ZrO2 Ceramics Research Articles

Preparation of super-strong ZrO2 ceramics using dynamic hot forging

In this study, dynamic hot forging (DHF) was proposed and applied to make super-strong zirconia (ZrO2) ceramics. The flexural strength of the sample forged at 1400 ℃ reached 1860 MPa, which was more than 3 times higher than that of the unforged commercial zirconia ceramics and 120 MPa higher than the conventional hot forged zirconia ceramics. The microstructural analysis suggested that the anomalously high strength is due to grain boundary strengthening caused by grain boundary sliding and grain strengthening caused by work hardening. It is anticipated that DHF could be applicable to other material systems to make super-strong materials.

Microstructure of Ultrafine-Grained Al2O3–ZrO2 Ceramics Produced by Two-Step Spark Plasma Sintering

Microstructure of Ultrafine-Grained Al2O3–ZrO2 Ceramics Produced by Two-Step Spark Plasma Sintering

Low temperature brazing between pre-metallized ZrO2 ceramics and medical pure titanium using AuSn20 filler metal

ZrO2 ceramic was metallized by Sn-xTi (x = 2, 3 and 4 at.%) foils at 1000 °C for 30 min. As the Ti content was 3%, the continuous and homogeneously distributed Ti–O compounds layer formed at the interface. The metallization layer metallized Sn–3Ti foil consisted of Ti2O3 layer + Ti11.31Sn3O10/β-Sn + Ti2Sn3+Ti6Sn5. During the metallization process, as the thickness of metallization layer increased, the Ti–O and Ti–Sn–O layers became continuous, Ti2Sn3 and Ti6Sn5 phases increased and aggregated obviously. Vacuum brazing using AuSn20 filler metal after pre-metallization were utilized to achieve the robust ZrO2/Sn–3Ti/AuSn20/Ti joints. The effect of the metallization layer thickness on microstructure and mechanical properties was systematically examined. The typical interfacial microstructure of ZrO2/Sn–3Ti/AuSn20/Ti joints brazed at 550 °C for 30 min was ZrO2/Ti2O3+Ti11.31Sn3O10/β-Sn + Ti6Sn5+AuSn2+AuSn4+AuTi5Sn3 layer/Ti. During brazing process, the Ti–O and Ti–Sn–O layers beside ZrO2 ceramic varied barely with the increasing of the thickness of metallization layer during brazing process. In the brazing seam, the AuSn2 phases and Ti6Sn5 phases attenuated whereas AuSn4 phases evidently increased with the thicker metallization layer. The AuTi5Sn3 layer nearby the Ti substrate thickened significantly varied with the thickness of metallization layer. Additionally, the ZrO2/Ti brazed joint with the maximum shear strength of 48.6 MPa was achieved by the metallization layer thickness of 150 μm. Furthermore, fracture analysis demonstrated that the cracks initiated at the Ti–O layer and propagated along the AuSn2 phases and layered Au–Ti–Sn phases.

Properties of porous zirconia ceramics fabricated by using various pore-forming agents

The present work proposes a new method for fabrication partially stabilized porous zirconia ceramics using monoclinic zirconia as raw material, yttrium nitrate and magnesium compounds as pore-forming agents and stabilizers. Effects of different pore-forming agents, firing temperatures and firing time on properties of samples were investigated. Thermal decomposition of yttrium nitrate and magnesium compounds creates a large number of pores, and thus porous zirconia ceramics were fabricated. ZrO2 can be partially stabilized by Y2O3 and MgO derived from the precursors. The porous ZrO2 ceramics obtained by using Y(NO3)3·6H2O and 4MgCO3·Mg(OH)2·6H2O as pore-forming agents had relatively high stabilization ratio, uniform pores and high strength. The optimum firing temperature and firing time are 1400 °C and 3 h, and the samples have the stabilization ratio of 45.7%, high cold crushing strength (26 MPa), uniform pores, and apparent porosity is about 40%.

Peculiarities of the Phase Formation during Electroconsolidation of Al2O3-SiO2-ZrO2 Powders Mixtures.

