纳米压痕条件下 MgB2 超导块材的压入力学行为研究

本文采用纳米压痕技术对固相烧结法制备的 MgB2 超导块材进行压入力学实验, 对不同压深的载荷-位移曲线和纳米压痕数据的再现性进行了分析, 实验数据使用 Oliver-Pharr 方法计算得出 MgB2 的硬度值, 借助经验方程拟合纳米压痕蠕变曲线求得蠕变速率敏感指数(m ) . 结果表明, 微观结构不均匀性会对材料在压头压入过程中抵抗外力作用时产生影响, 使压痕数据再现性变差; MgB2 的硬度表现出尺寸效应, 即随着压入深度的增加硬度逐渐下降;m 值随压入深度增加而增加是位错滑移速度加快的结果.     

考虑相界效应的Ni基单晶合金纳米压痕模拟

利用分子动力学方法分别模拟金刚石压头压入Ni模型和Ni基单晶合金γ/γ′模型的纳米压痕过程,通过计算得到两种模型[001]晶向的弹性模量及硬度.采用中心对称参数分析不同压入深度时两种模型内部位错形核、长大过程以及Ni基单晶合金γ/γ′(001)相界面错配位错对纳米压痕过程的影响.结果显示:压入深度0.641 nm之前,两种模型的压入载荷-压入深度曲线相似,说明此时相界面处的错配位错对纳米压痕过程的影响很小;压入深度0.995 nm时,在错配位错处发生位错形核,晶体在γ相中沿着{111}面滑移,随即导致Ni基单晶合金γ/γ′模型压入载荷的下降,并在压入深度达到1.487 nm之前低于Ni模型相同压入深度时的压入载荷;压入深度从1.307 nm开始,由于相界面错配位错的阻碍作用,Ni基单晶合金γ/γ′模型压入载荷上升速度较快.     

利用分子动力学方法铁素体-渗碳体相界面效应探究

珠光体是十分重要的组织结构,因此本文构建了含铁素体-渗碳体相界面的模型,并采用分子动力学模拟方法模拟纳米压入的过程。通过对模拟结果的力学性能和组织结构分析,探究了铁素体-渗碳体相界面效应。研究发现,距铁素体-渗碳体晶界不同距离（位置压入）,在压入最初阶段,压头载荷随着压头与晶界距离的增大而增大,当压入深度达到一定深度后,载荷随着距离的增大而减小。杨氏模量和最大剪切模量受压头尖端下方原子结构的直接影响,硬度受到结构完整性和类型的共同影响。铁素体-渗碳体相界面影响了纳米压入过程中位错形核、增殖和扩展,宏观表现为在相同压入深度下,不同压入位置压头载荷的差异。     

孔洞对FeCrNiCoCu高熵合金拉伸性能影响的分子动力学研究

孔洞是FeCrNiCoCu高熵合金在制备过程中常见的缺陷,为此本文利用分子动力学模拟方法构建含孔洞的FeCrNiCoCu模型进行单轴拉伸模拟,探究了孔洞位置、孔洞半径和变形温度对其力学性能的影响.研究发现,在Z轴晶向为[111]的晶体中和晶界处的孔洞会显著降低模型的屈服应变和屈服强度,但对模型的杨氏模量影响不大.随着晶界处孔洞半径的增大,在弹性阶段,孔洞半径增大使应力集中面积增大,有利于位错形核,模型的力学性能随之降低.在塑性变形阶段,随着孔洞半径的增大,初始位错更倾向于向Z轴晶向为[001]的晶体中扩展.在中、低温条件下(T<800K),模型保持良好的力学性能;在高温条件下,力学性能显著降低.在高温塑性变形阶段,模型中的总位错线长度较低,平均流变应力也较低.     

初始压入位置对Ni基单晶合金纳米压痕影响研究

针对Ni基单晶合金建立初始压入γ 相的γ /γ' 模型和初始压入γ'相的γ'/γ 模型, 采用分子动力学方法模拟金刚石压头压入两种模型的纳米压痕过程, 计算两种模型[001]晶向硬度. 采用中心对称参数分析两种模型(001)相界面错配位错对纳米压痕过程的影响. 结果显示: 弛豫后, 两种模型(001)相界面错配位错形式不同, 其中γ'/γ 模型(001)相界面错配位错以面角位错形式存在; 压入深度在0.930 nm 之前, 两种模型(001)相界面错配位错变化不大, 压入载荷-压入深度及硬度-压入深度曲线较符合; 压入深度在0.930 nm之后, γ'/γ 模型(001)相界面错配位错长大很多, 导致相同压入深度时γ'/γ 模型比γ /γ'模型压入载荷和硬度计算结果小; 压入深度在2.055 nm之后, γ /γ'模型(001)相界面错配位错对γ 相中位错进入γ'相有阻碍作用, 但仍有部分位错越过(001) 相界面进入γ' 相中, γ'/γ 模型(001)相界面处面角位错对γ' 相中位错进入γ 相有更明显的阻碍作用, 几乎无位错越过(001) 相界面进入γ 相中, 面角位错的强化作用更明显, 所以γ'/γ 模型比γ /γ'模型压入载荷上升速度快.     

