Yttria-doped Tetragonal Zirconia Polycrystals Research Articles

Intraoral low-temperature degradation of monolithic zirconia dental prostheses: Results of a prospective clinical study with ex vivo monitoring

ObjectiveTo investigate the intraoral development and kinetics of low-temperature degradation (LTD) in second-generation 3 mol.% yttria-doped tetragonal zirconia polycrystal (3Y-TZP) monolithic prostheses, as well as the influence of masticatory mechanical stress and glaze layer on it. MethodsA total of 101 posterior tooth elements were included in a prospective clinical study, which included ex vivo LTD monitoring (at baseline, 6 months, 1 year, and 2 years) using Raman spectroscopy (n = 2640 monoclinic phase measurement points per evaluation time) and SEM. Four types of areas (1–2 mm2 surface, 6 on molars, and 4 on premolars) were analyzed on each element surface: occlusal, axial, glazed, or unglazed. Raman depth mapping and high-resolution SEM were performed on the selected samples. ResultsLTD developed in 3Y-TZP monolithic restorations 6 months after intraoral placement and progressed with time. After two years, the tetragonal-to-monoclinic transformation was non-uniform, with the presence of localized clusters of transformed grains. In axial areas, the grain aspect was typical of the classical nucleation-growth process reported for LTD, which progresses from the surface to a depth of several tens of microns. However, in occlusal areas, tribological stress generated surface crushing and grain pull-out from the clusters, which induced an underestimation of the aging process when the evaluation was limited to monoclinic phase quantification. Glazing cannot be considered a protection against LTD. SignificanceIf LTD occurs in dental prostheses in the same way as in orthopedic prostheses, its clinical impact is unknown and needs to be further studied.

Effect of gadolinia addition on the mechanical and physical properties of zirconia/ceria ceramics

In this work, zirconia powders containing 5 and 10 mol % ceria were prepared by co-precipitation method followed by doping with small amounts (< 1 mol %) of gadolinia which stabilized the tetragonal phase. X-ray diffraction and scanning electron microscopy were used for phase analysis and microstructure examination, respectively. The sintered compacts showed fine grained, dense, tough and strong ceramics. Strength up to 795.8 MPa and toughness of 17 MPa√m have been obtained. These ceramics showed higher toughness than yttria-doped tetragonal zirconia polycrystals (Y-TZP) and a higher strength than the ceria tetragonal zirconia polycrystals (Ce-TZP). The high fracture toughness was attributed to the tetragonal-to-monoclinic phase transformation which was associated with ferroelastic domain switching. The domain switching toughening has been discussed in view of Chevalier's model. Four-point bending strengths for the samples tested in the as cut state are higher than those tested in the as-sintered or the ground states.

Preparation and properties of yttria doped tetragonal zirconia polycrystal/Sr-doped barium hexaferrite ceramic composites

Abstract The effects of substitution of Ba2+ by Sr2+ on the magnetic property of barium ferrite and addition barium ferrite secondary phase to the 3 mol% yttria-doped tetragonal zirconia polycrystal (3Y-TZP) matrix on the mechanical property of composites were investigated. The Sr-doped barium ferrite (Ba1−xSrxFe12O19, x = 0, 0.25, 0.50 and 0.75) was synthesized by solid-state reaction in advance. Then 3Y-TZP/20 wt% Sr-doped barium ferrite composites were prepared by means of conventional ceramic method. It was found that a moderate amount of Sr added to barium ferrite could boost the saturation magnetization by 15% compared with the composites without Sr-doping. Besides, the composite with x = 0.50 possessed the best mechanical properties, such as 11.5 GPa for Vickers hardness and 8.3 MPa m1/2 for fracture toughness, respectively. It was demonstrated that magnetic and mechanical properties of the composites could be harmonized by the incorporation of barium ferrite secondary phase.

