Importance of amorphous intergranular films in self-reinforced Si₃N₄ ceramics

High-fracture-strength and high-toughness β-Si₃N₄ ceramics can be obtained by tailoring the size and number of the elongated bridging grains. However, these bridging mechanisms rely on debonding of the reinforcing grains from the matrix to increase toughness. Interfacial debonding is shown to be influenced by sintering aids incorporated in the amorphous intergranular films. In one case, the interface strength between the intergranular glass and the reinforcing grains increases with the aluminum and oxygen content of an interfacial epitaxial β-SiAlON layer. In another, the incorporation of fluorine in the intergranular film allows the crack to circumvent the grains. Atomic cluster calculations reveal that these two debonding processes are related to (1) strong Si-O and Al-O bonding across the glass/crystalline interface with an epitaxial SiAlON layer and (2) a weakening of the amorphous network of the intergranular film when difluorine substitutes for bridging oxygen.