Nacre-like alumina composites reinforced by zirconia particles

Nacre-like alumina is a class of bio-inspired ceramic composite manufactured by field-assisted sintering of green bodies made primarily of alumina platelets with an anisotropic microstructure. Here we investigate the addition of zirconia particles to enhance the mechanical properties of the composite. The resulting structure is a nacre-like anisotropic structure which features deflection and reinforcement during crack propagation. Monoclinic zirconia has no impact on the mechanical properties of the composite while tetragonal zirconia improves its fracture resistance properties. Both types of zirconia seem to slow down grain growth during sintering. The addition of zirconia stabilised in the tetragonal phase is thus a good option to obtain a composite with a fine microstructure and higher mechanical properties than a standard nacre-like alumina, with a flexural strength of 626 ± 39 MPa and a crack initiation toughness of 6.1 ± 0.6 MPa.m^0.5.     

Simultaneous synthesis and densification of α-Zr(N)/ZrB2 composites by self-propagating high-temperature combustion under high nitrogen pressure

Simultaneous synthesis and densification of α-Zr(N)/ZrB₂ composites from a 85 mol% Zr/15 mol% B mixed-powder compacts have been achieved by self-propagating high-temperature under a nitrogen pressure of 10 MPa. Composites consist of fine and short rodlike ZrB₂ grains (0.1 μm^?–0.5 μm^l) dispersed into α-Zr(N) matrix (3 μm). Dense composite materials (96.5% of theoretical) exhibit excellent mechanical properties, in which their bending strength and H_v are 560 MPa and 6.5 GPa, respectively. This bending strength is much superior to those (205 and 480 MPa) of dense equi-axial α-Zr(N) (10 μm) and dense ZrB₂ (6 μm). Fine and rodlike ZrB₂ grains greatly enhanced their mechanical properties.     

Phase stability and mechanical properties of TZP with a low mixed Nd2O3/Y2O3 stabiliser content

The phase assembly of 1.0–5.0 mol% Nd₂O₃-doped ZrO₂ sintered at 1400 °C revealed that the tetragonal ZrO₂ phase could not be completely stabilised. Co-stabilising of 0.5–2.5 mol% Nd₂O₃ with 0.5–1.0 mol% Y₂O₃, however, allowed the preparation of fully dense (Nd,Y)-TZP ceramics by pressureless sintering in air at 1450 °C. The mixed stabiliser monoclinic zirconia nanopowder starting material was synthesized from a suspension of neodymium nitrate, yttrium nitrate and monoclinic zirconia powder in an alcohol/water mixture. A HV₃₀ hardness of 10 GPa combined with an excellent indentation toughness of 13 MPa m^1/2 could be achieved for the (1.0Nd,1.0Y)- and (1.5Nd,1.0Y)-TZP ceramics. The influence of the mixed stabiliser content on the phase stability and mechanical properties are investigated and discussed.     

Effect of calcia co-doping on ceria-stabilized zirconia

Ceria-stabilized zirconia ceramics are characterised by excellent hydrothermal stability and high fracture toughness, but the fracture strength and hardness are lower than that of conventional 3Y-TZP, which is sensitive to low temperature degradation in humid environments. In the present work, the influence of small concentrations of calcia on the microstructure, mechanical properties and hydrothermal ageing resistance of 10 and 12?mol% CeO₂ stabilised ZrO₂ has been assessed. The addition of only 1?mol% of CaO had a strong refining effect on the microstructure resulting in an increased hardness and strength but reduced stress activated tetragonal-to-monoclinic transformability. The addition of 3?mol% CaO however enhanced the transformability with respect to 1?mol% CaO and preserved the high resistance to hydrothermal degradation of Ce-TZP.     

