Yttria-stabilized Zirconia Ceramics Research Articles

Investigation on microstructures and thermo-physical properties of ferroelastic (Y1-xDyx)TaO4 ceramics

A series of ceramics of (Y1-xDyx)TaO4 monoclinic phases (space group: I2 (5)) are prepared by the solid-state reaction method. The high-resolution transmission electron microscopy (HRTEM) suggest the (Y1-xDyx)TaO4 ceramics possess ferroelastic domain, which is derived from reversible second-order ferroelastic transformation between the high-temperature tetragonal (t) and low-temperature monoclinic (m) phase. Similar to metastable tetragonal YSZ ceramics, the ferroelastic toughening mechanism may exist in m-phase (Y1-xDyx)TaO4 ceramics through the shift or reorientation of ferroelastic domain under the stress field of a crack. The energy dispersive spectroscopy reveals that the elements of Y and Dy are inhomogeneous in the grain of (Y1-xDyx)TaO4 ceramics, which breaks down the relationship of concave parabola between thermal conductivity and composition parameter (x). Moreover, the thermal conductivities (1.7–2.0 W/m/k at 900 ̊C) and thermal expansion coefficients (TECs) (10–11 × 10−6 K−1) of (Y1-xDyx)TaO4 ceramics are comparable to that of YSZ (yttria-stabilized zirconia). In view of the low thermal conductivity, high thermal expansion coefficients, good high-temperature phase stability and ferroelastic transformation, the (Y1-xDyx)TaO4 ceramics show great potentials in the application of high-temperature thermal insulator materials, particularly as thermal barrier coatings.

Roughness and its effects on flexural strength of dental yttria-stabilized zirconia ceramics

This study evaluated the effect of the surface roughness parameters on the flexural strength of Yttria-partially stabilized zirconia, Y-TZP ceramics: ZrO2 (3 mol%) or ZrO2 (5 mol%), as function of different surface finishing protocols: polishing or alumina (Al2O3) blasting. Zirconia discs were manufactured by CAD/CAM prototyping and sintered at different temperatures. The specimens were characterized by X-ray diffraction, scanning electron microscopy, biaxial flexural strength test and 3D profilometry. In all specimens tetragonal Y2O3-ZrO2 was observed to be the main crystalline phase. The 3D profilometry indicated that alumina blasting of ZrO2 (3 mol%) increased the roughness Ra around 16–26 times as opposed to the polished surfaces, and approximately 38 times for ZrO2 (5 mol%) specimens when compared to the polished samples. The results of flexural strength indicate that groups of ZrO2 (3 mol% Y2O3) ceramics present a reduction of 21–23% from polished to alumina blasted specimens. In the group of ZrO2 (5 mol% Y2O3) ceramics, a flexural strength reduction of 37.5% was observed. A correlation between flexural strength, failure size and different roughness parameters: Ra (arithmetic average of the absolute values of the profile heights over the evaluation length), Rz (measure of the ordinates of the five highest peaks and the five deepest valleys along the specimen's length) and PV (mean values of peak and valley measured in the specimen space) is proposed and indicates that Rz and PV should not be neglected in a systematic evaluation of the surface roughness effect on the bulk mechanical properties of dental ceramics.

Effects of He ion irradiation on the microstructures and mechanical properties of t' phase yttria-stabilized zirconia ceramics

Abstract The dense ZrO2-8 wt.% Y2O3 ceramics with tetragonal (t') phase were prepared by spark plasma sintering (SPS). The micro-structure modification in poly-crystalline t' phase YSZ ceramics irradiated with 60 KeV He ions over a wide fluence range were studied. It is found that, the t' → t + c phase transformation was detected for the samples irradiated at the fluence above 2 × 1017 cm−2. In addition, a three-steep model for damage stages has been proposed and discussed for interpreting the evolution of mechanical properties of t'-YSZ ceramics after irradiated, mainly revealed by the analysis from HRTEM results. Firstly, as the He+ fluences range from 0.5 × 1017 cm−2 to 1 × 1017 cm−2, the hardening effects were confirmed in t'-YSZ owing to the irradiation induced defects serve as obstacles to resist slipping of dislocations. As the fluence was increased to 2 × 1017 cm−2, the degradation of mechanical properties was evidenced, which could be ascribed to the deterioration effect of aggregated He bubbles inside t' YSZ grains. While the fluence increased to 10 × 1017 cm−2, a seriously damaged region was approximately located at 250 nm in depth, at which a great number of He bubbles with different shape and dislocation loops were evidenced, leading to a further reduction of hardness and elastic modulus. No micro-cracks emerge even for He+ fluences as high as 10 × 1017 cm−2, indicating high irradiation stability and resistance to micro-crack propagation in t'-YSZ.

