Novel technique for synthesis and consolidation of aluminium nitride nanopowders

Abstract

In this paper, the use of a microwave plasma method for the synthesis of aluminium nitride nanopowders is described. The powders were consolidated to near theoretical densities using the unique rapid consolidation technique, plasma pressure consolidation (P²C), developed by MMI. Rapid consolidation of nanopowders is an ideal requirement for better mechanical and thermal properties in the consolidated part, as it retains the fine microstructure preventing anomalous grain growth. Microwave plasma synthesis resulted in aluminium nitride nanopowders (85–200 nm), which were consolidated to near theoretical density using P²C in <5 min without sintering additives. The effect of yttria (3 wt-%) as a sintering additive on the thermal conductivity (TC) of aluminium nitride was also evaluated and compared with TC values obtained from additive free AlN consolidated samples.     

纳米粉末烧结的研究现状与前景

描述了纳米粉末烧结的特点，介绍了近年来发展的几种先进烧结技术，总结了国内外对各类纳米粉末烧结方法的研究工作进展与现状，对于研究和开发高性能的纳米材料具有一定的意义。指出以低温、快速、抑制晶粒生长为重点，乃是今后研究纳米粉末烧结的关键。     

Spark Plasma Sintering of Cryomilled Nanocrystalline Al Alloy - Part II: Influence of Processing Conditions on Densification and Properties

In this study, nanostructured Al 5083 powders, which were prepared via cryomilling, were consolidated using spark plasma sintering (SPS). The influence of processing conditions, e.g., the loading mode, starting microstructure (i.e., atomized vs cryomilled powders), sintering pressure, sintering temperature, and powder particle size on the consolidation response and associated mechanical properties were studied. Additionally, the mechanisms that govern densification during SPS were discussed also. The results reported herein suggest that the morphology and microstructure of the cryomilled powder resulted in an enhanced densification rate compared with that of atomized powder. The pressure-loading mode had a significant effect on the mechanical properties of the samples consolidated by SPS. The consolidated compact revealed differences in mechanical response when tested along the SPS loading axis and radial directions. Higher sintering pressures improved both the strength and ductility of the samples. The influence of grain size on diffusion was considered on the basis of available diffusion equations, and the results show that densification was attributed primarily to a plastic flow mechanism during the loading pressure period. Once the final pressure was applied, power law creep became the dominant densification mechanism. Higher sintering temperature improved the ductility of the consolidated compact at the expense of strength, whereas samples sintered at lower temperature exhibited brittle behavior. Finally, densification rate was found to be inversely proportional to the particle size.     

粉末冶金新技术

粉末冶金在超导体、纳米材料、高级磁性材料、生物工程材料、超硬材料、超微机械、梯度功能材料等领域已获得越来越广泛的应用。本文简要介绍了粉末冶金技术在粉末制取、压制、烧结、注射成形、快速原型制作及超硬材料学等领域的新技术     

Nanomaterials of SHS technology for tribological applications: A review

A review of results of applying the powder technology of self-propagating high-temperature synthesis (SHS) for obtaining various nanomaterials that can be used according to the tribotechnical indentation is given. First, these are low-cost nanopowders of sulfides, oxides, nitrides, carbides, borides, and metals which are suitable as solid lubricants and friction modifiers for liquid and plastic lubricating materials. Second, these are solid compact nanostructured ceramic and composite materials for the fabrication of tribotechnical construction. This type of nanomaterial can be fabricated both ex situ from SHS nanopowders by sintering or introduction into the melt both in situ in one stage from initial powders reagents by gasostatic SHS technology, forced SHS compaction, SHS casting, and SHS in the melt, which considerably simplifies and cheapens the production of such materials. Third, these are SHS materials for the deposition of nanostructured coatings of various thicknesses with a high wear resistance and low friction coefficients, such as nanostructured materials for surfacing and spraying, electrospark alloying electrodes, multicomponent targets for magnetron sputtering, cathodes for vacuum-arc evaporation, and nanodispersed fillers of electrochemical and chemical coatings.     

