纳米摩擦中极性有机分子超薄膜的结构、对称性及能量机理

采用分子动力学方法, 模拟了由脂肪酸C_nH_2n+1COOH和C₁₇H₃₅COOH (n=12,13,14,15,16,17) 按1:1比例组成的7种混合单层Langmuir-Blodgett (LB)膜和由C₁₆H₃₃COOH 分子组成的单层膜的摩擦性质. 结果显示: 1) 随着混合单层膜内的不同分子链长差的减小, 其剪切压随之减小, 摩擦力主要来自单层膜间的库伦作用; 2) 混合膜内的两种不同分子的尾基排列对其摩擦性能影响较大, 当混合LB膜中所有分子尾基全同排列时剪切压较小. 当分子链长差为1 个C-C键长时, 分子尾基排列对膜的摩擦性质影响较大. 3) 同种分子尾基全同排列组成的单层膜, 当上下两单层膜的尾基呈镜面对称时, 其剪切压随着分子链长的增加而减小, 摩擦力主要来自膜间的库伦作用; 当上下两单层膜的尾基呈中心对称时, 膜间摩擦力主要来自膜间的范德华 (VDW) 作用. 关键词： 分子动力学模拟 纳米摩擦 薄膜 库伦能     

有机分子超薄膜的结构对摩擦的影响

采用分子动力学方法, 模拟了由脂肪酸C_nH_2n+1COOH}和C₁₇H₃₁COOH (n=12,13,14,15,16,17)组成的混合单层Langmuir-Blodgett(LB)膜间的摩擦特性, 探究了膜结构的变化对超薄膜的摩擦的影响. 结果显示. 在滑动过程中, 随着n的增加, 膜内分子的运动受到邻近分子的约束逐渐增加, 膜结构的稳定性也逐渐增加, 其剪切压逐渐减小, n=17时的剪切压最小. 在两单层膜之间无氢键形成; 而混合膜内的分子之间形成的氢键是单层膜结构稳定的主要因素, 其中n=16时形成的氢键最稳定, 但全部由相同C₁₇H₃₁COOH分子组成的单层膜的滑动效果最好. 分子的弯曲形变能对剪切压影响非常小.     

萘酞菁LB膜取向液晶研究

利用二(3-叔丁基硅氧基)萘酞菁硅 (SiNc) Langmuir-Blodgett (LB) 膜对丝状液晶的定向进行了研究,发现在不同膜压下沉积的SiNc LB膜具有不同的定向效果.在15mN/m低膜压下沉积的LB膜上丝状液晶主要呈沿膜面水平排列,而30mN/m高膜压下沉积的LB膜则取垂直膜面取向.原子力显微镜(AFM)对相应的LB单层膜和多层膜进行的分子尺度上的形貌研究发现,高膜压条件下SiNc分子以分离的单体或聚集体形式非连续地在基片上呈线性堆积排列,形成一定的分子“纳米线”,并且大环平面垂直于基片 关键词：     

摩擦导致的聚合物表层微观结构改变

应用大规模分子动力学方法, 模拟了锥形探头在非晶态聚合物薄膜表面的滑动摩擦过程, 研究了摩擦导致的聚合物薄膜表层微观结构改变, 以及探头与基体间黏着作用、滑动速度和分子链长度对基体表层微观结构改变的影响. 当探头与基体之间为黏着作用时, 摩擦导致基体表面滑痕区域的键取向沿滑动方向重新取向, 导致表层分子链回转半径沿滑动方向伸长, 并且这些表层微观结构的改变程度随滑动速度的减小而增大. 在摩擦导致结构改变的过程中, 链端单体和链中单体的贡献作用不同, 形成了不同的分子链拉伸变形机制. 当样本缠结度较大或探头滑动速度较小时, 相比于链中单体, 探头对链端单体的拖曳作用使更多分子链发生拉伸变形. 研究还发现, 在探头与聚合物薄膜系统中, 使薄膜表层微观结构发生改变是摩擦能量耗散的重要途径.     

直流电场诱导LB膜中分子重新取向研究

用旋转二次谐波产生方法研究了半花菁/花生酸Y型交替LB多层膜中由外加法向直流电场诱导的分子的重新取向.外加法向直流电场能够有效地改变LB膜中光学活性半花菁分子的取向,迫使其长轴转向电场的方向.随着极化电场的增大,分子在基板平面内的各向异性逐渐减小,而二次谐波强度的增量则随外加电场的增大而呈现平方增长的规律.在室温下,用1.3×10⁶V/m的电场极化10min,可以使半花菁LB多层膜的二次谐波强度增大一个量级.     

