硬度合金基体上CVD金刚石薄膜的形态表征

采用ＳＥＭ、Ｒａｍａｎ光谱、ＸＲＤ等测试方法，对直流等离子体射流ＣＶＤ法在硬质合金基体上合成的金刚石膜进行了形貌和结构分析。结果表明，该方法合成的金刚石膜形貌和质量受基体表面上的温度梯度、化学物质（原子氢、碳氢基团等）浓度梯度的影响较大。膜层内存在ＧＰａ数量级的残余压应力，微观应力很小。嵌镶块尺寸为纳米数量级，且随甲烷浓度增高而减小，由此而估算的位错密度统计平均值达１０＾１０ｃｍ＾－２数量级。综合     

超细颗粒和超细颗粒材料

超细颗粒和超细颗粒材料韩仲琦（天津水泥工业设计研究院，天津３００４００）１什么是超细颗粒众所周知，我们把眼睛可以看见的物质体系叫作宏观体系（ｍａｃｒｏｓｃｏＰｉｃＳｙｓｔｅｍ），把理论研究中所接触到的原子、分子大小体系叫作微观体系（ｍｉｃｒｏｓｃｏｐ...     

钢球颗粒增强锌铝基复合材料界面微观应力场分析

有限元计算细观力学在复合材料应力传递机制的研究中得到了广泛应用。研究不同情况下复合材料内的应力场分布规律,对认识材料的损伤机制,指导复合材料设计,具有重要的理论与实际意义。本文主要通过有限元对锌铝基复合材料界面微观应力场进行研究,建立单向拉伸（或压缩）情况下的分析模型,计算界面处在拉应力与压应力两种情况下的基体和填充体的应力分布,分析复合材料破坏的根本原因。拉伸应力作用下复合材料经常于界面处产生脱粘,脱粘位置一般发生在极区。压缩时裂纹的产生与扩展机理同拉伸不一样,它不是主拉伸应力的作用,其裂纹的产生与扩展,可能是基体在塑性流变过程中大量的位错在增强相处受阻塞积的结果。     

涂层材料的断裂分析

和基体厚度相比，涂层很薄，因此细观力学模型可把基体作为半无限弹性体，由于涂层和基体材料的膨胀系数及弹性系数不匹配，涂层材料中残余热应力的解析解为Ｅｃ／（１－ｙｃ）·（αｓ－αｃ）△Ｔ。用有限元法校核，该应力和解析解吻合得好，通过对涂层产生裂纹驱动力和断裂韧性的讨论，提出了抗裂涂层厚度公式。     

陶瓷基复合材料（SiCw／Y—TZP）在循环压应力下的疲劳裂纹扩展

研究了碳化硅晶须（ＳｉＣｗ）增强，Ｙ２Ｏ３稳定的ＺｒＯ２四方多晶体（Ｙ－ＴＺＰ）复合材料（ＳｉＣｗ／Ｙ－ＴＺＰ）在循环压应力作用下的疲劳特性，单边缺口弯曲试样在纵向循环压应力作用下缺口根部产生垂直于压应力的Ｉ型裂纹，类似于金属材料，在室温下循环应力导致Ｉ型裂纹的稳定扩展。压应力在缺口根部产生的不可逆损伤区在循环卸载过程中形成较大的残余拉伸应力场，使裂纹萌生并长大，同时，裂纹面产生的碎粒及晶须拔出导     

Si晶体临界切应力的一种求法

研究了Ｓｉ晶体中微印压和氧沉淀应力场开动位错的临界切应力τｃ结果表明，在充分大的距离内，微印压或氧沉淀连同邻近的位错群可视为一个集中应力芯；其应力场随距离增大而趋于零．在此基础上获得了求τｃ的公式，求得区熔和直拉Ｓｉ单晶在９００℃时的τｃ为３．１×１０３和５．３×１０３Ｎ．ｃｍ－２．     

短纤维增强金属基复合材料应力分布的剪切滞后分析

使用弹性理论和剪切滞后分析, 推导出了基体和纤维应力场分布表达式, 研究了纤维体积分数、纤维长径比和基体屈服强度等对应力分布和应力传递的影响。研究表明, 基体和纤维应力分布及基体的塑性行为具有明显的不均匀性, 基体与纤维之间存在明显的应力传递和应变分配。关键词　金属基复合材料, 剪切滞后理论, 应力应变分布      

