Mechanical Properties of Monoclinic Zirconia

Fracture toughness and fracture strength data are presented for the first time for monoclinic zirconia. An undoped nanocrystalline zirconia powder was sintered at 1100°C and yielded a theoretical density of more than 90% with a grain size of about 150 nm. The surface crack in flexure (SCF) technique was deemed most suitable for nanocrystalline materials. Measurements of Young's modulus and the determination of the fracture origin are also provided.     

Reaction Sintering and Mechanical Properties of Hydroxyapatite–Zirconia Composites with Calcium Fluoride Additions

The effects of calcium fluoride (CaF₂) additions on the densification and mechanical properties of hydroxyapatite–zirconia composites (HA–ZrO₂) were investigated. When small amount of CaF₂ was added, the density of the composites was markedly enhanced. The reactions of HA with CaF₂, which led to the formation of fluorapatite (FA), were attributed to the observed improvements in densification. When HA–20-vol%-ZrO₂ composites were sintered, with the addition of 5 vol% of CaF₂, in air at 1300°C, the density of the specimen approached 98% of the theoretical value. The flexural strength and fracture toughness of the composites were also improved, as a result of the enhanced densification.     

烧成制度及杂质对Mg—PSZ材料极出体形态和力学性能的影响

以阳泉产ＺｒＯ２原料为研究对象,通过控制烧成制度,即在烧结温度前保温（１６００℃）,再在Ｃ相区烧结固溶,经慢速至１１００℃后再自然冷却到室温,可使基体中长出大小合适的析出体,获得较佳的力学性能,含１０ｍｏｌ％ＭｇＯ的Ｍｇ－ＰＳＺ材料强度 可ａ韧 可达１２．３ＭＰａ·ｍ＾１／２,与宜兴料相比较,可知步量杂质（Ｆｅ２Ｏ３）可略降低材料的烧结温度,但对材料的力学性能无影响,通过ＳＥＭ对选用不同烧成制度材     

Effect of Dynamic Compaction on the Retention of Tetragonal Zirconia and Mechanical Properties of Alumina/Zirconia Composites

Dynamic consolidation techniques were employed to investigate the retention of tetragonal zirconia and degree of consolidation in alumina/zirconia powder compacts. Heating the specimens prior to explosive shock compaction increased the tetragonal-phase retention significantly. Low shock pressures yielded no macrocracking, although final densities were low (60% to 70% of the theoretical density). Heat treatment following dynamic consolidation enhanced the retention of the tetragonal zirconia polymorph regardless of the shock pressure employed. Compact densities were increased to over 90% of theoretical at relatively low sintering temperatures (1300°C). Hardness, toughness, and Young's modulus of the compacts were comparable to those achieved in composites that were synthesized using more conventional techniques. Dynamic compaction offers an alternative method for the fabrication of zirconia-toughened alumina ceramics.     

氧化锆活性对烧结性能的影响

分别以化学法和电熔法制备的氧化锆及二者混合共磨粉为原料,通过造粒、成型、干燥、烧成制得样品,对烧成样品物理性能和微观结构进行分析,研究原料活性对烧结性能的影响.研究结果表明:在加入同样稳定剂的条件下,电熔氧化锆活性较差,烧结后气孔率较高,化学氧化锆烧结后收缩大,开裂严重,两者都无法满足作为制备氧化锆质定径水口基质原料要求,当二者的混合比例为1:1时,试样的烧结温度合适,气孔率、烧成收缩、耐压强度等各项理化指标较好,满足用作制备氧化锆质定径水口基质要求.     

二次烧结温度对牙科氧化锆陶瓷组织和力学性能的影响

研究二次烧结温度对氧化锆牙科陶瓷微观组织和力学性能的影响。方法：氧化锆亚微米粉经过干压、冷等静压成型后在1050ºC预烧结,然后将预烧结牙科氧化锆瓷块在1300ºC-1600ºC进行二次烧结。对不同二次烧结温度下材料的线收缩率、烧结密度、物相、三点抗弯强度进行测量分析,并通过扫描电镜观察试样的断面形貌。结果：结果表明随着二次烧结温度提高,氧化锆的密度、弯曲强度呈上升趋势。在1350ºC时体积密度达到6.10g/cm³,1500ºC时的机械性能最优,三点弯曲强度为852MPa,主晶相为四方相。结论：亚微米氧化锆粉体烧结活性高,力学性能优良,能够满足口腔全瓷修复材料的要求。     

