Study On Ultra Precision Machining Mechanism Of Monocrystalline Silicon Based On Molecular Dynamics

Monocrystalline silicon has many unique mechanical and physical properties such as high strength,corrosion resistance,antioxidation,high temperature resistance and abrasion resistance.It plays an important role in the integrated application of different fields,such as semiconductor devices,sensors,mechanical components and electronic products.Ultraprecision machining is a nanoscale modern ultraprecision mechanical removal technology,which has been applied to generate sub-nanometer level shape accuracy and nanometer surface quality.However,monocrystalline silicon with high hardness,high brittleness,low plasticity and micro crack limits its application.In the ultraprecision machining process of monocrystalline silicon,various types of damage such as surface/subsurface crack,dislocation,residual stress,phase transformation and so on can be generated.Ultraprecision machining is a complex engineering problem involving many disciplines such as mechanics,physics,tribology and mathematics.It is essentially a highly nonlinear and coupled thermomechanical process.The current experimental conditions are difficult to observe the nanoscale machining process.As one of most important methods to study the nanoscale machining process,the molecular dynamics(MD)simulations have been attracted more and more attention in recent decades,especially for hard and brittle materials.Thus,in this paper,molecular dynamics is used to study the nanoscale machining.In order to reduce subsurface damage and improve material removal rate during nanoscale machining of single crystal silicon,the influence of tool geometry,auxiliary laser parameters and structured tool parameters on subsurface and material removal rate are investigated by establishing the corresponding three dimensional MD model in this dissertation.The main research work of this dissertation is listed as follows:(1)A three dimensional molecular dynamics model is developed to investigate the nanoscale cutting process of monocrystalline silicon with diamond tool with different geometries.The effects of tool geometry on the deformation of material including rake angle,clearance angle,edge radius are carefully explored by analyzing the phase transformation,chips,hydrostatic stress,shear stress and workpiece deformation,which reveals that a positive rake angle tip,a larger clearance angle tip or a small edge radius tip would result in a smaller cutting force,a better subsurface and a larger chipping volume.A bigger negative rake angle tip cutting has a larger hydrostatic stress and shear stress.However,a positive rake angle tip cutting has a larger average friction coefficient,which means that negative rake angle tip cutting experiences a lower resistance rate.The results also indicate that a tip with positive rake angle or smaller edge radius will improve the smoothness of a ground surface.A bigger clearance angle tip cutting generates a lower temperature.The effect of half apex angle of conical diamond tool on subsurface damage and scratching surface integrity are investigated by analyzing phase transformation,chip,dislocation movement,hydrostatic stress,von mises stress and workpiece deformation.The results show that a bigger half apex angle of indenter scratching causes a higher hydrostatic stress,a larger chip volume,a higher temperature,and increases subsurface damage.The results also indicate that the evolution of crystalline phases is consistent with the distribution of hydrostatic stress and temperature.A bigger half apex angle tip scratching would result in a larger scratching force and a bigger phase transformation zone.Moreover,the established theoretical analytical model indicates that the partial dislocation emission is more likely to occur at a bigger half apex angle of indenter during nanoscratching process.(2)For the first time,the mechanism of laser assisted machining is explained on the nanoscale.A three dimensional MD model of laser assisted grinding is established.The effects of laser moving speed,laser pulse intensity and laser spot radius on grinding depth and material removal rate are thoroughly investigated according to atomic trajectories,phase transformation,temperature distribution,average workpiece temperature,grinding force and friction coefficient.The investigation indicates that a higher laser moving speed reduces the subsurface damage and improves the material remove rate.Cutting forces decrease as the laser moving speed increases.As the laser pulse intensity becomes larger,the friction coefficients became smaller,the material remove rate improves and the depth of grinding increases.However,larger laser pulse intensity may result in a larger thermal deformation of workpiece.A larger laser spot radius reduces the grinding depth,but increases the width of laser irradiation zone on machined surface.In addition,a comprehensive comparison was made between LAM and TM in this paper.The results reveal that LAM reduces the subsurface damage of workpiece,gets a better-qualified ground surface and improves the material removal rate.Moreover,LAM reduces the grinding forces and friction coefficients.Thus,it is possible to control and adjust the laser parameters according to laser moving speed,laser pulse intensity and laser spot radius,and it provides a potential technology to improve a surface integrity and a smoothness of ground surface.(3)For the first time,the advantage and disadvantage of using laser nano-structured diamond tool cutting are analyzed in detail by molecular dynamics simulation on nanoscale.According to the definition of laser structured tool,Pyramid tip tool,arc-shape groove tool(Pattern B),V-shape groove tool(Pat-tern C)and non-structured tools are established,respectively.The results indicate that a structured nanoscale tool causes a smaller hydrostatic stress,a less compressive normal stress ?xx and ?yy,a lower temperature and a smaller cutting force.However,the structured tool cutting results in smaller chip volume and more beta-silicon phase.The tool with V-shape groove can reduce the resistance to cutting during the nanoscale machining process.The potential energy and the temperature of subsurface atoms for nano-structured tool cuttings are analyzed.In addition,a theoretical analysis model is established to investigate the distribution of residual stress in workpece during the nanoscale machining process.(4)for the first time,the effect of groove direction,groove depth,groove width,groove factor and groove shape on the material removal behavior of workpiece are investigated deeply on the nanoscale by establishing a series of three-dimensional MD models.The results reveal that groove orientation 60° tool cutting has a smaller cutting force,less cutting heat,more beta-silicon phase,smaller von mises stress and hydrostatic stress.A small groove orientation tool,a small groove depth tool,a smaller groove width tool or a bigger groove factor tool would lead to a more ductile cutting mode and a larger material removal rate.However,a smaller groove width tool cutting results in more cutting heat.The average temperature of subsurface increases as the groove factor increases.Besides,V-shape groove cutting can improve the material removal ability in nanoscale cutting.