Final report

Abstract

In the DOE award, DE-FG02-00ER45823, we have used molecular dynamics (MD) computer simulations of the intergranular films (IGFs) present in alumina and silicon nitride materials to address specific questions such as: What is the atomistic structure of the glassy silicate phase? Because of the extremely thin nature of the IGF, do bulk-like glass structure and properties prevail? Does distortion exist in the silicate bonds (which affects bond strength and reactivity) and how is this structure affected by the separation distance between the crystals and/or by the composition of the IGF? Does a structural ordering caused by epitaxial adsorption occur at the IGF/crystal interface? What is the correlation length of this order perpendicular to the interface? How is this ordering affected by composition of the IGF or by the crystals in question?In all simulations, a specific number of ions in stoichiometric ratio were placed as the IGF between two similar crystals, with, in some cases, different crystallographic orientations. The IGF compositions coincided with some of those observed experimentally (calcium aluminosilicate (CAS) glasses in the alumina case, calcium silicon oxy-nitride in the nitride case). The number of ions in the IGF was varied to allow for different thicknesses, although the X and Y dimensions (parallel to the interface) were usually ~50Åx50Å. The IGF was melted at a high temperature between the crystals, followed by a slow quench to room temperature, where structural data were collected.The results can be summarized as follows: Results of these simulations show ordering of the ions in the IGF caused by epitaxial adsorption onto the crystal interface. This occurs for both the (0001) and (112 0) in alumina and the (0001) and (10 1 0) surfaces in silicon nitride. This order extends about 1-1.5nm from the IGF/crystal interface into the IGF. However, the simulations used nearly ideal crystal surfaces (for ease in computations and subsequent analysis) that may have affected the degree of interface order and hence extent of this order into the IGF. We had planned to introduce both compositional variations in the crystal surfaces and topographic, atomistically, roughened crystal surfaces in future computations to see the effect on ordering over larger scales and correlation lengths.Our results showed how the composition of the glassy calcium alumino-silicate (CAS) IGF between α-alumina crystals affects growth in a manner consistent with the experimental studies. Specifically, the simulations showed how Ca adsorption retarded growth of alumina on the (0001) crystallographic plane at several concentrations of Al in the IGF, up to the anorthite composition, but had no such effect on the (112 0) orientation, consistent with experimentally observed anisotropic grain growth in alumina. Figure 1 shows the results of simulations of the CAS IGF between dissimilar planes of α-Al 2 O 3 at different IGF compositions. The figure shows preferential epitaxial