Journal of Superhard Materials最新文献

We studied the plastic mode of processing brittle materials, the effect of the change in the working surface of the diamond wheel during processing, and ways to achieve such a mode. To assess adequately the energy intensity of processing with diamond wheels, we proposed to calculate the specific energy density of grinding, considering the volume of material spent during the processing of the wheel working layer. We derived an equation for calculating the specific energy density during the diamond grinding of ceramics. The plastic regime occurs precisely when the specific energy density of grinding becomes close to the specific heat of melting of ceramic materials.

The modern generation of instrumental composite materials with a fundamentally new mechanism of contact interaction with processed materials was created through the use of binder polymers with embedded metallocene fragments. The structure of hybrid oligomers obtained by the interaction of oligophenylene and complex compounds of vanadium, iron, and copper was determined; it is a combination of organic and inorganic fragments. Inorganic fragments are represented by metal ions and clusters, bonded by covalent and coordination bonds with organic fragments. Organic fragments have a metallocene structure connected by methylene sites with weakly conjugated benzene rings. Clusters and metal ions can be located both between the planes of metallocenes and weakly conjugated rings and between oligomer chains as their continuation.

In this study, composite nanoparticles of B₄C–TiB₂ were produced by combustion synthesis. Production was carried out by self-propagating high-temperature synthesis (SHS) method in atmospheric conditions by using oxide raw materials (B₂O₃, TiO₂), carbon black and magnesium as a reducing agent. The effect of Mg particle size on SHS efficiency was investigated. Single-stage and 2‑stage leaching processes were carried out to remove undesired phases in the SHS product. In the 1st HCl acid leaching process, the leaching temperature and leaching duration were optimized. As a result of the 2nd leaching process with the addition of carbonic acid and H₂O₂, commercial quality nanoparticle synthesis was performed. Results revealed that the increase in Mg particle size decreased the SHS efficiency, however very fine particle sized Mg usage decreased the SHS efficiency due to the evaporation and scatter of Mg. The optimum Mg particle size was determined as 75–150 µm. Since it has a significant effect on the removal of Mg-borate phases, 90°C was determined as the optimum leaching temperature. The optimum leaching duration was determined to be 60 min. As a result of optimized leaching processes, 99.11% purity B₄C–TiB₂ nanoparticle with 193.5 nm particle size and 30.65 m²/g surface area was synthesized.

As established from the results of studying the mechanism of nanorelief formation of the treated surface during polishing of polymer optical materials by means of dispersed systems based on micro- and nanoparticle polishing powders, roughness parameters Ra, Rq, and R_max increase linearly with an increase in the size of the sludge particles and decrease with an increase in the transfer energy. It is shown that they substantially increase with a decrease in the spectral separation between the processed material and the polishing powder particle and are extremely dependent on the dielectric constant differences between the processed material, the polishing powder, and the dispersed system. It is found that the roughness parameters of the treated surface decrease exponentially with an increase in the frequency index of Förster resonance energy transfer (FRET) efficiency and increase linearly with an increase in the time index of FRET efficiency. In the case of an increase in the Q factor of the resonator formed by the surfaces of the processed material and the polishing tool, the roughness parameters of the polished surfaces of parts made of polymeric optical materials increase linearly.

Two continuum models are proposed for predicting stress fields and elastic properties of nanosized diamond monocrystals. The first model is a boundary value problem of the elasticity theory formulated for a sphere with a thin shell, which ensures consideration of the effect of the free surface energy on the elastic behavior of a diamond nanocrystal. In the second model, the surface energy is taken into account through the boundary condition in accordance with the Gurtin–Murdoch theory of material surfaces. The geometric and material parameters of the models are determined by comparison with the results of classical molecular dynamics. A parametric analysis of the developed models is carried out, and the trends in the influence of the nanoparticle size on the lattice parameter, stress concentration, and bulk elastic modulus of diamond are determined.

The methodological details of the indirect analytical determination of the thickness of the diamond powder coating, which is an important characteristic of the latter, are analyzed. The key role of the method used for matching the actual geometric parameters of the grain to the parameters of its adopted 3D model as a factor that has an effect on the error in determining the thickness of the coating is proven. The results of a comparative study of the effect of the method of matching the actual geometric parameters of the test grain (cuboctahedron) to the parameters of its accepted 3D models (sphere, cube, and ellipsoid) on the thickness of the coating and the error of its determination are given.

The effect of the conditions of synthesis on the formation of micro- and nanoparticles of zirconium dioxide from a fluoride solution has been studied. It is shown that the finest particles are formed by precipitation from dilute solutions with a zirconium concentration of 0.02–0.04 mol/L and impurities of polyvinyl alcohol at a mass ratio of m_Zr : m_PVC 1 : 0.1. To sinter ZrO₂ nanoparticles, it is proposed to use the electroconsolidation method in a vacuum to obtain ceramics with high values of the hardness and elastic modulus.

First-principles calculations within the framework of density-functional theory (DFT) are implemented to investigate the high-pressure behavior of ultrahigh high-pressure phases of zirconia (ZrO₂) and hafnia (HfO₂) compounds. We have studied the phase relations among the highest-pressure phases of these dioxides: The previously observed OII (cotunnite) phase, Fe₂P-type phase, and the recently predicted Ni₂In-type phase. Our calculations, using the generalized gradient approximation (GGA), predict unusual phase transition of OII phase with respect to Fe₂P phase. In both dioxides, our enthalpy calculations show that OII phase transforms to Fe₂P phase at 96 GPa (122 GPa) for ZrO₂ (HfO₂), where Fe₂P phase remains stable up to 254 GPa (310 GPa) in ZrO₂ (HfO₂) before it transforms back to OII phase, indicating a reentrant transition behavior of OII phase. Our calculations show that OII → Fe₂P and Fe₂P → OII transitions are associated with a slight change in both volume and enthalpy. Consequently, we have concluded that the transition to Ni₂In phase likely occurs from OII phase rather than Fe₂P phase, and thus we provide an updated high-pressure phase transition sequence for zirconia and hafnia at such extreme pressures. The OII → Ni₂In transition is predicted to occur at 302 and 372 GPa in zirconia and hafnia, respectively. Furthermore, to obtain a deeper insight into the mechanism of the phase transitions in ZrO₂ and HfO₂, the effect of the components of the enthalpy difference across our predicted phase transitions has been thoroughly investigated.

Thermobaric formation of carbon solvent alloys under high pressures of 4–6 GPa and at temperatures of 1200–1300°C is used to grow structurally perfect diamond single crystals in the region of thermodynamic stability. It is shown that the formation of samples with minimum values of porosity in the range from 2.49 to 4.50% ensures a stable distribution of elements over the growth volume even in the presence of large differences in the density of elements. For comparison, the results of experiments on the formation of solvent alloys by the methods of powder metallurgy and direct isostatic pressing with subsequent sintering and by the methods of hot pressing of powders are given. It is shown that the samples obtained by classical methods have relatively large final porosity values and cannot be used in the future as carbon solvent alloys during directed growth of diamond single crystals at high pressures and temperatures.

The current state of research on the use of CVD diamonds in diamond tools is reviewed. Features of single crystal and polycrystalline CVD diamonds and CVD diamond films are shown. Their comparative properties are given. Their structure and the peculiarities of processing their surfaces are shown. Technological features of the preparation of diamond tools with a working layer of CVD diamonds and the use of such tools are discussed. Samples of polycrystalline CVD diamonds for the dressing tool and features of their application in the dressing rollers are shown.