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°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 (~1 h) as opposed to shorter durations (~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. [ABSTRACT FROM AUTHOR]