Molecular Dynamics Study Of Generation Mechanism Of Surface Layer In Nanomechanical Machining Of Crystalline Copper

The nanofabrication technology is one of six branches in the nanotechnology. It provides the base for fabricating physical, chemical and bio-nanostructures with nanometer size, and has been utilized to evaluate development level of the nanotechnology. As one of the top down approaches, the nanomechanical machining technology such as diamond tool cutting, diamond abrasive lapping and diamond probe-based nanoscratching, has provided novel opportunities to fabricate three-dimensional nanostructures due to the machining accuracy and size of fabricated nanostructure can be down to nanometer regime.However, the development of nanomechanical machining technology is hindered by many factors, such as machining mechanism, machining process, and measurement and evaluation of machining quality. Lacking fundamental understanding of machining mechanism is one of the most important issues. In addition to experimental study, molecular dynamics (MD) simulation is widely employed to investigate nanomechanical machining process. Although valuable insights have been obtained by previous studies, there is rather limited work that deals with MD simulation of mechanical nanomachining on polycrystalline materials. Since the nanomechanical machining is a highly coupled process between tool and workpiece, the difference of deformation mechanism between single crystalline and polycrystalline materials can affect machining results significantly. In addition, previous experimental and theoretical studies of subsurface damage layer induced during mechanical nanomachining process mainly focused on measuring the depth and distribution of subsurface damaged layer, less attention is paid to the formation mechanism of subsurface damage layer, especially for polycrystalline materials. It is well known that the microstucture evolution is one of the main reasons to cause subsurface damage. Therefore, regarding above problems, we perform MD simulations to investigate the formation mechanisms of surface layer during nanomechanical machining on single crystalline, bi-crystal, and nanocrystalline (nc) copper using theories of friction and wear, nanomechanics and crystal plasticity. The research content reads as follows.One is setting up MD simulation model of mechanical nanomachining on different kinds of materials (single crystalline copper, bi-crystal copper and nc copper). We first select suitable integration algorithm to solve Newton's equations of motion, empirical potentials for describing atomic interactions and ensemble. And then we construct single crystalline, bi-crytal and nc copper structures based on crystal structure, coincidence site lattice and Voronoi diagram. We also identify the type and location of defects generated using advanced defect analysis techniques.The second is exploring material deformation, machining force and formation of surface layer during mechanical nanomachining on single crystalline, bi-crystal and nc copper using MD simulation and theories of friction and wear, nanomechanics and crystal plasticity. For machining on single crystalline copper, we emphasize on deformation mechanism of material, formation mechanism of machined surface. We further evaluate effects of empirical potential, machining velocity, crystalline orientation and tool geometry on nanomachining process. For machining on bi-crystal copper, we focus on effect of dislocation-grain boundary (GB) interaction and dislocation-twin boundary (TB) interaction on material deformation and machining results. For machining on nanocrystalline copper, we reveal machining mechanism by full-three-dimensional nc copper and quasi-three-dimensional columnar nc copper simultaneously.Based on insights into the formation mechanism of subsurface damage layer, we establish a quantitative prediction method to characterize subsurface damage using nanoindentation technique. Using the criteria of determining deformation status of single atom according to its variation in potential energy, we quantitatively access the depth of subsurface deformed layer. We then set up a model of measuring hardness of materials based on nanoindentation technique, which can be employed to distinguish difference of indentation hardness between machined surface and original surface. In such a way the depth of subsurface damage layer can be quantitatively predicted.Based on above studies, we then propose a novel method to control and modify subsurface damage layer by deformation twinning. On the one hand we investigate effect of grain size on deformation twinning during mechanical nanomachining on nc copper; on the other hand we investigate effect of TB spacing on mechanical nanomachining on nanotwinned copper.In order to verify the accuracy of current MD simulation results, we conduct sequential comparisons of MD calculations with experimental results from following three aspects: fundamental principle of MD simulation, measuring mechanical property of material using nanoindentation technique, and machining result. We obtain the bulk modulus, shear modulus and Young's modulus of simulated system through compression, shear and tensile deformation respectively, and further conduct quantitative comparison with experimental data to evaluate accuracy of empirical potential. We calculate the Young's modulus from indentation force-indentation depth curve during MD simulation of nanoindentation on single crystal copper, and further conduct quantitative comparison with experimental values. We perform nanoindentation experiment on single crystal copper (010) surface and (111) surface, and further qualitatively compare the characteristic of surface pile up with MD simulation.