Keramički materijali opravdavaju sve češću uporabu u restauraciji oštećenoga žvačnog sustava. »isti keramički sustavi rabe se u izradbi inleja, krunica i mostova manjih raspona. Cirkonijev dioksid je poznat polimorf. Dodatkom magnezijeva ili itrijeva oksida u određenom postotku, ZrO2 je potpuno ili djelomično stabiliziran i time je omogućena njegova uporaba u stomatološkoj protetici. Tetragonski cirkonski polikristal (TZP) stabiliziran s 3mol% itrija ima izvanredna mehanička i estetska svojstva i ima sve veću uporabu u restauraciji endodontski saniranih zuba, izradbi krunica i mostova CAD/CAM sustavom. Očekivati je da će taj keramički materijal nadomjestiti kovinsko-keramičke nadomjestke i u sanaciji gubitka većega broja zuba.Dental ceramics justifies more frequent use in prosthetic restoration of damaged dental status. Inlays, crowns and three-unit bridges have been made of all-ceramic system. Zirconia dioxide is a well- known polymorph. The addition of stabilising oxides like MgO, Y2O3 to pure zirconia, makes it completely or partially stabilized zirconia which enables use in prosthodontics. Tetragonal Zirconia Polycrystals (TZP) stabilized with 3mol % yttria, has excellent mechanical and esthetical properties. Fixed prosthetic appliances of this ceramic have been made using CAD/CAM techniques. It can be expected that zirconium oxide ceramics will replace metal-ceramics in restorations that require high strength

Andreja Carek

Jasenka Živko-Babić

Marko Jakovac

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A S CActa Stomatol Croat, Vol. 39, br. 1, 2005. 25Zirconium Oxide Ceramics inProsthodonticsSummaryDental ceramics justifies more frequent use in prosthetic restorationof damaged dental status. Inlays, crowns and three-unit bridges havebeen made of all-ceramic system. Zirconia dioxide is a well- knownpolymorph. The addition of stabilising oxides like MgO, Y2O3 to purezirconia, makes it completely or partially stabilized zirconia whichenables use in prosthodontics. Tetragonal Zirconia Polycrystals (TZP)stabilized with 3mol % yttria, has excellent mechanical and estheticalproperties. Fixed prosthetic appliances of this ceramic have been madeusing CAD/CAM techniques. It can be expected that zirconium oxideceramics will replace metal-ceramics in restorations that require highstrength.Key words: Zirconia, mechanical properties, Tetragonal ZirconiaPolycrystals (TZP), partially stabilized zirconia (PSZ), In-Ceram Zir-conia.Jasenka Æivko-BabiÊAndreja CarekMarko JakovacDepartment of ProsthodonticsSchool of Dental MedicineUniversity of ZagrebActa Stomat Croat2005; 25-28PRELIMINARY REPORTReceived: March 23, 2004Address for correspondence:Prof. Jasenka Æivko-BabiÊZavod za stomatoloπkuprotetikuStomatoloπki fakultetGunduliÊeva 5, 10000 Zagrebtel: 01 4802 135fax: 01 4802 159IntroductionZircon has been known as a gem since ancienttimes. The name of metal zirconium, comes fromthe Arabic Zargon, which means golden colour. Zir-conia, the metal dioxide (ZrO2) ,was identified in1789 by the German chemist Klaproth (1). The devel-opment of zirconia as an engineering material wasdemonstrated by Garvie et al, who showed how tomake the best of T-M phase transformation of Par-tially Stabilized Zirconia (PSZ) improving mechan-ical strength and toughness of zirconia ceramics (2).ZrO2 exists in several modifications and shapes andit is known a lot of different kinds of processing fromcold to warm. Pure ZrO2 is of limited use for indus-trial applications because of the phase transforma-tions that occur.Zirconia is stable in oxidizing and poor reducingatmospheres. It is inert to acids and bases at roomtemperature (RT) with the exception of HF. It reactswith carbon, nitrogen and hydrogen at temperaturesabove 2200°C and does not react with the refracto-ry metals up to 1400°C. However, its reactivity isstrongly dependent upon the type and quantity of thestabilizer used in the