Yttria-stabilized Zirconia Ceramics Research Articles

Thermophysical Properties of (Y1-xErx)TaO4 Ceramics

A series of (Y1-xErx)TaO4 ceramics (x=0/6, 1/6, 2/6, 3/6, 4/6, 5/6, 6/6) were prepared using powders synthesized by the reverse co-precipitation technique. Investigations were conducted into the phase compositions, microstructure, and thermophysical characteristics. In contrast to yttria-stabilized zirconia (YSZ) ceramics, (Y1-xErx)TaO4 ceramics demonstrate reduced thermal conductivity and Young's modulus, along with greater coefficient of thermal expansions (CTEs). The impact of Er and Y elements on separation interferes with the parabolic relationship between thermal conduction and the composition of (Y1-xErx)TaO4 ceramics, modifying their thermal diffusion and conductivity patterns. Furthermore, the performance of (Y1-xErx)TaO4 ceramics was optimized by adjusting the ratio of Er/Y, from which (Y1-xErx)TaO4 ceramics exhibited lower thermal conductivity (1.38∼1.72 W·m-1·K-1 @ 800 °C), higher CTEs (9.24×10-6∼10.9×10-6 K-1 @ 1000 °C) and more excellent mechanical properties comparable to YSZ. Herein, (Y1-xErx)TaO4 ceramics show great potential for thermal barrier coating applications.

Digital light processing 3D printing of yttria stabilized zirconia ceramics: microstructures, characterizations, and cytocompatibility

Digital light processing 3D printing of yttria stabilized zirconia ceramics: microstructures, characterizations, and cytocompatibility

Enhanced densification and phase transformations during rapid microwave sintering of alumina – yttria-stabilized zirconia ceramics

Alumina – 7.5 wt% yttria-stabilized zirconia (YSZ) ceramic composites were sintered using 24 GHz microwave heating at rates of 10 – 200 °C/min with zero isothermal hold. The starting powders were nanophase η-Al2O3 and YSZ prepared by a laser evaporation method. The final densities of the sintered samples were up to 97.5 % of the theoretical value. The samples exhibited rapid densification until transformation to the α-Al2O3 phase. The temperature of the densification rate peak (and hence of the phase transformation) decreased consistently with increasing microwave electromagnetic field intensity (varied by using different susceptor materials). The densification peak temperature difference between microwave and conventional sintering experiments exceeded 200 °C.

Realizing near-full density monophasic tetragonal 1.5-mol% yttria-stabilized zirconia ceramics via current-ramp flash sintering

This study demonstrates the successful fabrication of near-full density monophasic tetragonal 1.5-mol% yttria-stabilized zirconia (1.5YSZ) ceramics using current-ramp flash (CRF) sintering technique. The process involved regulating Joule heating in the samples by fine-tuning the input power through an alternating current field (nominal current density: 50 mA·mm−2) within a 3-min timeframe at a furnace temperature of 1100°C. This approach effectively enhanced grain size uniformity, which is crucial for preventing sample cracking associated with spontaneous tetragonal-to-monoclinic (T→M) phase transformation, thereby promoting densification with average relative densities exceeding 99%. The highest average fracture toughness of the 1.5YSZ samples was measured at 9.2 MPa·m0.5 using the standardized single-edge pre-cracked beam method. This toughness is approximately double that of commonly used 3YSZ samples produced by CRF sintering, all of which exhibited comparable average grain sizes and relative densities. Additionally, the 1.5YSZ samples demonstrated nearly identical resistance to low-temperature degradation (LTD) compared to the 3YSZ samples after accelerated hydrothermal aging at 140°C for 15 h, roughly equivalent to 60 years at 37°C. The reduced yttria concentration in 1.5YSZ facilitates T→M phase transformation at lower stress thresholds, enhancing toughness through increased crack shielding from a larger volume fraction of transformed grains. Furthermore, the uniform yttria distribution in 1.5YSZ, revealed by scanning transmission electron microscopy, compensates for the reduced tetragonal phase stability and contributes to improved LTD resistance. Notably, these exceptional properties were achieved at a furnace temperature 300°C lower and with a sintering duration several hours shorter than those of conventionally-sintered counterparts.

