纳米压痕过程的多尺度准连续介质法研究

【作者】 黎军顽；

【导师】 倪玉山； 林逸汉；

【作者基本信息】 复旦大学 ， 流体力学， 2010， 博士

【摘要】 纳米材料由于其特殊的力学性能引起了人们的广泛关注,例如高的强度、硬度、耐磨性、延展性以及低温超塑性等。纳米材料之所以表现出这些特殊的性能与材料内部结构和变形机制密切相关。纳米压痕技术是一种相对简单有效地评估薄膜材料力学性能的方法,通过纳米压痕实验不仅可以获得材料的相关性能参量,而且能够反映材料弹塑性转变的机制,揭示微观组织结构与宏观力学性能的关系。然而,纳米压痕是一个复杂的接触问题,进行纳米压痕试验时受到诸多因素的影响,例如材料表面粗糙度,衬底效应,晶界效应,压头几何形状,晶格各项异性和压痕尺寸效应等,即便是在相同的设备和试验条件下纳米压痕试验过程的重复性也不能得到保证,因此需要采用多尺度方法对纳米压痕过程进行模拟。为了深入研究纳米压痕过程中材料的微观破坏过程和弹塑性转变的机制,本文采用多尺度准连续介质法(Quasicontinuum method, QC)对薄膜材料纳米压痕过程进行模拟,探讨材料初始表面缺陷、晶格各向异性、压头尺寸、层间界面以及界面结构对纳米压痕过程的影响,本文的主要研究内容如下：(1)初始表面缺陷对纳米压痕过程的影响。研究了初始缺陷对纳米压痕过程中位错形核与发射、Peierls应力以及位错发射临界载荷的影响。研究表明,在整个纳米压痕过程中出现了多次位错形核与发射现象,初始缺陷对第1和第3对位错的形核与发射影响较小,而对第2对位错的形核与发射具有明显地推迟作用,并伴随有裂纹扩展现象；由于初始缺陷引起薄膜材料内部严重的晶格畸变,导致系统应变能和位错运动Peierls应力增加；裂纹扩展前,发射第2对位错需要的临界载荷增加,裂纹失稳后,位错发射需要的临界载荷下降。多尺度模拟获得的纳米硬度和位错Peierls应力与实验结果吻合。(2)晶格各向异性和压头尺寸效应对纳米压痕过程的影响。研究了晶格各项异性和压头尺寸对纳米压痕过程中纳米硬度,压头接触应力分布,位错形核临界载荷以及系统应变能的影响。研究表明,不同晶面纳米压痕过程中呈现出完全不同的位错现象,纳米压痕获得的载荷-位移曲线呈现出的不连续性与这些不同的位错活动密切相关；沿有利于开动滑移系的晶体取向,由于在该晶体取向下具有较多的滑移方向,易激活滑移系产生滑移,因此获得的纳米硬度值,压头法向和切向接触应力,位错形核临界载荷以及系统应变能较小；沿不利于开动滑移系的晶体取向,由于在该晶体取向下无明显的滑移方向,难激活滑移系产生滑移,因此获得的纳米硬度值,压头法向和切向接触应力,位错形核临界载荷以及系统应变能较大；模拟结果与实验结果以及Rice-Thomson (R-T)位错模型计算结果吻合。(3)层间界面对纳米压痕过程的影响。从数值仿真和理论研究两方面揭示了Cu/Ag双层薄膜纳米压痕过程中界面对双层薄膜的强化或弱化效应的本质。研究表明,纳米压痕过程中层间界面不仅对Cu/Ag双层薄膜具有强化效应,而且还具有弱化效应。对于界面的强化效应,它主要是由滑动位错在传输过程中所受的阻力控制(例如,映像力,晶格阻力以及滑动位错与失配位错之间的相互排斥力)；滑动位错在传输过程中所受的阻力越大,界面对Cu/Ag双层薄膜的强化效果越好；对于界面的弱化效应,它主要是由于界面上失配位错形核及运动引起的严重应力集中和局部失配应变所致。然而,在Cu/Ag双层薄膜纳米压痕过程中,与界面对双层薄膜的弱化效应相比,界面对双层薄膜的强化效应占主导地位,主要表现为界面的强化作用。模拟结果与实验结果以及位错理论模型预测一致。(4)界面结构对纳米压痕过程的影响。详细分析了不同界面结构对Cu-Ag双层薄膜纳米压痕过程中滑动位错形核与发射、纳米硬度以及系统应变能的影响。研究表明,界面对上层薄膜内滑动位错的形核与发射具有明显的推迟作用；Cu-Ag双层薄膜系统的力学性能主要取决于其上层薄膜的力学性能；界面结构对滑动位错形核与发射临界压深和临界载荷以及系统纳米硬度具有显著影响,界面对双层薄膜系统具有软化效应,呈现出反Hall-Petch关系；不同的界面结构引起双层薄膜系统中呈现复杂的位错活动状态,导致系统应变能呈现显著变化。尤其是滑动位错与界面发生相互作用时,系统应变能将出现一次大幅度的跳跃。更多还原