ceramic (3).Structure and properties of zirconium dioxideZirconium dioxide appears as a monoclinic, cubicor tetragonal polymorph. At room temperature onlythe monoclinic ZrO2 exists. This phase is stable upto 1170°C when it inverts to a tetragonal, metastablephase, whereas above 2370°C it turns into a cubicA S C Acta Stomatol Croat, Vol. 39, br. 1, 2005.26Jasenka Æivko-BabiÊ et al. Zirconium Oxide Ceramics in Prosthodonticsphase. Cooling enables allotropic changes below1070°C, when the tetragonal phase is transformedinto monoclinic. It occurs in the temperature rangeof about 100°C. By the mentioned transformationof ZrO2, its volume increases by 3 - 4 % and its formis changed. Stress caused by volume increase formscracks in pure zirconia ceramics, which simply dis-integrates at RT after sintering at temperaturesbetween 1500 and 1700°C. To stabilise ZrO2, sev-eral oxides have been added (ZrO2-CaO, ZrO2--MgO, ZrO2-Y2O3) (1, 4-6). Stabilisation primari-ly means decreasing the temperature of T- to M-phase transformation. Oxide as a stabiliser, partial-ly or wholly prevents transformation and is a fac-tor of the transformational strengthening of the zir-conia-based ceramic structure. By the addition ofstabilising oxides such as CaO, MgO, CeO2 or Y2O3to pure zirconium dioxide, a multiphase material isobtained, known as partially stabilised zirconia(PSZ). At RT the PSZ matrix is of cubic structure,with monoclinic and tetragonal precipitates. Garvieand Nicholson demonstrated how PSZ's mechanicalproperties can be enhanced by the homogenous anduniform distribution of the monoclinic phase in acubic matrix (7). Ruff et al. proved in 1929. thatthe stabilisation of the cubic C-phase is possible atRT if a small amount of CaO is added to ZrO2 (quot.from Piconi and Maccauro, 1). The addition of largeamounts of a stabiliser such as MgO will induce acubic crystal structure during firing, which does notrevert to the monoclinic phase upon cooling (3). The addition of less than 10wt% of stabilisersresults in high-density of the ceramic particles, yield-ing thus transformation toughened zirconia or tetrag-onal zirconia polycrystals (TZP) (3, 8, 9). Extreme-ly fine crystals inhibit T M phase transformationduring cooling. Transformation toughened zirconiaexhibits superior strength and fracture toughness (10,11). Due to these properties, the load of this mate-rial can induce the phase transition of tetragonal tomonoclinic crystals, with a corresponding change involume. TZP exhibits optimal characteristics below500°C.Depending on the stabiliser, MgO or Y2O3, thereare two different microstructures. The addition ofMgO yields relatively coarse-grained (50-100μ)microstructure, known as Mg-PSZ. The grains are predominantly cubic morphologywith fine precipitates of tetragonal phase. Garvie etal. observed that tetragonal metastable precipitatesuniformly dispersed within the cubic matrix cantransform into the monoclinic phase when the matrixpressure decreases. This occurs at the formation andgrowth of cracks within the matrix which is there-by deformed (2). The stress induced by volumeincrease, caused by crystal transformation, acts inopposition to the strain, which accelerates crackpropagation. The material becomes more resistantdue to the lack of energy lost in suppressing com-pressive forces through volume increase. This isrelated to crack propagation during the TõM phasetransformation. Such tetragonal metastable precip-itates can be formed by adding 8mol% MgO to ZrO2.Tetragonal metastable phase can also be formed inthe matrix during controlled cooling.There are two basic types of Mg-PSZ. One (MS)exhibits superior fracture toughness, high mechan-ical strength and average thermal shock resistance.Application is optimal at temperature below 800°C,with minimal thermal cycling. The other type (TS)exhibits excellent thermal shock resistance, lowerstrength but higher toughness. TS can be optimallyprocessed