碳纳米管及石墨烯协同增强尼龙6摩擦学性能的分子动力学模拟

为研究碳纳米管（CNT）和石墨烯片(GNS)协同增强尼龙6（PA6）复合材料摩擦学及力学性能的微观机制,利用分子动力学方法模拟PA6及其复合材料的拉伸过程及摩擦学行为,分析CNT、GNS对PA6复合材料力学及摩擦学性能的影响。建立Fe原子与纯PA6和PA6/ CNT、PA6/GNS、PA6/GNS/CNT复合材料组成的摩擦学模型,并对模型进行几何优化、退火及动力学平衡,通过对Fe原子施加0.2 GPa应力及0.01 nm/ps速度进行摩擦模拟。研究结果发现,PA6/GNS/CNT复合材料摩擦因数在所有材料中最低为0.252;相比其他3种材料,PA6/GNS/CNT复合材料的抗剪切性能最好,且弹性模量及剪切模量均有提高。通过对比分析4种材料的径向分布函数、摩擦界面温度、材料总势能揭示了CNT和GNS协同增强PA6摩擦学及力学性能的作用机制,指出加入的CNT/GNS通过范德华及静电力作用降低了PA6与Fe原子摩擦副之间的相互作用,此外一维CNT与二维GNS通过π-π堆叠杂化作用形成了3D杂交堆叠体系,协同增强了PA6的摩擦学性能。     

碳纳米管对丁苯橡胶摩擦学性能的影响研究

碳纳米管作为填料,可以有效提高橡胶材料的摩擦学性能.采用分子动力学模拟方法研究碳纳米管对丁苯橡胶摩擦学性能的影响.在研究中,建立纯丁苯橡胶和碳纳米管丁苯橡胶两个无定型模型,优化橡胶分子内部构象.为分析摩擦学性能,建立铁原子层对摩副摩擦模型,对铁原子层施加一定的速度实现滑动摩擦.研究结果显示,纯丁苯橡胶的摩擦因数为0.77,碳纳米管丁苯橡胶的摩擦因数为0.628.碳纳米管可以增强对附近分子链的吸附,限制分子链向摩擦界面运动,使摩擦界面的原子浓度降低,减小粘附摩擦,减轻与对摩副的相互作用,减小摩擦界面温升,从而提高橡胶材料的摩擦学性能.     

基于分子动力学的MEMS器件微摩擦特征仿真研究

微机电系统(MEMS)的尺度深入到微-纳米量级时,表面力与体积力相比成为起主导作用的力,其黏着效应与表面效应渐成主导的或不可忽略的机制.分子动力学模拟的日趋成熟,使人们有可能研究一种最单纯的摩擦状态,即"界面摩擦",它是指把负荷引起的塑性变形、黏着和粗糙度的影响抑制到最低限度,实现原子级光滑表面的分子接触时的摩擦状态.对不同材料的界面摩擦过程进行了分子动力学(MD)模拟研究,采用基于Morse势函数描述原子间的相互作用,以牛顿方程建立力学运动方程,使用改进后的Verlet算法求解原子运动轨迹.将MD仿真结果得出的微观摩擦规律与文献[7]等实验对比,并进行了讨论.最后在此基础上探讨了分子动力学方法在微机械设计中的作用及发展方向.     

基于分子动力学的单晶硅压痕过程计算机仿真研究

对单晶Si的压痕过程进行了分子动力学模拟.采用Morse势函数描述原子间的相互作用,以牛顿方程建立力学运动方程,使用改进后的Verlet算法解原子运动轨迹,通过对MD仿真结果的分析研究,将压痕过程分为三个特征阶段,即初期弹性变形阶段、中期塑性变形阶段及非晶层形成阶段.并从原子角度分析了压痕过程中原子间势能、磨削力的变化、应力状态、磨削温度等特征,解释了微观材料的去除和表面形成机理.     

