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

Abstract

Reaction hot‐pressing behavior of α‐Si3N4, Al2O3, A1N, and M2OZ 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 Mg2Al4SisO18 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.

Machinable α‐SiAlON/BN Composites

“…There was also a rapid densification stage (between T 1 and T 2 in Fig. 7), which has been attributed to nitride dissolution 20,21 . Here the starting temperature ( T 1 ) was relatively constant but the finishing temperature ( T 2 ) systematically decreased with increasing magnitude of the low‐temperature step.…”

“…7), which has been attributed to nitride dissolution. 20,21 Here the starting temperature (T 1 ) was relatively constant but the finishing temperature (T 2 ) systematically decreased with increasing magnitude of the low-temperature step. The extra densification step was most likely related to BN dissolution, which was easier for smaller BN sizes, as in Si 3 N 4 /tBN where BN was well dispersed.…”

Rapid Densification and Deformation of Li‐Doped Sialon Ceramics

“…1. The sintering shrinkage starts at ϳ1100°C and stops at ϳ1450°C for "A" and ϳ1400°C for "B," 8 whereas we observed only two shoulders in our sintering curves with no evidence of any intermediate crystalline phase formation. We interpret the two shoulders in the shrinkage curves, marked by arrows at ϳ1100°and ϳ1300°C, as onsets of the formation of two types of liquid phases.…”

Liquid‐Phase Growth of Small Crystals for Seeding α‐SiAlON Ceramics

“…In the pelletized samples, relatively large dense regions are formed in the first stage of liquid-phase sintering, when the liquid content is high and very rapid densification is experienced. 32,33 Since the number density of ␣-SiAlON crystals in the dense regions is also high, such crystals must encounter impingement from the very beginning. Therefore, they have little chance for constraint-free growth, which is required for developing a high aspect ratio.…”