Densification of Al-2024 and Al-2024/Al2O3 Powders by Conventional P/M Route and ECAP: A Comparative Study

In this study, mechanically alloyed Al-2024 and Al-2024/Al₂O₃ powders are densified by conventional sintering and by equal channel angular pressing (ECAP) with and without back pressure. The powder was encapsulated in an aluminium can for consolidation through ECAP. The properties obtained in the compacts by conventional sintering route and by ECAP are compared. The effect of conventional sintering and ECAP on consolidation behaviour of powder, microstructure, density and hardness is discussed. Room temperature back pressure aided ECAP results in nearly full denser (97?% of its theoretical density) compact at room temperature. Nano Indentation technique was used to determine the modulus of the consolidated compacts.     

Bulk nanocrystalline aluminum 5083 alloy fabricated by a novel technique: Cryomilling and spark plasma sintering

Dense, bulk nanocrystalline aluminum 5083 alloy was fabricatedvia a combined technique: cryomilling (mechanical milling at cryogenic temperature) to achieve the nanocrystalline Al 5083 powder and spark plasma sintering (SPS) to consolidate the cryomilled powder. The results of X-ray diffraction analysis indicate that the average grain size in the SPS consolidated material is 51 nm, one of the smallest grain sizes ever reported in bulk Al alloys produced by powder metallurgy derived methods. In contrast, transmission electron microscopy (TEM) analysis revealed a bimodal grain size distribution, with an average grain size of 47 nm in the fine-grained regions and approximately 300 nm in the coarse-grained regions. Nanoindentation was used to evaluate the mechanical properties and the uniformity of the consolidated nanocrystalline Al 5083. The hardness of the material is greatly improved over that of the conventional equivalent, due to the fine grain size. The mechanisms for spark plasma sintering and the microstructural evolution are discussed on the basis of the experimental findings.     

Effect of Density and Processing Conditions on Oxide Transformations and Mechanical Properties in Cr–Mo-Alloyed PM steels

To improve the mechanical properties and performances of water-atomized powder metallurgy steels, it is necessary to enhance the density. Consolidating water-atomized steel powders via conventional pressing and sintering to a relative density level > 95 pct involves processing challenges. Consolidation of gas-atomized powders to full density by hot isostatic pressing (HIP) is an established process route but utilizing water-atomized powders in HIP involves challenges that result in the formation of prior particle boundaries due to higher oxygen content. In this study, the effect of density and processing conditions on the oxide transformations and mechanical properties from conventional press and sintering, and HIP are evaluated. Hence, water-atomized Cr–Mo-alloyed powder is used and consolidated into different density levels between 6.8 and 7.3 g cm⁻³ by conventional die pressing and sintering. Fully dense material produced through HIP is evaluated not only of mechanical properties but also for microstructural and fractographic analysis. An empirical model based on power law is fitted to the sintered material properties to estimate and predict the properties up to full density at different sintering conditions. A model describing the mechanism of oxide transformation during sintering and HIP is proposed. The challenges when it comes to the HIP of water-atomized powder are addressed and the requirements for successful HIP processing are discussed.

     

Effect of the conditions of sintering W-Ni-Fe-Co heavy alloy nanopowders on the structure and density of compacted samples

A compact material made of a heavy tungsten alloy W-Ni-Fe-Co nanopowder is produced. The nanopowders are synthesized by the treatment of a solid tungstic acid in aqueous solutions of Ni, Fe, and Co salts followed by the reduction of the solid residue by hydrogen at 800°C (the average size of the powder conglomerates is ∼300 nm, and the conglomerates consist of 100-nm particles). Solid-phase sintering is performed in stages. An increase in the temperature at the last stage from 1300 to 1350 and 1450°C increases the density from 16.7 to 17.2–17.4 g/cm³ and the average tungsten grain size to 2.4–4.6 μm. The samples after solid-phase sintering at 1350°C have no porosity. Liquid-phase sintering of nanopowders with high surface and interface energies occurs at 1480°C. Original Russian Text ? K.B. Povarova, M.I. Alymov, O.S. Gavrilin, A.A. Drozdov, E.V. Evstratov, A.I. Kachnov, A.E. Sal’ko, 2007, published in Metally, 2007, No. 6, pp. 65–72.     

