纳米压痕法测磁控溅射铝薄膜屈服应力

为了在考虑残余应力下测量出磁控溅射铝薄膜的屈服应力,提出了一种实验测量方法,通过曲率测试法和球形压头纳米压痕法测出磁控溅射铝薄膜的屈服应力．建立球形压痕力学模型,并用ANSYS对球形压痕进行力学有限元仿真,利用直流磁控溅射技术在硅基上淀积一层1 μm厚的铝薄膜,首先通过曲率测试法测量膜内等双轴残余应力,再利用最小二乘曲线拟合法从薄膜/基底系统的球形压头纳米压痕实验数据中提取出铝薄膜的屈服应力,测得磁控溅射铝薄膜的屈服应力为371 MPa．该方法也可以用来研究其他材料的薄膜和小体积材料的力学特性．     

氮化铬薄膜的径向纳动损伤机制研究

采用纳米压痕仪和曲率半径为20μm的金刚石球形压头研究了40Cr基体和CrNx薄膜的径向纳动行为,并探讨其损伤机制.结果表明：CrNx薄膜能够提高40Cr基体的抗压性能,提高其接触刚度,降低残余压痕深度,减少在纳动循环过程中的能量耗散;增大载荷或增加循环次数均可加剧材料的纳动损伤;40Cr基体的纳动损伤表现为压痕边缘的塑性堆积;而CrNx薄膜的纳动损伤由表面的径向和环向拉应力所引起,表现为环向裂纹和径向裂纹的萌生与扩展.     

血红细胞力学性能的纳米压痕实验和有限元模拟

采用纳米压痕技术和有限元方法研究了血红细胞的生物力学性能. 进行了血红细胞的纳米压痕实验, 得到了血红细胞的材料参数和变形形貌; 在实验基础上, 建立了血红细胞的三维有限元模型, 模拟了血红细胞的压痕载荷-位移曲线, 并考虑了参数效应. 数值模拟结果和实验数据符合很好. 通过改变压头与材料之间的摩擦系数和压头曲率半径等参数, 比较了载荷-位移曲线的变化情况. 研究表明摩擦系数对压痕载荷-位移曲线和应力分布影响很小, 而压头曲率对载荷-位移曲线的影响明显.     

纳米压痕残余应力场拉曼光谱实验研究

本文采用纳米压入仪在晶向为〈111〉和〈100〉的两种单晶硅片表面压入1000nm,卸载后得到深度约为550~570nm的压痕。使用共聚焦显微拉曼光谱仪对压痕周边区域进行测量,采用场扫描成像技术得到了压痕周边拉曼频移、半高宽、峰强等拉曼信息,通过分析由频移求得残余应力场的分布。在实验的基础上讨论了残余应力场的分布,以及晶向对应力场分布的影响,近似给出了压痕边缘最大压应力与微裂纹尖端最大拉应力。对其他拉曼信息的分析表明,半高宽和峰强信息与材料晶格结构的变化相关,在一定程度上也可以反映残余应力的作用。     

纳米压痕中位错运动的原子模拟

利用分子动力学方法模拟研究了金刚石压头压入Ni薄膜(111)晶面的纳米压痕过程中薄膜进入初始塑性后的纳观机制,采用中心对称参数(CSP)研究不同压入深度时薄膜内部的位错的萌生和生长情况.结果表明:压痕力-压痕深度曲线的每一次的剧烈的振荡,都是一次能量释放的过程,在薄膜内部的位错生长也最剧烈.加载过程中,压入深度为0.66nm时出现位错(层错),压入深度为0.93nm时出现明显的位错形核,随着压入深度的增加,多个位错形核相互作用形成梯杆位错.压入深度为1.4nm时,梯杆位错旁出现了棱柱形不全位错环,随着压入深度的增加,棱柱形不全位错环沿着{111}滑移面运动.在最大压入深度处,薄膜塑性形变达到最大.     

灵芝孢子孢壁的弹性模量和硬度研究

采用纳米压痕技术和数值模拟研究灵芝孢子孢壁的弹性模量和硬度.利用原位纳米力学测试与分析系统,测试灵芝孢子孢壁的弹性模量和硬度.得到了载荷--位移曲线图和硬度、弹性模量随压痕深度变化的值.并用有限元方法模拟压痕过程,利用ANSYS软件,按照灵芝孢子孢壁和Berkovich压头的结构,建立了二维计算模型,得到纳米压痕的等效应力分布以及压痕过程中加载和卸载时的载荷--位移曲线.考察了摩擦、压头尖端半径对模拟结果的影响.结果显示:灵芝孢子孢壁的平均弹性模量为2.0GPa,硬度为0.13GPa.模拟结果在趋势上与实验结果有较好的吻合,与理论分析的载荷--位移关系基本一致.摩擦、压头尖端半径小于100nm时对模拟结果不会造成明显影响.研究结果为分析孢子的破壁机理提供必要参数.     

