REACTIONS BETWEEN HOT-PRESSED CALCIUM HEXALUMINATE AND SILICON-CARBIDE IN THE PRESENCE OF OXYGEN

The reactions between hot-pressed calcium hexaluminate (CaAl12O19, hib onite) 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 140 0 degrees C. In air, the oxidation of SIC produced a layer of silica o r a multicomponent amorphous silicate (depending on impurities) that r eacted with CaAl12O19. At temperatures below 1300 degrees C, the react ion resulted in the stratification of two distinct interfacial layers: a partially devitrified CaO-Al2O3-SiO2 glass adjacent to SiC and a Ca Al2Si2O8 (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 w as 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 oxyg en diffusion through a calcium aluminosilicate glass, The reaction bet ween rare-earth hexaluminates and SiO2 was predicted to produce a more -viscous glass than CaAl12O19 and SiO2 and, therefore, have slower rea ction kinetics, because of lower mass transport in the glass.