Journal of Superhard Materials最新文献

Dense composite ceramics based on B₄C with an addition of 10 to 50 wt % of B₃Si were obtained by hot pressing. An increased amount of boron silicide decreases the hot pressing temperature to obtain a high-density material. We found a hot pressing temperature, optimal for each of the compositions and possess the highest hardness, which is associated with the formation of a fine-grained structure. Densification occurs during reactive hot pressing, which consists in the decomposition of B₃Si into Si and BSi_n (n = 12–23) and further interaction with B₄C to form SiC and B₁₂(Si,B,C)₃. The modification of boron carbide with silicon leads to the stabilization of hardness at ~24–26 GPa in the load range from 2 to 200 N. The highest bending strength of ~500 MPa is observed for samples with up to 20 wt % of B₃Si. The optimal composition, which combines high values of hardness and flexural strength, is B₄C–20 wt % B₃Si.

We considered the existing technologies for obtaining crystals of gallium nitride. The finite element method was used to create a model of the thermal state of a cell in a high-pressure apparatus that was used to examine the solubility of gallium nitride in iron. The calculation results are represented as temperature fields in different elements of the apparatus. The effect of varying the wall thickness of a tubular heater and changing the concentration of zirconium dioxide in axial heaters on the thermal state of a high-pressure cell is studied. When the wall thickness of the tubular heater is altered, the temperature in the center of the cell changes linearly. An increase in the heater thickness leads to an insignificant (~2°C) increase in the maximum temperature drop in the iron sample under study. The temperature difference at the characteristic points of the cell in the radial and axial directions decreases with an increase in the concentration of zirconium dioxide in the axial heaters. The optimal composition of axial heaters for conducting experiments on studying the solubility of gallium nitride in iron corresponds to the concentrations of ZrO₂ and graphite of 60 and 40 wt %, respectively.

A comparative quantitative analysis of the shape characteristics and abrasiveness of the grains of powders of natural impact and synthetic diamond is performed. The characteristics of the grain morphology of diamond powders, which have an effect on their abrasive properties, are studied. The shape and specific perimeter of the grain projection, the coefficient of their flattening, and the external specific surface area of the powder are accepted as such characteristics. Differential shape similarity indices of the grain projections of investigated powders and their uniformity with respect to these characteristics as signs of powder quality are determined. The tendencies and degrees of variation of the analyzed characteristics during the transition from impacted diamond powders to synthetic diamond powders are established. For two types of origin of the powder material, a quantitative analysis of the relationship between the detected trends of changes in the grain morphology characteristics of diamond powders and their abrasiveness is performed.

Hexagonal boron nitride (h-BN) solid waste recovered from the high pressure and high temperature (HP-HT) synthesis of cubic boron nitride was used as photocatalyst to study its degradation performance in organic pollutants. Results showed that the main phase of the solid waste was h‑BN and it contained a small amount of impurity phase. The grains were irregular, and the grain size was approximately several micrometers. Compared with commercial h-BN powder, h-BN waste had strong light absorption ability in the whole light range. Under visible-light irradiation, the degradation ability of h-BN waste to organic pollutants was very weak. Adding H₂O₂ could significantly improve the photocatalytic performance of h-BN waste and basically degrade methyl orange (MO), rhodamine B (RhB), methylene blue (MB) in 30, 60, and 80 min, respectively.

Polycrystalline cubic boron nitride (PcBN) with excellent mechanical properties was sintered by using high activity TiC_0.5 and metal aluminum as binder at high temperature and pressure. The effects of sintering temperature on phase composition, microstructure, relative density and mechanical properties of PcBN composites were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy spectrum analysis. The results show that, under the conditions of low reaction temperature of 1400°C and holding time of 10 min, excellent composite properties of PcBN sintered material were obtained. The reinforced phases such as Ti(C, N), TiB₂ and AlN were stably presented in the sintered body. To be specific, the microhardness, flexural strength and fracture toughness of the sintered body first increased and then decreased with the increasing sintering temperature, as well as posessing a high fracture toughness, which were 39.74 GPa, 1217 MPa and 8.78 MPa m^1/2, respectively.

We determined the thermal conductivity of the dense structure of reaction-sintered silicon carbide, a ceramic material obtained by impregnating the SiC framework with silicon, followed by liquid-phase carbidization. The resulting SiC material has a thermal conductivity of 177 W/(m K), which is 36% of the theoretical value.

