Simulations of the mechanical properties of crystalline, nanocrystalline, and amorphous SiC and Si

Molecular-dynamics simulations of crystalline (c), nanocrystalline (nc), and amorphous (a) silicon carbides and silicon were carried out to investigate their vibrational and mechanical properties. The atomic configurations, vibrational spectra, and stress-strain curves were calculated at room temperature. In the case of the nanocrystalline structures, these characteristics were analyzed as functions of grain size. Young's and bulk modul and yield and flow stresses were determined from uniaxial deformation of samples under periodic boundary constraints and from experiments on rod extension. For silicon carbides, Young's modulus and flow stress decrease in the sequence "c-nc-a," and with decreasing grain size, which is attributed to a weakening of the Si-C bonds in the amorphous matrix. The enhancement of the strength properties of the homopolar nc-Si structures is attributed to their deformation anisotropy. The calculated vibrational spectra and Young's moduli are in rather good agreement with the corresponding experimental characteristics.