Effect of SiC concentration on aero-thermal behavior of ZrB2-based ceramics in hypersonic environment

This paper presents an extensive experimental campaign over the effect of SiC concentration on the aero-thermal behavior of ultra-high temperature ceramics in the hypersonic atmospheric re-entry environment. Four compositions made of ZrB2 with different amount of SiC from 5 to 18 vol.% were exposed to the supersonic plasma flow of an arc-heated plasma wind tunnel, at specific total enthalpies up to 20 MJ/kg, measuring their surface temperature by non-intrusive diagnostic equipment, including two-color pyrometers and an infrared thermo-camera. As SiC content increases, maximum steady-state temperature reached on the surface decreased and emissivity value are higher. During some tests, a spontaneous temperature jump in the order of 400 K was observed, which only occurred on the front surface of the sample. Surface temperatures over 2800 K were measured after a temperature jump. The composition which experienced the jump showed an external surface reaction made of only zirconia layer on the front surface, probably appearing upon complete removal of liquid borosilicate glass which forms during exposure to oxygen and is still present on the surface of samples with higher SiC content. The experiments demonstrated that the temperature jump, closely related to the overall thermal stability of the material, appears favoured by lower SiC amount, but it can be triggered also in case of larger SiC content (up to at least 15% vol.), as long as the flow total enthalpy and the exposure time are sufficiently high