Spherical instrumented indentation of porous nanocrystalline zirconia

Nanoindentation tests with a spherical tip were performed to analyze the stress–strain response on 3-mol%-yttria-doped tetragonal zirconia polycrystals produced by spark plasma sintering (SPS) with porosities in the range 2–21% and nanometric average grain sizes in the range 65–120 nm. Indentation stress–strain curves were obtained by using load ( P)–displacement ( h) data. Onset of elasto-plastic transition was determined by Hertz fits on P– h curves. Elastic modulus obtained from spherical indentation was similar to the values obtained by Berkovich indentation, with a slight difference with increasing porosity. Microstructural characterisation shows no cracks around and beneath the indentations. Raman spectroscopic analysis of the residual indentation imprints reveals tetragonal to monoclinic phase transformation in the most porous material, whereas no phase transformation was detected in the dense material in spite of its larger grain size. The results are discussed in terms of porosity, grain size and tetragonal-monoclinic transformability.

Grain core and grain boundary electrical/dielectric properties of yttria-doped tetragonal zirconia polycrystal (TZP) nanoceramics

Bulk samples of nanocrystalline tetragonal zirconia polycrystal (TZP), with 3 mol.% Y 2O 3, were fabricated over a range of average grain sizes (16–70 nm) by partial sintering. The samples were measured using AC-impedance spectroscopy over a range of temperatures, and porosity-corrected electrical results were interpreted in terms of microstructural models. Whereas the conventional brick layer model (BLM) significantly overestimated the specific grain boundary conductivity at the nanoscale, our recently developed nano-Grain Composite Model (n-GCM) allowed accurate determination of local grain boundary and grain core conductivities, grain boundary dielectric constants, and electrical grain boundary widths. Grain core effective dielectric constants were also separately measured on microcrystalline samples over a range of temperatures for use in the n-GCM analysis. It was found that TZP exhibits an enhanced local grain boundary conductivity at the nanoscale, but the enhancement is insufficient to improve the total conductivity. Rather, total conductivity decreased with decreasing grain size. Results were compared with those for nanocrystalline yttria-stabilized zirconia (8 mol.% Y 2O 3, YSZ).

Superplastic flow stress and electronic structure in yttria-stabilized tetragonal zirconia polycrystals doped with GeO 2 and TiO 2

Superplatic flow in 3 mol%-yttria-doped tetragonal zirconia polycrystal (3Y-TZP) with tetravalent cation, Ge and Ti, has been examined by high temperature tensile testing. These cations are effective to suppress the flow stress of 3Y-TZP and to improve tensile ductility. First principles molecular orbital calculations using model clusters reveal that substituted Ge and Ti ions have strong covalent bonds with neighboring oxygen ions than Zr ion. Such high covalency of Ge–O and Ti–O bonds is due to large bonding components in valence band. Addition of Ti and Ge ions leads to reduction in the ionicity in 3Y-TZP. From our high temperature tensile tests and theoretical calculations for GeO 2- and TiO 2-doped TZP, flow stress is observed to correlate with an effective ionic bonding strength. It is expected that the ionicity is a major parameter to control the diffusivity of cation and to hence affect the superplastic flow of 3Y-TZP.

Effect of CoO Doping on the Sintering Ability and Mechanical Properties of Y-TZP

The effect of CoO addition into Y-TZP (Yttria doped Tetragonal Zirconia Polycrystals) was studied on the evolution of its sintering ability, grain size, grain boundary structure and mechanical properties. The doping of a small amount of CoO effectively reduced the sintering temperature. A small amount of CoO up to ~ 0.3 mol% was effective for the suppression of grain growth, but the addition of 1.0 mole % resulted in an enhanced grain growth. The hardness and toughness of the CoO doped TZP were about the same as those of undoped TZP. Furthermore, despite the grain refinement, CoO doped TZP did not exhibit improved mechanical properties. This may be suggesting that CoO dopant had weakened the grain boundary strength.

Materials dynamics under nanosecond pulsed pressure loading

A nanosecond pressure pulse is generated by focusing a nanosecond-pulsed laser onto an aluminum target with plasma confined geometry. A spatially uniform pressure pulse is generated by focusing laser beams with a flat-top spatial energy distribution. High-pressure pulse loading and recovery experiments were performed on yttria-doped (3 mol%) tetragonal zirconia polycrystals at 11 GPa. In the pressure-loaded region, the monoclinic phase was uniformely formed. The transition ratio was approximately 30%. Nanosecond time-resolved Raman spectroscopy was performed on polytetrafluoroethylene under high-pressure pulse loading at 1 GPa, and rapid structural phase transition within 10 ns was revealed.

