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

Abstract

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 β‐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 β‐Si3N4, α‐Si3N4, γ‐Si3N4, and Si2N2O. The total bond order density—a quantum mechanical metric characterizing internal cohesion—of the IGF model and crystalline β‐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.

Ceramic science of crystal defect cores

“…Even more complicated GB structures with amorphous secondary films in Si 3 N 4 GBs were addressed. 91), 92) In order to account for various phenomena and properties at GBs in ceramic materials, the space charge model has been long used. 93)96) This model also originates from ionicity of chemical bonding in ceramic materials such as oxides.…”

Mechanical Properties of Super-soft Biomolecular Systems: Application to Twenty Solvated Canonical Amino Acids

“…We have experience in calculating mechanical properties for different materials ranging such as suolunite crystal and silica binding peptide [44] , metal organic framework [45] , crystals [46] , glasses [47] , and some complex system [48] . Based on our experience, we used energy cut off 600 eV with electronic convergence of 10-5 eV, ionic convergence of -10 -3 eV, and a single k-point sampling.…”

Defect structures and dopant solution states of Hf‐doped Si3N4 ceramics

“…1,2 The mechanical property of Si 3 N 4 is strongly related to the existence of intergranular glassy film (IGF) between grains and the elementary properties of the IGF were investigated by experimental 3,4 and theoretical research. 5,6 Because of these excellent mechanical properties, Si 3 N 4 has been the subject of a vast amount of research from various perspectives, ranging from applied physics to basic material science. Thermodynamical properties at finite temperatures are theoretically investigated by firstprinciples lattice dynamics calculations combined with quasi-harmonic approximation.…”

“…In general, Si 3 N 4 ceramics exhibit excellent strength and fracture toughness up to high temperatures 1,2 . The mechanical property of Si 3 N 4 is strongly related to the existence of intergranular glassy film (IGF) between grains and the elementary properties of the IGF were investigated by experimental 3,4 and theoretical research 5,6 . Because of these excellent mechanical properties, Si 3 N 4 has been the subject of a vast amount of research from various perspectives, ranging from applied physics to basic material science.…”