Morphology and Toughness of Abrasive Particles and Their Effects on the Friction and Wear of Friction Materials: A Case Study with Zircon and Quartz

This study examined the friction and wear of brake friction materials containing two different abrasives: zircon and quartz. Commercial grade abrasives with two different sizes (fine and coarse) were compared in terms of the effects of the size, shape, and toughness of the abrasive particles on the friction and wear of the friction material and counter discs. The results showed that the morphology of the abrasives has a considerable effect on the friction effectiveness and wear of the friction couple. The level of friction was higher in the case of using quartz than zircon, and smaller particles were more effective in increasing the coefficient of friction. The toughness of the abrasives also played important roles in determining the friction effectiveness. Improved heat resistance at elevated temperatures was achieved when coarse zircon was used. The wear of the friction material was also dependent on the morphology and toughness of the abrasives and the large abrasive particles produced more wear on the gray iron disc.     

Tribological Properties of Potassium Titanate in the Brake Friction Material; Morphological Effects

The tribological characteristics of brake friction materials containing different shapes of potassium titanate were investigated. They contain typical ingredients of a non-asbestos organic based friction material, including potassium titanate in the shapes of whiskers, platelets, and splinters. A Krauss type friction tester is used to obtain thermal stability and wear resistance of the friction materials at elevated temperatures. The results showed that the morphology of potassium titanate plays an important role in the formation of contact plateaus and transfer films on the rubbing surfaces, which are closely associated with tribological properties. The friction material with splinter shape potassium titanate shows better friction stability and improved wear resistance compared to those containing other types of potassium titanate due to larger contact plateaus and stable friction films at the sliding interface. On the other hand, the transfer films produced by the friction materials with platelet or whisker potassium titanate are not sustainable at elevated temperatures since they are easily detached during sliding, resulting in poor wear resistance.     

旋转滑动摩擦高频噪声产生机理的实验研究

为了深入研究金属旋转滑动摩擦高频噪声的产生机理,以多功能摩擦磨损试验机为平台,采用单因素实验法来研究相对滑动速度、接触压力、表面形貌等影响因素对摩擦高频噪声的影响规律。实验发现：金属旋转滑动高频摩擦噪声多发生在相对滑动速度低、接触压力较大以及摩擦因数-速度负斜率处;摩擦副间相对滑动速度、接触压力和表面粗糙度的变化几乎不会对高频噪声的频率产生影响,但高频噪声声压级随表面粗糙度的增大呈现明显减小趋势。对摩擦噪声声压信号和法向、切向振动加速度信号进行互相关分析,结果表明,法向振动加速度信号和摩擦噪声声压信号之间具有更高的相关性,从而为进一步理解摩擦高频噪声的产生机理和预测模型的建立提供了参考。     

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.     

Tribological Characteristics of Rubber-Based Friction Materials

This article deals with the rubber-based friction materials (RBFMs) which can be used in brake system. The physico-mechanical and tribological properties of a series of fiber filled RBFMs containing steel wool and aramid pulp at different concentrations along with a fiber-free reference material were characterized. Rubber–glass transition induced at higher sliding velocities was identified based on the friction fade behavior of the RBFMs. The rubber–glass transition which is inherently originated by viscoelastic response of polymeric binder was found to be influential on the tribological properties of the RBFMs. It was revealed that steel wool increased coefficient of friction (COF) and improved friction recovery behavior at low volume percent (7.5 vol.%) but it aggravated the COF at high concentration of steel wool (15 vol.%) and severe sliding conditions because of harsh abrasive mechanism. Aramid pulp improved the fade behavior at high sliding velocities and increased COF due to formation of sticky contact patches. It was revealed that steel wool increased the wear rate while aramid pulp did not affect the wear rate significantly, contrary to the increase in the friction coefficient of RBFM. SEM analysis was proved to be useful in correlating the wear rates of composites to the topographical changes on the worn surfaces.     

