Oxidation Behavior of SiC‐Whisker‐Reinforced Alumina‐Zirconia Composites

During high-temperature oxidation in air, SiC-whisker-reinforced Al2O3—ZrO2 composites degrade by the formation of a whisker-depleted mullite-zirconia scale. The reaction kinetics have been studied as a function of time and temperature for composites with whiskers preoxidized for different times. The evolution of the microstructure has been investigated by optical, scanning and transmission electron microscopy. Possible reaction mechanisms have been discussed. A model compatible with our observations on Al2O3—ZrO2—SiC and the results reported in the literature for Al2O3—SiC whisker composites is proposed: The oxidation occurs at an internal reaction front. Oxygen diffuses along dislocations and grain boundaries through the mullite scale to react at this front with silicon carbide, thereby forming amorphous silica and graphite. Silica penetrates grain boundaries and further reacts with alumina and zirconia to form mullite and zircon, while the second reaction product, graphite, is oxidized into carbon monoxide when the reaction front moves deeper into the sample.