High strength YSZ/YSZ joints bonded with a matching thermal expansion coefficient sealing glass

A novel CaO–Al2O3–SiO2–Li2O (CASL) glass was developed for bonding yttria–stabilized zirconia ceramics (YSZ). The CASL glass possessed a coefficient of thermal expansion (CTE) that matched with YSZ and exhibited outstanding wettability on the surface of YSZ substrate. During the bonding process, mutual dissolution and diffusion between YSZ and CASL glass took place, which ensured good interface bonding. Furthermore, the rapid cooling speed led to a completely amorphous seam, which allowed the CTE of the seam to correspond with that of the original glass. The optimal flexural strength of YSZ/YSZ joints could reach more than 90 % of that of the YSZ.

Fabrication and properties of pink 3 mol% yttria‐stabilized zirconia ceramics with high toughness

AbstractTo broaden the applications of yttria‐stabilized zirconia (YSZ) in mobile phone back panels and high‐end decorative materials, the 3 mol% YSZ (3YSZ) ceramics doped with erbium oxide (Er2O3) as colorant were successfully sintered at 1250 and 1350°C in air. The influences of Er2O3‐doping concentration and sintering temperature on the phase, microstructure, color, and mechanical properties of the pink Er2O3‐3YSZ ceramics were investigated. As the Er2O3 concentration increased, the redness value a* gradually increased, while the yellowness value b* and the lightness value L* gradually decreased. The highest value a* and b* of Er2O3‐3YSZ ceramics reached absolute values of 15.2 and 4.5, respectively. The effect of Er2O3 concentration is also depicted in detail, which also affects the mechanical properties including hardness and fracture toughness of the ceramics. The maximum hardness and fracture toughness of Er2O3‐3YSZ ceramics reached 13.9 ± 0.3 GPa and 6.21 ± 0.20 MPa·m1/2, respectively. The hardness of the ceramics exceeds 12.5 GPa, which is the required market index for further practical application. This work provides a method for preparing 3YSZ ceramics with excellent mechanical properties and bright pink color, which has the potential to be used in the fields of advanced and superior electronics in particular.

Conventional sintering of nano-crystalline Yttria-Stabilized Zirconia enables high-strength, highly translucent and opalescent dental ceramics

Development of restorative materials capable of mimicking optical and mechanical performance of natural teeth is a quest in aesthetic density. Yttria-Stabilized Zirconia (YSZ) ceramics represent one of the most popular choices for dental restorations, owing to their biocompatibility, white colour, and the possibility to use CAD-CAM technologies. In particular, YSZ doped with 3 mol. % yttria (3YSZ) is popular because it presents high strength. Nonetheless, the limited light transmission of commercially available high strength 3YSZ does not meet the requirements of highly aesthetic cases. On the other side, YSZ presenting a larger portion of yttria are more translucent but exhibit modest strength. Here, we report on fabrication of dense zirconia nanostructures in bulk form via conventional pressure-less sintering at temperatures down to 1100–1200 °C, achieving highly translucent and strong 3YSZ with significant opalescent behaviour. Both Hall−Petch and inverse Hall-Petch relationship were observed in 3YSZ samples with average grain size in the range of 250 nm and 55 nm, demonstrating the importance of grain size control to enhance both optical and mechanical properties of zirconia ceramics, simultaneously. Maximum biaxial strength of 1980 ± 260 MPa, in-line light transmission of 38% in the visible spectrum and opalescence approaching that of enamel were obtained at optimum grain size of 80 ± 5 nm. The notable optical properties are linked to the miniaturization of the residual pores and refinement of grain size towards the nanoscale while the superior mechanical strength is justified by the activation of different energy dissipation processes at nano and macroscale.