短纤维增强金属基复合材料应力分布的剪切滞后分析

使用弹性理论和剪切滞后分析,推导出了基体和纤维应力场分布表达式,研究了纤维体积分数、纤维长径比和基体屈服强度等对应力分布和应力传递的影响。研究表明,基体和纤维应力分布及基体的塑性行为具有明显的不均匀性,基体与纤维之间存在明显的应力传递和应变分配。     

基于拉曼光谱mapping技术研究低温循环对炭纤维增强聚酰亚胺复合薄膜界面微观力学的影响

碳纳米管选用（CNT）作为拉曼应力传感器，通过建立拉曼光谱mapping技术研究了经多次低温循环（-198～25°C,0～300次）的炭纤维增强聚酰亚胺复合薄膜（CF/CNT-PI）的界面微观应力变化。研究发现：聚酰亚胺薄膜（CNT-PI）即使经300次低温循环，树脂内部应力依然为～175 MPa，循环次数对树脂内部应力影响较小，表明该材料具有良好的耐低温性。进一步研究了炭纤维（CF）增强的CNT-PI薄膜的内应力变化，获得了炭纤维、界面、树脂基体区域的微观应力mapping分布，发现CF区域的受力大于基体部分，表明CF在该体系中起到了对应力最主要载体的作用，并发挥了良好的增强效果。在循环次数<250次时，微观应力变化不大；但当循环次数高达300次时，炭纤维及界面区域应力值分别提高了21%和12.9%，应力在材料内部的集中增大会降低材料的力学性能。本研究有效地定量了外界温度循环变化下复合材料的增强材料、基体及界面的微观应力分布，这为检测复合材料服役过程中的使用安全性提供了一种理论依据与评判手段。     

增强体对复合材料接触性能的影响

基于等效夹杂方法,引入一种数值建模方法用于求解Hertz接触载荷作用下复合材料次表面应力场。通过与有限元方法的对比验证本方法的有效性和优越性;讨论不同形状增强体深度、材料、尺寸、体积分数、相对位置等分布参数对基体应力场的影响。分析结果表明,双增强体接触模型中,次表面最大von Mises应力随增强体深度和半径的增大呈先增大后减小趋势,随增强体与基体之间材料差异的增大而单调递增;增强体体积分数及相对位置将对基体应力分布产生较大影响。通过钛基复合材料滚动接触疲劳实验验证了本文方法处理复合材料接触性能的能力。     

Computer experiments on nano-indentation: A molecular dynamics approach to the elasto-plastic contact of metal copper

Molecular dynamics simulations are used to investigate the micro-mechanisms of nano-indentation for tip to substrate contact. The method combines a many-body interatomic potential derived from the nearest-neighbor EAM and brownian dynamics (BD) approach to simulate a rigid tip indenting Cu (001) surface. Elastic contact and plastic instability of the crystal are investigated through the loading-unloading cycle, the variations of the system potential energy versus the tip approach, the atomic stress distributions and the portraits of atomic trajectories and configurations. For elastic indentation, we find that atomistic stress distributions resembling roughly to those of the continuum Hertzian fields, except for a jump-to-contact phenomenon in the initial contact stage. When the tip approach is beyond some critical value, plastic instability of the substrate occurs, and both the contact load and potential energy decrease dramatically. Detailed calculations reveal that material yield at the atomic level is still governed by the von Mises shear strain-energy criterion, while atomistic trajectories show that the displacements in (010) plane of atoms near the contact region is similar to that in Johnson's cavity model, accompanied by atomic cross-layer movements in [010] direction to release the strain energy. The crystal defects after plastic indentation include subsurface cavities, surface atomic steps and plastic indent.     

