The Chip Formation Mechanism of Polycrystalline Copper in Nano-Cutting

Polycrystalline copper is widely used in multiple areas such as chip interconnects, nanoelectrodes, and nanoscale devices due to its unique electrical, thermal, and mechanical properties. Unlike monocrystalline materials, the presence of disordered grain boundaries and crystal orientations in polycrystalline materials significantly affects the chip formation mechanism during nanoscale machining. To investigate the influence of microstructural characteristics, such as grain size and grain boundary angle, on the chip formation process, a molecular dynamics model for polycrystalline copper was developed. The study systematically analyzed the evolution of the crystal structure, crystal orientation distribution, as well as the strain and stress during the chip formation process. The results indicated that the chips formed during the nano-cutting of polycrystalline copper are almost sawtooth-like, and the shear strain, crystallographic orientation, and crystal structure within the chip are periodically distributed. It is found that, an inclined grain boundary can effectively reduce shear strain on the machined surface, thereby improving the quality of the machined surface, and a highest surface quality can be achieved when the grain boundary angle approaches to 150°. Moreover, the results indicated that the chip was formed through two ways, namely, the extrusion and shear. The workpiece material near the tool tip was destroyed by the extrusion of the cutting tool, forming the flowing chip with amorphous atoms. On the other hand, the workpiece material near the free surface was separated by the periodically emerged shear slip bands. For these two chip formation processes, the extrusion process is not affected by grain boundary, but the shear process is dominated by the inclined angle of grain boundaries, the direction of the shear slip may be along the grain boundary or through the grain boundary.