纳米力学进展(续)

概述2002年度在纳米力学方面的若干新进展.在纳观计算力学方面,讨论了在微结构质流演化算法、纳米结构中应变的量子效应算法、LMPM并行算法等方面的进展.在纳观实验力学范畴,着重介绍了立体刻蚀的微加载系统.然后,我们展述了在纳米晶体力学、纳米管力学和纳米压痕力学等方面的新进展.      

Modeling and simulation of mechanical properties of nano-particle filled materials

With the recent advances in nanoscale science and engineering, materials containing reinforcement with superior mechanical properties can be found in many advanced products. The accurate prediction of the mechanical properties of this class of composite materials is important to ensure the reliability of the products. Characterization methods based contact probe such as nano-indentation and scratch tests havebeen developed in recent years to measure the mechanical properties of the new class of nanomaterials. This paper presents a constitutive modeling framework for predicting the mechanical properties of nanoparticle reinforced composite materials. The formulation directly considers the effects of inter-nanoparticle interaction and performs a statistical averaging to the solution of the problem of two-nanoparticle interaction. Final constitutive equations are obtained in analytical closed form with no additional material parameters. The predictions from the proposed constitutive model are compared with experimental measurement from nano-indentation tests. This constitutive model for nanoparticle reinforced composites can be used to determine the volume concentration of the reinforcing nanoparticles in nano-indentation test.     

测量界面软铬层力学性质的纳米压入法

为了测量双层铬的界面软铬层力学性质,提出了化学腐蚀基体法,通过溶解掉基体制备没有基体支撑的自由铬层,将在横截面内线状显示的界面转化为界面表面(铬层与基体相连接的面),避免了横截面不能显示界面表面的缺点。对界面表面进行纳米压入实验和借助于表征薄膜力学性质的表面压入能量法,测得了描述界面软铬层力学性质的弹性模量和压入弹、塑性功等参数。     

Ag implantation-induced modification of Ni–Ti shape memory alloy thin films

Nanocrystalline thin films of Ni–Ti shape memory alloy are deposited on an Si substrate by the DC-magnetron co-sputtering technique and 120?keV Ag ions are implanted at different fluences. The thickness and composition of the pristine films are determined by Rutherford Backscattering Spectrometry (RBS). X-Ray diffraction (XRD), atomic force microscopy (AFM) and four-point probe resistivity methods have been used to study the structural, morphological and electrical transport properties. XRD analysis has revealed the existence of martensitic and austenite phases in the pristine film and also evidenced the structural changes in Ag-implanted Ni–Ti films at different fluences. AFM studies have revealed that surface roughness and grain size of Ni–Ti films have decreased with an increase in ion fluence. The modifications in the mechanical behaviour of implanted Ni–Ti films w.r.t pristine film is determined by using a Nano-indentation tester at room temperature. Higher hardness and the ratio of higher hardness (H) to elastic modulus (E_r) are observed for the film implanted at an optimized fluence of 9?×?10¹⁵ ions/cm². This improvement in mechanical behaviour could be understood in terms of grain refinement and dislocation induced by the Ag ion implantation in the Ni–Ti thin films.     

基于纳米压痕试验的混凝土微观耐久性表征

利用纳米压痕和扫描电镜研究了不同冻融循环下混凝土的微观性能,揭示了冻融循环所导致混凝土退化的机理．纳米压痕结果显示,混凝土浆体的压痕模量与硬度的相关系数很大,并且都不满足正态分布．本文利用核密度估计来计算浆体微观性能的概率密度函数．根据浆体和骨料的性能差异,用阶跃函数来拟合界面过渡区的微观力学性能,同时利用扫描电镜观察界面过渡区的形貌．采用以上两种方法研究不同冻融循环下的混凝土,发现在不同的尺度下,冻融循环对混凝土的影响不同：在纳米尺度下,由于水的参与,高密度水凝胶会进一步形成;而在微米尺度下,由于冻融循环中水结冰膨胀和材料多相性,微裂纹进一步扩展,导致断裂能量降低,混凝土表现出宏观退化．     

电镀铜薄膜力学性能的实验研究

电子器件中大量使用铜膜作为电信号通道,而且一般采用电镀工艺制成.铜膜的力学性能参数对于其热疲劳可靠性的研究非常重要.目前有关该材料的力学性能研究尚不充分,而且数据极为不统一.本文借助于纳米压痕法、声发射等实验手段对电镀铜薄膜的静态力学性能(包括弹性模量和屈服强度等)及疲劳性能进行了测试.结果发现,与大块铜材料相比,电镀铜薄膜的弹性模量低很多,但屈服强度与大块铜材料相当,甚至高出200％.同时,本文采用弯曲疲劳实验,以电阻变化为失效判据,对镀铜材料的疲劳性能进行了测试,获取了该材料不同失效判据下的疲劳寿命预测模型的系数.     