Microstructure and Mechanical Properties of Porous Alumina Ceramics Fabricated by the Decomposition of Aluminum Hydroxide

The mechanical properties of Al₂O₃-based porous ceramics fabricated from pure Al₂O₃ powder and the mixtures with Al(OH)₃ were investigated. The fracture strength of the porous Al₂O₃ specimens sintered from the mixture was substantially higher than that of the pure Al₂O₃ sintered specimens because of strong grain bonding that resulted from the fine Al₂O₃ grains produced by the decomposition of Al(OH)₃. However, the elastic modulus of the porous Al₂O₃ specimens did not increase with the incorporation of Al(OH)₃, so that the strain to failure of the porous Al₂O₃ ceramics increased considerably, especially in the specimens with high porosity, because of the unique pore structures related to the large original Al(OH)₃ particles. Fracture toughness also increased with the addition of Al(OH)₃ in the specimens with higher porosity. However, fracture toughness did not improve in the specimens with lower porosity because of the fracture-mode transition from intergranular, at higher porosity, to transgranular, at lower porosity.     

Sintering Behavior of Fully Agglomerated Zirconia Compacts

Fully agglomerated superfine zirconia powders were prepared with the coprecipitation and spray-drying method. The compaction of such powders shows no fragmentation of the agglomerates. The sintering behavior of the compacts was studied and two sintering stages were identified: densification within agglomerates at temperatures not higher than 1250°C and the removal of interagglomerate pores at temperatures above 1600°C. The interagglomerate pores are difficult to remove, and sintering between agglomerates even at 1600°C is still insignificant. Heating of the compacts at temperatures above 1600°C leads only to grain growth and the entrapping of pores in large grains.     

Pressureless Sintering and Mechanical and Biological Properties of Fluor-hydroxyapatite Composites with Zirconia

Fluor-hydroxyapatite (FHA) fabricated by a reaction between fluorapatite (FA) and hydroxyapatite (HA) was mixed with ZrO₂ to produce FHA–ZrO₂ composites. When the relative amount of FA to HA increased, the decomposition of the composite was decreased gradually because of the formation of thermally stable FHA solid solutions. With such suppression of decomposition, the FHA–ZrO₂ composites retained fully densified bodies. As a result, significant enhancements in mechanical properties, such as hardness, flexural strength, and fracture toughness, were achieved as the relative amount of FA to HA increased. The highest values in strength and toughness were 220 MPa and 2.5 MPa·m^1/2, respectively, with FHA–40 vol% ZrO₂ composites. In vitro proliferation of osteoblast-like cells (MG63) on the composites showed behavior similar to that observed on pure HA and FHA. Alkaline phosphatase (ALP) activity of the growing cells (HOS) on the composites was slightly down-regulated compared with that on pure HA and FHA at prolonged periods.     

Microstructure and Mechanical Properties of Fine-Grained Magnesia-Partially-Stabilized Zirconia Containing Titanium Carbide Particles

Fine-grained Mg-PSZ has been fabricated by adding TiC particles. The average cubic grain size was smaller by more than an order of magnitude than without TiC when the TiC content was over 2.5 vol%. Pressurelessly sintered specimens contained numbers of relatively large pores while hot-pressed ones were fully dense. For hot-pressed specimens, addition of TiC particles did not affect the growth behavior of tetragonal precipitates during annealing. With increasing TiC content, the bend strength of hot-pressed specimens increased while the fracture toughness decreased. The bend strength and the fracture toughness of fine-grained Mg-PSZ containing 5 vol% TiC were 980 MPa and 8.2 MPa·m^1/2, respectively.     

