Molecular dynamics computer simulations of surface and interface phenomena in vitreous silica and faceted grain boundaries of alpha-alumina containing intergranular films

A classical molecular dynamics simulation technique was used to investigate self-diffusion in the bulk and on the surface of pristine vitreous silica and the effect of the composition of silicate intergranular films on the structure and energies of interfaces between those films and the basal plane of α-alumina. Previously developed multibody potential functions were used to describe the interatomic forces.; Activation energies were found to be similar for Si and O self-diffusion and did not vary appreciably for bulk and surface simulations. Average activation energies ranged from 113–115 kcal/mole for both species in both environments. Oxygen self-diffusion coefficients were only slightly higher than those for silicon in both environments. Self-diffusion coefficients in the top 3–7 angstroms of the surface were found to be higher than bulk coefficients by less than a factor of two. Self-diffusion on the surface was observed to occur by motion of SiO₃ and SiO₄ polyhedra over several angstroms, with little neighbor exchange. Significant neighbor exchange was observed over similar length scales in the bulk. Surface diffusion events through the vapor phase (via desorption and re-adsorption at a distant location on the surface) were not included in the surface data.; To model faceted grain boundaries in alumina containing glassy intergranular films (IGF), simulations of the basal plane of α-alumina in contact with silica, sodium silicate, calcium silicate, aluminosilicate, and calcium aluminosilicate glasses were performed. Ordered, cage-like structures were observed at all of the interfaces simulated. Sodium and calcium ions segregated to sites within the cages at the interfaces. These modifier ions were surrounded by more oxygen ions at the interface than in the bulk of the IGF. Aluminum and silicon ions also occupied sites within the cages at the interface, causing distortion of the structure of the cages and the crystal surfaces. As modifier ion concentration increased, Al and Si ions in cage sites were replaced by modifier ions. In general, interface energy decreased as the modifier ion content of the IGF increased.; In the aluminosilicate and calcium aluminosilicate interfaces, aluminum epitaxy was inhibited at high calcium and silicon concentrations. Thus, the growth rate of the basal plane in the ⟨0001⟩ direction would be expected to decrease as these concentrations increased.