Aging resistance, mechanical properties and translucency of different yttria-stabilized zirconia ceramics for monolithic dental crown applications

ObjectivesThe dental market moves towards high-translucency monolithic zirconia dental crowns, which are usually placed either with – or without – a thin glaze layer. The microstructural features and the mechanical performances of these materials are still controversial, as well as their susceptibility to aging. This paper aims at studying these aspects in the current generation of zirconia dental crowns showing different degrees of translucency. MethodsFour different commercial zirconia materials were investigated, including one standard ‘full-strength’ 3Y-TZP and three grades with improved translucency. The microstructural features (phase composition and assemblage, grain size) were carefully studied, as well as mechanical properties (biaxial bending strength and indentation toughness), translucency and aging behavior (in autoclave at 134°C). Aging was conducted on crowns with and without glaze to better represent clinical uses. ResultsImportant differences are found in terms of microstructures among the materials in terms of cubic phase content and yttria in the tetragonal phase, leading to different optical, mechanical and aging resistance properties. We show that higher cubic phase content leads to better translucency and stability in water steam, but at the expense of strength and toughness. A compromise is always inevitable between translucency and aging resistance on one side and mechanical properties on the other side. Significance-Even so called ‘high translucent’ zirconia ceramics tested in this work should be considered as medium translucency materials.-Aging occurs in standard state-of-the-art dental zirconia and glazing does not fully avoid this issue. However, aging did not compromise strength even after prolonged duration.-Aging is null in the ‘highly translucent’ zirconia grades but at the expense of strength, which is then comparable to glass-ceramics.

Design, Fabrication, and Characterization of Porous Yttria‐Stabilized Zirconia Ceramics for Hot Gas Filtration Applications

Hot gas filtration is a lucrative target for a number of high temperature industrial operations, but new materials must be developed to withstand intense high temperature conditions. Here, the authors put the emphasis on porous yttria‐stabilized zirconia (YSZ) ceramics for hot gas filtration, owing to their high thermodynamic stability and toughness. The authors design and fabricate porous YSZ ceramics using a pore‐forming inclusion technique, where a polymeric pore former is mixed and compacted with YSZ powder. After burning off, the polymer leaves behind a controlled amount of porosity before sintering commenced. The authors systematically investigate the sintering temperature, polymer particle size, and polymer‐to‐ceramic ratio to optimize the open porosity, pore size, gas permeability, and hardness of porous YSZ ceramics. The authors then evaluate the performance of porous YSZ ceramics under industrial conditions by cyclic heating/cooling testing and high temperature exposure testing. The authors show, for the first time, porous YSZ ceramics can perform in hot gas filtration applications at 1400 °C for extended thermal cycling or at 1700 °C for limited periods of time.

Effects of various surface treatments on the biaxial flexural properties of yttria-stabilized zirconia ceramics

Aim: The aim of the present study was to evaluate and compare the influence of different surface treatments and their cumulative effects on the biaxial flexural properties and phase transformation of yttria-stabilized zirconia ceramics. Materials and Methods: A total of fifty specimens were fabricated by computer-aided design/computer-aided manufacturing machining from Cercon®. The samples were divided into five groups following different surface treatments as control (C), air particle abrasion (Si), mechanical loading (ML), low-temperature degradation (LTD), and cumulative treatment (CT) groups. Statistical Analysis Used: The results were analyzed by two-way ANOVA and Tukey's honestly significant difference (HSD) test. Two-way ANOVA was used to find significance between the test and the control groups. Tukey's HSD test was carried out to determine any significant difference among the groups. Results: The highest biaxial flexural strength was observed in the Si group (950.2 ± 126.7 MPa) followed by the LTD group (861.3 ± 166.8 MPa), CT group (851.2 ± 126.5 MPa), and the least with the ML group (820 ± 110 MPa). Significant difference was observed in two-way ANOVA test. Tukey's HSD test showed that there was a significant difference (P ≤ 0.05) between the C and Si groups and C and LTD groups; however, no significant difference was observed (P ≥ 0.05) between the C and ML groups and C and CT groups. X-ray diffraction analysis showed that the control group consisted of 100% tetragonal zirconia while the maximum amount of monoclinic phase was obtained after the LTD treatment. Conclusions: Air particle abrasion with CoJet Sand, LTD, and CTs had no negative impact on biaxial flexural strength indeed it increased the biaxial flexural strength. Hence, these surface treatments can be done in routine clinical practice to improve the performance of ceramic restorations.

