汽车制动器摩擦材料的研究现状和发展

介绍汽车制动器摩擦材料的研究现状。探讨半金属基摩擦材料、非石棉有机摩擦材料和粉末冶金摩擦材料的优缺点。提出汽车制动摩擦材料研究的发展趋势。     

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.     

Performance and evaluation of eco-friendly brake friction materials

Eco-friendly brake friction materials were formulated without copper, lead, tin, antimony trisulfide, and whisker materials, to minimize their potential negative environmental impacts. A combination of scanning electron microscopy with energy dispersive microanalysis, profilometry, and thermogravimetry allows successful analysis of friction surface and thermal stability of friction materials. Extension evaluation method was applied to rank the friction materials using multi-parameter criteria, including friction, wear, thermal stability, cost of raw materials, and parameters from the brake effectiveness evaluation procedure (BEEP) assessment. The eco-pad (sample E) exhibited the best overall quality, expressed as the weighted average dependent degree in extension evaluation.     

Neural network prediction of brake friction materials wear

Wear of brake friction materials depends on many factors such as temperature, applied load, sliding velocity, properties of mating materials, and durability of the transfer layer. Prediction of friction materials wear versus their formulation and manufacturing conditions in synergy with brakes operating conditions can be considered as a crucial issue for further friction materials development. In this paper, the artificial neural network abilities have been used for predicting wear of the friction materials versus influence of all relevant factors. The neural model of friction materials wear has been developed taking into account: (i) complete formulation of the friction material (18 ingredients), (ii) the most important manufacturing conditions of the friction material (5 parameters), (iii) applied load and sliding velocity of the friction material both represented by work done by brake application, and (iv) brake interface temperature.     

Effects of ZrSiO4 in non-metallic brake friction materials on friction performance

The effects of ZrSiO₄ (zirconium silicate or zircon) as an abrasive on brake friction performance and friction layers of non-metallic brake friction materials were evaluated. The experimental results indicated that ZrSiO₄ enhances friction coefficient, but depresses wear rate. However, ZrSiO₄ can improve the negative wear rate of the friction materials. The formation and development of friction layers are complex so that the friction layers formed during friction process were carefully characterized using scanning electron microscopy (SEM), light microscopy (LM), and X-ray diffraction (XRD) methods. Following characteristics of friction layers were identified—(1) dynamic behavior: the structure of friction layers changes at the different surfacial positions and across sample's thickness; (2) friction condition dependence: formation of friction layers depends upon temperatures, time, and thermal history such as fade and recovery; and (3) compositional dependence: the compositions of friction surface and bulk differ, nevertheless the bulk's composition determine the friction layers. The phenomena as baryte films, altered layers, iron patches, and zircon loose areas formed on the friction surfaces were observed. Baryte films were detected on the friction surfaces of Zr-0 (sample without zircon). Baryte films have positive effect on wear property, but the films disappear in the presence of ZrSiO₄. The amount of carbonaceous materials decreases with the increase in ZrSiO₄. Only negligible thickness of altered layers was found on the friction surfaces of Zr-0 sample, while samples containing zircon show out relatively thick altered layers. Both iron-patches and zircon loose areas increase with the ZrSiO₄ contents. The relationships among formulation, friction performance, and friction surfaces were summarized.     

Effects of moisture adsorption on laboratory wear measurements of brake friction materials

As is the case for tribological systems in general, the wear of brake friction materials, sliding against brake discs or drums, is influenced by many factors. This dependency requires very complicated test matrices in order to characterize the range of lining wear properties. Furthermore, the wear rate is very low under normal operating conditions, and thus a substantial amount of test time is needed before the wear progresses to reach a measurable amount. One way to reduce the test time is to use small brake material samples and to measure their weight change with an accurate electronic balance. A laboratory-scale, block-on-disc brake testing machine was used to measure the wear rate of two brake friction materials against cast iron. While conducting these tests, the lining specimen weight changes caused by moisture adsorption were found to be similar in magnitude to the weight change caused by wear. These effects were studied using weight change experiments with both desiccants and heating. Gravimetric methods for wear measurement in porous materials like brake linings are problematic. Several alternatives are offered for mitigating this problem.     

Numerical simulation of typical contact situations of brake friction materials

In the paper, a model typical for contact situations of automotive brakes is established based on the method of movable cellular automata. The processes taking place at local contacts in an automotive brake system are analysed. Based on microscopic and micro-analytical observations, the following contact situations were simulated: (i) a couple of ferritic steel against pearlitic steel, both covered by an oxide layer mixed with graphite nanoparticles and (ii) the same situation but without oxide layers. The results of calculated mean coefficients of friction of the oxide-on-oxide contact correspond well to expected values for a real braking system, whereas steel-on-steel contact are twice as high. This allows one to make some conclusions; for example, oxide formation will take place more quickly than friction layer elimination, and finally this is responsible for the stabilisation of the coefficient of friction.     

