Molecular dynamics simulation of the nanometric cutting of silicon

Molecular dynamics simulations of nanometric cutting of single-crystal, defect-free, pure silicon were performed using the Tersoff potential over a wide range of rake angles (from -60degrees to +60degrees), widths of cut (1.1 to 4.34 nm), depths of cut (0.01 to 2.72nm) and clearance angles (10degrees to 30degrees) to investigate the nature of material removal and surface generation process in ultraprecision machining and grinding. The observed material removal mechanisms can be divided into four components: (i) compression of the work material ahead of the tools (ii) chip formation akin to an extrusion-like process (iii) side flow; and (iv) subsurface deformation in the machined surface. Unlike in conventional machining of most ductile materials, where no volume or phase change is observed in the plastic deformation process, significant volume changes (from 18.38 to 14.19Angstrom(3)), resulting in a densification of about 23% occur owing to phase transition from a diamond cubic (or alpha-silicon) to a bet (or beta-tin structure) in the case of machining silicon. Such a structural change is typical of silicon undergoing a pressure-induced phase transformation. The extent of structural changes and their contributions to each of the four material removal mechanisms depend on the tool rake angle and the width of cut. The ratio of the width of cut. to depth of cut w/d is the primary factor affecting the extent of side flow and subsurface compression. The tool rake angle and the w/d ratio are found to be dominant factors affecting the chip flow and shear zone compression ahead of the tool. Subsurface or near-surface deformation was observed with all rake angles and all cut depths down to 0.01 nm, indicating the need for an alternate final polishing process such as chemomechanical polishing to produce defect-free surfaces of silicon on an atomic scale.