Adhesion Between Surfaces Separated by Molecularly Thin Perfluoropolyether Films

The relationship between the adhesion of surfaces separated by a molecularly thin liquid film and the surface energy of the film was investigated. AFM-based force–distance curves were measured on a series of carbon surfaces coated with hydroxyl-terminated perfluoropolyether (PFPE) films. The surface energy of the PFPE films was varied by altering either the total film thickness or the bonding ratio of the film by changing the concentration of the PFPE film in the solution and/or the pull-rate during dip-coating. A linear relationship between adhesion force and surface energy was observed. Adhesion was found to vanish at non-zero values of surface energy. The experimental results indicate that the adhesive force between macroscopic bodies separated by molecularly thin liquid films is linearly proportional to the excess surface energy of the film.     

The effects of acid passivation,tricresyl phosphate presoak,and UV/ozone treatment on the tribology of perfluoropolyether‐lubricated 440C stainless steel couples

Research on the boundary lubrication performance of two perfluoropolyether (PFPE) thin films in the presence of passirated 440C stainless steel is presented. The study used a standard ball‐on‐disc (BoD) tribometer operating in dry nitrogen and a vacuum spiral orbit tribometer (SOT). Stainless steel surfaces were passivated using one of four techniques: a high‐temperature and low‐temperature chromic acid bath, a tricresyl phosphate (TCP) soak, or UV/ozone treatment for 15 min. After passivation, each BoD disc had a 400 Å film of Krytox® 16256 (PFPE) applied to it. The lives of these films were quantified by measuring the number of sliding cycles before an increase in friction occurred. The lubricated life of the 440C couple was not altered as a result of the various passivation techniques. The resulting surface chemistry of each passivation technique was examined using X‐ray photoelectron spectroscopy. The SOT was used to examine the effects of the TCP treatment on the lubricated life of another PFPE, Brayco® 815Z, under rolling conditions. None of the passivation techniques were found to increase dramatically the oxide film thickness or lubricated life.     

Direct observation of PFPE lubricant molecules by cryogenic AFM under ultra-high vacuum

Surface lubrication is one of the essential technologies in modern magnetic disk systems and improvement of the surface lubrication is very important in the development of next generation systems. In this study, we used AFM for the direct observation of perfluoropolyether (PFPE) lubricant molecules on atomically flat surfaces. We used a cryogenic non-contact AFM to observe the molecules in a frozen state of micro-Brownian motion of PFPE segments, because the glass transition temperature of PFPE is very low. To avoid freezing a trace amount of water vapor on the sample surface at liquid nitrogen temperatures, the AFM observation was performed under ultra-high vacuum. We observed that on a gold surface the size of the molecules increases with repeated AFM scans. This is because the mechanical stimulus causes the fusion of PFPE lubricant molecules to form reversed micelles at the non-polar surface. At a hydrophilic silicon wafer surface, however, we succeeded in observing single lubricant molecules. This is because almost all PFPE lubricant molecules are fixed to the hydrophilic solid surface by polar–polar bond formation and they cannot move around on the surface and thus they cannot fuse to each other. As formation of the reversed micelle structure is a rather general phenomenon in the PFPE lubricant thin layer at non-polar surfaces, we also will discuss briefly the expected molecular structures of PFPE lubricants at the surface of the carbon overcoat of magnetic disks.     

Autophobic Dewetting of Perfluoropolyether Films on Amorphous-Nitrogenated Carbon Surfaces

The dewetting of perfluoropolyether (PFPE) films on amorphous nitrogenated carbon, CNx, is investigated. An optical surface analyzer is used to image perfluoropolyether films on CNx-overcoated magnetic recording disks. An autophobic dewetting transition is observed to result when the PFPE film thickness applied to the disk surface exceeds a critical value. This critical dewetting thickness is linearly dependent on the PFPE molecular weight. Addition of the phosphazine, X-1P, to the PFPE film reduces the critical dewetting thickness compared to that of the neat lubricant. Dewetting in these molecularly-thin PFPE lubricant films is shown to occur at thicknesses where the total disjoining pressure is negative. The impact of this autophobic dewetting on the performance of a head--disk interface is inferred from take-off height measurements conducted as a function of PFPE film thickness. A steep reduction in the slider--disk clearance is observed when the PFPE film is present at thicknesses in excess of the critical dewetting thickness.     

