Effect of thickness of the intergranular film on fracture in Si ₃N₄

Molecular dynamics computer simulations were used to study the fracture behavior of silica intergranular films (IGFs) between silicon nitride crystals as a function of film thickness. Results showed a significant increase in fracture stress with decreasing IGF thickness. IGFs that are 2 nm thick fracture similarly to bulk silica glass, while the 1 nm IGF fractured at a much higher value. The simulations show bond rupture and rearrangement of siloxane rings that coalescence to form larger rings, or "voids." The delineating difference in the systems is the significantly larger concentration of six-membered rings in the 1 nm IGF in comparison to the 2 nm IGF or bulk silica glass. The Si-O bond is more stable in six-membered rings than other ring sizes. Rupture is more catastrophic in the 1 nm IGF than the other systems where stress decays more slowly with strain.