L. A. Krushinskaya

Formation of Diborides of Groups IV–VI Transition Metals During Mechanochemical Synthesis

Fine powders of diborides of groups IV–VI transition metals (TiB2, HfB2, NbB2, TaB2, CrB2, Mo2B5, and W2B5) were produced by mechanochemical synthesis and low-temperature heat treatment of activated charges. The particle size of the powders was ≤1 μm. It is revealed that processes of formation of diborides of groups IV–VI transition metals differ within period and group. Diborides Ti and Nb formed discontinuously. Diborides Hf and Ta formed after the formation of lower boride phases, whereas higher borides Cr, Mo, and W formed in almost a single-phase state only after a low-temperature treatment of preliminarily mechanoactivated charges. The differences were analyzed from the point of view of the donor-acceptor capacity of the atoms of boron and diboride-forming transition metals.

Use of Binary Titanium–Chromium Diboride for Producing Protective Coatings on a Nickel Substrate

The production of composite electrolytic coatings on a nickel substrate using binary titanium–chromium diboride obtained by mechanical synthesis was studied. The influence of heat treatment parameters on the phase composition and structure of the coatings was examined. It is shown that these coatings substantially increase the wear resistance of structural medium-carbon steel.

Phase Formation During Nitriding of Vanadium Disilicide

The evolution of microstructural and phase transformations during nitriding of mechanically preactivated vanadium disilicide powder is investigated by X-ray diffraction, chemical analysis, and transmission electron microscopy. It is established that, in the initial stage of nitriding (1000–1100°C), the phase formation is accompanied by the dispersion of near-surface zones of VSi2 particles and the formation of V2N and α-modification silicon nitride. With increase in the nitriding temperature, the phase formation is accompanied by the delamination of particles and the formation of mainly VN and silicon nitride of α- and α-modifications. Nitriding of a mechanically activated vanadium disilicide powder at 1400°C enables synthesizing a fine silicon nitride–vanadium nitride composite powder in a single process. The synthesized powder is formed as loose aggregates consisting of 50 nm particles.

Producing Nanodispersed Composite Nitride Powders by Nitriding of Precursors

Highly dispersed and nanodispersed composite powders based on Si3N4–TiN, Si3N4–CrN, BN–AlN, and BN–Si3N4non-metallic nitrides are produced by nitriding of precursors. The main criteria for selecting precursors for the synthesis of these powders are determined. Features of the processare analyzed for powders of TiSi2, CrSi2, B4Si, and AlB2precursors in both mechanically activated and non-activated conditions. It is established that the mechanical activation of precursors substantially decreases the temperature of nitride formation and enables synthesizing nanodispersed composite nitride powders with a particle size of 30–70 nm. In comparison with the specimens produced from the powders synthesized by conventional techniques, the specimens produced by hot pressing and spark plasma sintering possess a highly dispersed microstructure with a uniform distribution of phases and advanced physical and mechanical properties.

Production and Thermal Stability of Silver Nanoparticles in the Ag–O System

The paper examines the structure of porous Ag + NaCl condensates and analyzes the phase and chemical composition and sizes of Ag nanoparticles produced by electron-beam evaporation and vacuum condensation. It is shown that Ag nanoparticles are highly adsorptive to air oxygen in a porous salt matrix. Thermogravimetric analysis is used to study the kinetics of variation in the weight of porous NaCl and Ag + NaCl condensates during heating (to 650°C) and cooling in air. The results are considered in terms of physical and chemical adsorption. Stabilized colloidal systems of silver nanoparticles are obtained. Photon correlation spectroscopy is employed to determine the size of nanoparticles in aqueous solutions with surfactant agents.