This paper is devoted to the sintering process of Al2O3–SiO2–ZrO2 ceramics. The studied method was electroconsolidation with directly applied electric current. This method provides substantial improvements to the mechanical properties of the sintered samples compared to the traditional sintering in the air. The research covered elemental and phase analysis of the samples, which revealed phase transition of high-alumina solid solutions into mullite and corundum. Zirconia was represented mainly by tetragonal phase, but monoclinic phase was present, too. Electroconsolidation enabled samples to reach a density of 3.0 g/cm3 at 1300 °C, while the sample prepared by traditional sintering method obtained it only at 1700 °C. For the composite Al2O3—20 wt.% SiO2—10 wt.% ZrO2 fabricated by electroconsolidation, it was demonstrated that fracture toughness was higher by 20–30%, and hardness was higher by 15–20% compared to that of samples sintered traditionally. Similarly, the samples fabricated by electroconsolidation exhibited elastic modulus E higher by 15–20%. The hypothesis was proposed that the difference in mechanical and physical properties could be attributed to the peculiarities of phase formation processes during electroconsolidation.

Flash sintering of Al2O3–ZrO2 ceramics under alternating current electric field

In this study, the influence of alternating current (AC) electric field on flash sintering and microstructural evolution of alumina–zirconia (Al2O3–ZrO2) composite was systematically investigated at furnace temperature of 800 °C. Compared with direct current (DC) electric field, AC electric field not only promoted densification and grain growth of Al2O3–ZrO2 composite, but also improved the uniformity of microstructure of ceramics. Grain size of AC flash-sintered samples was found to be inversely related to electric field, and positive correlation was observed with current density limit. Dense Al2O3–ZrO2 composite ceramic was fabricated via AC flash sintering under 60 mA mm−2 at low furnace temperature within 120 s, and as-sintered samples exhibited relatively good mechanical properties. The mechanism involving synergistic effect of Joule heating and defects generation under the influence of electric field was proposed to explain rapid densification during AC flash sintering. These results indicate the feasibility of preparation of dense composite ceramic with homogeneous microstructure via AC flash sintering.

Influence of the Grain Size of ZrO2(Y2O3) Ceramics on the Hardness, Fracture Toughness and Formation of a Transformation-Deformation Relief in the Indentation Zone

Influence of the Grain Size of ZrO2(Y2O3) Ceramics on the Hardness, Fracture Toughness and Formation of a Transformation-Deformation Relief in the Indentation Zone

The Effect of Yttria Content on Microstructure, Strength, and Fracture Behavior of Yttria-Stabilized Zirconia.

Yttria-stabilized zirconia (YSZ) is well-known as a material with perfect mechanical, thermal, and electrical properties. It is used for manufacturing various high-temperature components for aerospace and energy generation, as well as wear- and corrosion-resistant devices in medicine. This work investigated the effect of a Y2O3 addition to ZrO2 on the microstructure and mechanical properties of YSZ ceramics produced by one sintering schedule. ZrO2 ceramics doped with 3, 4, 5, 6, 7, and 8 mol% Y2O3 (designated 3YSZ through to 8YSZ) were prepared by using conventional sintering at 1550 °C for 2 h in argon. The effect of yttria content was analyzed with respect to grain size, morphology of the microstructural features, phase composition, parameters of fracture surface, and flexural strength. The 7YSZ ceramics sintered at 1550 °C for 2 h showed the highest level of flexural strength due to the formation of the fine-grained microstructure containing mainly the monoclinic and tetragonal zirconia phases. The fracture micromechanism in the studied YSZ ceramics is discussed.

The Effect of Mullite Addition on Wear Properties of SiO2 Doped ZrO2 Ceramics

Mullite (3Al2O3.2SiO2) and 10 mol% silica added zirconia (10 mol% SiO2 - 90 mol% ZrO2) ceramic powders were synthesized by conventional ceramic processing route. The mixtures were prepared by mechanical alloying method using zirconia ball mill in acetone environment. To synthesize mullite, Al2O3 and SiO2 powders mixture was prepared with stoichiometric proportions and fired it in the air at 1600 oC for 3 h. And the silica added zirconia composites were fired at 1300 oC for 2 h. Thus, silica - zirconia and mullite composite phases were obtained and milling and sieving processes were carried out. Then, mullite-free and 10% by weight mullite reinforced silicon oxide added zirconia mixtures were prepared by powder metallurgy method. The powders were compacted by uniaxial pressing. The formed samples were sintered in a high temperature furnace in air conditions for 1 and 5 h at 1500 and 1600 oC sintering temperatures. Finally, microstructure examinations of the composites with SEM, phase analysis with XRD, hardness, three-point bending and wear tests were performed. In addition, the results of water absorption, porosity and density from physical properties and the effect of mullite additive on the mechanical and especially wear properties of this mixture were investigated.

