Investigation of Lubricant Transfer between Slider and Disk Using Molecular Dynamics Simulation

A model for lubricant transfer from a rotating magnetic recording disk to a magnetic recording slider is developed using molecular dynamics simulation. The combined effect of disk velocity and local air-bearing pressure changes on lubricant transfer is investigated. The simulation results indicate that local pressure changes in the absence of disk circumferential velocity can cause lubricant redistribution on the disk, while local pressure changes on a moving disk can result in lubricant transfer from the disk to the slider. The amount of lubricant transferred from the disk to the slider and the lubricant buildup on the disk are a function of the local pressure change and disk velocity. The amount of lubricant transferred from the disk to the slider and the height of lubricant buildup on the disk surface decrease with an increase in the number of functional groups of the disk, a decrease in the local pressure change, and a decrease in the disk circumferential velocity.     

Tribological Performance and Transfer Behavior of Lubricating Oils at Head-Disk Interface under Volatile Organic Contamination

Tribological performance of head-disk interface (HDI) under volatile organic contamination was investigated using a contact start/stop (CSS) tester. Slider and disk surfaces were analyzed using Time-of-Flight Secondary Ion Mass Spectroscopy (TOF-SIMS) after CSS tests. The CSS test results indicated that the friction forces were high and unstable under contamination. Transfer of lubricating oil onto the slider surface was detected after the CSS tests. The transfer amount of lubricating oil was revealed to be dependent on the chemical structure of the terminal group in the lubricating oil. Piperonyl (–CH₂−phe=(O)₂=CH₂) terminated AM3001 lubricating oil was lost more easily than two hydroxyl (–OH) terminated Tetraol lubricating oil, probably because of the weak attractive force of the piperonyl groups with carbon overcoat. TOF-SIMS chemical images indicated that the transferring behavior of the lubricating oil onto the slider surface during CSS tests was dependent on the chemical structure of volatile organic contaminants. The lubricating oil became built up on the slider surface when the dioctyl sebacate (DOS) pollutant used. In contrast, the lubricating oil distribution on the slider surface was uniform under a polydimethylsiloxane (PDMS) vapor. The different transfer behavior of lubricating oil onto the slider surface may be resulted from the changeable surface properties of slider and disk because of the coexistence with gaseous contaminants.     

Molecular Dynamics Simulation on the Aggregation of Lubricant Oxidation Products

The aggregation phenomenon of lubricant oxidation products, fatty acids, fatty alcohols, aliphatic aldehydes and aliphatic ketones in base oil, was probed by molecular dynamics (MD) simulation. The results show that there are aggregates formed in fatty acids or fatty alcohols, and that the aggregation does not occur in aliphatic aldehydes and aliphatic ketones in single component models. Fatty acids or fatty alcohols are connected by hydrogen bonds in the form of dimers or multimers. The hydrogen bond can be formed at a higher temperature in fatty acids, but not in fatty alcohols. In the mixture models, there are dimers and/or multimers constructed by different molecules of lubricant oxidation products, and the multimers have a chain or a ring structure. The number of molecules involved in the formation of aggregates decreases with the increase of temperature. The opportunity to construct hydrogen bonds rises with the increasing concentration of oxidation products. Obvious aggregation occurs at 25 and 100 °C in conditions of high concentration, while it happens only at 25 °C with lower concentration. It is easier for fatty acids and fatty alcohols to form hydrogen bonds and the formed hydrogen bonds are more stable, which result in a higher concentration of fatty acids and fatty alcohols in the aggregates. Fatty acids and fatty alcohols are possibly more significant in the formation of aggregates and it may be beneficial to prevent the aggregation by controlling the concentration of them in lubricating oil.     

