Ultra-high temperature (UHTC) transition metal borides can be used for a wide range of mechanical applications - as components of military and commercial equipment operating in extreme conditions, for rocket propulsion, hypersonic flights, atmospheric reentry, protective coatings on graphite, for using in abrasive, erosive, corrosive and high-temperature environments, which requires materials with significantly improved physical properties. To UHTC belongs TaB₂ which exhibits high melting point (3200 °C), hardness (24.5 GPa -25.6 GPa), fracture toughness (4.5 MPa m^0.5), bending strength (555 MPa), excellent chemical stability, electrical (308×10⁴  Ω^-1×ｍ^-1)  and thermal (0.160 -0.161 W×cm^-1×K^-1 at 300-1300 ^oC) conductivity, good corrosion resistance [1-5]. To increase the oxidation resistance and mechanical characteristics TaB₂ can be modified by silicides [1]. In the present study we investigated modification of TaB₂ by MoSi₂, ZrSi₂ and Si₃N₄ in the amount of 20-30 wt.% and its sintering process under hot pressing conditions (30 MPa, 1750-1950 ^oC) and high pressure (4.1 GPa) - high temperature (1800 ^oC) conditions. The highest Vickers hardness H_V=31.5 GPa and fracture toughness K_1C=6 MPa×m^0.5 under P= 9.8 N load was obtained for the composite sintered at 30 MPа, 1750 °C, 20 min from TaB₂+20 wt.% ZrSi₂, the increase of amount of ZrSi₂ up to 30 wt.% leads to further increase in K_1C= 6.9 MPa×m^0.5, but to the reduction of microhardness  down to H_V=23.5 GPa. The composite sintered under 30 GPa at 1950 °C for 40 min from TaB₂+20 wt.% MoSi₂ showed H_V=28.2 GPa and K_1C= 5.42 MPa×m^0.5. TaB₂+30 wt.% Si₃N₄ sintered at 4.1 GPa, 1800 ^oC for 7 min had H_V=18.8 GPa and K_1C= 4.82 MPa×m^0.5. The specific weight of the materials prepared from TaB₂+20 wt.% MoSi₂ was g=10.82 g/cm³, TaB₂+30 wt.% Si₃N₄ - g=8.77 g/cm³, TaB₂+20 wt.% ZrSi₂ - g=9.35 g/cm³, TaB₂+30 wt.% ZrSi₂ - g=10.12 g/cm³.

AcknowledgementS: This work was supported by the Project of the National Academy of Sciences of Ukraine III-5-23 (0786) “Study of regularities and optimization of sintering parameters of composite materials based on refractory borides and carbides, their physical and mechanical properties in order to obtain products of complex shape for high-temperature equipment with an operating temperature of up to 2000 ^oC”