Behavior of HfB2–30 vol% SiC UHTC obtained by sol–gel approach in the supersonic airflow

Abstract

Using the sol–gel process, a composite powder of HfB2–(SiO2–C) composition was synthesized and then utilized for reactive sintering (hot pressing, 30 MPa; 1800 °C, 15 min) of ultra-high-temperature ceramic HfB2–30 vol% SiC composites. It was established that nanocrystalline silicon carbide (36 ± 2 nm) in a cubic modification is formed during hot pressing; HfO2 and HfC impurities were not found. The relative density of the obtained materials was 89.2 ± 2.3%. The long-term (40-min) oxidation resistance of the HfB2–30 vol% SiC sample was studied under the exposure to the supersonic dissociated airflow on a high-frequency induction plasmatron (heat flux changed from 232 to 779 W/cm2, pressure in the plasmatron chamber was 15 hPa), in a configuration that prevented significant heat discharge from the sample to a water-cooled holder (with 1-mm overhang). It was shown that, with a stepwise increase in the heat load in the initial stages, the temperature distribution over the front surface of the sample was relatively uniform. However, at the heat flux of 598 W/cm2, local overheated areas appeared in the central region, which spread over almost the entire surface of the sample within 1–2 min; the average temperature was ~2560 °C. Using emission spectroscopy data from the boundary layer above the sample surface, as well as XRD and SEM of the sample after exposure, it was shown that the sharp increase in temperatures from ~1500–1600 to 2500–2600 °C was associated with a change in the chemical nature of the surface, due to evaporation of borosilicate glass components and the appearance of a porous highly catalytic HfO2 with low thermal conductivity on the surface. It was noted that these processes under the exposure to the supersonic flow started at lower temperature than under the exposure to subsonic dissociated airflows.