Microstructure and Bending Strength of 3Y-TZP/12Ce-TZP Ceramics Fabricated by Liquid-Phase Sintering at Low Temperature

With the addition of 1 wt% of MgO–Al₂O₃–SiO₂ glass as a sintering aid, 3Y-TZP/12Ce-TZP ceramics (composed from a mixture of 3Y-TZP and 12Ce-TZP powder) have been fabricated via liquid-phase sintering at 1250°–1400°C. In the sintered bodies, the grain growth of Y-TZP is almost unaffected, whereas that of Ce-TZP is inhibited. MgO·Al₂O₃ spinel and an amorphous phase that contains Al₂O₃ and SiO₂ (from the sintering aid) fully fill the grain junctions. The bending strength of 3Y-TZP/12Ce-TZP, when sintered at 1250°–1300°C, is ∼800–900 MPa, which is greater than that of 3Y-TZP ceramics without Ce-TZP particles. Ce-TZP grains and MgO·Al₂O₃ spinel in 3Y-TZP/12Ce-TZP ceramics may impede crack growth, and the bending strength is enhanced.     

Solid-Solution Effects of a Small Amount of Nickel Oxide Addition on Phase Stability and Mechanical Properties of Yttria-Stabilized Tetragonal Zirconia Polycrystals

Stability and mechanical properties of the tetragonal phase were investigated for NiO-doped yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) systems. Only 0.3 mol% of NiO in solid solution could be added to the Y-TZP while maintaining the tetragonal phase. Fracture toughness improved remarkably on addition of a small amount of NiO. Raman spectroscopy analysis around cracks introduced by Vickers indentation revealed that the amount of monoclinic phase transformed from tetragonal phase was increased. It was confirmed that fracture toughness improvement was due not only to increased grain size, but also to Y-TZP destabilization by solid solution of NiO.     

Sintering of Zirconia Nanopowder by Microwave-Laser Hybrid Process

A new hybrid sintering process has been developed by replacing all but one laser by microwaves in the existing simultaneous multiple laser process (SIMPLE). Microwave energy has been used to preheat the material before laser radiation, and the synergism between microwave and laser energies could effectively heat the material to temperatures of 1700°C and beyond in just a few minutes. Using this process, rapid sintering of 3Y–ZrO₂ (3Y–TZP) pellets has been achieved in a few minutes. Microstructural investigations reveal that the microwave–laser hybrid sintered pellets of 3Y–ZrO₂ have nanograins averaging about 20 nm. The microwave–laser hybrid sintering process can clearly be a new approach for fabrication of nanoceramics and nanocomposites.     

Effect of Hot-Pressing Temperature on Densification and Mechanical Properties of Titanium Diboride with Silicon Nitride as a Sintering Aid

The effect of hot-pressing temperature on the densification behavior and mechanical properties of titanium diboride (TiB₂) was investigated. TiB₂ specimens were hot-pressed for 1 h at temperatures in the range of 1500°–1800°C, with an addition of 2.5 wt% of silicon nitride (Si₃N₄) as a sintering aid. The density increased markedly at temperatures in the range of 1500°–1600°C and remained constant thereafter. The formation of a eutectic liquid at 1550°C was attributed to the steep increase in density. The hot-pressing temperature also improved the mechanical properties, such as the flexural strength, Vickers hardness, and fracture toughness of the specimens. Similar to the density, the mechanical properties improved remarkably at ∼1550°C, so that optimum properties were obtainable at temperatures as low as 1600°C.     

液相烧结8YSZ陶瓷的性能研究

以Bi2O3为烧结助剂,利用液相烧结法制备8YSZ陶瓷材料,研究了烧结助剂对材料致密化、相组成、显微结构、力学性能及电学性能的影响.结果表明:烧结助剂的引入显著促进了材料的致密化、降低了烧结温度;引入烧结助剂Bi2O3后,使得ZrO2中的Y2O3含量减少,以致出现了含有单斜相氧化锆的第二相;而材料的致密化和单斜相氧化锆的出现,又使其具有良好的力学性能,同时对电学性能也有一定的影响.     

纳米氧化锆添加量对氧化锆制品性能的影响

在电熔单斜氧化锆原料中添加不同数量的CaO稳定剂,制备部分稳定氧化锆,研究CaO加入量和稳定相数量的关系.在制备的CaO部分稳定氧化锆中添加纳米氧化锆粉体,经过造粒、200 MPa压力成型、干燥、1650 ℃×2 h烧成制得试样.测试试样的物理性能、分析矿物相组成、观察显微结构,研究纳米氧化锆粉体添加量对试样性能的影响.研究结果表明:2Ca-PSZ、3Ca-PSZ、4Ca-PSZ试样中,4Ca-PSZ试样稳定化程度最高;3Ca-PSZ试样显气孔率小,体积密度较大,耐压强度高.在3Ca-PSZ试样中加入纳米氧化锆粉体,随着加入量的增加,试样的显气孔率下降、烧成收缩率增加、耐压强度提高.其中纳米氧化锆粉体添加比例为8wt%时,试样气孔率为9.4%,体积密度为5.08 g/cm3,抗压强度达到381 MPa.与3Ca-PSZ试样相比,气孔率下降40%,体积密度提高5%,耐压强度提高70%.     

