G. S. Oleinik

The effect of the methods of recovering diamond nanopowders on their physicochemical properties

The effect has been considered of the methods of recovering (liquid oxidation of nondiamond carbon with a mixture of sulphuric and chromic acids, selective gas oxidation of nondiamond carbon in the presence of copper salts as a catalyst, and oxidation of nondiamond carbon with a mixture of concentrated sulfuric and nitric acids) diamond nanopowders from detonation-synthesized diamond-carbon products on physicochemical properties of diamond, for which the structural state of the diamond surface is responsible. It has been found that diamond nanopowders recovered using different technologies exhibit a decrease of the volume fraction of aggregates among particles. Nanodiamonds recovered by a catalytic gas-phase oxidation in the presence of copper have been found to be characterized by the highest adsorption activity, thermostability, and electroconductivity due to the carbon-containing cover of a complex composition. The recommendations have been given on the use of diamond nanopowders recovered by different technologies.

Structural transformations in the formation of superhard materials based on the wurtzitic boron nitride initial powders

Mechanisms of structural transformations, which occur in sintering materials produced on the basis of wurtzitic boron nitride at high pressures and temperatures, have been considered. It has been shown that depending on the sintering conditions, several types of microstructures, which differ in BN polymorphous modifications content, grain size and substructure, and structural state of interfaces, may be formed in the material. It has been concluded that a combination of high hardness (60–80 GPa) and fracture toughness (14–22 MPa·m1/2) of the wBN-based material (the trade mark is hexanit-R) is realized in the formation of a microstructure with a high portion of grains having a particular structural state that is responsible for the presence of intragranular interfaces, interphase boundaries of the composite type, and for high stresses caused by these boundaries.

The role of plastic deformation in compaction and decompaction processes in sintering materials based on wurtzitic boron nitride

The evolution of the microstructure of wurtzitic boron nitride-based polycrystals with sintering time (τ) increasing from 15 to 240 s at p = 7.7 GPa and T = 1800°C has been TEM examined. It has been concluded that the decisive role in compaction below 90 s has been played by the plastic deformation of initial particles caused by the basal slip and irregular, i.e. non-crystallographic, rotation. A dispersion of particles into disoriented fragments without distortion of their continuity, which defines their further deformation due to slipping along the fragment boundaries, is promoted by the deformation via the irregular rotation. The strengthening of the orientation factor for the development of basal slip and, hence, the activation of the wBN transition into the cubic phase is facilitated by the fragment disorientation. A decompaction of polycrystals in sintering for τ > 90 s is caused by the evolution of plastic deformation via the creep initiated by the formation of areas of the microstructure of a material based on the perfect singlephased (cBN) grains of sizes below 1 μm with well-developed (with no relief) boundaries between them. The creep process is assumed to proceed owing to intergranular slipping combined with diffusion atom-by-atom mass transfer in grain boundaries.

Mechanism of the rBN → hBN transformation in Nano-and Microtubes

The data first obtained by the authors on the transformation (at 1650 K) of the boron nitride rhombohedral phase into the hexagonal one in cut tubes have been discussed. The transformation occurs with the participation of basal shears and the formation of the intermediate structures in the form of multilayer polytypes. It has been concluded that the development of the transformation is caused by the relaxation of stresses in the tubes, which generate in them because of the anisotropy of thermal expansion and facilitate the development of plastic shear along the (0001) plane in rBN.

On the mechanism of forming nanosized particles of diamond detonation synthesized from explosive decomposition products

A brief generalization of the known data on the structural state of detonation_synthesized ultradispersed diamond (UDD) and the mechanism of its formation have been discussed. The data of our electron microscopy studies of UDD have been considered and on their basis the structural “marks” (like crystal geometry, sizes of independent particles, the nature of their organization and consolidation in flaky aggregates, etc.) have been revealed, which are diagnostic indications of the UDD formation from graphite by martensitic mechanism. A possible mechanism of the formation of graphite as a primary phase in the detonation synthesis of UDD has been suggested.

