Effect of starting powders on the sintering of nanostructured ZrO2 ceramics by colloidal processing

Abstract

The effect of starting powders on the sintering of nanostructured tetragonal zirconia was evaluated. Suspensions were prepared with a concentration of 10 vol.% by mixing a bicomponent mixture of commercial powders (97 mol.% monoclinic zirconia with 3 mol.% yttria) and by dispersing commercially available tetragonal zirconia (3YTZ, Tosoh). The preparation of the slurry by bead-milling was optimized. Colloidal processing using 50 μm zirconia beads at 4000 rpm generated a fully deagglomerated suspension leading to the formation of high-density consolidated compacts (62% of the theoretical density (TD) for the bicomponent suspension). Optimum colloidal processing of the bicomponent suspension followed by the sintering of yttria and zirconia allowed us to obtain nanostructured tetragonal zirconia. Three different sintering techniques were investigated: normal sintering, two-step sintering and spark plasma sintering. The inhibition of grain growth in the bicomponent mixed powders in comparison with 3YTZ was demonstrated. The inhibition of the grain growth may have been caused by inter-diffusion of cations during the sintering.     

Electrical resistivity in the titanium carbonitride-zirconia system

A mixture of conductive titanium carbonitride and insulating partially stabilized zirconia powders was hot-press sintered, and a theoretical dense composite material obtained. The electrical conductivity as a function of C/N ratio and up to 50 vol % dispersed zirconia, partially stabilized by 3–5 mol % yttria, was studied. The Landauer model was tested. For low sintering temperatures (1500 °C), and with 3 mol % yttria in zirconia, titanium carbonitride particles present bad electrical contacts, and the transformation of tetragonal to monoclinic zirconia induces microcracking, therefore the Landauer model is not followed. On the other hand, for high-temperature (1700 °C) sintering, and using a fully stabilized zirconia powder with 5 mol % yttria, good electrical contacts between grains are established, thus the Landauer's model is confirmed. The high electrical conductivity of this insulator-conductive ceramic-ceramic composite allows an electric discharge machining.     

Zirconia nanoceramic via redispersion of highly agglomerated nanopowder and spark plasma sintering

A 2.7 mol% yttria stabilizing tetragonal zirconia (2.7Y-TZP) nanopowder was synthesized and stored for five years. Humidity and unsatisfactory storage conditions gradually caused heavy agglomeration. Within a few months, 2.7Y-TZP nanopowder became useless for any technological application. A bead-milling deagglomeration technique was applied to recover the heavily agglomerated yttria-stabilized zirconia nanopowder. Low-temperature sintering and spark plasma sintering (SPS) were performed, resulting in fully dense nanostructured ceramics. Compacts formed with heavily agglomerated powder present low sinterability and poor mechanical properties. Bead-milling suspension formed compacts exhibit mechanical properties in the range of the values reported for nanostructured zirconia. This observation confirms the effectiveness of bead-milling in the deagglomeration of highly agglomerated nanopowders. The high value of Vickers hardness of 13.6 GPa demonstrates the success of the processing technique for recovering long-time-stored oxide nanopowders.     

Texture development in 3 mol% yttria-stabilized tetragonal zirconia

We have developed a method of forming textured tetragonal zirconia. A suspension containing 10 vol% solid loading of monoclinic ZrO₂ mixed with 3 mol% Y₂O₃ was prepared, and then a bead-milling process was performed using 50 μm diameter zirconia beads resulting in a well-dispersed suspension. The mixture suspension of monoclinic zirconia and yttria nanoparticles was slip cast under a magnetic field of 12 T to produce oriented monoclinic zirconia with yttria. The reaction sintering between yttria and the oriented monoclinic zirconia produces a final 3 mol% Y₂O₃ doped tetragonal zirconia that remains oriented.     

Strain localization in compressed ZrO<Subscript>2</Subscript>(Y<Subscript>2</Subscript>O<Subscript>3</Subscript>) ceramics

Spatiotemporal distributions of local components of the distortion tensor of a nonplastic material—yttria partially stabilized tetragonal zirconia (YTZ) ceramics—have been studied under active compressive straining conditions using double-exposure speckle photography techniques. The strain localization patterns are presented and the features of macroscopic strain inhomogeneity in the elastic state of YTZ ceramics are considered.     

Studies on crystallization behaviour of 3Y-TZP/Al2O3 composite powders

The present work deals with the characterization of 3.0 mol.% yttria stabilized zirconia (3Y-TZP) powders prepared as composite precursors containing up to 20.0 vol.% of alumina. Following the wet-chemistry route of co-precipitation, a homogeneous dispersion of component hydroxides was obtained. Effect of calcination treatment on t-phase and primary particle size evolution of zirconia was investigated by XRD technique. A marked difference with respect to inhibition of crystallization reaction of 3Y-TZP was observed in the presence of alumina. This is attributed to the increased diffusion lengths of the two developing phases. The powders could be retained in the ultrafine and quasi-amorphous state by a suitable choice of alumina content and calcination treatment.     

