The importance of amorphous intergranular films in self-reinforced Si3N4 ceramics

High-fracture-strength and high-toughness beta -Si3N4 ceramics can be obtai ned by tailoring the size and number of the elongated bridging grains. Howe ver, 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 f ilms. In one case, the interface strength between the intergranular glass a nd the reinforcing grains increases with the aluminum and oxygen content of an interfacial epitaxial beta -SiAlON layer. In another, the incorporation of fluorine in the intergranular film allows the crack to circumvent the g rains. Atomic cluster calculations reveal that these two debonding processe s are related to (1) strong Si-O and Al-O bonding across the glass/crystall ine interface with an epitaxial SiAlON layer and (2) a weakening of the amo rphous network of the intergranular film when difluorine substitutes for br idging oxygen. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Scienc e Ltd. All rights reserved.