Densification of zirconia-hematite nanopowders

The densification of dual-phase yttria-doped tetragonal zirconia polycrystals (Y-TZP) and -Fe2O3 (hematite) composite powders is described. Different powder synthesis methods, different forms of dry compaction processes, and two sinter methods (pressureless sintering and sinterforging) were compared. The homogeneity and average grain sizes of the sintered compacts were determined with SEM/EDX. Compacts produced from homogeneous powders that were prepared by the co-precipitation (CP) method showed large-scale phase segregation and grain growth during sintering. Compacts made from the less homogeneous sequentially precipitated (SP) powders showed far less phase segregation and relatively small grain growth during sintering. Dense (>96%) nano/nano zirconia-hematite composites were made with average grain sizes of 80 nm by sinterforging at 1000 °C and 100 MPa.

Quantitative characterization of various tetragonal zirconia polycrystals (TZPs)

The microstructures , Y 2 O 3 content and phases of 3 mol% yttria-doped tetragonal zirconia polycrystals (TZPs) were quantitatively investigated. Five yttria-doped zirconia samples were observed by scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS) or electron probe microanalyzer (EPMA) equipped with wavelength dispersive spectroscopy (WDS). The amount of phase transformation was determined by X-ray diffractometry (XRD). Operating conditions of EDS or WDS were controlled based on the needs of statistical analysis. The Student's t distribution in a confident interval of 99% was adopted and applied to reveal the level of homogeneity of Y 2 O 3 content in five TZP samples. Based on the results and analytical methods, the quality, including sintered density, the fraction of large grains, the transformation capability, and the homogeneity of Y 2 O 3 content of TZP can be distinctly verified.

Effects of material properties and testing parameters on wear properties of fine-grain zirconia (TZP)

The wear behaviors of yttria-doped tetragonal zirconia polycrystal (Y-TZP) ceramics with different microstructures worn against diamond wheel under various wearing conditions were investigated and compared to that of dense magnesia-doped partially stabilized zirconia (Mg-PSZ) and fine-grain Al 2O 3. A block-on-wheel tribometer with unidirectional sliding was used to carry out the wear experiments. The friction coefficient of dry sliding was recorded. The worn surfaces of these materials were also characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM) and surface profilometry. The wear resistance of worn pieces were analyzed and determined. The possible wear mechanisms and a master wear equation of the ceramics are proposed and discussed.

Novel processing of hydroxyapatite-zirconia composites using nano-sized particles

Hydroxyapatite, HA, is employed in the repair of skeletal defects, because it closely resembles the mineral phase in bone and is bioactive. However, its relatively poor strength has limited its use in major load-bearing applications. Attempts have been made to improve the strength of HA by the incorporation of a second ceramic phase, such as ZrO2 phases (e.g. yttria-doped tetragonal zirconia polycrystals, Y-TZP), but the addition of a second phase can have inherent disadvantages. Sintering HA {Ca10(PO4)6(OH)2} at high temperatures can result in the formation of decomposition products such as tri-calcium phosphate, TCP {Ca3(PO4)2} and tetracalcium phosphate, TeCP {Ca4(P2O9)}[1, 2]. TeCP can decompose further to TCP and calcium oxide [CaO] at higher temperatures. These secondary phases have, in certain instances, been reported to adversely affect the biological response [3, 4]. When a ZrO2 phase, such as Y-TZP, is added to HA, high temperatures of 1200–1400 ◦C are commonly required to sinter these composites to high density, but HA has been reported to be thermally unstable above 1300–1400 ◦C [5, 6]. It has commonly been observed that phase transformations/decompositions occur extensively during sintering of these composites at temperatures even below 1200 ◦C. For example, at 1150 ◦C, HA will react with the ZrO2 phase, with the formation of cubic calcia-stabilized ZrO2, and subsequently calcium zirconate (CaZrO3) [7]. Increasing the sinter temperature further increases the amounts of β-TCP and CaZrO3. At a sinter temperature of 1400 ◦C, virtually no HA remains and some α-TCP forms from β-TCP. This phase decomposition means that the advantage of combining a low-strength bioactive ceramic (HA) with a high-strength, bioinert ceramic (Y-TZP) is lost. This problem of decomposition reactions between the HA and the ZrO2 phases during sintering have limited the applications of this composite, as the mechanical and biological properties are compromised by the formation of undesirable secondary phases. In this study, a novel approach has been used to produce composites of HA with ZrO2 (Y-TZP) additions between 0 and 10 wt % at a low sinter temperature (750 ◦C), in order to limit the amount of reaction between the HA and ZrO2. The aim was to find a