Prediction of brake friction materials recovery performance using artificial neural networks

The brake friction materials in an automotive brake system are considered as one of the key components for overall braking performance of a vehicle. The sensitivity of friction material performance and accordingly brake performance, versus different operating regimes, has always been an important aspect of its functioning. In this paper, the influences not only on the brake operation conditions but also on the formulation and manufacturing conditions of friction materials have been investigated regarding friction materials recovery performance by means of artificial neural networks. A new neural network model of friction material recovery performance, trained by the Bayesian Regulation algorithm, has been developed.     

Comparison of Chase and inertial brake Dynamometer testing of automotive friction materials

For effective and safe automotive brake system applications, friction materials must meet certain minimum requirements for performance, noise and durability. To ensure this, friction materials are subjected to a series of vehicle tests on a test track or on the road before they are released as commercial products.While vehicle tests are expensive, time consuming and subject to road conditions and weather variability, brake dynamometer testing in the laboratory is faster and less costly to screen or verify friction material characteristics. Furthermore, because of the capability to control test conditions precisely, a brake dynamometer serves as an excellent tool for the research and development of friction materials.In the U.S.A. two major types of brake dynamometers are commonly used: the inertial dynamometer which evaluates a full size brake or a brake system and simulates vehicle braking well, but is time consuming and expensive, or a smaller Chase dynamometer that features low capital expenditure and shorter test time using a small friction material sample against a large drum. The Chase dynamometer does not simulate brake conditions as well as the inertial dynamometer, and therefore is used primarily for rapid screening and/or for quality control only.The correlation, or the lack of it, between these two brake dynamometers is discussed in this paper. Friction and wear data from both test systems on several friction pairs under various temperature, sliding speed and load conditions are compared and discussed. The materials used were (a) non-asbestos organic disc pads against cast iron rotors, (b) semimetallic disc pads against cast iron rotors and (c) non-asbestos organic disc pads against copper rotors.     

The effect of Al2O3 on the friction performance of automotive brake friction materials

This study consists of two stages. In the first stage, bronze-based break linings were produced and friction-wear properties of them were investigated. In the second stage, 0.5%, 1%, 2% and 4% alumina (Al₂O₃) powders were added to the bronze-based powders and Al₂O₃ reinforced bronze-based break linings were produced. Friction–wear properties of the Al₂O₃ reinforced samples were aslo investigated and compared to those of plain bronze-based ones. For this purpose, friction coefficient and wear behaviour of the samples were tested on the grey cast iron disc. The hardness and density of the samples were also determined. Microstructures of the samples before and after the sintering and the worn surfaces of the wear specimens were examined using a scanning electron microscope (SEM). The sample compacted at 350 MPa and sintered at 820 °C exhibited the optimum friction–wear behaviour. With increase in friction surface temperature, a reduction in the friction coefficient of the samples was observed. The lowest reduction in the friction coefficient with increasing temperature was for the 2% and 4% Al₂O₃ reinforced samples. The SEM images of the sample indicated that increase in Al₂O₃ content resulted in adhesive wear. With increase in Al₂O₃ content, a reduction in mass loss of the samples was also observed. Overall, the samples reinforced with 2% and 4% Al₂O₃ exhibited the best results.     

Effect of frictional heating on brake materials

An exploratory investigation was conducted concerning materials and their properties for use in aircraft brakes. The overall objective of the program is to develop improved brake materials primarily from a safety point of view. In this investigation a study was made in order to determine what improvements could be made in present brake materials. Primary consideration was given to the heat dissipation. Used brake pads and rotor disks, taken from aircraft at overhaul were analyzed to provide additional information on material behavior.A simplified analysis was conducted in order to determine the most significant factors which affect surface temperature. Where there are size and weight restrictions the specific heat and maintaining the contact area appear to be the most important factors. A criterion is suggested for determining the number and thickness of brake disks, within the limited space available in a wheel, to provide an optimal compromise between effective surface temperature of the disk and weight. This criterion is that the quantity ($\text{αt}{}_{\max }\text{l}{}^2\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ should be approximately equal to 1 where α is the thermal diffusivity, t_max is the time at the end of the braking cycle and l is half the disk thickness.For lower surface temperatures maximum benefit can be obtained by using materials of high specific heat (and density) and by maximizing the contact area.     

盘式制动器摩擦片热-结构耦合分析

采用间接耦合法对盘式制动器摩擦片进行热-结构耦合分析,较准确的反映了整个过程中摩擦片的瞬态温度和应力变化情况.最后得到了制动过程中摩擦片受温度、制动压力、摩擦力共同作用的强度分布.为摩擦片的材料选择,尺寸设计提供了理论依据.     