The structure, mechanical properties (hardness H and elastic modulus E), evaluation parameters for determining the resistance of the material to elastic (H/E) and plastic (H³/E²) deformations, resistance 1/(E²/H) to abrasive wear, and the ability of the hard alloy matrix to hold diamond grains from their precipitation in the 25 wt % C_diamond‒70.5 wt % WC‒4.5 wt % Co composite diamond-containing materials (CDMs) with different contents of ZrO₂ (in the range from 0 to 10 wt %), which are formed by the spark plasma sintering (SPS) method in the temperature range of 20‒1350°C under a pressure of 30 MPa for 3 min. The initial CDMs have a coarse grained structure with weak adhesion of diamond grains to the hard alloy matrix and are characterized by low values of H/E, H³/E², and 1/(E²H), which is the reason for the premature loss of diamond grains. The presence of ZrO₂ in the composition of CDMs interferes with the processes of Oswald ripening and acts as a growth inhibitor that ensures the reduction of WC grains, the formation of strong adhesion of diamond grains to the matrix, and a substantial increase in the H/E, H³/E², and 1/(E²H) parameters. As a result, the ability of the matrix to reliably hold diamond grains from premature loss during the CDMs operation increases. It is shown that the addition of zirconium dioxide in the amount of 10 wt % to the composition of the 25 wt % C_diamond‒70.5 wt % WC‒4.5 wt % Co composite leads to increases in the H/E parameter from 0.043 to 0.057, in the H³/E² parameter from 0.05 to 0.075 GPa, and in the 1/(E²H) parameter from 0.75 × 10^–7 to 2.75 × 10^–7 GPa^–3. Moreover, signs of strong adhesion between the diamond grains and the carbide matrix in the CDMs samples are revealed.

As found from the results of studying the mechanism of polishing precision surfaces of nonmetallic parts by dispersion systems based on micro- and nanoparticles of polishing powders, the formation of sludge particles, a polishing powder, and grinding particles is a consequence of the Förster resonant energy transfer (FRET). This occurs in the resonator formed by two parallel surfaces of the work piece and the polishing paste. It is shown that the productivity of polishing of nonmetallic materials increases nonlinearly with an increase in the quality factor of the resonator, while the roughness of the treated surfaces deteriorates. The relationship between the quality factor and the optical length of the resonator is established, and it is shown that the maximum values of the removal rate of the treated material, the intensity of wear of the polishing powder, and the wear of the grinding surface are achieved at a certain optical length of the resonator. It is equal to the number of half waves, which is characteristic of nanoparticles of the sludge, polishing powder, and grinding material. It is shown that the productivity of polishing the parts made of nonmetallic materials and the roughness of their polished surfaces are extremely dependent on the dielectric constant diffrenece between the processed material, the polishing powder, and the dispersion system.

Using the technology of vacuum compression sintering, which makes it possible to obtain hard alloys with a high level of mechanical and operational properties, the influence of gas pressure growth rates of 0.08, 0.2, and 0.5 MPa/min to 5 MPa during vacuum compression sintering on the structure and properties of the VK6M fine grained hard alloy is investigated. A positive effect of increasing the rate of gas pressure growth on the ultimate flexural strength and operational stability due to obtaining a structure with finer graines (d_WC = 1.24 μm), reducing the residual microporosity to A1 0.02, increasing the shape factor of carbide grains to 0.84, and decreasing the intensity of gas desorption by a factor of 2.5–7.0 is established. The ultimate flexural strength of the alloy sintered at a gas pressure growth rate of 0.5 MPa/min is 1.35 times higher compared to that of the alloy sintered at a gas pressure growth rate of 0.08 MPa/min and 1.5 times higher compared to that of the alloy sintered in a vacuum, while the relative operational stability against rough milling in the first case is higher by 1.35 and 1.8 times compared to those in the corresponding other two cases, respectively.

New data on the wetting of superhard materials based on dense BN polymorphs with braze alloy metallic Zr–Ti–Ni, Ti–Zr–Cu–Ni, and Cu–Sn–Ti melts by the sessile drop method within a temperature range of 900–1000°C were reported. Contact angles were varied from 5° to 25° depending on the active component (Zr, Ti) in the composition of a soler. The metallic melt/BN contact interface was studied. The formation of Zr and Ti nitrides and borides at this interface was established. The soldered joints of cBN specimens with each other were manufactured, and the physicomechanical characteristics of soldered joints under shear and bending were studied.