Journal of Superhard Materials最新文献

Melting of boron-rich chalcogenides, rhombohedral B₁₂S and B₁₂Se, has been studied at pressures up to 8 GPa using in situ electrical resistivity measurements. It was found that above 2.5 GPa both chalcogenides melt congruently. The melting curve of B₁₂Se has a negative slope of ‒30(3) K/GPa, indicating a higher density of the melt compared to the solid phase, while a very unusual zero slope of –1(2) K/GPa is observed for B₁₂S. The melting points at ambient pressure were estimated to be 2270(7) K for B₁₂S and 2250(16) K for B₁₂Se.

Ablation tests of ultrahigh-temperature HfB₂ and HfB₂–SiC based hot-pressed ceramic samples in the air under heating in the flame of a burner fed with a O₂/C₂H₂ mixture at a distance of 13 mm to the surface of a sample have demonstrated that HfB₂ ceramic with 30 wt % of SiC additive with a grain size of 30–50 and 5–10 µm and a mass loss of 0.25 mg/s is much more stable (up to 2066–2080°C) as compared to additive-free HfB₂ ceramic cracking at a temperature of 1870°C. Fracture toughness values are comparable for all the studied materials, and the hardness of HfB₂–SiC composites is slightly higher.

In this paper, Mo(Si, Al)₂-cBN superhard composite was designed and prepared successfully in lower-temperature infiltrating melt aluminum at 975°C for 20 min to overcome shortcomings in low decompose temperature and oxidation resistance, high preparing temperature of present cBN/MAX composite. XRD showed that the main phase of Mo(Si, Al)₂-cBN superhard composite were in-situ formed Mo (Si, Al)₂ and remained cBN, and the vacuum atmosphere was more advantageous for it in gaining high density and llinear shrinkage, and the relationship between molding pressure and shrinkage of this composite obeyed a parabolic form in vacuum atmosphere. The increasement of infiltrated Al was helpful to its bending strength, when Al increased to same level as reported in c-BN/Al superhard materials as 2.86 at % or 9.8 vol %, the bending strength of Mo (Si, Al)₂-cBN superhard composite reached the highest. And also, Mo(Si, Al)₂-cBN ceramic-based superhard material owned excellent wearing property which was evaluated by friction and wear experiment between W₁₈Cr₄V counter-grinding ring and prepared (Mo(Si, Al)₂-cBN ceramics block in dry friction model with a load of 49N.

For the diamond grinding of silicon nitride ceramic balls based in a groove with circular feed under oscillation, the results of experimental study on the effect of the processing regime on the accuracy of their shape and the shape of the worn surface of a diamond wheel were reported. The shape accuracy criteria were the diameter variation and out-of-roundness profile shape factor for the ground balls and the radial slope angle and curvature coefficient of the working surface profile for the worn surface. This effect was described as adequate linear functions of process parameters from the rotation speed of a diamond wheel and the feed speed of balls. The kinematic parameters of processing, at which the studied ball grinding scheme was reasonable to apply, were determined.

This study refines the calculation-experimental approach to evaluating residual stress in thin vacuum-plasma protective coatings on tools made of polycrystalline cubic boron nitride (PCBN), taking into account the thermal and structural components that arise during their formation. The stress values in a TiNbN coating were calculated using experimental data on the deformation of a model specimen—a thin steel plate—and the structural and thermally induced components of residual stress in the coating were identified. A functional relationship was found between the specimen deflection, coating deposition temperature, Young’s modulus, and residual stress in the coating. Using data obtained from model specimens, the residual stress was determined in a coating applied to a PCBN substrate of the Borsinit tool. The absolute values of the total residual stress significantly increased with coating thickness from 1 to 2.5 µm, reaching 1.9 GPa. The results demonstrate that the residual stress remains within safe limits regarding the potential cohesive failure of the coating.

The floatation separation of diamond powders, where some grains of synthetic diamond grinding powder were carried to the surface layer (“foam” product) with air bubbles, and the others diamond grains remained in the chamber of the floatation machine to be better wetted with water (“chamber” product), has been studied. It has been shown that the “foam” product diamonds, in contrast to the original and “chamber” powders, have a higher strength of grains and are more homogeneous in strength at the lowest mass content of impurities and minimum specific magnetic susceptibility. To increase the wear resistance of a diamond wheel, the most efficient is to use just the “foam” product and, notably, such efficiency grows with an increase in the machining rate. It has been revealed that the application of a tool with diamond powders after floatation separation (“foam” and “chamber” products) makes it possible to attain a lower value for the roughness parameter R_a as compared to the tools with original diamond powders. At the same time, the fullness of the ground rough surface according to such a bearing surface profile parameter as t₅₀ indicates that this parameter for different variants of applying the tools with diamond powders after floatation sorting is higher than for the tools with powders in the initial state.

