Yu. V. Blagoveshchenskii

Determination of oxygen in W-C-Co nanopowders

The oxygen content of W-C-Co nanopowders produced by plasma reduction of WO3, followed by low-temperature carburization in hydrogen has been determined by carrier gas hot extraction. The oxygen in adsorbed water, carbon-oxygen complexes weakly bound to the surface, and surface oxides (WOx and CoOx) has been determined separately. Freshly prepared, passivated WC and WC + 8% Co powders with a specific surface area of 6–11 m2/g were found to contain 0.03–0.07 μg/cm2 of oxygen, not counting adsorbed water. Strongly bound oxygen in the form of surface oxides accounts for at least 80% of the total oxygen. These oxygen contents are equivalent to surface coverages from 1.2 to 2.5 oxygen monolayers (monolayer density of 1015 at/cm2). The water content varies from 0.15 to 0.3%, which corresponds to a water film no thicker than a monolayer. All of the water is physisorbed. The products of the plasma reduction of WO3 have a complex phase composition (W2C, WC1 − x, W, α-WC) and a specific surface area from 21 to 24 m2/g. In spite of the high content of the readily oxidizable phases W and W2C, the plasma-synthesized mixtures have submonolayer surface coverages with oxygen. They are protected from air oxidation by thin (one to three monolayers) pyrolytic carbon films, while the small amount of oxygen present originates from unreacted particles. In dry air, the powders oxidize insignificantly. At 100% humidity, stoichiometric WC powders are the most stable, while WC + 8% Co shows the lowest stability. The oxidation rate of W-C powders is proportional to the overall content of W and W2C.

Study of the Structure and Mechanical Properties of Nano and Ultradispersed Mechanically Activated Heavy Tungsten Alloys

Mechanisms of sintering and the structure and mechanical properties of nano- and ultradispersed W-Ni-Fe (WNF) and W-Ni-Fe-Co (WNFC) heavy tungsten alloys are investigated. The effect of tungsten particle sizes on the optimal sintering temperature is studied. The size of particles has been changed by the mechanical activation (MA) of the source W-Ni-Fe coarse-grained (CG) charge and by adding ultradispersed particles obtained using plasmochemical synthesis. Nanodispersed powders and ultradispersed powders (UDPs) have been sintered using the techniques of free sintering and pulse plasma sintering (PPS). It has been revealed that the dependence of the alloy density on heating temperature is nonmonotonic, with the maximum corresponding to the optimum sintering temperature. It has been shown that an increase in the time of MA and acceleration of grinding bodies in the process of MA accompanied by a decrease in the size of alloy particles and formation of nonequilibrium solid solutions lead to a reduction in the optimal sintering temperature. It has been shown that, using planetary high-energy milling methods and high-rate spark plasma sintering, it is possible to obtain ultrastrong tungsten alloys whose mechanical properties (macroelasticity stress and yield stress) substantially exceed analogous properties of commercial alloys.

Superhard nanodisperse tungsten heavy alloys obtained using the methods of mechanical activation and spark plasma sintering

We have studied the structure and mechanical properties of nanodisperse tungsten-based heavy alloys of the W-Ni-Fe system. The temperature dependence of the density of compacted alloys exhibits a nonmonotonic character with a maximum that corresponds to the optimum temperature of sintering. The effect of the regime of solid-state pulsed spark plasma sintering (SPS) on the structure and mechanical properties of mechanically activated W-Ni-Fe heavy alloys has been studied. It is established that, using preliminary mechanical activation in a planetary ball mill and the subsequent high-rate SPS, it is possible to obtain superhard tungsten-based heavy alloys with mechanical properties that substantially exceed those of the analogous standard alloys.

Preparation of nanopowders of carbides and hard-alloy mixtures applying low-temperature plasma

The preparation of nanodimensional powders of tungsten carbides, as well as other transitionmetal carbides, using plasma-chemical reduction synthesis are investigated. The main regularities of preparing powders of specified dispersity and composition are revealed. Characteristic particle sizes of carbides are 40–80 nm. In order to prepare a homogeneous mixture of WC-Co nanoparticles with an exact weight content, the procedure of deposition of cobalt on the tungsten carbide powder with the simultaneous introduction of an additive of inhibitor carbides such as carbides of chromium, vanadium, and tantalum is developed. The prepared powders were investigated using modern methods, including high-resolution scanning electron microscopy and fractional gas analysis.

