Mk. Cinibulk et al., AMORPHOUS INTERGRANULAR FILMS IN SILICON-NITRIDE CERAMICS QUENCHED FROM HIGH-TEMPERATURES, Journal of the American Ceramic Society, 76(11), 1993, pp. 2801-2808
High-temperature microstructures of an MgO-hot-pressed Si3N4 and a Yb2
O3 + Al2O3-sintered/annealed Si3N4 were obtained by quenching thin spe
cimens from temperatures between 1350-degrees and 1550-degrees-C. Quen
ching materials from 1350-degrees-C produced no observable changes in
the secondary phases at triple-grain junctions or along grain boundari
es. Although quenching from temperatures of approximately 1450-degrees
-C also showed no significant changes in the general microstructure or
morphology of the Si3N4 grains, the amorphous intergranular film thic
kness increased substantially from an initial approximately 1 nm in th
e slowly cooled material to 1.5-9 nm in the quenched materials. The va
riability of film thickness in a given material suggests a nonequilibr
ium state. Specimens quenched froin 1550-degrees-C revealed once again
thin (1-nm) intergranular films at all high-angle grain boundaries, i
ndicating an equilibrium condition. The changes observed in intergranu
lar-film thickness by high-resolution electron microscopy can be relat
ed to the eutectic temperature of the system and to diffusional and vi
scous processes occurring in the amorphous intergranular film during t
he high-temperature anneal prior to quenching.