Study On Crack Initiation And Propagation Behavior Of Ultra-precision Cutting Of Single Crystal Germanium

Single crystal germanium is an extremely important semiconductor material,which has a wide range of important applications in the fields of infrared optics,aerospace measurement and control,nuclear physical detection,solar cells and other fields.However,it has the characteristics of high hardness,fragility,and easy to produce cracks or brittle fractures which are not conducive to processing.Therefore,in-depth study of the crack generation and propagation law of single crystal germanium materials during ultra-precision cutting processing,and then to suppress the crack generation and propagation behavior,is of great significance in improving the surface quality and product performance.The topic of this thesis is selected from the National Natural Science Foundation of China: Study on the mechanical mechanism of micro-nanoscale plastic cutting of crystalline germanium infrared optical hard and brittle materials(project number:51765027).The article first uses molecular dynamics simulation methods to simulate the cutting process of single crystal germanium under different cutting depths,different tool nose arc radii and different tool rake angles.A prominent deformation region is proposed,the stress changes near the region are calculated,and the causes and influencing factors of crack initiation during the cutting process are analyzed.In addition,the molecular dynamics simulation method is also used to simulate the process of tensile growth of single crystal germanium material prefabricated cracks at different crystal planes,different temperatures,different preformed crack lengths,and different orientations on the same crystal plane.The crack tip stress and crack growth rate during crack instability cracking are calculated,and the law of crack growth and the factors affecting the crack growth are analyzed.The results show that when the cutting depth reaches a certain level,a prominent deformation area appears,and the atomic distance in this area becomes larger,and the volume of the area expands.In the vertical direction perpendicular to the cuttingdirection,the stress state appears as tensile stress.Under this action,the material near the protruding deformation area will generate micro-cracks,thereby forming crack initiation areas.Under the condition that other cutting conditions are not changed,appropriately increasing the radius of the tool nose arc and the negative rake angle of the tool can help to suppress the occurrence of the protruding deformation zone and reduce the value of the vertical tensile stress during the cutting process,thereby suppressing the initiation of microcracks.Different orientation crack propagation ability on different crystal planes and isomorphic planes.The crack tip stress of crack instability propagation on the(111)crystal plane is greater,while the crack propagates faster on the(110)crystal plane.As the temperature increases,the crack cracking stress gradually decreases,and the crack propagation speed increases accordingly.Preparing the crack length in advance will reduce the crack cracking stress and increase the crack propagation speed.Finally,based on the nano-indentation and nano-scratch experiments,the observation of the surface micro-morphology of the material indentations and scratches under a scanning electron microscope,the mechanical characteristics of single crystal germanium materials and the crack generation and propagation behavior were further analyzed.From the experimental level,the obvious anisotropy of the single crystal germanium material and the orientation effect of crack propagation are demonstrated,and the correctness of the laws obtained by simulation research is verified.