The effects of subsonic and supersonic dissociated air flow on the surface of ultra-high-temperature HfB2-30 vol% SiC ceramics obtained using the solgel method

By hot pressing (1900 degrees C, 30 MPa, holding time 15 min) of HfB2-(SiO2-C) composite powder, an ultra-high temperature ceramic material of HfB2-30 vol% SiC composition with nanocrystalline silicon carbide has been obtained. The effects of subsonic and supersonic dissociated air flow on the surface of produced materials have been studied on a high-frequency induction plasmatron in the geometry of a cylindrical sample with a flat face, fixed in a copper water-cooled holder with a 1 mm protrusion. It has been shown that a sudden rise in the average surface temperature of the samples to -2600 degrees C is characteristic for both modes, which is associated with the occurrence of local sites with a temperature > 2000 degrees C and a subsequent increase in their area. It is a matter of evaporation of the borosilicate glass layer from the surface of the oxidized sample and formation of the ceramic layer of a highly catalytic and low thermal conductive porous HfO2 layer, which is confirmed by the emission spectroscopy data, XRD and elemental analysis of the material surface after the experiments. The features of heating the oxidized surface of the samples under the impact of subsonic and supersonic dissociated air flow have been noted: there are differences in the location of overheated sites, initiating a sharp temperature rise and the rate of growth of their area.