Attaining translucency in binder/additive-free, nanograined, tetragonal 1.5 mol.% yttria-stabilized zirconia ceramics

In the quest of minimum yttria content to stabilize tetragonal zirconia system, we report the synthesis of ultrafine (∼9 nm), near-spherical, phase-pure tetragonal 1.5 mol.% yttria-stabilized zirconia (1.5YSZ) nanoparticles. The pressureless sintering process (at 1100 °C) involved of 1.5YSZ nanopowders (without any slurry processing) and did not need any binders, additives, or sintering aid to attain high-density (>98.5%) and refined grain size (∼50 nm) for the consolidated fully tetragonal 1.5YSZ nanoceramics. A promising transmittance value of 46% (at 630 nm) was obtained, making it comparable/higher in optical properties with respect to the transmittance values of existing 3 mol.% yttria-doped ZrO2 compositions. This result is credited to our synthesized phase-pure (validated via X-ray Diffraction and Raman analysis) near-spherical nanoparticles with improved tetragonality (c/a ratio = 1.439, obtained via Rietveld analysis) that paved the way for low-temperature conventional sintering for possible large-scale industrial applications.

Sustainable Processing of Insulating Yttria Stabilized Zirconia Ceramic Using Wire Electrical Discharge Machining in Distilled Water

Sustainable Processing of Insulating Yttria Stabilized Zirconia Ceramic Using Wire Electrical Discharge Machining in Distilled Water

Investigation of the infrared radiation characteristics modulation mechanism of titanium dioxide composite yttria-stabilized zirconia ceramics

Yttria stabilized zirconia (YSZ) presents a promising option for infrared camouflage applications due to its lower emissivity within the 3–5 μm waveband. Nevertheless, its emissivity increases within the 8–14 μm waveband, which is attributed to lattice vibration-induced absorption characteristics, combined with an extended Christiansen wavelength (K-point). In this study, we regulate YSZ’s infrared radiation performance in the 8–14 μm band by combining a material with lower infrared emissivity in the 8–14 μm band and lower lattice resonance frequencies such as titanium dioxide (TiO2) based on lattice vibration theory and classical electromagnetic theory. The results showed that as the TiO2 content increases, the emissivity of the composite ceramic block remains relatively stable in the 3–5 μm band. Notably, With a TiO2 content of 80 %, the emissivity within the 8–14 μm band decreases from 0.55 to 0.48 and within the 5–8 μm band increases from 0.33 to 0.41, the K-point of ceramics shifted from 12.8 μm to 11.8 μm. It provides an effective approach for modulating the infrared emissivity and K-point of YSZ.

Grain boundary effects on thermal shock responses of yttria-stabilized zirconia

The rapid temperature change or thermal shock is inevitable in many engineering applications of polycrystalline yttria-stabilized zirconia (YSZ). Nevertheless, the thermal shock response of YSZ especially its dependence on the grain boundary (GB) structure in YSZ remains almost unknown to date. In this study, the effect of the GB on thermal shock behaviors of YSZ ceramics was investigated by molecular dynamics (MD) simulations in conjunction with experimental characterizations. According to the distribution of misorientation angles of GBs experimentally extracted from our sintered YSZ samples, four most common GBs with a misorientation angle ranging from 35° to 63° were constructed in MD simulations through the annealing technique. The thermal shock simulations on the bi-crystalline YSZ with such GBs show a weakening effect of the GB on the thermo-elastic strain wave, which is found to be the most significant in the [100]/[111] GB and almost vanish in its [110]/[111] counterpart. Our simulations also reveal that the weakening effect of GB on the thermo-elastic strain wave is majorly attributed to the impedance mismatch between its two adjacent grains, although the grain-boundary thermal resistance observed also can weaken the temperature wave after crossing GBs. These findings provide valuable atomic insights into the thermomechanical behaviors of YSZ under thermal shock, which will guide the design of microstructures of YSZ for some special applications in solid oxide fuel cells and thermal barrier coatings.