Effect of thermal treatment at high temperature on phase stability and transformation of Yb2O3 and Y2O3 co-doped ZrO2 ceramics

Y2O3 doped ZrO2 (YSZ) ceramic material is used to protect alloy components worked in high-temperature. But its phase transformation between tetragonal phase and monoclinic phase occurred at 1150 °C leads to YSZ invalid. Therefore, enhancing the phase stability of YSZ is necessary for meeting the demands of the development of thermal barrier coatings (TBC). In this study, X-ray diffraction and Raman spectra were used to explore the phase stability and phase transformation of Yb2O3 and Y2O3 co-doped ZrO2 (YbYSZ) ceramics after heat treatment at 1300 °C with different times. The stability of tetragonal phase is improved by tetragonality decreasing with Yb3+ doped. Simultaneously, the incorporation of Yb3+ leads to O–O coupling, which is beneficial for increasing the concentration of oxygen vacancies near the substituted ions, thereby improving the stability of the crystal. The 6.5YbYSZ ceramic has the best stability after heat treatment at 1300 °C for different times.

Longitudinal-torsional coupled rotary ultrasonic machining of ZrO2 ceramics: An experimental study

Zirconia (ZrO2) ceramics, as advanced structural ceramics, are gradually used in manufacturing, aerospace and other fields due to their excellent properties. However, it is problematic to process the ZrO2 ceramics by conventional machining. This study adopted the longitudinal-torsional coupled rotary ultrasonic machining to process the ZrO2 ceramics. Firstly, the material removal mechanism was clarified in the view of a single abrasive particle. Then, the longitudinal-torsional coupled rotary ultrasonic machining experiment was conducted to explore the effect of processing parameters on the cutting force, hole surface roughness, surface morphology, and single abrasive particle wear with those in the conventional machining. It was found that the cutting force reduced with the spindle speed and ultrasonic power, growing with the feed rate. The hole surface roughness declined with spindle speed, enhanced with feed rate, and decreased first and then increased with ultrasonic power. Compared with the conventional machining, both the cutting force and surface roughness were significantly reduced in the longitudinal-torsional coupled rotary ultrasonic machining. The test results proved that the hole surface quality was improved, and the single abrasive particle wear was reduced in the longitudinal-torsional coupled rotary ultrasonic machining. This study's results are considered instrumental in the longitudinal-torsional coupled rotary ultrasonic machining processing of hard and brittle materials.

Creep behavior of zirconia ceramics under a strong DC field

The tensile creep of 3 mol% Y2O3 stabilized tetragonal ZrO2 ceramics under a DC field was systematically investigated. The results show that the deformation mechanism of the material strongly depends on the current density and the applied stress. Exceptionally large uniform elongation can be obtained only when the creep is dominated by dislocation accommodated grain-boundary sliding. A maximum uniform elongation of ∼300% is achieved at a strain rate close to 10−3 s−1. At an even higher strain rate of about 10−2 s−1, uniform elongation is still near 100%. The current results suggest that high strain rate superplasticity can be induced by applying a strong electric field at proper stress and current density.

Rheological properties of magnetic field-assisted thickening fluid and high-efficiency spherical polishing of ZrO2 ceramics

Shear thickening polishing technology using non-Newtonian polishing fluids is a low-cost, low-damage polishing method for the ultra-precision machining of complex curved surfaces. However, the low polishing efficiency and poorly controlled viscosity of traditional shear thickening polishing fluids significantly limit their practical applications. In this study, a novel weak magnetic field-assisted shear thickening polishing fluid (WMFA-STPF) containing carbonyl iron particles, which utilized a weak magnetorheological effect to promote the shear thickening process, was developed, and its rheological characteristics were investigated. The obtained results revealed that WMFA-STPF exhibited good fluidity at low shear rates and enhanced thickening characteristics in the working shear rate range. To verify the high efficiency, high quality, and uniform polishing ability of WMFA-STP technology applied to the spherical surface of a zirconia ceramic workpiece, contrast polishing experiments were performed. After 75 min of polishing, the surface damage was effectively mitigated; the surface quality and uniformity were significantly improved; and the material removal rate increased by 156% up to 7.82 μm/h. Hence, the WMFA-STP method can be successfully utilized for the high-efficiency high-quality polishing of hard and brittle ceramics.

