BETA-SI3N4 GRAIN-GROWTH .2. INTERGRANULAR GLASS CHEMISTRY

The intergranular chemistry and the substitution level of the beta'-gr ains in a Si3N4 ceramic fabricated with 9.5 mol% Al2O3 and Y2O3 have b een 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 a nd O were incorporated into the Si3N4 structure during densification a nd a dilute beta'-Si3N4 formed. Al concentration gradients were not ob served 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 c ontent of the oxynitride liquid phase sintering medium resulted in a l imited formation of N-alpha-wollastonite (Y2SiAlO5N) during cooling fr om the densification temperature. Y and Al were not homogenously distr ibuted in the residual glass pockets; an enrichment close to the beta- Si3N4 grains could be observed. Both Y and Al were observed also in th e glassy films between adjacent beta'-grains. Growth ledges on {10 (1) over bar 0} facets of beta'-grains facing curved grain boundaries res ulted in a variation of the intergranular film thickness. These featur es are consistent with grain growth by coalescence. (C) 1997 Elsevier Science Limited.