First‐principles study in an inter‐granular glassy film model of silicon nitride

• Published in: Journal of the American Ceramic Society Vol. 101; no. 7; pp. 2673 - 2688
• Main Authors: Ching, Wai‐Yim, Yoshiya, Masato, Adhikari, Puja, Rulis, Paul, Ikuhara, Yuichi, Tanaka, Isao
• Format: Journal Article
• Language: English
• Published: Columbus Wiley Subscription Services, Inc 01-07-2018; Wiley-Blackwell
• Subjects: Atomic structure; Bonding; Bulk density; Charge distribution; Electronic structure; Electropositivity; first‐principle calculations; Fracture toughness; Grains; interatomic bonding; intergranular glassy film (IGF); Mechanical analysis; Quantum mechanics; Silicon nitride; β‐Si3N4
• ISSN: 0002-7820; 1551-2916
• DOI: 10.1111/jace.15538

Based on a previously constructed intergranular glassy film (IGF) model for bulk silicon nitride, a large periodic model of 3864 atoms containing 2 grains of different orientations from the main bulk β‐Si3N4 and 2 IGFs was fully relaxed using the ab initio density‐functional theory package VASP. The relaxed structure was then used to calculate the electronic structure, density of states, interatomic bonding, partial charge distribution, and electron localization using the OLCAO method. Analysis of the data focuses on the interfacial regions between bulk β‐Si3N4 and the Si‐O‐N glass layer with different orientations. The total bond order density (TBOD) is evaluated in different interfacial and bulk regions. We show minor differences in the internal cohesion between crystalline grains of different facets. However, the overall charges in the bulk crystal grains and in the glassy regions are electropositive which are balanced by the negatively charged interfacial region between the two. The presence of a less rigid glassy layer is the reason for structural flexibility in ceramics without a huge penalty in the steric energy. The optimization of the interfacial structure and bonding via the creation of defective sites is the atomic origin for the existence of the IGF in silicon nitride. The insights obtained from this detailed quantum mechanical analysis of a realistic IGF model are discussed including implications on the strength, fracture toughness, and processing methods. We also discuss the potential applications of our method to other complex materials systems.