at a temperature above 600°C.The addition of Y2O3 enhanced the characteris-tics of ZrO2 in applications. The main characteris-tic of yttria-zirconia ceramics is a fine-grained (<1 μ)microstructure known as Y-PSZ or tetragonal zir-conia polycrystals (TZP). If we consider PSZ, thematerial is multiphase. On the other hand, TZP is amonophase or almost monophase material. In theTZP material with approximately 2-3mol% Y2O3particles are tetragonal. The tetragonal TZP phaseat RT depends on the amount of yttrium oxide, thegrain size, and the matrix pressure. Lin et al. observedthat the aluminium-silicate matrix in TZP destroysyttrium ions on grain boundaries, causing the lossof tetragonal phase stability (12). Zirconium diox-ide with a higher Y2O3 concentration is complete-ly stabilised, less flexible and deformable than thepartially stabilised (13). It is less resistant to ther-mal shock than Mg-PSZ. Optimal temperature forits applications is below 500°C. At high temperatures (>900°C) yttria-stabilisedzirconia is prone to deformations due to the sus-pension of insoluble particles and high degree ofoxygen permeability, which results in creep (3, 12,14, 15). Rieth et al. (10) and Gupta et al. (11) firstreported obtaining this ceramic.Jasenka Æivko-BabiÊ et al. Zirconium Oxide Ceramics in ProsthodonticsA S CActa Stomatol Croat, Vol. 39, br. 1, 2005. 27The successful ZrO2 application required a newcomponent that would retain cubic phase resistanceat high temperatures. The mixing of ZrO2 and 5mol%Er2O3 stabilises the zirconia microstructure andincreases deformation and creep resistance at hightemperatures. The ZrO2-Er2O3 microstructure con-sists of stable and metastable tetragonal precipitates.They convert to a nanosized tetragonal lamellarstructure during thermal treatment. Stabilised or par-tially stabilised zirconia act as surface thermal bar-riers (16).There are several ceramic materials toughenedby lanthanum phosphate or zirconium dioxide (17,18). Zirconia toughened alumina (ZTA) is increas-ingly applied in dental restoration (14, 19-22). Theaddition of ZrO2 makes glass-infiltrated In-Ceramceramics tougher. In-Ceram Zirconia (ICZ) contains33wt% of partially stabilised zirconium dioxide,which increases tensile strength and fracture tough-ness. This material also contains 16wt% CeO2 (23). Zirconia ceramics have superior mechanical prop-erties to other biomaterials, e.g. aluminium oxideceramics (9, 23-27). It has by far the highest frac-ture strength values (x = 913 - 1000 MPa) and alsothe Weibull module (m = 18.4) (9, 26-33), whichensures its higher clinical durability compared withIn-Ceram ceramics. In spite of high strength beforefracture In-Ceram has a lower value of the Weibullmodule. This means that fractures in In-Ceram ceram-ics could also occur at smaller loads. The Young'smodule of ZrO2 ceramics is in the same order as theone of base metal alloys (33). ZrO2 ceramic is extremely hard (HV 1200), itsflexural strength is 1000-1300 MPa (three-point flex-ure test), and fracture toughness 9-10 MPa. Optimalmechanical properties are a result of ZrO2 powderpurity, the specific size and distribution of particles,industrial sintering, hot isostatic pressing (HIP), andfine-grained metastable microstructure (34). HIPtoughened ZrO2 is resistant to bite forces up to 1000N. External load causes phase transformation andvolume increase of up to 4%. This dimensionalchange caused by pressure, stops crack propaga-tion and thereby toughness the material. The so-called transformation toughening is another reasonfor the excellent mechanical properties of TZP (9,35). Sandblasting has the same positive effect. Elec-troplating of metal surfaces by zirconium dioxidereduces the release of metal ions (36). Thermal cyclingenhances the bond between the composite and zir-conia's ceramics (37).ZrO2 biocompatibility is similar to that of tita-nium (23). Y-PSZ as implants exhibit the same