基于分子动力学模拟的纳米多晶α-碳化硅变形机制

在考虑晶界和温度效应影响的条件下,基于分子动力学法使用Vashishta势函数研究多晶α-碳化硅基体在纳米压痕作用下的塑性变形机制,分析载荷位移曲线并通过识别变形结构描述了变形区域中的原子破坏和迁移轨迹变化。在下压过程中,因接触载荷不断增大在接触区的晶粒内产生无定型化相变并不断向晶体内部扩展,扩展到晶界处被阻碍住。随着载荷的持续增大,晶界作为位错发射源在高应力水平下出现1/2〈110〉全位错滑移。同时,随着温度的升高α-碳化硅多晶的承载能力下降,特别是材料内部出现塑性变形,位错从晶界处形核长大并向晶体内部扩展,最后形成‘U型’位错环。     

外应力场下NiAl合金微裂纹动态扩展的分子动力学模拟

目的 对NiAl合金中不同晶体取向的裂纹扩展动力学行为进行原子尺度研究,明晰在塑性变形过程或实际应用过程中裂尖的脆性解理和塑性变形行为,为研究NiAl合金的塑性变形行为和评估服役寿命提供理论基础。方法 建立了4种不同取向的裂尖模型,其扩展取向分别为(010)[001]、(0■1)[100]、(010)[101]、(01■)[011],用分子动力学方法对上述模型进行模拟,采用Gear算法计算原子在真实受力状态下的运动情况。结果 在NiAl合金中,微裂纹在外载作用下的裂尖反应强烈依赖于裂纹取向（裂纹面及裂纹前沿方向）。{110}裂纹面的裂纹构型易于脆性解理扩展;{100}裂纹面的裂纹构型具有一定的塑性,裂尖处可形成位错发射,位错的出现可以协调塑性变形,模拟结果与实验观察相一致。结论 裂纹的晶体取向对裂尖的马氏体相变行为有重要影响,当裂纹前沿为<100>方向时,原子在裂纹前端的{100}滑移面上运动,诱导B2相转变成L10相,产生马氏体相变,这种马氏体相变有利于相变增韧,能够促进裂尖处位错发射,可提升材料塑性和服役寿命。     

单晶硅（111）晶面纳米压痕过程分子动力学仿真及实验

为了研究脆性单晶材料单晶硅（111）晶面微观机械特征,应用分子动力学,对压痕过程进行仿真分析．本文应用平均势能和径向分布函数相结合方法对压痕过程进行考察．仿真结果表明,工件在压头附近的晶格结构发生变化．在仿真过程中通过确定工件内部的分界线来表明住错传播运动;在压痕结束后工件自身弛豫过程变形区域弹性恢复大约27％．应用Hysitron公司的Triboindenter纳米原位压痕仪进行压痕实验,并把实验得到的材料压痕折合模量与分子动力学仿真计算结果进行比较,结果表明仿真的误差率小于31％．     

Influence of contact geometry on hardness behavior in nano-indentation

In this paper the influence of contact geometry, including the round tip of the indenter and the roughness of the specimen, on hardness behavior for elastic-plastic materials is studied by means of finite element simulation. We idealize the actual indenter by an equivalent rigid conic indenter fitted smoothly with a spherical tip and examine the interaction of this indenter with both a flat surface and a rough surface. In the latter case the rough surface is represented by either a single spherical asperity or a dent (cavity). Indented solids include elastic perfectly plastic materials and strain hardening elastic–plastic materials, and the effects of the yield stress and strain hardening index are explored. Our results show that due to the finite curvature of the indenter tip the hardness versus indentation depth curve rises or drops (depending on the material properties of the indented solids) as the indentation depth decreases, in qualitative agreement with experimental results. Surface asperities and dents of curvature comparable to that of the indenter tip can appreciably modify the hardness value at small indentation depth. Their effects would appear as random variation in hardness.     

The brittle to ductile transition of single crystal materials—An indentation model

A model for the brittle to ductile transition of brittle single crystal materials under indentation has been investigated. Continuous dislocation pile-ups against the wedge tip are used to explain the plastic deformation. The indentation depth is attributed to the dislocation pile-ups. The critical indentation depth p _cof brittle to ductile transition is proposed. Thus, the single crystal material is in brittle mode during the indentation loading if the indentation depth is greater than p _c. Otherwise, it is ductile. Micrographs support this modeling. Indentation on the surfaces of (100) or (110) in fcc and bcc single crystal materials is compared. The parameter Sis proportional to the number of dislocations and to the reciprocal of wedge angle. The value of Sis smaller for (100) than for (110) in fcc structure, but the trend of bcc structure is opposite. The shape of indenter is similar to that of grinding particles consisting in cutting tools. In order to maintain cutting efficiency in ductile mode, the cutting tool must be replaced if the grinding particles are blunt.     

