Ultrahigh-temperature, corrosion-resistant materials based on HfB₂ (melting point of HfB₂ - 3380 ^oC) have high thermal conductivity, high level of mechanical characteristics, high corrosion resistance in oxidizing atmosphere due to the ability to form protective, oxidation-resistant scales at elevated temperatures. They are promising for many ultrahigh-temperature applications, for example, for the manufacture of nozzles for aircraft and rocket engines that are in contact with aggressive gases at high temperatures, as well as for the manufacture of wing edges and fairings for supersonic aircraft, etc. It is known that the addition of SiC to HfB₂ can increase the mechanical properties of composite. The results of present investigations (obtained in the framework of III-5-23 (0786) grant from the National Academy of Sciences of Ukraine) showed that on the densification, mechanical characteristics and resistance toward ablation important role play sizes and quality of SiC initial powder used as addition. Such effect we observed both for the composites prepared under hot pressing conditions (30 MPa pressure) and conditions of high pressure (2 GPa) – high temperature. Our previous studies have shown that the use of high pressures and temperatures and hot pressing, and the addition of SiC to HfB₂ allowed us to achieve a level of mechanical properties of the resulting ceramic materials that, in terms of hardness and crack resistance, surpass the best world analogues. It was also shown that the addition of SiC significantly reduces the melting point and accelerates the oxidation kinetics upon heating. The microhardness, H_V, and fracture toughness, K_1C, (at an indentation load of 9.8 N) of the HfB₂-30 wt.% SiC(5-10 µm) composite material which was hot pressed (under 30 MPa) were H_V =38.6 ±2.5 GPa and K_1C =7.7 ±0.9 MPa m^0.5 when specific density 6.54 g/cm³ (and near zero porosity) was attained. For HfB₂-30 wt.% SiC(30-50 µm) porosity was about 17 % and H_V = 28.1 ±11.3 GPa and K_1C = 6.1 ±2.2 MPa m^0.5. Hot-pressed HfB₂ without additives exhibits H_V = 18.9 ±0.1 GPa and K_1C = 7.65 ±0.6 MPa·m^0.5, porosity 2.4% and specific density 10.79 g/cm³. Ablation tests in air of the samples of ultrahigh-temperature hot-pressed ceramics based on HfB₂ and HfB₂-SiC when heated with a gas burner (into which an O₂/C₂H₂ mixture was fed, and the distance to the sample surface was 13 mm) showed that HfB₂ ceramics with an additive of 30% by weight of SiC with a grain size of 30-50 μm and 5-10 μm turned out to be significantly more stable (up to 2066-2080 °C, respectively, at an internal mass of 0.25 mg/s) than ceramics with HfB₂ without the additive (cracked at 1870 °C). 

The sintering processes of TaB₂ and TaB₂ mixtures with 20 and 30 wt. % SiC, ZrSi₂, Si₃N₄, and MoSi₂ were investigated under hot pressing (HotP) conditions at 30 MPa, 1750-1970 °C, 0.33 - 1.0 h, and high pressure–high temperature (HP-HT) conditions at 4.1 GPa, 1800 °C, 0.33 h,, as well as TaB₂ and its mixtures with 20 and 30 wt.% SiC under spark plasma sintering (SPS) at 45 MPa, 1500-1950 °C, 0.05 h. The highest values of mechanical characteristics of single-phase TaB₂ samples were achieved after sintering by the HotP (1900 °C, 1 h) – Vickers hardness Н_V(9.8 N) = 32.4 ± 0.1 GPa (density r =11.8 g/cm³) and SPS (1950 °C, 0.05 h) - Н_V(49 N) = 20.8 ± 2.0 GPa and K_1C(49 N)= 7.6 ± 1.6 MPa•m^0.5 (r =11.75 g/cm³). A significant improvement in Young's modulus from 532 GPa to 853 GPa was achieved by adding 20 wt.% SiC and HotP at 1900, 1 h. By sintering mixtures with 30 wt.% SiC using the HP-HT and SPS methods at 1800 °C for 0.13 and 0.05 h, respectively, the following materials were obtained: with Н_V(9.8 N)= 39.4 GPa and K_1C(9.8 N)=6.75 MPa•m^0.5 (HotP) and Н_V(49 N)=25.4±2.1 GPa and K_1C(49 N)=10.8±0.8 MPa•m^0.5 (SPS). The variation in the properties of the materials upon addition of additives is explained by the formation of solid solutions due to the diffusion during sintering of the present elements and different porosity. When adding 30 wt.% SiC after HotP (1900 °C, 1 h), the approximate stoichiometric composition of the matrix phase of the sample estimated by SEM EDX was TaB₂Si_0.5O_0.06.

The results of influence of amount of SiC additives to HfB₂ and the physical chemical characteristics of the additives will be under the discussion. Studies of the resistance to ablation of hot-pressed HfB₂ and HfB₂-SiC samples heated by a gas burner showed that HfB₂ ceramics with the addition of 30 wt.% SiC with average grain sizes of 30-50 μm (powder with fragmented grains with sharp edges with approximate average stoichiometry SiC_1.6O_0.1, and  6_H SiC structure) and 5-10 μm (single-crystal grains with a hypercubic shape, close to spherical, practically free of impurities, with approximate stoichiometry SiC_1.5, b-SiC) have significantly higher thermal resistance – up to temperatures of 2766 and 2780 °C, respectively (mass loss of 0.25 mg/s) than HfB₂ ceramics without additives, samples of which cracked already at 1870 °C. The formation of a framework from SiC when 40 wt.% SiC was added resulted in decrease of resistance to ablation, of Young's modulus, and the material cracking at low temperature during heating in air. The composite made from a mixture of HfB₂ - 30 wt.% b-SiC (5-10 μm) by hot pressing under a pressure of 30 MPa, 1950 °C, 30 min. with a specific gravity of 6.54 g/cm³ demonstrated the highest Vickers microhardness H_V(9.8 N)=38.6±2.5 GPa and fracture toughness, K_1c(9.8 N)=7.7 ± 0.9 MPa m^0.5, Young's modulus 510 GPa. The additions of SiC_6H with sharp fragment grains of 1 μm in size with a lamellar or strongly elongated in one direction grains, with an approximate stoichiometry of SiC_4.6O_0.75 or 3-10 μm with an approximate stoichiometry of SiC_2.3O_0.25 added in the same amount (30 wt.%) were cracked during heating in air at a temperature of 1787 and 1455 °C, respectively.