Ultra-fine Yttria-Stabilized Zirconia for dental applications: A step forward in the quest towards strong,translucent and aging resistant dental restorations

To decrease the light scattering caused by birefringence of the tetragonal phase of dental Yttria-Stabilized Zirconia (YSZ), two main strategies are followed: 1) increasing Y₂O₃ content to have a larger amount of non-birefringent cubic phase or 2) decreasing grain size below 100 nm to reduce their scattering coefficient. Both strategies might affect mechanical properties and aging resistance. This study shows that increasing the stabilizer content enhances both translucency and aging resistance, at the expense of mechanical properties. Nanometric-sized YSZ show, instead, very interesting compromise. Nanometric-sized zirconia stabilized with 3 mol.% of Y₂O₃ possesses high strength and toughness (above 1600 MPa and 3.3 MPa m^1/2), aging resistance and translucency, thanks to its fully-stabilized tetragonal nano-grains. Nanometric-sized zirconia stabilized with 1.5 mol.% of Y₂O₃ has the best mechanical performances (strength above 1500 MPa and toughness of 4.8 MPa m^1/2), still showing aging resistance and intermediate opacity. These results highlight the interest of moving towards Yttria-containing transformable tetragonal nano-ceramics for dental applications.     

Iron oxide colouring of highly-translucent 3Y-TZP ceramics for dental restorations

The influence of 0.01–2 mol% Fe₂O₃ powder addition on the microstructure, mechanical and optical properties, and hydrothermal stability of highly-translucent 3Y-TZP ceramics is assessed and compared with commercially available co-precipitated 0.18 mol% Fe₂O₃ doped ZrO₂ powder-based ceramics. Only those ceramics with up to 0.1 mol% Fe₂O₃ resulted in a proper shade for dental zirconia ceramics, with a typical composition of 87 vol% t-ZrO₂ and 13 vol% c-ZrO₂. The amount of cubic phase increased at higher Fe₂O₃ content. The hardness (∼13 GPa) and fracture toughness (∼3.6 MPa m^1/2) of the 0.01 mol% - 0.1 mol% Fe₂O₃ doped 3Y-TZP was comparable, whereas the hardness decreased above 0.5 mol% Fe₂O₃ and the fracture toughness decreased above 2 mol% Fe₂O₃. The hydrothermal ageing resistance slightly increased for Fe₂O₃ concentrations up to 1 mol%, whereas the translucency slightly decreased with increasing Fe₂O₃ content.     

Effect of CeO2 addition on densification and microstructure of Al2O3-YSZ composites

Alumina (Al₂O₃) and alumina-yttria stabilized zirconia (YSZ) composites containing 3 and 5 mass% ceria (CeO₂) were prepared by spark plasma sintering (SPS) at temperatures of 1350-1400 °C for 300 s under a pressure of 40 MPa. Densification, microstructure and mechanical properties of the Al₂O₃ based composites were investigated. Fully dense composites with a relative density of approximately 99% were obtained. The grain growth of alumina was inhibited significantly by the addition of 10 vol% zirconia, and formation of elongated CeAl₁₁O₁₈ grains was observed in the ceria containing composites sintered at 1400 °C. Al₂O₃-YSZ composites without CeO₂ had higher hardness than monolithic Al₂O₃ sintered body and the hardness of Al₂O₃-YSZ composites decreased from 20.3 GPa to 18.5 GPa when the content of ZrO₂ increased from 10 to 30 vol%. The fracture toughness of Al₂O₃ increased from 2.8 MPa m^1/2 to 5.6 MPa m^1/2 with the addition of 10 vol% YSZ, and further addition resulted in higher fracture toughness values. The highest value of fracture toughness, 6.2 MPa m^1/2, was achieved with the addition of 30 vol% YSZ.     

Dense and strong ZrO2 ceramics fully densified in <15 min

ABSTRACT

Crack-free zirconia ceramics were consolidated via sintering by intense thermal radiation (SITR) approach at 1600–1700°C for 3–5?min. The resulted ceramic bulks can achieve a relative density up to 99.6% with a grain size of 300–1200?nm. Their bending strength, Vickers hardness and indentation toughness values are up to 1244?±?139?MPa, 13.3?±?0.3?GPa and 5.5?±?0.1?MPa?m^1/2, respectively. Quantitative Raman and XRD analysis show the presence of minor m phase on the natural surface (<7%), fracture surface (<10%) and indentation areas (<15%). It reveals that the SITR method is efficient for rapidly manufacturing zirconia ceramics with desired density, fine grained microstructure and good mechanical properties that are strongly demanded in dental applications.     

