Atomistic structure of calcium silicate intergranular films between prism and basal planes in silicon nitride: A molecular dynamics study

Molecular dynamics simulations of approximately 15 Å thick intergranular films (IGFs) containing SiO₂ and CaO in contact with two surface terminations of the prism (101̄0) and basal planes (0001) of Si₃N₄ were performed using a multibody interatomic potential. Samples with the same composition (1.5 mol% CaO) and number of atoms but different crystals planes (i.e., the prism and basal planes of Si₃N₄) were studied. In both the prism and basal cases, the IGF in the final configuration is well-ordered in the interface region. A small number of N ions from the crystal moved into the IGF near the interface, and O ions moved into the N sites in the crystal, indicating the formation of a Si-O-N interface. In addition, Ca ions do not segregate to the IGF-crystal interface. The bonding characteristics of the O ions at the interface with neighbor Si ions are different in the prism and basal cases. Such difference may be explained by the difference in the two crystal Si₃N₄ surfaces. The Si-O bond length of the IGF has a range from 1.62 Å to 1.64 Å, consistent with recent experimental findings.