Yb2O3-fluxed sintered silicon nitride

Abstract

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 postsintering 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 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.