Synthesis of Highly Porous Yttria-Stabilized Zirconia by Tape-Casting Methods

Porous ceramics of Y₂O₃-stabilized ZrO₂ (YSZ) were prepared by tape-casting methods using both pyrolyzable pore formers and NiO followed by acid leaching. The porosity of YSZ wafers increased in a regular manner with the mass of graphite or polymethyl methacrylate (PMMA) to between 60% and 75% porosity. SEM indicated that the shape of the pores in the final ceramic was related to the shape of the pore formers, so that the pore size and microstructure of YSZ wafers could be controlled by the choice of pore former. Dilatometry measurements showed that measurable shrinkage started at 1300 K, and a total shrinkage of 26% was observed, independent of the amount or type of pore former used. Temperature-programmed oxidation (TPO) measurements on the green tapes demonstrated that the binders and dispersants were combusted between 550 and 750 K, that PMMA decomposed to methyl methacrylate between 500 and 700 K, and that graphite combusted above 900 K. The porosity of YSZ ceramics prepared by acid leaching of nickel from NiO–YSZ, with 50 wt% NiO, was studied as a function of NiO and YSZ particle size. Significant changes in pore dimension were found when NiO particle size was changed.     

Structural and Mechanical Property Changes in Toughened Magnesia-Partially-Stabilized Zirconia at Low Temperatures

The mechanical properties of high-toughness magnesia-partially-stabilized zirconia were found to be dramatically altered by a single cooling cycle between room temperature and − 196°C. Raman spectroscopy and X-ray diffraction were used to correlate the changes in mechanical properties with structural changes that occur at temperatures below ∼− 100°C. Most of the tetragonal precipitates that are responsible for toughening transformed to an orthorhombic phase with unit-cell volume intermediate between those of the tetragonal and monoclinic phases. The orthorhombic phase was stable with heating to 300°C, but it transformed back to the tetragonal structure when heated to 400°C. Surprisingly, the orthorhombic phase was not readily transformable by stress, with the consequence that, after the cooling cycle, most of the high-toughness properties of the original tetragonal-containing material were lost.     

Fabrication of Highly Porous Yttria-Stabilized Zirconia by Acid Leaching Nickel from a Nickel-Yttria-Stabilized Zirconia Cermet

Porous Y₂O₃-stabilized ZrO₂ (YSZ) samples were synthesized by preparing NiO/YSZ composites by tape casting and calcining at 1800 K, reducing the NiO to nickel in H₂ at 973 K, and finally leaching the nickel out of the structure with 2.2 M HNO₃ at 353 K. Porous YSZ was prepared from NiO/YSZ composites containing 0, 20, 40, and 50 wt% NiO. Complete removal of the nickel was demonstrated by XRD, weight changes, and porosity increases. Porosities >75% could be achieved without structural collapse of the YSZ phase. Finally, the method was applied to the fabrication of a solid oxide fuel cell with a copper-based anode operating on H₂ and n -butane.     

Properties and microstructural aspects of TiO2‐doped sintered Alumina‐Zirconia composite ceramics

The effect of titania content on the densification, the phase transformation, the microstructures, and mechanical properties of 50 wt% Al₂O₃‐50 wt% ZrO₂ (12 mol% CeO₂) was evaluated. Ceramic composites with different TiO₂ content (0.27, 5, 10 wt%) were prepared by pressureless sintering at low temperature (1400°C) for 2 hours in air. Dense ceramic was obtained by adding 5 wt% of TiO₂ loading to improved mechanical properties. The microstructure analysis provided lots of information about solid‐state reactivity in alumina‐zirconia‐titania ternary system. The content of TiO₂ strongly affected the phases evolution and the grain growth during sintering. Furthermore, a significant effect on mechanical properties and fracture behavior was also observed.