Yttria-stabilized Tetragonal Zirconia Polycrystalline Ceramics Research Articles

Rapid processes for the production of nanocrystal yttria-stabilized tetragonal zirconia polycrystalline ceramics: ultrasonic spray pyrolysis synthesis and high-frequency induction sintering

In this study, nanocrystalline 3 mol % yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) ceramic was produced by sintering with a high-frequency induction heating (HFIH) system of granular powders obtained by ultrasonic spray pyrolysis (USP) at 600 °C. The granular nano-sized powders (10-30 nm) were micron in size (average size: 700 nm), spherical in shape and amorphous. The influences of the HFIH sintering temperature (1400-1600 °C), applied current time (60-300 sec.) and the mechanical pressure (10 MPa and 20 MPa) on the final density and grain size of the products were investigated. The amorphous granular Y-TZP powders compressed with the HFIH system allow very rapid condensation in the tetragonal phase at high density and avoid grain growth. High density (relative density over 95) nanocrystalline Y-TZP ceramic with ∼70 nm size could be obtained from the simultaneous application of 20 MPa pressure and an induced current within 300 sec. of sintering time at 1500 °C. In this condition, the sample’s maximum hardness and fracture toughness values were reached at 14.9 GPa and 3.8 MPa·m1/2, respectively. Y-TZP powders produced in nano-micro structure with the USP system were sintered with the HFIH system and rapid production was achieved by preventing grain growth.

Optimized sintering and mechanical properties of Y-TZP ceramics for dental restorations by adding lithium disilicate glass ceramics

The novel dental ceramics can be fabricated at lower temperatures when sol-gel derived lithium disilicate glass ceramics (LDGC) was used as an additive for yttria stabilized tetragonal zirconia polycrystalline (Y-TZP) ceramics. The effect of LDGC on the sintering, mechanical, and translucent properties of Y-TZP ceramics was investigated in the present study. The results showed that the LDGC additive effectively improved the densification of Y-TZP at 1100 °C, which was much lower than the sintering temperature for pure Y-TZP. When sintered at 1100 °C, the Y-TZP with 1 wt% LDGC reached a relative density of 95.45%, and prossessed a flexural strength of 482.4 MPa and a fracture toughness of 5.94 MPa·m1/2. Moreover, its translucency was also improved. While, the addition of LDGC could result in an escape of yttrium atoms from the grain lattice of zirconia, which induced the tetragonal-monoclinic transformation of zirconia and abnormal growth of monoclinic grains. The escaped yttrium atoms diffused into the intergranular glass phase. The results indicated that the novel Y-TZP-LDGC ceramics has a great potential to be used for all-ceramic restorations.

Fracture toughness enhancement of yttria-stabilized tetragonal zirconia polycrystalline ceramics through magnesia-partially stabilized zirconia addition

Abstract In this work, for the first time, we report a novel method on the fracture toughness enhancement of 3 mol % yttria-stabilized tetragonal zirconia polycrystalline (3Y-TZP) ceramics through the incorporation of 8 mol % magnesia-partially stabilized zirconia (8 Mg-PSZ) powders having high fracture toughness. Highly densified composites (x3Y-TZP/y8Mg-PSZ; where x and y vary between 0.25 and 0.75 wt. %) were obtained with a relative density over 99% by pressureless sintering. Relative density, Vickers hardness (HV) and indentation fracture toughness (KIc) were significantly improved by sintering temperature and dwell-time increment. Specifically, HV and KIc values of 0.5(3Y-TZP)/0.5(8 Mg-PSZ) composite sintered at 1500oC-2h were increased by 7% and 30%, respectively, compared to that of 3Y-TZP. Sintered bodies consisted of c-ZrO2, t-ZrO2 and m-ZrO2 phases without any new phase formation. m-ZrO2/c-ZrO2+t-ZrO2 volumetric phase ratios changed with the increase of sintering temperature and time. Stress-induced t-ZrO2→m-ZrO2 phase transformation within c-ZrO2 grains in 8 Mg-PSZ was the main mechanism for toughness enhancement. Energy absorbing mechanisms, e.g., crack-bridging, crack-deflection and crack branching were also found to contribute the blunting of cracks. It is thought that our approach presented herein can be considered not only fracture toughness enhancement but also other properties in various materials for functional and structural purposes.