Dominant Factors in Lubricant Transfer and Accumulation in Slider-Disk Interface

Lubricant transfer from disk to slider and lubricant accumulation on slider are very important in designing a stable slider-disk interface of ultra-low spacing. In this article, the effects of different parameters on the lubricant transfer and accumulation are studied and the reasons behind the effects are explained. Furthermore, the time for the lubricant transfer to reach steady state is estimated. It is found that lubricant molecular weight plays a dominant role in the lubricant transfer and accumulation. Lubricant transfer and accumulation decrease dramatically with the increase in lubricant molecular weight. Lubricant transfer also strongly depends on lubricant thickness and bonding ratio on disk surface. A thinner lubricant and higher lubricant bonding ratio on disk surface reduce lubricant transfer obviously, which results in less lubricant accumulation. A diamond-like-carbon (DLC) overcoat of low adsorption area density on slider surface can reduce lubricant transfer and accumulation, especially for lubricant of low molecular weight. Lubricant accumulation increases with disk velocity and increases slightly with the decrease in slider flying height. Lubricant accumulation can be reduced by minimizing the area of slider pad. Lubricant transfer and accumulation become worse at higher ambient temperature. It takes seconds for lubricant of low molecular weight to reach steady transferred thickness and hours for lubricant of high molecular weight to reach the steady state.     

Effects of End Groups on the Tribochemical Reactions of Lubricants at Head-Disk Interface

Mass spectrometry was used to monitor in-situ gaseous species that were generated at the head-disk interface (HDI) in a high vacuum. It was found that the end groups of the lubricants significantly affected the wear durability at the HDI; piperonyl (–CH₂-phe = (O)₂ = CH₂) terminated Fomblin AM3001 lubricant exhibited longer life than hydroxyl (–OH) terminated Fomblin ZDOL lubricant. The continuous removal of the lubricants resulted in a continuously increasing friction coefficient. Further, the characteristics of tribochemical reactions of the lubricants (Fomblin Z series) with different end groups was investigated in details using Time-of-Flight Secondary Ion Mass Spectroscopy (TOF-SIMS) just after the sliding tests. It was found that the decomposition of the end groups was more significant than that of the backbone. The lubricants terminated with the following groups showed the following order of increasing decomposition: –CH₂O-CH₂-phe = (O)₂ = CH₂ (AM3001), –CH₂OH (ZDOL) < –CH₂OCH₂CH(OH)CH₂OH (Z Tetraol) < –CH₂(OCH₂CH₂)_nOH (ZDOL-TX), –COOH (Z Diac). The decomposition of the lubricants appeared to start from the end groups.     

Molecular Dynamics Simulation of Thermal-Induced Local Heating and Depletion of Ultrathin Perfluoropolyether Lubricant Under Moving Laser Heating

Molecular dynamics simulations of nanostructured perfluoropolyether lubricant are performed to investigate localized heating and lubricant depletion instability under moving laser heating. The evolution of the lubricant surface morphology indicates that a valley-like depletion track is formed along the path of the laser beam, resulting in aggravated depletion with heating time. The depletion profiles at various laser durations are characterized by the film thickness, in which a raised ridge is formed around the depletion zone signifying that the thermocapillary stress has a non-negligible effect on lubricant depletion. The recovery process is studied as well by further equilibrating the entire system with the laser beam turned off. During the cooling stage, the lubricant undergoes a slow recovery when compared with the laser-induced depletion. In addition to the attractive lubricant-to-disk interaction, the strong polar coupling between end beads prevents the recovery of lubricant beads back to the depleted surface. Moreover, the nonuniform surface tension and the nonequilibrium thermocapillary stress are expected to account for the mechanisms of lubricant depletion under moving laser heating.     

计算机磁头/磁盘超薄气膜润滑稳定性

以任意拉森数的超薄气体润滑方程为基础,给出磁头刚体小扰动对空气轴承滑块 (ABS)气膜压强摄动方程。采用算子分裂法求解气膜压强和非结构三角网格的有限元法解压强摄动方程,得到气膜的刚度系数和阻尼系数矩阵。模态分析得到磁头气固耦合系统的固有频率,衰减率和振型。以Ω型磁头为例,分析了在不同气膜厚度和磁盘转速下的磁头稳定性。研究结果表明,磁头稳定性对气膜厚度很敏感,在小气膜厚度运行时,系统固有频率高,稳定性好;磁头升沉和纵倾运动的动力耦合,使磁头系统固有频率和衰减率降低,对稳定性不利;高转速的磁盘对磁头稳定性不利,但影响不大。     

