Direct Observations of Debonding of Reinforcing Grains in Silicon Nitride Ceramics Sintered with Yttria Plus Alumina Additives

As in fiber-reinforced composites, debonding, which allows the elongated reinforcing grains to at least partially separate from the rest of the matrix, is a critical part of the toughening mechanism in self-reinforced silicon nitrides. In situ high-resolution electron microscopy observations reveal that the debonding path can occur at the interface between the grains and continuous nanometer-thick intergranular film (IGF) or within the IGF depending on the film's composition, which varies with the yttria to alumina ratio in the fixed total amount of sintering additives. Theoretical calculations reveal that the bonding across the interface can be weakened by decreasing the Al and O content ( z ) of the epitaxial Si_{6– Z} Al _Z O _Z N_{8– Z} layer on the grains, which is consistent with the observations of interfacial debonding. However, evidence also indicates that weakening of the amorphous network of the IGF occurs with increase in yttrium levels that can be responsible for the observed mixture of debonding by crack propagation along the interface and within the IGF when the sintering additive contains the highest yttria:alumina ratio.