速度对聚四氟乙烯摩擦系数影响的分子动力学模拟

基于分子动力学方法建立了双层聚四氟乙烯(polytetrafluoroethylene,PTFE)摩擦模型,研究了不同速度下PTFE的摩擦过程.通过分析不同速度下接触区内下层PTFE分子键长、键角、分子形状的变化及接触过程中摩擦力和正压力的变化,从微观角度研究了速度对PTFE摩擦系数的影响.研究结果表明:随着速度的增加,接触区内PTFE粒子间的键长变短,键角变小,分子链沿x;方向的变形量增加.变形后的PTFE分子产生的回弹力导致上、下层PTFE分子间相互作用力增加,从而增加了摩擦力.当速度进一步增大时,接触区内下层PTFE粒子间的键长和键角多处于平衡位置,分子链沿x方向的变形量减小.这很可能是由于接触区内下层PTFE分子沿速度方向倾斜,使上、下层PTFE分子趋于平行滑动,从而降低了摩擦力.不同速度下正压力几乎保持不变.因此,当上层PTFE所受外载荷固定时,摩擦系数随着速度的增加先增大后减小,临界速度为1.2 m/s,这与实验研究结果一致.     

MgO（001）表面上沉积MgO薄膜过程的分子动力学模拟

采用分子动力学方法模拟了MgO分子连续沉积于MgO（001）表面上的薄膜生长过程,分析了衬底温度和分子入射能对MgO分子在衬底表面上的扩散能力以及对衬底表面覆盖率的影响.模拟结果表明,随着衬底温度的升高,在衬底表面上沉积的MgO分子扩散能力增强,MgO薄膜层中空位缺陷变少.低温下,分子入射能的增大有助于提高衬底表面覆盖率;高温下,表面覆盖率随入射能增大到3.0 eV时达到最大值,入射能继续增大,表面覆盖率减小. 关键词： MgO薄膜生长 分子动力学 计算机模拟 表面扩散     

LJ流体非牛顿现象的分子动力学模拟

使用非平衡分子动力学模拟方法研究了单原子LJ流体的非牛顿流变行为,并在系统中分别施加稳态Couette流场和振荡剪切流场.在Couette流场的模拟中,流体出现剪切变稀和法向应力差效应,不同剪切率下的径向分布函数反映了流体分子由于剪切所导致的微观结构变化,通过分析势能函数发现当剪切率增大时,分子间排斥作用增强,吸引作用减弱.在振荡剪切流场的模拟中,发现剪切应力和剪切率之间的相位差随频率增加而增加,随频率增加复数粘度的实部先增大再减小,虚部单调增加,导致虚部粘度相对实部粘度比例增大,弹性模量和粘性模量之比也随频率增加而增加.这三点现象表明LJ流体出现粘弹性行为,且在高频率下,弹性所占比重增大.     

用ATR方法研究LB单分子膜层的等温热脱附过程

对由有机长链花生酸分子所组成的Langmuir-Blodgett单分子膜层(LB膜)的多层结构,利用衰减全反射方法进行了等温热脱附研究,提出了一个物理模型以描述LB膜的脱附过程,得到了LB膜中花生酸分子的不同吸附端的层间脱附能,疏水端为19kcal/mol;亲水端为27kcal/mol.     

一种制备单原子碳链的方案

考虑到迄今为止实验上尚不能制备含有上百个原子的自由单原子链,本文提出利用探针从graphene中拉伸较长单原子碳链的设想,并通过分子动力学计算发现,室温下可以利用C60探针以1 m/s的速度从graphene的zigzag边缘拉出较长的一维单原子碳链,为实验提供了一种制备单原子碳链的可能方案. 关键词： 碳链 一维 分子动力学 探针     

