Dr. Tetiana Prikhna

Abstract:

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).