A New Model Developed to Evaluate the Contact Area Arising During Nano-indentation Tests With Pileup Behavior of Metal Materials

A new mechanical model is developed in this paper for metal materials to investigate the behavior arising during the loading/unloading process of an indentation test. Two governing differential equations are derived for the depth solution of the indenter tip and the depth solution formed at the separation point, expressed in a power form. All spring/block and damping coefficients shown in these governing differential equations associated with the elastic/plastic and viscous behaviors are determined by the real-coded genetic algorithm. With the aid of experimental results of the depth at the indenter tip shown at large and small indentation depths, aluminum and steel were used as two examples of soft materials in this paper. The pileup behavior is implicitly included in the evaluation of the contact projected area (A) in the present model. A significant difference in is caused if the pileup is not considered in the model. Under a constant maximum load, the contact area is slightly increased by decreasing the loading/unloading rate.     

An atomic force microscope tip designed to measure time-varying nanomechanical forces

Tapping-mode atomic force microscopy (AFM), in which the vibrating tip periodically approaches, interacts and retracts from the sample surface, is the most common AFM imaging method. The tip experiences attractive and repulsive forces that depend on the chemical and mechanical properties of the sample, yet conventional AFM tips are limited in their ability to resolve these time-varying forces. We have created a specially designed cantilever tip that allows these interaction forces to be measured with good (sub-microsecond) temporal resolution and material properties to be determined and mapped in detail with nanoscale spatial resolution. Mechanical measurements based on these force waveforms are provided at a rate of 4 kHz. The forces and contact areas encountered in these measurements are orders of magnitude smaller than conventional indentation and AFM-based indentation techniques that typically provide data rates around 1 Hz. We use this tool to quantify and map nanomechanical changes in a binary polymer blend in the vicinity of its glass transition.     

Atomic force microscopy investigations on nanoindentation impressions of some metals: effect of piling-up on hardness measurements

In the indentation test, the hardness and the elastic modulus depend strongly on the estimate of the indenter-material contact area at peak load. However, many elastic–plastic behaviours such as elastic recoveries during unloading and piling-up or sinking-in of surface profiles during indentation affect the determination of the hardness and the elastic modulus. So, atomic force microscopy is a method of utmost importance to provide an accurate knowledge of the indentation impression especially when plastic deformations occur, that leads to errors in the determination of the contact area. Atomic force measurements of vanadium, tungsten, molybdenum and tantalum pure metals as well as stainless steels, often used as substrates for thin films depositions, highlight the difficulties to estimate the contact area. The variation of hardness values determined by atomic force microscopy measurements and nanoindentation test is correlated to the formation of folds of 150 and 100 nm high, around the residual impression of vanadium and tungsten indented at 0.1 N, respectively. Some folds which increase with increasing loads are detected on the residual impressions of both 35CD4 and 30NCD16 stainless steels indented under loads of 0.01 N, only. Such structures are related to piling-up of surface profiles that could lead to an underestimate of the contact area in the indentation test. So, the hardness value of tungsten could be closer to 6 than to 7 GPa whereas the effect of piling-up on the estimation of contact area of vanadium could be lower. Almost no deformation is seen on tantalum and molybdenum. So, the hardness values determined by the various methods are consistent. These results show that atomic force microscopy measurements are quite complementary of the nanoindentation test.     

极浅压入下纳米压痕仪针尖面积函数校准方法的研究

采用原子力显微镜直接扫描纳米压痕仪针尖法、球面拟合法和熔融石英标准样块的间接测量法对极浅压入下纳米压痕仪的针尖面积函数进行比较分析。实验表明,在极浅压入下,原子力显微镜直接法由于真实地反映了针尖尖端的几何形貌因而获得的面积函数更为准确可靠。建立了相应的数学模型,对于直接法测量中主要的误差,即由于原子力显微镜针尖曲率半径带来的误差进行了分析,结果表明在极小压入深度下压入深度越小,原子力显微镜针尖曲率半径带来的压痕仪针尖面积函数相对误差越大。