Vol 41, No 6 (2019)

Using the example of cyclic twins of CVD diamond, their pseudo-pentagonal and icosahedral forms have been described. It is shown that their general appearance is determined by simple habit forms of crystals (octahedron and cube) in the case of cyclic twinning in accordance with the spinel law on (111) plane. The varieties of cyclic twins are identified. A topographic characteristic of cube and octahedron faces of these complex twins is provided. Models for the formation of pseudo-pentagonal and icosahedral forms of diamond crystals are proposed.

The processes of structure formation, mechanical and tribological properties under vacuum hot pressing of a blend of powders of iron, copper, nickel and tin with a dispersion of 5–50 µm with the addition of a nanopowder of vanadium nitride depending on temperature are described. It was found that an increase in sintering temperature from 800 to 1000°C leads to grinding of ferrite grains to 20–400 nm, the release of primary (d = 10–100 nm) and secondary (d ≤ 10 nm) VN grains and an increase in hardness from 3.75 to 5.37 GPa and H/E parameters from 0.021 to 0.043 and H³/E² from 1.70 to 9.91 MPa, a decrease in the elastic modulus from 176 to 125 GPa and a friction force from 250 to 180 mN, a decrease in speed wear from 8.15 × 10⁻⁴ to 1.79 × 10⁻⁴ mm³·N⁻¹·m⁻¹. The mechanism of grain refinement and the improvement of physical and mechanical properties is due to α → γ → α transformations occurring under conditions of VN dissolution in α-Fe and subsequent cooling.

The paper addresses the problem of isolating single particles of detonation nanodiamonds from an ensemble of particles. The problem is solved through the use of the Langmuir-Blodgett technique. Particular attention is paid to the process underlying the formation of isolated particles of detonation nanodiamonds. The characteristics of the isolated particles are given.

In this paper, we investigate the elastic properties, bond stiffness, hardness and Debye temperatures for hexagonal P6₃/mmc WX₂ (X = B or N). It is observed that these two compounds are stable in mechanics. Both these two have three typical bonds, W-X bonds, X-X and W-W bonds. By investigating the bond stiffness of these three types of bonds, we found that the bulk modulus of WX₂ is mainly determined by W-X and W-W bonds, while the shear modulus is mainly determined by X-X bonds. In addition, using a theoretical model, we evaluate the hardness of these two compounds. Results showed that the Vickers hardness of WN₂ is much lower than that of WB₂. What’s more, by calculating the Debye temperatures, we found the melting point of WN₂ is much lower than WB₂, and the overall chemical bonds in WB₂ are stronger than that of WN₂.

The paper addresses the influence of grinding debris solid particles on the formation of a machined surface roughness, which is reflected on the parameter Rmax. A special feature of the dependence of the surface roughness ratio Rmax/Ra on the grinding cost parameters has been revealed. The probability of scratch formation on the workpiece surface in grinding has been theoretically assessed.