纳米压痕试验中压头尺寸效应的准连续介质法分析

采用准连续介质法模拟了单晶铝纳米压痕试验过程,分析了不同宽度的刚性矩形压头所引起的初始塑性变形特点,获得了载荷-压深、应变能-位移和硬度-压深曲线.从位错理论的角度分析了压头尺寸对纳米压痕测试结果的影响.研究发现:随着压头宽度的不断增大,压头下方位错形核所需要的载荷和压深程度增大,需要的应变能增加,应变能的变化速率递增,纳米硬度值减小,呈现出明显的尺寸效应.同时表明在一定的压入深度下,硬度与压头尺寸之间存在着一定的比例关系,不同尺寸压头获得的硬度值可以相互换算,但当矩形刚性压头宽度大于或等于120时这种尺寸效应消失.研究结果为纳米压痕实验过程中压头尺寸的选择提供了参考依据.     

聚甲基丙烯酸甲酯材料(PMMA)纳米压/划痕变形热恢复的实验研究

聚甲基丙烯酸甲酯(PMMA)是常见的光伏电池封装材料.本文采用锥形和球形两种压头,利用纳米压痕仪开展了PMMA的纳米压痕和划痕实验.基于表面形貌扫描得到的PMMA材料在不同温度热处理后压痕与划痕变形随时间恢复的演化规律,分析了时间、温度和压头形状等对PMMA材料压/划痕变形恢复过程的影响机理.结果表明,压/划痕变形恢复...     

碳化硅薄膜的力学性能测试分析

对利用射频磁控溅射及真空退火方法在(100)硅晶片衬底上制备的纳米晶碳化硅(SiC)薄膜,用纳米压痕仪进行了力学性能测试分析。纳米压痕技术测试给出两块SiC薄膜样品I和II的弹性模量/硬度分别约为106GPa/9.5GPa和175GPa/15.6GPa。纳米划痕技术测试两块SiC薄膜的摩擦系数分别约为0.02~0.15和0.05~0.18,显示出良好的润滑性能;对薄膜的临界附着力等进行测量以评价膜基结合强度,分析了划痕过程中薄膜近表面弹塑性变形和断裂信息。在原子力显微镜下对SiC薄膜样品的初始表面及残余压痕和划痕形貌进行了观察分析,与测试结果相符。综合比较,样品II的整体性能优于样品I。本文中薄膜的弹性模量和硬度值较低可归因于制膜技术的不同和表层碳含量偏高。     

纳米金刚石薄膜结构相变及摩擦性能的研究

论文用第一性原理分子动力学模拟研究了纳米金刚石薄膜的结构相变和表面重构,以及金刚石薄膜(110)表面不同方向上表面形态和抛光残余的本质.研究发现,纳米金刚石的表面碳团簇通过断开(111)面的σ键,形成具有碳六元环结构的石墨碎片;内部原子sp3杂化向sp2杂化转化的发生是从(111)面上成对C原子向石墨相转化时形成π键的过程中获得了能量,驱动石墨的转变由表层向心部逐渐进行.转变过程中存在一种洋葱状富勒烯和金刚石结构共存的过渡相—Bucky-diamond,表面悬空键的消除和表层的富勒烯外壳最大限度地降低了表面能和系统总能量,Bucky-Diamond结构稳定存在.sp2杂化碳的缓冲作用对薄膜中残余应力有较大的影响.     

Size effects in indentation response of thin films at the nanoscale: A molecular dynamics study

The indentation response of Ni thin films of thicknesses in the nanoscale was studied using molecular dynamics simulations with embedded atom method (EAM) interatomic potentials. A series of simulations were performed in films in the [1 1 1] orientation with thicknesses varying from 4 to 12.8 nm. The study included both single crystal films and films containing low angle grain boundaries perpendicular to the film surface. The simulation results for single crystal films show that as film thickness decreases larger forces are required for similar indentation depths but the contact stress necessary to emit the first dislocation under the indenter is nearly independent of film thickness. The low angle grain boundaries can act as dislocation sources under indentation. The mechanism of preferred dislocation emission from these boundaries operates at stresses that are lower as the film thickness increases and is not active for the thinnest films tested. These results are interpreted in terms of a simple model.     

