Behavior of HfB2-SiC (10, 15, and 20 vol %) ceramic materials in high-enthalpy air flows

HfB2-SiC ceramic samples containing 10, 15, and 20 vol % silicon carbide were prepared by spark plasma sintering. The samples were characterized by X-ray powder diffraction, SEM, and other methods. Their densities and calculated porosities were determined. The behavior of the materials under heating by a subsonic dissociated air flow was studied on a VGU-4 high-frequency inductive plasmatron. The average surface temperatures of the 10 and 15 vol % SiC samples were shown to increase up to 2550-2675A degrees C during heating, due to the generation of surface localities having temperatures of 2600-2700A degrees C (the initial surface temperature was similar to 1700-1900A degrees C) and the progressive growth of these regions in area. The overall time during which the average surface temperatures of these samples were higher than 2000A degrees C, was about 31-32 min. For the 20 vol % SiC sample, heat removal (when the sample touched a water-cooled holder) was shown to influence the surface temperature and surface temperature distribution. The variation in gas-phase composition over the central area of the sample surface during an experiment was studied using emission spectroscopy. Explanations are proposed to the variation of boron and silicon concentrations in the course of exposure to high-enthalpy flows. The elemental and phase compositions were determined and the microstructures were studied on the surface and sections of samples after long-term (similar to 40-min) exposure to high-enthalpy air flows.