Internal Nano Voids in Yttria-Stabilised Zirconia (YSZ) Powder.

Porous yttria-stabilised zirconia ceramics have been gaining popularity throughout the years in various fields, such as energy, environment, medicine, etc. Although yttria-stabilised zirconia is a well-studied material, voided yttria-stabilised zirconia powder particles have not been demonstrated yet, and might play an important role in future technology developments. A sol-gel synthesis accompanied by a freeze-drying process is currently being proposed as a method of obtaining sponge-like nano morphology of embedded faceted voids inside yttria-stabilised zirconia particles. The results rely on a freeze-drying stage as an effective and simple method for generating nano-voided yttria-stabilised zirconia particles without the use of template-assisted additives.

YSZ/Al2O3 multilayer thick films deposited by spin coating using ceramic suspensions on Al2O3 polycrystalline substrate

Yttria Stabilized Zirconia (YSZ) high chemical and thermomechanical stability under Solid Oxide Fuel Cells (SOFCs) standard operational conditions and its high electrolytic domain indicate that an interesting alternative for lowering SOFCs operating temperature would be the development of high conductive YSZ-based electrolytes compared to conventional electrolytes through microstructure engineering. In this work, YSZ/Al2O3 micrometric multilayer films obtained by spin coating presented electrical conductivity 3.5 times higher than traditional YSZ ceramics. The multilayer film was formed by three layers of YSZ with 3.7 µm of thickness each were intercalated by two 7.4 µm thick layers of Al2O3. Therefore, the high electrical conductivity observed in nanometric multilayer films obtained by advanced techniques was achieved, even with lower magnitude, with ceramic suspensions and in the micrometric scale. Computer simulation shows higher stresses on YSZ than on Al2O3 layers, correlating the heterofilms higher conductivity with the stress imposed on the YSZ layer during sintering.

Structure, densification and electrical properties of Gd3+ and Cu2+ co-doped ceria solid electrolytes for SOFC applications: Effects of Gd2O3 content

Ceria-based solid electrolytes exhibit superior electrical conductivity compared to traditional yttria-stabilized zirconia ceramics. However, they require high sintering temperatures to achieve full densification. Transition metal oxides exhibiting low melting points, such as CuO, have been used as additives to lower the sintering temperature of these materials. In this context, the present work is focused on the evaluation of the effects of gadolinium oxide (Gd2O3) content on the structure, densification and electrical properties of ceria co-doped with CuO. Nominal compositions of Ce0.99−xGdxCu0.01O2-δ (0 ≤ x ≤ 0.3) were synthesized by the polymeric precursor method. The precursor powders were characterized by simultaneous thermogravimetry and differential thermal analysis (TG/DTA) and the calcined powders were studied by X-ray diffraction (XRD) and Rietveld refinement to obtain crystallographic parameters. The sinterability of green bodies was evaluated by dilatometry up to 1200°C. The relative density was determined in samples sintered between 950 and 1050°C and the microstructural characterization was performed by scanning electron microscopy (SEM). The electrical properties were investigated by impedance spectroscopy (IS). The XRD results confirms the formation of a cubic fluorite type structure in the entire composition range. The lattice parameters obtained by Rietveld refinement showed a reduction in the crystallite size with increasing gadolinium content. Densification was improved with increasing Gd-content up to x = 0.15. The electrical conductivity was enhanced by gadolinium addition, reaching a maximum of 7.81mScm−1 at 600°C for the composition x = 0.15 sintered at a temperature as low as 1050°C.

