Nanostructured Composite Powders Research Articles

Effect of high energy ball milling on compressibility of nanostructured composite powder

Compressibility of a nanostructured Al–5AlN composite powder synthesised via high energy ball milling for various times was studied by means of a modified Heckel equation. Since workhardening and morphological changes take place by milling evolution, the compressibility was consequently affected. Strengthening of composite compacts was influenced by milling and compaction processes, i.e. strength of compacts increased at longer milling times and higher compaction pressures. It was found that, at the initial stages of milling and higher compaction pressures, the strengthening was mostly affected from compaction process, whereas the milling strengthening fraction was near to unity at lower compaction pressures as well as prolonged milling. Nevertheless, a sharp increase in milling strengthening fraction of unreinforced Al occurred at intermediate milling times. The rate of strengthening induced by milling raised at higher compaction pressures.

Nanostructured MgH 2 prepared by cold rolling and cold forging

Magnesium is a promising material for solid state hydrogen storage, since it has low cost and its hydride can store reversibly up to 7.6 wt.% of hydrogen. Fast H-sorption kinetics at around 300 °C can be achieved after processing Mg-based mixtures by high energy ball milling (HEBM), which produces nanostructured composite powders. Severe plastic deformation (SPD) processing techniques are being explored as an alternative to HEBM in order to obtain more air-resistant materials and to reduce the time and energy required for processing. In this paper, MgH 2 and MgH 2–Fe mixtures were severely mechanically processed by extensive cold forging (CF) and cold rolling (CR). A very significant grain refinement (up to 10 nm) was achieved, which is comparable to the values typically obtained after processing by HEBM. Enhanced H-sorption properties were observed for these mechanically processed MgH 2-based nanocomposites in comparison with commercial magnesium hydride. The obtained compacts after CR and CF presents a much lower specific surface area than the ball-milled powders and therefore show higher air-resistance. These results are promising from the point of view of applications since it reveals the potential of the use of low cost mechanical processing routes to produce Mg-based nanomaterials for hydrogen storage.

Ultrafine grained bulk ceramic composites consolidated from nanostructured composite powders by pressure less sintering

Nanostructured alumina–titania composite powders which are microsized particles composed of nanosized grains were prepared from nano alumina and titania powders with nanodopants (zirconia and ceria) by ball milling, spray drying and heat treating. Then, the as prepared nanostructured composite powders were consolidated into ultrafine grained alumina–titania bulk ceramic composites through powder compacting and pressure less sintering. The addition of nanodopants leads to a finer grain size in the ceramic and improves the densification of bulk ceramic composite. The active mechanisms of nanodopants were discussed in detail.

Si<SUB>3</SUB>N<SUB>4</SUB>–TiN Nanocomposite by Nitration of TiSi<SUB>2</SUB> and Consolidation by Hot Pressing and Spark Plasma Sintering

Homogeneous nanostructured Si3N4-TiN composite powder was obtained by nitration of a TiSi2 powder precursor in a nitrogen flow. Mechanoactivation of titanium disilicide increases the nitration rate and reduces the temperature of formation of the Si3N4 and TiN. The properties of hot pressing (HP) and spark plasma sintering (SPS) of the nanostructured Si3N4-TiN composite with Y2O3 and Al2O3 additives have been studied. In the case of the HP-prepared composite the processing conditions are sufficient to form a stable, equilibrated grain boundary framework. The SPS consolidation is extremely rapid, low-temperature process and the sintering temperature was 300-400 degrees C lower than that of the hot pressing temperature. As a result the grain boundary framework was underdeveloped. Post-sintering anneal of the SPS-prepared samples caused significant improvement of their mechanical properties. The SPS and HP-derived Si3N4-40 mass% TiN-6 mass% Y2O3-2 mass% Al2O3 nano-composite of 98.4% and 98.9% of relative density demonstrate the Vickers hardness values of 13.2 and 13.7 GPa, respectively. The grains of Si3N4 and TiN were much finer in the case of the SPS-derived ceramic composite. However, the better development of the grain boundary framework in the case of the higher temperature HP treatment in comparison to the SPS significantly reduced the advantage of nanocrystallinity. In both cases the fracture toughness was comparable even after the improvement of grain boundary framework during the SPS consolidation. The K(1c), of 7.83 MPax m(1/2) of the sample prepared according to the best SPS regime is also comparable to K(1c) of 8.30 MPa x m(1/2) of much coarser hot pressed ceramic with very similar relative density.