Thermal processing and properties of highly homogeneous alumina-zirconia composite ceramics

Alumina-zirconia composites were made using a novel processing technique involving nonsolvent precipitation of a dilute polymer solution containing dispersed ceramic powder particles. Seven identical alumina-nominally 15 wt% zirconia composite green bodies were fired at different temperatures. An optimal firing temperature of 1500° C was found, above and below which sample fracture strengths, densities, and tetragonal zirconia contents were lower. Fracture strengths correlated well with porosity. An alumina-14.06 wt% (zirconia-1.35 mol% yttria) composite had excellent resistance to abnormal grain growth upon high-temperature annealing. The tetragonal zirconia content increased with increasing annealing time; this was attributed to the equilibration of zirconia particles with widely varying yttria concentrations.     

Toughness tailoring of yttria-doped zirconia ceramics

Despite the impressive development in understanding transformation toughening, tailoring the toughness of yttria-doped zirconia ceramics remained a major challenge. In our research, a simple but innovative route based on the mixing and hot pressing (under identical conditions) of zirconia powders with varying yttria content (3 and 0 mol%) is developed to investigate this critical issue. The experimental results clearly reveal that the fracture toughness of yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) ceramics can be tailored by careful mixing of co-precipitated and yttria-free zirconia starting powders.     

Fabrication and sinterability in Y2O3-CeO2-ZrO2

CeO₂-stabilized tetragonal zirconia polycrystal (Ce-TZP) containing 1 to 6 mol % YO_1.5 have been fabricated as fine powders by a coprecipitation technique. The microstructure of the as-sintered surface and fracture surface were examined by electron microscopy. CeO₂ dopants reduced the phase transformation temperature from amorphous to tetragonal and stabilized the tetragonal phase at low temperature. The addition of Y₂O₃ to Ce-TZP inhibited the grain growth. The sintered density reached 99% theoretical for short sintering times at 1440 and 1540° C, but decreased slightly to 97 to 98% theoretical for longer sintering times. The decrease in density is attributed to the morphological development of agglomerates, which induce large pores during sintering. The average grain size decreased significantly as the yttrium content increased from 1 to 3 mol %. Specimens aged in water at low temperatures exhibited no phase transformation. This implies fairly good thermal stability in the Y₂O₃-doped Ce-TZP system.     

Preparation and sintering behaviour of nanostructured alumina/titania composite powders modified with nano-dopants

Nanostructured alumina/titania composite powders were prepared by doping with small amounts of nanosized zirconia and ceria. The nanosized raw materials powders were reconstituted into nanostructured particles by ball milling, spray drying and heat treating. Then, the nanostructured reconstituted powders were cool-isostatic pressed and pressureless-sintered into bulk ceramics. The phase composition and microstructures of reconstituted powders and as-prepared ceramic composites were characterized by using X-ray diffractometer (XRD), scanning electron microscope (SEM) and energy-dispersive spectrometer (EDS). The sintering behaviour of the nanostructured ceramic composite powders and the effects of nano-dopants and sintering temperatures on the microstructures of the ceramic composites were investigated and discussed. It was found that nano-dopants could lower the sintering temperature and accelerate densification of ceramic composites.     

等离子喷涂氧化锆纳米涂层显微结构研究

利用大气等离子喷涂（APS）技术,在不锈钢基体上制备了氧化锆纳米结构涂层.运用XRD、SEM与TEM等分析手段对喷涂用粉末原料和涂层的显微结构、物相组成进行了观察和确定.实验结果表明,纳米氧化锆粉末经喷雾造粒后的颗粒粒径主要分布在15～40μm之间,流动性好,适合于等离子喷涂用.等离子喷涂氧化锆纳米涂层颗粒分布在60～120nm之间,晶粒发育良好.涂层物相由四方和立方相氧化锆所组成.氧化锆纳米涂层的气孔率约为7％,结合强度为45MPa。     

Mechanical instability of a porous material

The deformation behavior of a ceramic material based on ZrO₂(Y₂O₃), yttrium partly stabilized tetragonal zirconia (YTZ), exhibits certain features related to a variety of the porous structure morphologies. In particular, the phenomenon of mechanical instability was observed in the YTZ rod and plate structures formed by ceramic synthesis.     

3Y—TZP陶瓷超塑性应变速率敏感性指数及晶界玻璃相

研究了应变量，温度及测试技术对３Ｙ－ＴＺＰ陶瓷超塑性应变速率敏感性指数ｍ的影响。     

Fabrication of hydrothermal ageing resistant of 3 mol % yttria-stabilised tetragonal zirconia polycrystalline via microwave sintering

The influence of microwave sintering on the densification, mechanical performances, microstructure evolution and hydrothermal ageing behaviour of pure 3 mol % yttria-stabilised tetragonal zirconia polycrystalline (3Y-TZP) ceramics was compared with conventional sintered samples. Green bodies were sintered via conventional pressure-less and microwave sintering method between 1200 °C to 1400 °C with dwelling time and firing rate at 120 min, 10 °C/min and 1 min, 20 °C/min. Result showed that reduced processing temperature and holding time is possible with microwave sintering technique for fabricating good resistant zirconia sample with bulk density, Young's modulus, and Vicker's hardness that are comparable to samples sintered with conventional method. However, the microwave sintered samples suffered from hydrothermal ageing where their average grain size is above critical size. The enhancement of hydrothermal ageing resistance of the sintered samples is associated with the decreasing grain size of the sintered samples instead of sintering method.     