osseointegration percentage as titanium (34). As the firstlayer around the wax object, zirconium dioxidereduces the reaction of titanium with investmentmaterial. At the same time it reduces the thicknessof the surface reaction layer on titanium cast (38).Processing and application of zirconium dioxideZirconium dioxide as a biomaterial has been usedin medicine as ball heads for Total Hip Replace-ments. ZrO2 is mostly used for inlays, crowns, onlays,bridges, implants, commercial posts and attachmentsbecause of its good esthetical, chemical, mechani-cal properties and dimensional stability. (9, 20, 25,31, 39-45). Silver-grey colours of tooth structure are anesthetical problem in reconstruction of anterior teethbecause of metallic posts. Non-metallic posts incombination with all-ceramic crowns are aestheti-cally preferable for the restoration of anterior teeth(9, 20, 35, 42-45). Industrially made ceramic mate-rial in the form of a blanket or ingots have betterfracture strength than those made in the dental lab-oratory (27). They are more suitable for clinicalusage, although CAM/CAD machining may causesurface and subsurface cracks (46). Zirconium diox-ide is known as a white, black and green one, withregard to its processing (47). Zirconia-based ceram-ic can be obtained by sintering procedure or in heat-pressed reaction (48). During sintering intermetalphases oxidise first (ZrSi2). It appears ZrSiO2 andSiO2 as amorphous matrix. By sintering of ZrO2 andSiO2, ingots of black zirconium dioxide are formed.Oxidation causes growth and expansion of particleswhich compensates contraction as a consequence ofsintering. HIP procedure is the sintering procedureof material under high pressure and at high temper-ature. These procedures reduce porosity of the mate-rial and ensure high values of toughness and translu-cency of zirconium ceramics. There is no addition-al sintering or infiltration.Ingots prefabricated by HIP are formed by Com-puter-Aided-Design/Computer-Aided-manufactur-A S C Acta Stomatol Croat, Vol. 39, br. 1, 2005.28Jasenka Æivko-BabiÊ et al. Zirconium Oxide Ceramics in Prosthodonticsing (CAD-CAM) techniques. The tooth should beconusly prepared with 5-15-degree taper. An opti-cal impression is made and the basic colour andcolour of tooth checked. Ingots are milled withinone hour with accuracy of 1 nm in detail reproduc-tion. Veneering can be obtained with one of the den-tal ceramics. It may be cemented like any otherceramic crown. Zirconium ceramic does not react toprotein in saliva. Plaque does not remain on the sur-face of these crowns (49).White ZrO2 can be processed by basic procedure.Enlarging crowns or bridge skeletons are milledfrom porous, presintered blanks. The objects willlater be sintered. Shrinkage is 20-30%, which enablesoptimal dimension of restorations. This process iscalled the Cercon system (13).Dimensional stability of core made by the Cer-con system is also retained during the fusing ceram-ic layers and glazing. Clinical application of Y-TZP ZrO2 in dentistrystarted more than ten years ago. Prosthetic appliances can be shaped by hard pro-cedure. Because of dimensional stability, sinteredceramic blanket may be milled in proportion 1:1. Itcan be coated with ceramic material and glazed.Infiltration type zirconium ceramic is based onAlO3/ZrO2. The white ingots are milled by whiteprocedure and infiltrated with lanthalium glass. There are few literature data on the success ofzirconium as construction and veneer material. Y--TZP ZrO2 have less thermal coefficient than con-ventional feldspathic ceramic. It is necessary to findceramic material with adjusted thermal coefficient.Today, zirconium ceramic has indication for crownsand three-unit bridges. Zirconium ceramics is one of the many materi-als which can ensure optimum aesthetics and func-tional permeability in restorations of long duration.

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