In situ alumina/aluminum titanate bulk ceramic composites prepared by SPS from different structured composite powders

Nanostructured Al 2O 3/TiO 2 composite powders (spherical particles composed of nanosized alumina and titania grains) were successfully prepared by ball-milling, spray drying and heat treatment. As-received microstructured and the as-prepared nanostructured Al 2O 3/TiO 2 composite powders (M and N powders, respectively) were then used to consolidate in situ alumina/aluminum titanate bulk ceramic composites by spark plasma sintering. The sintering behavior of the starting powders and the effect of the starting powders with different microstructure on the microstructure and mechanical properties of Al 2O 3/Al 2TiO 5 bulk ceramic composites were investigated. N powder showed enhanced sintering behavior compared with M powders. The obtained N bulk composite possesses much finer and more homogenous microstructure than M bulk composite. The mechanical properties of N bulk composite are significantly higher than that of M bulk composite, e.g. the flexural strength, fracture toughness and Vickers hardness of N bulk composite are 91.6%, 37.2% and 145.6% higher than that of M bulk composite, respectively. The improved mechanical properties of N bulk composite compared with that of M bulk composite are attributed to the enhanced densification and the finer and more homogeneous microstructure through the use of the novel nanostructured composite powders.

MICROSTRUCTURE AND MAGNETIC PROPERTIES OF NANOSTRUCTURED Al2O3-20vol%Fe70Co30 COMPOSITE PREPARED BY HIGH ENERGY MECHANICAL MILLING

Al 2 O 3-20 vol % Fe 70 Co 30 composite powders have been prepared by high energy ball milling a mixture of Al 2 O 3 powder and Fe 70 Co 30 alloy powder. The Fe 70 Co 30 alloy powder was also prepared by mechanical alloying of Fe and Co powders using the same process. The effects of milling duration from 8 to 48 hours on microstructure and magnetic properties of the nanostructured composite powders have been studied by means of X-ray Diffractometry (XRD), scanning electron microscopy (SEM) and vibrating sample magnetometry (VSM). It was found that the nanostructured composite powder particles with irregular shapes and Fe 70 Co 30 alloy particles being embedded in them formed after 8 hours of milling. The average grain size of the Al 2 O 3 matrix reduced drastically to less than 18nm after 16 hours of milling. On the other hand, the embedded alloy particles demonstrated almost unchanged average grain size in the range of 14-15nm. Magnetic properties of the powder compacts at room temperature were measured from hysteresis curves, and show strong dependence of the milling time, with the coercivity increasing from 67.1 up to 127.9kOe with increasing the milling time from 8 to 48 hours. The possible microstructural reasons for this dependence are discussed.

In situ porous alumina/aluminum titanate ceramic composite prepared by spark plasma sintering from nanostructured powders

In situ porous alumina/aluminum titanate ceramic composite was fabricated by spark plasma sintering of nanostructured alumina/titania composite powders. The phase composition and microstructure of the nanostructured composite powders and the fabricated porous bulk ceramic composite were investigated by X-ray diffraction and scanning electron microscopy, respectively. The results indicate that a large number of pores were distributed homogeneously in the porous alumina/aluminum titanate ceramic composite, which possessed excellent mechanical properties.

Tribological behavior of NiCr-base blended and nanostructured composite APS coatings by rig test

In the present paper, a nanostructured composite powder containing nano-sized Cr 2O 3 (<100 nm) is manufactured by spray drying and heat treatment. By wear and rig test at room temperature (RT), 200 °C, and 350 °C, respectively, the mechanical and tribological properties of atmospheric plasma spray (APS) blended (PS304) and composite coatings with same composition (60NiCr–20Cr 2O 3–10Ag–10BaF 2/CaF 2, wt.%) are compared. The results show that uniform microstructure of the composite coating could be obtained by eliminating the difference in physical and thermophysical properties of components in feedstock, which leads to the non-uniformity of blended coating due to their different in-flight trajectories of components in APS process. Therefore, the excellent wear resistance of composite coating benefits from its uniform and fine microstructure. Also, the wide and uniform distribution of plastic Ag lubricant could result in the formation of surface localized tribofilm as lubricant film to protect the coating from fracture and spallation.

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.

W-Diamond/Cu-Diamond nanostructured composites for fusion devices

ABSTRACTA novel material design for nuclear fusion reactors is proposed based on Cu-Diamond and W-Diamond nanocomposites. The proposed design involves the production of W/W-Diamond/CuDiamond/Cu functionally graded material. W, W-Diamond, Cu-Diamond and Cu nanostructured composite powders were produced independently by mechanical alloying and subsequently consolidated/welded through spark plasma sintering. Modulation of processing parameters allowed controlling the extent of unfavourable conversion of Diamond into tungsten carbide, as well as to overcome the Diamond intrinsically difficult bonding to copper. Microstructural features and microhardness of the as-produced materials are presented.