Microstructural Evolution and Gas-Tight Properties of Yttria-Stabilized Zirconia/Crofer 22H Stainless Steel Brazed Joints with the Ag-Ge-Si Filler for Use in Solid-Oxide Fuel Cells

In this paper, a novel 95Ag-2.5Ge-2.5Si (in wt %) filler is utilized for brazing yttria-stabilized zirconia (YSZ) electrolytes and commercial Crofer 22H interconnects for solid-oxide fuel cells’ (SOFCs) sealing application. Before brazing, surface metallization is applied on YSZ and Crofer 22H substrates to improve the wetting performance of the filler on YSZ and Crofer 22H substrates. The brazing procedure is performed at 900 °C for 10 min under a high vacuum (~10−6 torr) to prepare sandwiched YSZ/Crofer 22H brazed coupons. The metallization mentioned above can achieve reactive wetting toward YSZ ceramics. A Si/Ti-rich oxide layer and an Fe-Cr-Si alloying phase are formed at the brazed joints’ YSZ/filler and filler/Crofer 22H interfaces. After exposure to air at 750 °C for 100 h, Cu and Si contents suffer from oxidation and form CuO and SiO2, respectively, in the brazed zone and the YSZ/filler interface of the joints. The Fe-Cr-Si alloying phase at the filler/Crofer 22H interface is preserved without apparent oxidation. The pressure-drop test results show that the brazed joints’ gas tightness does not deteriorate significantly after thermal aging, which is attributed to the good interfacial integrity of thermal-aged joints.

Grain size effect on the plasma etching behavior of spark plasma sintered yttria-stabilized zirconia ceramics

Yttria-stabilized zirconia (YSZ) ceramics with various grain sizes were fabricated via a spark plasma sintering (SPS), and their plasma etching properties were investigated, particularly focusing on the effect of grain size on plasma etching behavior. All the sintered YSZ ceramics were fully densified (∼99.8 % relative density) and the grain size was varied by controlling the sintering conditions. The plasma etching test was conducted by an inductively coupled plasma (ICP) chamber using CF4. The sintered YSZ ceramics exhibited a low etching rate compared to that of the conventional plasma-resistant materials such as Al2O3 and SiC. The superior plasma erosion resistance of YSZ ceramics can be attributed to the yttrium (Y) enrichment and YF3 formation in the plasma etched surface. Interestingly, the grain size of the YSZ ceramics barely affected the etching depth and mass loss. On the other hand, the surface roughness increased with the increasing grain size. The YSZ ceramics with the large grain size showed a much rough surface after the plasma exposure. The obtained results indicated that the grain size could affect the surface properties of the ceramics during plasma etching, which should be considered when fabricating the future plasma-resistant materials.

High concentration Eu3+ doped yttria-stabilized zirconia ceramics

The series of yttria-stabilized zirconia (YSZ) ceramics doped with high Eu3+ ions concentration (up to 15 wt%) were successfully fabricated by Spark Plasma Sintering (SPS) technique for the first time. The effect of europium concentration and atmospheric annealing on the optical, structural, and luminescent properties has been studied in detail. The combination of XRD and EDS data allows to find two phases in tested YSZ:Eu ceramics such as ZrO2 and Zr2O. The highest transmission values were achieved about 63% at 600 nm for the 10 wt% of Eu2O3 doped YSZ ceramics without annealing. When the annealing temperature changes from 1300° to 1600°C the transmission values increase to 70%. An analysis of the photoexcitation and photoluminescence spectra showed that the main excitation bands are determined by the direct excitation of the 7F0 ground state to higher 4 f levels with following radiation transitions in Eu3+ ions. The optimal Eu3+- dopant concentration in YSZ ceramics was established and the process of emission quenching was discussed. The luminescence decay kinetics are approximated by the single or sum of two exponential functions in dependence of Eu3+ ions concentrations.

