beta-Si3N4 grain growth .2. Intergranular glass chemistry

The intergranular chemistry and the substitution level of the beta'-grains in a Si3N4 ceramic fabricated with 9.5 mol% Al2O3 and Y2O3 have been characterised by high resolution analytical transmission electron microscopy. The Y2O3/Al2O3 molar ratio of the starting powder mixture corresponded to that of the Y,Al-garnet (5Al(2)O(3) . 3Y(2)O(3)). Al and O were incorporated into the Si3N4 structure during densification and a dilute beta'-Si3N4 formed. Al concentration gradients were not observed in the beta'-grains, although the Al substitution level varied within and between beta'-grains (0.16 less than or equal to less than or equal to less than or equal to 0.24). The consequently reduced Al content of the oxynitride liquid phase sintering medium resulted in a limited formation of N-alpha-wollastonite (Y2SiAlO5N) during cooling from the densification temperature. Y and Al were not homogenously distributed in the residual glass pockets; an enrichment close to the beta-Si3N4 grains could be observed. Both Y and Al were observed also in the glassy films between adjacent beta'-grains. Growth ledges on {10 (1) over bar 0} facets of beta'-grains facing curved grain boundaries resulted in a variation of the intergranular film thickness. These features are consistent with grain growth by coalescence. (C) 1997 Elsevier Science Limited.