On the dry and wet sliding performance of potentially new frictional brake pad materials for automotive industry

In this work, dry and wet continuous sliding performances of newly developed four different non-commercial frictional brake pad materials (NF1, NF2, NF4, and NF5) were evaluated and compared with other two chosen commercial brake pad materials (CMA and CMB) using a small-scale tribo-tester of pad-on-disc type.Results showed that under dry continuous braking, friction coefficients for all non-commercial brake pad materials including the CMB were insensitive to the type of brake pad materials. In addition, all brake pad materials showed a slight increase in the friction coefficients (5–19%) with increasing pressure or speed. Meanwhile, the wear rates were substantially dependent on the type or ingredient of brake pad materials and the pressure. Conversely, under wet sliding condition, the friction coefficients were decreased by a factor of 2. Moreover, no evidence of HD water film could be evidenced as the measured friction coefficient values were in the order of dry friction. Thus, the wet results suggested that the friction behaviour was influenced by factors other than HD film, and the values of friction coefficient were in the range of dry friction, mixed and boundary lubrication friction. Qualitative assessment of the SEM morphologies of brake pad surfaces showed that tribofilms were easily formed in dry braking and hardly formed in wet braking. Besides, all brake pad rubbing surfaces showed contact plateaus “patches” and disintegrations of various sizes and locations depending on the braking condition. Furthermore, the removal of material was associated with either mechanical crushing action performed by entrapped wear debris or due to disintegration of plateaus which were accelerated by spraying the water.     

Contact Mechanics and Friction on Dry and Wet Human Skin

The surface topography of the human wrist skin is studied using an optical method and the surface roughness power spectrum is obtained. The Persson contact mechanics theory is used to calculate the contact area for different magnifications, for both dry and wet condition of the skin. For dry skin, plastic yielding becomes important and will determine the area of contact observed at the highest magnification. The measured friction coefficient [M.J. Adams et al., Tribol Lett 26:239, 2007] on both dry and wet skin can be explained assuming that a frictional shear stress σ_f ≈ 15 MPa acts in the area of real contact during sliding. This frictional shear stress is typical for sliding on polymer surfaces, and for thin (nanometer) confined fluid films. The big increase in the friction, which has been observed for glass sliding on wet skin as the skin dries up, can be explained as resulting from the increase in the contact area arising from the attraction of capillary bridges. This effect is predicted to operate as long as the water layer is thinner than ～14 μm, which is in good agreement with the time period (of order 100 s) over which the enhanced friction is observed (it takes about 100 s for ～14 μm water to evaporate at 50% relative humidity and at room temperature). We calculate the dependency of the sliding friction coefficient on the sliding speed on lubricated surfaces (Stribeck curve). We show that sliding of a sphere and of a cylinder gives very similar results if the radius and load on the sphere and cylinder are appropriately related. When applied to skin the calculated Stribeck curve is in good agreement with experiment, except that the curve is shifted by one velocity-decade to higher velocities than observed experimentally. We explain this by the role of the skin and underlying tissues viscoelasticity on the contact mechanics.     

水润滑橡胶/镀镍钢配副摩擦磨损机理研究

对水润滑轴承专用橡胶／镀镍钢配副分别在干摩擦、边界润滑条件下用数显式高速环块摩擦磨损试验机进行了摩擦磨损试验，并借助SEM分析了其摩擦磨损机理。结果表明：千摩擦条件下主要为粘着磨损，边界润滑条件下主要为磨粒磨损。     

Effects of Normal Load,Sliding Speed,and Surface Roughness on Tribological Properties of Niobium under Dry and Wet Conditions

