I. A. Petrusha

Structural mechanisms of rhombohedral BN transformations into diamond-like phases

Abstract Structural features and mechanisms of transformation of graphite-like rhombohedral boron nitride modification (rBN) into the diamond-like wurtzite (wBN) and zinc blende (zBN) types have been studied. The puckering mechanism of the rBN→zBN transformation in shock waves was first proved structurally. It has been found that in static compression of CVD rBN (8 GPa, 20–2600 °C), transformations occur in the following sequence: rBN→(S,T)→Ig→wBN→Id→cBN, where (S,T) are structures deformed via basal shears or twinning, and Ig and Id are intermediate graphite- and diamond-like structures, respectively. The structure of wBN formed from rBN has been shown to be much more disordered than that of wBN obtained from the BN hexagonal modification (hBN).

Features of a cBN-to-graphite-like BN phase transformation under pressure

Abstract Phase transformations of cubic boron nitride to graphite-like forms (cBN→gBN) have been studied within the 1670–2970 K temperature range and at high pressures, which suppresses the dissociation decomposition of the compound, by using a recessed-type high pressure apparatus with a toroid-type lock. To clarify the degree of structure imperfection effect on the type and mechanism of transformation, various types of bulk cBN initial materials were used. A partial or complete cBN→gBN transformation has been observed in the studied temperature range. It has been found that in a pure recrystallized material with a low level of the structural imperfection, a two-stage transformation occurs to form an intermediate metastable rhombohedral phase (rBN). Transformation into the hexagonal modification of graphite-like boron nitride (hBN) is realized only in material with a highly imperfect structure. From the analysis of the crystallographic correspondence between cBN, rBN and hBN lattices, and the differences in structure between the initial and forming materials, the possible structural mechanisms of the cBN→gBN transformation were proposed.

Production of the Y3Al5O12 transparent nanostructured ceramics

The nanocrystalline Y3Al5O12 ceramics with different phase compositions, microstructures, and optical characteristics have been prepared by low-temperature consolidation of the Y3Al5O12 powder at pressures from 6 to 7.7 GPa in the temperature range from 250 to 550°C. The conditions have been defined for obtaining transparent nanostructured ceramics Y3Al5O12 having grains of size 20–40 nm and a transmission coefficient in the visible region of 40–45%. The criteria for the transparency of the nanostructured ceramics have been formulated and the ways of an improvement of its optical characteristics are discussed.

Wurtzitic boron nitride thermal stability and transformation into the rhombohedral boron nitride modification when heated

Abstract Samples of wurtzitic boron nitride (wBN) were obtained from the rhombohedral BN (rBN) modification as a result of phase transformation under high pressures. Structure features and kinetics of wBN→graphite-like BN transformation in heating have been investigated. The nature of the low thermal stability of wBN is discussed.

Theoretical shear strength and the onset of plasticity in nanodeformation of cubic boron nitride

The nanoindentation in the continuous stiffness measurement mode was used to investigate the onset of plasticity at the nanodeformation of cubic boron nitride (cBN). This technique allows us to reveal an elastic-plastic transition in the contact and to measure the yield strength of cBN at the nanoscale. An abrupt elastoplastic transition (a pop-in) was observed in the (111) cBN single crystal as a result of a homogeneous or heterogeneous nucleation of dislocations in the previously dislocations-free region under the contact. The analysis of the data obtained at the homogeneous nucleation of dislocations in the contact region made it possible to experimentally estimate the theoretical shear strength of cBN and its ideal (elastic) hardness. In a sample of the fine-grained cBN with a nanotwinned substructure a smooth elastoplastic transition was observed in consequence of the propagation and multiplication of already existing dislocations in the contact region.

Dynamic phenomena in finish turning of hardened steels with cBN-based tools

The paper presents some findings of the investigation of finish turning of KhVG hardened steel (60–62 HRC) using a cutting tool with an insert made of a cubic boron nitride based composite (cBN-Si3N4 system). The influence of machining process variables on the cutting force components, vibrations, and machined surface roughness is clarified. The authors propose some practical recommendations of how to choose machining modes and conditions.

Preventive action of silicon nitride at HT-HP sintering of cubic boron nitride

Special features of sintering cBN at the pressure ~ 7.7 GPa and temperature ~ 2300°C with addition of Si3N4 (~3 wt %) have been studied. It has been found that the Si3N4 addition makes it possible to block the penetrating percolation of melts from the outer medium, which results in an intergranular destruction. A material with an extremely high density (3.46 ± 0.02 g/cm3) has been developed for a wide application in cutting tools, including tools to machine hard alloys of the WC–Co group. The mechanism of the preventive protective action of Si3N4 has been discussed.

Phase Diagram and Diamond Synthesis in the Aluminum—Carbon System at a Pressure of 8 GPa

Diamond crystallization in the Al—C system under high static pressure and temperatures has been investigated and diamond synthesis regularities have been established. Phase equilibria in the binary Al—C system at a pressure of 8 GPa have been studied using metallographic and X-ray diffraction analyses as well X-ray spectrum microanalysis. The samples were prepared by quenching. The experimental results have been used to define the unknown parameters in phenomenological models of the phases that compete at high pressures. The phase diagram of the Al—C system has been thermodynamically calculated and constructed at 8 GPa. It has been found that at a pressure of 8 GPa the incongruent mode of the melting of the Al4C3 carbide is retained and between 2470 and 2800 K the L + D two-phase region appears in the phase diagram.

The Influence of VC–Al Additive on Wear Resistance of cBN-based Composites

The paper addresses wear resistance of cBN–VC–Al cutting tool material under the conditions of high-speed continuous turning of AISI 316L stainless steel.

Composites of the cBN–Si3N4 system reinforced by SiCw for turning tools

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·m1/2, 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%.