Effect of AlN Substitutions on the Oxidation Behavior of ZrB2-SiC Composites at 1600°C

The oxidation behavior of ZrB2-SiC composites, with varying amounts of AlN substituting for ZrB2, was studied isothermally under static ambient air at 1600 degrees C for up to 5 h. Small amounts of AlN substitutions (<= 10 vol%) were found to result in marginal improvement in the oxidation resistance, whereas larger amounts resulted in a significant deterioration. The size of ZrO2 clusters formed on the oxidized surface was found to be a function of the AlN content. This effect was more pronounced after longer oxidation times (similar to 1 h) as opposed to shorter durations (similar to 5 min). It was postulated that presence of AlN results in the formation of Al2O3 during the oxidation process, subsequently resulting in a lowering of viscosity of the glassy silica scale, which facilitates the coarsening of ZrO2 clusters. This also increases oxygen permeation through the scale which adversely affects the oxidation resistance of the high AlN content composites.