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

The reactions between hot‐pressed calcium hexaluminate (CaAl 12 O 19 , 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 CaAl 12 O 19 . At temperatures below 1300°C, the reaction resulted in the stratification of two distinct interfacial layers: a partially devitrified CaO‐Al 2 O 3 ‐SiO 2 glass adjacent to SiC and a CaAl 2 Si 2 O 8 (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 SiO 2 was predicted to produce a more‐viscous glass than CaAl 12 O 19 and SiO 2 and, therefore, have slower reaction kinetics, because of lower mass transport in the glass.