晶粒尺寸对纳米多晶铁变形机制影响的模拟研究

利用分子动力学模拟方法研究了拉伸荷载作用下晶粒尺寸对纳米多晶铁变形机制的影响.研究结果表明杨氏模量随着晶粒尺寸的减小而减小.当晶粒尺寸小于15.50 nm时,纳米多晶铁的峰值应力和晶粒尺寸之间遵循反常的Hall-Petch关系,此时晶粒旋转和晶界迁移是其塑性变形的主要变形机制;随着晶粒尺寸的增大,变形孪晶和位错滑移在其塑性变形过程中逐渐占据主导地位.裂纹的形成是导致大晶粒尺寸模型力学性能降低的主要因素.纳米多晶铁在塑性变形中会出现孪晶界的迁移和退孪晶现象.此外还研究了温度对纳米多晶铁变形机制的影响.     

应力预释放对单晶硅片的压痕位错滑移的影响

单晶硅片的压痕位错在一定温度下的滑移距离反映了硅片的机械强度. 压痕位错的滑移是受压痕的残余应力驱动的, 因此研究残余应力与位错滑移之间的关系具有重要的意义. 本文首先采用共聚焦显微拉曼术研究了单晶硅片压痕的残余应力经过300或500 ℃ 热处理后的预释放, 然后研究了上述应力预释放对压痕位错在后续较高温度(700–900 ℃)热处理过程中滑移的影响. 在未经应力预释放的情况下, 压痕位错在700–900 ℃热处理2 h后即可滑移至最大距离. 当经过上述预应力释放后, 位错在900 ℃热处理2 h后仍能达到上述最大距离, 但位错滑移速度明显降低; 而在700和800 ℃时热处理2 h后的滑移距离变小, 其减小幅度在预热处理温度为500 ℃时更为显著. 然而, 进一步的研究表明: 即使经过预应力释放, 只要足够地延长700和800 ℃ 的热处理时间, 位错滑移的最大距离几乎与未经预应力释放情形时的一样. 根据以上结果, 可以认为在压痕的残余应力大于位错在某一温度滑移所需临界应力的前提下, 压痕位错在某一温度滑移的最大距离与应力大小无关, 不过达到最大距离所需的时间随应力的减小而显著增长.     

基于纳米压痕法的多孔硅硬度及杨氏模量与微观结构关系研究

采用电化学腐蚀制备多孔硅，利用场致发射扫描式电子显微镜(field emission scanning electron microscope，FESEM)观测多孔硅的二维微观形貌，利用Nano Indenter XP中的纳米轮廓扫描仪组件(nano profilometry, NP)得到其三维拓扑分析图像，分析了微观结构差异的原因并讨论了多孔硅内部微观结构对其机械性能的影响；利用MTS Nano Indenter XP纳米压入测量仪器，研究了多孔硅的显微硬度和杨氏模量随压入深度的变化规律，比较了不同孔隙率多孔硅的机械性能差别.实验结果测得40mA/cm²，60mA/cm²，80mA/cm²和100mA/cm²四个不同腐蚀电流密度条件下制备多孔硅样品的孔隙率在60%—80%范围内，孔隙率随着腐蚀电流密度的增加而增大；在氢氟酸(HF)浓度为20%的条件下制备出多孔硅样品的厚度在40μm—50μm范围内；测得多孔硅的平均硬度、平均杨氏模量分别在0.478GPa—1.171GPa和10.912GPa—17.15GPa范围内，并且其数值随腐蚀电流密度的增加而减小，在纳米硬度范围内随压入深度的增加而减小，在显微硬度范围内其数值保持相对恒定，分析了样品表面、厚度、微观结构，及环境对其机械性能的影响，得到了多孔硅力学性能随其微观尺度形貌的变化规律. 关键词： 多孔硅 微观结构 硬度 杨氏模量     

基于多尺度模拟研究Ni/Cu薄膜压痕塑性的原子机制

基于准连续介质多尺度模拟方法研究了Ni/Cu双层薄膜初始压痕塑性的原子机制,结果主要包括:(1)当Ni晶体层厚度小于10nm时,随着Ni晶体层厚度的减少,薄膜弹性极限所对应的临界接触力逐渐降低,即Ni/Cu薄膜随Ni晶体层厚度减小而变软;(2)压头下方晶格Shockley分位错的开动、界面位错的分解、以及界面位错与晶格位错的相互作用是Ni/Cu薄膜初始塑性的微观原子机制,(3)根据模拟结果观察和位错弹性理论计算,承载初始塑性的界面位错数目变少是Ni/Cu薄膜软化的主要原子机制.本文研究结果能够为异质界面力学行为研究提供有益参考.     

