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.     

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.     

Instabilities in the frictional behaviour of carbons and graphites

Both graphitic and non-graphitic carbons are widely used in a variety of tribological applications. A recent development is the use of carbons for aircraft brake discs and this has prompted an experimental investigation to determine the most important factors influencing the magnitude and stability of the coefficient of friction and rate of wear.Graphitic carbons sliding against themselves, or against metals, sometimes exhibit large and sudden transitions in friction and wear at critical conditions of sliding. These conditions are defined and the mechanisms involved are described. Carbons of low or negligible graphiticity are less prone to transitions than graphitic materials, but their friction and wear properties depend markedly on the way in which the conditions of sliding influence the structure and topography of the surface layers developed during sliding. Examples are given of typical variations of friction and wear with distance of sliding, speed and temperature. Discontinuities in friction and wear can also arise from localised surface misalignments induced either mechanically or thermally.The stability of the coefficient of friction and the rate of wear for both graphitic and non-graphitic carbons can be greatly improved by the incorporation of additives or by the presence of organic vapours in the environment. Examples are described.     

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。     

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.     

Tribological phenomena in steel-composite brake material friction pairs

The mechanism of the formation of iron layers on brake friction materials sliding over a steel surface has been investigated. The nature of the metallization of friction material specimens and the changes in the surfaces as a result of sliding were studied by optical and scanning electron microscopy, electron probe microanalysis, X-ray diffraction, thermogravimetric analysis, gas Chromatographic analysis and microhardness measurements. A mechanism for the metallization of friction linings (iron layer formation) is proposed. Models of a tribological system for a frictional brake and of the subsurface layers of composite brake materials are described.     

石油钻机盘式刹车副材料的摩擦磨损性能研究

为提高深井石油钻机盘式刹车副的摩擦学性能和使用寿命,研制开发了新型刹车盘表面堆焊材料和无石棉刹车块摩擦材料,并通过变温摩擦磨损性能实验,研究了刹车副的摩擦学性能。研究表明,刹车副具有良好的变温摩擦特性和高温抗热衰退性能,高温下的摩擦因数比较稳定;刹车块和刹车盘的磨损率均随温度的升高而增大,但刹车盘表现出相对稳定的耐磨性能。载荷对刹车副的摩擦因数影响不大,变化比较平稳;刹车块和刹车盘的磨损率均随载荷的增大而增大,但刹车块表现出相对稳定的耐磨性。刹车副的摩擦因数随滑动速度的增加而增大,并趋向平稳;但速度对刹车块和刹车盘的磨损率影响不大,变化相对稳定。研制的刹车副材料能够满足石油矿场钻机作业的要求。     

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.     

A Study of Tribological Phenomena in Friction Couple: Brake Composite Material—Steel

An investigation has been made of the mechanism of the iron layer formation on brake friction materials sliding over a steel surface. The nature of the metallization of specimens of friction materials, as well as changes in the surfaces as a result of sliding, were studied by optical and scanning-electron microscopy, electron-probe microanalysis, X-ray diffraction, thermogravimetric analysis, gas chromatograph analysis, and microhardness measurements. Based on this work and on reviews of some of the most recent results concerning the wear of friction materials, a hypothesis of the metallization of friction linings (iron layer formation) is presented. A model of a tribological system for a frictional brake as well as a model of the subsurface layers of composite brake materials is described.     

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.     

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.     

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.     

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.     

Tribological, rheological and mechanical characterization of polymer blends for ropes and nets

The tribological characterization of polymers is becoming popular due to the wide range of applications of these materials where friction and wear characteristics are relevant (e.g., packaging, medical, fishing). Microabrasion, a new tribo-testing system is particularly interesting, as tests are easy to perform and are highly reproducible.This work reports the mechanical and tribological characterization of polyethylene/polypropylene blends for filament manufacture. In view of marine related applications, microabrasion wear tests were performed under various sliding conditions—dry, aqueous and slurry. Sliding friction was also evaluated. The possible wear mechanisms and the polymers systems having better tribological performance were identified.     

