Effect of reinforcement volume fraction and size distribution on the tribological behavior of Al-composite/brake pad tribo-couple

Tribological behavior of aluminium matrix composite (AMC)/brake pad tribo-couple under dry sliding conditions was studied using Pin-on-Disc machine. Brake pad material was used as pins while the AMC formed the rotating disc. Series of experiments were performed to characterize the tribological nature of the tribo-couple. Load and sliding speeds were varied over a range to represent actual braking conditions in passenger cars. Effect of volume fraction and size distribution of reinforcement on wear and friction coefficient has been studied. It was observed that a heterogeneous tribo-layer was formed over the worn surfaces during the wear tests. Presence of tribo-layer was believed to cause two effects: acting as a lubricant layer and acting as a source of wear debris. Morphology and topography of worn surfaces and debris were studied using scanning electron microscope (SEM), electron probe micro analyzer (EPMA), and X-ray diffraction (XRD) techniques. When the reinforcement in the matrix has wide size distribution, wear rate and friction coefficients are found to be higher compared to composite containing mono-size reinforcement.     

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.     

Advanced Friction–Wear Behavior of Organic Brake Pads Using a Newly Developed System

The objective of this study was to investigate the influence of an advanced performance system on the tribological behavior of brake pad material using a specially designed brake pad tester system following standard SAE J-661. The tribological behavior and friction and wear characteristics of the organic brake pad samples were evaluated. During braking tests, the samples, in contact with a cast iron disk, were studied at different disc speeds, temperatures, and braking cycles under a constant pressure. In order to understand the friction and wear behavior, the unworn surfaces, worn surfaces, and wear debris were characterized by means of scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX). Furthermore, the surface characteristics and differences in the wear modes of the brake pad samples were examined. Wear debris was permitted to deform the brake pad surfaces, leading to friction layers and enabling the estimation of the friction behavior of the brake pads. The results showed that the best friction–wear behavior was obtained with lower braking cycles at low speeds and temperature. Thus, the newly developed brake pad tester system proved very effective in evaluating the performance of the brake pad samples.     

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.     

Simulation of wear and contact pressure distribution at the pad-to-rotor interface in a disc brake using general purpose finite element analysis software

Passenger car disc brakes are safety-critical components whose performance depends strongly on contact conditions at the pad-to-rotor interface. The interface can be classified as a conformal dry sliding contact. During braking both brake pad and rotor surfaces are worn, affecting the useful life of the brake as well as its behavior. This paper discusses how wear of the pad-to-rotor interface can be predicted using general purpose finite element analysis software. A three-dimensional finite element model of the brake pad and the rotor is developed to calculate the pressure distribution in the pad-to-rotor contact. A wear simulation procedure based on a generalized form of Archard's wear law and explicit Euler integration is used to simulate the wear of the brake pad under steady-state drag conditions.     

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。     

Tribological behavior of Al–Si–SiCp composites/automobile brake pad system under dry sliding conditions

Tribological behavior of stir-cast Al–Si/SiC_p composites against automobile brake pad material was studied using Pin-on-Disc tribo-tester. The Al-metal matrix composite (Al-MMC) material was used as disc, whereas the brake pad material forms the pin. It has been found that both wear rate and friction coefficient vary with both applied normal load and sliding speed. With increase in the applied normal load, the wear rate was observed to increase whereas the friction coefficient decreases. However, both the wear rate and friction coefficients were observed to vary proportionally with the sliding speed. During the wear tests, formation of a tribo-layer was observed, presence of which can affect the wear behavior, apart from acting as a source of wear debris. Tribo-layer formed over the worn disc surfaces was found to be heterogeneous in nature. Morphology and topography of worn surfaces and debris were studied using scanning electron microscope (SEM). Chemical composition of different wear products was obtained using electron probe micro analyzer (EPMA) and X-ray diffraction (XRD) techniques. Possible wear mechanisms operative in Al-MMC—brake pad tribo-couple have been discussed.     

