Brittle-ductile transition in monocrystalline silicon analysed by molecular dynamics simulation

Abstract

For a better understanding of essential mechanisms of material removal at extremely small depth of cut and of the brittle-ductile transition in the material removal process of monocrystalline silicon, nonometric deformation behaviour in three-point bending of defect-free monocrystalline silicon is analysed by molecular dynamics (MD) computer simulation. MD simulations show that plastic deformation takes place through a phase transformation from diamond to amorphous structures. The critical octahedral shearing stress for phase transformation is estimated to be 12–14 GPa. In the deformed region, a crack nucleus on atomic scale can be generated owing to thermally activated vibration of atoms. After the crack nucleus, the crack extension takes place under a certain stress field. The crack initition takes place when a tensile stress reaches a certain critical value of about 30 GPa at the crack nucleus. The critical values for plastic deformation and crack initiation depend on crystal orientation and hydrostatic pressure. It is shown that there can also be critical criteria of the stress field to determine whether plastic deformation or crack initiation would predomiantly take place. When the plastic deformation proceeds to a crack initiation, ductile mode machining can be realized.