Bonding mechanism in ultrasonic-assisted soldering of ZrO2 and 304 stainless steel using a micro-alloyed active solder alloy

Ceramic-metal soldering joints are widely needed in various industries and a key factor in soldering is the interfacial bonding between the ceramic and solder alloys. Active elements, such as Ti and rare earth, are often added to solder alloys to realize interfacial bonding. The content of the active elements is usually 2 ∼ 4 wt%, which can significantly increase the melting temperature of the solder alloys. In this research, a micro-alloyed reactive solder comprising Sn-3.0Ag-0.5Cu alloy with 0.2 wt% Ti was used to solder ZrO2 ceramics and 304 stainless steel. The effective interfacial reaction was achieved with the aid of the ultrasound. The reaction product was Zr0.5Ti0.35Sn0.15O2 and Ti11.3Sn3O10 at the Sn-Ag-Cu-Ti/ZrO2 interface, and the maximum shear strength of the soldered specimens was 30.75 ± 9.92 Mpa. The melting temperature of the soldering alloy was not significantly increased compared to regular Sn-Ag-Cu solder alloy.

Initial sintering mechanism and additive effect in zirconia ceramics

AbstractY2O3‐stabilized ZrO2 (YSZ) ceramics have been used for various engineering applications because of their excellent mechanical properties or oxide‐ion conductivity applicable to solid‐electrolyte. The performance of YSZ depends on the sintered texture that is directly determined by sinterability of raw powders. A new kinetic analysis method of diffusion mechanism based on an initial sintering theory (grain‐boundary or volume diffusion) is theoretically derived, the initial sintering mechanism of hydrolytic YSZ powder is experimentally determined, and the effects of powder characteristics on sinterability are discussed. Furthermore, the additive‐enhanced sintering is proposed. A small amount of Al2O3 significantly enhances the densification. Using the additive effect, the low‐temperature degradation that is the fault of zirconia ceramics can be improved by decreasing the sintering temperature.

As-Sintered Manganese-Stabilized Zirconia Ceramics with Excellent Electrical Conductivity

As promising electrolyte materials in intermediate-temperature solid oxide fuel cells (IT-SOFCs), Sc-stabilized ZrO2 (ScSZ) and Y-stabilized ZrO2 (YSZ) electrolytes continue to be plagued by high cost and low intermediate conductivity. To mitigate these problems, Mn has been chosen as a new stabilization element for the synthesis of Mn-stabilized ZrO2 ceramics (MnSZ) through solid state sintering. Microstructures and electrical properties of micron-crystalline Zr1−xMnxO2−δ (x = 0.15, 0.20 and 0.25) ceramics electrolytes for IT-SOFCs have been systematically evaluated. Within the applied doping content, Mn2+ ions can enter the ZrO2 crystal lattice, leading to the formation of single cubic phase samples. Electrical conductivity measurements in the temperature range between 400 °C and 1000 °C show a sharp increase in conductivity due to Mn doping. The highest conductivity is obtained from the Zr0.75Mn0.25O2−δ samples, being 0.0144 S/cm at 600 °C and 0.182 S/cm at 1000 °C. The electrical conductivity at 600 °C is twice higher than that of the YSZ and two orders of magnitude higher than that of the ScSZ. These properties can fulfill the conductivity requirement (∼1 × 10−2 S/cm) for the electrolyte. Therefore, based on this study, we propose that Mn stabilized ZrO2 is a promising candidate as a solid electrolyte for IT-SOFCs.

The Effect of Sintering Temperature on the Phase Composition, Microstructure, and Mechanical Properties of Yttria-Stabilized Zirconia.

It is known that the yttria-stabilized zirconia (YSZ) material has superior thermal, mechanical, and electrical properties. This material is used for manufacturing products and components of air heaters, hydrogen reformers, cracking furnaces, fired heaters, etc. This work is aimed at searching for the optimal sintering mode of YSZ ceramics that provides a high crack growth resistance. Beam specimens of ZrO2 ceramics doped with 6, 7, and 8 mol% Y2O3 (hereinafter: 6YSZ, 7YSZ, and 8YSZ) were prepared using a conventional sintering technique. Four sintering temperatures (1450 °C, 1500 °C, 1550 °C, and 1600 °C) were used for the 6YSZ series and two sintering temperatures (1550 °C and 1600 °C) were used for the 7YSZ and 8YSZ series. The series of sintered specimens were ground and polished to reach a good surface quality. Several mechanical tests of the materials were performed, namely, the microhardness test, fracture toughness test by the indentation method, and single-edge notch beam (SENB) test under three-point bending. Based on XRD analysis, the phase balance (percentages of tetragonal, cubic, and monoclinic ZrO2 phases) of each composition was substantiated. The morphology of the fracture surfaces of specimens after both the fracture toughness tests was studied in relation to the mechanical behavior of the specimens and the microstructure of corresponding materials. SEM-EDX analysis was used for microstructural characterization. It was found that both the yttria percentage and sintering temperature affect the mechanical behavior of the ceramics. Optimal chemical composition and sintering temperature were determined for the studied series of ceramics. The maximum transformation toughening effect was revealed for ZrO2-6 mol% Y2O3 ceramics during indentation. However, in the case of a SENB test, the maximum transformation toughening effect in the crack tip vicinity was found in ZrO2-7 mol% Y2O3 ceramics. The conditions for obtaining YSZ ceramics with high fracture toughness are discussed.