The Effect of Pr–Er Colorant on the Chroma of Dental Zirconia Ceramics

Three mol.% yttria-stabilized tetragonal zirconia polycrystalline ceramics (3Y-TZP) billets were dipped in dyeing solution and then sintered for external dyeing. The effect of Pr and Er single elements on 3Y-TZP coloring and the rule of multivariate influence on the overall coloring effect after mixing were investigated and clarified. The structure, morphology, color and physical properties of 3Y-TZP were characterized by x-ray diffraction, scanning electron microscopy and spectrophotometry. The results show that Pr and Er show yellow and red, respectively, in the process of coloring, and the effect of Pr coloring is confirmed. When the two elements are used for dyeing at the same time, the value of a*, b* of the mixed color shows the additivity of the value of a*, b*, and the value of L* increases. This mixed dyeing is of great significance to the development of dental ceramics.

Clinical Evaluation of All Ceramic Zirconia Framework Resin Bonded Bridges.

To evaluate the clinical longevity of 58 adhesively bonded single unit yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) ceramic Resin Bonded Bridges (RBB). Twenty six consecutive patients with at least one congenitally missing tooth in the maxilla or mandible were provided with 58 single unit Y-TZP RBBs. The cantilever RBBs were designed and milled using a CAD-CAM system to produce frameworks which were veneered using a glass-ceramic material and cemented with a self-etching dual-cure resin cement. Following a mean follow up period of 36.2 months (maximum 62.3 months, minimum 25.4 months), 48 restorations remain in service with a survival rate of 82.7%. One anterior retainer fracture was encountered and no fractures of the posterior framework or ceramic were noted within the time frame reported. All ceramic Y-TZP RBBs replacing maxillary and mandibular teeth in the anterior and posterior areas demonstrated an 82.7% Kaplan Meier survival rate over 3 years, which was comparable to previously published survival rates for the non-perforated metal framework RBBs. Replacement of anterior or posterior teeth using single unit cantilevered RBBs using Y-TZP ceramics should be considered a viable restorative option with a high survival rate.

Comparative radiopacity of conventional and full-contour Y-TZP ceramics.

The purpose of this study was to evaluate the radiopacity of different yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) ceramics used in the fabrication of fixed dental prostheses. Disk-shaped specimens (N=120, n=30 per group) were fabricated with thicknesses of 0.3, 0.5, and 1 mm from the following four Y-TZP ceramics: Cercon ht (Degudent) (CZ), Prettau (Zirkonzahn) (ZZ), Alliance (Kuraray Noritake Dental) (NA), and Ice Zirkon (Zirkonzahn) (ICE). The optical densities of each material were measured from radiographic images, and each material (N=1) were characterized by using XRD and SEM. The results were analyzed with oneway analysis of variance and the Tukey-Kramer test (α=0.05). In the case of the ICE and NA specimens, the radiopacity significantly differed with thickness (p<0.05). However, the radiopacities of 0.5- and 0.3-mm-thick CZ and ZZ specimens were not significantly different (p>0.05). Full-contour Y-TZP materials have higher radiopacities than those of the conventional Y-TZP materials in decreased thicknesses.

In vitro shear bond strength of Y-TZP ceramics to different core materials with the use of three primer/resin cement systems

