1994 Volume 102 Issue 1190 Pages 925-929
Oxidation of a carbon/B4C/SiC/ZrB2 composite was investigated at 900°C in moist air atmospheres. From mass change measurement, scanning electron microscopy (SEM) observation, X-ray powder diffraction analysis and ion microprobe mass analysis before and after oxidations, the structure of oxidized layer was examined. The mass of the sample decreased with oxidation time and linearly with an increase in partial pressure of the moisture. Cross sectional observation showed that the oxidized layer was formed on the sample and the oxidized layer was porous in comparison with the sample core. The oxidized layer consisted of SiC and ZrO2, but carbon and boron were not detected. To discuss the formation process of the oxidized layer, thermodynamic calculations were performed. The equilibrium partial pressure of boric acids was calculated as a function of temperature or partial pressure of moisture. It is clarified that B2O3(s, l) formed from B4C and ZrB2 by oxidation reacted with H2O(g) to produce HBO2(g), (HBO2)3(g) and H3BO3(g) at elevated temperatures and the equilibrium partial pressure of these boric acids became high at 900°C in H2O(g)-containing atmospheres. Based on the experimental results and thermodynamic considerations, the formation process of the oxidized layer at 900°C in moist air was discussed.
