Atomic Stick-Slip Friction Between Commensurate Self-Assembled Monolayers

Molecular dynamics (MD) simulations have been used to examine the friction between commensurate self-assembled monolayers (SAMs) on Au (111). The discrepancy between sawtoothed friction force and the discontinuous molecular movements reveals the complex dynamics of the closed-packing chains under shearing. Molecules in the lower monolayer can be divided into two groups with a phase difference of . The periodic motion of the molecules is not synchronous with the frictional stick-slip loops, which result in a second-order valley in friction curves.     

摩擦磨损自修复润滑油添加剂研究进展

阐述了摩擦磨损自修复润滑油添加剂的研究现状,归纳为软金属自修复添加剂、自补偿修复添加剂和矿物微粉自修复添加剂3类主要的研究内容。对今后的研究方向进行了展望,指出了摩擦磨损自修复添加剂的发展方向及思路。     

Wet Clutch Friction Interfaces under Water-Contaminated Lubricant Conditions

The performance of wet clutches used for automatic transmissions or other applications usually includes the desired positive friction characteristics and a shudder-free torque generation. Changes in the operating variables such as the lubricant conditions influence the formation of a tribofilm, and friction characteristics and can alter the degradation of the friction interfaces. In this work, the friction characteristics and degradation of the paper–steel friction interfaces were monitored when a commercial fully formulated automatic transmission fluid (ATF) was contaminated with water. It was found that water in ATF influenced the clutch stability by increasing the mean coefficient of friction (µ) and the negative friction–velocity slope. Surface studies of the posttest friction interfaces clearly indicated reduced surface porosity and permeability, increased wettability, and changed elemental composition on the contacting surfaces after tested with water-contaminated ATF. Moreover, water-contaminated paper liners' thermal decomposition shifted to a lower temperature compared to an uncontaminated liner during thermal analyses. These results displayed faster degradation and reduced service life of the clutch friction interfaces due to water contamination. The resultant surface condition can be associated with the observed unstable friction and negative friction–velocity slopes.     

氨基表面自组装膜成膜及摩擦性能研究

运用原子力显微镜与接触角测量仪研究了3-氨丙基三乙氧基硅烷(APTES)自组装膜不同成膜时间的表面形貌平均粗糙度、接触角,并进行了XPS(X射线光电子能谱)元素分析,表明APTES自组装膜以“岛”状生长,随着反应的时间的延长,岛的数量急剧增加,形成平整光滑的膜,进一步延长成膜时间,可能形成多层膜,从而平均粗糙度先增大再减小然后再增大,表面性质一直变化不大。微摩擦磨损实验研究表明,APTES自组装膜大大降低镀Cr钢球与硅片之间的摩擦因数,摩擦因数从0.71左右降低到0.16左右,在分子级润滑中有广阔的应用前景。     

Friction Reduction in Mixed Lubrication

Minimization of frictional losses in the drivetrain of heavy-duty vehicles is important from both consumer satisfaction and environmental perspectives. Approaches to friction reduction in these components can be evaluated using simulation-based investigations. However, nearly all drivetrain components operate in the mixed lubrication regime which is difficult to model because both hydrodynamic lubrication and surface contact are significant and therefore, the total friction consists of hydrodynamic friction due to lubricant shearing and boundary film friction at asperity contact locations. Recent advances in simulation methods for mixed elastohydrodynamic lubrication (EHL) have enabled improved virtual design tools, such as those developed by Zhu and Hu and further improved by Liu et al. Here, these simulation tools are used to evaluate friction reduction and predict the effects on a mixed EHL interface under severe operating conditions. Three practical means of friction reduction are discussed based on the experimentally validated mixed lubrication friction model and its predictions made for representative, sample cases.     

Dynamic Friction Control Using Dynamic Structured RFNN and Friction Parameter Estimator

A nonlinear dynamic friction control is dealt with using dynamic friction observer and intelligent control.The adaptive dynamic friction observer based on the LuGre friction is proposed to estimate the friction parameters and a directly immeasurable friction state variable.The dynamic structured Recurrent Fuzzy Neural Network(RFNN)is designed to give additional robustness to the control system under the presence of the friction model uncertainty.A proposed composite control scheme is applied to the position tracking control of the servo system.The performances of the proposed friction observer and the friction controller are demonstrated by simulation.     

