Research On Performance Evolution Process Of Machined Surface And Tensile Mechanical Properties Of Micro/Nanostructures

In nanomachining, formation of machined surface of micro/nanostructures bymaterial removal plays a very important role in manufacturing technology, and so doesthe performance evolution of machined surface after material removal; however it isoften tend to be ignored in previous studies. With the development of modern scienceand technology, the size requirements of microstructures become smaller and smaller,especially as the development of MEMS and NEMS, the size of the devices has reachednanometers. The properties of machined surface have become the main factors affectingthe surface performance. Surface properties, including surface roughness, defectstructures, residual stress, the plastic deformation and work hardening of the surfacelayer, have great impact on material properties and service performance ofmicrostructure. Therefore, it is exactly necessary for the research on the performanceevolution of the machined surface of micro/nanostructure, which will contribute to thedesign and manufacture of micro/nanostructures with theoretical and practicalsignificance.The machined workpiece has high mechanical storage energy because of the defectstructures formed in nanocutting. The workpiece can evolve to the low energy andstable state from high energy and unstable state by adjusting atom positionsautomatically, and this process is called surface-energy aging. In this paper, based onstochastic algorithm, molecular dynamics and Monte Carlo (MC) methods arecombined to construct the stochastic analysis model of surface-energy aging process ofmachined surface of micro/nanostructure, and GPU high-performance parallelcomputing simulation platforms are established.Firstly, MC simulations of surface-energy aging process of machined surface areperformed to analyze the surface-energy aging effect on the surface properties ofmonocrystalline copper workpiece. Performance evolution of machined surface insurface-energy aging process is investigated in detail from several aspects: the surfaceroughness, degree of order, defect structure, stress distribution, potential energydistribution and phase transition structures. The simulation results show thatsurface-energy aging effect can reduce the surface roughness, and increase the degree oforder of damaged layer, as well as decrease the residual stress and the average potentialenergy of workpiece. Defect structure and phase transition structures in metamorphiclayer significantly reduce in surface-energy aging process, in which part of thestructures change into some dislocation group and surface structure. Recrystallizationphenomenon occurs in surface-energy aging process. Surface-energy aging effects havea significant impact on the surface properties of the machined surface, and candramatically improve the service performance of micro/nanostructures. Secondly, the surface-energy aging effect on the surface properties ofmonocrystalline silicon workpiece is analyzed by MC simulations of machined surface.The surface roughness and degree of order, stress distribution, potential energydistribution and phase transition structure are investigated in detail before and aftersurface-energy aging process. It is shown that surface-energy aging effect can increasethe surface roughness and the degree of order of damaged layer, as well as decrease theresidual stress and the average potential energy of workpiece. Amorphous siliconstructures in metamorphic layer reduce and recrystallization phenomenon occurs insurface-energy aging process. It is found that some β-Si phase structures and BCT5-siphase structures transform into the diamond cubic structure of Si in surface-energyaging process. Surface-energy aging effects have a large impact on the surface propertyof the machined surface of monocrystalline silicon workpiece, and can improve theservice performance of micro/nanostructure.Finally, molecular dynamics (MD) simulation models of tension process of singlecrystal copper workpiece and single crystal silicon workpiece before and aftersurface-energy aging are established to investigate the aging effect on the deformationmechanism and mechanical properties of two materials micro-components. Whilemultiscale method, combining MD method with finite element method, is adopted toanalyze the anisotropy of mechanical properties of the monocrystalline copperworkpiece. It is shown that the surface-energy aging processes affect mechanicalproperties of the monocrystalline copper workpiece which also have obvious anisotropy.As a result of cutting process, the yield strength, tensile strength and ductility ofmonocrystalline copper workpiece reduce dramatically. However, they are allrebounded obviously in surface-energy aging process. The surface-energy aging effectalso has a small impact on the mechanical properties of the monocrystalline siliconworkpiece. The tensile strength and ductility of the workpiece reduce greatly in thecutting process, while rebound in surface-energy aging process.