陶瓷粉体表面化学镀技术

本文综述了陶瓷粉体表面化学镀技术的一般方法,研发现状与应用前景,介绍了化学镀技术的过程及其影响因素,展望了陶瓷粉体表面金属化学镀技术的发展前景.     

Dynamics of shear band instabilities in cutting of metals

We study the dynamics of flow instability and shear localization in cutting using direct high-speed imaging and low melting point alloy as a model material system. The onset of instability and departure from steady laminar flow is triggered by nucleation of shear band at the tool tip and subsequent propagation towards the free surface. The stress at the onset of shear band formation is found to be constant and a physical characteristic of the material. The shear band velocity and inhomogeneous strain field arising from the banding are quantitatively characterized using an image correlation method.     

Prediction of the fracture toughness of a ceramic multilayer composite – Modeling and experiments

A procedure to predict the fracture toughness of a ceramic multilayer composite made of different phases is presented. The procedure requires experiments to measure the intrinsic fracture toughness of the phases and to determine the required material data. The numerical modeling includes a conventional finite element stress analysis and the calculation of the crack driving force based on the concept of configurational (material) forces. The procedure yields the fracture toughness of the composite as a function of the crack length. A bending bar consisting of layers made of alumina and an alumina–zirconia composite is investigated. The bar has a crack perpendicular to the interfaces. The spatial variations of both the thermal residual stresses and the elastic modulus induce shielding and anti-shielding effects on the crack, which are quantified. The numerically predicted fracture toughness is compared with the experimental values.     

Preparation of zirconium-based ceramic and composite fine-grained powders

Zirconium-based ceramics and composites such as ZrC, ZrB₂, ZrC-SiC, ZrB₂-SiC-ZrC, and ZrB₂-SiC-ZrC-ZrSi were synthesized in fine powder form via combustion synthesis (CS) using ZrSiO₄, Mg, C, B, and NaCl as raw materials. Temperature distributions in the combustion wave were measured by thermocouples and used to estimate the combustion temperature and wave propagation velocity. The influence of the NaCl mole fraction on the combustion parameters, phase composition, and particle size of the composite powders was investigated. The experimental results revealed that the combustion temperature and particle size of the composites have a stable decreasing tendency with increase in the NaCl mole fraction in the starting mixture. It was found that near the combustion limit (1.5 mol NaCl), the combustion temperature drops below 1500 °C and the particle size reaches the nanometer scale.     

Effect of particle size in aggregated and agglomerated ceramic powders

This work describes the compaction of agglomerated and aggregated ceramic powders with special emphasis on the role of primary particle size. Discrete element simulations are used to model weakly bonded agglomerates as well as strongly bonded aggregates. Crushing tests are carried out to obtain the characteristic strength of single agglomerate and aggregate. Microstructure evolution and stress–strain curves indicate that aggregates undergo a brittle to plastic-like transition as particle size decreases below 50 nm. It is shown that agglomerates made of nanoparticles exhibit much greater strength than those made of micron-sized particles, with an approximately inverse linear relationship with primary particle size. Simulation of the uniaxial compaction of a representative volume element of powder demonstrates that adhesive effects are responsible for the difficulty to compact nanopowders and for the heterogeneity of microstructure prior to sintering.     

Strengthening of HT250 by modified ultra-fine ceramic powders

In this study, three kinds of modified ultra-fine ceramic powders marked A, B and C, which were prepared by each of three different modifiers mixing with a commercial SiC, were added to HT250 cast iron, respectively, and the effects of the modified ultra-fine ceramic powders on microstructure, mechanical properties and wear resistance were studied. Metallographic examination, tensile test, scanning electron microscopy, and three-dimensional surface topography were applied to analyze and compare the samples containing modified powder with the original samples. The results showed that the most obvious modification effect among the powders was seen in the sample containing powder A, with the graphite and eutectic cells being refined, the tensile strength being increased by 36.9%, and the wear resistance being improved by 45.5% and 47.2% under loads of 150 N and 300 N, respectively. The improvements of mechanical properties and wear resistance in the HT250 cast iron with the modified ultra-fine ceramic powders were attributed to the synergistic effect of the grain refinement with the powder acting as a hard particle phase and the lubrication by the graphite.     

Microstructural refinement of ceramic powders under mechanical processing conditions

The present work discusses the preparation of nanostructured ceramics by mechanical processing. Coarse-grained powders of different oxides, carbides, borides and silicides have been subjected to mechanical treatment inside a commercial ball mill under controlled experimental conditions. Ceramic powders underwent a microstructural refinement process down to the nanometer range, accompanied by the accumulation of structural disorder. The analysis of X-ray diffraction patterns according to the Rietveld method permitted the estimation of both the average crystallite size and strain content as a function of the milling time. Opposite trends are observed, the decrease of the crystallite size being paralleled by the increase of the strain content. The processes of crystallite size decrease and strain accumulation were then phenomenologically characterized in terms of apparent rate constants and asymptotic final values of crystallite size and strain. It has been thus possible to point out an intimate connection between the processes of crystallite size decrease and strain accumulation.     

