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 degrees-1400 degrees 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 degrees 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 degrees 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 degrees 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.