聚甲基硅氧烷微球润滑油添加剂的制备与摩擦行为研究

采用水解-缩聚两步法以甲基三甲氧基硅烷为原料制备了粒径在0.5~10.0μm之间的聚甲基硅氧烷微球。以黄油脂为介质考察了聚甲基硅氧烷微球的添加量和尺寸对黄油脂减摩抗磨性能的影响,同时对其抗磨机制进行了探讨。结果表明:聚甲基硅氧烷微球的添加量和尺寸对黄油脂减摩抗磨性能有显著的影响,添加聚硅氧烷微球的润滑油的减摩和抗磨性能比未添加提高20%以上;聚甲基硅氧烷微球的减摩抗磨机制是由于微球表面含有大量的羟基,可以与钢球表面发生摩擦化学反应生成Fe2O3,沉积在钢球表面形成保护膜,同时羟基在摩擦副间形成吸附薄膜,保护了钢球表面,提高了润滑脂的耐磨性。     

润滑剂对轮轨摩擦与磨损的影响

利用MMS-2A磨损试验机模拟轮轨系统在润滑油、二硫化钼锂基脂、植物油和石墨钙基脂4种润滑剂润滑下的摩擦与磨损行为,研究润滑剂对轮轨副摩擦、磨损特性的影响.结果表明:与干态相比,4种润滑剂均使摩擦副的摩擦因数减小,表面磨痕深度减小,磨损量降低,其中石墨钙基脂的减摩和抗磨效果最好;试验结束后,轮轨试样接触表面的硬度均有不同程度的增加,其中涂有石墨钙基脂的轮轨试样的表面硬度增加最小.     

润滑油摩擦系数的不确定度初步研究

润滑油的摩擦系数的精确测试对节能减排及设备寿命有重要影响。针对目前常用的润滑剂摩擦因数四球测定标准方法误差较大问题,利用不确定度理论对摩擦系数测量的影响因素进行了分析,并使用改进的MS-800四球试验机,对某润滑油在特定的实验条件下进行实验,实验表明其摩擦系数的不确定度均能控制在5%之内。     

基于摩擦补偿的交流伺服工作台爬行控制

低速工况下由摩擦引起的爬行现象是影响机械加工精度的重要因素。建立了XY交流伺服工作台爬行的数学模型。设计了一种变结构控制和基于摩擦模型前馈补偿相结合的综合控制策略,用于减小交流伺服工作台爬行对定位精度的影响。仿真和实验结果表明,采用提出的控制策略可以明显降低爬行产生的跟踪误差。     

Lubricant Additives Based on Carbon Nanotubes Produced from Carbon-Rich Fly Ash

Carbon-rich fly ash has been reported to be a suitable precursor and catalyst for carbon nanotube (CNT) growth. In this work, CNTs grown from carbon-rich fly ash were evaluated as a lubricant oil additive to reduce the friction coefficient of metallic surfaces using a ball-on-disk tribometer. Different concentrations of the as-grown CNTs in the range 0.005–0.5 wt% were dispersed in a base sunflower oil. The value of the friction coefficient was also investigated as a function of load. Excellent results were obtained for the value of the friction coefficient, where it drastically decreased to around 58% of its original value without additives. This was achieved at a very low concentration of CNTs; that is, 0.1 wt%. The obtained result was compared with that of a commercial multiwalled CNT at the same concentration and found to be superior. This superiority of CNTs produced from fly ash could be attributed to the existence of active radical sites on their side wall. Moreover, the friction coefficient value was observed to decrease with increasing load, which might be due to the formation of protective graphitic carbon layers on antagonist surfaces. The viscosity of pure and 0.1 wt% CNTs-impregnated base oil was also studied in the 25–100°C temperature range. No significant changes are observed in the viscosity of the CNTs-impregnated base oil. These results suggest that the low-cost CNTs produced from fly ash are excellent nanomaterials as additives for lubricant oil.     

手动变速箱油摩擦特性评定试验台研制

为了评定手动变速箱油摩擦特性对同步器性能的影响,研制了手动变速箱油摩擦特性评定试验台。阐述了试验台的总体结构与工作原理,设计了试验台的关键子系统,包括动力与传动子系统、换挡子系统、运动控制与数据采集子系统等。在研制的试验台上,对手动变速箱的换挡特性和工况参数进行了分析与测量,对试验台的整体性能进行了测试与评估。结果表明：试验台能够模拟变速箱同步器的实际运行工况,试验结果具有良好的重复性和区分性,能够评定出手动变速箱油的摩擦特性。     

摩擦力不确定的机器人液压驱动器的精确控制

基于H∞鲁棒控制器来改善机器人液压系统位置控制性能。首先经液压动力学和机械动力学分析,建立液压驱动系统的数学模型,摩擦力采用Dahl模型。然后按系统参数标称值设计基于混合灵敏度的H∞鲁棒控制器,对权值函数进行优化选择。最后经仿真和实验验证得出如下结论：该控制方法可以有效抑制摩擦力参数误差对位置跟踪控制的影响,尤其在速度换向时摩擦力模型的不确定性产生的影响,显著减小了位置跟踪误差,且该控制器稳态和过渡性能都优于传统PID控制器。     

Effect of Graphite on Friction and Wear Characteristics of Molybdenum Dithiocarbamate

The effects of graphite powder on the friction and wear characteristics of molybdenum dithiocarbamate MoDTC were studied in reciprocated sliding contact using a ball-on-plate type tester. The oil used was squalane, a pure hydrocarbon. The addition of MoDTC alone to the oil showed a high coefficient of friction at an early stage of a rubbing test, since the extent of the formation of a surface film containing MoS₂ was insufficient to decrease the coefficient of friction. The high friction at the early stage brought about noticeable wear. On the other hand, the addition of graphite powder together with succinimide-type dispersant to the oil containing MoDTC considerably improved the friction and wear performance not only at the early stage but also at the steady stage. The added graphite powder seems to cover the part of rubbing surfaces without a film containing MoS₂ to reduce the friction and wear, not only at the steady stage after the running-in process, but also in the running-in process or at the early stage.     