Study of the densification of a nanostructured composite powder: Part 1: Effect of compaction pressure and reinforcement addition

The compressibility behavior of Al-AlN nanostructured composite powder with different amount of reinforcement content was studied. The composite powder was synthesized by blending and high-energy milling process for 25 h. Williamson–Hall method was applied to determine the crystallite size after milling process. To investigate the role of reinforcement particles in consolidation of composite powders, monolithic aluminum powder was examined. Samples were made at different pressures and relationships were established between the compaction pressure and the density of the compacts. The modified Heckel equation was used to assume the pressure effect on yield strength and then was compared with other common equations. The best correlation was obtained when the effect of compaction pressure was considered. Due to lower plastic deformation capacity, the powders with higher content of hard reinforcements exhibited lower compressibility. The results also revealed that the yield strength of compacts, predicted by the mathematical equation, increases with reinforcement addition.

Parametric Study of an HVOF Process for the Deposition of Nanostructured WC-Co Coatings

Nanocrystalline WC-Co coatings were deposited by high velocity oxyfuel from commercial nanostructured composite powders. Processing parameters were optimized for maximal retention of the nanocrystalline size and for minimal decarburation of the ceramic reinforcement. Thermochemical and gas-dynamical properties of gas and particle flows within the combustion flame were identified in various operating conditions by computational fluid-dynamics (CFD) simulation. Significant improvements of the mechanical properties of the coatings were obtained: a decrease of the friction coefficient was measured for the nanostructured coatings, together with an increase of microhardness and fracture toughness.

Microstructures and properties of nanostructured thermal sprayed coatings using high-energy milled cermet powders

This study demonstrates the potential of high-energy milling to use nanostructured cermet powders for thermal spraying utilizing a TiC–Ni-based composite as model material. The microstructure of coatings processed by VPS and HVOF spraying of nanostructured composite powders is characterized and compared to the initial microstructure of the feedstock. Thus, the effect of different microstructures, which can be produced by high-energy milling, on the microstructural evolution during spraying is evaluated. Results show that partial dissolution and reprecipitation of hard phase material as well coarsening of the binder phase crystallite size occur during the spraying process. However, a homogeneously dispersed hard phase distribution similar to that of the nanostructured precursor powder with hard phase sizes in the range of 100 nm is formed. Additionally, hard phase particles bigger than of 300 nm are retained during spraying. First results concerning hardness and wear resistance of respective coatings are shown and discussed.

Stacking Faults Created by Mechanical Milling in Nanostructured WC-Co Composite Powder

ABSTRACTNanostructured WC-Co powders obtained by mechanical milling were investigated by combination of X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) techniques. HRTEM image analysis shows that in the as-milled nanostructured powder, many WC grains contain stacking faults lying on the plane{10.0}. Analysis of phase images showed that these defects were nearly periodically ordered along the [10.0] direction. Based on these observations, a structural model is proposed for the WC grains with ordered stacking faults, which is in fact equivalent to a superstructure of WC with space group Amm2. When this model is introduced together with the normal WC structure (space group P6m2) into the Rietveld refinement, a much better agreement between the calculated and experimental XRD profiles is obtained. This study allowed obtaining the lattice parameters, grain size, microstrain and other structural information on the as-milled powders.

Synthesis and Processing of Nanograined Fe - (Fe, Mo)6C Composite Powders

ABSTRACTSpray Conversion Processing was used to synthesize high volume fractions (0.52 - 0.74) of nanograined (Fe, Mo)6C carbide dispersions in iron, starting from water soluble precursors. The essential features of the process are, (1) preparation of a chemically homogeneous precursor powder, and (2) thermochemical conversion of the precursor powder to the desired nanostructured composite powder through controlled gas-solid reactions. The thermodynamic and kinetic features of the gas-solid reactions, and the influence of various processing parameters on the structures developed, are discussed. The powders were consolidated to near theoretical density by pressureless sintering and hot pressing. All the consolidated samples had bicontinuous structures. Compared to M2 high speed tool steels, these high volume fraction carbide strengthened iron alloys display superior hardness values up to 500°C.

Sintering of nanograined Fe(Fe, Mo) 6C powder compacts

High strength in two phase materials can be achieved by uniformly dispersing a high volume fraction of ultrafine (preferably nanometer size) hard particles in a softer matrix. Prealloyed micron-sized iron powder, with 52 vol% (Fe, Mo) 6C dispersoid, was synthesized by a novel spray conversion process, starting from water soluble precursors. The nanostructured composite powder ( ∼ 50 nm grain size both phases was subsequently compacted and sintered in vacuum at different temperatures. The influence of as-compacted density on the sinterability of powder compacts is discussed.