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.     

Plasma spraying of an indigenous yttria stabilized zirconia powder prepared by the sol-gel technique

An indigenous sol-gel derived yttria-partially stabilized zirconia (Y-PSZ) powder has been characterized and its suitability for plasma spraying applications evaluated. The powder, determined to have about 5·1% yttria content, predominantly consisted of spherical particles with an average equivalent particle diameter close to 25μm. Furthermore, it was found that the powder did not contain any particles >50μm, which is considered the ideal upper size limit for spray-grade ceramic powders in order to ensure complete melting during spraying. The sol-gel produced powder exhibited good flow characteristics and the plasma sprayed coatings developed using this powder were also found to have excellent thermal shock resistance. The corresponding results obtained using an imported Y-PSZ powder are also presented for the purpose of comparison.     

共沉淀法制备部分钇稳定的二氧化锆

采用共沉淀法制备了部分钇稳定的二氧化锆粉体（YSZ）,研究了不同钇含量、不同锆液浓度和不同煅烧条件对粉体的相结构的影响。实验结果表明YSZ粉体中四方相含量变化对钇的掺杂量极为敏感,锆液浓度必须高于0．2mol／L才能得到部分稳定的二氧化锆,煅烧条件对粉体的相结构无明显影响。通过实验获得了YSZ粉体的较佳制备条件。     

Growth of yttria stabilized zirconia thin films by metallo-organic, ultrasonic spray pyrolysis

Randomly and preferentially oriented thin films of yttria stabilized zirconia (YSZ) have been prepared on fused quartz substrates by ultrasonic spray pyrolysis using zirconium octylate and yttrium octylate as metallo-organic precursors at a substrate temperature of 873–1023 K. The as-deposited films composed of fine columnar grains were found to be crystalline and transparent with a cubic fluorite structure. With increasing substrate temperature the growth rate and diameter of the columnar grains increased and the crystal habit showed a preferential orientation at the (111) plane.     

燃料电池YSZ薄膜凝胶-流延成型的制备研究

结合流延法和凝胶法的优点,形成了水溶液丙烯酰胺体系的凝胶－流延成型方法。以固相含量为５４ｖｏｌ％的ＹＳＺ 体为浆料,利用此方法制备了厚度小、致密度高、气孔少的固体电解质ＺｒＯ２薄片,并从粉体特性、分散剂和ｐＨ值对悬浮体性能的影响、素坯及烧结体密度等多角度探讨了工艺的特点。研究表明与干压法、流延法相比,用凝胶－流延成型的薄片成型的薄片致密度高,气孔少、适合制备固体电解质ＹＳＺ薄膜。     

Effect of powder structure on microstructure and electrical properties of plasma-sprayed 4.5 mol% YSZ coating

Electrolyte coatings by atmospheric plasma spraying were prepared by 4.5 mol% ytrria-stabilized zirconia (YSZ) powders manufactured by agglomerate-sintered (A-S) and fusing-crashed (F-C) processes. Microstructure of the powders and the coatings were characterized using scanning electron microscopy. The electrical conductivity of the coatings was investigated using both impedance spectroscopy and DC methods. The results showed that the electrical conductivity of coating prepared with A-S powder was lower than that with F-C powder. It was found from the impedance analysis that both the grain and grain boundary resistances were large in the coating formed by A-S powder. This fact resulted from deposition of partially melting of spray particles. A model was proposed to explain the effect of powder structure and melting state on the coating microstructure and properties.     

New Y-TZP powders for medical grade zirconia

There is interest in using zirconia for biomedical applications as ballheads for total hip prostheses. Two potential types are under discussion:partially stabilized zirconia (PSZ) and tetragonal zirconia polycrystals (TZP)materials. Because of its enhanced material properties, TZP stabilized withyttria is favourable. To eliminate high amounts of natural radioactiveimpurities, the precursors are purified. The kind of precursor and purificationmethod determine the powder impurity level. The disadvantage of Y-TZP is thatthe hydrothermal decomposition reaction method is that it depends very stronglyon the grain size and the distribution of the stabilizing yttria within thezirconia grains. Thermodynamical and kinetic investigations on high puritycoprecipitated and yttria-coated zirconia powders show different behaviours.Y-TZP materials based on yttria-coated zirconia powders show excellentmechanical strength of more than 1000 MPa, a Weibull modulus of up to 20 a!nd a fracture toughness of 9 MPam. The material properties of Y-TZP ceramicsbased on coprecipitated powders and prepared under the same conditions are lessattractive. It is expected that materials based on yttria-coated zirconia willshow enhanced properties compared to materials derived from coprecipitatedpowders. Therefore Y-TZP materials derived from yttria-coated powders are veryattractive as medical grade zirconia.     

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.     