Production and properties of tooth-colored yttria stabilized zirconia ceramics for dental applications

Dense zirconia stabilized with 3mol% yttria ceramics were produced in disc shape by first cold isostatically pressing at 100MPa and then sintering at 1450°C at ambient laboratory conditions. Coloring was accomplished by immersion the discs in NiCl2, MoCl3, and NiCl2 + MoCl3 solutions for 5, 30, and 60s. Different concentrations (0.1, 0.25, and 0.5wt%) were applied to get the color of natural tooth. The density, color, microhardness, fracture toughness, compressive strength, and wear rate of the discs were measured to evaluate the suitability of the colored discs for dental applications. Color assessments were made by measuring CIE Lab L*, a*, b*, and ΔE* values. Low temperature degradation of the samples was evaluated by aging sensitivity tests in autoclave for 2, 4, and 6h. Results have shown that color produced depends on the kind and concentration of the colorant solution while time of immersion has no significant effect on coloring process. Coloring solutions containing 0.1 and 0.25wt% MoCl3 provided clinically acceptable color with the ΔE* value ranging from 5.16 to 6.42 for dental applications.

Towards the prediction of hydrothermal ageing of 3Y-TZP bioceramics from processing parameters

Hydrothermal ageing of yttria-stabilized tetragonal zirconia ceramics can have a strong influence on the lifetime of zirconia devices. Ageing kinetics are often described by the Mehl–Avrami–Johnson equation, most often used as a phenomenological description. This work seeks to relate the parameters of MAJ equations (Vmax, n, b0 and Q) to microstructural characteristics of the zirconia material: grain sizes, Y2O3 partitioning, monoclinic, tetragonal and cubic phases ratio. Samples with identical nominal composition of 3Y-TZP were prepared with grain sizes ranging from 190 nm to 773 nm. From their microstructural parameters, a relationship between microstructural parameters and sintering cycles was first proposed, followed by a relationship between ageing parameters and microstructural parameters. These results provide a convenient framework to better develop the sintering cycle of zirconia biomaterial in order to maximize their resistance to hydrothermal ageing.

Combined Microwave and Laser Heating for Glazing of 8Y–ZrO2 and 8Y–ZrO2/ZrSiO4–Composites

Sintered porous yttria‐stabilized zirconia and zirconia composite ceramics with zirconium silicate are surface glazed by Laser‐Assisted Microwave Plasma Processing (LAMPP). Suitable process parameters for surface glazing are determined for those ceramics. The plasma process is monitored by means of pyrometry and optical emission spectroscopy. In order to prove the quality of the surface glazing and to characterize hot corrosion resistance, tests with molten vanadium pentoxide are performed. After 4 h of exposure, the penetration depth of the molten salt is investigated as a function of ceramic composition and pre‐treatment by glazing. Upon hot corrosion testing of glazed and non‐glazed ceramics, the molten vanadium pentoxide reacts selectively with yttrium oxide, forming yttrium vanadate, and causes crack formation in the zirconia ceramics due to transition to monoclinic zirconia. The results for LAMPP‐glazed ceramics show, that a surficial melting phase is achieved because process temperatures exceed 3000 °C. Hence, a dense, crack‐free and hardness‐enhanced surface layer achieves a better resistance to hot corrosion as compared to non‐glazed ceramics. Due to LAMPP‐glazing, the vanadium ingress is reduced from 33 to 7 μm for yttria‐stabilized zirconia and from 104 to 17 μm for zirconia composite ceramic. Reactions and microstructural changes taking place upon LAMP‐Processing are discussed.

The influence of stresses on ageing kinetics of 3Y- and 4Y- stabilized zirconia

Hydrothermal ageing is one of the most important limiting factors for the use of yttria-stabilized zirconia ceramics in contact with water-containing environments. It consists in the transformation of tetragonal phase to monoclinic phase, initiates on the surface of zirconia in the presence of water, and leads to roughening and potentially to micro-cracking and loss of integrity. The present work seeks to explore the influence of applied and residual mechanical stresses on the ageing kinetics of 3Y- and 4Y-TZP. Residual stresses were obtained by rough polishing. A subsequent Annealing step was employed for the preparation of samples free of residual stresses. All samples were submitted to in situ 3-points bending tests in water vapour atmosphere inside an autoclave at 134°C, allowing surfaces with a mechanical stress gradient to be exposed to hydrothermal ageing. The evolution of the monoclinic fraction with time and stress was then analyzed using Mehl-Avrami-Johnson equation.