Study on the cyclotriphosphazene film on magnetic head surface

Stable lubrication is very important to the slider/disk interface with the increasing demand on the life of computer hard disk drive (HDD). The inert lubricant perfluoropolyether (PFPE) on the surface of magnetic hard disk is still prone to be catalyzed to decomposition by the slider material Al₂O₃. The properties of a partial fluorinated hexaphenoxy cyclotriphosphazene, X-1P, are investigated and its function to reduce the catalytic decomposition of PFPE is discussed. The results of contact start–stop (CSS) tester indicate that the thermal stability of the lubricant was greatly improved in the presence of X-1P, and its film thickness has a great influence on the lubrication properties of the HDD.     

Reversed Micelle Structures in PFPE Lubricant Thin Films on Magnetic Disk Surface Observed by Non-Contact AFM

PFPE lubricants (Fomblin Z-dol) for hard disk surface lubrication have two hydroxyl groups, one at each end of the molecules, and form stable insoluble monolayers at the water surface. In this study, molecular weight-fractionated PFPE lubricant monolayers were transferred from the water surface to solid substrates such as a hydrophilized silicon wafer, gold-sputtered mica, and a hard disk after adjusting the two-dimensional density of the lubricant molecules. The molecular structures of the PFPE lubricant molecules at the solid surfaces were observed by the cryogenic non-contact AFM under ultra-high vacuum. At the hydrophilic silicon wafer surface we could observe a single lubricant molecule in a random coil sphere shape. However, at the non-polar gold surface we confirmed the formation of reversed micelle structures. At the hard disk surface we detected various sizes of reversed micelles of PFPE lubricant in a flat oval shape.     

计入热变形影响的内燃机主轴承热流体动力润滑分析

根据动载滑动轴承热流体动力润滑理论,结合热变形矩阵法,提出一种考虑热变形因素影响时的内燃机主轴承热流体动力润滑分析方法,阐述该方法的基本理论和控制方程,探讨热变形因素对主轴承工作时的轴心轨迹、润滑油流量、最大油膜压力和最小油膜厚度等状态参量的影响情况.结合一主轴承实例进行数值仿真分析,仿真分析结果发现,计入热变形影响因素后,同未考虑热变形影响时分析得到的结果相比,轴心运动轨迹发生了很大变化,平均润滑油流量和一个载荷周期内的最大油膜压力均明显增加,一个载荷周期内的最小油膜厚度明显减小,润滑油平均温升则稍有减小.内燃机主轴承在工作时受各种热源因素的影响会产生热变形,在主轴承设计以及内燃机润滑系统供油量设计过程中考虑这种变形因素的影响是很有必要的.     

Tribological evaluation and analysis of the head/disk interface with perfluoropolyether and X1-P phosphazene mixed lubricants

The retention characteristics of magnetic thin film media coated with perfluoropolyether (PFPE) lubricants and a phosphazene additive, X-1P, were investigated in this study. The retention performance was evaluated by a drag test with a waffle head sliding against the disk that was designed to mechanically wear out the lubricant layer. An IR beam was aligned on the test track to directly measure the amount of PFPE lubricants and X-1P left on the media surfaces for determining the retention characteristics of the lubricants. The drag test results show that under ambient and hot/wet conditions the media coated with AM3001 PFPE lubricant have higher retention ratio on the test track than those coated with ZDOL 2000 PFPE lubricant. The phosphazene additive X-1P was observed to strongly anchor on the surface and not easily removed as PFPE lubricants (ZDOL and AM3001). The retention characteristics of X-1P are independent of lube combination, either AM or ZDOL lubricants. It is demonstrated that X1-P exhibits a good antiwear property and excellent retention performance. This revised version was published online in July 2006 with corrections to the Cover Date.     

点接触润滑状态转化的实验观察

利用球-盘接触润滑油膜厚度的光干涉测量法，通过卷吸速度或载荷的改变实验研究了弹性流体动力润滑与流体动力润滑转化过程中油膜厚度的变化规律。实验结果显示这2种润滑状态之间存在明显的过渡区。与已有的理论一致，在弹性流体动力润滑区和流体动力润滑区，油膜厚度与卷吸速度或载荷在对数坐标中呈直线关系。在两者的过渡区，固体表面的弹性变形量随卷吸速度或载荷的变化发生明显的变化，油膜厚度与速度或载荷的关系不再为对数坐标中简单的线性关系。使用已有的润滑状态区理论不能得到实验观测到的润滑状态的转化过渡区。     

Adhesion Properties of Nanometer-Thick Perfluoropolyether Films Confined Between Solid Surfaces: A Coarse-Grained Molecular Dynamics Study