Frictional Properties and Mechanisms of an Organic–Metal Brake Pair Braking Repeatedly in Magnetic Field

With the aim to investigate repeated braking of organic–metal brake pairs, tribological and scanning electron microscopy (SEM) experiments were performed to reveal the influence of a magnetic field on the tribological performance of brakes. A nonasbestos copper-based brake pad and gray cast iron brake disc were selected as the brake pair. The X-DM pad-on-disc friction tester was improved to set up a tribological tester under a magnetic field. The worn surfaces were observed by SEM to reveal the friction mechanisms. It was found that a magnetic field can ameliorate the dynamic friction and wear. In addition, the global mean friction coefficient increases and the wear resistance of brake materials improves. A magnetic field promotes surface oxidation and aggravates the surface heat emission condition. As a result, the mean temperature on the friction surface increases obviously. An appropriate magnetic field can improve the dynamic temperature rise and decrease the global temperature rise on the friction surface. It is considered that a magnetic field has important influences on tribological performance in repeated braking. Therefore, this research could provide theoretical references for studying the tribological performance in repeated braking and/or under a magnetic field.     

摩擦副组合对摩擦磨损性能的影响

在1：1惯性力矩制动试验台上研究了两种不同石墨形态的铸铁制动盘与两种混杂纤维增强的酚醛基制动闸片配副时的摩擦磨损性能。结果表明，对于某一配方的制动闸片，使用灰口铸铁盘的摩擦副具有较高的摩擦系数，但制动盘表面温度较高，闸片磨损量较大；对于某一种制动盘，使用B配方制动闸片时，制动盘表面的温度较高，但闸片的磨损量较小；在所有四种组合中，B配方制动闸片与灰口铸铁盘配副的瞬时摩擦系数能够完全满足有关技术要求。     

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。     

Investigation of disc/pad interface temperatures in friction braking

Maintaining appropriate levels of friction interface temperature is important for the overall operating effectiveness of modern friction brakes, and implicitly the safety of the vehicle. Measurement and prediction of the distribution and magnitude of brake friction interface temperatures are difficult. A thermocouple method with an exposed hot junction configuration was used for interface temperature measurement, and the magnitude and distribution of the friction interface temperature were investigated in this study. Using a designed experiment approach, the factors affecting the interface temperature, including the number of braking applications, sliding speed, braking load and type of friction material were studied. It was found that the number of braking applications had the strongest effect on the friction interface temperature. The real contact area between the disc and pad, i.e. pad regions where the bulk of the kinetic energy is dissipated via friction, had a significant effect on the braking interface temperature. For understanding the effect of real contact area on local interface temperatures and friction coefficient, finite element analysis (FEA) was conducted, and it was found that the maximum temperature at the friction interface does not increase linearly with decreasing contact area ratio. This finding is potentially significant in optimising the design and formulation of friction materials for stable friction and wear performance.     

Coefficient of Friction Measurements for Thermoplastics and Fibre Composites Under Low Sliding Velocity and High Pressure

Friction materials for typical brake applications are normally designed considering thermal stability as the major performance criterion. There are, however, brake applications with very limited sliding velocities, where the generated heat is insignificant. In such cases it is possible that friction materials which are untypical for brake applications, like thermoplastics and fibre composites, can offer superior performance in terms of braking torque, wear resistance and cost than typical brake linings. In this paper coefficient of friction measurements for various thermoplastic and fibre composite materials running against a steel surface are presented. All tests were carried out on a pin-on-disc test-rig in reciprocating operation at a fixed sliding speed and various pressure levels for both dry and grease lubricated conditions. Moreover, a generic theoretical framework is introduced in order to interpret the changes of friction observed during the running-in phase.     

涡轮式粘性制动器结构优化仿真研究

黏性制动器也称液黏制动器,相比于传统的干式摩擦制动器以及湿式摩擦制动器,黏性制动器由于将制动时产生的热通过内部液体转动耗散,且转子与定子间相隔距离较大,因此其散热较为均匀,且对转子和定子的磨损较小,制动器使用寿命较大。研究的目的在于明确涡轮式黏性制动器的制动过程影响因素。通过对现有的涡轮黏性制动器产品进行结构分析,建立一个合理的简化模型;通过涡轮式黏性制动器工作原理,推导其制动力矩公式;通过改变移动挡板位置转子输入转速分析各个工况对应的制动力矩;通过比较各个工况下的制动力矩以及制动器内部压力场、气穴分布提出一个较优的制动配置方案,有利于促进对于涡轮式黏性制动器的深入研究。     