Zirconia ceramics from coprecipitated powders

The possibility was explored of making dense and strong partly stabilized zirconia (PSZ)-based ceramic materials from coprecipitated zirconium and yttrium hydroxide powders of both factory and laboratory preparation. The effect of dry and wet grinding, powder burning, cold isostatic pressure (CIP) at≤0.8 GPa, and sintering at ≤1600°C on the physicochemical properties of the material was investigated. It was found that the properties (a density of 5.7–5.8 g/cm³, a bending strength of 600 to 800 MPa, and a crack resistance of 7–9 MPa·m^1/2) of the resulting ceramic material would not be reproduced unless the tendency of the PSZ powder to agglomerate spontaneously in storage is overcome or avoided. It is shown that in contrast to other similar materials the ceramic material from a deagglomerated powder has a higher optimal CIP pressure (0.6 GPa), which implies that the material has an improved thermal endurance and a better mechanical stability.     

Effect of Y-TZP addition on the microstructure and properties of titania-based composites

To maintain the bioactivity and to improve the mechanical properties of titania, both pure titania ceramics and titania–yttria-stabilized tetragonal zirconia (Y-TZP) composites with 5, 10, and 15 vol.%Y-TZP were prepared via a sol–gel precipitation method. A titania precursor (titanium butoxide) was mixed with a submicron-sized Y-TZP powder, followed by hydrolysis-condensation reactions, green compact forming, and sintering in air at 1200–1350 °C. It was found that the addition of Y-TZP resulted in reduced rutile titania grain size from 13 to 3 μm. The Y-TZP tetragonal phase also resulted in improved mechanical properties of the titania–Y-TZP composites. For instance, the titania–15 vol.%Y-TZP composite had a hardness value of 983 kg/mm², a bending strength of 160 MPa, and a fracture toughness of 3.79 MPa m^0.5. While the addition of Y-TZP increased the mechanical properties, it also decreased the bioactivity of the composites.     

Microstructure and mechanical properties of in-situ grown mullite toughened 3Y-TZP zirconia ceramics fabricated by gelcasting

In-situ grown mullite toughened zirconia ceramics (mullite-zirconia ceramics) with excellent mechanical properties for potential applications in dental materials were fabricated by gelcasting combined with pressureless sintering. The effect of sintering temperature on the microstructure and mechanical properties of mullite-zirconia ceramics was investigated. The results indicated that the columnar mullite produced by reaction was evenly distributed in the zirconia matrix and the content and size of that increased with the increase of sintering temperature. Mullite-zirconia ceramics sintered at 1500 °C had the optimum content and size of the columnar mullite phase, generating the excellent mechanical properties (the bend strength of 890.4 MPa, the fracture toughness of 10.2 MPa.m^1/2, the Vickers hardness of 13.2 GPa and the highest densification). On the other hand, zirconia particles were evenly distributed inside the columnar mullite, which improved the mechanical properties of columnar mullite because of pinning effect. All of this clearly confirmed that zirconia grains strengthened columnar mullite, and thus the columnar mullite was more effective in enhancing the zirconia-based ceramics. Simultaneously, the residual alumina after reaction was distributed evenly in the form of particle, which improved the mechanical properties of the sample because of pinning effect. Overall, the synergistic effect of zirconia phase transformation toughening with mullite and alumina secondary toughening improved the mechanical properties of zirconia ceramics.     

