Molecular Dynamics Study Of The Mechanism In Micro Laser And Elliptical Vibration Assisted Nanoscale Cutting Of Single-crystal Silicon

Optical elements based on single-crystal silicon are widely applied in precision optical systems in the field of aerospace and defense industry.The performance of the system is greatly determined by the manufacturing quality of the optical elements.At present,fieldassisted nanocutting based on laser heating and tool vibration has been proposed and is gradually becoming an important method to achieve nanoscale surface roughness and complex surface structure on brittle materials like single-crystal silicon.However,the deformation mechanism of the material is sophisticated.The lack of knowledge in the material removal and subsurface damage mechanism is one of the bottlenecks that restricts further improvement of the manufacturing quality of silicon optical components.This dissertation aims to solve the common and difficult problems in nanocutting mechanism of single-crystal silicon.The material removal mechanism of nanocutting and its transition processes are investigated.The mechanism of material removal and subsurface damage formation in thermal and elliptical vibration assisted nanocutting is proposed.By combining of thermal and vibration effect,the deformation and subsurface damage mechanism in hybrid machining is investigated.The main contents of this dissertation are summarized as follows:The MD cutting model of single-crystal silicon is established.According to the basic principle in MD,the potential energy function and simulation ensemble are selected to establish the atomic model.Based on the classical cutting simulation method and the assistive field,a MD cutting model for field-assisted cutting is established.A variety of crystal defect analysis techniques and self-developed post-processing codes are used to identify the internal defects of single-crystal silicon in nanocutting.Compared with the traditional model,the proposed cutting model has effectively improved the accuracy and efficiency of cutting simulation and defect characterization,which is advantageous for improving the understanding of the material removal mechanism in field-assisted nanocutting.The transition of material removal mechanism in nanocutting of single-crystal silicon is proposed.The cutting characteristics under different removal mechanisms are clarified based on the atomic flow and stagnation region with various tool rake angles.The dominant factors in material removal mechanism transition are investigated with consideration of the tool rake angle and workpiece crystal orientation.The subsurface damage formation mechanism under different material removal mechanisms is investigated.The proposed transition model elucidates the dominant factors of material removal mechanism and explains the phenomenon of crystal anisotropy weakening from extrusion to no-removal transition.The material removal mechanism and subsurface damage formation in micro laser assisted nanocutting are investigated.The influence of temperature on the machined surface morphology and material removal mechanism is studied based on cutting simulation.The mechanism of lubrication and recrystallization of the amorphous silicon at high temperature are revealed.The suppression mechanism of the subsurface damage at elevated temperature is investigated.The simulation results are verified by micro laser assisted cutting experiments and TEM observations.The proposed lubrication and recrystallization effects could explain the flow state and phase transition behavior of the amorphous atoms,and illustrate the mechanism of subsurface damage formation in thermal assisted cutting.The transition of the material removal mechanism in one vibration cycle during elliptical vibration cutting is proposed.The differences of chip formation and subsurface damage evolution between traditional cutting and elliptical vibration cutting are compared.Based on the nanocutting model established in Chapter 2,the transient mechanism of material removal and subsurface damage formation in a single vibration cycle are investigated.The effective tool rake angle model is established based on the geometrical relationship between the cutting tool and workpiece.The material removal mechanism transition and critical tool rake angle are determined under different cutting parameters.The proposed law of the material removal mechanism transition and tool effective angle model could explain the transient removal process in elliptical vibration cutting of single-crystal silicon.The combined effect of the thermal and vibration assistants in the material removal mechanism and subsurface damage evolution during hybrid machining is proposed.The influence of assistive field on material removal behavior in a vibration cycle is investigated.The formation mechanism of surface swelling and subsurface damage are clarified.The recrystallization process and vacancy formation mechanism are studied,and the effect of tensile stress on recrystallization is revealed.The influence of the thermal and vibration parameters on the recrystallization is investigated.The proposed mechanism of surface swelling and recrystallization could improve the understanding of the coupling effect of thermal and vibration assistant in hybrid machining.