Density‐modified structure and mechanical properties of amorphous Si 2 BC 3 N by ab initio molecular dynamics calculations

Abstract Density‐modified structural features and mechanical properties of the amorphous Si 2 BC 3 N are studied by ab initio molecular dynamics simulations. The chemical bonds of Si 2 BC 3 N are insensitive to the density variation, which is reflected by the negligible changes in bond lengths, peak locations in density of states and Bader charge values under different densities. Instead, the composition of chemical bonds is altered. Meanwhile, high‐density condition induces the transition of polyhedral units from the sp 2 ‐like trigonal configuration to the tetrahedral configuration, especially for B, C, and N. This variation is the main structural responding mechanism of Si 2 BC 3 N to increased density, which is different from the stretching and/or shrinkage of bonds as occurred in crystals. The increased tetrahedron content shall further benefit the amorphous structure stability of Si 2 BC 3 N by impeding the separation of turbostratic BN(C), and enhance the second‐order elastic constants, elastic moduli and tensile/shear strengths of Si 2 BC 3 N. The high density may decrease the debonding capability of the fiber/Si 2 BC 3 N interface but will not change the preference for crack deflection at interfaces. These results suggest the promising prospect of mechanical property optimization of SiBCN ceramics through density tailoring in experiments.