Friction and wear performance of MoDTC‐containing and ester‐containing lubricants over steel surfaces under reciprocating conditions

Lubricant oil can be regarded as a complex mixture of base oils and additives, each one with its specific functions and behaviour. In this paper, the interaction of a molybdenum dialkyldithiocarbamate (MoDTC)‐based additive and combinations of a polyalphaolefin and a synthetic ester is investigated. A reciprocating ball‐on‐disc configuration was used for tribological tests. The effect of MoDTC is seen as a sharp drop in the coefficient of friction. This friction reduction is affected by the base fluid: the effect is more intense and lasts longer when the ester content is decreased. The applied normal force also affects the MoDTC effect, which is not sustainable at higher loads. Copyright © 2014 John Wiley & Sons, Ltd.     

车轮踏面不饱和聚酯摩擦控制剂的工艺优化及性能研究

以不饱和聚酯、锆英砂、萤石、石墨、二硫化钼为原料,制备一种火车车轮踏面摩擦控制剂,并探究温度、压力和时间因素对摩擦控制剂性能的影响。结果表明:在110~160℃之间,随温度升高,摩擦控制剂的固化时间减少,硬度先增大后降低;在20~70 MPa之间,随压力增大,固化时间减少,硬度先增大后降低。选定温度140~160℃,压力40~60 MPa,时间2~3 h进行正交试验,得到最优的工艺条件为:压力60 MPa,时间2 h,温度150℃。通过极差分析,得到3种因素的影响由大到小依次为压力、时间、温度。采用优化条件制备的摩擦控制剂的弯曲强度为17.13 MPa,压缩强度为25.73 MPa,硬度为65,密度为2.12 g/cm^3,摩擦因数稳定在0.3~0.4之间,有较好的抗磨性能。扫描电镜分析结果显示,摩擦控制剂中无机填料被树脂均匀包裹,其结构稳定;热重测试结果显示,摩擦控制剂200℃开始失重,800℃完全失重,满足性能要求。     

Tribological Properties of Friction Pairs in Lubricant Contaminated with Particles under High Temperature

A liquid–solid lubricant with sand particles of different sizes and concentrations is prepared in advance. The viscosity of the lubricant is measured by a capillary viscometer to determine its relationship to the concentration or size of the sand particles. The relationships between friction and concentration or size of the sand particles are also identified with a UMT2 tribometer. Results indicate that the size of sand particles plays an important role in the lubrication performance; when the size of sand particles is 1–5 μm, the friction coefficient of the liquid–solid lubricant is reduced at low concentration and low load. Contaminant concentration greatly influences the tribological behavior of such a lubricant. The failure probability of the part surface decreases with a reduction in particle concentration; moreover, a high temperature aggravates the friction and wear of this surface. The friction coefficient is 0.14 at 200°C, which is well above the friction coefficient at room temperature (0.078), and the wear volume also increases by 30% compared to the normal temperature. When the temperature is 300°C the wear volume is two times that under room temperature.     

Friction,Wear, and Aging of an Alkoxy-monolayer Boundary Lubricant on Silicon

We study the friction, wear, and aging of a model boundary lubricant, an alkoxy monolayer covalently bonded to a Si(111) surface, using an interfacial force microscope with a spherical diamond probe. The robust alkoxy bond creates a film that effectively lubricates and prevents wear of Si at stresses comparable to those found in microelectromechanical systems devices. Sliding on the monolayer over 50 nm produced friction approximately three times greater than that of sliding over molecular length scales (∼2 nm); this is attributed to deformation dynamics of the experiment. By repeated scanning over the same location, we observed wear on a level that reduces the friction by thinning and/or reordering the monolayer film.     

低温下润滑剂对微型轴承启动摩擦力矩的影响

对于微型轴承启动摩擦力矩敏感的应用场合,测试润滑剂性能的影响十分重要,介绍了一种微型轴承低温启动摩擦力矩测试装置,其可根据要求改变轴向载荷,测试结果表明:该装置室温下与用YZC-II摩擦力矩测试仪测得的结果吻合;在不同的超低温下测试了几种润滑剂对微型轴承启动摩擦力矩的影响,测得的数据有较好的重复性。     

碳纳米管在矿物油中的高温摩擦性能研究

用多功能SRV试验机考察了碳纳米管在不同试验条件下的减摩抗磨性能。试验发现,碳纳米管能明显改善矿物油在中等条件(如温度100～300℃,速度0.08～0.16m/s和负荷100～300N)下的减摩性能,但对苛刻条件如高温(500℃)和高负荷(500N)、高速(0.20m/s)下的减摩性能没有明显改善。碳纳米管对矿物油的抗磨性能的影响与试验条件有关,碳纳米管能明显改善矿物油在较为苛刻条件下的抗磨性能。