Study On Brittle-ductile Transition Process In Nanometric Cutting Of Single Crystal Silicon Based On Dislocation Theory

There are brittle and ductile removal modes in the ultra-precision machining of single crystal silicon as the change of the undeformed chip thickness.Once single crystal silicon is cut in brittle removal mode,there will be cracks,pits and other defects on the machined surface,which will affect the processing quality and performance of single crystal silicon.The cutting properties of brittle materials are closely related to the evolution of defects in the microstructure.In order to control the removal mode of single crystal silicon and improve the processing quality furtherly,the effects of dislocation defects on the brittle-ductile transition process were studied by using the dislocation theory at the micro and nano scale in this paper.Firstly,in order to simulate the removing process of the single crystal silicon from ductile mode to brittle mode,a molecular dynamics model of single crystal silicon nanometric cutting process whose undeformed chip thickness increases with the increase of cutting distance is constructed.The maximum undeformed chip thickness is 30 nm.Secondly,the dislocation mechanism of two typical deformation modes including shear slip and cleavage fracture of the single crystal silicon is analyzed.The effects of two kinds of dislocation behavior including dislocation reaction and dislocation pile up on the mechanical properties of single crystal silicon are investigated.The relationship between dislocation behavior and deformation of single crystal silicon is analysed from the angle of force,stress and strain.It is found that the dislocation can influence the stress state of the single crystal silicon and change the deformation mode through different dislocation behaviors,which lays the foundation for the subsequent analysis of the dislocation mechanism during the cutting process.Then,in order to analysis nanometric cutting process of single crystal silicon expediently,the cutting process is divided into different stages according to the deformation mechanism.The deformation mechanisms of single crystal silicon at different stages are analyzed from the angle of force,energy and stress combining with instantaneous position figures.The mechanism of the minimum cutting thickness and the bonding between the tool and the workpiece are discussed.During the whole cutting process,the number of phase change atoms increase from 0 to 80000,and the number of dislocations increases from 0 to 40.It is found that the dislocation nucleation is an important mechanism to release the strain energy and change the deformation mode in the cutting process,and the sessile dislocation in the front of the tool is the important factor that leads to the formation of the shear band,which changes the removal mode of single crystal silicon from ductile mode to brittle mode.The critical undeformed chip thickness for brittle-ductile transition is 15 nm.Finally,the integrity of machined surface is evaluated from the aspects of surface roughness,metamorphic layer and residual stress.From the point of view of the quality of the process,the accuracy of different stages in the cutting process is verified by analyzing the differences of machined surfaces in different cutting stages.The degree of damage caused by the dislocation generated in the cutting process is observed,which shows that the dislocation will not only cause the brittle-ductile transition,but also increase the damage degree of the sub surface layer.