Molecular dynamics simulations of the effect of the composition of intergranular films on grain growth

Molecular dynamics simulations were performed to study the effect of the composition of the intergranular film (IGF) on grain growth in α-Al ₂O ₃ and β-Si ₃N ₄ In each case, the IGF is formed while in contact with two differently oriented crystals. Results show preferential growth along the [11̄20] direction of the (11̄20) surface in comparison to growth along the surface normal on the (0001) surface for certain calcium alumino-silicate compositions in the alumina system. However, in the silicon nitride system, preferential growth occurred along the surface normal on the (0001) surface in comparison to that in the (10̄10) surface. Such preferential growth in each system is consistent with experimentally observed anisotropic grain growth in each of the different systems. Platelets form in the alumina system because of faster outward growth of the prism orientations than the basal orientation, while the more rapid growth along the basal surface normal in the nitride system causes anisotropic growth to manifest itself as whisker growth. In the alumina system, the simulations show the mechanism by which Ca ions in the IGF inhibit growth on the basal surface and the Ca/Al ratio in the IGF plays an important role in the change from isotropic to anisotropic grain growth. At compositions with high or low Ca/Al ratios, growth along each surface normal is equivalent, indicating isotropic grain growth. The simulations provide an atomistic view of attachment onto crystal surfaces, affecting grain growth in alumina.