The effects of normal load, sliding speed, and surface roughness on the friction and wear of high-purity niobium (Nb) during sliding without and with an introduction of water were systematically investigated. Increasing the normal load or sliding speed decreased the friction of the Nb under the both dry and wet conditions because the increased wear of the Nb decreased the interfacial shear strength between the steel ball and Nb by promoting the surface roughening and the production of wear debris. However, the Nb tested at the lowest sliding speed under the lowest normal load with water exhibited the lowest friction and wear due to the formation of oxide layer on the wear track. The friction and wear of the Nb tested under the dry condition decreased with increased surface roughness because the higher interfacial shear strength between the steel ball and smoother Nb resulted in the earlier breakdown of the native oxide layer and direct contact between the steel ball and Nb. However, increasing the surface roughness of the Nb increased its friction and wear under wet conditions, probably due to the easier breakdown of the oxide layer that formed on the rougher surface during sliding. The tribological results clearly showed that the introduction of water during sliding had a significant influence on the tribological properties of the Nb.     

车轮滚动接触疲劳与磨耗耦合关系数值模拟

滚动接触疲劳和磨耗是车轮失效的主要方式。通过三维弹性体非赫兹滚动接触理论得到接触斑内的法向、切向应力和材料上不同深度处的最大切应力分布,以CL60钢和贝氏体车轮钢为例,基于"layer"滚动接触疲劳失效模型和Zobory车轮磨耗模型,分析LM型车轮踏面和75 kg.m–1钢轨型面匹配时轮轨接触条件和车轮材质对车轮滚动接触疲劳和磨耗竞争关系的影响。计算结果表明,摩擦因数为0.3时,CL60钢在小蠕滑条件下会发生滚动接触疲劳损伤,在大蠕滑条件下只有轴重大于30 t时才会出现滚动接触疲劳损伤,而贝氏体车轮钢只有在大蠕滑条件且轴重为30 t时,载荷循环次数小于1×105的情况下才会出现滚动接触疲劳损伤;摩擦因数为0.6时,CL60钢和贝氏体车轮钢在各种工况下的滚动接触疲劳损伤速度都小于相同条件下的磨耗速度。     

Friction and wear thresholds of alumina-chromium steel pairs sliding at high speeds under dry and wet conditions

Friction and wear of alumina sliding against two chromium steels and against itself under dry and wet conditions are reported. Tests were performed using a pin-on-disc device. Loads and speeds, respectively, ranged from 5 to 300 N and 3 to 10.7 m/s. The evolution of the friction coefficient and wear rates point to the existence of a load threshold correlated with the sudden degradation of the ceramic under both wet and dry conditions. Below the threshold, the ceramic remains undamaged while steels exhibit a moderate wear rate. Worn surfaces show thickening metallic transfer on the ceramic and abrasion grooves on steels. Above the threshold, the ceramic damage is induced by grain boundaries failure which leads to a coarse roughness and the release of abrasive particles. The circulation of the latter increases the steel wear and reduces the metallic transfer thickness on the ceramic. Water in the contact zone significantly lowers the threshold value and the friction coefficient value. The mechanical, thermal and chemical effects of load, speed and water are discussed with regard to damage undergone by the sliding bodies. A particular emphasis is focused on the calculation of average and maximum temperatures occuring in the contact area in relation to ceramic grain boundary damage.     

Tribological Behavior of NiAl–1.5 wt% Graphene Composite Under Different Velocities

The progress in aerospace field requires a new NiAl matrix composite that can stand against wear and decrease the energy dissipation through decreasing friction. In this study, the tribological behavior of NiAl–1.5 wt% graphene composite is investigated at room temperature under a constant load of 12 N and different sliding velocities. The results show that the friction coefficient and wear rate increase with increasing sliding velocity from 0.2 to 0.4 m/s due to the adhesion between the sliding bodies and tearing of the graphene layer. The friction coefficient and wear rate tend to decrease at a sliding velocity of 0.6 m/s as a result of severe plastic deformation and grain refinement of the worn surface. However, at 0.8 m/s the friction coefficient reaches a minimum value and the wear rate increases and changes the wear mechanism to fatigue wear. It can be concluded that various wear mechanisms lead to different tribological performance of NiAl–1.5 wt% graphene composite.     

