掺硅类金刚石薄膜摩擦过程的分子动力学模拟

掺硅类金刚石(Si-DLC) 薄膜表现出优异的摩擦学性能, 在潮湿空气和高温中显示出极低的摩擦系数和很好的耐磨性, 但是许多实验表明Si-DLC膜的摩擦性能受其硅含量的影响很大. 因此, 本文采用分子动力学模拟的方法分别研究干摩擦和油润滑两种情况下不同硅含量的Si-DLC膜的摩擦过程. 滑移结果表明干摩擦时DLC膜和掺硅DLC膜之间生成了一层转移膜, 而油润滑时则为边界膜. 因此干摩擦时的摩擦力明显大于油润滑时的摩擦力. 少量添加硅确实能降低DLC膜的摩擦力, 但是硅含量大于20%后对DLC膜的摩擦行为几乎无影响. 干摩擦时硅含量对转移膜内键的数量影响很大, 转移膜内CC键和CSi键都先增加后减少, 滑移结束时几乎不含CSi键.     

Effect of surface physicochemical properties on the lubricating properties of water film

Effect of surface physicochemical properties on the water film confined within a nanogap was investigated. The film thickness and friction force were measured by the Relative Optical Interference Intensity (ROII) method and a UMT-2MT tribotester. It was found that the confined water film formed the thicker lubricate film than the prediction of elastic-isoviscous lubrication theory. Experimental results indicate that the higher the solid/water interfacial energy is, the thicker lubricate film the highly viscous “interphase” water layer forms and the lower the friction force is.     

Hydrogenated diamond-like carbon film deposited on UHMWPE by RF-PECVD

In this work, investigations were conducted to analyze the properties of diamond-like carbon (DLC) film deposited on ultra-high molecular weight polyethylene (UHMWPE) by radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) at a low temperature of 50 °C. Composition and structure of the films were characterized by scanning electron microscopy (SEM) and Raman spectroscopy. Hardness and wettability of the film were tested. Tribological characterizations were carried out on a universal micro-tribometer, and reciprocating friction against ZrO₂ ball was adopted with 25% bovine serum as lubrication. Results show that DLC film was successfully deposited on UHMWPE surface by RF-PECVD and the sp³ content was about 20% in the film. The film increased the macrohardness of the substrate by about 42% and the wettability was improved too. Tribology test showed a higher friction coefficient but a much smaller wear volume after the deposition due to the surface roughening and strengthening.     

涂覆聚甲基丙酸甲酯的磁性膜外磁场作用下的往复滑动摩擦行为研究

磁性材料被广泛应用于磁记录和磁润滑等领域,聚甲基丙烯酸甲酯因其良好的介电性,能够用作磁性材料的表面涂层.本文对外磁场作用下,外加载荷和磁场强度对往复滑动的聚甲基丙烯酸甲酯/磁性薄膜双膜系摩擦性能的影响开展了研究.实验结果表明:聚甲基丙烯酸甲酯/磁性双膜体系的摩擦性能随载荷和磁场强度改变而变化;但在干摩擦和硅油润滑两种模式下,磁场对其摩擦学性能的影响规律不同.理论分析了磁场作用下磁场诱发的磁性力与摩擦副物理性质变化对摩擦力和摩擦系数的影响,与实验结果符合良好.研究结果为磁性薄膜的界面介质设计与控制提供了依据.     

不同织构形状及分布下滑动摩擦副的多目标驱动优化及敏感性分析

为改善滑动摩擦副润滑性能,引入了表面微织构技术。首先采用计算流体力学(Computational Fluid Dynamics,CFD)方法建立了四种不同织构形状下滑动摩擦副的数值计算模型,并对其流动状态进行研究,其次以承载力最高和摩擦力最低为目标,基于非支配排序遗传算法(NSGA-Ⅱ)对织构结构及分布进行多目标驱动优化,最后采用局部敏感性分析方法研究了目标参数对各设计变量的敏感度。结果表明:优化后滑动摩擦副润滑性能明显提高,最优分布参数与织构形状无关,而最优织构深度随着织构形状不同有所变化,且优化前后矩形织构型滑动摩擦副润滑性能均最优;环向距离对承载力影响程度最大,织构深度次之,而摩擦力对各设计变量均不敏感。     

Friction,adhesion and wear durability of an ultra-thin perfluoropolyether-coated 3-glycidoxypropyltrimethoxy silane self-assembled monolayer on a Si surface

