Melakh L. Structure and physical and mechanical properties of ceramics based on borides and boron-containing compounds.

Thesis is devoted to the study of physical and mechanical properties of ultrahigh-temperature ceramics based on zirconium diborides and of impact resistant ceramics based on borides and boron-containing compounds depending on the structural state and strength of interphase and grain boundaries. For the purpose of analyzing the activation mechanisms of refractory compounds sintering in eutectic systems, thermal characteristics of the crystals have been studied. It is shown that at high temperatures in eutectic systems Debye temperature and the value of dynamic thermal displacements of atoms differ significantly from those in single-phase components: bonding weakens and the amplitude of the thermal vibrations of the atoms increases. This leads to an increase in near-boundary diffusion activity and has to accelerate the proceeding a number of high-temperature processes. Regularities of structure formation and mechanical properties of zirconium diboride-based ceramics were investigated. Taking into account peculiarities of the mechanism of brittle solids destruction with microcracking, method of calculating grain-boundary strength has been developed, and the relationship between grain-boundary strength and a complex of other mechanical properties of the ceramics has been analyzed. B4C-CaB6 system ceramics possess high strength and hardness values (HVP=200N = 25GPa). The calculation of B4C-CaB6 two-phase system contact strength at compression revealed its maximum value of 6.25 GPa and, consequently, the promise of these ceramics use as a material with high resistance to penetration at an impact. B4C-TiSi2 system has a high resistance to penetration upon the impact, as well, which allows to use it as impact-resistant materials. Another advantage of these ceramics is a low temperature of obtaining. W2B5-TiB2-B4C ceramics have a sufficiently high level of bending strength (~800 MPa) because of tensile stresses in the W2B5-TiB2 matrix and compressive stresses in B4C grains.