轮轨接触界面摩擦管理研究进展

列车向着高速与重载方向迅速发展,显著加剧了轮轨接触界面间的损伤。通过在轮轨接触界面进行摩擦管理能够有效地降低轮轨之间的磨损、显著提高列车的运行安全性以及降低运营成本。对轮轨接触界面摩擦管理研究现状进行综述,并介绍轮轨界面摩擦控制对轮轨作用力、黏着、磨耗、滚动接触疲劳以及振动与噪声影响的研究进展;展望了轮轨接触界面摩擦管理未来研究方向,即应针对不同应用环境和接触部位,研发合理的摩擦控制材料,以克服摩擦管理过程中对轮轨损伤及使用局限性等问题;应探究车轮踏面/轨顶面和轮缘/轨距面摩擦控制方式,严格控制摩擦材料喷涂量使两接触面不相互干扰,优化改进轮轨接触界面摩擦管理的最佳应用参数;应研发环境友好型的轮缘/轨距面润滑剂与车轮踏面/轨顶面摩擦控制剂,稳定调控轮轨接触界面的黏着特性。     

湿滑状态下轮胎路面摩擦特性的数值分析方法

为研究湿滑状态下轮胎路面的摩擦特性,以胎面橡胶和沥青路面作为研究对象,利用谐波叠加法建立三维粗糙路面模型,采用“伪”流体动力轴承作用等效反映路面水膜“密封”作用,综合使用有限元软件ABAQUS和计算流体动力学软件Fluent得到湿滑状态下橡胶与路面滑动接触时的橡胶接触压力、滞后摩擦力及路面水膜承载力,由此形成了综合兼顾橡胶材料、接触压力、滑动速度、路面形貌和路面水膜等多因素的轮胎与湿路面摩擦特性的仿真方法。通过橡胶与干-湿路面摩擦特性的变化与公开的试验对比,证明本方法的合理性和可行性,并进一步分析滑动速度、接触压力和路面特征对湿滑状态下轮胎路面摩擦特性的影响规律。研究结果为轮胎和路面的抗湿滑性设计及优化提供理论依据。     

超音速火焰喷涂WC-Co涂层耐磨性研究

利用超音速火焰喷涂（HVOF）工艺制备了WC-Co涂层，测定了涂层孔隙率、显微硬度及干摩擦磨损过程中涂层材料失重，得出涂层干摩擦因数随时间的变化关系，分析了涂层摩擦磨损机制。结果表明，WC-Co涂层致密，平均孔隙率为1．29％，显微硬度达1140HV（测试载荷2．94N），干摩擦条件下材料失重低于电镀Cr镀层2个数量级；摩擦初期，干摩擦因数迅速增加，主要磨损特征是粘结相富Co区的犁沟切削，摩擦中后期，摩擦副间实际接触面积增大，摩擦因数变化较小，磨损趋于稳定。WC-Co涂层的主要磨损机制是疲劳磨损和犁沟切削。     

Oil-Lubricated Fretting Behaviors of 304 Stainless Steels under Different Fretting Strokes and Normal Loads

The influence of oil lubrication on the fretting wear behaviors of 304 stainless steel flat specimens under different fretting strokes and normal loads has been investigated. The results proved that fretting regimes and fretting wear behaviors of 304 stainless steels were closely related to the fretting conditions. In general, the increase in normal load could increase wear damage during sliding wear. However, according to the results, a significant reduction in wear volume and increase in friction coefficient was observed when the normal load was increased to critical values of 40 and 50 N at a fretting stroke of 50 μm due to the transformation of the fretting regime from a gross slip regime to partial slip regime. Only when the fretting stroke further increased to a higher value of 70 μm at 50 N, fretting could enter the gross slip regime. There was low wear volume and a high friction coefficient when fretting was in the partial slip regime, because oil penetration was poor. The wear mechanisms were fatigue damage and plastic deformation. There was high wear volume and low friction coefficient when fretting was in the gross slip regime, because the oil could penetrate into the contact surfaces. Unlike the wear mechanisms in the partial slip regime, fretting damage of 304 stainless steels was mainly caused by abrasive wear in the gross slip regime.     