基于分子动力学的单晶硅压痕过程计算机仿真研究

对单晶Si的压痕过程进行了分子动力学模拟。采用Morse势函数描述原子间的相互作用,以牛顿方程建立力学运动方程,使用改进后的Verlet算法解子运动轨迹,通过对MD仿真结果的分析研究,将压痕过程分为三个特征阶段,即初期弹性变形阶段,中期塑性变形阶段及非晶层形成阶段。并从原子角度分析了压痕过程中原子间势能,磨削力的变化,应力状态,磨削温度等特征,解释了微观材料的去除和表面形成机理。     

裙部型线对内燃机活塞二阶运动和活塞裙部润滑的影响研究

以某多缸内燃机为研究对象,研究活塞裙部纵向型线对活塞二阶运动和活塞裙-缸套摩擦副润滑性能的影响。考虑活塞裙部和缸套的表面粗糙度与微凸体接触,建立了活塞二阶运动方程与平均Reynolds方程相结合的活塞裙-缸套摩擦副润滑分析模型。活塞二阶运动方程采用Broyden方法求解,应用有限差分法进行活塞裙-缸套摩擦副的润滑分析。结果表明,采用二次曲线型活塞裙部有利于改善活塞裙-缸套摩擦副润滑性能、降低内燃机摩擦损失、减轻活塞对气缸套的冲击和提高内燃机工作的平顺性。     

沟槽型织构摩擦学性能模拟及试验研究

为揭示沟槽型织构的摩擦学特性,以沟槽型表面织构化的止推圈式摩擦副为研究对象,基于雷诺方程和微凸体接触方程建立微沟槽织构混合润滑理论计算模型,模拟摩擦因数随沟槽面积占有率、角度和深宽比的变化规律,并由摩擦性能试验验证其准确性。在假设面积占有率和角度两个参数互不干涉的前提下,模拟两者共同作用下摩擦因数的变化情况。结果表明,经过表面沟槽织构后,摩擦副的摩擦学性能得到了明显改善;摩擦因数随沟槽面积占有率和深宽比的增加呈现先减小后增大,随角度的增加呈现先减小后增大再减小的趋势;当沟槽与运动方向夹角为65°～70°,宽度为30μm,深宽比为4/15,面积占有率接近8%时,摩擦副具有最佳摩擦性能,相对于未织构情况,摩擦因数可降低约55%。     

含有冗余支链的3-RPS/3n-SPS并联机构逆动力学建模

提出一类含有冗余支链的3-RPS/3n-SPS并联机构,为解决该类机构的逆动力学问题,首先对机构进行运动学分析,建立支链与动平台末端的速度、加速度映射关系,并对运动学模型进行仿真验证;之后对3-RPS/3n-SPS机构进行受力分析,列出的平衡方程数目少于未知量个数,基于机构分支作用力与动平台末端位姿误差间的协调关系构建动力学补充方程,求解出该类机构中所有运动副的约束反力和机构分支作用力;基于达朗贝尔原理对机构进行逆动力学建模,建立了机构末端力和各杆件惯性力与机构分支作用力间的关系表达式,求解了3-RPS/3-SPS机构分支作用力、运动副约束反力及动平台位姿误差,通过分析机构分支作用力、运动副约束反力、动平台位姿误差随时间的变化规律,验证本文建立的3-RPS/3n-SPS并联机构动力学模型的正确性。所得到的动力学结果为该类机构的进一步研究和工程应用奠定了理论基础,此外该逆动力学建模方法也可用于其他含有冗余支链的并联机构中。     

碳纳米管/丁腈橡胶复合材料力学及摩擦性能的分子动力学模拟*

采用分子动力学模拟技术,从分子水平研究碳纳米管(CNTs)增强丁腈橡胶(NBR)复合材料的力学性能及摩擦学性能。运用恒应变法计算材料的力学性能,分别建立纯NBR和CNTs/NBR复合材料的3层模型,并对顶层和底层的铁摩擦副施加剪切载荷,研究材料的摩擦学性能。研究结果表明：在摩擦过程中,由于CNTs表面存在很强的吸附力,抑制了NBR分子链的迁移率,使得CNTs和聚合物分子链间的相互作用增强;CNTs/NBR复合材料具有更高的致密性以及更强的结构,从而表现出了比纯NBR更加优异的力学和摩擦学性能。     