The effect of Fe addition on the densification of B4C powder by spark plasma sintering

Boron carbide is a low-density ceramic with high hardness and stiffness values that make it a valuable candidate for light armor applications. Fully dense boron carbide is fabricated by hot pressing of fine (<2 µm) powder at a relatively high temperature (2150–2200°C). Fully dense boron carbide can be processed from an initial mixture of 5.5 vol.% Fe and low-cost B₄C powder by spark plasma sintering (SPS) at 2000°C. At this temperature, Fe-free boron carbide can be consolidated only to 96% of the theoretical density. The effect of the Fe addition on the densities is even more pronounced at lower processing temperatures and is related to the presence of a liquid phase in the Fe-containing material. The resulting microstructure and mechanical properties of the Fe-containing boron carbide are presented and discussed.     

Syntheses of full-density nanocrystalline titanium nitride compacts by plasma-activated sintering of mechanically reacted powder

Nearly equiatomic nanocrystalline titanium nitride (Ti₅₆N₄₄) powder with an average grain size of 5 nm has been synthesized by ball milling elemental Ti powder under nitrogen gas flow at room temperature. During the first stage of reactive ball milling (RBM) (time <3.6 ks), the metallic Ti powder tends to agglomerate to form powder particles with a larger diameter. At the second stage (3.6 to 22.0 ks), the agglomerated particles of Ti fragment to form smaller particles. These smaller particles that have new or fresh surfaces begin to react with the milling atmosphere (nitrogen) during the third stage of milling (22 to 86 ks) to form TiN powder coexisting with unreacted Ti powder. Toward the end of milling (86 to 173 ks), a single phase of nanocrystalline TiN (NaCl structure) is obtained. The powder of this end-product has a spherical-like morphology with an average particle size of about 0.4 μm diameter. A sintering procedure using plasma activation has been employed to consolidate the powder particles at several stages of the RBM. The as-milled and as-consolidated powders have been characterized as a function of the RBM time by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), optical metallography, and chemical analyses. Density measurements of the consolidated samples show that after 86 to 173 ks of the RBM time, the compacted samples are essentially fully dense (above 96 pct of the theoretical density for TiN). The results also show that the consolidated TiN compacts still maintain their unique nanocrystalline properties with an average grain size of about 65 nm. The hardness and some mechanical properties of the consolidated TiN compacts have been determined as a function of the RBM time.     

Particle bonding,annealing response,and mechanical properties of dynamically consolidated type 304 stainless steel powders

The nature of interparticle bonding in explosively consolidated, centrifugally atomized (CA), and vacuum gas-atomized (VGA) Type 304 stainless steel powders has been examined. Stress waves with sufficient amplitude to produce full density do not necessarily produce metallurgical bonds between particles; the local strain and strain rate are found to determine the degree of local heating and, in turn, the degree of particle fusion. Particle interaction is found to be limited to nearest neighbors. The as-consolidated CA material has approximately twice the ultimate tensile strength of mill-annealed wrought Type 304 stainless steel. Consolidated CA powder has a higher defect density than VGA powder consolidated under the same conditions; however, the VGA material recrystallizes at a lower temperature due to a lower concentration of carbides. Annealing explosively consolidated material produced from either powder results in sintering, improved particle bonding, and greater ductility.     

贵金属纳米材料及其产业化过程

对贵金属纳米材料的种类、用途、产业化方法和发展趋势等内容进行了综合评述。贵金属纳米材料包括贵金属单质和化合物纳米粉体材料、贵金属新型大分子纳米材料和贵金属膜材料等几大类，随着国家对黄金和白银专控政策的放开和纳米技术与传统的贵金属深加工产业的结合，贵金属纳米材料作为一类在工业生产中起着重要作用的新材料，具有良好的发展前景。非负载型贵金属纳米粉末常用化学还原法、光化学合成法、电化学沉积法以及热物理法等方法进行生产，负载型贵金属纳米粉体材料则一般采取化学法（浸渍法）、离子交换法和吸附法等）生产，这两类贵金属粉体材料是目前获得工业应用最多的贵金属纳米材料，贵金属纳米材料的产业化过程有其特殊性，其发展趋势可以概括为改造、隆本、集约化和多功能化。     

THE CORRELATION BETWEEN RAW MATERIALS,PREPARATION CONDITIONS,AND PROPERTIES OF SINTERED IRON

Abstract

Three plain iron powders of different types (sponge-iron, atomized and electrolytic iron powder) were studied with respect to their sintering behaviour and to the influence of manufacturing parameters—i.e., compacting pressure, sintering temperature, and sintering atmosphere—on the microstructure and the properties of sintered compacts. The changes of length, electric conductivity, and strength during sintering are explained in physical and chemical terms. Technical sintering diagrams are presented. The influence of sintering atmospheres on the mechanical properties of sintered compacts is shown for the three types of powder. The correlation between pore structure and strength is discussed; analytical relationships are developed which are in agreement with the experimental results.     

