β-Si 3N 4 grain growth, part II: Intergranular glass chemistry

Abstract

The intergranular chemistry and the substitution level of the β′-grains in a Si 3N 4 ceramic fabricated with 9.5 mol% Al 2O 3 and Y 2O 3 have been characterised by high resolution analytical transmission electron microscopy. The Y 2O 3 Al 2O 3 molar ratio of the starting powder mixture corresponded to that of the Y,Al-garnet (5Al 2O 3·3Y 2O 3). Al and O were incorporated into the Si 3N 4 structure during densification and a dilute β′-Si 3N 4 formed. Al concentration gradients were not observed in the β′-grains, although the Al substitution level varied within and between β′-grains (0.16 ≤ z ≤ 0.24). The consequently reduced Al content of the oxynitride liquid phase sintering medium resulted in a limited formation of N-α-wollastonite (Y 2SiAlO 5N) during cooling from the densification temperature. Y and Al were not homogenously distributed in the residual glass pockets; an enrichment close to the β-Si 3N 4 grains could be observed. Both Y and Al were observed also in the glassy films between adjacent β′-grains. Growth ledges on facets of β′-grains facing curved grain boundaries resulted in a variation of the intergranular film thickness. These features are consistent with grain growth by coalescence.