Friction Instability Induced by Corrosion of Gray Iron Brake Discs

This study examined the frictional force oscillation induced by the corrosion of brake discs when two different types of brake friction materials (low-steel and non-steel types) were used. Corrosion of the disc was carried out in an environmental chamber using burnished discs to simulate disc corrosion in a parked vehicle. The thickness of the oxide layers on the discs after corrosion was examined using non-contacting distance probes, and the change in brake torque was analyzed using a single-end brake dynamometer. The results showed that the oxide thickness on the disc was affected by the friction film on the burnished disc surface, and the friction force oscillation was closely related to the removal of the oxide layers while applying the brake. The low-steel friction material removed the oxide layer faster in the early stage than the non-steel friction materials so that it produced small oscillations in the friction force. However, the low-steel friction material increased the amplitude of the friction force in the later stage of the extended brake tests due to the excessive DTV (disc thickness variation). On the other hand, the non-steel friction material produced large friction force oscillations in the early stage with the amplitude decreasing in the later stage of brake application due to removal of the oxide film.     

Friction and Wear of Fiber Composites with Abrasive Particles on Contact Surface

A friction and wear model of fiber composite material with abrasive particles on a contact surface has been proposed. The problem of a composite material friction in which fibers are perpendicular to a friction surface and a rigid counterbody separated by a layer of spherical rigid particles has been considered. The effect of the material and abrasive particles’ properties on the wear rate and surface degradation has been determined. A comparison of the obtained results with the experimental data has been performed.     

Influence of Thermal Fatigue on the Wear Behavior of Brake Discs Sliding against Organic and Semimetallic Friction Materials

ABSTRACT

In this article, brake discs are exposed to high thermal stress, causing thermal fatigue damage. The aim of this work is to study the evolution of the wear behavior of brake disc materials, such as cast iron, chromium steel, and metal matrix composites, under the influence of thermal fatigue. The brake disc specimens are heated and then cooled rapidly. Then, wear tests are carried out using a pin-on-disc-type tribometer. Organic and semimetallic friction materials are used for all wear tests. The results show that thermal fatigue affects the structure of the contact surfaces of all of the disc specimens by increasing their roughness. Furthermore, the wear rate of the friction materials increased, except a reduction of the wear rate is noted for the semimetallic friction material rubbing against cast iron. Moreover, thermal fatigue has no significant influence on the coefficient of friction. The worn surface of the metal matrix composite sliding against semimetallic friction material is characterized by abrasive and adhesive wear mechanisms.     

On the nature of tribological contact in automotive brakes

The tribological contact in automotive brakes involves dry sliding contact at high speeds and high contact forces. The commonly used organic binder-type brake pad friction materials are extremely inhomogeneous and exhibit very low bulk strengths. Despite the low strength, the specific contact surfaces that form during the use render the pads very good friction and wear characteristics. This paper gives a general view of the contact situation of organic binder brake friction materials against cast iron discs, with special emphasis on many mechanisms for contact surface variations and the corresponding variations of the coefficient of friction.     

磨屑对润滑油摩擦性能的影响

通过实验和模拟研究磨粒对润滑油摩擦性能的影响。首先通过微纳米压/划痕试验测量含磨屑润滑油的摩擦因数。同时,建立边界润滑体系模型,采用分子动力学方法模拟含磨屑润滑油膜在不同载荷下沿膜厚方向的压缩率和密度分布;对体系的上下固体壁面施加方向相反的剪切速度,计算出壁面原子的应力、摩擦力、正压力和摩擦因数;分析不同粒径磨屑的动态行为特征;通过减少润滑油分子数量,探究乏油工况下含磨屑润滑体系的摩擦性能。结果表明,润滑体系摩擦因数的模拟值与试验值一致;磨屑的存在会降低油膜的压缩率,同时在高载下磨屑的存在会对油膜的分层产生破坏,影响磨屑附近的密度分布;含小粒径磨屑的润滑体系的摩擦因数比含大粒径磨屑的润滑体系的小,表明磨粒聚集长大现象会恶化润滑油的润滑性能;磨屑在剪切过程中同时存在滚动和滑动,含小粒径磨屑的润滑体系剪切过程中表现出波动幅度更大的角速度;随着载荷的增大,磨屑角速度减小,波动幅度降低;在乏油工况下,磨屑会在剪切过程中出现变形破碎现象。     

