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 debated for decades with no firm conclusion. Herein, we report the result of quantum mechanical modeling on a realistic IGF model in beta-Si3N4 that unravels the fundamental 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 beta-Si3N4, alpha-Si3N4, gamma-Si3N4, and Si2N2O. The total bond order density-a quantum mechanical metric characterizing internal cohesion-of the IGF model and crystalline beta-Si3N4 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.