Lower air-crystallization temperature and nanostructured yttria-doped tetragonal zirconia polycrystals ceramics by seeding assisted chemical coprecipitation

The crystallization temperature of yttria-doped tetragonal zirconia polycrystals (Y-TZP) amorphous precursors can be lowered 150 °C below that currently used (≥500 °C), if “seeding assisted chemical coprecipitation” is used. Completely crystallized Y-TZP nanocrystalline powder was obtained by calcining at 350 °C in air; the Y-TZP precursors seeded with 10 wt% of nanometric (∼8 nm) Y-TZP particles. The seed particles enhanced both the nucleation and the crystallization rates at lower temperatures. From such a powder, 99% dense and nanostructured (grain size, &lt;90 n m) Y-TZP bodies can be prepared by sintering below 1050 °C.

Liquid Phase Sintering of Y-TZP with CuO and Y&lt;sub&gt;2&lt;/sub&gt;Cu&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;5&lt;/sub&gt; Dopants

Effects of CaO and Y 2 Cu 2 O 3 addition on the sintering ability and the phase stability of Y-TZP (Yttria doped Tetragonal Zirconia Polycrystals) were studied. Both of the dopants were found to be quite effective in reducing the sintering temperature to attain full density and refining the grain size. Cu ions were easily dissolved into the matrix during sintering leaving no grain boundary phase. Since the dissolved Cn ions have a destabilizing effect on the tetragonal lattice. the maximum allowable concentration of the dopants were limited to 0.3 mol mol% CuO and 0.5 mol% for YCuO 2.5 to maintain fully tetragonal phase. Wit h t he addit ion of these dopant s. the bending strength decreased by 15% in comparaison with the undoped specimen but the fracture toughness increased by 10 to 20%.

Isothermal Tetragonal to Monoclinic Phase Transition around Room Temperature in 2.0 mol% Yittria-doped Zirconia Ceramics.

The tetragonal to monoclinic (t-m) phase transition in 2.0 mol% yttria-doped tetragonal zirconia polycrystals (2Y-TZP) was studied by use of XRD and FT-IR diffuse scattering (DRIFT) measurements. The phase transition occurred isothermally when the 2Y-TZP powder after calcining at temperatures below 1200°C was annealed in air even at room temperature, and more than 90% of the monoclinic phase was obtained after a long period of annealing. However only about several percent and about 40% of the monoclinic phase were formed after the annealing treatments under vacuum and 1 atm air separately sealed in a 5 ml Pyrex tube, respectively. On the other hand, no phase transition occurred in air around room temperature in the 2Y-TZP powder after being calcined at temperatures above 1300°C. A broad band around 3300 cm-1 due to the existence of adsorbed water was observed in the FT-IR DRIFT spectra of the 2Y-TZP powder only after calcination below 1200°C. The t-m transition behavior of 2Y-TZP powder after calcination below 1200°C differed with that after calcination above 1300°C. The existence of adsorbed water on 2Y-TZP was essential for the t-m phase transition of 2Y-TZP to take place in air around room temperature. The amount of transformed monoclinc phase was determined by the amount of adsorbed water on 2Y-TZP and supplied water from annealing atmosphere.

