Emissivity, catalycity and microstructural characterization of ZrB2–SiCfiber based UHTC at high temperature in a non-equilibrium air plasma flow

ZrB2-based Ultra-High Temperature Ceramics (UHTC) could be used at temperatures above 2000 K, a typical limit of most structural ceramics such as SiC-coated C/C composites. In particular, UHTC are candidate materials for thermal protection systems (TPS) for the new sharp-shaped hypersonic re-entry vehicles. Among the open points of research on UHTC materials is the characterization of surface properties like emissivity and catalycity in the high temperature range. High emissivity and low surface catalycity towards the exothermal recombination of oxygen atoms will improve the material performance in re-entry conditions by reducing temperature gradients and thermal stresses. In this paper are presented and discussed the experimental results of emissivity and catalycity of two different UHTC materials based upon the ZrB2–SiCfiber composition with different sintering additives (Si3N4 or ZrSi2). Microstructural characterization by SEM, XRD and XPS have shown oxidation-induced surface modifications with oxide layers composed of silica with trace amounts of boron oxide and zirconia if the maximum reached temperature is lower than about 1800 K and only zirconia for higher temperatures. The differences in the oxide layer composition may account for the different thermal radiative properties and catalytic behavior.