Study On Nano-cutting Mechanism Of Monocrystalline Germanium

Nano cutting is an important approach to achieve complex surfaces with nanometric accuracy.Exploring in depth the nano-cutting mechanism can provide the theoritic basis for the ultra-precision machining in nanometric accuracy,and has great significance for improving the level of the ultra-precision machining.Monocrystalline germanium is widely used in the field of infrared optics and play an important role in many areas such as high-tech products.Understanding the nanometric cutting mechanism and machined surface characteristics is of great significance for machining high quality germanium components.In this study,molecular dynamics(MD)analysis,ultra-precision machining experiments and nano-machined surfaces characterization are conducted to investigate the nanometric cutting mechanism of monocrystalline germanium and the tool-edge-related limitation of nano cutting.The detailed contents of this thesis are as follows:1.Large-scale three-dimentional MD simulations are carried out to study the nano cutting process of monocrystalline germanium.Studies are focused on the crystal structure deformation of surface and subsurface,material flow of the cutting area,cutting force and energy.The material deformation mechanisms of germanium are comparatively analysed with those of silicon,copper and aluminium alloy.2.The nanoindentations on different crystal planes and the nano cutting along different combinations of crystal plane and direction of monocrystalline germanium are investigated.The anisotropic behaviors in subsurface defects are discussed and the cutting directions to achieve the minimum depth of deformed layer on the machined surface with different crystal planes are suggested.3.MD simulations of nano-cutting germanium and copper are conducted to study the material flow model and the position of stagnation region.A new approach based on the maximum displacement of atoms in cutting direction is proposed to estimate the chip formation in MD simulation.It is found that the minimum rake angle for chip formation is-60°~-70°,which is in good agreement with the experimental results.The available minimum tool edge radius is suggested to be not less than 10 nm,which can remove the most materials in a single atomic layer in theory.Finally,the MD simulations of nano turning process with different feedrates are carried out to investigate the turning surface topography,cutting force and deformed layers on machined surface of germanium.4.The machinability of different crystal planes of germanium are analysed by ultra-precsion machining experiments.The sub-surface structural deformations on machined surfaces are characterized by micro-Raman spectrometer and TEM,which verify the results of MD simulation.According to the extrusion deformation mechanism of hard-brittle materials in nano cutting such as silicon and germanium,a novel method based on mechanical surface modification for nano-cutting brittle materials is proposed.The MD simulation and experiment results prove that the method is viable to reduce the crack defect and increase the depth of brittle-to-ductile transition of germanium.