Predictive modeling of nanoindentation-induced homogeneous dislocation nucleation in copper

Nanoscale contact of material surfaces provides an opportunity to explore and better understand the elastic limit and incipient plasticity in crystals. Homogeneous nucleation of a dislocation beneath a nanoindenter is a strain localization event triggered by elastic instability of the perfect crystal at finite strain. The finite element calculation, with a hyperelastic constitutive relation based on an interatomic potential, is employed as an efficient method to characterize such instability. This implementation facilitates the study of dislocation nucleation at length scales that are large compared to atomic dimensions, while remaining faithful to the nonlinear interatomic interactions. An instability criterion based on bifurcation analysis is incorporated into the finite element calculation to predict homogeneous dislocation nucleation. This criterion is superior to that based on the critical resolved shear stress in terms of its accuracy of prediction for both the nucleation site and the slip character of the defect. Finite element calculations of nanoindentation of single crystal copper by a cylindrical indenter and predictions of dislocation nucleation are validated by comparing with direct molecular dynamics simulations governed by the same interatomic potential. Analytic 2D and 3D linear elasticity solutions based on the Stroh formalism are used to benchmark the finite element results. The critical configuration of homogeneous dislocation nucleation under a spherical indenter is quantified with full 3D finite element calculations. The prediction of the nucleation site and slip character is verified by direct molecular dynamics simulations. The critical stress state at the nucleation site obtained from the interatomic potential is in quantitative agreement with ab initio density functional theory calculation.     

Quasicontinuum simulation of single crystal nano-plate with a mixed-mode crack

The quasicontinuum (QC) method is employed to simulate a nickel single crystal nano-plate with a mixed-mode crack. Atomic stresses near the crack tip are fitted according to the elastoplastic fracture mechanics equations. It is found that the atomic stress fields neighboring the crack tip are also singular and controlled by the atomic stress intensity factors. And then the critical energy release rates for brittle and ductile fracture are computed and compared in order to predict crack propagation or dislocation emission. Four possible slip directions at the crack tip are pointed out. Finally, the slip direction around the crack tip is determined by the shear stress and it is well consistent with the atomic pictures from the QC simulation.     

Wedge indentation into elastic-plastic single crystals, 1: Asymptotic fields for nearly-flat wedge

Asymptotic stress and deformation fields under the contact point singularities of a nearly-flat wedge indenter and of a flat punch are derived for elastic ideally-plastic single crystals with three effective in-plane slip systems that admit a plane strain deformation state. Face-centered cubic (FCC), body-centered cubic (BCC), and hexagonal-close packed (HCP) crystals are considered. The asymptotic fields for the flat punch are analogous to those at the tip of a stationary crack, so a potential solution is that the deformation field consists entirely of angular constant stress plastic sectors separated by rays of plastic deformation across which stresses change discontinuously. The asymptotic fields for a nearly-flat wedge indenter are analogous to those of a quasistatically growing crack tip fields in that stress discontinuities can not exist across sector boundaries. Hence, the asymptotic fields under the contact point singularities of a nearly-flat wedge indenter are significantly different than those under a flat punch. A family of solutions is derived that consists entirely of elastically deforming angular sectors separated by rays of plastic deformation across which the stress state is continuous. Such a solution can be found for FCC and BCC crystals, but it is shown that the asymptotic fields for HCP crystals must include at least one angular constant stress plastic sector. The structure of such fields is important because they play a significant role in the establishment of the overall fields under a wedge indenter in a single crystal. Numerical simulations—discussed in detail in a companion paper—of the stress and deformation fields under the contact point singularity of a wedge indenter for a FCC crystal possess the salient features of the analytical solution.     

Use of thin polyparaxylene films to study the plastic deformation bismuth single crystals

The effect of thin polyparaxylene films on the mechanical twinning of bismuth single crystals with the (111) surface subjected to local deformation. It is found that the number of twins formed near the stress concentrator increases in the presence of the film. Possible mechanisms are proposed to explain an increase in the mobility of twin dislocations in a deformable crystal whose surface is coated with a polyparaxylene film. Spalling of bismuth is found in the regions deformed by the indenter. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 47, No. 4, pp. 162–166, July–August, 2006.     

An atomistically-informed dislocation dynamics model for the plastic anisotropy and tension-compression asymmetry of BCC metals

Atomistic simulations have shown that a screw dislocation in body-centered cubic (BCC) metals has a complex non-planar atomic core structure. The configuration of this core controls their motion and is affected not only by the usual resolved shear stress on the dislocation, but also by non-driving stress components. Consequences of the latter are referred to as non-Schmid effects. These atomic and micro-scale effects are the reason slip characteristics in deforming single and polycrystalline BCC metals are extremely sensitive to the direction and sense of the applied load. In this paper, we develop a three-dimensional discrete dislocation dynamics (DD) simulation model to understand the relationship between individual dislocation glide behavior and macro-scale plastic slip behavior in single crystal BCC Ta. For the first time, it is shown that non-Schmid effects on screw dislocations of both {110} and {112} slip systems must be implemented into the DD models in order to predict the strong plastic anisotropy and tension-compression asymmetry experimentally observed in the stress-strain curves of single crystal Ta. Incorporation of fundamental atomistic information is critical for developing a physics-based, predictive meso-scale DD simulation tool that can connect length/time scales and investigate the underlying mechanisms governing the deformation of BCC metals.     