铜基复合制动材料摩擦磨损性能研究进展*

我国高速列车的不断提速,对制动盘材料的性能提出了更高的要求。铜基复合制动盘材料由于具有高比刚度、高比强度、优良的高温性能,以及良好的摩擦磨损性能等优点,被认为是最有应用前景的制动盘材料。在介绍高速列车制动盘材料发展的基础上,进一步论述了铜基复合制动盘材料的构成组元、制备方法及发展历程;阐述了铜基复合制动盘材料摩擦磨损性能的研究现状;最后展望了铜基复合制动盘材料的发展趋势,为高性能铜基复合制动盘材料的研制提供参考。     

四种车辆制动闸瓦材料摩擦特性试验研究

使用MM-1000型摩擦试验机，在不同的压力和速度下作了4种铁路车辆制动闸瓦材料与车轮钢的摩擦试验，测试它们的制动摩擦特性。试验结果表明，闸瓦材质对制动摩擦性能有较大的影响。高磷铸铁A、B两种材料的摩擦因数比较不稳定，在制动过程中摩擦因数出现了较大的波动，而且易受制动压力和速度的影响。高分子树脂复合材料C的摩擦因数比较稳定，受制动速度的影响较小但是受压力的影响较大。高分子树脂复合材料D的摩擦因数受制动速度的影响较大，但是受制动压力的影响则较小。     

On the role of copper in brake friction materials

Copper is a major ingredient in friction materials used for automotive braking. The purpose of this study was to find out how copper contributes to good brake performance properties in addition to providing good thermal conductivity. Microstructural investigations of copper chips at the surfaces of brake pads revealed a zone of severe plastic deformation which provides high hardness, but there is also evidence of recrystallized copper nano-particles which are incorporated into friction layers as soft ingredient once detached from the pad surface. Thus copper seems to play a dual role, firstly as reinforcing element of the brake pad providing primary contact sites, and secondly as solid lubricant by contributing to the formation of a layer of granular material providing velocity accommodation between the rotating disc and fixed pad. Confirmation for this hypothesis was obtained by modelling contact sites on the nanometre scale with the method of movable cellular automata. Results show both, the similarity of steel fibres and copper macro-particles in respect to providing primary contact sites, as well as similar sliding behaviours of friction layers containing either copper or graphite as soft inclusions. Furthermore, it is shown that not only material properties, but also the concentration of solid lubricant particles in the friction layers, determine conditions for friction force stabilization and smooth sliding behaviour.     

磨粒微观滑动接触过程热效应的有限元分析

利用ANSYS有限元软件分析了磨粒与被磨损材料表面滑动接触过程中,在摩擦热和力场的耦合作用下,接触区表现出的局部温度变化、应力变化等特性。结果表明,在磨粒滑移过程中,磨粒相当于接受固定热源作用,接触区温度逐渐上升,温度存在起伏波动现象,瞬现温升最高点在磨粒接触区两侧,反映出接触状态的不连续性,接触区状态的非稳定性;被磨材料表面的各点在进入接触前、经历接触时、脱离接触时,接触区温度存在先升高再下降的变化过程,同时,接触区的应力、剪应力、接触压力也发生变化。磨粒滑动过程的热效应问题研究将有助于揭示接触过程中材料表面损伤机制。     

干摩擦条件下树脂刹车片磨损机制研究

为了优化拖缆机刹车部件的设计参数,同时进一步提高刹车片的耐磨性能,采用MPV-600型磨粒磨损试验机研究无石棉树脂摩擦片和黄铜试样与45#钢配副在干摩擦条件下的摩擦学性能,利用体式显微镜观察试样的磨损形貌并分析其磨损机制。结果表明:摩擦热引起的温升导致的硬度下降及磨损机制的改变是干摩擦条件下摩擦片磨损的主要原因;树脂刹车片的耐热性能、耐磨性能均好于黄铜试样,树脂刹车片与钢配副的摩擦因数主要是由树脂刹车片中的铜纤维材料决定的;干摩擦条件下树脂摩擦片的磨损机制是以磨粒磨损和氧化磨损为主,而黄铜试样以磨粒磨损和黏着磨损为主。     

Sliding wear and friction of electroplated and clad connector contact materials: effect of surface roughness