车用传动装置润滑系统的流动与传热仿真

提出了车用传动装置润滑系统流动与传热的稳态仿真模型。基于一维不可压缩流动方程,建立了润滑油流动的仿真模型;研究了传动润滑系统的传热机理,采用热网络法建立了润滑系统的传热模型;对某型坦克液力机械传动装置润滑系统的流动与传热进行了仿真,预测了润滑系统的流量、压力和温度分布,仿真结果与试验值基本吻合。本研究为车用传动装置润滑系统的流动与传热性能提供了一种有效的理论分析手段和仿真方法。     

分子动力学模拟方法概述

介绍了分子动力学模拟的基本原理及常用的原子间相互作用势,如Lennard-Jones势;论述了几种常用的有限差分算法,如Verlet算法;说明了分子动力学模拟的几种系综及感兴趣的宏观统计量的提取。     

分子动力学计算机模拟技术进展

阐述了分子动力学模拟以及进行分子动力学模拟的基本条件及步骤,介绍了国内外分子动力学计算机模拟的研究现状,指出了分子动力学模拟方法应用的进一步发展方向。展望从原子尺度方面出发来研究材料力学的力学行为,特别对裂纹、错位和晶界以及它们的相互作用的研究是可行的。     

纳米加工分子动力学仿真应用及实例

详细介绍了国外分子动力学仿真技术在纳米加工中的应用情况.合理选择了超高速纳米磨削单晶铜和单晶铝的分子动力学仿真势函数,进行了纳米磨削分子动力学仿真实验,并对仿真结果进行了分析和研究,得出了超高速纳米磨削单晶铜和单晶铝时工件所能获得的极限表面粗糙度值,预测了超高速纳米磨削单晶铜时工件表面变形层的极限深度和所能达到的极限加工精度.     

分子动力学模拟研究具有"荷花效应"的微流通道

采用非平衡分子动力学模拟(NEMD)分析了流体在具有"荷花效应"的微流通道中的边界滑移现象.选择不同尺度的纳米级图样、通道管壁厚度、不同的密度和温度分别进行了多次模拟分析,根据模拟结果对比分析了以上各参数对流体流动方向的速度分布、速度最大值及其在通道中流动垂直方向上的位置的影响,确定了适宜的条件,使得通道中流体的速度滑移达到最大,流速这到最快.从对模拟结果的分析可知,流体流动方向的速度在通道上下边界的值及其最大值受到纳米图样高度和通道内径影响显著;在对不同密度的流体的模拟分析中发现,最大速度滑移与最大流速对应不同的流体密度.     

金属铜纳米切削的分子动力学模拟

微纳米科技的发展和器械的小型化对精细加工过程提出了更高的要求,深入理解微纳米的切削规律至关重要。本文运用分子动力学方法,对大尺寸的单晶及多晶铜进行了切削深度为0.1μm的计算模拟,并就切削过程中单晶和多晶内部微结构演化、温度及切削力变化进行了分析及对比。结果表明:多晶铜在切削过程中位错滑移集中在刀具前沿几个晶粒内,对未切削工件区域的影响较小,温度分布更为集中,切削力也略小于单晶。     

纳米切削多晶铜的分子动力学仿真研究

采用Voronoi方法建立了多晶铜切削模型,基于分子动力学方法实现了多晶铜纳米切削加工的二维分子动力学仿真。分别选用EAM势函数和Morse势函数来计算工件原子间以及工件原子和刀具原子间的相互作用。对切削过程和切削力的变化进行了分析,发现晶界会阻止位错向晶粒内部传播,在已加工区域表面,前一晶粒中的原子会随刀具运动到下一晶粒中形成晶界,切削过程中切削力随时间波动剧烈,并在晶界处会出现瞬时的峰值。     

热超声倒装键合界面的运动传递过程

采用改进的激光多普勒振动测量系统,获得小直径激光斑,解决了大量数据采集和分析、同步触发等技术问题,实时精密测量热超声倒装键合过程中工具和芯片的运动.通过分析运动曲线,发现"速度分离"是表征键合过程状态改变的重要临界现象;发现在"速度分离"前工具/芯片粘着在一起运动,之后它们间是"粘滑"交替的过程;在"速度分离"后的运动过程中,工具能量一部分通过芯片传递到键合界面,形成键合强度;另一部分消耗在芯片/工具间的摩擦上,磨损芯片和工具,损害键合界面结构和强度.根据研究结果提出了新的超声加载过程思路,以期减小磨损,提高键合强度.     