单晶硅的纳米力学响应及其相变机制

硅在大规模集成电路、MEMS/NEMS、半导体工业中具有不可替代作用,但是目前对硅的塑性变形及其相变机制的理解远未成熟.采用大规模分子动力学模拟研究(100)面的单晶硅在球形金刚石压头纳米压入过程中的纳米力学响应、相变过程和相分布规律.结果表明:在弹性变形阶段载荷-压深曲线与Hertz接触理论预测结果相吻合.两者的分离点准确地预示了塑性变形的发生.金刚石结构的Si-I相向体心结构的BCT5相转变导致了单晶硅初始的塑性变形.初始形成的BCT5相在次表面形成了一个倒置的金字塔形结构.Si-II相的形成则稍微滞后一些.在较大的载荷下BCT5在压入面上形成一个四重对称的图案分布.相对于小压头条件下大的BCT5相区,大压头更有利于SiII相的发展.卸载后生成的高压Si-II相和BCT5相全部转变为非晶硅.研究结果确认了单晶硅纳米压入中BCT5相的存在;揭示了单晶硅塑性变形的相变机理,即Si-I转变为BCT5和Si-II相;并强调了Si-I相向BCT5相转变对于单晶硅塑性变形的重要作用.     

Li2O-Al2O3-SiO2微晶玻璃表面粗糙度对纳米硬度测试的影响

 介绍了纳米压痕测试技术的基础理论及纳米压痕法常用的Oliver -Pharr方法的计算原理。采用纳米压痕试验测得不同表面粗糙度的Li2O-Al2O3-SiO2微晶玻璃样品的纳米硬度、弹性模量和载荷-位移曲线。结果表明样品表面粗糙度会降低纳米压痕测试结果的稳定性、准确性和可靠性：样品表面粗糙度越小,测得的纳米硬度和弹性模量值波动越小,载荷-位移曲线重合性越高。随着最大载荷的增大,测得的弹性模量逐渐减小,其原因是压痕边缘材料发生了塑形变形。在超光滑表面样品（Ra=0.9 nm）上测得较为准确的Li2O-Al2O3-SiO2微晶玻璃纳米硬度和弹性模量值分别为8.8 GPa和7.79 GPa。纳米压痕测试结果的重合度对于评价超光滑表面完整性研究具有指导意义。     

纳米孪晶界对可动位错演化特性与金属Al强化机理探究

位错是金属塑性变形普遍形式,对其可动位错演化特性与规律探寻并充分利用,将在金属强韧化提升中有着潜在基础前瞻性研究价值.本文基于分子动力学法对金属Al塑性变形的可动位错迁演特性展开研究,洞悉纳米压痕诱导的可动位错与孪晶界面间作用规律,揭示出金属强化微观机制,并分析单层孪晶界高度与多层孪晶界层间距对可动位错迁演、位错密度、硬度、黏着效应的影响.研究发现:高速变形下的金属非晶产生和密排六方结构的出现会协同主导Al基塑性变形,而孪晶界会阻碍可动位错滑移、诱导可动位错缠绕及交滑移产生,在金属承载提升中扮演了位错墙和诱导位错胞形成的微观作用.通过在孪晶界形成钉扎位错和限制位错迁移,在受限域形成高密度局域可动位错,显著强化了金属硬度和韧性,降低了卸载时黏附于探针表面的原子数.结果表明:Al基受载会诱导上表面局部非接触区原子失配斑出现;单层孪晶界高度离基底上表面距离减小时,位错缠绕和交滑移作用越明显,抗黏着效应也随之下降;载荷持续增加会诱驱孪晶界成为位错萌生处与发射源,并伴随塑性环的繁衍增殖.     

Stiffness based technique to probe cyclic damage accumulation in micro-structurally graded bond coats via micro-beam bending tests

This paper highlights a novel technique to delineate the fatigue response of different regions within thin microstructurally graded platinum nickel aluminide bond coats. Notched clamped beam structures fabricated from distinct microstructural zones of these coatings are subjected to programmed cyclic bending using the nano-indentation system. A methodical approach is established herein to quantify the cyclic damage preceding crack pop-in by using the cyclic stiffness of the beam as an indicator to mark failure. Preliminary results from these tests show that there is a characteristic change in the stiffness of the beam before a crack pop-in event occurs and different regions within the coating show different stiffening characteristics. Factors affecting the measured stiffness such as offsets in the loading position and blunting of the notch tip have been estimated using the finite element method. A graded flow stress model has been proposed and implemented in FEM to quantify the local flow stress changes accompanying the measured rise in stiffness. Electron transparent foils lifted off from the notched region of the beam post-testing suggests that the cyclic stiffening of the beams occurs due to dislocation hardening in the plastically deformed region close to the notch tip. Toughening mechanisms active in the crack wake have thus been investigated and correlated to the measured cyclic stiffness.