【作者】 张俊杰；

【导师】 孙涛； 闫永达；

【作者基本信息】 哈尔滨工业大学 ， 机械制造及其自动化， 2011， 博士

【摘要】 作为纳米技术分支之一的纳米加工技术是在纳米尺度范围内制造物理、化学和生物等功能结构与器件的基础,已经成为衡量纳米技术发展水平的重要标志。纳米机械加工技术如金刚石刀具车削、金刚石磨粒加工以及金刚石微探针纳米刻划等可以使加工精度达到纳米量级、加工形成的结构与器件的尺寸达到纳米量级,为制造三维纳米结构与器件提供了创新的途径,是一种重要的由上而下的纳米加工技术。然而,纳米机械加工技术的发展目前受到加工理论、加工工艺及加工质量的测量与评定等诸多因素的制约。其中,缺乏对纳米机械加工机理特别是表层形成机理的理解是制约纳米机械加工质量提高的重要因素之一。除了实验研究外,国内外学者普遍采用分子动力学模拟来研究纳米机械加工机理,并得到了许多有价值的研究结果。然而,在前人的研究中大部分工件材料为单晶形式,而对多晶材料纳米机械加工的分子动力学模拟研究较少。纳米机械加工是一个刀具与材料高度耦合的过程,多晶材料不同于单晶材料的变形机理直接影响纳米机械加工的结果。此外,前人对纳米机械加工中引入的亚表面损伤层的实验和仿真研究主要是检测亚表面损伤层的深度及分布,而对多晶材料的亚表面损伤层的形成机理研究较少。在纳米机械加工中工件材料内部的结构和组织状态的变化是形成亚表面损伤层的一个重要原因。因此,针对上述存在的问题,本文基于摩擦磨损、晶体塑性和纳米力学等理论,使用分子动力学模拟研究了单晶铜、双晶铜和纳米晶体铜纳米机械加工表层形成机理,具体研究内容包括如下几个方面:建立了包含多种结构材料(单晶铜、双晶铜、纳米晶体铜)的纳米机械加工的分子动力学模型。从分子动力学基本原理角度出发选择了经典牛顿运动方程的积分方法、精确描述体系的势能函数和模拟真实环境的系综等参数,得到了分子动力学模拟的精确原子模型;结合晶体结构、重位点阵和维诺图等建立了单晶铜、双晶铜和纳米晶体铜的原子结构模型;结合晶体塑性理论和先进的晶体缺陷分析技术辨别了工件内部形成的缺陷的种类和位置。基于摩擦磨损、晶体塑性和纳米力学等理论,使用分子动力学模拟研究了单晶铜、双晶铜、纳米晶体铜纳米机械加工机理,包括工件材料变形、加工力变化、表层形成等。在纳米机械加工单晶铜中研究了工件材料变形机理、加工表面形成机理,以及势能函数、加工速度、晶向、刀具几何形状等参数对纳米机械加工的影响;在纳米机械加工双晶铜中研究了位错-晶界交互作用特别是位错-孪晶界交互作用对工件材料变形以及加工结果的影响;在纳米机械加工纳米晶体铜中,分别采用全三维纳米晶体铜和准三维柱状纳米晶体铜原子结构模型来揭示纳米晶体铜纳米机械加工机理。基于晶体铜纳米机械加工亚表面损伤层的形成机理的研究,提出了一种基于纳米压痕的亚表面损伤层深度的定量预测方法。使用前人建立的不同变形区域的原子势能变化模型实现了亚表面变形层深度的定量预测;建立了基于纳米压痕的亚表面损伤层的硬度检测模型,用来辨别亚表面损伤层中工件材料与未加工的工件材料的压痕硬度的差异,实现了亚表面损伤层深度的定量预测。在发现纳米机械加工亚表面损伤层产生孪晶现象的基础上,提出了一种利用孪晶变形来控制和改善亚表面损伤层的新设想。使用分子动力学模拟研究了晶粒尺寸对纳米晶体铜纳米机械加工中孪晶变形的影响;使用分子动力学模拟研究了纳米孪晶铜纳米机械加工机理,以及孪晶界间距对纳米机械加工的影响。为了使纳米机械加工的分子动力学模拟的结果可信、有用,分别从分子动力学模拟基本原理、材料机械性能的纳米压痕检测、纳米机械加工结果这三个方面逐级开展分子动力学模拟与实验的对比。检测了当前使用的经验势能函数描述的体系的体积模量、剪切模量和杨氏模量,并与实验测得的值进行了定量对比;基于单晶铜纳米压痕的分子动力学模拟获得的压痕深度-压痕硬度曲线求得了单晶铜的杨氏模量,并与纳米压痕实验测得的值进行了定量对比;进行了单晶铜(010)晶面和(111)晶面的纳米压痕实验,并从工件表面形貌的角度与分子动力学模拟的结果进行了定性的对比。更多还原

【Abstract】 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.更多还原