Fatigue crack growth in some PM low alloy steels consolidated by rotary forging and sintering

Abstract

Fatigue crack propagation rates under plane strain conditions have been investigated for three PM low alloy steels consolidated to high densities by rotary compaction followed by sintering and heat treatment. It is shown that the densities and properties are intermediate between those of pressed and sintered materials and of powder forged materials. Threshold stress intensities compare satisfactorily with those for wrought counterparts, but resistances to crack growth are inferior to those of wrought steels. Possible reasons for the properties of the rotary compacted materials are considered in the light of their microstructures and the behaviour of other PM materials.     

京唐烧结铁矿粉的同化性测试研究

烧结过程中铁矿粉的同化性是研究高温特性的重要部分,具有适宜同化性的铁矿粉在烧结过程中液相粘结良好,有利于烧结矿质量指标的改善.对京唐烧结用料中有代表性的5种主料矿粉：巴卡粉、杨迪粉、巴烧粉、澳粉和地方粉的同化性能进行了测试;并按两两搭配组合的方式进行了同化性测试,发现不同矿粉两两搭配后,混合粉的同化性均介于单种矿粉的同化性之间,并且混合矿的同化性基本与单种矿比例呈线性相关关系.     

Investigation into the compaction of nanopowders and micropowders of silicon carbide in high-pressure apparatus

The results of the investigation into the compaction (sintering) of silicon carbide nanopowders and micropowders in a DO-138 high-pressure apparatus are presented. Compaction modes for both types of materials are identical (a pressure of 3.5–4.0 GPa, a temperature of 1600—1700°C, and a holding time of 10 s). The influence of cladding of SiC nanopowders and micropowders with titanium and titanium nitride on the properties of compacts (cakes) formed under the same sintering modes is investigated. It is established that, when compacting the silicon carbide nanopowder, cakes differ in regards to higher density, hardness, and lower porosity compared with the samples made of finely dispersed technical silicon carbide. A higher activity of titanium relative to SiC makes it possible to chemically associate the grains of the latter due to the formation of intermediate layers of titanium carbide between them. The resulting ceramics possesses a higher density, hardness, and wear resistance. The wear resistance of synthesized composites based on nano-SiC is higher than for a polycrystalline material based on silicon carbide micropowder by a factor of 4.5.     

碳／铜复合材料摩擦磨损性能的研究

将粉末冶金法制备的碳／铜复合材料进行放电等离子烧结。对烧结后的样品进行了摩擦磨损性能研究。结果表明,复合材料的摩擦系数均随含碳量的增加呈下降趋势;当含碳量为6％时,磨损率最低。     

The role of surface phenomena in powder metallurgy processes— A review

Conclusions A survey is presented of the role of surface phenomena at all stages of formation of sintered bodies by the methods of powder metallurgy.The properties of powders are shown to be determined chiefly by the surface properties. The role of surface phenomena in obtaining metal powder by various methods (reduction, electrolytic, mechanical, dispersion) is discussed in the paper. Surface phenomena are also important in pressing processes, and are a motive force in sintering processes. This is distinctly seen on sintering in the presence of the liquid phase and in impregnation processes. An attempt is made to link the strength of binding and microstructure in cermets with the surface properties of their components.     

放电等离子烧结制备铁基大块非晶材料

以惰性气体雾化的非晶铁基粉末为原料,采用放电等离子烧结技术(SPS)制备大块非晶材料.探索了SPS烧结温度对烧结体的物相、相对密度、微观结构和性能的影响.试验表明,采用优化的SPS烧结工艺,用粉末冶金的方法,可以获得致密的大块非晶材料,部分性能与铸态相当.