Influence of some precipitation variables on thermal behavior of ZrO2-Y2O3 and ZrO2-CeO2 precipitated gels

The scope of this work consisted of producing pure zirconia powders, yttria stabilized zirconia powders and ceria stabilized zirconia powders from zirconium oxychloride and yttrium and cerium chlorides using urea as precipitating agent and polyacrylic acid as dispersing agent. A factorial analysis was designed to study the effects of some precipitation variables (precipitation temperature, precipitation time, urea concentration, yttria concentration, ceria concentration and polyacrylic acid concentration) on the thermal behavior of zirconia gels using differential scanning calorimeter (DSC). It was observed that the addition of yttrium and cerium in the solution raised the crystallization temperature of the gels, due to changes on the characteristics of the Zr–OH bonds caused by the substitution of zirconium by yttrium or cerium. For the ZrO₂-Y₂O₃ system, the presence of polyacrylic acid increased the crystallization temperature. It was suggested that polyacrylic acid promoted the formation of a random polymeric structure of zirconia gels that required higher temperatures to crystallize.     

Influence of the Thermal Barrier Coatings Design on the Oxidation Behavior

The properties of two different types of thermal barrier coatings(TBCs) were compared to improve the surface characteristics on high temperature components.These TBCs consisted of a duplex TBC and a five-layered functionally graded TBC.NiCrAlY bond coats were deposited on a number of Inconel-738LC specimens using high velocity oxy-fuel spraying(HVOF) technique.For duplex coating,a group of these specimens were coated with yttria stabilized zirconia(YSZ) using plasma spray technique.Functionally graded NiCrA...     

Plasma Spraying of Al2O3 and ZrSiO4 to Form ZrO2- Mullite Composites

Zirconia is effective in improving fracture toughness of a number of ceramics when introduced as a reinforcement either in the form of participates, dispersed phase or whiskers because of its unique tetragonal-monoclinic (t → m) transformation. In this paper, the authors attempt to prepare ZrO₂, reinforced mullite by plasma spraying mixtures of zircon and alumina. Pre-mixed powders of zircon and alumina are injected onto a D.C. plasma jet. The plasma sprayed particles are collected in distilled water and analyzed. The results indicate that the plasma sprayed powders consist of zirconia, zircon, and alumina. It was found that fine grained, even amorphous and chemically homogeneous composite powders could be obtained by ball milling and plasma spraying. Recrystallization of amorphous phases and formation of mullite occurred at about 1OOO^°C in plasma sprayed powders. This value is more than 500^°C lower than the formation of mullite in asmilled powders. Uniform coatings with good structural integrity were obtained by plasma spraying. The relative quantity of mullite in coatings after heat treatment is about 4 times as much as that obtained in the spheroidized powders. Preheat treatment of the spheroidized powder promoted dissociation of zircon. Zirconia remained as tetragonal under 1000^°C in the sprayed coatings.     

Phases in Plasma Sprayed Thermal Barrier Coatings from Yttria Partially Stabilized Zirconia

The application of thermal barrier coatings (TBC) is increasing in aeroengines. Surface temperatures up to 1450°C require the application of ceramic TBCs because the temperature capability of metallic substrate materials is not high enough. The service life of turbine components could be improved by the use of yttria partially stabilized zirconia top coatings. The most successfull TBCs are made from 7–9 wt‐% yttria partially stabilized zirconia. One of the most discussed reasons of damages of such TBCs is the transformation between monoclinic and tetragonal phase in zirconia in connection with a dramatic change in volume. Thus in this work resulting phases of plasma sprayed zirconia coatings were investigated. It was found that no monoclinic phase could be detected after heat treatments at 1300, 1400 and 1466°C with cooling rates > 2°/min. Only with cooling rates < 2°/min monoclinic phases occured. It can be concluded that the metastable tetragonal high temperature configuration of yttria partially stabilized zirconia is “very stable”. The conditions in aeroengines with cooling rates > 2°C prevent the formation of the monoclinic phase in zirconia.     

喷涂距离对Fe基非晶涂层孔隙影响的研究

热喷涂涂层中孔隙的存在会降低涂层的耐蚀性,减少涂层寿命,而热喷涂工艺参数很大程度上影响涂层的孔隙率。本文采用计算机数值模拟和设计验证实验的分析方法,重点研究了JP-8000超音速火焰喷涂系统(HVOF)制备Fe基非晶涂层工艺参数中喷涂距离与涂层孔隙的关联性。利用商用计算软件Fluent计算平台,研究加入粉末粒子前,喷枪内火焰温度和速度的变化规律,以及加入非晶粉末后,不同喷涂距离条件下颗粒飞行过程的温度和速度的变化规律。仿真结果表明,喷涂距离为360~380 mm时,非晶粉末颗粒在撞击基板时处于半融化状态,颗粒在基板上具有良好的流动性,可获得孔隙率较低的涂层。验证实验结果与仿真结果一致。X射线衍射结果表明,粉末、不同喷涂距离所制备的涂层以及同成分的非晶条带均为完全非晶态结构。SEM和孔隙率统计结果表明,喷涂距离为370 mm时,涂层截面的孔隙较少,且孔隙率最低,为0.57%,验证了计算模拟优化的最佳喷涂距离范围。     

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.     

