The oxidation resistance of the heterophase ZrSi2-MoSi2-ZrB2 powders – Derived coatings

Abstract

In this work, we investigate the oxidation resistance of the coatings manufactured by reactive impregnation of Cf/C-SiC composites with a melt derived from layers of ZrSi2-MoSi2-ZrB2 powder composition synthesized by SHS and alternating layers of SHS powders and elemental Si. Oxidation resistance tests were conducted in static conditions at 1650 °C in air and under an air plasma torch at 2200 °C. The performance of the coatings in these conditions is defined by the emergence and evolution of the heterogeneous oxide films based on zirconia-modified borosilicate glass. The protective oxide layers effectively inhibit the oxygen influx and further oxidation of the substrate. The increase of temperature above 1750−1800 °C resulted in evaporation of the glass phase from the surface and formation of porous thermally insulating ZrO2-based layer with Mo/MoO2, Mo3Si, and Mo5Si3 inclusions. The gradient of temperature across the coatings’ thickness allows for a partial retainment of the glassy phase in the coatings’ lower layers due to the decrease of partial pressures of boron and silicon suboxides, resulting in an additional degree of protection against oxygen diffusion. The specimens were characterized in terms of the temperature and time boundaries of their performance, mass loss kinetics, catalytic activity, and spectral emissivity of the coatings. The limiting factors of the coatings’ performance are outlined.