含纳米氧化锌丁腈橡胶摩擦学行为的分子动力学模拟

运用分子动力学模拟方法研究纳米氧化锌改善丁腈橡胶摩擦学行为的微观机制,考察纳米氧化锌增强丁腈橡胶的原子运动速度、剪切动力学与摩擦因数。结果表明:在剪切条件下,纳米氧化锌增强丁腈橡胶分子链段沿剪切方向分布完整连续,纳米氧化锌的存在提高了丁腈橡胶分子链的刚性;纳米氧化锌增强丁腈橡胶上下摩擦界面的原子运动速度峰值分别比纯丁腈橡胶材料的低11.8%与37.1%,说明纳米氧化锌的存在限制了胶料摩擦表面原子的热力学运动,减小了胶料摩擦剪切变形的程度;与纯丁腈橡胶材料相比,纳米氧化锌增强丁腈橡胶具有更低的摩擦因数,纳米氧化锌的存在增强了橡胶分子链的结合力,提高了胶料抵御剪切变形的能力,改善了胶料的摩擦学性能。     

Roughness and Lubricant Effect on 3D Atomic Asperity Contact

Large-scale molecular dynamics simulations were performed to study the sliding process of rough surfaces with and without lubricant. In the dry contact, a linear relationship has been observed between the load and the contact area for surfaces with large root mean square (RMS) roughness. However, it becomes nonlinear when the RMS is small. In the presence of adhesion, small roughness results in a large friction force when the surfaces are flattened and the contact area reaches 60 %. In order to confirm this observation, nonadhesive models have been established with an observation that under the combined influence from roughness and adhesion, the contact area plays a crucial role to determine whether the dry sliding is under the domination of roughness or adhesion. In the lubricated sliding, an increase in friction force has been found for the partially lubricated condition because the asperity contact still accounts for a great deal of resisting force. Besides, the lubricant exerts a comparable resisting force to the sliding.     

高速、高精度数控伺服工作台摩擦误差的研究

建立了一种考虑摩擦影响的PID控制下的高速、高精度进给伺服工作台的数学模型。伺服机构中的摩擦力是采用一种“两维混合摩擦模型”来描述的。该模型可以根据滑动速度、表面接触物体的油膜厚度等计算摩擦力。通过数值仿真与实际测量 ,在大范围的工况下 ,研究了圆运动过象限时出现凸起的误差现象。通过计算与试验结果的比较 ,证明了提出的考虑摩擦影响的数学模型能够精确地模拟PID控制下的高速、高精度进给伺服工作台的动力学过程 ,能够正确地预测进给运动过程中摩擦误差大小及特征。     

Molecular dynamics simulation of the contact process in AFM surface observations

In the present work, several molecular dynamics simulations have been performed to clarify dynamically the contact mechanism between the specimen surface and probe tip in surface observations by an atomic force microscope (SFM) or friction force microscope (FFM). In the simulation, a three‐dimensional model is proposed where the specimen and the probe are assumed to consist of monocrystalline copper and rigid diamond or a carbon atom, respectively. The effect of the cantilever stiffness of the AFM/FFM is also taken into consideration. The surface observation process is simulated on a well‐defined Cu{100} surface. From the simulation results it has been verified that the surface images and the two‐dimensional atomic‐scale stick‐slip phenomenon, just as is the case for real AFM/FFM surface observations, can be detected from the spring force acting on the cantilever. From the evaluation of the behaviour of specimen surface atoms, the importance of the specimen stiffness in deciding the cantilever properties can also be understood. The influence of the probe tip shape on the force images is also evaluated. From the results it can be verified that the behaviour of the specimen surface atoms as well as the solid surface images in AFM/FFM surface observations can be understood using the molecular dynamics simulation of the model presented.     

Atomic-scale study of friction and energy dissipation

This paper presents an analysis of the interaction energy and various forces between two surfaces, and the microscopic study of friction. Atomic-scale simulations of dry sliding friction and boundary lubrication are based on the classical molecular dynamics (CMD) calculations using realistic empirical potentials. The dry sliding of a single metal asperity on an incommensurate substrate surface exhibits a quasi-periodic variation of the lateral force with two different stick-slip stage involving two structural transformation followed by a wear. The contact area of the asperity increases discontinuously with increasing normal force. Xe atoms placed between two atomically flat Ni surfaces screen the Ni-Ni interaction, decrease the corrugation of the potential energy as well as the friction force at submonolayer coverage. We present a phononic model of energy dissipation from an asperity to the substrates.     

