Effects of the Amorphous Oxide Intergranular Layer Structure and Bonding on the Fracture Toughness of a High Purity Silicon Nitride

The microstructural evolution and structural characteristics and transitions in the thin grain-boundary oxide films in a silicon nitride ceramic, specifically between two adjacent grains and not the triple junctions, are investigated to find their effect on the macroscopic fracture properties. It is found that by heat treating a model Si₃N₄-2wt% Y₂O₃ ceramic for ∼200 hr at 1400°C in air, the fracture toughness can be increased by ∼100%, coincident with a change in fracture mechanism from transgranular to intergranular. Structural phase transformations occur in the thin grain boundaries during oxidation that are revealed by XRD, EDX, Raman and EELS analyses. They affect the local bonding of atoms. It is concluded that only specific crystal "building blocks", i.e., tetrahedra, are transformed along the grain boundary and the resulting difference in the atomic structure of the oxide interface is seen directly to alter the macroscopic fracture behavior.