Rare earth oxide CeO2 enhanced reactive air brazing of YSZ ceramics

Cerium oxide (CeO2) in the range of 0–16 mol.% is used to enhance the joint of Ag-based filler for brazing yttria-stabilized zirconia (YSZ) ceramic in the air. Ag–CeO2 filler is first adopted to braze YSZ ceramic (3 mol.% yttria). The CeO2 is found to cluster at the interface of YSZ and Ag seam, improving the wetting behavior of Ag-based fillers. The maximum shear strength of YSZ joints can reach 35.1 MPa using Ag–1CeO2 fillers at 1050 °C/30 min. Moreover, CeO2 is used to enhance the Ag–CuO filler for brazing YSZ ceramic. As the increase of CeO2 content, the contact angle of Ag–CuO on the YSZ surface decreases first and then increases. The average shear strength could reach 76.5 MPa using Ag–CuO–8CeO2 at 1050 °C/30 min. The CeO2 could decrease residual stress by adjusting the linear expansion coefficient of Ag-based fillers. Therefore, this study provides a new strategy for exploring air brazing fillers.

Thermoplastic bonding of TiAl-based alloy and YSZ ceramic utilizing a Ni60Nb40 metallic glass as the filler metal

Achieving a reliable bond between advanced ceramics and high-temperature application alloys for high-temperature applications can greatly enhance the operating temperature range of vital components like engines and turbines. In this work, we have achieved success in bonding Yttria-stabilized zirconia (YSZ) ceramics with TiAl-base alloys through the use of a novel ceramic-metal joining method called thermoplastic bonding (TPB). Compared with traditional brazing methods, TPB requires a lower bonding temperature that is below the melting point of the filler. This advantage enables TPB to use fillers with higher melting points, resulting in improved high-temperature performance of the joint. Furthermore, TPB substitutes the conventional liquid phase bonding method in brazing with a solid phase bonding process. This not only diminishes residual stress on the joint but also prevents the generation of fragile oxides, which effectively enhances the strength of the joint. Shear tests have demonstrated that TPB joints possess excellent room temperature strength, reaching up to 141 MPa. Remarkably, the TPB joint retains remarkable strength, with a value of approximately 84 MPa even when exposed to a high temperature of 600 °C. These test results proved that TPB was an effective strategy for successfully realizing high-strength and high-temperature resistant metal-ceramic joins.

Residual stress associated with crystalline phase transformation of 3–6 mol% yttria-stabilized zirconia ceramics induced by mechanical surface treatments

Monolithic dental prostheses made of 3–6 mol% yttria-stabilized zirconia (3–6YSZ) have gained popularity owing to their improved material properties and semi-automated fabrication processes. In this study, we aimed to evaluate the influence of mechanical surface treatments, such as polishing, grinding, and sandblasting, on the residual stress of 3–6YSZ used for monolithic prostheses in association with crystalline phase transformation. Plate specimens were prepared from five dental zirconia blocks: Aadva Zirconia ST (3YSZ), Aadva Zirconia NT (6YSZ), Katana HT (4YSZ), Katana STML (5YSZ), and Katana UTML (6YSZ). The specimens were either polished using 1, 3, or 9 μm diamond suspensions, ground using 15, 35, or 55 μm diamond discs, or sandblasted at 0.2, 0.3, or 0.4 MPa. The residual stress, crystalline phase, and hardness were analyzed using the cosα method, X-ray diffraction (XRD), and Vickers hardness test, respectively. Additionally, we analyzed the residual stress on the surfaces of monolithic zirconia crowns (MZCs) made of 4YSZ, 5YSZ, and 6YSZ, which were processed using clinically relevant procedures, including manual grinding, followed by polishing using a dental electric motor on the external surface, and sandblasting on the internal surface. Residual stress analysis demonstrated that grinding and sandblasting, particularly the latter, resulted in the generation of compressive residual stress on the surfaces of the plate specimens. XRD revealed that the ground and sandblasted specimens contained a larger amount of the rhombohedral phase than that of the polished specimens, which may be a cause of the residual stress. Sandblasting significantly increased the Vickers hardness compared to polishing, which may possibly be due to the generation of compressive residual stress. In the case of MZCs, compressive residual stress was detected not only on the sandblasted surface, but also on the polished surface. The difference in the residual stress between the plate and crown specimens may be related to the force applied during the automated and manual grinding and polishing procedures. Further studies are required to elucidate the effects of the compressive residual stress on the clinical performance of MZCs.