用内凝胶法制备球形Y2O3-ZrO2等离子喷涂粉料

以无机物为前驱物,用内凝胶法制备了适合于等离子喷涂的、粒径在10～50μm范围内的、用氧化钇稳定的氧化锆（YSZ）球形粉末,实验结果表明,通过改变乳化条件可以控制这种粉末的形貌、粒径和粒度分布．利用SEM和光学显微镜观察发现,用该方法得到的粉末主要是球形的,并且符合粒径分布的要求（5～50μm）．致密的球形粉末显示出良好的流动性．差热分析表明,用47mol％氧化钇稳定的氧化锆（YSZ）粉末在468℃有一放热峰,是YSZ粉末的相转变温度．XRD分析表明,上述粉末在800℃条件下煅烧可以得到100％非平衡四方相（t’）的YSZ粉末,没有单斜相存在．     

Nanocrystalline zirconia-yttria system–a Raman study

Nano sized zirconia (ZrO₂) powders doped with different amount of yttria (Y₂O₃) (3, 5 and 8 mol%) were prepared through coprecipitation method. The crystallite size estimated from the X-ray peak broadening is around 10 nm. Phase identification was carried out using XRD and Raman spectroscopy. Raman spectroscopic study of the synthesized materials show clear evidence of the presence of single phase cubic structure in the case of 8 mol% Y₂O₃ doped fully stabilized zirconia (8Y-FSZ); tetragonal phase in the case of zirconia doped with 3 mol% Y₂O₃ (3Y-TZP-tetragonal zirconia polycrystal) and a mixture of cubic and tetragonal phases for 5 mol% Y₂O₃ doped partially stabilized zirconia (5Y-PSZ). Raman technique is therefore an effective tool to distinguish the phases present in the calcined nano sized powders of zirconia.     

Microwave sintering of yttria-containing tetragonal zirconia polycrystal (Y-TZP) ceramics

Sintering of yttria-containing tetragonal zirconia polycrystal (Y-TZP) ceramics was performed in a single-mode cylindrical cavity applicator CMPR-250 operating at 2·45 GHz in the TM₀₁₂ mode. High heating rates at low power levels were achieved. Rapid heating and cooling resulted in a fine-grain microstructure. High-purity submicron Y-TZP powders were sintered from an initial green density of 60% to final sintered density close to the theoretical density. Microwave sintering offers potential for improving microstructural properties through controlled development of uniform microstructure.     

The effects of copper and iron oxide additions on the sintering and properties of Y-TZP

Yttria-tetragonal zirconia polycrystals (Y-TZP) with a range of yttria contents were prepared from powders neutralized during processing with ammonium hydroxide and sintered at temperatures of 1300–1700 °C. Iron or copper oxide was ádded and studies made of body characteristics, mechanical properties and ageing resistance. Densification was aided by higher yttria concentrations. The effects of the oxide additives were dependent on amounts present and sintering conditions, including ramp rates and holding temperatures. Ageing resistance was significantly improved for both oxide additions fired to lower temperatures but rapid transformation to monoclinic phase was observed for materials with larger grain sizes associated with higher sintering temperatures.     

Thermal stability improvement on pore and phase structure of sol-gel derived zirconia

Thermal stability improvement of mesoporous zirconia is important for its applications as catalyst supports or membrane layers at high temperatures. Yttria-coated zirconia is synthesized by doping a small amount of yttrium nitrate solution in zirconia sol. The sol-gel derived undoped and 2 and 3 mol% yttria-doped zirconia samples after calcination at 450 °C are in the metastable phase. Undoped zirconia starts to transform to thermodynamically stable monoclinic phase at 600 °C. The yttria-doped zirconia samples remain in the metastable phase after heat treatment in air at 900 °C for 30 h. Doping yttria also retards the sintering and grain-growth rates of the sol-gel derived zirconia. Doping with 2 mol% yttria gives the most effective results with respect to retarding the surface area loss of zirconia at high temperature. Yttria with a loading up to 2 mol % is possibly present on the grain surface of zirconia. The improvement of the thermal stability of zirconia by coating yttria is explained in terms of the effects of dopant on the mobility and concentration of the surface defects.     

Electron microscopy studies of nanocrystalline zirconia

Pure and yttria, calcia doped zirconia powders were prepared in nanostructured form by the method of co-precipitation by hydrolysis. As prepared and heat treated powders were characterized by X-ray diffraction and transmission electron microscopy. Features and variations in crystallite/particle size measurements by both XRD and TEM are discussed.