冲击条件下含纳米空洞的单晶铜的塌缩

利用分子动力学方法研究了单晶铜中不同大小的球形空洞在冲击波下的演化过程.模拟结果表明不同大小空洞的塌缩过程不同.模拟中冲击波由空洞左边扫向空洞右边.在较大尺寸的空洞塌缩过程中会产生系列的位错环.当空洞半径较小时，先在空洞的右侧形成位错环，当空洞半径增大到某一临界大小时，在空洞左右两侧同时产生位错环，当空洞半径较大时，先在空洞左侧形成位错环.当空洞左右两侧的位错环均形成以后，其右侧位错环前端的生长速度大于其左侧的.空洞半径增大，相应的位错环前端的生长速度变化不大.当空洞半径增大时，空洞中心指向位错源的矢量方     

Reverse analysis in depth-sensing indentation for evaluation of the Young's modulus of thin films

This paper presents an approach to reverse analysis in depth-sensing indentation of composite film/substrate materials, which makes use of numerical simulation. This methodology allows the results of experimental hardness tests, acquired with pyramidal indenter geometry, to be used to determine the Young's modulus of thin film materials. Forward and reverse analyses were performing using three-dimensional numerical simulations of pyramidal and flat punch indentation tests to determine the Young's modulus of the thin films. The pyramidal indenter used in the numerical simulations takes into account the presence of the most common imperfection of the tip, so-called offset. The contact friction between the Vickers indenter and the deformable body is also considered. The forward analysis uses fictitious composite materials with different relationships between the values of the Young's modulus of the film and substrate. The proposed reverse analysis procedure provides a unique value for the film's Young's modulus. Depending on material properties, the value of the Young's modulus of the film can be more or less sensitive to the scatter of the experimental results obtained using the depth-sensing equipment. The validity of the proposed reverse analysis method is checked using four real cases of composite materials.     

The composition-dependent mechanical properties of Ge/Si core–shell nanowires

The Stillinger–Weber potential is used to study the composition-dependent Young's modulus for Ge-core/Si-shell and Si-core/Ge-shell nanowires. Here, the composition is defined as a ratio of the number of atoms of the core to the number of atoms of a core–shell nanowire. For each concerned Ge-core/Si-shell nanowire with a specified diameter, we find that its Young's modulus increases to a maximal value and then decreases as the composition increases. Whereas Young's modulus of the Si-core/Ge-shell nanowires increase nonlinearly in a wide compositional range. Our calculations reveal that these observed trends of Young's modulus of core–shell nanowires are essentially attributed to the different components of the cores and the shells, as well as the different strains in the interfaces between the cores and the shells.     

The effect of temperature,defect and strain rate on the mechanical property of multi-layer graphene: Coarse-grained molecular dynamics study

In this work, we investigate the effect of temperature, defect, and strain rate on the mechanical properties of multi-layer graphene using coarse-grained molecular dynamics (CGMD) simulations. The simulation results reveal that the mechanical properties of multi-layer graphene tend to be less sensitive to temperature as the layer increases, but they are sensitive to the distribution and coverage of Stone-Wales (SW) defects. For the same number of defect, there is less decline in the fracture stress and Young's modulus of graphene when the defects have a regular distribution, in contrast to random distribution. In addition, Young's modulus is less influenced by temperature and defect, compared to fracture stress. Both the fracture stress and Young's modulus have little dependence on strain rate.     

单层石墨烯杨氏模量的理论模型

石墨烯力学性能的研究对其在半导体技术中的应用是十分重要的,本文基于半连续体模型并结合石墨烯纳米结构特性,通过对原子的描述构建了石墨烯形变分量和位移分量的新关系,从而给出了单层石墨烯结构形变能,并计算了不同尺寸单层石墨烯的杨氏模量值.通过对不同方向杨氏模量的分析,讨论了单层石墨烯的手性行为.结果表明:随着尺寸的增加,单层石墨烯两个方向的杨氏模量分别趋于0.746 TPa和0.743 TPa,当尺寸相同时,两方向杨氏模量的最大差值不超过0.003 TPa,此结果与文献报道结果相符.在小应变情况下,单层石墨烯薄膜呈各向同性,且薄膜尺寸变化对该特性影响不大.该计算结果对研究石墨烯的其它力学特性提供一定的参考价值.     