Phase stability,microstructural evolution and room temperature mechanical properties of TiO2 doped 8 mol% Y2O3 stabilized ZrO2 (8Y-CSZ)

The effect of TiO₂ dopant on phase stability, microstructural evolution and room temperature mechanical properties of 8 mol% yttria-stabilized cubic zirconia (8Y-CSZ) was studied. The results show that TiO₂ (up to 10 wt%) can be dissolved in solid solution in the zirconia matrix. When the dopant amount is less than 5 wt%, TiO₂ doped 8Y-CSZ remains single phase cubic zirconia. Increased additions of TiO₂ destabilize the cubic phase and cause the formation of tetragonal zirconia with a resultant microstructure consisting of large cubic zirconia grains and small tetragonal zirconia grains. EDS analyses show that yttria is partitioned between these two types of grains. The solubility of TiO₂ is the highest for cubic grains which also have higher yttria concentrations. Room temperature mechanical property measurements show that hardness does not change significantly with additions of TiO₂, but fracture toughness is more than doubled for 10 wt% TiO₂ doped 8Y-CSZ.     

Effects of carbonyl nickel powder addition on the mechanical properties,microstructure, and yttrium segregation of sintered 3YSZ composites

In this study, the effects of the addition of carbonyl nickel powder on the density, microstructure, and mechanical properties of sintered yttria-stabilized zirconia (3YSZ) were investigated. Sintering at 1300 °C resulted in the optimum comprehensive mechanical properties. The addition of 5 vol% carbonyl Ni increased the fracture toughness and flexural strength from 9.51 MPa m^1/2 to 14.5 MPa m^1/2 and from 747 MPa to 873 MPa, respectively. The addition of carbonyl nickel showed greater improvement than did the addition of spherical Ni powder. The dendritic morphology improved the interface bonding between the ceramic and the metal, enabling a bridging mechanism of the ductile phase. However, further Ni addition decreased the mechanical properties. X-ray diffraction results showed that the amounts of the monoclinic phase (M) and cubic phase (C) of 3YSZ increased, whereas the amount of the tetragonal phase (T) decreased. The Y segregation near the Ni particles, which was confirmed by an energy dispersive spectrometer (EDS), caused the phase changes. The segregation of Y occurred during the cooling stage, rather than the holding stage, of sintering. During the cooling stage, the heat mismatch between Ni and ZrO₂ resulted in strong elastic strain energy, which promoted Y segregation.     

Theoretical and experimental investigations on high temperature mechanical and thermal properties of BaZrO3

The perovskite BaZrO₃ has high phase stability from room temperature to its melting point and therefore is regarded as a promising candidate for various high-temperature applications. In this work, the mechanical and thermal properties of BaZrO₃ at high temperatures are investigated by combining first-principles calculations and experimental approaches. BaZrO₃ has moderate mechanical properties and low thermal conductivity, being comparable to other zirconium-based and silicate structural ceramics. Its remaining Young's modulus of 174.4?GPa at 1523?K is 81.6% of 213.8?GPa at room temperature. The residual flexural strength of 127.8?MPa at 1273?K is 74% of 172.4?MPa at room temperature, while the residual value at 1673?K is still 53.4?MPa. The thermal conductivity of BaZrO₃ is 5.75?W?m^?1 K^?1 at 298?K and decreases to 2.81?W?m^?1 K^?1 at 1473?K. The good high temperature mechanical and thermal properties ensure the potential high temperature applications of BaZrO₃ and our results are expected to arouse the design of BaZrO₃-based ceramics in the near future.     

Tough and damage-tolerant monolithic zirconia ceramics with transformation-induced plasticity by grain-boundary segregation