A constitutive model for sintering of granulated ceramic powders

Sintering behavior of granulated powder is investigated to develop a constitutive model for deformation analysis of ceramic powder compacts during sintering. Spray-dried alumina is compacted by CIPing (cold isostatic pressing) and sintered at various temperatures. Shrinkage and the change in grain size of the compacts during sintering are revealed in relation to the inhomogeneous microstructure consisting of fractured and unfractured granules as a consequence of the compaction. A constitutive model for the ceramic powder compacts having the internal structure is presented; The difference in grain growth in dense and sparse regions of the compacts is taken into consideration to the model. The results calculated by the model show good agreement with that obtained by experiment.     

PZT陶瓷粉末低电压电磁压制实验研究

以PZT陶瓷粉末为研究对象,采用间接加工方式对其进行低电压电磁压制成形,在成功压制出高密度制品的基础上,分析了电压、电容、毛坯尺寸、压制次数等参数对压实密度的影响。研究结果表明,PZT陶瓷粉末低电压电磁压制存在一个可成形的放电电压范围,在此范围内,压制密度先随电压增加而增加,烧结性能也不断得到改善,最终使得烧结后陶瓷制品密度提高;但电压越高,制品密度增幅趋缓,至最佳放电电压后,压制密度开始下降,而制品密度则急剧下降;同样,增加电容也能一定程度地提高PZT陶瓷粉末压实密度并改善其烧结性能,降低最佳放电电压;其他放电参数不变的条件下,制品高径比越大,压实密度越低,且最佳放电电压提高,烧结后制品密度降低;两次压制可以有效提高压制密度,降低最佳放电电压,改善陶瓷坯体的烧结性能,并最终提高功能陶瓷制品致密度,在设备能量不足的情况下,两次压制是一种获得高致密PZT陶瓷制品的有效方法;相比静力压制,低电压电磁压制能提高PZT陶瓷坯体密度,并改善陶瓷体的烧结性能。     

Modeling of plasma spraying of two powders

The behavior of metal and ceramic powders co-sprayed through a plasma jet was simulated using a commercial fluid dynamics model in which the particles are considered as discrete Langrangian entities. Computations were carried out for the plasma jet and the injected particles using (a) a steady-state three-dimensional (3-D) jet and (b) a simplified two-dimensional (2-D) model. An analytical method was used to estimate the appropriate injection velocities for the metal and ceramic particles, injected through opposing nozzles perpendicular to the plasma flow, so that their “mean” trajectories would impinge on the same area on the target surface. Comparison of the model projections with experimental measurements showed that this method of computation can be used to predict and control the behavior of particles of widely different properties.     

改性SiC陶瓷浆料分散流动性的研究

本项目采用疏水预处理和接枝聚合的方法对SiC微粉进行表面改性,通过研究SiC微粉颗粒表面的zeta电位,pH值对浆料的分散性和流动性的影响,制备出了固相体积分数为50%,黏度低于0.42mPa.s,在水基体系中分散流动性能较好的SiC悬浮浆料。测试结果表明:改性SiC微粉在酸性(pH=2～4)或碱性(pH=8～12)条件下,均可获得较大的zeta电位,其分散稳定性均得到了较大的改善,可以有效的改善水浇注陶瓷磨具的成型性能。     

一种切头剪液压缸升级改造的实施方法

根据产品升级需要,对某机组切头剪液压缸进行了改造设计,校核了关节轴承承载能力,针对现场情况,提供了一种简单的液压缸升级改造的实施方法。     

Generation of a high-velocity jet in the electrothermal explosion of conductive ceramic powders

The electrothermal explosion spraying of conductive ceramic powders was characterized according to the electric and gas dynamic behavior of heated powders. Optical observation revealed the generation of the jetting with a leading velocity over 3 km/s. The velocity depended on the shape of the jet and the energy supplied to the powder. The heating process was related to the jetting process of the heated powders. The total electric energy supplied to the powder was two to three times the theoretical amount needed to melt the powder. Such electric energy was used for heating and accelerating the powder. This spray technique is characterized with the high-velocity jet consisting of high-pressure gas and molten ceramic particles.     

Development of new ceramic cutting tools with alumina coated carbide powders

An experimental investigation is carried out to coat two types of carbide powders, TiC and (W, Ti)C, with an alumina ceramic using a sol-gel technology. The coated carbide powders are then fabricated into two kinds of new ceramic tool materials by the hot pressing method. A scanning electron microscope (SEM) observation reveals that in general the matrix (carbide) grains are uniformly coated with the alumina ceramic and the microstructure of the new tool materials is more homogeneous than that of conventionally made ceramics. The tests of mechanical properties and wear resistance in machining are finally conducted. It is shown that when machining a mild carbon steel the new tool materials can increase the tool-life by up to 100% as compared to other two ceramic tool materials that have the same matrix but fabricated in the conventional way, while the fracture toughness is improved by up to 33%. When compared with a hard coated carbide tool, the new materials exhibit a superior ability in maintaining the wear resistance during the entire tool-life.     

