EFFECTS OF OXIDATION ON INTERGRANULAR PHASES IN SILICON-NITRIDE CERAMICS

The effects of oxidation on changes in the secondary phases of two Si3N4 ceramics were investigated by transmission electron microscopy. The Si3N4 materials were oxidized at 1400-degrees-C for 168 h in laboratory air. One material, sintered with 5 vol % Yb2O3 + 0.5 vol % Al2O3, containing a Yb2Si2O7 crystalline secondary phase, displayed no gross changes following oxidation. However, the thickness of the amorphous intergranular film was observed to have decreased by approximately 20% from its initial thickness of 1.0 nm. The second Si3N4 material, sintered with 5 Wt% Y2O3 + 1 wt% MgO, had a completely amorphous secondary phase. Devitrification of the secondary phase at multiple-grain junctions to beta-Y2Si2O7 accompanied the outward diffusion of additive and impurity cations occurring in the residual amorphous intergranular films during oxidation. Substantial cavitation and intergranular phase depletion was observed at both multiple-grain junctions and two-grain boundaries. The equilibrium thickness of the amorphous intergranular film consequently decreased from 1.2 to 0.9 nm following oxidation. Purification of the amorphous intergranular films by diffusion of cations to the surface led to a reduction in impurity concentration, resulting in the observed thinning of grain-boundary films.