Conventional ceria-stabilized tetragonal zirconia (Ce-TZP) with modest flexural strength has rarely been used as compared to yttria-stabilized zirconia, even though it has excellent hydrothermal stability and high toughness. Ce-TZP-based composites were recently developed, being tough and remarkably in combination with transformation-induced plasticity. However, distinct from the widely applied composite approach to improve the strength of Ce-TZP, in this study, a simpler and easily tailorable method was proposed by doping aliovalent oxides that are able to segregate at the zirconia-grain boundaries. 0.2–1 mol% divalent oxides with different cation size (Mg²⁺, Ca²⁺, Ba²⁺ and Sr²⁺) were selected to dope 10 mol% ceria-stabilized zirconia. CaO and MgO dopants were able to enter the tetragonal Ce-TZP lattice and showed a grain-boundary segregation effect, thereby tailoring the microstructure and transformation behavior. At a higher dopant concentration of 1 mol% MgO or CaO, the ceramics were strong but brittle with a typical elastic linear fracture behavior, whereas at a low dopant concentration of 0.1–0.2 mol% CaO or 0.2–0.4 mol% MgO doping, the ceramics deformed in-elastically to a certain degree without changing the Young’s modulus. At the transition between both fracture behaviors, the best combination of toughness (>10 MPa m^1/2), biaxial strength (≥1200 MPa), reliability (Weibull modulus up to 30) and damage-tolerance was obtained. In-situ fracture testing revealed that transformation in such tough and deformable zirconia ceramics took place well before crack initiation with a large transformation zone of ～100 µm ahead of the crack tip having been formed before failure.     

Role of cold isostatic pressing in the formation of the properties of ZrO2-base ceramics obtained from ultradisperse powders

The physicomechanical properties of ceramics obtained from plasmachemical and sol-gel powders of partially stabilized (3% Y₂O₃) zirconia (PSZ) and its compositions with 20% Al₂O₃ by cold isostatic pressing (CIP) at a pressure of at most 2 GPa and sintering at 1300–1650°C are investigated. It is established that plasmachemical PSZ exhibits its best properties (K _lc=7.8 MPa · m^1/2, a strength of 650 MPa) only after complete disintegration at a CIP of 0.1 GPa and a sintering temperature of 1650°C, when the material is sintered to a density of 5.5 g/cm³. After partial stabilization and CIP at 0.1 GPa the plasmachemical composition of PSZ+20% Al₂O₃ is sintered at 1650°C to a density of 4.7 g/cm³, but hasK _lc=8.5 MPa · m^1/2 and a strength of 700 MPa. The deagglomerated sol-gel powder exhibits properties at a level ofK _lc=12.4 MPa · m^1/2 and a strength of 950 MPa at a density above 6.0 g/cm³ after CIP at 0.3 GPa and sintering at 1450°C. The latter obviously has the best mechanical properties of all the investigated materials.Translated from Ogneupory, No. 2, pp. 12 – 19, February, 1995.     

Effect of cation doping on lattice and grain boundary diffusion in superplastic yttria-stabilized tetragonal zirconia

Lattice diffusion coefficients D_l and grain boundary diffusion D_gb coefficients of hafnium were studied for 0.5 and 1 mol% cation-doped yttria-stabilized tetragonal zirconia at the temperature range from 1283 to 1510 °C. The diffusion profiles were determined by two experimental techniques: secondary ion mass spectroscopy and electron microprobe analysis. Additionally the first principle calculations of the electronic states of Zr⁴⁺, dopant cations and O^2? anions and elastic properties in 3Y-TZP were performed. Superplastic strain rate versus stress and inverse temperature was also measured. For 1 mol% doped samples the significant increase of the grain boundary diffusion and superplastic strain rate was observed. Correlations between the calculated ionic net charges and D_gb indicate that enhancement of D_gb was caused by the reduction of ionic bonding strength between metal cation and oxygen anion in zirconia. The new constitutive equation for superplastic flow of yttria-stabilized tetragonal zirconia ceramics was obtained.     