The tribological properties, such as coefficient of friction, adhesion and wear durability of an ultra-thin (<10?nm) dual-layer film on a silicon surface were investigated. The dual-layer film was prepared by dip-coating perfluoropolyether (PFPE), a liquid polymer lubricant, as the top layer onto a 3-glycidoxypropyltrimethoxy silane self-assembled monolayer (epoxy SAM)-coated Si substrate. PFPE contains hydroxyl groups at both ends of its backbone chain, while the SAM surface contains epoxy groups, which terminate at the surface. A combination of tests involving contact angle measurements, ellipsometry, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) was used to study the physical and chemical properties of the film. The coefficient of friction and wear durability of the film were investigated using a ball-on-disk tribometer (4?mm diameter Si₃N₄ ball as the counterface at a nominal contact pressure of ～330?MPa). AFM was used to investigate the adhesion forces between a sharp Si₃N₄ tip and the film. This dual-layer film had a very low coefficient of friction, adhesion and wear when compared to epoxy SAM-coated Si only or bare Si surface. The reasons for the improved tribological performance are explained in terms of the lubrication characteristics of PFPE molecules, low surface energy of PFPE, covalent bonding between PFPE and epoxy SAM coupled with reduced mobile PFPE. The low adhesion forces coupled with high wear durability show that the film has applications as a wear resistant and anti-stiction film for microcomponents made from Si.     

Nanopolishing by colloidal nanodiamond in elastohydrodynamic lubrication

In this paper, the feasibility of using explosion synthesized diamond nanoparticles with an average particle size (APS) of 3–5 nm with a concentration of 1 % by weight for improving lubrication and friction in elastohydrodynamic lubrication (EHL) was investigated. Owing to the orders of magnitude increase in the viscosity of the lubricant in the EHL contact zone, diamond nanoparticles in the lubricant polish the surfaces at the nanoscale which decreases the composite roughness of contacting surfaces. The reduced composite roughness results in an increased film thickness ratio which yields lower friction. In the numerical analysis, governing equations of lubricant flow in the full elastohydrodynamic lubrication were solved, and the shear stress distribution over the fluid film was calculated. Using an abrasion model and the shear stress distribution profile, the material removal by the nanofluid containing nanoparticles and the resultant surface roughness were determined. The numerical analysis showed that in full EHL regime, the nanolubricant can reduce the composite roughness of moving surfaces. Experimental results from prior studies which exhibited surface polishing by such nanolubricants in boundary, mixed, and full elastohydrodynamic lubrication were used for comparison to the numerical model.     

Atomistic simulations of the lubricative mechanism of a nano-alkane lubricating film between two layers of Cu-Zn alloy

We describe simulations of lubrication by a hexadecane molecular lubricating film during the shearing process of a Cu-Zn alloy performed using the atomistic method. The results indicate that with increasing Zn contents, the interface slip between the alloy wall and the lubricating film first decreases and then increases, according to variations of the radius distribution function (RDF), while the interface slip reaches its lowest value of 0.12 during the shearing of CuZn30 alloy. We also discuss the relationship between interface roughness and the lubricating film. During film lubrication, the interface's roughness effectively inhibits interfacial slip. For the convex contact model, the presence of the hexadecane lubricating film reduces the interfacial contact pressure from 11.9 GPa to 8.7 GPa and the friction coefficient from 0.81 to 0.52.     

A dynamic rheological model for thin-film lubrication

In this study, the effects of the non-Newtonian rheological properties of the lubricant in a thin-film lubrication regime between smooth surfaces were investigated. The thin-film lubrication regime typically appears in Stribeck curves with a clearly observable minimum coefficient of friction (COF) and a low-COF region, which is desired for its lower energy dissipation. A dynamic rheology of the lubricant from the hydrodynamic lubrication regime to the thin-film lubrication regime was proposed based on the convected Maxwell constitutive equation. This rheology model includes the increased relaxation time and the yield stress of the confined lubricant thin film, as well as their dependences on the lubricant film thickness. The Deborah number (De number) was adopted to describe the liquid-solid transition of the confined lubricant thin film under shearing. Then a series of Stribeck curves were calculated based on Tichy's extended lubrication equations with a perturbation of the De number. The results show that the minimum COF points in the Stribeck curve correspond to a critical De number of 1.0, indicating a liquid-to-solid transition of the confined lubricant film. Furthermore, the two proposed parameters in the dynamic rheological model, namely negative slipping length b (indicating the lubricant interfacial effect) and the characteristic relaxation time λ 0 , were found to determine the minimum COF and the width of the low-COF region, both of which were required to optimize the shape of the Stribeck curve. The developed dynamic rheological model interprets the correlation between the rheological and interfacial properties of lubricant and its lubrication behavior in the thin-film regime.     