金属陶瓷闸片配对碳陶制动盘的摩擦副1:3台架试验

按《动车组闸片暂行技术条件》TJ/CL307-2019中C6程序,对金属陶瓷闸片配对碳陶制动盘的摩擦副在1:3台架机上进行摩擦磨损试验。试验结果表明,金属陶瓷闸片配对碳陶制动盘的摩擦副具备了基本摩擦学适配性、制动稳定性和抗磨性,特别是金属陶瓷闸片耗量为0.024cm³/MJ,表现出优异的耐磨性。     

Tribological properties of ultrananocrystalline diamond films in various test atmosphere

Transformation from higher to ultra low friction coefficient was observed in ultrananocrystalline diamond film (UNCD) while changing the test atmospheric conditions. High friction coefficients were observed in dry argon and nitrogen atmosphere, however, low and ultra low friction coefficients were obtained in dry oxygen and in ambient atmospheric conditions, respectively. Wear rates follow the same trends as the friction coefficients. This fascinating behavior of friction and wear of UNCD film is explained by the chemical changes of sliding surfaces and extent of passivation of dangling covalent bonds.     

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.     

制动过程中闸片材料与制动盘温度关系试验研究

为探讨碳/陶制动盘与不同闸片材料的匹配性,对碳/陶制动盘分别与碳/陶复合闸片、铜基粉末冶金闸片和铁基粉末冶金闸片组成的摩擦副进行制动试验,研究了在制动过程中盘面各点瞬时温度、最高温度、闸片温度与制动工况的关系。结果表明：碳/陶制动盘与碳/陶复合闸片摩擦副温度及温度梯度均高于其他2种摩擦副,其温度梯度在低速制动时随压力的增加而明显增加,当制动速度较高时,温度梯度并没有随压力的增加而增加;对于碳/陶制动盘与铜基和铁基粉末冶金闸片摩擦副,随制动速度和压力的提高,盘面温度梯度变化不明显。原因在于材料导热性和起始摩擦因数决定了盘面的散热能力和制动功率,碳/陶制动盘与碳/陶复合闸片摩擦副因较高的起始摩擦因数以及较低的导热性,其制动功率高和散热能力低,导致盘面温度持续升高。     

Simulation of Temperature Distribution in Disk Brake Considering a Real Brake Pad Wear

This paper presents a methodology for the modeling of the transient thermal behavior of the disk brake of the vehicles using finite element methods. The influence of the wear properties of friction materials on thermoelastic behaviors is investigated to facilitate the conceptual design of the model. The coupled characteristics of the friction heat flow between the disk and pad as well as the effects of the brake disk thermal stress because of the variable applied pressure was considered. At the same time, the model was optimized by the experiment. Repeated brake processes with varying load, sliding speeds and temperature are applied in the simulation of the disk brakes. Experimental dependencies of the coefficient of friction and wear rate on the temperature of brake pad were approximated and applied to the model. The temperature and pressure on the contact surface of the pad/disk brake system obtained for constant and speed/pressure sensitivity applications were confronted and compared. The thermo-distribution is operated to visualize the disk temperature.     

Friction and Wear Behaviour of Brake Pads Dry Sliding Against Semi-Interpenetrating Network Ceramics/Al-alloy Composites

Semi-interpenetrating network composites containing 40 vol.% ceramics (5Al₂O₃·8SiO₂) and 60 vol.% Al-alloy were fabricated in place of cast iron available for automotive brake rotors. The friction and wear performances of brake pads dry sliding against the composites were measured using a SRV testing machine. The test procedures include friction fade and recovery, load sensitivity at 100 and 250°C, and wear. The friction was found to increase first and then decrease with increasing temperature, followed by the inverse recovery upon cooling. Wear showed an incremental tendency over a wide temperature range. For loads from 40 to 160 N, the friction decreased at 100 and 250°C. At load below 128 N, the former friction was inferior to the latter while at load above 128 N the friction exhibited an inverse tendency. Wear mildly increased with load at 100 °C and decreased dramatically at 250 °C. SEM and EDS investigations revealed that the worn pad surfaces at 250 °C were covered by more tribofilms, including more coke and graphite with friction-reducing action as well as fewer compounds (corresponding to Si and Al) with friction-increasing action in comparison with those at 100 °C. The compression of the tribofilms contributed to a large decrease in the friction and wear with increasing load. However, at 100 °C E-glass fibers exposed at the worn surfaces inhibited the excessive wear of the pad despite lack of more tribofilms. Their glossy surfaces decreased the friction. The proposed friction models explain some friction and wear behaviour better.     

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。