Statement of problemDurability of the bond between different core materials and zirconia retainers is an important predictor of the success of a dental prosthesis. Nevertheless, because of its polycrystalline structure, zirconia cannot be etched and bonded to a conventional resin cement. PurposeThe purpose of this in vitro study was to compare the effects of 3 metal primer/resin cement systems on the shear bond strength (SBS) of 3 core materials bonded to yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) ceramic retainers. Material and methodsZirconia ceramic (Cercon) disks (5×3 mm) were airborne-particle abraded, rinsed, and air-dried. Disk-shaped core specimens (7×7 mm) that were prepared of composite resin, Ni-Cr, and zirconia were bonded to the zirconia ceramic disks by using one of 3 metal primer/cement systems: (Z-Prime Plus/BisCem, Zirconia Primer/Multilink Automix, or Clearfil Ceramic Primer/Clearfil SA). SBS was tested in a universal testing machine. Stereomicroscopy was used to evaluate the failure mode of debonded specimens. Data were analyzed using 2-way ANOVA and post hoc analysis using the Scheffe procedure (α=.05). ResultsClearfil SA/Clearfil Ceramic Primer system with an Ni-Cr core yielded the highest SBS value (19.03 MPa), whereas the lowest SBS value was obtained when Multilink Automix/Zirconia Primer system was used with the zirconia core group (4.09 MPa). Differences in mean SBS values among the cement/primer groups were statistically significant, except for Clearfil SA and BisCem with both composite resin and zirconia cores. Differences in mean SBS values among the core subgroups were not statistically significant, except for zirconia core with BisCem, Multilink, and Clearfil SA. The predominant failure mode was adhesive, except for Clearfil SA and BisCem luting agents with composite resin cores, which displayed cohesive failure, and Multilink Automix with a composite resin, core as well as Clearfil SA with Ni-Cr cores, where the debonded specimens of each group displayed a mixed (adhesive/cohesive) failure pattern. ConclusionsUse of the Clearfil SA/Clearfil Ceramic Primer system, based on methacryloyloxydecyl dihydrogen phosphate (MDP), increased the bond strength of Y-TZP ceramics to core materials.

High speed end-milling characteristics of pre-sintered Al2O3/Y-TZP ceramic composites for dental applications

Yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) ceramics have been utilized as a restorative dental material. Pre-sintered Y-TZP ceramics are machined using the Computer-Aided Design and Computer-Aided Manufacturing (CAD/CAM) techniques. In this study, pre-sintered 3 mol % yttria-stabilized zirconia (3Y-TZP) bioceramics reinforced with Al2O3 particles were prepared by sintering at relatively low temperature. The machining characteristics of the developed 3Y-TZP composites were investigated using an end-milling process. A series of experiments were performed on a high speed spindle with several sensors: a tool dynamometer, an acceleration sensor, and an acoustic emission sensor. The tool dynamometer was installed below the work-piece, a three-axis acceleration sensor was installed on the spindle head, and the acoustic emission sensor was installed on the fixture. The machining characteristics of the composites material were evaluated in a high speed machining process under various cutting conditions by changing the cutting speed, depth-of-cut, and feedrate. The machined surface was investigated using SEM photographs.

Model analysis of boundary residual stress and its effect on toughness in thin boundary layered yttria-stabilized tetragonal zirconia polycrystalline ceramics

The average thermal residual stress in a continuous boundary phase in polycrystalline ceramic composites was calculated with a simple thin boundary layer model, and a criterion for the self-cracking of the boundary phase was derived under a certain assumption. From the proposed model, the toughness of the materials can be increased by both tensile and compressive stress at boundaries when the crack propagates transgranularly. The toughness will be increased when the stress at boundary is compressive for intergranular fracture mode. The maximum increase is predicted to be achieved at boundary phase contents below 33%. The experimental results for yttria-stabilized tetragonal zirconia polycrystalline ceramics doped with different kinds of grain-boundary phase is in a qualitative agreement with the prediction by the model, but the toughness increase is largely dependent on the distribution feature of glass phases.

Correlation between microstructure, phase transformation during fracture and the mechanical properties of Y-TZP ceramics

The mechanical properties of Y-TZP (yttria-stabilized tetragonal zirconia polycrystalline) materials with different microstructures have been studied and related to the phase transformation capability during fracture. It has been found that the microstructure of the materials affects the phase composition and also the phase transformation volume (tetragonal to monoclinic) on the fracture surface during fracture. Higher densities lead to higher phase transformation volume; and both of them contribute to higher fracture toughness and strength.