Blackening Mechanism and Mechanical Properties Variation of Zirconia Ceramic Induced by Active Metal Brazing

Blackening of zirconia (ZrO2) ceramics often occurs in active metal brazing, however, its discoloration mechanism and induced variation in mechanical properties remain unclear. Herein, ZrO2 ceramic and Ti are successfully brazed by Ag–Cu–Pd filler in vacuum along with the blackening phenomenon. In the brazed joints, the reaction layers of TiO2 + Ti3(Cu, Pd)O are formed adjacent to ZrO2, indicating that partial oxygen in ZrO2 is taken by active Ti in the liquid filler that dissolved from the Ti substrate. Furthermore, the height of the blackened ZrO2 ceramic near the brazing seam is in line with the amount of Ti content added to the filler, which infers that bulk‐sized black ZrO2 can be efficiently obtained by the interfacial reaction between Ti and ZrO2. Additionally, the black ZrO2 is having abundant Zr3+ defects and more tetragonal phase content than the original ZrO2. The formation of Zr3+ defects results in the blackening of ZrO2, while the phase transformation leads to reduced hardness, improved impact toughness, and enhanced wear resistance of black ZrO2. Thus, this blackening phenomenon of ZrO2 ceramics provides a new way to tune the mechanical properties of ZrO2.

Making Ultra-Tough Nanoceramics by Columnar Submicrocrystals with Three-Level Micro-Nano Structures.

The low fracture toughness of equiaxed nanocrystalline ceramics is the main bottleneck of its wide range of commercial applications. Here, the authors report a method to overcome this limitation for preparing ultra-tough nanoceramics from using amorphous and supersaturated Al2 O3 /ZrO2 solid solution micro-powders, which is fabricated by Al-O2 ultrahigh-temperature combustion synthesis assisted rapid water cooling. The Al2 O3 /ZrO2 micro-powders containing amorphous and metastable dendritic solid solutions can induce the three-level micro-nano structure (submicro/nano/supra-nano) of the high-content (up to 70-90%) columnar submicro-crystals accompanied with high-density nanoprecipitation after sintering or annealing, which makes the fracture toughness of Al2 O3 /ZrO2 ceramics with a unique combination of high-strength and high-hardness increased by 50-100%. This method is beneficial to microstructural design of high-performance ceramics and can be widely applied to various ceramic systems, coupled with simplicity, low-cost, and high-efficiency, making it suitable to industrially produce large-sized nanoceramics with specific grain geometry in large quantities.

Catalytic Removal of NOx on Ceramic Foam-Supported ZnO and TiO2 Nanorods Ornamented with W and V Oxides

Energy consumption steadily increases and energy production is associated with many environmental risks, e.g., generating the largest share of greenhouse gas emissions. The primary gas pollution concern is CO2, CH4, and nitrogen oxides (NOx). Environmental catalysis plays a pivotal role in NOx mitigation (DeNOx). This study investigated, for the first time, a collection of ceramic foams as potential catalyst support for selective catalytic NOx reduction (SCR). Ceramic foams could be an attractive support option for NOx removal. However, we should functionalize the surface of raw foams for such applications. A library of ceramic SiC, Al2O3, and ZrO2 foams ornamented with nanorod ZnO and TiO2 as W and V oxide support was obtained for the first time. We characterized the surface layer coating structure using the XPS, XRF and SEM, and TEM microscopy to optimize the W to V molar ratio and examine NO2 mitigation as the SCR model, which was tested only very rarely. Comparing TiO2 and ZnO systems reveals that the SCR conversion on ZnO appeared superior vs. the conversion on TiO2, while the SiC-supported catalysts were less efficient than Al2O3 and ZrO2-supported catalysts. The energy bands in optical spectra correlate with the observed activity rank.