Density-modified structure and mechanical properties of amorphous Si₂BC₃N by ab initio molecular dynamics calculations

Density-modified structural features and mechanical properties of the amorphous Si₂BC₃N are studied by ab initio molecular dynamics simulations. The chemical bonds of Si₂BC₃N 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 sp²-like trigonal configuration to the tetrahedral configuration, especially for B, C, and N. This variation is the main structural responding mechanism of Si₂BC₃N 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 Si₂BC₃N by impeding the separation of turbostratic BN(C), and enhance the second-order elastic constants, elastic moduli and tensile/shear strengths of Si₂BC₃N. The high density may decrease the debonding capability of the fiber/Si₂BC₃N 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.