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。     

Wear mechanism of disc-brake block material for new type of drilling rig

To improve friction and wear performance and service life of the disc-brake pair material of a drilling rig, a new type of asbestos-free frictional material with better performance for disc-brake blocks is developed, and its wear mechanism is investigated by friction and wear experiments. Topography and elementary components of the brake block’s wear surface are analyzed by employing SEM and EDAX patterns, revealing its tribological behaviour and wear mechanism. When the frictional temperature is lower, the surface film of the brake block is thinner, dense, smooth with plasticity, and divided into the mixture area, Feabundant area, carbon-abundant area and spalling area. The mixture area consists of various constituents of frictional pairs without ploughing and rolling trace. The Fe-abundant area mainly consists of iron and other constituents. The carbon-abundant area is the zone where graphite and organic fibre are comparatively gathered, while the spalling area is the zone where the surface film is spalled and its surface is rough and uneven, with a loose and denuded state. During the period of high frictional temperature, the frictional surface is also divided into the mixture area, Feabundant area and spalling area. In this case, the mixture area consists of abrasive dust from friction pairs, and the surface film is distributed with crumby hard granules, exiguous oxide, carbide granules and sheared slender fibre. The Fe-abundant area is mostly an oxide layer of iron with a flaky distribution. Fracture and spalling traces as well as an overlapping structure of multilayer surface films can be easily found on the surface film. The components of the spalling area are basically the same as that of the matrix. At the beginning of wear, the hard peaks from the friction surface of the disc-brake plough on the surface of the brake block. With increasing frictional temperature, the friction surface begins to soften and expand, and oxidized wear occurs at the same time. During the high-temperature wear period, severely influenced by friction heat, obvious softening and plastic flow can be found on the friction surface of the brake block, its anti-shearing ability is weakened, and adhesive wear is intensified. Thermal decomposition of cohesive material in the brake block is simultaneously strengthened, so that constituents shed due to loss of adhesion. Organic fibre is in a flowing state and obviously generates drawing, shearing, carbonization and oxidization. In addition, thermal cracking, thermal oxidization, carbonization and cyclization of organic substances on the surface of brake block can make the friction surface produce pores or cracks, thus fatigue wear occurs.     

Wear mechanism of disc-brake block material for new type of drilling rig

To improve friction and wear performance and service life of the disc-brake pair material of a drilling rig, a new type of asbestos-free frictional material with better performance for disc-brake blocks is developed, and its wear mechanism is investigated by friction and wear experiments. Topography and elementary components of the brake block’s wear surface are analyzed by employing SEM and EDAX patterns, revealing its tribological behaviour and wear mechanism. When the frictional temperature is lower, the surface film of the brake block is thinner, dense, smooth with plasticity, and divided into the mixture area, Fe-abundant area, carbon-abundant area and spalling area. The mixture area consists of various constituents of frictional pairs without ploughing and rolling trace. The Fe-abundant area mainly consists of iron and other constituents. The carbon-abundant area is the zone where graphite and organic fibre are comparatively gathered, while the spalling area is the zone where the surface film is spalled and its surface is rough and uneven, with a loose and denuded state. During the period of high frictional temperature, the frictional surface is also divided into the mixture area, Fe-abundant area and spalling area. In this case, the mixture area consists of abrasive dust from friction pairs, and the surface film is distributed with crumby hard granules, exiguous oxide, carbide granules and sheared slender fibre. The Fe-abundant area is mostly an oxide layer of iron with a flaky distribution. Fracture and spalling traces as well as an overlapping structure of multilayer surface films can be easily found on the surface film. The components of the spalling area are basically the same as that of the matrix. At the beginning of wear, the hard peaks from the friction surface of the disc-brake plough on the surface of the brake block. With increasing frictional temperature, the friction surface begins to soften and expand, and oxidized wear occurs at the same time. During the high-temperature wear period, severely influenced by friction heat, obvious softening and plastic flow can be found on the friction surface of the brake block, its anti-shearing ability is weakened, and adhesive wear is intensified. Thermal decomposition of cohesive material in the brake block is simultaneously strengthened, so that constituents shed due to loss of adhesion. Organic fibre is in a flowing state and obviously generates drawing, shearing, carbonization and oxidization. In addition, thermal cracking, thermal oxidization, carbonization and cyclization of organic substances on the surface of brake block can make the friction surface produce pores or cracks, thus fatigue wear occurs.     

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.     

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.     

铜基粉末冶金/Q235B摩擦副载流摩擦磨损性能*

磁浮列车中部分制动闸片在服役时一直处于受流状态,导致材料磨损加剧,影响闸片的服役寿命。为研究中低速磁悬浮列车制动闸片在受流工况下的摩擦磨损性能,以制动闸片使用的铜基粉末冶金材料和刹车盘使用的Q235B材料为摩擦副,研究不同制动速度下铜基粉末冶金/Q235B摩擦副的载流摩擦磨损行为。结果表明：无电流时随着滑动速度的增大,摩擦因数及磨损率整体呈现下降的趋势,载流时随着滑动速度的增大,摩擦因数整体呈现下降的趋势,而磨损率则整体呈现上升的趋势;无电流时磨损后的铜基粉末冶金材料表面覆盖着一层靛色的第三体层,该第三体层低速时主要以颗粒状为主,随着速度的增加逐渐被压实成连续致密状,高速时因黏着磨损加剧使得连续致密状第三体被破环,导致材料的摩擦因数和磨损率呈现反向增长的趋势;载流下磨损后的铜基粉末冶金材料表面出现了以机械磨损为主和以电弧烧蚀为主的2个区域,其中以机械磨损为主的区域依然是由靛色的第三体层组成,而以电弧烧蚀为主的区域表面则覆盖了一层金色熔融状物质,并且随着速度的增大,烧蚀区面积也逐渐增大。