计入固液界面作用的润滑热力学模型与分析

摩擦与润滑过程是典型的能量耗散过程, 在机理上与非平衡热力学中的熵增、耗散结构等理论颇有相似之处. 通过热力学分析可以对一些典型的摩擦磨损过程做出合理的机理揭示与推测. 本文利用热力学理论对典型的润滑过程进行了建模分析. 采用分离压模型表征和计入了微尺度下的固液界面作用, 揭示分析了润滑热力学模型与润滑状态Stribeck曲线的联系. 从分析计算结果来看, 润滑Stribeck曲线的摩擦系数最低点与系统热力学上的熵增率最低点具有相当好的对应关系, 而润滑状态从弹流润滑向薄膜润滑的转变过程, 可以用耗散结构理论加以机理解释. 文中的热力学模型和方法能够有效地体现出润滑过程中多物理要素跨尺度非线性耦合的作用, 对实际工程与实验有着重要的指导作用.     

Preparation and tribological tests of thin fluoroorganic films

Adhesion, friction and consequent wear of sliding surfaces are the basic problems that limit the performance and reliability of microelectromechanical devices. Lubrication of these nano- and microscale contacts is different from traditional lubricants. Self-assembled monolayers (SAMs) chemically bonded to the substrate are considered to be the best solution of lubrication. The majority of these monolayers are hydrophobic providing low friction, adhesion and wear.Chemical vapor deposition was used to grow a fluorosilane film on silicon Si(1 0 0) and a condensed monolayer of 3-mercaptopropyltrimethoxysilane (MPTMS) on Au(1 1 1). The films were characterized by means of a contact angle analyzer for hydrophobicity, and time-of-flight secondary ion mass spectrometry (ToF-SIMS) for identification of thin fluoroorganic monolayers deposited on silica surfaces and condensed monolayer MPTMS. Adhesion and friction measurements were performed using atomic force microscopy (AFM) and compared with measurements performed using a microtribometer operating in millinewton (mN) normal load range. Nanotribological measurements indicated that silica and MPTMS modified by fluorosilanes have the lowest friction coefficient and indicated a decrease friction coefficient with increasing fluoric alkyl chain length.     

Study on effect of dimples on friction of parallel surfaces under different sliding conditions

The influence of rectangle dimples with flat bottom on the friction of parallel surfaces at different sliding conditions is investigated based on lubrication equations. The elastic deformation of rough surfaces is evaluated using continuous convolution fast Fourier transform (CC-FFT). The friction coefficients for dimpled and non-dimpled parallel surfaces by simulation are compared with experimental results. Results show that this kind of dimples can reduce the friction coefficient for cases with the smaller ratio of film thickness to roughness (h/R_q), small roughness or large applied load. The friction force for the parallel surfaces can decrease due to the dimple effect over the range of the larger sliding speed, larger load or smoother surfaces.     

New slip regimes and the shape of dewetting thin liquid films

We compare the flow behavior of liquid polymer films on silicon wafers coated with either octadecyl-(OTS) or dodecyltrichlorosilane (DTS). Our experiments show that dewetting on DTS is significantly faster than on OTS. We argue that this is tied to the difference in the solid/liquid friction. As the film dewets, the profile of the rim advancing into the undisturbed film is monotonically decaying on DTS but has an oscillatory structure on OTS. For the first time we can describe this transition in terms of a lubrication model with a Navier-slip condition for the flow of a viscous Newtonian liquid.     

Studies on tribology

In high speed rotating machines such as turbines and generators, vibrations of a rotating shaft often hinder the smooth operation of the machine or even cause failure. Oil whip is one of such vibrations due to oil film action of journal bearing. Its mechanism and preventive method is explained and proposed in this paper. Further theoretical and experimental analyses are made for considering heat generation and temperature rise in hydrodynamic lubrication. The usefulness of the lubrication theory based on the k-epsilon model is also shown for bearings with high eccentricity ratios. In the latter half of this paper, water lubrication, nitrogen gas lubrication and tribo-coated indium lubrication are shown as new promising methods, and their mechanisms are discussed and the importance of tribo-layer is explained. Some mechanisms of wear are introduced for better understanding of tribo-layer. In the last part of this paper, the mechanisms of generating static friction are shown for the cases of plastic contact and elastic contact, which is the base for understanding the mechanism of initiation of macroscopic sliding.     

Microstructure, chemical bonds, and friction properties of nanocrystalline diamond films deposited in two different plasma media

Nanocrystalline diamond films with the properties dependent on the composition of the gaseous medium have been prepared using the microwave plasma enhanced chemical vapor deposition (MPECVD) method. A nanocrystalline film formed in the Ar/CH₄ plasma is characterized by a high crystallinity factor, a small grain size, a large fraction of sp ²-amorphous carbon, and, consequently, by an increase in the hardness and elastic modulus. The low value of the friction coefficient of this film is associated with the small grain size and large fraction of the sp ²-amorphous carbon boundary phase that ensures an easy sliding. The contact angle of the film is small (hydrophilic properties) in the case when the plasma consists of an Ar/CH₄ mixture. It has been shown that the wetting properties of the films are provided by a thin layer of carboxyl and hydroxyl functional groups passivating the dangling bonds at the surface that are responsible for the boundary lubrication mechanism. It has also been found that the friction coefficient of these films is inversely proportional to the contact pressure dependent on the diameter of the sliding counterbody ball.     