Synthesis and characterization of nanocrystalline CoCr coatings by plasma spraying

The present paper describes the synthesis and characterization of nanocrystalline CoCr (ASTM F75) coating produced by plasma spraying for possible surgical implant applications. The feedstock powders were synthesized by mechanical milling to produce irregular agglomerates with an average grain size of less than 100 nm. The powders were then introduced into an argon plasma spray to successfully produce a nanocrystalline coating. Scanning electron microscopy and transmission electron microscopy were used to study the morphology of the nanometric particles and the resultant sprayed coatings. Microhardness and porosity measurements were performed on the conventional and the nanocrystalline coatings to characterize and compare the physical and mechanical properties.     

The formation of hollow spherical ceramic oxide particles in a d.c. plasma

Investigations have been carried out to determine the conditions that lead to the production of spherical hollow ceramic oxide particles during melting in a d.c. plasma jet. Reports in the literature indicated that such ceramic particles were formed by plasma spraying spray dried agglomerates, but precise details of the conditions necessary for their formation were not stated. In this study it is shown that for hollow particles to be formed several conditions had to be met. Spherical spray dried agglomerates had to be used as starting materials, the material being sprayed had to melt over a narrow temperature range and the size of the particles had to exceed a certain diameter. Experiments, using yttria, showed that the relative size of the pore was dependent on particle diameter, and it has been proposed that the major controlling factor that influences this dependence is the escape of gas trapped in the spray dried agglomerate during melting rather than surface tension or undercooling which were shown to produce only minor effects. In addition, the results also showed that the nature of porosity within the hollow particles as well as the surface morphology was dependent on the material being sprayed.     

Sintering characteristics of microfine zirconia powder

In this paper the sintering characteristics of yttria-doped zirconia microfine powder are investigated by measuring shrinkage and shrinkage rate curves of powder compacts. The experimental results show that there is a transition point in the shrinkage-temperature curves and two peaks appear in the shrinkage rate-temperature curves. These results provide further support for the idea that the sintering of microfine zirconia powders is a two-stage process. In the first stage, sintering of the powder compact is mainly dominated by inner-sintering in agglomerates and in the second stage, sintering will occur among densified agglomerates. The microstructure of agglomerates during sintering is also studied by TEM and provides evidence that the transition point in the shrinkage-temperature curve is the end of the first sintering stage. Furthermore, the relationships between sintering characteristics and powder parameters are investigated. It is found that the shrinkage rate in the first stage is related to the primary grain size of the powder, and in the second stage it is related to agglomerate size. The effect of agglomerates on the sinterability of microfine Zr0₂ powder can be directly evaluated by the difference of shrinkage rate between the two sintering stages. Experiments also show that the shrinkage rate can be influenced by heating rate. A rapid heating rate would result in difficulties in the second sintering stage.     

Preparation of Porous Ceramics from Nanocrystalline Zirconia and Their Microstructure

Data are presented on the compaction behavior of nanocrystalline yttria partially stabilized zirconia powder and the effects of compaction pressure, sintering temperature, and sintering time on the microstructure of the resultant ceramics. It is shown that even relatively low (50 MPa) compaction pressures lead to the disintegration of powder particles and aggregates. The compaction behavior of the powder points to changes in the densification mechanism: from quasi-liquid sliding of powder particles at the beginning of the process to the breakdown of large microstructural constituents at the end. In the initial stages of sintering, a robust skeleton forms, which plays a key role in determining the pore structure of the ceramic.     

Preparation of nanocrystalline YSZ powders by the plasma technique

A plasma synthesis method has been devised to produce nanosize YSZ powders with various yttria contents. The powders are synthesized by introducing a mixture of coarse-grained zirconia and yttria into an r.f. inductively coupled plasma flame. The average particle size of the as-prepared powders is in the range 20–40 nm and the specific surface area is 18–50 m²g^–1. The phase and granulometric composition of the produced powders depend on the degree of evaporation of raw powders, reagent concentration in the gas flow and quenching rate, and on the content of Y₂O₃. Up to 5.5 mol% yttria, the major phase of nanosize powders is tetragonal ZrO₂, mostly as the non-transformable (t) form. For yttria contents higher than 6 mol%, the major phase is cubic ZrO₂.     

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。