Microstructural and Mechanical Properties of Cubic Silicon Nitride: Insights from Molecular Dynamics Simulation

Molecular dynamics (MD) simulations have been used to study the mechanical behavior as well as the structural change of cubic (c-) Si₃N₄ upon uniaxial tensile loading. The Si₃N₄ sample is simulated under the cooling process and high pressure. At temperature of 300 K, most nitrogen (N) atoms arrange into fcc lattice, and the rest of N atoms have hcp and amorphous (a-) structures. The hcp and a- N atoms gather into a band. Upon uniaxial tensile tests, this c- Si₃N₄ exhibits elastic and plastic deformations. The fcc and hcp N atoms transform into a- N atoms under tensile loading. The band of a- structure develops and spreads across the sample with increasing strain. The transformation of SiN_x units occurs from high to low coordination number in the a- structure. The large simplexes, with radii of 2.0 Å and above, grow and coalesce only in the a- structure during tensile loading, leading to form the porous structure. The formation of porous a- structure causes ductile deformation at high strain.