EFFECTS OF OXIDATION ON INTERGRANULAR PHASES IN SILICON-NITRIDE CERAMICS

The effects of oxidation on changes in the secondary phases of two Si3 N4 ceramics were investigated by transmission electron microscopy. The Si3N4 materials were oxidized at 1400-degrees-C for 168 h in laborato ry air. One material, sintered with 5 vol % Yb2O3 + 0.5 vol % Al2O3, c ontaining a Yb2Si2O7 crystalline secondary phase, displayed no gross c hanges following oxidation. However, the thickness of the amorphous in tergranular film was observed to have decreased by approximately 20% f rom its initial thickness of 1.0 nm. The second Si3N4 material, sinter ed with 5 Wt% Y2O3 + 1 wt% MgO, had a completely amorphous secondary p hase. Devitrification of the secondary phase at multiple-grain junctio ns to beta-Y2Si2O7 accompanied the outward diffusion of additive and i mpurity cations occurring in the residual amorphous intergranular film s during oxidation. Substantial cavitation and intergranular phase dep letion was observed at both multiple-grain junctions and two-grain bou ndaries. The equilibrium thickness of the amorphous intergranular film consequently decreased from 1.2 to 0.9 nm following oxidation. Purifi cation of the amorphous intergranular films by diffusion of cations to the surface led to a reduction in impurity concentration, resulting i n the observed thinning of grain-boundary films.