This study presents a review and comparative analysis of existing approaches, methodologies, and findings related to the thermal conductivity of nanostructured solids. The authors developed theoretical models to predict the effective thermal conductivity of nanopolycrystals and nanocomposites with imperfect grain boundaries, incorporating the effects of isotopic composition, crystal size, interfacial thermal resistance, and temperature. These models rely on the current understanding of the physical mechanisms of lattice heat transfer in covalent crystals and phonon scattering at structural defects. The developed theory offers a straightforward method for estimating the thermal conductivity of nanocomposites and provides insight into the dominant factors governing heat transfer in crystalline structures at the nanoscale. Comparison with experimental data confirms the model’s validity and its applicability to the prediction of thermal conductivity in real-world nanopolycrystalline and nanocomposite materials, including those containing diamond.

The study of the mechanisms governing material removal and nanoprofile formation on polished surfaces during the polishing of optical components made of glass, semiconductors, and copper using dispersed systems composed of micro- and nanopowders reveals that the generation of slurry nanoparticles, resulting from energy transfer from abrasive particles to the processed surface, proceeds via the Förster resonance energy transfer (FRET) mechanism in the case of glass, or quantum dot-mediated FRET (QD-FRET) in the case of semiconductors and copper. Quantum dots form on these surfaces during polishing. The material removal rate decreases with increasing bonding energy in glass or with the effective bandgap width of semiconductor or copper oxide quantum dots that form on the surface. The relationship between the energy of slurry nanoparticles and their most probable size follows a linear function for K8 glass and polymethyl methacrylate (PMMA), and a parabolic function for germanium, indium antimonide, and copper. The material removal rate during the polishing of optical components made of K8 glass, PMMA, germanium, indium antimonide, and copper increases linearly with the quality factor of the microresonator and the excited-state lifetime of clusters or quantum dots on the treated surface, in accordance with general polishing trends. Surface roughness parameters R_a, R_q, R_max, and R_z, together with the material removal rate, serve as effective criteria for evaluating polishing efficiency. Theoretical predictions of the material removal rate demonstrate good agreement with experimental measurements of polishing performance for glass, semiconductor crystals, and copper, with deviations ranging from 2 to 5%.

This study presents a computational investigation into the effects of high-temperature dwell time, sintering temperature, and applied pressure on the densification kinetics and grain growth of micrometer-scale boron carbide–based powder mixtures during high-speed pressure-assisted sintering (HSPAS) at pressures ranging from 250 to 1200 MPa. The simulations employed numerical models of electric heating and densification. The densification model relies on the Skorokhod–Olevsky–Stern theory of sintering for porous materials and incorporates grain growth kinetics throughout the sintering process. The results demonstrate that by adjusting the sintering temperature, dwell time, and pressure during HSPAS, one can control the densification behavior and grain evolution. Specifically, appropriate parameter selection shortens the time required for complete densification, significantly suppresses grain growth, and yields a dense microstructure in the sintered sample. At lower-temperature HSPAS conditions, increasing pressure from 250 to 1200 MPa markedly enhances the densification rate and reduces the time for full densification by a factor of 2 to 3. Notably, by optimizing the heating rate and pressure during HSPAS, the densification time can be reduced by two to three times compared to spark plasma sintering under pressures up to 100 MPa, while simultaneously preventing grain growth.

Extending the investigation of the ‘graphite-to-diamond’ G2D-like changes of carbon crystal systems implying 2D planar trigonal C(sp²)-like paving to 3D tetrahedral sp³-like, to 3D trigonal → tetrahedral transformation, original body centered tetragonal BCT C₂₀ allotrope with dia (diamond) topology was devised from crystal chemistry rationale and demonstrated to behave like diamond for all the physical properties: cohesive, mechanical, dynamical and thermal. The investigations were based on crystal chemistry rationale and first principles investigations within the Density Functional theory with comparisons to available experimental observations. A holistic assessment of the results let assign BCT C₂₀ a “pseudo-diamond” label.