Impact of machining on the flexural fatigue strength of glass and polycrystalline CAD/CAM ceramics

ObjectivesTo assess the effect of machining on the flexural fatigue strength and on the surface roughness of different computer-aided design, computer-aided manufacturing (CAD/CAM) ceramics by comparing machined and polished after machining specimens. MethodsDisc-shaped specimens of yttria-stabilized polycrystalline tetragonal zirconia (Y-TZP), leucite-, and lithium disilicate-based glass ceramics were prepared by CAD/CAM machining, and divided into two groups: machining (M) and machining followed by polishing (MP). The surface roughness was measured and the flexural fatigue strength was evaluated by the step-test method (n=20). The initial load and the load increment for each ceramic material were based on a monotonic test (n=5). A maximum of 10,000 cycles was applied in each load step, at 1.4Hz. Weibull probability statistics was used for the analysis of the flexural fatigue strength, and Mann-Whitney test (α=5%) to compare roughness between the M and MP conditions. ResultsMachining resulted in lower values of characteristic flexural fatigue strength than machining followed by polishing. The greatest reduction in flexural fatigue strength from MP to M was observed for Y-TZP (40%; M=536.48MPa; MP=894.50MPa), followed by lithium disilicate (33%; M=187.71MPa; MP=278.93MPa) and leucite (29%; M=72.61MPa; MP=102.55MPa). Significantly higher values of roughness (Ra) were observed for M compared to MP (leucite: M=1.59μm and MP=0.08μm; lithium disilicate: M=1.84μm and MP=0.13μm; Y-TZP: M=1.79μm and MP=0.18μm). SignificanceMachining negatively affected the flexural fatigue strength of CAD/CAM ceramics, indicating that machining of partially or fully sintered ceramics is deleterious to fatigue strength.

Effect of specific absorbed power on microwave sintering of 3YSZ ceramics

Samples of 3 % yttria-stabilized zirconia (3YSZ) ceramics have been sintered to near full density with no appreciable grain growth using an ultra-rapid microwave sintering process. The sintering experiments were carried out on a 24 GHz / 6 kW gyrotron system for microwave processing of materials with automatic process control. By varying the properties of the thermal insulation surrounding the samples it was possible to vary the microwave power required for heating. The final relative density of 3YSZ ceramic samples microwave heated at a rate of 50 °C/min to a temperature of 1400 °C without isothermal hold varied from 91.6 % when the specific absorbed microwave power was 4 W/cm3 to 99.4 % when the specific absorbed microwave power was 90 W/cm3. The specific absorbed power is therefore demonstrated to be the key parameter determining the achievable density in ultra-rapid field-assisted sintering processes.

In-situ coagulation of yttria-stabilized zirconia suspension via dispersant hydrolysis using sodium tripolyphosphate

A novel in-situ coagulation method without coagulation agent and adjusting pH value for yttria-stabilized zirconia (YSZ) suspension via dispersant hydrolysis is reported. Sodium tripolyphosphate (STPP) is used as dispersant to prepare electrostatic stabilized YSZ suspension. Influences of STPP contents on the dispersion and pH value of YSZ suspension were investigated. It indicated that there was a well-dispersed YSZ suspension with the addition of 0.3wt% STPP at pH=10. Influence of coagulation temperature on coagulation process and properties of green body was investigated. The sufficiently high viscosity suspension to coagulate was achieved at 60–80°C. The coagulation mechanism was different from traditional direct coagulation casting. The suspension was coagulated by directly shifting the isoelectric point to the original state without increasing the ionic strength and adjusting the pH value. It was proposed that the YSZ suspension could be destabilized via decrease of zeta potential by sodium tripolyphosphate hydrolyzing at elevated temperature. Coagulated samples with wet compressive strength of 3.60MPa could be demolded without deformation by treating 50vol% YSZ suspension with 0.3wt% STPP at 60°C for 30min. Dense YSZ ceramics with flexural strength of 887±110MPa and relative density of 98.9% had been prepared by this method sintered at 1450°C for 3h.

