N. D. Bega

Sintering of Zirconium Diboride and Phase Transformations in the Presence of Cr3C2

The kinetics of sintering and phase interactions in the diffusion contact zones of Cr3C2 and ZrB2 are investigated. It is found that using Cr3C2 as a sintering activator for ZrB2 sintering reduces the hot pressing temperature from 2200°C to 1500–1750°C, depending on the Cr3C2 content. The phase formation accompanied with the formation of new high-temperature refractory compounds, such as zirconium carboborides, chromium borides and carboborides, and zirconium-chromium carboborides, rapidly develops at the Cr3C2 and ZrB2 interface within the diffusion zone. An intense diffusion of chromium and carbon into zirconium diboride accompanied with the formation of nonporous states in zirconium diboride (to a depth of 50–100 μm at 1360°C) near diffusion contact zone is observed. It is established that the contact interaction during sintering in the diffusion zone results in the formation of the vanishing eutectic liquid phase, which is consumed to forming new high-refractory compounds (phases based on zirconium carbide and chromium borides).

Structural Features and Solid-Solution Hardening of High-Entropy CrMnFeCoNi Alloy

The phase composition, microstructure, and mechanical properties of the single-phase CrMnFeCoNi high-entropy alloy with fcc lattice produced by argon-arc vacuum melting are studied. The alloy solid-solution hardening mechanism is analyzed. The abnormally high athermic solid-solution hardening of the alloy is due to variation in the Burgers vector along the dislocation line and a vector component perpendicular to the slip plane. The activation energy of dislocation movement and activation volume are calculated. The activation energy of dislocation movement is close to the activation energy of pure metals with fcc lattices, and the activation volume is significantly lower compared to that of pure metals because picosized distortions grow in the crystal lattice of the multicomponent alloy. The ratio of hardness and yield stress for this alloy is examined.

Kinetics of Nonisothermal Pressure Sintering of Zirconium Diboride Powder with Additives of Boron and Chromium Carbides in Vacuum

The analysis of the experimental data on the kinetics of nonisothermal pressure sintering of zirconium diboride-based powder mixtures with the addition of boron and chromium carbides in vacuum showed a division of the sintering temperature mode into low-temperature and hightemperature areas. The division is clearly observed when the temperature increases at a rate of 20°C/min. In low-temperature area, the densification occurs slowly and hardly depends on the temperature. In high-temperature area, the viscous flow of the matrix of the porous material is controlled by the power-law creep of the matrix forming the porous body with an activation energy of 394 kJ/mol. Twofold increase in heating rate leads to acceleration of low- and high-temperature strain of the matrix and increase in the apparent activation energy to 581 kJ/mol, indicating the series engagement of additional mechanisms of material flow. The investigation of structure and properties of materials has showed that the formation of new boron–carbide phases and eutectic that disappears during sintering activates the densification of the porous material. It is found out that under low pressure nonisothermal sintering, a superplasticity phenomenon can occur, which results in the densification of the porous body to the virtually non-porous state.

Kinetics of Shrinkage, Structurization, and the Mechanical Characteristics of Zirconium Boride Sintered in the Presence of Activating Additives

Compaction kinetics, structurization, and mechanical characteristics of ceramics based on zirconium boride during the combined addition of sintering activating carbon additives, tungsten boride, and zirconium and tungsten silicides into the system are investigated. It is shown that the efficient additives for ZrB2 ceramics are tungsten silicide and boride, which allow not only reducing the sintering temperature and activating the ceramic compaction, but also provide the maximum strength of the samples. The shrinkage of activator-free ZrB2 occurs in the restructuring of boride that sinters with the formation of an azimuth texture. It is established that these processes do not occur in the ceramics sintered with activators. When such processes are developing, the sintering occurs under the conditions of intensive grain-boundary phase interactions. The mechanical characteristics are studied mainly by the indentation method with the determination of a certain number of ceramic strength characteristics. The differences in the characteristics of ceramics produced by hot pressing in vacuum and by hot pressing in the CO–CO2 atmosphere are studied. It is established that the grain-boundary strength of forming materials and their defective state show the greatest differences. The data obtained allow optimizing the production, composition, and structure of ceramics to ensure the necessary mechanical characteristics (compressive strength, tensile strength, bending strength, hardness, and fracture toughness).

The Mechanical Properties of Sialon–Boron Nitride Composite Ceramics

The production of sialon and sialon–boron nitride composites using oxidized (up to ~15 wt.% O) Si3N4–18 wt.% AlN composite powders resulting from plasma chemical synthesis is studied. Hot pressing of these powders to nonporous state at 1620–1775°C is accompanied by the formation of composites consisting mainly of a mixture of β-sialon (z = 1.5–2) and o′-sialon. Boron nitride additions in an amount of up to 15 wt.% lead to the formation of composites that contain BN lamellar grains in the matrix of single-phase β-sialon (z = 1.5–2). The mechanical and tribological characteristics of the composites with micron and submicron grains are examined. It is observed that the dependence of scratch hardness on contact strength is almost linear for the sialon ceramics and sialon–boron nitride composites. The material is removed during machining through intensive brittle fracture. The introduction of boron nitride into sialon ceramics in an amount of more than 5% is accompanied by approximately a tenfold increase in the material removal in machining, making possible turning operations.

Phase composition and properties of hot-pressed AlN–BN materials

The effect of hot-pressing temperature on the phase composition and properties of AlN–BN materials with addition of Y2O3 or SiC is studied. The composites have higher density and mechanical strength than the hot-pressed AlN–BN material without additions and retain its good insulating properties, thermal and chemical resistance, and adequate machinability.