Research Of Tool Wear In Nano-cutting Of Single Crystal Silicon Based On Molecular Dynamics Simulation

Single crystal silicon has been applied widely in optical,mechanical,electronic and other high-tech fields because of its excellent optical properties,high mechanical strength and chemical stability.However,the fracture strength and yield strength of single crystal silicon are so close that the material can easily break in the traditional processing methods and thus make it hard to obtain high quality surface.At present,as one of the advanced manufacturing technology,nano-cutting has become a research focus in single crystal silicon processing field because of its high precision,high efficiency,controlling easily and machining of complex surface.During the nano-cutting process of single crystal silicon,the diamond tool will be worn heavily which leads to the quickly deteriorating of surface quality.Besides,when the machining has reached the nanometer scale,the theory involved is beyond the continuous medium mechanics of conventional cutting theory with removal objects of discrete atoms or molecules.So huge disagreements of diamond tool worn mechanisms and machining mechanisms of nano-cutting have been existed around the researchers all over the world.Problems above have become urgent issues for further development of nano-cutting technology.Considering that Molecule Dynamics(MD)is able to identify the transient processes during nano-cutting at atomic level and analyze the mechanism of nano-cutting,the effects of machining parameters,materials aeolotropism and tool geometry on the machining process and tool worn during single crystal silicon processing are discussed in this dissertation.The research work is concluded as follows:Firstly,the MD simulation models of nano-cutting for single crystal silicon are built based on MD theories.The appropriate potential function,integration methods and system ensembles are chosen.The aeolotropism behavior of diamond graphitizing is studied with crystal structure and thermodynamic properties,and thus the changing rules of behaviors varying with temperature and crystal orientation is obtained.In order to analyze the graphitizing process precisely,a new method named as 6-ring method is proposed in this dissertation which can distinguish diamond atoms and carbon atoms correctly.Furthermore,revised MD tool model is illustrated which can reflect tool states more realistically during nano-cutting process.The revised model is turned to be more suitable for research on tool wear than traditional method by comparing the wear pattern of revised model and traditional model.Secondly,material removal and surface formation mechanisms in nano-cutting process of single crystal silicon are studied based on the MD simulation model of the large scale silicon workpiece(100×50×3nm3).The cutting processes on crystal surfaces(100),(110)and(111)are studied to analyze the effect of aeolotropism of single crystal silicon.And the rules are concluded from the effect of direction changing on chip and surface damage layer.Better orientation has gained by comparing the integrity of machined surface.Coordination method is used to indicate the damage structure of machined surface,and based on the analysis of static pressure and maximum shear stress,the formation mechanisms of amorphous surface layer and subsurface damage layer are revealed.The influences of tool geometry and back engagement of the cutting edge on single crystal silicon nano-cutting is studied in detail.Material By analyzing the shear plane structure in chip,the removal mechanism of monocrystalline silicon in nano cutting process is discussed in detail.Next,graphitization wear of diamond tool will make the tool surface soften and wear heavily under the carving of hard particles.The effect of temperature and shear stress on tool graphitization wear is studied on the basis of revised method mentioned above and its formation mechanisms are revealed.Meanwhile,the functional mechanisms of low coordination carbon atoms adsorbed in front of tool are studied and the crystal directions combination which has the strongest wear-resistance ability is obtained.Finally,diamond tools with different geometries are built in order to simulate the geometry effects on tool wear.The main reasons for tool wear and the effects of tool rake and clearance angles on the front and flank surfaces are analyzed systematically.Meanwhile,the expanded wear of diamond tool during the cutting process and the relationship between carbon atom hybrid structure in wear zone and thermal conductivity are studied,and the mechanism is also researched.Besides,micro-ditches are created on the flank surface of tool and the influences of micro-ditches depth and density on tool wear are studies,and its working mechanisms are revealed.Researches above can provide theoretical guidance and technology support for diamond tool design in nano-cutting process.