Effect of the SiC Content and Physicochemical Properties on the Heat Resistance and Mechanical Characteristics of Ultra-High-Temperature HfB2–SiC Composites Synthesized by Hot Pressing

Abstract

The effect of the content of SiC additives to HfB₂ and their physicochemical properties (grain morphology and size of SiC powders, content and composition of their impurities, 6H or β crystal structure) on the level of compaction in hot-pressed ultra-high-temperature HfB₂–SiC composites, their mechanical properties (hardness, fracture toughness, Young modulus), and heat resistance (resistance to ablation) under heating in air to high temperature with a gas burner was studied. The Vickers microhardness H_V and fracture toughness K_Ic of the best from the developed composites were, respectively, H_V = 38.6 ± 2.5 GPa and K_Ic = 7.7 ± 0.9 MPa m^0.5 (after the indentor load of 9.8 N), and the Young modulus is 510 GPa. This composite was synthesized from a HfB₂–30 wt % SiC (5–10 µm) mixture under hot pressing (at a pressure of 30 MPa, 1950°C, 30 min) and had a density of 6.54 g/cm³. The studies of resistance to ablation in air for hot-pressed HfB₂ and HfB₂–SiC specimens heated with a gas burner show that HfB₂ ceramic with addition of 30 wt % SiC and an average grain size of 30–50 µm (clastic grains with sharp edges and an approximate stoichiometry SiC_1.6O_0.1, 6_H SiC) and 5–10 µm (single-crystal grains with a hypercubic nearly spherical shape almost free from impurities with an approximate stoichiometry SiC_1,5, β-SiC) are highly refractory: they are resistant to the temperatures of 2766 and 2780°C, respectively, at a mass loss of 0.25 mg/s as compared to the HfB₂ ceramic free from additives, the specimens from which were cracked as soon as at a temperature of 1870°C, and also more resistant than the HfB₂–30 wt % SiC ceramic synthesized with addition of SiC with sharp clastic grains with a size of 1 µm (with a shape lamellar or strongly elongated in one direction and an approximate stoichiometry SiC_4.6O_0.75, 6H-SiC) or 3–10 µm (with an approximate stoichiometry SiC_2.3O_0.25, 6H-SiC), which was cracked as soon as at a temperature of 1787 and 1455°C, respectively. A better heat resistance (resistance to ablation) exhibited by the HfB₂–SiC ceramic with addition of certain SiC types can be explained by high hardness and Young modulus values, the formation of solid solutions on the basis of HfB₂ and SiC phases with a small quantity of impurity oxygen, and the distribution of the present phases over the volume of the composite.