Journal of Superhard Materials最新文献

In this study, the formation conditions of TiB₂, whose main production method is carbothermal reduction, at lower temperatures with additional processes (vacuum, mechanical activation and functional addition effect) were investigated. In this study, firstly the 100% stoichiometric mixture was reduced at 1200°C under vacuum for varying experimental durations. Afterwards, the effect of mechanical activation amount on TiB₂ formation was determined by keeping the 4-h reduction time, which was determined as optimum in the first set of experiments. At the last stage of the experiments, the addition of 2.5 and 5% NaCl effect on the mixtures in the first set of experiments was determined. The samples obtained after the experiment were characterized by XRD and SEM techniques, and it was observed that the undesired phases were completely removed as a result of the reduction performed after 16 h of ball mile operation, and this experiment was determined as the optimum experimental condition.

Crystal chemistry analysis and accurate calculations of the mechanical properties of the recently proposed cubic diamondlike BC₂N indicate that this hypothetical phase is not related to the experimentally synthesized cubic BC₂N.

The resonant frequencies of natural oscillations in disk-shaped specimens are determined as a function of the elasticity modulus and dimensions of a specimen. The effect produced by the specimen fastening place on the precision of measuring the elasticity modulus and the decrement of oscillation in the series of cubic boron nitride/diamond polycrystal specimens are estimated. All the considered variants of sintered polycrystals can be classified into three groups depending on the diamond powder content. The properties (elasticity modulus, oscillation decrement, sound velocity, Q factor) are different for each of these groups of polycrystals.

Hexagonal carbon allotropes C₆, C₉, and C₁₂ with qtz, sta and lon topologies, respectively, were predicted on the basis of crystal chemistry and first principles (DFT) calculations. The new allotropes are mechanically (elastic properties) and dynamically (phonons) stable phases and are characterized by ultra-high Vickers hardness, exceptionally high for qtz C₆ and C₁₂, close to the previously studied qtz C₃. The electronic band structures of all new allotropes show semi-conducting to insulating behavior. lon C₁₂ can be considered as novel “superlonsdaleite.”

ZrO₂–Y₂O₃ composites with different content of Al₂O₃–Yb₂O₃ additives were fabricated by spark plasma sintering (SPS). The effect of Al₂O₃ and Yb₂O₃ additives on the mechanical performance, phase transformation, thermal shock resistance and microstructure of ZrO₂–Y₂O₃ composites was researched. The addition of Al₂O₃ and Yb₂O₃ contributed to enhancing the phase transformation and densification of ZrO₂–Y₂O₃ composites fabricated by SPS. ZrO₂–Y₂O₃ composites possessed the optimum comprehensive performances as the content of Al₂O₃ and Yb₂O₃ is 10 and 4 wt %, respectively. The thermal shock resistance (TSR), hardness, toughness, strength and relative density could achieved of 344°C, 13.6 GPa, 5.7 MPa m^1/2, 900 MPa, and 99.5%. The combination of refinement of grain size, changing of fracture mode, enhancement of density, phase transformation contributed to the improvement in TSR and mechanical performances of ZrO₂–Y₂O₃ composites fabricated by SPS.

The results of an experimental study of the effect of the processing mode on the wear of the diamond wheel surface and the accuracy of the ball shapes are described for diamond grinding of ceramic balls made of silicon nitride with a circular feed. The angle of inclination of the working surface in the radial direction, the curvature coefficient, and the accuracy of the shape of the polished balls, i.e., the instability of the ball diameter and the form factor of the round profilogram, are taken as wear indices of the wheel surface. The observed effects are described by adequate linear dependences of the angle of inclination and the curvature coefficient on the frequency of feeding the workpieces, the instability of the ball diameter on the wheel rotation frequency, and the form factor on both parameters. Combinations of the processing mode parameters within the studied range of changes are determined, which served as a basis for the subsequent optimization of the indicated parameter values.