YB2O3-Fluxed Sintered Silicon-Nitride.1. Microstructure Characterization

Microstructural development and crystallization behaviour of
Yb2O3-fluxed sintered silicon nitride materials was investigated using
CTEM and HREM. The materials contained 5 and 10 vol% Yb2O3 as sintering
additives. After densification, both compositions were subsequently
heat treated to crystallize the residual amorphous secondary phases
present at triple-grain regions. In the material doped with 5 vol%
Yb2O3, only an amorphous secondary phase was observed after sintering,
which was about 80% crystalline (Yb2Si2O7) after the post-sintering
heat treatment. A metastable phase was formed in the material with 10
vol% additives after sintering, with about 70% crystallinity in the
triple-point pockets. Upon post-sintering heat treatment, the material
could be completely crystallized. During heat treating, the metastable
phase combined with the remaining glass to form Yb2SiO5 plus Yb2Si2O7
and a small amount of Si3N4 which deposited epitaxially on pre-existing
Si3N4 grains in areas of low-energy within the triple-point pockets.
All materials contained thin amorphous films separating the grains. The
amorphous intergranular films along grain boundaries (homophase
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boundaries) revealed excess ytterbium and oxygen. The thickness of the
intergranular films was about 1.0 and 2.5 nm for the grain boundaries
and the phase boundaries, respectively, independent of additive content
and heat-treatment history.