铁基合金激光熔覆层高温润滑磨损性能

 为提高40Cr合金钢的表面耐磨性,采用预置激光熔覆法在40Cr基体表面制备铁基合金涂层, 利用扫描电镜观察分析熔覆层显微组织形貌,用显微硬度仪测试熔覆层截面显微硬度,用摩擦磨损试验机测定在润滑条件下基体、熔覆层的摩擦系数随温度变化的规律。研究结果表明:熔覆层与基体实现良好冶金结合,熔覆层横截面微观组织呈现平面晶、树枝晶和胞状晶分布;熔覆层硬度值介于617.5~926.6 HV0.2之间,基体硬度介于205.2~278.2 HV0.2之间;在200 ℃以下,熔覆层摩擦系数在磨程中趋于平稳,在0.1附近轻微波动,小于基体平均摩擦系数;当温度超过200 ℃,油膜分解,引发润滑失效,磨损方式向干摩擦转化,磨损机理从微切削磨损主导向粘着磨损、磨粒磨损和氧化磨损复合磨损方式转化。     

Microstructure and tribological properties of WS2/MoS2 multilayer films

In this paper, a novel method, namely, magnetron sputtering and low temperature ion sulfurizing combined technique was used to fabricate the solid lubrication WS₂/MoS₂ multilayer films. Scanning Electron Microscopy (SEM) was used to observe the surface and worn scar morphologies. X-ray diffraction (XRD) was utilized to analyze the phase structure. The nano-hardness and elastic modulus of WS₂/MoS₂ multilayer films were surveyed by the nano-indentation tester. The friction and wear test were conducted on a ball-on-disk wear tester under dry sliding condition. The results obtained showed that the WS₂/MoS₂ multilayer films exhibited a lower friction coefficient and better wear-resistance when compared with single WS₂ film and original 1045 steel.     

Noise Reduction in Spur Gear Systems

This article lists some tips for reducing gear case noise. With this aim, a static analysis was carried out in order to describe how stresses resulting from meshing gears affect the acoustic emissions. Different parameters were taken into account, such as the friction, material, and lubrication, in order to validate ideas from the literature and to make several comparisons. Furthermore, a coupled Eulerian–Lagrangian (CEL) analysis was performed, which was an innovative way of evaluating the sound pressure level of the aforementioned gears. Different parameters were considered again, such as the friction, lubrication, material, and rotational speed, in order to make different research comparisons. The analytical results agreed with those in the literature, both for the static analysis and CEL analysis—for example, it was shown that changing the material from steel to ductile iron improved the gear noise, while increasing the rotational speed or the friction increased the acoustic emissions. Regarding the CEL analysis, air was considered a perfect gas, but its viscosity or another state equation could have also been taken into account. Therefore, the above allowed us to state that research into these scientific fields will bring about reliable results.     

Investigation on friction and wear behaviors of FeS films on L6 steel surface

Low temperature ion sulfuration technique was utilized to prepare the solid lubrication iron sulfide (FeS) film on the surface of die-steel AISI L6. The friction and wear behaviors of sulfurized L6 steel were investigated on the ring-on-block tester of MM-200 under dry sliding condition. Atomic force microscopy and scanning electron microscopy were adopted to analyze the morphologies and compositions of surfaces and wear scars of the FeS film. X-ray diffractometer was used to study the film phase structure. Scanning Auger microprobe was employed to detect the elements distribution with depth. The results showed that the tribological properties of sulfurized L6 steel were superior to that of the unsulfurized one.     

A history of neutrons and nuclei. Part 2

The application of physical principles in some studies of lubrication and wear is described. The states of boundary and hydrodynamic lubrication are contrasted and the importance of identifying the state of lubrication of a system is emphasized. Experiments which demonstrate that the lubrication of gears and roller bearings is essentially hydrodynamic are described and the consequences of solid-solid contacts through the hydrodynamic film are discussed. The function of boundary additives such as are present in E.P. oils in mitigating the damage consequent upon solid-solid encounters is described and so is the influence upon damage of the surfaces themselves. Lastly the importance is indicated of wear experiments conducted without lubrication. Such experiments have a relevance to solid-solid encounters through hydrodynamic films and are of direct application to systems in which fluid lubrication cannot be tolerated.

So often a little oil suffices to still a squeal or to restore a neglected machine to use that sometimes, without conscious effort, the scientific interest of lubrication and wear and their economic importance pass unmarked.