A modeling study with bench apparatus was conducted with connector contact materials consisting of electroplated hard gold and gold flashed palladium on nickel underplatings mated to a clad noble metal. The clad metal contact was the ‘rider’, i.e., had the smaller surface involved in sliding compared to the plated ‘flat’ surface. This configuration is consistent with good engineering practice in commercial products. It was found that although a lubricant could reduce friction significantly and virtually eliminate adhesive transfer of metal, a requirement for negligible wear, i.e., the absence of loose particles, was that the plated surface be very smooth. If this contact was rough, such as might occur due to it having a nodular nickel underplate, tool marks, or burrsm abrasive wear of the cladding occurred in proportion to the magnitude of roughness. However, when both contacts were plated, lubricated wear was low and surface roughness was not a significant factor. Unlubricated plated contacts were severely with high friction by the prow formation adhesive wear mechanism regardless of their surface roughness.     

惯性制动条件对铜-石墨材料摩擦性能及第三体的影响

采用粉末冶金技术制备铜-10%石墨烧结材料,通过GF150D型摩擦试验机,在干摩擦状态及制动压力为0.51 MPa的条件下,研究不同制动方式对材料摩擦磨损性能的影响。结果表明,采用从高速到低速分段制动方式(摩擦方式A)时,随着制动速度降低,摩擦表面形成的致密第三体破碎、剥落,机械啮合力增加,摩擦因数提高;同时,摩擦表面温度下降,基体强度提高,磨损率降低。采用从高速到低速连续制动方式(摩擦方式B)的摩擦因数和磨损量均大于摩擦方式A。     

Effects of Ceramic Fiber on the Friction Performance of Automotive Brake Lining Materials

Friction material containing aluminum-silicon fiber was prepared. The effects of the aluminum-silicon fiber content on fade, recovery, and wear properties of the friction material were studied using a friction tester with a constant speed. Morphologies of the wear surfaces were observed by scanning electron microscopy (SEM). It was found that the heat fading resistance property of friction material was clearly improved when the content of aluminum-silicon wool was more than 5 wt.%, but the property of recovery declined and the wear rate increased slightly at the same time. The wear mechanisms were adhesive and abrasive, caused by the zircon sand, for the semi-metal friction material, while the abrasive wear of hybrid fiber reinforced composites was caused by cracked ceramic fibers and zircon sand.     

苎麻纤维增强环境友好型制动摩擦材料的研究

新型环境友好制动摩擦材料以天然植物纤维苎麻、天然矿物纤维玄武岩和硅灰石为增强体,以石墨为固体润滑剂,锆英石为磨料,蛭石为降噪剂,重晶石为空间填料,腰果酚型苯并噁嗪原位增韧酚醛树脂为基体。用定速摩擦试验机研究了苎麻纤维含量对材料摩擦因数和磨损率的影响,并应用可拓理论对摩擦性能进行了评价。结果表明:苎麻纤维可降低摩擦材料的摩擦因数,降低摩擦材料在中、高温下的磨损率;苎麻纤维在制动摩擦材料中起弱固体润滑剂的作用;苎麻纤维体积分数为5.6%左右时制动摩擦材料具有最佳的摩擦因数、磨损率及综合关联度。     

A test method to investigate the tribological behaviour of friction materials under ultrasonic fretting conditions

To investigate and understand the tribological behaviour of high-frequency tribosystems such as ultrasonic motors, a specific test method is necessary. This work reports on the construction of a test machine to evaluate the friction and wear behaviour of friction materials under ultrasonic fretting conditions, as well as giving some representative experimental results. Hard/soft (steel/polymer) and hard/hard (steel/alumina, alumina/alumina) couples were studied with respect to their application as contact materials in ultrasonic motors. Investigation of friction behaviour at high frequencies showed that friction-induced vibrations lead to friction forces of much lower magnitude than predicted by quasistationary friction coefficients obtained for sliding friction. The wear behaviour is characterised by abrasive, adhesive, fatigue and oxidative mechanisms, depending on the mating materials. For polymeric friction materials, the influence of fibre reinforcement and the incorporation of PTFE as a solid lubricant were evaluated. The presence of PTFE resulted in a strong improvement of both friction and wear behaviour.