The influence of oxygen and water vapour on the friction and wear of an aluminium alloy under fretting conditions

An investigation was conducted to determine the effect of oxygen and water vapour on the friction and wear behaviour of an Al-Zn-Mg alloy under fretting conditions. Fretting wear experiments were carried out in wet air, dry air and in dry argon. In this case the peak-to-peak relative slip amplitude was varied from 20 to 260 μm to determine the critical slip amplitude of fretting wear in these environments.The experimental results indicated that the wear rates in dry air and in dry argon under macroslip conditions were almost the same and quite lower than the wear rate in wet air. This revealed that the effect of oxygen on fretting wear was not large but that water vapour accelerated the fretting wear of the aluminium alloy. The cyclically softened material due to overaging was observed below the contact surface during fretting in wet air. The mechanism involved rapid fretting wear in wet air which caused the removal of a heavily work-hardened layer as it was formed but the softened material below it was not removed.     

石墨烯/MoS2复合涂层多环境下的摩擦学性能

以MoS2作为润滑剂,以石墨烯(GE)作为润滑添加剂,采用喷涂法在GCr15钢样片表面制备不同含量的GE/MoS2复合涂层.利用HSR-2M型高速往复式摩擦磨损试验机测试涂层在干摩擦及海水环境中的摩擦磨损性能,并分析了磨痕形貌及磨损机制.结果表明:添加适量石墨烯可明显改善MoS2涂层的摩擦磨损性能,且海水环境中涂层的摩...     

The Standard Friction Test Condition between Woven Fabric and Skin in Wet States

In order to recommend a suitable friction test condition in a wet state, the friction force and its fluctuation trend between woven fabric and human skin during the fabric hanging process were investigated. It was found that the friction between fabric and skin first increased and then reached a maximum value in the second time node. Then the friction began to decrease gradually to a minimum value in a dry state. It was noted that the large drops of water on the fabric surface had already scattered and the operability was relatively strong at the second time node. Thus, this suitable relative hanging time (T = 0.2) was recommended as the standard friction test condition between woven fabric and skin in a wet state. It was also found that friction in the recommended wet state was three times that in the dry state. The friction between woven fabric made of natural fibers and skin was generally greater than that for woven fabric made of chemical fibers. Under the recommended friction test conditions, at least nine friction tests were required for each friction pair of woven fabric and human skin.     

Influence of Calf Serum on Fretting Behaviors of Ti–6Al–4V and Diamond-Like Carbon Coating for Neck Adapter–Femoral Stem Contact

Influences of newborn calf serum on the fretting behaviors of Ti–6Al–4V and diamond-like carbon coating were investigated using a fretting-wear test rig with a cylinder-on-flat contact. The results indicated that, for the Ti–6Al–4V/Ti–6Al–4V contact, the friction coefficients were high (0.8–1.2) and the wear volumes presented an increase with the increase in the displacement amplitude under dry laboratory air conditions. Under serum-liquid conditions, the Ti–6Al–4V/Ti–6Al–4V contact presented significantly larger wear volumes under the displacement of ±?40 µm; however, it presented significantly lower friction coefficients (0.25–0.35) and significantly smaller wear volumes under the displacement of ±?70 µm. For the DLC coating/Ti–6Al–4V contact, the coating response wear maps could be divided into two areas: the coating working area (low normal force conditions) and the coating failure area (high normal force conditions). In the coating working area, the DLC coating could protect the substrate with low friction, low wear volume, and mild damage in the coating. The presence of serum had a positive influence on the tribological performance of the DLC coating. Furthermore, the positive influence was more significant under larger displacement amplitudes condition.     

