur ace

roughness, as well as ypesA andB (nonpolishedspecimens)( able ).

able clearly shows that sur ace polishing causes astrong increase in the average values o exural strength,as well as in Weibull characteristic strength. able alsoshows that the exural strength o the polished specimensis characterized by smaller standard deviations (and higherWeibull moduli) with respect to the unpolished ones. Teseresults are mainlycaused by microscopic sur ace sharp cracksand scratches, which act as crack initiation sites. Teseobservations are also con rmed by Students -test whichindicatesa signi cantdifference in strengthbetween polishedand nonpolished specimens, while the coloring process does

not signi cantly affect the mechanical strength.

. . Mechanical Behavior of wo-Layered Zirconia-Veneer Sys-tems. en two-layered specimens or each veneering ceramicwere analyzed by three-point bending tests. Tree different

ailure mechanisms have been observed as shown in Figure .

(a) Simultaneous ailure o zirconia and veneer ( ype F).

(b) Complete inter acial debonding o the veneer ( ypeD).

(c) Serrated racture o the ceramic veneer ( ype S),that is, the crack spreads in the veneer, approaches


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: Failure mechanism o two-layered zirconia-veneer speci-mens.

Veneer type Failure mechanism

Lava Ceram F and SCeramco PFZ F Vita VM F and S

riceram F and SZirox S and DGC Initial ZR F and SIPS e.max DSakura Interaction F F : simultaneous ailure,S: serrated racture,and D: complete delamination.

0.35

0.30

0.25

0.20

0.15

0.10

0.05

0.00

Veneer damage

Complete failure

V M 9

T r i c e r a m

Z i r o x

G C I n i t i a

l P F Z

L a v a

I P S e . m a x

S a k u

r a

S p e c

i c s t r a

i n e n e r g y ,

( J / m m

3 )

F : Strain energy per unit volume at ailure and veneerdamage or the two-layered specimens.

4. Discussion

Te zirconia-based ceramic materials are the most promis-ing or dental application, because o their trans ormationtougheningcapabilities [ , ]. Inparticular, this action can bemainly ascribed to a stress-induced tetragonal-to-monoclinicphase trans ormation ( ) and to the corresponding volume expansion [ ]. More speci cally, the latter inducescompressive internal stresses, thereby leading to crack growtharrest and to an increase in racture toughness [ , ]. Itis worth noting that zirconia-based ceramics are usually doped with stabilizing oxides, such as yttrium (Y- ZP,Yttria etragonal Zirconia Polycrystal), in order to ensurea tetragonal structure at room temperature and, there ore,improve this toughening effect. As a direct consequence o these interesting eatures, many research activities have beencarried out on this subject so ar, with the aim o analyzingthe mechanical strength o Y- ZP dental ceramics, as wellas their interaction with different ceramic veneers. In detail,due to the brittle behavior o this class o materials, several

experiments have been conducted in order to understand theeffects o different sur ace conditions on their mechanicalstrength [ ]. Furthermore, as Y- ZP ceramics are used incombination with veneering ceramics to produce all-ceramicrestorative systems, the mechanical behavior o two-layeredstructures has also been recently investigated by suitable

experimental tests [ ].In the present study, the authors analyzed the mechanicalbehavior o a commercial yttria-stabilized zirconia or dentalapplication (Kavo Everest Bio ZS Blank), as well as itsinteraction with eight commercial veneering ceramics.

Te effects o sur ace roughness as well as o the coloringprocess on the mechanical strength o the stabilized zirconia,analyzed by standard three-point bending tests, were rstly examined. Te results indicate that sur ace roughness plays acritical role in the mechanical strength o zirconia structures,because a strong increase in the average exural strength,

romabout MPa to MPa, isobserved afer a mechan-ical polishing treatment o the test specimens. Te resultsalso show that the exural strength o thepolished specimensis characterized by smaller standard deviations (and higherWeibull moduli). More accurate sur ace polishing, that is,smaller values o sur ace roughness, could urther increasethe mechanical strength o the material [ ]; however, thishas a limited practical implication due to the higher values o sur ace roughness produced by the CAM milling processes,as well as by the common postmilling laboratory procedures[ ].

Furthermore, the results show that the coloring process,carried out by using a commercial coloring liquid ( ZirkonZahn ), has no signi cant effects on the mechanical strengtho the zirconia specimens. As zirconia-based structures arenormally used in combination with veneering ceramics orthe production o all-ceramic restorations, the mechanicalbehavior o eight commercial ceramic veneers has been ana-lyzed by three-point bending tests o two-layered zirconia- veneer specimens, and comparative studies o the selectedceramics have been carried out. Te results achieved by means o bending tests show the ollowing three different

ailure mechanisms or the selected ceramic veneers [ ]:(i) simultaneous ailure o zirconia and veneer, indicatinga good adhesion strength between core and veneer; (ii)complete inter acial delamination, indicating a lower adhe-sion strength; (iii) serrated racture, characterized by mixedcohesive and adhesive ailure mechanisms. Furthermore,the total strain energy per unit volume o the two-layeredspecimens was calculated to give an overall measure o the mechanical behavior o the eight selected core/veneersystems.

It is worth noting that the effects o sur ace roughness,at the core/veneer inter ace, on the adhesion mechanismswere not analyzed in this investigation; however, even i it is expected to play a signi cant role in ailure mecha-nisms o all-ceramic restorations, the sur ace morphology o the zirconia dental rames, obtained rom CAD-CAMtechniques, is not modi ed prior to ceramic veneering, dueto both economic and technological issues. Tere ore, thiswork wasaimed at the identi cation o the best combinationso core/veneer, based on a re erence sur ace condition, with


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roughness values ( ) close to the that obtained rom CAMmilling process [ ].

On the contrary, the gingival sur aces o dental bridgesare normally not veneered and, consequently, the roughnesso the zirconia rame plays a very important role in themechanical strength, because they are subjected to tensile

stresses during chewing; there ore, high care should bedevoted to both milling and/or possible postmilling labo-ratory procedures o zirconia rames in order to avoid the

ormation o rough sur aces, especially in the connectorarea, where geometric discontinuities and sources o stressconcentration are present.

5. Conclusions

Te results o this research indicate that close attentionshould be paid to the preparation procedure o zirconia-based crowns and bridges by CAD/CAM process, with theaim o obtaining smooth sur aces, because a strong effect o

sur ace roughness on the mechanical strength was observed.Furthermore, no signi cant effects o the coloring process onthemechanicalbehaviorweremeasured.Finally, comparativestudies o several commercial ceramic veneers, to be used

or the realization o all-ceramic systems, show differentmechanical behavior and ailure modes between the selected veneers. Further studies should be carried out to measurethe wear properties o the ceramic veneers, as well as theinteraction with natural enamel.

Acknowledgments

Tis study was supported by ecnologica Srl (Crotone, Italy).

Te authors wish to thank Mr. Roberto Marrelli and histechnical team(OdontoiatricCenter Calabrodental, Crotone,Italy) or the valuable support in specimen preparation.

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