High-strength ultrafine-grained tungsten-carbide-based materials obtained by spark plasma sintering

Ultrafine-grained (UFG) tungsten carbide (WC) samples with high hardness (up to 34 GPa) and increased cracking resistance have been obtained by the method of spark plasma sintering (SPS). Initial powders have been prepared by two-stage plasmachemical synthesis. The influence of the initial size of WC nanoparticles on the density, structural parameters, and mechanical properties of UFG tungsten carbide obtained by SPS has been studied. It is established that the phenomenon of accelerated sintering of WC powder is related to enhanced grain-boundary diffusion.

Preparation and investigation of ultrafine-grained tungsten carbide with high hardness and fracture toughness

High-density samples of ultrafine-grained tungsten carbide with high hardness (up to 31–34 GPa) and increased fracture toughness (up to 5.2–6.4 MPa m1/2) are obtained using the technology of electropulse plasma sintering. The influence of the initial size of nanoparticles of α-WC prepared by plasmachemical synthesis on the density, structural parameters, and mechanical properties of tungsten carbide is investigated.

Sintering of nano- and ultradispersed mechanically activated W-Ni-Fe powders and the manufacture of ultrahigh-strength heavy tungsten alloys

The structure and mechanical properties of nano- and ultradispersed mechanically activated heavy W-Ni-Fe and W-Ni-Fe-Co tungsten alloys (VNZh and VNZhK alloys, respectively) are studied. Mechanically activated nano- and ultradispersed charge powders are sintered by free sintering (thermally activated) and spark plasma sintering. The dependence of the density of the alloys made of the mechanically activated powders on the sintering temperature is found to have a nonmonotonic character with a maximum corresponding to the optimum sintering temperature. It is shown that an increase in the mechanical activation time and the acceleration of the milling bodies during mechanical activation lead to a decrease in the alloy particle size and the formation of nonequilibrium solid solutions and are accompanied by a decrease in the optimum sintering temperature of heavy tungsten alloys. Ultrahigh-strength tungsten alloys the mechanical properties of which are substantially higher than those of standard coarse-grained analogs are fabricated due to the optimization of the conditions of ball milling and high-rate spark plasma sintering of W-Ni-Fe powders.

High-speed electropulse plasma sintering of nanostructured tungsten carbide: Part 1. Experiment

The high-temperature consolidation of nanopowders of pure tungsten carbide by electropulse plasma sintering (spark plasma sintering) is investigated. The influence of the initial size of WC nanoparticles and their preparation modes on the density, structural parameters, and mechanical properties of tungsten carbide are investigated. Samples of high-density nanostructured tungsten carbide with high hardness (to 31 GPa) and crack resistance (5.2 MPa m1/2) are fabricated.

Effect of hydrogen annealing on the composition and particle size of molybdenum nanopowders

We have studied the behavior of molybdenum nanopowder consisting of particles with an average size of 23.7 nm. The powder was prepared by hydrogen plasma reduction of molybdenum trioxide and contained considerable amounts of oxygen in the form of amorphous molybdenum oxides and hydroxides. Annealing in hydrogen for 1 h at temperatures from 300 to 1000°C is shown to cause crystallization of oxides, which then vaporize to form the volatile hydroxide MoO2(OH)2. Most of the oxygen is removed by annealing below 700°C. Starting at this temperature, particle growth is observed. The main mechanism behind the coagulation of molybdenum particles is vapor transport. Annealing at 1000°C for 30–50 min in hydrogen with a dew point of −5°C increases the particle size by about one order of magnitude.

Coagulation of tungsten nanopowders under annealing in hydrogen

Coagulation of tungsten nanoparticles is investigated under annealing in hydrogen flow at 600–1000°C and duration of holding up to 1 h. It is established that the main mechanism of coagulation is the gasphase transport, and mass carriers are the WO3 · H2O molecules. Evaluative calculations of the coagulation rate are performed and their satisfactory agreement with experimental results is shown. Recommendations for optimization of conditions of thermal treatment of tungsten nanopowders are given.