乳化液泵曲柄轴承润滑特性研究

由乳化液泵曲柄连杆机构的动力学分析,建立起曲柄轴承流体动力润滑的基于雷诺边界条件的雷诺方程,利用Holland法求得曲柄轴承在不同磨损状态下的轴心轨迹运动规律。指出随着曲柄轴承的不断磨损,最小油膜厚度所对应的偏心率将逐渐减小,最小油膜处的油膜将发生破裂,轴颈与轴承金属表面发生直接碰磨,形成刚性冲击。     

Influence of the Molecular Level Structure of Polyethylene and Polytetrafluoroethylene on Their Tribological Response

Crosslinks occur in polymers following irradiation and are used in computational simulations to mimic the effects of chain tangling. Here, the effect of crosslink density on the tribological behavior of atomic-scale models of polyethylene and polytetrafluoroethylene is determined using classical molecular dynamics simulations. In the simulations, oriented crosslinked surfaces are slid in different directions over a range of applied normal loads. The results indicate that, at the same normal load, the friction force increases with increased crosslink density. In addition, the influence of randomized versus ordered crosslinking on the simulated tribological behavior is investigated. Finally, the influence of crosslink density on the simulated wear mechanisms of polyethylene and polytetrafluoroethylene is elucidated. The results have important implications for the atomic-scale modeling of friction at the interfaces of polymers that have been irradiated or contain entangled chains.     

Computer simulation of sliding hydroxylated alumina surfaces

A molecular dynamics simulation is performed to investigate the frictional force and energy transfer dynamics associated with sliding hydroxylated alumina surfaces. The calculated coefficient of friction is in good agreement with a recent experimental study. The dynamics of energy transfer from the interface of the sliding surface is investigated by calculating the surface–surface intermolecular potential and the energy in surface hydroxyl groups. The simulations indicate the experimental friction force arises from energy relaxation. A transition from stick–slip to smooth sliding is observed as the sliding velocity is increased. This revised version was published online in September 2006 with corrections to the Cover Date.     

Prediction of the occurrence of wear by friction force-displacement curves

If a contacting body is moved in the tangential direction in very small steps a rapid increase of the friction force can be observed during the early stages of motion. No transition to a constant value representing the kinetic friction was observed during the investigation of steel surfaces. The friction force increased, though at a more moderate rate. At this stage of movement wear will occur. Investigations with oil-lubricated surfaces showed similar increases. No increase was observed with MoS₂ or PTFE lubrication. It is suggested that the behaviour of the friction force after the transition displacement indicates whether deterioration of the surfaces will occur. If the movement does not exceed the transition displacement no wear occurs.     

Investigation of the Tribological Behaviors of Polymer Molecular Deposition Films by Tribometer Based on Interferometer

The polymer molecular deposition films including polyelectrolyte molecular deposition (PEMD) film and nanoparticles composite molecular deposition (NPs/MD) film have been prepared using the molecular deposition method and the in situ synthesize method. The polymer molecular deposition films were characterized by atomic force microscope (AFM) and X-ray photoelectron spectroscopy (XPS). The tribological behaviors of the substrate and polymer molecular deposition films were investigated by a tribometer based on interferometer. It is found that the NPs/MD film has a lower friction force and a better anti-wear property than the PEMD film under the dry friction. The poly alpha olefin (PAO2) and water films confined between samples and steel ball surfaces have been investigated using thin film interferometry. The friction force of substrate was lower than the polymer molecular deposition films under PAO2 lubrication. The friction forces alteration of PEMD film and NPs/MD film were similar and consistent, and lower than that for substrate under water lubrication.     

Recent progress in microtribology

Sliders having very small mass (less than 10 mg) are beginning to be used in magnetic recording devices and very light contacting loads (less than 1 μN) are expected to be used in ultrahigh-density recording devices using the point-recording technique. These newer devices will require wear rates that are virtually zero. The wear on such sliding surfaces is primarily due to the surface interaction forces rather than the load. The ultimate goal of microtribology is to create practical zero-wear devices with very small mass and very light load. Computer simulation of molecular dynamics is used to trace the movement of each atom in sliding surfaces and of each molecule in lubricant films, New tools, such as scanning probe microscopes, are being used to evaluate sliding surfaces and lubricant films. Experimental studies of microwear processes on solid surfaces and of lubrication using very thin lubricant films have recently begun. Microtribology is an important technology for development of new microdevices, and also an important science for understanding the origin of friction and wear. Close cooperation between scientists and engineers is necessary. However, as we have very little of the knowledge needed for microtribology, we must obtain much more data.