Understanding the oxidation behavior of a ZrB2-MoSi2 composite at ultra-high temperatures

This basic research investigates the microstructure evolution of a composite based on ZrB2-MoSi2 from the as-sintered features to the changes occurring upon oxidation at ultra-high temperatures, 1650 and 1800 degrees C, in a bottom-up loading furnace for 15 min. Scanning and transmission electron microscopy evidenced the formation of a matrix typified by ZrB2-cores surrounded by (Zr,Mo)B-2-rims with dispersed MoSi2 particles and SiO2 glass trapped at the triple junctions. The oxidation at 1650 degrees C induced the migration of silica to the surface, which formed a continuous and protective scale. Below this scale, the matrix evolved into ZrO2 grains encasing MoB nano-inclusions, as a result of the oxygen and boron oxide partial pressures established in the subscales. Underneath, a MoSi2-depleted boride region, but substituted by SiO2 and MoB was found. The same phases were observed upon oxidation at 1800 degrees C, but a thicker and more turbulent oxidized layer formed as a consequence of the rapid evolution of MoO3, SiO and B2O3 gases from the scales beneath the outermost silica-layer. According to the observed phases and the calculated phase stability diagrams, the partial pressures gradient within the oxide layer were defined and the effect of Mo-doping in boride matrices on the oxidation behavior was compared to that of other transition metals to establish a criterion design for the realization of ceramics with improved oxidation resistance. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.