Theoretical study on the structure and energetics of intergranular glassy film in Si3N4-SiO2 ceramics

Molecular dynamics simulations have been performed to investigate intergranular glassy film in Si3N4-SiO2 ceramics. A 90 degrees-twist grain boundary was employed as a representative random grain boundary and an amorphous structure of the intergranular glassy film was introduced in the calculations. It is found that the presence of the inter,granular glassy film at the grain boundary significantly decreases the number of dangling bonds at the interface originating from geometric strain due to misorientation of adjacent Si3N4 grains. Calculations with a several of thicknesses of the intergranular glassy film revealed that the excess energy significantly decreased with the increase in the thickness smaller than 7.0 angstrom, while further increase in the thickness did not contribute to a release in the geometric strain energy. Equilibrium thickness of the intergranular glassy film was estimated from the calculations. A new approach to estimated the equilibrium thickness is proposed. A possible explanation of the disparity between intergranular glassy film thicknesses of the intergranular glassy film measured by high-resolution electron microscopy and electron energy-loss spectroscopy is given in terms of ordering of atoms at the interface.