Material removal mechanism and subsurface characteristics of silicon 3D nanomilling

Abstract

Atomic force microscope (AFM) tip-based nanomilling has drawn significant attention because it can greatly improve the material removal rate. Previous studies have mainly focused on the machining process of two-dimensional (2D) nanomilling. However, the material removal mechanism and subsurface damage in three-dimensional (3D) ultrasonic vibration-assisted nanomilling (UVAN) are still unclear. Thus, by considering single-crystal silicon as the sample material, we investigate 3D-UVAN using experiments and molecular dynamics (MD) simulations. The material removal mechanisms for 3D-UVAN are uncovered first. We find that the material removal is dominated by shearing and extrusion, respectively, when the maximum undeformed chip thickness (UCT) is larger and smaller than a critical value. The strain rates generated by 3D-UVAN and 2D nanomilling are calculated by considering the machining velocity. It is higher for 3D-UVAN than for 2D nanomilling, which induces a fracture chip. The effects of vertical vibration amplitude and frequency in 3D-UVAN on the subsurface damage are investigated by MD simulation. Results show that the induced immobile dislocations lead to embrittlement of the material. Furthermore, a high strain rate occurs at a large vertical vibration amplitude and high frequency, which contributes to a shallow subsurface damage. This is also verified by TEM experiments. Thus, our findings enrich understanding of the material removal mechanism in 3D-UVAN and suggest a new route for suppressing subsurface damage.