Lubrication with thin liquid films is essential to ensure the tribological reliability of technologically advanced devices, such as micro-electro-mechanical systems and hard disk drives. However, the adhesion and friction properties of thin films and the underlying mechanism remain elusive due to our limited understanding of film structures and motions at the molecular scale. Here, we investigate the adhesion behavior of nanometer-thick perfluoropolyether (PFPE) films confined between two solid surfaces as a function of film thickness using coarse-grained molecular dynamics simulations. Consistent with typical experimental results, our simulations show that the adhesive force exerted by the PFPE films reaches a maximum and then decreases with increasing solid–solid spacing. The maximum adhesive force increases sharply for PFPE films thinner than 4 nm. When exhibiting the maximum adhesive force, PFPE films are slightly stretched within a solid–solid spacing a little larger than the initial film thickness and thereby show lower density than the original equilibrium density. Conventional theories of adhesion, which assume equilibrium density for liquid films, are not applicable in such case. Therefore, we construct a theoretical model that takes decreasing liquid density into account to discuss the underlying mechanism of the adhesive force exerted by nanometer-thick PFPE films on solid surfaces. We infer from the theoretical analyses that the maximum adhesive force originates mainly from solid–liquid interaction for thin films and liquid–liquid interaction for thick films.     

Kelvin probe study on the perfluoropolyether film on metals

The effect of mineral oil on surface potentials of copper and aluminum coated with a perfluoropolyether (PFPE) film is studied with a non-vibrating Kelvin probe. The probe measures the contact potential difference (CPD) between the probe surface and the coated copper and aluminum surfaces. The PFPE film was applied to the Cu and Al surfaces by a dip-coating technique. The Kelvin probe signals are especially sensitive to the interface between the coated and uncoated regions of the metal surfaces. It is found that the PFPE film causes the surface potentials of Cu and Al to drop by 150 and 98 mV, respectively. Immersion of the PFPE-coated surfaces in mineral oil causes the surface potential difference of the PFPE/Cu and PFPE/Al to increase, and removing the mineral oil re-establishes the surface potential.     

The effect of space irradiation on the lubricating performance of perfluoropolyether greases in simulated space environment

In this paper, 2 kinds of commercial perfluoropolyether (PFPE) greases were coated on the polyimide (PI) blocks, which were placed within simulated space environment including atomic oxygen (AO), proton (Pr), ultraviolet (UV), and electron (El) irradiations, and then the tribological performance has been investigated with a ball‐on‐disc tribometer. Results indicated that the MoS₂‐grease showed better lubrication performance than the PTFE‐grease. The changes in infrared spectroscopy induced by Pr and El irradiations were more obvious than that by AO and UV irradiations. Results of energy dispersive X‐ray spectroscopy indicated that Pr and El irradiations caused carbonation of greases, and AO and UV irradiations induced oxidation of greases. Referred to the tribological properties of PI coated with PFPE oil, PI coated with PFPE greases showed minor changes in friction coefficient and wear rate, and the MoS₂ additives could significantly improve the lubrication properties of PFPE greases in simulated space environment.     

Changes in Friction Properties of Monolayer Lubricant Films Induced by Development of Molecules’ Bonding

Functional perfluoropolyether (PFPE) films consisting of mobile and bonded molecules are widely used for lubrication of magnetic disks. In order to clarify the influence of film composition (mobile/bonded) on tribological performance, we measured the friction properties of two types of 2 nm-thick PFPE films (functional Zdol2000 and nonfunctional Z03) under lightly loaded (loading force: 0–1 mN) and quasi-static (low rotational speed: 2.1 mm/s) conditions as a function of elapsed time. The friction force of Z03 remained unchanged with time and increased linearly with loading force as described by Amontons’ law. In contrast, induced by the development of the molecules’ bonding in time, the friction force of Zdol2000 increased and transited to a nonlinear increase with loading force as time proceeded. The nonlinear friction-load relationship of Zdol2000 in the equilibrium state was characterized by the Johnson-Kendall-Roberts model.     

核电循环泵行星齿轮热弹流润滑研究

核电齿轮箱的良好润滑性能是核电循环泵可靠运行的重要保障,充分考虑齿面形貌和齿廓修形等因素对内/外啮合齿轮副的影响是准确评估其润滑特性的前提。建立典型工况下核电循环泵行星传动系统斜齿轮热弹流润滑模型,首先将斜齿轮副的啮合状态几何等效为圆锥滚子的接触问题,然后考虑斜齿轮接触变形和齿廓偏差,计算得到内/外啮合齿轮副接触区域不同位置的油膜厚度、承载压力、摩擦应力和闪温等参数。考虑齿面磨合作用,采用移动平均滤波方法对未经磨合的初始形貌进行光滑处理,分析了齿面形貌对润滑行为的影响,最后采取齿廓修形改善润滑特性。结果表明：粗糙度和齿廓修形均会对润滑特性产生明显的影响,齿面粗糙形貌会造成油膜厚度减小,进而影响其润滑特性,弱化润滑油膜的承载能力;通过齿廓修形可以改善齿轮啮合边界处的几何过渡,降低该区域的应力集中和表面温度,从而明显改善啮合线终端的润滑状态。     

