Molecular Dynamics Simulations of Calcium Silicate Intergranular Films between Silicon Nitride Crystals

Molecular dynamics simulations were used to study the structure of calcium silicate intergranular films (IGFs) formed between the basal planes of silicon nitride crystals. A multi-body potential was used to describe the interactions between ions. Samples with different film thickness and CaO contents were studied. Epitaxial adsorption of Si and O atoms from the intergranular films onto N and Si, respectively, in the crystal surface was observed. This epitaxial order induced a structural order into the nominally amorphous IGF that decreased as a function of distance from the IGF/crystal interface. A higher concentration of strained siloxane bonds was observed at the IGF/crystal interface in comparison to the amorphous interior of the IGF. While Ca ions were observed to segregate to the IGF/crystal interface in simulations of calcium silicate glass IGFs between alumina crystals, no segregation of calcium to the first adsorbed layer on the nitride was found in these simulations using silicon nitride crystals. Planar alignment of Ca ions parallel to the IGF/crystal interface occurred with either the largest concentrations of CaO or with the thinnest IGFs studied here. This alignment creates localized non-bridging oxygen that would affect the stability of the IGF/crystal system.