Vol 38, No 1 (2016)

The cBN–Si₃N₄–SiCw composites with different SiCw contents up to 20 vol % have been produced at high pressure of 8.0 GPa and high temperature of 2500 K. It has been defined that the Young modulus of the composites were within 740–846 GPa, the Vickers hardness and fracture toughness values were 37.5–42.0 GPa and 11.4–12.9 MPa·m^1/2, respectively. An important feature of the composite microstructure is the breaking of SiCw as a result of HPHT action. It has been shown that at the addition of 10 vol % SiCw to the structure of a cBN–Si₃N₄ composite the interrupted turning of hardened steel results in the flank wear reduction up to 20%.

Using mathematical modeling the kinetics of the rim-core structure formation of TiC grains in an isothermal liquid-phase sintering of TiC–Ni hard alloy, doped with a transition metal carbide has been investigated. The algorithm that has been used to study the nature of the concentration distribution of the diffusant in a TiC grain allows for the structural characteristics of the alloy; diffusion coefficients of alloying additives in the metal melt and a grain, boundary solubility of the diffusant in the binder, and its concentration in the alloy as well as the sintering temperature and time.

The performance of PcBN cutting tool during its application in the mass production of components made from AISI 440B stainless steel has been considered. The experimental tests have been performed at cutting speed ranging between 350–500 m/min at dry cutting conditions. The machining operations that have been explored included facing, turning, grooving and boring and the 3D topography of the machined surface are presented. The results show that good surface finish similar to grinding and dimensional accuracy can be achieved with PcBN tools.

The paper addresses the physical meaning and tendencies of variation of the abrasive cutting factor f_a = P_z/P_y as applied to the processes of grinding with superabrasive wheels. The author shows in which cases one should try to increase the f_a value. A classification of work materials by this factor is put forward.

The paper addresses the influence of cutting conditions on acoustic emission signals in fine turning of aluminum alloys. Each AE signal was split into two sections: the first one is associated mostly with the chip formation and the second one with the tool–workpiece friction. The tool materials were single crystals of natural and synthetic diamond as well as hardmetal WC–6Co. The experimental data demonstrate that in diamond turning the main signal is emitted during the chip formation, while in the case of hardmetal turning the portion of the signal emitted due to the tool flank friction is often larger and depends on the cutting conditions.

The authors compare IR spectra of diamonds of different origins, both natural and synthetic ones—produced by detonation, dynamic, and static syntheses, and subjected to purifying treatment under various conditions. It is shown that in the IR-spectral range there are absorption bands at 421, 945, 1022, 2854, 2920, and 3368 cm^–1, which are typical of the majority of the samples studied. Whatever the origin of diamonds, their IR spectra have been found to contain absorption bands of S–S, C–S bonds (upon treatment with sulfuric acid), and C–N bonds (after treatment with nitric acid). The presence of functional C–H and–OH groups has been confirmed.

Understanding of the covalent clathrate formation is a crucial point for the design of new superhard materials with intrinsic coupling of superhardness and metallic conductivity. It has been found that silicon clathrates have the archetype structures, which can serve an existent model compounds for superhard clathrate frameworks Si–B, Si–C, B–C and C with intercalated atoms (e.g., alkali metals or even halogens) that can assure the metallic properties. Here we report our in situ and ex situ studies of high-pressure formation and stability of clathrates Na₈Si₄₆ (structure I) and Na_24+xSi₁₃₆ (structure II). Experiments have been performed using standard Paris–Edinburgh cells (opposite anvils) up to 6 GPa and 1500 K. We have established that chemical interactions in the Na–Si system and transition between two structures of clathrates occur at temperatures below silicon melting. The strong sensitivity of crystallization products to the sodium concentration has been observed. A tentative diagram of clathrate transformations has been proposed. At least up to ~6 GPa, Na_24+xSi₁₃₆ (structure II) is stable at lower temperatures as compared to Na₈Si₄₆ (structure I).