弹性材料表面织构对摩擦副润滑性能的影响

为了研究弹性材料表面微织构对摩擦副空化现象和润滑特性的影响,建立考虑空化效应的二维弹性织构计算模型,采用流固耦合方法计算润滑流场与材料变形之间的相互作用。对比刚性材料表面微织构,从弹性模量、滑动速度、微织构深度以及织构间距等方面分析弹性材料表面织构对摩擦副润滑性能的影响,通过实验验证模拟结果的准确性。结果表明：弹性织构摩擦副比刚性织构摩擦副摩擦因数更小,润滑性能更好;存在最优织构深度,使得弹性织构摩擦副的摩擦力最小且承载力最大;适当增大滑动速度以及织构间距可以提高弹性摩擦副的润滑性能;随着弹性模量的降低,弹性变形和油膜厚度增加,空化现象更为显著,摩擦副的润滑性能得到提升。     

考虑粗糙度的船舶水润滑高分子轴承弹流润滑性能研究

为了揭示表面粗糙度对船舶水润滑高分子材料轴承润滑性能的影响规律,开展水润滑轴承弹流混合润滑理论研究;建立考虑内衬材料粗糙度和弹性变形的水润滑轴承混合润滑模型,并对模型进行仿真验证;分析内衬粗糙峰对水膜厚度、水膜压力分布和承载能力的影响规律。研究结果表明：在转速增大的过程中,内衬粗糙度的增大会减缓水膜厚度的增幅比,使轴承需要更高的转速来进入流体动压润滑状态;减小轴承内衬粗糙度能有效降低轴承起飞转速,加快轴承由混合润滑转变为流体动压润滑的过程,减小轴承与轴颈的局部接触,降低轴承异常振动噪声发生的可能性。研究结果揭示了内衬粗糙度变化对轴承润滑特性的影响机制,为水润滑轴承的优化设计提供理论参考。     

Nanotribological characterization of molecularly thick lubricant films for applications to MEMS/NEMS by AFM

Molecularly thick perfluoropolyether (PFPE) films are considered to be good protective films for micro/nanoelectromechanical systems (MEMS/NEMS) to reduce stiction, friction, and improve their durability. Understanding the nanotribological performance and mechanisms of these films are quite important for efficient lubrication for MEMS/NEMS devices. These devices are used in various operating environments and their effect on friction, adhesion and durability needs to be clarified. For this purpose, mobile and chemically bonded PFPE films were deposited by dip coating technique. The friction and adhesion properties of these films were characterized by atomic force microscopy (AFM). The effect of rest time, velocity, relative humidity, and temperature on nanotribological properties of these films was studied. Durability of these films was also measured by repeated cycling tests. The adhesion, friction mechanisms of PFPE at molecular scale, and the mechanisms of the effect of operating environment and durability are subject of this paper. This study found that adsorption of water, formation of meniscus and its change during sliding, viscosity, and surface chemistry properties play a big role on the friction, adhesion, and durability of the lubricant films.     

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.     

石墨干膜润滑剂在碳钢表面摩擦磨损性能试验研究*

传统油或脂润滑剂在极端工况环境下无法满足碳钢类零件的减摩要求,采用干膜润滑剂是提高极端工况环境下碳钢表面摩擦磨损性能的可行性方法。采用超声波分散方法制备以石墨粉末为基体的干膜润滑剂,使用压力喷涂技术使其沉积在碳钢试件表面,在端面摩擦试验仪中开展干摩擦和石墨干膜润滑剂润滑下摩擦磨损性能对比性试验研究。试验结果表明：石墨干膜润滑剂在碳钢表面的沉积效果较好,沉积的石墨干膜润滑剂具有较好的润滑性能,可以有效地保护碳钢表面不被过度磨损;喷涂石墨干膜润滑剂的碳钢试件的工作寿命随着压力载荷和主轴转速的增大而缩短,负载和滑动速度的联合作用会加速涂层向稳定方向的过渡;磨损过程中形成的微观润滑剂颗粒会形成颗粒流润滑,适当添加石墨颗粒粉末可能会延长润滑剂正常发挥减摩作用的时间。制备的石墨干膜润滑剂为碳钢在极端工况环境下的减摩提供了支持。