Effect of Silica Doping on the Electrical Conductivity of 3 mol % Yttria-Stabilized Tetragonal Zirconia Prepared by Colloidal Processing

Silica-doped (SiO2 = 0 - 1.0 wt%) 3Y-TZP (3 mol % yttria-doped tetragonal zirconia polycrystal) ceramics are prepared from hetero-coagulated aqueous suspension by colloidal processing. Consolidation of the suspension was carried out by pressure filtration at 10 MPa followed by cold isostatic pressing (CIP) at 400 MPa. Consolidated compacts are densified to a relative density over 99% by sintering at 1573 K for 2 h. The formation of glass pockets at grain boundary multiple junctions was observed by SEM for ≥0.5 wt % silica-doped samples. Electrical conductivity measurements were performed to evaluate the modification of grain-boundaries by silica. The apparent grain boundary conductivity decreased with an increase in silica content and became nearly constant above 0.3 wt % of silica, while the bulk conductivity was constant with silica content.

Microstructure and Phase Stability of Yttria‐Doped Tetragonal Zirconia Polycrystals Heat Treated in Nitrogen Atmosphere

The low‐temperature degradation of zirconia (ZrO2) that was doped with 3 mol% yttria (Y2O3) (3Y‐TZP) was prevented by the heat treatment of sintered specimens in nitrogen. The heat treatment of sintered specimens resulted in a surface layer that was stabilized by nitrogen ions, whereas the interior was only slightly affected by the heat treatment. X‐ray diffractometry and transmission electron microscopy analyses revealed that the stabilized surface layer consisted of cubic grains with tetragonal precipitates. Although the presence of the surface layer decreased the strength of the sintered 3Y‐TZP, the strength of nitrified specimens was maintained when low‐temperature annealing was applied.

Grain‐Size Dependence of Sliding Wear in Tetragonal Zirconia Polycrystals

Using a pin‐on‐plate tribometer with the reciprocating motion of SiC against yttria‐doped tetragonal zirconia polycrystal (Y‐TZP) plates, the friction and wear of Y‐TZP ceramics were investigated as a function of grain size in dry N2 at room temperature. The results showed that the overall wear resistance increased as the grain size of Y‐TZP ceramics decreased. For grain sizes ≤0.7 μm, the wear results revealed a Hall‐Petch type of relationship (d−1/2) between wear resistance and grain size. In this case, the main wear mechanisms were plastic deformation and microcracking. For grain sizes ≥0.9 μm, the wear resistance was proportional to the reciprocal of the grain diameter. In this regime, delamination and accompanying grain pullout were the main mechanisms. In this case, the phase transformation to monoclinic zirconia had a negative effect on the wear resistance of TZP ceramics. The coefficient of friction tended to be higher for fine‐grained TZP‐SiC couples than for coarse‐grained TZP‐SiC couples, whereas, for a specific regime of grain size, the coefficient of friction was almost independent of the grain size.

高強度高靭性ジルコニア

Characteristics of high fracture strength and high fracture toughness of zirconia in yttria doped tetragonal zirconia polycrystal (Y-TZP), Y-TZP/Al2O3 composites and ceria doped tetragonal zirconia polycrystal (Ce-TZP) are described in this paper. The maximum fracture strength of Y-TZP/Al2O3 composites is 2, 400MPa at 30°C The maximum fracture toughness of Ce-TZP is 20 MPam1/2It is found that hot isostatic pressing is effective to eliminate fracture origins such as pore. From the results of thermal shock behavior of Y-TZP with various grain sizes by the water quenching method and the tetragonal to monoclinic phase transformation by using Raman microprobe spectroscopy, it is found that the critical quenching temperature different of Y-TZP increases from 250°C to 425°C with increasing grain size from 0.4 to 3.0μm.

Preparation of Y-TZP/AI2O3 whisker preform by an in situ method

In situ growth of AI2O3 whiskers into the matrix of Y-TZP (yttria-doped tetragonal zirconia polycrystals) was examined in order to prepare Y-TZP/AI2O3 whisker preform for the composites. Various shapes of AI2O3 particles were grown by the reaction of AI2O3 and AIF3 powders with moist nitrogen or oxygen gases at high temperature. They showed a trend to change the particle shapes from massive → rhombohedron → whisker → platelet as the processing temperature was increased. These particles, however, grew only on the surface and not inside the pellets It was found necessary to introduce the carrier gas inside the pellets for particle growth to occur internally. AI2O3 whiskers can be synthesized inside the pellets by mixing with an organic space-forming agent having a relatively large particle size.