【Abstract】 Nanostructural materials have been the subject of intensive research in recent years due to its unique mechanical properties, such as high strength, hardness, superior wear resistance, high tensile ductility and superplasticity at relatively low temperatures. Research has shown that these unique mechanical properties are closely related to internal structure of nanomaterials and deformation mechanism. Nanoindentation has become a standard technique for evaluating the mechanical properties of thin film. The load-displacement response obtained from nanoindentation test can be used to predict the material properities, understand the nature of the elastic-plastic deformation mechanism, and reveal the relationships between the microstructure and the macroscopic mechanical properties. However, nanoindentation is a complicated contact problem, which can be strongly influenced by surface roughness, substrate effects, grain boundaries effects, indenter geometry, crystalline anisotropy and indentation size effect. The repeatability and reproducibility of nanoindentation test result are poor, even if the experimental equipment and condition are the same. Therefore, it is very important to study the incipient plasticity during nanoindentation by multiscale atomic simulation.To further study the microscopic failure process and the elastic-plastic deformation mechanism of thin film, the quasicontinuum method (QC) is employed to elucidate the details of incipient plasticity during nanoindentation. The influences of initial defect, crystalline anisotropy, indenter size, interface between adjacent layers and interfacial structure on nanoindentation are discussed, respectively. The main contents of this paper are as follows:(1) Effects of initial defect on nanoindentation. The nanoindentation processes under influences of the initial defect are investigated about dislocation nucleation, dislocation emission, Peierls stress, and load necessary for dislocation emission. The results demonstrate that the load versus displacement response curves experience many times abrupt drops with the emission of dislocations beneath the indenter. The initial defect is found to be insignificant on nucleation and emission of the 1st and 3r dislocation dipoles, but has a distinct effect on the 2nd dislocation dipole. The nucleation and emission of the 2nd dislocation dipole is postponed obviously because of the effect of initial defect, and then crack propagation is accompanied. The strain energy of thin film and Peierls stress of dislocation dipole beneath the indenter are increase with deformation processes due to the severe lattice distortion in the thin film. Before the cleavage occurs, the load necessary for the 2nd dislocation dipole nucleation and emission increases in nanoindentation with initial defect, on the contrary, it decreases after the cleavage occurred. The nanohardness and Peierls stress in this simulation show a good agreement with relevant theoretical and experimental results.(2) Effects of crystalline anisotropy and indenter size on nanoindentation. The nanoindentation deformation processes under influences of crystalline anisotropy and indenter size are investigated about hardness, load distribution, critical load for first dislocation emission and strain energy under the indenter. It is shown that entirely different dislocation activities are presented under the effect of crystalline anisotropy during nanoindentation. The sharp load drops in the load-displacement curves are caused by the different dislocation activities. Both crystalline anisotropy and indenter size are found to have distinct effect on hardness, contact stress distribution, critical load for first dislocation emission and strain energy under the indenter. The above quantities are decreased at the indenter into Ag thin film along the crystal orientation with more favorable slip directions that easy trigger slip systems; whereas those will increase at the indenter into Ag thin film along the crystal orientation with less or without favorable slip directions that hard trigger slip systems. The results are shown to be in good agreement with experimental results and Rice-Thomson dislocation model solution.(3) Effects of interface between adjacent layers on nanoindentation. The nature of strengthening and weakening effects of interface on Cu/Ag bilayer film and the underlying deformation mechanisms during nanoindentation are revealed from both the numerical and theoretical aspects. The investigations show that there is not only a strengthening effect of interface on Cu/Ag bilayer film system, but also a weakening effect. Concerning the strengthening effect, it is governed primarily by the resistance to the glide dislocation transmission, such as Image force, Peierls-Nabarro force and the repulsive force between the glide dislocation and the misfit dislocation. The bigger resistance will lead to the stronger strengthening effect. With regard to the weakening effect, it is produced by the stress concentration and local misfit strain in the core region of the misfit dislocations due to the nucleation and propagation of misfit dislocations along the interface, which can impair significantly the binding strength between adjacent layers. However, compared with the weakening effect induced by the nucleation and motion of misfit dislocations, it must be emphasized that the strengthening effect of interface on Cu/Ag bilayer film system is predominant. The multiscale simulation results are in good agreement with the experimental results and dislocation theory model.(4) Effects of interfacial structure on nanoindentation. The influences of interfacial structure on nanoindentation of Cu-Ag bilayer film system are analyzed systematically about glide dislocation nucleation and emission, nanohardness and strain energy. The results show that the nucleation and emission of glide dislocations are postponed obviously because most of the indentation energy is absorbed by the interface between adjacent layers. The mechanical property of Cu-Ag bilayer film system strongly depends on the performance of upper thin film. The interfacial structure is found to have distinct effect on critical load and critical indentation depth for first glide dislocation emission and nanohardness of bilayer film system. Due to the softening effect of interface, a reverse Hall-Petch phenomenon is typically observed in Cu-Ag bilayer film system. The complicated dislocation configurations caused by different interfacial structure during nanoindentation can lead to a greatly change in strain energy. In particular, during the interaction between glide dislocations and interface, the strain energy-displacement curve will represents an abrupt jump.更多还原