Grand canonical Monte Carlo simulations of intergranular glassy films in β silicon nitride

Grand canonical Monte Carlo simulations have been performed on intergranular glassy films in β silicon nitride to determine their equilibrium structure, width and composition at 2000 K. A reactive force field for Si O N systems has been developed and tested, and chemical potentials of oxygen and nitrogen were obtained by requiring that they reproduce, in conjunction with the reactive force field, the experimentally determined compositions of the glass pockets at triple junctions and the interior of the adjoining β silicon nitride grains. Two silicon oxynitride films developed during the course of very long simulations, at the end of which their composition and structure were analysed. The thermodynamic equilibration of the film thickness was explored to give insight into the experimental uniformity of these films. Incorporation of a significant concentration of oxygen into the adjoining Si3 N4 grains was observed, producing somewhat diffuse interfaces on either side of the film. We believe this is the first thermodynamically consistent, atomic-scale, simulation of intergranular glassy films capable of exchanging matter and heat with reservoirs.