Reaction Densification of α′‐SiAION: II, Densification Behavior

Reaction hot‐pressing behavior of α‐Si 3 N 4 , Al 2 O 3 , A1N, and M 2 O Z powder mixtures (M = Li, Mg, Ca, Y, Nd, Sm, Gd, Dy, Er, and Yb) forming α′‐SiAlON has been studied. Five characteristic temperatures are found to control the densification behavior of these materials. The densification proceeded in three major stages. The first two stages were formation of ternary oxide eutectic and wetting of majority nitride powder. The third stage involved dissolution/melting of intermediate phase. Variation from this behavior sometimes occurs due to localization of wetting liquid at A1N, extremely high melting/dissolution temperature of Mg 2 Al 4 Si s O 18 and Nd and Sm melilite, and secondary precipitation of Dy‐α′‐SiAlON. The dominant densification mechanism was found to be massive particle rearrangement, irrespective of the wetting and dissolution/melting behavior. The efficiency of this mechanism is mostly affected by the amount of available liquid and less by its viscosity. Fully dense, single‐phase ceramics were obtained in all cases except Mg when hot‐pressed at a constant heating rate to 1750°C, and considerably lower temperatures for Li, Ca, Gd, Dy, Er, and Yb‐SiAlON when held isothermally.