On the nonlocal nature of dislocation nucleation during nanoindentation

Using static atomistic simulations, we study the full details of the mechanism by which dislocations homogeneously nucleate beneath the surface of a initially defect-free crystal during indentation. The mechanism involves the collective motion of a finite disk of atoms over two adjacent slip planes, the diameter of which depends on the indenter size. The nucleation mechanism highlights the need for nonlocal considerations in the development of a nucleation criterion. We review three nucleation criteria from the literature, each of which is based on purely local measures of the state of stress, and show that none are sufficiently general to predict nucleation in realistic atomic systems. We then propose a criterion based on an eigenmode analysis of the atomic-scale acoustic tensor. We demonstrate the accuracy of the criterion, which also works in the presence of existing topological defects like free surfaces or dislocation cores. The dependence of the size of the nucleated disk on the indenter radius leads to a self-similar nucleation process and virtually no indentation size effect (ISE), suggesting that homogeneous nucleation is only possible for very small indenters.     

Modeling the plastic response of thin metal membranes

Using a dislocations-based model of slip and crystal plasticity, we show by illustrative examples that the experimentally observed increase in the yield stress of very thin metallic membranes most likely is due to the variation of grain orientations through the thickness of the membrane, as well as the surface hardness due to oxidation or contamination, both of which generally are insignificant when there is a sufficient number of interior crystals through the membrane thickness; the overall effect may well be produced by a combination of these two causes. We show that crystal plasticity models can account for such size effects without a need for resorting to phenomenological strain-gradient models. We illustrate this using Nemat-Nasser's dislocations-based slip-induced crystal plasticity model that inherently includes length scales, although other rate-dependent slip models, e.g., the classical power-law slip model, most likely would qualitatively produce similar results. Our numerical results, based on the experimentally supported dislocation-induced slip model and the values of the model parameters given in Nemat-Nasser and Li [1998. Flow stress of F.C.C. polycrystals with application to OFHC Cu. Acta Mater. 46, 565-577], correlate well, both qualitatively and quantitatively, with the experimental results reported by Hommel and Kraft [2001. Deformation behavior of thin copper films on deformable substrates. Acta Mater. 49, 3935-3947] and Espinosa et al. [2004. Plasticity size effect in free-standing submicron polycrystalline FCC films subjected to pure tension. J. Mech. Phys. Solids 52, 667-689] for thin copper membranes, suggesting that, for submicron-sized samples, the classical crystal plasticity with slip models, does qualitatively account well for the small-size effects, and that quantitative predictions are obtained when, in addition, a physics-based dislocation model that includes length scales, is used. It is thus concluded that the length-scale effect and the size effect are two separate issues in metal plasticity, both of which are nicely accounted for by physics-based dislocation models of crystal plasticity without a need to include the plastic strain gradient.     

A rheological theory for liquid crystal thin films

A macroscopic rheological theory for compressible isothermal nematic liquid crystal films is developed and used to characterize the interfacial elastic, viscous, and viscoelastic material properties. The derived expression for the film stress tensor includes elastic and viscous components. The asymmetric film viscous stress tensor takes into account the nematic ordering and is given in terms of the film rate of deformation and the surface Jaumann derivative. The material function that describes the anisotropic viscoelasticity is the dynamic film tension, which includes the film tension and dilational viscosities. Viscous dissipation due to film compressibility is described by the anisotropic dilational viscosity. Three characteristic film shear viscosities are defined according to whether the nematic orientation is along the velocity direction, the velocity gradient, or the unit normal. In addition the dependence of the rheological functions on curvature and film thickness has been identified. The rheological theory provides a theoretical framework to future studies of thin liquid crystal film stability and hydrodynamics, and liquid crystal foam rheology. Received: 9 October 2000 Accepted: 6 April 2001     

Nanocontact between BCC tungsten and FCC nickel using the quasicontinuum method

The nanocontact between BCC tungsten and FCC nickel is investigated by using the multiscale simulation method (quasicontinuum method). The new proposed interatomic potential – Extended Finnis-Sinclair potential is adopted and compared with the conventional Ackland Finnis-Sinclair potential in the nanoindentation model. A series of multiscale simulations of contact formation, indentation, subsequent pulling, and separation between a tungsten indenter and a nickel substrate are performed by means of the quasicontinuum method. Different plastic mechanisms are observed for different crystal directions and indenter shapes. Besides the adherence between the tungsten indenter and the nickel substrate, the geometrical shape, which is formed by the slip planes inside the substrate, plays a vital role in the material transfer between the indenter and the substrate. The formation and evolution of necking and cracking in the nanocontact model are also discussed.