Effects of aluminum and zirconia contents on the reaction bonded aluminum oxide process

The effects of aluminum and ZrO₂ contents on the reaction and sintering of reaction bonded aluminum oxide (RBAO) were investigated. It was apparent that ZrO₂-containing RBAO powders with higher initial aluminum contents (>45 vol%) were increasingly more difficult to react and sinter. During oxidation in air, samples often underwent a self-propagating high-temperature synthesis (SHS) reaction which led to catastrophic failure. This reaction and cracking behavior was more pronounced with increasing aluminum and ZrO₂ contents of the powders. Subsequently, it was shown that the SHS reaction was actually two combustion phenomena: a thermal explosion reaction on the surface of the sample between aluminum and oxygen, which (in ZrO₂-containing samples) triggered a self propagating aluminothermic reduction of ZrO₂, forming Al₂O₃ and Al ₃Zr. Therefore, methods for controlling the rate of the initial oxidation reaction were effective since both SHS reactions were prevented.; Despite the use of controlled firing, initial samples with increasing aluminum contents proved difficult to densify. It was found that in all RBAO samples (regardless of ZrO₂ content), the reactively formed Al ₂O₃ underwent the γ to α-Al₂O ₃ transformation, which resulted in the development of a vermicular microstructure. In ZrO₂-containing RBAO samples, this transformation was inhibited and occurred concurrently with the start of densification. In addition, the start of bulk shrinkage in these samples was delayed and the densification rates were decreased in comparison to samples without ZrO ₂. This ultimately resulted in a decrease in the limiting density to which ZrO₂-containing RBAO samples could be sintered. Surprisingly, in samples without ZrO₂, increasing the aluminum content did not appear to have any effects upon the densification behavior of RBAO. In examining RBAO samples with similar aluminum contents but increasing ZrO₂ contents, it became apparent that the grain growth inhibiting action of the ZrO₂ prevented the complete removal of the vermicular microstructure. Thus, in ZrO₂ containing RBAO samples with higher aluminum contents, where there was more reactively formed Al₂O ₃, the vermicular structure was even more difficult to eliminate.