A proposal of wheel/rail contact model for friction control

Controlling the friction between wheel and rail is direct and very effective measures to improve the curiving performances of railway trucks, because the curving performances depend much on friction characteristics Authors have proposed a method, “fnction control”, which utilizes friction modifier (KELTRACK™ HPF) with onboard spraying system With the method, not only friction coefficient, but also fuction characteristics can be controlled as expected In this study, MBD simulation is very valuable tool to foresee the effect of the control in advance of experiment with real car And the creep characteristics of wheel/rail contact with the fuction modifier takes very important role in the simulation In this paper, authors propose a theoretical model of wheel/rail contact condition considering the creep characterstics of friction modifier, which is derived the application of principle tribological theories

     

Ultra-Low Friction with a Protic Ionic Liquid Boundary Film at the Water-Lubricated Sapphire–Stainless Steel Interface

The sapphire/AISI 316L contact has been lubricated with water, with the protic ionic liquid (PIL) bis (2-hydroxyethylammonium) succinate (MSu), and with a 1 wt% solution of MSu in water. Neat water evaporates after the running-in period to give a transition to dry contact. Neat MSu reduces the running-in period, reducing the friction coefficient to 0.09, an 88 % lower than water. Water + 1 wt% MSu reduces the running-in period, and after water evaporation, a PIL boundary film is formed, with an ultralow minimum friction coefficient of 0.0001, and a mean friction value of 0.02, a 97 % lower than that of water, and a 78 % lower than that of neat MSu. Surface interactions have been studied by 3D profilometry, FTIR, SEM-EDX, and XPS analysis.     

Experimental characterization and analysis of wet belt friction and the vibro-acoustic behavior

The robustness and noise warranty costs of rubber belts used for power transmission are directly affected by the frictional properties under varying environmental conditions. This paper presents an experimental characterization and analysis of the friction and vibro-acoustic behavior of automotive ribbed rubber belts under wet conditions. The experimental results show that the static friction under wet condition is higher than the corresponding kinetic friction by 40%-1040% for different belts; and the wet static friction is also much higher than the dry static friction. The wet kinetic friction is lower than the dry kinetic friction by about 30-40%. The occurrence of wet static friction is associated with the strong noise of the belt system. The spectrogram analysis of recorded sound demonstrates that the sound exhibits an impulsive sound pattern with broadband frequency extending to 20 kHz. In this study, the belt vibration is also measured and the spectrum results correlated with those of the sound measurement. The capillary effect, dry adhesive effect and the boundary lubrication effect are discussed based on adhesion models, which are used to correlate with experimental results and to interpret the effects of relevant parameters. The presented results are based on the start-up running of a newly developed belt-pulley test rig, which are different from some published results based on SAE Standard J2432. The test rig based on SAE Standard J2432 is actually operated as water lubricated coast-down, which is not applicable to characterizing the friction properties of belt in wet start-up running.     

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.     

钢-钢摩擦副在不同润滑和接触形式下的高温减摩性能的研究

用多功能SRV试验机评价了在干摩擦和油润滑条件下,试验参数对钢-钢摩擦副在点接触和线接触形式下的高温减摩性能的影响。结果表明,在试验范围内,随着试验负荷的增大,钢-钢摩擦副在干摩擦条件下线接触摩擦因数和点接触摩擦因数呈逐渐下降的趋势,这种下降趋势在线接触时尤为明显;但在油润滑条件下,试验负荷的增大对钢-钢摩擦副之间的线接触和点接触摩擦因数的影响不显著。在干摩擦条件下,不论是在点接触还是线接触形式下,试验速度对钢-钢摩擦副之间的高温点接触和线接触摩擦因数的影响并不显著;但在油润滑条件下,试验速度对钢-钢摩擦副的高温线接触和点接触摩擦因数的影响都比较显著,特别是在较高的试验速度时影响更为明显。