Grain-Boundary Engineering for Aging and Slow-Crack-Growth Resistant Zirconia

Ceramic materials are prone to slow crack growth, resulting in strength degradation over time. Although yttria-stabilized zirconia (Y-TZP) ceramics have higher crack resistance than other dental ceramics, their aging susceptibility threatens their long-term performance in aqueous environments such as the oral cavity. Unfortunately, increasing the aging resistance of Y-TZP ceramics normally reduces their crack resistance. Our recently conducted systematic study of doping 3Y-TZP with various trivalent cations revealed that lanthanum oxide (La2O3) and aluminum oxide (Al2O3) have the most potent effect to retard the aging kinetics of 3Y-TZP. In this study, the crack-propagation behavior of La2O3 and Al2O3 co-doped 3Y-TZP ceramics was investigated by double-torsion methods. The grain boundaries were examined using scanning transmission electron microscopy and energy-dispersive spectroscopy (STEM-EDS). Correlating these analytic data with hydrothermal aging studies using different doping systems, a strategy to strongly bind the segregated dopant cations with the oxygen vacancies at the zirconia-grain boundary was found to improve effectively the aging resistance of Y-TZP ceramics without affecting the resistance to crack propagation.

Influence of zirconia surface treatments on resin cement bonding and phase transformation

It evaluated the effects of different zirconia surface treatments on the bond strength of a resin cement to Y-TZP (yttria-stabilized tetragonal zirconia) ceramics, as well as their phase-transformations. 75 blocks (5 mm × 5 mm × 4 mm) of Y-TZP were assigned into five groups (n = 15): (tribochemical silica coating - TBS) zirconia surface was abraded by silica coated alumina particles followed by silanization; (GLZ1) zirconia surface received the application of a thin layer of low-fusing porcelain glaze, followed by hydrofluoric acid (HF) etching for 1 min; (GLZ5) glaze application + HF etching by 5 min; (GLZ10) glaze application + HF etching by 10 min; (GLZ15) glaze application + HF etching by 15 min. After etching, all the specimens were washed, dried and silanized. Cylinders of composites (diameter: 3.25 mm; height: 3 mm) were cemented to the Y-TZP blocks using a resin cement. All the specimens were subjected to aging (10,000 thermal cycles and 90 days storage), tested under shear conditions, and finally analyzed by a stereomicroscope (failure analysis). In addition, we also performed topographical and phase transformation analyses of the treated zirconia surfaces. The TBS group presented the highest bond strength value (23.34 MPa). The glazed groups presented low bond values and high prevalences of pretest failures. X-ray diffraction analysis showed a phase transformation for the TBS group (13.14%); however, there was no clear phase change observed for the GLZ groups. From our results, we concluded that tribochemical silica coating is the main Y-TZP surface conditioning for resin bond improvements.

Novel Baddeleyite-Based Zirconia Ceramics for Biomedical Applications

For the first time nanostructured engineering ceramics were prepared from natural zirconia mineral (baddeleyite) with CaO as a tetragonal phase stabilizer. The effect of synthesis conditions on microstructure and mechanical properties of the baddeleyite-based ceramics is reported, furthermore, the effect of calcia content on hardness and fracture toughness is studied. Optimal calcia concentration and synthesis conditions are found, corresponding hardness and fracture toughness values are 10,8 GPa and 13,3 MPa×m1/2. The reported mechanical properties are comparable to those typically reported for yttria-stabilized engineering zirconia ceramics, prepared from chemically synthesized zirconia.

Investigating the effect of yttria contents on the structure and mechanical properties of zirconia ceramics

In this study, the co- precipitation method and annealing process were used to prepare the yttria stabilized zirconia ceramics. Data of Raman and XRD showed that an increase of yttria doping leads to an decrease of the ratio between monoclinic and tetragonal phases. Howerver, it existed a critical concentration of yttrium at which the highest value of hardness was about 14.5 GPa and grain sizes of 0.2–0.5 m. An investigation of low temperature degradation indicated that theaging process only occurred in the first 5 hours and then virtually negligible changes.