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 sp(2)-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.

Automated summary

Density-modified structural features and mechanical properties of amorphous Si2BC3N were investigated using ab initio molecular dynamics simulations. The chemical bonds of Si2BC3N were found to be unaffected by density changes, with only minor alterations in bond lengths, peak positions in density of states, and Bader charge values. However, the composition of chemical bonds was altered. Under high-density conditions, the polyhedral units of Si2BC3N transitioned from a sp(2)-like trigonal configuration to a tetrahedral configuration, particularly for B, C, and N. This structural response mechanism was different from the stretching or shrinking of bonds observed in crystals. The increased tetrahedron content improved the amorphous structure stability of Si2BC3N, inhibiting the separation of turbostratic BN(C) and enhancing the second-order elastic constants, elastic moduli, and tensile/shear strengths. While high density may reduce the debonding capability of the fiber/Si2BC3N interface, it will not affect the preference for crack deflection at interfaces. These findings suggest the potential for optimizing the mechanical properties of SiBCN ceramics through density tailoring in experiments.