Origin of the existence of inter‐granular glassy films in β‐Si3N4

Inter‐granular glassy films (IGFs) are ubiquitous in structural ceramics and they play
a critical role in defining their properties. The detailed origin of IGFs has been de-
bated for decades with no firm conclusion. Herein, we report the result of quantum
mechanical modeling on a realistic IGF model in β‐Si3N4 that unravels the funda-
mental reason for its development. We calculate the electronic structure, interatomic
bonding, and mechanical properties using ab initio density functional theory with
parallel calculations on crystalline β‐Si 3N4, α‐Si 3N4, γ‐Si 3N4, and Si 2N2O. The total
bond order density—a quantum mechanical metric characterizing internal cohe-
sion—of the IGF model and crystalline β‐Si 3N4 are found to be identical. Detailed
analysis shows that weakening of the bonds in the glassy film is compensated by
strengthening of the interfacial bonds between the crystalline grain and the glassy
layer. This provides a natural explanation for the ubiquitous existence of IGFs in
silicon nitride and other structural ceramics. Moreover, the mechanical properties
of this IGF model reveal its structural flexibility due to the presence of the less rigid
glassy layer. This work demonstrates that high‐level computational modeling can
now explain some of the most intriguing phenomena in nanoscale ceramic materials