Nanoindentation of gold nanoparticles functionalized with proteins

The hardness and Young's modulus of 10 and 20 nm gold nanoparticles (Au NPs) modified with bovine serum albumin and streptavidin were measured using a nanoindenter. The Au NPs were immobilized on a semiconductor surface through organic self-assembled monolayers. Changes in mechanical properties occurred when the Au NPs were immobilized on the surface. The hardness and Young's modulus were dependent on the size of the NPs, and the proteins on the particles showed highly plastic and elastic behavior compared to flat surfaces modified with self-assembled monolayers.     

分子动力学研究表面缺陷对硅纳米线杨氏模量的影响

硅纳米线因受量子尺寸效应与表面效应的影响而具有奇特的力、电及其耦合特性,成为了纳米电子器件的核心构件.然而在硅纳米线的制备过程中,表面产生缺陷不可避免.因此本文采用分子动力学方法着重研究了表面缺陷浓度对不同横截面形状(正方形、六角形和三角形)的[110]晶向和[111]晶向硅纳米线杨氏模量的影响.研究结果表明,当硅纳米线仅有单一表面缺陷时,不同晶向硅纳米线的杨氏模量均随表面缺陷浓度增加而迅速单调减小.当表面缺陷浓度为10%时,杨氏模量的减小幅度在10%-20%之间,减小幅度的差异与硅纳米线的晶向以及横截面形状密切相关.当存在多个表面缺陷时,杨氏模量随着缺陷浓度的增加表现出了不同程度的波动趋势.三角形截面硅纳米线的杨氏模量波动幅度最大,正方形截面的波动较小,即表面缺陷分布的不同对正方形截面硅纳米线的杨氏模量影响较小,这表明表面缺陷的影响与其分布及硅纳米线的横截面形状密切相关.通过与实验结果对比,本文的研究结果揭示了表面缺陷是导致硅纳米线杨氏模量实验值变小的重要因素,因此在表征硅纳米线的力学性能时,需要考虑表面缺陷的影响.     

Elastic properties of SWCNTs with curved morphology: Density functional tight binding based treatment

The self-consistent charge density functional based tight-binding method is used to calculate the effect of curvature on the structure, average energy of atoms and Young's modulus of armchair single-wall carbon nanotubes (SWCNTs) under axial strains. We found that as the amount of curvature increases, the average energy of atoms and the Young's modulus decrease and the equilibrium CC distance increases for (7,7) SWCNTs. However, we also found that the average energy of atoms and Young's modulus of (5,5) SWCNTs are weakly affected by increasing the amount of curvature. Our results also show that the average energy of atoms and Young's modulus of smaller diameter armchair nanotubes are smaller than that of the larger diameter ones.     

Mechanical examination of PZT microfibre defects structure

The major problem in the production process of efficient ultrasonic transducer is the preparation of defect-free PZT fibres. A considerable amount of empirical work is presently in progress to achieve this goal of special importance for high-sensitive transducers. However, there is a lack of basic research on the detection of residual stress and defects areas in these fibres due to difficulties in mechanical examination of such flexible elements. This work presents use of the nanoindenter for material characterisation of PZT fibres of 140 µm radius obtained by extrusion method. The sudden depth-excursions during indentation on the edge of fibres have been clarified using Piezoresponse Mode Atomic Force Microscopy method and XRD measurements. The nanoindentation method proves to be the efficient tool capable to detect contribution of defects along the radius, properly estimate hardness as well as corresponding Young's modulus and concluding on structural properties of the micrometre-range ceramics fibres.     

Modification of the Mechanical Properties of Core-Shell Liquid Gallium Nanoparticles by Thermal Oxidation at Low Temperature

Gallium nanoparticles (Ga NPs) are attracting increasing attention because of their appealing physical-chemical properties. In particular, their mechanical properties play a key role in the implementation of these core-shell structures on certain applications, such as soft and stretchable electronics. Thus, efforts are being addressed to modulate them mainly by chemical means. In contrast, this study investigates how the mechanical properties of the outer gallium thin oxide shell change when its thickness is increased through a thermal oxidation strategy. Specifically, as-deposited Ga NPs, as well as those subjected to thermal oxidation at 300 °C for three different times, are studied by performing single-particle indentations by atomic force microscopy over a wide range of NP radius. This analysis helps to confirm that the Reissner's thin-shell model for small deformations within the elastic regime is obeyed. From these data, the dependence of the shell stiffness and the Young's modulus of the gallium oxide on the thermal treatment is obtained. It is found that the shell stiffness increases with the annealing time, even by a factor of 50 under prolonged thermal oxidation, while the gallium oxide Young's modulus, close to 30 GPa, does not change significantly.