Reactions between Hot-Pressed Calcium Hexaluminate and Silicon Carbide in the Presence of Oxygen

The reactions between hot-pressed calcium hexaluminate (CaAl₁₂O₁₉, hibonite) and silicon carbide (SiC) at 1100°-1400°C in air and nominal argon atmospheres were investigated. In inert atmospheres, there was no evidence of reaction at temperatures up to at least 1400°C. In air, the oxidation of SiC produced a layer of silica or a multicomponent amorphous silicate (depending on impurities) that reacted with CaAl₁₂O₁₉. At temperatures below 1300°C, the reaction resulted in the stratification of two distinct interfacial layers: a partially devitrified CaO-Al₂O₃-SiO₂ glass adjacent to SiC and a CaAl₂Si₂O₈ (anorthite) layer adjacent to hibonite. At 1400°C, a large amount of liquid was formed, the majority of which was squeezed out from between the reaction couple. No distinct layer of anorthite was present; instead, the anorthite was replaced by a layer of alumina between the glass-rich layer and hibonite. An activation energy of 290 kJ/mol was determined for the reaction, which is consistant with oxygen diffusion through a calcium aluminosilicate glass. The reaction between rare-earth hexaluminates and SiO₂ was predicted to produce a more-viscous glass than CaAl₁₂O₁₉ and SiO₂ and, therefore, have slower reaction kinetics, because of lower mass transport in the glass.