Ideal shear banding in metallic glass

As the most fundamental deformation mechanism in metallic glasses (MGs), the shear banding has attracted a lot of attention and interest over the years. However, the intrinsic properties of the shear band are affected and even substantially changed by the influence of non-rigid testing machine that cannot be completely removed in real compression tests. In particular, the duration of the shear banding event is prolonged due to the recovery of the stressed compliant frame of testing machine and therefore the temperature rise at the operating shear band is, more or less, underestimated in previous literatures. In this study, we propose a model for the ‘ideal’ shear banding in metallic glass. The compliance of the testing machine is eliminated, and the intrinsic shear banding process is extracted and investigated. Two important physical parameters, the sliding speed and the temperature of shear band, are calculated and analysed on the basis of the thermo-mechanical coupling. Strain-rate hardening is proposed to compensate thermal softening and stabilise the shear band. The maximum value of the sliding speed is found to be on the order of 10 m/s at least, and the critical temperature at which strain-rate hardening begins to take effect should reach as high as 0.9T_g (T_g is the glass transition temperature) for a stable shear banding event in metallic glass according to the early experimental data. This model can help to understand and control the shear banding and therefore the deformation in MGs.     

A TiB2 metal matrix composite coating enriched with nitrogen: Microstructure and wear properties

Metal matrix composites containing titanium nitrides or titanium borides raise great interest to researchers due to their high wear resistance and enhanced corrosion properties. In the present investigation composite coatings containing both titanium nitrides/carbonitrides and titanium diborides were produced on plain steel substrates using the plasma transferred arc (PTA) technique with argon-nitrogen mixtures in the plasma and shielding gas. The microstructure of the metal matrix composites (MMC) obtained was thoroughly studied and found to consist of primary titanium diboride particles surrounded by a eutectic matrix containing, apart from ferrite, both titanium diboride and titanium carbonitride particles. The wear behavior of the composite coatings was assessed by pin on disk experiments. The wear rate against both a tool steel counterbody and an alumina counterbody is of the order of 10⁻⁴ mm³/m. The friction coefficient for both the alloyed layer-tool steel system and the alloyed layer-alumina system increases up to sliding speed of 0.30 m/s and then decreases, when the sliding speed increases further. Specifically, the friction coefficients are varied between the values 0.5 and 0.65. The wear mechanism for the tribosystem alloyed layer-tool steel is characterized by plastic deformation and adherence of material coming from the alloyed layer to the surface of the ball, while for the tribosystem alloyed layer-alumina ball, severe plastic deformation and formation of oxide layer are observed.     

LiIO3-NaIO3赝二元系相图及其非晶态晶化动力学的研究

本文对LiIOO₃-NaIO₃赝二元系的相图,相变,非晶态的形成和稳定性,以及晶化的动力学过程等进行了仔细的研究,LiIO₃-NaIO₃赝二元系属共晶体系,共晶温度为325℃,共晶点成份为含50m/o LiIO₃,用共晶点附近成份的试样,在超过熔点150℃的情况下首次获得碘酸盐的非晶态,非晶态的存在降低了LiIO₃的表观相变温度,强X射线的辐照,各种空气湿度,细粒度等因素都加 关键词：     

Phase composition, microstructures and magnetic properties of melt-spun Nd1.2Fe10.5Mo1.5 ribbons and their nitrides

The effect of wheel speed on the phase compositions and microstructures of melt-spun Nd_1.2Fe_10.5Mo_1.5 ribbon was investigated. It is found that the Nd(Fe,Mo)₁₂ phase can be obtained at the wheel speed of 10 m/s, and TbCu₇-type Nd(Fe,Mo)₇ phase is formed with the wheel speed higher than 10 m/s. The amorphous phase is achieved at 65 m/s. The average grain size of phases decreases linearly with increasing wheel speed. The Nd(Fe,Mo)₁₂N_1.0 nitride obtained from annealed ribbons quenched at 65 m/s shows a coercivity much higher than that from the ribbons quenched at 10 m/s, which is due to the smaller grain size in the former ribbons.     

Dry sliding wear behavior of Al-SiO2 composites

Abstract

Al-base composites with different amount of silica (5, 10, 15 and 20 wt.%) were developed using powder metallurgy route and compacts were sintered at 550 °C for 2 h. XRD analysis of all compositions was conducted for phases and amount of the second phase present. Morphology of the composites shows quite uniform distribution of the SiO₂ particles but at higher percentage of SiO₂ particles the clustering starts. Mechanical properties such as uniaxial compressive strength (UCS) and hardness were evaluated and it is seen that among all compositions, composite with 10 wt.% SiO₂ has maximum UCS and hardness. Wear behavior of all composites was studied with sliding distance, applied loads, sliding velocity and composition. All composites show a linear increase in cumulative wear with distance and load. Wear rate with load increases continuously for all compositions, however, composite with 10 wt.% SiO₂ revealed minimum wear rate with distance, sliding velocity and loads. Wear rate with sliding velocity increases sharply after attaining minima at 3 m/s sliding velocity. SEM analysis of wear tracks is in agreement with wear results. Al-10 wt.%SiO₂ also shows minimum wear coefficient values for all loads, however, wear coefficient decreases with load for all compositions.     