A reduced-scale brake dynamometer for friction characterization

Friction behavior is a critical factor in brake system design and performance. For up-front design and system modeling it is desirable to describe the frictional behavior of a brake lining as a function of the local conditions such as contact pressure, temperature, and sliding speed. Typically, frictional performance is assessed using brake dynamometer testing of full-scale hardware, and the average friction value is then used for the remaining brake system development. This traditional approach yields a hardware-dependent, average friction coefficient that is unavailable in advance of component testing, ruling out true up-front design and leading to redundant lining screening tests. To address this problem, a reduced-scale inertial brake dynamometer was developed to determine the frictional characteristics of lining materials. Design of a reduced-scale dynamometer began with the choice of a scaling relation. In this case, the energy input per unit contact area was held constant between full-scale and reduced-scale hardware. All linear variables were thereby scaled by the square root of the scaling factor, while the pressure, temperature, sliding velocity, and deceleration were kept constant. Experimental validation of the scaling relations and the reduced-scale dynamometer focused on comparisons with full-scale dynamometer data, particularly the friction coefficient. If similar trends are observed between reduced-scale and full-scale testing, the reduced-scale dynamometer will become an important tool in the up-front design and modeling of brake systems.     

Studying thermal state of friction pairs of multidisc brake

A dimensionless heat problem for a friction pair of a multidisc brake was formulated under conditions of a linear decrease in the friction power of discs with time. Thermal state of friction pairs of the multidisc brake under load and velocity conditions of the friction of the discs that simulate service brakings of a wheel tractor was experimentally and theoretically studied. The validity of a mathematical model of the temperature field in the discs of the brake has been experimentally confirmed and functional dependences of the maximum temperature of friction surfaces of the discs on values of the main performance parameters have been obtained.     

湿式多片制动器摩擦偶件温度场的研究

用二维有限元法研究湿式多片制动器摩擦偶件温度 ,并指出其优缺点 ,然后提出了摩擦偶件三维有限元模型的分析方法 ,最后论述了该法应用存在的困难和可行性。     

风电制动片材料冲击性能及摩擦性能分析

基于对风力发电机制动片现用摩擦材料的分析,研制了纤维增强复合材料,并利用分离式霍普金森压杆试验装置和MM-1000磨粒磨损试验机,研究制动片摩擦材料的冲击性能和耐磨性能.通过对铜基粉末冶金、颗粒增强复合材料、纤维增强复合材料的应力-应变和摩擦性能比较和分析,得出纤维增强复合材料具有较好的耐冲击性能和良好的塑性性能,并且其摩擦性能高于其他两种材料,故可以取代其他两种材料成为风电制动片材料.     

The importance of static friction characteristics of brake friction couple,and methods of testing

Detailed knowledge about static friction materials is required for the accurate calculation of the braking torque needed to hold a load at rest. This is particularly important for brakes in cranes, elevators, hoists and mining winding machines, which must meet specifications such as the definite value of the static safe braking factor. The study of static friction is also a useful supplement to the dynamic testing of brake friction materials. In such a study, precise control of the temperature on the surfaces is possible, as well as surface roughness, and existence of the third-body can be accurately identified. It is an important fact that the coefficient of static friction, μ_s, is not an invariant, and it cannot be adequately represented in many engineering applications as a single number. The study of static friction dependence upon factors such as stationary contact time, rate of tangential loading, and surface temperature, contributes to a better understanding of friction phenomena. In this paper, a test apparatus is presented, and a series of experiments is described. The experiments reveal the static friction characteristics of some brake friction materials.     

无石棉制动带摩擦磨损性能预测系统的研究

以实验为基础,利用Matlab软件平台上的BP神经网络工具箱,建立并训练了无石棉制动带复合纤维优化设计中所需的各纤维组分体积百分含量与制动带摩擦磨损性能指标之间的人工神经网络映射模型。研究结果表明,该BP网络映射模型的网络误差小,映射结果与实际实验结果的吻合度高,能较为准确地预测复合纤维组成对应制动带的摩擦磨损性能指标,解决了二者关系无法用显函数表征的问题,为制动带无石棉复合纤维组成的优化设计奠定了基础。     

基于马尔可夫链的飞机刹车片检查时间确定

为结合飞机实际运行工况,科学合理地制定刹车片检查时间,笔者提出一种基于马尔可夫链理论的预测模型。以某一机群刹车片检查时间确定为实例,分别论述了刹车片磨损状态的划分,转移概率矩阵的构造与估计,并对该机群刹车片磨损状态进行了预测,其预测结果与实际检查情况较吻合。根据刹车片磨损状况的预测结果,制定了该机群在实际运行工况下的刹车片检查时间。实际运行情况表明,所制定的刹车片查检时间可行、有效。     

基于ANSYS的湿式制动器摩擦盘温度场分析

湿式制动器工作时摩擦盘与对偶钢盘之间因摩擦产生大量热量,摩擦盘应力场和温度场的研究是其重要的研究方面。运用有限元软件ANSYS,采用间接耦合,在不同的制动强度下,研究该类型摩擦盘压应力的分布情况;根据摩擦盘在不同工况下的温度分布情况,考核制动器的热负荷、研究热弹性失稳现象。研究结果表明,由于摩擦盘所受压应力的不均匀分布,导致输入摩擦盘的热流密度不均,引起热机失稳现象发生。