Cutting of Al/Si bilayer systems: molecular dynamics study of twinning, phase transformation, and cracking
Using the molecular dynamics simulation, we study the cutting of Al/Si bilayer systems. While the plasticity of metals is dominated by dislocation activity, the deformation behavior of Si crystals is governed by phase transformations here to the amorphous phase. We find that twinning adds as a major deformation mechanism in the cutting of Al crystals. Cutting of Si crystals requires thrust forces that are larger than the cutting forces in order to induce amorphization; in metals, the thrust forces are relatively smaller than the cutting forces. When putting an Al top layer on a Si substrate, the thrust force is reduced; the opposite effect is observed if a Si top layer is put on an Al substrate. Covering an Al substrate with a thin Si top layer has the detrimental effect that the hard Si requires high pressures for cutting; as a consequence, twinning planes with intersecting directions are generated that ultimately lead to cracks in the ductile Al substrate. The crystallinity of the Si chip is strongly changed if an Al substrate is put under the Si top layer: With decreasing thickness of the Si top layer, the Si chip retains a higher degree of crystallinity.