Influence of the wurtzitic boron nitride initial structural state on the formation of the granular structure of the wBN-Based materials. II. Structural transformations in the microstructure formation of the samples of materials based on various types of wBN

The studies of the microstructures of polycrystals produced of five varieties of wurtzitic boron nitride powders at p = 8 GPa in a range of 1600–1800°C have been considered. Particles-platelets of powders differed in size of the developed surface, thickness, and substructure. It has been found that the basic factors responsible for the granular structure and phase composition of polycrystals are the thickness of the initial particles and orientation caused by the formation of the morphological texture in the course of the compaction of the particles-platelets system in sintering. The following structural transformations in particles of wurtzitic boron nitride in sintering: dispersion of the initial particles in mechanical fracture by spalling, plastic fragmentation of them, and as a result the development of slip processes of disoriented fragments at the interface have been affected by the above factors. The maximum grain size and the lowest degree of the phase transformation of the wurtzitic boron phase into the cubic one has been observed in polycrystals based on powders with particles of the highest thickness. And vice versa, these characteristics have the minimum and maximum values, respectively, in polycrystals based on powders with particles of the lowest thickness.

Influence of the Wurtzitic Boron Nitride Initial Structural State on the Formation of the Granular Structure of the wBN Based Materials. I. Structural Characteristics of Particles of the Initial wBN Powders

The results of the studies of the initial structure of wurtzitic boron nitride particles produced from graphite-like BN in shock waves with and without addition of water have been discussed. The structure of wBN particles produced with an addition of water after the deformation processing by high pressure (3 GPa) at room temperature and rolling with a shear has also been considered. It has been shown that the size in the prismatic section of wBN particles synthesized with an addition of water is always larger than that of wBN particles synthesized without additions. The high pressure-room temperature processing of the wBN powder produced with a water addition has been shown to contribute to the grinding particles mainly in the developed (basal) surface. During the rolling of such a powder the particles have been reduced both in the developed surface and through the thickness, and the substructure has been transformed due to plastic deformations (lattice, rotational, by twinning).

Structural transformations in the HP-HT formation of monophase superhard materials based on carbon and boron nitride dense phases

The known data on the mechanisms of structural transformations in the formation of bulk monophase materials based on diamond and cubic boron nitride at high pressures and temperatures have been briefly summarized. Two groups of materials have been defined: those based on the initial micron powders of diamond, cBN, and graphite-like BN as powder and pyrolytic deposits. The structure formation of the first group materials has been shown to be governed by the development of the lattice plastic deformations (by translational slip and twinning) and rotational deformation, while the structure formation of the cBN-based materials is governed by the primary recrystallization occurring at T > 1900°C and p = 7.7 GPa. The rotational deformation governs a plastic fragmentation of grains in materials. The formation of materials based on the initial graphite-like BN has been found to depend on the crystal-oriented phase transformation into BN dense (wurtzitic and sphaleritic) phases and recrystallization of the forming cBN phase. The mechanisms of the cBN primary recrystallization have been discussed.

Structural transformations of the wurtzitic boron nitride-titanium nitride composition at high pressure and temperature

The results of the electron microscopy and XRD examination of structural transformations of the wurtzitic boron nitride-titanium nitride powder composite obtained by plasmachemical synthesis under a pressure of 8 GPa and temperature of 1100–1700°C have been considered. The interaction in this system has been shown to yield Ti-B-N solid solutions. It has been found that at 1300°C continuous BN-TiN interfaces form and at T ≥ 1500°C a monolithic layer, which is a nanodispersed TiN-TiB2 composite, is formed between nitride boron grains.

On manufacturing self-bonded diamond

AstractThe possibility has been shown to obtain a self-bonded diamond material by the high-pressure sintering of statically synthesized diamond micron powders and detonation-synthesized diamond nanopowders.