Numeric Simulation of Friction Phenomenon at Soil-Tool Interface

Tillage is an essential agricultural operation in which friction occurs at the soil-tool interface. The distribution of normal force, tangential force and tangential displacement at the soil-tool interface at different travel speeds was studied by using the finite element method. In addition, a method was proposed to evaluate the maximum potential savings of the tool draft.     

分子动力学仿真技术在超精密加工领域中的应用

分子动力学仿真在超精密及微细加工中的作用已显示出良好的前景，较系统描述了分子动力学中采用的势函，仿真过程中的切屑形成、切削力、切削温度及表面质量，仿真算法与边界条件等内容；并结合实际分析了仿真图形的物理意义。最后给出了存在的问题和改进措施。     

Molecular Dynamics Simulation of Rolling Friction Using Nanosize Spheres

The rolling resistance of a sphere on an atomically flat surface was studied by molecular dynamics simulation carried out by rolling a rigid Ni sphere on a copper (100) surface. Spheres of 6 and 12 nm diameters were used for rolling simulations after indentation up to 10 Å in depth and the computations carried out using embedded atom potentials of Ni and Cu, assuming either active molecular interaction at the contacts (normal potentials) or the presence of a passivation layer on the sphere. Results showed that the sphere size, penetration depth, and adhesion at the rolling interface strongly affected the rolling friction. When molecular interactions were allowed at the rolling contacts, the average rolling friction coefficient was higher and severe oscillations in the friction force was observed. On the other hand, a sphere with a passivation layer produced more dislocations in the copper substrate during rolling and the motion of the dislocations affected the coefficient of rolling friction and the size of the friction force oscillations. This work also suggested that rolling friction at the nanoscale level was similar to the macroscopic rolling condition during strain hardening of metals due to severe dislocation multiplication underneath the nanosphere in nanorolling.     

纳米有序体系膜分子动力学模拟系统的研究

纳米有序体系膜(超薄膜即膜厚趋于分子量级)的摩擦特性与宏观薄膜有很大的不同,但与纳米有序膜的微观结构有密切的关系。利用分子动力学模拟方法可以研究超薄膜(纳米有序体系膜)宏微观特性,本文主要研究了用VC 6．0与OpenGL结合模拟纳米有序膜的微观运动状态和特性,模拟纳米有序膜的失效机理,显示分子膜结构的变化对分子膜摩擦力的影响。同时,纳米有序膜系统中的能量损失也可得到。     

Molecular Dynamics of Thin Lubricant Films on Sol-Gel SiO2 Surfaces for Thin Film Magnetic Disks

Mobility of molecularly thin lubricant film is an important issue in understanding boundary lubrication mechanisms and to develop reliable magnetic disk media. Intra-molecular mobility for a perfluorinated poly ether (PFPE), which is used as a disk lubricant, with two hydroxyl groups on a sol-gel SiO₂ surface, which is used for a protective overcoat for plated magnetic disks, was studied using nuclear magnetic resonance (NMR). Thin film viscosities for molecular segments were derived from a relaxation time. The viscosity for the hydroxyl segment is 1.8 to 11 times as much as that for a bulk lubricant at room temperature, and the viscosity rate increased with increasing temperature. For example, it increased 15 times at 100°C. The viscosities for the segments in a main chain were not different from that of bulk PFPE.

A spin-off calculation for the molecularly thin lubricant film with thin film viscosity, derived from the NMR method, shows that there is no thickness decrease after seven years.