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.
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.
Understanding the oxidation behavior of a ZrB2-MoSi2 composite at ultra-high temperatures
Silvestroni L;Sciti D;
2018
Abstract
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.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