Effect of Fe2O3 doping on colour and mechanical properties of Y-TZP ceramics

Yttria-stabilized zirconia (Y-TZP) samples with different Fe concentrations were prepared aiming to study the effects of Fe₂O₃ doping on colour and mechanical properties. Since colour is an important optical property for jewellery and watchmaking, the investigation of colour in zirconia ceramics has a great scientific and technological interest. An investigation of the mechanical and optical properties, specifically the colour, was developed starting from commercial partially yttria-stabilized zirconia (Y-TZP) powders produced by Emulsion Detonation Synthesis (EDS). Within the strategies to get colours, the use of colouring oxides such as iron oxide (Fe₂O₃) was the chosen approach. The addition of specific ions into the ZrO₂ matrix can be used to tune zirconia colour without compromising its outstanding mechanical properties. Doping with iron oxide has proved to be a suitable, reproducible and irreversible colouring mechanism, allowing the development of a chromatically beige stable material with respect to its use in different processing conditions such as different atmospheres and temperature ranges. XRD results suggested that iron ions dissolved into tetragonal zirconia phase are at interstitial positions since the unit-cell volume of the tetragonal zirconia increases with increasing iron content. The effect of dopant addition on the mechanical properties of Y-TZP ceramics was also assessed. Compared to the undoped samples, doped ones exhibit a similar Vickers hardness (>1200?MPa) and biaxial flexural strength (>1000?MPa). However, it was observed that Fe₂O₃ doping slightly decreased the fracture toughness of Y-TZP ceramics.     

Low temperature degradation resistant nanostructured yttria-stabilized zirconia for dental applications

When used in prosthetic dentistry, zirconia encounters severe durability issues due to low temperature degradation: exposure to humidity results in a transition from tetragonal to monoclinic phase, associated to disruptive integrity loss. Recently it has been shown that size-induced stabilization helps maintaining zirconia in tetragonal form, when the grain size is reduced to the nano-range. Objective of this work is to demonstrate the applicability of High Pressure Field Assisted Sintering (HP-FAST) to the preparation of dense, nanostructured samples of tetragonal yttria stabilized zirconia, with yttria content between 0.5 and 3 mol% and showing resistance to low temperature degradation. The yttria stabilized zirconia nanopowders were prepared by a hydrothermal method. Sintering by HP-FAST was performed at 900 °C in 5 min, under a pressure of 620 MPa. Resistance to low temperature degradation was tested at 134 °C, under vapor pressure, for up to 40 h. Both pristine and aged samples were characterized by X-ray diffraction, high-resolution scanning electron microscopy and nanoindentation tests in continuous stiffness measurement mode. The sintered samples presented a grain size between 20 and 30 nm and low or null monoclinic content. Both parameters resulted unaffected by ageing. The best results in terms of phase composition and mechanical properties have been obtained with the material containing 1.5 mol% of yttria. These results induce to reconsider the use of yttria stabilized zirconia as material for dental prosthetic systems requiring long-term durability.     

Microstructure and mechanical properties of zirconia stabilized with increasing Y2O3, for use in dental restorations

The present in vitro study aims at characterizing dental zirconia ceramics, which are stabilized with a high amount of Y₂O₃. Two groups of specimens were fabricated by computer-aided design/computer-aided manufacturing technique. The specimens of each group were divided into two subgroups (SGs): SGs 1a and 2a contained a relatively low amount of Y₂O₃ (6–8 wt.%), whereas SGs 1b and 2b contained a higher amount of Y₂O₃ (8–10 wt.%). The influence of yttria content on their microstructure and mechanical properties was experimentally determined. The statistical significance of the differences in the mechanical properties between the SGs was evaluated by the t-test (p < 5% was considered statistically significant). Homogeneous and dense ceramics with fine nanostructure, comprising grains of yttria-stabilized tetragonal and cubic zirconia, sized between ∼160 and ∼800 nm, were produced. The increase of yttria content, which causes an increase in grain size, favors the formation of cubic zirconia, resulting in mechanical properties’ slight reduction; yet, the differences were not statistically significant. Consequently, the mechanical properties (HV 11.74–12.91 GPa, and K_IC 2.66–4.25 MPa m^0.5) and the good esthetics of the investigated zirconia ceramics stabilized with high yttria content qualify these zirconia materials for fabricating dental restorations, because they can approach the properties and the esthetics of dental hard tissues as well as the tooth structure.