V. F. Britun

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).

The influence of the shock compression conditions on the graphite transformations into lonsdaleite and diamond

Shock compression-induced phase transformations of graphite into carbon dense modifications in cylindrical recovery containers (pshc = 20–36 GPa, Tshc = 1800–3500 K) have been studied. The dependences of the lonsdaleite and diamond yields on the compression conditions have been established. The results obtained have been analyzed taking into account the formation mechanisms of the dense phases and special features of their shock-wave synthesis.

Structure and Properties of Wear-Resistant Spark-Deposited Coatings Produced with a Titanium Carbide Alloy Anode

The paper examines the phase and structure formation during hot pressing of titanium carbide electrode materials as well as the structurization and properties of spark-deposited coatings on steel substrates. The influence of the operating current used for erosion processing on the phase composition and hardness of the coatings is established. It is shown that the electrode materials with a certain composition and structure enable the electrospark deposition of 100 μm-thick coatings with hardness to 14 GPa. The mass of the material deposited on a steel substrate is three times higher than that formed using standard titanium carbide alloy TN20. The wear resistance of the coatings in abrasive and dry sliding friction is high, hence they may be recommended for protection of steel parts for severe wear applications.

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.

Phase transformations of carbon-black in high-temperature shock compression

The carbon-black transformations into diamond and amorphous carbon phase having an intermediate density of 2.9 g/cm3 in high-temperature shock compression at 20–32 GPa and 2500–3500 K have been studied. The conditions of compression that ensure the maximum yield of these phases have been defined. The transformation regularities have been analyzed under the assumption that the amorphous phase is an intermediate structure on the way to the transformation of turbostratic carbon into diamond.

Phase composition and structure of composite powders based on solid solutions of SiC and AlN

The structure of SiC–AlN powders is investigated by x-ray diffraction and transmission electron microscopy methods. The powders were produced by joint carbon reduction and nitriding of silicon and aluminum oxide mixtures. The results show that a mixture of solid solutions forms during joint SiC and AlN synthesis at 1700°C, with SiC forming β (3C) and α (2H) modifications with different grain morphology. The fiber form is characteristic of β-SiC, whereas the grains of the solid solution based on SiC have a predominantly equiaxed form. α-SiC grain dimensions are considerablys smaller than those of AlN.

Structure of polycrystals produced by sintering nanocrystalline powders of cubic and wurtzitic boron nitrides

The structure and some properties of polycrystals produced by sintering nanocrystalline powders of the dense modifications of shock—wave-synthesized BN have been studied. The sintering was conducted at a static pressure of 7.7 GPa and temperatures from 1100 to 1800° C. The highest density (3 g/cm3) and microhardness (up to 20 GPa) have been exhibited by polycrystals produced by sintering the powder containing wurtzitic and cubic modifications in amounts that are approximately equal. In the temperature range from 1100 to 1300° C the wurtzitic phase transformed into the cubic one. In this temperature range the average size of cBN grains changed from 20 to 50 nm. The structure of compacts is characterized by the presence of grain (grain-boundary) interlayers 2–5 nm in thickness.

Physical principles of shock-wave synthesis of superhard phases and their structure

This paper reviews work carried out at the Institute for Problems of Materials Science, Ukraine National Academy of Sciences, in developing physical bases for shock-wave synthesis of superhard phases of carbon (diamond, lonsdaleite) and boron nitride (wurtzite and sphalerite modifications). The effect of phase transformation mechanisms on structure features of the phases that are obtained under shock compression conditions is considered.

Nonisothermal Pressure Sintering Kinetics of the B4C–SiC Powder Mixture, Structure and Fracture Behavior of Sintered Composite

The kinetics of nonisothermal pressure sintering of boron carbide powder mixed with 20 wt.% silicon carbide in the controlled heating mode is studied. The isothermal sintering kinetics of the mixture at temperature of 2240 K under applied pressures of 36.1, 49.6, 63.2, and 72.2 MPa was analyzed to determine the Laplace pressure. It is found that the kinetics is controlled by steady-state creep mechanism in the matrix forming the porous body, with the viscous flow rate being proportional to the square of stress. The relatively low value of the evaluated Laplace pressure (5.6 MPa) explains the difficulties in producing boron carbide composites with pressureless sintering. The current values of temperature and height of the samples during pressure sintering were used to determine the heating rate and the temperature derivatives of relative density, which enabled to describe the pressure sintering kinetics in the terms of the theory of bulk viscous flow of the porous bodies in a die. The evaluated activation energy of the intermediate and late stages of pressure sintering of the composite for different heating rates ranges from 670 to 710 kJ/mol. These values indicate that the sintering kinetics is controlled by dislocation climb mechanism. The structure and fracture behavior of the sintered samples are shown to depend on the heating rate. The higher the heating rate during B4C–20% SiC sintering, the more heterogeneous is the distribution of powder components and the larger the portion of transcrystalline fracture of sintered samples.

Effect of the Structural State of Graphitic Materials on Their Phase Transformations under Shock Compression

The effect of structural disordering of graphitic materials on their phase transformations into dense modifications of carbon under shock compression conditions (Pshock = 30 GPa, Tshock = 3000 K) is studied. It is shown that a lower degree of three-dimensional ordering of initial structure (P3) first decreases and then increases the overall yield of dense phases, reaching the maximum at P3 = 0. The content of lonsdaleite simultaneously decreases and that of the dense amorphous phase (Cam) increases. The results obtained are attributed to gradual change of the predominant martensite transformation mechanism to predominant diffusion-controlled mechanism, and also to the fact that the metastable phases (lonsdaleite and Cam) that form at the initial transformation stage partly transform into a stable diamond phase.