Friction and normal forces of model friction modifier additives in simulations of boundary lubrication

ABSTRACT

Interaction forces between solid surfaces are often mitigated by adsorbed molecules that control normal and friction forces at nanoscale separations. Molecular dynamics simulations were conducted of opposing semi-ordered monolayers of united-atom chains on sliding surfaces to relate friction and normal forces to imposed sliding velocity and inter-surface separation. Practical examples include adsorbed friction-modifier molecules in automatic transmission fluids. Friction scenarios in the simulations had zero, one, or two fluid layers trapped between adsorbed monolayers. Sliding friction forces increased with sliding velocity at each stable separation. Lower normal forces were obtained than in most previous nanotribology molecular simulations and were relatively independent of sliding speed. Distinguishing average frictional force from its fluctuations showed the importance of system size. Uniform velocities were obtained in the sliding direction across each adsorbed film, with a gradient across the gap containing trapped fluid. The calculated friction stress was consistent with measurements reported using a surface forces apparatus, indicating that drag between an adsorbed layer and trapped fluid can account sufficiently for sliding friction in friction modifier systems. An example is shown in which changes in molecular organisation parallel to the surface led to a large change in normal force but no change in friction force.     

急冷条件下NiAl-Mo三元共晶合金的组织形成机制

采用单辊急冷技术实现了NiAl-Mo三元两相共晶合金的快速凝固, 同时与常规条件下的凝固组织进行了对比研究. 实验发现, 单辊急冷的合金条带与常规条件的凝固样品均由B2结构的NiAl金属间化合物和bcc结构的Mo固溶体两相组成, 两相均具有(110)晶面优先生长的趋势, 并呈现出(110)NiAl//(110)Mo取向关系. 常规条件下得到的微观结构主要由规则的两相共晶组织组成, 形成了类似菊花状的共晶胞. 而单辊急冷条件下形成的组织结构主要是由近辊面的柱状晶区和近自由面的等轴晶区组成的凝固组织. 理论计算发现, 合金熔体的单辊辊速由10 m/s增大至50 m/s后, 其冷却速率从1.01×10⁷ K/s逐渐增大到2.46×10⁷ K/s, 冷却速率明显高于常规铸造过程, 因而形成了差别很大的凝固组织. 随着辊速(冷却速率)的增加, 合金条带的厚度从54.4 μm减小至22 μm, 近辊面柱状晶区的厚度所占比例也逐渐增大, 晶粒发生了明显细化. 关键词： 快速凝固 三元共晶 共晶转变 冷却速率     

Systematic study of the effect of disorder on nanotribology of self-assembled monolayers

The adhesion and friction between pairs of ordered and disordered self-assembled monolayers on SiO2 are studied using molecular dynamics. The disorder is introduced by randomly removing chains from a well ordered crystalline substrate and by attaching chains to an amorphous substrate. The adhesion force between monolayers at a given separation increases monotonically with chain length at full coverage and with coverage for fixed chain length. Friction simulations are performed at shear velocities between 0.02-2 m/s at constant applied pressures between 200 and 600 MPa. Stick-slip motion is observed at full coverage but disappears with disorder. With random defects, the friction becomes insensitive to chain length, defect density, and substrate.     

Atomistic simulation of the 60∘ dislocation mobility in silicon crystal

Dislocation velocity and mobility are studied via molecular dynamics simulation for a 60^∘ dislocation dipole in silicon crystal. The atomic interactions are described using the Stillinger–Weber potential and the external stress is applied by means of the Parrinello–Rahman algorithm. It is found that the dislocation begins to move when the applied stress is larger than the Peierls stress, and the calculated Peierls stress decreases as the temperature increases, which is in agreement with the Peierls–Nabarro model. The dislocation velocity at relatively low temperature is insensitive to variation of temperature. In fact, the velocity increases monotonically as the stress increases, and eventually approaches its plateau velocity which is about 2900 m/s. At higher temperature, however, the velocity no longer increases monotonically as the stress increases and the plateau velocity decreases as the temperature increases. In general, the dislocation velocity decreases as the temperature increases, which is consistent with the phonon drag model.