Influence of sintering temperature on the thermal conductivity of digital light processing 3D-printed yttria-stabilized zirconia ceramic

In this work, digital light processing (DLP) – common 3D-printed technology – was used to shape yttria-stabilized zirconia (YSZ) ceramic green bodies. Influences of sintering temperature on thermal conductivity of YSZ ceramics were studied systematically. Results showed that thermal conductivity of ceramics increased from 1.89 W/(m·K) to 2.99 W/(m·K) with increasing sintering temperature from 1200 °C to 1600 °C. Porosity of YSZ ceramics decreased from 15.00% to 1.59%, and grain size increased from 0.13±0.03 μm to 0.70±0.20 μm. At lower sintering temperature from 1200 °C to 1400 °C, excessive pores exerted an impact on thermal conductivity due to phonon scattering processes. With further increase in temperature to 1600 °C, the amount of grain boundaries controlled by grain size was main factor affecting thermal conductivity of ceramics. In particular, fewer grain boundaries reduced phonon scattering, resulting in the increase in thermal conductivity. Therefore, sintering temperature is key parameter influencing thermal conductivity of YSZ ceramics through porosity, grain size and grain boundary at different stages.

Microstructure and permeability of porous YSZ ceramics with aligned and gradient pore channels fabricated by freeze casting

Porous yttria-stabilized zirconia (YSZ) ceramics with aligned and gradient pore channels were fabricated by the unidirectional freezing of aqueous suspensions with distinct solid loadings under different sintering temperatures. The results indicate that the microstructure and response properties could be tailored by adjusting the solid loading and sintering temperature. The pore channel size increased with the increase of solid loading or the decrease of sintering temperature, regardless of microstructure changes in an individual sample. With increasing the solid loading and sintering temperature, the compressive strength (7.18–30.76 MPa) increased owing to the lower porosity of individual samples. The fabricated samples had increasing Darcian and non-Darcian constants along the freezing direction due to the enhanced pore size and open porosity. The Darcian and non-Darcian constants in this paper were in the range of 7.06 × 10−13 to 5.92 × 10−12 m2 and 1.66 × 10−7 to 8.83 × 10−7 m, respectively. These results are of practical value for the fabrication and optimization of porous ceramics in the applications of filters and membrane supports.

Cold sintering process assisted sintering for 8YSZ ceramic: A way of achieving high density and electrical conductivity at a reduced sintering temperature

Yttria-stabilized zirconia (YSZ) is a well-studied and practically applied electrolyte for solid oxide fuel cells (SOFC), but the high sintering temperature causes some problems. In this work, 8 mol% yttria-stabilized zirconia (8YSZ) ceramics were prepared by a cold sintering process (CSP) technique associated with post-heat-treatment (PHT). The CSP was beneficial for getting high initial density for the green bulk, while the PHT could further improve the bulk density of the 8YSZ ceramics at a reduced sintering temperature. Electrochemical impedance spectra showed that the electrical conductivity was 0.060 S/cm at 850 °C for the 8YSZ ceramic prepared by CSP-PHT at 1300 °C, about twice of that prepared by CS. The improved electrical conductivity was attributed to the high density and the reduced activation energy relevant to refined grain size and increased bulk density. Our study demonstrates that the CSP-PHT is a promising technique for preparation of high-dense YSZ ceramics with improved electrical conductivity at a reduced sintering temperature.