Modeling and prediction of surface roughness in ceramic grinding

Surface quality of workpiece during ceramic grinding is an ever-increasing concern in industries now-a-days. Every industry cares to produce products with supposedly better surface finish. The importance of the surface finish of a product depends upon its functional requirements. Since surface finish is governed by many factors, its experimental determination is laborious and time consuming. So the establishment of a model for the reliable prediction of surface roughness is still a key issue for ceramic grinding. In this study, a new analytical surface roughness model is developed on the basis of stochastic nature of the grinding process, governed mainly by the random geometry and the random distribution of cutting edges on the wheel surface having random grain protrusion heights. A simple relationship between the surface roughness and the chip thickness was obtained, which was validated by the experimental results of silicon carbide grinding.     

Modeling of shear induced coarsening effects in semi-solid alloys

In long-term rheological shear experiments with semi-solid alloys, coarsening of the particles will falsify the interpretation of the experimental results. The coarsening is intensified by the shear induced convection and the mean size of the particles is changed significantly during the experiments. A simple model has been set up which takes the influence of the convection into account. The resulting growth law has been simplified for diffusion and convection dominated growth. The growth law was verified with shear experiments in a Searl-rheometer with A356 and tin-lead alloys. The experiments demonstrated that under convection the growth follows a linear time law and that the rate constant depends on the root of the shear rate. The correction of experimental results to gain the true viscosity function is demonstrated for a shear jump experiment with A356.     

Delamination failure in ceramic matrix composites: Numerical predictions and experiments

The cohesive-zone finite element approach is used to predict initiation and propagation of delamination in relatively complex ceramic matrix composite sub-elements. Two different generic attachment sub-elements are analyzed and tested under applied uniaxial load. Pre-test analyses predict that delamination initiation and growth are the predominant failure mechanism for both of the sub-elements. Experimental results confirm the finite element predictions, and a good qualitative and quantitative agreement is found between the two.     

Rapid plasma quenching for the production of ultrafine metal and ceramic powders

The rapid plasma quench concept used to produce ultrafine titanium hydride, magnesium, and aluminum powders involves the thermal dissociation of liquid reactants into gaseous components followed by rapid quenching of the products of the subject reaction to prevent back reactions. For example, in the case of titanium hydride powder production, titanium tetrachloride dissociates into titanium and chlorine atoms at 5,000 K. Expansion through a Delaval nozzle accelerates the gas to supersonic speed, cooling it very rapidly at rates as high as 7¹⁰ K/s. Injected hydrogen reacts with condensed titanium particles to form titanium hydride and with the chlorine to form hydrogen chloride. Titanium powder has been produced at 20 kg/h in a continuous reactor. Costs are projected to be lower than the Kroll process at a sufficiently large scale. Magnesium and aluminum production based upon the rapid plasma quench concept are also discussed. For more information, contact Alan Donaldson, Idaho Titanium Technologies, Inc., 101 Technology Drive, Idaho Falls, ID 83401; (208) 522-9909; fax (208) 523-6685; e-mail al@aljeanie226.myrf.net.     

钎料对TiC陶瓷/铸铁钎缝处剪应力的影响

采用有限元数值模拟方法研究了采用不同钎料对冷却过程中TiC陶瓷／铸铁焊缝处剪应力的影响。结果表明，无论采用Ni基钎料、Ti基钎料还是他基钎料，剪应力均主要集中在TiC陶瓷／铸铁的钎缝端点上。当采用Ni基和Ag基钎料时，剪应力的最大值出现在钎料／TiC陶瓷界而；而采用Ti基钎料时，剪应力的最大值出现在铸铁／钎料界面。当采用Ni基和Ti基钉料时，冷却到室温的钎缝最大剪应力值较大，因此接头的连接强度较低；当采用Ag基钎料时，冷却到室温的钎缝最大剪应力值较小，因此接头的连接强度较高。     

基于SIMULINK的阀控液压缸运行稳定性建模与仿真分析

在分析阀控液压缸系统工作原理的基础上,应用流量和力平衡方程,建立了锻压机构工作过程中阀控三位四通单活塞液压缸系统的数学模型,在MATLAB/SIMULINK计算机软件平台上进行了仿真计算,分析系统的稳态特性、动态特性以及频率特性。结果表明:三位四通阀控液压缸在运行过程中出现振荡和超调,并且随着时间的推移系统处于稳定状态,稳态误差为0,系统的响应速度较快,上升时间约为0.4 s,调节时间约为0.55 s,相位裕度约为86.7°,幅值裕度约为29.2 d B,为系统结构参数的优化提供了必要的理论依据;冲击末速度约为6.22 m·s-1,回程最大速度为1.13 m·s-1,增加了阀控液压缸对活塞伸出的助推作用和对回缩的缓冲作用。